Retinoid regulation of PEPCK gene expression in transgenic mice

Date of Completion

January 1999

Keywords

Biology, Molecular|Health Sciences, Nutrition

Degree

Ph.D.

Abstract

Phosphoenolpyruvate carboxykinase (PEPCK) catalyzes a rate-determining step in hepatic gluconeogenesis. In hepatoma cells, the PEPCK gene is induced by retinoids. Retinoids mediate their actions through the retinoic acid receptors (RAR) and the retinoid X receptors (RXR) that bind to retinoic acid response elements (RARE) and regulate transcription of target genes. The objective of this study was to examine the molecular basis of retinoid regulation of PEPCK gene expression in the whole animal and determine its physiologic relevance. Mice were made vitamin A deficient by initiating a vitamin A-deficient diet on gestation day 10. At 7 wk of age, vitamin A-deficient mice were treated with 50 mg/kg body weight of all-trans or 9-cis retinoic, acid (RA) and killed for analysis of gene expression in liver. In vitamin A-deficient mice, the PEPCK gene was not induced by food deprivation or cAMP, as it was in vitamin A-sufficient mice. All-trans RA treatment restored the inhibition of PEPCK gene expression. Expression of a PEPCK/bGH transgene directed by a PEPCK promoter fragment containing RARE1 and RARE2 was reduced by vitamin A deficiency and induced by all- trans or 9-cis RA treatment in the periportal region of the liver. Expression of a PEPCK/bGH transgene directed by a PEPCK promoter fragment containing only RARE2 was reduced by vitamin A deficiency and induced by all-trans RA, but not by 9-cis RA in the periportal region. Hepatic RARβ mRNA levels were decreased by vitamin A deficiency and increased by all-trans or 9- cis RA treatment. Hepatic RARα and RXRβ mRNA levels were essentially unaffected by vitamin A deficiency or by all-trans or 9-cis RA treatment. RARα, RARβ, and RXRα mRNA was predominantly localized in the periportal and intermediate regions of the liver, as determined by in situ hybridization. Nuclear RARβ protein was predominantly detected in the periportal region, nuclear RARα protein was predominantly detected in the intermediate region, and nuclear RXRα protein was predominantly detected in the periportal and intermediate regions of the liver, as determined by immunohistochemistry. These results indicate that all-trans and 9-cis RA induce PEPCK gene expression, possibly by RARβ/RXRα and RXRα/RXRβ binding, via RARE2 and RARE1, respectively. ^

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