Growth hormone (GH) and prolactin (PRL) regulate IGFBP-3 gene expression in rat beta-cells

We examined the effects of growth hormone (GH), prolactin (PRL) and insulin-like growth factor-I (IGF-I) on DNA synthesis, and insulin-like growth factor binding protein 3 (IGFBP-3) gene expression, in a rat beta-cell line INS-1. GH and PRL were equally effective in increasing DNA synthesis and IGFBP-3 gene expression. IGF-I also stimulated DNA synthesis, but its effect on IGFBP-3 mRNA accumulation was moderate. The GH activities in this cell line were not mediated by IGF-I production: (1) no IGF-I mRNA could be detected in poly-A RNA preparation, and IGF-I level was very low; (2) neither IGF-I mRNA nor the protein, were increased upon GH treatment; (3) DNA synthesis stimulatory activities of GH and IGF-I were additive; (4) the GH-induced IGFBP-3 mRNA increase was not inhibited by cycloheximide treatment. Thus, the activities of GH in this cell line are not mediated by a protein synthesis.