Negative regulation of glucocorticoid-dependent induction of c-fos by ras in intestinal epithelial cells

In order to investigate the regulatory mechanisms involved in the expressio n of fos and jun family members by glucocorticoids, and the effect of ras t ransformation in intestinal epithelial cells, we used the rat cell line IEC -6. Dexamethasone treatment induced transiently c-jun mRNAs, in contrast to the sustained expression of c-fos, whereas its effect on junB expression r esulted in a later increase. Dexamethasone-dependent stimulation of c-fos a nd c-jun was modulated predominantly at the level of transcription. Sustain ed levels of induced c-fos and c-jun proteins were observed after dexametha sone treatment. AP-1 DNA-binding capacity of c-fos, and to a smaller extent c-jun, was increased by glucocorticoids later than after serum treatment. To analyse the effect of ras on the glucocorticoid response of AP-1 compone nts, we studied several IEC-6 cell clones transformed by the Ha-ras oncogen e. In comparison to normal cells, these transformants displayed increased A P- 1 DNA-binding activity with higher levels of junB and variable levels of c-jun in the AP-1 complex. Ras transformation repressed the growth-inhibit ory properties of glucocorticoids. Furthermore, ras inhibited the glucocort icoid-dependent induction of c-fos protein and mRNA, leading to changes in AP-1 composition as compared to normal cells. As assessed by transient tran sfection luciferase assays, glucocorticoids induced significantly a minimal promoter containing 3 copies of an AP-1 DNA-binding site as well as the mu rine c-fos -276 to +112 promoter in non-transformed cell lines. In contrast , glucocorticoid addition did not induce these constructs in two ras transf ormed cells. These results suggest that ras negatively modulates specific r esponses of intestinal epithelial cells to glucocorticoids.