Differential Expression and Regulation of Nucleoside Transport Systems in Rat Liver Parenchymal and Hepatoma Cells

Abstract

Primary cultures of rat–liver parenchymal cells show carrier–mediated nucleoside uptake by a mechanism that mainly involves concentrative, Na⁺–dependent transport activity. In contrast, the hepatoma cell line FAO shows high nucleoside transport activity, although it is mostly accounted for by Na⁺–independent transport processes. This is associated with a low amount of sodium purine nucleoside transporter (SPNT) mRNA. SPNT encodes a purine–preferring transporter expressed in liver parenchymal cells. To analyze whether SPNT expression is modulated during cell proliferation, SPNT mRNA levels were determined in the early phase of liver growth after partial hepatectomy and 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 uptake and occurs before the peak of ³H–thymidine incorporation into DNA. FAO cells 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 transporter 1 (rENT1) and rENT2, respectively. Interestingly, rENT2 mRNA levels follow a similar pattern to that described for SPNT when FAO cells are induced 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 transport 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 appears 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 delayed early response gene.