GROWTH-HORMONE ATTENUATION OF EPIDERMAL GROWTH FACTOR-INDUCED MITOGENESIS

Growth hormone (GH) has previously been reported to influence the adip ose conversion of 3T3-F442A murine fibroblasts, partly by causing thes e cells to exit the cell cycle and to become unresponsive to serum-sti mulated mitogenesis. To better understand this process, quiescent fibr oblasts were treated with fully stimulatory doses (50 nM) of epidermal growth factor (EGF) in the presence or absence of pituitary human CH (hGH) or the phorbol ester phorbol 12-myristate 13-acetate (PMA), whic h is known to down-regulate EGF receptor activity. EGF-induced DNA syn thesis was attenuated by hGH in a dose-dependent manner with an ED50 o f approximately 0.1 nM and a maximally effective dose of 10-30 nM. Thi s effect appeared to be the result of inhibition of DNA synthesis and exclusive of a time shift in the initiation of the S phase of the cell cycle. Additionally, insulin-like growth factor-1 (IGF-1), which can act as an important in vivo mediator of GH, failed to mimic the anti-m itogenic effects of GH. The ability of hGH to antagonize EGF-stimulate d mitogenesis did not appear to be due to the down-regulation of EGF r eceptor mass or to pronounced changes in EGF-induced tyrosine kinase a ctivity. Furthermore, when GH was administered at various times after EGF addition, GH continued to be effective at inhibiting EGF-induced D NA synthesis for up to 9 hr after EGF treatment. Modulation of EGF-ind uced cell cycle progression was further evidenced by the ability of GH to promote a marked decrease in the EGF-induced expression of D cycli ns. In comparison, PMA inhibited EGF-induced DNA synthesis for up to 1 8 hr after EGF addition and also down-regulated EGF receptor mass and activity; these observations suggest that the mechanism of GH action i s largely distinct from that of PMA. We conclude that CH can selective ly and dose-dependently modulate EGF receptor-mediated DNA synthesis e xclusive of any rapid or extensive effects on EGF receptor mass or tyr osine kinase activity. Furthermore, the capacity of GH to attenuate EG F-induced mitogenesis, even when administered 9 hr after EGF addition, and the CH modulation of EGF-induced expression of D cyclins, suggest that there are GH-induced effects on systems involved in the transiti on of these fibroblasts through the G(1) phase of the cell cycle. In s um, these data support a specific interaction of this somatotropic hor mone/cytokine with EGF in the control of cell cycle progression in 3T3 -F442A fibroblasts. (C) 1997 Wiley-Liss, Inc.