Nitric oxide produced in response to engagement of β2 integrins on human neutrophils activates the monomeric GTPases Rap1 and Rap2 and promotes adhesion

We found that engagement of beta 2 integrins on human neutrophils increased the levels of GTP-bound Rap1 and Rap2. Also, the activation of Rap1 was blocked by PP1, SU6656, LY294002, GF109203X, or BAPTA-AM, which indicates that the downstream signaling events in Rap1 activation involve Src tyrosine kinases, phosphoinositide 3-kinase, protein kinase C, and release of calcium. Surprisingly, the beta 2 integrin-induced activation of Rap2 was not regulated by any of the signaling pathways mentioned above. However, we identified nitric oxide as the signaling molecule involved in beta 2 integrin-induced activation of Rap1 and Rap2. This was illustrated by the fact that engagement of beta 2 integrins increased the production of nitrite, a stable end-product of nitric oxide. Furthermore, pretreatment of neutrophils with N-omega-mono-methy1-L-arginine, or 1400W, which are inhibitors of inducible nitric- oxide synthase, blocked beta 2 integrin-induced activation of Rap1 and Rap2. Similarly, R-P-8pCPT-cGMPS, an inhibitor of cGMP-dependent serine/threonine kinases, also blunted the beta 2 integrin-induced activation of Rap GTPases. Also nitric oxide production and its downstream activation of cGMP-dependent serine/threonine kinases were essential for proper neutrophil adhesion by beta 2 integrins. Thus, we made the novel findings that beta 2 integrin engagement on human neutrophils triggers production of nitric oxide and its downstream signaling is essential for activation of Rap GTPases and neutrophil adhesion.