Repression of the Rat Steroidogenic Acute Regulatory Protein (StAR) by Dexamethasone is Mediated Through by the Negative Transcription Factor DAX-1

• Main Authors: Chih-Chuan Yu, 余智娟
• Other Authors: P. Shirley Li
• Format: Others
• Language: en_US
• Published: 2001
• Online Access: http://ndltd.ncl.edu.tw/handle/13184201458656311332

碩士 === 國立成功大學 === 生理學研究所 === 89 === Stress and other conditions that elevate levels of circulating glucocorticoids lead to depressed serum testosterone in all known animal species. This glucocorticoid-mediated suppression of testosterone can have profound physiological consequences, including muscular atrophy and sexual dysfunction. Leydig cells possess glucocorticoid receptors and are the primary targets of glucocorticoid action in the testis. The steroidogenic acute regulatory protein (StAR) is thought to mediate the rapid increase in steroid hormone biosynthesis by facilitating cholesterol transport to the inner mitochondria membrane. DAX-1 (dosage-sensitive sex reversal, adrenal hypoplasia congenita, critical region on the X chromosome gene 1), a negative transcription factor, inhibits transcription of StAR gene expression in vitro. In the present study, we investigated whether the effects of dexamethasone on testosterone production were related to regulatory action on StAR gene expression and determined whether dexamethasone altered DAX-1 protein levels. Male Wistar rats (250-300 g) were treated with dexamethasone (0.4 or 4 mg/kg body weight per day) by ip administration using PBS as the vehicle for 7 days. Testosterone was measured in serum; StAR mRNA and protein and DAX-1 protein levels were measured in isolated testicular cells. Serum testosterone levels were decreased by dexamethasone (4 mg/kg) treatment. Response to luteinizing hormone and 8-bromo-cyclic AMP (8-Br-cAMP) in vitro was significantly reduced in testicular cells prepared from the testes of rats treated with dexamethasone. The conversion of 22(R)-hydroxycholesterol, pregnenolone, 17-hydroxypregnenolone, dehydroepiandrosterone and androstenedione to testosterone was not affected by dexamethasone. This indicates that dexamethasone affects the cholesterol transport into mitochondrial inner membrane. Dexamethasone significantly decreased StAR mRNA, StAR protein, and StAR promoter activity. Treatment with dexamethasone (4 mg/kg body wt) increased the level of DAX-1 protein, but had no effect on the level of steroidogenic factor 1 (SF-1). The increase in DAX-1 protein corresponded to a 57% reduction in StAR mRNA levels concomitant with a 79% reduction in serum testosterone levels. Dexamethasone also increased the amount of complexed DAX-1-SF-1. To determine whether DAX-1 could negatively regulate expression of the StAR gene, a mouse StAR promoter linked to a luciferase reporter gene (p-966 StAR) was cotransfected into Y-1 adrenal tumor cells with DAX-1 cDNA. Treatment with 8-Br-cAMP for 24 h resulted in a 5.4-fold increase in StAR promoter activity. When DAX-1 was cotransfected with the StAR promoter, 8-Br-cAMP-induced StAR gene transcription was repressed in a dose-dependent manner with 0.75 ug of DAX-1 decreasing luciferase level to 7% of that in the non-DAX-1 expressing 8-Br-cAMP treated cells. Dexamethasone attenuated 8-Br-cAMP-induced StAR promoter activity at all doses of DAX-1 transfected. These results demonstrated that dexamethasone inhibits StAR gene expression via DAX-1 in testicular cells. The inhibitory action could be due, at least in part, to the induced amount of DAX-1-SF-1 complex.