REGULATION OF ADRENAL CELL-DIFFERENTIATED FUNCTIONS BY GROWTH-FACTORS

Adrenal cell-differentiated functions and, therefore, their steroidoge nic capacity can be regulated in an opposite direction by insulin-like growth factor I (IGF-I) and transforming growth factor-beta(1) (TGF b eta(1)). The enhanced steroidogenic responsiveness of bovine and ovine adrenal cells treated with IGF-I can be explained by its positive eff ects on the number of corticotropin (ACTH) and angiotensin II (A-II) r eceptors associated with an increase in the alpha(s) and alpha(i) subu nits of G proteins but also by its effects on several steps of the ste roidogenic pathway. In contrast, TGF beta(1) is a potent inhibitor of differentiated functions of both bovine and ovine adrenal cells. TGF b eta(1) reduces ACTH and A-II receptor number, inhibits cAMP formation, and decreases several steroidogenic enzyme activities. The physiologi cal role of these peptides in adrenal cells is strengthened by the fac t that both are synthetized and secreted by these cells and that their secretion can be regulated by the specific hormones, ACTH and A-II.