Apoptosis induced by inhibitors of nucleotide synthesis in deoxyadenosine-resistant leukemia L1210 cells that lack...

An L1210 cell line (Y8) selected for resistance to deoxyadenosine contains ribonucleotide reductase that is not subject to inhibition by dATP. In addition, the Y8 cells have other phenotypic expressions that include increased sensitivity to apoptosis induced by various agents such as radiation, doxorubicin, anisomycin and roscovitine. The Y8 cells were found to be more sensitive to apoptosis induced by methotrexate (MTX), tiazofurin (TZ), deoxyguanosine (dGuo) and N-(phosphonoacetyl)-L-aspartate (PALA). Deoxyguanosine, at concentrations that did not cause apoptosis in the Y8 cells, prevented the apoptotic response of the Y8 cells to MTX and TZ. Deoxycytidine had no effect. Since caspase-3 activation is involved in apoptotic pathways, the effects of the caspase-3 inhibitor, Ac-DEVD-CHO, were studied on the dGuo-, MTX- or TZ-induced apoptosis in the Y8 cells. Ac-DEVD-CHO caused a marked decrease in the fraction of cells in the early phase of apoptosis. However, there was a corresponding increase in the fraction of cells in the late apoptotic/necrotic stages of cell death. This is in marked contrast to the dGuo-induced decrease in apoptosis seen in the MTX- and TZ-treated Y8 cells in which there were no increases in the late apoptotic/necrotic fraction of cells. These data show that alterations of nucleotide pools in the Y8 cells cause marked increases in the apoptotic response which may indicate that the Y8 cells are much more susceptible to the effects of misincorporation of nucleotides into DNA than are the parental WT L1210 cells.