Enhancing Immunotherapy in Ovarian Cancer: The Emerging Role of Metformin and Statins

Abstract

Ovarian cancer metastization is accompanied by the development of malignant ascites, which are associated with poor prognosis. The acellular fraction of this ascitic fluid contains tumor-promoting soluble factors, bioactive lipids, cytokines, and extracellular vesicles, all of which communicate with the tumor cells within this peritoneal fluid. Metabolomic profiling of ovarian cancer ascites has revealed significant differences in the pathways of fatty acids, cholesterol, glucose, and insulin. The proteins involved in these pathways promote tumor growth, resistance to chemotherapy, and immune evasion. Unveiling the key role of this liquid tumor microenvironment is crucial for discovering more efficient treatment options. This review focuses on the cholesterol and insulin pathways in ovarian cancer, identifying statins and metformin as viable treatment options when combined with standard chemotherapy. These findings are supported by clinical trials showing improved overall survival with these combinations. Additionally, statins and metformin are associated with the reversal of T-cell exhaustion, positioning these drugs as potential combinatory strategies to improve immunotherapy outcomes in ovarian cancer patients.

Keywords: T-cell exhaustion; clinical trials; drug repurposing; ovarian cancer metabolomics.

Figure 1

Metabolism of cholesterol within an ovarian cancer cell. (a) De novo cholesterol biosynthesis starts with acetyl-coenzyme A (ACoA) and is then synthesized in more than 20 enzymatic steps, while 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR), farnesyl-diphosphate farnesyltransferase 1 (FDFT1), and squalene epoxidase (SQLE) act as rate-limiting enzymes. (b) Cholesterol uptake is mediated by the ligation of low-density lipoprotein (LDL) to its receptor (LDLR), which is followed by endocytosis of LDL. This uptake creates high cholesterol accumulation, leading to intracellular lipo-toxicity and suppressing sterol-regulatory element binding protein 2 (SREBP2) transcription factor activity, thereby restricting the expression of enzymes involved in cholesterol synthesis and LDLR-mediated cholesterol uptake. (c) Excess cholesterol is converted into cholesterol esters (CE) by acyl-CoA: cholesterol acyltransferase 1 (ACAT1), also known as sterol-O-Acyl transferase 1 (SOAT1) enzyme, is then stored in lipid droplets. (d) Excess cholesterol is also converted to oxysterol through multiple enzymatic or non-enzymatic processes, which then activates liver X receptor (LXR)-retinoid X receptor (RXR) signaling and results in gene expression of ATP-binding cassette (ABC) subfamily A member 1 (ABCA1) and ABC subfamily G member 1 (ABCG1), which promote the (e) cholesterol efflux pathway. The cholesterol exported by ABCA1 is transported by lipid-free apolipoprotein A-1 (APOA1), producing immature high-density lipoprotein (HDL) that is converted into mature HDL by lecithin-cholesterol acyltransferase (LCAT) in the plasma. The cholesterol exported by ABCG1 can directly become mature HDL, which can be consumed by liver cells or steroidogenic cells (e.g., ovarian cells) by binding to HDL receptor-scavenger receptor type B1 (SR-B1), activating downstream pathways involved in cancer cell proliferation, growth, and migration. Figure created in BioRender.com (accessed on 24 November 2023). ER—Endoplasmic reticulum; FPP—Farnesyl pyrophosphate; INSIG—insulin-induced gene; PDZK1—PDZ Domain Containing 1; SCAP—SREBP-cleavage activating protein; S1P—site-1 protease; SRE—steroid response element.

Figure 2

The insulin-like growth factor 1 (IGF1) signaling pathway and major downstream effects on cancer cells. The activation of IGF1 by the ligation to its receptor (IGF-1R) activates two pathways: phosphatidylinositol 3-kinase/Akt (PI3K/AKT) and Ras/mitogen-activated protein kinase (Ras/MAPK). (a) PI3K/AKT activates nuclear factor-κB (NFκB) and mouse double minute 2 (MDM2) for cell survival and inhibits apoptosis through inhibition of BCL2 associated agonist of cell death (BAD) and Forkhead transcription factor FOXO1 (FKHR), resulting in decreased apoptosis, increased protein synthesis, cell growth, and cell proliferation, among various other effects not represented here. (b) Ras/MAPK contains an elaborate kinase cascade that ultimately leads to increased cellular proliferation by promoting the activity of transcription factors, such as ELK1. The ligation of IGF-1 to IGF-1R is modulated by IGFBPs through direct binding in the extracellular space. IGFBPs also exert several IGF-independent effects via direct interaction with cell membrane-bound proteins, such as integrins. Image created in BioRender.com (accessed on 24 November 2023). Akt—Ak strain transforming; Erk—extracellular-signal-regulated kinase; ELK1—ETS Transcription Factor like-1; IGF-1—insulin-like growth factor 1; IGF-1R—insulin-like growth factor 1 receptor; IGFBP—insulin-like growth factor binding protein; IRSs—insulin receptor substrate proteins; MEK—mitogen-activated protein kinase; mTOR—mammalian target of rapamycin; P—phosphate; PI3K—phosphatidylinositol 3-kinase; PIP2—phosphatidylinositol 3, 4 phosphate; PIP3—phosphatidylinositol 3, 4, 5 phosphate; Raf—rapidly accelerated fibrosarcoma; Ras—rat sarcoma; SHC—Src homology/collagen.

Figure 3

Mechanisms by which pitavastatin and metformin act to (A) turn cancer cells sensitive to chemotherapy and (B) help to revert the exhaustion state of CD8⁺ T cells. The pitavastatin blocks drug efflux pumps and inhibits HMGCR, leading to a cascade of inhibition, including cholesterol biosynthesis. The uptake of cholesterol by the exhausted CD8⁺ T cells increases the stress of the endoplasmic reticulum (ER), leading to the augmented expression of transcription factor X-box binding protein 1 (XBP1), which increases the expression of inhibitory receptors (PD-1). The metformin inhibits the respiratory-chain complex 1, which mediates the activation of adenosine monophosphate-activated protein kinase (AMPK), which inhibits the mammalian target of rapamycin (mTOR) and its downstream signaling pathways. This leads to increased T cell longevity and mitochondrial oxidative phosphorylation. AMPK activation induces PPAR-gamma coactivator 1α (PGC1α), which increases mitochondrial activity and synergistically suppresses tumor growth by phosphorylation of programmed cell death protein ligand-1 (PD-L1). CD8⁺ T cells are immune active when they have the ligation MHC II with TCR/CD4; on the contrary, when they express the receptor PD-1, they are immune inactive. So, for example, using an antibody to PD-1 allows the inhibition of ligation to PD-1 with PDL-1, and the CD8⁺ T cell stays active. Figure created in BioRender.com (accessed on 24th of November 2023).