Loss of N-Myc interactor promotes epithelial-mesenchymal-transition by activation of TGF-β/SMAD signaling

Epithelial-Mesenchymal-Transition (EMT) is one of the critical cellular programs that facilitate the progression of breast cancer to an invasive disease. We have observed that the expression of N-myc interactor (NMI) decreases significantly during progression of breast cancer, specifically in invasive and metastatic stages. Recapitulation of this loss in breast cell lines with epithelial morphology [MCF10A (non-tumorigenic) and T47D (tumorigenic)] by silencing NMI expression causes mesenchymal-like morphological changes in 3-D growth, accompanied by up-regulation of SLUG and ZEB2 and increased invasive properties. Conversely, we found that restoring NMI expression attenuated mesenchymal attributes of metastatic breast cancer cells accompanied by distinctly circumscribed 3-D growth with basement membrane deposition and decreased invasion. Further investigations into the downstream signaling modulated by NMI revealed that NMI expression negatively regulates SMAD signaling, which is a key regulator of cellular plasticity. We demonstrate that NMI blocks TGF-β/SMAD signaling via up-regulation of SMAD7, a negative feedback regulator of the pathway. We also provide evidence that NMI activates STAT signaling which negatively modulates TGF-β/SMAD signaling. Taken together, our findings suggest that loss of NMI during breast cancer progression could be one of the driving factors that enhance invasive ability of breast cancer by aberrant activation of TGF-β/SMAD signaling.