SILAC identifies LAD1 as an oncogenic filamin binder regulating actin dynamics in response to EGF and marking aggressive breast tumors

Mutations mimicking growth factor-induced proliferation and motility characterize aggressive subtypes of mammary tumors. To unravel novel players, we applied phosphoproteomics on untransformed mammary cells, which were pre-stimulated with the epidermal growth factor (EGF). This analysis identified ladinin-1 (LAD1), a hitherto poorly characterized protein, as a phosphor-effector of the EGF-to-ERK pathway. We report that LAD1 is essential for mammary cell proliferation and migration. LAD1 is transcriptionally induced, undergoes phosphorylation by EGF and partly co-localizes with actin stress fibers. Yeast 2-hybrids, proximity ligation and co-immunoprecipitation assays revealed that LAD1 binds with filamins, actin cross-linking proteins. Co-sedimentation analyses attribute to LAD1 a role in actin treadmilling, probably in collaboration with SFN/14-3-3sigma. Depletion of LAD1 led to a decrease in transcripts relevant to cell survival, and inhibited growth of mammary xenografts in an animal model. Furthermore, LAD1 is highly expressed in two aggressive subtypes of breast cancer, as well as predicts poor patient prognosis. This study identifies a new cytoskeletal component essential for cell migration and for the acquisition of oncogenic attributes by human mammary tumors.