Differential Effects of Octreotide and Pasireotide on Somatostatin Receptor Internalization and Trafficking in Vitro

Objective: The clinically used somatostatin analogs, octreotide and lanreotide, act primarily by binding to somatostatin receptor 2 (sst(2)). In contrast, the novel multireceptor ligand pasireotide (SOM230) binds with high affinity to somatostatin receptor subtypes sst(1), sst(2), sst(3), and sst(5). SOM230 is currently under clinical evaluation for treatment of acromegaly, Cushing's disease, and octreotide-resistant carcinoid tumors. However, the effects of SOM230 on internalization and postendosomal sorting of individual human somatostatin receptor subtypes have not been determined so far. Results: Here we show that SOM230 was less potent than octreotide in inducing internalization and signaling of sst(2) receptors expressed in human embryonic kidney cells. In contrast, SOM230 was more potent than octreotide in inducing internalization and signaling of sst(3) and sst(5) receptors. Both SOM230 and octreotide stimulated a rapid down-regulation of sst(3) but not of sst(2) or sst(5) receptors. SOM230 and octreotide profoundly differed in their patterns of sst(2)-stimulated beta-arrestin mobilization. Whereas octreotide-mediated receptor activation led to the formation of stable complexes facilitating the internalization of sst(2) and beta-arrestin-2 into the same endocytic vesicles, SOM230-mediated receptor activation led to the formation of unstable complexes that dissociated at or near the plasma membrane. Consequently, sst(2) receptors recycled rapidly to the plasma membrane after endocytosis in SOM230-treated cells, but not in octreotide-treated cells. Conclusion: We show that SOM230 modulates somatostatin receptor trafficking in a manner clearly distinct from octreotide and somatostatin. These findings may provide an explanation for the differential regulation of somatostatin receptor responsiveness during long-term administration of stable somatostatin analogs. (J Clin Endocrinol Metab 94: 654-661, 2009)