Sterol regulatory element binding protein-1a regulation of the steroidogenic acute regulatory protein gene

The binding of tropic hormones to their specific receptors in steroidogenic cells stimulates the cAMP second-messenger system in the presence of steroidogenic factor-1 (SF-1) to increase expression of steroidogenic acute regulatory (StAR) protein, facilitating the transfer of cholesterol to the inner mitochondrial membrane. The increased use of cholesterol in steroidogenesis triggers activation of sterol- sensitive genes through a second regulatory pathway involving the binding of sterol regulatory element (SRE)-binding proteins (SREBP) to SREs located in the promoter regions of these genes. A search of the rat StAR promoter revealed five potential SRE sites, which demonstrated specific binding with recombinant SREBP-1a. Overexpression of SREBP-1a, -1c or -2 in HTB-9 cells cotransfected with the rat StAR promoter resulted in an increase in promoter-driven luciferase activity. In addition, SREBP-1a was able to activate the StAR promoter through an E-box but only in a promoter construct lacking SREs. SREBPs are known to be weak transcriptional activators and require the presence of additional coactivators like Sp1 and nuclear factor-Y (NF-Y) to elicit maximum activation. Electrophoretic mobility shift assays demonstrated that Sp1, SF-1, and NF-Y enhanced SREBP-1a binding to SREs in the StAR promoter. There was a 4-fold increase in StAR promoter luciferase reporter gene expression when HTB-9 cells were cotransfected with expression vectors for SREBP-1a and NF-Y. In addition, the combined action of SREBP-1a and SF-1 increased both basal (1.6-fold) and cAMP-induced (3.5-fold) activation of the rat StAR promoter. Although Sp1 enhanced SREBP-1a binding to an SRE, Sp1 was not able to increase StAR promoter activity in the presence of SREBP-1a. These results suggest that SREBP-induced regulation of the rat StAR gene is responsive to selective combinations of transcriptional cofactors that could necessitate the convergence of multiple regulatory pathways to enhance gene transcription.