The role of O6-alkylguanine DNA alkyltransferase in limiting nitrosourea-induced sister chromatid exchanges in proliferating human lymphocytes.

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 less than 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.