Growth hormone stimulates sequential induction of c-myc and insulin-like growth factor I expression in vivo

The effect of GH administration to hypophysectomized rats on expression of the c-myc proto-oncogene and insulin-like growth factor I (IGF-I) in the liver and kidney was examined. In both tissues maximal expression of c-myc occurred by 1 h after a single injection of human GH (100 micrograms/100 g bw). In the liver the maximal c-myc mRNA level was increased 12 +/- 3.9-fold (mean +/- SEM; n = 4), while the maximal c-myc level in the kidney was increased 3.4-fold (n = 2) compared to levels in basal hypophysectomized rats. In both the liver and kidney the IGF-I cDNA hybridized under stringent conditions to three transcripts with apparent sizes of 7.0, 1.8, and diffuse group of transcripts of 0.7-1.1 kilobases. Each of these transcripts demonstrated some degree of GH dependence. Under the hybridization condition used, the 7.0-kilobase IGF-I mRNA was virtually undetectable in the hypophysectomized control rat liver, while the smaller transcripts were easily detectable. Peak expression of each of the IGF-I transcripts occurred 6-12 h after GH administration. Maximal IGF-I expression in the kidney occurred 9 h after the GH injection. To determine whether the increase in c-myc expression following GH could result from a small but undetectable increase in IGF-I expression in these tissues, we administered human recombinant IGF-I to hypophysectomized rats (50 micrograms/100 g bw, ip). Despite a significant increase in serum IGF-I concentrations to levels greater than those present in the first 3 h after GH administration, no increase in c-myc expression was apparent in either the liver or kidney. These observations suggest that GH itself, rather than IGF-I, initiates the mitogenic response in the liver and kidney that follows GH administration to hypophysectomized rats.