The Role of O⁶-Alkylguanine DNA Alkyltransferase in Limiting Nitrosourea-induced Sister Chromatid Exchanges in Proliferating Human Lymphocytes¹

Although induction of sister chromatid exchanges (SCEs) following nitrosourea exposure may be greater during cell proliferation, the increase could be offset by the action of the DNA repair protein O⁶-alkylguanine DNA alkyltransferase (alkyltransferase). To evaluate these factors in resting and proliferating (phytohemagglutinin stimulated) human lymphocytes, we studied the effect of changes in alkyltransferase activity on 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU)-induced SCEs. Phytohemagglutinin stimulation resulted in induction of alkyltransferase activity (5.9 ± 0.3 units, resting, versus 9.2 ± 0.2 units, proliferating). In both resting and proliferating lymphocytes the alkyltransferase activity was inactivated by 85–88% after an 18-h exposure to 0.5 mm of the modified base O⁶methylguanine (O⁶mGua). However, 48 h after removal of O⁶mGua, proliferating lymphocytes recovered alkyltransferase activity while resting cells did not. In the absence of O⁶mGua, both resting and proliferating lymphocytes were equally sensitive to BCNU-induced SCEs. Following inactivation of the alkyltransferase by O⁶mGua, BCNU-induced SCEs were markedly increased, but the increase was much greater in resting than proliferating cells, 4-fold vs. 2.6-fold at each dose of BCNU (P < 0.001). The factors providing partial protection against BCNU-induced SCEs in proliferating lymphocytes appear to include the proliferation-dependent increase in alkyltransferase activity and the ability of proliferating lymphocytes to rapidly recover alkyltransferase activity after its inactivation. Thus, the alkyltransferase appears to provide an important mechanism of resistance to SCE induction in human lymphocytes.