News and Publications

Signal Transduction by Integrins

M.A. Schwartz, V. Baron, M. del Pozo, W.B. Kiosses, J.M. Lewis, N. Meller, J.E. Meredith, L.S. Price, X.-D. Ren, M.W. Renshaw

In addition to its structural role in tissue organization and cell adhesion, the extracellular matrix regulates cell functions, including growth, survival, and gene expression. Transduction of signals across the plasma membrane by integrins apparently accounts for many of these regulatory effects.

Much of our work is directed toward understanding why cells require both growth factors and integrin-mediated adhesion to proteins in the extracellular matrix to progress through the cell cycle. This fact implies that signals from growth factors and integrins must converge at some point to regulate key events in the cell cycle.

One example is the joint regulation of the mitogen-activated protein (MAP) kinase pathway by integrins and growth factors. We found that serum or growth factors could activate the MAP kinase ERK2 in adherent cells but did so poorly in suspended cells. The point in the MAP kinase cascade that is sensitive to adhesion is MEK1. More recent work has shown that the integrin component of this pathway is dependent on focal adhesion kinase (FAK), a tyrosine kinase that is regulated by integrins. We found that serum-induced MAP kinase activity is restored by constitutive activation of FAK in suspended cells and blocked by inhibition or deletion of FAK in adherent cells. Current work is aimed at further elucidation of this pathway.

Previously, we showed that hydrolysis of phosphoinositides is also strongly dependent on both growth factors and cell adhesion. Nonadherent cells have low levels of 4,5-phosphatidylinositol bisphosphate because of integrin regulation of the enzyme that produces this lipid, phosphatidylinositol 4-phosphate 5-kinase. Growth factors can activate phospholipase C in suspended cells, but production of second messengers is minimal because of the low levels of substrate for the enzyme. Thus, at least 2 important growth control pathways (inositol lipids and MAP kinase) require both growth factors and cell adhesion.

A second major area of interest is the involvement of the small GTPases Rho, Cdc42, and Rac in integrin signal transduction. We found that Rac and Cdc42 are activated by cell adhesion and mediate cell spreading, migration, and possibly growth. Rho, which mediates formation of focal contacts and actin stress fibers, may also play a role in cell adhesion. Although published data suggest that both integrins and growth factors contribute to the function of Rho, the precise contributions are controversial. To address this question, we developed an assay for the activation state of Rho. We found that growth factors can trigger Rho activation independent of cell adhesion, but during cell adhesion and spreading, complex interactions occur that determine Rho activity, so that integrins appear to inactivate or activate Rho under different conditions. We plan further studies to understand in more detail how integrins contribute to the regulation of Rho.

We are also studying the nonreceptor tyrosine kinase c-Abl. We showed that c-Abl transiently localizes to focal adhesions during cell spreading and that integrin-mediated adhesion regulates c-Abl kinase activity. We recently found that c-Abl interacts with the focal adhesion protein paxillin and participates in the integrin-induced activation of MAP kinase.

Finally, we are studying an alternatively spliced variant of the integrin ß₁ subunit. In this variant, termed ß_1C, the C-terminal 20 residues of the ß₁ cytoplasmic domain are replaced with 42 unique residues. The variant does not localize to focal adhesions and is a potent inhibitor of progression of the cell cycle, leading to a block in late G₁ near the restriction point. We are continuing to investigate the mechanism by which integrin ß_1C inhibits cell growth.

PUBLICATIONS

Lewis, J.M., Schwartz, M.A. Integrins regulate the association and phosphorylation of paxillin by c-Abl. J. Biol. Chem., in press.

Price L.S., Leng, J., Schwartz, M.A., Bokoch, G.M. Activation of Rac and Cdc42 by integrins mediates cell spreading. Mol. Biol. Cell, in press.

Ren, X.-D., Schwartz, M.A. Regulation of inositol lipid kinases by Rho and Rac. Curr. Opin. Genet. Dev. 8:63, 1998.

Renshaw, M.W., Ren, X.-D., Schwartz, M.A. Growth factor activation of MAP kinase requires cell adhesion. EMBO J. 16:5592, 1997.

Schwartz, M.A. Integrins, oncogenes and anchorage independence. J. Cell Biol. 139:575, 1997.

Schwartz, M.A., Meredith, J.E., Kiosses, W.B. An activated Rac mutant functions as a dominant negative for membrane ruffling. Oncogene, in press.