Rat Protein Tyrosine Phosphatase η Suppresses the Neoplastic Phenotype of Retrovirally Transformed Thyroid Cells through the Stabilization of p27(Kip1)

The r-PTPη gene encodes a rat receptor-type protein tyrosine phosphatase whose expression is negatively regulated by neoplastic cell transformation. Here we first demonstrate a dramatic reduction in DEP-1/HPTPη (the human homolog of r-PTPη) expression in a panel of human thyroid carcinomas. Subsequently, we show that the reexpression of the r-PTPη gene in highly malignant rat thyroid cells transformed by retroviruses carrying the v-mos and v-ras-Ki oncogenes suppresses their malignant phenotype. Cell cycle analysis demonstrated that r-PTPη caused G(1) growth arrest and increased the cyclin-dependent kinase inhibitor p27(Kip1) protein level by reducing the proteasome-dependent degradation rate. We propose that the r-PTPη tumor suppressor activity is mediated by p27(Kip1) protein stabilization, because suppression of p27(Kip1) protein synthesis using p27-specific antisense oligonucleotides blocked the growth-inhibitory effect induced by r-PTPη. Furthermore, we provide evidence that in v-mos- or v-ras-Ki-transformed thyroid cells, the p27(Kip1) protein level was regulated by the mitogen-activated protein (MAP) kinase pathway and that r-PTPη regulated p27(Kip1) stability by preventing v-mos- or v-ras-Ki-induced MAP kinase activation.