INSULIN-LIKE-GROWTH-FACTOR-II (IGF-II) IS MORE POTENT THAN IGF-I IN STIMULATING CORTISOL SECRETION FROM CULTURED BOVINE ADRENOCORTICAL-CELLS - INTERACTION WITH THE IGF-I RECEPTOR AND IGF-BINDING PROTEINS

Although the stimulating effect of insulin-like growth factor I (IGF-I ) on adrenal steroidogenesis has been well established, the rob of IGF -II in the adult adrenal gland remains unknown. We, therefore, investi gated the effect of recombinant human IGF-II on cortisol and cAMP synt hesis from adult bovine adrenocortical cells. IGF-II, time and dose de pendently, stimulated basal cortisol secretion maximally 3-fold. In co mbination with ACTH, IGF-IT (13 nM) synergistically increased cortisol secretion from 18-fold (10(-8) M ACTH) to 28-fold of untreated contro l levels. In contrast, IGF-I at equimolar concentrations did not show an effect on basal cortisol secretion, and in combination with ACTH el icited a significant weaker stimulatory effect than IGF-II (22-fold in crease). The synergistic effect of IGF-II on ACTH-promoted cortisol se cretion was paralleled by accumulation of cAMP in the culture medium. Although both IGF receptors are present in adult bovine adrenocortical cells, the effect of IGF-II seems to be mediated through interaction with the IGF-I receptor, as [Arg(54,55)]IGF-II, which only binds to th e IGF-I receptor, was equipotent to native IGF-II, whereas [Leu(27)]IG F-II, which preferentially binds to the type II IGF receptor, did not show any effect. By Western ligand blotting, four different molecular forms of IGF-binding proteins (IGFBPs) were identified in conditioned medium of bovine adrenocortical cells with apparent molecular masses o f 39-44, 34, 29, and 24 kilodaltons. ACTH treatment increased the abun dance of all binding proteins, on the average, 2.3-fold, except for th e 29-kDa band, which was predominantly induced 6.8-fold. Additionally, [des(1-3)]IGF-I, a truncated IGF variant that exhibits only minimal b inding to IGFBPs, was significant more potent than IGF-I and elicited the same maximum stimulatory effect on cortisol secretion as IGF-II an d [des(1-6)]IGF-II. In conclusion, these results demonstrate that 1) I GF-II stimulates basal as well as ACTH-induced cortisol secretion from bovine adrenocortical cells moro potently than IGF-I; 2) this effect is mediated through interaction of TGF-II with the TGF-I receptor; 3) bovine adrenocortical cells synthesize various IGFBPs that are induced differentially by ACTH; and 4) IGFBPs apparently play a modulatory ro le in IGF-induced stimulation of adrenal steroidogenesis. Therefore, b ovine adult adrenocortical cells provide a useful tissue culture model in which the interactions among locally produced IGFs, IGFBPs, and th e IGF-I receptor can be evaluated.