Adenosine-mediated killing of cultured epithelial cancer cells

Because micromolar concentrations of adenosine (Ado) have been documented r ecently in the interstitial fluid of carcinomas growing in animals, we exam ined the effects of low concentrations of Ado on the growth of cultured hum an carcinoma cells. Ado alone had little effect upon cell growth. In the pr esence of one of a number of Ado deaminase (ADA) inhibitors, Ado led to sig nificant growth inhibition of all cell lines tested. Similar effects were f ound when ATP, ADP, or AMP was substituted for Ado, Surprisingly, the ADA i nhibitor coformycin (CF) had a much greater potentiating effect than did 2' -deoxycoformycin (DCF), although DCF is a more potent ADA inhibitor. The gr owth inhibition of the Ado/CF combination was not abrogated by pyrimidines or caffeine, a nonspecific Ado receptor blocker. Toxicity was prevented by the addition of the Ado transport inhibitor dipyridamole or the Ado kinase inhibitor 5'-amino 5'-deoxyadenosine. S-Adenosylhomocysteine hydrolase is n ot involved because neither homocysteine thiolactone nor an S-adenosylhomoc ysteine hydrolase inhibitor (adenosine dialdehyde) potentiated toxicity of the Ado/CF combination. Unexpectedly, substitution of 2'-deoxyadenosine (th e toxic moiety in congenital ADA deficiency) for Ado, did not lead to equiv alent toxicity. The Ado/CF combination inhibited DNA synthesis and brought about morphological changes consistent with apoptosis. Together, these find ings indicate that the Ado-mediated killing proceeds via an intracellular r oute that requires the action of Ado kinase. The enhanced cofactor activity of CF may be attributable to its being a more potent inhibitor of AMP deam inase than is DCF.