Aldosterone action: induction of p21rasandfra-2 and transcription-independent decrease inmyc,jun, andfos

Abstract

Adrenal steroids induce an increase in transcellular Na+reabsorption across Xenopus laevis A6 cell epithelia that requires the action of transcriptionally regulated gene products. In a previous study we identified K- ras2 as an aldosterone-upregulated mRNA in A6 epithelia. Here, we show that in vivo injection of aldosterone in Xenopus (2.5 h) increases K- ras2 mRNA specifically in the kidney (2.5-fold) and that in A6 epithelia aldosterone (2.5 h) increases Ras protein synthesis (∼6-fold). Xl- ras, another ras mRNA expressed at a low level in A6 cells, was also induced (2-fold). Aldosterone was shown to regulate the mRNA levels of several transcription factors as well. After 2 h of aldosterone treatment, fra-2 mRNA was upregulated by 130%, whereas c- myc, c- jun, c- fos, and glucocorticoid receptor mRNAs were downregulated by 23–43%. After 16 h, c- fos and GR mRNAs were further decreased, whereas levels of fra-2, c- jun, and c- myc began to return to control levels. Interestingly, the downregulation of the protooncogene mRNAs was independent of transcription. These results support the view that aldosterone exerts complex pleiotropic transcriptional and nontranscriptional actions that involve the regulation of signaling cascade elements (i.e., K-Ras2) as well as that of transcription factors.