Effects of methomyl on cell cycle and apoptosis in leukocytic cells

Abstract

Methomyl is a methyl carbamate insecticide widely used in Thailand and many agricultural countries for crop protection protection. Effects of methomyl on modifying apoptosis in plant has been reported to induce a 0.8-1.5 nm sized-mitochondrial pore generation and cell death in male sterile maize expressing hihg levels of URF13 protein in its inner mitochondrial membrane (Rhoads et al., 1994; Chaumont et al., 1995). A study of Klotz et al. (1997) suggested methomyl as an endocrine modulator. In addition, its cardiotoxicity from the developed depressor responses may not be related to the anti-acetylcholinesterase action (Saiyed et al., 1992; Futagawa et al., 200). Lohitnavy and Sinhaseni (1998) suggested that methomyl could generate oxidative stress, as shown by the blocking effect of N-acetylcysteine on methomyl-induced cell death in spleen cells. This oxidative damage induced by methomyl is in agreement with a report from IPCS (1996), which showed that a metabolite of methomyl in the biological systems could generate oxidative stress from a glutathione depletion. In this study, rats orally exposed to single dose of methomyl at 8 mg/kg body weight for 6 hours showed lymphocyte apoptosis. The apoptosis induced by methomyl was also shown with different sensitivities in leukocytic cell lines. In these cell lines, methomyl reduced mitochondrial transmembrane potential ( ) and induced apoptosis at concentrations of 18 mM (MM6), 12 mM (THP-1), and 12 mM (Jurkat) after 6 hour-exposure, and at concentrations of 12 mM (MM6), 6 mM (THP-1), and 6 mM(Jurkat) after 24 hour-exposure. Methomyl was not shown to induce nucher apoptosis in Raji cells since DNA fragmentation was not detected, instead, the Raji cells showed a cell cycle arrest in the G1/G1 phase The apoptosis occurred could be blocked by zVAD-fmk, which indicated a caspase dependent process. In addition, IL-6 could partly prevent methomyl-induced apoptosis in these cell lines, suggesting that the apoptosis was partially related to either the reduction of IL-6 or the interference in IL-6-related signaling pathways. Thansmission electron microscopic sections of spleens collected from rats after the 24 hour-oral exposure to single dose of methomyl at 8 mg/kg illustrated the mitochondrial swelling and degenerative changes with cristae loss. Cell death was shown under light microscope at the sme dose of methomyl. Energy metabolism related to mitochondrial function was investieated. The results showed that the increasing levels of 2,3-DPG in blood and the increase in red blood cell NADH-DCIP reductase activity were affected and exhibited in dose-response relationship It is concluded that methomyl affects mitochondrial function via the interference with energy metabolism. It can induce apoptosis at hihg dose treatments both in vitro and in vivo. In addtion, its effects on spleen show mitochondrial degeneration and cell death. Mechanisms of these effects are postulated as a possible relationship to the reduction of IL-6 or the interference with IL-6-related signaling pathways. These changes may suggest the further investigation into possible modulatory effects of methomyl on IL-6 related signals in the immune response.