Somatostatin Receptor Type 5 Modulates Somatostatin Receptor Type 2 Regulation of Adrenocorticotropin Secretion

Abstract

Somatostatin inhibits adrenocorticotropin (ACTH) secretion from pituitary tumor cells. To assess the contribution of somatostatin receptor subtype 5 (SST5) to somatostatin receptor subtype 2 (SST2) action in these cells, we assessed multipathway responses to novel highly monoreceptor-selective peptide agonists and multireceptor agonists, including octreotide and somatostatin-28. Octreotide and somatostatin-28 cell membrane binding affinities correlated with their respective SST2-selective peptide ligand. Although octreotide had similar inhibiting potency (picomolar) for cAMP accumulation and ACTH secretion as an SST2-selective agonist, somatostatin-28 exhibited a higher potency (femtomolar). Baseline spontaneous calcium oscillations assessed by fluorescent confocal microscopy revealed two distinct effects: SST2 activation reduced oscillations at femtomolar concentrations reflected by high inhibiting potency of averaged normalized oscillation amplitude, whereas SST5 activation induces brief oscillation pauses and increased oscillation amplitude. Octreotide exhibits an integrated effect of both receptors; however, somatostatin-28 exhibited a complex response with two separate inhibitory potencies. SST2 internalization was visualized with SST2-selective agonist at lower concentrations than for octreotide or somatostatin-28, whereas SST5 did not internalize. Using monoreceptor-selective peptide agonists, the results indicate that, in AtT-20 cells, SST5 regulates the dominant SST2 action, attenuating SST2 effects on intracellular calcium oscillation and internalization. This may explain superior somatostatin-28 potency and provides a rationale for somatostatin ligand design to treat ACTH-secreting pituitary tumors.Somatostatin (somatotropin-release inhibiting factor, SRIF), 1 suppresses hormone release from secreting cells, including pituitary corticotrophs (1, 2). Selective SST receptor subtype ligands regulate pituitary hormones in vitro (3-5). Deletion of mouse somatostatin receptor type 2 (SST2) (6) resulted in increased pituitary ACTH release (7) underscoring the physiologic role for SRIF in regulating ACTH (8, 9). Mouse pituitary AtT-20 corticotroph cells are the only viable cell line available to study ACTH secretion in vitro. These cells express endogenous somatostatin receptors (SSTs) and respond to SRIF with a dose-dependent inhibition of ACTH secretion. AtT-20 cells express specific binding sites for SST2 and SST5, but not for SST1, -3, or -4 (10, 11). Through these receptors, SRIF inhibits pituitary cell adenylate cyclase activity and blocks inward Ca 2ϩ currents (12-14). Unfortunately, the currently available SRIF agonist, octreotide, which recognizes primarily SST2, is ineffective in treating ACTH-secreting pituitary adenomas (15). Therefore, elucidating pathways whereby SRIF activates corticotroph receptors will facilitate design of effective SRIF analog ligand-selective pharmacotherapy (1) for the inexorable ACTH hypersecretion-characterizing patients with Cushing disease.Baseline spontaneous Ca 2ϩ os...

In vitro testing of new somatostatin analogs on pituitary tumor cells

“…And indeed there is evidence indicating that SSTR5 is important in transducing SRIF inhibitory effects in a mouse ACTH-secreting pituitary adenoma cell line, the AtT20 cells (Richardson et al 1981;van der Hoek et al 2005). Indeed, upon SRIF treatment, SSTR5 persists at the membrane level while SSTR2 rapidly internalizes and desensitizes (Ben-Shlomo et al 2005). Further studies on human ACTH-pituitary adenomas in vitro demonstrated that pasireotide (SOM230, a novel SRIF analog interacting with all SSTRs except for SSTR4) at 10 nM significantly inhibits ACTH release in primary cultures predominantly expressing SSTR5.…”

“…Indeed, upon SRIF treatment, SSTR5 persists at the membrane level while SSTR2 rapidly internalizes and desensitizes (Ben-Shlomo et al 2005). Further studies on human ACTH-pituitary adenomas in vitro demonstrated that pasireotide (SOM230, a novel SRIF analog interacting with all SSTRs except for SSTR4) at 10 nM significantly inhibits ACTH release in primary cultures predominantly expressing SSTR5.…”

Acromegaly pathogenesis and treatment

“…GH-secreting adenomas exhibit heterogenous SSTR expression (SSTR 2 >SSTR 5 > SSTR 1 > SSTR 3 ), while SSTR 4 is notably undetectable in pituitary tumors (91, 92, S27). Several lines of evidence point to a cooperative functionality of SSTR 2 and SSTR 5 in suppressing GH and ACTH secretion (93)(94)(95). SSTR 5 may also heterodimerize with SSTR 2 to enhance availability of cell membrane receptors (96).…”

Identification of transcriptional regulatory elements in the human somatostatin receptor sst2 promoter and regions including estrogen response element half-site for estrogen activation

“…There exist five distinct somatostatin receptor subtypes (sst1-sst5), among which sst2 is well known to be involved in many physiological functions (Patel 1999, Reubi 2003, Krantic et al 2004, Olias et al 2004. These include inhibitory regulation of endocrine and exocrine secretions (Patel 1999), such as GH, adrenocorticotrophin, glucagon, and gastric acid (Martinez et al 1998, Ren et al 2003, Ben-Shlomo et al 2005. In central nervous system of an sst2-deficient mouse, GH-induced feedback suppression of GH secretion in GH-releasing hormone-secreting arcuate neurons is impaired (Zheng et al 1997), and fine motor control, spatial learning, and neuronal plasticity are disordered (Dutar et al 2002, Krantic et al 2004, Olias et al 2004.…”