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 stero idogenic factor-1 (SF-1) to increase expression of steroidogenic acute regu latory (StAR) protein, facilitating the transfer of cholesterol to the inne r mitochondrial membrane. The increased use of cholesterol in steroidogenes is triggers activation of sterol-sensitive genes through a second regulator y 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 de monstrated specific binding with recombinant SREBP-1a. Overexpression of SR EBP-1a, -1c or -2 in HTB-9 cells cotransfected with the rat StAR promoter r esulted 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 transcrip tional activators and require the presence of additional coactivators like Spl and nuclear factor-Y (NF-Y) to elicit maximum activation. Electrophoret ic mobility shift assays demonstrated that Spl, SF-l, and NF-Y enhanced SRE BP-1a binding to SREs in the StAR promoter. There was a B-fold increase in StAR promoter luciferase reporter gene expression when HTB-9 cells were cot ransfected 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 cA MP-induced 13.5-fold) activation of the rat StAR promoter. Although Spl enh anced SREBP-1a binding to an SRE, Spl was not able to increase StAR promote r activity in the presence of SREBP-1a. These results suggest that SREBP-in duced regulation of the rat StAR gene is responsive to selective combinatio ns of transcriptional cofactors that could necessitate the convergence of m ultiple regulatory pathways to enhance gene transcription.