Inhibition of a phosphodiesterase III in the lysis-sensitive target-induced elevation of cyclic AMP (cAMP) in human natural killer cells.
Natural killer (NK) cells are lymphocytes that are capable of destroying tumor cells and virally infected cells (cytolysis) without prior sensitization. When cyclic AMP (cAMP) is elevated artificially in NK cells, it is a potent inhibitor of their cytolytic function. Recently, we have shown that when NK cells are exposed to a range of lysis-sensitive (LS) tumor target cells, there is an increase in intracellular cAMP levels in the NK cells over a 60-min period. There is no increase in NK-cell cAMP in response to lysis-resistant (LR) tumor target cells. We determined that this cAMP elevation is due, in part, to an LS target-induced activation of adenylyl cyclase (AC), and that the AC-activation component appears to require a protein tyrosine kinase (PTK) activity. In the present study, we demonstrated that an LS target-induced inhibition of phosphodiesterase (PDE) is also contributing to the overall elevation of cAMP. Direct measurement of PDE activity showed an inhibition in lymphocytes that were exposed to LS targets but not in those exposed to LR targets. The inhibition of PDE activity was maximal by 30 min. Lymphocytes were exposed to targets and then lysed, so that PDE activity could be measured. Addition of class-selective inhibitors of PDE (at levels sufficient to completely block that class of PDE) to the lysate focused the measurement of PDE activity on those classes of PDE that were unaffected by the selective inhibitor. Using the PDE IV selective inhibitor rolipram and the PDE III selective inhibitors trequinsin and milrinone, we showed that a PDE III is being inhibited in lymphocytes by exposure to LS targets. As PDE III is known to be inhibited by elevated cyclic GMP (cGMP) levels, increased cGMP in NK cells following exposure to LS targets was a possible mechanism by which a PDE III in NK cells might be inhibited. However, when we measured cGMP levels in control and LS target-stimulated lymphocytes, we saw no change.