Regulation of fatty acid synthase on tumor and progress in the development of related therapies

Abstract

Fatty acid synthase (FASN) is an essential molecule in lipid metabolic pathways, which are crucial for cancer-related studies. Recent studies have focused on a comprehensive understanding of the novel and important regulatory effects of FASN on malignant biological behavior and immune-cell infiltration, which are closely related to tumor occurrence and development, immune escape, and immune response. FASN-targeting antitumor treatment strategies are being developed. Therefore, in this review, we focused on the effects of FASN on tumor and immune-cell infiltration and reviewed the progress of related anti-tumor therapy development.

Introduction

Rapidly proliferating cancer cells exhibit metabolic demands that are quite different from those of normal cancer cells.^[1] In addition to increased demands for glucose, glutamine, and some amino acids, cancer cells undergo lipid metabolism reprograming to obtain the energy stores and membrane production required for rapid proliferation.^[2] Unlike normal cells, which take up lipids mainly from the microenvironment, tumor cells exhibit aberrant activation of the de novo fatty acid synthesis (FAS) pathway, as shown by enhanced fatty acid (FA) synthesis and palmitoleic acid production, providing sufficient energy for the survival of cancer cells. Fatty acid synthase (FASN) is a key lipogenic enzyme that catalyzes the terminal step in the de novo biosynthesis of endogenous FAs and is essential for maintaining FA metabolic balance, energy metabolism, and membrane structural homeostasis within cancer cells.^[3] FASN has been shown to play an important role in tumor cell metabolism and energy supply. Recently, FASN has been recognized as an oncoprotein that is highly expressed in most tumor cells and can promote the malignant biological behavior of tumors through a variety of mechanisms, such as promoting epithelial-mesenchymal transition and angiogenesis, and is closely related to tumor proliferation, invasion, metastasis, and prognosis.^[4,5] Inhibitors targeting FASN have been comprehensively developed and characterized, and several novel molecules targeting FASN have been developed. In contrast, FASN also regulates tumor immunity,^[6,7] which is closely related to immune cell infiltration, tumor immune escape, and immune responses in the body and tumor microenvironment. This suggests that inhibitors targeting tumor FASN may have synergistic effects with immunotherapy. Accordingly, we focused on the effects of FASN on tumor and immune cell infiltration and reviewed the progress in the development of anti-tumor therapeutics targeting FASN.

Regulation and Mechanisms of FASN in Tumor Development

FASN-mediated tumor metabolic alterations

Increased de novo FAS provides cancer cells with FAs that are immediately required for membrane and lipid raft generation,^[8] as well as long-chain FAs, including palmitate, which serve as ligands for transcription factors, such as peroxisome proliferator-activated receptors (PPARs).^[9] FASN is an essential molecule in the lipid metabolism pathway, and 90% of fat synthesis in tumor cells arises from de novo FAS catalysis by FASN. Overexpression of FASN has been observed in multiple human cancers,^[4,5] highlighting the aberrant activation of the de novo FA pathway. Under energy stress, these FAs provide the high energy required by tumor cells, through β-oxidative breakdown. In addition to regulating lipid metabolism, FASN plays a role in glycolysis, amino acid metabolism, and other metabolic processes. Notably, FASN inhibition impairs glycolytic processes. Inhibition of FASN expression impairs glycolytic capacity and cancer cell reserves in colorectal cancer cell lines and downregulates mitochondria-bound hexokinase activity.^[10] The inhibition of FASN by cerulenin in the breast cancer cell line SKBR-3 was shown to significantly reduce glucose uptake and lactate production.^[11] Cross-talk between glycolysis and FASN-mediated lipid synthesis is dependent on the phosphatidylinositol-3′-kinase (PI3K)/AKT (also known as protein kinase B [PKB]) pathway in non-Hodgkin lymphoma cell lines, where the inhibition of FASN activity impairs cellular glycolytic flux.^[12] The novel manganese complex PdpaMn ([(Pdpa)MnCl₂]) was demonstrated to selectively inhibit cancer cell and tumor growth while blocking FASN expression and inhibiting free FA content and glycolysis by downregulating the PI3K/AKT signaling pathway, revealing a direct link between FASN and glycolysis.^[13] Inhibition of FASN activity induced broad changes in tumor metabolism, as evidenced by gene set enrichment analysis, including down-regulation of glycolysis/gluconeogenesis and Krebs cycle pathways.^[13] In addition, downregulation of the Krebs cycle pathway is consistent with the global repression of mitochondrial genes involved in energy metabolism and the oxidative phosphorylation system (OXPHOS).^[14] In addition to glycolysis, FASN is functionally related to other metabolic pathways in tumor cells with low glycolytic activity. The inhibition of FASN activity in tumor cells was found to induce multiple adaptive changes in FA synthesis and other related metabolic pathways, including ketone metabolism and glutaminolysis.^[15]

In addition, FASN regulates tumor oxidative metabolism and development by affecting mitochondrial function and dynamics. Mitochondria are responsible for OXPHOS and cell motility and are key sites of oxidative metabolism in cells. FASN levels are positively correlated with OXPHOS rates in various cancer cell types and immune cells.^[10,16] Overexpression of FASN produces excess free FA, which breaks down into acetyl-CoA, thus promoting mitochondrial respiration via FA oxidation (FAO), which is positively associated with metastatic cancer cells with high energy requirements.^[17] FASN inhibition was shown to decrease mitochondrial respiration and FAO in an in vitro colorectal cancer model. Supplementation of the medium with nutrients, such as glucose and glutamine, did not rescue mitochondrial respiration and FAO^[17] in FASN-inhibited cells. In a study of siRNA knockdown FASN, researchers found that the transcription of genes involved in energy metabolism in tumor cells was affected by siRNA knockdown, as demonstrated by downregulation of lipid metabolism, glycolysis, Krebs cycle or tricarboxylic acid cycle (TCA cycle), and OXPHOS.^[15] The authors reported that this loss of FASN was correlated with tumor growth suppression through upregulation of cell cycle arrest and death receptor-mediated apoptotic pathways and also caused changes in genes regulating transcription and ubiquitin-dependent protein degradation.^[15] In conclusion, FASN can regulate multiple metabolic pathway alterations in tumor cells.

Mitochondria are dynamic organelles; in addition to performing metabolic reactions, they undergo frequent cycles of fusion and fission facilitated by specific proteins,^[18] which allows for adaptation to changes in cellular metabolic demands. FASN promotes mitochondrial fusion in cancer cells. As mentioned earlier, FASN activity is closely related to the survival function of the epidermal growth factor receptor (EGFR) family of receptor tyrosine kinases.^[19] In pancreatic and breast cancer cell lines, EGFR-overexpressing cancer cells contain two distinct EGFR localization subsets: plasma EGFR (pmEGFR) and mitochondrial EGFR (mtEGFR). Phosphorylation of FASN has been shown to promote de novo palmitic acid synthesis, with increased production of the latter promoting palmitoylation of mtEGFR at cysteine residues 781 and 797.^[20] Palmitoylation of mtEGFR induces its phosphorylation and activity, which in turn induces the expression of prohibitin 2 (PHB2) and OPA-1, molecules essential for the fusion of the inner mitochondrial membrane. Blocking the palmitoylation process via an FASN inhibitor (cerulein) reduced mtEGFR phosphorylation and mitochondrial fusion, suggesting that palmitate synthesized by FASN regulates palmitoylation and mtEGFR activation, thereby promoting mitochondrial fusion.^[20] In addition, cytochrome c oxidase subunit 4 isoform 1, mitochondria (COXIV), a mitochondria-specific marker, and peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), the major transcriptional coactivator of mitochondrial biogenesis, were significantly increased in FASN-overexpressing senescent cells, and mitochondrial mass was increased, which was reversed by the FASN inhibitor C75.^[21] It can be seen that FASN can affect tumor development by regulating mitochondrial function and dynamics in tumor cells. In conclusion, FASN can regulate multiple metabolic pathway changes in tumor cells, affecting tumor development.

FASN regulates tumor epithelial-mesenchymal transition and angiogenesis

Upregulation of FASN expression has been reported in various human cancers, and FASN overexpression not only provides a metabolic advantage favoring cancer cell survival and proliferation but also supports particularly aggressive tumor phenotypes and is thought to be associated with increased tumor burden and poorer prognosis in multiple cancer species.^[4,5,22,23] Palmitic acid (C16:0), a major FASN metabolite, enhances the invasiveness of pancreatic cancer cells.^[24] The absorption of FAs or selective release of specific FAs from membrane lipids may contribute to the formation of signaling molecules that promote cell migration and invasion when tumor cells shift from a proliferative to a migratory state.^[25] FASN is reported to be significantly overexpressed in gastric cancer (GC), and its high expression leads to poor survival outcomes in GC patients, making it a putative prognostic biomarker for GC.^[6] High FASN expression can increase the recurrence or metastasis rate of breast cancer and is associated with poor survival, including overall survival (OS), relapse-free survival (RFS), distant metastasis-free survival (DMFS), and post-progression survival (PPS) in breast cancer patients with HER2-negative subtypes/mesenchymal tissues.^[26,27] Chang et al^[28] demonstrated that downregulation of FASN inhibits the invasion and migration of A549 cells in vitro, whereas inhibition of FASN resulted in significant tumor shrinkage in animal models. Therefore, FASN may be a critical regulator of non-small cell lung cancer (NSCLC) cell migration and invasion. Mechanistically, FASN may increase tumor invasion and metastasis by affecting multiple pathways. FASN has been shown to promote EMT via transcriptional regulation of different cadherins.^[29] FASN promotes lipid raft composition; activates the CD44/c-Met complex; and induces the activation of Src, focal adhesion kinase, and paxillin, in addition to reorganizing the actin cytoskeleton, leading to changes in cell morphology and increased mobility.^[30] Research has shown that FASN silencing reverses the effect of hyperglycemia on EMT marker levels, increases E-cadherin expression, decreases vimentin and fibronectin expression, promotes mesenchymal to epithelial transition, and inhibits tumor cell growth.^[31] Investigators have reported that FASN might exert its effects on invasion and metastasis by regulating the Wnt or transforming growth factor-β (TGF-β) signaling pathways. Knockdown of FASN in CRC cell lines has been reported to attenuate the Wnt signaling pathway by downregulating the Wnt5a, Wnt5b, and Fzd2 genes, thereby inhibiting cell migration and invasion.^[22] In addition, FASN affects the expression of a range of angiogenesis-related factors that promote tumor angiogenesis and metastatic behavior. FASN has been reported to promote angiogenesis in colorectal cancer by stimulating the secretion of angiogenic factors and proliferation of ECs,^[32] and its expression is positively correlated with increased vascular endothelial growth factor (VEGF) levels.^[33]

FASN regulates tumor DNA damage repair and apoptosis

FASN can indirectly affect the development and apoptosis of tumor cells, and the inhibition of FASN expression can promote tumor cell apoptosis. In MCF7 breast cancer cells, microarray analysis of gene expression profiles following FASN knockdown revealed increased expression of several pro-apoptotic proteins, including BNIP3, TRAIL, and DAPK2 plus.^[34] FASN knockdown also significantly increased ceramide levels in tumor cells, which may mediate the upregulation of these pro-apoptotic genes. In another microarray analysis, the expression of pro-apoptotic genes, such as apoptotic protease activating factor, was found to be upregulated.^[14] In addition, FASN overexpression likely contributes to intrinsic and acquired resistance by regulating gene expression and diverse cellular processes. In pancreatic cancer, increased FASN is associated with disease progression and poor survival, as well as resistance to gemcitabine through enhanced endoplasmic reticulum (ER) stress.^[35] Orlistat inhibition by FASN increases the susceptibility of breast cancer cells to anticancer therapies.^[36] Targeting FASN may represent a novel therapeutic strategy for reversing oxaliplatin resistance in GC.^[37] In a recent study, it was suggested that FASN plays an important role in regulating pro-apoptotic proteins, PARP-1 expression, and DNA repair pathways, which in turn contribute to resistance to drug- and radiation-induced apoptosis. The knockdown of FASN expression in MCF7/AdVp3000 cells significantly decreased the activity of non-homologous end-joining repair.^[38]

FASN regulates tumor signaling pathways

In addition to playing a critical role in cellular metabolism and tumor biological activity, FASN integrates signaling pathways that contribute to post-translational palmitoylation and affect membrane structure and function.^[39] FASN signaling regulates multiple signaling pathways during tumor progression, including cell-cell adhesion, migration, proliferation, and chemokine transcription.^[40–42] The mechanism of FASN transcriptional upregulation has been proposed to involve the activation of growth factor receptor (GFR) signaling pathways in tumor cells, such as epidermal growth factor receptor (EGFR), human epidermal growth factor receptor-2 (HER2), and platelet-derived growth factor receptor (PDGFR).^[19] The effects of GFR signaling on FASN regulation are complex and include crosstalk with other signal transduction pathways, such as PI3K, extracellular regulated kinase 1/2 (ERK1/2), and mitogen-activated protein kinase (MAPK), to regulate the malignant phenotype of tumor cells.^[19] FASN gene expression is activated downstream of the PI3K/AKT/mTOR signal transduction pathway in response to cellular metabolic and growth signals and is driven by SREBP-1, ZBTB7A, and p53 family transcription factors.^[43] AKT can significantly increase the expression of FASN in the mouse liver, increase the accumulation of abnormal lipids in the liver tissue, and promote the occurrence and development of mouse liver cancer.^[44] Qiu and Zhao^[45] investigated the association of the PI3K/AKT signaling pathway with FASN overexpression in osteosarcoma and observed that upregulation of FASN expression upregulated mTOR, an important downstream molecule of the PI3K/AKT signaling pathway, and was associated with soft tissue invasion, metastasis, and poor prognosis. FASN inhibition decreases AKT phosphorylation, and AKT inhibition similarly downregulates FASN mRNA and protein expression in vitro.^[46] FASN acts as a novel regulator of GC cell proliferation and metastasis through the mTOR/Gli1 signaling pathway. The expression levels of HIF-1α, FASN, and SREBP-1c are consistently upregulated in GC cell lines, tissues, and serum and are associated with adverse clinicopathological features.^[47,48] As a transcriptional coactivator, spindling-1 (SPIN1) co-stimulates SREBP1c to promote FASN expression. SPIN1/SREBP1c/FASN signaling pathway regulates abnormal lipid metabolism and promotes hepatoma cell growth.^[49]

Role of FASN in Tumor Immunity and Tumor Microenvironment

Immune-targeted therapy for cancer has gained widespread use in basic and clinical applications and serves as an alternative or complementary approach to radiotherapy and cytotoxic chemotherapy. FASN is a key metabolic enzyme involved in the formation of new fats that directly provides cancer cells with the ability to proliferate and metastasize. However, the impact of aberrant FASN activation on the host tumor immune environment remains unclear, and elucidating its relevance could contribute to the effective application of anti-tumor therapeutic strategies targeting FASN. In a previous study, it was shown that FASN is closely associated with tumor immune cell infiltration and tumor immune escape. Statistically significant differences were found between FASN and CD8⁺ T cells in HNSC, KIRC, and ovarian cancer.^[6] Dendritic cells (DCs) are required to initiate and maintain T cell-dependent anti-tumor immunity. Jiang et al^[7] reported that constitutive activation of FASN in ovarian cancer cells induces an increase in lipids present at high concentrations in the tumor microenvironment (TME), and increases lipid uptake in the TME, resulting in the inhibition of the ability of tumor-infiltrating DC (TIDC) to present antigens and prime T cells. The application of FASN inhibitors partially restores the immunostimulatory activity of TIDC and prolongs the control of tumors by eliciting protective antitumor immune responses. Similarly, FASN has been reported to be associated with survival and activation of resident T cells^[50] and CD4⁺ T cells^[51] and lipid accumulation in lymphomas, leading to DC dysfunction.^[52] FASN plays a critical role in regulating GC immunology, and FASN expression has been found to be closely related to immune epidemic infiltration levels of gastric adenocarcinoma tumor-infiltrating immune cells (TIICs); CD8⁺ T cells, CD4⁺ T cells, neutrophils, macrophages, and DCs, as well as DC infiltration levels and DC signature genes, such as HLA-DQB1, HLA-DRA, HLA-DPB1, and CD11c, are significantly associated with FASN expression.^[6]

In contrast, FASN exerts oncogenic effects through immunomodulatory mechanisms that may regulate the phenotype and function of regulatory immune cells, such as cancer-associated fibroblasts (CAFs), tumor-associated macrophages (TAMs), and Tregs. Favorable relationships were found between FASN expression and CAFs in cervical squamous cell carcinoma (CESC), kidney renal clear cell carcinoma (KIRC), kidney renal papillary cell carcinoma (KIRP), ovarian cancer, and uveal Melanoma (UVM).^[6] The characteristic gene signatures of M1 macrophages (interferon regulatory factor 5 [IRF5] and Prostaglandin endoperoxide synthase 2 [PTGS2]) show a weak association with FASN expression, whereas those of M2 macrophages (membrane-spanning 4-domains subfamily A member 4A [MS4A4A], CD163 [a cell surface receptor], and v-set and Ig domain-containing 4 [VSIG4]) show a relatively stronger association with FASN, suggesting that FASN may regulate the polarization of TAMs.^[6] In addition, FASN may activate Tregs and lead to T-cell exhaustion, the expression of which is inversely correlated with the genetic markers of Tregs and T-cell exhaustion (signal transducer and activator of transcription 5B [STAT5B], forkhead box P3 [FOXP3], chemokine receptor 8 [CCR8], TGF-β1, lymphocyte activation gene-3 [LAG-3], and T cell immunoglobulin 3 [TIM-3]).^[6] Similarly, transcriptome analysis of intra-tumoral Tregs revealed the downregulation of both the FA metabolism pathway and specific T cell receptor (TCR)-dependent gene signature in FASN-deficient Tregs, and FASN signaling is presumed to contribute to the functional maturation^[53] of TCR-induced Tregs. In addition, significant correlations between FASN expression and signature markers of helper T cells (Th1, Th2, Th17, and Tfh) have been reported in GC.^[6] Overall, FASN expression negatively correlated with positive immune cell infiltration and positively correlated with regulatory or negative immune cell infiltration.

Taken together, these observations suggest a closely linked role for FASN in the regulation of TME phenotype and tumor immune response. The endogenous FASN oncogenic pathway in tumor cells induces impaired anti-tumor immune responses through multiple mechanisms, indicating that targeting the FASN oncogenic pathway while enhancing anti-tumor immunity is a unique approach to anti-tumor immunotherapy. Investigating the FASN inhibitor orlistat, CD80/CD81-positive and IL-12-positive DC populations, granzyme b/NKG2D-positive NK populations, and perforin/granzyme b-positive CD8 T lymphocytes increased, whereas Treg counts decreased, supporting an anti-tumor response in experimental melanoma.^[54] Therefore, targeting FASN in combination with immunotherapy may have clinical benefit.^[7] In the era of personalized and precise medicine, next-generation sequencing and multi-omics analyses have become increasingly important for understanding human cancers, and the exploration of novel biomarkers based on survival outcomes and different therapies is crucial.

Inhibitory Effect of FASN and Development of FASN-Inhibitors

Effects of inhibiting FASN

In addition to the regulation of lipid metabolism, glucose metabolism, oxidative metabolism, and other pathways, FASN can affect the malignant behavior of tumor cells through a variety of mechanisms and is closely related to changes in the tumor immune microenvironment. Therefore, FASN has emerged as a potential target for anti-tumor therapy. In the fat bodies of flies with knocked down NAD kinase (NADK) or fatty acid synthase 1 (FASN1), a large number of mitochondrial metabolism-related proteins were significantly downregulated, and the number of mitochondria was reduced and dysfunctional.^[55] FAS mediated by NADK and FASN1 orchestrates lipid storage and mitochondrial function, and de novo FAS regulates mitochondrial mass, at least in part, by controlling PGC-1α acetylation and cardiolipin (CL) synthesis.^[55] The inhibition of FASN or acetyl-CoA carboxylase (ACC) reduces mitochondrial oxygen consumption, alters mitochondrial morphology, and affects the levels of mitochondrial proteins and metabolites in cancer and stem cells.^[56] These findings suggest that mitochondrial function, regulated by FAS, is critical for cancer cell growth and stem cell differentiation.

Lipid profiling showed that the inhibition of FASN led to a significant reduction in sphingolipids and phosphatidylcholine (PC), a key metabolite of hPSCs cell survival, and increased cleaved caspase-3 levels in hPSCs, which suggests that it could induce cell death in undifferentiated hPSCs via apoptosis mediated by the intrinsic mitochondrial pathway, suggesting the utility of FASN inhibition in regenerative medicine.^[57] This is consistent with a previous study showing that reductive carboxylation of glutamine supports de novo FA synthesis during mitochondrial dysfunction.^[58] Researchers have demonstrated that FASN activity determines the ability of cells to approach the apoptotic threshold, a property known as mitochondrial priming.^[59] FASN inhibitors may not appreciably affect the viability of cancer cells but increase mitochondrial priming and shift cancer cells toward a death-inducing state that opposes apoptosis protein addiction. When B-cell lymphoma-2 (BCL-2) homology 3 (BH3) mimetic that antagonize BCL-2 oppose the binding of pro-apoptotic proteins, they lead to increased apoptosis of cancer cells that have been inhibited by FASN.^[60] In one study, the authors added FASN-dependent endogenous lipogenesis to the list of metabolic pathways tightly intertwined with apoptotic cell death in cancer cells and concluded that enhanced mitochondrial priming underlies the apoptotic hypersensitivity of nascent FA-starved cancer cells induced by FASN.^[60] Thus, FASN activity regulates cancer cell survival by fine-tuning the threshold for mitochondrial apoptosis, supporting the mitochondrial basis for exploring the actionable synergy between FASN inhibitors and BH3 mimics targeting BCL-2 in a clinical setting. Novel manganese (Mn) complexes labeled with PdpaMn have been found to bind to the thio-esterase domain of FASN and reduce the activity and expression level of FASN, followed by the development of intrinsic apoptotic pathways in tumor cells, as shown by the permeability of the outer mitochondrial membrane to pro-apoptotic proteins (e.g., cytochrome c), formation of apoptotic complexes through caspase-9 interactions, eventual collapse of the mitochondrial membrane potential, reduction of mitochondrial oxygen consumption, and profound inhibition of ATP release.^[61] Thus, the inhibition of FASN-induced apoptotic events in cancer cells by PdpaMn is dependent on mitochondrial dysfunction. Sulforaphane depolymerizes microtubules, downregulates FASN, acetyl CoA carboxytransferase (ACACA), and ATP citrate lyase (ACLY) by activating the proteasome and downregulating the transcription factor SREBP1, inhibits the interaction between α-tubulin and FASN, ACACA, and ACLY, and decreases the amount of intracellular FA and mitochondrial phospholipids^[62]; knockdown of FASN decreases mitochondrial membrane potential (ΔΨm), increases reactive oxygen species, mitochondrial abnormalities, and apoptosis^[62]; further, sulforaphane downregulates mitophagy-related proteins Bnip3 and NIX and upregulates mitochondrial LC3II/I.^[62] This series of reactions mediates mitophagy, ultimately leading to apoptosis.^[62] In conclusion, the inhibition or downregulation of FASN expression may cause mitochondrial dysfunction in tumor cells through multiple mechanisms, ultimately leading to increased apoptosis. Therefore, the findings suggest that inhibitors targeting FASN may become effective strategies for anti-tumor therapy.

Development and progress of FASN inhibitors

In recent years, there has been renewed interest in the development of lipid inhibitors as anticancer therapies. FASN has received considerable attention as a therapeutic target in personalized medicine. Several FASN inhibitors have been fully developed and characterized^[63–65], including cerulenin, C75, orlistat, and others. Cerulenin, a natural metabolite of cephalosporin cyanobacteria, has shown promising anticancer effects in mice bearing OVCAR-3 ovarian cancer.^[66] C75 is more chemically stable than cerulein and strongly inhibits a variety of human cancer cell lines, including breast, prostate, mesothelioma, and ovarian cancers.^[67] However, these early small-molecule inhibitors have shown side effects in mice, including severe weight loss and marked changes in food intake,^[68] as well as non-specific binding to other proteins, such as carnitine palmitoyltransferase 1 (CPT-1)^[69] and glyceraldehyde 3-phosphate dehydrogenase (GAPDH)^[70]. These characteristics limit the use of these small molecule inhibitors.^[71] Orlistat contains a highly active β-lactone that generates covalent adducts with the FASN thio-esterase domain and irreversibly inhibits its enzymatic activity,^[72] followed by caspase-8-mediated activation of the apoptotic cascade, negative regulation^[73] of the mTOR pathway, and cell cycle arrest in the G1/S phase.^[74] Although the anticancer properties of orlistat are supported by the results of many in vivo and in vitro studies, orlistat is highly unstable, poorly water-soluble, and poorly absorbed from the gastrointestinal tract due to the presence of β-lactones, which hinders its advancement in clinical trials.^[75] Notably, orlistatin-loaded micellar nanoparticles and orlistatin-loaded folate receptor-targeted micellar nanoparticles have recently been reported, which increase the solubility, stability, and efficacy of drugs^[76] and allow for more selective targeting of tumor cells while reducing potential toxic effects on normal tissues. These findings are worth further investigation.

In recent years, a better understanding of the structure of FASN has led to the development of more specific and less worrisome FASN inhibitors. Several generations of novel molecules targeting FASN are currently being developed, including GSK837149A, TVB2640, and plant-derived polyphenols, making accurate and effective inhibition of tumor lipid metabolism a potential new anti-tumor therapeutic strategy. Currently, one of the most promising candidates is TVB-2640, which responds to various tumor types, including ovarian, kirsten rat sarcoma viral oncogene (KRAS)-mutated non-small cell lung, and breast cancers.^[77] A phase 2 clinical trial (NCT03179904) evaluating TVB-2640 in combination with trastuzumab and paclitaxel for the treatment of advanced HER2-positive breast cancer is ongoing. Positive results were obtained in phase 2 clinical trials of TVB-2640 combined with bevacizumab for the treatment of high-grade astrocytomas. The China National Medical Products Administration (NMPA) has approved a phase 3 clinical trial of TVB-2640 (also known as ASC40) in combination with bevacizumab in patients with recurrent glioblastoma. In addition, another FASN inhibitor, TVB-3166, rapidly inhibits neopalmitate synthesis. When the latter is depleted, gene expression changes in tumor cells, lipid raft architecture is disrupted, and the PI3K/AKT/mTOR and β-catenin signaling pathways are inhibited, ultimately leading to apoptosis in tumor cells.^[78]

Tumor cells exhibit metabolic heterogeneity, and not all tumor cells are adversely affected by FASN inhibition. Therefore, identifying cancer cell populations that are susceptible to FASN inhibitors is critical. Researchers have identified three subsets of patients associated with glycolysis, lipolysis, and redox balance based on the metabolite profiles of pancreatic ductal adenocarcinoma, each displaying distinct metabolic profiles and sensitivities to different classes of metabolite inhibitors; for example, lipogenic cells are generally sensitive to FASN inhibitors, whereas glycolytic cells are sensitive to glycolytic inhibitors; they do not respond to FASN inhibitors.^[79] However, tumor cell metabolism is plastic, as shown by several lipogenic cell types that are not affected by FASN inhibitors and may undergo additional stages of adaptation against FASN inhibitors.^[79] In addition, the sensitivity of tumor cells to FASN inhibitors is influenced by specific genetic mutations. Studies in a colorectal cancer patient-derived xenograft (PDX) model have shown that sensitivity to TVB-3166 was influenced by KRAS and TP53 mutations, activation of adenosine 5′-monophosphate (AMP)-activated protein kinase (AMPK) and AKT pathways, and stored lipid content.^[80] In addition, FASN-positive tumor markers can serve as predictive biomarkers to select cancer patients who may benefit from FASN-targeted therapy.^[79] More importantly, further studies are needed to better understand the relationship and dynamics between cancer metabolism, oncogenic mutations, and the activation of other signaling pathways in FASN-positive tumors. In conclusion, in-depth exploration of the extensive crosstalk between FASN and poor metabolic/oncogenic pathways, as well as the identification of FASN-targeted therapies, is critical for their successful clinical use.

Figure 1 shows FASN-mediated FA metabolism in tumor cells and its regulation of tumors and immune microenvironment.

Figure 1.

FASN-mediated FA metabolism in tumor cells and its regulation on tumor and immune microenvironment. (1) In tumor cells, d e novo FAS is mediated by key metabolic enzymes (including FASN) and is mainly regulated by the PI3K/AKT/mTOR signaling pathway. Various inhibitors targeting FASN have been developed (brownish-yellow boxes). (2) Abnormal FASN activation in tumor cells is closely related to the immune cell infiltration phenotype of the TME, tumor immune escape, and immune response. Immune effector cells, including T and NK cells, are in an inhibitory state in the TME, and immunosuppressive cells, including MDSCs, CAF, and TAM, are activated. (3) The glycolytic process is upregulated in tumor cells and regulated by the PI3K/AKT/mTOR signaling pathway. In addition to regulating lipid metabolism, FASN has regulatory effects on glycolysis, amino acid metabolism, mitochondrial oxidative metabolism, and mitochondrial dynamics. AKT: Known as protein kinase B (PKB); ACC: Acetyl-CoA carboxylase; ACLY: ATP citrate lyase; BCL-2: B-cell lymphoma-2; BH3: BCL-2 homology 3; CAF: Cancer-associated fibroblast; CCR8: Chemokine receptor 8; CD163: a cell surface receptor; CESC: Cervical squamous cell carcinoma; CPT1: Carnitine palmitoyl transferase 1; CSF-1: Colony stimulating factor-1; Drp1: Dynamin-related protein 1; FA: Fatty acid; FAO: FA oxidation; FAS: Fatty acid synthesis; FASN: Fatty acid synthase; FOXP3: Forkhead box P3; GLUT1: Glucose transporter 1; HK2: Hexokinase 2; IRF5: Interferon Regulatory Factor 5; KIRC: Kidney renal clear cell carcinoma; KIRP: Kidney renal papillary cell carcinoma; KRAS: Kirsten rat sarcoma viral oncogene; LAG-3: Lymphocyte activation gene-3; MDSCs: Myeloid-derived suppressor cells; MHC: Major histocompatibility complex; MS4A4A: Membrane-spanning 4-domains subfamily A member 4A; mTOR: Mechanistic target of rapamycin; NAD: Nicotinamide adenine dinucleotide; NADK1: NAD kinase 1; NK: Natural killer cell; NSCLC: Non-small cell lung cancer; PD-1: programed death-1; PD-L1: Programed death-ligand-1;PGC-1α: Peroxisome proliferator-activated receptor-γ coactivator-1α; PI3K: Phosphatidylinositol-3′-kinase; PTGS2: Prostaglandin endoperoxide synthase 2; RTK: Receptor tyrosine kinase; STAT5B: Signal transducer and activator of transcription 5B; TAM: Tumor-associated macrophage; TCA: Tricarboxylic acid; TCR: T cell receptor; TGF-β: Transforming growth factor-β; TIDC: Tumor-infiltrating DC; TIM-3: T cell immunoglobulin 3; TME: Tumor microenvironment; UVM: Uveal Melanoma; VSIG4: V-set and Ig domain-containing 4.

Summary and Prospects

Lipid metabolism is recognized as an important pathway in cancer and can provide an additional source of energy required for metastasis, proliferating assembly blocks, and secondary messengers in various signaling pathways. FASN is an essential molecule in the lipid metabolism pathway, and its mediated lipid uptake and accumulation play key roles in metabolic and functional reprograming in a variety of cancers, enabling the rewiring of tumor cells for greater energy flexibility and higher energy requirements. In recent years, the role of FASN as a tumor oncogene has been gradually recognized, and its impact on the occurrence and development of tumors and malignant biological behavior is multifactorial and involves many mechanisms. Therefore, FASN has emerged as a potentially advantageous target for tumor metabolic therapy. FASN inhibitors have gradually become attractive targets in clinical practice and are currently undergoing phases II/III clinical trials. However, poor broad crosstalk between FASN and metabolic/oncogenic pathways may hinder the successful application of FASN inhibitors in clinical practice, making it particularly important to screen and identify predictive biomarkers to identify patients with cancer who may benefit from FASN-targeted therapy. Moreover, FASN has a unique regulatory effect on tumor immune cell infiltration. FASN signaling intrinsic to tumor cells may be a dual driver of malignant tumor manifestations and immune escape, providing a combined opportunity to simultaneously target tumor metabolism and immunity. More importantly, further studies are needed to better understand the relationship and dynamics of FASN in cancer metabolism, oncogenic mutations, and activation of other signaling pathways and to fully combine the complex mutual regulation of body immunity and the TME to ultimately accurately and effectively develop innovative therapeutic strategies targeting tumor lipid metabolism.

Funding

This work was supported by grants from the Jilin Provincial Science and Technology Department (No. 20190303146SF) and Jilin Provincial Department of Finance Project (No. JLSWSRCZX2020-0023).

Conflicts of interest

None.