Insig Proteins Mediate Feedback Inhibition of Cholesterol Synthesis in the IntestineThis work was supported, in whole or in part, by National Institutes of Health Grant HL-20948

Enterocytes are the only cell type that must balance the de novo synthesis and absorption of cholesterol, although the coordinate regulation of these processes is not well understood. Our previous studies demonstrated that enterocytes respond to the pharmacological blockade of cholesterol absorption by ramping up de novo sterol synthesis through activation of sterol regulatory element-binding protein-2 (SREBP-2). Here, we genetically disrupt both Insig1 and Insig2 in the intestine, two closely related proteins that are required for the feedback inhibition of SREBP and HMG-CoA reductase (HMGR). This double knock-out was achieved by generating mice with an intestine-specific deletion of Insig1 using Villin-Cre in combination with a germ line deletion of Insig2. Deficiency of both Insigs in enterocytes resulted in constitutive activation of SREBP and HMGR, leading to an 11-fold increase in sterol synthesis in the small intestine and producing lipidosis of the intestinal crypts. The intestine-derived cholesterol accumulated in plasma and liver, leading to secondary feedback inhibition of hepatic SREBP2 activity. Pharmacological blockade of cholesterol absorption was unable to further induce the already elevated activities of SREBP-2 or HMGR in Insig-deficient enterocytes. These studies confirm the essential role of Insig proteins in the sterol homeostasis of enterocytes. Background: Insig proteins are required for feedback regulation of cholesterol synthesis in cultured cells and liver. Insig may play a similar role in intestine. Results: Insig deficiency in enterocytes leads to constitutively elevated cholesterol synthesis in intestine. Conclusion: Intestinal regulation of cholesterol homeostasis requires Insig. Significance: This work provides insight into how enterocytes coordinately balance de novo cholesterol synthesis and absorption.