Concentrative nucleoside transporters (CNTs) in epithelia: from absorption to cell signaling.

Concentrative and Equilibrative Nucleoside Transporter proteins (CNT and ENT, respectively) are encoded by gene families SLC28 and SLC29. They mediate the uptake of natural nucleosides and a variety of nucleoside-derived drugs, mostly used in anticancer therapy. CNT and ENT proteins are mostly localized in the apical and basolateral sides, respectively, in (re)absorptive epithelia. This anatomic distribution determines nucleoside and nucleoside-derived vectorial flux. CNT expression (particularly CNT2) is associated with differentiation and is also nutritionally regulated in intestinal epithelia, whereas ENT protein amounts (mostly ENT1) are increased when cells are exposed to proliferative stimuli such as EGF, TGF-alpha or wounding. Although all these features suggest a role for NT proteins in nucleoside salvage and (re)absorption, recent data demonstrate that CNT2 might be under purinergic control, in a manner that is dependent on energy metabolism. A physiological link between CNT2 function and intracellular metabolism is also supported by the evidence that extracellular adenosine can activate the AMP-dependent kinase (AMPK), by a mechanism which relies upon adenosine transport and phosphorylation. Thus the complex pattern of NT isoform expression in mammalian cells can fulfill physiological roles other than salvage.