Ecto‐5'‐nucleotidase (CD73) in multidrug‐resistant cell lines generated by doxorubicin

Cytochemical screening for a panel of enzymes revealed increased 5'nucleotidase (5'NT) expression in 3 of 3 Pglycoprotein 170 (Pgp 170)‐positive multidrug‐resistant (MDR) variants of the murine EL4 T‐lymphoma cell line (EL4/ADM, ER2 and ER13). Electron microscopic localization established the presence of the membrane‐bound ecto‐form of the enzyme. Nine other murine, human and Chinese hamster cell lines and their MDR variants were tested for ecto‐5'NT. Of these, 4 MDR variants (human cell lines MCF7A6, MCF7A2, HeLaJ2C and the murine cell line LI2I0A) showed increased expression of ecto‐5'NT, when compared with their parental cell lines. The findings with cells of human origin were confirmed by immunofluorescent localization with a specific monoclonal antibody (MAb) (27.2) against the human ecto‐5'NT. All MDR cell lines with elevated ecto‐5' NT expression were generated by doxorubicin treatment. These cells were more sensitive than their parental cell lines to AMP at concentrations of 1.5–3.0 mM, confirming that the expressed ecto‐5'NT was biologically active. The parental and MDR cells did not differ, in general, in their sensitivity to adenosine. An inhibitor of ecto‐5'NT, α,β‐methyleneadenosine 5'‐diphosphate, completely reversed the resistance of the EL4/ADM cell line to doxorubicin. The possibility exists of a functional relationship between the ecto‐5'NT molecule and the members of the ATP‐binding cassette transporter superfamily, important components of MDR, in some cell types.