Divergent effects of epidermal growth factor and transforming growth factors on a human endometrial carcinoma cell...

Epidermal growth factor (EGF), at concentrations ranging from 0.83 to 4.98 nM, markedly inhibited the proliferation of RL95-2 cells that were seeded at low plating densities (4.7 X 10(3) cells/cm2). Under the same incubation conditions, 16.6 pM EGF enhanced cell proliferation. At high plating densities (2.5 X 10(4) cells/cm2) 0.83 nM EGF also stimulated cell proliferation. Both the inhibitory and stimulatory effects of EGF were mimicked by transforming growth factor-alpha (TGF-alpha). However, the inhibitory action of TGF-alpha was always greater that of EGF. Binding studies with 125I-labeled TGF-alpha indicated that maximal cell surface binding of TGF-alpha occurred at 15 min, whereas maximal internalization occurred at 45 min. Both cell surface and internalized radioactivity declined sharply thereafter. Analysis of radioactivity released into the incubation medium during pulse-chase experiments indicated that RL95-2 cells extensively degraded both TGF-alpha and EGF. The lysosomotropic compound methylamine arrested the generation of low-molecular-weight degradation products of EGF, but not of TGF-alpha. In contrast to EGF and TGF-alpha, transforming growth factor-beta (TGF-beta) inhibited the proliferation of RL95-2 cells that were seeded at either low or high plating densities. Further, transforming growth factor-beta induced the appearance of large cuboidal cells that were readily distinguished from cells treated with either EGF or TGF-alpha. These findings point to complex regulatory actions of growth factors on the proliferation of RL95-2 cells and suggest that the processing of TGF-alpha following EGF receptor activation is distinct from the processing of EGF.