Phorbol ester regulation of Ca2+ flux during natural, lectin and antibody-dependent killing.

Phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), an activator of protein kinase C, suppresses natural, lectin and antibody-dependent killing by normal human lymphocytes in short-term radioisotope release assays. Fifty percent inhibition of killing of lymphoid target cells was seen at approximately 5 ng/ml TPA and inhibition was further potentiated by the presence of monocytic cells. In contrast, TPA increased killing of K-562 erythroleukaemic cells by non-adherent NK cells with optimal activity around I ng/ml. Two anti-estrogenic drugs, tamoxifen and clomiphene, known to inhibit protein kinase C, gave near to complete inhibition of NK killing at concentrations 12 and 30 μM, respectively. Retinal, another protein kinase C inhibitor, inhibited both antibody- dependent killing and lectin-dependent killing. An influx of ⁴⁵Ca²⁺ into the effector population was found during effector-target cell conjugation and this flux was suppressed at TPA concentrations similar to those that suppressed killing. The results suggest that killing depends on a co-ordinated activation of protein kinase C together with a channel-dependent calcium influx. TPA may suppress killing by a negative feedback effect of protein kinase C on the hydrolysis of inositol phospholipids, as demonstrated in many other systems, or through the down-regulation of cell surface receptors required for triggering of lysis. [ABSTRACT FROM AUTHOR]