Beyond Mortality: Exploring the Influence of Plant Phenolics on Modulating Ferroptosis-A Systematic Review.

Ferroptosis is a new mechanism of regulated cell death which is characterized by the accumulation of ferrous iron, depletion of GSH, and overproduction of ROS through the Fenton reaction. This leads to lipid peroxidation in the cell membrane and eventually to cell death. Ferroptosis has been connected with many diseases and pathologies such as neurodegenerative diseases, ischemia/reperfusion injury, and liver fibrosis. In some of these conditions, the selective initiation of ferroptosis can be beneficial, for example in tumors and liver fibrosis. On the other side, the inhibition of ferroptosis can be beneficial in cases of neurodegenerative diseases for the prevention of loss of neurons and in case of ischemia/reperfusion injury. Plants have been used in traditional medicine for centuries, and with the development of science, new disciplines have emerged such as rational phytotherapy. Plants produce an abundance of secondary metabolites, such as polyphenols, which have numerous bioactivities such as antioxidant, anti-inflammatory, anti-cancer, neuroprotective, cardioprotective, immunomodulatory, and other properties. The multitude of bioactivities exhibited by polyphenols presents significant potential for the exploration and development of novel medications and therapeutic approaches. According to the systematic review carried out within this study, it is evident that the exploration of the connection between ferroptosis and polyphenols, both in vitro and in vivo, is a vibrant ongoing direction for research. It has been confirmed that many polyphenols can modulate ferroptosis, whether through initiation and promotion, or inhibition, which both could have practical implications for health and disease. Yet, as per the review, there is 2.5 times more research on the inhibitory effects of polyphenols on ferroptosis compared to their potential to induce it. This study encompasses the results of research on 143 phenolic compounds. In total, 53 compounds showed an ability to induce ferroptosis, and 110 compounds were able to inhibit ferroptosis, and out of those compounds, 20 showed both abilities depending on the model system. In terms of the inhibition and activation of ferroptosis, the greatest number of papers are on the influence of flavonoids (quercetin, baicalein, baicalin, erianin, puerarin), followed by diarylheptanoids (curcumin) and stilbens (resveratrol). Out of 53 compounds with the ability to induce or promote ferroptosis that were included in this review, 63% belong to flavonoids (the most abundant subclasses are flavones—17%, flavonols—13%, bioflavonoids—9%, and flavanones—8%), followed by coumarins (9%). The frequency of compounds with pro-ferroptotic activity across polyphenol subclasses is shown in Figure 10 . A similar trend can be seen for phenolic compounds with anti-ferroptotic activity ( Figure 10 ). Out of the 110 examined compounds that were included in this review, 55% were accounted for by flavonoids (chalcones—12%, flavones—10%, flavonols—10%, flavanones—5%, and isoflavones—5%), followed by stilbenes and hydroxycinnamic acids with 7% each, and tannins (6%). From this, it can be seen that flavonoids were the most studied class of polyphenols especially in subclasses that are not so common in plants such as isoflavones and chalcones. Biflavonoids are present among pro-ferroptotic compounds but are absent from anti-ferroptotic compounds which can be of interest for further research of their mechanisms, especially for diseases where ferroptosis induction would be favorable. Additionally, quercetin and rutin are one of the most studied compounds in this review. When it comes to ferroptosis inhibitions, many stilbenes, cinnamic acids and their derivatives, and tannins have shown promising results. This highlights the importance of research on other classes not just flavonoids (probably highly researched due to their wide-ranging activities and long-lasting use in traditional medicines). The most extensively studied compounds as inducers of ferroptosis include curcumin, erianin, and shikonin. On the flip side, the most extensively studied inhibitors of ferroptosis include baicalein, curcumin, quercetin, and resveratrol. All the compounds listed are already recognized in natural product research for their beneficial pharmacological properties, and the plants that naturally produce them have been extensively documented in traditional medicine for centuries [ 37 , 46 , 50 , 60 , 78 , 184 , 316 ]. Curcumin is a polyphenolic compound derived from the rhizomes of the turmeric plant ( Curcuma longa L.). It is a natural phenolic pigment that imparts the characteristic yellow color to turmeric. In scientific terms, curcumin is a diarylheptanoid, belonging to the curcuminoid class. It is not surprising that curcumin is so widely investigated regarding its potential to modulate ferroptosis, since it has significant attention in scientific research due to its diverse pharmacological properties, including antioxidant, anti-inflammatory, anti-cancer, and neuroprotective effects. Its versatile biological activities make it a subject of interest in various fields, including pharmacology, medicine, and nutrition. Researchers frequently investigate curcumin’s bioavailability, metabolism, and safety profile to enhance its therapeutic potential. It is essential to note that despite its promising attributes, challenges related to curcumin’s low bioavailability have prompted ongoing efforts to develop formulations that improve its absorption and effectiveness in biological systems, what would also be an issue for ferroptosis modulation in rational therapy [ 45 , 50 , 56 , 222 , 225 ]. Eranin, is a biphenyl compound, which has a historical application as an antipyretic and analgesic agent. It is a constituent of plants from the Dendrobium genus, recognized among the 50 foundational herbs in traditional Chinese medicine. Researchers are particularly interested in eranin due to its demonstrated potential for treating conditions such as inflammation, diabetic nephropathy, retinopathy, psoriasis, and various cancers. Given that ferroptosis is implicated in the development of listed conditions, it come as no surprise that eranin demonstrated potential as a modulator of ferroptosis [ 37 , 41 , 57 , 66 , 67 ]. Furthermore, shikonin is a naphthoquinone derivative obtained from the roots of plants, particularly Lithospermum erythrorhizon Siebold & Zucc. 1846. The compound’s distinctive red color makes it easily identifiable. Shikonin and its enantiomeric analogue, alkannin, are prevailing natural lead compounds in the drug discovery and development of anticancer agents. Despite having numerous biological effects, the most important activity reported for shikonin derivatives is their antitumor effect which is exerted through various mechanisms such as the induction of apoptosis and autophagy. The listed body of evidence in this review supports adding ferroptosis to this list [ 73 , 74 , 182 , 184 ]. Moreover, both baicalein and quercetin are classified within the flavonoid group of polyphenols, and they are abundantly present in a variety of fruits and vegetables. Numerous studies have substantiated their positive impact on human health, underscoring their potent antioxidant, anti-inflammatory, and anticancer properties. Consequently, it is unsurprising that researchers have shown considerable interest in these compounds, as they demonstrate substantial potential in inhibiting ferroptosis [ 95 , 118 , 197 , 306 ]. Wine and grape juice, particularly leaves and skins, are rich sources of resveratrol, a stilbenoid polyphenolic compound. Additionally, resveratrol can be derived from various foods, including peanuts, pistachios, blueberries, strawberries, etc. Research has demonstrated that resveratrol possesses antitumor, antioxidant, anti-inflammatory, and anti-apoptotic effects. This anti-apoptotic effect can be partially explained by its great potential to inhibit ferroptosis [ 111 , 175 , 180 , 307 , 319 ]. Even though polyphenols show promising results for the development of drugs and therapeutics for many diseases, their application can be difficult. Namely, many polyphenols have low bioavailability which can make drug delivery difficult. Curcumin and flavonoid aglycones have high hydrophobicity and are poorly absorbed in the intestine, this can be enhanced through newly developed drug delivery strategies such as the formulation of liposomes or nanoparticles. The next problem with the application is the extensive metabolic transformation that polyphenols can go through in an organism. Hydrophobic compounds such as flavonoid aglycons can be glucuronidated and sulfated which enhances their solubility in body fluids, but at the same time results in their fast excretion through the kidneys. This not only makes it difficult to predict in which state the compounds would be delivered to the target and in which concentration but also poses a question of whether the metabolites will have the same activity as pure compounds. Sometimes metabolites are completely inactive while in other cases metabolites can show greater or different activities. This is a big flaw of in vitro assays on cells, because absorption, bioavailability, and metabolic changes that happen in the body are not encompassed by these experiments. Animal studies, being performed on living organisms, give a better idea about the efficacy of examined compounds and their pharmacokinetics and can be a great stepping stone for the design of new compounds with more desirable properties [ 6 , 346 , 347 , 348 ]. Another thing that needs to be taken into consideration when considering the application of polyphenolics as medicines is their potential interaction with other drugs. Polyphenols can bind iron in complexes from which it is not bioavailable or influence the systemic metabolism of iron which needs to be taken into consideration when iron supplementation is being used for the treatment of iron-deficiency anaemia [ 349 ]. Another thing that needs to be taken into consideration is the ability to influence the P450 enzyme. P450 is a group of liver enzymes that play a crucial role in the detoxication, metabolism, and removal of different molecules, including drugs. The rate of their activity is crucial for the metabolism and pharmacokinetics of drugs. Polyphenols can lower the activity or inhibit the activity of P450 enzymes, which can result in the accumulation and prolonged activity of drugs, as well as increase the risk of side effects or toxic effects due to higher doses of drugs in the body. On the other hand, some polyphenols can induce P450 activity, which leads to faster metabolism and removal of drugs and can result in low drug efficacy [ 350 ]. All this highlights the importance of studying not just bioavailability but also the metabolism and metabolic effects of polyphenolics before their clinical application. The use of artificial intelligence (AI) and machine learning in different areas has become a hot topic, and interest is still growing. This is due to the various possibilities of applications such as searching for potential targets for therapeutics or design and the development of new drugs. If trained well through machine learning, AI can make this process much easier, faster, and economically affordable because then only the most promising compounds would be synthesized and tested. Recently, there have been some promising results for the use of machine learning to train AI to discern whether the mechanism of cell death was through apoptosis or ferroptosis. This was completed through the synthesis of a database of results obtained in the lab and the use of certain biomarkers for each cell death mechanism [ 351 , 352 ]. Deep learning was also applied to identify new potential targets for treatment of lung cancer through the modulation of ferroptosis [ 353 ]. These studies are just stepping stones for future research and showcase the potential and power that AI and machine learning have to offer.

Cell death is a fundamental process present in all living organisms, executed through various mechanisms. For a long time, cells were thought to be removed in two ways—regulated cell death, called apoptosis, and unregulated cell death, called necrosis [ 1 ]. Recently, many processes of controlled cell death have been discovered, such as intrinsic apoptosis, extrinsic apoptosis, anoikis, autophagy-dependent cell death, programmed cell death (physiological cell death), entotic cell death, necrosis, necroptosis, oxytosis/ferroptosis, pyroptosis, paraptosis, parthanatos, oxeiptosis, and NETosis. These processes of cell death are characterized by different methods of induction and the involvement of specific signaling pathways [ 2 ]. Apoptosis, often referred to as controlled or programmed cell death, is a highly regulated process that plays a crucial role in various physiological and pathological situations. It can be triggered by different factors, including significant cellular damage or the activation of specific receptors on the cell membrane, all characterized by the activation of caspases [ 3 ]. Ferroptosis, on the other hand, is a mechanism of cell death that occurs due to significant oxidative damage to cells. It is characterized by the depletion of glutathione, increased levels of free intracellular iron, and elevated lipid peroxidation in the cell membrane [ 4 , 5 ]. Ferroptosis is believed to play a role in various pathological conditions, including neurodegenerative diseases, chronic kidney disease, cardiovascular diseases, as well as ischemia/reperfusion injury [ 6 , 7 , 8 ]. Conversely, inducing ferroptosis is considered a potential therapeutic approach for addressing liver fibrosis and cancer—conditions characterized by disrupted normal cell activity leading to pathology [ 9 , 10 ]. Plants generate a countless number of compounds through secondary metabolism, exhibiting protective and hormonal functions within the plant itself. Interestingly, these compounds also possess high bioactivity in animals and humans. These characteristics have made plants integral to traditional medicinal practices spanning centuries. In contemporary times, the advancement of scientific knowledge has given rise to new disciplines like rational phytotherapy. The primary objective of such fields is to formulate safe and scientifically substantiated medicines derived from plants. Consequently, there is a substantial and growing interest in the research of medicinal plants and their bioactive compounds [ 11 ]. The term apoptosis was proposed in 1972, in a paper written by Kerr, Wyllie, and Currie. They described apoptosis as programmed cell death that is complementary to mitosis. Apoptosis is defined by two phases: the first phase is characterized by the formation of spheroid apoptotic bodies and the second by their phagocytosis and finally degradation by other cells [ 12 ]. There are two distinct apoptotic pathways—intrinsic and extrinsic. Both pathways culminate with the activation of caspases, which are proteases responsible for cleaving numerous vital proteins within the cell. This process leads to the manifestation of apoptotic cell morphology and eventual cell death, typically occurring within a time span of minutes to hours. The intrinsic pathway is activated when there is significant damage caused by different types of stress inside the cell, such as DNA damage, endoplasmic stress, and lack of growth factors. Signaling pathways activated under stress can disturb the equilibrium between the BCL2 family of proteins with anti-apoptotic functions and proapoptotic proteins from family BH3-only. Equilibrium can be pushed towards the proapoptotic side by overexpression of BH3-only proteins, while simultaneously posttranslational modifications and proteolytic processing of BCL2 proteins occurs, and excess of BH3-only proteins leads to the initiation of apoptosis. BH3-only proteins bind to BCL2 proteins, as well as to BAK and BAX proteins which are necessary for apoptosis. Following the activation of BAX and BAK, pores in the outer mitochondrial membrane are formed. That process is termed mitochondrial outer membrane permeabilization, and it results in the outflow of many molecules from the mitochondrial intermembrane space, among which is cytochrome c (Cyt-c). Cyt-c is necessary for the activation of the apoptotic protease-activating factor 1 (APAF1). APAF1 contains the CARD (caspase recruitment domain) which interacts with the CARD of pro-caspase 9 (pro-CASP9) forming apoptosome. Inside this complex, apoptosis initiator caspase 9 (CASP9) is formed from pro-caspase 9, and subsequently, it leads to the activation of caspases 3 and 7 (CASP3 and CASP7) ( Figure 1 ) [ 1 , 13 ]. The initiation of the extrinsic pathway occurs external to the cell, triggered by environmental conditions that signify the necessity for the cell to undergo apoptosis. The extrinsic pathway is activated by the binding of particular ligands to cell surface death receptors and consists of three signaling pathways: tumor necrosis factor receptor (TNFR), TNF-related apoptosis-inducing ligand (TRAIL), and factor-associated suicide (Fas)/Fas ligand (FasL) pathways [ 14 ]. When FasL binds to the Fas receptor, it leads to activation of the signaling pathway. The Fas-associated death domain (FADD) activates pro-caspase 8 (pro-CASP8), after which, active caspase 8 (CASP8) activates CASP3 and CASP7 ( Figure 1 ) [ 15 ]. Activation of CASP3 and CASP7, by both intrinsic and/or extrinsic pathways, marks a point of no return and the cell has no choice but to undergo suicide ( Figure 1 ). These caspases are involved in processes that define apoptosis, such as the fragmentation of DNA, exposure of phosphatidylserine (PS) on the outer lipid bilayer, blebbing of the cell membrane, and formation of apoptotic bodies. CASP3 promotes DNA fragmentation through the proteolytic inactivation of DNA fragmentation factor subunit alpha (DFFA or ICAD), which results in the release of active DFFB (also known as CAD). CASP3 is also involved in the externalization of PS through the activation of enzymes called phospholipid scramblases. Additionally, CASP3 inhibits enzymes—flippases—involved in PS internalization [ 3 ]. The mechanism of apoptosis initiation is shown in Figure 1 . Ferroptosis, initially proposed by Dr. Brent R. Stockwell and collaborators in 2012, represents a pivotal advancement in cell biology. Its discovery emerged from a comprehensive investigation spanning several years. Between 2001 and 2003, Dr. Stockwell’s laboratory delved into the exploration of small molecules exhibiting HRAS V12 -selective lethality. This intensive research culminated in the identification of a compound named erastin, characterized by its capacity to induce cell death via a non-apoptotic mechanism. Subsequent investigations further elucidated the properties of erastin and led to the discovery of RAS-selective lethal-3 (RSL3). Both compounds demonstrated the ability to trigger cell death, not through apoptosis, but rather via an iron-oxidative stress-dependent pathway. These groundbreaking findings, along with complementary discoveries by other research groups, collectively defined ferroptosis as a novel form of regulated cell death. Ferroptosis is distinguished by its dependence on intracellular iron accumulation and the production of reactive oxygen species (ROS). Moreover, they identified it as a cell death mechanism that is morphologically, biochemically, and genetically distinct from previously known cell death mechanisms [ 4 , 5 ]. This paradigm shift in our understanding of cellular demise underscores the intricate interplay between iron metabolism, oxidative stress, and cell fate regulation. The elucidation of ferroptosis mechanisms holds profound implications for various fields, including cancer biology, neurodegenerative diseases, and therapeutic development. Ferroptosis is characterized by the accumulation of free iron in cells which leads to high production of ROS and the subsequent lipid peroxidation of plasma membrane lipids. In the following text, mechanisms of ferroptosis induction are explained in detail. In ferroptosis, oxidized polyunsaturated fatty acids (PUFAs), from membrane phospholipids, play an important role. ACSL4 (acyl-CoA synthetase long-chain family member 4) is an enzyme that catalyzes the biosynthesis of PUFA-CoA, among others also from arachidonic acid (AA) and adrenic acid (AdA). PUFA-CoA are then used for the synthesis of phosphatidylethanolamines (PE) containing AA (PE-AA) and AdA (PE-AdA), catalyzed by LPCAT3 (lysophosphatidylcholine acyltransferase 3). These PEs are substrates for ALOX (arachidonic acid lipoxygenases) enzymes (ALOX15 and ALOX12), which catalyze peroxidation reactions in which PE-peroxides are formed (PE-OOH, Figure 2 ). PE-OOH are very unstable and can initiate lipid peroxidation which can damage the lipid bilayer ( Figure 3 ) [ 16 ]. Iron uptake by cells (apart from intestinal cells) occurs via transferrin receptors (TfR) which are positioned on the cell membrane. These receptors bind transferrin (Tf), the iron transporter in plasma, after which they are internalized as endosomes. In endosomes, under acidic conditions, Fe 3+ is released from the Tf-TfR complex. After release, Fe 3+ is reduced to Fe 2+ by the STEAP3 (six-transmembrane epithelial antigen of prostate 3), and exported in the cytosol by DMT1 (divalent metal transporter 1). In the cytosol, free Fe 2+ can initiate the Fenton reaction, which is prevented by binding to ferritin. Ferritin is a protein that stores excess iron in the form of Fe 3+ . PCBP1 and PCBP2 (poly rC-binding protein) are cytoplasmic proteins that are involved in the transportation of iron to ferritin. NCOA4 (nuclear receptor coactivator 4) mediates ferritinophagy, the autophagy degradation of ferritin in lysosomes, resulting in the release of iron into the cytosol ( Figure 3 ) [ 17 ]. It has been shown that, in the first phases of ferroptosis, ferritinophagy plays a crucial role and that inhibitors of ferritinophagy can slow down the ferroptosis. However, in later stages, this inhibition does not exhibit a significant effect [ 18 ]. Iron can be exported from the cell, such as in enterocytes, hepatocytes, and macrophages, into the bloodstream by ferroportin-1 (FPN1). FPN1 exports iron as Fe 2+ , which is immediately oxidized to Fe 3+ by hephaestin (HEPH) or ceruloplasmin. Hephaestin is expressed in enterocytes, while ceruloplasmin is expressed in macrophages and hepatocytes [ 19 ]. Once oxidized to Fe 3+ , iron can be bound to Tf ( Figure 3 ). Hepcidin is a peptide hormone secreted by the liver that plays a key role in the systemic regulation of iron metabolism. Hepcidin binds to FPN1, and the complex is internalized in the cell and degraded. Higher levels of hepcidin will lead to the inhibition of iron export and subsequent iron accumulation [ 20 ]. The Xct- transportation system is responsible for the supply of cystine inside the cell, which is necessary for the synthesis of glutathione (GSH). This system consists of a regulatory unit (SLC3A2) and a transportation unit (SLC7A11). The availability of cystine determines the rate of GSH synthesis. GSH is a substrate for glutathione peroxidase 4 (GPX4), the enzyme that is responsible for the reduction of lipid peroxides, such as PE-OOH, to lipid alcohol, PE-OH. Xct-/GSH/GPX4 is the axis that is responsible for the inhibition of ferroptosis. Inhibition of the Xct- system and/or GPX4 results in the initiation of ferroptosis, and vice versa ( Figure 3 ) [ 21 ]. Blocking the Xct- system also results in elevated intracellular free iron levels, heightening the cell’s vulnerability to ferroptosis. Specifically, labile iron forms a complex with GSH and binds to PCBP1. Depletion of GSH inhibits iron binding to PCBP1, leading to the accumulation of free iron, which can generate ROS through the Fenton reaction [ 4 , 22 ]. Likewise, GCH1 (GTP cyclohydrolase-1) is the rate-limiting enzyme for the synthesis of tetrahydrobiopterin (BH4). Lower levels of BH4 indicate higher sensitivity to ferroptosis ( Figure 4 ). BH4 is not only a reducing agent in cells but also serves as a cofactor of many enzymes. BH4 is necessary for the conversion of phenylalanine into tyrosine, which is necessary for the production of 4-OH-benzoate—a precursor of CoQ 10 [ 23 ]. DHFR (dihydrofolate reductase) is the enzyme responsible for the regeneration of BH4. Inhibition of this enzyme, along with inhibitors of GPX4 can cause ferroptosis. The GCH1/DHFR/BH4 axis can neutralize lipid peroxides and lipid radicals, protecting cells from ferroptosis ( Figure 4 ) [ 20 ]. FSP1/NAD(P)H/CoQ10 is a GPX4-independent system for cell protection against ferroptosis ( Figure 4 ). Namely, FSP1 (ferroptosis suppressor protein 1) belongs to the quinone oxidoreductase NDH-2 family and is associated with the plasma membrane. It reduces ubiquinone to ubiquinol via NAD(P)H and is included in the synthesis of CoQ10-ubiquinol. Namely, CoQ10 is located in membranes where it serves as an antioxidant. It can neutralize lipid radicals and protect cells from ferroptosis. Moreover, FSP1 can also regenerate α-tocopherol (vitamin E), another potent antioxidant in the plasma membrane [ 20 ]. Nuclear factor erythroid 2-related factor 2 (Nrf2, Figure 4 ) is a transcriptional factor that under normal conditions is bound to Kelch-like ECH-associated protein 1 (Keap1) and degraded through the ubiquitin-proteasomal pathway. Inhibition of Keap1 leads to the release and activation of Nrf2 [ 24 ]. Nrf2 translocates to the nucleus, interacts with antioxidant-responsive elements (AREs) and promotes the expression of many proteins. Some targets of Nrf2 are responsible for the prevention of ferroptosis, such as GPX4, ferritin light and heavy chains (FTL, FTH1), and FPN1. On the other hand, Nrf2 promotes the expression of proteins that can enhance ferroptosis such as heme-oxygenase 1 (HO-1), which is responsible for the degradation of heme from hemoglobin, products of this reaction are biliverdin, CO, and Fe 2+ [ 25 ]. Tumor suppressor protein p53 plays an important role in the suppression of tumor formation and growth by regulating the cell division and response of the cell to different types of stress. p53 can inhibit the expression of SLC7A11, making cells more susceptible to ferroptosis. p53 P47S polymorphism leads to increased levels of GSH and the inhibition of ferroptosis, probably due to the inability to suppress the expression of SLC7A11. p53 can activate the expression of SAT1 (Spermidine/spermine N1-acetyltransferase 1), which leads to the induction of the expression of ALOX15 which can lead to ferroptosis and tumor suppression. The exact mechanism of activation of this p53/SAT1/ALOX15 metabolic pathway, and the induction of ferroptosis through it, is still not fully elucidated. Additionally, p53 can translocate in mitochondria in hepatic stellate cells and cause the accumulation of redox-active iron and induction of ferroptosis. On the other hand, p53 can also act as a ferroptosis suppressor. Namely, p53 regulates the localization of DPP4 (dipeptidyl peptidase 4). The proposed mechanism is that depletion of p53 leads to decreased levels of DPP4 in the nucleus, which in turn leads to membrane-associated DPP4- binding to NOX1 (NADPH oxidase 1), lipid peroxidation, and ferroptosis. Another pro-survival effect of p53 is through p21 also known as CDKN1A (cyclin-dependent kinase inhibitor 1A). It has been shown that the p53-dependent expression of p21 and the production of GSH can delay ferroptosis, but this mechanism is not fully elucidated. All this highlights the importance of p53 in tumor progression and shows that p53 can be an important ferroptosis modulator in cancer cells and the loss of function of p53 can make cancer cells more resistant to ferroptosis, thus promoting cancer growth [ 26 , 27 ]. Numerous plants have been historically employed in traditional medicinal practices over the centuries. Presently, there exists a substantial interest in the utilization of medicinal plants and harnessing plant secondary metabolites for medicinal purposes. This has led to the development of fields such as rational phytotherapy and pharmacognosy, as well as the design and approval of several methods of phytotherapeutics by national health regulatory bodies [ 11 ]. Plants produce a wide range of secondary metabolites. Their concentration is higher when plants are under stress, suggesting that they have a protective role. They are divided into three big groups, alkaloids, phenolics (PC), and terpenoids, and show many bioactivities that can be beneficial for humans and animals, such as antioxidant, anti-inflammatory, antihypertensive, anti-aging, and insulin-sensitizing properties [ 28 , 29 ]. Phenolics or polyphenols represent a class of numerous compounds that possess one or more phenolic group in their structure. They can be divided into subclasses such as phenolic acids, flavonoids, tannins, coumarins, stilbenes, and lignans. Phenolic acids are polyphenols that are derived from hydroxybenzoic and hydroxycinnamic acids, and they differ by the number and position of hydroxy and methoxy groups. The most common benzoic acids are p-hydroxybenzoic, vanillic, and protocatechuic, while the most common hydroxycinnamic acids are caffeic, ferulic, and p-coumaric ( Figure 5 ). Hydroxycinnamic acids are often found in esters with quinic acid called chlorogenic acids, while the most common are esters with caffeic acid. They possess antioxidant, anti-inflammatory, and antitumor activities [ 30 ]. Flavonoids are one of the biggest classes of plant polyphenols. They have a C6-C3-C6 skeleton made of the three rings denoted as A, B, and C ( Figure 6 ). They are divided into seven subclasses: flavanones, flavanols, anthocyanidins, flavones, flavonols, and chalcones. Flavonols have a 2-phenyl-chromen-4-one backbone and hydroxyl group bound at position C3, and representatives of this subclass are quercetin, rutin, and kaempferol. Flavones differ from flavonols due to their lack of hydroxyl group at position C3; apigenin and luteolin are the most common members of this subclass. Flavanones are also termed dihydroflavones because they do not have double bonds between C2 and C3. They are present in citrus fruits, with hesperidin, naringin, and eriodyctiol as representatives. Flavanols, known as flavan-3-ols, do not have double bond between C2 and C3 and a keto group at C4 but possess a hydroxyl group at C3. They are present as monomers or as polymers making proanthocyanidins. They are present in different fruits with (+)-catechin and (−)-epicatechin as the most common ones. Isoflavones are characterized by a 3-phenyl-chromen-4-on backbone, and thanks to this structural characteristic they are similar to animal estrogens and show affinity for estrogen receptors. They are known as phytoestrogens, and daidzein and genistein are members of this flavonoid subclass. Anthocyanidins are characterized by having flavylium cation as their backbone which makes them unstable and present as anthocyanins. In plants, they are pigmented with red, purple, and blue colors in flowers and fruits. The most common anthocyanidins are cyanidin, malvidin, delfinidin, and pelargonidin. Chalcones are a specific subclass of flavonoids because they have an open C-ring. They are precursors for the biosynthesis of flavonoids and isoflavonoids. Flavonoids have strong antioxidant activity and other beneficial properties for human health such as anti-hypertensive, cardioprotective, anti-inflammatory, anti-cancer, and neuroprotective properties [ 31 ]. Coumarins are a class of polyphenols that, at the core of their structure, have a 2H-chromene-2-on structure. They are divided into several classes: simple coumarins (including hydroxycoumarins—scopoletin, esculetin), furanocoumarins (methoxsalen), pyranocoumarins (samidin, braylin), benzocoumarins (urolithin A and B), phenylcoumarins, and biscoumarins ( Figure 7 ). They possess numerous biological properties such as anticoagulant, antioxidant, antiangiogenic, anticancer, and antibacterial properties [ 32 ]. Stilbenes are polyphenols with a diphenylethylene (C6-C2-C6) structure. In plants, they are produced as a defense mechanism against infections and UV radiation [ 33 ]. A representative of stilbenes is resveratrol ( Figure 8 ). It is a polyphenolic stilbene with a double bond connecting two phenolic rings. Under UV radiation, it can undergo geometric isomerization. The trans-isomer is more abundant than the cis-isomer, and also more bioactive. Resveratrol possesses many activities that can be beneficial for humans such as cardioprotective, antioxidant, anti-inflammatory, anti-fungal, anti-viral, and other activities [ 34 ]. Tannins are polymers, and they are divided into hydrolyzable and non-hydrolyzable condensed tannins. Hydrolysable tannins are composed of ellagic and gallic acids with a sugar core. On the other hand, condensed tannins are composed of flavonoids (flavan-3-ol and flavan-3,4-diol). Tannins have anticancerogenic, antiviral, anticancer, and antioxidant activities [ 35 ]. As can be seen from the above text, plants produce a plethora of polyphenolic compounds with many positive impacts on human health. In recent years, many papers have been published with evidence that plant secondary metabolites can modulate ferroptosis. In this paper, we will try to give some overview of those results and insights for future research in this field.

This review work was undertaken under the guidance of the Preferred Reporting Items for Systematic Review and Meta-analysis (PRISMA) [ 36 ]. A comprehensive search of multiple databases was performed. The searched databases were Web of Science, PubMed, ScienceDirect, Scopus, and Springer in the time span from 2017 to November 2023. The search was limited to research articles written in English and the keyword used was “ferroptosis”, in combination with “polyphenol”, “flavonoid”, “phenolic acid”, “coumarin”, “stilbene”, “chalcone”, “anthraquinone”. Studies investigating the influence of pure, naturally occurring plant polyphenols on ferroptosis in any type of cells, tissue, or organism were considered. From each paper, the following information was extracted: investigated compound, study design, proposed mechanism of action, first author, and year of publication. Studies investigating the influence of semi-synthetic or synthetic compounds not naturally occurring in plants; studies investigating extracts; and studies with in silico investigation without experimental confirmation of activity toward ferroptosis were excluded.

The selection process according to PRISMA guidelines is shown in Figure 9 and the PRISMA checklists are given in the Supplementary Materials (Tables S1 and S2) The research protocol for this study has been registered at INPLASY (registration number INPLASY202410104, https://doi.org/10.37766/inplasy2024.1.0104 , accessed on 7 March 2024). A comprehensive search of databases resulted in the identification of 508 papers. After removing 134 duplicates and 58 studies that did not meet the inclusion criteria or that were not relevant, 4 papers were not retrieved and 312 studies were fully read. In total, one paper, previously included, was removed because of a retraction notice. In the end, 311 papers were included in this review. From the included papers, data were extracted and summarized in tables. Data extraction was performed by two independent researchers (N.Z. and M.L.). Research on the ferroptosis influence on cells can be carried out in vitro and in vivo. In vitro research is carried out on many different cell lines, mostly cancer cells [ 37 , 38 , 39 , 40 ]. In vivo studies are carried out on animals, most commonly mice and rats, but other species can also be used like gerbils and zebrafish, as shown in Table 1 and Table 2 [ 41 , 42 , 43 , 44 , 45 ]. According to this review, research into the inhibition of ferroptosis is conducted both at the cellular and animal levels. In cell studies, ferroptosis is initiated, followed by the analysis of the inhibitory potential of the tested compounds. Ferroptosis initiation is induced by incubating cells with substances like erastin (Xct- system inhibitor), RSL3 (GPX4 inhibitor), hemin, or others [ 46 , 47 ]. Standard ferroptosis inhibitors, such as liproxstatin-1 (Lip-1) and ferrostatin-1 (Fer-1), are employed for comparison with the treatments under examination [ 47 ]. In animal studies, chemicals or surgical procedures are employed to induce ferroptosis, followed by the administration of phytochemicals to assess their anti-ferroptosis activity. Parameters examined include GSH, oxidized GSH, malondialdehyde (MDA), free cellular iron levels, and the expression of genes and proteins involved in the regulation of ferroptosis, such as GPX4, HO-1, Nrf2, FTH1 [ 7 , 48 , 49 , 50 ]. For ferroptosis initiation/promotion studies, similarly to inhibition studies, cells are grown under the usual conditions until treatment with examined compounds. Parameters that are being examined are the same as for inhibition studies, levels of GSH, MDA, free iron, ROS, expression of GPX4, HO-1, FTH-1, Nrf2, among others. The inclusion of standard ferroptosis inhibitors in the treatment serves as an additional validation of ferroptosis initiation. Ferroptosis initiators (erastin and RSL3) can be used as a standard for evaluating the potency of the examined compound as a ferroptosis initiator. In ferroptosis initiation studies, animals are often inoculated with cancer cells in order to induce tumor formation, in which ferroptosis induction can be a potential treatment. Animals are usually divided into groups that receive treatment with the examined compound and control which receives saline or buffer solution. In addition to the standard parameters examined in tissues, these studies also track changes in tumor size [ 41 , 51 , 52 , 53 , 54 ]. The involvement of ferroptosis in diverse diseases positions it as a promising target for therapeutic intervention. Plant secondary metabolites, particularly polyphenols, exhibit significant promise in the potential development of novel treatments centered on the modulation of ferroptosis. Cancer stands out as a major concern in contemporary society and medicine research. Existing therapies lack the desired selectivity, causing significant harm to healthy tissues and organs, thereby posing substantial challenges for patients. The emerging concept of ferroptosis presents a novel and promising path in cancer treatment. Considerable research has been dedicated to exploring plant secondary metabolites, especially polyphenols, as inducers of ferroptosis, thereby serving as potential anticancer drugs. Numerous cancer cell lines undergo scrutiny in this research, complemented by a substantial body of in vivo studies utilizing tumor xenograft models [ 54 , 55 , 56 , 57 ]. Specifically, a variety of polyphenols have been verified as potential anti-cancer agents, and notably, curcumin, whose potency to promote ferroptosis consequently demonstrated significant anti-cancer effects across various cancer cell types. It has the ability to reduce the expression of GPX4, SLC7A11, and FPN1, while simultaneously increasing the expression of ACSL4. This modulation resulted in the accumulation of iron, overproduction of ROS, and cell death through ferroptosis [ 10 , 51 , 58 , 59 , 60 , 61 ]. Furthermore, amentoflavone [ 62 , 63 , 64 ], baicalin [ 54 , 55 , 65 ], erianin [ 37 , 41 , 57 , 66 , 67 ], gambogenic acid [ 68 ], auriculasin [ 69 ], wogonin [ 70 ], quercetin [ 71 , 72 ], shikonin [ 73 , 74 ], scoparone [ 75 ], osthole [ 76 ], also have shown to be able to inhibit cancer growth though the induction of ferroptosis. All of this suggests the potential for developing novel cancer treatments grounded in natural products acting as ferroptosis inducers. Diabetes is a significant burden of contemporary society as another prevalent disease. It is defined by the body’s incapacity to regulate normal blood sugar levels, stemming from either insulin resistance or insufficient insulin production. Additionally, diabetes can contribute to the development of other conditions, such as diabetic nephropathy, cardiomyopathy, and neuropathies [ 77 , 78 , 79 ]. Interestingly, polyphenols quercetin, rhein, and glabridin have been shown to attenuate diabetic nephropathy through inhibition of ferroptosis. Quercetin acted through the activation of the Nrf2/HO-1 pathway, rhein acted via the Rac1/NOX1/β-catenin pathway, while glabridin acted on the VEGF/Act/Erk pathway [ 78 , 80 , 81 ]. Furthermore, curcumin showed the ability to ameliorate diabetic cardiomyopathy by inhibiting ferroptosis. It was able to activate the Nrf2 pathway, raising levels of GPX4 and HO-1 and finally inhibiting ferroptosis caused by high glucose levels [ 82 ]. It is crucial to underscore that certain polyphenols exhibit a dual effect on ferroptosis, acting as both inhibitors and enhancers of this process. For instance, curcumin exemplifies this phenomenon. In cancer research, curcumin has demonstrated the ability to promote ferroptosis in cancer cells, thereby inducing cell death. Conversely, in diabetes, curcumin has been shown to inhibit ferroptosis, leading to cell revival. It is conceivable that curcumin operates through cell-specific mechanisms concerning ferroptosis. While these findings may be confusing, ongoing research endeavors aim to elucidate these phenomena further. Dihydromyricetin, ferulic acid, and naringin protected neurons from death and prevented cognitive impairment associated with diabetes and a high-fat diet. Dihydromyricetin acted as an inhibitor of the JNK-inflammatory pathway, oxidative stress, and lipid peroxidation, and caused downregulation of ACSL4 and upregulation of GPX4 expression [ 83 ]. On the other side, ferulic acid and naringin are proven to activate the Nrf2/GPX4 axis which leads to the inhibition of ferroptosis and neuroprotection [ 77 , 84 ]. Neurodegenerative diseases such as Parkinson’s and Alzheimer’s are caused by neuronal degeneration, accompanied by synaptic dysfunction, inflammation, oxidative stress, and the accumulation of proteins such as synuclein, Aβ, and tau proteins. Studies have shown that neuron death in these diseases is at least partially caused by iron metabolism imbalance and ferroptosis. Polyphenols have shown potential to at least partially improve and alleviate these conditions [ 85 , 86 , 87 ]. Specifically, it was shown that chrysophanol protected neurons from ferroptosis in Alzheimer’s disease by decreasing ROS levels, inhibiting lipid peroxidation, and increasing GPX4 expression [ 86 ]. Eriodictyol showed potential in the prevention and treatment of neurodegenerative disease thanks to its ability to guard neurons from oxidative stress and ferroptosis through activation of the Nrf2 pathway [ 88 , 89 ]. Echinatin [ 90 ], gastrodin [ 47 , 91 , 92 ] icariin [ 93 ], isoforthiaside [ 85 ], quercetin [ 94 , 95 ], salidroside [ 96 , 97 ], sennoside A [ 87 ], and tetrahydroxy stilbene glycoside [ 98 ] have been shown to prevent neurodegeneration and ameliorate neurodegenerative diseases, such as Alzheimer’s, through activation of the Nrf2 signalling pathway, which protects against oxidative stress and ferroptosis. Unlike them, silibinin protects against Alzheimer’s disease and neurodegeneration by inhibiting ferroptosis and inflammation through modulation of the STING pathway [ 99 ]. A traumatic brain injury can cause damage to blood vessels and brain tissues leading to haemorrhage and neuron death. Similar to an intracranial haemorrhage after a traumatic brain injury, a subarachnoid haemorrhage, which can be caused by a ruptured aneurism, leads to damage of surrounding brain tissues. The neuronal damage is at least partially caused by the degradation of released haemoglobin by HO-1, which leads to the accumulation of free iron, oxidative stress, lipid peroxidation, and neuronal death by different mechanisms, including ferroptosis [ 46 , 100 , 101 , 102 ]. There is no treatment for these conditions, but some polyphenols show promising results in this regard. Baicalein and baicalin have been shown to also alleviate brain injury through the inhibition of ferroptosis. They were able to increase the expression of GPX4 and SLC7A11 which led to protection against oxidative damage [ 46 , 103 , 104 ]. Puerarin is another compound that showed potential in the treatment of a brain injury caused by an intracerebral haemorrhage. It activated the AMPK/PGC1α/Nrf2 pathway, which was followed by the alleviation of iron accumulation, lipid peroxidation inhibition, an increase in the expression of GPX4, and a decrease in the expression of ACSL4 [ 101 ]. Ischemia/reperfusion injury arises when blood flow is interrupted, leading to hypoxia, followed by the restoration of blood flow and reoxygenation. This phenomenon can be manifested in the myocardium, brain, and various other organs. It is common for this to occur during major surgeries, myocardial infarction, stroke, and organ transplantations [ 105 ]. It is marked by elevated production of ROS, diminished levels of antioxidants, inflammation, and cell death. New discoveries suggest that ferroptosis plays a role in ischemia/reperfusion injury, offering a fresh approach to treating this condition. Plant secondary metabolites, especially polyphenols, have exhibited encouraging outcomes as potential therapeutic agents in this context. In vivo studies revealed that isorhamnetin and apigenin-7-O-(6″-p-coumaroyl)-glucoside can attenuate ischemia/reperfusion injury by inhibiting ferroptosis [ 106 , 107 ]. Galangin was able to attenuate ischemia/reperfusion injury through inhibition of ferroptosis by activating Nrf2/GPX4, SLC7A11/GPX4, and PI3K/AKT/CREB signaling pathways [ 44 , 108 , 109 ]. Resveratrol also showed potential to attenuate ischemia/reperfusion injury through anti-ferroptotic activity [ 110 , 111 , 112 ]. Some other compounds that displayed promising results for treatment of ischemia/reperfusion injuries were rhein [ 113 ], vitexin [ 114 ], naringenin [ 115 ], loureirin C [ 116 ], caffeic acid [ 117 ], baicalein [ 118 , 119 ], luteolin [ 119 ], kaempferol [ 120 ], and others. The predominant mechanism underlying this phenomenon involves the upregulation of GPX4 and SLC7A11 expression, along with the modulation of iron metabolism. The liver serves as a central hub in the body’s defense system, playing a crucial role in metabolizing and detoxifying various chemicals and drugs. Some substances, such as acrylamide and acetaminophen, have been identified as potential contributors to liver injury [ 121 , 122 , 123 , 124 ]. Nevertheless, certain polyphenols have demonstrated significant potential in mitigating liver injury. For instance, salidroside has been shown to alleviate acetaminophen-induced acute liver injury. Its mechanism involves inhibiting endoplasmic reticulum stress-dependent ferroptosis through the activation of AMPK/SIRT1 and the inhibition of the ATF4/CHAC1 axis [ 124 ]. Similarly, kaempferol was proven effective in ameliorating acetaminophen-induced liver injury. It achieved this by activating the Nrf2 pathway, leading to the upregulation of GPX4 and SOD2 expression, the reduction of ACSL4 expression, alleviation of intracellular iron accumulation, and ultimately the inhibition of ferroptosis [ 125 ]. Additionally, epigallocatechin-gallate has demonstrated its ability to alleviate liver injury induced by a high-fat diet. The accumulation of triglycerides in the liver due to a high-fat diet renders the organ susceptible to damage from oxidative stress, inflammation-induced lipid peroxidation, and ferroptosis. Epigallocatechin-gallate protects the liver by mitigating iron accumulation, increasing the expression of GPX4, decreasing the expression of ACSL4 and COX2, and ultimately inhibiting ferroptosis [ 126 ]. Liver fibrosis is a prevalent condition associated with numerous chronic liver diseases, marked by the excessive production and build up of extracellular matrix proteins, including α-smooth muscle actin and collagens. Hepatic stellate cells play a pivotal role in this process, as they undergo activation and contribute to the synthesis of matrix proteins. Research has indicated that specifically targeting and inducing the death of these cells can mitigate fibrosis, with ferroptosis emerging as a potential targeted mechanism for this purpose [ 127 , 128 ]. Chrysophanol has expressed some potential to attenuate liver fibrosis by removing activated hepatic stellate cells through the induction of ferroptosis through the downregulation of GPX4 and SLC7A11 expression [ 128 ]. Danshensu, isoliquiritigenin, and wogonoside are phenolic compounds with the proven potential to alleviate liver fibrosis through induction of ferroptosis in hepatic stellate cells. They downregulated the expression of GPX4 and SLC7A11, and promoted ROS production and lipid peroxidation which leads to cell death through ferroptosis [ 9 , 127 , 129 ]. Acute lung injury can be caused by different mechanisms such as infections, non-chest traumas, massive blood transfusions, acute pancreatitis, and prolonged oxygen supplementation during hospitalization. It is characterized by the production of inflammatory mediators, increased permeability of the barrier between the capillary endothelium and alveolar epithelial cells and pulmonary oedema. It has been shown that non-apoptotic cell death mechanisms, such as ferroptosis, are implicated in acute lung injury. The most common treatments for acute lung injury are glucocorticoids, which come with serious side effects. Ferroptosis inhibition is seen as a new potential target for the development of new therapeutics [ 130 , 131 , 132 ]. Chicoric acid was able to alleviate LPS-induced lung injury in mice thanks to its anti-inflammatory and anti-oxidative activities. It reduced the production of inflammatory cytokines and ROS, inhibited lipid peroxidation, increased levels of GSH and activated the Nrf2/HO-1 pathway [ 133 ]. Ferulic acid and Naringenin are other polyphenolics with a proven ability to alleviate acute lung injury through activation of the Nrf2/HO-1 signaling pathway and subsequent inhibition of ferroptosis [ 134 , 135 ]. Also, quercetin alleviated lung injury through ferroptosis inhibition by activating the Sirt1/Nrf2/GPX4 pathway [ 130 ]. Significantly, as described before, both the induction and inhibition of ferroptosis by polyphenols have demonstrated therapeutic benefits in different pathological states. As this review aims to provide the latest data on the potential use of polyphenols in the context of ferroptosis, it endeavors to contribute to the exploration of these compounds as potential therapeutics for the future. By presenting up-to-date information, this review seeks to elucidate the nuanced role of polyphenols in modulating ferroptosis and their potential applications in addressing various pathological conditions. The multifaceted effects of polyphenols on ferroptosis pave the way for a comprehensive understanding of their therapeutic potential, marking them as promising candidates for future medical interventions. Specifically, Table 1 presents a compilation of polyphenols capable of inducing ferroptosis, accompanied by their verified mechanisms of action, the model systems in which these effects were examined, and corresponding references. Additionally, Table 2 provides parallel information on polyphenols identified as inhibitors of ferroptosis. These tables serve as comprehensive up-to-date references, offering valuable insights into the diverse actions of polyphenols in modulating ferroptosis. List of polyphenols as inducers and enhancers of ferroptosis, verified mechanisms of their action, the model systems in which these effects were examined, and corresponding references. List of polyphenols with inhibitory activity on ferroptosis, verified mechanisms of their action, the model systems in which these effects were examined, and corresponding references.

From the data gathered within this review, it can be summed up that the parameters examined during the evaluation of the potential of polyphenols to promote or inhibit ferroptosis in most cases include the evaluation of levels of GSH, oxidized GSH, MDA, free cellular iron levels, ROS, and the expression of genes and proteins involved in the regulation of ferroptosis, such as GPX4, HO-1, Nrf2, FTH1, and others. Although there’s been a lot of research on the potential of polyphenols to modulate ferroptosis, very little is known about the influence of plant extracts that contain numerous polyphenols, with some of them possibly possessing antagonistic activities. Additionally, the effects of polyphenols on ferroptosis can vary depending on factors such as concentration and bioavailability. It would be of interest to construct structure–activity relation studies to gain insight into what structural elements of polyphenols are responsible for their desired activities toward ferroptosis, which can open the door to the development of new drugs based on naturally occurring phytochemicals. Also, there is a need for further study of the role that ferroptosis plays in some diseases which would provide additional targets for potential treatments. According to the author’s opinion, the emerging field of ferroptosis and its modulation by natural compounds like polyphenols presents a fascinating and promising area of research with profound implications for human health and disease, notably for cancer, cardiology, neurology, and transplantation. The intricate interplay between ferroptosis and various physiological processes underscores the complexity of cellular regulation and underscores the potential for innovative therapeutic interventions. Furthermore, the duality of polyphenols in either promoting or inhibiting ferroptosis, as exemplified by compounds like curcumin, highlights the complicated nature and wide range of natural product pharmacology. This complexity underscores the importance of comprehensive research efforts to unravel the precise mechanisms underlying these effects and their potential applications in different disease contexts. As we delve deeper into understanding ferroptosis and its modulation, it becomes increasingly clear that exploring not only the molecular pathways involved but also the practical aspects of delivering these compounds into specific cells is essential for translating research findings into effective clinical interventions. Overall, the study of ferroptosis and its interaction with natural compounds like polyphenols represents an exciting frontier in biomedical research, offering new insights and opportunities for improving human health and combating various diseases.