Pituitary insulin-like growth factor-I gene expression: regulation by triiodothyronine and growth hormone

Most of the growth-promoting effects of GH are mediated through insulin-like growth factor-I (IGF-I). Pituitary GH gene expression is, in turn, inhibited by IGF-I. Since rat pituitary tissue and GH3 pituitary tumor cells express both the GH and the IGF-I genes, we have attempted to clarify their potential interactions in the somatotroph by examining hormonal factors involved in the regulation of pituitary IGF-I gene expression. IGF-I mRNA was measured in GH3 cells by a solution hybridization/RNase protection assay, using riboprobes to differentially protect the IGF-I variant mRNAs arising by alternative splicing at both the 5' untranslated (UT) and 3' ends of the primary transcript. GH3 cells contained both class A and class C 5' UT variant mRNAs, with a relative abundance similar to that found in the liver. Sixty-five percent of the total IGF-I mRNA in GH3 cells was processed at the 3' end to IGF-Ia, and 35% to IGF-Ib mRNAs, whereas in the liver the proportions were 85% and 15%, respectively. GH3 cells grown in thyroid hormone-depleted medium for 4 days contained low levels of IGF-I mRNA. T3 and human (h) GH induced total IGF-I mRNA content in thyroid hormone-depleted cells, with both 5' and 3' alternative transcripts regulated coordinately, an effect that was maximal at 48-72 h. T3 stimulation of GH3 IGF-I mRNA over 48 h was dose dependent (0.01-5 nM). Similarly, hGH (0.5-10 micrograms/ml) evoked a dose-dependent induction of IGF-I mRNA in the thyroid hormone-deficient GH3 cells. The effects of T3 (5 nM) and hGH (10 micrograms/ml) on IGF-I mRNA were not additive. Furthermore, the effects of both T3 and hGH were selective for IGF-I mRNA, as neither of these treatments stimulated PRL mRNA, and treatment with hGH decreased GH3 cell GH mRNA content. This model does not discriminate whether T3 has an independent effect on IGF-I gene expression or if its action is mediated solely through induction of GH. In conclusion, IGF-I mRNA transcripts are present in GH3 cells and are modulated by T3 and GH. Local paracrine or autocrine interactions may, therefore, be involved in the feedback control of GH secretion.