Specificity and Complexity of Integrin Signaling

	The Src Family of tyrosine Kinases (SFKs) are dedicated to integrin signaling. These enzymes localize to focal adhesions, which are sites of integrin-mediated cell attachment to the matrix, and phosphorylate several of their target-effectors at these sites.

Most integrins activate Focal Adhesion Kinase (FAK) and thereby Src Family Kinases (SFKs), causing phosphorylation of, and hence signaling from, p130-CAS and paxillin. A subset of integrins -α1β1, α5β1, and αvβ3 - also activate the adaptor protein Shc. Further complexity arises from the existence of integrin-specific mechanisms of signaling, such as those exemplified by α6β4. Despite this complexity and specificity, the essence of integrin signaling - what integrins do - is simple: they promote cell survival and impart positional control to the action of receptor tyrosine kinases (RTKs), determining whether cells proliferate and migrate in response to soluble growth factors and cytokines.

The shared integrin signaling machinery (adapted from Giancotti and Tarone. 2003. Annu. Rev. Cell Dev. Biol. 19:173-206).

We have elucidated several integrin signaling pathways. Maja Oktay provided evidence that FAK/SFK signaling induces activation of JNK and cell cycle progression through phosphorylation of p130-CAS and recruitment of Crk. Laura Barberis showed that CAS/Crk signaling can lead to activation of ERK if a cell expresses B-Raf. Kishore Wary and Fabrizio Mainiero demonstrated for the first time signaling differences between integrins. They showed that a subset of integrins -including α1β1, α5β1, αvβ3, and α6β4 - activate SFKs, and thereby ERK, independently of FAK. Kishore Wary demonstrated that α1β1, α5β1, and αvβ3 activate ERK through the SFK/Shc pathway. In this pathway, caveolin - or another lipid microdomain organizer - couples the transmembrane segment of the integrin α subunit to a palmytoylated SFK. Upon integrin-mediated activation, the palmytoylated SFK undergoes a conformational change and its newly exposed SH-3 domain recruits Shc. Shc is then phosphorylated on tyrosine and combines with the Grb2/SOS complex, causing activation of ERK and thereby AP-1-dependent transcription. In primary fibroblasts and endothelial cells, the integrins that activate this pathway are able to cooperate with RTKs to promote cell survival and cell proliferation, whereas other integrins are not able to do so. Amel Mettouchi provided evidence that the Shc-linked integrins promote cell cycle progression predominantly by inducing - through Rac - translation of the Cyclin D1 mRNA.

Integrin-specific signaling (adapted from Giancotti and Tarone. 2003. Annu. Rev. Cell Dev. Biol. 19:173-206).

We are using iRNA-mediated knock-down followed by reconstitution with wild-type and mutant cDNAs to examine the mechanisms by which integrin signaling controls translation of Cyclin D, and the role of various Rho family proteins in this process.