Concerted Transcriptional Activation of the Low Density Lipoprotein Receptor Gene by Insulin and Luteinizing Hormone in Cultured Porcine Granulosa-Luteal Cells: Possible Convergence of Protein Kinase A, Phosphatidylinositol 3-Kinase, and Mitogen-Activated Protein Kinase Signaling Pathways

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Abstract
Insulin and insulin-like growth factor I (IGF-I) can amplify gonadotropin-stimulated steroidogenesis by augmenting the expression of key sterol regulatory genes in ovarian cells, viz. low density lipoprotein (LDL) receptor, steroidogenic acute regulatory protein, and P450 cholesterol side-chain cleavage enzyme (CYP11A). The mechanisms underlying the foregoing bihormonal interactions are not known. Accordingly, in relation to the LDL receptor gene, the present study tests the hypothesis that insulin/IGF-I and LH can act via concerted transcriptional control of promoter expression. To this end, we transiently transfected primary monolayer cultures of porcine granulosa-luteal cells with a reporter vector containing the putative 5′-upstream full-length (pLDLR1076/luc) regulatory region (−1076 to+ 11 bp) of the homologous LDL receptor gene driving firefly luciferase in the presence or absence of insulin (or IGF-I) and/or LH (each 100 ng/ml). Combined exposure to LH and insulin (or IGF-I) stimulated LDL receptor transcriptional activity maximally at 4 h by 8- to 20-fold, as normalized by coexpression of Renilla luciferase. Further analysis of multiple 5′-nested deletional constructs of the LDL receptor gene promoter showed that deletion of− 139 bp upstream of the transcriptional start site virtually abolished basal expression and promoter responsiveness to LH and insulin/IGF-I. In contrast, full basal activity and 60–80% of maximal monohormonal and bihormonal drive were retained by the −255 to +11 bp fragment. As LDL receptor gene expression in other tissues is negatively regulated by the abundance of intracellular free cholesterol, we assessed the impact of concomitant pretreatment of granulosa-luteal cells with an exogenous soluble sterol (25-hydroxycholesterol, 1 and 10μ m). Excess sterol markedly (50–70%) attenuated bihormonally and, in lesser measure, LH-stimulated and basal LDL receptor promoter expression, thus affirming a feedback-sensitive sterol-repressive region in this gene. Non-LH receptor-dependent agonists of protein kinase A (PKA), 8-bromo-cAMP (1 mm), and forskolin (10 μm) with or without insulin/IGF-I costimulation likewise augmented LDL receptor promoter expression with similar strong dependency on the −255 to −139 bp 5′-upstream region. To assess more specific PKA-dependent mediation of LH’s contribution to combined hormonal drive, the LDL receptor (−1076 to +11 bp) reporter plasmid was cotransfected with a full-sequence rabbit muscle protein kinase inhibitor (PKI) minigene driven constitutively by a Rous sarcoma virus promoter. Expression of the latter PKA antagonist blocked transcriptional stimulation by LH alone as well as that by LH combined with insulin (or IGF-I) by 70–85% without reducing basal transcriptional activity. Transfection of a mutant inactive (Arg to Gly) Rous sarcoma virus/PKI gene confirmed the specificity of the PKI effect. To investigate the convergent role of the insulin/IGF-I effector pathway mediating bihormonal stimulation of LDL receptor promoter expression, transfected granulosa-luteal cells were pretreated for 30 min with two specific inhibitors of phophatidylinositol 3-kinase, wortmannin (100 nm) and LY 294002 (10μ m), or of mitogen-activated protein kinase kinase, PD 98059 (50 μm), U0126 (10 μm), or the latter’s inactive derivative, U0124 (10 μm). Both classes of antagonists impeded the ability of insulin or IGF-I to enhance LH-stimulated LDL receptor promoter expression by 60–80%. In conclusion, the present analyses indicate that LH and insulin (or IGF-I) can up-regulate LDL receptor transcriptional activity supraadditively in porcine granulosa-luteal cells 1) via one or more agonistic cis-acting DNA regions located between −255 and −139 bp 5′- upstream of the transcriptional start site, 2) without abrogating sterol-sensitive repressive of this promoter, and 3) by way of intracellular mechanisms that include the PKA, phophatidylinositol 3-kinase, and mitogen-activated protein kinase signaling pathways.

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