tBHQ-Induced HO-1 Expression Is Mediated by Calcium through Regulation of Nrf2 Binding to Enhancer and Polymerase II to Promoter Region of HO-1

Induction of Nrf2-mediated detoxifying/antioxidant enzymes is an effective strategy for cancer chemoprevention. The goal of this study was to examine the role of calcium [Ca2+] in regulating a well-known phenolic chemopreventive compound tertiary-butylhydroquinone (tBHQ) activation of Nrf2 and induction of Nrf2 downstream target gene heme-oxygenase (HO-1). tBHQalone caused Nrf2 nuclear localization and induced HO-1 mRNA and protein expression in a dose-dependent manner. Using RT-PCR and Western blotting, we showed that tBHQ-induced transcription of HO-1 is Ca2+ dependent. Chelation of [Ca2+](ext) or [Ca2+](intra) by EGTA or BAPTA attenuated tBHQinduced HO-1. Cotreatment of tBHQ with inhibitors of [Ca2+]-sensitive protein kinase C and camodulin kinase did not attenuate HO-1 induction. Nuclear translocation of Nrf2 induced by tBHQ was also not affected by treatment of EGTA or BAPTA. Additionally, EGTA and BAPTA treatments decreased basal nuclear phosphorylation of CREB and decreased tBHQinduced Nrf2-CBP binding and Nrf2 binding to enhancer as well as polymerase II binding to the promoter of HO-1 gene. Furthermore, tBHQ in combination with higher [Ca2+], augmented HO-1 induction both in vitro and in vivo, indicating that the modulation of [Ca2+](int), could be used as an adjuvant to increase the efficacy of chemopreventive agents. Taken together, our results indicated that in addition to tBHQinduced oxidative stress-mediated Nrf2 translocation, HO-I induction by tBHQ also appears to be dependent on a series of Ca2+ -regulated mechanisms.