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Protein tyrosine phosphatase PRL-3 expression and transcriptional regulation in cancer and metastasis Wong, Patrick Chun Wei


Elevated expression of protein tyrosine phosphatase PRL-3 is implicated in cell migration, invasion, and metastasis, although the molecular targets of this phosphatase remain unidentified. PRL-3 expression is strikingly upregulated in metastatic colon carcinoma, compared with much lower expression in primary colon tumours and virtually undetectable expression in normal colon epithelia. Increased transcriptional activity is suggested to account for increased PRL-3 expression in 75% of the metastatic samples. We used bioinformatics analyses and cell-based promoter assays to investigate the tumour- and metastasis-specific transcriptional activity o f the ~4.3 kb promoter region upstream of the PRL-3 transcription start site. Through bioinformatics analyses, sequence conservation (human/mouse) and multiple putative transcription factor binding sites were identified in various regions upstream of the PRL-3 transcription start site. A 518bp CpG island (CpG-64) was predicted to have high regulatory potential. The transcriptional activities of the PRL-3 promoter were analyzed by luciferase reporter assays in two cell lines derived from (i) a primary colon adenocarcinoma (SW480) and (ii) a metastasis of the same tumour (SW620). These assays revealed that the full-length promoter region (which contains the CpG-64 island) was 4.5-fold more transcriptionally active in the metastatic colorectal carcinoma cells than in the primary colorectal carcinoma tumour cells. The C pG island itself directed 2.3-fold higher transcriptional activity in the metastatic colorectal carcinoma cells than in the primary colorectal tumour cells. In prostate cancer cells, this CpG-64 island was 8.7-fold more active in the highly invasive androgen-independent DUI45 cells compared to the less invasive androgen-dependent LNCaP cells. However, the full length promoter was 3.6-fold more active in the LNCaP cells than in the DUI45 cells. Additionally, this CpG-64 island was significantly more active in malignant muscle cells (RD and RMS 13) than in normal muscle cells (C2C12). These results suggest that the CpG-64 island contains regulatory elements responsible for the tumour- and metastasis-specific transcriptional upregulation of PRL-3. Identification of these elements and their interacting factors may provide insight into the molecular pathways that are involved in the tumour-to-metastatic transition. Using tissue-microarray technology, PRL-3 expression was detected in colorectal, uveal melanic, breast, and certain pediatric tumours. Immunohistochemical localization of PRL-3 appeared to be associated with tissue differentiation status. More specifically, nuclear PRL-3 was mainly present in poorly differentiated tissues, whereas cytoplasmic PRL-3 was predominantly found in well differentiated tissues. Also, high levels of PRL- 3 in clinical tumour samples correlated with an unfavorable trend in uveal melanic and breast cancer patient prognosis. In conclusion, the evidence from both the basic science and the clinical sides argue strongly for the importance of PRL-3 as a tumourigenic and/or metastatic biomarker. Thus, it is imperative to study this phosphatase at the molecular level to understand the clinical>'ramifications o f its activities.

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