Schoenheimer effect explained – feedback regulation of cholesterol synthesis in mice mediated by Insig proteins
Luke J. Engelking, … , Joseph L. Goldstein, Michael S. Brown
Luke J. Engelking, … , Joseph L. Goldstein, Michael S. Brown
Published September 1, 2005
Citation Information: J Clin Invest. 2005;115(9):2489-2498. https://doi.org/10.1172/JCI25614.
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Research Article Metabolism

Schoenheimer effect explained – feedback regulation of cholesterol synthesis in mice mediated by Insig proteins

Abstract

End-product feedback inhibition of cholesterol synthesis was first demonstrated in living animals by Schoenheimer 72 years ago. Current studies define Insig proteins as essential elements of this feedback system in mouse liver. In cultured cells, Insig proteins are required for sterol-mediated inhibition of the processing of sterol regulatory element–binding proteins (SREBPs) to their nuclear forms. We produced mice with germline disruption of the Insig2 gene and Cre-mediated disruption of the Insig1 gene in liver. On a chow diet, these double-knockout mice overaccumulated cholesterol and triglycerides in liver. Despite this accumulation, levels of nuclear SREBPs and mRNAs for SREBP target genes in lipogenic pathways were not reduced. Whereas cholesterol feeding reduced nuclear SREBPs and lipogenic mRNAs in wild-type mice, this feedback response was severely blunted in the double-knockout mice, and synthesis of cholesterol and fatty acids was not repressed. The amount of HMG-CoA reductase protein was elevated out of proportion to the mRNA in the double-knockout mice, apparently owing to the failure of cholesterol to accelerate degradation of the enzyme. These studies indicate that the essential elements of the regulatory pathway for lipid synthesis function in liver as they do in cultured cells.

Authors

Luke J. Engelking, Guosheng Liang, Robert E. Hammer, Kiyosumi Takaishi, Hiroshi Kuriyama, Bret M. Evers, Wei-Ping Li, Jay D. Horton, Joseph L. Goldstein, Michael S. Brown

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Figure 1

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Targeted disruption of Insig1 and Insig2 genes in mice. (A) Schematic of...
Targeted disruption of Insig1 and Insig2 genes in mice. (A) Schematic of Insig1 gene-targeting strategy. Cre-mediated excision of the sequences between loxP sites deletes exon 1. The location of the probe used for Southern blot analysis is denoted by the horizontal filled rectangle labeled “probe.” (B) Representative Southern blot analysis of NcoI-digested DNA from livers of mice with the indicated genotypes that were treated with 4 intraperitoneal injections of pIpC (300 μg/injection). (C) Northern blot analysis of hepatic RNA of mice indicated in B. Total RNA from liver was pooled, and 20-μg aliquots were subjected to electrophoresis and blot hybridization with 32P-labeled cDNA probes for mouse Insig1 and mouse cyclophilin. (D) Immunoblot analysis of livers of mice indicated in B. Liver membrane fractions were prepared as described in Methods, and aliquots (45 μg) were subjected to SDS-PAGE and immunoblot analysis. (E) Schematic of Insig2 gene-targeting strategy. The Insig2 allele was disrupted by replacement of exons II and III of the Insig2 gene with a polIIsneopA expression cassette. The DNA probe used for Southern blot analysis is denoted by the horizontal filled rectangle labeled “probe.” (F) Representative Southern blot analysis of NcoI-digested tail DNA of the offspring from mating of Insig2+/− mice. (G) Northern blot analysis of hepatic RNA of mice described in F. Total RNA from livers of mice was subjected to electrophoresis and blot hybridization with 32P-labeled cDNA probes for mouse Insig2 and mouse cyclophilin. (H) Immunoblot analysis of liver membranes from mice with the indicated Insig2 genotype, as described above.