Identification of the Mitochondrial Targeting Signal of the Human Equilibrative Nucleoside Transporter 1 (hENT1)

Abstract

We have previously shown that the human equilibrative nucleoside transporter 1 (hENT1) is expressed and functional in the mitochondrial membrane and that this expression enhances the mitochondrial toxicity of the nucleoside drug, fialuridine ( The human nucleoside transporter, hENT1, 2 plays an important role in modulating the physiological activity of nucleosides and in the transport of many therapeutic nucleoside drugs used in the treatment of cancer (e.g. 5-fluorouridine) and viral diseases (e.g. ribavirin) (2). hENT1 is an equilibrative nucleoside transporter that is ubiquitously expressed (3). Two additional members of the equilibrative nucleoside transporter family have been identified, hENT2 and hENT3 (4, 5). hENT2 is expressed in a selective number of tissues, such as the muscle (3), whereas hENT3 appears to be an intracellular transporter, expressed primarily in the lysosomes (5). We have recently reported that hENT1 is also functionally present in intracellular compartments, specifically in the mitochondrial membrane (1). In addition, we have shown that the presence of hENT1 on the mitochondrial membrane facilitates the entry of fialuridine (FIAU) into the mitochondria, where it is phosphorylated to produce its mitochondrial toxicity.FIAU is a uridine analog that was developed for treatment of hepatitis B. Administration of this antiviral nucleoside to 15 patients with hepatitis B resulted in severe multisystem toxicity, due to mitochondrial damage, including fatal hepatotoxicity, pancreatitis, neuropathy, or myopathy (6). Among seven patients showing severe FIAU-mediated hepatotoxicity, five died, and two survived only after emergency liver transplant (6). hENT1, expressed in the mitochondrial membrane, facilitates the entry of the hydrophilic FIAU (1). Once in the mitochondria, FIAU is metabolized by thymidine kinase 2, a mitochondrial specific enzyme, to the monophosphate that is the rate-limiting step in the formation of the triphosphate (TP), a potent inhibitor of mitochondrial DNA polymerase-␥ (7). The mitochondrial hepatotoxicity of FIAU is widely accepted to be due to inhibition by FIAU-TP of the mitochondrial DNA polymerase ␥, resulting in depletion of mitochondrial DNA and cell death (8 -10).Interestingly, the lethal mitochondrial toxicity of FIAU observed in the clinic was not predicted from preclinical toxicity studies in rodents (rats or mice), even at doses that were 1,000-fold of those used in the human study (8,(11)(12)(13). This lack of predictive accuracy is troubling and important as the rodents are extensively used in drug development to predict human toxicity. These preclinical data on FIAU toxicity raise an important question. What is the mechanistic basis for this dramatic interspecies difference in FIAU toxicity? Understanding the mechanistic basis of the interspecies differences in FIAU toxicity will guide the use of appropriate species for future development of nucleoside drugs, including for preclinical toxicity studies. The lack of hepatotoxicity of FIAU in the rodents canno...

The Role of the Equilibrative Nucleoside Transporter 1 (ENT1) in Transport and Metabolism of Ribavirin by Human and Wild-Type or Ent1(-/-) Mouse Erythrocytes

“…The kinetics of ribavirin transport was measured in MDCK cells stably overexpressing mouse Ent1 using a method similar to that described previously (Lai et al, 2005;Lee et al, 2006). The Ent1-MDCK cells were grown in monolayers in complete media to 85% confluence in 24-well polystyrene plates.…”

Progesterone Receptor (PR) Isoforms PRA and PRB Differentially Regulate Expression of the Breast Cancer Resistance Protein in Human Placental Choriocarcinoma BeWo Cells

“…We identified two putative PREs, PRE1 (between Ϫ1143 and Ϫ1129 bp) and PRE2 (between Ϫ187 and Ϫ173 bp), using the NUBIScan program (University of Basel, Basel, Switzerland). PRE1 or PRE2 was deleted, one at a time, from the Ϫ1285/ϩ362 construct using polymerase chain reaction mutagenesis as described previously (Lee et al, 2006). The Ϫ243/ϩ362 construct was used as template for polymerase chain reaction mutagenesis to generate point (Mut1 and Mut2) or double (Mut3) mutations in PRE2 and deletion of PRE2.…”

Targeted Transgenic Overexpression of Mitochondrial Thymidine Kinase (TK2) Alters Mitochondrial DNA (mtDNA) and Mitochondrial Polypeptide Abundance

“…4 In humans, mitochondria are capable of importing unphosphorylated pyrimidine deoxyribonucleosides (and NRTIs) directly via the equilibrative nucleoside transporter (ENT1). [5][6][7][8][9] Three sequential phosphorylations follow intramitochondrially to generate the active deoxyribonucleoside-or NRTI triphosphate. The critical first intramitochondrial phosphorylation of pyrimidines is performed by the mitochondrial isoform thymidine kinase 2 (TK2, a nuclear-en-coded, mitochondrially localized TK in mitotically quiescent cells).…”