Chlorpyrifos Induces Apoptosis in Rat Cortical Neurons that is Regulated by a Balance Between p38 and ERK/JNK MAP Kinases

Chlorpyrifos, an acetylcholinesterase (AChE) inhibitor, is a widely used organophosphate pesticide. Recent concern has fo-cused on its neurotoxicity that is not attributable to AChE inhi-bition. Here, we report that chlorpyrifos and chlorpyrifos-oxon, but not 3,5,6-trichloro-2-pyridinol (TCP; the breakdown product of chlorpyrifos and chlorpyrifos-oxon), induce apoptosis in pri-mary cortical neurons cultured from embryonic day 17 or newborn rats. It is generally agreed that chlorpyrifos-oxon is approximately three orders of magnitude more potent than chlorpyrifos in inhi-bition of brain acetylcholinesterase activity. However, our data demonstrate that chlorpyrifos-oxon is only slightly more potent than chlorpyrifos in inducing apoptosis. This indicates that chlor-pyrifos-induced apoptosis may occur independently of AChE in-hibition, although AChE activity was not measured in this study. Furthermore, chlorpyrifos activates the ERK1/2 and p38 MAP kinases. Surprisingly, blocking ERK1/2 activation by the MEK inhibitor SL327 caused a small but statistically significant inhibi-tion of apoptosis, while blocking p38 with SB202190 significantly accelerated apoptosis induced by chlorpyrifos. This suggests a pro-and anti-apoptotic role for ERK1/2 and p38, respectively. Al-though chlorpyrifos did not stimulate total JNK activity, it caused a sustained activation of a sub-pool of JNK in the nucleus and stimulated phosphorylation of c-Jun, a downstream target of JNK. Transient expression of a dominant negative c-Jun mutant inhibited chlorpyrifos-induced apoptosis, suggesting a role for JNK and JNK-mediated transcription in this cell death. Together, our data suggest apoptosis as a novel toxic endpoint of chlorpyr-ifos neurotoxicity in the brain that may be independent of AChE inhibition. Furthermore, activation of the ERK1/2 and JNK MAP kinases contributes to, while activation of the p38 MAP kinase counteracts chlorpyrifos-induced apoptosis in cortical neurons