C/EBPβ regulates metastatic gene expression and confers TNF-α resistance to prostate cancer cells

BACKGROUND CCAAT/enhancer‐binding protein β (C/EBPβ) is a transcription factor and consists of three isoforms, transcription‐activating A/B (C/EBPβ‐AB) and transcription inhibitory C (C/EBPβ‐C). We previously reported that C/EBPβ‐C was predominantly expressed in hormone‐dependent LNCaP cells, while C/EBPβ‐AB forms were predominant in hormone‐independent prostate cancer (HI‐PCa) cells. METHODS Reporter gene analysis was performed to investigate transcriptional activity of C/EBPβ on metastatic gene expression upon TNF‐α treatment. NF‐κB activation and C/EBPβ protein upregulation were determined by immunoblotting. WST assay was used to determine the role of C/EBPβ in TNF‐α‐induced cell death. RESULTS We first determined that the C/EBPβ‐C overexpression or siRNA‐mediated C/EBPβ depletion decreased TNF‐α‐induced promoter activities of Bfl‐1, IL‐6, and IL‐8 genes. IL‐6 and IL‐8 are autocrine growth factors of HI‐PCa cells and Bfl‐1 is an anti‐apoptotic protein whose function in prostate cancer is yet to be determined. Secondly, we determined differential regulation of C/EBPβ by TNF‐α. In DU‐145 cells, C/EBPβ was upregulated by TNF‐α, but downregulated in LNCaP cells, although NF‐κB was activated in both cells. This result suggested cell‐type specific activation of signaling pathways leading to C/EBPβ upregulation, which was distinct from that leading to NF‐κB activation. Most importantly, C/EBPβ depletion decreased cell growth and sensitized DU‐145 cells to TNF‐α‐induced cell death. Conversely, overexpression of C/EBPβ‐A in LNCaP cells increased resistance to TNF‐induced cell death and TNF‐induced promoter activities of IL‐6 and Bfl‐1. CONCLUSION Our study, for the first time, demonstrated that C/EBPβ regulated cell growth and conferred TNF‐α resistance to PCa cells, in part, via regulation of metastatic gene expression. Prostate 69: 1435–1447, 2009. © 2009 Wiley‐Liss, Inc.