Constitutive expression of human Bcl-2 modulates nitrogen mustard and camptothecin induced apoptosis

Bcl-2 is a novel protooncogene which prolongs cell survival and suppresses apoptosis. We examined whether constitutive expression of transfected human bcl-2 conferred resistance to two different DNA damaging drugs, nitrogen mustard (HN2) and camptothecin (CPT) in a murine, IL-3 dependent cell line (FL5.12). HN2 treatment produced 2-fold less cell death and DNA degradation in cells overexpressing bcl-2 relative to control cells transfected with a construct bearing only the neoR gene. DNA degradation was characterized by oligonucleosomal length fragments indicating that programmed cell death or apoptosis had occurred. Equimolar HN2 produced similar extents of interstrand cross-link formation and repair in each cell line. Cell cycle characteristics were similar for both cell lines following equimolar HN2 treatment, exhibiting a brief S phase delay followed by a longer G2 arrest. Time course studies indicated that DNA fragmentation occurred following peak G2 arrest in control cells and 12 h later in bcl-2 transfected cells. Equimolar CPT exposure also induced 2-fold less death and apoptotic DNA fragmentation in bcl-2 transfected compared to control cells. DNA single strand break formation and resealing kinetics were comparable in both cell lines following equimolar CPT treatment. CPT caused similar cell cycle perturbations in both cell lines, with a brief S phase block detectable 12 h after an equimolar drug dose. Kinetic studies showed apoptosis occurred following maximal S phase arrest in control and 12 h later in bcl-2 transfected cells. By contrast, IL-3 withdrawal produced rapid and extensive DNA degradation and apoptosis in controls 24 h postwithdrawal, and this process was inhibited 3-4-fold in bcl-2 transfectants. Cell cycle analysis showed both cell lines arrested in G0/G1 following IL-3 removal. In summary, bcl-2 transfection affords a 2-fold protection from HN2 and CPT cytotoxicity and decreases drug induced apoptosis in FL5.12 cells, despite the different mechanisms of action and cell cycle effects of each agent. Bcl-2 overexpression appears to represent a novel drug resistance mechanism of potential clinical significance.