Nrf2, a Cap'n'Collar transcription factor, regulates induction of the hemeoxygenase-1 gene

Stress response elements, which mediate induction of the mouse heme oxygena se-l (HO-1) gene by several agents, resemble the binding site for the activ ator protein-1 (Jun/Fos), Maf, and Cap'n'Collar/basic leucine zipper (CNC-b ZIP) families of proteins. In L929 fibroblasts, significant activation of a n HO-1 enhancer-reporter fusion gene was observed only with the CNC-bZIP cl ass of proteins with Nrf2 exhibiting the highest level of transactivation, between 25- and 30-fold. To further examine the role of this factor in HO-1 gene regulation, a dominant-negative mutant, Nrf2M, was generated and cond itionally expressed in L929 cells. The mutant protein was detected in cytop lasmic and nuclear fractions but did not affect cell growth. Under conditio ns of Nrf2M overexpression, HO-1 mRNA accumulation in response to heme, cad mium, zinc, arsenite, and tert-butylhydroquinone was inhibited by 85-95%. I n contrast, overexpression of a dominant-negative mutant of c-Jun decreased L929 cell growth but did not inhibit HO-1 gene activation. Nrf2 does not h omodimerize, but CNC-bZIP small Maf protein heterodimers and Nrf2 Jun prote in complexes are proposed to function as trans-activators. Coexpression of Jun proteins or p18, however, had no significant affect or inhibited Nrf2-m ediated trans-activation. Taken together, these results implicate Nrf2 in t he induction of the HO-1 gene but suggest that the Nrf2 partner in this fun ction is a factor other than p18 or Jun proteins.