In vivo effects of cis- and trans-diamminedichloroplatinum(II) on SV40 chromosomes: differential repair, DNA-protein crosslinking, and inhibition of replication

Abstract

The mechanism of action of the antitumor drug cis-diamminedichloroplatinum(II), cis-DDP, was investigated by using the ~5200 base pair (bp) chromosome of simian virus 40 (SV40) as an in vivo chromatin model. Comparative studies were also carried out with the clinically ineffective isomer trans-DDP. Although 14 times more trans- than cis-DDP in the culture medium is required to inhibit SV40 DNA replication in SV40-infected green monkey CV-1 cells, the two isomers are equally effective at inhibiting replication when equimolar amounts are bound to SV40 DNA in vivo. Since both isomers are transported into CV-1 cells at similar rates, differential uptake cannot account for the greater ability of cis-DDP to inhibit SV40 DNA replication. Rather, this result is explained by the finding that cis-DDP-DNA adducts accumulate continuously over the incubation period, whereas trans-DDP binding to DNA reaches a maximum at 6 h and thereafter decreases dramatically. We suggest that the different accumulation behavior of cis-DDP and trans-DDP on DNA is due to their differential repair in CV-1 cells. A variety of non-histone proteins, including SV40 capsid proteins but virtually no histones, are cross-linked to SV40 DNA in vivo by either cis- or trans-DDP. More DNA-protein cross-links are formed by trans-DDP than by cis-DDP at equivalent amounts of DNA-bound platinum. Since the cis-DDP analogue dichloro(ethylenediamine)platinum(II) inhibits SV40 DNA replication as efficiently as cis- and /trans-DDP but does not form DNA-protein cross-links, platinum-mediated DNA-protein cross-linking is not directly responsible for the inhibition of DNA replication. We discuss these findings in relation to the known cytotoxic and antitumor properties of cis-DDP.

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