Mutation of Vav1 adaptor region reveals a new oncogenic activation.
Bottom Line: We show that the oncogenicity of activated Vav1 proto-oncogene is associated with a non-degradative phosphorylation of β-catenin at residues important for its functions and its redistribution along the cell membrane in fibroblasts.In these cells, Vav1 is also involved in the modulation of β-catenin phosphorylation.Altogether, our data highlight that only a single mutation in the proto-oncogene Vav1 enhances tumorigenicity.
Affiliation: INSERM, UMR 978, Bobigny, France.
Vav family members function as remarkable scaffold proteins that exhibit both GDP/GTP exchange activity for Rho/Rac GTPases and numerous protein-protein interactions via three adaptor Src-homology domains. The exchange activity is under the unique regulation by phosphorylation of tyrosine residues hidden by intra-molecular interactions. Deletion of the autoinhibitory N-terminal region results in an oncogenic protein, onco-Vav, leading to a potent activation of Rac GTPases whereas the proto-oncogene barely leads to transformation. Substitution of conserved residues of the SH2-SH3 adaptor region in onco-Vav reverses oncogenicity. While a unique substitution D797N did not affect transformation induced by onco-Vav, we demonstrate that this single substitution leads to transformation in the Vav1 proto-oncogene highlighting the pivotal role of the adaptor region. Moreover, we identified the cell junction protein β-catenin as a new Vav1 interacting partner. We show that the oncogenicity of activated Vav1 proto-oncogene is associated with a non-degradative phosphorylation of β-catenin at residues important for its functions and its redistribution along the cell membrane in fibroblasts. In addition, a similar interaction is evidenced in epithelial lung cancer cells expressing ectopically Vav1. In these cells, Vav1 is also involved in the modulation of β-catenin phosphorylation. Altogether, our data highlight that only a single mutation in the proto-oncogene Vav1 enhances tumorigenicity.
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Mentions: Since subcellular localization and phosphorylation on particular residues of β-catenin are crucial events that modulate its different functions, we further investigated whether Vav1 proteins could have an impact on these modifications. Western blot analysis with anti pS675 β-catenin, a residue phosphorylated by the JNK kinase [29, 40] confirmed increased phosphorylation in Vav1 expressing cells, and more significantly with the two oncogenic forms. Similar increases of pY654- and non phosphorylated- β-catenin were also observed for all Vav1 expressing cells (Figure 6a). A non specific increase of all phosphorylated forms in these cells was ruled out by using anti pY142-, pS552 and pY489- β-catenin that remained with constant levels in all the transfectants (data not shown).