Effects of gold coordination complexes on neutrophil function are mediated via inhibition of protein kinase C

Previous studies have shown that the gold compounds auranofin (AUR) and gold sodium thiomalate (GST) inhibit responses of various cells and tissues. We found that superoxide anion generation induced in human neutrophils by the chemotactic tripeptide fmet-leu-phe (1 microM), fluoride (18 mM), or phorbol myristate acetate (PMA, 100 nM) was inhibited by pretreatment of cells with 5-100 microM AUR. The extent of inhibition was dependent on AUR concentration and duration of the preincubation. GST was much less potent, inasmuch as only weak effects were observed at 5 times higher concentrations. The ineffectiveness of GST was attributed to its slower rate of penetration into cells, compared with AUR. The finding that mobilization of internal Ca2+ stores was not blocked in AUR-treated cells suggests that phospholipase C-mediated hydrolysis of polyphosphoinositides to inositol 1,4,5-trisphosphate was not inhibited by the drug. Because PMA is known to mimic the action of diacylglycerol in activating protein kinase C (PKC), we investigated the possibility that gold compounds might be interfering with signal transduction at this level. Enzymatic assays indicated that both gold compounds reduced the level of PKC activity associated with the cytosol; however, translocation of PKC to the plasma membrane was not found. Immunoblot analyses carried out with polyclonal anti-PKC antisera revealed that the gold compounds did not cause degradation of PKC or increase translocation to the membrane. Further studies indicated that enhanced endogenous protein phosphorylation resulting from PMA stimulation was attenuated in cells co-treated with AUR. Finally, in vitro enzymatic assays showed that both AUR and GST inhibited partially purified PKC in a concentration-dependent manner. It is suggested that modulation of PKC represents a mechanism of action of gold coordination complexes at the cellular level.