A single cycle of treatment with temozolomide, alone or combined with O(6)-benzylguanine, induces strong chemoresistance in melanoma cell clones in vitro: role of O(6)-methylguanine-DNA methyltransferase and the mismatch repair system

Int J Oncol. 2006 Oct;29(4):785-97.


Clinically achievable concentrations of temozolomide (TMZ) produce cytotoxic effects only in mismatch repair (MMR)-proficient cells endowed with low O6-methylguanine-DNA methyltransferase (MGMT) activity. Aim of the present study was to investigate the molecular mechanisms underlying acquired resistance of melanoma cells to TMZ and the effect of O6-benzylguanine (BG), a specific MGMT inhibitor, on the development of a TMZ-resistant phenotype. Three MMR-proficient melanoma cell clones with low or no MGMT activity were treated daily for 5 days with 50 micromol/l TMZ, alone or in combination with 5 micromol/l BG. Parental clones and sublines established after one or four cycles of treatment were analyzed for sensitivity to TMZ or TMZ+BG and for other parameters. The sublines established after one cycle of TMZ or TMZ+BG exhibited a marked increase in MGMT activity and resistance to TMZ alone. BG only partially reversed acquired resistance to the drug. In some cases, alterations in the MMR system accounted for MGMT-independent resistance to TMZ. Up-regulation of MGMT activity was associated with either demethylation of the MGMT promoter or hypermethylation of the body of the gene, and partially reversed by 5-aza-2'-deoxycytidine. The sublines established after four cycles of TMZ or TMZ+BG did not show a further increase in resistance to TMZ alone. However, two out of three sublines established after TMZ+BG treatment exhibited increased resistance to TMZ+BG. In conclusion, our data demonstrate that a single cycle of TMZ is sufficient to induce high levels of drug resistance in melanoma clones, principally, but not exclusively, via up-regulation of MGMT expression. Exposure to TMZ+BG favors the development of MGMT-independent mechanisms of TMZ resistance.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Antineoplastic Agents / pharmacology*
  • Base Pair Mismatch / physiology*
  • DNA Methylation
  • DNA Repair
  • Dacarbazine / analogs & derivatives*
  • Dacarbazine / pharmacology
  • Drug Resistance, Neoplasm* / genetics
  • Enzyme Inhibitors / pharmacology*
  • Guanine / analogs & derivatives*
  • Guanine / pharmacology
  • Humans
  • Melanoma / enzymology*
  • Melanoma / genetics*
  • O(6)-Methylguanine-DNA Methyltransferase / antagonists & inhibitors
  • O(6)-Methylguanine-DNA Methyltransferase / genetics
  • O(6)-Methylguanine-DNA Methyltransferase / physiology*
  • Promoter Regions, Genetic
  • Temozolomide
  • Tumor Cells, Cultured
  • Up-Regulation


  • Antineoplastic Agents
  • Enzyme Inhibitors
  • O(6)-benzylguanine
  • Guanine
  • Dacarbazine
  • O(6)-Methylguanine-DNA Methyltransferase
  • Temozolomide