Saturated fatty acids promote cholesterol biosynthesis: Effects and mechanisms

Saturated fatty acids promote cholesterol biosynthesis: Effects and mechanisms

Obesity Medicine 18 (2020) 100201 Contents lists available at ScienceDirect Obesity Medicine journal homepage: www.elsevier.com/locate/obmed Review...

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Obesity Medicine 18 (2020) 100201

Contents lists available at ScienceDirect

Obesity Medicine journal homepage: www.elsevier.com/locate/obmed

Review

Saturated fatty acids promote cholesterol biosynthesis: Effects and mechanisms

T

Yunjie Gua, Jun Yinb,c,∗ a

School of Clinical Medicine, Bengbu Medical College, Bengbu, Anhui Province, 233000, China Department of Endocrinology and Metabolism, Shanghai Jiao Tong University Affiliated Sixth Peopleʼs Hospital, Shanghai Clinical Center for Metabolic Diseases, Shanghai Key Laboratory of Diabetes Mellitus, Shanghai Diabetes Institute, Shanghai, China c Department of Endocrinology and Metabolism, Shanghai Eighth People's Hospital, Shanghai, China b

ARTICLE INFO

ABSTRACT

Keywords: SFAs Cholesterol biosynthesis SREBP2 HMGCR SM LDLR

It's considered that the amount of dietary saturated fatty acids (SFAs) intake is positively correlated with plasma cholesterol concentration. In mammals, de novo biosynthesis is the major resource of cholesterol, which could be promoted by SFAs. In this article, we discuss the probable mechanisms by which SFAs promote cholesterol synthesis. In human cells, the activation of sterol regulatory element-binding protein 2 pathway and degradation of 3-hydroxy-3-methyl-glutaryl coenzyme A reductase elaborately regulate the synthesis by sensing cholesterol fluctuations in cells. SFAs lower intracellular cholesterol concentration via suppressing low density lipoprotein (LDL) endocytosis mediated by LDL receptor and retarding cholesterol transport from plasma membrane to endoplasmic reticulum.

Cholesterol is a crucial component of eukaryotic membranes and a precursor molecule involved in various metabolism activities. Previous studies have suggested that disrupted cholesterol metabolism has a great effect on human health, and it causes manifold diseases, from acquired illnesses to congenital disorders. The popular opinion that the deposition of low density lipoprotein (LDL) cholesterol in the endothelium is a critical culprit of atherosclerosis has been supported by various studies. And numerous researches have suggested that SFAs have the considerable potential to increase plasma total cholesterol in human. American Heart Association has proposed that one remedy to reducing the incidence of cardiovascular disease (CVD) is to replace saturated fatty acids (SFAs) with unsaturated fatty acids. However, not all the SFAs have cholesterol-raising property. Those SFAs with chain of 4–10 carbon do little to affect the cholesterol concentration. In contrast, long-chain SFAs such as palmitic acid (C16:0), myristic acid (C14:0) and lauric acid (C12:0) may elevate plasma cholesterol. How do SFAs modulate plasma cholesterol concentration? On the one hand, they may prevent cholesterol being cleared from blood. On the other hand, they may increase the resources of circulating cholesterol. De novo biosynthesis is the major resource of cholesterol compared with uptake from exogenous lipids in the intestine. In this review, we mainly discuss the probable mechanisms by which long chain SFAs stimulate the biosynthetic pathway to raise plasma cholesterol. Cells

have two elaborate feedback systems to regulate cholesterol synthesis: activation of sterol regulatory element-binding protein 2 (SREBP2) pathway and degradation of 3-hydroxy-3-methyl-glutaryl coenzyme A reductase (HMGCR), both of them are sensitive to the changes of cholesterol in cells. SFAs reduce intracellular cholesterol, as a result, it gives a cholesterol depletion signal to cells and thus triggers cholesterol biosynthetic pathway. 1. Intracellular cholesterol reduction Cholesterol synthesis is a complex and expensively energy-consuming process which is rigorously regulated. The intracellular cholesterol content is the main factor to modulate this process. SFAs reduce intracellular cholesterol by two ways: suppressing endocytosis mediated by LDL receptor (LDLR) and blocking cholesterol transport from plasma membrane (PM) to endoplasmic reticulum (ER). This decline of cholesterol caused by SFAs will be sensed by certain domains in cells and promote cholesterol production. 1.1. LDLR endocytosis pathway LDLR, a cell surface glycoprotein, uptakes LDL in blood via its extracellular ligand binding domain. After capturing LDL, its surface

∗ Corresponding author. Department of Endocrinology and Metabolism, Shanghai Jiao Tong University Affiliated Sixth Peopleʼs Hospital, 600 Yishan Road, Shanghai, 200233, China. E-mail address: [email protected] (J. Yin).

https://doi.org/10.1016/j.obmed.2020.100201 Received 25 January 2020; Received in revised form 20 February 2020; Accepted 21 February 2020 2451-8476/ © 2020 Published by Elsevier Ltd.

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membrane pouches inward and incorporates LDL into the clathrincoated vesicles which are subsequently dissociated from the membrane and travel to lysosomes (Goldstein and Brown, 2009). After vesicles have entered the lysosomal system, exogenous cholesteryl esters carried by LDL are hydrolysed and cholesterol is exported from lysosomes by interaction between two couples of proteins, LAMP1, LAMP2 and NPC1, NPC2 (Li et al., 2016; Luo et al., 2017). The LDL-derived cholesterol travels first to PM to interact with adjacent lipids to form nanoscale micro-domains and then transports from PM to ER to regulate cholesterol biosynthesis (Infante and Radhakrishnan, 2017). Previous studies have indicated that dietary SFAs are able to regulate LDLR activity. Fox et al. demonstrated that SFAs retarded LDLR expression by reducing hepatic LDLR mRNA in baboons (Fox et al., 1987). By providing healthy subjects with three kinds of diets including different SFAs contents: Average American Diet with 34% fat, 15% SFAs; Step-One Diet with 29% fat, 9% SFAs; and Low SAT Diet with 25% fat, 6% SFAs, Mustard et al. found the number of LDLR was in inverse proportion to the content of SFAs in diets (Mustard, 1997). Reduction of LDLR is partially responsible for the increase of plasma cholesterol concentration as well as the subsequent decline of intracellular cholesterol since the LDLR-mediated endocytosis is suppressed.

Its tripartite structure consists of an NH2-terminal domain, a middle hydrophobic region, and a COOH-terminal domain which interacts with SREBP cleavage activating protein (SCAP) (Brown, 1997; Horton, 1999). SCAP is sensitive to ER cholesterol fluctuations and regulates the transport of SCAP-SREBP2 complex from ER to Golgi (Gong et al., 2016). The specific process of SREBP2-mediated cholesterol synthesis works as follows. SCAP sterol-sensing domain (SSD) confers SCAP sensitivity to cholesterol levels in ER membrane. When ER cholesterol concentration is lower than threshold point, the SCAP-SREBP2 complex responds by binding to COPII vesicles and translocating from ER to the Golgi apparatus where SREBP2 is hydrolysed (Luo et al., 2019). The SREBP2 is cleaved for sequential two times in Golgi and a NH2-terminal fragment is formed, namely nuclear SREBP2 (nSREBP2) which enters the nucleus and binds to the sterol regulatory element (SRE) sequence in the promoters of the genes for cholesterol synthesis (Brown et al., 2018). ER tightly regulates cholesterol biosynthesis through precise feedback systems. There are two cholesterol-mediated regulators of SREBP2 pathway: insulin-induced gene protein (Insig) and progestin and adipoQ receptor 3 (PAQR3). Insig serves as anchor protein to retain SCAP-SREBP2 complex in ER, which terminates biosynthetic pathway of cholesterol, while PAQR3 helps the complex anchor at Golgi, which has the opposite effect of Insig. Take Chinese hamster ovary cells, once ER cholesterol exceeds 5% of total ER lipids, SSD receives the information and prompts SCAP-SREBP2 complex to bind to Insig. Accordingly, the ER-to-Golgi transport via COPII proteins is blocked (Radhakrishnan et al., 2008). Under cholesterol depletion condition, Insig bound to SCAP will be degraded through the ubiquitin-proteasome pathway and PAQR3 will interact with SCAP and SREBP2 to promote the formation of SCAP-SREBP2 complex and potentiate the SREBP2 processing (Gong et al., 2006; Xu et al., 2015) (Fig. 1).

1.2. Cholesterol transport from PM to ER Membranes adopt distinct phases: liquid disordered (Ld) phase induced by low lipid saturation, liquid ordered (Lo) phase promoted by cholesterol, and solid ordered (So) phase favored by high lipid saturation. SFAs have an important effect on the compositions and phases of the membranes. For example, palmitic acid, a raw material for sphingomyelin (SM), promotes synthesis and accumulation of saturated lipids in ER, which potentiates the formation of solid-like domains (Shen et al., 2017). Cholesterol is known to have the ability to eliminate So phase of the membrane and spontaneously transform it into Lo one, as once exported from lysosomes cholesterol preferentially inserted into phospholipid with long chain SFAs, like SM. Although in human cells, PM contains ~80% of total cellular cholesterol and ER contains only ~1%, the sterol sensors involved regulating cholesterol metabolism are located in the ER membrane (Lange and Steck, 2016). How do these sensors detect cholesterol needs from PM? Cholesterol interacts with the lipids in membranes to meet PM needs first, and the remnants can be sensed after translocating from PM to ER and then regulate cholesterol metabolism. The amount of transferred cholesterol has an inverse relationship with the level of lipids saturation in PM. That means saturated lipids in PM can regulate cholesterol synthesis, which is in line with the evidence that increasing membrane saturation induced by the loss of lysophosphatidylcholine acyltransferase 3 (Lpcat3), a phospholipid-remodeling enzyme, enhances cholesterol biosynthesis (Wang et al., 2018). Recently, a study gains a further insight of the mechanism by which SM sequestrates and prevents cholesterol to transport to ER. Cholesterol tends to form a stable complex with SM, whose constitution is relatively fixed. In the complex, the radio of SM and cholesterol is about 1:1.5, which is the same with that of SM and SM-sequestered cholesterol (Endapally et al., 2019). It means that once combined with SM, this part of cholesterol fails to travel to ER and cannot be sensed, which gives a cholesteroldeficiency signal to cells.

3. Degradation of HMGCR The 3-hydroxy-3-methyl-glutaryl coenzyme A reductase (HMGCR), an ER-localized glycoprotein, is a rate-limiting enzyme for cholesterol synthesis. At transcriptional level, the HMGCR gene is regulated by SREBP2 activation as one of the target genes of the pathway. In addition, ER cholesterol has a direct effect on the degradation of HMGCR, which is also a critical feedback regulatory mechanism sustaining intracellular cholesterol homeostasis. HMGCR consists of two domains: the NH2-terminal domain containing eight membrane spanning regions and anchoring the protein to ER membrane; the COOH-terminal domain projecting into the cytosol and exerting catalytic activity. The sterol-sensing domain of HMGCR shares resemblance with SCAP, which is also composed of transmembrane domains 2–6, and it enables HMGCR to respond to the changes of ER cholesterol level. Depletion of cholesterol stabilizes HMGCR (Sever et al., 2003). As cholesterol accumulates, sterol intermediates such as lanosterol and oxysterols from biosynthetic pathway of cholesterol stimulate Insig associating with E3 ubiquitin ligases designated gp78, TRC8 and RNF145 and triggering HMGCR degradation (Jo et al., 2011; Jiang et al., 2018). 4. Conclusion Dietary lipids have long been studied as significant factors associated with several diseases. SFAs have a recognized relationship with plasma cholesterol. In this review, we discuss the probable mechanisms by which SFAs stimulate cholesterol biosynthesis, a major resource of circulating cholesterol. Activation of SREBP2 pathway and degradation of HMGCR are two critical cholesterol-mediated feedback regulatory systems for cholesterol biosynthesis. SFAs suppress LDLR activity and block cholesterol PM-to-ER transport, disrupting the balance between intracellular and extracellular cholesterol. In consequence, the depletion of cholesterol in cells triggers SREBP2 pathway for cholesterol synthesis and prevents HMGCR degradation, both of which have a

2. SREBP2 cholesterol biosynthetic pathway Sterol regulatory element-binding proteins (SREBPs), a family of transcriptional regulators, which play a critical role in lipids metabolism have three isoforms in human, designated SREBP1a, SREBP1c, and SREBP2 respectively. SREBP2 acts as an ER-anchored precursor and predominantly modulates cholesterol synthesis by directly regulating expression of multiple genes involved in biosynthesis (Horton, 2002). 2

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Fig. 1. Regulation of SREBP2 pathway. Low density lipoprotein (LDL) is captured by LDL receptor (LDLR) from blood and incorporated into the clathrin-coated vesicles. Lysosome hydrolyzes the vesicles and exports cholesterol carried by LDL. The cholesterol exported from lysosome preferentially travels to plasma membrane (PM) and combines with sphingomyelin (SM), and the rest of them transports to endoplasmic reticulum (ER). Sterol regulatory element-binding protein cleavage activating protein (SCAP) can sense the fluctuation of ER cholesterol. Under the cholesterol repletion condition, SCAP interacts with insulin-induced gene protein (Insig), which blocks SCAP-SREBP2 complex transport to Golgi, otherwise the complex will bind to progestin and adipoQ receptor 3 (PAQR3) at the Golgi. The fragment cleaved from the complex at the Golgi, namely nuclear SREBP2 (nSREBP2) enters nucleus and regulates cholesterol synthesis.

concerted effect on cholesterol synthesis. Through SREBP2-mediated biosynthetic pathway, Insig and PAQR3 are two regulators modulating the synthetic process following the changes of cholesterol in cells. Cholesterol of different states and sources plays dissimilar roles in human. Figuring out how SFAs raise plasma cholesterol will help us have a further knowledge of the relationship between diseases and dietary lipids and provide guidance for diets. However, direct evidences to confirm the mechanisms above are still lack. Up to now, few clinical trials on human have been conducted in the field. Consequently, it is unclear if these mechanisms work in the same way as in the animals or in in vitro experiments.

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Funding information National Natural Science Foundation of China (81670790). Declaration of competing interest The authors declare no conflict of interest. References Brown, M.S., 1997. The SREBP pathway: regulation of cholesterol metabolism by proteolysis of a membrane-bound transcription factor. Cell 89 (3), 331–340. Brown, M.S., Radhakrishnan, A., Goldstein, J.L., 2018. Retrospective on cholesterol homeostasis: the central role of Scap. Annu. Rev. Biochem. 87, 783–807. Endapally, S., Frias, D., Grzemska, M., et al., 2019. Molecular discrimination between two conformations of sphingomyelin in plasma membranes. Cell 176 (5), 1040–1053.

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