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 in sulin-like growth factor-I (IGF-I) on DNA synthesis, and insulin-like growth factor binding protein 3 (IGFBP3) gene expression, in a rat bet a-cell line INS-1. GH and PRL were equally effective in increasing DNA synthesis and IGFBP3 gene expression. IGF-I also stimulated DNA synth esis, but its effect on IGFBP3 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 incr eased upon GH treatment; (3) DNA synthesis stimulatory activities of G H and IGF-I were additive; (4) the GH-induced IGFBP-3 mRNA increase wa s not inhibited by cycloheximide treatment. Thus, the activities of GH in this cell line are not mediated by a protein synthesis.