Analysis of ion transport perturbations caused by hu MDR 1 protein overexpression Journal Article


Authors: Hoffman, M. M.; Roepe, P. D.
Article Title: Analysis of ion transport perturbations caused by hu MDR 1 protein overexpression
Abstract: In previous work [Luz et al. (1994) Biochemistry 33, 7239-7249; Roepe et al. (1994) Biochemistry 33, 11008-11015] we measured changes in Cl- and HCO3--dependent pH(i) regulation for LR73 Chinese hamster ovary fibroblasts overexpressing mu MDR 1 protein. However, only one clonal cell line overexpressing the protein but not previously exposed to chemotherapeutic drug (i.e., a 'me' transfectant) was examined, since very few MDR cell lines of this nature have been constructed. Recently [Hoffman et al. (1996) J. Gen. Physiol. 108, 295-313] we derived a series of true LR73/hu MDR 1 transfectants that are valuable for defining the MDR phenotype mediated by MDR protein alone, without the additional complexities introduced by exposing cells to chemotherapeutic drugs. Several independently derived clones from these and additional transfection experiments exhibit expression of MDR protein that is higher than that found in other true transfectants, and that is similar to the highest level of overexpression yet recorded for drug selected MDR cells. We examined altered Cl--dependent phi regulation for these clones using improved single-cell photometry (SCP) techniques. Short- term isotonic Cl- substitution experiments performed in the presence of CO2/HCO3- reveal that mild overexpression of hu MDR 1 protein alters anion exchange (Cl-/HCO3- exchange or AE) for LR73 cells, as expected on the basis of previous work [Luz et al. (1994/Biochemistry 33, 7239-7249]. Interestingly, we now find that several independently selected high-level MDR 1 overexpressing clones acidify quite extensively upon isotonic exchange of Cl- and then rapidly alkalinize upon restoring normal [Cl-]. These data suggest that MDR protein may effectively compete against AE. The MDR protein effect is not dependent on HCO3-/CO2 or K+, is partially inhibited by verapamil, is completely inhibited by substituting K+ or N-methylglucamine (NMG+) for Na+ in the SCP perfusate but is not affected by 100 μM levels of amiloride, bumetanide, chlorothiazide, or stilbene. ATP depletion inhibits the MDR 1 effect. We are unable to restore normal AE activity for the MDR clones via manipulation of Cl- or HCO3- gradients. We thus suggest that MDR protein overexpression provides a novel Na+- and Cl--dependent pathway for transmembrane H+ transport. From analysis of ion dependency and inhibitor sensitivities, we conclude the transport is not via altered regulation of any known K+/H+, Na+/H+, or Cl-/HCO3- antiporters, Na+: K+:2Cl-, Na+:K+:2HCO3-, K+:HCO3-, or Na+:HCO3- co-transporters, or any combination of these. Thus, it appears to represent a novel ATP and Na+-dependent Cl-/H+ antiport process that (1)may be directly mediated by the MDR protein, (2) may represent the modulation of one or more currently unidentified ion transport proteins by MDR protein. (3) may be due to some combination of direct ion transport and regulation of ion transport, or (4) may represent unusual passive H+ movement in response to a novel Cl-- dependent electrical perturbation that occurs during our Cl- substitution protocol. The results have important implications for understanding drug resistance mediated by MDR 1 overexpression, as well as the physiologic function of endogenously expressed MDR protein.
Keywords: protein expression; nonhuman; animal cell; animals; analytic method; cell line; animalia; genetic transfection; fibroblast; sodium; verapamil; hydrolysis; models, chemical; proton transport; hydrogen-ion concentration; p-glycoprotein; potassium; cell membrane transport; carbon dioxide; cricetinae; cricetulus griseus; competitive inhibition; chlorides; ion transport; bumetanide; calcium channel blockers; chlorothiazide; amiloride; priority journal; article; antiporters; bicarbonates; chinese hamster; chloride-bicarbonate antiporters; meglumine; stilbene
Journal Title: Biochemistry
Volume: 36
Issue: 37
ISSN: 0006-2960
Publisher: American Chemical Society  
Date Published: 1997-09-16
Start Page: 11153
End Page: 11168
Language: English
DOI: 10.1021/bi970530g
PUBMED: 9287158
PROVIDER: scopus
DOI/URL:
Notes: Article -- Export Date: 17 March 2017 -- Source: Scopus
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  1. Paul D. Roepe
    26 Roepe