Targeting cholesterol metabolism: a promising therapy strategy for cancer

Abstract

Cholesterol is a crucial structural component of cell membranes, playing a vital role in maintaining membrane fluidity and stability. Cholesterol metabolism involves four interconnected processes: de novo synthesis, uptake, efflux, and esterification. Disruptions in any of these pathways can lead to imbalances in cholesterol homeostasis, which are significantly associated with cancer progression. In recent years, traditional Chinese medicine (TCM) has emerged as a comprehensive therapeutic approach with multi-target and multi-pathway effects, demonstrating significant potential in regulating cholesterol metabolism. Research has shown that certain components of TCM can modulate enzymes, transport proteins, and signaling pathways involved in cholesterol metabolism, effectively interfering with survival and migration of cancer. These mechanisms highlight the unique advantages of TCM in inhibiting tumor progression. In this review we systematically describe the execution and regulation of the four key cholesterol metabolism processes, highlights the roles of critical proteins involved, and provides a comprehensive overview of natural products from TCM that modulate cholesterol metabolism. This review provides valuable insights for the development of novel drugs and cancer therapeutic strategies targeting cholesterol metabolism.

Introduction

In cancer treatment, metabolic reprogramming has emerged as a focal point of research, with cholesterol metabolism receiving widespread attention due to its critical role in tumor. Cholesterol, as an amphiphilic sterol molecule, is an essential component of cellular membrane. It not only maintains membrane fluidity and electrophysiological properties but also supports tumor cell proliferation by regulating signal transduction and membrane biogenesis [1]. Cholesterol homeostasis is maintained by a sophisticated regulatory network involving the processes of cholesterol de novo biosynthesis, uptake, efflux and esterification, as illustrated in Fig. 1.

Fig. 1. The process of cholesterol metabolism.

Cholesterol is the most abundant sterol compound in the human body, both as a component of cellular biofilms and for its conversion into substances with a variety of physiological effects. Cholesterol is synthesized in all tissues of the adult human body except the brain, with the liver and intestinal mucosa being the main sites of synthesis, involving more than 30 enzymatic reactions. Synthesized cholesterol is transferred to the cell membrane or other organelles by multiple cholesterol transport proteins such as NPC1L1, ABCA1. The main degradation of cholesterol is through the liver to bile salts, which are then secreted through the bile into the intestines to aid in the digestion and absorption of fat-soluble substances. Excess cholesterol then combines with fatty acids to form cholesteryl esters for storage. In conclusion, cholesterol metabolism is a four-part process consisting of biosynthesis, uptake, efflux and esterification. HMG-CoA 3-hydroxy-3-methylglutaryl-coenzyme A, SREBP Sterol-regulatory element binding protein, HMGCR HMG-CoA reductase, LDLR low-density lipoprotein receptor, NPC1L1 Niemann–Pick type C1-like 1, ABCA1 ATP-binding cassette transporter A1, ACAT acyl-coenzyme A, cholesterol acyltransferase, CE cholesteryl ester.

Alterations in cholesterol homeostasis, whether altered by pharmacology or genetic factors, are closely associated with cancer progression. For example, cholesterol synthesized via 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) and transported by low-density lipoprotein receptor (LDLR) increases melanogenesis and promotes uncontrolled proliferation of melanoma cells in a time- and dose-dependent manner [2]. Additionally, high expression of ATP-binding cassette transporter A1 (ABCA1) has been shown to promote cholesterol efflux and enhance membrane fluidity, facilitating metastasis in triple-negative breast cancer [3]. These findings indicate that the dysfunction of cholesterol metabolism plays a crucial role in cancer development and could serve as a promising therapeutic target. Reprogramming cholesterol metabolism may offer an effective strategy for inhibiting tumor growth and progression.

Existing therapeutic strategies targeting cholesterol metabolism, such as enzyme inhibitors and transport protein modulators, have demonstrated significant potential in controlling cancer progression [4]. However, these therapies are often accompanied by challenges, including adverse side effects, the emergence of drug resistance, and limited long-term efficacy in certain patient populations [5, 6]. This underscores the need for novel therapeutic approaches to overcome these limitations. In this context, traditional Chinese medicine (TCM), with multi-target and multi-pathway mechanisms, has emerged as a promising alternative for regulating cholesterol metabolism. Additionally, TCM-derived compounds are typically characterized by low toxicity and natural origins, reducing side effects and making them attractive candidates for integration into existing treatment regimens, with the potential to enhance overall therapeutic efficacy.

Recent studies have further revealed the significant role of TCM-derived compounds in regulating cholesterol metabolism and combating cancer. For example, quercetin can reduce cholesterol synthesis by activating adenosine monophosphate-activated protein kinase (AMPK) and inhibiting 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR) activity [7]. Similarly, platycodin D can help lower blood cholesterol levels by regulating transcription factors, enzyme activity, and other signaling molecules that increase LDLR synthesis [8]. Additionally, the natural compound isoglycyrrhizin has been shown to inhibit ABCA1, thereby reducing cholesterol efflux [9]. These studies highlight the diverse mechanisms by which TCM compounds regulate cholesterol metabolism, providing a solid scientific basis for their application as anti-cancer strategies.

Building upon these promising findings, further studies are needed to comprehensively investigate the effects of natural products on cholesterol metabolism and their specific mechanisms of action, thereby providing a robust scientific foundation for the development of cholesterol-modulating strategies and new drugs based on natural ingredients. This review systematically described the execution and regulation of the four key processes involved in cholesterol metabolism and the roles of critical proteins implicated in these pathways. And it provided a comprehensive overview of natural products that modulate cholesterol metabolism, highlighting their underlying mechanisms. These insights pave the way for the development of novel drugs and cancer therapeutic strategies targeting cholesterol metabolism.

Targeting cholesterol biosynthesis

Most cells, except brain tissue and mature red blood cells, can synthesize cholesterol, with approximately 50% occurring in the liver [10]. Cholesterol synthesis is a complex ATP-intensive process requiring nicotinamide adenine dinucleotide phosphate (NADPH) as a hydrogen donor. It begins with thiolase catalyzing the condensation of two acetyl coenzyme A (CoA) molecules to form acetoacetyl-CoA, which then combines with another acetyl CoA to produce HMG-CoA, catalyzed by HMG-CoA synthetase (HMGCS). HMGCR reduces HMG-CoA to methyl valproate (MVA), which undergoes multiple enzymatic steps to form farnesyl pyrophosphate (FPP), followed by squalene synthetase and its eventual conversion to cholesterol through intricate pathways involving lanosterol as an intermediate [2, 11]. With nearly 30 enzymatic steps and over 15 proteins involved, disruptions in genes or enzymes involved at any stage can disturb cholesterol homeostasis and promote cancer. This section highlights the roles of sterol-regulatory element binding protein 2 (SREBP2), HMGCR, and squalene epoxidase (SQLE) in cholesterol synthesis and reviews natural products that inhibit cholesterol synthesis and describe their underlying mechanisms (Fig. 2).

Fig. 2. Mechanisms of action of natural products targeting cholesterol biosynthesis in cancer.

Cancer cells normally upregulate cholesterol biosynthesis by activating SREBP2 signaling to gain a sufficient base for proliferation. Therefore, inhibition of cholesterol synthesis may be a promising pathway to inhibit cancer progression. SREBP Sterol-regulatory element binding protein, SCAP SREBP cleavage-activating protein, INSIG insulin-induced gene protein, COPII coat protein complex II, S1P site-1 protease, S2P site-2 protease, HMGCS HMG-CoA synthetase, HMGCR HMG-CoA reductase, SQS Squalene Synthase, SQLE squalene epoxidase.

SREBP2

SREBP is an intracellular cholesterol sensitizer located in the endoplasmic reticulum [12]. There are three mammalian isoforms of SREBPs: SREBP1a, SREBP1c and SREBP2. SREBP1a regulates adipogenesis and cholesterol production, while SREBP1c controls lipid metabolism, both encoded by the SREBF1 gene [13]. SREBP2, encoded by the SREBF2 gene, regulates cholesterol synthesis and uptake [14]. At high intracellular cholesterol levels, the insulin-induced gene protein (INSIG) binds SREBP and SREBP cleavage-activating protein (SCAP), retaining the complex in the endoplasmic reticulum [15]. When cholesterol level decreases, the complex is transported to Golgi via coat protein complex II (COPII) vesicles, where site-1 protease (S1P) and site-2 protease (S2P) cleave SREBP2, allowing its nuclear translocation to promote cholesterol synthesis and uptake [16].

High SREBP2 expression correlates with poor prognosis in breast, prostate, liver, and colorectal cancers [17]. Overexpression in hepatocytes promotes lipid peroxides in natural killer T cells (NKT cells), impairing cytotoxicity, while SREBP2 knockdown reduces hepatic steatosis, restores NKT function, and inhibits tumorigenesis in mice [18]. Additionally, melanoma-derived lactate activates SREBP2, converting dendritic cells (DCs) into mature regulatory phenotypes, promoting immunosuppression [19]. Targeting SREBP2 to inhibit cholesterol synthesis represents a promising strategy for cancer therapy.

Artesunate (ART), a derivative of artemisinin, reduces SREBP2 nuclear localization, disrupts its interaction with p53, and upregulates the expression of p53 target gene p21, including glioma cell senescence, though its effect on SCAP-SREBP2 remains unclear [20]. Gypenoside L modulates the MVA pathway by targeting SREBP2 and inhibiting the key enzymes, restoring cholesterol homeostasis, enhancing CD8⁺ T cells immunity, and promoting pro-inflammatory cytokine release [21].

In addition, SREBP2-mediated cholesterol biosynthesis also supports hepatic tumor stem cells growth and contributes to drug resistance in cancer [22]. Simvastatin, an approved cholesterol-lowering drug, can inhibit hepatocellular carcinoma (HCC) growth and enhance sorafenib sensitivity but cause toxicity with long-term use. Natural products, such as emodin (derived from the dried rhizomes and roots of Reynoutria japonica Houtt) and osthole (a coumarin-like compound extracted from Cnidium monnieri (L.) Cuss.), synergistically reduce SREBP2 activity, enhance sorafenib’s cytotoxicity, and affect pathways like NF-κB, MAPK, and PI3K/Akt [23, 24]. However, their precise mechanisms on SREBP2 and its downstream signaling require further investigation.

HMGCR

Clinical data revealed that HMGCR is abundantly expressed in tumor samples from advanced prostate cancer and HCC, making it a potential prognostic marker [25]. HMGCR inhibition activates p38 and suppresses programmed cell death 1 (PD-1) expression in regulatory T cells, enhancing anti-tumor immune responses [26]. Additionally, HMGCR correlates with smoothened receptor expression and promotes stemness and metastasis in HCC by activating the Hedgehog signaling pathway, leading to nuclear translocation of the transcriptional activator GLI family zinc finger 1 [27]. While statins, competitive HMGCR inhibitors, have shown promise in cancer treatment, challenges with high-dose administration limit their use as monotherapy [28]. Therefore, developing novel HMGCR inhibitors is still necessary to overcome these limitations and enhance anticancer efficacy.

The natural product alpineisoflavone (AIF) has significant anticancer effects on LNCaP (androgen-sensitive) cells by inhibiting androgen receptor (AR) expression, targeting HMGCR-mediated cholesterol biosynthesis, and inducing caspase-associated apoptosis, although its specific mechanism remains unknown [29]. Similarly, quercetin treatment in C6 glioma cells significantly inhibited cholesterol and fatty acids synthesis, decreasing the mRNA and protein levels of HMGCR and acetyl CoA carboxylase 1 (ACC1). This effect is associated with transcriptional downregulation of SREBP1, SREBP2, and carbohydrate response element binding protein (ChREBP), a bHLH-LZ transcription factor that regulates glucose-responsive genes [30].

SQLE

SQLE, also known as squalene monooxygenase, is the rate-limiting enzyme in the first oxygenation step of cholesterol synthesis, converting squalene to 2,3(S)-oxidized squalene [31]. RNA sequencing revealed that SQLE was the most highly expressed metabolic gene in patients with non-alcoholic fatty liver disease-induced HCC (NAFLD-HCC) [32]. SQLE overexpression in colon promotes intestinal dysbiosis and metabolic changes, causing intestinal barrier damage and increased colon cell proliferation [33]. Furthermore, SQLE inhibition reduced the levels of calcitriol (the active form of vitamin D3) and CYP24A1, increased intracellular Ca²⁺ concentrations, and suppressed MAPK signaling, thereby suppressing CRC cell growth [34].

Interestingly, cholesterol level above a certain threshold can trigger SQLE degradation, while low SQLE level promotes epithelial-mesenchymal transition (EMT), enabling cancer stem cells to overcome growth constraints. Soo Young Jun et al. demonstrated that reduced SQLE expression accelerates CRC progression by activating the β-catenin oncogenic pathway and inactivating the p53 tumor suppressor pathway. In a spontaneous CRC mouse mode, they identified the interactions between SQLE, GSK3β, and p53. Active GSK3β stabilizes SQLE, raising cellular cholesterol levels, while SQLE depletion disrupts the GSK3β/p53 complex, promoting metastasis [35]. Thus, the role of SQLE in cancer is a double-edged sword. Although some natural products such as resveratrol, curcumin, and green tea polyphenols have shown SQLE inhibitory activity and anticancer effects, these findings remain preliminary and require further mechanistic studies and clinical validation.

DHCR24

24-dehydrocholesterol reductase (DHCR24) is a key enzyme in the cholesterol biosynthesis pathway, responsible for reducing desmosterol to cholesterol. Beyond its role in maintaining cholesterol homeostasis, DHCR24 is closely associated with the development and progression of various cancers. Studies have shown that DHCR24 is highly expressed in certain tumors, such as endometrial cancer, where its elevated levels are significantly correlated with advanced clinical stages, higher histological grades, vascular invasion, lymphatic metastasis, and reduced overall survival [36]. This association may stem from DHCR24’s role in regulating cholesterol metabolism and its involvement in promoting tumor cell survival and proliferation through signaling pathways, such as the Hedgehog pathway [37]. Furthermore, overexpression of DHCR24 has been found to counteract the antitumor effects induced by the knockdown of SRY-box transcription factor 9 (SOX9) in diffuse large B-cell lymphoma, further highlighting its critical role as a regulator of cancer stem cells [38]. Notably, genkwadaphnin, an active ingredient isolated from the pistil of Daphne genkwa Siebold & Zucc (Thymelaeaceaeae), specifically targets DHCR24 to disrupt cholesterol biosynthesis, thereby inhibiting the growth of HCC cells in vitro and in vivo [39]. However, the study did not elaborate on how disrupting cholesterol biosynthesis specifically impacts tumor cell proliferation, apoptosis, or associated signaling pathways. Further research is needed to elucidate these mechanisms.

Targeting cholesterol uptake

While cholesterol biosynthesis is the primary pathway for maintaining cellular cholesterol levels, cancer cells also rely on exogenous cholesterol uptake to support their rapid proliferation demands. Studies have shown that in certain tumor microenvironments, cholesterol uptake mechanisms are just as crucial as biosynthesis, and in metabolically flexible cancer cells, they may even be more critical. This uptake is facilitated by proteins such as niemann-pick type C1-like 1 (NPC1L1) and LDLR, which help cancer cells adapt to nutrient limitations and meet their metabolic demands. Therefore, this discussion focuses on regulating NPC1L1 and LDLR by small molecule compounds to reduce cholesterol uptake and exert anti-cancer effects.

NPC1L1

NPC1L1, a 1332 amino acid membrane protein located at the brush border of small intestinal epithelium cells, plays a key role in dietary cholesterol absorption [40]. Cholesterol binding induces a conformational shift in NPC1L1, forming transport tunnels that allow cholesterol uptake via its steroid-sensing domain [41]. Elevated NPC1L1 expression reduces oxidative stress by increasing the uptake of vitamin E, protecting cancer cells from chemotherapeutic damage and promoting drug resistance [42]. Ezetimibe, the only approved NPC1L1 inhibitor, treats hypercholesterolemia but shows limited efficacy in cancer therapy, highlighting the need for novel NPC1L1 inhibitors for anticancer treatment.

Curcumin, the main active component of Curcuma longa L. (turmeric) rhizome, has demonstrated anticancer efficacy in breast, lung, prostate, and colorectal cancers [43]. When colorectal cancer Caco-2 cells were exposed to a hypercholesterolemic environment, curcumin enhanced intracellular cholesterol transport, a process requiring transient receptor potential A1 (TRPA1) activation, as evidenced by reduced transport rate with the TRPA1 inhibitor HC-030031. Curcumin also decreased SREBP2 and NPC1L1 levels in a dose-dependent manner [44]. Isoglycyrrhizin, a chalcone-structured flavonoid from Glycyrrhiza uralensis Fisch., binds to NPC1L1 in vitro, downregulating its expression and competitively inhibiting cholesterol uptake with negligible cytotoxicity, making it a promising candidate for further exploration in vivo [45]. Chrysanthemone, a natural small molecule from Tanacetum parathenium L., is a potent NPC1L1 inhibitor with effects comparable to ezetimibe, though its potential role in cancer remains underexplored [46].

LDLR

LDLR facilitates LDL uptake through endocytosis and plays a pivotal role in cancer progression. High LDLR expression in breast cancer correlates with reduced recurrence-free survival, especially in patients undergoing systemic therapy. Conversely, LDLR expression decreases from normal liver tissue to primary and metastatic liver cancer sites, underscoring its variable role in different cancers [47]. Platycodin D, a triterpene saponin from Platycodon grandiflorus, inhibits NPC1L1-mediated lysosomal cholesterol export and upregulates LDLR in glioblastoma cells. This results in lysosomal cholesterol accumulation, disrupted autophagy, and tumor cells death, highlighting the potential of targeting cholesterol metabolism and autophagy for cancer therapy (Fig. 3) [48].

Fig. 3. Mechanisms of action of natural products targeting cholesterol uptake in cancer.

Curcumin competitively binds NPC1L1 to cholesterol thereby inhibiting cellular uptake of cholesterol. LDL and HDL act as carriers to transport cholesterol from the liver throughout the body, which is subsequently recognized by LDLR. Several natural products such as platycodin D accelerate the uptake of excess cholesterol by tumor cells by up-regulating LDLR, resulting in a metabolic burden that promotes tumor cell death. This means that targeting cholesterol uptake to block cancer development is also a viable pathway. VLDL very low-density lipoprotein cholesterol, NPC1L1 Niemann–Pick type C1-like 1, PCSK9 proprotein convertase subtilisin/kexin type 9, SR-B1 scavenger receptor class B type I.

Apolipoprotein (apo) B, a key component of LDL, exists in two subtypes: apoB-48 and apoB-100, with the latter serving as the sole protein component of LDL and facilitating its recognition and uptake by LDLR [49]. Isoflavones such as genistein and daidzein, abundant in soy, inhibit apoB secretion by targeting cholesterol synthesis and esterification. These compounds also upregulate LDLR expression and reduce apoB secretion in HepG2 cells, presenting a promising therapeutic strategy for cancer and hypercholesterolemia [50].

The effects of LDLR on CD8⁺ T cell immune response and anti-tumor immunity have been extensively studied. Beyond its role in mediating cholesterol uptake, LDLR also interacts with the CD3 subunit of T cell receptor (TCR) complex, regulating TCR recycling and signal transduction. Furthermore, the proprotein convertase subtilisin/kexin type 9 (PCSK9) is highly expressed in tumors and impairs the immune response of CD8⁺ T cells by downregulating LDLR levels, ultimately hindering TCR signaling and effector function [51]. PCSK9 binds LDLR, inducing lysosomal degradation and decreasing surface LDLR levels [52, 53]. PCSK9 inhibition enhances the infiltration of CD8⁺ T cells and reduces myeloid-derived suppressor cells, linking cholesterol metabolism and tumor immunity [54]. Pseurotin A (PS), a spiro-heterocyclic γ-lactam alkaloid isolated from the fungus Aspergillus fumigatus, has been identified as a dual inhibitor of PCSK9 and PCSK9-LDLR interactions. Daily oral administration of 10 mg/kg PS in mice reduced PSCK9 secretion, increased LDLR expression, and inhibited prostate and breast cancer progression and recurrence [55, 56]. Additionally, tanshinone IIA, an active component isolated from Salvia miltiorrhiza Bunge, suppresses PCSK9 expression in HepG2 cells by enhancing FOXO3a/PCSK9 promoter complex formation while weakening HNF-1α/PCSK9 complex formation. Interestingly, it increases LDLR protein levels via post-transcriptional regulation without affecting LDLR promoter activity or mRNA expression [57].

SR-B1

Scavenger receptor class B type I (SR-B1), encoded by SCARB1, and sharing sequence homology with CD36, is a B-class scavenger receptor and HDL receptor that mediates selective uptake of cholesteryl esters (CEs) into cells [58]. Analysis from prostate cancer patients revealed that approximately 50% of cancer samples exhibited high SR-B1 expression, which correlated with reduced disease-free survival [59]. In castration-resistant prostate cancer C4-2 cells, SR-B1 antagonism impairs cholesterol uptake, de novo steroid synthesis, androgen activity, and cell proliferation, leading to endoplasmic reticulum stress and autophagy induction [60]. These findings suggest that SR-B1 is a crucial receptor in human malignancies, contributing to cancer phenotypes such as metastasis, and underscore its potential as a therapeutic target in cancer treatment.

A highly biocompatible HDL-mimetic peptide-phospholipid scaffold (HPPS) nanocarrier has been synthesized, enabling efficient absorption by nasopharyngeal carcinoma cells with high SR-B1 expression. HPPS mimics HDL function by binding to SR-B1 and competitively inhibiting HDL interaction, thereby reducing HDL-cholesteryl esters uptake and inhibiting tumor growth both in vitro and in vivo [61]. Similarly, synthetic HDL nanoparticles loaded with gemcitabine prodrug and apoA-II selectively target SR-B1, enhancing targeted delivery to pancreatic ductal adenocarcinoma compared to free gemcitabine [62]. Transcriptome analysis revealed significant alterations in pathways such as TGF-β, MAPK, cAMP, and cGMP-PKG between APC ^min/+ SR-B1^−/+ mice and APC ^min/+ mice, indicating SR-B1’s involvement in lipid metabolism and immune modulation via genes like Cyp27a1, Soat1, Tiam1 and Fyn [63]. However, research on small molecule inhibitors targeting SR-B1, particularly through traditional Chinese medicine, is limited, warranting further exploration of downstream pathways and mechanisms.

Targeting cholesterol efflux

Excess intracellular cholesterol, derived from both de novo synthesis and exogenous uptake, is primarily excreted via reverse cholesterol transport (RCT), a process mediated by ATP-binding cassette (ABC) transporter [64]. These transporters facilitate cholesterol transfer from cells to the liver, where it is secreted as bile acids or other derivatives. This section focuses on the role played by ABC transporter and liver X receptor (LXR) in cholesterol efflux and how small molecules enhance this process to achieve anticancer effects.

ABCA1

ATP-binding cassette transporter A1 (ABCA1) is a transmembrane protein widely expressed in tissues, facilitating nascent HDL formation by binding to apolipoproteins, such as A-I, and mediating the efflux of free cholesterol and phospholipids. In LNCaP prostate cancer cells, hypermethylation of ABCA1 promoter region significantly reduces its expression. This cancer-specific epigenetic modification leads to decreased cholesterol efflux, increased intracellular cholesterol accumulation, and a tumor-promoting microenvironment [65].

Isoliquiritigenin acts as an ABCA1 inhibitor, primarily by reducing the interaction between YY1 and proliferator-activated receptor gamma coactivator 1α (PGC-1α), which is highly expressed in CRC tissues and correlates with metastases. It also downregulates EMT linked to ABCA1-mediated cholesterol efflux [9]. Celastrol, a triterpenoid compound derived from Tripterygium wilfordii Hook F., inhibits the growth of clear cell renal carcinoma (ccRC) xenograft tumors in a dose-dependent manner, along with the upregulation of ABCA1 expression in tumor tissues. By enhancing ABCA1 expression, celastrol promotes cholesterol efflux, improving lipid metabolism and reducing cholesterol accumulation within tumor cells. ABCA1-mediated cholesterol efflux not only helps regulate intracellular cholesterol homeostasis but also contributes to anticancer effects by slowing cell proliferation and inhibiting EMT [66].

ABCG1

ATP binding cassette subfamily G member 1 (ABCG1), a hemitransporter protein involved in RCT, facilitates cholesterol efflux and HDL maturation by forming functional complexes via homodimerization or heterodimerization [67]. ABCG1 specifically promotes cholesterol efflux to HDL, but not to lipid-free apoA-I. ABCA1 mediated lipid efflux converts apoA-I into a substrate suitable for ABCG1-dependent cholesterol efflux, with efficiency linked to the phospholipid content of lipidized apoA-I. Thus, ABCA1 and ABCG1work synergistically to mediate the efflux of lipids from macrophages to apoA-I [68].

ABCG1 also connects cholesterol metabolism with tumor immunity by modulating macrophage polarization. In studies involving subcutaneous injections of MB49 bladder cancer and B16F10 melanoma cells into ABCG1^−/− mice and wild-type mice, tumor volumes were significantly smaller in ABCG1^−/− mice, suggesting that ABCG1 deficiency inhibits tumor growth. Tumor macrophage analysis revealed elevated expression of the M1 macrophage marker CD11c in ABCG1^−/− mice, indicating that ABCG1 inhibition promotes M1 macrophage polarization, which contributes to tumor suppression [69]. These findings highlight the pivotal role of macrophage phenotype, regulated by intracellular cholesterol levels, in tumor progression, offering novel therapeutic potential targeting ABCG1.

Despite the high levels of CEs and free cholesterol in lung tumor tissues, tumor-associated macrophages (TAMs) exhibit lower cholesterol concentrations. This correlates with reduced expression of cholesterol synthesis and uptake genes like LDLR and HMGCR, alongside increased cholesterol efflux transporters ABCA1 and ABCG1, suggesting a shift toward cholesterol export rather than import [70].

LXR

LXR is a nuclear receptor transcription factor that forms heterodimers with retinoid X receptor (RXR). Human cells express two LXR isoforms: LXRα, predominantly found in the liver and adipose tissue, and LXRβ, which is more universally expressed. LXR is activated by specific oxidative sterol metabolites and regulates genes involved in adipogenesis and cholesterol efflux, the initial step in the reverse cholesterol transport from macrophages to the liver [71].

Activation of LXR has been shown to regulate cholesterol efflux, thereby reducing membrane cholesterol, weakening membrane integrity, and inhibiting the proliferation and migration of breast cancer cells [72]. This process is similarly observed when LXR agonists are co-cultured with primitive plasma cell-like dendritic cell tumors, highlighting the broader relevance of LXR-mediated cholesterol efflux in tumor suppression [73]. Moreover, LXR activation significantly reduces cholesterol uptake by promoting ubiquitin-dependent LDLR degradation. In highly proliferative glioblastoma (characterized by EGFR mutations and PTEN loss), activation of LXR through synthetic ligands induces molecular changes that ultimately downregulate LDLR protein expression. This series of mechanisms reduces cholesterol influx and intracellular cholesterol accumulation, leading to decreased cell proliferation [74].

Regarding the inherent activity of LXR, researchers compared the effects of LXR agonists and LXR inverse agonists on clear renal cell carcinoma. Their findings indicate that the LXR agonist LXR623 decreases the expression of LDLR and increases the expression of ABCA1, which lowers intracellular cholesterol levels and promotes apoptosis in cancer cells. Conversely, the LXR inverse agonist SR9243 reduces the synthesis of fatty acid (FA)-related proteins, including SREBP1c, fatty acid synthase (FASN), and stearyl CoA desaturase 1 (SCD1). This reduction in FA content also leads to apoptosis in clear kidney cell carcinoma cells. Importantly, compound adversely affects normal kidney cells, suggesting that LXR is a promising and safe target for cancer treatment [75].

Bergamot, a natural analogue of psoralen found in various plants, has demonstrated the ability to reduce lipid accumulation in liver cancer cells through LXR activation in vitro. This effect is associated with the modulation of key signaling molecules, including PI3K and Akt, as well as lipogenic genes such as FASN and SCD1, leading to a reduction in lipid droplets level in liver cancer cells [76]. Additionally, the LXR agonist GW3965 significantly increases the expression of LDLR degradation products by targeting LDLR and regulates lipid metabolism, effectively promoting tumor cell death in an in vivo GBM model [77]. Unlike the chemically synthesized GW3965, the natural product flindissone retains its intact core structure, does not significantly induce SREBP1 expression or activate new lipogenesis, but exhibits LXR agonist activity, thereby regulating lipid metabolism [78]. Surprisingly, low concentrations of lycopene synergize with the synthetic LXR agonist T0901317 to inhibit the proliferation of androgen-independent prostate cancer DU145 cells and exert anticancer effects through the activation of LXRα and PPARγ signaling pathways [79]. These studies highlight the critical role of LXR activation in regulating cancer cell metabolism and promoting tumor cell death, emphasizing the potential of combining natural phytochemicals with existing drugs or using them as adjuvants in cancer therapy.

Targeting cholesterol esterification

To mitigate cytotoxicity caused by cholesterol overload, cholesterol can be converted to CEs by acyl-coenzyme A: cholesterol acyltransferase (ACAT), an intracellular membrane-binding enzyme. ACAT utilizes cholesterol and long-chain fatty acyl CoA as substrates to produce CEs, as illustrated in Fig. 4 [80]. Serving as raw materials for cell membrane synthesis, the accumulation of CEs provides tumor cells with an ample cholesterol supply for membrane formation and for maintaining membrane fluidity and stability. This, in turn, promotes tumor cell growth and proliferation. CEs have been found to be highly prevalent in tissue samples obtained from glioblastoma (GBM) patients, with their presence negatively correlated with patient survival [81]. This observation is not coincidental. CEs accumulate significantly in various types of cancer tissues, including pancreatic cancer [82], colorectal cancer [83], and prostate cancer [84]. This accumulation of CEs is closely associated with the loss of the tumor suppressor factor PTEN. PTEN is an important negative regulator of cell growth and survival, and its loss or functional impairment typically leads to the overactivation of the PI3K/Akt/mTOR signaling pathway. The upregulation of this pathway not only promotes the proliferation and survival of cancer cells but also further activates the transcription factor SREBP and LDLR, thereby providing cancer cells with the lipid resources necessary for their growth [84]. These findings suggest that CEs may serve as a potential biomarker for diagnosing invasive cancer and could offer therapeutic opportunities for targeting cholesterol metabolism in advanced tumors.

Fig. 4. Mechanisms of action of natural products targeting cholesterol efflux and esterification in cancer.

Cholesterol is not a bad thing in the traditional sense. On the one hand, high cholesterol levels in tumor cells can cause metabolic dysregulation and thus impede cancer development. On the other hand, some natural products such as bergamot activate LXR to promote the expression of cholesterol efflux genes such as ABCA1, ABCA5, etc. to reduce cellular energy supply. And ACAT converts cholesterol into cholesteryl esters to avoid cytotoxicity brought by cholesterol overload. Therefore, ACAT inhibitor avasimibe can act as a tumor “killer”. CE cholesteryl ester, ACAT acyl-coenzyme A, cholesterol acyltransferase, ABCA ATP-binding cassette transporter A, ABCG ATP Binding Cassette Subfamily G Member.

There are two types of human ACATs: ACAT1 and ACAT2. ACAT1 exists in all nucleated eukaryotic cells, where its products are incorporated into cytoplasmic lipid droplets. In contrast, ACAT2 is predominantly found in liver and intestinal epithelial cells, with its products transferred to the ER cavity and incorporated into the secreted lipoproteins [85]. Overexpression of ACAT1 is a distinct characteristic of HCC. Research indicates that p53 not only suppresses cholesterol synthesis by downregulating the mevalonate pathway but also directly binds to the promoters of ubiquitin-specific peptidase 19 (USP19) and ACAT1, resulting in transcriptional repression [86]. In other words, ACAT1 promotes cholesterol esterification and fatty acid synthesis in a p53-dependent manner to support HCC growth. Additionally, inhibiting ACAT1 enzyme activity or knocking down ACAT1 to eliminate cholesterol esterification results in increased intracellular free cholesterol levels, disrupting cholesterol homeostasis. This disturbance is associated with heightened endoplasmic reticulum stress, ultimately leading to cell apoptosis. Furthermore, this intervention significantly reduces the growth and metastasis of pancreatic tumors in situ mouse models [82].

Avasimibe is an ACAT inhibitor that has previously undergone clinical trials for atherosclerosis, with its safety profile well established. In a murine melanoma model, avasimibe demonstrated significant antitumor efficacy by reducing CEs levels through the inhibition of ACAT1 activity. This mechanism not only enhanced the effector function and proliferation of CD8⁺ T cells but also promoted T cell receptor clustering and signal transduction by increasing membrane cholesterol, thereby forming more effective immune synapses [87]. Furthermore, avasimibe inhibited the growth of primary and metastatic prostate cancer by reducing Wnt3a secretion and blocking the Wnt/β-catenin pathway [88]. In addition to avasimibe, K604 is a selective ACAT1 inhibitor that inhibits the proliferation of U251-MG cells and downregulates the activation of Akt and extracellular signal-regulated kinases in glioblastoma cells [89]. Moreover, treatment of triple-negative breast cancer MDA-MB-231 cells with bitter melon extract (BME) significantly reduced CEs levels, indicating that BME inhibits cholesterol esterification by targeting ACAT1, thereby suppressing tumor cell growth [90].

Studies have shown that the expression of ACAT2 is significantly upregulated in uterine fibroid tissue compared to normal myometrial tissue, making ACAT2 a potential therapeutic target. Manzamine A, a bioactive compound, can specifically target ACAT2 to inhibit cholesterol esterification, thereby significantly disrupting the nuclear localization of β-catenin and the activation of its downstream target genes, including extracellular matrix (ECM) proteins such as fibronectin and matrix metalloproteinases (MMPs) such as MMP-2 and MMP-9. This inhibitory effect is achieved by reducing lipid droplet accumulation and decreasing the availability of monounsaturated fatty acids required for Wnt protein acylation, ultimately blocking the β-catenin signaling pathway. Additionally, manzamine A reduces the acylation level of Wnt3a, limiting its secretion and further weakening Wnt signaling activity, highlighting its potential value in treating uterine fibroids [91]. In contrast, flavonoids such as citrus flavonoids, naringin, and hesperidin reduce the accumulation of apoB in a dose-dependent manner, indirectly lowering cholesterol esterification levels. However, unlike manzamine A, these flavonoids do not selectively inhibit specific forms of ACAT, suggesting that their regulation of cholesterol metabolism may involve broader pathways [92].

Conclusion and outlook

Tumor cells exhibit metabolic abnormalities to meet the increased energy and biosynthetic demands of rapid growth. A hallmark of cancer cells is the acquisition of oncogenes and the loss of tumor suppressor factors, which lead to the reprogramming of cholesterol metabolism pathways. Recent evidence suggests that cholesterol and its metabolites play crucial roles as signaling molecules that promote tumor development. Given the essential role of cholesterol metabolism in cancer progression, there has been an increased focus on identifying novel molecules and strategies to target this metabolic pathway, which has made significant strides in recent years.

Natural products have garnered significant attention in cancer research due to their enhanced biocompatibility and reduced toxicity. Their pleiotropic nature allows them to modulate various components of the tumor microenvironment, thus improving their anticancer effect. In contrast, synthetic compounds, often designed for high specificity towards a single target, may not effectively address the multifactorial and multipathway nature of tumors. Therefore, it is important to continue developing novel, effective, and non-toxic natural products that target cholesterol metabolism, with the aim of converting these into clinically available drugs to improve the quality of life and survival of cancer patients.

This review discusses in detail several important targets in cholesterol metabolism, including SREBP2, HMGCR, LDLR, and LXR, which are involved in cholesterol synthesis, uptake, efflux, and esterification. It also examines the roles of natural anti-cancer products at various stages of cholesterol metabolism (Table 1). This review not only highlights potential cholesterol metabolism targets that may serve as prognostic markers for future cancer therapies, but also explores the mechanisms through which cholesterol metabolism influences selective tumor susceptibility. This is critical for the development of targeted metabolic therapies. However, the regulation of cholesterol metabolism is not limited to these targets, but also includes some specific cholesterol metabolites. One such metabolite is 7-dehydrocholesterol (7-DHC), an intermediate in the distal cholesterol biosynthesis pathway. 7-DHC has been shown to exert anti-lipid peroxidation effects by interacting with conjugated dye, thereby protecting plasma and mitochondrial membranes from lipid peroxidation. This mechanism helps cancer cells evade ferroptosis, contributing to cancer metastasis [93]. Additionally, 7-DHC, as a natural ferroptosis inhibitor, is also a precursor to vitamin D, which is known for its anti-tumor properties, including the inhibition of cancer cell proliferation and the induction of differentiation [94]. The dual role of 7-DHC, acting both as a source of harmful oxidative stress and as a beneficial precursor to vitamin D3, complicates its function in cancer development. Therefore, future research could focus on developing targeted drugs that modulate the metabolic pathways of cholesterol metabolites, such as 7-DHC or utilize 7-DHC as a biomarker to predict disease progression and treatment response. In the study of cholesterol metabolism inhibitors, exploring combinations of different inhibitors will be crucial. These approaches may involve blocking cholesterol synthesis, uptake, esterification, or transport in cancer, potentially paving the way for the next generation of metabolic therapies.

Table 1.

Natural products targeting cholesterol metabolism and mechanisms of action.

	Ingredients	Structure	Effect	Mechanism	Ref.
Targeted cholesterol biosynthesis	Artesunate		Anti-glioblastoma	Affecting SREBP2 nuclear localization, disrupting SREBP2-p53 interactions and inducing p21 expression.	[20]
	Gypenoside L		Anti-hepatoma	Inhibiting the expression of HMGCS1, HMGCR and MVK, enhancing the immune response of CD8+ T cells and promoting the release of pro-inflammatory immune factors IL-2, IL-6, IFN-γ.	[21]
	Emodin		Anti-hepatoma	Down-regulating SREBP2 to inhibit the expression of HMGCS1, HMGCR, FDPS, DHCR7 and DHCR24.	[23]
	Osthole		Anti-hepatoma	Down-regulating SREBP2 and HMGCR to inhibit cholesterol biosynthesis and increase sensitivity of doxorubicin to hepatocellular carcinoma.	[24]
	Alpinumisoflavone		Anti prostate cancer	Down-regulating HMGCR to inhibit cholesterol biosynthesis.	[29]
	Quercetin		Anti-glioma	Inhibiting SREBP and ChREBP transcription to downregulate HMGCR and ACC1, thereby inhibiting cholesterol and fatty acid synthesis.	[30]
	Genkwadaphnin		Anti-hepatoma	Targeting DHCR24 to block cholesterol biosynthesis.	[39]
Targeted cholesterol uptake	Curcumin		Anti-colorectal cancer	Activating TRPA1 and downregulating NPC1L1 to reduce cholesterol absorption.	[44]
	Platycodon D		Anti-glioblastoma	Upregulating LDLR level to inhibit autophagy.	[48]
	Pseurotin A		Anti-prostate and breast cancer	Inhibiting PSCK9 to upregulate LDLR level.	[55, 56]
	Tanshinone IIA		Anti-hepatoma	Inhibiting PSCK9 to upregulate LDLR level through FoxO3a and HIF-1α pathways.	[57]
Targeted cholesterol efflux	Isoliquiritigenin		Anti colorectal cancer	Downregulating ABCA1 to inhibit cholesterol efflux thereby inhibiting EMT by weakening the interaction between YY1 and PGC-1α.	[9]
	Celastol		Anti-renal clear cell carcinoma	Upregulating ABCA1 to promote cholesterol efflux to inhibit tumor growth and migration by activating LXR expression.	[66]
	SR9243		Anti-clear cell renal cell carcinoma	Downregulating the synthesis of SREBP-1c, FASN, and SCD1 to decrease intracellular FA content and induce apoptosis.	[75]
	LXR623		Anti-clear cell renal cell carcinoma	Activating LXR by inhibiting the expression of LDLR and upregulating the expression of ABCA1.	[75]
	Bergapten		Anti-hepatoma	Regulating LXR/PI3K/Akt and IDOL/LDLR pathways to inhibit lipid accumulation.	[76]
	GW3965		Anti- glioblastoma	Regulating the LXR/LDLR/IDOL pathway and increasing ABCA1 expression to promote glioblastoma death.	[77]
	Lycopene		Anti prostate cancer	Activating PPARγ-LXRα-ABCA1 pathway to inhibit proliferation.	[79]
Targeted cholesterol esterification	Avasimibe		Anti-melanoma	Targeting ACAT1 to inhibit cholesterol esterification in T cells, leading to increased plasma membrane cholesterol levels and enhanced T cell receptor aggregation and signal transduction in CD8+ T cells.	[87]
			Anti-prostate cancer	Blocking Wnt/β-catenin pathway to inhibit cholesterol esterification.	[88]
	K604		Anti-glioblastoma	Inhibiting cell proliferation and downregulating Akt and extracellular signal-regulated kinase activation by inhibiting ACAT1.	[89]
	Manzamine A		Anti-uterine fibroids	Targeting ACAT2 to inhibit the nuclear localization of β-catenin and the activation of its downstream target genes reduces the acetylation level of Wnt3a, further blocking its secretion and disrupting Wnt signaling activity.	[91]

In summary, this review not only provides a theoretical foundation for targeting cholesterol metabolism in cancer research but also offers valuable insights for drug discovery. It underscores the importance of monitoring the rapid advancements in cholesterol metabolism research to foster the development and application of innovative anticancer drugs targeting this pathway.

Competing interests

The authors declare no competing interests.