Involvement of alpha-PAK-interacting exchange factor in the PAK1-c-Jun NH2-terminal kinase 1 activation and apoptosis induced by benzo[a]pyrene

Benzo[a]pyrene [B(a)P], a potent procarcinogen found in combustion products such as diesel exhaust and cigarette smoke, has been recently shown to act ivate the c-jun NH2-terminal kinase 1 (JNK1) and induce caspase-3-mediated apoptosis in Hepa1cIc7 cells. However, the molecules of the signaling pathw ay that control the mitogen-activated protein kinase cascades induced by B( a)P and the interaction between those and apoptosis by B(a)P have not been well defined. We report here that B(a)P promoted Cdc42/Rac1, p21-activated kinase 1 (PAK1), and JNK1 activities in 293T and HeLa cells. Moreover, alph a-PAK-interacting exchange factor (alpha PIX) mRNA and its protein expressi on were upregulated by B(a)P. While overexpression of an active mutant of a lpha PIX (Delta CH) facilitated B(a)P-induced activation of Cdc42/Rac1, PAK 1, and JNK1, overexpression of mutated alpha PIX (L383R, L384S), which lack s guanine nucleotide exchange factor activity, SH3 domain deleted alpha PIX (Delta SH3), which lacks the ability to bind PAK, kinase-negative PAK1 (K2 99R), and kinase negative SEK1 (K220A, K224L) inhibited B(a)P-triggered JNK 1 activation. Interestingly, over expression of alpha PIX (Delta CH) and a catalytically active mutant PAK1 (T423E) accelerated B(a)P-induced apoptosi s in HeLa cells, whereas alpha PIX (Delta SH3), PAK1 (K299R), and SEK 1 (K2 20A, K224L) inhibited B(a)P-initiated apoptosis. Finally, a preferential ca spase inhibitor, Z-Asp-CH2-DCB, strongly blocked the alpha PIX (Delta CH)-e nhanced apoptosis in cells treated with B(a)P but did not block PAK1/JNK1 a ctivation. Taken together, these results indicate that alpha PIX plays a cr ucial role in B(a)P-induced apoptosis through activation of the JNK1 pathwa y kinases.