REGULATION OF INSULIN-LIKE GROWTH FACTOR-I (IGF-I) AND IGF-BINDING PROTEIN-1 GENE-TRANSCRIPTION BY HORMONES AND PROVISION OF AMINO-ACIDS IN RAT HEPATOCYTES

Synthesis of insulin-like growth factor-I (IGF-I) and IGF binding protein-1 (IGFBP-1) is altered in diabetes and malnutrition, but underlying processes are poorly understood. To study molecular mechanisms, we examined regulation of IGF-I and IGFBP-1 gene transcription in primary cultures of rat hepatocytes. Transcription of the IGF-I and IGFBP-1 genes was measured as incorporation of [alpha-P-32]UTP into prein itiated message in isolated nuclei. IGFBP-1 gene transcription was not sensitive to reduction in amino acid concentration from 5x to 0.5x rat arterial plasma levels. However, IGF-I gene transcription fell 60-70% in response to reduced provision of amino acids. Culture with 10(-9) M insulin lowered IGFBP-1 gene transcription 50% below control levels (10(-11) M) but did not affect IGF-I gene transcription; 10(-6) M insulin raised IGF-I gene transcription P-fold. After an acute reduction in insulin concentration, IGFBP-1 transcription began to rise within 30 min, but IGF-I gene transcription was unchanged over 120 min. Similarly, 3-6 h were required for stimulation of IGF-I gene transcription by insulin, but a 40% decrease in IGFBP-1 gene transcription could be detected within 15 min after adding 10(-6) M insulin, and suppression of IGFBP-1 transcription by insulin was unaffected by the presence of cycloheximide. Effects of insulin on IGFBP-1 gene transcription were not mimicked or antagonized by phorbol ester. An acute increase in dexamethasone (10(-10) M to 2 x 10(-7) M) produced a 4- to 5-fold rise in IGFBP-1 gene transcription within 30 min; stimulation of IGFBP-1 transcription was decreased 50% by the presence of cycloheximide. An acute increase in dexamethasone also led to a rise in IGF-I transcription, but stimulation was not observed in the presence of cycloheximide. It is concluded that transcription of the IGFBP-1 gene responds rapidly to insulin and dexamethasone, and such hormonal regulation appears to be independent of ongoing protein synthesis. In contrast, IGF-I gene transcription is less sensitive to insulin and dexamethasone but responds to availability of amino acids; delayed stimulation by insulin may indicate a requirement for protein synthesis. Different regulatory mechanisms appear to control expression of these two genes in hepatocyte primary culture.