Orexin A Stimulates Cortisol Secretion from Human Adrenocortical Cells through Activation of the Adenylate Cyclase-Dependent Signaling Cascade

Abstract

Orexins A and B are two hypothalamic peptides that increase food intake and body weight and probably play a role in the sleep regulation. They act through two subtypes of G protein-coupled receptors, called OX1-R and OX2-R. OX1-R selectively binds orexin-A, whereas OX2-R is nonselective for both orexins. Orexins did not affect the in vitro secretion of either catecholamine or aldosterone from human adrenals. Conversely, orexin A, but not orexin B, concentration dependently increased basal cortisol secretion from dispersed adrenocortical cells; the maximal effective concentration was 10(-8) mol/L. Orexin A (10(-8) mol/L) enhanced the cortisol response to maximal effective concentrations (10(-9) mol/L) of angiotensin II and endothelin-1, but only to low concentrations of ACTH (10(-12)/10(-11) mol/L). Orexin A (10(-8) mol/L) increased basal cAMP release by dispersed adrenocortical cells, and the effect was blocked by the adenylate cyclase inhibitor SQ-22536. The cortisol response to 10(-8) mol/L orexin A was unaffected by the ACTH receptor antagonist corticotropin-inhibiting peptide, but was abolished by either SQ-22536 or the protein kinase A inhibitor H-89. RT-PCR demonstrated high levels of OX1-R messenger ribonucleic acid and very low levels of OX2-R messenger ribonucleic acid in human adrenal zona fasciculata-reticularis and adrenal medulla. Collectively, our findings suggest that orexins selectively stimulate glucocorticoid secretion from human adrenocortical cells, acting through OX1-R coupled with the adenylate cyclase-dependent signaling pathway.

OX1 Orexin Receptors Couple to Adenylyl Cyclase Regulation via Multiple Mechanisms

“…10). This coupling may be able to explain the orexin-stimulated cAMP accumulation and glucocorticoid synthesis in the adrenal cortex, which is unlikely to occur via Ca 2ϩ -or PKC-stimulated adenylyl cyclases (27,55,56). The general of role of adenylyl cyclase in orexin receptor signaling is speculative, because almost no studies have addressed this question.…”

“…Activation of adenylyl cyclase has also been shown to be important for orexin receptor signaling, although this pathway has seldom been investigated. In rat and human adrenal cortex, orexins strongly elevate cAMP, leading to activation of protein kinase A and increased synthesis and release of glucocorticoids (26,27), probably, at least in man, via OX 1 receptors (27). Although this effect appears to be very similar to the adrenocorticotropic hormone, which utilizes the G s pathway, the mechanisms of orexin signaling to adenylyl cyclase has not been investigated.…”

Orexins and their receptors: structural aspects and role in peripheral tissues

“…A single mRNA transcript encodes for prepro-orexin containing both orexin-A and orexin-B [2,3,37]. The human prepro-orexin gene has been localized on chromosome 17q21 (table 1) [3].…”

“…Mature orexin-A and orexin-B peptides issue from post-translational modifications of preproorexin involving specific proteolytic processing. The orexin-A sequence starts with Gln 33 , which is cyclized into an N-terminal pyroglutamyl residue presumably by enzymatic transamidation [3,37]. The last residue (Leu 65 ) of the mature orexin-A peptide is followed by a Gly residue which presumably serves as NH 2 donor for C-terminal amidation by sequential actions of peptidylglycine monooxygenase and peptidylamidoglycolate lyase [3,37].…”

“…The orexin-A sequence starts with Gln 33 , which is cyclized into an N-terminal pyroglutamyl residue presumably by enzymatic transamidation [3,37]. The last residue (Leu 65 ) of the mature orexin-A peptide is followed by a Gly residue which presumably serves as NH 2 donor for C-terminal amidation by sequential actions of peptidylglycine monooxygenase and peptidylamidoglycolate lyase [3,37]. As expected, Gly 66 is followed by a pair of basic amino acid residues, Lys 67 -Arg 68 , which constitute a recognition site for prohormone convertases [3,37].…”