Natural compounds in the regulation of proteostatic pathways: An invincible artillery against stress, ageing, and diseases.

Humans have learned the art of extraction and effectively utilizing naturally occurring bioactive components and chemical molecules towards medical purposes for centuries. Many groundbreaking discoveries about the inherent medicinal properties of natural compounds against numerous life-threatening diseases have been awarded Nobel Prizes in the past. The mid-nineteenth century discoveries of antibiotics penicillin and streptomycin have thoroughly changed the idea of drug discovery and accelerated the pursuit of more such compounds for other medicinal purposes in the following decades. Technological advancements, the inclusion of computational approaches, and the reincarnation of the vast literature of ancient Indian and Chinese medicine have substantially assisted and overwhelmed the field of drug discovery. The recent Nobel Prize for recognizing the medical importance of avermectins and artemisinin has again pressed upon the hidden potential of the small molecule-based drug substances. In previous sections, we have discussed how the formation of inclusion bodies follows an aberrant protein aggregation. The inefficiency of cellular QC mechanisms to fight back and address the loss of proteostasis-like conditions may lead to an array of systemic and non-systemic diseases 103 . With the available knowledge and experimental evidence, it could be understood that reestablishing the lost activities of these pathways, by inducing chaperone function or enhancing the activities of proteolytic machinery (proteasome and autophagy), etc., may exhibit the tremendous potential to delay the onset of pathologies or aging-associated changes inside the organisms 110 . Results from many studies converge towards the consensus advocating for small molecule-based therapies as beneficial and low-cost strategic tools to suppress the aggregation of most kinds of disease-associated amyloidogenic aggregates 111 . Notably, many recently recognized molecules, termed as pharmacological chaperones, have a strong potential of precisely facilitating the folding and stabilization of aberrant proteins, thereby assist in restoring their native functions 112 , 113 . The bulk degradation pathway of the cells, termed as ‘autophagy’ soon after its discovery by Christian de Duve, derived its name from Greek words meaning self-eating 114 . In later years, it was found that the autophagic degradation, which was initially considered a non-specific degradation system of the cells, could also be a part of much-targeted protein degradation pathways in association with chaperones or UPS components 115 , 116 . It joins hands with the UPS and plays a balancing act of degradation with the protein synthesis and folding machinery concurrently working in the cells 89 , 117 . Many studies also suggest that autophagy and proteasome pathways may also compensate each other under different stress conditions; therefore, a few drugs suppressing UPS activity, e . g ., MG132 and lactacystin, may lead to activation of autophagic responses 118 , 119 , 120 . Contrarily, inhibition of autophagy overwhelms the cells with accumulating protein inclusions causing impairment of proteasomal degradation 121 . In truth, a clear understanding of how the two systems balance each other while protecting the cells from proteotoxic stresses is not known. Here, a comprehensive overview is provided for those molecules or drug candidates, which can bind and modulate the activity or functions of one or multiple cellular PQC machinery components. Chaperones are essential regulators of cell homeostasis, and their anomalous functioning can lead to perturbance of many normal and stress-related pathways. The primary functions that the HSP family proteins perform inside the cells are recognizing any unusual change in the cellular homeostasis, encountering spontaneous stress condition, and providing a piece of machinery to monitor and establish the structures and functioning of other cellular proteins. The term ‘chemical chaperone’ has been widely used in the past decade for a group of potentially active molecules that can stabilize cellular proteins in a non-specific way and help in reversing the mislocalization or aggregation 122 , 123 . These molecules mostly act on the proteins' active domains or sites, providing them an increased opportunity to form hydrogen, electrostatic, and van der Waals interactions and potentially stabilize the overall structure of the proteins 124 . Additionally, an array of naturally occurring substances and their derivatives have shown modulatory potential over inherent chaperoning capacity inside the cells 125 . These bioactive chemical molecules can bind and alter the structure, activity, and overall functions of the most active HSP70 and HSP90 chaperone complexes, along with many of their co-chaperones and accessory factors 126 , 127 . They also provide cushion for structural rearrangements of unfolded or misfolded proteins inside the cytosol, thus help in ameliorating the accumulation of aberrant proteins. However, the initial attempts to exploit chaperones for therapeutic purposes started with identifying the inhibitory activity of radicicol against HSP90 ATP-binding pockets 128 , 129 . It was initially used against malignant fibroblasts. Although promising, the drug failed in delivering the promises because of several pharmacokinetic challenges. The other prominent molecule in this category is a bacterial isolate geldanamycin that was later proved to be toxic to the liver 130 . In later years, advancements in the medicinal chemistry tools have led to the synthesis of many derivatives of these less successful drug candidates, e . g ., monocillin I, pochonins, 17-allylamino-geldanamycin (17-AAG), etc. 131 , 132 , 133 . Small molecules can shatter the interaction of major chaperones with their co-chaperones, thereby affecting chaperoning activities. For example, celestrol, a triterpene, and gambogic acid, a xanthonoid, can interfere with the interaction of HSP90 with its co-chaperone CDC37; while curcumin blocks HSP90–P23 binding, leading to the induction of cell death signaling pathways 134 , 135 . Other drug candidates with similar cell death-inducing effects are herbimycin A and derrubone 130 , 136 . Quercetin, one of the most studied flavonoids, shows an upstream regulation of heat-shock response inside the cells by suppressing the heat shock factor (HSF1), the major transcription factor that regulates the intracellular levels of most of the chaperones 137 . A green tree extracted molecule, epigallocatechin-3-gallate (EGCG), can also inhibit multiple chaperones, including HSP90, HSP70, and ER-resident GRP78, and suppress the growth of cancer cells 138 , 139 . Interestingly, other mechanisms of functional suppression of HSP90 are increased ubiquitination (by hypericin), destruction of chaperone cycle (by sansalvamide A), and oxidation (by tubocapsenolide A) of HSP90 itself 140 , 141 , 142 . All these can interfere with the turnover of the substrate proteins of the chaperones, thus deregulating the proteostasis balance of the cell. Modulation of HSP70 functions by myricetin and spergualin may also help suppress cancerous cells’ growth, possibly by inhibiting the ATPase activity of the chaperone 143 , 144 . Few reports further suggest the possible activation of upstream regulator HSF1 in response to drug-mediated suppression of one or the other molecular chaperones; however, more work is required to understand the feedback mechanisms involved in this mechanism 145 . A few studies have shown that geldanamycin-mediated HSP90 inhibition may, in turn, upregulate the activities of HSP70 and HSP40, which could be helpful and may benefit the neuronal cells under different stress or pathology conditions, e . g ., HD, ALS, cerebral ischemia, etc. 146 , 147 , 148 , 149 , 150 . Similarly, treatment of curcumin and withaferin A may also exert neuroprotective effects on the cells and mouse models; the effects could be due to improved activities of HSP70, HSP27, and α -crystallin chaperones 151 , 152 . A summarized overview of various such kinds of molecules of natural origin that can help in reestablishing the proteostasis inside the cells by modulating the inherent chaperoning capacity of the cell has been presented in Table 3 . Table 3 Natural molecules affecting the activities and functions of the proteasome. These molecules have been identified as the putative modulators, including inducers and inhibitors, of the three known protease activities, including chymotrypsin, trypsin, and caspase-like specificity of the proteasome subunits. The associated diseases and studied model systems are also presented in adjacent columns. Table 3 Compound Source Subunit Pathway/disease Model system Ref. Proteasomal inducers Betulinic acid Betula sp. β 5 Neurodegeneration MT4 cells 84 Canthin-6-one Ailanthus altissima β 5 Parkinson's disease Mice 85 Fatty acids Animal sources β 1 Ageing Rats 86 Harmine Peganum harmala β 1, β 2, β 5 Parkinson's disease Mice 87 Heparin Animal sources β 2 Ageing Human erythrocytes 88 Lysophospholipids Animal sources β 5 Acrosome formation Sea urchin sperm 89 Oleuropein Olea europea β 1, β 2, β 5 Ageing IMR90, WI38 cells 90 Oxyphylla A Alpinia oxyphylla β 5 Parkinson's disease Mice 91 Sulfatides Animal sources β 5 Ageing Human erythrocytes 88 Sulforaphane Brassica oleracea β 1, β 2, β 5 Neurodegeneration Mice 92 Zerumbone Zingiber zerumbet β 5 Neurodegeneration Hepa1c1c7 cells 93 Proteasomal inhibitors Microbial sources Aaptamine Aaptos suberitoides β 1, β 5 Cancer HeLa cells 94 Aclarubicin Streptomyces galilaeus β 5 Cancer Bovine pituitary 95 Agosterol C Spongia sp. β 5 Cervical carcinoma HeLa cells 96 Antiprotealide Salinispora tropica β 5 Multiple myeloma RPMI 8226 cells 97 Argyrin A Archangium gephyra β 1, β 2, β 5 Cancer HeLa, SW480 cells, mice 98 Belactosin A/C Streptomyces sp. β 5 Muscle wasting Rats 99 Carmaphycin-17 Symploca sp. β 1, β 5 Trichomoniasis Trichomonas vaginalis 100 Ciclosporine A Tolypocladium inflatum β 5 Inflammation RAW, murine brain 101 Cinnabaramides Streptomyces sp. β 5 Cancer PBMC cells 102 Cystargolide A Kitasatospora cystarginea β 5 Cancer Purified 20S proteasome 103 Dibromophakellin Phakellia flabellata β 1, β 5 Cancer HeLa cells 104 Eponemycin Streptomyces sp. β 5 Murine thymoma EL4 cells 105 Epoximycin Actinomycetes sp. β 2, β 5 Inflammation HUVEC cells 106 Fellutamide B Penicillium fellutanum β 5 Nerve injury Mouse fibroblasts 107 Glidobactins Polyangium brachysporum β 2, β 5 Cancer Phaseolus vulgaris 108 Gliotoxin Aspergillus fumigatus β 5 Cancer HeLa cells 109 Halicyclamine B Haliclona sp. β 1, β 2, β 5 Cancer HeLa cells 110 Heteronemin Hyrtios sp. β 2, β 5 Leukemia K562, Jurkat T cells 111 Lactacystin Streptomyces lactacystinicus β 1, β 2, β 5 Neuroblastoma Neuro2a 112 Lovastatin Pleurotus ostreatus β 5 Breast cancer MDA-MB-157 cells 113 Marizomib Salinospora sp. β 5 Colon carcinoma HCT-116 114 Mevastatin Penicillium citinium β 5 Neuroblastoma NBP2 cells 115 Mycalolides Mycale sp. β 5 Melanoma B-16 cells 116 Omuralide Streptomyces sp. β 5 Neuroblastoma Neuro2a 112 Palau'amine Stylotella agminata β 5 Cancer HeLa cells 104 Petrosaspongiolide M Petrosaspongia nigra β 1, β 5 Inflammation THP cells 117 Rhabdastrellic acid-A Rhabdastrella globostellata β 2, β 5 Leukemia HL-60 cells 118 Syringolins Pseudomonas syringae β 1, β 2, β 5 Cancer Phaseolus vulgaris 108 TMC-95 Apiospora montagnei β 1, β 2, β 5 Cancer HCT-116, HL-60 cells 119 Tetradehydrohalicyclamine B Acanthostrongylophora ingens β 1, β 2, β 5 Cancer HeLa cells 110 Tyropeptin A Kitasatospora sp. β 2, β 5 Cancer PC-12 cells 120 Plant products Ajoene Allium sativum β 2, β 5 Leukemia HL-60 cells 121 Apigenin Portulaca oleracea β 5 Breast cancer MDA-MB-231, mice 122 Bisbibenzyls Bryophytes β 5 Prostate cancer LNCaP cells 123 Capsaicin Capsicum annuum β 1, β 2, β 5 Prostate cancer PC-3 cells 124 Celestrol Tripterygium wilfordii β 5 Prostate cancer PC-3 cells, mice 125 Chrysin Passiflora caerulea β 2, β 5 Cancer HepG2, HL-60, A549 126 Curcumin Curcuma longa β 1, β 2, β 5 Cancer Neuro 2a cells 127 Catechin-gallate Camellia sinensis β 5 Cancer Jurkat T cells 128 Emodin Rheum palmatum β 1, β 2, β 5 Cancer HeLa cells, mice 129 Fangchinoline Stephania tetrandra β 1 Prostate cancer LNCaP, PC-3 cells 130 Genistein Glycine max β 5 Cancer LNCaP, MCF-7 cells 131 Ginsenosides Panax ginseng β 5 Cancer Pig RBCs 132 Isoginkgetin Ginkgo biloba β 1, β 2, β 5 Cancer HeLa cells 133 Kaempferol Fruits and vegetables β 5 Leukemia Jurkat T cells 134 Luteolin Cichorium endivia β 2, β 5 Cancer HepG2, HL-60, A549 126 Marchantin M Marchantia sp. β 1, β 5 Prostate cancer PC-3 cells 135 Myricetin Fruits and vegetables β 5 Leukemia Jurkat T cells 134 Pectolinarin Cirsium chanroenicum β 1, β 5 Tuberculosis M. tuberculosis 136 Physalin B Physalis angulata β 1, β 2, β 5 Colon cancer DLD-1 cells 137 PMI5011 Artemisia dracunculus β 1, β 5 Diabetes C2C12 cells, mice 138 Pristimerin Maytenus ilicifolia β 5 Prostate cancer PC-3 cells, mice 139 Quercetin Aesculus indica β 1, β 2, β 5 Atherosclerosis Rabbits 140 Resveratrol Vitis viniferae β 5 Neurodegeneration N27 cells 141 Tannic acid Caesalpinia spinosa β 5 Cancer Jurkat T cells 142 Vinblastine Vinca rosea β 1, β 2, β 5 Leukemia HL-60 cells 143 Withaferin A Withania somnifera β 5 Prostate cancer LNCaP cells, mice 144 Other natural compounds Arenobufagin Toad venom β 1, β 2, β 5 Cervical carcinoma HeLa cells 145 Natural molecules affecting the activities and functions of the proteasome. These molecules have been identified as the putative modulators, including inducers and inhibitors, of the three known protease activities, including chymotrypsin, trypsin, and caspase-like specificity of the proteasome subunits. The associated diseases and studied model systems are also presented in adjacent columns. UPS is the next line of defense in most subcellular compartments and works continuously to regulate the proteostasis inside these organelles 75 . As described previously, ubiquitination and proteasomal degradation are a kind of intracellular regulatory mechanisms that often is crucial for many cellular pathways. Therefore, any disturbances in these systems may have deleterious effects on cellular health 153 . The proteasomal system comprises several components that could be regulated by different mechanisms and may exert varying effects on cellular physiology. For example, regulating the activities of proteasomal subunits has been shown to have a direct effect on the overall cellular protein degradation scheme and the overall proteostasis 19 . Many proteasome modulators have been proposed, and a few of them are under clinical trials for diseases like cancer and neurodegeneration 154 , 155 . A plethora of naturally-derived chemicals has been reported over the years, which have shown a substantial modulation of the activities of various enzymes of the pathway. Thus, their use may enhance or suppress the proteostasis provided by these enzymes 19 , 156 . The proteasomal system is very specific in its activity and takes part in the precise regulation of the majority of physiological pathways; therefore, very tightly-controlled modulation is needed in order to exploit it for therapeutic purposes 157 , 158 . Bortezomib was the initial drug having the proteasomal inhibitory potential and has been widely used as an anticancer drug for long 159 . Later, another synthetic molecule, carfilzomib, was also approved by the U.S. Food and Drug Administration (FDA) for anti-cancer therapy 160 . Following the identification of these two FDA approved drugs, many other drugs with similar inhibitory activity against different proteolytic subunits ( β 1, β 2, and β 5) of 20S proteasome have been identified and thoroughly investigated for their therapeutic applications in many diseases 161 , 162 . Lactacystin is the most well-known natural molecule of this class that was initially reported to be effective against neuroblastoma cells and is currently one of the widely used drugs in the research 163 . Eponemycin and epoximycin specifically target chymotrypsin-like activity containing β 5 subunits of the 20S core and help in suppressing the inflammation in cancer cells 164 , 165 . Mevastatin, belactosin A, and fellutamide B are other similar bacterial isolates that have been presented with the anti-protease activity of the proteasome in different experimental model systems 166 , 167 , 168 . Fungi and marine animals are other prominent sources of many biologically active molecules having critical therapeutic properties. Many proteasomal inhibitors have been isolated from these animals also. For example, gliotoxin and cyclosporine A from fungal sources and agosterol C and aaptamine from sponges are prominent inhibitors of 20S proteases 169 , 170 , 171 , 172 . These molecules could affect one or the multiple protease subunits of the 20S core particle of the proteasome. Interestingly, the toad venom contains a compound called arenobufagin that has the potential to inhibit all three activities simultaneously 173 . An exhaustive list of such natural molecules obtained from various biological sources has been presented in the form of Table 3 . Plant-based molecules have specifically dragged lots of attention for their proteasome-modulatory activity and have been widely covered in other descriptive reviews 156 , 174 . Flavonoids make the most comprehensively explored class and have shown tremendous potential to be used in therapeutics against many diverse kinds of diseases. For example, genistein, EGCG, and physalin B have anti-cancerous roles, while pectolinarin has positive effects on tuberculosis due to its anti-inflammatory potential 175 , 176 , 177 , 178 . Apigenin, myricetin, quercetin, and luteolin are anti-atherogenic and may also help suppress tumor growth 179 , 180 , 181 , 182 . PMI5011 is an ethanolic preparation obtained from a herb, Artemisia dracunculus , and shows pathological improvements in diabetes mice 183 . Polyphenols like vinblastine, capsaicin, resveratrol, tannic acid, and curcumin 184 , 185 , 186 , 187 , 188 , along with some well-known terpenoids, e . g ., celestrol, pristimerin, etc., further adds up to the list 189 , 190 . The compounds like anthraquinones, saponins, sulfur-derivatives, and plant-derived lactones come next into this long list ( Table 3 ) of compounds with different types of inhibitory potential against β 1, β 2, or β 5 activities of proteasome. Contrary to proteasomal suppression, which is widely exploited in cancer therapeutics, enhancing the proteasomal activities could be useful in many stressful conditions and in the diseases associated with protein misfolding and aggregation. Two widely explored terpenoids, zerumbone and betullinic acid, have activated the β 5 activities and thus presented neuroprotective effects 191 , 192 . Myricetin, oleuropein, and sulforaphane are other plant-derived molecules representing the proteasomal activators that may upregulate one or multiple 20S core subunits 193 , 194 . Few other molecules were identified that might delay the aging and neurodegeneration processes by increasing proteasomal degradation of the substrate proteins. These are heparin, sulfatides, and lysophospholipids, a few metabolic byproducts or those obtained from other animal sources 195 , 196 . Unlike proteasome inhibition, the effects of proteasome activation are not widely explored and need a more rigorous investigation to identify new molecules with a positive effect on proteasome functioning and their downstream impact on protein clearance. A few recent studies have given clear insights into Parkinson's disease models that activation of proteasome function by hermine, oxyphylla A, and canthin-6-one can significantly upregulate the clearance of alpha-synuclein, the major constituent of the Lewy bodies formed in the substantia nigra 197 , 198 , 199 . Apart from protease subunits of 20S particle, many other components involved in protein ubiquitination have been looked for their applicability as a possible drug target in aging, neurodegeneration, and many other diseases. Modulation of the major enzymes involved in the ubiquitination process, e . g ., E1, E2s, E3s, and deubiquitinases (DUBs), could be a vital strategy to regulating several critical signaling and metabolism pathways 19 , 200 . E1 ubiquitin-activating enzyme is a unique protein required for the ubiquitination of all the possible cellular substrate proteins. Therefore, interfering with its activity may compromise the whole UPS and may have devastating effects 71 . However, this observation can be utilized in anticancer therapeutics as previously exemplified by hyrtioreticulins largazole, himeic acid A and panepophenanthrin 201 , 202 , 203 , 204 . The next line of drug targets is E2 ubiquitin-conjugating enzymes, which transfer ubiquitin molecules from the E1 enzymes to the E3 ligases. Not too many drugs have been identified, which can interfere with the enzymatic activities of E2; however, a few known naturally-occurring compounds are vitexin, a polyphenolic extract from Byrsonima crassifolia , and a few poriferan-derived leucettamol A, manadosterols A and B, etc. 205 , 206 , 207 . Deubiquitinases (DUBs) are a group of enzymes that are crucial for breaking down the ubiquitin chains, replenishing the ubiquitin pool of the cells, and playing regulatory roles in many biological pathways 208 , 209 . Betulinic acid and one curcumin analog are a few known inhibitors of this class of enzymes, which have shown tremendous promises as anti-cancer molecules 210 , 211 . Cruciferous vegetables have a group of compounds called isothiocyanates, which are prominent inhibitors of DUBs, and have shown significant anti-tumor properties 212 . A diterpenoid candidate, 15-oxospiramilactone, is another DUB inhibiting molecule that has a positive effect on the restoration of the mitochondrial network 213 . Interestingly, the molecules that have the potency to modulate the most diverse class of enzymes of this pathway, the E3 ubiquitin ligases, has widely been explored for specific regulation of substrates and related pathways 214 . However, some molecules may inhibit multiple E3s simultaneously. Heclin is a recently developed molecule that can suppress many HECT domain-containing E3 ligases. Additionally, a few ubiquitin variants were prepared, which have shown tremendous inhibitory potential against RING and U-box domains of the E3 ligases 215 , 216 , 217 . A line of studies proposes several natural molecules as probable drug candidates against many life-threatening diseases. Inhibition of Mdm2 by matrine at the RNA level and by berberine via self-ubiquitination mechanism are prominent examples of regulating the turnover of P53, the primary tumor suppressor protein 218 , 219 . Oroxylin-A, apigenin, and genistein are plant flavonoids that may initiate a high apoptotic response in cancerous cells 220 , 221 , 222 . Many terpenoids ( e . g ., triptolide, inulanolide, etc.), saponins, chalcones, and polyphenols extracted from plants and other natural sources have also shown promising effects against cancerous cells by inhibiting the MDM2–P53 interaction and degradation of the tumor suppressor 191 , 223 , 224 . Enhancing the functions of the anaphase-promoting complex (APC) by crosslinking CDC27 also exerts a similar effect by acting at the spindle assembly checkpoint of the proliferating cells 225 . Similarly, inducing the functioning of crucial E3 ligases like CHIP by lanosterol, a sterol molecule, as we found in our previous study, may help ameliorate the wide-spread proteotoxicity and related cellular deaths 226 . Enhancing the E3 ligase activities may elevate the clearance of accumulated proteins inside the cells, which could be a promising strategy against neurodegeneration. Recently, we found a similar effect of myricetin on the E6-AP and HSP70-mediated clearance of the substrate proteins 227 . Trehalose, an autophagy inducer, has also been shown to improve the clinical deficits caused by mutated CHIP in ataxia patient-derived fibroblasts 228 . Other studies from our group and possibly from many other labs are undergoing to identify other similar molecules with potency to modulate different E3 ubiquitin ligases so that specific molecular pathways could be targeted for disease therapeutics and drug development. The autophagic pathway was initially identified as an intracellular lysosomal degradation mechanism that targets consumed, unusable, or toxic cell material using protease enzymes present within membrane-bound organelles 114 . Autophagic clearance pathways may have many variants that select and degrade cellular proteins and debris differentially through varying mechanisms using multiple selections and targeting mechanisms using several adapters and membrane-bound receptor proteins 55 , 229 . In a way, this leads to a variety of opportunities to regulate these pathways of degradation at various points. An array of reports has shown that autophagy regulation using small natural molecules could also be achieved and used for drug discovery purposes 229 , 230 . Modulation of autophagic pathways is proposed for therapeutics against cancer and neurodegeneration in a large number of studies 231 , 232 . As shown in Table 4 , different types of proteinopathies, neurodegenerative disorders, cancers, and several systemic diseases could be targeted by derivatives of natural molecules with modulatory effects on various effectors of the autophagy pathway. Several reports could still not be included in the present article due to space restrictions. The most prominent members of this class of natural autophagy inducers are resveratrol and trehalose 233 , 234 . Both these inducers have shown the tremendous potential of relieving neurons from various stresses by reducing free radicals and degrading protein aggregates 234 , 235 . Table 4 Small natural molecules affecting the cellular autophagy pathway. A concise representation of the potential candidates that can alter the autophagic flux, increase the protein degradation or interfere with different steps of autophagosome biogenesis or lysosome fusion, therefore can target specific molecular targets and pathways that are involved in many harmful diseases. Table 4 Compound Source Target pathway Physiological condition Model system Ref. Autophagy inducers Marine/microbial products Actinonin Streptomyces sp. AMPK, mtRNA Cancers HeLa cells 146 Araguspongine C Xestospongia sp. PI3K/AKT/mTOR Breast cancer BT-474 cells 147 Chromomycin A2 Streptomyces sp. LC3 Melanoma MALME-3M cells 148 Clionamine B Cliona celata LC3 Breast cancer MCF-7 cells 149 Coibamide A Leptolyngbya sp. LC3 Glioblastoma U87-MG cells 150 Hirsutanol A Chondrostereum sp. LC3 Hepatic carcinoma Hep3B cells 151 Ilimaquinone Hippospongia sp. p53 Colon cancer RKO cells 152 Isoaaptamine Aaptos sp. LC3 Breast cancer T-47D cells 153 Monanchocin D Monanhora pulchra P38, ERK Germ cell tumors NCCIT cells 154 Ovothiol A Paracentrotus lividus Beclin-1, LC3 Hepatic carcinoma HepG2 cells 155 Papuamine Haliclona sp. LC3, JNK Breast cancer MCF-7 cells 156 Psammaplin A Psammaplysilla sp. P73 Glioblastoma U87-MG cells 157 Rapamycin Streptomyces hygropicus mTOR Polyglutamine diseases PC12, Cos7 cells 158 Rhabdastrellic acid A Rhabdastrella sp. AKT Various human cancers Hep3B, A549 cells 159 Salinosporamide A Salinospora tropica eIF2 α Prostate cancer LNCaP-Pro5 160 Stellettin B Jaspis stellifera PI3K/AKT/mTOR Lung cancer A549 cells 161 SD118-xanthocilin-X Penicillium commune MEK/ERK Hepatic carcinoma HepG2 cells 162 Trehalose Streptomyces cerevisiae mTOR Neurodegeneration SK-N-SH, PC12 cells 163 Urolithin A Gut microbiome AMPK Ageing C. elegans 164 Xestospongin B Xestospongia exigua IP 3 R Cervical adenocarcin HeLa cells 165 Plant products Terpenes Bigelovin Inula helianthus AKT/mTOR/S6K Liver cancer HepG2, mice 166 Eriocalyxin B Isodon eriocalyx AKT/mTOR/S6K Breast cancer MCF-7, MDA-MB-231 167 Gossypol Gossypium sp. Beclin-1, ATG5, Breast adenocarcinoma MCF-7, HeLa cells 168 Grifolin Albatrellus confluence AKT/mTOR/S6K Ovarian cancer A2780, SKOV3 cells 169 Oridonin Rabdosia rubescens P21 Prostate cancer PC-3, LNCaP cells 170 Platycodin-D Platycodon grandiflorum PI3K/AKT/mTOR Lung cancer NCI–H460, A549 cells 171 Triptolide Tripterygium wilfordii SQSTM1, LC3 Parkinson's disease MN9D cells, rats 172 Ursolic acid Ocimum sanctum JNK, BCL-2 Colorectal carcinomas HCT-15 cells, mice 173 Flavonoids Ampelopsin Ampelopsis sp. AKT/mTOR/S6K Breast cancer MDA-MB-231, MCF-7 174 Apigenin Fruits, vegetables mTOR, S6 Leukemia HL60, TF1 cells 175 Curcumin Curcuma longa FOXO1, beclin-1 Oxidative stress HUVEC cells 176 Delicaflavone Selaginella doederleinii AKT/mTOR/S6K Lung cancer A549, PC-9 177 5-Demethylnobiletin Sideritis tragoriganum JNK Lung cancer A549 and CL1-5 cells 178 Galangin Alpinia officinarum P53 Hepatic carcinoma HepG2 cells 179 Glabridin Glycyrrhiza glabra JNK1/2, P38, ERK Hepatoma Huh7 cells 180 Juglanin Juglans mandshurica JNK Breast cancer MCF-7 cells, mice 181 Kaempferol Fruits and berries AMPK, AKT Hepatic cancer SK-HEP-1 cells 182 Licochalcone A Glycyrrhiza sp. PI3K/AKT/mTOR Cervical cancer SiHa cells 183 Luteoloside Gentiana macrophylla AKT/mTOR/S6K Lung cancer A549, H292 cells 184 Myricetin Fruits, vegetables mTOR Hepatic carcinoma HepG2 cells 185 Quercetin Fruits and berries PI3K, beclin-1 Leukemia P39 cells, mice 186 Resveratrol Vitis viniferae SIRT1, RAB7 Oxidative stress Mice 187 Alkaloids Berberine Coptidis Rhizoma AKT/mTOR, beclin-1 Hepatic carcinoma HepG2, MHCC97-L cells 188 Capsaicin Capsicum annuum Beclin-1, LC3 Hepatic carcinoma HepG2 cells 189 Corynoxine B Uncaria rhynchophylla Beclin-1 Parkinson's disease N2a,SHSY-5Y cells 190 Fangchinoline Stephania tetrandra Sestrin2 Hepatic carcinoma HepG2 cells 191 Harmol Peganum harmala Survivin Glioma U251MG cells 192 Isorhynchophylline Uncaria rhynchophylla Beclin-1 Parkinson's disease N2a, PC12, SH-SY5Y 193 Matrine Sophora flavescens mTOR, P53 Hepatic carcinoma HepG2, SMMC-7721 194 Piperlongumine Piper longum AKT/mTOR Various cancers 786-O, PC-3, MCF7 195 Vinblastine Vinca rosea Cathepsin D Stress conditions Rat hepatocytes 196 Other natural molecules Arenobufagin Toad venom PI3K/AKT/mTOR Hepatic carcinoma HepG2 cells 197 Benzyl isothiocynate Lepidium sativum AKT, mTOR Prostate cancer Rv-1, PC-12 cells 198 Bisbibenzyls Bryophytes LC-3 Prostate cancer LNCaP cells 123 Bufalin Bufo gargarizans JNK, ATG5, beclin-1 Colorectal cancer HT-29 and Caco-2 cells 199 Cinobufagin Bufo gargarizans PARP, JNK/P38 Osteosarcoma U2OS cells 200 Concanavalin A Canavalia ensiformis LC3, BNIP3, AKT Hepatoma ML-1 cells 201 Daucosterol Smilax glabra Roxb. Beclin-1, LC-3 Breast cancer MCF-7 cells 202 Docosahexaenoic acid Metabolic intermediate NFE2L2 Neurodegeneration ARPE-19 203 Embelin Embelia ribes ATG-5, ATG-12 Oral cancer Ca9-22 cells 204 Lanosterol Metabolic intermediate CHIP Neurodegeneration Cos-7 49 Noggin Xenopus LC3, beclin-1 Acute pancreatitis AR42J cells, mice 205 Ophiopogonin B Radix ophiopogon var. PI3K/AKT/mTOR Lung cancer NCI-H157, NCI-H460 206 Polyphyllin G Paris yunnanensis AKT, MAPK Nasopharyngeal carcinoma HONE-1 and NPC-039 207 Rottlerin Mallotus philippinensis PI3K/AKT/mTOR Pancreatic cancer Cancer stem cells 208 6-Shogaol Zingiber officinale AKT/mTOR Lung cancer A549 209 Sitosterol Plant sterols P38 Sitosterolemia Mice macrophages 210 Spermidine Natural polyamine ATG7 Ageing Yeast, fly, worm, PBMC 211 Sulforaphane Brassica oleracea ERK Huntington's disease Mice 212 Autophagy inhibitors Aspargine Natural amino acid Lysosome fusion Proteopathies Rat hepatocytes 213 Cytochalasins Aspergillus sp. Microfilaments Proteopathies Rat kidney cells 214 Emodin Fallopia japonica LC3, beclin-1 Acute pancreatitis Rats 215 Estrogen Natural hormone CXCL12 Endometriosis Endometrial stromal cells 216 Leupeptin Streptomyces sp. Serine proteases Proteopathies Rat hepatocytes 217 3-Methyladenine Metabolic intermediate PI3K Proteopathies Hepatocytes 218 Pepstatin A Streptomyces sp. Aspartyl peptidases Proteopathies Rat livers, hearts 219 Vinblastine Catharanthus rosea Microtubules Proteopathies Rat fibroblasts 220 Vincristine Catharanthus rosea Microtubules Proteopathies Rat fibroblasts 220 Wortmannin Penicillium sp. PI3K Acute pancreatitis Rats 221 Small natural molecules affecting the cellular autophagy pathway. A concise representation of the potential candidates that can alter the autophagic flux, increase the protein degradation or interfere with different steps of autophagosome biogenesis or lysosome fusion, therefore can target specific molecular targets and pathways that are involved in many harmful diseases. Interestingly, autophagy plays very crucial roles in the clearance of many infectious agents, including HIV, Mycobacterium , or other parasites 236 . Triggering this pathway by vitamin D or starvation mechanisms have shown improvements in various pathological conditions, ranging from viral/bacterial infections to tuberculosis and malaria 237 , 238 , 239 , 240 . Autophagy also performs vital roles in cell metabolism and signaling, as evidenced by multiple lines of studies, which are covered in detail in several previous articles 241 , 242 . The influence of autophagy induction has been investigated in many metabolism-related disorders, including diabetes, glucose intolerance, obesity, and atherosclerosis 115 . It was evident from the past studies that modulation of autophagy may have enormous potential to counter the stress conditions and protect from several incurable diseases 243 , 244 , 245 . Likewise, the autophagy inducers, e . g ., bigelovin, oridonin, and stellettin B may accelerate the apoptotic pathways in various types of cancer cells 246 , 247 , 248 . The majority of molecules ( e . g ., cinobufagin, juglanin, ursolic acid, ampelopsin, etc.) act on the target proteins, like PI3K, AKT, mTOR, S6K, MAPK, JNK, P38, ERK, etc., which are explicitly involved in the autophagy regulation 249 , 250 , 251 , 252 . For the past many decades attempts to upregulate the autophagic degradation of large aggregates of proteins have been made, and considerable success has been achieved. The research on exogenous autophagy induction using exercise/starvation like lifestyle changes or natural molecule-based food habits has shown enormous promises to deliver in many stress-related changes like neurodegeneration and aging 253 , 254 . Use of curcumin and triptolide in oxidative stress conditions in cells and Parkinson's disease animal models have shown neuroprotective effects of these drugs via the upregulation of autophagy 255 , 256 . Docosahexaenoic acid, sulforaphane, and lanosterol are other natural inducers of autophagy, which have shown multifactorial effects in ameliorating the stress conditions of the cells and alleviate the degenerative conditions in the brain 194 , 226 , 257 . Although a vast literature is available on the induction of autophagy by small molecules, there are limited reports of inhibitors that can demonstrate beneficial effects on disease conditions. Emodin, wortmannin, and 3-methyladenine are few known autophagy suppressors with disease modulating potential 258 , 259 , 260 . A comprehensive list of naturally derived inducers and inhibitors of the autophagy pathway is prepared in Table 4 .

A battery of multifaceted enzymes is involved in the replication and transcriptional processes, exhibiting highly efficient proof-reading activity to preserve the genomic contents of the cell 9 , 10 , 11 . Similarly, in association with an array of extremely proficient molecular chaperones, a well-organized ribosomal quality control (RQC) machinery maintains the robustness of the cellular proteome 12 , 13 , 14 . Additionally, a specialized pathway of quality assurance of newly synthesized polypeptides (called ERAD) operates inside the endoplasmic reticulum and associated secretory pathways 15 . Several molecular chaperones and additional proteins get involved in these QC pathways, regulating the folding and degradation processes inside the cells and maintain a healthy and functional cellular proteome 16 , 17 . All the cellular proteins have their unique turnover rate regulated by the ubiquitin–proteasome system (UPS) that involves a few hundred E3 ubiquitin ligase enzymes to provide the substrate specificity 18 , 19 . Under some physiological conditions, the E3 ubiquitin ligases, along with few other adapter proteins, may take part in identifying and redirecting aberrant or aggregated forms of intracellular proteins to another proteolytic pathway, called autophagy, which is not as specific as UPS and is chiefly take part in the degradation of the bulk of cellular debris 20 , 21 . Similarly, heat shock proteins (HSPs) or molecular chaperones also play crucial roles in the triage of polypeptides inside the cytoplasm by switching among different quality control pathways. Here, we are providing a very brief outline of these major QC pathways in this section. An intracellular overview of these significant components of the cellular QC pathways is presented in Fig. 1 . Figure 1 A eukaryotic cell showing the major cellular components constituting the cellular protein quality control machinery. Molecular chaperones are the immediate interactors of the newly synthesized proteins, which help them to achieve their natural active conformation and provide additional opportunities during lifetime to attain the same if they lose their native conformation. The UPS is comprised of three primary classes of enzymes: E1 ubiquitin-activating enzymes, in an ATP-dependent manner, activate small ubiquitin molecules that are later conjugated to E3 ubiquitin ligases by E3 conjugating enzymes. E3 ligases provide substrate specificity to transfer these conjugated ubiquitin molecules to the aberrant proteins, unfolded, misfolded, or aggregated inside the cytoplasm and other cell compartments. The ubiquitinated proteins are subjected to proteasomal degradation. Autophagy is a bulk degradation system in which a large amount of cytoplasmic waste material is packaged in membranous structures. The membrane-bound waste material is subjected to degradation by lysosomal proteases, which is accomplished by the fusion of the membranous vesicles to the lysosomes. Figure 1 A eukaryotic cell showing the major cellular components constituting the cellular protein quality control machinery. Molecular chaperones are the immediate interactors of the newly synthesized proteins, which help them to achieve their natural active conformation and provide additional opportunities during lifetime to attain the same if they lose their native conformation. The UPS is comprised of three primary classes of enzymes: E1 ubiquitin-activating enzymes, in an ATP-dependent manner, activate small ubiquitin molecules that are later conjugated to E3 ubiquitin ligases by E3 conjugating enzymes. E3 ligases provide substrate specificity to transfer these conjugated ubiquitin molecules to the aberrant proteins, unfolded, misfolded, or aggregated inside the cytoplasm and other cell compartments. The ubiquitinated proteins are subjected to proteasomal degradation. Autophagy is a bulk degradation system in which a large amount of cytoplasmic waste material is packaged in membranous structures. The membrane-bound waste material is subjected to degradation by lysosomal proteases, which is accomplished by the fusion of the membranous vesicles to the lysosomes. Proteins are large (macro-) molecules inside the cells, which orchestrate most of the physiological and metabolic tasks and are inclusively involved in the structural organization of the cellular components. Therefore, the maintenance of their native conformations is a prerequisite for the cells to be healthy. Such a condition of a stable and healthy set of proteins is called proteostasis 22 , 23 . Chaperones are the first line of molecules that start their work immediately after the newly synthesized peptide exits from the ribosome 24 . Different classes of molecular chaperones have already been reported in various forms of life across different kingdoms, including prokaryotes and eukaryotes 25 , 26 . The de novo folding of nascent polypeptides is orchestrated by family chaperones and is accomplished by multiple cycles of ‘binding and release’ in an energy-dependent manner 27 , 28 . Folding of a proportion of proteins is governed by HSP70 and HSP40, whereas the rests of the proteins are transferred to HSP90 proteins 29 . These chaperones are also implicated in refolding and disaggregating aberrantly folded polypeptides, or unfolding and degrading aggregated proteins 30 , 31 . In fact, a large number of chaperones and chaperonins are coherently involved in the folding, refolding, and disaggregation processes of all the cellular proteins 32 , 33 . Chaperones can guide the substrate proteins towards two well-established systems of proteins degradation, i . e ., UPS and autophagy 34 . They may interact with crucial proteins implicated in these two pathways, e . g ., sequestosome-1 (SQSTM1/P62), BCL2 associated athanogene 1 or 3 (BAG1/3), carboxy-terminus of heat shock cognate 70 (HSC70) interacting protein (CHIP), next to BRCA1 gene 1 (NBR1), and several E3 ubiquitin ligases 35 , 36 . The mechanisms that are driven by chaperones in concerted action with the other pathways are chaperone-mediated autophagy (CMA), chaperone-assisted selective autophagy (CASA), and chaperone-assisted proteasomal degradation (CAP) 37 , 38 . The idea of autophagy originated in the 1960s when Christian de Duve identified lysosome, an organelle that contains hydrolytic enzymes, and got involved in removing cytoplasmic waste materials 39 , 40 . Nobel Prize in Medicine to Christian De Duve in 1974, and Yoshinori Ohsumi in 2014 for the discovery of the lysosome and detailed investigation of this degradation pathway confirm the importance of the autophagy machinery for the cells. This lysosomal degradation process targets not only the damaged organelles but also different forms of cellular proteins, either ubiquitylated or non-ubiquitylated 41 , 42 . Multiple lysosomal degradation pathways have been identified in the past with different roles and specificities; for example, the formation of a double-membrane bound structure, called the autophagosome, is a characteristic of macroautophagy that engulfs a large amount of cellular debris along with bulky proteinaceous inclusions 43 , 44 . Aggrephagy is often used to describe selective targeting of bulky protein aggregates or inclusion bodies for degradation through macroautophagy in a process facilitated by adapter proteins, like P62 and NBR1 and light chain 3 (LC3), a phagophore membrane receptor 45 , 46 . A double-membrane structure called autophagosome is formed as a result of the closure of phagophore, which is followed by fusion with late endosomal vesicle or lysosomal sacs 47 , 48 . The contents within this newly formed structure, referred to as amphisome, are degraded by various lysosomal enzymes 49 , 50 . Similar to aggrephagy, few other pathways of selective degradation of cytoplasmic proteins are orchestrated by cytosolic chaperones HSC70 along with its regulatory co-chaperones 51 , 52 . For example, microautophagy involves selective transport of cytosolic proteins to vesicles using endosomal sorting complexes required for transport (ESCRT I and III) in the HSC70-dependent manner 53 , 54 . However, the microautophagy pathway involves invagination and tube formation, followed by vesicle expansion and degradation 55 , 56 . Another highly selective proteolytic pathway is CMA that could be defined as a process of selective identification of the KFERQ motif-containing cellular proteins by HSC70 and co-chaperones 57 , 58 . The HSC70-conjugated substrates are internalized after binding to LAMP-2a (a lysosome-associated membrane protein) and later degraded by membrane-bound proteases 59 , 60 . BAG3-mediated selective degradation pathway, CASA is also governed by chaperones HSC70 and HSPB8, in concerted action with CHIP (an E3 ligase) that mediates the ubiquitination of the proteins before their disposal to the lysosomal compartment in a P62-dependent manner 61 , 62 . The ubiquitin–proteasomal pathway is a multistep process of protein degradation, in which a series of enzymes sequentially catalyze the substrate proteolysis inside a large barrel-shaped, cylindrical protein complex called proteasome 63 , 64 . The 26S proteasome is a multi-subunit complex containing a 20S core particle and one or two regulatory 19S sub-particles to regulate the entry of the ubiquitylated chains into the core 65 , 66 . The 20S core proteasome subunit contains three types of protease activities governing the cleavage of incoming polypeptides into smaller fragments 67 , 68 . Out of four heptameric rings forming the core, two inner rings, termed β -rings, contain the proteolytic activities of different types: post-glutamyl peptide hydrolase ( β 1), trypsin ( β 2), and chymotrypsin ( β 5) 69 , 70 . In the first ATP-dependent step, an E1 ubiquitin-activating enzyme activates the small 8 kDa ubiquitin molecule (Ub) and forms a thioester bond 71 , 72 . A transacylation reaction transfers this ubiquitin to the thiol group present on another class of enzymes called E2 ubiquitin-conjugating enzyme 73 , 74 . These thiol esters (ubiquitin-E2 conjugates) provide ubiquitin molecules to the third class of enzymes called E3 ubiquitin ligases for tagging the substrate proteins 73 , 75 . The C-terminus glycine of the ubiquitin polypeptide forms an isopeptide bond with one of the lysine residues present on the cellular proteins 76 . According to the long-standing notion, attachment of single ubiquitin (monoubiquitination) generally does not target substrate proteins for proteolytic pathways; however, recent advancements also oppose this belief 77 . In addition, more than one ubiquitin molecules might get attached to the substrate proteins, independently (multi-monoubiquitination) or one over the other (polyubiquitination) through lysine residues present in the already conjugated ubiquitin or the N terminal methionine residue of the ubiquitin 78 . This may result in an array of signals, and ubiquitin codes interpreted and dealt in different manners by cellular subsystems 79 , 80 . The patterns of attachment of subsequent ubiquitin moieties may govern differential fates of the targeted proteins. For example, a Lys-63 linked ubiquitin chain preferably directs the proteins towards autophagic degradation 81 , 82 . Contrarily, highly abundant K-48 linked polyubiquitin chains are majorly targeted for proteasomal degradation 83 . Other ubiquitin chains formed with K6, K11, K27, K29, and K33 linkages form different kinds of signals and regulate multiple physiological processes, including cell cycle control, cellular transport, and DNA repair 84 , 85 , 86 . Altogether, the involvement of the UPS has been reported in immune pathways, hormonal signaling, cellular metabolism, apoptosis, etc. 19 , 87 . Considering the coexistence of all these proteolytic processes inside the eukaryotic cells, we can assume that maintenance of proteostasis requires a very tightly regulated coordination between different components and arms of the cellular protein quality control 88 , 89 . Their involvement in the pathologies of cancer, neurodegeneration, and aging processes has led scientists to identify their therapeutic potential and devise methods or ways to modulate them for exploitation for remedial purposes. Natural molecules have remained a primary therapeutic tool over the years showing enormous potential to modulate crucial regulatory proteins inside the cells. Several reports over the past few years, as shown in Table 2 , have been published describing various kinds of possible regulation of different UPS components, which ultimately govern many disease-associated pathways. Table 2 Small natural compounds having chaperone-modulating activities. A broad array of natural molecules have been identified over the years, which can enhance or suppress the cellular chaperoning activity by elevating the expression or interfering with the functioning of major chaperones belonging to HSP70, HSP90, small HSPs or co-chaperones. Table 2 Compound Source Target protein Target disease Model system Ref. Inducers of chaperone machinery Actinomycin D Streptomyces parvullus HSP70 Huntington's disease S. cerevisiae 42 Celastrol Tripterygium wilfordii HSF1, SSA3/4 Stress response S. cerevisiae 43 Compound A Salsola tuberculatiformis HSP70 Inflammation A549 cells 44 Curcumin Curcuma longa HSF1, HSP70 Stress response C6 cells, rats 45 Geldanamycin Streptomyces spp. HSP70 Neurodegeneration H4 cells 46 Glycyrrhizin Glycyrrhiza glabra HSP70 Stress response HeLa cells 47 Handelin Handelia trichophylla HSP70 Neuroinflammation BV2, HEK293T 48 Lanosterol Metabolic intermediate CHIP Neurodegeneration Cos7 cells 49 Myricetin Fruits and berries HSP70, HSF1 Neurodegeneration Cos7 cells 50 Paeoniflorin Paeonia lactiflora HSF1, HSP70 Stress response HeLa cells 47 Prostaglandins Human HSF1, HSP70 Stress response C6 cells 51 Withaferin A Withania somnifera HSP25, HSP70 ALS Mice 52 HSP90 inhibitors Argenteoside A Tabebuia argentea HSP90 Epithelial carcinoma HeLa cells 53 Celastrol Tripterygium wilfordii HSP90 Prostate cancer LNCaP cells 54 Clorobiocin Streptomyces spp. HSP90 Breast cancer SKBR3, MCF7 55 Coumermycin A1 Streptomyces spp. HSP90 Breast cancer SKBR3, MCF7 55 Cruentaran A Byssovorax cruenta HSP90 Lung, breast cancer A549, MCF-7 56 Curcumin Curcuma longa HSP90 Viral infection HELF cells 57 Deguelin Derris trifoliata HSP90 Cancer Mice 58 Derrubone Derris robusta HSP90 Breast cancer SKBR3, MCF-7 59 EGCG Camellia sinensis HSP90 Hepatoma HePa, HspG2 60 Gambogic acid Garcinia harburyi HSP90 Cancer SKBR3, MCF7 61 Gedunin Azadirachta indica HSP90 Prostate cancer LNCaP cells 54 Geldanamycin Streptomyces spp. HSP90, HSF1 Cancer 3T3 cells 62 Herbimycin A Streptomyces spp. HSP90 Cancer 3T3 cells 62 Hypericin Hypericum spp. HSP90 Squamous carcinoma SQ2 cells 63 Kotschyn D Pseudrocedrela kotschyi HSP90 Prostate cancer PC-3 cells 64 Lentiginosine Astragalus lentiginosus HSP90 Cancer In silico 65 Macbecin Actinomyces spp. HSP90 Prostate, lung cancer DU145, H460 66 Monocillin I Monocillium nordinii HSP90 Breast cancer MCF-7 cells 67 Novobiocin Streptomyces niveus HSP90 Breast cancer SKBR3, MCF-7 55 Pochonins Pochonia chlamydosporia HSP90 Cancer In vitro 68 Radicicol Monosporium bonorden HSP90 Cancer NIH3T3 cells 69 Sansalvamide A Fusarium spp. HSP90 Colon cancer HCT-116 70 Withanolides Withania somnifera HSP90, HSF1 Thyroid cancer DRO, NPA cells 71 Quercetin Fruits and berries HSP90, HSF1 Breast cancer HeLa 72 Triptolide Triptergium wilfordii HSP90 Cancers HeLa cells 73 HSP70 inhibitors Apidaecin Insect peptides DNAK, GROEL Microbial infection E. coli 74 Cantharidin Epicauta funebris HSP70 Colorectal cancer HCT-116 cells 75 Drosocin Insect peptides DNAK, GROEL Microbial infection E. coli 74 Fisetin Fruits and berries HSP70, HSF1 Colorectal cancer HCT-116 cells 76 Myricetin Fruits and berries DNAK Proteostasis E. coli 77 Novolactone Fungal metabolites HSP70 Proteostasis HCT-116 cells 78 Pyrrhocoricin Insect peptides DNAK, GROEL Microbial infection E. coli 74 Quercetin Fruits and berries HSP70 Lung cancer A549, H460 cells 79 adaSGC Human HSP70 Proteostasis BHK cells 80 Spergualin Bacillus subtilis HSC70 Immune reaction Jurkat cells 81 Triptolide Triptergium wilfordii HSF1, HSP70 Cancers HeLa, HEK293T 82 Tubocapsenolide A Tubocapsicum anomalum HSP90-HSP70 Breast cancer MDA-MB-231 83 Small natural compounds having chaperone-modulating activities. A broad array of natural molecules have been identified over the years, which can enhance or suppress the cellular chaperoning activity by elevating the expression or interfering with the functioning of major chaperones belonging to HSP70, HSP90, small HSPs or co-chaperones.

Aberrant protein's accumulation inside cells is very well-described as a leading factor of aging, neurodegeneration, and multiple other pathologies, including cancer, diabetes, cystic fibrosis, etc. Researchers and clinicians have made multiple efforts to understand the underlying causes and mechanisms for these diseases. The molecular mechanisms whose failures can lead to inappropriate protein folding events and the common features across all these pathologies are still unclear. Some unique features across all these diseases and a noticeable genetic diversity in various conditions have prevented the scientific community from reaching a common conclusion and devising possible solutions for these life-threatening diseases 93 , 261 . However, continuous efforts are made worldwide to identify underlying causes, including the most common genetic mutations and contributing environmental or lifestyle associated factors. A comprehensive picture of all these factors, associated changes, and the pathological conditions caused by them is presented in Fig. 3 . Unfortunately, none of the hypotheses and explanations addressing the mechanisms and causes behind such detrimental changes leading to the age-associated decline in the efficiency of physiological systems have led us to develop a proper understanding and possible solutions to these conditions. Figure 3 An overview of intricate association of natural compounds and proteostasis machinery. (A) A summary of important natural sources of bioactive compounds that can affect the activities and functioning of chief players of protein quality control machinery inside the cells; examples of small molecules from each source are also presented in respective boxes. (B) A central block shows how crucial is the precise balance of protein synthesis, maintenance, trafficking and degradation of cellular proteins for the maintenance of healthy proteome. Some internal or external perturbance factors may alter the homeostasis of the cells, while multiple key actors of cellular proteostasis machinery continuously maintain the health of the proteome. (C) Multiple kinds of intracellular regulatory subsystems are identified, which are required to synthesize and maintain the healthy set of proteins in various cellular compartments to perform all kinds of cellular functions. The key proteins of each subcellular pathway are mentioned in each box. (D) Many intra- and extracellular factors and stressors could be responsible for generating different kinds of stresses during the lifetime of the organisms, which are counter-balanced by factors described in (C). (E) A number of proteinopathies, which are directly and indirectly linked with perturbance in cellular proteostasis machinery, have been reported. These diseases could be associated with one or multiple pathways, tissues, and organs specified in each box. Figure 3 An overview of intricate association of natural compounds and proteostasis machinery. (A) A summary of important natural sources of bioactive compounds that can affect the activities and functioning of chief players of protein quality control machinery inside the cells; examples of small molecules from each source are also presented in respective boxes. (B) A central block shows how crucial is the precise balance of protein synthesis, maintenance, trafficking and degradation of cellular proteins for the maintenance of healthy proteome. Some internal or external perturbance factors may alter the homeostasis of the cells, while multiple key actors of cellular proteostasis machinery continuously maintain the health of the proteome. (C) Multiple kinds of intracellular regulatory subsystems are identified, which are required to synthesize and maintain the healthy set of proteins in various cellular compartments to perform all kinds of cellular functions. The key proteins of each subcellular pathway are mentioned in each box. (D) Many intra- and extracellular factors and stressors could be responsible for generating different kinds of stresses during the lifetime of the organisms, which are counter-balanced by factors described in (C). (E) A number of proteinopathies, which are directly and indirectly linked with perturbance in cellular proteostasis machinery, have been reported. These diseases could be associated with one or multiple pathways, tissues, and organs specified in each box. In the past, many attempts, both successful and unsuccessful, have been made for devising novel therapeutic approaches against various diseases. Numerous molecules have been proposed for their efficacy for mitigating the proteotoxicity generated by intracellular protein aggregates or inclusion bodies 106 , 110 , 111 . One consensus that most of the studies meet is that natural products could be medicinally very active and useful. They were used for centuries in ancient traditional natural medicinal approaches in old-world countries. Based on all the observations mentioned above, it could be stated that targeting cellular PQC machinery by modulating their activities using small molecules may have vast potential. Plant extracts were used by ancient researchers and physicians to cure deadly infections and diseases, described in many primitive Indian, Unani, and Chinese literature 262 , 263 . Most natural molecules posit lesser toxicity and side effects than synthetic chemicals when administered to cells or animals in laboratory tests 264 . This makes them preferred choices over costly synthetic chemicals in experimental studies. Many less-invaded human territories, like the Himalayas, are the homeland of such medicinally rich natural resources and are yet to be explored and utilized for treatments of life-threatening diseases in these regions. A thorough and well-managed exploration could be done in order to identify and delve into more effective and easily derived drugs. Several naturally extracted drugs obtained from microbial and fungal isolates, marine and land animals, many aquatic and terrestrial plants, are currently in research allowing us to identify and investigate more such drugs. These small natural molecules may have several unexplored applications that need further studies. The primary benefit of these naturally derived molecules is a low-cost therapeutic alternative to many treatment strategies, which are in the pipeline against these diseases and may need a much higher cost, although many challenges remain unaddressed 265 . The identification, isolation, purification, and characterization of new molecules are a highly tedious and lengthy process, requiring lots of hard work, funds, and time 19 , 266 . Many times, designing or synthesizing some derivatives of already known drugs seems a more straightforward and cost-effective strategy in comparison to looking for new molecules. Repurposing older drugs could also be a beneficial drug discovery model to save much time, effort, and cost. Additionally, these small plant-based molecules could be used as food supplements to reduce the overall risk of many diseases.

A eukaryotic cell represents a well-evolved architecture, made up of many small components working independently and coherently. A highly efficient and coordinated way of functioning of multiple subsystems towards the fitness and survival of individual cells and the organism is a highly complex biological phenomenon. It remains a great challenge to understand the intricacies and complexities of the living systems. For a very long period, the central dogma, i . e ., the idea of sequential flow of information from DNA to RNA, followed by its retrieval into the form of proteins, remained a mystery for the scientists. However, the improvements in the techniques and adaptations of the newer approaches to decipher the molecular arrangements have led to a higher understanding of the fine details of the cells’ structure and functional arrangements. The involvement of biochemical and molecular biology approaches has led to the deduction of most of the metabolic pathways and their subsequent impact on the cellular physiology. At the same time, structural and computational biologists have played a critical role in providing crucial insights about the mysteries of the genetic codes, amino acid sequences, and structural plans of the proteins. Many other tools have also helped in devising various ways of visualizing and interpreting the intermolecular interactions involved in different cellular pathways and mechanisms. With all the advancements and our current understanding of cellular architecture, we can believe that a functional proteome is a prerequisite for regulating the essential physiological pathways and maintaining good cellular health 1 . To preserve an advantageous proteome, the cells have a well-developed protein quality control (PQC) machinery that ensures a healthy set of protein repertoires to execute all the necessary cellular tasks 2 , 3 . First, it helps the nascent polypeptide chains to attain their native conformations. Second, it identifies any dysfunctional or aberrant protein that remains present into our cellular milieu; and third, it removes all such unwanted proteins from the system. However, under certain kinds of stress conditions, their inefficiency may lead to the generation of unwanted proteinaceous species inside the cytoplasm, leading to misfolding and aggregation 4 . These events of failure of proteostasis and formation of large molecular weight aggregates or cytoplasmic inclusion bodies underlie the causal mechanism behind several systemic and non-systemic diseases, as summarized in Table 1 . Table 1 Major amyloidosis and the associated aggregatory proteins. The table summarizes major systemic and non-systemic diseases associated with misfolding and aggregation of certain proteins. Table 1 Disease/pathological condition Proteins misfolded/aggregated Ref. Alzheimer's disease Amyloid β 42, Tau protein 1 , 2 Amyotrophic lateral sclerosis SOD1, TDP43, 3 , 4 Atrial amyloidosis Atrial natriuretic factor 5 BriPP amyloidosis Amyloid-Bri 6 Cancer Tumor protein 53 7 Cataracts Crystallins 8 Creutzfeldt Jakob disease Prion 9 Cystic fibrosis CFTR 10 Diabetes Amylin, IAPP 11 , 12 Familial amyloid polyneuropathy I Transthyretin 13 Familial amyloid polyneuropathy III Apolipoprotein AI 14 Familial hypercholesterolemia LDL receptor 15 Fibrinogen α -chain amyloidosis Fibrinogen α -chain variants 16 Finnish hereditary systemic amyloidosis Gelsolin 17 Fronto-temporal dementias Tau protein 18 Haemodialysis-related amyloidosis β 2-Microglobulin 19 Hereditary cerebral amyloid angiopathy Cystatin C 20 Lysozyme-related amyloidosis Lysozyme 21 Hereditary renal amyloidosis Fibrinogen α -A chain, lysozyme 16 , 22 Huntington's disease Huntingtin 23 IBMPFD Valosin-containing protein 24 Insulin-related amyloidopathy Insulin 25 Leprechaunism Insulin receptor 26 Marfan syndrome Fibrillin 27 Medullary carcinoma of the thyroid Calcitonin 28 Osteogenesis imperfecta Type I procollagen pro α 29 Parkinson's disease/Lewy body dementia α -Synuclein 30 Primary systemic amyloidosis Ig light chains 31 Retinitis pigmentosa Rhodopsin 32 Scrapie Prion protein 33 Secondary systemic amyloidosis Serum amyloid A 34 Senile systemic amyloidosis Transthyretin 35 Spinal and bulbar muscular atrophy Androgen receptor 36 Spinocerebellar ataxias Ataxin proteins 37 Spinocerebellar ataxia 17 TATA box-binding protein 38 Spongiform encephalopathies Prion protein 39 Tay-Sachs disease β -Hexosaminidase 40 α 1-Antitrypsin deficiency α 1-Antitrypsin 41 CFTR, cystic fibrosis transmembrane receptor; IBMPFD, inclusion body myopathy associated with Paget disease of bone and frontotemporal dementia; IAPP, islet amyloid polypeptide. Major amyloidosis and the associated aggregatory proteins. The table summarizes major systemic and non-systemic diseases associated with misfolding and aggregation of certain proteins. CFTR, cystic fibrosis transmembrane receptor; IBMPFD, inclusion body myopathy associated with Paget disease of bone and frontotemporal dementia; IAPP, islet amyloid polypeptide. In the past many years, substantial efforts have been made to affect the functional efficiency of many components of the PQC systems to establish and maintain the homeostatic conditions inside cells 5 , 6 . Small molecules obtained from plants and other natural sources may have diverse medicinal values, as they can modulate many cellular proteins and affect several associated signaling pathways 7 , 8 . The primary sources of these molecules of immense medicinal values include bacterial or fungal isolates, extracts of marine animals or plant sources, and few specific mammalian tissue secretions, etc. The upcoming sections describe the significance of these crucial cellular subsystems in regulating multiple molecular networks. The article further provides a brief overview of the available reports describing various naturally-derived molecules with the proposed medicinal values.

Aging, neurodegeneration, and cancer have always remained significant challenges before the scientific community. Many theories and hypotheses have been formulated and postulated to explain these pathologies, but none has succeeded in understanding why these pathological changes occur. Genetic, environmental, infections and metabolic alterations are among the many possible reasons behind most proteopathies 90 , 91 . However, none of these could solely be held responsible for pathological conditions; instead, a blend of multiple factors contribute towards a highly diverse disease condition. This diversity among the individual cases of these pathologies further complicates the research processes and leads to failure of treatment options 92 , 93 , 94 . However, in the past few decades, tremendous progress is observed in our understanding of many of these pathologies. At the same time, these advancements have led to the evolution of multiple lines of research methodologies and approaches to understand a given problem. This has given rise to speculations and multiple lines of theories behind the origin, development, sustenance, and progression of these pathologies. The declined competence of cellular defense mechanisms and pathways are suggested to be one such notion that has attained wide acceptance in recent decades 4 , 5 . Inefficient functions of quality control systems that regularly monitor the well-being of the genomic and proteomic repertoire of the cells could be a possible cause of instigating multiple pathways leading towards aging 1 . The compromised capacity of molecular chaperones to fold the nascent polypeptides into the proper three-dimensional shape and deficient functioning of autophagy and the proteasomal system could be credited for over-burdening the cytoplasmic milieu with misfolded proteins 95 , 96 . Aggregation of multiple types of aberrant proteins could lead to the formation of large perinuclear/cytoplasmic inclusion bodies that may further mount a heightened reaction by initiating immunological responses 97 . The increased burden of the aggregates may lead to increased neuronal deaths, as observed in many disease models of neurodegeneration 98 , 99 . Aging encompasses several other attributes or hallmarks, which may include but is not limited to the genomic instability, mitochondrial loss, telomere shortening, metabolic alterations, etc. 91 , 100 . These pathways and alterations in their physiological conditions are also among the crucial factors responsible for most types of cancers 101 , 102 . Altogether, the conditions discussed above have many common features. One of the similarities is the compromised proteostasis caused due to the inefficient protein folding and degradation in cells 103 , 104 . Many other diseases, like diabetes, cataract, cystic fibrosis, myopathies, etc. are directly affected by the aggregation of one or more proteins 22 , 105 . An imbalanced proteostasis may directly or indirectly link with many other life-threatening diseases associated with lungs, heart, liver, kidneys, etc. 19 , 106 . Based on the recommendations made by the International Society of Amyloidosis, a depiction of various amyloidogenic proteins, their aggregatory forms, and the affected organs in many associated diseases is presented in Fig. 2 107 , 108 , 109 . However, drawing a common line across all these diseases would be difficult at the present state of our understanding of these intracellular systems. Based on their common connecting links, i . e ., perturbed proteostasis and the cellular PQC machinery, various strategies have been postulated in the past, while some are currently under trial. Figure 2 An overview of amyloidosis. Various amyloid-forming proteins (left), their normal precursor protein forms (middle), and tissues or organs affected in one or more similar diseases caused by individual proteins 106 , 107 . The left column shows a list of amyloidic forms of various proteins shown in the center as precursors. These proteins may aggregate in such amyloidic structures in their full, cleaved, or modified forms, while several mutations contribute to their amyloidogenicity. The structural modification of these proteins may lead to abnormal metabolic or signaling alterations at the molecular level in different tissues and organs. These changes may lead to a possible functional loss or decline, causing multiple pathological conditions. Many proteins are found to be involved in multiple diseases of different organs, whereas some diseases may have several proteins involved together in the pathogenesis. The figure was prepared using RAWGraphs, an open source platform for data representation ( http://rawgraphs.io ) 108 . Figure 2 An overview of amyloidosis. Various amyloid-forming proteins (left), their normal precursor protein forms (middle), and tissues or organs affected in one or more similar diseases caused by individual proteins 106 , 107 . The left column shows a list of amyloidic forms of various proteins shown in the center as precursors. These proteins may aggregate in such amyloidic structures in their full, cleaved, or modified forms, while several mutations contribute to their amyloidogenicity. The structural modification of these proteins may lead to abnormal metabolic or signaling alterations at the molecular level in different tissues and organs. These changes may lead to a possible functional loss or decline, causing multiple pathological conditions. Many proteins are found to be involved in multiple diseases of different organs, whereas some diseases may have several proteins involved together in the pathogenesis. The figure was prepared using RAWGraphs, an open source platform for data representation ( http://rawgraphs.io ) 108 .