Apoptosis and the cell cycle in Xenopus: PMA and MPMA exposure of splenocytes

Spontaneous and induced cancers are rare in non-isogeneic or inbred amphibi ans. Neoplastic cells become immortalized through loss of a normal capacity to die by apoptosis. Mature lymphocytes of mammals require activation and entry into the cell cycle in order to become susceptible to apoptosis. Whet her Xenopus lymphocytes differ from mammalian lymphocytes in this regard is examined. In vitro exposure of PMA, or its analogue, MPMA, to adult spleno cytes of Xenopus laevis was used to affect apoptosis. Flow cytometric analy sis of FITC-Annexin V/propidium iodide (PI) fluorescence (apoptosis) and Br dU uptake (DNA synthesis) were assayed concurrently in the same lymphocyte population over time. Significant increases in apoptotic levels were induce d throughout a 72 hour period in PMA-treated cells only. Lymphocytes were a lso separated by size for analysis. Several sub-populations of lymphocytes were identified, the most interesting of which was small and apoptotic with in 4 hours, after PMA exposure. PMA-induced DNA synthesis did not become el evated until after 24 hours. "Direct" apoptosis, i.e. without cell cycle en try, was found only in these small, mature lymphocytes. Since small lymphoc ytes make up the vast majority of those being analyzed, "direct" apoptosis may be a determining mechanism in the resistance to neoplasia observed in A mphibia. Cells that die more readily are less likely to transform into neop lastic cells.