PRL-3 Expression in Metastatic Cancers

Purpose: Expression of the PRL-3 tyrosine phosphatase is elevated in liver metastases derived from colorectal cancer (CRC). We sought to determine the cellular basis of this elevation and assess the expression of PRL-3 in metastatic lesions derived from cancers of the colon and other tissues. Experimental Design: We developed modifications of in situ hybridization methods that facilitated the study of paraffin-embedded sections. We also evaluated PRL-3 gene copy numbers using fluorescence in situ hybridization and developed antibodies to assess PRL-3 subcellular localization. Results: PRL-3 mRNA expression was elevated in nearly all metastatic lesions derived from CRCs, regardless of the site of metastasis (liver, lung, brain, or ovary). Expression was found in neoplastic cells, although tumor endothelium also expressed the gene. In contrast, little or no PRL-3 expression was observed in normal colon, nonmetastatic primary cancers, or metastatic lesions derived from cancers other than those of the colon (pancreas, stomach, or esophagus). Interphase fluorescence in situ hybridization confirmed that gene amplification was not the major cause of PRL-3 overexpression. Immunohistochemical analysis with anti-PRL-3 antibodies showed a cell membrane localization, consistent with the predicted isoprenylation of the protein. Conclusions: These studies establish an unexpected and unprecedented specificity in metastatic gene expression profiles: PRL-3 is apparently expressed in CRC metastases to any organ but is not expressed in metastases of other cancers to the same organs or in nonmetastatic CRCs. PRL-3 is also expressed in tumor vasculature, regardless of the tumor source. These data raise intriguing questions about the role of protein phosphorylation in angiogenesis and cell-type-specific metastatic processes.