Lipoproteins and cancer: The role of HDL-C, LDL-C, and cholesterol-lowering drugs

Abstract

Cholesterol is an amphipathic sterol molecule that is vital for maintaining normal physiological homeostasis. It is a relatively complicated molecule with 27 carbons whose synthesis starts with 2-carbon units. This in itself signifies the importance of this molecule. Cholesterol serves as a precursor for vitamin D, bile acids, and hormones, including estrogens, androgens, progestogens, and corticosteroids. Although essential, high cholesterol levels are associated with cardiovascular and kidney diseases and cancer initiation, progression, and metastasis.

Although there are some contrary reports, current literature suggests a positive association between serum cholesterol levels and the risk and extent of cancer development. In this review, we first present a brief overview of cholesterol biosynthesis and its transport, then elucidate the role of cholesterol in the progression of some cancers. Suggested mechanisms for cholesterol-mediated cancer progression are plentiful and include the activation of oncogenic signaling pathways and the induction of oxidative stress, among others. The specific roles of the lipoprotein molecules, high-density lipoprotein (HDL) and low-density lipoprotein (LDL), in this pathogenesis, are also reviewed. Finally, we hone on the potential role of some cholesterol-lowering medications in cancer.

1. Introduction

Cholesterol is a 27-carbon amphipathic sterol molecule that is of physiological importance. Cholesterol stabilizes and maintains the permeability and fluidity of cell membranes independent of temperature changes. Cholesterol also has a vital role in forming and stabilizing lipid microdomains, known as lipid rafts, along with phospholipids, sphingolipids, and glycosylphosphatidylinositol-anchored proteins [1]. These rafts are in an inactivated state but become activated when grouped, allowing functional proteins of a signaling cascade to interact; cholesterol has a pivotal role in this aggregation process [2]. Cholesterol also serves as a precursor for vitamin D, bile acids, and hormones, including estrogens, androgens, progestogens, and corticosteroids. Cholesterol stores come from two sources, endogenously synthesized and dietary via the small intestine.

Although cholesterol is vital for maintaining normal physiological homeostasis, high cholesterol levels are associated with various pathologies, including cardiovascular disease (CVD). Its accumulation in the vasculature causes plaque formation and atherosclerosis, kidney disease, and cancer. Several studies show that many oncogenic signaling pathways modulate cholesterol synthesis, suggesting a role in its progression [3]. In this review, we first present a brief overview of the cholesterol synthetic pathway and its transport, then elucidate the role of cholesterol in the progression of some cancers, the specific roles of the lipoprotein fractions, high-density lipoprotein (HDL), and low-density lipoprotein (LDL) in this pathogenesis, and finally evaluate the potential role of some cholesterol-lowering medications in the management of cancer.

2. Cholesterol synthesis

2.1. Endogenous pathway

Cholesterol synthesis primarily occurs in the liver, which synthesizes about 50% of the cholesterol produced in humans. It is an energy-consuming process requiring acetyl-CoA, ATP, and NADH/NADPH [1]. Acetyl-CoA is produced in the glycolytic pathway from beta-oxidation of free fatty acids following lipolysis and amino acid catabolism [4]. The latter two processes produce energy during fasting or starvation. In the fed state, acetyl-CoA from glycolysis enters the mitochondria, where it reacts with oxaloacetate to form citrate, the first step in the Krebs/tricarboxylic acid (TCA) cycle. This citrate is then transported back to the cytosol, where ATP citrate lyase regenerates the acetyl-CoA that is used in fatty acid and cholesterol synthesis [4]. Figure 1 gives an overview of cholesterol synthesis where acetyl-CoA is converted to 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA); this is then converted to mevalonate by the enzyme HMG-CoA reductase, the rate-limiting enzyme in the cholesterol synthetic pathway. Mevalonate is then converted to farnesyl-pyrophosphate, then to squalene, which is converted to lanosterol; this is then converted to cholesterol through a series of 19 reactions, reviewed in [5].

Figure 1:

Overview of Endogenous Cholesterol Synthesis. Endogenous cholesterol synthesis begins with two molecules of acetyl-CoA, which are used to synthesize acetoacetyl-CoA by the enzyme thiolase. Acetoacetyl-CoA is then used to synthesize HMG-CoA, by the enzyme HMG-CoA synthase. The rate-limiting enzyme of the cholesterol synthesis pathway is HMG-CoA reductase, which converts HMG-CoA to mevalonate. This enzyme is stimulated or inhibited by several physiologic molecules, such as insulin, glucagon and AMP. It is also a target of the cholesterol lowering medication, the statins. Further enzymatic reaction produces IPP, which is then converted to FPP. FPP is converted to squalene, followed by lanosterol and finally cholesterol by a series of enzymatic reactions. FPP is also converted to GGPP with both having roles in protein prenylation, as discussed in the text. FPP is also a chemotherapeutic target and it links K-ras, which is mutated in many human cancers [182], to the membrane. Cholesterol is also a precursor for several important molecules, as depicted above. Cholesterol negatively regulates its own synthesis by inhibiting SREBP processing, via interactions with the SCAP protein. The role of the SREBP gene in cholesterol synthesis is discussed in the text. Created with BioRender.com

Cholesterol synthesis is under both short-term and long-term regulation. Short-term regulation is through HMG-CoA reductase, which is phosphorylated by an AMP-dependent protein kinase (HMG-CoA reductase kinase) that is active when cellular AMP is high (ATP is low). So, when cellular ATP is low, energy is not spent in synthesizing cholesterol. Long-term regulation is by varied formation and degradation of HMG-CoA reductase and other enzymes of the pathway. Degradation is stimulated by cholesterol, oxidized metabolites of cholesterol, mevalonate, and farnesol (dephosphorylated farnesyl pyrophosphate). HMG-CoA reductase also has a transmembrane sterol-sensing domain that has a role in activating the enzyme’s degradation. The enzyme is also regulated at the transcription level by a sterol regulatory element-binding protein #2 (SREBP-2). Three other proteins are involved in the activation of SREBP-2 in response to sterol levels; SREBP cleavage-activating protein (SCAP), an integral protein of the endoplasmic reticulum; protease S1P (site one, serine protease), an integral protein of the Golgi; and protease S2P [1, 6].

2.2. Exogenous cholesterol absorption

Cholesterol is absorbed by the small intestine through the brush border membranes (BBM), either passively or facilitated by proteins [7] such as the Niemann-Pick C1 like 1 protein (NPC1L1). The proposed mechanism of NPC1L1 suggests that it recruits dietary cholesterol to form a lipid raft surrounding it on the brush border membrane; once a certain level of cholesterol has aggregated into the lipid raft, NPC1L1 can sense the sterol molecule via its sterol sensing domain (SSD), and trigger clathrin-mediated endocytosis of itself and the cholesterol molecules in its vicinity to the endoplasmic reticulum [8]. Once internalized within the endoplasmic reticulum, cholesterol is modified into cholesterol ester by the enzyme acyl-CoA cholesterol acyltransferase 2 (ACAT2) and packaged into chylomicrons in conjunction with dietary derived triglycerides. Alternatively, the absorbed cholesterol can also be secreted back into the intestinal lumen by the heterodimer proteins, ABCG5 and ABCG8 [7, 9]. On average, about 50% of dietary cholesterol is ultimately absorbed into the circulation [7]. Chylomicrons, with a large concentration of triglycerides, are secreted into the lymphatic circulation through which adipose tissue and muscle cells absorb the triglycerides through the actions of lipoprotein lipase (LPL), the remaining chylomicron remnants are endocytosed into the liver. Here, the chylomicron remnants are metabolized, and cholesterol is re-packaged into either very low-density lipoprotein (VLDL) for secretion into the circulation or secreted into the lumen as bile.

3. Cholesterol Transport and Lipoproteins

Cholesterol is transported by packaging it into carrier molecules called lipoproteins. Cholesterol is a nonpolar, hydrophobic molecule, and thus, cannot travel in the serum. Lipoprotein contains a surface layer of phospholipids and free cholesterol, both of which are amphipathic. The core of lipoproteins is composed of varying compositions of the hydrophobic triglyceride and cholesterol ester molecules. The conversion of free cholesterol to the hydrophobic cholesterol ester is completed by the enzyme lecithin cholesterol acyltransferase (LCAT) in peripheral tissues [10], as well as ACAT2 in the enterocytes bordering the intestinal lumen [8]; this conversion allows for the storage of a greater amount of cholesterol molecules in individual lipoproteins than would be possible with just free cholesterol.

Also, the key to the formation of lipoprotein molecules are apolipoproteins, which are found on the surface of the lipoprotein molecules, and allow interactions with enzymes and cell membrane receptors. Different types of lipoproteins contain different apolipoproteins, which help distinguish their functionality. Many different lipoproteins are composed of a mixture of triglycerides and cholesterol involved in cholesterol transport; these are classified by their relative size, densities, and apolipoproteins associated with them. Traditionally the classifications are chylomicrons, VLDL, low-density lipoprotein (LDL), intermediary density lipoprotein (IDL), and high-density lipoprotein (HDL). Another lipoprotein is lipoprotein (a), which is an LDL particle with an added apolipoprotein (a) (apo (a) attached to the apolipoprotein B-100 (ApoB-100) component of the LDL particle via a disulfide bridge) [11].

LDL functions to deposit cholesterol into cells in need of repair, and in states of excess, can deposit cholesterol into the vasculature, a precursor for cardiovascular pathologies. LDL particles can also be endocytosed by the liver, releasing their cholesterol and triglyceride content for excretion and metabolization. Most lipoproteins are atherogenic; that is, they promote the formation of atherosclerotic plaques in the vasculature. One lipoprotein that is not atherogenic is HDL, which can interact with both LDL particles and chylomicrons and muscle cells and adipose tissue to receive cholesterol for transport back to the liver, where cholesterol can be excreted as bile [12].

4. Cholesterol and cancer

Hypercholesterolemia is correlated with an increased risk of developing cancers via multiple mechanisms; these include breast, prostate, and colon, among others [13–16], summarized in Table 1. Various metabolites of cholesterol have been positively correlated with the growth and metastasis of cancers. In a transgenic mouse model of breast cancer, 27-hydroxycholesterol, a cholesterol metabolite, promoted tumor growth and metastasis [17]. These findings have translated to humans with estrogen receptor-positive breast cancer [13].

Table 1:

Cholesterol-associated molecular pathways implicated in cancer

Molecular Target	Mechanism	Cancer Types	References
Hedgehog proteins	Cholesterol mediates the activation of hedgehog proteins, which then activates tumor formation and metastasis.	Prostate Cancer	[18, 20]
IL6-STAT3	Lipid rafts composed of cholesterol are key to the activation of STAT3 by IL-6.	Advanced prostate cancer	[18, 19]
Rho GTPase	Cholesterol precursors prenylate and activate Rho GTPases, causing dysregulation of proliferation, differentiation and migration. Rho GTPase upregulation has been observed in many cancer types.	Breast Cancer, Melanoma, Gastric Cancer	[24–26]
STARD1, STARCH, HIF-1α	Increased presence of mitochondrial cholesterol causes dysregulation of cellular functions, and may activate HIF-1α by increasing mitochondrial ROS levels; this leads to upregulation of angiogenesis and other tumorigenic characteristics.	Hepatocellular Carcinoma	[28, 184]
PI3K/Akt overactivity, leading to mTORC1 overactivity	Cholesterol precursor (mevalonic acid) upregulates mTORC1 activity, leading to cell growth and proliferation.	Breast Cancer, Ovarian Cancer, Tuberous Sclerosis (mTORC1 mediation)	[14, 34, 36, 185, 186]
SREBP2 (by Mutant p53)	P53 mutations increase activity of SREBP2, increasing cholesterol synthesis	Breast Cancer, Prostate Cancer, Glioblastoma, Ovarian Cancer	[26, 37, 42, 43]
Bile Acids	Derived from cholesterol, these increase ROS.	Colorectal Cancer	[15, 45–49]
Dendrogenin A	A natural cholesterol metabolite that has anticancer properties; its levels are decreased cancer in patients.	Breast Cancer	[50, 52, 53]
22-Hydroxycholesterol	A cholesterol metabolite which has immunosuppressive effects via mechanisms such as activation of CXCR2 and reduced recruitment of CD8+ T-Cells.	LLC, RMA and AB1 cell lines (mouse models) Melanoma Lung Cancer Colon Cancer Kidney Tumors	[57, 58]

Lipid rafts containing cholesterol located on cell membranes express many proteins that can transduce oncogenic signals; thus, manipulating these lipid rafts by altering their cholesterol levels can affect tumor development [14, 18]. Some oncogenic pathways activated by increased cholesterol levels include the Hedgehog pathway and IL6-STAT3, both observed in prostate cancer [18–20].

The Hedgehog pathway, which is mostly inactive in healthy adults, can be activated by cholesterol binding to the smoothened receptor (Smo), a G-protein [14, 21, 22]. This pathway can also be modulated by the Ptch1 and Gli1 proteins [23]. In a normally functional Hedgehog pathway, the absence of the hedgehog ligand-protein leads to Ptch1 inhibition of the Smo, which normally leads to the activation of this signaling pathway. When activated, these signaling pathways are involved in the survival, proliferation, and migration of stem cells in tumors [22, 23].

Isoprenoids, which are precursors of cholesterol in the cholesterol synthesis pathway, are also implicated in oncogenic signaling pathways. Isoprenoids can mediate the activity of Rho GTPase proteins by a process called prenylation. Rho GTPase over-activity can help in the morphogenesis of cancer by dysregulation of processes such as cellular migration and division [24]. Prenylation of Rho GTPase molecules allows for increased interaction and activation by GTPase activating proteins (GAPs) [25]. Prenylated RhoA molecules can also inhibit P27kip1 and prevent its translocation to the cell nucleus, where it would typically regulate stem cell fate [26].

High levels of cholesterol in the mitochondria have also been implicated in cancer progression. In several cancers, the expression of the steroidogenic acute regulatory domain proteins (StARD) on the mitochondrial inner membrane is upregulated [3, 27]. In particular, upregulation of the StARD1 protein and mutations in the StARD3 endosomal protein permit increased cholesterol import into the mitochondria [28]. Increased cholesterol levels in the mitochondria are linked to the activation of hypoxia-inducible factor 1-alpha (HIF-1α), a protein that is activated in hypoxia and in conditions of high oxidative stress. Upon its activation, HIF-1α translocates to the nucleus and heterodimerizes with a complex of HIF-1β and Arnt to drive the upregulation of genes that have roles in angiogenesis, autophagy, and metabolism [28, 29]. In cancer, HIF-1α-angiogenesis helps drive tumor progression and anaerobic glycolysis – a characteristic of most tumor cells, which preferentially use oxygen in anabolic processes, the so-called Warburg effect [30]. Increased cholesterol in the mitochondrial membrane impairs its fluidity [31]. Consequently, it might impair the function of the mitochondrial protein 2-oxoglutarate carrier (2-OGC), which normally exchanges 2-OG of the mitochondria with GSH, an important antioxidant [28]. Therefore, cholesterol may deplete mitochondrial GSH leading to reactive oxygen species (ROS) generation and HIF-1α stabilization [28]. Another interesting effect of increased cholesterol in the mitochondrial membrane is its anti-apoptotic effects. Apoptosis can occur when a complex of Bax/Bak proteins creates a pore in the mitochondrial matrix, known as mitochondrial outer matrix permeabilization (MOMP). This pore allows for the release of apoptosis-inducing factors, including cytochrome C. Mitochondria with increased membrane cholesterol levels have some resistance to MOMP, which would protect them from apoptosis. These anti-apoptotic effects of cholesterol underscore its role in cancer progression and resistance to chemotherapy [28, 32]. In addition, StARD3 overexpression is associated with a more carcinogenic phenotype and less adhesive cellular mass in human epidermal growth factor receptor 2 (HER2) negative breast cancer cell lines [33].

Cholesterol can also increase the activity of the main target of rapamycin complex 1 (mTORC1) protein, which can increase the activity of SREBP1C and SREBP2 proteins, driving fatty acid and sterol synthesis that modulate cell proliferation and growth [34, 35]. Using a Rag-GTPase heterodimer, mTORC1 is anchored to lysosomal membranes and activated by interacting with the Rheb-GTPase protein. Cholesterol has been shown to increase this activation by promoting the binding proteins [36]; the role of cholesterol in this activation process appears to be mediated by SLC38A9 transporters [34]. Thus, increased activation of Rag-GTPases can increase the activation of mTORC1 and subsequently positively regulate fatty acid synthesis, cholesterol synthesis, and cellular proliferation. Furthermore, the cholesterol precursor mevalonic acid also indirectly increases mTORC1 expression by activating the phosphatidylinositol 3-kinase/protein kinase Akt (PI3K/Akt pathway) [14].

Just as cholesterol can increase the activity of the oncogenic pathways, cholesterol levels can be increased in cancer due to oncogene effects on the SREBP gene; mutations in tumor suppressor gene p53, as well as over-activation of signaling pathways such as RTK/Ras and PI3K/Akt can increase SREBP transcription, which can subsequently lead to an increase in cholesterol synthesis [14, 37]. SREBP1 activity is upregulated in prostate cancer, glioblastomas, and ovarian cancers [38–40]. The downregulation of SREBP1 can inhibit tumor growth, presumably by significantly altering the metabolism of cancer cells [41]. In addition, mutant p53 interacts with and activates SREBP2, causing downstream activation of mevalonate pathway enzymes and cholesterol synthesis, as seen in breast, prostate, and ovarian cancers as well in glioblastoma [26, 37, 42, 43]. The mevalonate pathway enzymes are similarly overexpressed in many other cancer cell lines, including melanoma, breast, lung, and colorectal cancers [43].

Metabolites of cholesterol have also been implicated in cancer progression. Bile acid, the primary excretory form of cholesterol, has been implicated as a promoter of colorectal cancer. Primary bile acids, cholic acid (CA), and chenodeoxycholic acid (CDCA) are produced primarily in hepatocytes by the enzyme cholesterol 7-α hydroxylase (CYP7A1). The enterohepatic system reabsorbs these bile acids; those that are deconjugated in the distal intestine are subsequently converted to the secondary bile acids deoxycholic acid (DCA) and lithocholic acid (LCA) by bacterial 7-α dehydroxylase [44]. DCA, along with the taurine-conjugated form of CDCA, can induce oxidative stress as evidenced by decreased cellular viability and increased thioredoxin reductase expression [15]. Increased levels of fecal bile acids have been associated with a higher risk of developing colorectal cancer and have been shown in multiple studies to increase the dysplastic potential and proliferation of colorectal cancer cells [45–47]. Bile acids can also influence the gut microbiome, potentially affecting tumorigenesis. In APC^min/+ mice, DCA administration resulted in greater cell proliferation and tumor formation than the control group. DCA treatment also resulted in dysbiosis with proliferation of microbes such as Clostridium (c-diff) and Escherichia-Shigella, with subsequent upregulation of inflammatory cytokines [48]. Furthermore, transplantation of fecal microbiota of DCA-treated mice into untreated mice promoted tumor formation in APC^min/+ mice, but not in wild-type mice [48]. The inability of fecal microbiota transplantation to promote tumor formation in wild-type mice suggests that susceptibility also plays a role in DCA-mediated tumor formation, as summarized by [49].

Cholesterol does, however, have some tumor suppressant activity as well. Dendrogenin A (DDA) is a natural cholesterol metabolite that exhibits anti-tumor properties in vivo [50]. It induces tumor cell differentiation and death and significantly reduces tumor growth. This metabolite is found in nanomolar quantities per gram of tissue and can also be found in trace quantities in the serum [50]. DDA has been identified as the most potent natural inhibitor of the enzyme cholesterol epoxide hydrolase (ChEH), an enzyme that normally produces a toxic metabolite, cholestane-3β, 5α, 6β-triol, which is more cytotoxic than the substrates of the enzyme [51]. Decreased DDA levels have been found in patients with various cancers; for example, in patients with breast cancer, DDA levels were 15 ± 6 ng g⁻¹ of breast tissue compared to 72 ± 28 ng g⁻¹ in patients without cancer [50, 52, 53].

Cholesterol and its metabolites may modulate cancer progression by influencing the immune response against cancer cells. Cholesterol has both pro-cancer and anti-cancer effects in this context. Activation of toll-like receptors (TLR) by high levels of cholesterol can lead to an inflammatory state that may promote tumor progression [54]. On the other hand, the formation of pro-inflammatory cytokines can also be controlled in part by cholesterol derivatives such as geranyl-geranyl pyrophosphate, via activation of the PI3K pathway [54, 55]. As with other cellular membranes, the cholesterol composition of the immune cells impacts the functionality of membrane proteins; increased cholesterol to cholesterol ester ratio increases the cytotoxic ability of CD8 T-cells by improving T-cell receptor transduction ability [54, 56]. Oxysterols may promote the progression of tumors by inhibiting an immune response. For example, 22-hydroxycholesterol may recruit pro-angiogenic or immunosuppressive neutrophils via CXCR2 activation [57]. Oxysterols also suppress immune response by activation of the liver-x-receptors (LXR). LXR-α activation by oxysterols may also promote tumor progression, by inhibiting the expression of CCR7, which is responsible for the migration of dendritic cells [58]. However, this effect is also variable, depending on the differentiation stage of the dendritic cell [59, 60]. The role of cholesterol in the immune response against cancer is further reviewed in [54].

4.1. Role of LDL-C in cancer

Distinct lipoproteins can provide a better perspective into the potential role of cholesterol in cancer. LDL is a lipoprotein most responsible for distributing cholesterol amongst the extrahepatic tissues and cells of the body. The sole apolipoprotein associated with LDL is apolipoprotein B-100 (ApoB-100); each LDL lipoprotein has only one ApoB-100. LDL binds to the LDL receptor (LDLR, apo B-100 receptor) present on most tissues, including the liver [12]. After binding, it facilitates the uptake of LDL via clathrin-mediated endocytosis. It can also bind ApoE of chylomicron molecules. Once LDL is endocytosed into the cell, it is fused with lysosomes, where it is hydrolyzed to release cholesterol, fatty acids, and amino acids. The SREBP2 protein mediates LDLR levels. When cellular cholesterol levels are low, SREBP2 is cleaved and translated to the nucleus, where it upregulates the expression of LDLR; when cellular cholesterol levels are high, SREBP2 remains inactive, and LDLR expression is downregulated [12, 61]. LDL-C levels of <100 mg/dL are considered optimal while levels of >160 mg/dL are considered elevated.

4.1.1. LDL-C and breast cancer

Increases in LDL-C have been positively associated with tumor size of at least 20 mm, and lymph node metastasis [62]. Patients with LDL-C levels greater than 144 mg/dL were less likely to have disease-free survival (DFS) [32, 62], although overall survival (OS) rates did not differ significantly. Patients with the highest cholesterol levels (>144 mg/dL) were found to be HER2 positive [62]. In cell culture studies, higher LDLR expression was observed in HER2/neu/ERBB2 (HER2) positive and some triple-negative cell lines compared to in ER+ cells [32]. LDLR silencing and lower circulating levels of LDL-C retard tumor growth in HER2 positive and triple-negative breast cancer mouse models [32]. LDL-C also promotes the migration of breast cancer cells by decreasing the expression of various adhesion proteins, such as claudin and occludin [63]. A potential mechanism for LDL-promoted tumorigenesis could is elevated intracellular ROS levels and subsequent cellular inflammation and damage. LDL-C can activate HER2, which increases the phosphorylation of oncogenic signaling pathways, Akt and extracellular signal-regulated kinase (ERK) [63]. Activation of these pathways by LDL-C can mediate cell proliferation in cancer. ERK phosphorylates the transcription factor FOXO3a, which leads to its degradation; thus, it will not cause cell cycle arrest. In normal cells, FOXO3a induces Bim and FasL, causing apoptosis. It also has a role in cell cycle regulation through the induction of p27kip1 and Cyclin D. Therefore, increased degradation of FOXO3a by activation of the ERK pathway can stimulate cell survival and cell proliferation by deregulation of the cell cycle [64]. Akt activation has many implications in cell survival, including the suppression of p53 gene expression, increased Bcl-xL expression and the activation of mTOR signaling; it can also induce cell proliferation by inhibiting p21 and p27 [65].

4.1.2. LDL-C and colorectal cancer

Increased LDL-C levels have been associated with an increased risk of developing colorectal cancer and poor prognoses [66–69]. Increased LDLR expression is also seen in patients with colorectal cancer (CRC), suggesting an association between tumorigenesis and LDL-C absorption [70]. Using the TNM classification system [71], LDLR expression was higher in patients with higher N and M stages of cancer, although not in patients with higher T stages [70]. LDL promotes the migration of colorectal cancer cells and upregulates cancer stem cells [70]. LDL also promotes the MAPK signaling pathway gene expression, mediated by increased expression of the p38 protein. LDL can also increase stem-ness genes such as Sox2, Oct4, Nanog, and Bmi1 in CRC cells [70]. High-Fat Diets may also play a role in colorectal tumorigenesis, as the use of these diets in mice models produced significant inflammatory effects [70]. In this regard, diet-induced weight loss in obese individuals largely decreased colorectal inflammation and downregulated some carcinogenic markers such as STAT3, STAT5A, and STAT5B [72]. Dietary inflammation of the colon underscores the importance of cholesterol in the pathogenesis of colorectal cancer. In CRC patients, a positive association in a subclass of the LDL-C known as small-dense low-density lipoprotein (sdLDL) and smaller diameter HDL particles was observed [73]. Furthermore, increases in sdLDL may be a predictor of CRC as elevated levels were found in CRC patients despite lower total LDL-C levels [73]. One reason for higher sdLDL levels in CRC is due to the preference of LDLR for binding LDL rather than sdLDL, allowing sdLDL levels to remain elevated. This allows sdLDL to be a predictor of cancer even in more advanced tumors, in which serum LDL-C levels may be decreased due to increased metabolic demands of cancer. Another implication for higher sdLDL concentration is the increased likelihood for sdLDL to be oxidized into oxidized LDL (oxLDL) in comparison to its normal-sized LDL-C counterpart [73]. The oxidative stress caused by increased oxLDL can therefore play a role in cancer development.

4.1.3. LDL-C and Prostate Cancer

A similar pattern has been observed in patients with prostate cancer (PCa), with increases in total cholesterol and LDL-C being associated with higher cancer prevalence and more advanced tumor presentation [16, 74–76]. The growth of prostate cancer cells is dependent on the steroidal hormone androgen, which controls cellular proliferation in PCa through agonism of the androgen receptor (AR). Cholesterol is a precursor of androgens and is positively associated with tumor size and intratumoral testosterone levels [16]. oxLDL levels also have a positive correlation with N stage prostate cancer and stimulate proliferative and invasiveness in PCa cell lines [77]. Higher LDL-C in PCa may be due to a loss of negative feedback; PCa cells had no negative down-regulation of LDLR in response to increased LDL-C levels, in contrast with normal prostate tissue [78].

4.1.4. Potential chemo-preventative effects of LDL-C in cancer

Although several studies show a positive association between LDL-C and cancer risk, several epidemiologic studies do not support these observations [79–84]. A population-based study demonstrated no significant association between any lipoprotein or apolipoprotein level and the risk for developing cancer in both men and women [85]; however, in men, high ratios of ApoB/ApoA1 and higher levels of ApoB were associated with an increased risk for developing cancer [85]. ApoB concentrations indicate a measure of all atherogenic lipoproteins, including VLDL, IDL, and predominantly LDL [86]. This study also shows that higher levels of ApoA1 are associated with a greater incidence of breast cancer and that higher levels of ApoB are inversely related to breast cancer risk [85]; other studies have also shown an inverse association between LDL-C/ApoB levels and breast cancer risk [87, 88], reviewed in [89]. These studies dispute the general notion that higher LDL-C levels increase the risk for cancer, although the mechanism for this unique finding in breast cancer is not known. In another study of patients with advanced CRC, serum LDL-C levels were significantly lower than in healthy patients and patients with benign colorectal disease [90]. This finding is presumably due to an increased metabolic demand for cholesterol from the cancer cells.

4.2. HDL-C and cancer

HDL is the major lipoprotein responsible for transporting cholesterol from the periphery back to the liver for clearance. The apolipoproteins associated with HDL include apolipoproteins A1, A2, C-1, and E. Of these, ApoA1 and A2 are the most prevalent, initiating HDL synthesis. ApoA1, which is primarily synthesized in the liver and the small intestine, is expressed in most HDL particles and usually compromises 70% of its lipoprotein content[10, 91]. ApoA2 is synthesized in the liver comprising 20% of its lipoprotein content, and is expressed on about 67% of HDL particles [10]. Once secreted, apo-A1 is lipidated by free cholesterol and phospholipids to form nascent HDL particles. The early lipidation process is primarily mediated by the liver and intestines, which secrete these necessary lipids using the ABCA1 transporter. Cholesterol and phospholipids can also be derived from macrophages during this phase, also by the action of the ABCA1 transporter [92]. This is the major mechanism by which HDL plays a protective role against atherosclerosis. HDL also acquires a significant amount of cholesterol from extrahepatic tissues that cannot metabolize cholesterol independently. Several mechanisms mediate this cholesterol transfer. Passive, aqueous diffusion between cell membranes and HDL particles accounts for significant cholesterol transfer [92]; this is further enhanced by the activity of the ABCG1 transporter, which mobilizes cholesterol from organelles to increase passive diffusion [92]. Cholesterol transfer can also be mediated in a passive, non-aqueous manner by scavenger receptor BI proteins (SR-BI). The maturation of HDL particles from their nascent forms is marked by the formation of cholesterol esters which can expand the HDL core [10]. This esterification utilizes the enzyme LCAT, which is activated by ApoA1 [93]. The ability of LCAT to convert free cholesterol to cholesterol esters increases the capability of HDL to carry cholesterol. It helps to maintain the concentration gradient required for the passive transfer of cholesterol from cellular membranes to HDL [92]. In the liver, mature HDL can deposit accumulated cholesterol for metabolism and excretion. The SR-BI receptor is also vital for this process, as it allows for the selective uptake of cholesterol from HDL without metabolizing the lipoprotein. After depositing its cholesterol content, the HDL is re-secreted for more cholesterol accumulation [10].

In contrast to LDL-C levels, generally high HDL-C levels indicate good health since the accumulation of excess cholesterol in the extrahepatic tissues and vasculature are less; this potentially translates to better cardiovascular health and disease prevention. As such, low HDL-C levels are positively associated with an increased risk of cancer. In women, serum HDL-C <30 mg/dL was associated with overall increased mortality due to cancer [94]. Similarly, in patients with colorectal cancer, serum HDL-C levels were positively associated with increased OS and DFS [95]. The relationship between HDL-C and cancer mortality is not an inverse-linear association but rather J-shaped, with the lowest mortality occurring at HDL-C values of 50-70 mg/dL [96]. Although HDL has generally been acknowledged as having many protective effects on the cardiovascular system, studies have also suggested that the composition of HDL particles is critical; HDL particles with altered composition can even have harmful effects, reviewed in [97]. Many of the proteins commonly found on HDL particles have been observed to be altered in cancer patients; the most important of these proteins will be discussed in the following sections.

4.2.1. ApoA1 association with multiple cancers

ApoA1 is a 243 amino acid polypeptide whose levels are inversely associated with a primary cancer diagnosis, progression, and prognosis in most organs [93]. The addition of ApoA1 and ApoA1 mimetics to several different cancer cell lines has also provided greater insight into the mechanism by which ApoA1 and HDL-C might be protective against cancer.

ApoA1 decreased cell viability in both ovarian and colorectal carcinoma cell lines by binding and inhibiting lysophosphatidic acid (LPA). This phospholipid increases migration, invasion, and tumorigenesis of cancer cells and induces VEGF production [98–102]. ApoA1 mimetics can also decrease the activation of the Vascular Endothelial Growth Factor (VEGF) and basic Fibroblast Growth Factor (bFGF) signaling pathways, each of which is involved in epithelial angiogenesis. They accomplish this by decreasing the activation of Fibroblast Growth Factor Receptor 1 (FGFR1) and Vascular Endothelial Growth Factor Receptor 2 (VEGFR2) receptors and by inhibiting phosphorylation of Akt and ERK1/2 [102, 103]. ApoA1 also has roles in inhibiting cancer cell proliferation by inducing pro-apoptotic proteins, caspase 5, Tumor Necrosis Factor (TNF) superfamily 10B, and apoptosis protein activating factor 1 (APAF-1), most commonly through inhibition of the MAPK cellular signaling pathway [104]. ApoA1 also downregulates the expression of ABCA1, which, when overexpressed, has been linked to colorectal malignancies by the induction of cell proliferation, migration, and invasiveness. This downregulation is mediated by decreasing cyclooxygenase-2 (COX-2) [105]. ApoA1 mimetics also inhibit cellular proliferation by decreasing cyclin A and D1 expression and by decreasing serum oxLDL levels [100, 106]. These mimetics have demonstrated a greater impact on inhibiting tumor progression than increased HDL particles containing ApoA1 [106]. The tumor-suppressive activity of ApoA1 and ApoA1 mimetics has been supported in multiple animal model studies, reviewed in [93].

Although ApoA1 generally has protective effects against cancer risk and progression, other reports suggest an opposite effect, promoting retinoblastomas and squamous cell carcinoma of the head and neck [93, 107, 108]. There have also been conflicting reports on whether ApoA1 is protective or harmful in the pathogenesis of gastric, colorectal, nonsmall cell lung, breast, prostate, and transitional cell bladder cancers, summarized in [93, 109–112]. These apparent contradictions may be explained by measurements of ApoA1 at different times in each pathology. For example, in the case of lung cancer, levels of pro-lipoprotein A1 have been observed in patients with metastatic brain cancer [110]; in gastric cancer, ApoA1 was notably elevated in patients in the early stages of the disease but was comparatively normal later in the disease process [111]. However, ApoA1 is sometimes upregulated regardless of cancer staging or metastasis, as observed in colorectal carcinoma [112, 113].

4.2.2. ApoA2 in Pancreatic cancer

Associations between ApoA2 and carcinogenesis have also been explored. ApoA2 is a 77 amino acid protein that primarily exists as a homodimer of equal subunits that are linked together by a disulfide bond between cysteine residues at the 6^th position [114]. Dimer isoforms of ApoA2 can be classified by their C-terminal amino acid residues; 5 dimer phenotypes are ApoA2-ATQ/ATQ, ApoA2-ATQ/AT, ApoA2-AT/AT, ApoA2-AT/A, and ApoA2-A/A. Plasma expressions of ApoA2-AT/A and ApoA2-A/A are correlated with a greater risk of developing pancreatic cancer and are found in pancreatic cancer patients [115]. Furthermore, while the levels of ApoA2-ATQ/ATQ and ApoA2-AT/AT usually are equally distributed in healthy patients, these levels are disproportionate in either direction in patients with pancreatic ductal adenocarcinoma and other pancreatic diseases and are indicative of either hypo-processing or hyper-processing by the exocrine pancreas. Specifically, it has been suggested that the altered activity of carboxypeptidase A is at the root of this change [116]. These disproportionate ratios are reflected in the overall ApoA2-ATQ/AT levels, which are significantly decreased in these diseases [117]. This association shows that in addition to apolipoproteins being stimulatory or preventative factors for carcinogenesis, cancer can also in part modify apolipoproteins. Thus these changes may be used as markers for cancer development and progression.

4.2.3. ApoE phenotypes affecting cancer

Although apolipoprotein E (ApoE) is a minor component of HDL, it too may be associated with cancer incidence. ApoE is a 299 amino acid polypeptide that has a role in cholesterol transport; it binds to its receptor located on the liver and mediates rapid uptake of HDL and VLDL [114]. There are 3 different polymorphisms of the ApoE gene, which produce apolipoprotein E2 (ApoE2), apolipoprotein E3 (ApoE3), and apolipoprotein E4 (ApoE4). These three polymorphisms give rise to 6 different potential genotypes of ApoE; (E4/4, E3/3, E2/2, E4/2, E3/2, E4/3) [114]. Meta-analysis of 25 studies in Asian populations showed that the ε4 allele of the ApoE gene was associated with an increase in cancer incidence compared to the ε3 and ε2 alleles; this allele was also associated with lower levels of serum HDL-C [118]. This association points to the fact that phenotypic variations in apolipoproteins amongst individuals can confer an altered risk of cancer development via modulation of serum lipoprotein levels. Functional ApoE activity may play a role in maintaining an intact immune response through agonist activity of Liver-X-nuclear receptors (LXR) [119]. ApoE binds to and inhibits myeloid-derived suppressor cells (MDSCs), which generally suppress immune T-cell activation [119]; this explains how potentially non-functioning ApoE polymorphisms might promote tumor activity.

4.2.4. Apo-C1 in cancer

Although many studies have identified ApoC-1 as a biomarker for several cancers, the mechanism(s) by which this apolipoprotein affects cancer progression is not yet clear. In patients with triple-negative breast cancer, there is conflicting data. One study identified decreased levels of ApoC-1 as a potential biomarker in these patients, and in a xenograft model using MCF-7 cells (estrogen receptor-positive breast cancer cell line), an ApoC-1 peptide suppressed tumor growth and increased apoptosis [120]. However, another study reported that ApoC-1 levels were increased in patients with triple-negative breast cancer [121]. In pancreatic cancer, ApoC-1 expression was upregulated and was associated with a poor prognosis [122]. In patients with clear cell renal cell carcinoma (ccRCC), ApoC-1 was increased and was highly associated with poor survival time [123]; it also promoted metastasis of ccRCC cells via the EMT pathway. Low levels of ApoC-1 have been observed in patients with gastric, colorectal, prostate, ovarian, and non-small cell lung cancers, as well as in Wilm’s Tumor, among others [124–128].

Although many studies analyze the potential diagnostic role of ApoC-1 levels in identifying various cancers, further research is needed to elucidate the mechanistic role of ApoC-1 in cancer progression.

4.2.5. SR-BI in breast cancer

Tumors may take advantage of the cholesterol transfer mechanisms of HDL to receive cholesterol esters. Tumor cells can facilitate HDL-mediated cholesterol absorption by overexpressing SR-BI; this receptor serves the same role on cancer cells as it does at the liver, allowing cholesterol ester efflux from HDL to tumor cells [129]. The phenomenon of SR-BI overexpression in tumors has been observed in several studies, including cancers of the lung, prostate, breast, and the endocrine system [130–133]. SR-BI expression is regulated by the SCARB1 gene, which itself can be upregulated by several pathways. SREBP2 that induces cholesterol synthesis can also induce SCARB1 gene upregulation [129]. The MAPK/ERK kinase (Mek1/2) signaling pathway can also be responsible for SR-BI accumulation due to inhibition of its proteasome-mediated degradation [129]. In addition to being induced in cancer, SR-BI may also promote further progression of cancer. SR-BI and HDL interaction has also been shown to activate the PI3K and ERK1/2 pathways in breast cancer [129]. In addition, interactions with PDZ domain containing 1 (PDZK1), a molecule that stabilizes SR-BI and promotes its translocation to the plasma membrane, may mediate several other oncogenic effects [129]. PDZK1 expression is associated with Akt activation; the interaction of SR-BI with PDZK1 also promotes eNOS activation and endothelial cell migration [129]. These findings might provide insights into how higher HDL-C levels might be associated with higher cancer mortality.

4.2.6. PON-1 in breast, ovarian, gastric, esophageal, pancreatic, head and neck, and bladder cancers

A systematic review shows that paraoxonase-1 (PON-1) activity levels were consistently lower in patients with breast and ovarian, gastric, esophageal, pancreatic, head and neck, and bladder cancers [134]. This enzyme is an antioxidant enzyme whose activity is enhanced when HDL is bound [135]. PON1 enzymatic activity is categorized into three groups; lactonase activity, arylesterase activity, and paraoxon activity [134, 135]. These activities were found to be significantly decreased across many cancer types [134]. Decreases in PON-1 activity could be a risk factor in cancer development that allows higher levels of oxidant stress. Other studies suggest that altered PON-1 activity results from cancer, rather than presenting a risk factor; higher utilization of PON-1 by cancer cells can lead to the appearance of low serum PON-1 activity [134].

5. The role of cholesterol-lowering therapeutics in cancer

Given the association between high cholesterol levels and cancer progression, treatments that aim to lower serum cholesterol levels may have beneficial effects on cancer. Research on the impact of both classic and alternative cholesterol treatments in cancer therapy has led to interesting results, some of which are discussed below.

5.1. Statins

Statins are cholesterol-lowering drugs that are widely used in both men and women. As a class, they competitively and reversibly inhibit HMG-CoA reductase, which is the rate-limiting enzyme for cholesterol biosynthesis in the liver; this leads to decreases in serum LDL-C, triglyceride, and cholesterol levels [136]. Statins can be either lipophilic (cerivastatin, simvastatin, lovastatin, fluvastatin, and atorvastatin) or hydrophilic (pravastatin and rosuvastatin) [137].

Statins may prevent various types of cancers, including colon [138–140], breast [141–143], ovarian, pancreatic, lung, and lymphomas [144]. As a class, they inhibit cancer cell proliferation by arresting the cell cycle at the G1-S phase and inducing apoptosis [145, 146], and decreasing tumor progression [42, 147].

Statins decrease isoprenoid formation, which tether proteins involved in signaling pathways to cellular membranes; these include Ras GTPase and Rho GTPase. Decreased production of isoprenoids and subsequent protein prenylation can reduce tumor proliferation via downregulation of these signaling pathways [147, 148]. Specifically, statin administration can increase the localization of Ras to the cytoplasm and likely affect the phosphorylation of downstream signaling such as EGF-mediated proliferation, MAPK, ERK1/2, and Akt [149, 150]. Similarly, statins can prevent the localization of Rho molecules to the membrane by preventing geranylgeranylation, which influences downstream activation of the Akt pathway [151–153]. Inhibition of prenylation also activates the tumor suppressor protein, AMP-activated protein kinase (AMPK), and inactivates mTOR [152–154].

Statins have been shown to inhibit tumor growth at clinically relevant doses and diminish metastasis in animal models [155, 156]. Of interest, simvastatin and fluvastatin have been shown to enhance NO levels through increases in the inducible form of nitric oxide synthase (iNOS) mRNA and protein expression. Statin-induced NO and tumor cell cytotoxicity was inhibited by 1400W [N-(3-aminomethyl)benzylacetamidine], a specific iNOS inhibitor [157].

It has been proposed that statins exert both LDL-dependent and LDL-independent or pleiotropic effects [158, 159]. One such pleiotropic effect is the upregulation of the endothelial form nitric oxide synthase (eNOS), resulting in higher production of endothelial NO, which has roles in vasodilation and platelet aggregation, vascular smooth muscle proliferation, and endothelial–leukocyte interactions [159, 160]. Statins upregulate eNOS activity through multiple mechanisms, including Rho/ROCK signaling. Statins also increase eNOS activity by post-translational activation of the PI3K/Akt pathway as eNOS is phosphorylated by Akt [161]. On the other hand, it has also been shown that statins decreased the levels of phosphorylated ERK1/2 and Akt [149, 150]. Other pleiotropic effects of statins, which increase eNOS activity, include effects on caveolin 1, transcription factor kruppel-like factor-2, and mobilization of endothelial progenitor cells [159]. Of note, NO can increase the responsiveness of tumors to chemotherapy [162]; in this respect, statins may be used as adjuvants.

Statins also have pro-apoptotic properties through activation of caspase-3 [150], Bax, caspase 8, and caspase 9 and downregulate the expression of the anti-apoptotic Bcl-2 [137, 163]. The effects of statins on angiogenesis are variable; at low doses, they induce angiogenesis, while at higher doses, they inhibit it [148].

Statins also can induce the degradation of mutant p53, which are implicated in the activation of the mevalonate pathway, whilst not affecting wild-type p53, and do not demonstrate a significant effect on non-cancer cells which express wild-type p53 [42, 147]. These effects are specific to statins as opposed to other inhibitors of the mevalonate pathway, likely due to a reduction in mevalonate phosphate (MVP) levels leading to decreased mutant p53 stability, as supplementation of MVP reverses these effects but supplementation of a downstream product of the mevalonate pathway, mevalonate-5-pyrophosphate does not; p53 stability is likely affected by regulation of its binding with the Hsp40 protein, DNAJA1 [147]. There is also evidence that statins can affect tumorigenesis via mechanisms separate from their influence on the mevalonate pathway, such as by inhibiting the purinergic P2X7 receptor [152].

There are some concerns that statins could potentially accelerate cancer progression. Lipophilic statins have greater apoptotic effects and reduce cancer risk more significantly than hydrophilic statins [137, 164]. This may be due in part to their better ability to diffuse into extrahepatic tumors [148, 149]. It is believed that hydrophilic statins do not decrease susceptibility to cancer [148]. In this regard, it is proposed that hydrophilic statins will increase extrahepatic cholesterol synthesis rather than reduce it [148]. This would occur as a compensatory mechanism for decreased hepatic cholesterol synthesis, paired with the inability of hydrophilic statins to effectively inhibit extrahepatic HMG-CoA reductase because of decreases in translocating the plasma membrane [148].

5.2. Fibrates

Fibrates are drugs that can reduce cholesterol levels by inducing the transcription factor PPAR alpha (PPAR-α), which promotes the synthesis of LPL, increases HDL levels by increasing ApoA1 and ApoA2 synthesis, and decreases LDL production. In a transgenic intestinal PPAR-α deficient mouse model, administration of the carcinogen azoxymethane resulted in larger colon tumors than control; decreases in PPAR-α was associated with reductions in levels of Retinoblastoma 1 protein (RBI), an increase in DNA methyltransferase 1 (DNMT1), and Protein Arginine Methyltransferase 6 (PRMT6), which themselves decrease the expression of tumor suppressor genes p21 and p27 [165]. This was consistent with findings of low PPAR-α in human patients with colorectal cancer, which became progressively lower with disease progression [165]. Although these findings suggest a potential role for fibrates as cancer therapeutics, other results suggest that fibrates do not meaningfully change cancer prognosis. In a meta-analysis of 17 randomized clinical trials, fibrates did not manifest any positive or negative effects on cancer incidence or mortality [165, 166].

5.3. Ezetimibe

Ezetimibe is a cholesterol-lowering medication that decreases the absorption of cholesterol from the intestinal lumen. Ezetimibe inhibits the NPC1L1 protein, which is responsible for cholesterol absorption from the lumen; one proposed mechanism of action is that ezetimibe prevents NPC1L1 from endocytosing into the enterocytes [7]. In mice that were fed different concentrations of cholesterol/fat and were harboring human prostate cancer xenografts, ezetimibe-mediated decrease in cholesterol inhibited angiogenesis through upregulation of the TSP-1 protein. This protein is usually downregulated by the activation of the Akt pathway [167]. Ezetimibe induced significant changes in apoptosis and cellular proliferation [167]. Although little clinical research has been conducted regarding the chemo-preventative effects of ezetimibe in patients with cancer, these in vivo findings clearly suggest that hypercholesterolemia directly accelerates the growth of prostate carcinomas and that pharmacological reductions of serum cholesterol levels may slow prostate cancer growth by inhibiting tumor angiogenesis [167].

5.4. Novel cholesterol-based therapies for cancer treatment

Novel therapies that target the key enzymes in fatty acid, cholesterol, lipoprotein, and apolipoprotein metabolism are being evaluated with regards to their effects on various cancers. Two novel targets are the SREBP1 and SREBP2 proteins, which contribute to fatty acid and cholesterol synthesis. Since SREBP1 and SREBP2 are upregulated in many cancers, inhibitors of these proteins are of clinical interest. Betulin, a drug that inhibits SREBP1 activation by binding to SCAP, thereby preventing SCAP-mediated SREBP cleavage [168] in combination with sorafenib when administered to hepatocellular carcinoma patients who did not respond to initial sorafenib therapy, helped enhance the effects of Sorafenib [168]. Fatostatin, another SREBP inhibitor, which prevents SCAP from exiting the ER [169], has been shown to reduce the growth of prostate [170], pancreatic [171], and endometrial [172] cancers. In addition to its inhibition of SREBP, fatostatin also has anti-tumorigenic activity independent of SREBP-inhibition; it destabilizes the mitotic microtubule spindle causing mitotic arrest [173]. Natural agents can also directly target SREBP-2 to downregulate key enzymes for the mevalonate pathway, thus reducing tumor growth, reviewed in [174]. Tocotrienol, a minor form of vitamin E, can degrade mature SREBP-2 in prostate cancer [175]. Artesunate, initially developed as an anti-malaria drug, effectively inhibits gliomas and distant metastasis and further induces cell senescence by regulating the nuclear localization of SREBP-2 and the expression of HMG-CoA reductase [176]. Emodin, a plant-derived anthraquinone, inhibits SREBP-2 transcriptional activity to suppress cholesterol metabolism and Akt signaling, which sensitizes hepatocarcinoma cells to the anti-cancer effect of sorafenib [177].

Several geranylgeranyl-transferase inhibitors (GGTI) are being evaluated as adjuvants in the treatment of breast, prostate, and nonsmall cell lung cancers, as they directly inhibit prenylation processes [178–180]. GGTI monotherapy in patients with metastatic tumors was investigated in 2009 but discontinued due to a lack of efficacy [181]; because prenylation can occur via other isoprenoids, namely farnesyltransferase, it is likely that monotherapy with GGTIs will continue to lack efficacy [181]. In Table 2 below, we have detailed some of the clinical outcomes of statins and other cholesterol-lowering medications in various cancers.

Table 2:

Effects of some cholesterol-targeting therapies in cancer patients

Drug/Study Name	Cancer Type	Findings	References
I. Statins
Lovastatin, Pravastatin, Simvastatin, Atorvastatin: Relationship between statin use and colon cancer recurrence and survival: results from CALGB 89803 Simvastatin, Pravastatin, Atorvastatin, Rosuvastatin, Fluvastatin: Statin use is not associated with improved progression free survival in cetuximab treated KRAS Mutant metastatic colorectal cancer patients: results from the CAIRO2 study	Stage 3 Colorectal Cancer	Statin use was not associated with improved cancer prognoses, as measured by overall and disease-free survival. Patients with metastatic colorectal cancer, treated with chemotherapy and other pharmacological treatment, did not have a significant change in overall survival after statin administration, as compared to other patients.	[187, 188]
Atorvastatin: Statin-induced anti-proliferative effects via cyclin D1 and p27 in a window-of-opportunity breast cancer trial Atorvastatin: Targeting HMG-CoA reductase with statins in a window-of-opportunity breast cancer trial	Primary Invasive Breast Cancer	Statin use led to a decrease in Cyclin D1 expression, and an increase in the expression of the tumor suppressor p27. Statin use led to a decrease in the expression of the tumor marker Ki67.	[189, 190]
Atorvastatin, Pravastatin, Fluvastatin, Simvastatin: Statin use is associated with improved survival in multiple myeloma: A Swedish population-based study of 4315 patients	Multiple Myeloma	A dose dependent relationship between statin use and survival was observed in patients with multiple myeloma.	[191]
Atorvastatin, Pravastatin, Fluvastatin, Simvastatin: Statin attenuates cell proliferative ability via TAZ (WWTR1) in hepatocellular carcinoma Pravastatin: Pravastatin combination with sorafenib does not improve survival in advanced hepatocellular carcinoma	Hepatocellular Carcinoma	Statin use was associated with longer recurrence free survival in patients with hepatocellular carcinoma. The use of pravastatin in combination with the current therapy for hepatocellular carcinoma, sorafenib, did not improve survival rates when compared to sorafenib monotherapy.	[192, 193]
Atorvastatin: Global transcriptional changes following statin treatment in breast cancer	Breast Cancer	Statins induced upregulation of many pro-apoptotic genes.	[194]
Fluvastatin, Atorvastatin, Lovastatin, Pravastatin, Simvastatin, Cerivastatin: Statin drug use is not associated with prostate cancer risk in men who are regularly screened Lovastatin: Use of statins and prostate cancer recurrence among patients treated with radical prostatectomy Atorvastatin: Atorvastatin versus placebo before radical prostatectomy—A randomized, double-blind, placebo-controlled clinical trial	Prostate Cancer	Statin use was not significantly associated with decreased risk or progression of prostate cancer. Treatment with atorvastatin did not significantly change the expression of tumor proliferation markers such as Ki67; however, a nonsignificant positive improvement was noted in patients who continued treatment for 28 or more days.	[195–197]
Simvastatin: Randomized phase II study of Afatinib plus Simvastatin versus Afatinib alone in previously treated patients with advanced nonadenocarcinomatous non-small cell lung cancer	Non-Small-Cell Lung Cancer	Patients taking a combination therapy of simvastatin and afatinib did not have a significant change in survival rate or tumor response as compared to monotherapy.	[198]
Pravastatin: Multicenter, phase III, randomized, double-blind, placebo-controlled trial of Pravastatin added to first-line standard chemotherapy in small-cell lung cancer (LUNGSTAR)	Small-Cell Lung Cancer	The administration of Pravastatin in Small-Cell Lung Cancer patients does not significantly change overall survival time or tumor response in patients.	[199]
II. Fibrates
Fibrate, fibric acid, fenofibrate, bezafibrate, ciprofibrate, clofibrate, gemfibrozil: Use of fibrates and cancer risk: a systematic review and meta-analysis of 17 long-term randomized placebo-controlled trials.		A meta-analysis, showing that fibrate use did not have an impact on the incidence or mortality of cancer	[166]
III. Ezetimibe
Ezetimibe is an inhibitor of tumor angiogenesis	Prostate Cancer	The study showed that ezetimibe use (in animal models) worked to lower tumor angiogenesis via lowering serum cholesterol levels.	[167]
IV. Novel Therapies
SREBP-1 inhibitor betulin enhances the antitumor effect of sorafenib on hepatocellular carcinoma via restricting cellular glycolytic activity.	Hepatocellular Carcinoma	Betulin increases the efficacy of sorafenib in patients who do not respond to sorafenib monotherapy.	[168]
Fatostatin blocks ER exit of SCAP but inhibits cell growth in a SCAP-independent manner Fatostatin suppresses growth and enhances apoptosis by blocking SREBP-regulated metabolic pathways in endometrial carcinoma. Fatostatin displays high antitumor activity in prostate cancer by blocking SREBP-regulated metabolic pathways and androgen receptor signaling. Fatostatin inhibits cancer cell proliferation by affecting mitotic microtubule spindle assembly and cell division.	Endometrial Carcinoma, Prostate Cancer	Fatostatin has antitumor properties, as observed in multiple in vitro and animal model studies.	[169, 170, 172, 173]
Mevalonate metabolism regulates basal breast cancer stem cells and is a potential therapeutic target. Geranylgeranyl transferase 1 inhibitor GGTI-298 enhances the anticancer effect of gefitinib. Synergistic effect of Geranylgeranyltransferase Inhibitor, GGTI, and Docetaxel on the growth of prostate cancer cells. Prenyltransferase inhibitors: treating human ailments from cancer to parasitic infections.	Prostate Cancer	Geranylgeranyl-transferase inhibitors favorably prevent the growth of cancer cells, although working better with other prenyloid inhibitors.	[178–181]

6. Conclusions and Perspectives

High serum cholesterol levels have a strong association with all stages of cancer development. There is evidence that cholesterol is both upregulated due to the over-activation of oncogenic signaling pathways and that cholesterol and the intermediates in its biosynthesis can stimulate these signaling pathways. Other mechanisms related to its role in carcinogenesis include prevention of apoptosis, promotion of angiogenesis, and ROS generation. Cholesterol also likely plays a role in providing cancer cells with the metabolic nutrients needed to grow. Regarding the roles of specific lipoproteins as predictive factors in cancer, LDL is positively associated with the risk of cancer development and progression. However, there are some contradictory data here. Generally, cancer cell lines overexpress and fail to negatively regulate LDLR, suggesting increased internalization of LDL-C in these cells. HDL levels are inversely associated with cancer, suggesting a protective role, although some studies suggest that HDL can also contribute to cancer progression as evidenced by an increased expression of SRB-1 cells on some tumor cells. Mutations and phenotypic variants in the ApoA1, ApoA2, and ApoE are also associated with the risk of developing cancer, further elucidating the typical role of HDL in preventing cancer. Of all conventional cholesterol-lowering therapy, the role of statins in cancer therapy has been the most investigated, and it seems to be the most promising; lipophilic statins are more effective as antitumor therapeutics than hydrophilic statins.

Future studies in this arena can further elucidate the mechanisms by which particular lipoprotein abnormalities modify cancer risk. In HDL particles, the mechanism by which modification of minor apolipoproteins such as ApoC-1 and ApoE increases cancer risk has not been studied extensively. The efficacy of all cholesterol-lowering therapeutics needs to be further appraised with clinical trials to determine whether in vitro effects can translate to the clinic.

Figure 2:

Effects of LDL on tumor signaling pathways. LDL-C can be a potent activator of tumorigenic pathways by multiple mechanisms. In vitro, many potential pathways explaining an association between LDL and cancer have been characterized. ROS generation and ERBB2 activation mediated by the apolipoprotein B-100 induce the ERK-mediated MAPK, which induces cellular proliferation and survival. ERBB2 can also induce cellular proliferation via activation of the PI3K/Akt pathway. The p38-dependent MAPK pathway increases cellular senescence in certain cell lines, and also has a pro-inflammatory effect via proteins such as COX-2 [183]. LDL decreases the expression of adhesion proteins, which can assist tumor cell motility and metastasis. They can also increase the expression of stem-cell inducing genes. Tumor cells also upregulate LDL receptors in order to maximize cholesterol transfer. Created with BioRender.com

Figure 3:

Effects of HDL on tumor signaling pathways. HDL inhibits the development and progression of cancer by multiple mechanisms, and conversely, genetic variants of HDL-associated apolipoproteins have been linked to cancer development. The ApoA1 protein is the major protein involved in inhibition of cancer progression. Cellular cholesterol uptake inhibits downstream activation of oncogenic pathways such as Akt. ApoA1 directly inhibits MAPK signaling in cancer cells resulting in induction of apoptosis and decreases in cellular proliferation; this is mediated by altering downstream MAPK signaling of apoptotic factors (Caspase 5, TNF Superfamily 10B, and APAF-1) and cell cycle regulators (Cyclin A and Cyclin D1). ApoA1 mediates inhibition of LPA1, an oxidant stress molecule associated with ovarian and colorectal carcinomas, which acts via HIF-1α mediated VEGF activation; ApoA1 also downregulates pro-inflammatory COX-2 expression. PON1, a protein associated with HDL lowers oxidant stress, thereby decreasing the risk of tumor development. ApoE also facilitates HDL uptake by the liver to prevent its utilization by cancer cells. SR-B1 receptors are upregulated in cancer cells resulting in further cholesterol uptake. Created with BioRender.com

Abbreviations

2-OGC

mitochondrial 2-oxoglutarate carrier

ACAT2

acyl-CoA cholesterol acyl transferase 2

AMPK

AMP-activated protein kinase

APAF-1

apoptosis protein activating factor 1

ApoA1

apolipoprotein A1

ApoA2

apolipoprotein A2

ApoB-100

apolipoprotein B-100

ApoC-1

apolipoprotein C1

ApoE

apolipoprotein E

AR

androgen receptor

bFGF

basic fibroblast growth factor

BBM

brush border membrane

CA

cholic acid

CCR7

C-C chemokine receptor 7

ccRCC

clear cell renal cell carcinoma

CDCA

chenodeoxycholic acid

ChEH

cholesterol epoxide hydrolase

COX-2

cyclooxygenase-2

CRC

colorectal cancer

CVD

cardiovascular disease

CXCR2

C-X-C motif chemokine receptor 2

DCA

Deoxycholic acid

DDA

dendrogenin A

DFS

disease free survival

DNAJA1

DNAJ heat shock protein family (Hsp40) member A1

DNMT1

DNA Methyltransferase 1

eNOS

endothelial nitric oxide synthase

ERK

extracellular signal-regulated kinase

FGFR1

fibroblast growth factor receptor 1

FPP

farnesyl pyrophosphate

GAP

GTPase activating protein

GGTI

geranylgeranyl-transferase inhibitors

GGPP

geranylgeranyl pyrophosphate

HDL

high density lipoprotein

HER2

human epidermal growth factor receptor 2

HIF-1α

hypoxia inducible factor 1-alpha

HMG-CoA

3-hydroxy-3-methylglutaryl-CoA

IDL

intermediate density lipoprotein

IPP

isopentenyl pyrophosphate

LCA

lithocholic acid

LCAT

lecithin cholesterol acyltransferase

LDL

low density lipoprotein

LDLR

low density lipoprotein receptor

LPA

lysophosphatidic acid

LPL

lipoprotein lipase

LXR

liver-x-nuclear receptor

MAPK

mitogen-activated protein kinase

MDSC

myeloid derived suppressor cells

MEK

MAPK/ERK kinase

MOMP

mitochondrial outer matrix permeabilization

mTORC1

main target of rapamycin complex 1

MVP

mevalonate phosphate

NO

nitric oxide

NPC1L1

Niemann-Pick C1 like 1 protein

OS

overall survival

oxLDL

oxidized low-density lipoprotein

PCa

prostate cancer

PDZK1

PDZ domain containing 1

PI3K

phosphatidylinositol 3-kinase

PON-1

paraoxonase-1

PRMT6

protein arginine methyltransferase 6

RB1

retinoblastoma 1 protein

ROS

reactive oxygen species

RTK

receptor tyrosine kinase

SCAP

SREBP cleavage-activating protein

sdLDL

small-dense low density lipoprotein

Smo

smoothened receptor

SR-BI

scavenger receptor BI proteins

SREBP-2

sterol regulatory element binding protein #2

S1P

site one serine protease

S2P

site two serine protease

SSD

sterol sensing domain

StARD

steroidogenic acute regulatory domain proteins

TCA

tricarboxylic acid

TNF

tumor necrosis factor

VEGF

vascular endothelial growth factor

VEGFR2

vascular endothelial growth factor receptor 2

VLDL

very low-density lipoprotein