FAK and Phosphatidyl Inositol in Cell Polarity
After the past three days of blogging focal adhesion kinase (FAK), each focusing on an important regulator of cell adhesion dynamics and cell motility, I’m going to turn my attention to phosphatidyl inositol-3 kinase (PI3K). PI3K has a regulatory subunit (p85), and a catalytic subunit (p110) capable of catalyzing the phosphorylation of the D3 position of the inositol ring of a class of lipid components of the cell membrane. In the inactive state, p85 binds to p110, blocking its activity; but when the Src homology 2 (SH2) domain of p85 finds a phosphotyrosine-containing sequence in another protein that it likes, it dissociates from p110, which is then active.
Among the phosphotyrosine-containing activators of PI3K are FAK (through integrin receptors) and growth factor receptors (or receptor tyrosine kinases, RTK’s). In a recent study, Teet Velling and colleagues in Sweden sought to distinguish between FAK and RTK mechanisms of PI3K activation in EGFR and ?1 integrins utilize different signaling pathways to activate Akt. (Akt is activated as a product of the inositol phosphorylation)
So, what did Velling et al. show?
Through a variety of immunoblots, they demonstrated that tyrosine phosphorylation of FAK is not required for activation of Akt during EGF-stimulated cell adhesion, and indeed that FAK/Src-specific inhibitors inhibited FAK-related but not EGF-stimulated activation of Akt. The immunoblots examined in vitro Akt activity under the effects of kinase inhibitors, EGF, fibronectin-like matrix components, and analysis of protein expression and phosphorylation levels following those stimuli.
The striking observation, from my perspective, is the neat delineation between the integrin and RTK signal pathways. My personal observations in the lab lead me to an unsupported conclusion that the two synergistic pathways were complementary and synergistic, when in fact it turns out here that they appear as alternatives.
For the figure shown above (Fig1A), Velling et al. used GD25 cells deficient for beta-1 integrin and transfected with splice forms that can (beta-1A) and cannot (beta-1B) activate FAK. Akt kinase assay was performed by immunoprecipitating Akt and measuring phosphorylation of a recombinant fragment of glycogene synthase kinase-3 (pGSK).
- Velling T, Stefansson A, Johansson S. (2008) Exp Cell Res., 314(2):309-16. doi:10.1016/j.yexcr.2007.08.018
molecular biology
