Inhibition of protein kinase C mediated signal transduction by tamoxifen. Importance for antitumour activity.

Recent studies have demonstrated tamoxifen inhibition of the enzyme protein kinase C (PKC) in vitro. The aim of this study was to investigate the effects of tamoxifen on PKC function in intact human cells. As PKC activates the neutrophil oxidase mechanism the neutrophil was chosen as an experimental model to assess PKC-tamoxifen interaction in these experiments. Neutrophils from healthy volunteers were separated by centrifugation through Ficoll Hypaque. Two separate parameters of oxidase activation; oxygen consumption and reactive oxygen metabolite production were monitored by a Clark electrode chamber and luminol dependent chemiluminescence respectively. Neutrophil chemiluminescence was markedly stimulated by 4 Phorbol-12 myristate-13 acetate (PMA). This stimulation was inhibited by tamoxifen; IC50 = 6.1 +/- 1.6 microM (means +/- S.E.M.) N = 6. Neutrophil oxygen consumption was similarly stimulated by PMA and inhibited by tamoxifen. The tamoxifen inhibition was not due to cell toxicity as assessment of cell integrity by the exclusion of trypan blue and measurement of intracellular concentrations of ATP showed no significant differences before and after treatment. Tamoxifen also inhibited neutrophil chemiluminescence which was stimulated by oleoyl acetyl glycerol and mezerein excluding interaction with PMA as an explanation of its inhibitory effect. These results are consistent with tamoxifen inhibition of PKC function in intact human cells. This may be central to its antitumour action.