Regulation of cholesterol homeostasis in health and diseases: from mechanisms to targeted therapeutics

Abstract

Disturbed cholesterol homeostasis plays critical roles in the development of multiple diseases, such as cardiovascular diseases (CVD), neurodegenerative diseases and cancers, particularly the CVD in which the accumulation of lipids (mainly the cholesteryl esters) within macrophage/foam cells underneath the endothelial layer drives the formation of atherosclerotic lesions eventually. More and more studies have shown that lowering cholesterol level, especially low-density lipoprotein cholesterol level, protects cardiovascular system and prevents cardiovascular events effectively. Maintaining cholesterol homeostasis is determined by cholesterol biosynthesis, uptake, efflux, transport, storage, utilization, and/or excretion. All the processes should be precisely controlled by the multiple regulatory pathways. Based on the regulation of cholesterol homeostasis, many interventions have been developed to lower cholesterol by inhibiting cholesterol biosynthesis and uptake or enhancing cholesterol utilization and excretion. Herein, we summarize the historical review and research events, the current understandings of the molecular pathways playing key roles in regulating cholesterol homeostasis, and the cholesterol-lowering interventions in clinics or in preclinical studies as well as new cholesterol-lowering targets and their clinical advances. More importantly, we review and discuss the benefits of those interventions for the treatment of multiple diseases including atherosclerotic cardiovascular diseases, obesity, diabetes, nonalcoholic fatty liver disease, cancer, neurodegenerative diseases, osteoporosis and virus infection.

Fig. 1

The pathway for cholesterol biosynthesis. In cholesterol biosynthesis, all the carbon atoms are derived from acetyl-CoA. The biosynthesis of cholesterol can be divided into four stages. (I) Synthesis of mevalonate (MVA). Two molecules of acetyl-CoA are reversely catalyzed by thiolase to form acetoacetyl-CoA. Acetoacetyl-CoA and acetyl-CoA are catalyzed to form 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) by HMG-CoA synthase (HMGCS). Finally, the HMG-CoA is catalyzed by HMG-CoA reductase (HMGCR) to convert to MVA, a step that requires two molecules of NADPH and H⁺ and determines the rate of cholesterol biosynthesis. (II) Production of isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP). MVA is sequentially phosphorylated twice by mevalonate kinase and phosphomevalonate kinase to produce 5-pyrophosphate mevalonate, which is further decarboxylated by 5-pyrophosphatemevalonate decarboxylase to produce isopentenyl pyrophosphate (IPP). IPP is converted to dimethylallyl pyrophosphate (DMAPP) catalyzed by isopentanoyl pyrophosphate isomerase, and DMAPP is used together with IPP as the starting materials for the third step of cholesterol synthesis. (III) Synthesis of squalene. IPP and DMAPP are condensed by farnesyl transferase to form geranyl pyrophosphate (GPP), followed by a second condensation reaction between GPP and IPP to form farnesyl pyrophosphate (FPP), and finally two molecules of FPP are condensed by squalene synthase to form squalene. (IV) Squalene cyclizes to form lanosterol and subsequently to synthesize cholesterol. Squalene forms a closed loop catalyzed by squalene monooxygenase and 2,3-oxidosqualene lanosterol cyclase to form lanosterol. Lanosterol is converted into cholesterol in more than twenty steps totally

Fig. 2

SREBP2 pathway in regulation of cholesterol biosynthesis. The process of cholesterol biosynthesis is strictly regulated by negative feedback, of which the sterol regulatory element binding protein (SREBP) pathway and the HMG-CoA reductase (HMGCR) degradation pathway are the two main mechanisms of negative feedback regulation. a SREBP2 forms a complex with SREBP cleavage activating protein (SCAP) at the ER. When sterol depletion occurs to cells, SCAP binds to COPII vesicles, allowing the SCAP-SREBP complex to translocate from the ER to the Golgi for cleavage. SREBP2 is sequentially cleaved by S1P and S2P in the Golgi, and the N-terminal of SREBP2 is subsequently transported to the nucleus, where the N-terminal of SREBP2 recognizes and binds to the SRE sequence on the target gene promoter to activate the target gene transcription. In addition, HMGCR is also prevented from binding to INSIGs and gp78 (ubiquitin ligase) during cholesterol depletion, thereby stabilizing HMGCR to activate cholesterol biosynthesis. b When the cell sterol is replete, it triggers the interaction of SCAP with INSIGs, resulting in blocking the binding of SCAP to COPII and keeping the SCAP-SREBP2 complex in the ER. At the same time, HMGCR also binds to INSIGs and gp78, which catalyzes the ubiquitination of HMGCR. The ubiquitinated HMGCR is eventually degraded in the proteasome via ER-related degradation (ERAD). Ub ubiquitin

Fig. 3

Regulation of cholesterol transport. Daily food and the hepatic endogenous synthesis are the two main sources of human cholesterol, of which dietary free cholesterol (FC) uptake is mediated by Niemann-Pick C1 Like 1 (NPC1L1) protein in enterocytes. The endocytosis of cholesterol by NPC1L1 responds to the change of cellular cholesterol concentration. FC taken up by NPC1L1 in enterocytes is esterified to cholesteryl ester (CE) by acyl-CoA:cholesterol acyltransferase 2 (ACAT2), which is loaded into ApoB-48 with triglycerides (TG) mediated by microsomal triglyceride transfer protein (MTP), to form chylomicron (CM). After TG in CM is hydrolyzed and utilized, most of the remaining cholesterol will be absorbed through low-density lipoprotein receptor (LDLR) in the liver. In contrast, some unesterified cholesterol is pumped back to the intestinal lumen by ATP-binding cassette (ABC) transport proteins G5 and G8 (ABCG5/ABCG8) or synthesized into pre-β-HDL by ABCA1 and released into circulation. Cholesterol synthesized endogenously in the liver is converted into VLDL with TG, ApoB-100, and most of VLDL is then converted into LDL, which is the main carrier for transporting endogenous cholesterol. LDL is taken up by scavenger receptors in macrophages, where expression of CD36, scavenger receptor A1 (SR-A1), and LDL receptor 1 (LOX1) is increased in atherosclerosis, further promoting cholesterol accumulation. LDL is endocytosed into macrophages and hydrolyzed by lipase (LAL) to produce FC. Excess FC is esterified by ACAT1 and stored as lipid droplets, and the excess accumulation of CE in macrophages can contribute to formation of foam cells. To mediate cholesterol efflux, macrophages hydrolyze CE into FC by the neutral cholesteryl ester hydrolase (NEH). Macrophage-mediated cholesterol efflux includes simple diffusion, SR-BI-facilitated diffusion, and ABCA1/ABCG1-mediated efflux. Among them, simple diffusion dominates cholesterol efflux in normal macrophages, regulated by cholesterol concentrations. In cholesterol overloaded macrophages, ABCA1 and ABCG1 are critical for cholesterol efflux. ABCA1 is able to bind to ApoA-I to mediate the production of pre-β-HDL, lecithin cholesterol acyltransferase (LCAT) further matures pre-β-HDL particles into HDL3, while ABCG1 and SR-BI mediate cholesterol flow directly to HDL3. HDL3 is further esterified by LCAT to produce HDL2, in which CE is eventually taken up by SR-BI in the liver and converted to FC. In addition, CE in HDL2 particles can be exchanged by cholesteryl ester transfer protein (CETP) to LDL particles, which are subsequently taken up by LDLR. Excess cholesterol in the liver is excreted into the bile mediated by ABCG5/ABCG8 and eventually enters the intestinal lumen for excretion in feces. Some other cholesterol in the blood can be excreted directly into the intestinal lumen via transintestinal cholesterol excretion (TICE) pathway in enterocytes

Fig. 4

Inhibition of atherosclerosis by cholesterol-lowering interventions. Bempedoic acid and statins reduce acetyl-CoA and HMG-CoA production by inhibiting ACLY and HMGCR, respectively, thereby lowering cholesterol synthesis. Ezetimibe inhibits intestinal uptake of cholesterol by inhibiting NPC1L1. PCSK9 inhibitors reduce LDLR degradation by inhibiting PCSK9 expression/function. Bile acid sequestrants bind to BA in the small intestine, thus preventing BA from being reabsorbed into the liver. Lomitapide reduces the assembly of ApoB-containing lipoproteins in intestine and liver. Evinacumab restores LPL activity by inhibiting ANGPTL3. Fibrates reduce TG levels. All of the above interventions can reduce plasm LDL-C levels, which is the base for the development of atherosclerosis. The arterial wall consists of three layers: adventitia, media, and intima. The outermost layer, adventitia, is mainly composed of connective tissues. The middle layer, media, consists of smooth muscle cells. The innermost layer, intima, is bounded by endothelial cells (ECs) on the inner side of the lumen and internal elastic membrane on the outer side. Atherosclerotic plaques form in the intima. In the early stage of atherosclerosis, LDL particles enter the intima through EC layer and undergo oxidation and other modifications to form oxLDL, which makes it pro-inflammatory and immunogenic. ECs secrete adhesion molecules and chemokines after activation, and monocytes circulating in the blood bind to adhesion molecules and enter the intima under the promotion of chemokines. After entering the intima, the infiltrated monocytes then differentiate into macrophages and express scavenger receptors to bind and internalize oxLDL to form foam cells. A subset of smooth muscle cells from the media can also differentiate into a macrophage-like phenotype, which in turn phagocytoses oxLDL to form foam cells. As the lesion progresses, dead foam cells and SMCs aggregate with free lipoprotein and cholesterol crystals in the intima to form a necrotic core. SMCs migrate to endothelium and forms fibrous cap during the evolution of atherosclerotic plaque. As cholesterol crystals grow, they eventually penetrate the intima, causing plaque instability and further rupture of the plaques. Acetyl CoA acetyl coenzyme A, ACLY ATP citrate lyase, ANGPTL3 angiopoietin-like protein 3, BA bile acid, CE cholesteryl ester, CM chylomicron, EC endothelial cell, FA fatty acid, FC free cholesterol, HMGCR 3-hydroxy-3-methylglutaryl coenzyme A reductase, HMG-CoA 3-hydroxy-3-methylglutaryl coenzyme A, LDL low-density lipoprotein, LDLR LDL receptor, LPL lipoprotein lipase, MTP microsomal triglyceride transfer protein, NPC1L1 Niemann-Pick C1 like 1, oxLDL oxidatively modified low-density lipoprotein, PCSK9 proprotein convertase subtilisin/kexin type 9, SMC smooth muscle cell, TG triglyceride, VLDL very low-density lipoprotein