Sequential modifications of topoisomerase I activity in a camptothecin-resistant cell line established by progressive adaptation

Biochem Pharmacol. 1993 Jan 26;45(2):339-48. doi: 10.1016/0006-2952(93)90069-9.


The DNA-topoisomerase I (Topo I) inhibitor, camptothecin (CPT), is a plant alkaloid with an important antitumor activity. In order to investigate the cellular mechanism leading to the development of the resistance to this agent, we have established by progressive adaptation a P388 subline resistant to CPT. After 5 months of continuous drug exposure, the resistance index reached a value of 20 and the resistant cell line, P388CPT0.3, was maintained in the presence of CPT. CPT-induced single strand breaks measured by alkaline elution were found drastically reduced in the resistant cell line. Topo I activity and CPT-induced DNA cleavage were measured on cells at different steps of resistance. We first observed that the Topo I activity was strongly decreased. In addition, the resistant cells recovered the ATP-independent relaxation activity after 3 months of exposure to CPT, but still kept a reduced CPT-induced DNA cleavage. Further evaluations at the final stage of the resistance induction have indicated that cells presented a CPT-resistant form of Topo I. Rearranged Topo I gene on one allele and a reduced Topo I transcription were also observed in resistant cells. The putative role of the rearrangement was discussed. These data show that the resistance mechanism has evolved from a decreased Topo I activity to an altered form of the enzyme, and suggest that multiple mechanisms of Topo I modifications could contribute to CPT resistance.

MeSH terms

  • Animals
  • Antineoplastic Agents / pharmacology*
  • Camptothecin / pharmacology*
  • Cell Division / drug effects
  • DNA Damage
  • DNA Topoisomerases, Type II / genetics
  • DNA Topoisomerases, Type II / metabolism*
  • DNA, Single-Stranded / drug effects
  • Drug Resistance
  • Gene Rearrangement
  • Mice
  • RNA, Messenger / analysis
  • Topoisomerase II Inhibitors
  • Tumor Cells, Cultured / drug effects


  • Antineoplastic Agents
  • DNA, Single-Stranded
  • RNA, Messenger
  • Topoisomerase II Inhibitors
  • DNA Topoisomerases, Type II
  • Camptothecin