We have found that cultured differentiated astrocytes pretreated with N⁶, 2^'-O-dibutyryladenosine 3',5'-cyclic monophosphate (DBcAMP), a permeable analogue of cAMP, incorporate thymidine, but not uridine, via nucleoside transporters including equilibrative nucleoside transporters (ENTs） into TCA insoluble fraction for repair on DNA injury in the presence of hydrogen peroxide (H₂O₂) at an early time, and these phenomena are specific in differentiated astrocytes, but not undifferentiated astrocytes and neurons.
We studied expression of ENT3 and LIMPII (Lysosomal Integral Membrane Protein II) in cultured astrocytes by RT-PCR and western blot analysis and immunocytochemistry. ENT3 mRNA and protein expression were found by RT-PCR and western blot analysis.
Astrocytes were double stained by anti-GFAP antibody or anti-LIMPII antibody, and anti-ENT3 antibody. ENT3 was co-expressed with GFAP or LIMPII. We could confirm ENT3, that is assumed to be presented in lysosomes on cultured astrocytes.
These results indicate that ENT3 expressed in lysosomes in astrocytes.　Lysosomes incorporate foreign matters by phagocytosis and disassemble them and decomposition products including nucleosides were transported from lysosomes to cytosol for reuse.　
When H₂O₂-induced thymidine incorporation into astrocytes was increased, delayed cell death was suppressed for DNA repair involved in nucleosides supply via nucleoside transporters, including ENTs. ENT3 existed in lysosomes might transport nucleosides from lysosomes to cytosol for DNA repair in astrocytes exposed by H₂O₂.