Research ArticleCancer

SILAC identifies LAD1 as a filamin-binding regulator of actin dynamics in response to EGF and a marker of aggressive breast tumors

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Sci. Signal.  30 Jan 2018:
Vol. 11, Issue 515, eaan0949
DOI: 10.1126/scisignal.aan0949

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LAD1 marks aggressive breast cancer

The actin cytoskeleton is a framework of filaments that gives cells shape. Dynamic regulation of the cytoskeleton coordinates complex cell behaviors, such as cell-cell communication, migration and invasion, and cell division, the regulation of which is critical to development, tissue homeostasis, and disease. Roth et al. showed that signaling by the epidermal growth factor receptor (EGFR) promoted cell migration–associated actin dynamics through the phosphorylation of the protein LAD-1 (see also the Focus by Chiasson-MacKenzie and McClatchey). LAD1 was also a marker of aggressive subtypes or cases of breast tumors in patient samples; thus, it might be a useful biomarker to inform clinical decisions.

Abstract

Mutations mimicking growth factor–induced proliferation and motility characterize aggressive subtypes of mammary tumors. To unravel currently unknown players in these processes, we performed phosphoproteomic analysis on untransformed mammary epithelial cells (MCF10A) that were stimulated in culture with epidermal growth factor (EGF). We identified ladinin-1 (LAD1), a largely uncharacterized protein to date, as a phosphorylation-regulated mediator of the EGF-to-ERK pathway. Further experiments revealed that LAD1 mediated the proliferation and migration of mammary cells. LAD1 was transcriptionally induced, phosphorylated, and partly colocalized with actin stress fibers in response to EGF. Yeast two-hybrid, proximity ligation, and coimmunoprecipitation assays revealed that LAD1 bound to actin–cross-linking proteins called filamins. Cosedimentation analyses indicated that LAD1 played a role in actin dynamics, probably in collaboration with the scaffold protein 14-3-3σ (also called SFN). Depletion of LAD1 decreased the expression of transcripts associated with cell survival and inhibited the growth of mammary xenografts in an animal model. Furthermore, LAD1 predicts poor patient prognosis and is highly expressed in aggressive subtypes of breast cancer characterized as integrative clusters 5 and 10, which partly correspond to triple-negative and HER2-positive tumors. Thus, these findings reveal a cytoskeletal component that is critically involved in cell migration and the acquisition of oncogenic attributes in human mammary tumors.

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