Abstract
In monocytes and macrophages, activation of the tyrosine kinase Syk is an essential step in the biochemical cascade linking aggregation of receptors for immunoglobulin G (FcγR) to initiation of effector functions. An increase in Syk activation during differentiation of myeloid cells by different agents has been reported. We studied the activation state of Syk in response to FcγRII crosslinking in monocytic cells before and after in vitro differentiation with 1α, 25‐dihydroxy‐vitamin D3. We show here that while in undifferentiated THP‐1 cells clustering of FcγRII induces significant phosphorylation and activation of Syk, in THP‐1 cells differentiated in vitro by 1α, 25‐dihydroxy‐vitamin D3, FcγRII crosslinking induced a decrease in Syk activity. In vitro differentiation did not induce changes in the expression of FcγRII isoforms. The observed effect on Syk activation though FcγRII could be mediated by differentiation‐induced changes in the expression and basal activation level of Syk, as well as changes in the association of Syk with the tyrosine phosphatase SHP‐1. These results suggest that the biochemical signaling pathways induced by FcγRII could be dependent on the differentiation state of the cell. J. Cell. Biochem. 89: 1056–1076, 2003. © 2003 Wiley‐Liss, Inc.