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J. Biol. Chem., Vol. 281, Issue 30, 21256-21265, July 28, 2006

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JNK- and p38 Kinase-mediated Phosphorylation of Bax Leads to Its Activation and Mitochondrial Translocation and to Apoptosis of Human Hepatoma HepG2 Cells^*

Bong-Jo Kim, Seung-Wook Ryu, and Byoung-Joon Song¹

From the Laboratory of Membrane Biochemistry and Biophysics, National Institute on Alcohol Abuse and Alcoholism, and the Biochemistry Section, Surgical Neurological Branch, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20892-9410

Mitochondrial translocation of pro-apoptotic Bax prior to apoptosis is well established after treatment with many cell death stimulants or under apoptosis-inducing conditions. The mechanism of mitochondrial translocation of Bax is, however, still unknown. The aim of this work was to investigate the mechanism of Bax activation and mitochondrial translocation to initiate apoptosis of human hepatoma HepG2 and porcine kidney LLC-PK1 cells exposed to various cell death agonists. Phosphorylation of Bax by JNK and p38 kinase activated after treatment with staurosporine, H₂O₂, etoposide, and UV light was demonstrated by the shift in the pI value of Bax on two-dimensional gels and confirmed by metabolic labeling with inorganic [³²P]phosphate in HepG2 cells. Specific inhibitors of JNK and p38 kinase significantly inhibited Bax phosphorylation and mitochondrial translocation and apoptosis of HepG2 cells. A specific small interfering RNA to MAPKK4 (the upstream protein kinase of JNK and p38 kinase) markedly decreased the levels of MAPKK4 and MAPKK3/6, blocked the activation of JNK or p38 kinase, and inhibited Bax phosphorylation. However, the negative control small interfering RNA did not cause these changes. Confocal microscopy of various Bax mutants showed differential rates of mitochondrial translocation of Bax before and after staurosporine treatment. Among the Bax mutants, T167D did not translocate to mitochondria after staurosporine exposure, suggesting that Thr¹⁶⁷ is a potential phosphorylation site. In conclusion, our results demonstrate, for the first time, that Bax is phosphorylated by stress-activated JNK and/or p38 kinase and that phosphorylation of Bax leads to mitochondrial translocation prior to apoptosis.

Received for publication, September 29, 2005 , and in revised form, April 25, 2006.

^* This work was supported by the Intramural Research Program of the National Institute on Alcohol Abuse and Alcoholism and the Gift Fund of SK Chemicals (Korea). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed: Laboratory of Membrane Biochemistry and Biophysics, National Institute on Alcohol Abuse and Alcoholism, 9000 Rockville Pike, Bethesda, MD 20892-9410. Tel.: 301-496-3985; Fax: 301-594-3113; E-mail: bjs{at}mail.nih.gov.

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