The activation of the phosphotyrosine phosphatase eta (r-PTP eta) is responsible for the somatostatin inhibition of PCCI3 thyroid cell proliferation

The aim of this study was the characterization of the intracellular effecto rs of the antiproliferative activity of somatostatin in PC Cl3 thyroid cell s. Somatostatin inhibited PC Cl3 cell proliferation through the activation of a membrane phosphotyrosine phosphatase. Conversely, PC Cl3 cells stably expressing the v-mos oncogene (PC mos) were completely insensitive to the s omatostatin antiproliferative effects since somatostatin was unable to stim ulate a phosphotyrosine phosphatase activity. In PC mos cells basal phospho tyrosine phosphatase activity was also reduced, suggesting that the express ion of a specific phosphotyrosine phosphatase was impaired in these transfo rmed cells. We suggested that this phosphotyrosine phosphatase could be r-P TP eta whose expression was abolished in the PC mos cells. To directly prov e the involvement of r-PTP eta in somatostatin's effect, we stably transfec ted this phosphatase in PC mos cells. This new cell line (PC mos/PTP eta) r ecovered somatostatin's ability to inhibit cell proliferation, showing dose -dependence and time course similar to those observed in PC Cl3 cells. Conv ersely, the transfection of a catalytically inactive mutant of r-PTP eta di d not restore the antiproliferative effects of somatostatin. PC mos/PTP eta cells showed a high basal phosphotyrosine phosphatase activity which, simi larly to PC Cl3 cells, was further increased after somatostatin treatment. The specificity of the role of r-PTP eta in somatostatin receptor signal tr ansduction was demonstrated by measuring its specific activity after somato statin treatment in an immunocomplex assay. Somatostatin highly increased r -PTP eta activity in PCCl3 and PC mos/PTP eta (+300%, P < 0.01) but not in PCmos cells. Conversely, no differences in somatostatin-stimulated SHP-2 ac tivity, (similar to +50%, P < 0.05), were observed among all the cell lines . The activation of r-PTP eta by somatostatin caused, acting downstream of MAPK kinase, an inhibition of insulin-induced ERK1/2 activation with the su bsequent blockade of the phosphorylation, ubiquitination, and proteasome de gradation of the cyclin-dependent kinase inhibitor p27(kip1). Ultimately, h igh levels of P27(kip1) lead to cell proliferation arrest. In conclusion, s omatostatin inhibition of PC Cl3 cell proliferation requires the activation of r-PTP eta which, through the inhibition of MAPK activity, causes the st abilization of the cell cycle inhibitor p27(kip1).