Background

Cholesterol

Cholesterol is a steroid ring molecule found in both animals and plants [1]. In humans, about 80% of total cholesterol is biosynthesized in the body, while the remaining 20% is received through diet. Cholesterol comes in two forms: either free or esterified with fatty acids [2••]. Cholesterol ester – the predominant form of dietary cholesterol – is carried by Apo proteins as part of an Apo-lipoprotein complex (cholesterol carrier/transporter) in the body to form lipoproteins. Plasma lipoproteins are classified as chylomicrons, chylomicron remnants, very-low-density lipoprotein (VLDL), intermediate low-density lipoprotein (ILDL), low-density lipoprotein (LDL), lipoprotein A (Lp (a)), and high-density lipoprotein (HDL) [3, 4].

Cholesterol plays a role in cancer incidence, prognosis, and treatment outcomes. Cancer cells require ample cholesterol for rapid growth. Increased serum cholesterol levels have been linked to an increased risk of developing malignancies such as colon, rectal, prostatic, and testicular cancers [5]. Overexpression of LDLR and ACAT is found in most tumor tissue from cancer patients, which supports rapid cancer cell proliferation [1]. Most cancer tissues have higher LDL receptor expression than normal [6]. Using a combination of drugs to target cholesterol production and uptake was found to decrease cancer cell survival.

As hypercholesterolemia is also one of the major risk factors for hypertension, Cho et al. [7] showed that there was a substantial correlation between the long-term usage of ARBs and a decreased incidence of cancer. When compared to users of other kinds of antihypertensive pharmaceuticals, there was no overall increased risk of cancer for those who took typical antihypertensive medications. On the other hand, other analyses showed that there is no significant association was found between cancer and other antihypertensives [8, 9].

As obesity is one of the main risk factors for several types of cancer [10, 11], investigating one of the most related comorbidities such as hypercholesterolemia is essential to knowing how obesity induces cancer. Several recent studies have emphasized altering tumor progression by targeting cholesterol metabolic pathways [12,13,14]. From there, several studies were made to find how cholesterol is essential for cancer growth, especially on proliferation, apoptosis, and oxidative stress. On the other hand, few studies randomly discussed the immunomodulatory effect of cholesterol. As the immune microenvironment is a recent promising axis for cancer-targeting, investigating the role of cholesterol on the cancer immune milieu will open a new era of better immune-metabolome interaction and thus novel targeting approaches. The immune mediators and immune cells discussed here are among the most studied immune-related biomarkers related to the tumor immune microenvironment.

Antihypercholesterolemia

Statins are FDA-approved drugs that prevent cholesterol synthesis by inhibiting the enzyme 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase [15, 16]. Statins appear to inhibit the growth and survival of several cancer cells in vitro and in vivo experiments. Furthermore, LDLR downregulation/inhibition increases the efficacy of chemotherapeutics [17].

Cholesterol and Tumor Pathobiology

Cholesterol’s Effect on Cancer Proliferation

Enhanced proliferation is one of the main hallmarks of tumors. Cholesterol induces cancer proliferation by affecting multiple pathways. Afrin et al. [18•] found that simvastatin, one of the statins, inhibits uterine leiomyoma stem cell proliferation and induces apoptosis. In a patient-derived xenograft mouse model, El Sabeh et al. [19] showed that simvastatin significantly reduced tumor volume and inhibited the proliferation marker Ki67 expression when compared to the control group. Borahay et al. [20] revealed that simvastatin decreased tumor development and Ki67 expression in xenograft tumor tissue. Afrin et al. [21] found that simvastatin dramatically decreased proliferating cell nuclear antigen (PCNA) expression and E2-induced proliferation in leiomyoma cells. El Sabeh et al. [22] showed that both primary and immortalized human leiomyoma cells showed decreased levels of β-catenin following simvastatin treatment.

Cholesterol’s Effect on Cancer Apoptosis

Cholesterol inhibits cancer apoptosis by affecting multiple pathways. Caspase-3 is one of the apoptosis-inducing enzymes. Malik et al. [23] found that in simvastatin-treated cells, caspase-3 level decreased in a concentration-dependent manner. Borahay et al. [24] showed that simvastatin strongly induced leiomyoma cell death.

Cholesterol’s Effect on Cancer Oxidative Stress

Cholesterol induces oxidative stress and thus cancer progression by affecting multiple pathways. Homma et al. [25] found that long-term feeding of cholesterol induced atypical prostatic hyperplasia and increased tissue oxidative stress. Rauchbach et al. [26] found that in a model of non-alcoholic steatohepatitis, cholesterol may trigger hepatic stellate cells lipid peroxidation and death in the liver.

Cholesterol and Tumor Immune Microenvironment

Cholesterol and Immune Mediators

Cholesterol is found to suppress the immune microenvironment by suppressing immunostimulant cytokines and stimulating immunosuppressive cytokines. Immune mediators and cells discussed here are chosen based on their significance for cancer progression (Fig. 1).

Fig. 1
figure 1

Effect of cholesterol on cancer progression. Cholesterol increases cancer progression through different mechanisms such as increased proliferation, oxidative stress, metastasis, and immunosuppressive cells and mediators. It also inhibits apoptosis and immunostimulant cells and mediators

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Transforming Growth Factor-beta (TGF-β)

Transforming growth factor-beta (TGF-β) is an immunosuppressive cytokine that encourages cell growth and promotes cancer growth [27]. In HCC, TGF-β is linked to immune cell exhaustion, whereas inactivated TGF-β is linked to inadequate DNA repair [28, 29]. By controlling immune cells in the liver, TGF-β maintains a balance between immunological tolerance and activation. TGF-β is also a growth factor that regulates immune cells [30, 31] (Fig. 2).

Fig. 2
figure 2

Effect of cholesterol on tumor immune mediators. Cholesterol alters the tumor microenvironment in favor of immunosuppressive activity via a variety of mechanisms, including (1) Stimulation of both PDL1, IL-23, and TGF-β1, which stimulates both Treg cells and tolerogenic DCs while inhibiting cytotoxic T cells and mature DCs, (2) Stimulation of FOXP3, and CTLA-4, which stimulates Treg cells and inhibits cytotoxic T cells, and (3) Inhibition of IL-12 which stimulates Treg cells and M2 macrophages and inhibits cytotoxic T cells and M1 macrophages. CTLA-4: Cytotoxic T lymphocyte antigen-4; Il-12: interleukin 12; PD-L1: Programmed death-ligand 1; TGF-β: Transforming growth factor-β; DCS: Dendritic cells; FOXP3: Forkhead box protein P3; Treg: Regulatory T cells

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Cholesterol induces TGF-β expression. Zhou et al. [32••] found that the levels of plasma TGF-β1 and cholesterol were positively correlated. Feeding high cholesterol elevated glomerular TGF-β1 and fibronectin mRNA levels in a nephrosis rat model. Furthermore, Statins decreased TGF-β activity as well as expression of TGF-β targets ZYX and SERPINE1. These effects were observed in GBM and GBM-initiating cell (GIC) lines [33]. In UL stem cells, simvastatin has been observed to suppress the production of TGF-β1 (Fig. 2) [18•].

Programmed Cell Death 1 (PD-1)

Programmed cell death 1 (PD-1) and its ligand, PD-L1, deplete T cells and prevent the action of proinflammatory mediators. PD-L1 induces tumor growth, activated T-cell immune suppression [34, 35]. Cholesterol induces PD-L1 expression. Anti–p PD–1 immunotherapies may be more effective if cholesterol is reduced [36]. Simvastatin was found to inhibit PD-L1 expression promoting anti-tumor in colorectal cancers (CRCs) [37]. In melanoma and lung cancer cells, simvastatin, atorvastatin, lovastatin, and fluvastatin reduced PD-L1 expression [38••]. Cholesterol increased PD-1, decreased interferon-gamma, and granzyme B production, and increased apoptosis in T cells [39] (Fig. 2).

Interleukin 12 (IL-12)

IL-12, a heterodimeric cytokine consisting of p40 and p35 subunits, is mostly thought to be pro-inflammatory. It is produced by antigen-presenting cells (APCs) – including macrophages and dendritic cells (DCs) – and is essential for CD8+ T and NK cell recruitment and effector functions. Thus, IL-12 plays a significant role in promoting anti-tumor immune responses [40, 41]. Cholesterol was found to inhibit IL-12 expression. In a central nervous system (CNS) autoimmune illness model, atorvastatin decreased STAT4 phosphorylation and suppressed the release of IL-12 [42]. Coward et al. [43] found that the primary mechanisms by which statins induce a proinflammatory response in activated peripheral blood monocytes (PBMCs) are the activation of caspase-1 and IL-18 production in the monocytes, with IL-12 playing a secondary role (Fig. 2).

Forkhead Box Protein P3 (FOXP3)

Forkhead box protein P3 (FOXP3) is a transcription factor and member of the forkhead box (FOX) protein family. FOXP3 acted as a master regulator in the maturation and operation of the immunosuppressive regulatory T cells (Tregs) [44]. Tregs can suppress several immunostimulant cells as NK cells, macrophages, DCs, and B cells by generating immunosuppressive substances and thus encourage tumor growth [45,46,47]. A large number of tumor-infiltrating Tregs are associated with HCC [48]. Li et al. [49] found that in stage B HCC patients, Treg levels independently predicted a poor prognosis.

Cholesterol induces FOXP3 expression. High-cholesterol diets increase the CD4+ FOXP3+ Treg cell population in the liver [50]. Mailer et al. [51] found that a high-cholesterol diet increased FOXP3 expression. Wen et al. [52] noted that cholesterol replenishment could prevent nicotine-induced p-STAT5/ FOXP3 pathway suppression, and Treg frequency (Fig. 2).

Cytotoxic T-lymphocyte Associated Antigen 4 (CTLA-4)

As a T cell suppressor, cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) was the first molecule effectively targeted for immune checkpoint therapy [53,54,55]. Cholesterol induces CTLA-4 expression. By targeting Ras-activated mTOR signaling, atorvastatin has been shown to suppress the expression of CTLA-4, implying an indirect link between cholesterol and immune checkpoint expression [56]. According to Zeng et al. [57], the mevalonate pathway, which is responsible for cholesterol synthesis, is necessary to produce CTLA-4 (Fig. 2).

IL-23

Interleukin-23 is an immunosuppressive cytokine that is overexpressed in many human malignancies, consistent with its involvement in increasing tumor growth in mice [58]. Infiltration of the immunosuppressive M2 macrophages, neutrophils, TGF-β, IL-10, and VEGF into tumor tissues is promoted by IL-23. IL-23 also raises the expression of the endothelial and proliferative markers CD31 and Ki67 in malignancies. Furthermore, IL23 suppresses the immune system by lowering the invasion of CD4+ and CD8+ T lymphocytes into tumor tissues [59]. Mice lacking IL-23p19 were resistant to DMBA/TPA-induced skin papillomas [58]. Cholesterol induces IL-23 expression. In obese patients, adiposity is a potential biological source of IL-17 and IL-23, as well as a source of pro-inflammatory mediators and invading immune cells [60]. In the synovitis, acne, pustulosis, hyperostosis, and osteitis group, serum IL-23 was associated positively with total cholesterol and HDL cholesterol [61].

Ma et al. [62] found that one week after starting statin medication, there was a decrease in IL-23 levels in peripheral blood [62]. Furthermore, statin inhibits the phosphorylation of the transcription factors STAT3 and STAT1, which are implicated in the control of IL-6 and IL-23 [63] (Fig. 2).

All data mentioned in the immune mediators’ part and their correlations to cholesterol are summarized in Table 1.

Table 1 Effect of cholesterol on immune mediators
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Cholesterol and Immune Cells

Cholesterol is found to suppress the immune microenvironment by suppressing immunostimulant cells and stimulating immunosuppressive cells.

Neutrophils

Neutrophils comprise 50–70% of human leukocytes in the bloodstream. They serve as the host's first line of defense, protecting against pathogen attacks by phagocytosis, granule release, and cytokine synthesis. Tumor-associated neutrophils (TANs) that infiltrate the body exhibit either protumorigenic (N2) or anttumorigenic (N1) properties [65]. Xiong et al. [66] found that cholesterol metabolite causes an increase in neutrophil counts. Akinyemi et al. [67] and Grzywa et al. [68] found that arginase activity, one of the common inducers of N2 immunosuppressive neutrophils, was significantly increased in rats fed a high-cholesterol diet. Guasti et al. [69] demonstrated that patients on long-term statin therapy consistently showed a reduction in AT1-R expression in primed PMNs and a reversion of the pro-inflammatory oxidative functional response.

Cytotoxic CD8+ T Cells

Cytotoxic T lymphocytes (Cytotoxic CD8+ T Cells; CTLs) induce cancer cell death through temporary cell-cell interaction and paracrine distribution of cytotoxic effector chemicals [45, 54, 70]. Interleukin-2 and IFN-γ promote T cell priming, activation, and cytotoxicity, which culminates in anti-tumor action [71]. Upregulated T cell expression of PD-1, 2B4, TIM-3, and LAG-3 was positively and gradually linked with T cell exhaustion in tumor tissues enriched with cholesterol and cholesterol content in tumor-infiltrating CD8+ T cells [39]. Picarda et al. [72] found that T cell exhaustion was induced by tumor-derived or exogenous cholesterol via upregulating immunological checkpoints on CD8 T cells and inducing death (Fig. 2).

Dendritic Cells

Dendritic cells are APCs that can recognize diseases and alert the immune system, mainly T cells, of their presence. Major histocompatibility molecules (MHCs) are required for naive T cells to locate, identify, capture, and process pathogens in peripheral tissues before conveying antigenic peptides from pathogens to naive T cells in lymphoid organs. These methods result in a critical role for DCs in the development of antigen-specific immune responses [73]. Cholesterol shifts the equilibrium in DCs from the mature type (immunostimulant) to the tolerogenic (immunosuppressive) type (Fig. 3). Ramakrishnan et al. [74] found that cross-presentation was inhibited by oxidized lipids. DCs with a high lipid content could not efficiently excite allogeneic T lymphocytes or present tumor-associated antigens, and their ability to digest antigens was diminished [75]. Please see Table 2 for a summary of the data of Cholestertol’s correlation to immune cells.

Fig. 3
figure 3

Effect of cholesterol on tumor immune archetype. Cholesterol alters the tumor microenvironment by various mechanisms, including (1) Shifting T cell equilibrium away from the cytotoxic type and towards the Treg cell type; (2) Shifting DCs equilibrium away from the mature type and towards the tolerogenic cell type; (3) Shifting Neutrophils equilibrium away from the N1 type and towards N2 cell type. DCs: Dendritic cells; Tregs: regulatory T cells

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Table 2 Effect of cholesterol on immune cells
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Conclusion

Despite decades of considerable research on cancer immunophenotyping that has yielded intriguing results, there is still a need to investigate the pathobiological role of different metabolites in tumor immune microenvironment.

Several studies proved the carcinogenic role of cholesterol through its pro-proliferative, pro-oxidant, and anti-apoptotic properties. Cholesterol was found to suppress tumor immune fitness. Cholesterol decreases immunostimulant mediators like IL-12 while increasing immunosuppressive mediators, including TGF-β, FOXP3, IL-23, PD-L1, and CTLA-4. Furthermore, immune-stimulatory cells such as cytotoxic T cells, DCs, and neutrophils were blocked by cholesterol, whereas immunosuppressive cells like Treg cells were activated. Based on our review, we suggest more research be done on different types of cancer, studying the effect of cholesterol and cholesterol-lowering medication on cancer immune milieu.