Insulin-like growth factor-II in human fetal adrenals: regulation by ACTH, protein kinase C and growth factors

Insulin-like growth factor-II (IGF-II) may be one of the most important local growth factors in human fetal adrenals (HFAs), where its mRNA levels are upregulated by ACTH. We have investigated whether protein kinase C (PKC)-dependent mechanisms and various polypeptide growth factors participate in the regulation of IGF-II gene expression in cultured HFA cells, and whether HFA cells secrete IGF-II peptide into the culture medium. ACTH enhanced IGF-II mRNA accumulation dose- and time-dependently, maximally four- to sixfold, and this increase was inhibited dose-dependently (0.01-100 micrograms/l) by 12-O-tetradecanoyl phorbol-13-acetate (TPA), a PKC activator. TPA decreased basal IGF-II mRNA levels by approximately 55%. Staurosporine, a PKC inhibitor, abolished the inhibitory effects of TPA and induced accumulation of IGF-II mRNA. Dibutyryl cyclic AMP, cholera toxin and forskolin increased IGF-II mRNA accumulation as much as ACTH, and TPA inhibited these stimulations in a similar way. ACTH increased the IGF-II peptide concentration in most experiments, but this increase was modest in comparison with IGF-II mRNA changes. TPA, although it decreased IGF-II mRNA levels, tended to increase IGF-II peptide in the medium. Additions of GH, IGF-I and IGF-II to the cell culture medium also increased IGF-II mRNA accumulation. Transforming growth factor-beta 1 inhibited IGF-II mRNA accumulation to the same extent as TPA. Epidermal growth factor and basic fibroblast growth factor did not change IGF-II mRNA levels. Our results confirm previous reports that ACTH is an important regulator of IGF-II in human fetal adrenals, and show that IGF-II gene expression is under multifactorial control, which includes the PKC system and polypeptide growth factors.