Epigenetic and metabolic interplay in cutaneous squamous cell carcinoma

Abstract

With the ageing of the population and increased levels of recreational sun exposure and immunosuppression, cutaneous squamous cell carcinoma (cSCC), is both an enormous and expanding clinical and economic issue. Despite advances in therapy, up to 5000–8000 people are estimated to die every year from cSCC in the U.S., highlighting the need for both better prevention and treatments. Two emerging areas of scientific discovery that may offer new therapeutic approaches for cSCC are epigenetics and metabolism. Importantly, these disciplines display extensive crosstalk, with metabolic inputs contributing to the chromatin landscape, while the dynamic epigenome shapes transcriptional and cellular responses that feedback into cellular metabolism. Recent evidence suggests that indeed, epigenetic and metabolic dysregulation may be critical contributors to cSCC pathogenesis. Here, we synthesize the latest findings from these fast-moving fields, including how they may drive cSCC, yet also be harnessed for therapy.

1 |. INTRODUCTION

Keratinocyte cancers (KCs) outnumber all other human malignancies combined, with cutaneous squamous cell carcinoma (cSCC) being the second most common cancer worldwide. One million cases of cSCC are treated in the United States each year with fatalities reaching up to 8000, rivaling those attributed to melanoma.^1–3 Climbing incidence rates show a 200% increase over the past three decades. However, accurate numbers are difficult to know for certain as reporting to national cancer registries is not required for KCs.^4,5 These data underscore the need for novel therapeutics and improved risk assessment. Epigenetic modifiers have been discovered as highly mutated in all forms of cancer, including cSCC.^6,7 Moreover, these enzymes can be easily targeted with small molecule inhibitors as the modifications catalysed by epigenetic modifiers are highly reversible.^8,9 Many of these compounds are already in clinical trials while some are already approved for the treatment of cancer.¹⁰ Adapting and developing epigenetic inhibitors may provide new therapies for these incredibly common cancers. Additionally, while cancer metabolism has been studied for almost a century, it is still poorly understood in the context of cSCC.¹¹ In recent years, connections between epigenetic modifiers and metabolites have emerged as one of the most exciting areas of study in all of biology and medicine, and in particular in cancer. Given the high rate of epigenetic mutations in cSCC and the predicted consequences of altered metabolism on its initiation and maintenance, the overlap between these two areas of study holds promise for improved treatments and disease outcomes.

Epigenetic modifiers are integral regulators of gene expression and chromatin organization, which in turn is crucial for cellular development and differentiation.¹² Over the past several years, epigenetic modifiers have been discovered as some of the most highly mutated genes in all forms of cancer, especially cSCC where they are mutated in >50% of all cases.¹³ Among the most frequent are those that write and erase histone post-translational modifications and DNA methylation. These enzymes facilitate the deposition or removal of chemical moieties, or modifications, on the histone proteins and DNA that ultimately result in heritable changes in gene expression.¹⁴ When these enzymes are dysregulated, it can create broad and lasting changes on the transcriptome of the cell. These enzymes rely on several cofactors to facilitate their enzymatic reactions, many of which are intimately involved in metabolism. While changes in metabolism during carcinogenesis are well documented, how these changes affect the function of critical epigenetic enzymes and gene regulation is just beginning to be elucidated.¹⁵ This review will discuss understanding of how epigenetic and metabolic dysfunction may contribute to cSCC pathogenesis.

2 |. EPIGENETIC DYSREGULATION: KEY DRIVER IN THE PATHOGENESIS OF SCC?

In addition to the canonical cancer genes TP53, NOTCH1/2 and HRAS, histone modifiers and DNA methylation enzymes are some of the most frequently mutated genes in all forms of SCC. According to data from both The Cancer Genome Atlas (TCGA) and the Catalogue of Somatic Mutations in Cancer (COSMIC), KMT2D (MLL4) (~52–62%), KMT2C (MLL3) (~32–37%), CREBBP (~30–32%), EP300 (~29%), TET1 (~27–31%), TET3 (~23–30%), TET2 (~19–24%), KMT2A (~19–30%), DNMT1 (~16–17%), SETD2 (~15%), KDM6A (~10–11%) and EZH2 (~7.5%) are among the most highly mutated epigenetic modifiers in cSCC.^6,7,16–18 Many of these modifiers function to activate gene expression programmes involved in development and differentiation (Figure 1). This process is critically important in somatic self-renewing epithelia like the skin that is constantly regenerating and consequently undergoing drastic transcriptional changes.¹⁹ When this coordinated balance is disrupted it can promote a variety of diseases, including cSCC.²⁰ In fact, a poor degree of differentiation in cSCC is indicative of higher stage tumors and poor prognosis.²¹

FIGURE 1.

Epigenetics can impact numerous mechanisms to promote cSCC. Epigenetic chromatin modifying enzymes regulate gene transcription through the control of chromatin accessibility, which can promote or inhibit access to the DNA template by transcriptional machinery. Many of these enzymes are highly mutated or dysregulated in cSCC. Many transcriptional activators undergo presumed loss of function mutations in cSCC (KMT2C, KMT2D, KDM6A, SETD2, CREBBP (CBP) and EP300 (p300)), while several transcriptional repressors are overexpressed (LSD1, HDACs). Disruption of the normal function of these enzymes has been associated with a variety of downstream effects ranging from altering normal transcription, metabolism and immunosurveillance to impairing cellular differentiation and DNA repair systems

2.1 |. Histone methylation and acetylation

EZH2, for example, is the main histone lysine methyltransferase of the Polycomb Repressive Complex 2 (PRC2) which catalyses the repressive histone mark H3K27me3. A murine model of Ezh2 deletion in the epidermis demonstrated that Ezh2 plays a critical role in maintaining epidermal proliferative potential through its ability to repress Cdkn2a and Cdkn2b expression. Consistent with these findings, several studies have implicated EZH2 mutations in differentiation defects and it is shown to be overexpressed in a number of cancers.^22–24 In addition, data from one study in cSCC cells suggested that EZH2 may repress innate immunity to repress antitumor immune responses and promote an increased risk of metastasis in cSCC.²⁵ In line with these data, EZH2 inhibitor treatment of oral SCCs has demonstrated gene expression changes consistent with reduced proliferation.²⁶

While EZH2 mutations frequently result in a gain of function, individual mutations can have unique, context-dependent effects. For example, a recent study demonstrated that loss of function mutations in FAT1 inactivate EZH2, leading to derepression of SOX2 and promoting an epithelial to mesenchymal hybrid state and cSCC.²⁷ On a metabolic level, a deficiency of EZH2 in leukaemia led to an increase in branched-chain amino acid transferase (BCAT1) though reduced repressive H3K27me3. This resulted in increased intracellular branched amino acids and activation of the mTORC1 signalling pathway and progression of the leukaemia.²⁸

Opposing EZH2 and the PRC2 complex, the histone demethylase KDM6A is a member of the COMPASS-like complex that demethylates the repressive H3K27me3 mark and is shown to act as a tumor suppressor in several epithelial cancers.^29–31 In a various forms of SCC, KDM6A undergoes loss of function mutations as well as deletions.^16,17 The COMPASS-like complex also consists of KMT2C and KMT2D, which both catalyse H3K4me1 and recruit the histone acetyltransferase p300 (EP300) to deposit H3K27ac, which collectively result in enhancer activation.^32,33 This coordinated transition of histone modifications activates the enhancer landscape and causes a massive switch in gene regulation that facilitates the expression of critical differentiation genes. Importantly, recent work has shown that mutations in KMT2D and KDM6A are amongst the earliest carcinogenic events in normal epithelial tissues and are critical for the early formation of clones that precede full malignant transformation.^34,35 Data from cSCC tumors likewise suggests that these mutations result in loss of function, as they typically consist of missense, nonsense and truncating mutations, in addition to frequent deletions as well.^16,17 In normal human keratinocytes and 3D skin organoid models, KMT2D has been shown to directly regulate the activation of the target enhancers of p63, a master epidermal transcription factor with complex roles in gene expression throughout development and differentiation.^36,37

In addition to a role in promoting differentiation, evidence from some other cancer types suggests that KMT2D dysfunction can also have effects on metabolism. In recent work utilizing a murine model of lung cancer, Kmt2d loss disrupted enhancer function, reducing the expression of the circadian regulator and inhibitor of glycolytic genes, Per2. This drove the activation of genes involved in both glycolysis and oxidative phosphorylation to promote lung tumorigenesis. Importantly, the authors found that inhibitors of glycolysis effectively extended survival in mice with Kmt2d deficiency, though not in in tumors with wildtype Kmt2d.³⁸ Other studies in models of pancreatic cancer as well as B-cell lymphoma have identified that loss of KMT2D expression or function could drive both increased glycolysis and oxidative phosphorylation, respectively.^39,40 In each of these studies, these epigenetic-metabolic links revealed therapeutic vulnerabilities which could be harnessed to inhibit these cancers and highlight how future research should address whether similar metabolic dysfunction may result in cSCC models from loss of KMT2D. Opposing KMT2C and KMT2D is LSD1, a histone demethylase that removes the H3K4 methylation. LSD1 has been shown to repress epidermal differentiation in primary human keratinocytes and suppress cSCC in a human organoid model.⁴¹ Consistent with this, LSD1 has been shown to be overexpressed in numerous cancers, including SCCs.⁴² This delicate balance between all these histone modifiers allows for normal differentiation of the epidermal layers.

Data from regulators of histone acetylation suggests a similarly complex, yet important role in cSCC. CBP (CREBBP) complexes with p300 (EP300) to form a co-activator complex that binds numerous transcription factors and possesses acetyltransferase activity. CREBBP and EP300 are both thought to undergo loss of function mutations in cSCC. However, while one study which involved staining 165 human patient cSCCs suggested that overexpression of p300 may be a marker of poor prognosis, other work has suggested the CBP/p300 complex is actually an important suppressor of EGFR-Ras-ERK signalling to repress cSCC in a murine model, again suggesting a potentially context-dependent role.^43,44 Amongst the histone deacetylases (HDACs), pan-HDAC inhibitors have been shown to be effective in inhibiting cSCC both in vitro and in vivo previously.^45,46 For example, inhibition of cSCC lines with vorinostat, a pan-HDAC inhibitor was attributed to its ability to reduce mTOR signalling, which is frequently activated in cSCC.⁴⁷ More recent work in cSCC cell lines has shown that a specific inhibitor of HDAC3 could inhibit cSCC epithelial mesenchymal transition (EMT).⁴⁸

SETD2 is a well-established tumor suppressive histone modifier and is mutated in a range of different cancers including clear cell renal cell carcinoma, leukaemia, bladder carcinoma, melanoma, lung adenocarcinoma, colorectal adenocarcinoma and cSCC among others.^7,49,50 This is perhaps unsurprising as SETD2 has been shown to be involved in the DNA damage response, alternative splicing, RNA processing and transcription elongation.^51–54 This is particularly significant given that DNA-damaging UV-radiation is the major carcinogen driving cSCC development. Amongst its diverse roles, SETD2 functions as a methyltransferase, known to complex with RNA pol II and serve as the only enzyme known to deposit the H3K36me3. This histone mark is recognized by several different reader enzymes that regulate gene expression, as well as both DNA (DNMT3A and DNMT3B) and RNA (METTL14) methyltransferase complexes, highlighting the intricate complexity of its functions.^55–57 Further cementing its importance in cancer biology, it is known to complex with the major tumor suppressor p53 and regulate downstream p53 target genes.⁵⁸ Indeed future studies should address the potential role SETD2 dysfunction may play in the development of cSCC.^{52–54,58–60}

2.2 |. DNA methylation

Histone modifications are not the only epigenetic marks responsible for alterations in gene expression. Modification to the DNA itself via DNA methylation also plays a major role in epigenetic gene regulation and carcinogenesis. It has long been known that DNA methylation is decreased in a wide variety of cancers when compared to surrounding tissues.^61–63 DNA methylation is largely repressive of gene expression and plays a major role in development. While DNA methylation states largely remain static from cell to cell, there are discrete dynamic regions that alter gene expression during development.⁶⁴ Disruptions in the methylation states of DNA leads to genome instability and altered gene expression that can result in diseases like cancer.⁶⁵

Genome-wide studies profiling human cSCC patient samples, as well as cSCC premalignancies (actinic keratoses, or AKs), has demonstrated that there may be distinct subsets of AKs and cSCC tumors, some that display methylomes reminiscent of stem cell and cancer methylomes, while others resembled methylomes observed in normal skin.⁶⁶ The authors reasoned that this may reflect distinct cells of origin based on differentiation state. In contrast, a more recent genome-wide methylation profiling study found that unique methylation signatures could be identified and distinguish classes of cSCC ranging from AK to ‘initial invasive’ to ‘high-risk non-metastatic’ to ‘metastatic’ carcinoma.⁶⁷

Regarding the role of specific modifiers of DNA methylation, the de novo DNA methyltransferases, DNMT3A and DNMT3B, were discovered to both play important roles in epidermal stem cell enhancer regulation, as well as serve to suppress squamous carcinogenesis in a murine chemical carcinogenesis model.^68,69 For its part, the maintenance DNA methyltransferase, DNMT1, was recently shown to be upregulated in human UVB-radiation induced cSCC patient samples while the DNA demethylation enzymes TET1 and TET2 were downregulated resulting in silencing of tumor suppressors.⁷⁰ TET enzymes are responsible for active DNA demethylation through a series of oxidation events that are replication independent. TET enzymes are highly mutated in hematopoietic cancers as well as solid tumors, including cSCC. Notably, TET2 expression has been shown to be lost in human oral head and neck SCC patient samples, and restoration of TET2 was able to inhibit tumor cell growth and migration.^71,72 Despite this and frequent dysregulation observed in cSCC, the role of TET enzymes in cSCC is poorly understood. Filling these gaps will provide significant insight towards uncovering why epigenetic disruption is such a common feature of cSCC. Furthermore, while some studies have linked epigenetic loss of function mutations to early clonal expansion in cancer other studies have suggested that mutations in epigenetic modifiers are more common in aggressive and metastatic disease.^6,7,34,35 Therefore, it will be imperative to decipher how different mutations and mutational signatures may ultimately contribute to these variable disease states, findings which will likely offer new therapeutic insights and strategies for both prevention and treatment.

3 |. METABOLIC DISRUPTION IN CSCC PATHOGENESIS

3.1 |. Cancer metabolism

For nearly a century we have known about altered metabolism driving the initiation and maintenance of cancer since Otto Warburg published his findings that cancer cells show an increase in glycolysis and lactic acid production relative to normal tissues.^73–75 Today, we understand that this paradigm of increased glycolysis is just one manner through which cancer cells derive energy. Indeed, numerous lines of evidence have now identified that cancer cells also actively rely on upregulation of oxidative phosphorylation (OXPHOS) and display increased rates of oxygen consumption, collectively fuelling the enormous need for energy and anabolism.^76–82

There are several factors that contribute to metabolic changes in cancer, one of which is adaption to chronic hypoxia. HIF-1α is the main transcriptional regulator of the hypoxic response in cells. It is directly regulated by oxygen concentration and under hypoxic conditions, it translocates to the nucleus where it activates a host of hypoxia response genes. Among these are the glucose transporter GLUT-1, the rate-limiting enzyme in glycolysis hexokinase-2 (HK-2), lactate dehydrogenase (LDH) which catalyses the conversion of lactate to pyruvate, and pyruvate dehydrogenase kinase-1 (PDK1) that inhibits the conversion of pyruvate to acetyl-coA by inactivating pyruvate dehydrogenase.^83–85 Through this, hypoxia and HIF-1α expression effectively upregulate glucose uptake and glycolysis while limiting the amount of acetyl-coA available to feed into the citric acid (TCA) cycle and OXPHOS. This is especially relevant for the skin which exhibits a gradation of oxygen throughout the epidermal layers that is inversely proportional to differentiation. Thus, the most undifferentiated layers exist in a state of mild hypoxia while the terminally differentiated dead squames are in direct contact with atmospheric oxygen. Consistent with this, hypoxia is known to promote a more proliferative state in keratinocytes while oxygen exposure triggers differentiation.^86–89 Additionally, recent evidence suggests that HIF-1α promotes UVB-mediated cSCC in mouse models through enhancing DNA repair and increasing levels of reactive oxygen species (ROS).⁹⁰

3.2 |. Evidence for the role of altered metabolism in keratinocyte cancers

Accumulating evidence suggests that altered metabolism may play an essential role in the initiation and maintenance of cSCC.¹¹ For example, recent studies suggest that nicotinamide adenine dinucleotide (NAD+), an essential co-enzyme of redox reactions for ATP production and for other metabolic processes, promotes PARP1-mediated DNA repair in the setting of UV-radiation.^91,92 Oral administration of its precursor, nicotinamide, has been shown to prevent KCs in high-risk populations.⁹³ Other recent work utilized an in vivo UVB-radiation-induced murine cSCC model and found that while glycolysis, TCA cycle and fatty acid β-oxidation are decreased in the earliest stages of UVB-induced cSCC tumor formation, mitochondrial ATP synthesis and the distal part of the ETC are increased due to upregulation of dihydroorotate dehydrogenase (DHODH).⁹⁴ This activation of the ETC and DHODH were both shown to be essential for subsequent DNA repair and the avoidance of apoptosis by the cancer cells in this model, underscoring the importance of metabolic rewiring as an essential aspect of UVB-mediated cSCC development.

Further supporting the role that metabolism has in cSCC is its influence in the closely related basal cell carcinoma (BCC). Metabolic connections can be made to the hedgehog pathway (Hh), the most frequently dysregulated pathway and main driver of BCC. The activity of the Hh pathway can be affected through both intra- and extracellular metabolites like sterols and lipids.^95–97 Several recent studies have highlighted that cholesterol metabolism in particular may have a critical influence on Hh signalling.^98,99 In this way, altered metabolism can directly impact the balance between tumor suppression and carcinogenesis in BCC. In both BCC and cSCC, TP53 is one of the most highly mutated genes. P53 has been shown to downregulate the expression of GLUT-1 while also increasing OXPHOS by inhibiting the conversion of pyruvate to lactate and maintaining the cytochrome c oxidase complex.^100–102 In contrast, upregulation of oncogenes can cause the opposite metabolic changes. HRAS mutations upregulate glycolysis by increasing expression of glycolytic enzymes while KRAS mutations can increase the expression of GLUT1, HK-1, HK-2, PFK-1, ultimately affecting glutamine metabolism.^103–106 Together, these findings demonstrate how either the loss of a major tumor suppressor or activation of an oncogene can directly alter metabolism in the cell predisposing it to carcinogenesis.

Other work has further examined the links between the DDR and metabolism, given its critical role in epidermal homeostasis as well as the prevention of cSCC. One study found that cell lines deficient in DDR (via deficiency of the nucleotide excision repair protein, XPC) show increased rates of glycolysis and decreased rates of OXPHOS consistent with several examples of cancer metabolism. The authors attributed this metabolic change to AKT and NOX1 activation alongside accumulation of ROS leading to mitochondrial DNA (mtDNA) deletions.^107,108 This is consistent with previous studies showing that UV-irradiated, sun exposed skin and keratinocyte cancers demonstrate significant mutations and/or deletions of mtDNA.^109,110 Supporting these links between metabolism, DDR and cSCC, several studies have also found that delivering the biguanide diabetes drugs metformin or phenformin can activate AMP-activated kinase (AMPK) to both inhibit keratinocyte proliferation and promote DNA repair and differentiation. In one report, metformin was found to reduce UVB-induced skin tumors in mice through both enhanced DNA repair and reduced ERK signalling, while another group observed that metformin could block mTOR/AKT signalling and reduce tumor growth in a human cSCC mouse xenograft model.^111,112 Most recently, utilizing the DMBA-TPA murine cSCC model, it was shown that phenformin could both inhibit tumor growth and promote keratinocyte differentiation through the AMPK-mediated activation of calcineurin and NFATC1.¹¹³

Collectively these studies highlight the multiple ways metabolic pathways may interface with the balance between proliferation, differentiation and carcinogenesis. No doubt future studies will further reveal the extent that altered metabolism impacts the initiation and maintenance of cSCC as well as the mechanisms by which it most frequently occurs. Most importantly, the interplay between metabolism and key tumor suppressors and oncogenes is necessary to fully understand the progression of this disease and to develop more effective treatments.

4 |. THE CONVERGENCE OF EPIGENETIC AND METABOLIC DISRUPTION IN CSCC PATHOGENESIS

While epigenetic disruption may indeed promote cSCC development through a variety of mechanisms (Figure 1), one area increasingly recognized as being a potentially critical driver of cancer is through its interplay with metabolism (Figure 2).¹¹⁴ Importantly, many epigenetic modifiers are dependent on metabolite cofactors to carry out their various enzymatic reactions.

FIGURE 2.

The interplay between metabolism and epigenetic modifiers. Key metabolites act as cofactors for several epigenetic enzymes, and fluctuations in the abundance of these metabolites can affect the function of these enzymes. Histone lysine acetyltransferases require acetyl-coA from glycolysis and other pathways to facilitate acetylation reactions. Histone, DNA and RNA methyltransferases use the methyl donor SAM to carry out their enzymatic functions. Histone, DNA and RNA demethylases use both molecular oxygen and α-ketoglutarate cofactors in their demethylase reactions creating carbon dioxide and succinate byproducts. When these pathways are altered in carcinogenesis the abundance of these metabolites shifts, resulting in dysfunctional epigenetic modifying enzymes and altered gene expression

4.1 |. Histone methyltransferases

Methylation of both DNA and histone proteins by methyltransferases are dependent on the abundance of S-adenosylmethionine (SAM) and one-carbon metabolism. In turn, histone modifications can serve as metabolic energy stores in times of nutrient abundance.¹¹⁵ All of those methyltransferases highly mutated and dysregulated in cSCC (ie KMT2D, KMT2C, SETD2, EZH2, DNMT1 and DNMT3A/B) require a methyl donor to facilitate their methyltransferase reactions. SAM is an important intermediate of one-carbon metabolism, and these pathways are in turn critical for anabolic metabolism that is often hijacked in cancer to fulfil the needs of hyperproliferative cancer cells.¹¹⁶ Serine-dependent one-carbon metabolism, for example, is a promising cancer target.¹¹⁷ This pathway promotes the de novo synthesis of ATP through serine catabolism, providing the adenosine necessary to form ATP. SAM is synthesized by the transfer of adenosine from ATP to a molecule of methionine. Then, after a methylation reaction, SAM loses a methyl group and becomes S-adenosylhomocysteine (SAH). Adenosine from SAH is recycled back into ATP synthesis as homocysteine is able to be re-methylated by the same serine catabolism pathway and fed back into SAM production (Figure 2).¹¹⁶ As long as there is abundant serine, cells are able to recycle SAM and increase de novo ATP production. Consistent with this, a recent study utilized a mouse cSCC model to demonstrate that extracellular serine was required for tumor initiation, and that inhibition of serine synthesis not only blocked tumor initiation but did this in part through the secondary activation of pro-differentiation epigenetic modifiers (that require and utilize α-ketoglutarate, αKG).¹¹⁸ Another study found that by manipulating the metabolic flux of the methionine cycle though nutrient availability they were able to change the levels of SAM and SAH in the cell and consequently change histone methylation levels resulting in altered gene expression.¹¹⁹ This demonstrates clear crosstalk between metabolism and the epigenome through the metabolic cofactors of epigenetic modifying enzymes and how the abundance of SAM in the cell directly impacts the function of epigenetic enzymes that are also frequently dysregulated and/or mutated in cSCC.

4.2 |. Histone demethylases

These findings are not exclusive to methyltransferases. Demethylase enzymes use a host of metabolic cofactors to facilitate their enzymatic reactions. The Jumonji C histone lysine demethylases such as KDM6A, as well as the enzymes that perform DNA (TET family) and RNA (FTO, ALKBH5) demethylation are all members of the 2-oxoglutarate-dependent oxygenase (2-OGX) family that require αKG and O₂ cofactors to demethylate histone, DNA and RNA substrates, respectively, resulting in succinate and CO₂ byproducts.^120,121 Notably, these enzymes can also be inhibited by metabolism-derived structural analogues of αKG including succinate, fumarate and the oncometabolite D-2-hydroxyglutarate (D-2HG), demonstrating an additional mechanism by which cancer metabolism can result in the inactivation of these important tumor suppressive enzymes.^114,122

To further underscore the interrelationships between epigenetics and metabolism, it has recently been shown that KDM6A, which is frequently mutated in cSCC, is also able to sense cellular oxygen. This is due to the exceptionally low-affinity KDM6A has for oxygen compared to the other members of the 2-OGX family. This study demonstrated that hypoxia can induce increased levels of H3 K27 methylation specifically through the loss of KDM6A activity, and that this hypoxia-induced inactivation of KDM6A promotes the persistence of H3K27me3 and results in the inhibition of cellular differentiation¹²³ Given that there are variable levels of oxygenation in the epidermis, coupled with the fact that hypoxia can promote keratinocyte proliferation and is present in most solid tumors, including cSCC, these results suggest a potential link between cancer metabolism and cSCC initiation and maintenance through the inactivation of epigenetic enzymes.

4.3 |. Histone acetylation

Beyond regulators of methylation, the histone acetyltransferases CBP and p300 are also involved with HIF1 and the hypoxic response. CBP (CREBBP) and p300 (EP300), mutated in approximately one third of cSCCs, were previously shown to complex with HIF1 to induce the expression of hypoxia response genes including those important for tumor angiogenesis.¹²⁴ More recent studies have corroborated those findings and further reported that p300 acetylates and stabilizes HIF-1α under hypoxic conditions.^125,126 Although p300 usually functions as a tumor suppressor, targeting this function with inhibitors has been shown to be beneficial in treating some cancers, and a range of HIF-1α-p300/CBP inhibitors are currently being tested for their antitumor efficacy.^127–129

In addition to their association with the hypoxic response, histone acetyltransferases also require the metabolic cofactor acetyl-coA to facilitate their acetylation reaction. Acetyl-coA is central to many metabolic processes and is integral in both glycolysis and OXPHOS. Acetyl-coA can be synthesized through several different pathways and can also be shuttled through different cellular compartments depending on the needs of the cell.¹³⁰ In a nutrient poor state, acetyl-coA is fed into the mitochondria and the TCA cycle where it can be used to synthesize ATP and ketone bodies, while in a nutrient rich state acetyl-coA is transported into the cytosol for sterol and fatty acid catabolism as well as into the nucleus.¹³¹ Once in the nucleus, the abundance of acetyl-coA directly affects histone acetylation, with excess acetyl-coA triggering the activation of cellular growth genes.¹³² The enzymes responsible for the generation of acetyl-coA from several metabolic pathways have also been shown to affect levels of histone acetylation.^133–136 Seeing as acetyl-coA is a key metabolite manipulated in cancer metabolism, its control over certain epigenetic enzymes raises many questions as to its potential role in cSCC.

5 |. CONCLUSIONS AND FUTURE PERSPECTIVES

Metabolism and its role in promoting cancer has been extensively studied over the past century. However, there remains a poor understanding of its function in cSCC. Furthermore, epigenetic enzymes are becoming increasingly popular targets in cancer therapeutics and may prove useful in cSCC as epigenetic modifiers are among its most highly mutated genes. Current evidence that epigenetic modifiers both have an affect on and are affected by metabolism reveals an intriguing avenue for cSCC research, and how it may be exploited to improve treatment outcomes. Fully understanding what metabolic changes most frequently occur in cSCC will also provide a more complete picture of what epigenetic changes are occurring and allow for the use of more targeted, effective therapies to improve risk assessment and disease outcome for these extremely common cancers.