PROTEIN TYROSINE PHOSPHORYLATION INDUCED VIA THE IGG RECEPTORS FC-GAMMA-RI AND FC-GAMMA-RII IN THE HUMAN MONOCYTIC CELL-LINE THP-1

We have investigated the role of protein tyrosine phosphorylation in transmembrane signaling via the IgG receptors Fc-gamma-RI and Fc-gamma-RII in the human monocytic cell line THP-1. Fc-gamma-RI and Fc-gamma-RII were selectively engaged using the anti-Fc-gamma-RI mAb 197 (IgG2a) and the anti-Fc-gamma-RII mAb IV.3 (IgG2b). Addition to cells of mAb 197, but not addition of IgG2a mAb of irrelevant specificity, resulted in the rapid induction of cytoplasmic protein tyrosine phosphorylation as assessed by antiphosphotyrosine immunoblotting. A similar pattern of tyrosine phosphorylation was induced by mAb IV.3, but not by control IgG2b mAb. The induction of tyrosine phosphorylation by anti-Fc-gamma-R mAb was not dependent on antibody Fc region-FcR interactions, because tyrosine phosphorylation was also induced by cross-linked anti-Fc-gamma-RI F(ab')2 fragments and by cross-linked anti-Fc-gamma-RII Fab fragments. To investigate the relationship of Fc-gamma-R-induced tyrosine phosphorylation and activation of phospholipase C, which is known to follow Fc-gamma-R engagement, we assessed the effect of the tyrosine kinase inhibitor herbimycin A on Fc-gamma-R-induced Ca2+ flux. Herbimycin A strongly inhibited cellular Ca2+ flux induced by mAb 197, but did not inhibit Ca2+ flux induced by aluminum fluoride, suggesting that tyrosine phosphorylation may be important in regulating Fc-gamma-R-mediated activation of phospholipase C. Consistent with this, mAb 197 induced rapid phosphorylation of the gamma-1 isoform of phospholipase C. Finally, herbimycin A strongly inhibited the induction of TNF-alpha mRNA accumulation by Fc-gamma-R cross-linking. These results suggest that protein tyrosine phosphorylation may play an important role in the activation of phospholipase C and in the induction of monokine gene expression that follows engagement of Fc-gamma-R in human monocytes.