Regulation of CYLD activity and specificity by phosphorylation and ubiquitin-binding CAP-Gly domains

Non-degradative ubiquitin chains and phosphorylation events govern signaling responses by innate immune receptors. The deubiquitinase CYLD in complex with SPATA2 is recruited to receptor signaling complexes by the ubiquitin ligase LUBAC and regulates Met1- and Lys63-linked polyubiquitin and receptor signaling outcomes. Here, we investigate the molecular determinants of CYLD activity. We reveal that two CAP-Gly domains in CYLD are ubiquitin-binding domains and demonstrate a requirement of CAP-Gly3 for CYLD activity and regulation of immune receptor signaling. Moreover, we identify a phosphorylation switch outside of the catalytic USP domain, which activates CYLD toward Lys63-linked polyubiquitin. The phosphorylated residue Ser568 is a novel tumor necrosis factor (TNF)-regulated phosphorylation site in CYLD and works in concert with Ser418 to enable CYLD-mediated deubiquitination and immune receptor signaling. We propose that phosphorylated CYLD, together with SPATA2 and LUBAC, functions as a ubiquitin-editing complex that balances Lys63- and Met1-linked polyubiquitin at receptor signaling complexes to promote LUBAC signaling.

Loss of Tumor Suppressor CYLD Expression Triggers Cisplatin Resistance in Oral Squamous Cell Carcinoma

Cisplatin is one of the most effective chemotherapeutic agents commonly used for several malignancies including oral squamous cell carcinoma (OSCC). Although cisplatin resistance is a major obstacle to effective treatment and is associated with poor prognosis of OSCC patients, the molecular mechanisms by which it develops are largely unknown. Cylindromatosis (CYLD), a deubiquitinating enzyme, acts as a tumor suppressor in several malignancies. Our previous studies have shown that loss of CYLD expression in OSCC tissues is significantly associated with poor prognosis of OSCC patients. Here, we focused on CYLD expression in OSCC cells and determined whether loss of CYLD expression is involved in cisplatin resistance in OSCC and elucidated its molecular mechanism. In this study, to assess the effect of CYLD down-regulation on cisplatin resistance in human OSCC cell lines (SAS), we knocked-down the CYLD expression by using CYLD-specific siRNA. In cisplatin treatment, cell survival rates in CYLD knockdown SAS cells were significantly increased, indicating that CYLD down-regulation caused cisplatin resistance to SAS cells. Our results suggested that cisplatin resistance caused by CYLD down-regulation was associated with the mechanism through which both the reduction of intracellular cisplatin accumulation and the suppression of cisplatin-induced apoptosis via the NF-κB hyperactivation. Moreover, the combination of cisplatin and bortezomib treatment exhibited significant anti-tumor effects on cisplatin resistance caused by CYLD down-regulation in SAS cells. These findings suggest the possibility that loss of CYLD expression may cause cisplatin resistance in OSCC patients through NF-κB hyperactivation and may be associated with poor prognosis in OSCC patients.