Insig-Mediated Regulation of Hepatic Lipid Synthesis
Cholesterol synthesis in mammals is tightly regulated by end-product feedback inhibition. 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR) catalyzes a rate determining reaction that is highly regulated by transcriptional and post-transcriptional mechanisms. As cellular cholesterol accumulates, the transcription of HMGR mRNA is suppressed and the proteosomal degradation of HMGR protein is accelerated. The sterol-regulated transcription of HMGR and other lipogenic genes is controlled by sterol regulatory element binding proteins (SREBPs). These membrane-bound transcription factors are escorted by SREBP cleavage activating protein (SCAP) from the endoplasmic reticulum (ER) to the Golgi apparatus where SREBPs are proteolytically processed to their active forms. In cultured cells, feedback inhibition of SREBP processing is mediated by Insigs. When sterols accumulate, Insigs block SREBP activation by retaining SCAP in the ER. Insigs also mediate rapid, sterol-dependent turnover of HMGR protein. When sterols accumulate, Insigs bind to HMGR and stimulate its ubiquitination and degradation. Although Insigs are key regulators of cholesterol homeostasis in cultured cells, their role in the intact mammal was undefined. To explore this question, gain-of-function and loss-of-function analyses were performed by studying the livers of genetically engineered mice. First, transgenic mice that overexpress Insig-1 in liver (TgInsig-1) were generated. In the livers of TgInsig-1 mice, nuclear SREBPs (nSREBPs) were reduced and SREBP processing was supersensitive to inhibition by feeding high-cholesterol diets. The block in SREBP processing reduced the mRNA levels of SREBP target genes. Levels of HMGR protein were reduced and declined further with cholesterol feeding. Next, knockout mice that lack Insig-1, Insig-2, or both Insigs were generated. In the livers of Insig double knockout mice, cholesterol and triglycerides accumulated to high levels, and despite their accumulation, nSREBPs and mRNAs of SREBP target genes were not suppressed. SREBP processing was insensitive to inhibition by feeding high-cholesterol diets. HMGR protein levels were increased and failed to decline with cholesterol feeding. As a consequence of Insig overexpression or deficiency and the respective effect on SREBPs and HMGR, hepatic cholesterol and fatty acid synthesis in living animals was decreased in TgInsig-1 mice and increased in Insig double knockout mice. These studies indicate that Insigs are essential regulators of hepatic lipid synthesis.