O⁶-methylguanine-DNA methyltransferase activity, p53 gene status and BCNU resistance in mouse astrocytes

We observed previously that wild-type p53 rendered neonatal mouse astrocytes resistant to 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) in a gene dose-dependent fashion. This effect of p53 appeared to be unrelated to its cell cycle regulation or apoptotic functions. Because in many cell types O⁶-methylguanine-DNA methyltransferase (MGMT)-mediated DNA repair is an important mechanism of resistance to nitrosoureas, we measured MGMT activity in mild-type, heterozygous and p53 knockout neonatal mouse astrocytes. Wild-type p53 astrocytes had significantly greater MGMT activity than either heterozygous or p53 knockout astrocytes: MGMT activity was ~ 5-fold greater in wild-type p53 astrocytes than in p53 knockout cells. However, despite successful depletion of MGMT activity in wild-type astrocytes by O⁶-benzylguanine (BG), resistance to BCNU persisted unchanged. Moreover, we excluded the possibility that continued resistance to BCNU at the concentrations used could be explained by a compensatory induction of MGMT triggered by exposure to either BCNU or BG. Although these studies support a role for p53 regulation of MGMT in neonatal mouse astrocytes, BCNU resistance in wild-type cells appears to be mediated by a non-MGMT mechanism. Nevertheless, regulation of DNA repair by MGMT may be another mechanism by which alterations of the p53 gene promote tumor initiation or progression.