Role of a mitogen-activated protein kinase pathway in the induction of phase II detoxifying enzymes by chemicals

Mitogen-activated protein kinase (MAPK) cascades are activated by diverse e xtracellular signals and participate in the regulation of an array of cellu lar programs. In this study, we investigated the roles of MAPKs in the indu ction of phase II detoxifying enzymes by chemicals. Treatment of human hepa toma (HepG2) and murine hepatoma (Hepalclc7) cells with tert-butylhydroquin one (tBHQ) or sulforaphane (SUL), two potent phase II enzyme inducers, stim ulated the activity of extracellular signal-regulated protein kinase 2 (ERK 2) but not c-Jun N-terminal kinase 1. tBHQ and SUL also activated MAPK kina se. Inhibition of MAPK kinase with its inhibitor, PD98059, abolished ERK2 a ctivation and impaired the induction of quinone reductase, a phase II detox ifying enzyme, and antioxidant response element (ARE)-linked reporter gene by tBHQ and SUL. Overexpression of a dominant-negative mutant of ERK2 also attenuated tBHQ and SUL induction of ARE reporter gene activity. Interestin gly, although expression of Ras and its mutant forms showed distinct effect s on basal ARE reporter gene activity, they did not affect the activation o f reporter gene by the inducers. Furthermore, a dominant-negative mutant of Pas had little effect on ERK2 activation by tBHQ and SUL, implicating a Ra s-independent mechanism. Indeed, both tBHQ and SUL were able to stimulate R af-1 kinase activity in vivo as well as in vitro. Thus, our results indicat e that the induction of ARE-dependent phase II detoxifying enzymes is media ted by a MAPK pathway, which may involve direct activation of Raf-1 by the inducers.