Ectopic alpha IIb beta 3 integrin signaling involves 12-lipoxygenase- and PKC-mediated serine phosphorylation events in melanoma cells

Abstract

Megakaryocytic genes such as alpha IIb beta3 can be expressed by malignant cells as part of the disturbances in their gene regulation. However, the function of the gene product greatly depends on the interaction of the ectopic protein with the new environment. The outside-in signaling of the ectopically expressed alpha IIb beta3 integrin was studied in B16a murine melanoma cells using a monoclonal antibody, specifically directed to the activated conformation of alpha IIb beta3, PAC-1 and the physiological ligand, fibrinogen. Ligation of alpha IIb beta3 induced downregulation of FAK but serine phosphorylation of three protein bands, 20/21, 85 and 140 kDa within 1-15 min. Flow cytometry indicated that the ligation of the receptor in B16a cells induces similar to 50%. increase in phosphoserine positive cells within 5-15 min. 12-lipoxygenase is placed downstream in the signaling pathway, since ligation of alpha IIb beta3 induces 12-HETE production within 5 min and pretreatment of tumor cells with select lipoxygenase inhibitior, Baicalein, prevents the increase in serine phosphorylation. Confocal microscopy of adherent tumor cells demonstrated rearrangement of actin filaments upon alpha IIb beta3 ligation paralleled by downregulation of p125FAK and phoshotyrosine(+) adhesion plaques and translocation of PKC alpha to stress fibers and cortical actin. PKC appears to be the major effector serine kinase of the alpha IIb beta3-coupled signaling pathway, since pretreatment of tumor cells with a select PKC inhibitor, Calphostin C, prevents the ligation-induced serine phosphorylation. Previous studies have indicated a role for the 12-lipoxygenase-PKC signaling pathway in platelet aggregation as well as tumor invasion, therefore the involvement of this cascade in the signaling of the ectopic alpha IIb beta3 integrin may partially explain its role in tumor progression.