Differential effects of BCL-2 on survival and proliferation of human B-lymphoma cells following γ-irradiation

Bcl-2 can inhibit apoptosis induced by a variety of stimuli, including radiation and its presence in tumour cells would be expected to indicate poor prognosis. Bcl-2-expressing tumours, however, are often low-grade and highly responsive to therapy. To investigate this apparent paradox, we analysed in vitro the responses of Burkitt lymphoma (BL) cells to gamma-irradiation in the presence and absence of Bcl-2. High-level expression of Bcl-2 was shown to promote BL cell survival following irradiation. However, a significant proportion of Bcl-2-rescued cells subsequently underwent apoptosis after an extended period in culture. In addition, in different BL lines, Bcl-2 was found either to promote or to inhibit long-term proliferative activity following gamma-irradiation. This differential regulation of proliferation correlated both with differential effects of Bcl-2 on the cell cycle and with differences in p53 status. Thus, by one week after irradiation, BL cells expressing only wild-type p53 (wt/wt) had arrested in G1, whereas those with a mutant allele (wt/mu) were arrested in all phases of the cell cycle. The proportion of Bcl-2-rescued cells that subsequently underwent apoptosis was reduced by ligation of CD40 at the time of irradiation in wt/wt BL cells, but not in wt/mu cells. CD40-ligation reduced both G1-arrest and apoptosis in parallel. These results indicate that, whilst Bcl-2 can delay apoptosis in BL cells following gamma-irradiation, the protein can also cause growth-arrest and thereby promote apoptosis. Long-term survival following Bcl-2-mediated rescue of gamma-irradiated cells may depend on p53 status and require additional death-repressing or growth-promoting signals.