Activation of human lymphocyte high affinity cyclic AMP phosphodiesterase by culture with 1-methyl-3-isobutylxanthine

When 1-methyl-3-isobutylxanthine (MIX), a potent inhibitor of human lymphocyte cyclic AMP phosphodiesterase in vitro, was added to intact lymphocyte cultures, MIX caused an activation of this enzyme. Activation by MIX which showed an EC ₅₀ = 0.2 mM, required 24 hr in culture, reached a maximum near 48 hr, and remained at maximum through 144 hr. The effect of MIX was to increase the maximum velocity of the 3.5-4S form but not the 5.5-6S form of the lymphocyte high affinity enzyme system. Activation required serum, occurred in calcium depleted media, and was unaffected by sodium azide, adenosine, cycloheximide, or actinomycin D. Further, the effect of MIX was not mimicked by cyclic AMP, cyclic GMP, or by a variety of cyclic nucleotide derivatives with the exception of N ⁶-monobutyryl- and N ⁶-2'0 dibutyryl cyclic AMP in the culture medium. No correlation was observed between the in vitro IC ₅₀ value and the amount of intact cell activation with other phosphodiesterase inhibitors. Although MIX prevented the mitogenic response of lymphocytes to phytohemagglutinin (PHA), a synergistic effect to activate cyclic nucleotide phosphodiesterase was obtained when both agents were included in the culture medium. These data are discussed as an example of a novel modulation of a specific high affinity form of cyclic AMP phosphodiesterase by an apparent non-genetic regulatory mechanism.