Evidence that expression of c-fos protein in amnion cells is regulated by external signals.

While c-fos expression is normally very low in certain cell types but transiently inducible by growth factors (e.g. in fibroblasts), other cells (e.g. amnion cells) exhibit an apparently constitutive expression in vivo. Here we show that in primary amnion cells c-fos protein expression rapidly drops to undetectable levels following plating in normal growth medium. However, c-fos expression is inducible by, and maintained at high levels in the presence of, dialyzed placenta- or embryo-conditioned medium. These observations suggest that c-fos expression in primary amnion cells is regulated by placenta- and embryo-derived factor(s), providing further evidence for the hypothesis that transcription of the c-fos gene may generally be controlled by external signals. We also show that proliferation of primary amnion cells is not dependent on a high c-fos expression, suggesting that the function of c-fos is more likely to be associated with other cellular functions in the differentiated amnion cell.