Julie Wilsbacher, Sheri Moores and Joan Brugge have an interesting paper recently in Cell Communication & Signaling that’s worth noting, based upon observations of cell migration in MCF-10A mammary epithelial cells expressing an active form of Vav1.
Wilsbacher et al. found that expression of an activated form of Vav1 (Vav1Y3F) increased cell migration regardless of epidermal growth factor (EGF) expression, as well as increase Rac1 activation and phosphorylation of PAK and ERK. The Dbl homology, pleckstrin homology, and cysteine-rich domains of Vav1Y3F were all required for Rac1 and ERK activation in the absense of EGF; whereas the Src Homology SH2 and SH3 domains were not required for Rac1, but were necessary for a full increase in migration and constitutive ERK phosphorylation. Further, EGF-Receptor function was necessary for the observed EGF-independent Vav1Y3F-promoted migration, and conditioned media collected from Vav1Y3F expressing cells stimulated migration of parental MCF-10A cells. Lastly, treatment of cells with the EGF receptor inhibitory antibody blocked the Vav1Y3F-induced, EGF-independent stimulation of ERK phosphorylation, but had no effect on Rac1 activation or PAK phosphorylation.
All of this points to a dual activity of Vav1 in mediating both Rac-mediated cytoskeleton rearrangements necessary for cell migration and the autocrine EGF stimulation associated with a wide range of epithelial cancers. As mutation of the third amino acid of Vav1 from Tyr to Phe is associated with Vav1 activation, clearly phosphorylation of Vav1 at this site has a negatively regulating effect.
Wilsbacher et al. don’t mention whether it is known already or not, and I’m not sure, but it seems likely that this pY3 site of Vav1 might act as an auto-inhibitory site, that interacts intra-molecularly with Vav1’s SH2 domain to keep the protein in a folded-closed position – much like the role that pY527 of Src plays in regulation of Src activity. If that’s the case, the upstream activator of Vav1 will likely have a phosphotyrosine motif of higher specificity for Vav1’s SH2 domain than its own pY3 motif.
And the search for more mechanisms of cancer cell behavior continues…
Vav proteins are guanine nucleotide exchange factors (GEF) for Rho family GTPases. Overexpression of Vav proteins enhances lamellipodium and ruffle formation, migration, and cell spreading, and augments activation of many downstream signaling proteins like Rac, ERK and Akt. Vav proteins are composed of multiple structural domains that mediate their GEF function and binding interactions with many cellular proteins.
- An active form of Vav1 induces migration of mammary epithelial cells by stimulating secretion of an epidermal growth factor receptor ligand. Wilsbacher JL, Moores SL, Brugge JS. Cell Commun Signal. 2006 May 18; 4(1):5. Pubmed