Growth stimulation by retinol apparently independent of EGF receptor

A respiratory epithelial cell line M3E3/C3, derived from fetal Syrian hamster lung, grew very slowly in RPMI 1640 medium with serum concentrations as low as 1.5%. Under these conditions the simultaneous action of epidermal growth factor (EGF) and vitamin A (retinol) was apparently a combination of growth enhancement attributable to each compound. Both compounds also affected the morphological growth pattern considerably. Binding experiments with 125I-labelled EGF showed that retinol had only a small (if any) reducing effect on the numbers of EGF receptors. Nonetheless, retinol showed a marked growth stimulation. As far as we are able to judge from most of the other reports that retinoids inhibited growth of cells and increased the EGF receptor number, this type of growth regulation is unusual.