Cancer to Cataracts: The Mechanistic Impact of Aldo-Keto Reductases in Chronic Diseases.

The biochemical foundation of life encompasses the ability to evade entropy (death) and trigger its successful duplication. This is guaranteed by the specificity of enzymes that participate in streamlined regulation of metabolites, thereby securing the organism’s survival. However, promiscuity also plays an important role in housekeeping, as it involves the detoxification of extraneous molecules that could hamper survival and growth [ 1 , 2 ]. Aldo-keto reductases (AKRs) are one of the promiscuous superfamilies along with other detoxifying enzymes like short-chain (SDR), glutathione-S-transferases (GSTs), medium-chain dehydrogenases (MDRs), cytochrome P450s (CYP450s) etc. AKRs cause multiple chronic diseases like cataracts, diabetes, liver disease, various cancers etc. [ 3 , 4 ]. Despite numerous studies detailing their metabolic functions, detoxification processes, and involvement in chronic diseases, a comprehensive correlation among these areas remains largely unexplored. AKRs play a crucial role in several fundamental metabolic and detoxification pathways, which are vital for the survival of higher eukaryotes [ 5 ]. However, the rise in consumption of synthetic molecules (drugs, narcotics, flavoring agents, processed foods) sometimes lead to unfavorable consequences due to the promiscuity of AKRs. While other oxidoreductases like CYP450s, SDRs, and MDRs also participate in core metabolic pathways, AKRs are comparatively more ubiquitous [ 6 - 8 ]. For example, in core biosynthetic pathways like retinoic acid biosynthesis, MDRs, SDRs, and AKRs act as isozymes and oxidize retinol to retinaldehyde, and then to retinoic acid. The three superfamilies share comparable and low Km values for retinoids, ranging from 0.12 to 1.1µM. However, they display substantial differences in their kcat values. MDRs are more effective retinol dehydrogenases while RDH11 (an SDR) and AKR1B10 are efficient retinaldehyde reductases in vitro . Interestingly, cell culture studies demonstrate RoDH-4 as the main retinol dehydrogenase and AKR1B1 as the main retinaldehyde reductase hinting at differing expression profiles [ 9 ]. However, the rest of the pathway is regulated by specific enzymes that are not housekeeping proteins. Interestingly, no diseases are associated with this pathway concerning AKRs or other isozymes due to specificity of other enzymes. On the other hand, AKRs are present in multiple stages during steroid synthesis and are a major cause for breast, prostate, and testicular cancer due to over-production of androgens and estrogens leading to cell proliferation [ 10 ]. Additionally, AKRs are major participants of detoxification specifically redox reactions due to their biodistribution. The intestine and liver are responsible for the first-pass metabolism and bioavailability of orally administered drugs. mRNA and protein expression studies of all human AKRs (AKR1A1, AKR1B1, AKR1B10, AKR1B15, AKR1C1, AKR1C2, AKR1C3, AKR1C4, AKR1D1, AKR1E2, AKR7A2, and AKR7A3) and SDRs (CBR1, CBR3, CBR4, DCXR, DHRS4, HSD11B1, and HSD17B12) revealed that AKRs are highly expressed in the liver and the upper regions of the intestine (duodenum and jejunum), with expression declining toward the rectum. Among SDRs, CBR1 and DHRS4 are highly expressed in the upper regions, while the expression levels of the other isoforms are almost uniform across all regions. In the case of AKRs, AKR1B10 exhibits the highest expression, followed by AKR7A3 and CBR1. Each of these contributes to more than 10% of the total AKR and SDR levels in the small intestine. The expression pattern in the human intestine is markedly different from that in the human liver, where the AKR1C isoforms are primarily expressed. It is clear that AKRs have a more uniform distribution and expression throughout the alimentary canal and its associated organs compared to MDRs and SDRs [ 10 , 11 ]. As a result, they are major participants in performing redox reactions against many chemotherapeutic agents resulting in chemo resistant cancers. Indeed, AKRs play a significant role in essential pathways and detoxification reactions. Their ubiquitous biodistribution makes them a major contributor to many chronic diseases. This is evident even in lower organisms like Synechocystis sp. PCC 6803. Here, the native MDR (DMDR) and AKR (DMAKR) detoxify reactive carbonyls like acrolein. This is demonstrated by the inhibition of growth in DMDR and DMAKR double knockouts when exposed to the compound. However, knocking out these genes also inhibits the organism’s ability to perform photosynthesis. This illustrates the adverse effects of enzymes that participate in multiple pathways, a direct result of their ability to catalyze various compounds [ 12 ]. The current review describes the multifaceted roles of AKRs and their implications in various chronic diseases, as discussed, are largely influenced by their ability to catalyze various compounds, a characteristic that is closely tied to their promiscuous nature.

The human microbiome not only encompasses the actively residing microbes in the body of a healthy human being rather, it also contains bacteria that reside in various disease states as well. As described before, the human genome expresses only three families of AKRs viz AKR1, AKR6 and AKR7 however, microbes contain the rest of the known families. The participation of external AKRs is not well studied as multiple enzymes, ie, human and microbial AKRs can perform same reactions due to promiscuity. For example, human AKRs reduce the bioavailability of noncompetitive inhibitors like tolrestat and epalrestat, resulting in their limited clinical use. Interestingly, these compounds also bind to AKRs from distantly related microbes like Tm1743 from Themotoga maritima [ 86 ]. Another interesting example, albeit partially related to AKRs, is the dysfunction of human UDP Glucuronosyltransferase Family 2 Member B17 (UGT2B17). The enzyme plays a crucial role in the glucuronidation process, an intermediate step in the metabolism of steroids. In individuals with UGT2B17 gene deletion, the gut bacterial β-glucuronidases significantly impact testosterone disposition. Normally, the gene deletion results in the upregulation of AKR1D1 and AKR1C4, which convert testosterone to 5β-dihydrotestosterone and 3α, 5β-tetrahydrotestosterone. However, gut bacterial enzymes can reactivate testosterone glucuronide into testosterone resulting in higher amount of the hormone and metabolic dysregulation [ 87 ]. Hence, the overall effect of gut or pathogenic microbiome is quite hard to decipher. They could be essential for pathogenicity due to their participation in metabolic pathways or they can enhance pathogenicity by acting as detoxifying enzymes for drugs. Additionally, the promiscuity of AKRs makes this even more difficult to pinpoint significant functions. For example, babesiosis is a tick-borne disease caused by the Babesia microti . Symptoms range from none to severe, including fever, fatigue, and anemia. It is most common in the Northeastern and Midwestern US and parts of Europe. Its native AKR (BmAKR) is upregulated on day 8 post-infection and downregulated later. It is found in the cytoplasm of B. microti merozoites in mouse models. Its expression increased under oxidant stress and in response to anti-babesiosis (Atovaquone) and anti-coccidiosis drugs (Robenidine), suggesting a role in anti-parasite drug response. However, the relation of the AKR corresponding to the pathogenesis of the protozoan needs further studies [ 88 ]. On the other hand, the Staphylococcus aureus AKR IolS is the target of thymol, a natural compound used to treat S. aureus infections. Its binding increases the AKR activity of IolS and depletes NADPH within S. aureus cells, leading to a bactericidal effect making it a promising target for developing new antimicrobials [ 89 ]. Interestingly, recent studies have discovered AKRs that play a role in chronic diseases, but no ensemble study has been reported in any review yet. The bacteria Helicobacteria pylori infects the stomach, weakening its protective mucus lining. They produce urease, which neutralizes stomach acid, creating a more hospitable environment. This allows the bacteria to multiply and damage stomach tissue. The resulting irritation, combined with the stomach’s own acid, can lead to ulcers. Long-term infection may even lead to stomach cancer. Interestingly, its native AKR13C1 (HpAKR) functions over a broad pH range (4-9), with an optimum at pH 5.5. It performs cinnamyl alcohol dehydrogenase activity in H. pylori , enabling the organism to reduce a wide range of aldehydes. An isogenic HpAKR helps the organism to survive and grow under acidic conditions, suggesting its crucial role in adapting to the gastric mucosa [ 90 ]. AKRs are also involved in pathogenesis like Chagas disease caused by Trypanosoma cruzi . Chagas disease becomes chronic when the parasites inhabit heart and digestive muscles. This phase can last for years, with 30-40% of patients developing organ dysfunction. The disease often remains asymptomatic during this phase, making diagnosis and treatment challenging. It is treated using O-napthoquinone derivatives and Benzonidazole which are activated by a parasitic NADH-dependent type I nitroreductase (NTR I) but are rendered ineffective by native AKR (TcAKR) leading to drug-resistant T. cruzi and chronicity of the disease [ 91 , 92 ].

The complexity of human metabolic processes is such that even a single malfunction can lead to significant disruptions. The AKR superfamily, known for its functional diversity and involvement in numerous essential pathways, is particularly susceptible to mutagenesis. Consequently, it is often implicated in various chronic diseases ( Table 1 ). There is a clear correlation between their bioavailability and diseases caused by the enzymes. As aforementioned, AKR1A1, a conventional AKR, is known for its detoxification function. It is primarily located in the liver, and its malfunction leads only to alcohol toxicity. This underscores the importance of having detoxifying enzymes in specific organs, such as the liver. This is further supported by the presence of orthologs in rats and mice (see Figure S1 and Table S2). Additionally, AKR1A1 participates only in comparatively lesser number of metabolic reactions. It enables modification of glucoronate and retinaldehyde which encompass aliphatic ketone chains ( Figure 3 ). This is reflective of the enzyme’s comparatively narrow substrate site. Interestingly, AKR1A1 reduces a broad spectrum of carbonyl-containing compounds [ 36 ]. AKR1B1 also reduces many AKR1A1 substrates but with less activity. However, this is untrue as ketoreductases with larger substrate sites are better at catalyzing aromatic ketones compared to aliphatic ketones [ 93 , 94 ]. However, this notion is hypothetical at best as AKRs have not been extensively compared against the same substrates and warrants further exploration. Furthermore, AKR1A1’s role in detoxification and its presence in detoxifying organs (liver and kidney) portrays its distribution in the “right” place and does not cause any diseases in adults [ 3 ]. However, their presence in the fetal developmental stage is less, thereby resulting in fetal alcohol syndrome [ 4 ]. Although AKR1A1 is a known catalyst for producing L-gluconate, it is not able to convert the product further. This is not surprising as the forward reaction km is much lower than the reverse reaction. However, microbial AKRs are known to synthesize vitamin C in large amounts. For example, Corynebacterium sp. AKR is used to convert L-gluconate to Ascorbate (vitamin C) in industries [ 44 ]. Furthermore, gut bacteria are known source for vitamin C in humans [ 95 ]. Therefore, it would not be overly optimistic to speculate the contribution of gut bacteria AKRs for synthesizing vitamin C. Another AKR which has no reported promiscuity is the htAKR, which is confined to testes and is not known to cause any diseases. Comparatively AKR1Bs are more distributed and participate in central carbon metabolism thereby resulting in metabolic “ubiquity” causing cataracts and diabetic complications like retinopathy and neuropathy. Comparatively the AKR1Ds and 7As are sparse but are involved in specific metabolic diseases which are more treatable ( Figure 3 ) [ 96 , 97 ]. Similarly, AKR6As, which are not reductases but act as potassium channels, are found exclusively in the brain and heart, aiding in nerve and ion conduction ( Figure 3 ) [ 98 ]. Diseases associated with these proteins are typically due to enzyme malfunction, indicating their crucial role in maintaining metabolic and housekeeping functions. Although AR6As are known biomarkers for gastric cancer, their specific function in this context remains elusive [ 99 ]. In contrast, AKR1Bs are more widely distributed and participate in central carbon metabolism, resulting in metabolic “ubiquity” causing cataract and diabetic complications like retinopathy and neuropathy. The AKR1Ds and 7As are sparse but are involved in specific metabolic diseases which are more treatable, for example, diabetic neuropathy, retinopathy, obesity, etc. [ 42 , 68 , 75 ]. Lastly, the metabolic map of AKRs shows that AKR1Cs are the most widely dispersed of all AKRs and are involved in multiple pathways ( Figure 3 ). Due to their promiscuity they either take role of chemo resistant enzymes (liver, gut, and lung) or direct (breast, prostate) perpetrators of cancer (steroid metabolism) [ 41 ]. Furthermore, their overexpression often leads to higher production of steroids. Although there are isozymes that synthesize specific steps, noticeably, AKR1Cs and AKR1Ds are present throughout the synthesis of steroids such as testosterone, progesterone and androsterone [ 8 ]. The former is crucial for the development of male reproductive tissues and the manifestation of secondary sexual characteristics. Some studies suggest that testosterone therapy may elevate the risk of prostate cancer. Furthermore, an association between elevated testosterone levels and an increased risk of melanoma, a type of skin cancer, has been reported [ 100 ]. Similarly, androsterone an androgen pheromone, can influence the development of breast cancer. Androgens bind to androgen receptors, which are expressed in many breast cancers, potentially promoting tumor growth. Similarly, progesterone, a hormone vital for the menstrual cycle and pregnancy in women, has also been linked to cancer and is synthesized from AKR1C1 through 20α hydroxy progesterone. However, as seen in Figure 3 AKR1Cs participate not only in synthesizing the hormone but participate in the multiple steps in the pathway. High levels of progesterone are known to increase the risk of breast cancer. Conversely, some studies propose that naturally occurring progesterone may confer protection against breast cancer. The relationship between progesterone and cancer is multifaceted, influenced by factors such as the type of progesterone (synthetic or natural), its interaction with estrogen during HRT, and individual genetic variables [ 101 ]. Notably, AKR1Cs do not act as rate limiting steps for hormone synthesis, rather they are involved in “essential” function throughout the pathway thereby are major contributors of biochemical flux towards the production of steroids and their diseases. Hence, unsurprisingly they are known biomarkers for multiple cancers [ 102 ]. AKR1C1 and AKR1C4 do not have high reductase activity towards aliphatic aldehydes, aromatic aldehydes, aldoses, or dicarbonyls but can oxidize 1-acenaphthenol. AKR1C4 can also oxidize di- and tri-hydroxylated bile acids [ 37 ]. In conclusion, designing inhibitors for AKR1Cs that are closely related but have distinct functions presents a significant challenge for medicinal chemists, particularly when the goal is to design novel small molecules for inhibition. Future research is needed for developing methods to differentiate substrates and inhibitors that can bind to different conformers of AKR, a critical step towards resolving this issue [ 103 ]. Potential strategies could include structural characterization or molecular docking studies with a range of current inhibitors, the use of artificial intelligence to differentiate molecules, and the creation of a physiological atlas of AKRs in different subsets of cell populations within a tissue. Such an atlas could aid in the design of antibodies carrying specific small molecules.