MECHANISM OF IGF-I EFFECTS ON CHROMAFFIN CELL FUNCTION

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Insulin-like growth factor-I (IGF-I) has been reported to act as a trophic factor which enhances functions associated with the differentiated phenotype of many cells. Often IGF-I appears to modulate the activity of the cells with little, or no, effect on proliferation. In adrenal chromaffin cells, IGF-I enhances secretagogue-stimulated catecholamine secretion and Ca2+ uptake into the cells. The long-term objective of this project is to elucidate the mechanism by which IGF-I exerts its trophic effects on cell function. The studies described in this grant will investigate the mechanism by which chronic exposure to IGF-I enhances secretagogue-stimulated catecholamine secretion from chromaffin cells. We will examine the hypothesis that IGF-I enhances secretion by activating one of the isozymes of protein kinase C. The generality of the role of protein kinase C in IGF-I action will be explored using the effect of IGF-I on steroidogenesis in adrenocortical cells as another model system. Because catecholamines are important as hormones and neurotransmitters in the peripheral and central nervous system, this work will increase our understanding not only of the regulation of chromaffin cell function, but also of the regulation of those processes, such as blood pressure and cardiovascular function, that are regulated by catecholamines. In addition, understanding the trophic effects of IGF-I, such as steroidogenesis. The specific aims of the proposed research are: 1. To determine whether protein kinase C is required for IGF-I enhanced catecholamine secretion from chromaffin cells. Cells in which protein kinase C activity is suppressed either by inhibitors or by down regulation will be used to determine whether protein kinase C is involved in IGF-I action. 2. To determine how protein kinase C is involved in the effect of IGF-I on secretion from chromaffin cells. The effect of IGF-I on the activity of protein kinase C, and the effect of phorbol esters and protein kinase C inhibitors on the function of IGF-I receptors will be examined. 3. To determine whether the effect of IGF-I on protein kinase C is responsible for enhanced Ca2+ uptake in IGF-I treated chromaffin cells and to determine whether such an effect causes the enhanced secretion seen in IGF-I treated cells. Ca2+ uptake and efflux experiments will be performed in untreated and IGF-I treated cells made deficient in protein kinase C activity. 4. To determine whether protein kinase C is involved in the effect of IGF-I on ACTH stimulated cAMP accumulation and cortisol synthesis in adrenocortical cells. Adrenocortical cells in which protein kinase C activity is suppressed either by inhibitors or by down regulation will be used to determine whether protein kinase C is involved in IGF-I action in these cells.