Transcriptional inhibition of the human insulin receptor gene by aldosterone

In earlier studies, we reported reduced human insulin receptor (hIR) mRNA levels, insulin binding and insulin responsiveness in U-937 human promonocytic cells treated with aldosterone. The mechanism for this inhibition could be diminished IR gene transcription, since aldosterone did not affect hIR mRNA stability. All the effects were mediated by a downregulation of the mineralocorticoid receptor (MR, NR3C2) expressed at both the RNA and protein levels, suggesting that MR could act as a transcription factor that binds to hormone response elements in the hIR gene promoter. Indeed, MR has been shown to bind glucocorticoid response elements (GREs) in target genes. Given that five GREs have been characterized in the hIR promoter, we decided to test whether these elements could mediate the aldosterone-elicited inhibition of hIR expression detected by us in U-937 cells. In the present report, we demonstrate that aldosterone inhibits the activity of the hIR wild-type promoter by 23%, and causes 23 and 31% reductions in the activity of progressive deletions of this promoter comprised of fragments up to -1473 and -876bp, respectively. This indicates that the -876 to -271bp region of the hIR promoter may be sufficient for this transcriptional inhibition by aldosterone. We also provide evidence for direct MR interaction with some of the GREs of this promoter region, specifically with the cGRE1 and cGRE3, presumably as MR-MR homodimers, and with pGRE as a MR-GR heterodimer. This heterodimer may play the most relevant role and participate in the cross-talk between mineralocorticoids, glucocorticoids and insulin signalling in U-937 cells.