PBK/TOPK enhances aggressive phenotype in prostate cancer via ß-catenin-TCF/LEF-mediated matrix metalloproteinases production and invasion.

A current challenge in prostate cancer treatment is how to differentiate aggressive disease from indolent prostate cancer. There is an urgent need to identify markers that would accurately distinguish indolent prostate cancer from aggressive disease. The aim of this study was to evaluate the role of PDZ Domain-binding kinase (PBK) in prostate cancer and to determine if PBK expression enhances aggressiveness in prostate cancer. Using archival tissue samples, gain-of-function and loss-of-function studies, we show that PBK expression is up-regulated in prostate cancer, and its expression level is commensurate with invasiveness. Modulation of PBK expression and function causally regulates the invasive ability of prostate cancer cells. Production of matrix metalloproteinases-2 and -9, which are key players in metastatic invasion, is up-regulated, and the promoters of these genes are transcriptionally activated by PBK via increased ß-catenin-TCF/LEF signaling. Prostate cancer tissue specimens show that PBK's expression correlates with aggressive disease and distant metastasis in bone, lymph node and abdomen. Our in vitro and in situ data are in agreement that PBK could be a prognostic biomarker for prostate cancer that would discriminate aggressive prostate cancer from indolent disease, and is a potential target for the therapeutic intervention of aggressive prostate cancer in men.

Attenuation of DNA damage checkpoint by PBK, a novel mitotic kinase, involves protein-protein interaction with tumor suppressor p53.

Pathways adopted by developing cancer cells for evasion of cellular surveillance mechanism deserve attention for therapeutic exploitation as well as for better prognosis. A novel mitotic kinase, PDZ-binding kinase or PBK, which is upregulated in a variety of neoplasms including hematological malignancies, has been the focus of our attention with a goal to understand its role in malignant conversion and to examine as a possible new therapeutic target in disparate types of cancer. Earlier, we reported that PBK expression was downregulated during macrophage differentiation of HL60 promyelocytic leukemia cells, during doxorubicin-induced growth arrest in G2/M phase and that PBK was regulated by cell cycle-specific transcription factors E2F and CREB/ATF. Here, we demonstrate that HT1080 fibrosarcoma cells become adapted to doxorubicin-induced DNA damage checkpoint upon ectopic expression of a phosphomimetic mutant of PBK as indicated by the accumulation of polyploid cells. Aberrant entry into the mitotic phase by these cells is suggested by the appearance of a mitotic phase-specific marker, MPM-2. We propose that the effect is due to downregulation of p53 caused by direct physical interaction with PBK as detected by both a biochemical means as well as by yeast two-hybrid analysis. Together, our studies provide a plausible explanation for the role of PBK augmenting tumor cell growth following transient appearance in different types of progenitor cells in vivo as reported.

Expression of PDZ-binding kinase (PBK) is regulated by cell cycle-specific transcription factors E2F and CREB/ATF.

Earlier we reported that a novel mitotic protein kinase, PDZ-binding kinase (PBK), is expressed in primary hematopoietic neoplasms. Recent reports have suggested a role for PBK in mitotic progression. In the present study, we demonstrate that PBK is down regulated during doxorubicin induced growth arrest of HL60 promyelocytic leukemia cells at least partly due to cell cycle-specific transcriptional regulation. Furthermore, we show that transcriptional control is mostly due to binding of transcription factors E2F and CREB/ATF to two distinct binding sites within the PBK promoter. This was demonstrated by: (i) electrophoretic mobility shift assays showing transcription factor binding within the PBK promoter at the putative E2F (-146bp) and CREB/ATF (-312bp) binding sites; (ii) Western immunoblot analysis of knockdown extracts from siRNA inhibition of transcription factor expression showing that PBK protein expression is dependent upon the presence of these transcription factors; (iii) codistribution of CREB factor and PBK in cell lines of disparate tissue origin; and (iv) luciferase reporter assays showing that PBK promoter activity is dependent on factor binding at intact E2F and CREB/ATF sites. These findings may provide insight into the mechanisms that upregulate PBK expression in proliferative hematologic malignancies and down regulate its expression following growth arrest of leukemic cells.

NKX-3.1 interacts with prostate-derived Ets factor and regulates the activity of the PSA promoter.

The NKX-3.1 homeobox gene maps to human chromosome 8p21, a region that undergoes frequent loss of heterozygosity in prostate tumors. Loss of Nkx-3.1 function in mice leads to epithelial overgrowth. To further elucidate the molecular basis of NKX-3.1 function, a genetic screen for proteins that interact with NKX-3.1 was performed. Prostate-derived Ets factor (PDEF) was identified as a potential partner of NKX-3.1. Coimmunoprecipitation analyses demonstrated that NKX-3.1 and PDEF are physically associated in prostate epithelial cells. Cotransfection analyses demonstrated that NKX-3.1 can abolish the transcriptional activation function of PDEF on the prostate-specific antigen (PSA) promoter. These data identify PSA as a target gene for NKX-3.1 and provide new insights into the function of this candidate tumor suppressor.