Agonist-induced up-regulation of human somatostatin receptor type 1 is regulated by β-arrestin-1 and requires an essential serine residue in the receptor C-tail

We have previously shown that the human somatostatin receptor type 1 (hSSTR1) does not undergo agonist-induced internalization, but is instead up-regulated at the membrane upon prolonged somatostatin (SST) exposure. The deletion of the carboxyterminal C-tail of the receptor completely abolishes up-regulation. To identify molecular signals that mediate hSSTR1 up-regulation, we created mutant receptors with progressive C-tail deletions. Up-regulation was found to be absent in mutants lacking residues Lys³⁵⁹-Ser³⁶⁰-Arg³⁶¹. Moreover, point mutation of Ser³⁶⁰ to Ala completely abolished up-regulation. The coexpression of wild type hSSTR1 with V53D, a dominant negative mutant of β-arrestin-1, completely blocked hSSTR1 up-regulation. Further analysis demonstrated that calcium-calmodulin (CaM) dependent kinases were essential for the SST-induced up-regulation response. Like wild type receptors, all mutants failed to internalize after agonist exposure and were able to inhibit forskolin-stimulated cAMP accumulation. Taking these data together, we suggest that SST-induced hSSTR1 up-regulation is critically dependent upon a specific Lys-Ser-Arg sequence in the C-tail of the receptor, with Ser³⁶⁰ being essential. Up-regulation also requires the participation of CaM protein kinases and interactions with β-arrestins. In contrast, coupling to adenyl cyclase (AC) and internalization occur independently of molecular signals in the receptor's C-tail.