EVIDENCE FOR THE INVOLVEMENT OF ECTO-5-NUCLEOTIDASE (CD73) IN DRUG-RESISTANCE

Increased ecto-5'-nucleotidase (ecto-5'NT) protein expression in sever al multidrug-resistant (MDR) cell lines, documented previously by our group, suggests that this enzyme is involved in drug resistance. Here, Northern blot analysis of selected cell lines and their MDR variants positively correlated ecto-5'NT protein with its mRNA expression. An i nhibitor of ecto-5'NT enzymatic activity, alpha,beta-methyleneadenosin e 5'-diphosphate (AMP-CP), was used to determine if functionally activ e enzyme had a role in drug resistance. AMP-CP (0.3 mM) reversed the r esistance of ecto-5'NT-positive MDR cells (MCF7/AL, L1210/A) to doxoru bicin, whereas it did not affect the doxorubicin sensitivity of the ec to-5'NT-negative parental cell lines or that of 2 ecto-5'NT-negative M DR cell lines (HL60/VCR and A2780/DX5). Furthermore, AMP-CP increased rhodamine uptake and inhibited rhodamine efflux from ecto-5'NT-positiv e MDR cells without affecting ecto-5'NT-negative MDR cells. The presen ce of exogenous adenosine (0.5 mu M) circumvented AMP-CP-induced inhib ition of rhodamine efflux from EL4/ADM cells. AMP-CP inhibited the gro wth of the ecto-5'NT-positive L1210/A MDR cells but had no effect on t he growth of the parental cell line. Determination of intracellular AT P levels indicated that MDR cells which had increased ecto-5'NT expres sion also had a lower intracellular ATP level than their parental cell s. Our results suggest that, in certain MDR cell lines, ecto-5'NT serv es as a required accessory molecule in resistance mediated by ATP-depe ndent mechanisms and that growth-sustaining nucleosides are provided b y this salvage pathway. (C) 1996 Wiley-Liss, Inc.