Effects of azadirachtin on insect and mammalian cultured cells
The aim of this project was to try to find the mode of action of azadirachtin in cultured cells, and to compare its cytotoxicity with some well-known phytochemical pesticides. The results presented here showed that azadirachtin was toxic to the cultured insect cells used (Sf9 and C6/36, derived respectively from Spodoptera frugiperda and Aedes albopictus) even in very low concentrations with an EC50 for the Spodoptera cells estimated at 5x10-9 M, but that the mammalian fibroblast cell line L929 was little affected except at concentrations greater than 10-4M. The other major neem terpenoids, nimbin and salannin, showed low toxicity towards the cultured cells. The neurotoxic pyrethrum showed little effect against the cultures, except for some slight stimulatory effect on growth at 10-8 M. Rotenone, known to inhibit the electron transport chain, effectively inhibited the growth of both insect and mammalian cells. Nicotine, another neurotoxic phytochemical, had little effect on the growth of the cultured cells. It was concluded that while cell growth assessment is not appropriate for all phytochemical pesticides, it is useful for those, such as azadirachtin and rotenone, whose effect is on the essential mechanisms of insect cells in general. Rotenone was used as a positive control to investigate if azadirachtin had its effect on respiration of the cells. Only at the highest feasible concentration of azadirachtin, was there a slight but significant (15%) reduction of respiration which was the same in both insect and mammalian cells. As expected, rotenone inhibited both insect and mammalian cells even at concentrations as low as 10-11 M. When the effects of azadirachtin on the cell cycle were examined by means of cell cytometry, it was shown that the compound arrested the cell-cycle in G2/M phase, and that the effect was related to the concentration. Microscopy confirmed that there was a three-fold increase in the mitotic index after 2 hours of exposure of 2x10-6 M azadirachtin. The similarity of the of the nuclear profiles and cell-cycle distribution to Sf9 cells treated with colchicine, a well-known antimitotic phytochemical, suggested there was a similarity of action between the two compounds.