Regulation of the TGF-ß pathway by deubiquitinases in cancer.

The transforming growth factor-ß (TGF-ß) pathway regulates diverse cellular processes. It signals via serine/threonine kinase receptors and intracellular Smad and non-Smad effector proteins. In cancer cells, aberrant TGF-ß signalling can lead to loss of growth inhibition and an increase in invasion, epithelial-to-mesenchymal transition (EMT) and metastasis. Therapeutic targeting of the pro-oncogenic TGF-ß responses is currently being explored as a potential therapy against certain invasive and metastatic cancer types. The ubiquitin post-translational regulation system is emerging as a key regulatory mechanism for the control of TGF-ß pathway components. In this review, we focus on the role of deubiquitinases (DUBs), which counteract the activity of E3 ubiquitin ligases. We will discuss the mechanisms by which specific DUBs control Smad and non-Smad TGF-ß signalling routes, and how perturbation of the expression and function of DUBs contributes to misregulation of TGF-ß signalling in cancer.

Smad6 determines BMP-regulated invasive behaviour of breast cancer cells in a zebrafish xenograft model.

The transforming growth factor-ß (TGF-ß) family is known to play critical roles in cancer progression. While the dual role of TGF-ß is well described, the function of bone morphogenetic proteins (BMPs) is unclear. In this study, we established the involvement of Smad6, a BMP-specific inhibitory Smad, in breast cancer cell invasion. We show that stable overexpression of Smad6 in breast cancer MCF10A M2 cells inhibits BMP signalling, thereby mitigating BMP6-induced suppression of mesenchymal marker expression. Using a zebrafish xenograft model, we demonstrate that overexpression of Smad6 potentiates invasion of MCF10A M2 cells and enhances the aggressiveness of breast cancer MDA-MB-231 cells in vivo, whereas a reversed phenotype is observed after Smad6 knockdown. Interestingly, BMP6 pre-treatment of MDA-MB-231 cells induced cluster formation at the invasive site in the zebrafish. BMP6 also stimulated cluster formation of MDA-MB-231 cells co-cultured on Human Microvascular Endothelial Cells (HMEC)-1 in vitro. Electron microscopy illustrated an induction of cell-cell contact by BMP6. The clinical relevance of our findings is highlighted by a correlation of high Smad6 expression with poor distant metastasis free survival in ER-negative cancer patients. Collectively, our data strongly indicates the involvement of Smad6 and BMP signalling in breast cancer cell invasion in vivo.

Key role for ubiquitin protein modification in TGFß signal transduction.

The transforming growth factor ß (TGFß) superfamily of signal transduction molecules plays crucial roles in the regulation of cell behavior. TGFß regulates gene transcription through Smad proteins and signals via non-Smad pathways. The TGFß pathway is strictly regulated, and perturbations lead to tumorigenesis. Several pathway components are known to be targeted for proteasomal degradation via ubiquitination by E3 ligases. Smurfs are well known negative regulators of TGFß, which function as E3 ligases recruited by adaptors such as I-Smads. TGFß signaling can also be enhanced by E3 ligases, such as Arkadia, that target repressors for degradation. It is becoming clear that E3 ligases often target multiple pathways, thereby acting as mediators of signaling cross-talk. Regulation via ubiquitination involves a complex network of E3 ligases, adaptor proteins, and deubiquitinating enzymes (DUBs), the last-mentioned acting by removing ubiquitin from its targets. Interestingly, also non-degradative ubiquitin modifications are known to play important roles in TGFß signaling. Ubiquitin modifications thus play a key role in TGFß signal transduction, and in this review we provide an overview of known players, focusing on recent advances.