Agonist-biased signaling at the sst2A receptor: the multi-somatostatin analogs KE108 and SOM230 activate and antagonize distinct signaling pathways

Structural determinants of agonist-selective signaling at the sst(2A) somatostatin receptor

The clinically used somatostatin (SS-14) analogs octreotide and pasireotide (SOM230) stimulate distinct species-specific patterns of sst(2A) somatostatin receptor phosphorylation and internalization. Like SS-14, octreotide promotes the phosphorylation of at least six carboxyl-terminal serine and threonine residues, namely S341, S343, T353, T354, T356, and T359, which in turn leads to a robust endocytosis of both rat and human sst(2A) receptors. Unlike SS-14, pasireotide fails to induce any substantial phosphorylation or internalization of the rat sst(2A) receptor. Nevertheless, pasireotide is able to stimulate a selective phosphorylation of S341 and S343 of the human sst(2A) receptor followed by a clearly detectable receptor sequestration. Here, we show that transplantation of amino acids 1-180 of the human sst(2A) receptor to the rat sst(2A) receptor facilitates pasireotide-induced internalization. Conversely, construction of a rat-human sst(2A) chimera conferred resistance to pasireotide-induced internalization. We then created a series of site-directed mutants leading to the identification of amino acids 27, 30, 163, and 164 that when exchanged to their human counterparts facilitated pasireotide-driven S341/S343 phosphorylation and internalization of the rat sst(2A) receptor. Exchange of these amino acids to their rat counterparts completely blocked the pasireotide-mediated internalization of the human sst(2A) receptor. Notably, octreotide and SS-14 stimulated a full phosphorylation and internalization of all mutant sst(2A) receptors tested. Together, these findings suggest that pasireotide activates the sst(2A) receptor via a molecular switch that is structurally and functionally distinct from that turned on during octreotide-driven sst(2A) activation.

Effect of PD98059 on the proliferation and apoptosis of pancreatic cancer Panc-1 cells and tumor growth in Panc-1-xenografted nude mice

AIM: To investigate the effect of PD98059, a specific inhibitor of the mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase 1/2 (ERK1/2) signaling pathway, on the apoptosis and proliferation of pancreatic cancer Panc-1 cells and tumor growth in Panc-1-xenografted nude mice.

METHODS: After Panc-1 cells were treated with different doses of PD98059, cell proliferation was measured by MTT assay, cell cycle and apoptosis were detected by flow cytometry, and apoptotic cells were examined by Hoechst 33258 staining. The expression of phosphorylated ERK1/2 in tumors in Panc-1-xenografted nude mice was detected by Western blot.

RESULTS: PD98059 suppressed the proliferation of pancreatic cancer Panc-1 cells in a concentration- and time-dependent manner (P < 0.05). Compared with control cells, the absorbance at 490 nm (A₄₉₀) was significantly lower (0.391 ± 0.029 vs 0.994 ± 0.057, P < 0.05) and the apoptosis rate was significantly higher (11.77% ± 1.33% vs 1.13% ± 0.19%, P < 0.05) in Panc-1 cells treated with 50 μmol/L PD98059 for 96 h. However, no significant difference was noted in the apoptosis rate between Panc-1 cells treated with 20 μmol/L PD98059 and control cells. PD98059 could restrain the expression of P-ERK1/2 proteins in tumors in Panc-1-xenografted nude mice, and the maximum inhibition was achieved with 50 μmol/L PD98059.

CONCLUSION: The MAPK/ERK1/2 signaling pathway may play an important role in regulating the proliferation and apoptosis of pancreatic cancer cells.

Role of somatostatins in gastroenteropancreatic neuroendocrine tumor development and therapy

The incidence and prevalence of gastroenteropancreatic neuroendocrine tumors (GEP-NETs) have increased in the past 20 years. GEP-NETs are heterogeneous tumors, in terms of clinical and biological features, that originate from the pancreas or the intestinal tract. Some GEP-NETs grow very slowly, some grow rapidly and do not cause symptoms, and others cause hormone hypersecretion and associated symptoms. Most GEP-NETs overexpress receptors for somatostatins. Somatostatins inhibit the release of many hormones and other secretory proteins; their effects are mediated by G protein-coupled receptors that are expressed in a tissue-specific manner. Most GEP-NETs overexpress the somatostatin receptor SSTR2; somatostatin analogues are the best therapeutic option for functional neuroendocrine tumors because they reduce hormone-related symptoms and also have antitumor effects. Long-acting formulations of somatostatin analogues stabilize tumor growth over long periods. The development of radioactive analogues for imaging and peptide receptor radiotherapy has improved the management of GEP-NETs. Peptide receptor radiotherapy has significant antitumor effects, increasing overall survival times of patients with tumors that express a high density of SSTRs, particularly SSTR2 and SSTR5. The multi-receptor somatostatin analogue SOM230 (pasireotide) and chimeric molecules that bind SSTR2 and the dopamine receptor D2 are also being developed to treat patients with GEP-NETs. Combinations of radioactive labeled and unlabeled somatostatin analogues and therapeutics that inhibit other signaling pathways, such as mammalian target of rapamycin (mTOR) and vascular endothelial growth factor, might be the most effective therapeutics for GEP-NETs.