PGA₂-induced expression of HO-1 is mediated by transcriptional upregulation of Nrf2

Backgrounds: Prostaglandin (PG) A₂ reportedly stimulated expression of heme oxygenase(HO)-1 at the level of transcription via the activation of p38MAPK. Details of the mechanism, however, have not been provided, and this includes identification of the transcription factors responsible for PGA₂-induced HO-1 expression. Herein is described an analysis of the role of nuclear factor erythroid 2 related factor 2 (Nrf2) and how PGA₂ increases the activity of Nrf2 during PGA₂-induced HO-1 expression. Methods: Expressions of HO-1 and Nrf2 were analyzed at the levels of both mRNA and protein. Nrf2 siRNA, SB203580, an inhibitor of p38MAPK, and scavengers of reactive oxygen species (ROS) were used to identify the effects of Nrf2, p38MAPK and ROS on PGA₂-induced HO-1 expression. Results: Although SB203580 suppressed PGA₂-induced HO-1 expression, genetic activation of p38MAPK could not stimulate the transcription of HO-1. Cycloheximide (CHX), an inhibitor of protein translation, almost completely prevented PGA₂-induced increase of HO-1 transcription, but it did not prevent the phosphorylation of p38MAPK, which suggests that both de novo protein synthesis and p38MAPK activity are required to induce the transcription of HO-1 in response to PGA₂ treatment. In addition, PGA₂ increased the level of both Nrf2 mRNA and protein in a dose-dependent manner. Knockdown of Nrf2 using small interfering RNA (siRNA) suppressed PGA₂-induced HO-1 expression. The PGA₂-induced transcription of Nrf2 was prevented by ROS scavengers such as n-acetyl-l-cysteine and tempol but not CHX. Furthermore, siRNA against p38MAPK did not change the level of nuclear Nrf2 protein. Conclusion: These findings suggest that PGA₂ induces HO-1 transcription via an increase in Nrf2 protein, the transcription of which is initiated by an accumulation of ROS that is independent of the p38MAPK activation pathway.