Reprogramming of fatty acid metabolism in cancer

A common feature of cancer cells is their ability to rewire their metabolism to sustain the production of ATP and macromolecules needed for cell growth, division and survival. In particular, the importance of altered fatty acid metabolism in cancer has received renewed interest as, aside their principal role as structural components of the membrane matrix, they are important secondary messengers, and can also serve as fuel sources for energy production. In this review, we will examine the mechanisms through which cancer cells rewire their fatty acid metabolism with a focus on four main areas of research. (1) The role of de novo synthesis and exogenous uptake in the cellular pool of fatty acids. (2) The mechanisms through which molecular heterogeneity and oncogenic signal transduction pathways, such as PI3K–AKT–mTOR signalling, regulate fatty acid metabolism. (3) The role of fatty acids as essential mediators of cancer progression and metastasis, through remodelling of the tumour microenvironment. (4) Therapeutic strategies and considerations for successfully targeting fatty acid metabolism in cancer. Further research focusing on the complex interplay between oncogenic signalling and dysregulated fatty acid metabolism holds great promise to uncover novel metabolic vulnerabilities and improve the efficacy of targeted therapies.

In mammalian cells, FAs can either be obtained through direct exogenous uptake from the surrounding microenvironment or synthesised de novo by using nutrients, such as glucose or glutamine. It is widely accepted that a metabolic hallmark of cancer cells is lipidomic remodelling, which broadly encompasses alterations in FA transport, de novo lipogenesis, storage as lipid droplets (LDs) and β-oxidation to generate ATP. 4 However, the specific mechanisms driving particular lipid phenotypes are nuanced, and may be dependent on tumour type or molecular sub-classifications. Exogenous uptake of fatty acids allows for metabolic flexibility in cancer cells In terms of exogenous FA uptake, specialised transporters are required to facilitate efficient movement across the plasma membrane. The most well characterised of these include CD36, also known as fatty acid translocase (FAT), fatty acid transport protein family (FATPs), also known as solute carrier protein family 27 (SLC27) and plasma membrane fatty acid-binding proteins (FABPpm), all of which display increased gene and protein expression in tumours (Fig. 2 ). 5 In particular, high CD36 expression has been correlated with poor prognosis across several tumour types, including breast, ovarian, gastric and prostate. 6 , 7 In the case of highly aggressive Pten −/ − prostate cancers, CD36 promotes increased FA uptake and storage in LDs, as well as significant alterations in lipid composition encompassing elevations in acyl-carnitines (ACs), monoacylglycerols (MAGs) and other lysophospholipids, all of which are products of FA oxidation (FAO). 8 Notably, deletion of Cd36 is sufficient to rescue the aforementioned phenotypes and mitigate tumour growth, indicating that this FA transporter is integral for promoting the lipidomic remodelling of Pten- deficient prostate cancers. 8 Collectively, these findings demonstrate that CD36 plays a key role in tumour microenvironment metabolic crosstalk, ultimately shifting the dependency of tumour cells towards exogenous lipid uptake. Fig. 2 Cancer cells obtain fatty acids (FAs) from de novo lipogenesis and exogenous uptake. The exogenous uptake of FAs from the surrounding microenvironment is facilitated by specialised transporters, including CD36, FATPs and FABPpm. FAs and their synthetic products can be subsequently stored as LDs, and used for NADPH and acetyl-CoA production through β-oxidation. In terms of carbon sources for de novo lipogenesis, cancer cells rely on glucose, glutamine and acetate to synthesise citrate. Palmitate is ultimately generated from citrate through the enzymatic activities of ACLY, ACC and FASN, and can subsequently be desaturated and elongated to form a diverse group of lipid species. An alternative pathway for palmitate desaturation exists, which generates sapienate through FADS2, instead of palmitoleate. Abbreviations: GLUT1, glucose transporter 1; MCT, monocarboxylate transporter; CD36, cluster of differentiation 36; FATPs, fatty acid transport proteins; FABPpm, fatty acid-binding protein; GLS, glutaminase; IDH1/2, isocitrate dehydrogenase; ACLY, ATP–citrate lyase; ACSS2, acyl-CoA synthetase short-chain family member 2; ACC, acetyl-CoA carboxylase; FASN, fatty acid synthase; MUFAs, monounsaturated fatty acids; PUFAs, polyunsaturated fatty acids; SCD, stearoyl-CoA desaturase-1; FADS2, fatty acid desaturase 2; ELOVLs, elongation of very long-chain fatty acid protein; PA, phosphatidic acid; TAG, triacylglycerol; DAG, diacylglycerol; PE, phosphatidylethanolamine; PG, phosphatidylglycerol; PS; phosphatidylserine. An intriguing observation during the metastatic dissemination of breast, prostate and ovarian cancer cells is their preferential homing to adipocyte tissue located in periglandular regions and the visceral omentum. 9 It has long been appreciated that adipose tissue is implicated in metabolic syndromes such as obesity, and functions as an endocrine system that secretes growth factors, cytokines and free FAs following lipolysis. 10 In this respect, co-culture, as opposed to isolated, cell systems have been instrumental in our understanding of the impact of adipocytes on the FA metabolism of cancer cells. 11 Importantly, these seminal studies have demonstrated that the migration and proliferation of human ovarian cancer cells are significantly increased following co-culture either directly with human-derived omentum adipocytes, or their conditioned media. 11 In these models, adipocytes were shown to activate endogenous lipolysis of triglycerides to produce free FAs that could be subsequently secreted and taken up by metastatic cells overexpressing FABP4. 11 The transfer of FAs from surrounding adipocytes to metastatic ovarian cancer cells also potentiated AMP-activated protein kinase (AMPK) signalling in the latter, culminating in increased β-oxidation through carnitine palmitoyltransferase 1 (CPT1) and acyl-CoA oxidase 1 activation. 11 The overexpression of the FA translocase CD36 is also essential f

Several metabolic processes in cancer cells are directly regulated by oncogenes and tumour suppressors. For instance, mutant KRAS enhances glycolysis in pancreatic cancer cells through upregulation of the genes encoding hexokinase 1/2 ( Hk1 and Hk2 , respectively), and redirects glutamine flux to malate for the production of pyruvate and reducing power in the form of NADPH. 58 , 59 Moreover, amplification of the MYC oncogene drives increased glutamine metabolism and anaplerosis by transcriptionally activating mitochondrial glutaminase (GLS1) and the SLC1A5 glutamine transporter. 60 , 61 Hyperactivation of the phosphoinositide 3-kinase and AKT (PI3K–AKT) pathway has also been implicated in the rewiring of specific metabolic processes, including increased glucose uptake through stabilisation of glucose transporter 1 (GLUT1), enhanced glutamine anaplerosis via activation of glutamate pyruvate transaminase 2 (GPT2) and remodelling of the cellular lipidome. 62 – 64 It is also becoming increasingly appreciated that the complex regulatory networks involved in FA metabolism must be considered in specific molecular and metabolic contexts. These include the impact of molecular heterogeneity, as exemplified by the intrinsic subtypes of breast cancer, as well as the oncogenic processes that drive malignant transformation in different cancers. Molecular heterogeneity and lipid reprogramming Gene expression analyses have long suggested that upregulation of several enzymes involved in lipid metabolism is a near-universal metabolic hallmark of cancer cells. 4 , 65 However, more nuanced observations from these genomic studies highlight differences in the expression of lipid enzymes across different tumour types and molecular sub-classifications. 4 For instance, long-chain acyl-CoA synthetase 3 (ACSL3) is overexpressed in androgen-dependent cancers, such as prostate tumours, where it activates cholesterol synthesis and steroidogenesis, but it is downregulated in triple-negative breast cancers. 66 , 67 Furthermore, enzymes involved in β-oxidation such as α-methylacyl-CoA racemase (AMACR) and CPT1B are specifically overexpressed in colorectal, hepatic and prostate cancers, whilst CPT1A is upregulated in breast cancer. 4 , 68 The upregulation of AMACR in prostate cancer is highly significant to the extent that it is used as a bona fide biomarker for early detection and diagnosis of prostate cancer. 68 Furthermore, it also suggests that β-oxidation, particularly of branched-chain FAs, is the main bioenergetic pathway for obtaining ATP and NADPH in prostate cancer cells. 69 Clinically, prostate cancers display low rates of glycolysis, and are therefore poor candidates for diagnostic imaging by using fludeoxyglucose positron emission tomography (FDG-PET). 70 The higher dependence on FAO could serve as an alternative metabolic signature for prostate cancer diagnosis, and indeed this dependency is currently being explored as a novel imaging application through the use of C11-acetate PET tracers. 70 Thus, although dysregulated lipid metabolism is a broad feature of cancer, different tumour types may exhibit unique metabolic adaptations that contribute to the remodelling of their lipidome. The regulatory complexity of lipid metabolism in the context of molecular heterogeneity is perhaps best exemplified in breast cancer, which comprises multiple different subtypes characterised by unique hormone/growth-factor receptor expression and genetic profiles. 71 Comparative mRNA expression analyses between receptor-positive and triple-negative breast cancers (RPBC and TNBC, respectively) have revealed notable differences between these subtypes in terms of lipid procurement, storage and oxidation. 4 Interestingly, RPBCs are associated with a gene signature encompassing elevated de novo lipogenesis, FA mobilisation and oxidation, whilst TNBCs overexpress genes involved in exogenous lipid uptake – including FABP5 and FABP7 – and storage. 4 It is also notable that TNBCs are dependent on different ACSL isoforms when compared with RPBCs to mobilise FAs that have been stored in LDs. 4 Specifically, overexpression of ACSL4 in TNBCs is associated with the enhanced metabolism of arachidonic acid to arachidonyl-CoA that can be used as a substrate for cyclo-oxygenase 2 (COX2) for prostaglandin synthesis. 4 The co-ordinated synthesis of pro-inflammatory prostaglandins from LDs may, in turn, contribute to the more aggressive phenotypes generally observed in TNBCs compared with RPBCs. 4 , 72 Although gene expression analyses may not necessarily provide a direct reflection of enzyme activity or dependencies on specific metabolic pathways, studies have validated unique lipid-associated genetic signatures that can be used to guide therapeutic intervention. 67 , 73 For instance, a subset of TNBCs with MYC overexpression have been shown to upregulate several genes involved in β-oxidation, such as PGC1α , CPT1B and CDCP1 , whilst downregulating FASN and ACACB . 73

Thus far, rewiring of the tumour lipidome has been largely discussed as a consequence of hyperactive oncogenic signalling and genetic aberrations. However, it is also important to acknowledge that overexpression of several lipogenic enzymes occurs relatively early in tumorigenesis, and can be observed in both hyperplastic and preinvasive lesions. 65 , 121 Therefore, it is conceivable that upregulation of de novo lipogenesis and the enzymatic network that regulates it may also dynamically and reciprocally potentiate oncogenic signalling throughout malignant transformation, rather than representing a secondary phenomenon. In support of this, mechanisms linking the bidirectional crosstalk between FASN and oestrogen receptor α (ERα) signalling have been elucidated in hormone-dependent breast cancers. In these models, both genetic and pharmacological inhibition of FASN hypersensitises ERα to oestrogen-dependent transactivation, thus leading to a synergistic induction of both oestrogen receptor element (ERE) transcriptional activity and mitogen-activated protein kinase (MAPK)–ERK signalling. 122 Surprisingly, oestrogen stimulation following FASN inhibition has significant cytotoxic and cytostatic effects, despite activation of the seemingly pro-tumorigenic aforementioned processes. 122 Whilst this may seem paradoxical at first, these findings highlight a novel role for FASN in modulating the thresholds required for triggering hormone receptor signalling, and consequently regulating the balance between the opposing pro-tumorigenic and anti-proliferative effects of ERα stimulation. For instance, although oestrogen signalling contributes to the upregulation of genes involved in proliferation, invasion and metastasis – such as Twist and matrix metalloproteinases 2/9 (MMP2/9) – FASN blockade leads to the nuclear accumulation of the oestradiol (E 2 ) targets p21 WAF/CIP1 and p27 Kip1 , culminating in cell-cycle inhibition and induction of cytostatic and cytotoxic effects in response to E 2 exposure. 122 , 123 Moreover, it is interesting to note that while E 2 hyperactivates MAPK–ERK signalling following FASN inhibition, PI3K–AKT activity is attenuated. 122 Indeed, there exists significant crosstalk between MAPK–ERK and PI3K–AKT signalling encompassing both positive- and negative-feedback loops, and this may explain at least one of the mechanisms responsible for the observed effects of FASN blockade and E 2 exposure. 124 In terms of cross-inhibition, ERK has been shown to phosphorylate the scaffold protein GRB2-associated-binding protein 1 (GAB1), thus compromising the recruitment of PI3K to the plasma membrane and downstream activation of AKT. 125 Therefore, in addition to its role in de novo lipogenesis, FASN is also at the nexus of pathway integration between MAPK–ERK and PI3K–AKT pathways, and their interaction with E 2 –ERα signalling. 122 Molecular connections have also been made between FASN and HER2 overexpression. 121 These have largely been described at the transcriptional level, with inhibition of FASN leading to upregulation of the ets -DNA-binding protein PEA3 – a negative regulator of HER2 gene transcription – and consequent reduction in HER2 mRNA expression. 82 , 126 An additional layer of regulation involves the cellular localisation of HER2 in response to FASN levels and activity. Notably, small-interfering RNA (siRNA)-mediated silencing of FASN expression or chemical inhibition by using C75 markedly attenuates the membrane accumulation of HER2 and is also associated with broader alterations in cell morphology. 82 Given that FASN is predominantly involved in the synthesis of saturated FAs, and these, in turn, modulate cell membrane dynamics including fluidity and lipid raft formation, inhibiting this lipogenic enzyme not only impairs the appropriate localisation of HER2 to the plasma membrane, but also impinges on the dimerisation of HER2 with epidermal growth-factor receptor (EGFR) that drives resistance to lapatinib and trastuzumab. 82 , 127 Indeed, dual treatment with cerulenin and trastuzumab synergistically increases apoptosis in HER2-amplified breast cancer cells, indicating that FASN inhibition, and its associated effects on HER2 expression and localisation, could be an attractive combinatorial therapeutic target in this breast cancer subtype. 82 Furthermore, as HER2-amplified tumours are particularly dependent on PI3K signalling, FASN could play a central role in modulating the initiation of growth-factor-dependent oncogenic signalling that is required for malignancy. 128 – 132

It is widely appreciated that FAs are essential to cancer cells because they sustain membrane biosynthesis during rapid proliferation, and provide an important energy source during conditions of metabolic stress. However, more intricate oncogenic roles of FAs and their by-products are beginning to be uncovered. These predominantly focus on the latter acting as signalling molecules that can directly regulate cellular homoeostasis, by modulating the surrounding microenvironment to create conditions that are conducive for tumour progression. The complex interplay of these multifaceted and diverse biological functions is discussed below. Membrane structure and fluidity FAs are essential building blocks for maintaining the structure and fluidity of the cell membrane. One of the advantages of the elevated rate of de novo lipogenesis in cancer cells is the synthesis of saturated and monounsaturated FAs, which are inherently more stable than polyunsaturated FAs because they contain fewer double bonds that can be targeted for peroxidation (Fig. 4a ). 143 Cancer cells with a higher degree of membrane saturation are less susceptible to oxidative stress induced by chemotherapeutic agents such as doxorubicin, whilst combinatorial inhibition of de novo lipid synthesis, either through siRNA knockdown of lipogenic enzymes or treatment with soraphen (an ACC inhibitor), synergistically increases cytotoxicity. 143 Fig. 4 Fatty acids (FAs) regulate membrane architecture and oncogenic signalling pathways. a Membrane fluidity is largely determined by cholesterol levels and degree of FA desaturation. Cancer cells displaying elevated de novo lipogenesis can synthesise saturated phospholipids, which not only increases membrane rigidity, but also protects against peroxidation induced by reactive oxygen species. Conversely, highly migratory cells display more fluid membranes as a result of increased desaturation and cholesterol abundance, thus contributing to the epithelial-to-mesenchymal transition and metastasis. b FAs and their synthetic products also function as secondary messengers in signalling pathways, with the best characterised being the phosphoinositides. PI(3,4,5)P 3 activates oncogenic AKT, and contributes to hyperactivation of mTORC1 and mTORC2. In addition, PI(3)P stimulates SGK3 that promotes tumorigenesis independently of AKT. Finally, phosphatidic acid can also directly bind to and activate the mTOR complexes. Abbreviations: PI3Kα, phosphoinositide 3-kinase α; PI(4,5)P 2 , phosphatidylinositol (4,5)-bisphosphate; PI(3,4,5)P 3 , phosphatidylinositol (3,4,5)-trisphosphate; PI(3,4)P 2 , phosphatidylinositol (3,4)-bisphosphate; PI(3)P, phosphatidylinositol 3-phosphate; mTORC, mammalian target of rapamycin complex; PA, phosphatidic acid; PTEN, phosphatase and tensin homologue; SGK3, serum- and glucocorticoid-induced protein kinase-3; INPP4B, inositol polyphosphate-4-phosphatase type II B. Intracellular levels of cholesterol can also dramatically modulate membrane architecture that impacts cell migration and ultimately metastatic dissemination. Interestingly, incorporation of cholesterol in membranes generally reduces fluidity and consequently inhibits metastasis by limiting the capacity for a cell to change its shape, a process that is essential during the epithelial-to-mesenchymal transition (EMT) and intra-/extravasation from blood vessels (Fig. 4a ). 144 In support of this, cells undergoing EMT display increased cholesterol efflux through upregulation of the ATP-binding cassette transporter ABCA1, whilst human tumours overexpressing ABCA1 have strikingly higher rates of distant metastases. 144 It is important to note, however, that increasing membrane rigidity can also be advantageous for cancer cells. This is particularly relevant for the development of multi-drug-resistant tumours, which typically contain lower levels of endogenously synthesised cholesterol and are consequently less permeable to anticancer agents. 145 Epidemiological studies have highlighted controversial roles for aberrant cholesterol metabolism in cancer, thus raising important considerations for how tumours actually utilise cholesterol, and how it may be therapeutically exploited. 146 For instance, elevated serum cholesterol levels are associated with increased incidence and recurrence of prostate cancer, whilst inhibition of its biosynthesis by using statins reduces colorectal, breast and endometrial cancer mortality. 147 – 149 Conversely, numerous studies have shown either no association between cholesterol and cancer, or potential carcinogenic effects of statins. 150 , 151 Therefore, resolving these disparate clinical outcomes remains a significant challenge and source of uncertainty for pursuing therapies targeting cholesterol metabolism. Perhaps insights can be gained from preclinical studies that consistently demonstrate a role for cholesterol in tumorigenesis, but in specific contexts. As discussed previously, during metastatic disseminati

An important subclass of bioactive lipid molecules are eicosanoids. The omega-6 FA arachidonic acid (AA) serves as the main precursor of several eicosanoid species, including prostaglandins, thromboxanes and leukotrienes, each of which has pro-inflammatory and pro-tumorigenic effects. 72 There are three main mechanisms through which cancer cells can obtain AA: (1) direct exogenous uptake by using specific transporters, such as CD36 and TWIK-related AA-stimulated K + (TRAAK) channels, (2) de novo synthesis from linoleic acid (LA) and (3) cleavage of the sn-2 position of existing membrane phospholipids through the enzymatic activity of phospholipases. Each of these processes has been extensively reviewed previously. 72 , 180 The predominant enzymes involved in prostaglandin production are the prostaglandin G/H synthetases COX1 (PTGS1) and COX2 (PTGS2). The former is constitutively active and regulates normal cellular processes including angiogenesis and blood clotting, whilst COX2 expression is selectively induced by growth factors and chemokines, and is, therefore, more closely linked with inflammation. 72 The cyclo-oxygenase and peroxidase activities of COX enzymes sequentially convert AA into prostaglandin G2 (PGG2) and prostaglandin H2 (PGH2), respectively. 72 Other eicosanoids including PGE2, PGD2, prostacyclins and thromboxanes can be derived from PGH2. 72 Prostaglandins exert pro-inflammatory and pro-tumorigenic effects in both autocrine and paracrine fashions by activating G-protein-coupled prostanoid receptors including EP1–4, DP1, PGF receptor (FP), PGI receptor (IP) and TX receptor (TP) (Fig. 5 ). 181 Tumour-initiating events triggered by prostaglandins include PGE2-mediated activation of PI3K signalling and induction of ERK following binding to the EP4 receptor, thus stimulating proliferation, as well as cell migration through stabilisation of β-catenin and c-MET induction. 182 , 183 In addition, in models of colorectal cancer, intracellular PGE2 can induce phosphorylation and activation of EGFR, leading to induction of MMPs and a consequent promotion of cell invasiveness. 184 Prostaglandins can also promote adaptation to microenvironmental stress conditions such as hypoxia. 185 Under hypoxic conditions, HIF1α induces the expression of COX2, leading to a concomitant overproduction of PGE2 that stimulates tumour angiogenesis in a VEGF- and chemokine (c–X–c motif) ligand 1 (CXCL1)-dependent manner. 186 , 187 It is interesting to note that at least in colorectal cancer, HIF1 can actually induce cell-cycle arrest by binding to β-catenin and displacing transcription factor 4 (TCF4), thereby blocking the formation of a functional β-catenin–TCF4 transcriptional complex and inhibiting the transcription of genes involved in proliferation. 185 , 188 This mechanism is particularly intricate because it demonstrates how the crosstalk between HIF and prostaglandin signalling can induce a ‘hibernation’ state in cancer cells that is characterised by sustained nutrient acquisition through vascularisation, and reduced energy expenditure. 185 , 188 The main implications of this could be that cancer cells are better equipped to recover from hypoxia and resume proliferation immediately following reoxygenation. Further to the direct effects on tumour cells, prostaglandins can also modulate the surrounding microenvironment, and this has been extensively demonstrated in the context of inhibition of the anti-tumour immune response (Fig. 5 ). One mechanism through which elevated PGE2 promotes tumour immune evasion is by abrogating co-stimulation and complete activation of CD8 + T lymphocytes mediated by binding of intercellular adhesion molecule 1 (ICAM-1) expressed on tumour cells and the lymphocyte receptor LFA-1. 189 In addition, accumulation of conventional type 1 dendritic cells (cDC1s) is essential for initiating the anti-tumour immune response, and requires the infiltration of natural killer cells within the tumour microenvironment. 190 However, in models of BRAF V600E mutant melanomas, overproduction of COX2-derived PGE2 directly inhibits the production of CXCL1 and chemokine (c–C motif ligand 5 (CCL5) chemokines by natural killer (NK)) cells, thereby attenuating the migration of cDC1s to the tumour site. 190 Importantly, the accumulation of NK and cDC1 cells is associated with better prognosis in melanoma and breast cancers, indicating that the immunomodulatory properties of prostaglandins could have significant clinical implications. 190 Indeed, the response of melanomas harbouring BRAF V600E mutation to anti-programmed cell death protein 1 (PD1) immune checkpoint inhibitors is significantly improved following combinatorial therapy with COX inhibitors including aspirin and celecoxib. 191 An additional pathway for AA metabolism is its conversion into arachidonyl-CoA following the ligation of acetyl-CoA catalysed by the acyl-CoA synthetase ACSL4. 192 Arachidonyl-CoA can be subsequently esterified to form TAGs and i

Increased expression and activity of ACLY is observed across several tumour types including glioblastoma, colorectal, breast and hepatocellular carcinomas. 34 Considering its role in generating acetyl-CoA, which is the main substrate for lipogenesis and cholesterol synthesis, targeting ACLY has proven to be an effective therapy for treating hypercholesterolaemia and hyperlipidaemia. 206 Importantly, several ACLY inhibitors including ETC-1002 (Phase 2/3 clinical trials) and hydroxycitrate (randomised control trials) have shown high efficacy in lowering low-density lipoprotein–cholesterol and good tolerability in patients with cardiovascular disease and type 2 diabetes (Table 1 ), thus indicating that ACLY inhibition could represent a well-tolerated therapeutic strategy. 207 – 211 There is considerable interest in ACLY as a target for anticancer drugs; however, clinical trial studies in this context are far more limited when compared with dyslipidaemia cases. Nevertheless, there is a plethora of preclinical evidence to support the integral role of ACLY in tumorigenesis and the potential clinical benefits for its selective inhibition. In particular, both genetic and pharmacological targeting of ACLY significantly reduces the growth of lung and prostate tumour xenografts, and this anti-tumorigenic effect is more pronounced in highly glycolytic cells. 34 Perhaps this is not surprising given that ACLY largely generates acetyl-CoA from glucose-derived citrate, but it does suggest that metabolic stratification of tumours may improve the potency of ACLY inhibitors such as SB-204990 and hydroxycitrate. 34 This is particularly relevant when one considers the current limitations of ACLY-targeted cancer therapeutics, which predominantly include the relatively high drug concentrations required for complete inhibition of ACLY activity. Thus, by identifying tumours with high rates of glucose consumption that are consequently more dependent on ACLY activity for glycolysis-fuelled lipogenesis, dosing regimes with lower drug concentrations could be implemented. 34 Furthermore, structural studies focussing on characterising the protein domains of the ACLY tetramer have revealed novel hydrophobic regions near the citrate-binding site that have untapped potential for improving the drugability of ACLY. 212 Another approach for implementing ACLY inhibition as a therapeutic strategy for cancer is to consider its role in signalling networks that regulate cellular responses to energy stress. For instance, in CRPC cells, inhibition of ACLY disrupts ER and energy homoeostasis in CRPC cells, leading to AMPK activation and sensitisation to androgen receptor inhibitors. 213 Intriguingly, supplementation of exogenous FAs is sufficient to restore hormone resistance in CRPC cells by alleviating the ER and energy stress instigated by ACLY inhibition, demonstrating the importance of an ACLY–AMPK network in regulating energy homoeostasis that could be exploited pharmacologically. 213 Indeed, several AMPK-activating therapies such as metformin have shown anticancer and anti-inflammatory properties, and importantly are well tolerated in patients. 214 Notably, some ACLY inhibitors currently used for the treatment of hypercholesterolaemia, such as ETC-1002, are also dual activators of AMPK, and although not yet investigated, it would be interesting to assess the applicability of these drugs in the cancer therapy context. 207 While dual targeting of ACLY and AMPK does seem like an attractive therapeutic strategy, it is necessary to acknowledge the instances in which the latter may actually promote tumorigenesis. For instance, the presence of functional LKB1 is essential for complete AMPK activation, and given that this kinase is frequently mutated – including in 34% of lung carcinomas and 20% of cervical cancers – it may be necessary to first stratify patients based on LKB1 status to evaluate the true therapeutic potential of AMPK activators. 215 Furthermore, the extent of mTORC1 hyperactivation should also be considered, as tumours that are dependent on mTOR signalling are more likely to have a better response to AMPK activation. 214 Closely linked with this idea are the regulatory feedback mechanisms that exist between mTORC1 and mTORC2–AMPK is a well-characterised negative regulator of mTORC1; however, the potential compensatory effects leading to mTORC2 hyperactivation and initiation of pro-tumorigenic signalling, such as induction of AKT, must be carefully considered. 214 Finally, AMPK activation promotes calcium-induced migration of prostate cancer cells through calmodulin-dependent protein kinase (CaMKKB) signalling. 216 Thus, it appears that therapeutically exploiting ACLY and AMPK activities requires extensive molecular characterisation of a patient’s tumour, and may be context-dependent. Although the effects of ACLY inhibition predominantly focus on FA synthesis, an additional biological feature of reducing acetyl-CoA synthesis is remodell

Finally, there has been a resurgent interest in studying the role of dietary interventions in cancer therapy. 76 , 222 , 223 For instance, it has been demonstrated that a low-carbohydrate ketogenic diet dramatically increases the efficiency of PI3K inhibitors and synergistically reduces the growth of PIK3CA- mutant tumours. 76 Since humans can only obtain essential omega-3 and omega-6 FAs from the diet, it is tempting to speculate that dietary modifications based on lipid consumption could also have an impact on tumorigenesis. Omega-3 FAs, including eicosapentaenoic and docosahexaenoic acids (EPA and DHA, respectively), are widely accepted to have anti-inflammatory properties by competing with AA for COX2 binding, and subsequently producing PGE3 instead of PGE2. 224 In contrast, omega-6 FAs, such as LA and AA, are the precursors for pro-inflammatory eicosanoids. 225 The recommended dietary ratio of omega-6:3 FAs is 1:1; however, the Western diet, which is significantly enriched in omega-6s, has a ratio of 15:1, 225 and this has significant implications for the progression of several cancers including breast and colorectal. 226 Conversely, a diet rich in omega-3 FAs has been associated with the pathological improvement of several inflammatory diseases, such as arthritis and asthma 224 , 227 , 228 (Table 1 ), as well as reduced risks of developing breast, colorectal and prostate cancers. 224 , 229 It is important to note, however, that excessive omega-3 consumption can also have undesirable side effects including immunosuppression. 230 Therefore, it is essential that careful optimisation of omega-3:omega-6 ratios is undertaken for each patient. By taking these factors into account, modulation of dietary fat intake either alone or in combination with existing therapies could have tremendous potential in therapy response.