The adrenocorticotropic hormone receptor:

Steroidogenesis-adrenal cell signal transduction

The purpose of this article is to review fundamentals in adrenal gland histophysiology. Key findings regarding the important signaling pathways involved in the regulation of steroidogenesis and adrenal growth are summarized. We illustrate how adrenal gland morphology and function are deeply interconnected in which novel signaling pathways (Wnt, Sonic hedgehog, Notch, β-catenin) or ionic channels are required for their integrity. Emphasis is given to exploring the mechanisms and challenges underlying the regulation of proliferation, growth, and functionality. Also addressed is the fact that while it is now well-accepted that steroidogenesis results from an enzymatic shuttle between mitochondria and endoplasmic reticulum, key questions still remain on the various aspects related to cellular uptake and delivery of free cholesterol. The significant progress achieved over the past decade regarding the precise molecular mechanisms by which the two main regulators of adrenal cortex, adrenocorticotropin hormone (ACTH) and angiotensin II act on their receptors is reviewed, including structure-activity relationships and their potential applications. Particular attention has been given to crucial second messengers and how various kinases, phosphatases, and cytoskeleton-associated proteins interact to ensure homeostasis and/or meet physiological demands. References to animal studies are also made in an attempt to unravel associated clinical conditions. Many of the aspects addressed in this article still represent a challenge for future studies, their outcome aimed at providing evidence that the adrenal gland, through its steroid hormones, occupies a central position in many situations where homeostasis is disrupted, thus highlighting the relevance of exploring and understanding how this key organ is regulated. © 2014 American Physiological Society. Compr Physiol 4:889-964, 2014.

The renaissance of corticotropin therapy in proteinuric nephropathies

Refractory nephrotic syndrome continues to be a therapeutic challenge despite advances in immunosuppression and blockade of the renin-angiotensin-aldosterone cascade. Adrenocorticotropic hormone (ACTH), a pituitary neuroimmunoendocrine polypeptide, was widely used in the 1950s as an effective therapy for childhood nephrotic syndrome, but has since been replaced by synthetic glucocorticoid analogues. In addition to controlling steroidogenesis, ACTH also acts as an important physiological agonist of the melanocortin system. Clinical and experimental evidence now suggests that ACTH has antiproteinuric, lipid-lowering and renoprotective properties, which are not fully explained by its steroidogenic effects. ACTH therapy is effective in inducing remission of nephrotic syndrome in patients with a variety of proteinuric nephropathies, even those resistant to steroids and other immunosuppressants. This Perspectives article describes the biophysiology of ACTH, with an emphasis on its melanocortin actions, particularly in renal parenchymal cells, which could potentially explain the therapeutic effects of ACTH in nephrotic glomerulopathies.

Mechanisms of melanocortin-2 receptor (MC2R) internalization and recycling in human embryonic kidney (hek) cells: identification of Key Ser/Thr (S/T) amino acids

ACTH is the most important stimulus of the adrenal cortex. The precise molecular mechanisms underlying the ACTH response are not yet clarified. The functional ACTH receptor includes melanocortin-2 receptor (MC2R) and MC2R accessory proteins (MRAP). In human embryonic kidney 293/Flp recombinase target cells expressing MC2R, MRAP1 isoforms, and MRAP2, we found that ACTH induced a concentration-dependent and arrestin-, clathrin-, and dynamin-dependent MC2R/MRAP1 internalization, followed by intracellular colocalization with Rab (Ras-like small guanosine triphosphate enzyme)4-, Rab5-, and Rab11-positive recycling endosomes. Preincubation of cells with monensin and brefeldin A revealed that 28% of the internalized receptors were recycled back to the plasma membrane and participated in total accumulation of cAMP. Moreover, certain intracellular Ser and Thr (S/T) residues of MC2R were found to play important roles not only in plasma membrane targeting and function but also in promoting receptor internalization. The S/T residues T131, S140, T204, and S280 were involved in MRAP1-independent cell-surface MC2R expression. Other mutants (S140A, S208A, and S202D) had lower cell-surface expressions in absence of MRAPβ. In addition, T143A and T147D drastically impaired cell-surface expression and function, whereas T131A, T131D, and S280D abrogated MC2R internalization. Thus, the modification of MC2R intracellular S/T residues may positively or negatively regulate its plasma membrane expression and the capacity of ACTH to induce cAMP accumulation. Mutations of T131, T143, T147, and S280 into either A or D had major repercussions on cell-surface expression, cAMP accumulation, and/or internalization parameters, pointing mostly to the second intracellular loop as being crucial for MC2R expression and functional regulation.

Adrenocorticotropin hormone (ACTH) effects on MAPK phosphorylation in human fasciculata cells and in embryonic kidney 293 cells expressing human melanocortin 2 receptor (MC2R) and MC2R accessory protein (MRAP)β

Adrenocorticotropin hormone (ACTH) exerts trophic effects on adrenocortical cells. We studied the phosphorylation of mitogen-activated proteins kinases (MAPKs) in human embryonic kidney cells stably expressing the ACTH receptor, MC2R, and its accessory protein MRAPβ and in primary cultures of human adrenal fasciculata cells. ACTH induced a maximal increase in p44/p42(mapk) and of p38 MAPK phosphorylation after 5min. Neither the overexpression of wild-type arrestin2, arrestin3 or their respective dominant negative forms affected p44/p42(mapk) phosphorylation. However, preincubation with the recycling inhibitors brefeldin A and monensin attenuated both cAMP accumulation and p44/p42(mapk) phosphorylation proportionally. Cyclic AMP-related PKA inhibitors (H89, KI(6-22)) and Rp-cAMPS decreased p44/p42(mapk) phosphorylation but not ACTH-mediated cAMP production. The selective Epac1/2 activator, 8-pCPT-2'-O-MecAMP, did not modify the effect of ACTH. Thus, cAMP/PKA, but not cAMP/Epac1/2 pathways, or arrestin-coupled internalization of MC2R is involved in ACTH-induced p44/p42(mapk) phosphorylation by human MC2R. Together, ACTH binding to MC2R stimulates PKA-dependent p44/p42(mapk) phosphorylation.

Molecular characterization, tissue-specific expression, and regulation of melanocortin 2 receptor in rainbow trout

ACTH, the primary secretagogue for corticosteroid biosynthesis, binds to melanocortin 2 receptor (MC2R) and activates the signaling cascade leading to steroid biosynthesis in the adrenal cortex. Whereas MC2R regulation has been studied using mammalian models, little is known about the molecular mechanisms involved in ACTH signaling in nonmammalian vertebrates. A full-length cDNA encoding MC2R was sequenced from rainbow trout (Oncorhynchus mykiss) interrenal tissue (analogous to the adrenal cortex in mammals) and showed about 60 and about 44% amino acid sequence similarity to teleosts and humans, respectively. Phylogenetic analysis confirmed that MC2R from all species clustered together and was distant from other MCRs. Quantitative real-time PCR revealed a marked tissue-specific difference in MC2R mRNA abundance, with the highest levels observed in the interrenal tissue, ovary, and testis. Acute ACTH, but not alpha-MSH or [Nle4, d-Phe7]-MSH, stimulation resulted in a time- and dose-related elevation in MC2R mRNA abundance in the interrenal tissue. This corresponded with higher steroidogenic acute regulatory protein and cytochrome P450 side-chain cleavage enzyme gene expression as well as elevated cortisol production. An acute stressor transiently elevated plasma ACTH and cortisol levels at 1 h, and this was followed by a significant increase in MC2R mRNA abundance at 4 h after stressor exposure. Taken together, our results demonstrate that ACTH regulation of MC2R is highly conserved in vertebrates, whereas the tissue-specific distribution of this receptor transcript level leads us to propose a role for ACTH signaling in the stressor-mediated suppression of sex steroid levels in fish.

Differential regulation of the human adrenocorticotropin receptor [melanocortin-2 receptor (MC2R)] by human MC2R accessory protein isoforms alpha and beta in isogenic human embryonic kidney 293 cells

The ACTH receptor [melanocortin-2 receptor (MC2R)] is the smallest known G protein-coupled receptor (GPCR). Herein, human MC2R accessory protein (MRAP) isoforms alpha and beta, cloned from a human fetal adrenal gland, were expressed with c-Myc-tagged MC2R (Myc-MC2R) in 293/Flp recombinase target site cells by homologous recombination. Although insertion of Myc-MC2R at the plasma membrane occurred without MRAP assistance, ACTH stimulation of cAMP production was only detected in cells coexpressing MC2R with either MRAP isoform. On the other hand, a MC2R-green fluorescent protein fusion transfected with either MRAPalpha or MRAPbeta was impaired both in cell membrane localization and signaling. MRAP isoforms were also tagged with either Flag or 6xHis epitopes. In cell populations coexpressing transiently and/or stably Myc-MC2R with MRAPalpha or MRAPbeta, stimulation with ACTH induced production of cAMP with EC(50) values lower in MRAPalpha- than in MRAPbeta-expressing cells. ACTH only bound Myc-MC2R in the presence of MRAP. Higher Myc-MC2R cell surface density was observed in the presence of MRAPbeta comparatively to MRAPalpha, possibly contributing to higher ACTH binding capacity and higher maximal cAMP responses observed in MRAPbeta-expressing cells. Immunofluorescence studies indicated that MRAP isoforms were localized near the plasma membrane and in the vicinity, but not colocalized, with Myc-MC2R. In summary, through the generation of a new all-human experimental model devoid of endogenous MCRs, we present evidence that human MRAP isoforms, although not essential for MC2R localization at the plasma membrane, are essential for ACTH binding and ACTH-induced cAMP production and that they differentially regulate, although modestly, cell membrane density and functional properties of MC2R.

Design, synthesis, and evaluation of proline and pyrrolidine based melanocortin receptor agonists. A conformationally restricted dipeptide mimic approach

The design, synthesis, and structure-activity relationships (SAR) of a series of novel proline and pyrrolidine based melanocortin receptor (MCR) agonists are described. To validate a conformationally constrained Arg-Nal dipeptide analogue strategy, we first synthesized and evaluated a test set of cis-(2R,4R)-proline analogues (21a-g). All of these compounds showed significant binding and agonist potency at the hMC1R, hMC3R, and hMC4R. Potent cis-(2S,4R)-pyrrolidine based MCR agonists (35a-g) were subsequently developed by means of this design approach. A SAR study directed toward probing the effect of the two chiral centers in the pyrrolidine ring on biological activity revealed the importance of the (S) absolute configuration at the 2-position for binding affinity, agonist potency, and receptor selectivity. Among the four sets of the pyrrolidine diastereomers investigated, analogues with the (2S,4R) configuration were the most potent agonists across the three receptors, followed by those possessing the (2S,4S) configuration.

Expression of the human melanocortin-2 receptor in different eukaryotic cells

The human melanocortin-2 receptor (hMC2R) is mainly present in the adrenal cortex and has been difficult to express in heterologous cells. The hMC2R fused to the EGFP at its C-terminus has been stably transfected in the murine M3 melanoma and HEK293 cells. In the M3 cells, the hMC2R-EGFP was well-addressed to the cell membrane and functional whereas in the HEK293 cells, the hMC2R-EGFP was retained intracellularly. These results suggest that some specific factors, missing in cells, which do not express any melanocortin receptor, are involved in the correct addressing of the hMC2R to the cell membrane.

New substituted piperazines as ligands for melanocortin receptors. Correlation to the X-ray structure of "THIQ"

A series of piperazine analogues of the melanocortin 4 receptor (MC4R) specific small-molecule agonist "THIQ" was synthesized and characterized structurally and pharmacologically. First, several THIQ imitations lacking the triazole moiety were prepared. Syntheses included acylation of 4-phenylpiperazine or 4-cyclohexylpiperazine. In two cases the tertiary amine function was replaced by the corresponding N-oxide. To obtain more complex structures, a 4-substituted piperazine ring was formed by alkylation of the primary amino group of cyclohexane-derived amino alcohols with N,N-bis(2-chloroethyl)benzylamine. The hydroxylic group of the intermediate was first activated with methanesulfonyl chloride, and the sulfonic ester formed in situ was introduced into the reaction with the sodium salt of 1,2,4-triazole. In one case (i.e., preparation of 23c) introduction of the 1,2,4-triazole moiety was performed at a carbon of the cyclohexane ring. In addition, this intermediate contained a piperazine moiety connected via its nitrogen atom to a cyclohexane ring carbon neighboring the reaction center. As established in NMR and X-ray investigations herein, this substitution proceeded with retention of the initial trans configuration of 1,2-disubstituted cyclohexane. To obtain pure enantiomers of 23c, its precursor 21c was subjected to chiral chromatography on a Chirobiotic V column. The derivatives (R,R)-21cand (S,S)-21c obtained were introduced into further syntheses steps, giving (R,R)-23c and (S,S)-23c, respectively. Melanocortin MC(1,3-5) receptor binding studies showed that all tested piperazine derivatives were active. Several compounds showed clear selectivity for MC4R, with submicromolar affinities being obtained. Among them, one substance, (R,R)-23c, displayed a biphasic curve in displacement of [125I]NDP-MSH on MC4R [K(i)high = 1 nM and K(i)low = 260 nM]. This biphasic competition curve was similarly biphasic to the competition curve obtained herein using THIQ. An X-ray study performed on crystals of the THIQ sulfate salt revealed two closely related conformations, which resemble the shape of the letter "Y", where piperidine and 4-chlorophenyl groups are situated close to each other, but the 1,2,3,4-tetrahydroisoquinoline residue is remote, the triazole function being highly exposed to the environment. The crystals of the dinitrate salt of (R,R)-23c showed a different conformation, where parts of the molecule are spread out almost symmetrically around the central section. Molecular modeling, based on the THIQ crystal structure and the functional similarity of THIQ and (R,R)-23c, allowed us to suggest a possible "bioactive" conformation of (R,R)-23c that is similar to the crystal conformation of THIQ.