Aberrant phosphorylation of SMAD4 Thr277-mediated USP9x-SMAD4 interaction by free fatty acids promotes breast cancer metastasis

摘要

Obesity increases the risk of distant metastatic recurrence and reduces breast cancer (BC) survival. However, the mechanisms behind this pathology and identification of relevant therapeutic targets are poorly defined. Plasma free fatty acids (FFA) levels are elevated in obese individuals. Here we report that TGF-β transiently activates ERK and subsequently phosphorylates SMAD4 at Thr277, which facilitates a SMAD4-USP9x interaction, SMAD4 nuclear retention, and stimulates TGF-β /SMAD3-mediated transcription of Twist and Snail. USP9x inhibited the E3 ubiquitin-protein ligase TIF1γ from binding and monoubiquitinating SMAD4, hence maintaining SMAD4 nuclear retention. FFA further facilitated TGF-β-induced ERK activation, SMAD4 phosphorylation and nuclear retention, promoting TGF-β-dependent cancer progression. Inhibition of ERK and USP9x suppressed obesity-induced metastasis. Additionally, clinical data indicated that phospho-ERK and -SMAD4 levels correlate with activated TGF-β signaling and metastasis in overweight/obese patient BC specimens. Altogether, we demonstrate the vital interaction of USP9x and SMAD4 for governing TGF-β signaling and dyslipidemia-induced, aberrant TGF-β activation during BC metastasis.