Functional intracellular P-glycoprotein

Int J Cancer. 1998 Jun 10;76(6):857-64. doi: 10.1002/(sici)1097-0215(19980610)76:6<857::aid-ijc15>;2-#.


Efflux of chemotherapy drugs by P-glycoprotein (P-gp) at the plasma membrane is thought to be a major cause of cancer multidrug resistance. In this report, we show by flow cytometry that P-gp also concentrates large amounts of 2 different drugs, Hoechst 33342 and daunorubicin, within a cytoplasmic compartment of multidrug resistant CHRC5 cells. A quantitative assay of Hoechst 33342 revealed that cytoplasmic sequestration by P-gp in CHRC5 cells accounted for about half of the amount of Hoechst 33342 accumulated by the drug-sensitive parental Aux BI cells. Daunorubicin sequestered in the cytoplasm of CHRC5 cells could be released by inhibiting P-gp function with cyclosporin A, resulting in cell death. A likely site of drug sequestration is P-gp-containing cytoplasmic vesicles, in which the P-gp is oriented so that drugs are transported and concentrated in the interior of the vesicles. P-gp was detected in the membranes of cytoplasmic vesicles of CHRC5 cells by confocal immunofluorescence microscopy and immunoelectron microscopy with anti-P-gp monoclonal antibodies (MAbs). Vesicular localization of daunorubicin was observed by epifluorescence microscopy. The origin and nature of the P-gp-containing vesicles are unknown, but they do not correspond to endocytic vesicles. Our results directly demonstrate that chemosensitizer-induced release of drugs sequestered in cytoplasmic vesicles by P-gp can be used to overcome multidrug resistance.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • ATP Binding Cassette Transporter, Subfamily B, Member 1 / analysis
  • ATP Binding Cassette Transporter, Subfamily B, Member 1 / physiology*
  • Benzimidazoles / pharmacokinetics
  • Cell Line
  • Cyclosporine / pharmacology
  • Daunorubicin / pharmacokinetics
  • Drug Resistance


  • ATP Binding Cassette Transporter, Subfamily B, Member 1
  • Benzimidazoles
  • Cyclosporine
  • bisbenzimide ethoxide trihydrochloride
  • Daunorubicin