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 α4β1 in lymphocytes proceeds through phosphatidylinositol‐specific PLC and PKC, whereas the outside‐in signal utilizes phosphatidylcholine‐specific PLC and PKC. J. Leukoc. Biol. 65: 127–136; 1999.