Growth hormone synergizes with serum growth factors in inducing c-fos transcription in 3T3-F442A cells

Abstract

GH is a major regulator of growth and metabolism, but cellular effects of GH alone have been difficult to demonstrate. Accordingly, suggestions have arisen that GH works in conjunction with other agents in producing its characteristic long term biological effects. In 3T3-F442A cells, in addition to eliciting long term changes, GH rapidly increases the transcription of c-fos. The present study uses this rapid response to examine whether GH interacts with other factors early in its action, and whether such interactions lead to changes in gene expression. The induction of c-fos mRNA in response to the combination of GH (2.2 nM) and 10% calf serum or fetal calf serum was more than 3 times the additive effects of GH and either of the sera alone, indicating synergism between GH and serum. Insulin-like growth factor-I (IGF-I), which also induces c-fos, had an effect with GH that was greater than the additive responses to the two agents. Nuclear run-off experiments indicated that the synergism between GH and IGF-I occurred at the level of transcription of c-fos. However, synergism between GH and serum in inducing c-fos transcription was greater than synergism between GH and IGF-I, suggesting that factors in addition to IGF-I contribute to the interaction of GH with serum. Insulin and fibroblast growth factor also synergized with GH in inducing c-fos expression. Platelet-derived growth factor and epidermal growth factor appeared to induce c-fos additively with GH, suggesting that different types of interactions occur between GH and the various growth factors. In inducing c-jun, which works coordinately with c-fos in transcriptional regulation, the effect of GH was additive with that of IGF-I and synergistic with that of serum. These findings indicate that early in its action, GH interacts with other growth factors in inducing protooncogene expression in 3T3-F442A cells. Such interactions between GH and serum or specific growth factors result in synergistic induction of the expression of c-fos. These findings suggest a generalized mechanism in which a major contribution of GH to cellular growth regulation is to synergize with other growth-promoting signals early in transduction of such signals in targets cells, resulting in enhanced gene transcription.