THE CELL-CYCLE RELATED DIFFERENCES IN SUSCEPTIBILITY OF HL-60-CELLS TO APOPTOSIS INDUCED BY VARIOUS ANTITUMOR AGENTS

The studies were aimed to detect the cell cycle-associated differences in the susceptibility of HL-60 cells to apoptosis induced by diverse agents. Exponentially growing HL-60 cells were treated with the DNA to poisomerase I inhibitor camptothecin; the DNA topoisomerase II inhibit ors teniposide, m-AMSA, Mitoxantrone, or Fostriecin; the presumed tyro sine kinase inhibitor genistein; a serine/threonine kinase inhibitor H 7; the protein synthesis inhibitor cycloheximide; the DNA replication inhibitor hydroxyurea; the nucleoside antimetabolites 1-beta-D-arabino furanosylcytosine and 5-azacytidine; and the alkylating agent nitrogen mustard, cisplatin, hyperthermia, and gamma irradiation. Endonucleoly sis, which accompanied apoptosis induced by these agents, was assessed by two different flow cytometric methods, one based on DNA content me asurements following extraction of low molecular weight DNA, and anoth er using exogenous terminal deoxynucleotidyl transferase to label in s itu DNA strand breaks. Each method allowed for both identification of apoptotic cells and analysis of the cell cycle distribution of the una ffected cell population; the method using terminal transferase also al lowed for identification of the cell cycle position of apoptotic cells . Confirmed by analysis of DNA degradation by gel electrophoresis and changes in cell morphology, apoptosis was observed as early as 3 h aft er administration of most drugs and for some drugs was cell cycle phas e specific. Cells progressing through S phase were selectively suscept ible when treated with camptothecin, teniposide, m-AMSA, Mitoxantrone, H7, hydroxyurea, and 1-beta-D-arabino-furanosylcytosine. Cells in G2- M Preferentially underwent apoptosis in cultures treated with H7 or wi th gamma-irradiation. Cells in G1 phase were preferentially affected b y 5-azacytidine, nitrogen mustard, and hyperthermia. No significant ce ll cycle specificity was observed in the case of Fostriecin, genistein , cycloheximide, or cisplatin. The cell cycle related difference in su sceptibility to apoptosis may be a reflection of both the severity of the lesion induced by a given drug and the ability of the cells to rep air that lesion; both can vary depending on the cell cycle phase.