Ki-ras mRNA regulation in untransformed mouse lung cells.

Although the Ki-ras gene is an often-observed transforming gene in lung tumors, little is understood of the factors that regulate the expression of the normal gene in lung cells. Therefore, we used untransformed mouse lung epithelial cells to determine the effect of serum, growth factors, and cell confluence on the regulation of Ki-ras mRNA expression. In subconfluent cells synchronized by 24-h serum deprivation, the refeeding of media containing serum resulted in the expression of both Ki-ras and H4 histone mRNA. No change in the expression of either gene was observed in cells refed with media alone. In confluent cell cultures, the refeeding of media with serum had no effect on the expression of these genes, suggesting that cell-density-dependent mechanisms can override the serum-induced stimuli for Ki-ras and H4 histone mRNA expression. Confluent cells expressed low but detectable Ki-ras mRNA levels consistent with constitutive expression of this gene independent of its role in mitogenic stimuli. EGF (10 ng/mL) and TGF-alpha (10 ng/mL) were found to induce transient increases in Ki-ras mRNA but large increases in H4 histone mRNA levels. Similarly, 48-h-conditioned media obtained from two transformed mouse lung cells containing activated Ki-ras genes and overexpressing Ki-ras mRNA were observed to increase both Ki-ras and H4 histone mRNA in the untransformed mouse lung cells. The expression of these genes in mouse lung cells therefore appeared to be linked to stimuli provided by specific growth factors as well as by autocrine factors elaborated by lung tumor cells. The studies reported here provide insight into the regulation of the Ki-ras mRNA in untransformed lung cells and the factors that may contribute to the elevated levels of Ki-ras mRNA often observed in transformed lung cells.