Complex regulation of nucleoside transporter expression in epithelial and immune system cells

Nucleoside transporters have a variety of functions in the cell, such as the provision of substrates for nucleic acid synthesis and the modulation of purine receptors by determining agonist availability. They also transport a wide range of nucleoside-derived antiviral and anticancer drugs. Most mammalian cells co-express several nucleoside transporter isoforms at the plasma membrane, which are differentially regulated. This paper reviews studies on nucleoside transporter regulation, which has been extensively characterized in the laboratory in several model systems: the hepatocyte, an epithelial cell type, and immune system cells, in particular B cells, which are non-polarized and highly specialized. The hepatocyte co-expresses at least two Na+-dependent nucleoside transporters, CNT1 and CNT2, which are up-regulated during cell proliferation but may undergo selective loss in certain experimental models of hepatocarcinomas. This feature is consistent with evidence that CNT expression also depends on the differentiation status of the hepatocyte. Moreover, substrate availability also modulates CNT expression in epithelial cells, as reported for hepatocytes and jejunum epithelia from rats fed nucleotide-deprived diets. In human B cell lines, CNT and ENT transporters are co-expressed but differentially regulated after B cell activation triggered by cytokines or phorbol esters, as described for murine bone marrow macrophages induced either to activate or to proliferate. The complex regulation of the expression and activity of nucleoside transporters hints at their relevance in cell physiology.