Wk. Shea-eaton et al., Sterol regulatory element binding protein-1a regulation of the steroidogenic acute regulatory protein gene, ENDOCRINOL, 142(4), 2001, pp. 1525-1533
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.