Differential activation of MAPK and ICE/Ced-3 protease in chemical-inducedapoptosis - The role of oxidative stress in the regulation of mitogen-activated protein kinases (MAPKs) leading to gene expression and survival or activation of caspases leading to apoptosis

Chemical-induced oxidative stress to a cell can signal many cellular respon ses which include proliferation, differentiation, hemeostasis, apoptosis or necrosis. To better understand the underlying molecular mechanisms after e xposure to chemicals, we investigated the signal transduction pathways, in particular the mitogen-activated protein kinase (MAPK) pathway and the ICE/ Ced-3 protease (caspase) pathway, activated by different agents. Butylated hydroxyanisol (BHA) and its metabolite, t-butyl-hydroquinone (tBHQ), both a re well known phenolic antioxidants used in food preservatives, strongly ac tivated c-Jun N-terminal kinase 1 (JNK1) and/or extracellular signal-regula ted protein kinase 2 (ERK2) in a dose- and time-dependent fashion. Pretreat ment with free radical scavengers N-acetyl-L-cysteine (NAC), glutathione (G SH), or vitamin E, inhibited ERK2 activation and, to a much lesser extent, JNK1 activation by BHA and tBHQ, implicating the role of oxidative stress. Under conditions where JNK1 and ERK2 were activated, BHA also activated tra nscription factors nuclear factor kappa B (NF-kappa B), activated-protein-1 (AP-1), and anti-oxidant response element (ARE), leading to induction of g enes such as c-jun, and c-fos. At relatively high concentrations, BHA and t BHQ stimulated proteolytic activity of ICE/Ced3 cysteine proteases, and cau sed apoptosis, which was blocked by pretreatment with NAG. Further increase in concentrations lead to rapid cell death predominantly occurred via necr osis. Some naturally occurring phytochemicals, such as phenylethyl isothioc yanate (PEITC), green tea polyphenols (GTP), and sulfarophane, which have b een shown to be potent inducers of Phase II enzymes, also differentially re gulated the activities of JNK, ERK, or CPP-32, in a time- and dose-dependen t manner. Our data, together with the work of others, enable us to propose a model in which low concentrations of these chemicals (e.g., BHA, PEITC) a ctivate MAPKs leading to induction of gene expression (e.g., c-jun, c-fos, GST) which may protect the cells against toxic insults and enhance cell sur vival. At relatively high concentrations, these agents activated both MAPKs , and the ICE/Ced-3 caspase pathway, leading to apoptosis. The exact mechan isms by which MAPK and caspases are activated by these agents are currently unknown, but may involve oxidative modification of glutathione (GSH) and/o r protein thiols, and/or generation of secondary messengers, ceramide and c alcium, which further activate downstream events. Taken together, our resul ts suggest that chemicals including phenolic antioxidants activate MAPK pat hways which may lead to the induction of genes producing protection and sur vival mechanisms, as well as the ICE/Ced-3 protease pathway, leading to apo ptosis. The balancing amongst these pathways may dictate the fate of the ce lls upon exposure to chemicals.