Endocrine Abstracts

The human somatostatin receptor 3 (hsst3) is expressed in about 50% of all neuroendocrine tumours. The sst3 receptor is unique among somatostatin receptors which can initiate apoptosis of tumour cells through activation of the tumour suppressor p53. Furthermore, treatment of the sst3 receptor with somatostatin or stable somatostatin analogues such as octreotide or pasireotide can inhibited tumour cell proliferation. However, at present little is known about the agonist-induced regulation of the human sst3 receptor. We have generated a series of phosphorylation-deficient mutants of the receptor and determined important sites for agonist-induced internalisation. Based on this information we generated phosphosite-specific antibodies for the carboxyl-terminal serine 337, threonine 341 and threonine 348, which enabled us to investigate the temporal patterns of sst3 phosphorylation and dephosphorylation. Here we demonstrate that pasireotide and octreotide were not able to promote a phosphorylation to the same extent as natural somatostatin. Similar the sst3-selective ligand L-796,778 did not promote any detectable phosphorylation or internalisation. We also show that sst3 phosphorylation occurred within minutes, whereas dephosphorylation and recycling of the sst3 receptor occurred at a considerably slower rat. We also identify G protein-coupled receptor kinases 2 and 3 (GRK2/3) and protein phosphatase 1 (PP1) as key regulators of sst3 phosphorylation and dephosphorylation.