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

T cell receptor (TCR) ligation and protein kinase C (PKC) activation stimul ate proliferation and modulate apoptosis in both mammalian and amphibian ly mphocytes. The potential relationship between apoptosis and the cell cycle in mature Xenopus laevis splenic lymphocytes is addressed by monitoring apo ptosis and DNA synthesis over rime, using incorporation of propidium iodide (PI) and flow cytometry. Aliquots of the same populations of cells are fol lowed after exposure in vitro to phytohemagglutinin (PHA) or phorbol 12-myr istate 13-acetate (PMA). Significant increases in apoptosis preceed those i n DNA synthesis by 12 to 16 h following exposure to both reagents. Since ap optosis preceeds DNA synthesis, these dying cells clearly do not need to en ter the S phase of the cell cycle before becoming apoptotic, in contrast to mammalian T cells. Another striking difference is that the reagent with we aker mitogenic properties in this species, PHA, is significantly a more pot ent apoptogen, than the strong mitogen, PMA. The two phenomena then appear to be inversely related in Xenopus cells. Data on DNA synthesis suggest ind ependence of the two phenomena, as DNA synthesis is stimulated in direct pr oportion to the strength of each reagent as a mitogen. Mature mammalian T-c ells undergo apoptosis only when previously activated. The Xenopus lymphocy tes examined were not deliberately activated by exposure to antigen or lect in. PMA, a cancer promoter in mammals, usually rescues' mammalian cells fro m apoptosis, but stimulates apoptotic increases in Xenopus cells, Thus, mat ure Xenopus lymphocytes may be more readily stimulated to die by cancer ind ucing agents than mammalian lymphocytes. This could make them less suscepti ble to transformation into immortalized cancer cells. This characteristic m ay considerably contribute to the observed resistance to spontaneous and ch emically-induced neoplasia in wild type, non-isogeneic or non-inbred Xenopu s. (C) 1999 Elsevier Science B.V. All rights reserved.