The integrins bind to distinct, although partially overlapping, subsets of extracellular matrix proteins and transmit both mechanical and chemical signals. In addition to imparting polarity to the cell and organizing and remodeling its cytoskeleton during adhesion and migration, these signals exert a stringent control on cell survival and cell proliferation.
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, 5β1, 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.
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 β subunit to a palmitoylated SFK. Upon integrin-mediated activation, the palmitoylated 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 by inducing — through Rac — translation of the mRNA encoding Cyclin D1.
Miguel Lopez-Lago examined the underlying mechanism and found that Rac promotes PAK phosphorylation and thereby inactivation of Merlin. Loss of Merlin in turn activates mTORC1 and translation of cyclinD and other mRNAs involved in cell cycle progression and cell survival.
We are currently using affinity purification in conjunction with mass spectrometry to identify membrane-proximal components involved in integrin signaling. After biochemical validation, all novel integrin interactors are subjected to functional analysis. The results of these experiments suggest that the integrin signaling system is characterized by an unanticipated degree of complexity and specificity.