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.