title: Constitutive expression of functional P-glycoprotein in rat hepatoma cells.
abstract: P-glycoprotein is a plasma-membrane glycoprotein involved in multidrug resistance. P-glycoprotein overexpression has been demonstrated to occur in tumor cells after cytotoxic drug exposure, but also in some cancers including hepatocellular carcinomas before any chemotherapeutic treatment. In order to better analyze this constitutive type of tumoral drug resistance, we have investigated P-glycoprotein expression and function in rat liver tumors induced experimentally by administration of diethylnitrosamine and in two cell clones derived from one of these tumors designated as RHC1 and RHC2. High levels of P-glycoprotein mRNAs were found in both liver tumor samples and the two hepatoma cell clones as assessed by Northern blotting; both RHC1 and RHC2 cells displayed altered liver functions commonly observed in rat hepatoma cells, particularly the decreased expression of albumin and overexpression of the fetal glutathione S-transferase 7-7. The use of specific multidrug resistance (mdr) probes revealed a major induction of the mdr1 gene in liver tumor samples while RHC1 and RHC2 cells expressed both mdr1 and mdr3 genes without displaying a major alteration in the number of mdr gene copies as assessed by Southern blotting. High amounts of P-glycoprotein were also demonstrated in RHC1 and RHC2 cells by Western blotting. These cells were strongly resistant to doxorubicin and vinblastine, two anticancer drugs transported by P-glycoprotein. Doxorubicin intracellular retention was low in RHC1 and RHC2 cells, but was strongly enhanced in the presence of verapamil, a known modulator agent of P-glycoprotein; low retention appeared to occur via a drug efflux mechanism, indicating that P-glycoprotein was fully active. These results show that rat hepatoma cells can display elevated levels of functional P-glycoprotein without any prior cytotoxic drug selection and suggest that these cells represent a useful model for analyzing P-glycoprotein regulation in intrinsically clinical drug-resistant cancers.
authors.affiliation: Unité de Recherches Hépatologiques U 49 de l'Institut National de la Santé et de la Recherche Médicale, Hôpital Pontchaillou, Rennes, France.
authors.forename: O
authors.initials: O
authors.lastname: Fardel
authors.forename: P
authors.initials: P
authors.lastname: Loyer
authors.forename: V
authors.initials: V
authors.lastname: Lecureur
authors.forename: D
authors.initials: D
authors.lastname: Glaise
authors.forename: A
authors.initials: A
authors.lastname: Guillouzo
mesh_terms: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000818:Animals; D006528:Carcinoma, Hepatocellular / Q000139:chemically induced / Q000235:genetics / Q000378:metabolism*; D002352:Carrier Proteins / Q000096:biosynthesis* / Q000235:genetics; D002999:Clone Cells; D003513:Cycloheximide / Q000494:pharmacology; D004052:Diethylnitrosamine / Q000633:toxicity; D004317:Doxorubicin / Q000378:metabolism; D004351:Drug Resistance; D005260:Female; D015870:Gene Expression; D005982:Glutathione Transferase / Q000096:biosynthesis; D008113:Liver Neoplasms / Q000139:chemically induced / Q000235:genetics / Q000378:metabolism*; D008562:Membrane Glycoproteins / Q000096:biosynthesis* / Q000235:genetics; D012333:RNA, Messenger / Q000096:biosynthesis* / Q000302:isolation & purification; D012334:RNA, Neoplasm / Q000096:biosynthesis / Q000302:isolation & purification; D051381:Rats; D017207:Rats, Sprague-Dawley; D012709:Serum Albumin / Q000096:biosynthesis; D014158:Transcription, Genetic / Q000187:drug effects; D014407:Tumor Cells, Cultured; D014700:Verapamil / Q000494:pharmacology
chemical_list: D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D002352:Carrier Proteins; D008562:Membrane Glycoproteins; D012333:RNA, Messenger; D012334:RNA, Neoplasm; D012709:Serum Albumin; D004052:Diethylnitrosamine; D004317:Doxorubicin; D003513:Cycloheximide; D014700:Verapamil; D005982:Glutathione Transferase