Reversible ubiquitination regulates the Smad/TGF-β signalling pathway

TGF-β (transforming growth factor-β) signals through serine/threonine kinase receptors and intracellular Smad transcription factors. An important regulatory step involves specific ubiquitination by Smurfs (Smad– ubiquitin regulatory factors), members of the HECT (homologous to E6-associated protein C-terminus) ubiquitin ligase family, which mediate the proteasomal degradation of Smads and/or receptors. Recently, we have defined a novel interaction between Smads and UCH37 (ubiquitin C-terminal hydrolase 37), a DUB (de-ubiquitinating enzyme) that could potentially counteract Smurf-mediated ubiquitination. We have demonstrated specific interactions between UCH37 and inhibitory Smad7, as well as weaker associations with Smad2 and Smad3. Importantly, Smad7 can act as an adaptor able to recruit UCH37 to the type I TGF-β receptor. Consequently, UCH37 dramatically up-regulates TGF-β-dependent gene expression by deubiquitinating and stabilizing the type I TGF-β receptor. Our findings suggest that competing effects of ubiquitin ligases and DUBs in complex with Smad7 can serve to fine-tune responses to TGF-βs under various physiological and pathological conditions. Studies are currently under way using activity-based HA (haemagglutinin)-tagged ubiquitin probes to identify the full spectrum of DUBs that impact on Smad/TGF-β signalling activity. Introduction TGF-β (transforming growth factor-β) is a multifunctional cytokine that is implicated in a broad range of human diseases. For example, aberrant TGF-β signalling can cause fibrosis, cancer, congestive heart failure, chronic obstructive pulmonary disease, liver cirrhosis, kidney disease and cataracts. Understanding the molecular and cellular mechanisms by which TGF-β signals from the cell surface through the cytoplasm to the nucleus is of paramount importance if this pathway is to represent a useful therapeutic target. Following major scientific breakthroughs in the past decade, it is now known that TGF-β signals through cell-surface serine/threonine kinase receptors that phosphorylate R-Smad (receptorregulated Smad) transcription factors in the cytoplasm to effect specific modulation of gene expression [1]. Upon activation, the TGF-β pathway is rapidly ‘switched-off’ by the I-Smads (inhibitory Smads), Smad6 and Smad7. Smads and the Smurf (Smad–ubiquitin regulatory factor) ubiquitin ligases Ubiquitination-mediated proteolysis regulates the activity of diverse receptor systems and is involved in many biological processes including signal transduction, cell cycle and gene expression [2]. Recently, HECT (homologous to