Induction of gene expression of the catecholamine-synthesizing enzymes by insulin-like growth factor-I

The effects of insulin-like growth factor-I (IGF-I) on gene expression and the activities of the three enzymes specific for catecholamine biosynthesis, tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DBH), and phenylethanolamine N-methyltransferase (PNMT), were determined in bovine adrenomedullary chromaffin cells primary cultured in serum-free medium. The mRNA level of TH was maximally elevated in the presence of IGF-I by 3.1 +/- 0.4-fold after 48 h, DBH by 5.1 +/- 0.3-fold in 24 h, and PNMT by 2.8 +/- 0.5-fold in 72 h. In addition, the activity of TH was increased by 77%, DBH by 70%, and PNMT by 23% in IGF-I-exposed cultures. In the absence of the growth factor, the mRNA levels of TH and DBH were decreased to 45 +/- 10% and 35 +/- 12% of the time-zero control within 48 h while PNMT mRNA was decreased to 82 +/- 5% only after 72 h. When the cells were cotreated with the protein tyrosine kinase inhibitor genistein, DBH induction by IGF-I was suppressed, confirming that the effect is mediated by tyrosine kinase. Cotreatment with the protein kinase A (PKA) inhibitor H89 caused complete reversal of the IGF-I-induced DBH increase and the effects of IGF-I treatment and PKA activation by forskolin were not additive, suggesting that PKA is involved in the signaling initiated by IGF-I in these cells.