Heme acts through the Bach1b/Nrf2a-MafK pathway to regulate exocrine peptidase precursor genes in porphyric zebrafish

Using a zebrafish model for hepatoerythropoietic porphyria (HEP), we found a new mechanism underlying heme regulation of exocrine zymogens. Zebrafish bach1b, nrf2a and mafK are all expressed in the zebrafish exocrine pancreas. Overexpression of bach1b and knockdown of nrf2a result in downregulation of the zymogens, whereas overexpression of nrf2a and knockdown of bach1b cause their upregulation. In vitro luciferase assays demonstrated that heme activates the zymogens in a dosage-dependent manner, and the zymogens promoter activities require the integral MARE (Maf Recognition Element) motif. The Bach1b:MafK heterodimer represses the zymogens, whilst the Nrf2a:MafK heterodimer activates them. Furthermore, chromatin immunoprecipitation (ChIP) assays showed that MafK binds the MARE sites in the 5'; regulatory regions of the zymogens, heme triggers replacing Bach1b with Nrf2a within the MafK-occupied MARE sites in the regulatory regions of the zymogens, and particularly in heme-deficient porphyria, the repressing Bach1b:MafK heterodimer dominates, which can be switched to the activating Nrf2a:MafK heterodimer by hemin treatment. These results provide novel insights into regulation of exocrine function as well as the pathogenesis of porphyria, and should be useful for designing therapies for diseases of both categories.