Effect of malathion on apoptosis of murine L929 fibroblasts: a possible mechanism for toxicity in low dose exposure. L. Masoud a,1 , C. Vijayasarathy a , M. Fernandez-Cabezudo a , G. Petroianu b , A.M. Saleh a,  a Department of Biochemistry, Faculty of Medicine & Health Sciences, United Arab Emirates University, P.O. Box 17666, Al Ain, United Arab Emirates b Department of Pharmacology, Faculty of Medicine & Health Sciences, United Arab Emirates University, P.O. Box 17666, Al Ain, United Arab Emirates Received 5 July 2002; received in revised form 21 October 2002; accepted 21 October 2002 Abstract While acute organophosphorous compound poisoning due to inhibition of acetylcholinesterase is a well-established clinical entity, the existence of chronic poisoning due to exposure to low levels of organophosphorous compounds (below the threshold required for cholinergic clinical symptoms) is a hotly debated issue. In this study, we have evaluated the effects of noncholinergic doses of malathion (0.01  /20 mM) on apoptosis of murine L929 fibroblasts. Employing flow cytometric and caspase activation analyses we demonstrate that malathion induces apoptosis in L929 cells in a dose- and time-dependent manner. The initiator caspases (caspase-8 and caspase-9) as well as the effector caspase (caspase-3) were activated by the treatment of L929 cells with malathion. Exposure of L929 cells to malathion in the presence of a general inhibitor of caspase, z-VAD-FMK abolished the apoptotic effect of the compound. In addition, malathion induced an increase in the expression of the pro-apoptotic protein p53. However, the induction of p53 expression was subsequent to activation of the caspase cascades. The present findings suggest, that the cytotoxicity of malathion at noncholinergic doses is mediated through caspase-dependent apoptosis. # 2002 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Apoptosis; Caspases; p53; Organophosphorus compounds; Malathion Abbreviations: AChE, acetylcholinesterase; Caspase, cysteine specific aspartate protease; IC 50 , inhibitory concentration 50 (concentration of a chemical required to reduce the enzyme activity by 50%); OPCs, organophosphorus compounds; PARP, poly(ADP-ribose) polymerase; PI, propidium iodide; z-VAD-FMK, benzyloxy-valine-alanine-aspartate-O -methyl- fluoromethylketone.  Corresponding author. Fax:  /971-3-767-2033 E-mail address: asaleh@uaeu.ac.ae (A.M. Saleh). 1 This author and A.M. Saleh contributed equally to the work. Toxicology 185 (2003) 89  /102 www.elsevier.com/locate/toxicol 0300-483X/02/$ - see front matter # 2002 Elsevier Science Ireland Ltd. All rights reserved. PII:S0300-483X(02)00596-6