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

Epithelial-mesenchymal transition 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 3D growth, accompanied by upregulation of SLUG and ZEB2 and increased invasive properties. Conversely, we found that restoring NMI expression attenuated the mesenchymal attributes of metastatic breast cancer cells, accompanied by distinctly circumscribed 3D 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-ß/SMAD signaling via upregulation of SMAD7, a negative feedback regulator of the pathway. We also provide evidence that NMI activates STAT signaling, which negatively modulates TGF-ß/SMAD signaling. Taken together, our findings suggest that loss of NMI during breast cancer progression could be one of the driving factors that enhance the invasive ability of breast cancer by aberrant activation of TGF-ß/SMAD signaling.

DNAJB6 chaperones PP2A mediated dephosphorylation of GSK3ß to downregulate ß-catenin transcription target, osteopontin.

Elevated levels of the oncoprotein, osteopontin (OPN), are associated with poor outcome of several types of cancers including melanoma. We have previously reported an important involvement of DNAJB6, a member of heat-shock protein 40 (HSP40) family, in negatively impacting tumor growth. The current study was prompted by our observations reported here which revealed a reciprocal relationship between DNAJB6 and OPN in melanoma specimens. The 'J domain' is the most conserved domain of HSP40 family of proteins. Hence, we assessed the functional role of the J domain in activities of DNAJB6. We report that the J domain of DNAJB6 is involved in mediating OPN suppression. Deletion of the J domain renders DNAJB6 incapable of impeding malignancy and suppressing OPN. Our mechanistic investigations reveal that DNAJB6 binds HSPA8 (heat-shock cognate protein, HSC70) and causes dephosphorylation of glycogen synthase kinase 3ß (GSK3ß) at Ser 9 by recruiting protein phosphatase, PP2A. This dephosphorylation activates GSK3ß, leading to degradation of ß-catenin and subsequent loss of TCF/LEF (T cell factor1/lymphoid enhancer factor1) activity. Deletion of the J domain abrogates assembly of this multiprotein complex and renders GSK3ß inactive, thus, stabilizing ß-catenin, a transcription co-activator for OPN expression. Our in-vitro and in-vivo functional analyses show that silencing OPN expression in the background of deletion of the J domain renders the resultant tumor cells less malignant despite the presence of stabilized ß-catenin. Thus, we have uncovered a new mechanism for regulation of GSK3ß activity leading to inhibition of Wnt/ß-catenin signaling.