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Integrin-Stimulated Tyrosine Phosphorylation and Signaling

C.R. Hauck, K.C. Jones, S.R. Reider, D.J. Sieg, D.D. Schlaepfer

Members of the integrin family of transmembrane receptors have long been recognized for their structural roles in linking extracellular matrix proteins with the actin cytoskeleton to regulate cell shape, cell migration, and tissue architecture. Recently, it has become clear that integrin receptors can initiate intracellular signals that can either accentuate cell growth or enhance cell survival under adverse conditions. Because integrin receptors do not have intrinsic catalytic activity, we are interested in the mechanisms by which integrins can stimulate intracellular signaling.

Integrins associate with or activate a number of different nonreceptor protein-tyrosine kinases, and it is thought that activation of these kinases initiates many of the downstream integrin-stimulated signaling events. The focal adhesion kinase (FAK) associates with integrin receptors, and its activity is enhanced by cell binding to extracellular matrix proteins such as fibronectin. Fibronectin-stimulated autophosphorylation of FAK at Tyr397 promotes the transient Src homology 2 (SH2) domain--dependent binding of Src family nonreceptor protein-tyrosine kinases to FAK. We found that the complex composed of FAK and Src protein-tyrosine kinases can facilitate fibronectin receptor--stimulated signals to downstream targets such as the ERK2/mitogen-activated protein kinase (MAPK) and the JNK/MAPK through the phosphorylation of and interaction with multiple SH2 domain--containing adaptor proteins such as Shc and Grb2.

Grb2 is a small modular SH2/SH3 domain--containing adaptor protein that can bind to the motif surrounding phosphorylated FAK Tyr925 and to multiple tyrosine phosphorylation sites in the Shc adaptor protein. Grb2 SH3 domain--mediated recruitment of the Sos GDP-GTP exchange factor for the small G protein Ras is a direct link to the activation of the ERK2/MAPK kinase pathway. We found that it is the independent and combined protein-tyrosine kinase activity from both fibronectin-stimulated FAK and Src-family activation events that leads to Grb2-mediated activation of the ERK2/MAPK pathway. Our studies show that fibronectin-mediated signaling upstream of Ras and ERK2 does not follow a linear pathway; instead, multiple Grb2-mediated interactions with Shc, FAK, and perhaps other yet-to-be-determined phosphorylated targets represent parallel pathways that cooperate to promote maximal ERK2/MAPK activation. Ongoing efforts are directed at elucidating the biological significance of the connections between FAK, Src, and ERK2/MAPK, because these connections may be associated with fibronectin-stimulated cell growth or migration and tumor cell metastasis (Fig. 1).

PUBLICATIONS

Li, S., Kim, M., Hu, Y.-L., Jalali, S., Schlaepfer, D.D., Hunter, T., Chien, S., Shyy, J.Y.-J. Fluid shear stress activation of FAK: Linking integrins to mitogen-activated protein kinases. J. Biol. Chem. 272:30455, 1997.

Schlaepfer, D.D., Hunter, T. Integrin signaling and tyrosine phosphorylation: Just the FAKs? Trends Cell Biol. 8:151, 1998.

Schlaepfer, D.D., Jones, K.C., Hunter, T. Multiple Grb2-mediated integrin-stimulated signaling pathways to ERK2/mitogen-activated protein kinase: Summation of both c-Src and FAK-initiated tyrosine phosphorylation events. Mol. Cell. Biol. 18:2571, 1998.