Chronic exposure to bryostatin-1 increases the radiosensitivity of U937 leukaemia cells ectopically expressing Bcl-2 through a non-apoptotic mechanism

Purpose: Ionizing radiation (IR) produced a dose-dependent increase in apoptosis in U937/pCEP4 cells which was attenuated by the stable over expression of Bcl-2 (U937/Bcl-2). A dose of 2 Gy IR was selected for further analyses to determine if subsequent exposure to 10 nM bryostatin-1 would overcome the resistance to IR-induced apoptosis conferred by Bcl-2 over expression. Methods and results: Although bryostatin-l did not increase IR-induced apoptosis in U937/pCEP4 or U937/Bcl-2 cells, it impaired mitochondrial function and increased the antiproliferative effects of IR in both cell lines. The effects were more pronounced in U937/Bcl-2 cells. Bryostatin-l also exerted differential effects on cell-cycle distributions of U937 transfectant cells, producing a significant G(0)/G(1) arrest in U937/Bcl-2 cells, while decreasing IR-induced G(2)/M arrest in U937/pCEP4 cells. Although Bcl-2 over expression attenuated IR-induced apoptosis, clonogenic survival was similar in U937/pCEP4 and U937/Bcl-2 cells following 2 Gy IR treatment. Treatment with 10 nM bryostatin-l after 2 Gy IR further reduced clonogenic survival in both cell lines. Moreover, U937/Bcl-2 cells were more susceptible to the growth-inhibitory effects of IR/bryostatin-1 than U937/pCEP4 cells. Conclusions: Bryostalin-1 increased the radiosensitivity of U937 transfectant cell lines without enhancing apoptosis; furthermore, U937/Bcl-2 cells were more susceptible to IR/bryostatin-1-mediated antiproliferative effects than their empty-vector counterparts.