Separation of integrin-dependent adhesion from morphological changes based on differential PLC specificities

In normal lymphocytes an inside-out signal up-regulating integrin adhesion is followed by a ligand-mediated outside-in cell spreading signal. Protein kinase C (PKC) inhibition blocks lymphocyte adherence to and spreading on fibronectin. In contrast, putative PLC inhibitors yield distinct differences with respect to adhesion and morphology. The phosphatidylinositol-specific phospholipase C (PLC) inhibitor neomycin blocked spreading of CD3/CD28-activated T cells on fibronectin by disrupting adhesion. Furthermore, when an additional inside-out signal for fibronectin adhesion is unnecessary such as with HPB-ALL T leukemic or phorbol-myristate-acetate-treated normal T cells, neomycin treatment does not alter adhesion or morphology. However, the phosphatidylcholine-specific PLC inhibitor D609 abrogates cell spreading without affecting adhesion to fibronectin in these cells as well as the CD3/CD28-activated T cells. These results strongly suggest that inside-out signaling for the integrin alpha4beta1 in lymphocytes proceeds through phosphatidylinositol-specific PLC and PKC, whereas the outside-in signal utilizes phosphatidylcholine-specific PLC and PKC.