Elevated levels of SREBP-2 and cholesterol synthesis in livers of mice homozygous for a targeted disruption of the SREBP-1 gene.

Abstract

The synthesis of cholesterol and its uptake from plasma LDL are regulated by two membrane-bound transcription factors, designated sterol regulatory element binding protein-1 and -2 (SREBP-1 and SREBP-2). Here, we used the technique of homologous recombination to generate mice with disruptions in the gene encoding the two isoforms of SREBP-1, termed SREBP-1a and SREBP-1c. Heterozygous gene-disrupted mice were phenotypically normal, but 50-85% of the homozygous ( Ϫ / Ϫ ) mice died in utero at embryonic day 11. The surviving Ϫ / Ϫ mice appeared normal at birth and throughout life. Their livers expressed no functional SREBP-1. There was a 1.5-fold upregulation of SREBP-2 at the level of mRNA and a two-to threefold increase in the amount of mature SREBP-2 in liver nuclei. Previous studies showed that SREBP-2 is much more potent than SREBP-1c, the predominant hepatic isoform of SREBP-1, in activating transcription of genes encoding enzymes of cholesterol synthesis. Consistent with this observation, the SREBP-1 Ϫ / Ϫ animals manifested elevated levels of mRNAs for 3-hydroxy-3-methylglutaryl coenzyme A synthase and reductase, farnesyl diphosphate synthase, and squalene synthase. Cholesterol synthesis, as measured by the incorporation of

Adipogenesis

“…Transgenic mice with adipose tissue-specific knockout of SREBP-1 had minimal inhibition of adipose tissue development, with no overt effects on the expression of adipocyte marker genes (Shimano et al 1997b). Ectopic expression of a constitutively active SREBP1c surprisingly conferred lipodystrophy in mice .…”

Overexpression of Insig-1 in the livers of transgenic mice inhibits SREBP processing and reduces insulin-stimulated lipogenesis

“…Cholesterol and triglycerides in plasma and the liver were measured as previously described (19). Immunoblot analyses of mouse SREBP-1 and -2 were carried out as previously described (20). Monoclonal antibody against transferrin receptor (13-6800) was obtained from Zymed Laboratories Inc. (San Francisco, California, USA).…”

Regulation of Cholesterol and Fatty Acid Synthesis

“…Complete lethality was observed following germline deletion of the SREBP-2 gene in mice with mortality occurring at embryonic days 7-8 (Shimano et al 1997b). Homozygous disruption of the SREBP-1 gene, which eliminated both the SREBP-1a and SREBP-1c transcripts, led to partial lethality; 15% -45% of the mice survive past embryonic day 11.…”