The activation of the phosphotyrosine phosphatase eta is responsible for the somatostatin inhibition of PCCl3 thyroid cell proliferation

BACKGROUND: This study was aimed to identify possible intracellular effectors of the somatostatin (SST) antiproliferative activity, in PCCl3 thyroid cells. METHODS: To prove the involvement of r-PTPeta in SST's effect, we studied th proliferative activity of subclones of PCCl3 cells that do or do not express this PTP. RESULTS: SST inhibited PCCl3 TSH+insulin-dependent cell proliferation through the induction of a phosphotyrosine phosphatase (PTP) activity, detected using the synthetic substrate pNPP (+150%, p<0.01). Conversely, PCCl3 cells stably expressing the v-mos oncogene (PCmos) were completely insensitive to SST antiproliferative effects due to the incapability of SST to increase PTP activity, that correlated with the abolishment of the expression of the receptor-like PTP, r-PTPeta. In the cells in which r-PTPeta was transfected (PCmos/ PTPeta) SST inhibited cell proliferation showing a dose-dependence similar to that observed in PCCl3 cells. Conversely, the transfection of a catalytically inactive mutant of r-PTPeta did not restore the responsivity to SST. Also in PCmos/PTPeta cells SST, treatment increased membrane PTP activity. CONCLUSIONS: SST inhibition of PCC13 cell proliferation requires the activation of r-PTPeta.