BCNU-sequestration by metallothioneins may contribute to resistance in a medulloblastoma cell line.
OBJECTIVE
Resistance of neoplastic cells to the alkylating drug BCNU [1,3-bis(2-chloroethyl)-1-nitrosourea] has been correlated with expression of O (6)-methylguanine-DNA methyltransferase, which repairs the O (6)-chloroethylguanine produced by the drug. Other possible mechanisms of resistance include raised levels of glutathione or increased repair of the DNA interstrand cross-links formed by BCNU. Transcriptional profiling revealed the upregulation of several metallothionein (MT) genes in a BCNU-resistant medulloblastoma cell line [D341 MED (OBR)] relative to its parental line. Previous studies have shown that MTs, through their reactive thiol groups can quench nitrogen mustard-derived alkylating drugs. In this report, we evaluate whether MTs can also quench BCNU.


METHODS
To demonstrate the binding of BCNU to MT, we used an assay that measured the release of the MT-bound divalent cations (Zn(2+), Cd(2+)) upon their displacement by the drug. We also measured the decomposition rates of BCNU at those reaction conditions.


RESULTS
The rate of release of the cations was higher in pH 7.4 than at pH 7.0, which is likely a result of more rapid decomposition of BCNU (thus faster release of MT-binding intermediate) at pH 7.4 than at pH 7.0.


CONCLUSIONS
We demonstrate that resistance to BCNU may be a result of elevated levels of MTs which act by sequestering the drug's decomposition product(s).
Bacolod
Manny D
MD
Department of Surgery, Duke University Medical Center, Durham, NC 27710, USA. mdb2005@med.cornell.edu
Fehdrau
Randy
R
Johnson
Stewart P
SP
Bullock
Nancy S
NS
Bigner
Darell D
DD
Colvin
Michael
M
Friedman
Henry S
HS
D002330:Carmustine / Q000378:metabolism*; D002528:Cerebellar Neoplasms / Q000378:metabolism*; D019008:Drug Resistance, Neoplasm; D006801:Humans; D008527:Medulloblastoma / Q000378:metabolism*; D008668:Metallothionein / Q000378:metabolism*; D014407:Tumor Cells, Cultured
D008668:Metallothionein; D002330:Carmustine