[Proliferation and death of thymocytes caused by phospholipase A2 activator--melittin]

The effects of mellitin, a component of bee venom activating phospholipase A2, on proliferation and death of the rat thymocytes were studied in a wide concentration range. Cell proliferation was estimated by the accumulation of colchicine metaphases, Necrosis was estimated by cell lysis and Trypan blue staining. Apoptosis was estimated by the type of DNA fragmentation, amount of fragmented DNA, and percentage of cells with hypodiploid DNA set. Low concentrations of mellitin (below 5 micrograms/ml) stimulated proliferation. At higher mellitin concentrations, the thymocytes die by the primary necrosis type. Mellitin did not induce apoptosis in the thymocytes within the concentration range used: on the contrary, at high concentrations, it inhibited apoptosis of the thymocytes in the control and after irradiation. Actinomycin D, inhibitor of RNA synthesis, exerted no effect on the thymocyte death in the presence of mellitin. It has been concluded that activation of phospholipase A2 may induce necrosis, rather than apoptosis, and consequently, activation of phospholipase A2 is not a necessary step in the signalling cascade that initiated apoptosis in the thymocytes.