Increased expression of DNA topoisomerase II alpha has been associated
with resistance to certain DNA-damaging alkylating agents, but no cau
sal relationship or mechanism has been established, To investigate thi
s observation, we developed a model of topoisomerase II overexpression
by transfecting a full-length Chinese hamster ovary topoisomerase II
alpha into EMT6 mouse mammary carcinoma, Topoisomerase II alpha-transf
ected cell lines demonstrated continued topoisomerase II alpha mRNA an
d protein expression, which were undetectable in vector-only lines, in
stationary phase (G(0)-G(1)). The topoisomerase II transfectants were
similar to 5-10-fold resistant to the alkylating agents cisplatin and
mechlorethamine, Upon release from G(0)-G(1), the topoisomerase II tr
ansfectants demonstrated more rapid thymidine incorporation and shorte
r cell-doubling times than control cells, Purified topoisomerase II an
d nuclear extracts with topoisomerase II-decatenating activity bound t
o cisplatin-treated DNA with significantly greater affinity than to un
treated DNA in a cisplatin concentration-dependent manner, These obser
vations suggest that expression of topoisomerase II alpha may have a r
ole in cellular resistance to antineoplastic alkylating agents, The me
chanism for this may involve increased binding of topoisomerase II alp
ha to alkylating agent-damaged DNA.