Alteration of cholesterol distribution at the plasma

1 IntroductionAs a ubiquitous sterol found in vertebrate organisms, cholesterol (chol) exerts pleiotropic biological actions in cell physiology. From maintaining the structural integrity and regulating the biophysical properties of the plasma membrane (PM) to serving as a precursor for steroid hormones, vitamin D and oxysterols, chol is involved at many subcellular levels. Therefore, its homeostasis must be tightly regulated as any disbalance could lead to cancer development. The metabolism of chol, trafficking and its related intracellular functions have been the subject of many investigation over the years (Ikonen 2008; Afonso et al., 2018; Luo et al., 2020). However, despite several years of research, whether this lipid plays a role in oncogenesis is still an open question. This could result from differential sometimes contradictory association between chol levels and different types of cancers ((Asano et al., 2008; Llaverias et al., 2011; Pelton et al., 2012; Murai 2015; Heir et al., 2016; Radisauskas et al., 2016); reviewed in (Vona et al., 2021)). In addition, although chol-enriched domains are nowadays proposed to contribute to cancer cell proliferation, survival, death and invasion with important implications in tumor progression, how those domains are modified in malignant cells remains poorly understood. Another concern is whether and how the PM transversal distribution of chol is also impaired in malignant cells and whether it is coupled to chol-enriched domain alteration. These are key questions that need to be answered before we move forward implementing a chol-enriched domain-mediated approach. In the present review we discuss the current knowledge on the homeostasis, roles and membrane distribution of chol in non-tumorigenic cells. We then integrate documented alterations of chol distribution in domains at the surface of cancer cells and the mechanisms behind their contribution in cancer processes. We finally provide an overview on the potential strategies developed to target chol-enriched domains in cancer therapy.2 Physiological cholesterol homeostasis, roles and membrane distributionIn this section we summarize the key findings regarding chol homeostasis (Section 2.1), physiological roles (Section 2.2) and membrane distribution (Section 2.3) in non-tumorigenic cells.2.1 Cholesterol homeostasisIn normal cells, the metabolism of chol is tightly regulated and crucial for cellular integrity and biological functions. Any dysregulation in one or many stages of the chol homeostasis (import, synthesis, export and esterification) has been associated with pathological conditions such as cardiovascular disease, atherosclerosis and cancer. Nucleated cells use their endoplasmic reticulum (ER) chol levels as sensors to control the intracellular chol homeostasis. Simply, decreased ER chol levels activate sterol regulatory element binding proteins (SREBPs) that increase the transcription of genes involved in chol synthesis and import into cells. Conversely, increased intracellular chol levels activate another nuclear receptor system, the liver X receptors (LXRs) which facilitate chol export (Tontonoz and Mangelsdorf 2003; Goldstein et al., 2006; Ikonen 2008; Goedeke and Fernandez-Hernando 2012; Luo et al., 2020). This section will give a simplified overview of the complex protein network that regulates these different stages of chol homeostasis (Figure 1A).FIGURE 1. Physiological homeostasis, roles and cellular/membrane distribution of cholesterol. (A) Stages of chol homeostasis involving de novo synthesis, import, export, esterification and storage. (B,C) Pleiotropic actions of chol. (D) Differential levels of heterogeneity of membrane chol distribution in cells. PM, plasma membrane. See the Section 2 of the text for further details.2.1.1 De novo synthesisIn chol-poor conditions, nucleated cells activate the synthesis of new chol through the mevalonate pathway in the ER. In brief, two molecules of acetyl-coenzyme A (CoA) condense to form acetoacetyl-CoA which is added to a third acetyl-CoA molecule to produce one molecule of 3-hydroxy-3-methylglutaryl CoA (HMG-CoA) upon HMG-CoA synthase catalysis. Next, HMG-CoA is reduced to mevalonate by the integral ER membrane and rate-limiting HMG-CoA reductase (HMGCR). Then follows a succession of nearly 30 enzymatic reaction steps that convert mevalonate to squalene and afterwards to lanosterol and to chol (Ikonen 2008; Cerqueira et al., 2016; Shi et al., 2022). In the ER, chol can further be fatty acylated to form cholesteryl esters (CEs) or oxidized to form oxysterols. Additionally, chol can also be oxidized to bile acids and steroid hormones in hepatocytes and steroidogenic cells, respectively (Ikonen 2008). The chol biosynthetic pathway is tightly regulated by three key players, i.e. SREBP2, which regulates the transcription of genes encoding cholesterologenic enzymes, and HMGCR and squalene monooxygenase, two rate-limiting enzymes of the biosynthetic pathway [reviewed in (Brown and Goldstein 1997; Burg and Espenshade 2011; Chua et al., 2020; Luo et al., 2020)]. 2.1.2 ImportBesides the de novo synthesis, cells can acquire chol from the extracellular milieu through a receptor-mediated endocytic mechanism. Chol-carrying low-density lipoprotein (LDL) particles bind to LDL receptors (LDL-R) associated with clathrin-coated pits at the PM and are then delivered into early sorting endosomes. The LDL-R is recycled back to the cell surface while the chol-LDL complex is transported through compartments of the endocytic pathway where the low pH environment triggers hydrolysis of cholesteryl esters to provide free chol for cellular needs (Jeon and Blacklow 2005; Ikonen 2008; Goldstein and Brown 2009). The subsequent increase in intracellular chol generates a feedback regulation to stabilize the cell chol content (Goldstein and Brown 2009). Indeed, LDL-derived chol acts at different levels including suppression of the LDL-R gene transcription (Brown and Goldstein 1999), suppression of the HMGCR activity either by suppressing its gene transcription (Brown and Goldstein 1999) or accelerating the enzyme degradation (Gil et al., 1985) and activation of the chol-esterifying enzyme, acyl CoA: chol acyltransferase (ACAT) to store chol as CE-enriched droplets in the cytoplasm.2.1.3 ExportExcess chol is exported to the blood via ATP-binding cassette (ABC) subfamily A member 1 (ABCA1) or ABC subfamily G member 1 (ABCG1) to lipid-poor apolipoprotein A-I (ApoA-I), generating high-density lipoproteins (HDLs) (Gelissen et al., 2006; Rosenson et al., 2012; Daniil et al., 2013; Phillips 2014). Chol can also be exported to the intestinal lumen and bile ducts via ABCG5 and ABCG8 heterodimer (Luo et al., 2020). Nuclear receptor system LXRs are important regulators of chol export. In fact, high intracellular level of oxysterols activates the nuclear receptors of oxysterols LXRs which upregulates the transcription of ABCA1, allowing chol export (Ouvrier et al., 2009; Kuzu et al., 2016; Vona et al., 2021). On the other hand, expression of ABCA1 is downregulated by miR-33, which is co-transcribed with SREBP mRNAs during chol biosynthesis (Rayner et al., 2010).2.1.4 EsterificationA buffering mechanism takes places in normal cells to prevent free chol accumulation. Excess unesterified chol in the ER is transformed by integral membrane proteins ACAT into less toxic CEs to be stored in cytoplasmic lipid droplets (Chang et al., 2009; Luo et al., 2020). These lipid droplets are used for the production of plasma lipoproteins such as HDLs. The latter are then delivered from peripheral tissues, either to the liver and intestine for recycling or elimination, or to steroidogenic organs for steroid hormones production (Vona et al., 2021). Transcription of the two ACAT isozymes, ACAT1 and ACAT2, is not regulated by SREBPs or LXRs binding on their promoters but instead depends on various factors such as interferon-γ, all-trans-retinoic acid, synthetic glucocorticoid dexamethasone and tumor necrosis factor for ACAT1, as well as hepatocyte nuclear factors (HNF) 1α and 4α and homeobox protein CDX2 for ACAT2 (Chang et al., 2009; Luo et al., 2020).2.2 Cholesterol physiological rolesChol is involved in a large variety of
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