PISCATAWAY, New JerseyThree aspects of topoisomerase I are
currently under intensive investigation by researchers hoping to improve cancer
chemotherapy: the mechanism of topoisomerase I poisoning, cellular processing
and repair, and mechanisms of resistance to topoisomerase I inhibitors. These
issues were reviewed at the Vanderbilt University Symposium by Leroy F. Liu,
PhD, who heads the Department of Pharmacology, University of Medicine and
Dentistry of New Jersey-Robert Wood Johnson Medical School, in Piscataway.
Dr. Liu described some of the many possible mechanisms of
topoisomerase I poisoning. These include:
Enzyme/DNA binders such as camptothecin
DNA binders such as actinomy- cin D, protoberberines, and
Enzyme modification such as T722A creation of mutations in
DNA modification such as abasic sites, base mismatches,
benzo[a]pyrene-DNA adducts, or araC-substituted DNA
Dr. Liu said that studies of human xenografts in animal models
show that camptothecins are more effective than other drug classes at inducing
complete responses in solid tumors. "However, in the clinic the results
are not so dramatic," he said. New derivatives such as silatecan and
homocamptothecin have been developed to address this problem, which is thought
to be related in part to interactions with human serum albumins.
Dr. Liu said that the topoisomerase-I cleavable complexes
produced by camptothecins are reversible, so that a cellular process (eg, DNA
replication and RNA transcription) must occur to produce permanent damage.
‘Fork Collision Model’
Many experts believe that topoisomerase I inhibitors ultimately
cause tumor cell damage by the process described as the "fork collision
"The replication fork is one of the major events that
processes this complex into a double-strand break," Dr. Liu said.
"Upon the collision, the 5' hydroxyl end is displaced away from the active
site and the broken end of the now irreversible complex is available for
cellular recognition. In addition to DNA replication, RNA transcription can
also process the reversible topoisomerase cleavable complexes into
protein-linked DNA strand breaks. This event probably triggers destruction of
the topoisomerase cleavable complex.
Cellular Processing Pathways
There are two pathways for cellular processing of the complex:
the small ubiquitin-related modifier (SUMO/UBC9) pathway and the ubiquitin/26S
proteasome pathway. The topoisomerase I cleavable complexes are SUMO-conjugated
"within moments," according to Dr. Liu.
"In the presence of camptothecins, you trap the cleavable
complex, which is rapidly SUMO-lated and then destroyed. If you block
transcription, this destruction does not occur. The transcription collision
reveals the potentially lethal strand break. In order to repair this damage,
the cell uses its machinery to destroy topoisomerase I, then cellular repair
mechanisms can come into action," Dr. Liu said.
Dr. Liu concluded that topoisomerase I cleavable complexes can
be converted into DNA lesions by either DNA replication, RNA transcription, or
other cellular processes, and that topoisomerase I cleavable complexes are
modified by SUMO and ubiquitin. "Topoisomerase I down-regulation is a new
resistance mechanism to camptothecins," he said.