Cloning and Expression of Murine Sister of P-Glycoprotein Reveals a More Discriminating Transporter ThanMDR1/P-Glycoprotein
- Valerie Lecureur1,
- Daxi Sun1,
- Philip Hargrove2,
- Erin G. Schuetz1,
- Richard B. Kim3,
- Lu-Bin Lan1 and
- John D. Schuetz1
- Departments of 1Pharmaceutical Sciences (V.L., D.S., E.G.S.,L.-B.L., J.D.S.) and 2Experimental Hematology (P.H.), St. Jude Children's Research Hospital, Memphis, Tennessee; and the 3Department of Medicine & Pharmacology (R.B.K.), Vanderbilt University, Nashville, Tennessee.
Abstract
Sister of P-glycoprotein (SPGP), a novel murine cDNA and member of the ATP-binding cassette superfamily highly homologous to P-glycoprotein (Pgp), was cloned. Moreover, its genomic clone was isolated and localized to chromosome 2 by fluorescence in situ hybridization. SPGP was functionally evaluated relative to MDR1 after subcloning SPGP cDNA into a retroviral bicistronic vector capable of expressing both SPGP and the green fluorescent protein. LLC-PK1 and MDCKII cells were transduced with this retrovirus and SPGP-positive clones were isolated. Drug uptake and efflux was compared in cells ectopically expressing either SPGP or human MDR1. SPGP cells had decreased uptake of taurocholate and vinblastine compared with LLC-PK1 cells. Additional studies revealed that vinblastine efflux was accelerated by SPGP compared with LLC-PK1. Further comparison revealed that although MDR1 easily impaired uptake of vincristine, daunomycin, paclitaxel, and digoxin, SPGP had no effect on uptake of these drugs. However, further studies demonstrated that, like MDR1, SPGP effluxed calcein-acetoxymethyl ester (AM). Unlike MDR1, SPGP was incapable of effluxing rhodamine 123. Although cyclosporine A and reserpine blocked calcein-AM transport by MDR1, these drugs had either minimal or no effect, respectively, on blocking SPGP efflux of calcein-AM. In contrast, ditekiren, a linear hexapeptide, readily and preferentially inhibited SPGP efflux of calcein-AM. Further studies with three structural analogs of ditekiren revealed that one analog inhibited SPGP efflux of calcein-AM, although not as potently as ditekiren. These are the first studies to reveal that SPGP has distinct transport properties compared with MDR1.
Footnotes
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Send reprint requests to: Dr. John D. Schuetz, Department of Pharmaceutical Sciences, St. Jude Children's Research Hospital, 332 N. Lauderdale Ave., Memphis, TN 38105. E-mail:john.schuetz{at}stjude.org
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This work was supported by National Institutes of Health Grants ES/GM 5851, ES/GM 8568, CA 21765, CA 23099, and GM 31304, and by the American Lebanese Syrian Associated Charities.
- Abbreviations:
- Pgp
- P-glycoprotein
- SPGP
- sister of P-glycoprotein
- ABC
- ATP-binding cassette
- MDR
- multidrug resistance
- AM
- acetoxymethyl ester
- kb
- kilobase
- EST
- expressed sequence tag
- BAC
- bacterial artificial chromosome
- PCR
- polymerase chain reaction
- FISH
- fluorescence in situ hybridization
- GST
- glutathione transferase
- GFP
- green fluorescence protein
- BSEP
- bile salt export pump
- CsA
- cyclosporine A
- TM
- transmembrane
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- Received July 6, 1999.
- Accepted September 23, 1999.
- The American Society for Pharmacology and Experimental Therapeutics
