To evaluate the role of O6-alkylguanine-DNA
alkyltransferase (AGT) in colon
tumor chloroethylnitrosourea (
CENU) resistance, AGT-deficient VACO 8 cells were transfected with a vector containing or lacking the human O6-methylguanine-DNA
methyltransferase (MGMT)
cDNA. VACO 8MGMT (V8MGMT) sublines possessed high levels of AGT activity in cell culture and were > 10-fold resistant to the
CENU 1,3-bis(2-chloroethyl)-1-nitrosourea (
BCNU). V8MGMT cells, VACO 8neo cells, and mixtures of both were grown as xenografts in nude mice. MGMT expression in VACO 8 xenografts reflected the percentage of V8MGMT cells present in the
tumor inoculum. Xenografts originally containing 0-10% V8MGMT cells were sensitive to
BCNU, although partial resistance was observed as the percentage of V8MGMT cells increased.
Tumors containing 30-100% V8MGMT cells were completely resistant to
BCNU with no regressions and no growth delays. Pretreatment with
O6-benzylguanine (BG) depleted
tumor AGT activity for at least 6 h and sensitized xenografts containing 1 and 100% V8MGMT cells to
BCNU. After
BCNU or BG +
BCNU, xenografts growing from inoculums 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 expression influences both intrinsic and acquired colon
tumor CENU resistance, that selective expansion of AGT+ colon
tumor cells commonly occurs after
CENU exposure, and that BG is effective in sensitizing colon
tumors to CENUs, even when only a small fraction of the cells in a heterogeneous
tumor express MGMT.