Omeprazole induces heme oxygenase-1 in fetal human pulmonary microvascular endothelial cells via hydrogen peroxide-independent Nrf2 signaling pathway

Omeprazole (OM) is an aryl hydrocarbon receptor (AhR) agonist and a proton pump inhibitor that is used to treat humans with gastric acid related disorders. Recently, we showed that OM induces NAD (P) H quinone oxidoreductase-1 (NQO1) via nuclear factor erythroid 2-related factor 2 (Nrf2)-dependent mechanism. Heme oxygenase-1 (HO-1) is another cytoprotective and antioxidant enzyme that is regulated by Nrf2. Whether OM induces HO-1 in fetal human pulmonary microvascular endothelial cells (HPMEC) is unknown. Therefore, we tested the hypothesis that OM will induce HO-1 expression via Nrf2 in HPMEC. OM induced HO-1 mRNA and protein expression in a dose-dependent manner. siRNA-mediated knockdown of AhR failed to abrogate, whereas knockdown of Nrf2 abrogated HO-1 induction by OM. To identify the underlying molecular mechanisms, we determined the effects of OM on cellular hydrogen peroxide (H₂O₂) levels since oxidative stress mediated by the latter is known to activate Nrf2. Interestingly, the concentration at which OM induced HO-1 also increased H₂O₂ levels. Furthermore, H₂O₂ independently augmented HO-1 expression. Although N-acetyl cysteine (NAC) significantly decreased H₂O₂ levels in OM-treated cells, we observed that OM further increased HO-1 mRNA and protein expression in NAC-pretreated compared to vehicle-pretreated cells, suggesting that OM induces HO-1 via H₂O₂-independent mechanisms. In conclusion, we provide evidence that OM transcriptionally induces HO-1 via AhR - and H₂O₂ - independent, but Nrf2 - dependent mechanisms. These results have important implications for human disorders where Nrf2 and HO-1 play a beneficial role.