Fibroblast growth factor-2 inhibits the maturation of pro-insulin-like growth factor-II (Pro-IGF-II) and the expression of insulin-like growth factorfinding protein-2 (IGFBP-2) in the human adrenocortical tumor cell line NCI-H295R

The IGF system is thought to play a major role in adrenocortical tumorigene sis. In this study, we used the NCI H295R cell line as a model to investiga te the effects of fibroblast growth factor-2 (FGF-2), a potent mitogen for normal adrenal cells, on the proliferation and on the expression of the IGF system in cultured adrenocortical tumor cells. Three immunoreactive FGF-2 isoforms of molecular masses 18, 22, and 24 kDa were detected in H295R cell extracts. Recombinant human FGF-2 stimulated the proliferation of adrenoco rtical tumor cells in a dose- and time-dependent manner, with a maximal eff ect at a concentration of about 1 ng/ml. Treatment of H295R cells with 10 n g/ml FGF-2 for 7 days had no significant effect on IGF-II messenger RNA lev els. However, a marked increase in levels of intracellular IGF-II protein w as detected by immunoblotting. In contrast, FGF-2 induced a marked decrease in the amount of IGF-II protein secreted, with the disappearance of mature IGF-II and secretion of higher molecular forms of the growth factor, sugge sting modifications of IGF-II processing. Cell cultures in the presence of brefeldin A (1 mu g/ml), a specific inhibitor of protein secretion, suggest ed that FGF-2 did not increase IGF-II synthesis but instead inhibited the s ecretion of pro-IGF-II from H295R cells, thereby impairing the final steps of IGF-II processing to the mature 7.5-kDa peptide. At the same concentrati ons, FGF-P also decreased both IGFBP-2 messenger RNA and secreted protein, which might increase IGF-II bioavailability. No proteolysis of IGFBP-2 was detected in FGF-2-conditioned medium. Altogether, these results indicate th at FGF-2 is mitogenic for NCI H295R tumor cells and regulates the expressio n of both IGF-II and IGFBP-2 in this tumor model. Moreover, this study show s a novel effect of FGF-2, by which this growth factor modulates the proces sing of pro-IGF-II.