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 stimulate proliferation and modulate apoptosis in both mammalian and amphibian lymphocytes. The potential relationship between apoptosis and the cell cycle in mature Xenopus laevis splenic lymphocytes is addressed by monitoring apoptosis and DNA synthesis over time, using incorporation of propidium iodide (PI) and flow cytometry. Aliquots of the same populations of cells are followed after exposure in vitro to phytohemagglutinin (PHA) or phorbol 12-myristate 13-acetate (PMA). Significant increases in apoptosis preceed those in DNA synthesis by 12 to 16 h following exposure to both reagents. Since apoptosis preceeds DNA synthesis, these dying cells clearly do not need to enter the S phase of the cell cycle before becoming apoptotic, in contrast to mammalian T cells. Another striking difference is that the reagent with weaker mitogenic properties in this species, PHA, is significantly a more potent apoptogen, than the strong mitogen, PMA. The two phenomena then appear to be inversely related in Xenopus cells. Data on DNA synthesis suggest independence of the two phenomena, as DNA synthesis is stimulated in direct proportion to the strength of each reagent as a mitogen. Mature mammalian T-cells undergo apoptosis only when previously activated. The Xenopus lymphocytes examined were not deliberately activated by exposure to antigen or lectin. PMA, a cancer promoter in mammals, usually 'rescues' mammalian cells from apoptosis, but stimulates apoptotic increases in Xenopus cells. Thus, mature Xenopus lymphocytes may be more readily stimulated to die by cancer inducing agents than mammalian lymphocytes. This could make them less susceptible to transformation into immortalized cancer cells. This characteristic may considerably contribute to the observed resistance to spontaneous and chemically-induced neoplasia in wild type, non-isogeneic or non-inbred Xenopus.