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 G₁ 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.