Aldosterone Induces a Rapid Increase in the Rate of Na,K-ATPase Gene Transcription in Cultured Kidney Cells

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Abstract
Aldosterone (300 nM) induces a 4-fold increases over a 6-hr stimulation in A6 kidney cells from Xenopus laevis in the abundance of mRNA .beta.1 and a 2-fold in that of mRNA .alpha.1 coding for each Na,K-ATPase subunit, which is in agreement with a previous report. After a 3-hr stimulation already, aldosterone elicited a significant increase of mRNA .beta.1 (2.51-fold .+-. 0.47, n = 3, P < 0.05) and a nonsignificant increase of mRNA .alpha.1 (1.26-fold .+-. 0.30, n = 3, NS). Increasing doses of cycloheximide up to 3 .mu.g ml-1 led to 90% inhibition of protein synthesis, but failed to block the differential effect of aldosterone on mRNA abundance over a 6-h incubation period. The rate of transcription was measured by a nuclear run-on assay. The basal rate of mRNA .alpha.1 transcription exceeded that of mRNA .beta.1 by 2.8-fold. Aldosterone (300 nM) stimulated the transcription of the two subunit genes. Fifteen minutes after aldosterone addition there was a significant and parallel increase in the rate of transcription of the .alpha.1 subunit (1.98-fold .+-. 0.20, n = 3, P < 0.02) and that of the .beta.1 subunit (2.13 .+-. 0.32, n = 3, P < 0.04). After a 45-min stimulation period the transcription rate of the .alpha.1 subunit remained at the level observed at 15 min (1.84-fold .+-. 0.14, n = 4, P < 0.01), while the transcription rate of the .beta.1 subunit increased further (2.89-fold .+-. 0.38, n = 4, P < 0.01). At 45 min the difference in transcription rate between the two subunits was significant (.DELTA..beta.-.alpha. = 1.05 .+-. 0.25, n = 4, P < 0.03). Stringent inhibition of protein synthesis (> 97%) by cycloheximide (20 .mu.g ml-1) blocked the aldosterone-induced increase in relative transcription rate of both genes over a 45-min stimulation time and the aldosterone-induced accumulation of mRNA .beta.1 over a 3-hr incubation period. Our data suggest that aldosterone acts indirectly on Na,K-ATPase gene expression or that a direct effect of the activated hormone-receptor complex on the gene promoter requires the presence of a short-lived protein.