Adenosine transport: Recent advances in the molecular biology of nucleoside transporter proteins

Adenosine is a hydrophilic molecule that requires specialized transport pro teins for permeation of cell membranes. Functional studies have identified two types of nucleoside transport processes: equilibrative bidirectional pr ocesses driven by chemical gradients and inwardly directed concentrative pr ocesses driven by the sodium electrochemical gradient. The equilibrative nu cleoside transport processes (es, ei) are widely distributed among various cell types, whereas the concentrative nucleoside transport processes (cit, cif, cib, csg, cs) are present primarily in specialized epithelia. Using mo lecular cloning techniques and functional expression of cDNAs in oocytes of Xenopus laevis, we have isolated and characterized cDNAs encoding integral membrane proteins of the four major nucleoside transport processes of rat and human cells (es, ei, cit, cif). The equilibrative and concentrative nuc leoside transport processes are mediated by members of two previously unrec ognized groups of integral membrane proteins, which we have designated the Equilibrative Nucleoside Transporter (ENT) and the Concentrative Nucleoside Transporter (CNT) protein families. This review summarizes the current sta te of knowledge of the molecular biology of the ENT and CNT protein familie s, focusing on the role of these proteins (r/hENT1, r/hENT2, r/hCNT1, r/ hC NT2) in the transport of adenosine. Drug Dev. Res. 45:277-287, 1998. (C) 19 98 Wiley-Liss, Inc.