Inhibition of Lymphocyte-Mediated Cytolysis and Cyclic amp Phosphodiesterase by Erythro-9–(2-Hydroxy-3-Nonyl)Adenine

Abstract

The adenosine deaminase (ADA) inhibitor erythro-9–(2-hydroxy-3-nonyl)adenine (EHNA), at low concentrations (<10 µM), enhances the inhibitory activity of adenosine against lymphocyte-mediated cytolysis (LMC) without itself being inhibitory. At higher concentrations, EHNA alone is inhibitory to LMC with an IC₅₀ of 160 µM. This inhibition is reversible upon washout, appears to affect an early stage of the lytic process, and does not appear to involve changes in basal levels of cyclic AMP (cAMP), ribonucleoside 5′-triphosphate pool sizes, S-adenosylhomocysteine levels, or protein carboxy-methylation. EHNA does enhance the cAMP response of cytolytic lymphocytes (CL) to activators of adenylate cyclase such as prostaglandin E_l. EHNA inhibits lymphocyte high-affinity cAMP phosphodiesterase at immunosuppressive levels, exhibiting hyperbolic mixed-type inhibition (K_i = 83 µM, α = 0.47, β = 0.18). Whereas inhibition of intralymphocytic ADA is complete at low concentrations (<25 µM) of EHNA, inhibition of LMC and intralymphocytic cAMP phosphodiesterase increases linearly with EHNA concentration to at least 200 µM. The presence of 200 µM EHNA during the centrifugation of mixtures of CL and EL4 leukemia target cells leads to increased CL cAMP levels. 2′-Deoxycoformycin, a more potent ADA inhibitor than EHNA, is not inhibitory to LMC and shows none of these cAMP-related effects. These results suggest that CL-target cell contact stimulates adenylate cyclase in the CL and that EHNA inhibits LMC due to its enhancement of this target cell-stimulated elevation of cAMP.