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

The present study was designed to test the hypothesis that hypoxia-inducibl e factor-1 alpha (HIF-1 alpha)-mediated transcriptional activation contribu tes to increased expression of heme oxygenase (HO) genes in renal medullary interstitial cells (RMICs). By Northern blot analysis, HO-1 mRNA expressio n was found to significantly increase in response to reduction of PO2 in cu lture medium. However, HO-2 mRNA was not altered by hypoxia. This hypoxia-i nduced upregulation of HO-1 mRNA was significantly blocked by HIF-1 alpha i nhibition with ferrous ammonium sulfate. To further determine the role of H IF-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 expressio n. Both desferrioxamine and CoCl2 markedly increased HIF-1 alpha mRNA and p rotein levels and resulted in the upregulation of HO-1 mRNA but not HO-2. F urthermore, 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 bindi ng, increased mRNA expression of HO-1 in response to hypoxia and CoCl2 was attenuated. In vitro nuclear run-on assays further confirmed that hypoxia a nd 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 alp ha -mediated mechanism in RMICs. This hypoxia-induced transcriptional activ ation may be one of the important mechanisms mediating increased expression of HO-1 in the renal medulla.