Carboxyl-terminal multi-site phosphorylation regulates internalization and desensitization of the human sst2...

Area of Science:

• Pharmacology
• Molecular Biology
• Endocrinology

Background:

• The somatostatin receptor 2 (sst2) is a key target for neuroendocrine tumor diagnosis and treatment.
• Somatostatin analogs, such as octreotide and pasireotide, interact with sst2 but elicit different cellular responses.
• Previous work indicated distinct phosphorylation and internalization patterns for sst2 upon stimulation with octreotide and pasireotide.

Purpose of the Study:

• To elucidate the specific phosphorylation sites on the sst2 receptor carboxyl-terminal tail responsible for agonist-selective internalization.
• To determine the role of distinct serine and threonine residues in mediating the differential effects of somatostatin, octreotide, and pasireotide on sst2 receptor trafficking.

Main Methods:

• Site-directed mutagenesis was employed to replace specific serine and threonine residues in the sst2 carboxyl-terminal tail with alanine.
• HEK293 cells expressing wild-type and mutant sst2 receptors were treated with somatostatin, octreotide, and pasireotide.
• Receptor internalization and β-arrestin mobilization were assessed to evaluate the functional consequences of the mutations.

Main Results:

• Pasireotide-induced internalization was blocked by mutating S341 and S343, while octreotide and somatostatin required mutations at T353, T354, T356, and T359 for significant inhibition.
• Mutating T353, T354, T356, and T359 alone did not fully prevent somatostatin-mediated β-arrestin mobilization and receptor desensitization.
• Complete abrogation of sst2 receptor internalization and β-arrestin mobilization was observed when all fourteen carboxyl-terminal serine and threonine residues were mutated to alanine.

Conclusions:

• Agonist-selective internalization of the somatostatin receptor 2 is regulated by multi-site phosphorylation of its carboxyl-terminal tail.
• Specific phosphorylation patterns dictate the distinct trafficking responses of sst2 to different somatostatin analogs.
• These findings provide critical insights into the molecular mechanisms governing sst2 receptor signaling and drug response in neuroendocrine tumors.