Disease relevance of SLC29A2

• Equilibrative nucleoside transporter-2 (hENT2) protein expression correlates with ex vivo sensitivity to fludarabine in chronic lymphocytic leukemia (CLL) cells [1].
• Only a few endometrial carcinomas (15%) were found to be negative for hCNT1, but they all retained hENT1 and hENT2 expression [2].
• Demonstration of equilibrative nucleoside transporters (hENT1 and hENT2) in nuclear envelopes of cultured human choriocarcinoma (BeWo) cells by functional reconstitution in proteoliposomes [3].
• The human and rat nucleoside transport proteins hENT1, rENT1, hENT2 and rENT2 were produced in Xenopus oocytes and investigated for their ability to transport three 3'-deoxy-nucleoside analogues, ddC (2'3'-dideoxycytidine), AZT (3'-azido-3'-deoxythymidine) and ddI (2'3'-dideoxyinosine), used in human immunodeficiency virus (HIV) therapy [4].
• Thus, mutations in the coding segments of the HNP36 gene are not the cause of the MEN I syndrome [5].

High impact information on SLC29A2

• Examination of expressional levels of the equilibrative nucleoside transporter (ENT)1 and ENT2 revealed a transcriptionally dependent decrease in mRNA, protein, and function in endothelia and epithelia [6].
• Despite a huge individual variability in the mRNA amounts for every transporter gene expressed in CLL cells (CNT2, CNT3, ENT1, and ENT2), no relationship between mRNA levels and in vitro fludarabine cytotoxicity was observed [7].
• The first mammalian examples of the equilibrative nucleoside transporter family to be characterized, hENT1 and hENT2, were passive transporters located predominantly in the plasma membranes of human cells [8].
• To verify the roles of these residues in dipyridamole interactions, hENT2, which naturally exhibits low dipyridamole sensitivity, was mutated to contain side chains favorable for high affinity dipyridamole binding (i.e. a Met at the TM 1 and/or an Ile at the TM 11 positions) [9].
• The single mutants exhibited increased hENT2 sensitivity to inhibition by dipyridamole, and the double mutant was the most sensitive, with an IC50 value that was only 2% of that of wild type [9].

Chemical compound and disease context of SLC29A2

• Competition studies with HPX and thymidine transport via ENT2 indicated an overlap between nucleoside and nucleobase transport transporters in the breast cancer cell lines (MCF7 and T-47D) [10].

Biological context of SLC29A2

• The low overall genetic diversity in SLC29A2 makes it unlikely that variation in the coding region contributes significantly to clinically observed differences in drug response [11].
• This 456-residue protein is 46% identical in amino acid sequence with hENT1 and corresponds to a full-length form of the delayed-early proliferative response gene product HNP36, a protein of unknown function previously cloned in a form bearing a sequence deletion [12].
• The best-characterised members of the family, hENT1 and hENT2, possess similar broad substrate specificities for purine and pyrimidine nucleosides, but hENT2 in addition efficiently transports nucleobases [13].
• These data suggest that the delayed early response gene HNP36 is a truncated form of ENT2 and that the full-length open reading frame of ENT2 is required for production of a functional plasma membrane ei transporter [14].
• Residue 33 of human equilibrative nucleoside transporter 2 is a functionally important component of both the dipyridamole and nucleoside binding sites [15].

Anatomical context of SLC29A2

• In hENT1- and hENT2-containing HeLa cells, initial rates of uptake (10 microM; picomoles per cell per second) of [3H]TaraC, [3H]araC, and [3H]deoxycytidine were low (0.30 +/- 0.003, 0.42 +/- 0.03, and 0.51 +/- 0.11, respectively) and mediated primarily by hENT1 (approximately 74, approximately 65, and approximately 61%, respectively) [16].
• We now report the molecular cloning and functional expression in Xenopus oocytes of a cDNA from the same tissue encoding a homologous ei-type transporter, which we designate hENT2 [12].
• Uptake studies demonstrated that the majority of adenosine transport was mediated by hENT1, which was localized to both apical and basolateral membranes, with a smaller hENT2-mediated component in basolateral membranes [17].
• Human hepatocytes express hCNT1, hCNT2, hENT1, and hENT2 [18].
• All four isoforms are widely distributed in mammalian tissues, although their relative abundance varies: ENT2 is particularly abundant in skeletal muscle [13].

Associations of SLC29A2 with chemical compounds

• Like hENT1, hENT2 mediates saturable transport of the pyrimidine nucleoside uridine (Km 0.2+/-0.03 mM) and also transports the purine nucleoside adenosine [12].
• These results suggest that ribavirin is taken up by BeWo cells via both the high-affinity Na(+)-dependent transporter hCNT3 and the low-affinity Na(+)-independent transporters hENT1 and hENT2 [19].
• Transient expression studies with the full-length ENT2 and a 5'-truncated construct that lacks the first start codon (predicted protein 99% identical to HNP36) demonstrated that only the full-length construct conferred uridine transport activity to the cells [14].
• The human (h) and rat (r) equilibrative (Na(+)-independent) nucleoside transporters (ENTs) hENT1, rENT1, hENT2, and rENT2 belong to a family of integral membrane proteins with 11 transmembrane domains (TMs) and are distinguished functionally by differences in sensitivity to inhibition by nitrobenzylthioinosine and coronary vasoactive drugs [20].
• In contrast, the affinity of hENT2 for inosine is 4-fold higher than hENT1 [21].

Other interactions of SLC29A2

• The concentration dependence of inhibition of [(3)H]uridine transport in S. cerevisiae by TR exhibited lower K(i) values than BR: hCNT3 (5.4 versus 226 microM), hENT2 (16 versus 271 microM), hENT1 (57 versus 168 microM), and hCNT1 (221 versus 220 microM) [22].
• DESIGN AND METHODS: The relative amounts of hENT1 and hENT2-related mRNA and protein were analyzed in five MCL cell lines and 20 primary MCL tumors by real-time quantitative reverse transcriptase polymerase chain reaction and western blots [23].
• Consequently, the HNP36 gene was studied as a candidate for the MEN1 gene [5].

Analytical, diagnostic and therapeutic context of SLC29A2

• Moreover, analysis by RT-PCR showed that BeWo cells express mRNA of hCNT3, hENT1 and hENT2 [19].
• Quantitative PCR showed a transient decrease in the expression of both hENT1 (human ENT1) and hENT2 mRNAs within 4-12 h of induction of the inactive CK2alpha' subunit, but both transcripts had returned to control levels by 24 h [24].
• In this study, using polyclonal monospecific antibodies we have observed a significant correlation between the expression of hENT2 by Western blot and fludarabine uptake via hENT carriers and also with ex vivo sensitivity of CLL cells to fludarabine [1].
• To determine whether the reciprocal mutation in hENT2 (Ile33 to Met) also altered sensitivity to dilazep and dipyridamole, hENT2-I33M was created by site-directed mutagenesis [25].
• The primary goal of this study was to localize these transporters in polarized renal epithelia. hENT1 and hENT2 were tagged with green fluorescence protein, stably expressed in renal epithelial cells, and localized by immunofluorescence and functional analysis [26].

References