DAX-1 represses the high-density lipoprotein receptor through interaction with positive regulators sterol regulatory element-binding protein-1a and steroidogenic factor-1.
The high-density lipoprotein receptor (HDL-R) mediates the selective uptake of high-density lipoprotein cholesterol in nonplacental steroidogenic tissues. We have previously demonstrated that sterol regulatory element-binding protein-1a (SREBP-1a) and steroidogenic factor-1 (SF-1) positively regulate HDL-R gene transcription. In the present study, we examined whether DAX-1 (dosage-sensitive sex adrenal hypoplasia congenital critical region on the X chromosome, gene-1) could influence the expression of the HDL-R gene. Cotransfection studies demonstrated that DAX-1 was able to repress SREBP-1a and SF-1-dependent activation of the HDL-R promoter. Mammalian two-hybrid assays demonstrated that DAX-1 could interact with SREBP-1a. In addition, electrophoretic mobility shift assays demonstrated that initial incubation of DAX-1 with SREBP-1a protein in the absence of DNA prevented subsequent binding of SREBP-1a to the HDL-R sterol regulatory elements in a dose-dependent manner, whereas, in the case of SF-1, DAX-1 formed a complex with SF-1 protein on the DNA. These data suggest that DAX-1 inhibits SREBP-1a- and SF-1-dependent activation of the HDL-R promoter through different mechanisms. This investigation confirms that DAX-1 has an important role in regulating steroidogenesis by interfering with SREBP-1a and SF-1 induction of a gene involved in the transport of cholesterol, thereby limiting the amount of substrate available for steroid hormone production.