Cloning and expression of murine sister of P-glycoprotein reveals a more discriminating transporter than MDR1/P-glycoprotein

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 chromoso me 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. L LC-PK1 and MDCKII cells were transduced with this retrovirus and SPGP-posit ive clones were isolated. Drug uptake and efflux was compared in cells ecto pically expressing either SPGP or human MDR1. SPGP cells had decreased upta ke of taurocholate and vinblastine compared with LLC-PK1 cells. Additional studies revealed that vinblastine efflux was accelerated by SPGP compared w ith LLC-PK1. Further comparison revealed that although MDR1 easily impaired uptake of vincristine, daunomycin, paclitaxel, and digoxin, SPGP had no ef fect on uptake of these drugs. However, further studies demonstrated that, like MDR1, SPGP effluxed calcein-acetoxymethyl ester (AM). Unlike MDR1, SPG P was incapable of effluxing rhodamine 123. Although cyclosporine A and res erpine blocked calcein-AM transport by MDR1, these drugs had either minimal or no effect, respectively, on blocking SPGP efflux of calcein-AM. In cont rast, ditekiren, a linear hexapeptide, readily and preferentially inhibited SPGP efflux of calcein-AM. Further studies with three structural analogs o f ditekiren revealed that one analog inhibited SPGP efflux of calcein-AM, a lthough not as potently as ditekiren. These are the first studies to reveal that SPGP has distinct transport properties compared with MDR1.