Transport of physiological nucleosides and anti-viral and anti-neoplastic nucleoside drugs by recombinant Escherichia coli nucleoside-H⁺ cotransporter (NupC) produced in Xenopus laevis oocytes

Abstract

The recently identified human and rodent plasma membrane proteins CNT1, CNT2 and CNT3 belong to a gene family (CNT) that also includes the bacterial nucleoside transport protein NupC. Heterologous expression in Xenopus oocytes has established that CNT1-3 correspond functionally to the three major concentrative nucleoside transport processes found in human and other mammalian cells (systems cit, cif and cib, respectively) and mediate Na⁺-linked uptake of both physiological nucleosides and anti-viral and anti-neoplastic nucleoside drugs. Here, one describes a complementary Xenopus oocyte transport study of Escherichia coli NupC using the plasmid vector pGEM-HE in which the coding region of NupC was flanked by 5′- and 3′-untranslated sequences from a Xenopus ß-globin gene. Recombinant NupC resembled human (h) and rat (r) CNT1 in nucleoside selectivity, including an ability to transport adenosine and the chemotherapeutic drugs 3′-azido-3′-deoxythymidine (AZT), 2′,3′-dideoxycytidine (ddC) and 2′-deoxy-2′,2′-difluorocytidine (gemcitabine), but also interacted with inosine and 2′,3′-dideoxyinosine (ddI). Apparent affinities were higher than for hCNT1, with apparent K_m values of 1.5–6.3 μm for adenosine, uridine and gemcitabine, and 112 and 130 μm, respectively, for AZT and ddC. Unlike the relatively low translocation capacity of hCNT1 and rCNT1 for adenosine, NupC exhibited broadly similar apparent V_max values for adenosine, uridine and nucleoside drugs. NupC did not require Na⁺ for activity and was H⁺-dependent. The kinetics of uridine transport measured as a function of external pH were consistent with an ordered transport model in which H⁺ binds to the transporter first followed by the nucleoside. These experiments establish the NupC-pGEM-HE/oocyte system as a useful tool for characterization of NupC-mediated transport of physiological nucleosides and clinically relevant nucleoside therapeutic drugs.

• Nucleoside transporters
• Xenopus oocytes
• 3′-deoxy-nucleoside drugs
• CNT
• NupC

AIDS, acquired immunodeficiency syndrome; HIV, human immunodeficiency virus; NT, nucleoside transporter; CNT, concentrative nucleoside transporter; ENT, equilibrative nucleoside transporter; MIP, major intrinsic protein; AZT, 3′-azido-3′-deoxythymidine; ddC, 2′,3′-dideoxycytidine; ddI, 2′,3′-dideoxyinosine; gemcitabine, 2′-deoxy-2′,2′-difluorocytidine; bp, base pair(s); kb, kilobase(s); PCR, polymerase chain reaction; kDa, kilodaltons; TM, transmembrane helix

AIDS, acquired immunodeficiency syndrome; HIV, human immunodeficiency virus; NT, nucleoside transporter; CNT, concentrative nucleoside transporter; ENT, equilibrative nucleoside transporter; MIP, major intrinsic protein; AZT, 3′-azido-3′-deoxythymidine; ddC, 2′,3′-dideoxycytidine; ddI, 2′,3′-dideoxyinosine; gemcitabine, 2′-deoxy-2′,2′-difluorocytidine; bp, base pair(s); kb, kilobase(s); PCR, polymerase chain reaction; kDa, kilodaltons; TM, transmembrane helix