ADP stimulates the respiratory burst without activation of ERK and AKT in rat alveolar macrophages

Alveolar macrophages (AM) are the first line of defense against infection i n the lungs. We previously showed that the production of superoxide and hyd rogen peroxide, i.e., the respiratory burst, is stimulated by adenine nucle otides (ADP >> ATP) in rat AM through signaling pathways involving calcium and protein kinase C. Here, we further show that ADP induces a rapid increa se in the tyrosine phosphorylation of several proteins that was reduced by the tyrosine kinase inhibitor genistein, which also inhibited the respirato ry burst. Interestingly, ADP did not trigger the activation of the mitogen- activated protein kinases ERK1 and ERK2, or that of protein kinase B/AKT, a downstream target of the phosphatidylinositol 3-kinase (PI3K) pathway. Thi s is in contrast to another stimulus of the respiratory burst, zymosan-acti vated serum (ZAS), which activates both the ERK and PI3K pathways. Thus, th is study demonstrates that the receptor for ADP in rat AM is not coupled to the ERK and AKT pathways and, that neither the ERK pathway nor AKT is esse ntial to induce the activation of the NAPDH oxidase by ADP in rat AM while tyrosine kinases appeared to be required. The rate and amount of hydrogen p eroxide released by the ADP-stimulated respiratory burst was similar to tha t produced by ZAS stimulation. The absence of ERK activation after ADP stim ulation therefore suggests that hydrogen peroxide is not sufficient to acti vate the ERK pathway in rat AM. Nonetheless, as hydrogen peroxide was neces sary for ERK activation by ZAS, this indicates that, in contrast to ADP, ZA S stimulates a pathway that is targeted by hydrogen peroxide and leads to E RK activation. (C) 2001 Elsevier Science Inc.