Apoptosis by antitumor agents and other factors in relation to cell cycle checkpoints

More than a dozen cancer patients died in 1993 after treatment with antineoplastic derivatives of 5-fluorouracil and the antiherpes drug Sorivudine. This paper gives a short review of previous reports showing that killing of cells by 5-fluorouracil and other antitumor agents, including radiation at high doses, results from activation of apoptosis in the G2 phase. On the other hand, apoptosis of lymphocytes by radiation at low doses and treatment with other agents is known to occur in the G1 phase. The cells dying in the G1 or G2 phase could share the same final self-killing steps. For these common steps, I assume a mitotic catastrophe model, in which commitment to self-killing results from premature activation of the mitosis machinery, and propose a concept of a 'G1/G2 death circuit' for cells dying in the G1/G2 phase by short circuit to the M phase. Based on this model, reported modes of cell death, spontaneously occurring or after treatment with various agents, are classified by the phase of dying cells. The associations of incomplete apoptosis with production of chromosomal aberrations and prevention of tumorigenesis by complete apoptosis of carcinogen-treated cells are discussed. A presumptive rule for differentiation of G1 apoptosis and G2 apoptosis is proposed.