Epithelial protein-tyrosine phosphatase 1B contributes to the induction of mammary tumors by HER2/Neu but is not essential for tumor maintenance.

Protein-tyrosine phosphatase 1B (PTP1B), a well-established metabolic regulator, plays an important role in breast cancer. Using whole-body PTP1B knockout mice, recent studies have shown that PTP1B ablation delays HER2/Neu-induced mammary cancer. Whether PTP1B plays a cell-autonomous or a noncell-autonomous role in HER2/Neu-evoked tumorigenesis and whether it is involved in tumor maintenance was unknown. We generated mice expressing HER2/Neu and lacking PTP1B specifically in the mammary epithelium. We found that mammary-specific deletion of PTP1B delays the onset of HER2/Neu-evoked mammary tumors, establishing a cell autonomous role for PTP1B in such neoplasms. We also deleted PTP1B in established mouse mammary tumors or depleted PTP1B in human breast cancer cell lines grown as xenografts. PTP1B inhibition did not affect tumor growth in either model showing that neither epithelial nor stromal PTP1B is necessary for tumor maintenance. Taken together, our data show that despite the PTP1B contribution to tumor onset, it is not essential for tumor maintenance. This suggests that PTP1B inhibition could be effective in breast tumor prevention.

Loss of the protein-tyrosine phosphatase DEP-1/PTPRJ drives meningioma cell motility.

DEP-1/PTPRJ is a transmembrane protein-tyrosine phosphatase which has been proposed as a suppressor of epithelial tumors. We have found loss of heterozygosity (LOH) of the PTPRJ gene and loss of DEP-1 protein expression in a subset of human meningiomas. RNAi-mediated suppression of DEP-1 in DEP-1 positive meningioma cell lines caused enhanced motility and colony formation in semi-solid media. Cells devoid of DEP-1 exhibited enhanced signaling of endogenous platelet-derived growth factor (PDGF) receptors, and reduced paxillin phosphorylation upon seeding. Moreover, DEP-1 loss caused diminished adhesion to different matrices, and impaired cell spreading. DEP-1-deficient meningioma cells exhibited invasive growth in an orthotopic xenotransplantation model in nude mice, indicating that elevated motility translates into a biological phenotype in vivo. We propose that negative regulation of PDGF receptor signaling and positive regulation of adhesion signaling by DEP-1 cooperate in inhibition of meningioma cell motility, and possibly tumor invasiveness. These phenotypes of DEP-1 loss reveal functions of DEP-1 in adherent cells, and may be more generally relevant for tumorigenesis.