Differential expression and regulation of nucleoside transport systems in rat liver parenchymal and hepatoma cells

Primary cultures of rat-liver parenchymal cells show carrier-mediated nucle oside uptake by a mechanism that mainly involves concentrative, Na+-depende nt transport activity. In contrast, the hepatoma cell line FAO shows high n ucleoside transport activity, although it is mostly accounted for by Nat-in dependent transport processes. This is associated with a low amount of sodi um purine nucleoside transporter (SPNT) mRNA. SPNT encodes a purine-preferr ing transporter expressed in Liver parenchymal cells. To analyze whether SP NT expression is modulated during cell proliferation, SPNT mRNA levels were determined in the early phase of liver growth after partial hepatectomy an d in synchronized FAO cells that had been induced to proliferate. SPNT mRNA amounts increased as early as 2 hours after partial hepatectomy. FAO cells induced to proliferate after serum refeeding show an increase in SPNT mRNA levels, which is followed by an increase in Na+-dependent nucleoside uptak e and occurs before the peak of H-3-thymidine incorporation into DNA. FAO c ells also express significant equilibrative nucleoside transport activity, which may be accounted for by the expression of the nitrobenzylthioinosine (NBTI)-sensitive and -insensitive isoforms, rat equilibrative nucleoside tr ansporter I (rENT1) and rENT2, respectively. Interestingly, rENT2 mRNA leve ls follow a similar pattern to that described for SPNT when FAO cells are i nduced to proliferate, whereas rENT1 appears to be constitutively expressed . Liver parenchymal cells show low and negligible mRNA levels for rENT1 and rENT2 transporters, respectively, although most of the equilibrative trans port activity found in hepatocytes is NBTI-resistant. It is concluded that: 1) SPNT expression is regulated both in vivo and in vitro in a way that ap pears to be dependent on cell cycle progression; 2) SPNT expression may be a feature of differentiated hepatocytes; and 3) equilibrative transporters are differentially regulated, rENT2 expression being cell cycle-dependent. This is consistent with its putative role as a growth factor-induced delaye d early response gene.