title: Intracellular P-glycoprotein in multidrug resistant tumor cells.
abstract: The overexpression of the drug-efflux molecular pump P-glycoprotein (P-gp) may confer to tumor cells the multidrug resistant (MDR) phenotype, which is one of the causes of cancer chemotherapy failure. By investigating several in vitro models of human tumor cells, we observed that P-gp, in addition to its localization on the plasma membrane, can also be found intracellularly. In particular, by using immunocytochemical and cytofluorimetric methods, we revealed that in MDR breast cancer cells (MCF-7) a significant level of P-gp was expressed in the Golgi apparatus, which is the major site of accumulation of the antitumoral compound doxorubicin. Moreover, we demonstrated the intracellular location of P-gp in three stabilized human melanoma cell lines which had never undergone cytotoxic drug treatment and did not express the transporter molecule on the plasma membrane. Double immunofluorescence labelling and immunoelectron microscopy revealed, also in this tumor cell type, the location of P-gp in the Golgi apparatus where it seems to play a pivotal role in intracellular drug transport. Finally, we analyzed the expression, localization and function of drug transport proteins in human colon carcinoma lines (LoVo) exhibiting different degrees of intrinsic or drug-induced resistance. We found that only MDR LoVo cells expressed P-gp on the plasma membrane while both low-level drug resistant clonal LoVo cells and MDR LoVo cells appeared to be positive for intracellular P-gp. Our findings suggest a functional role of the intracytoplasmic P-gp in the transport and sequestration of drugs. This represents a complementary protective mechanism of tumor cells against cytotoxic agents.
authors.affiliation: Department of Ultrastructures, Istituto Superiore di Sanita', Rome, Italy. arancia@iss.it
authors.forename: G
authors.initials: G
authors.lastname: Arancia
authors.forename: A
authors.initials: A
authors.lastname: Molinari
authors.forename: A
authors.initials: A
authors.lastname: Calcabrini
authors.forename: S
authors.initials: S
authors.lastname: Meschini
authors.forename: M
authors.initials: M
authors.lastname: Cianfriglia
mesh_terms: D018435:ATP Binding Cassette Transporter, Subfamily B / Q000378:metabolism*; D000230:Adenocarcinoma / Q000188:drug therapy / Q000378:metabolism / Q000473:pathology; D000970:Antineoplastic Agents / Q000493:pharmacokinetics*; D001943:Breast Neoplasms / Q000188:drug therapy / Q000378:metabolism / Q000473:pathology; D002451:Cell Compartmentation / Q000187:drug effects / Q000502:physiology*; D003110:Colonic Neoplasms / Q000188:drug therapy / Q000378:metabolism / Q000473:pathology; D019008:Drug Resistance, Neoplasm / Q000502:physiology*; D005260:Female; D005434:Flow Cytometry; D006056:Golgi Apparatus / Q000378:metabolism* / Q000648:ultrastructure; D006801:Humans; D008545:Melanoma / Q000188:drug therapy / Q000378:metabolism / Q000473:pathology; D008856:Microscopy, Fluorescence; D009369:Neoplasms / Q000188:drug therapy / Q000378:metabolism* / Q000503:physiopathology; D014407:Tumor Cells, Cultured / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*
chemical_list: D018435:ATP Binding Cassette Transporter, Subfamily B; D000970:Antineoplastic Agents