Abstract: 58

SYNERGISTIC ROLE OF INSULIN/IGF-I IN LH-INDUCED StAR PROMOTER EXPRESSION IN PORCINE GRANULOSA CELLS.

Natesampillai Sekar^{1 *}, HA LaVoie¹ , Johannes Veldhuis^{1 *}
Dept. of Internal Medicine, University of Virginia Health Sciences Center, VA 22908 ¹

The Steroidogenic Acute Regulatory (StAR) protein is indispensable for maximal biosynthesis of steroid hormones by steroidogenic cells in the adrenal gland and ovary. Previously, we observed that luteinizing hormone (LH) and insulin synergistically enhance the expression of low-density lipoprotein receptor and StAR mRNA in porcine granulosa cell cultures. Based on this notion, studies were designed to identify the potential sites of insulin/insulin-like growth factor-I (IGF-I) actions and/or LH or a protein kinase A agonist (8 Br-cAMP) on the StAR promoter. IGF-I or insulin synergistically increased LH-dependent stimulation of progesterone (P₄) and cAMP synthesis by 2-fold above that observed in LH-stimulated cultures (P<0.001). Granulosa cells were transfected with the porcine full-length StAR promoter (1.423 kb) as well as with 5'-nested deletions driving a luciferase reporter (luc) and treated with LH and/or insulin or IGF-I for 24 h. In full-length or variably deleted pStARluc transfected cultures, insulin or IGF-I further augmented StAR promoter gene expression as stimulated by LH (100 ng/ml) or 8 Br-cAMP. Deletion of the proximal putative steroidogenic factor-1 [(SF-1); (-48 to -41)] site completely abolished StAR promoter expression. However, neither IGF-I nor insulin alone stimulated StAR promoter activity. This observation suggests that the synergistic induction of StAR promoter by insulin or IGF-I combined with LH may be mediated by IGF-I/insulin's interactions with LH/cAMP-directed pathway(s). In conclusion, the present observations establish that LH and insulin or IGF-I act synergistically to stimulate P₄ and cAMP synthesis and StAR promoter expression. Post-cAMP mediated pathways likely modulate this steroidogenic synergy between insulin or IGF-I and LH.

    This abstract is being presented on Sunday, August 1 at 8:00 AM to 10:15 AM at CUB 2nd Floor Ballroom.