The proto-oncogene Bcl-2 inhibits cellular toxicity of dopamine: possible implications for Parkinson’s Disease Apoptosis Vol 2 No 2 1997 I. Ziv, D. Offen, R. Haviv, R. Stein, H. Panet, R. Zilkha-Falb, A. Shirvan, A. Barzilai and E. Melamed Department of Neurology and the Felsenstein Medical Research Center, Beilinson Medical Center, Petah-Tiqva, The Sackler School of Medicine (I. Ziv, D. Offen, H. Panet, A. Shirvan, E. Melamed) and the Department of Neurobiochemistry, George Wise Faculty of Life Sciences, (R. Haviv, R. Stein, R. Zilkha-Falb, A. Barzilai), Tel-Aviv University, Tel-Aviv, Israel It is currently believed that excessive oxidant stress induced by metabolism of dopamine (DA), plays a major role in the pathogenesis of the selective nigrostriatal neuronal loss in Parkinson’s disease. We recently showed that the neurotransmitter DA, in physiological concentrations, is capable of initiating apoptosis in cultured, post-mitotic sympathetic neurons. Bcl-2 is a proto-oncogene that blocks apoptosis. We now report that Bcl-2 is a powerful inhibitor of DA toxicity in PC-12 pheochromocytoma cells. We induced stable expres- sion of Bcl-2 in PC-12 cells by transfection with recombinant pCMV5 expression vector, containing mouse bcl-2 (coding-sequence) cDNA. Cells express- ing Bcl-2 manifested marked resistance to otherwise lethal (300 uM) in vitro concentrations of DA. This pro- tective effect was reflected in the trypan-blue test of cell survival, 3 H-thymidine incorporation and inhibition of the characteristic apoptotic morphologic alterations in scanning electron microscopic studies. Bcl-2 and associated control systems of apoptosis may have an important physiological role in restraining the apop- tosis-triggering potential of DA in nigrostriatal neurons. This novel field of research may yield insights into the pathogenesis of Parkinson’s disease and lead to development of novel therapeutic approaches. Key words: Apoptosis; bcl-2; dopamine; Parkinson’s disease; PC-12; proto-oncogene. (Received 25 February 1997; accepted 4 March 1997) Introduction The cause for the progressive and rather selective degen- eration of substantia nigra pars compacta dopaminergic (DA) neurons in Parkinson’s disease (PD) is still enigmatic. A major current hypothesis (though not unequivocally proven) suggests that nigral neuronal death in PD is due to excessive oxidant stress generated by auto-and enzymatic oxidation of DA, formation of neuro- melanin and presence of high concentrations of iron. 1 We recently showed that DA, the endogenous neuro- transmitter of nigrostriatal neurons, can trigger apoptosis, a unique, active, genetically-controlled mode of cell self-destruction, in cultured, post-mitotic sympa- thetic neurons. 2 DA, in physiological concentrations initiated the highly characteristic apoptotic cascade of cell shrinkage, thinning and disruption of the neuritic network, extensive membrane blebbing and nuclear frag- mentation. In PD, nigral histopathology is characterized by a slow, protracted degeneration of individual neurons 3 that may indicate a process of apoptosis rather than a necrotic mode of cell death. Based on these observations, we hypothesized that nigrostriatal neuronal degeneration in PD may be due to an active process of apoptosis, triggered, at least in part, by genotoxicity of DA 4 or the reactive free radical species generated via its intra- or extra-neuronal oxidation. For instance, this may result from a failure of the neuronal control systems that normally restrain the apoptosis-triggering potential of their own neurotransmitter DA. Growth factors and several proto-oncogenes (e.g. , bcl-2) are major, recently- discovered systems that act at the level of the trigger of apoptosis. Bcl-2 (acronym for the B-cell lymphoma/leu- kemia-2 gene) is a proto-oncogene that was first discovered in association with B-cell malignancies, where its overexpression, induced by 18–14 chromosomal Supported, in part, by Teva Pharmaceutical Industries, Ltd., the National Parkinson Foundation, Miami, Florida, USA and the National Institute of Psychobiology, Israel. Correspondence to I. Ziv, Department of Neurology, Beilinson Medical Center, 49100, Petah-Tiqva, Israel. Tel: 972-39376358; Fax: 972-39223352. © 1997 Rapid Science Publishers Apoptosis 1997; 2: 149–155 Apoptosis . Vol 2 . No 2 . 1997 149