C/EBPβ Regulates Metastatic Gene Expression and Confers TNF-α Resistance to Prostate Cancer Cells

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