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Hypomorphic ASGR1 Modulates Lipid Homeostasis via SREBPs Network

ABSTRACT: A population genetic study identified that the asialoglycoprotein receptor 1 (ASGR1) mutation carriers had significantly lower non-HDL-c levels and reduced risks of cardiovascular diseases. However, the mechanism behind this phenomenon remained unclear. Here, we established Asgr1 knockout mice that represented a plasma lipid profile with significantly lower non-HDL-c and triglyceride caused by decreased secretion and increased uptake of VLDL/LDL. These two phenotypes were linked with the decreased expression of microsomal triglyceride transfer protein (MTTP) and proprotein convertase subtilisin/kexin type 9 (PCSK9), two key target genes of the sterol regulatory element-binding proteins (SREBPs). Furthermore, there were less nSREBPs on account of more SREBPs being trapped in endoplasmic reticulum, which was caused by an increased expression of insulin-induced gene 1 (INSIG1), an anchor of SREBPs. Logically, two rescue experiments were conducted in ASGR1 deficient condition. Both INSIG1 knockdown and ASGR1 rescue independently reversed the ASGR1-mutated phenotypes, restoring the SREBP signaling manifested by improved APOB secretion and reduced LDL uptake. Our observation demonstrated a novel axis of ASGR1-INSIG1-SREBPs regulating lipid hemostasis, and hypomorphic manipulation of ASGR1 leads to significant reduction of lipid content via less nSREBPs. It provides multiple potential targets, including ASGR, for lipid-lowering drug development.

ORGANISM(S): Mus musculus

PROVIDER: GSE178370 | GEO | 2021/12/31

REPOSITORIES: GEO

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Project description:The sterol regulatory element binding proteins (SREBPs) are transcription factors that govern cholesterol and fatty acid metabolism. Owing to their central role in controlling hepatic lipid and lipoprotein metabolism their activity is tightly coordinated, and accordingly dysregulation of the SREBP pathway is associated with development of dyslipidemia and non-alcoholic fatty liver disease. Using a suite of genome-wide genetic screens we have recently identified SPRING (C12ORF49) as a novel post-transcriptional regulator of SREBP activation in vitro. Our previous work demonstrated that constitutive ablation of Spring in mice is embryonically lethal. Here we show that inducible global deletion of Spring is also untolerated, and therefore to interrogate the physiological role of SPRING in controlling hepatic lipid metabolism we developed liver-specific Spring knockout mice (LKO). Liver transcriptomics and proteomics analysis revealed severely attenuated SREBP signaling in livers and in hepatocytes of LKO mice, which was associated with marked effects on both plasma and hepatic lipid levels. In plasma, total cholesterol levels were dramatically reduced in both male and female LKO mice, apparent in both the LDL and HDL fractions, while triglyceride levels remained largely unaffected. In liver, loss of Spring diminished cholesterol and triglyceride biosynthesis resulting in decreased hepatic cholesterol and triglyceride content. This coincided with reduced secretion of VLDL into the circulation. Consistent with diminished hepatic de novo lipogenesis, LKO mice were protected from developing hepatosteatosis when challenged with a fructose-rich diet. Supporting the significance of our findings in mice, we identified common and rare SPRING genetic variants that are strongly associated with circulating HDL-c and ApoA1 levels in humans. Collectively, our study positions SPRING as a core component of hepatic SREBP signaling, and consequently of systemic lipid metabolism in mice and humans.

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Hypomorphic ASGR1 Modulates Lipid Homeostasis via SREBPs Network

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