Elevated levels of SREBP-2 and cholesterol synthesis in livers of mice homozygous for a targeted disruption of the SREBP-1 gene.
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 (-/-) mice died in utero at embryonic day 11. The surviving -/- 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 -/- 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 [3H]water, was elevated threefold in livers of the -/- mice, and hepatic cholesterol content was increased by 50%. Fatty acid synthesis was decreased in livers of the -/- mice. The amount of white adipose tissue was not significantly decreased, and the levels of mRNAs for lipogenic enzymes, adipocyte lipid binding protein, lipoprotein lipase, and leptin were normal in the -/- mice. We conclude from these studies that SREBP-2 can replace SREBP-1 in regulating cholesterol synthesis in livers of mice and that the higher potency of SREBP-2 relative to SREBP-1c leads to excessive hepatic cholesterol synthesis in these animals.
Shimano
H
H
Department of Molecular Genetics, University of Texas Southwestern Medical Center, Dallas, Texas 75235, USA.
Shimomura
I
I
Hammer
R E
RE
Herz
J
J
Goldstein
J L
JL
Brown
M S
MS
Horton
J D
JD
D000273:Adipose Tissue / Q000737:chemistry; D000311:Adrenal Glands / Q000378:metabolism; D000595:Amino Acid Sequence; D000818:Animals; D022762:CCAAT-Enhancer-Binding Proteins; D002478:Cells, Cultured; D002784:Cholesterol / Q000096:biosynthesis*; D003001:Cloning, Molecular; D003433:Crosses, Genetic; D004268:DNA-Binding Proteins / Q000096:biosynthesis* / Q000235:genetics*; D005227:Fatty Acids / Q000096:biosynthesis; D005260:Female; D005822:Genetic Vectors; D006720:Homozygote; D006801:Humans; D007668:Kidney / Q000166:cytology; D008099:Liver / Q000378:metabolism*; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D008969:Molecular Sequence Data; D009687:Nuclear Proteins / Q000235:genetics*; D012333:RNA, Messenger / Q000032:analysis; D051780:Sterol Regulatory Element Binding Protein 1; D051782:Sterol Regulatory Element Binding Protein 2; D014157:Transcription Factors / Q000096:biosynthesis*
D022762:CCAAT-Enhancer-Binding Proteins; D004268:DNA-Binding Proteins; D005227:Fatty Acids; D009687:Nuclear Proteins; D012333:RNA, Messenger; C497387:SREBF1 protein, human; C497390:SREBF2 protein, human; C497388:Srebf1 protein, mouse; C497391:Srebf2 protein, mouse; D051780:Sterol Regulatory Element Binding Protein 1; D051782:Sterol Regulatory Element Binding Protein 2; D014157:Transcription Factors; D002784:Cholesterol