Molecular Cloning, Cellular Distribution and Regulation of Rat Steroidogenic Acute Regulatory Protein (StAR) in the Ovary.

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Rat ovarian genes induced by the treatment of immature rats with pregnant mare serum gonadotropin (PMSG) were isolated by a subtraction cloning method. Amongst them was obtained a probable rat homologue of steroidogenic acute regulatory protein (StAR), which has been recently identified as a protein that is an acute regulator of the rate limiting transfer of cholesterol from the outer to the inner mitochondrial membrane. Structure of rat StAR was determined by nucleotide sequence analysis. Northern blot analysis revealed that StAR mRNA levels were rapidly and strongly increased by PMSG/hCG. In situ hybridization revealed that the expression of StAR mRNA was strongly induced by PMSG in theca interna cells as well as in corpora lutea. These findings indicate that expression of StAR mRNA is restricted to and induced in the ovarian steroidogenic cell types where cholesterol is used as a substrate for synthesis of steroid hormones. To study the mechanisms of regulation of StAR in rat granulosa cells, we used granulosa cells obtained from diethylstilbestrol-treated immature rats. Northern blot analysis revealed two major transcripts of about 3.6 kb and 1.6 kb of rat StAR mRNA. Rat StAR mRNA had strongly increased within 2 h due to the treatment of FSH or 8-Br-cAMP in this culture, a parallel increase of transcripts of both sizes was observed. Compared to the control, StAR mRNA levels increased in a dose-dependent manner in the presence of increasing concentrations of FSH (1-100 ng/ml) and 8-Br-cAMP (0.25-5 mM). Although co-treatment of rat granulosa cells with FSH and TGF-β did not change FSH-induced StAR mRNA levels, these levels in granulosa cells were markedly increased by pretreatment with TGF-β before being acutely (2 h) stimulated with an effective dose of FSH. The stimulatory effect of TGF-β was concentration-dependent (1-30 ng/ml).

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