Research paperObesity-induced overexpression of miRNA-24 regulates cholesterol uptake and lipid metabolism by targeting SR-B1
Introduction
Scavenger receptor class B type 1 (SR-B1) is a well-characterized, high-density lipoprotein (HDL) receptor which has been shown to mediate the selective transfer of cholesteryl ester, a process known as the selective HDL-cholesteryl ester uptake (Swarnakar et al., 1999; Shen et al., 2014; Shen et al., 2018). In addition to HDL, SR-B1 could bind and regulate cellular metabolisms of LDL, modified LDL, VLDL and vitamins (Swarnakar et al., 1999; Shen et al., 2018). Based on its function, SR-B1 is implicated in atherosclerosis, lipid mechanism, glucose intolerance and steroidogenesis which involves the catalysis of cholesterol into different steroid hormones through diverse steroidogenic steps (Kozarsky et al., 2000; Azhar and Reaven, 2002; El Bouhassani et al., 2011; Karavia et al., 2015). SR-BI has also been shown to be involved in many other cellular processes including apoptosis, immune responses, hepatitis C virus (HCV) and dengue virus entry, malaria parasite infection (Shen et al., 2014; Pal et al., 2016). Current studies shown that SR-B1 also played an important role in the recognition, binding, and uptake of endogenous and exogenous ligands, inducing an anti-inflammatory or pro-inflammatory response (Vasquez et al., 2017). Additionally, SR-B1 is reported to be involved in tumor aggressiveness and prognosis in breast cancer (Gutierrez-Pajares et al., 2016).
SR-B1 is primarily expressed in the liver and steroidogenic cells of the adrenal and gonads with functional contributions to cholesterol trafficking and lipid mechanism. In steroidogenic cells, specific trophic hormones, such as luteinizing hormone (LH/hCG), follicle-stimulating hormone (FSH), PMSG and adrenocorticotropic hormone (ACTH), significantly up-regulated SR-B1 mRNA and protein expression levels (Azhar and Reaven, 2002). Our previous study showed that hormone/cAMP down-regulated SR-B1 gene promoter DNA methylation and stimulated SR-B1 expression (Hu et al., 2016). On the other hand, several diabetic subjects can regulate hepatic SR-BI expression (Wang et al., 2013; Shen et al., 2014). At the cellular and molecular levels, several kinds of transcriptional, posttranscriptional or posttranslational regulation mechanisms regulate the expression of SR-B1 (Azhar and Reaven, 2002; Hu et al., 2016). PDZK1 has been reported to interact with the C terminal end of SR-B1, and subject to the posttranscriptional regulation of hepatic SR-B1 stability and function (Ikemoto et al., 2000; Shen et al., 2016). Two other NHERF family members, NHERF1 and NHERF2, that negatively regulate the expression and function of SR-B1 partially via increasing SR-B1 ubiquitination and proteasome degradation was evidenced by our previous studies (Hu et al., 2013a; Lu et al., 2017). We also found that SIK1 functions as a positive regulator of HDL-CE transport by stimulating the phosphorylation of SR-B1 (Hu et al., 2015).
miRNA is 20–22 nt small RNA which targets 3’ UTR of target mRNA and results in the degradation of mRNA or inhibits its translation (Ha and Kim, 2014). miRNA has been reported to be involved in different biological process, such as biological development, differentiation, carcinogenesis, immune response, lipogenesis, steroidogenesis and other important cellular and metabolic processes (Krol et al., 2010; Hu et al., 2013b; Ha and Kim, 2014; Hu et al., 2017). Several kinds of miRNAs including miR-125a, -185, -455, -96 and -223, have been shown to repress the SR-B1 expression by directly binding to the 3’ UTR of SR-B1 mRNA (Hu et al., 2012; Wang et al., 2013). In our former study, we detected the miRNA expression profile related to steroidogenesis and found that miR-24 was little upregulated after ACTH treatment (Hu et al., 2013b). Using in silico analysis and functional studies, we demonstrated that miR-24 repressed SR-B1 expression by directly binding to specific sites in the 3′UTR of SR-BI mRNA. Overexpression of miR-24 actively regulated HDL uptake, triglycerides (TG) synthesis and SR-B1-supported steroid hormone synthesis, respectively. Furthermore, our results indicate that miR-24 affects the expression of a number of genes involved in cholesterol and fatty acid catabolism and synthesis in liver cells.
Section snippets
Animals and feeding
All animal experiments were performed according to the procedures approved by the Laboratory Animal Care Committee at Nanjing Normal University. Male C57/BL6 mice and OB/OB mice was purchased from the Model Animal Research Center of Nanjing University (Nanjing, Jiangsu, China). Ten C57/BL6 mice were separated randomly into two groups and fed with either normal laboratory diet or high fat diet (D12492; Research Diets, New Brunswick, NJ) for 12 weeks and body weights were monitored every week.
Cell culture, treatment and transfection
Regulation of miR-24 expression
SR-B1 is primary expressed in hepatocyte and steroidogenic cells (Shen et al., 2018). Dietary fatty acids regulated the expression of SR-B1 in liver cells, while hormonal treatment stimulated SR-B1 expression in steroidogenic cells (Azhar and Reaven, 2002; Hu et al., 2012; Shen et al., 2014; Hu et al., 2016). Thus, we detected the miR-24 expression profile in two typical steroidogenic cells MLTC and Y1 cells, as well as in HepG2 and mice liver with different treatments. We found that palmitic
Discussion
Cholesterol is an important constituent of mammalian cell membranes and plays a key role in membrane trafficking and transmembrane signaling processes. As the precursor of steroid hormones and bile acids, cholesterol is also critical for several important biological functions, while disorders of in vivo cholesterol homeostasis may lead to diseases such as atherosclerosis (van der Wulp et al., 2013). Through interacting and uptake of HDL-CE into cells, SR-B1 delivers cholesterol to the liver
Conflict of interest
The authors declare that they have no conflict of interest.
Acknowledgment
This work was supported by National Natural Science Foundation of China (31400659), Priority Academic Program Development of Jiangsu Higher Education Institutions. The authors thank Dr. Wen-Jun Shen at Stanford University Medicine School for discussion with this work.
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These authors contribute equally to this work.