Deoxyadenosine blockade of G₀ to G₁ transition in lymphocytes: Possible involvement of protein kinases

Immunodeficiency in adenosine deaminase deficiency has been attributed to the lymphotoxicity of deoxyadenosine that accumulates to high levels in patients. To gain insight into the mechanism of deoxyadenosine toxicity, we investigated the dose-response and time course of its toxic effects on concanavalin A-stimulated mouse splenic lymphocytes by thymidine incorporation and flow cytometry. Deoxyadenosine at a level as low as 0.3 μM inhibited the progression of G₀. In contrast, higher concentrations of the nucleoside, i.e., in the range of 1 to 3 μM, were needed to block transition of the stimulated lymphocytes from G₀ to G₁. The inhibition of their S entry and progression required even higher concentrations. Furthermore, staurosporine, a potent inhibitor of protein kinases, was found to potentiate the toxicity of deoxyadenosine in mitogen-stimulated lymphocytes. Calcium mobilization in mitogen-activated lymphocytes was inhibited by deoxyadenosine. Our data suggest that, while ribonucleotide reductase inhibition by dATP could explain the blockade of S entry and progression by deoxyadenosine in cycling lymphocytes or leukemic cells, more important effects of this compound on antigen-activated lymphocytes occur at the early G₀ phase. A possible mechanism of deoxyadenosine lethality is its inhibition of protein phosphorylation.