EPS8 phosphorylation by Src modulates its oncogenic functions

BACKGROUND: EPS8 is a scaffolding protein that regulates proliferation, actin dynamics and receptor trafficking. Its expression is increased in cancer, enhancing mitogenesis, migration and tumorigenesis. Src phosphorylates EPS8 at four tyrosine residues, although the function is unknown. Here we investigated the pro-oncogenic role of EPS8 tyrosine phosphorylation at Src target sites in HNSCC. METHODS: Plasmids expressing EPS8 Src-mediated phosphorylation site mutants (Y485F, Y525F, Y602F, Y774F and all four combined [FFFF]) were expressed in cells containing a normal endogenous level of EPS8. In addition, cells were treated with dasatinib to inhibit Src activity. EPS8 downstream targets were evaluated by western blotting. Wound closure, proliferation, immunofluorescence and tumorgenicity assays were used to investigate the impact of phenylalanine mutations on EPS8 biological functions. RESULTS: FOXM1, AURKA, and AURKB were decreased in cells expressing FFFF- and Y602F-EPS8 mutants, while cells harbouring the Y485F-, Y525F- and Y774F-EPS8 mutants showed no differences compared to controls. Consistent with this, dasatinib decreased the expression of EPS8 targets. Moreover, Y602F- and FFFF-EPS8 mutants reduced mitogenesis and motility. Strikingly though, FFFF- or Y602F-EPS8 mutants actually promoted tumorigenicity compared with control cells. CONCLUSIONS: Phosphorylation of EPS8 at Y602 is crucial for signalling to the cell cycle and may provide insight to explain reduced efficacy of dasatinib treatment.