Steroidogenesis and StAR
1) Steroidogenesis and StAR- The rate-limiting and acutely regulated step in steroidogenesis is the delivery of cholesterol, the precursor for all steroid hormones, to the mitochondrial inner membrane for the first enzymatic step in the pathway, the conversion of cholesterol to pregnenolone by the cytochrome P450 side chain cleavage enzyme (encoded by CYP11A1). Cholesterol transfer across the mitochondrial membranes is dependent upon tropic hormone-stimulated increase in the Steroidogenic Acute Regulatory protein (StAR). Dr. Clark was part of the group that identified StAR as the mediator for cholesterol transfer across the mitochondrial membranes in steroidogenesis (Clark et. al. The Purification, Cloning, and Expression of a Novel LH-Induced Mitochondrial Protein in MA-10 Mouse Leydig Tumor Cells: Characterization of the Steroidogenic Acute Regulatory Protein (StAR). The Journal of Biological Chemistry 269, 28314-28322.) and her lab has been investigating the molecular mechanisms that control StAR gene expression by divergent signaling pathways.
In particular the focus has been on the transcriptional and post- transcriptional regulation of StAR by cAMP- PKA- dependent pathway in mouse adrenal and Leydig cells as well as the angiotensin II -Ca2+-pathway in human adrenal cells. Our goal is to establish the cell-specific regulatory mechanisms for StAR expression required for normal steroid output in order to provide insight to potential underlying causes leading to aberrant StAR expression and thus either over or under production of steroid hormones in response to trophic hormones.
In Leydig cells we have demonstrated that the mouse gene has the mSin3A-HDAC repressor complex constitutively associated with the promoter and cAMP-PKA activation results in recruitment of phosphoCREB and CBP for transactivation. Current questions to address include defining the interactions between CREB and AP-1 with SF-1 and or GATA-4 to coordinate the cAMP-dependent response and whether repressor complexes function in non-steroidogenic cell types to maintain low to absent StAR expression.
We have used the H295R human adrenocortical cell line to define a novel signaling pathway for angiotensin II regulation of StAR and steroidogenesis. In brief, we have shown Jak2 is activated by Ang II and that blocking Jak2 activation inhibits Ang II stimulated increase in StAR mRNA and steroid output. Current work is focused on identifying the Ang II responsive transcription factor that is downstream of Jak2-STAT signaling in regulating StAR.