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Disruption of the oxysterol 7alpha-hydroxylase gene in mice
J Li-Hawkins1, E G Lund, S D Turley
1Departments of Molecular Genetics and Internal Medicine, University of Texas Southwestern Medical Center, Dallas, Texas 75235-9046, USA.
Abstract:
Mice without oxysterol 7alpha-hydroxylase, an enzyme of the alternate bile acid synthesis pathway with a sexually dimorphic expression pattern, were constructed by the introduction of a null mutation at the Cyp7b1 locus. Animals heterozygous (Cyp7b1(+/-)) and homozygous (Cyp7b1(-/-)) for this mutation were grossly indistinguishable from wild-type mice. Plasma and tissue levels of 25- and 27-hydroxycholesterol, two oxysterol substrates of this enzyme with potent regulatory actions in cultured cells, were markedly elevated in Cyp7b1(-/-) knockout animals. Parameters of bile acid metabolism as well as plasma cholesterol and triglyceride levels in male and female Cyp7b1(-/-) mice were normal. The cholesterol contents of major tissues were not altered. In vivo sterol biosynthetic rates were unaffected in multiple tissues with the exception of the male kidney, which showed a approximately 40% decrease in de novo synthesis versus controls. We conclude that the major physiological role of the CYP7B1 oxysterol 7alpha-hydroxylase is to metabolize 25- and 27-hydroxycholesterol and that loss of this enzyme in the liver is compensated for by increases in the synthesis of bile acids by other pathways. A failure to catabolize oxysterols in the male kidney may lead to a decrease in de novo sterol synthesis.
Insights
Mice lacking the oxysterol 7alpha-hydroxylase (Cyp7b1) enzyme showed elevated oxysterol levels but maintained normal bile acid and cholesterol metabolism. Kidney sterol synthesis was reduced in males, suggesting a specific role for this enzyme.
Area of Science:
- Biochemistry
- Genetics
- Metabolic Pathways
Background:
- Oxysterols, cholesterol metabolites, have regulatory roles in cellular processes.
- The alternate bile acid synthesis pathway involves oxysterol 7alpha-hydroxylase (CYP7B1).
- CYP7B1 exhibits sexually dimorphic expression, suggesting potential gender-specific functions.
Purpose of the Study:
- To investigate the physiological role of CYP7B1 by creating and analyzing knockout mice.
- To determine the impact of CYP7B1 deficiency on oxysterol levels and bile acid metabolism.
- To assess alterations in cholesterol homeostasis and sterol synthesis in vivo.
Main Methods:
- Generation of Cyp7b1 knockout mice (Cyp7b1(-/-)) using gene targeting.
- Quantification of plasma and tissue oxysterol levels (25- and 27-hydroxycholesterol).
- Analysis of bile acid metabolism, plasma lipids, tissue cholesterol, and in vivo sterol biosynthesis rates.
Main Results:
- Cyp7b1(-/-) mice exhibited significantly elevated plasma and tissue levels of 25- and 27-hydroxycholesterol.
- Despite elevated oxysterols, bile acid metabolism, plasma cholesterol, and triglyceride levels remained normal in knockout mice.
- A notable decrease (~40%) in de novo sterol biosynthesis was observed specifically in the male kidney of Cyp7b1(-/-) mice.
Conclusions:
- The primary function of CYP7B1 is the metabolism of 25- and 27-hydroxycholesterol.
- Hepatic compensation mechanisms, likely involving alternative bile acid synthesis pathways, maintain cholesterol homeostasis in the absence of CYP7B1.
- Impaired oxysterol catabolism in the male kidney may impair de novo sterol synthesis, highlighting a sex-specific metabolic role.