Transcriptional regulation of heme oxygenases by HIF-1α in renal medullary interstitial cells

The present study was designed to test the hypothesis that hypoxia-inducible factor-1 alpha (HIF-1 alpha)-mediated transcriptional activation contributes to increased expression of heme oxygenase (HO) genes in renal medullary interstitial cells (RMICs). By Northern blot analysis, HO-1 mRNA expression was found to significantly increase in response to reduction of PO(2) in culture medium. However, HO-2 mRNA was not altered by hypoxia. This hypoxia-induced upregulation of HO-1 mRNA was significantly blocked by HIF-1 alpha inhibition with ferrous ammonium sulfate. To further determine the role of HIF-1 alpha in the activation of HO-1, the inducers of HIF-1 alpha were used to address whether induction of HIF-1 alpha stimulates HO-1 mRNA expression. Both desferrioxamine and CoCl(2) markedly increased HIF-1 alpha mRNA and protein levels and resulted in the upregulation of HO-1 mRNA but not HO-2. Furthermore, inhibition of HIF-1 alpha degradation by CBZ-LLL, an inhibitor of ubiquitin-proteasome, significantly increased HIF-1 alpha protein and HO-1 mRNA but not HO-2 in these cells. Using cis-element oligodeoxynucleotide transfection to specifically decoy HIF-1 alpha and block HIF-1 alpha binding, increased mRNA expression of HO-1 in response to hypoxia and CoCl(2) was attenuated. In vitro nuclear run-on assays further confirmed that hypoxia and alterations of HIF-1 alpha mRNA or protein levels significantly affected the formation of HO-1 mRNA. Taken together, our results indicate that HO-1, but not HO-2, is transcriptionally activated by hypoxia through HIF-1 alpha -mediated mechanism in RMICs. This hypoxia-induced transcriptional activation may be one of the important mechanisms mediating increased expression of HO-1 in the renal medulla.