Inactivation of the Tumor Suppressor CYLD Sensitizes Mice to Breast Cancer Development.

BACKGROUND/AIM: Breast cancer is a leading cause of cancer-related deaths among women. Down-regulation of the tumor suppressor gene Cyld in breast cancer has been linked to a poor prognosis. This study investigated the role of Cyld in breast cancer using conditional mutant mouse models carrying a Cyld mutation, which inactivates the deubiquitinating activity of its protein product CYLD in mammary epithelial cells. MATERIALS AND METHODS: We examined the potential of CYLD inactivation to induce mammary tumors spontaneously or modify the susceptibility of mice to mammary tumorigenesis by DMBA treatment or ErbB2 over-expression. RESULTS: CYLD inactivation significantly increased susceptibility to breast cancer induced by either DMBA treatment or ErbB2 over-expression. Moreover, while CYLD inactivation alone did not lead to spontaneous mammary tumorigenesis, it did contribute to the formation of multifocal hyperplastic lesions in virgin mice of predominantly FVB/NJ background. CONCLUSION: Our study demonstrates the tumor enhancing potential of CYLD inactivation in mammary tumorigenesis in vivo and establishes novel relevant mouse models that can be exploited for developing prognostic and therapeutic protocols.

The Tumor Suppressor CYLD Inhibits Mammary Epithelial to Mesenchymal Transition by the Coordinated Inhibition of YAP/TAZ and TGF Signaling.

Downregulation of the cylindromatosis (CYLD) tumor suppressor has been associated with breast cancer development and progression. Here, we report a critical role for CYLD in maintaining the phenotype of mammary epithelial cells in vitro and in vivo. CYLD downregulation or inactivation induced an epithelial to mesenchymal transition of mammary epithelial cells that was dependent on the concomitant activation of the transcription factors Yes-associated protein (YAP)/transcriptional coactivator with PDZ-binding motif (TAZ) and transforming growth factor beta (TGF)signaling. CYLD inactivation enhanced the nuclear localization of YAP/TAZ and the phosphorylation of Small Mothers Against Decapentaplegic (SMAD)2/3 proteins in confluent cell culture conditions. Consistent with these findings were the hyperplastic alterations of CYLD-deficient mouse mammary epithelia, which were associated with enhanced nuclear expression of the YAP/TAZ transcription factors. Furthermore, in human breast cancer samples, downregulation of CYLD expression correlates with enhanced YAP/TAZ-regulated target gene expression. Our results identify CYLD as a critical regulator of a signaling node that prevents the coordinated activation of YAP/TAZ and the TGF pathway in mammary epithelial cells, in order to maintain their phenotypic identity and homeostasis. Consequently, they provide a novel conceptual framework that supports and explains a causal implication of deficient CYLD expression in aggressive human breast cancers.

Activation of peroxisome proliferator-activated receptor gamma in mammary epithelial cells upregulates the expression of tumor suppressor Cyld to mediate growth inhibition and anti-inflammatory effects.

Several studies have implicated the downregulation of the tumor suppressor Cyld expression in breast cancer development. However, the mechanisms that regulate Cyld expression in mammary epithelial cells are largely unknown. In order to investigate them, a bioinformatic analysis of the promoter region of Cyld was performed and identified putative nuclear hormone receptor response elements that included peroxisome proliferator-activated receptor gamma (PPAR-γ)-responsive elements. In the present study, we showed that upon activation of the nuclear hormone receptor PPAR-γ by the agonist troglitazone (TZD), there was a significant increase in Cyld mRNA in human mammary epithelial cell lines. The effect of TZD could be attributed to the transactivation of the Cyld promoter as indicated by the upregulation of a luciferase reporter that was driven by the -1995 to +95 region of the human Cyld gene. Furthermore, the upregulation of Cyld expression by TZD was dependent on PPAR-γ since downregulation of PPAR-γ expression by RNAi compromised the induction of Cyld expression by TZD. CYLD induction mediated, at least in part, the TZD-mediated downregulation of tumor necrosis factor α (TNFα)-induced interleukin 8 (IL-8). In addition, downregulation of CYLD compromised the cytotoxic effects of TZD in immortalized mammary epithelial cells. Our results demonstrated that PPAR-γ is a novel regulator of Cyld transcription and identified CYLD as a mediator of the PPAR-γ-dependent anti-inflammatory and anti-proliferative activity in mammary epithelial cells, which underscores its potential to be used as a target for the development of breast cancer therapeutic approaches.