Translationally regulated C/EBP beta isoform expression upregulates metastatic genes in hormone-independent prostate cancer cells.

C/EBP beta is a transcription factor regulating key biological processes including cellular growth and differentiation and its increased expression correlates with tumor invasiveness. Recently, the increased expression of C/EBP beta was reported in proliferative inflammatory atrophy of the prostate, associating with increased COX-2 expression and androgen receptor (AR) downregulation. C/EBP beta expression was determined in DU-145, PC-3 and LNCaP cells by immunoblotting. Transient transfection of C/EBP beta expression vectors was performed to investigate translational regulation of its isoform expression. Reporter gene analysis was performed to investigate transcriptional activity of C/EBP beta on metastatic gene expression. We determined that transcriptionally active, full-length C/EBP beta isoforms were dominantly expressed in hormone-independent DU-145 and PC-3 cells, while transcription-repressing truncated isoform was dominant in hormone-dependent LNCaP cells. Our results further showed lack of full-length isoform expression from the transiently transfected C/EBP beta expression vector in LNCaP cells compared to that in PC-3 cells transfected with the same vector, while the expression of truncated isoform was comparable in both cell lines. Interestingly, however, the most upstream initiation site A null mutation restored translation of full-length isoform in LNCaP cells. These results suggest that full-length C/EBP beta isoform expression in LNCaP cells may be suppressed at the upstream initiation sites, likely at site A. Most importantly, C/EBP beta overexpression significantly upregulated promoter activities of IL-8, COX-2, and anti-apoptotic Bfl-1 genes. Our study demonstrates that C/EBP beta is an important transcription factor upregulating metastatic gene expression and that its isoform expression is differentially regulated at the translational level in prostate cancer cells.