Induction of DNA ligase I by 1-beta-D-arabinosylcytosine and aphidicolin in MiaPaCa human pancreatic cancer cells

Exposure of MiaPaCa cells to 1-beta-D-arabinosylcytosine (ara-C) resulted in an increase in DNA ligase levels up to threefold compared to that in the untreated control cells, despite significant growth inhibition. Increased levels of DNA ligase I protein appear to correlate with the appearance of increased mRNA levels. The [(3)H]thymidine incorporation experiment and the biochemical assay of total polymerase activity revealed that an increase in DNA ligase I levels after treatment with ara-C was not accompanied by an increase of DNA synthesis or an increased presence of DNA polymerase activity inside cells. When cells resumed DNA synthesis after drug treatment, DNA ligase I levels began to drop, indicating that increased DNA ligase I is not required for DNA synthesis. An increase in DNA ligase I was also observed in cells treated with aphidicolin, another inhibitor of DNA synthesis that inhibits DNA polymerases without incorporating itself into DNA, indicating that an increase in DNA ligase I levels could be caused by the arrest of DNA replication by these agents. Interestingly, caffeine, which is a well-known inhibitor of DNA damage checkpoint kinases, abrogated the increase in DNA ligase I in MiaPaCa cells treated with ara-C and aphidicolin, suggesting that caffeine-sensitive kinases might be important mediators in the pathway leading to the increase in DNA ligase I levels in response to anticancer drugs, including ara-C and aphidicolin. We propose that ara-C and aphidicolin induce damage to the DNA strand by arresting DNA replication forks and subsequently increase DNA ligase I levels to facilitate repair of DNA damage.

Activity of aphidicolin glycinate alone or in combination with cisplatin in a murine ovarian tumor resistant to cisplatin

Aphidicolin, a reversible inhibitor of DNA polymerase alpha and delta, has recently been reported to reverse the resistance to cisplatin (DDP) of an ovarian cancer cell line. We investigated the pharmacokinetics of aphidicolin in mice and examined its activity either alone or in combination with DDP in the DDP-sensitive M5076 (M5) murine reticular cell sarcoma as well as in a DDP-resistant subline (M5/DDP). The drug was cleared from plasma very rapidly (clearance, 41.6 ml min-1 kg-1), showing a half-life of 15 min. Aphidicolin concentrations in the tumor were approximately 50% of those found in plasma at steady state. Using several dose schedules and continuous infusions we failed to detect significant antitumor activity for aphidicolin glycinate. Potentiation of the activity of DDP by aphidicolin glycinate was moderate in mice bearing M5 tumor as well as in those bearing M5/DDP tumor. These data do not support the possible clinical use of aphidicolin in combination with DDP. However, further studies should be carried out in different tumor models before this possibility is conclusively ruled out.