O-6-METHYLGUANINE-DNA METHYLTRANSFERASE (MGMT) TRANSFECTANTS OF A 1,3-BIS(2-CHLOROETHYL)-1-NITROSOUREA (BCNU)-SENSITIVE COLON-CANCER CELL-LINE SELECTIVELY REPOPULATE HETEROGENEOUS MGMT(+) MGMT(-) XENOGRAFTS AFTER BCNU AND O-6-BENZYLGUANINE PLUS BCNU/

To evaluate the role of O-6-alkylguanine-DNA alkyltransferase (AGT) in colon tumor chloroethylnitrosourea (CENU) resistance, AGT-deficient V ACO 8 cells were transfected with a vector containing or lacking the h uman O-6-methylguanine-DNA methyltransferase (MGMT) cDNA. VACO 8MGMT ( V8MGMT) sublines possessed high levels of AGT activity in cell culture and mere >10-fold resistant to the CENU 1,3-bis(2-chloroethyl)-1-nitr osourea (BCNU), V8MGMT cells, VACO 8neo cells, and mixtures of both we re grown as xenografts in nude mice. MGMT expression in VACO 8 xenogra fts reflected the percentage of V8MGMT cells present in the tumor inoc ulum, Xenografts originally containing 0-10% V8MGMT cells were sensiti ve to BCNU, although partial resistance was observed as the percentage of V8MGMT cells increased, Tumors containing 30-100% V8MGMT cells wer e completely resistant to BCNU with no regressions and no growth delay s, Pretreatment with O-6-benzylguanine (BG) depleted tumor AGT activit y for at least 6 h and sensitized xenografts containing 1 and 100% V8M GMT cells to BCNU, After BCNU or EG + BCNU, xenografts growing from in oculums containing as low as 0.1% V8MGMT cells had high AGT activities similar to that found in V8MGMT xenografts, with the majority of the cells expressing MGMT. These results provide evidence that MGMT expres sion influences both intrinsic and acquired colon tumor CENU resistanc e, that selective expansion of AGT(+) colon tumor cells commonly occur s after CENU exposure, and that BG Is effective in sensitizing colon t umors to CENUs, even when only a small fraction of the cells in a hete rogeneous tumor express MGMT.