Functional characterization of two human receptor activity-modifying protein 3 variants

Adrenomedullin (AM) and amylin are involved in angiogenesis/lymphangiogenesis and glucose homeostasis/food intake, respectively. They activate receptor activity-modifying protein (RAMP)/G protein-coupled receptor (GPCR) complexes. RAMP3 with the calcitonin receptor-like receptor (CLR) forms the AM(2) receptor, whereas when paired with the calcitonin receptor AMY(3) receptors are formed. RAMP3 interacts with other GPCRs although the consequences of these interactions are poorly understood. Therefore, variations in the RAMP3 sequence, such as single nucleotide polymorphisms or mutations could be relevant to human health. Variants of RAMP3 have been identified. In particular, analysis of AK222469 (Homo sapiens mRNA for receptor (calcitonin) activity-modifying protein 3 precursor variant) revealed several nucleotide differences, three of which encoded amino acid changes (Cys40Trp, Phe100Ser, Leu147Pro). Trp56Arg RAMP3 is a polymorphic variant of human RAMP3 at a conserved amino acid position. To determine their function we used wild-type (WT) human RAMP3 as a template for introducing amino acid mutations. Mutant or WT RAMP3 function was determined in Cos-7 cells with CLR or the calcitonin receptor (CT((a))). Cys40Trp/Phe100Ser/Leu147Pro RAMP3 was functionally compromised, with reduced AM and amylin potency at the respective AM(2) and AMY(3(a)) receptor complexes. Cys40Trp and Phe100Ser mutations contributed to this phenotype, unlike Leu147Pro. Reduced cell-surface expression of mutant receptor complexes probably explains the functional data. In contrast, Trp56Arg RAMP3 was WT in phenotype. This study provides insight into the role of these residues in RAMP3. The existence of AK222469 in the human population has implications for the function of RAMP3/GPCR complexes, particularly AM and amylin receptors.

The role of peptides and receptors of the calcitonin family in the regulation of bone metabolism

The 'calcitonin family' is a group of peptide hormones that share structural similarities with calcitonin, and includes calcitonin gene-related peptide (CGRP), amylin, adrenomedullin and adrenomedullin 2 (intermedin). These hormones are produced by different tissues, with calcitonin being produced in thyroid C cells, alphaCGRP predominantly in neural tissue, amylin in beta-islet cells of the pancreas and adrenomedullin in many tissues and cell types. Bone appears to be a common target for all the peptides of the calcitonin family, although the specific bone effects of the peptides vary. Administration of calcitonin produces rapid lowering of serum calcium levels, mainly through inhibition of bone resorption by osteoclasts. In vitro and in a number of animal experimental models, amylin and CGRP are also effective in inhibiting osteoclast activity and bone resorption. Amylin, adrenomedullin and CGRP can also affect cells of the osteoblast lineage, inducing osteoblast proliferation and promoting bone formation. Receptors for the peptides of the calcitonin family are formed by heterodimerization of the calcitonin receptor (CTR) or calcitonin receptor-like receptor (CLR) with receptor activity modifying proteins (RAMPs). Although the different combinations of these proteins create receptors with distinct ligand specificities, there is a degree of cross-reactivity and the receptors are able to bind other ligands from the family, usually with lower affinity. Analysis of the expression of the receptors for the calcitonin family in 16 samples of human osteoblasts showed high levels of CLR and RAMP1, low levels of RAMP2 and no expression of RAMP3 or CTR. Recent studies of the bone phenotype of knockout animals lacking the calcitonin, alphaCGRP or amylin gene indicated that in this experimental system the main physiological role of amylin in bone is the inhibition of bone resorption, that of CGRP is the activation of bone formation, while calcitonin, unexpectedly appears to be inhibiting bone formation without affecting bone resorption. Further investigations will be required to determine the mechanisms of action of calcitonin peptides in bone and their significance to human bone physiology.

Receptor activity-modifying proteins differentially modulate the G protein-coupling efficiency of amylin receptors

Receptor activity-modifying proteins (RAMPs) 1, 2, and 3 are prototypic G protein-coupled receptor accessory proteins that can alter not only receptor trafficking but also receptor phenotype. Specific RAMP interaction with the calcitonin receptor (CTR) generates novel and distinct receptors for the peptide amylin; however, the role of RAMPs in receptor signaling is not understood. The current study demonstrates that RAMP interaction with the CTRa in COS-7 or HEK-293 cells leads to selective modulation of signaling pathways activated by the receptor complex. There was a 20- to 30-fold induction in amylin potency at CTR/RAMP1 (AMY1) and CTR/RAMP3 (AMY3) receptors, compared with CTR alone, for formation of the second-messenger cAMP that parallels an increase in amylin binding affinity. In contrast, only 2- to 5-fold induction of amylin potency was seen for mobilization of intracellular Ca++ or activation of ERK1/2. In addition, in COS-7 cells, the increase in amylin potency for Ca++ mobilization was 2-fold greater for AMY3 receptors, compared with AMY1 receptors and this paralleled the relative capacity of overexpression of Galphaq proteins to augment induction of high affinity 125I-amylin binding. These data demonstrate that RAMP-complexed receptors have a different signaling profile to CTRs expressed in the absence of RAMPs, and this is likely due to direct effects of the RAMP on G protein-coupling efficiency.

Unaltered mRNA expression of calcitonin-like receptor and receptor activity modifying proteins in human arteries in stroke and myocardial infarction

Calcitonin-like receptor (CL-R) is a functional CGRP1-receptor when complexed with RAMP1 or an adrenomedullin-receptor or when complexed with RAMP2 or RAMP3. This study was carried out 1. to set up a method to examine the relative quantity of mRNA of CL-R, RAMP1, RAMP2 and RAMP3 in human coronary (CA), pulmonary (PA) and middle cerebral arteries (MCA), and 2. to examine the level of mRNA expression in cerebra- and cardiovascular diseases. The method was validated with respect to the use of postmortem tissue and we compared beta-actin and GAPDH as housekeeping genes. There was no time-dependent change in total RNA and level of mRNA for p-actin or GAPDH could be detected in vessels removed from 1 and 5 days post mortem. The expression of beta-actin appears lower in coronary artery than in pulmonary artery and middle cerebral artery with no significant difference for GAPDH; both worked well. There were some differences in mRNA expression for CL-R (higher) and RAMP3 (lower) in middle cerebral artery compared to coronary artery and pulmonary artery. There was no significant difference in mRNA for RAMP1 and RAMP2 in the three types of arteries. We did not observe any difference in mRNA for CL-R and RAMPs in arteries from patients with hemorrhagic stroke, arteriosclerosis and acute myocardial infarction when compared to patients without these diagnoses. Thus the mRNA expression seems to be unaltered in these disorders.

Expression of adrenomedullin in human epicardial adipose tissue: role of coronary status

Epicardial white adipose tissue (eWAT) is in close contact with coronary vessels and therefore could alter coronary homeostasis. Adrenomedullin (AM) is a potent vasodilatator and antioxidative peptide which has been shown to play a cytoprotective role in experimental models of acute myocardial infarction. We studied, using immunohistochemistry and qRT-PCR, the expression of AM and its receptors calcitonin receptor-like receptor (CRLR), and receptor activity-modifying protein (RAMP)2 and -3 in paired biopsies of subcutaneous WAT (sWAT) and eWAT obtained from patients with coronary artery disease (CAD) or without CAD (NCAD). In eWAT obtained from NCAD or CAD patients, immunoreactivity for AM, CRLR, and RAMP2 and -3 was detected in blood vessel walls and isolated stromal cells close to adipocytes. Some of the AM positive stromal cells colocalized CD68 immunoreactivity. eWAT from CAD patients showed increased AM immunoreactivity and AM gene expression. CRLR mRNA levels were comparable in sWAT of both groups and decreased by 40-50% in eWAT, irrespectively of the coronary status. RAMP2 mRNA concentrations did not change while RAMP3 mRNA levels increased in sWAT from CAD patients. There was a positive linear relationship between eWAT 11beta-hydroxysteroid dehydrogenase type 1 mRNA (11beta-HSD-1, the enzyme that converts inactive to active glucocorticoids) and AM mRNA. In conclusion, we demonstrate that AM and its receptors are expressed in eWAT. Our data suggest that eWAT AM, which could originate from macrophages, is related to 11beta-HSD-1 expression. AM synthesis, which is increased in eWAT during chronic CAD in humans, can play a cardioprotective role.

Rapid zygosity determination in mice by SYBR Green real-time genomic PCR of a crude DNA solution

We examined whether crude DNA extracts prepared from gene-engineered mouse tissues are suitable as a template for zygosity determination by SYBR Green real-time genomic PCR. A crude DNA solution was prepared by brief incubation with lysis buffer containing ear, tail, or fetus of ROSA26 mouse, a gene-trapped strain carrying the beta-galactosidase (beta-gal) gene. Five serially diluted crude DNA samples (original, 2-, 4-, 8-, 16-diluted) were next prepared and then subjected to three-step (95 degrees C, 60 degrees C and 72 degrees C) reactions of real-time PCR to detect the beta-gal gene and the receptor-activity-modifying protein 3 (ramp3) gene (as an internal reference gene). The slopes of standard curves obtained from the real-time PCR indicated that amplification efficiency was approximately 99%, and the efficiencies of target and reference were almost equal. With this system, we next determined the zygosity of mice derived from mating heterozygous ROSA26 females and males, and found a sharp distinction in zygosity, wild-type, heterozygous and homozygous. Assessment of crude DNA samples from other gene-engineered mice including B6ZP3Cre-Tg, B6rAM-Tg, and Ramp2-gene-targeted strains revealed that our method was effective for determination of zygosity. The present method is more convenient and rapid than formerly published methods employing purified genomic DNA as a template. Our method will be particularly useful for experiments requiring rapid and accurate genotyping of gene-modified animals/fetuses.

The effects of C-terminal truncation of receptor activity modifying proteins on the induction of amylin receptor phenotype from human CTb receptors

Receptor activity modifying proteins (RAMPs) interact with calcitonin receptors to produce novel amylin receptor phenotypes. We have recently demonstrated that the short intracellular C-terminus of RAMPs plays a key role in the function of amylin receptors derived from the CTa calcitonin receptor through the use of chimeric RAMPs and RAMPs that are truncated at the C-terminus [15, Udawela M, Christopoulos G, Morfis M, Christopoulos A, Ye S, Tilakaratne N, Sexton PM. A critical role for the short intracellular C terminus in receptor activity modifying protein function. Mol Pharmacol 2006;70:1750-60., 18, Udawela M, Christopoulos G, Tilakaratne N, Christopoulos A, Albiston A, Sexton PM. Distinct receptor activity-modifying protein domains differentially modulate interaction with calcitonin receptors. Mol Pharmacol 2006;69:1984-89.]. The calcitonin receptor in humans is expressed as two major alternatively spliced isoforms termed CTa and CTb. Relatively little is known about how alternate splicing of the receptor affects the interaction between calcitonin receptors and RAMPs. We have examined the effect of RAMP truncation, through use of mutant constructs that delete the last 8 amino acids of each of the 3 known human RAMPs, and characterised these for interaction with CTb receptors through co-expression in COS-7 cells. As seen with the CTa receptor isoform, RAMP truncation caused a marked loss in induction of AMYb receptor phenotypes as characterised by (125)I-rat amylin radioligand binding assays and cAMP accumulation assays; the latter as a marker of receptor signalling. The effect was most pronounced for RAMP1 and RAMP2 deletion mutants, but attenuated responses were also observed with co-expressed RAMP3 deletion mutants. These data support a direct role for the RAMP C-terminus in the interaction of RAMP/calcitonin receptor complexes with intracellular accessory proteins involved in signalling and/or receptor trafficking.

Calcitonin receptor-like receptor and receptor activity modifying protein 1 in the rat dorsal horn: localization in glutamatergic presynaptic terminals containing opioids and adrenergic alpha2C receptors

Calcitonin gene-related peptide (CGRP) is abundant in the central terminals of primary afferents. However, the function of CGRP receptors in the spinal cord remains unclear. CGRP receptors are heterodimers of calcitonin receptor-like receptor (CRLR) and receptor activity modifying protein 1 (RAMP1). We studied the localization of CRLR and RAMP1 in the rat dorsal horn using well-characterized antibodies against them, which labeled numerous puncta in laminae I-II. In addition, RAMP1 was found in cell bodies, forming patches at the cell surface. The CRLR- and RAMP1-immunoreactive puncta were further characterized using double and triple labeling. Colocalization was quantified in confocal stacks using Imaris software. CRLR did not colocalize with primary afferent markers, indicating that these puncta were not primary afferent terminals. CRLR- and RAMP1-immunoreactive puncta contained synaptophysin and vesicular glutamate transporter-2 (VGLUT2), showing that they were glutamatergic presynaptic terminals. Electron microscopic immunohistochemistry confirmed that CRLR immunoreactivity was present in axonal boutons that were not in synaptic glomeruli. Using tyramide signal amplification for double labeling with the CRLR and RAMP1 antibodies, we found some clear instances of colocalization of CRLR with RAMP1 in puncta, but their overall colocalization was low. In particular, CRLR was absent from RAMP1-containing cells. Many of the puncta stained for CRLR and RAMP1 were labeled by anti-opioid and anti-enkephalin antibodies. CRLR and, to a lesser extent, RAMP1 also colocalized with adrenergic alpha(2C) receptors. Triple label studies demonstrated three-way colocalization of CRLR-VGLUT2-synaptophysin, CRLR-VGLUT2-opioids, and CRLR-opioids-alpha(2C) receptors. In conclusion, CRLR is located in glutamatergic presynaptic terminals in the dorsal horn that contain alpha(2C) adrenergic receptors and opioids. Some of these terminals contain RAMP1, which may form CGRP receptors with CRLR, but in others CRLR may form other receptors, possibly by dimerizing with RAMP2 or RAMP3. These findings suggest that CGRP or adrenomedullin receptors modulate opioid release in the dorsal horn.

Adrenomedullin is induced by hypoxia and enhances pancreatic cancer cell invasion

Adrenomedullin (ADM) is synthesized by different types of cells and acts by binding calcitonin receptor-like receptor (CRLR) and members of the receptor activity-modifying protein (RAMP) family. In this study, the expression and functional role of ADM and its signaling components were investigated in pancreatic adenocarcinoma (PDAC). By QRT-PCR, median mRNA levels of ADM and CRLR were 1.5- and 2.4-fold higher, respectively, in PDAC tissues compared to normal pancreatic tissues. By immunohistochemistry, ADM, CRLR, RAMP1 and RAMP2, but not RAMP3, were expressed in pancreatic cancer cells. ADM serum levels were significantly increased in PDAC patients compared to healthy controls and chronic pancreatitis (CP) patients, with an area under the ROC curve of 0.83 and 0.98, respectively. At a cut-off level of 30.6 ng/ml, the specificity of ADM to differentiate PDAC from controls and CP patients was 85.5 and 83.6%, with a sensitivity of 80 and 100%. All 5 evaluated pancreatic cancer cells lines expressed ADM, CRLR, RAMP1 and RAMP2, whereas RAMP3 was expressed in only 1/5 pancreatic cancer cell lines. ADM was strongly induced by hypoxia and significantly increased invasiveness in 3/5 human pancreatic cancer cells. Blocking of CRLR decreased invasiveness in 4/5 human pancreatic cancer cells. In addition, rADM slightly up-regulated vascular endothelial growth factor secretion in 3/5 cell lines. In conclusion, ADM is induced by hypoxia and over-expressed in PDAC and might therefore serve as a potential tumor marker. Furthermore, ADM increases invasiveness of some pancreatic cancer cells and might influence angiogenesis, suggesting that blocking this pathway might have a therapeutic potential.

Receptor Activity-modifying Proteins 2 and 3 Have Distinct Physiological Functions from Embryogenesis to Old Age

RAMPs (receptor activity modifying proteins) impart remarkable effects on G protein-coupled receptor (GPCR) signaling. First identified through an interaction with the calcitonin receptor-like receptor (CLR), these single transmembrane proteins are now known to modulate the in vitro ligand binding affinity, trafficking, and second messenger pathways of numerous GPCRs. Consequently, the receptor-RAMP interface represents an attractive pharmacological target for the treatment of disease. Although the three known mammalian RAMPs differ in their sequences and tissue expression, results from in vitro biochemical and pharmacological studies suggest that they have overlapping effects on the GPCRs with which they interact. Therefore, to determine whether RAMP2 and RAMP3 have distinct functions in vivo, we generated mice with targeted deletions of either the RAMP2 or RAMP3 gene. Strikingly, we found that, although RAMP2 is required for survival, mice that lack RAMP3 appear normal until old age, at which point they have decreased weight. In addition, mice with reduced expression of RAMP2 (but not RAMP3) display remarkable subfertility. Thus, each gene has functions in vivo that cannot be accomplished by the other. Because RAMP2, RAMP3, and CLR transduce the signaling of the two potent vasodilators adrenomedullin and calcitonin gene-related peptide, we tested the effects of our genetic modifications on blood pressure, and no effects were detected. Nevertheless, our studies reveal that RAMP2 and RAMP3 have distinct physiological functions throughout embryogenesis, adulthood, and old age, and the mice we have generated provide novel genetic tools to further explore the utility of the receptor-RAMP interface as a pharmacological target.

Gene silencing of human RAMP2 mediated by short-interfering RNA

Adrenomedullin (AM) is a regulatory peptide widely expressed, along its receptors, in cells and tissues, of which it controls many basic and specific functions acting in an autocrine-paracrine manner. However, the unequivocal demonstration of the physiological relevance of the regulatory role of AM would require the study of cells where the endogenous AM system has been suppressed. For this task we developed a technique to silence the AM gene in human umbilical vein endothelial cells (HUVECs) and the human embryonal kidney cell line (HEK-293). AM acts via two subtypes of receptor, named AM1 and AM2, which derive from the interaction of the calcitonin receptor-like receptors with two chaperones, called receptor activity modifying proteins (RAMP2 and RAMP3). Hence, we developed a protocol to suppress the human AM1 receptor by silencing the RAMP2 gene by transfection with short interfering RNAs (siRNAs). HUVECs were transfected using a new Ambion transfection reagent. RAMP2 gene silencing was determined in HUVECs by measuring RAMP2 mRNA levels in transfected and control cells by real-time polymerase chain reaction. The RAMP2 gene silencing was approximately 60% and was observed 48 h after transfection. Matrigel assay in vitro was carried out to evaluate the effects of siRNA sequences. HUVECs cells were plated on matrigel and the analysis of capillary-like tubule formation showed that the cells were viable. The knockdown of the RAMP2 gene decreased the formation of tubes in response to 10(-8) M AM. The conclusion is drawn that siRNA technology can be successfully used in the investigations on AM and AM receptor functions.

Intermedin1-53 protects the heart against isoproterenol-induced ischemic injury in rats

Intermedin is a novel member of the calcitonin/calcitonin gene-related peptide (CGRP) family peptide, which has vasodilatory and hypotensive actions identical to those of adrenomedullin and CGRP. Cleavage sites located between 2 basic amino acids at Arg93-Arg94 result in the production of prepro-intermedin95-147, namely intermedin1-53. The bioactive action of intermedin1-53 and its physiological significance are unclear. In this work, we aimed to explore the effects of intermedin1-53 on acute myocardial injury induced by isoproterenol. Myocardial ischemia injury in rats was induced by subcutaneous injection of a high dose of isoproterenol, and the therapeutic effect of intermedin1-53 was observed. Plasma lactate dehydrogenase activity, myocardial and plasma malondialdehyde content were higher in the isoproterenol group than that in controls. Isoproterenol-treated rats showed lower maximal rate of increase and decrease of left-ventricle pressure development (+/-left-ventricle dp/dtmax) and higher left-ventricle end-diastolic pressure (all P<0.01), which suggested severe heart failure and myocardial injury. Semi-quantitative RT-PCR analysis showed that the gene expression of calcitonin receptor-like receptor and receptor-activity-modifying protein (RAMP)1, RAMP2 and RAMP3 in ventricular myocardia were up-regulated by 79% (P<0.01), 48% (P<0.01), 31% (P<0.05) and 130% (P<0.01), respectively, compared with controls. In myocardial sarcolemmal membranes, the maximum binding capacity for [125I]-intermedin1-53 was increased by 118% (P<0.01) in the isoproterenol group compared with controls. Rats treated with low dosage intermedin1-53 (5 nmol/kg/day, 2 days) showed 21% (P<0.05) higher myocardial cAMP content, 18% and 31% higher+left-ventricle dp/dtmax and -left-ventricle dp/dtmax respectively, 288% lower left-ventricle end-diastolic pressure (all P<0.01), and attenuated myocardial lactate dehydrogenase leakage and malondialdehyde formation (all P<0.01). Treatment with high dosage intermedin1-53 (20 nmol/kg/day, 2 days) gave better results than that with low dosage intermedin1-53. These results suggest that the intermedin receptor system was up-regulated in isoproterenol-induced myocardial ischemic injury and intermedin1-53 might play a pivotal cardioprotective role in such injury.

A critical role for the short intracellular C terminus in receptor activity-modifying protein function

Receptor activity-modifying proteins (RAMPs) interact with and modify the behavior of the calcitonin receptor (CTR) and calcitonin receptor-like receptor (CLR). We have examined the contribution of the short intracellular C terminus, using constructs that delete the last eight amino acids of each RAMP. C-Terminal deletion of individual RAMPs had little effect on the signaling profile induced when complexed with CLR in COS-7 or human embryonic kidney (HEK)293 cells. Likewise, confocal microscopy revealed each of the mutant RAMPs translocated hemagglutinin-tagged CLR to the cell surface. In contrast, a pronounced effect of RAMP C-terminal truncation was seen for RAMP/CTRa complexes, studied in COS-7 cells, with significant attenuation of amylin receptor phenotype induction that was stronger for RAMP1 and -2 than RAMP3. The loss of amylin binding upon C-terminal deletion could be partially recovered with overexpression of Galpha(s), suggesting an impact of the RAMP C terminus on coupling of G proteins to the receptor complex. In HEK293 cells the c-Myc-RAMP1 C-terminal deletion mutant showed high receptor-independent cell surface expression; however, this construct showed low cell surface expression when expressed alone in COS-7 cells, indicating interaction of RAMPs with other cellular components via the C terminus. This mutant also had reduced cell surface expression when coexpressed with CTR. Thus, this study reveals important functionality of the RAMP C-terminal domain and identifies key differences in the role of the RAMP C terminus for CTR versus CLR-based receptors.

Adrenomedullin Peptide: Gene Expression of Adrenomedullin, its Receptors and Receptor Activity Modifying Proteins, and Receptor Binding in Rat Testis—Actions on Testosterone Secretion1

Adrenomedullin (ADM) has been shown to be present in the human and rat male reproductive systems. This study demonstrates the expression of ADM in the rat testis and its effect on the secretion of testosterone. Whole testicular extracts had 5.43 +/- 0.42 fmol of immunoreactive ADM per milligram of protein and 84 +/- 8 fg of ADM mRNA per picogram of Actb (beta-actin) mRNA. Immunocytochemical studies showed positive ADM immunostaining in the Leydig cells and in the Sertoli cells. Gel filtration chromatography of testicular extracts showed two peaks, with the predominant one eluting at the position of the ADM precursor. Furthermore, the testis was shown to coexpress mRNAs encoding the calcitonin receptor-like receptor and receptor activity modifying protein 1 (Ramp1), Ramp2, and Ramp3. These account for the specific binding of ADM to the testis, which was partially inhibited by human ADM (22-52) and by human calcitonin gene-related peptide (8-37), the ADM and calcitonin gene-related peptide receptor antagonists, respectively. Administration of ADM to testicular blocks in vitro resulted in a dose-dependent inhibition of hCG-stimulated release of testosterone, which was abolished by the administration of ADM (22-52). Our results suggest a paracrine effect of ADM on testicular steroidogenesis.

The gene expression of adrenomedullin, calcitonin-receptor-like receptor and receptor activity modifying proteins (RAMPs) in CCl4-induced rat liver cirrhosis

This study was undertaken to determine AM expression in carbon tetrachloride (CCl4)-induced liver cirrhosis developed with peritoneal ascites. Sprague-Dawley rats received subcutaneous injections of CCl4 twice weekly in olive oil (1:1, 0.3 ml per kg body weight) for 6 or 12 weeks until ascites developed, or saline in olive oil as control. At 6 weeks, fibrosis developed and at 12 weeks cirrhosis developed with ascites formation. At both 6 and 12 weeks, increases in plasma renin and AM were evident, as was the gene expression of AM. At 12 weeks after CCl4 injection, the gene expression of calcitonin-like-receptor (CRLR) and receptor activity modifying proteins (RAMP1, RAMP2 and RAMP3) were all elevated when compared to the control. The results suggest that liver cirrhosis increases mRNA expressions of AM, CRLR and RAMP1, RAMP2 and RAMP3 and that the increase in AM gene expression precedes the development of cirrhosis. The increase in AM synthesis as reflected by an increase in AM gene expression, together with a lack of increase in AM peptide at both 6 and 12 weeks may suggest an elevation of AM release. Given the potent vasodilatory action of AM, the increase in the synthesis and release of AM in the cirrhotic liver may also contribute to peripheral vasodilatation in liver cirrhosis.

Expression of calcitonin receptor in rat mammary gland during lactation

Calcitonin (CT) and calcitonin receptor (CTR) have been reported to play an important role in mammary tissue during pregnancy, lactation, and involution. In the present study, the expression and distribution of CTR mRNA in rat mammary tissue during pregnancy and lactation were investigated. As measured by real-time RT-PCR, CTR mRNA levels were increased only slightly during pregnancy, but increased markedly immediately postpartum and remained elevated through lactation, with the highest levels observed 14 days postpartum. In situ hybridization analysis showed that intense CTR mRNA signals were detected in the whole mammary gland. We performed immunohistochemistry to determine distribution of CTR in the mammary epithelium. CTR has been reported to act as an amylin receptor when heterodimerized with receptor activity modifying protein-1 (RAMP1) or RAMP3. mRNA expression of RAMP1 and RAMP3 in mammary tissue decreased during pregnancy and lactation, and amylin mRNA was undetectable, suggesting that up-regulated CTR in lactating mammary tissues binds CT rather than amylin. In primary cultures of mammary cells isolated from rat dams 14 days postpartum, CT produced a statistically significant decrease in thymidine incorporation. These results suggest that up-regulation of CTR during lactation may contribute to inhibition of mammary epithelial cell proliferation.

Effects of calcitonin on the function of human osteoclast-like cells formed from CD14-positive monocytes

Calcitonin inhibits bone-resorbing activity of osteoclasts. Expression of mRNA of calcitonin receptor (CTR) and its related proteins was examined in human osteoclasts and their progenitors. CD14-positive (CD14 + macrophages) in the monocytes prepared from human peripheral blood cells differentiated into macrophages (CD14 +) presence of macrophage colony-stimulating factor (M-CSF) or into osteoclast-like cells (OCLs) in the presence of M-CSF plus receptor activator of NFkappaB ligand. CD14 macrophages expressed mRNA of CTR-like receptor (CRLR), receptor activity modifying protein (RAMP) 1, RAMP2, and RAMP3, but not CTR. In contrast, OCLs expressed mRNA of CTR but not CRLR or RAMPs. Human OCLs cultured on dentine slices formed actin rings (corresponding to clear zones) and resorption pits on the slices. Calcitonin disrupted actin rings and inhibited the pit-forming activity of OCLs. CTR is known to couple to cAMP-dependent protein kinase (PKA) and protein kinase C (PKC). The effect of calcitonin on actin ring disruption was partially blocked by adding H-7, an inhibitor of both PKA and PKC. Both forskolin, an activator of PKA, and phorbol myristate, an activator of PKC, disrupted actin rings in OCLs. These results suggest that both PKA- and PKC-mediated signals are involved in calcitonin-induced inhibition of human OCL function.

Potentiated response to adrenomedullin in myocardia and aortas in spontaneously hypertensive rat

Adrenomedullin (AM) is a multifunctional regulatory peptide, and endogenous AM is an important factor in regulating cardiovascular and renal homeostasis as a potent cardio-reno-protective factor. To illustrate the protective mechanism of adrenomedullin (AM) on the cardiovascular system by observing (1) the changes in mRNA and protein levels of AM and its receptor-calcitonin receptor-like receptor (CL) and receptor activity-modifying proteins (RAMPs)-in myocardia and aortas of spontaneously hypertensive rats (SHRs) and (2) the response of cardiovascular tissue to AM. The AM content and cyclic adenosine monophosphate (cAMP) production in myocardia and aortas were measured in SHRs and Wistar Kyoto (WKY) rats (11-week-old) by radioimmunoassay (RIA). The mRNA levels of brain natriuretic peptide (BNP), AM, CL, RAMP1, -2, -3 were determined by semi-quantitative RTPCR. Protein levels of CL, RAMP1, -2, -3 were assayed by Western blotting. SHRs had severe hypertension, and the tail-blood pressure was 76.7% higher, the ratio of heart weight to body weight (heart coefficient) 45.5% higher, and the BNP gene expression 4.5-fold higher than that of WKY rats (all p < 0.01). The AM-ir content in plasma, myocardia and aortas of SHRs increased by 42.5%, 68.3% and 80.4%, respectively (all p < 0.01) compared with WKY rats. Furthermore, the mRNA levels of AM, CL, RAMP1, RAMP2 and RAMP3 were elevated by 46% (p < 0.01), 62% (p < 0.05), 51.2% (p < 0.01), 41% (p < 0.01) and 54% (p < 0.01), respectively, in myocardia and by 72%, 87%, 155%, 53% and 74% (all p < 0.01), respectively, in aortas. The elevated mRNA level of CL, RAMP1 RAMP2 and RAMP3 correlated positively with that of AM mRNA in hypertrophic myocardia (r= 0.943, 0.621, 0.688 and 0.633, respectively, all p < 0.01) and aortas (r = 0.762, 0.892, 0.828 and 0.736, respectively, all p < 0.01). The protein levels of CL, RAMP1, RAMP2 and RAMP3 in myocardia and aortas of SHRs were increased compared with that of WKY rats. The response to AM was potentiated in myocardia and aortas in SHRs, and the production of cAMP was increased by 47% and 65% (both p < 0.01), respectively. AM-stimulated cAMP generation in myocardia and aortas was blocked by both AM(22-52), the specific antagonist of AM, and calcitonin gene-related peptide (CGRP)(8-37), the antagonist of the CGRP1 receptor. In myocardia and aortas of SHRs, the gene expressions and protein levels of AM, CL, RAMP1, RAMP2 and RAMP3 were increased, and the response to AM was potentiated. AM-stimulated cAMP generation in myocardia and aortas was blocked by both AM(22-52) and CGRP(8-37). The results suggest that the changes of AM and its receptors in cardiovascular tissue, and the increased response of cardiovascular tissue to AM might importantly impact the pathogenesis of hypertension.

The estrogen-responsive adrenomedullin and receptor-modifying protein 3 gene identified by DNA microarray analysis are directly regulated by estrogen receptor

Recent studies have revealed that hundreds of genes in the uterus are activated by estrogen. Their expression profiles differ over time and doses and it is not clear whether all these genes are directly regulated by estrogen via the estrogen receptor. To select the genes that may be regulated by estrogen, we treated mice with several doses of estrogen and searched for those genes whose dose-response expression pattern mirrored the uterine growth pattern. Among those genes, we found that the dose-dependent expression of the adrenomedullin (ADM) gene correlated well with the uterotrophic effect of estrogen. ADM expression is induced early after estrogen administration and is restricted to the endometrial stroma. The spatiotemporal gene expression pattern of ADM was similar to that of receptor-modifying protein 3 (RAMP3). RAMP3 is known to modify calcitonin gene-related receptor (CRLR) so that it can then serve as an ADM receptor. Chromatin immunoprecipitation assays indicated that the estrogen receptor binds directly to the ADM promoter region and RAMP3 intron after estrogen administration. It was also shown that neither the ADM nor RAMP3 gene could be activated in estrogen receptor-alpha-null mouse. Although uterine ADM expression has been reported to occur in the myometrium, our observations indicate that estrogen-induced ADM is also expressed in the uterine stroma and that such variable, spatiotemporally regulated ADM expression contributes to a wider range of biological effects than previously expected.

Induction of BAALC and down regulation of RAMP3 in astrocytes treated with differentiation inducers

Genes controlling proliferation and differentiation of astrocytes are likely to play an important role in the pathogenesis of astrocytic gliomas and could serve as therapeutic targets. mRNA differential display analysis identified two genes, viz. BAALC and RAMP3, whose expression is altered in primary astrocytes treated with differentiation inducers. BAALC, which has been reported to be expressed in neural progenitor cells, was found to be up regulated in both normal and malignant astrocytes on inhibition of proliferation. RAMP3 functions as a modulator of the receptor for adrenomedullin (AM). AM has been suggested to act as autocrine/paracrine growth factor for gliomas. Our studies show RAMP3 down regulation on staurosporine treatment of astrocytes, suggesting protein kinase C inhibition as a possible strategy for inhibiting AM activity and thereby growth of glioma cells.

Differential expression of components of the cardiomyocyte adrenomedullin/intermedin receptor system following blood pressure reduction in nitric oxide-deficient hypertension

Adrenomedullin (AM) and intermedin (IMD; adrenomedulln-2) are vasodilator peptides related to calcitonin gene-related peptide (CGRP). The actions of these peptides are mediated by the calcitonin receptor-like receptor (CLR) in association with one of three receptor activity-modifying proteins. CGRP is selective for CLR/receptor activity modifying protein (RAMP)1, AM for CLR/RAMP2 and -3, and IMD acts at both CGRP and AM receptors. In a model of pressure overload induced by inhibition of nitric-oxide synthase, up-regulation of AM was observed previously in cardiomyocytes demonstrating a hypertrophic phenotype. The current objective was to examine the effects of blood pressure reduction on cardiomyocyte expression of AM and IMD and their receptor components. Nomega-nitro-L-arginine methyl ester (L-NAME) (35 mg/kg/day) was administered to rats for 8 weeks, with or without concurrent administration of hydralazine (50 mg/kg/day) and hydrochlorothiazide (7.5 mg/kg/day). In left ventricular cardiomyocytes from L-NAME-treated rats, increases (-fold) in mRNA expression were 1.6 (preproAM), 8.4 (preproIMD), 3.4 (CLR), 4.1 (RAMP1), 2.8 (RAMP2), and 4.4 (RAMP3). Hydralazine/hydrochlorothiazide normalized systolic blood pressure (BP) and abolished mRNA up-regulation of hypertrophic markers sk-alpha-actin and BNP and of preproAM, CLR, RAMP2, and RAMP3 but did not normalize cardiomyocyte width nor preproIMD or RAMP1 mRNA expression. The robust increase in IMD expression indicates an important role for this peptide in the cardiac pathology of this model but, unlike AM, IMD is not associated with pressure overload upon the myocardium. The concordance of IMD and RAMP1 up-regulation indicates a CGRP-type receptor action; considering also a lack of response to BP reduction, IMD may, like CGRP, have an anti-ischemic function.

Localization of calcitonin receptor-like receptor and receptor activity modifying protein 1 in enteric neurons, dorsal root ganglia, and the spinal cord of the rat

Calcitonin receptor-like receptor (CLR) and receptor activity modifying protein 1 (RAMP1) comprise a receptor for calcitonin gene related peptide (CGRP) and intermedin. Although CGRP is widely expressed in the nervous system, less is known about the localization of CLR and RAMP1. To localize these proteins, we raised antibodies to CLR and RAMP1. Antibodies specifically interacted with CLR and RAMP1 in HEK cells coexpressing rat CLR and RAMP1, determined by Western blotting and immunofluorescence. Fluorescent CGRP specifically bound to the surface of these cells and CGRP, CLR, and RAMP1 internalized into the same endosomes. CLR was prominently localized in nerve fibers of the myenteric and submucosal plexuses, muscularis externa and lamina propria of the gastrointestinal tract, and in the dorsal horn of the spinal cord of rats. CLR was detected at low levels in the soma of enteric, dorsal root ganglia (DRG), and spinal neurons. RAMP1 was also localized to enteric and DRG neurons and the dorsal horn. CLR and RAMP1 were detected in perivascular nerves and arterial smooth muscle. Nerve fibers containing CGRP and intermedin were closely associated with CLR fibers in the gastrointestinal tract and dorsal horn, and CGRP and CLR colocalized in DRG neurons. Thus, CLR and RAMP1 may mediate the effects of CGRP and intermedin in the nervous system. However, mRNA encoding RAMP2 and RAMP3 was also detected in the gastrointestinal tract, DRG, and dorsal horn, suggesting that CLR may associate with other RAMPs in these tissues to form a receptor for additional peptides such as adrenomedullin.

Antiapoptotic effect of calcitonin gene-related peptide on oxidative stress-induced injury in H9c2 cardiomyocytes via the RAMP1/CRLR complex

Calcitonin gene-related peptide (CGRP) plays an important role in the mediation of protective effects observed in situations such as ischemic preconditioning in rat hearts. In this study, we investigated in H9c2 rat cardiomyoblasts if the protective effect of CGRP could be linked to an inhibitory effect on the apoptotic pathway. We also determined the specificity of observed effects by treatment with adrenomedullin (ADM) in stress conditions generated by 100 microM hydrogen peroxide. Using MTT assays, we demonstrate that a pretreatment with CGRP decreases by half the loss of cell viability induced by H(2)O(2). CGRP inhibits phosphatidylserine externalization, caspase 3 activation and DNA fragmentation due to oxidative stress. Using RT-PCR, we observed an increase in Bcl-2 mRNA expression induced by CGRP treatment. Dot blotting experiments showed that, in stress conditions, Bcl-2 protein level decreases while Bax is increased. CGRP administration prior to stress prevents these effects. The three-receptor activity modifying protein (RAMP) isotypes were detected by RT-PCR in H9c2 cells and in left ventricle rat tissue, RAMP1 and RAMP3 being the most abundant in both cases. RAMP1 expression was upregulated by CGRP while RAMP3 mRNA level was decreased. Cell viability assessed by MTT indicates that, contrary to CGRP, pretreatment of stressed cells with ADM, a RAMP2 agonist, fails to protect them while treatment with CGRP(8-37) (a RAMP1 and 2 inhibitor) abolished CGRP protective effect. Taken together, these data suggest that CGRP has antiapoptotic properties through the RAMP1/CRLR complex. CGRP could be used to prevent apoptosis in an ischemia-reperfusion context.

Receptor-activity-modifying proteins are required for forward trafficking of the calcium-sensing receptor to the plasma membrane

The calcium-sensing receptor (CaSR) is a class III G-protein-coupled receptor (GPCR) that responds to changes in extracellular calcium concentration and plays a crucial role in calcium homeostasis. The mechanisms controlling CaSR trafficking and surface expression are largely unknown. Using a CaSR tagged with the pH-sensitive GFP super-ecliptic pHluorin (SEP-CaSR), we show that delivery of the GPCR to the cell surface is dependent on receptor-activity-modifying proteins (RAMPs). We demonstrate that SEP-CaSRs are retained in the endoplasmic reticulum (ER) in COS7 cells that do not contain endogenous RAMPs whereas they are delivered to the plasma membrane in HEK 293 cells that do express RAMP1. Coexpression of RAMP1 or RAMP3, but not RAMP2, in COS7 cells was sufficient to target the CaSR to the cell surface. RAMP1 and RAMP3 colocalised and coimmunoprecipitated with the CaSR suggesting that these proteins associate within the cell. Our results indicate that RAMP expression promotes the forward trafficking of the GPCR from the ER to the Golgi apparatus and results in mature CaSR glycosylation, which is not observed in RAMP-deficient cells. Finally, silencing of RAMP1 in the endogenously expressing HEK293 cells using siRNA resulted in altered CaSR traffic. Taken together, our results show that the association with RAMPs is necessary and sufficient to transfer the immature CaSR retained in the ER towards the Golgi where it becomes fully glycosylated prior to delivery to the plasma membrane and demonstrate a role for RAMPs in the trafficking of a class III GPCR.

Parathyroid hormone induces receptor activity modifying protein-3 (RAMP3) expression primarily via 3',5'-cyclic adenosine monophosphate signaling in osteoblasts

Parathyroid hormone (PTH) has significant anabolic and catabolic effects on bone. We hypothesize that PTH-induced primary response genes are important determinants of osteoblast function. PTH induces osteoblastic gene expression through PTHR1, a heptahelical receptor that triggers cyclic adenosine monophosphate (cAMP)-protein kinase A (PKA), protein kinase C (PKC), and calcium signaling. By using representational difference analysis we found that receptor activity modifying protein-3 (RAMP3) is a PTH-induced primary response gene in osteoblastic cells. RAMP3 is a coactivator that directs calcitonin receptor (CTR) and CTR-like receptor (CRLR) glycosylation, trafficking, and ligand-binding specificity. Our purpose was to characterize PTH-induced RAMP3 messenger ribonucleic acid (mRNA) levels in primary mouse osteoblasts (MOBs) and to determine which signaling pathway mediates this effect. 10 nM PTH maximally induced RAMP3 mRNA levels in MOBs at 4 hours. Protein synthesis inhibition with 3 microg/mL cycloheximide did not affect PTH-induced RAMP3 mRNA levels. Selective activation of cAMP-PKA signaling with, 10 microM forskolin (FSK) and PKC signaling with 1 microM phorbol 12-myristate 13-acetate (PMA) significantly increased RAMP3 mRNA levels, whereas 1 microM ionomycin (a calcium ionophore) had no effect. Pretreatment with 30 microM H89, a PKA inhibitor, significantly blocked PTH- and FSK-induced RAMP3 mRNA levels. Pretreatment with 1 microM PMA, which depletes PKC, had no effect on PTH- and FSK-induced RAMP3 mRNA levels but blocked PMA-induced RAMP3 mRNA levels. 100 nM PTH (3-34), which activates PKC and calcium but not PKA, had no effect on RAMP3 mRNA levels. These findings indicate that RAMP3 is a PTH-induced primary response gene in primary MOBs and that PTH regulates RAMP3 gene expression primarily through the cAMP-PKA pathway.

Receptor Activity-modifying Protein (RAMP) Isoform-specific Regulation of Adrenomedullin Receptor Trafficking by NHERF-1

Receptor activity-modifying proteins (RAMPs 1-3) are single transmembrane accessory proteins critical to various G-protein coupled receptors for plasma membrane expression and receptor phenotype. A functional receptor for the vasodilatory ligand, adrenomedullin (AM), is comprised of RAMP2 or RAMP3 and calcitonin receptor-like receptor (CRLR). It is now known that RAMP3 protein-protein interactions regulate the recycling of the AM2 receptor. The major aim of this study was to identify other interaction partners of RAMP3 and determine their role in CRLR-RAMP3 trafficking. Trafficking of G-protein-coupled receptors has been shown to be regulated by the Na+/H+ exchanger regulatory factor-1 (NHERF-1), an adaptor protein containing two tandem PSD-95/Discs-large/ZO-1 homology (PDZ) domains. In HEK 293T cells expressing the AM2 receptor, the complex undergoes agonist-induced desensitization and internalization. However, in the presence of NHERF-1, although the AM receptor (CRLR/RAMP3) undergoes desensitization, the internalization of the receptor complex is blocked. Overlay assays and mutational analysis indicated that RAMP3 and NHERF-1 interact via a PDZ type I domain on NHERF-1. The internalization of the CRLR-RAMP complex was not affected by NHERF-1 when CRLR was co-expressed with RAMP1 or RAMP2. Mutation of the ezrin/radixin/moesin (ERM) domain on NHERF-1 indicated that NHERF-1 inhibits CRLR/RAMP3 complex internalization by tethering the complex to the actin cytoskeleton. When examined in a primary culture of human proximal tubule cells endogenously expressing the CRLR-RAMP3 complex and NHERF-1, the CRLR-RAMP complex desensitizes but is unable to internalize upon agonist stimulation. Knock-down of either RAMP3 or NHERF-1 by RNA interference technology enabled agonist-induced internalization of the CRLR-RAMP complex. These results, using both endogenous and overexpressed cellular models, indicate a novel function for NHERF-1 and RAMP3 in the internalization of the AM receptor and suggest additional regulatory mechanisms for receptor trafficking.

Novel Function for Receptor Activity-modifying Proteins (RAMPs) in Post-endocytic Receptor Trafficking

RAMPs (1-3) are single transmembrane accessory proteins crucial for plasma membrane expression, which also determine receptor phenotype of various G-protein-coupled receptors. For example, adrenomedullin receptors are comprised of RAMP2 or RAMP3 (AM1R and AM2R, respectively) and calcitonin receptor-like receptor (CRLR), while a CRLR heterodimer with RAMP1 yields a calcitonin gene-related peptide receptor. The major aim of this study was to determine the role of RAMPs in receptor trafficking. We hypothesized that a PDZ type I domain present in the C terminus of RAMP3, but not in RAMP1 or RAMP2, leads to protein-protein interactions that determine receptor trafficking. Employing adenylate cyclase assays, radioligand binding, and immunofluorescence microscopy, we observed that in HEK293 cells the CRLR-RAMP complex undergoes agonist-stimulated desensitization and internalization and fails to resensitize (i.e. degradation of the receptor complex). Co-expression of N-ethylmaleimide-sensitive factor (NSF) with the CRLR-RAMP3 complex, but not CRLR-RAMP1 or CRLR-RAMP2 complex, altered receptor trafficking to a recycling pathway. Mutational analysis of RAMP3, by deletion and point mutations, indicated that the PDZ motif of RAMP3 interacts with NSF to cause the change in trafficking. The role of RAMP3 and NSF in AM2R recycling was confirmed in rat mesangial cells, where RNA interference with RAMP3 and pharmacological inhibition of NSF both resulted in a lack of receptor resensitization/recycling after agonist-stimulated desensitization. These findings provide the first functional difference between the AM1R and AM2R at the level of post-endocytic receptor trafficking. These results indicate a novel function for RAMP3 in the post-endocytic sorting of the AM-R and suggest a broader regulatory role for RAMPs in receptor trafficking.

Osteoclast-like cells express receptor activity modifying protein 2: application of laser capture microdissection

Receptor activity modifying proteins (RAMPs) act as receptor modulators that determine the ligand specificity of receptors for the calcitonin (CT) family. The purpose of this study was to analyze the expression of RAMPs in osteoclast-like cells using the laser capture microdissection (LCM) technique. Mouse bone marrow and spleen cells were co-cultured on a film designed for LCM. After 10 days, 250 osteoclast-like cells were captured using the LCM system. Total RNA from these cells was used to synthesize cDNA and RT-PCR analysis was performed. Osteoclast-like cells expressed CT receptor (CTR), CT receptor-like receptor (CRLR) and RAMP2, but did not express RAMP1 or RAMP3. These results indicated (1) that a pure population of osteoclast-like cells can be prepared by LCM and gene expression of this population can be analyzed by RT-PCR and (2) that RT-PCR shows that osteoclast-like cells express RAMP2, CTR and CRLR, suggesting the potential for adrenomedullin binding to osteoclast-like cells. This is the first report that osteoclast-like cells express RAMP2.

Response of adrenomedullin system to cytokine in cardiac fibroblasts-role of adrenomedullin as an antifibrotic factor

OBJECTIVE

The adrenomedullin system acts as an autocrine or paracrine factor (or both) in the development of cardiac hypertrophy and in the regulation of cardiac function. However, several aspects of the local action of adrenomedullin remain unclear. We studied the effects of interleukin 1-beta (IL-1beta) on the adrenomedullin system in cardiac fibroblasts and also examined the pathophysiological significance of such effects.

METHODS

We cultured rat neonatal cardiac fibroblasts with or without IL-1beta and measured (1) two molecular forms of adrenomedullin in culture medium by specific immunoradiometric assay; (2) gene expression of adrenomedullin, calcitonin receptor like receptor (CRLR), receptor activity modifying protein2 (RAMP2), and RAMP3, components of the adrenomedullin receptor, by Northern blot analysis or RT-PCR analysis; (3) intracellular cAMP levels in response to exogenously administered adrenomedullin; and (4) (3)H-proline incorporation with and without a specific adrenomedullin antisense oligodeoxynucleotide.

RESULTS

(1) IL-1beta time-dependently increased the levels of two molecular forms of adrenomedullin, adrenomedullin-mature and adrenomedullin-glycine (P<0.01). In contrast to known levels in plasma (about 10%), adrenomedullin-mature was a major molecular form in the culture medium of cardiac fibroblasts and myocytes (65-80%). (2) IL-1beta significantly increased gene expression of adrenomedullin and its receptor components (adrenomedullin: +46%, CRLR: +460%, RAMP2: +32%, RAMP3: +350%, all P<0.01). (3) Preincubated IL-1beta elevated the intracellular cAMP response to exogenous adrenomedullin administered at a concentration of 10(-7) M (+26%, P<0.05). (4) Adrenomedullin antisense oligodeoxynucleotide treatment significantly lowered adrenomedullin-mature levels in culture medium (-50%). Adrenomedullin nonsense oligodeoxynucleotide treatment did not change (3)H-proline incorporation or mRNA levels of collagen I and III, whereas adrenomedullin antisense oligodeoxynucleotide treatment significantly increased (3)H-proline incorporation and mRNA levels of collagen I and III in IL-1beta-treated cardiac fibroblasts.

CONCLUSION

These results provide evidence that the adrenomedullin system acts as an autocrine antifibrotic factor in the regulation of collagen synthesis in cardiac fibroblasts exposed to higher cytokine levels. This may beneficially modulate the pathophysiology of certain cardiac diseases.

Receptor and secreted targets of Wnt-1/beta-catenin signalling in mouse mammary epithelial cells

BACKGROUND

Deregulation of the Wnt/ beta-catenin signal transduction pathway has been implicated in the pathogenesis of tumours in the mammary gland, colon and other tissues. Mutations in components of this pathway result in beta-catenin stabilization and accumulation, and the aberrant modulation of beta-catenin/TCF target genes. Such alterations in the cellular transcriptional profile are believed to underlie the pathogenesis of these cancers. We have sought to identify novel target genes of this pathway in mouse mammary epithelial cells.

METHODS

Gene expression microarray analysis of mouse mammary epithelial cells inducibly expressing a constitutively active mutant of beta-catenin was used to identify target genes of this pathway.

RESULTS

The differential expression in response to DeltaNbeta-catenin for five putative target genes, Autotaxin, Extracellular Matrix Protein 1 (Ecm1), CD14, Hypoxia-inducible gene 2 (Hig2) and Receptor Activity Modifying Protein 3 (RAMP3), was independently validated by northern blotting. Each of these genes encodes either a receptor or a secreted protein, modulation of which may underlie the interactions between Wnt/beta-catenin tumour cells and between the tumour and its microenvironment. One of these genes, Hig2, previously shown to be induced by both hypoxia and glucose deprivation in human cervical carcinoma cells, was strongly repressed upon DeltaNbeta-catenin induction. The predicted N-terminus of Hig2 contains a putative signal peptide suggesting it might be secreted. Consistent with this, a Hig2-EGFP fusion protein was able to enter the secretory pathway and was detected in conditioned medium. Mutation of critical residues in the putative signal sequence abolished its secretion. The expression of human HIG2 was examined in a panel of human tumours and was found to be significantly downregulated in kidney tumours compared to normal adjacent tissue.

CONCLUSIONS

HIG2 represents a novel non-cell autonomous target of the Wnt pathway which is potentially involved in human cancer.

Receptor pharmacology

Despite clear evidence for a distinct amylin pharmacology and localization of such pharmacology to sites such as the nucleus accumbens,efforts to clone an amylin receptor were fruitless for over a decade. This enigma led many to doubt the status of amylin as a bona fide hormone. Yet it became apparent during those cloning efforts that, whatever the amylin receptor was, it was somehow similar to a calcitonin receptor. The enigma of the amylin receptor was solved following the identification of receptor activity modifying proteins (RAMPs). These single transmembrane spanning molecules, when associated with a calcitonin receptor, altered its pharmacology from calcitonin-preferring to amylin-preferring. With at least two forms of the calcitonin receptor and three forms of RAMP, there is the potential for six subtypes of amylin receptors. Of these, two appear to predominate. The CTa (shorter form) calcitonin receptor, dimerized with RAMP1 [amylin 1 (a) receptor], appears to represent binding sites at the nucleus accumbens and the subfornical organ. Binding sites at area postrema appear to be composed of CTa + RAMP3 [amylin3 (a) receptors]. Thus far, RAMP proteins have been associated in vivo only with the CT/CLR receptor system. It is presently unknown whether RAMPs are more general modulators of receptor function, dynamically modifying responsivity with time or across other receptor classes. The largest and first identified amylin-binding field was in the nucleus accumbens. The function of these receptors is yet undetermined, but because the nucleus accumbens is within the blood-brain barrier, the cognate ligand is unlikely to be circulating amylin. Dense amylin binding is present at the circumventricular organs, including the subfornical organ, the organum vasculosum lateralis terminalis (OVLT), and the area postrema. There is no diffusional (blood-brain) barrier at these structures, so they most likely respond to circulating (beta-cell-derived) amylin. Despite pharmacological evidence of amylin sensitivity in several peripheral tissues, selective amylin binding outside of the brain is observed only in the renal cortex. The newly designated amylinomimetic drug class was defined on the basis of its unique pharmacology prior to the molecular characterization of amylin receptors. Currently, the class includes any agent that acts as antagonist at characterized amylin receptors. Several peptides, typically analogs of truncated salmon calcitonin, have been developed as potent and selective amylin antagonists and have been useful in identifying amylinergic responses. Of these, AC187 (30Asn32Tyr[8-32]sCT; Amylin Pharmaceuticals Inc.) is particularly selective and potent, and has been most often cited in studies using amylin antagonists. Antagonism of a response with an order of potency of AC187> AC66 > CGRP[8-37] is suggestive that it is mediated via amylin receptors. Activation of a response with salmon calcitonin (sCT) > amylin >calcitonin gene-related peptide (CGRP) > mammalian CT suggests activation via the amylinl (a) receptor, while sCT = amylin >> CGRP >mammalian CT suggests activation via amylin3 (a) receptors. Absence of response to other ligands (e.g., adrenomedullin) is useful for excluding certain pharmacologies.

RAMP2 and RAMP3 mRNA levels are increased in failing rat cardiomyocytes and associated with increased responsiveness to adrenomedullin

Adrenomedullin (AM) is a potent vasorelaxing peptide with natriuretic and diuretic actions. Recent data indicate that AM may function as an endogenous regulator of cardiac function. We investigated to what extent AM, the AM receptor subtypes, and AM receptor-associated proteins were regulated in cardiomyocytes and non-cardiomyocytes of rats with congestive heart failure (CHF), and whether such regulation was paralleled by corresponding alterations of functional responses to AM. Cardiomyocytes and non-cardiomyocytes were isolated from myocardial tissue of rats 7 days after induction of myocardial infarction or sham operation. AM immunoreactivity was found in cardiomyocytes, endothelial cells, and fibroblasts. Robust increase of AM mRNA levels was observed both in the cardiomyocytes and in the non-cardiomyocytes of CHF rats compared to that of sham-operated rats (2.7-fold and 3.7-fold, respectively, P <0.05). Fairly high mRNA levels and immunoreactivity against the AM receptor chaperone receptor activity-modifying protein-2 (RAMP2) were also detected in the cardiomyocytes and non-cardiomyocytes. However, induction of RAMP2 mRNA expression was restricted to cardiomyocytes (1.8-fold increase in cardiomyocytes from CHF rats vs. sham rats; P <0.05). In contrast, very low levels of RAMP3 mRNA were observed. RAMP3 mRNA levels, however, were elevated in both cardiomyocytes and non-cardiomyocytes from CHF rats (6.5-fold and 2.4-fold increase vs. sham rats, respectively; P <0.05). Parallel increases of specific AM receptor binding sites and of AM-stimulated adenylyl cyclase activities were observed in failing cardiomyocytes compared to cardiomyocytes from sham rats (fivefold and sixfold increase, respectively; P <0.05). Thus, this study demonstrates that AM mRNA levels, AM receptor binding sites, and AM-stimulated adenylyl cyclase activities are increased in cardiomyocytes from failing rat hearts. Furthermore, our data suggest that induction of RAMP2 and RAMP3 contributes to the increased responsiveness to AM in failing cardiomyocytes.

Heterodimers and family-B GPCRs: RAMPs, CGRP and adrenomedullin

RAMPs (receptor activity-modifying proteins) are single-pass transmembrane proteins that associate with certain family-B GPCRs (G-protein-coupled receptors). Specifically for the CT (calcitonin) receptor-like receptor and the CT receptor, this results in profound changes in ligand binding and receptor pharmacology, allowing the generation of six distinct receptors with preferences for CGRP (CT gene-related peptide), adrenomedullin, amylin and CT. There are three RAMPs: RAMP1–RAMP3. The N-terminus appears to be the main determinant of receptor pharmacology, whereas the transmembrane domain contributes to association of the RAMP with the GPCR. The N-terminus of all members of the RAMP family probably contains two disulphide bonds; a potential third disulphide is found in RAMP1 and RAMP3. The N-terminus appears to be in close proximity to the ligand and plays a key role in its binding, either directly or indirectly. BIBN4096BS, a CGRP antagonist, targets RAMP1 and this gives the compound very high selectivity for the human CGRP1 receptor.

Amylin receptors: molecular composition and pharmacology

Several receptors which bind the hormone AMY (amylin) with high affinity have now been identified. The minimum binding unit is composed of the CT (calcitonin) receptor at its core, plus a RAMP (receptor activity modifying protein). The receptors have been named AMY1(a), AMY2(a) and AMY3(a) in accordance with the association of the CT receptor (CT(a)) with RAMP1, RAMP2 and RAMP3 respectively. The challenge is now to determine the localization and pharmacological nature of each of these receptors. Recent attempts to achieve these aims will be briefly discussed.

Progesterone upregulates calcitonin gene-related peptide and adrenomedullin receptor components and cyclic adenosine 3'5'-monophosphate generation in Eker rat uterine smooth muscle cell line

Calcitonin gene-related peptide (CGRP) and adrenomedullin (AM), two potent smooth-muscle relaxants, have been shown to cause uterine relaxation. Both CGRP- and AM-binding sites in the uterus increase during pregnancy and decrease at labor and postpartum. These changes in binding sites appear to be related to the changes in calcitonin receptor-like receptor (CRLR), receptor activity-modified protein 1 (RAMP1), RAMP2, and RAMP3 mRNA levels. It is not clear, however, whether the changes in the receptor components occur in the myometrial cells and whether the steroid hormones can directly alter these receptor components in the muscle cells. In addition, the mechanism of CGRP and AM signaling in the rat myometrium is not well understood. Therefore, we examined the mRNA expression of CGRP- and AM-receptor components, G protein Galphas, CGRP, and AM stimulation of cAMP and cGMP, and the effects of progesterone on these parameters in the Eker rat uterine myometrial smooth-muscle cell line (ELT3). ELT3 cells expressed CGRP- and AM-receptor components CRLR, RAMP1, RAMP2, and RAMP3. Expression of CRLR and RAMP1 mRNA increased with progesterone treatment and decreased with estradiol-17beta treatment. However, RAMP2 and RAMP3 mRNA expressions were unaltered by both progesterone and estradiol. Progesterone increased (P<0.05) Galphas expression and augmented CGRP- and AM-induced increases in cAMP levels. In uterine smooth-muscle cells, the antagonist to Galphas protein NF449 decreased basal as well as CGRP- and AM-stimulated cAMP levels. None of the cell treatments affected cyclic GMP production. Our results suggest that the progesterone-stimulated increases in CGRP and AM receptors, Galphas protein levels, and cAMP generation in the myometrial cells may be responsible for increased uterine relaxation sensitivity to CGRP and AM during pregnancy.

The role of adrenomedullin and receptors in glomerular hyperfiltration in streptozotocin-induced diabetic rats

BACKGROUND

Since adrenomedullin (AM) elicits vasodilatation by binding to specific AM receptors consisted of calcitonin-receptor-like receptor (CRLR)/receptor-activity-modifying protein 2 (RAMP2) or CRLR/receptor-activity-modifying protein 3 (RAMP3) on endothelial cells and stimulating nitric oxide production, AM possibly involves in glomerular capillary dilatation in early phase of diabetic nephropathy.

METHODS

Streptozotocin (STZ)-induced diabetic Sprague-Dawley rats at 4 weeks after the injection were employed for expression studies of AM, RAPM2, and RAMP3. The measurement of AM peptide levels in kidney tissue, plasma, and urine was performed. Human aortic endothelial cells (HAEC) were used to investigate functional link between glucose-induced AM production and nitric oxide release.

RESULTS

STZ rats showed glomerular hypertrophy and increased urinary NO2- and NO3- excretion. By Northern blot analyses, AM and RAPM2 mRNAs significantly increased in the kidneys of STZ rats, while RAMP3 mRNA was not altered. In STZ rats, AM peptide was actively secreted into urine (1280 +/- 360 fmol/day vs. control 110 +/- 36 fmol/day). AM peptide was mainly detected on cortical and medullary collecting duct cells in control rat kidneys and AM peptide and mRNA were up-regulated on afferent arterioles and glomeruli of STZ rats. RAMP2 expression was detected on afferent arterioles and not in glomeruli in control rats and it was up-regulated on glomerular endothelial cells in STZ rats. In HAEC culture, d-glucose stimulated AM and nitric oxide production and they were suppressed by addition of AM antisense oligodeoxynucleotides.

CONCLUSION

Up-regulated expression of AM and RAMP2 in afferent arterioles and glomeruli may be related to selective dilatation of glomerular capillary in acute phase of type 1 diabetes.

RAMPs and CRLR expressions in osteoblastic cells after dexamethasone treatment

Adrenomedullin (ADM) is a potent stimulator of osteoblastic activity and promotes bone growth in vivo. ADM receptors are formed by heterodimerization of the CRLR and a RAMP2 or RAMP3 molecule. Since glucocorticoid responsive elements were recently identified in the human CRLR promoter and that glucocorticoids exert a major action in bones, we investigated the acute effect of dexamethasone (Dex) treatment on ADM receptor components in osteoblastic cell types: the MC3T3-E1 cells and calvaria-derived osteoblastic cells. Changes in expression of CRLR and RAMPs molecules were evaluated at mRNA levels using RT-PCR and at protein levels by Western blot analysis. We found that Dex increased expression of RAMP1 and RAMP2 mRNA in a time-dependent but dose-independent manner, while RAMP3 was unchanged. In contrast, Dex decreased the CRLR mRNA expression and these changes were reflected at protein levels. We suggest that Dex, in osteoblastic cells, altered ADM receptor by inhibition of CRLR expression and consequently could impair the ADM anabolic effect on bone. Our findings could explain in part, the detrimental side effects observed at bone level during glucocorticoid therapy.

Estrogen and phytoestrogen regulate the mRNA expression of adrenomedullin and adrenomedullin receptor components in the rat uterus

The serum estrogen surge in the uterus triggers precisely-timed physiological and biochemical responses required establishing and maintaining pregnancy. Previous reports have shown that consumption of phytoestrogen-containing plants may disrupt the precise control of pregnancy. To evaluate the effects of phytoestrogens in the uterus, we screened for estradiol (E2)-inducible genes in immature rat uteri. We identified the gene for receptor-activity-modifying protein 2 (Ramp2), known to be a component of the adrenomedullin (ADM) receptor, as responsive to both E2 and the phytoestrogen coumestrol (Cou). We further examined the expression of ADM and ADM signaling components Ramp2, Ramp3, and CRLR in the immature rat uterus and found that both E2 and Cou regulated these genes expression. In addition, treatment with ADM increased uterine weight and edema similar to that observed after Cou treatment. Our findings indicated that the phytoestrogen caused the abnormal induction of vasoactive factors in the uterus.

Regulation of adrenomedullin signaling in kidney interstitial fibroblasts

Adrenomedullin (AM), a potent vasodilatory peptide has beneficial effects in the kidney IN VIVO. The major aim of the present study was to determine the presence of AM receptor and the biological outcomes of AM on kidney interstitial fibroblasts in culture. Utilizing RT-PCR we found that NRK-49F cells express calcitonin receptor like receptor (CRLR) and receptor activity modifying protein 2 (RAMP2) but not RAMP3. Treatment of these cells with AM resulted in a concentration-dependent increase in cAMP activation. The activation of adenylate cyclase system was enhanced by over-expression of CRLR, RAMP2 and RAMP3. Furthermore, AM-stimulated adenylate cyclase activity was inhibited by AM-[22-52] the AM receptor antagonist. AM also caused a PKA-dependent increase in CRE-luciferase activity. To test the biological consequences of AM treatment and the signaling pathways mediating them, we examined the effect of AM on proliferation of NRK-49F cells and the desensitization of AM receptor. AM caused a significant decrease in proliferation that was AM-receptor mediated but was PKA independent. In addition, AM also caused desensitization of cAMP response within a few minutes of treatment. This effect of AM was also not mediated via cAMP pathway as forskolin failed to desensitize AM receptor, and a PKA-inhibitor did not inhibit the desensitization. Taken together these results demonstrate that NRK-49F cells express functional AM receptor that when activated by AM results in a significant reduction of cell proliferation. Although cAMP activation by AM, as in other systems, is also observed in NRK-49F cells, PKA-independent pathways lead to some of the biological responses observed in these cells.

[Increased atria expression of receptor activity-modifying proteins in heart failure patients]

OBJECTIVE

Receptor activity-modifying proteins (RAMPs) determine the ligand specificity of the calcitonin receptor-like receptor (CRLR); co-expression of RAMP1 and CRLR results in a calcitonin gene related peptide (CGRP) receptor, whereas the association of RAMP2 or RAMP3 with CRLR gives an adrenomedullin(ADM) receptor. As CGRP and ADM may play a beneficial role in heart failure, this study aimed at the question whether RAMPs mRNAs are changed in heart failure.

METHODS

Semi-quantitative reverse transcription-PCR (RT-PCR) was used to detect and quantify the mRNAs of RAMP1 and RAMP3 in the atria of heart failing patients.

RESULTS

It was found that the expressions of RAMP1, RAMP2 and RAMP3 mRNAs increased with the worsening of heart function, but the expressions of RAMP1 and RAMP2 mRNA decreased at level IV of heart failure.

CONCLUSION

The above results demonstrated in the atria of heart failure patients an up-regulation of CGRP receptor by an increase of RAMP1 in association with CRLR and an up-regulation of ADM receptor by an increase of RAMP2 expression in association with CRLR, thus suggesting that CGRP and ADM receptors be playing a functional role in compensating the chronic heart failure in human.

Expression of calcitonin gene-related peptide type 1 receptor mRNA and their activity-modifying proteins in the rat nucleus accumbens

The calcitonin receptor-like receptor (CRLR) and the orphan receptor RDC-1 have been proposed to be calcitonin gene-related peptide type 1 (CGRP1) receptors, and receptor activity-modifying proteins (RAMPs) determine the ligand specificity of CRLR. Coexpression of RAMP1 and CRLR resulted in functional CGRP1 receptors; the complex of RAMP2 or RAMP3 and CRLR created functional adrenomedullin receptor. Although high levels of CGRP binding sites in the nucleus accumbens have been reported, little is known about the expression of these novel CGRP receptors. In the present study, we used real-time quantitative RT-PCR to detect and quantitate the relative expression of CGRP, CRLR, RAMP1-3 and RDC-1 in the nucleus accumbens of intact rats and rats with inflammation. Our results demonstrate that CGRP, CRLR, RAMP1 and RAMP2 exist in the nucleus accumbens of intact rats, and that they were significantly upregulated in rats with inflammation. In contrast, no expression was detected for RDC-1 and RAMP3. These findings indicated a functional role for CGRP and its receptors in inflammation and pain modulation.

Differential gene expression of adrenomedullin receptors in pressure- and volume-overloaded heart--role of angiotensin II

Left ventricular (LV) adrenomedullin (AM) gene expression differs between pressure overload (POL) and volume overload (VOL) and angiotensin II could be a critical stimulator of AM gene expression in POL and VOL models. Calcitonin receptor-like receptor (CRLR) co-expressed with receptor activity modifying protein 2 (RAMP2) or RAMP3 functions as an AM receptor. Levels of CRLR, RAMP2 and RAMP3 mRNA that were significantly increased within 24 h returned to the basal level at 5 days after the imposition of POL in the present study. In contrast, mRNA levels of CRLR and RAMP2 gradually increased over 6 weeks after the imposition of VOL. Continuous infusion of angiotensin II stimulated LV AM gene and AM receptor gene expression independently of LV peak-systolic and LV end-diastolic pressure. The gene expression of LV AM receptors increased in different types of cardiac overload. The present study revealed an intimate association between the AM signaling system and angiotensin II.

The role of adrenomedullin and its receptor system in cardiovascular calcification of rat induced by Vitamin D(3) plus nicotine

Adrenomedullin (ADM) is a potent vasodilatory peptide which regulates blood pressure, cell growth and bone formation. Our work was aimed to explore the production of ADM, changes and pathophysiological significance of ADM mRNA and ADM receptor components--calcitonin receptor like receptor (CRLR) and receptor activity modifying proteins (RAMPs) mRNA in calcified myocardium and aorta of rats induced by Vitamin D3 plus nicotine. Contents of ADM in plasma, myocardium and aorta were measured by radioimmunoassay (RIA). The amount of ADM, CRLR and RAMPs mRNA was determined by semi-quantitative RT-PCR. The calcium content and alkaline phosphatase activity in myocardium and aorta of rats were measured. The results showed that the contents of calcium in calcified myocardium and aorta were increased by 3.5- and 6-fold (all P < 0.01), respectively, and alkaline phosphatases activity in calcified myocardium and aorta were increased by 66.5 and 82.7% (all P < 0.01 ), respectively, compared with control. Contents of ADM in plasma, myocardium and aorta were increased by 58% (P < 0.01), 14.3% (P < 0.01) and 27.8% P < 0.05). Furthermore, it was found that the amount of ADM, CRLR and RAMP2 mRNA in calcified myocardium was elevated by 90.6, 157.5 and 119.6% (all P < 0.01), RAMP3 mRNA was decreased by 14.1% (P < 0.01), respectively, compared with control. The amount of ADM, CRLR, RAMP2 and RAMP3 mRNA in calcified aorta was elevated by 37.7% (P < 0.01), 41.4% (P < 0.01), 60.1% (P < 0.05) and 13% P < 0.01), respectively, compared with control. The elevated level of CRLR and RAMP2 mRNA were in positive correlation with that of ADM mRNA (r = 0.992 and 0.882, respectively, P < 0.01) in calcified myocardium. The elevated level of CRLR and RAMP3 mRNA were also in positive correlation with that of ADM mRNA (r = 0.727, P < 0.05 and 0.816, P < 0.01, respectively) in calcified aorta. These results demonstrated that calcified myocardium and aorta generated an increased amount of ADM, up-regulated gene expressions of ADM, CRLR and RAMP2 mRNA. While the alteration of RAMP3 mRNA in calcified myocardium and aorta was different. These suggested that ADM and its receptor system might involve in the regulation of calcification in heart and aorta.

Characterization of the Human Calcitonin Gene-Related Peptide Receptor Subtypes Associated with Receptor Activity-Modifying Proteins

Coexpression of receptor activity-modifying proteins (RAMPs) with calcitonin receptor 2 (CTR2) or calcitonin receptor-like receptor (CRLR) leads to the formation of four functional heterodimeric receptors for human calcitonin gene-related peptide (hCGRP). In this study, we transfected hCGRP receptors into human embryonic kidney 293 cells and examined their pharmacological profiles using three dominant-negative (DN) RAMP mutants and various hCGRPalpha analogs. Fluorescence-activated cell-sorting analysis revealed that their cotransfection with CTR2 induced cell surface expression of all three RAMPs, and the three CTR2/RAMP heterodimers mediated equivalent levels of cAMP production in response to hCGRPalpha that were approximately 50-fold greater than were seen with CTR2 alone. By contrast, [Tyr0]hCGRPalpha binding and signaling were markedly weaker with CTR2/RAMP2 or -3 than with CTR2/RAMP1 or CRLR/RAMP1; likewise, 125I-[His10]hCGRPalpha bound most potently to CTR2/RAMP1. When CTR2 was coexpressed with DN RAMP1 or -2, hCGRPalpha-evoked responses were similar to those seen with CTR2 alone, despite the expression of both CTR2 and DN RAMP at the cell surface. But coexpression of DN RAMP3 with CTR2 significantly diminished hCGRPalpha signaling compared with that seen with CTR2 alone, indicating that DN RAMP3 is able to function as a negative regulator of CTR2 function. Competition experiments showed the relative agonist sensitivity of the four receptors to be hCGRPalpha > [Tyr0]hCGRPalpha > [Cys(Et)2,7]hCGRPalpha > [Cys(ACM)2,7]hCGRPalpha. Of the linear analogs, [Cys(ACM)2,7]hCGRPalpha (ACM, acetylmethoxy) enhanced cAMP formation only via CTR2/RAMP1, whereas [Cys(Et2,7)]hCGRPalpha acted via CRLR/RAMP1 and somewhat less potently via CTR2/RAMP1. Thus, among the three CGRP8-37-insensitive receptors, CTR2/RAMP1 is most sensitive to the two linear analogs, suggesting that it could be classified as a CGRP2 receptor. Moreover, the combined use of iodinated CGRPalpha analogs may be useful for defining the CGRP1 receptor.

The Pharmacology of Adrenomedullin Receptors and Their Relationship to CGRP Receptors

Adrenomedullin (AM) has two specific receptors formed by the calcitonin-receptor-like receptor (CL) and receptor activity-modifying protein (RAMP) 2 or 3. These are known as AM1 and AM2 receptors, respectively. In addition, AM has appreciable affinity for the CGRP1 receptor, composed of CL and RAMP1. The AM1 receptor has a high degree of selectivity for AM over CGRP and other peptides, and AM22-52 is an effective antagonist at this receptor. By contrast, the AM2 receptor shows less specificity for AM, having appreciable affinity for betaCGRP. Here, CGRP8-37 is either equipotent or more effective as an antagonist than AM22-52, depending on the species from which the receptor components are derived. Thus, under the appropriate circumstances it seems that betaCGRP might be able to activate both CGRP1 and AM2 receptors and AM could activate both AM1 and AM2 receptors as well as CGRP1 receptors. Current peptide antagonists are not sufficiently selective to discriminate between these three receptors. The CGRP-selectivity of RAMP1 and RAMP3 may be conferred by a putative disulfide bond from the N-terminus to the middle of the extracellular domain of these molecules. This is not present in RAMP2.

Effects of adrenomedullin on endothelial cells in the multistep process of angiogenesis: Involvement of CRLR/RAMP2 and CRLR/RAMP3 receptors

Recently, we demonstrated that U87 glioblastoma xenograft tumors treated with anti-adrenomedullin (AM) antibody were less vascularized than control tumors, suggesting that AM might be involved in neovascularization and/or vessel stabilization. Angiogenesis, the sprouting of new capillaries from preexisting blood vessels, is a multistep process that involves migration and proliferation of endothelial cells, remodeling of the extracellular matrix and functional maturation of the newly assembled vessels. In our study, we analyzed the role of AM on human umbilical vein endothelial cell (HUVEC) phenotype related to different stages of angiogenesis. Here we report evidence that AM promoted HUVEC migration and invasion in a dose-dependent manner. The action of AM is specific and is mediated by the calcitonin receptor-like receptor/receptor activity-modifying protein-2 and -3 (CRLR/RAMP2; CRLR/RAMP3) receptors. Furthermore, AM was able to induce HUVEC differentiation into cord-like structures on Matrigel. Suboptimal concentrations of vascular endothelial growth factor (VEGF) and AM acted synergistically to induce angiogenic-related effects on endothelial cells in vitro. Blocking antibodies to VEGF did not significantly inhibit AM-induced capillary tube formation by human endothelial cells, indicating that AM does not function indirectly through upregulation of VEGF. These findings suggest that the proangiogenic action of AM on cultured endothelial cells via CRLR/RAMP2 and CRLR/RAMP3 receptors may translate in vivo into enhanced neovascularization and therefore identify AM and its receptors acting as potential new targets for antiangiogenic therapies.

CL/RAMP2 and CL/RAMP3 produce pharmacologically distinct adrenomedullin receptors: a comparison of effects of adrenomedullin22-52, CGRP8-37 and BIBN4096BS

Adrenomedullin (AM) has two known receptors formed by the calcitonin receptor-like receptor (CL) and receptor activity-modifying protein (RAMP) 2 or 3: we report the effects of the antagonist fragments of human AM and CGRP (AM22-52 and CGRP8-37) in inhibiting AM at human (h), rat (r) and mixed species CL/RAMP2 and CL/RAMP3 receptors transiently expressed in Cos 7 cells or endogenously expressed as rCL/rRAMP2 complexes by Rat 2 and L6 cells. AM22-52 (10 microM) antagonised AM at all CL/RAMP2 complexes (apparent pA2 values: 7.34+/-0.14 (hCL/hRAMP2), 7.28+/-0.06 (Rat 2), 7.00+/-0.05 (L6), 6.25+/-0.17 (rCL/hRAMP2)). CGRP8-37 (10 microM) resembled AM22-52 except on the rCL/hRAMP2 complex, where it did not antagonise AM (apparent pA2 values: 7.04+/-0.13 (hCL/hRAMP2), 6.72+/-0.06 (Rat2), 7.03+/-0.12 (L6)). On CL/RAMP3 receptors, 10 microM CGRP8-37 was an effective antagonist at all combinations (apparent pA2 values: 6.96+/-0.08 (hCL/hRAMP3), 6.18+/-0.18 (rCL/rRAMP3), 6.48+/-0.20 (rCL/hRAMP3)). However, 10 microM AM22-52 only antagonised AM at the hCL/hRAMP3 receptor (apparent pA2 6.73+/-0.14). BIBN4096BS (10 microM) did not antagonise AM at any of the receptors. Where investigated (all-rat and rat/human combinations), the agonist potency order on the CL/RAMP3 receptor was AM approximately betaCGRP>alphaCGRP. rRAMP3 showed three apparent polymorphisms, none of which altered its coding sequence. This study shows that on CL/RAMP complexes, AM22-52 has significant selectivity for the CL/RAMP2 combination over the CL/RAMP3 combination. On the mixed species receptor, CGRP8-37 showed the opposite selectivity. Thus, depending on the species, it is possible to discriminate pharmacologically between CL/RAMP2 and CL/RAMP3 AM receptors.

Changes in the expression of calcitonin receptor-like receptor, receptor activity-modifying protein (RAMP) 1, RAMP2, and RAMP3 in rat uterus during pregnancy, labor, and by steroid hormone treatments

Calcitonin gene-related peptide (CGRP) and its related peptide, adrenomedullin (AM), are potent smooth muscle relaxants in a variety of tissues. The CGRP has been reported to play an important role in maintaining uterine relaxation during pregnancy. We have previously reported that CGRP-induced uterine relaxation was gestationally regulated. Calcitonin receptor-like receptor (CRLR), a seven-domain transmembrane protein functions as CGRP-A receptor, in association with receptor activity-modifying protein (RAMP) 1, a single-domain transmembrane protein, whereas CRLR and RAMP2 or RAMP3 constitute a receptor for AM. In the present investigation, we examined the mRNA expression of CRLR, RAMP1, RAMP2, and RAMP3 in rat uterus (n = 8) by reverse transcriptional analysis and polymerase chain reaction to assess the changes in the expression of CGRP-A- and AM-receptor components during pregnancy and labor and by steroid hormone treatments in adult ovariectomized rats. The changes in mRNA are expressed relative to the 18S mRNA in the uterus of rats at various stages: nonpregnant, pregnant on Day 18, spontaneous labor at term, Day 2 postpartum, and in pregnant rats on treatment with RU486. Ovariectomized rats treated for 3 days twice daily s.c. with estradiol-17beta (2.5 microg/injection), progesterone (2 mg/injection), and the combination of estradiol-17beta and progesterone (same doses as above) were also examined for the expression of various receptor components. Results showed that mRNA expression of the receptor components was significantly higher (P < 0.001 for CRLR, P < 0.01 for RAMP1, P < 0.05 for RAMP2, and P < 0.01 for RAMP3) in pregnant compared to nonpregnant rats. Except for RAMP3, expression of all the other three genes decreased significantly (P < 0.05) during labor. A progesterone antagonist, RU486 significantly decreased (P < 0.01 for CRLR, P < 0.05 for RAMP1, RAMP2, and RAMP3) all the receptor components during pregnancy. In adult ovariectomized rats, progesterone caused significant increases in CRLR (P < 0.001), RAMP1 (P < 0.05), and RAMP2 (P < 0.01). Levels of RAMP3 were unaffected by the progesterone treatment. Estradiol-17beta treatment decreased all of the four receptor components significantly (P < 0.01 for CRLR, P < 0.05 for RAMP1, RAMP2, and RAMP3). Our results demonstrate that both CGRP and AM may play a role in uterine quiescence during pregnancy and that their receptor components are regulated by the steroid hormones.

Chronic salt loading upregulates expression of adrenomedullin and its receptors in adrenal glands and kidneys of the rat

The vasodilator peptide adrenomedullin (AM) elicits diuresis and natriuresis and inhibits aldosterone secretion. The aim of this study was to better understand the role of AM in maintaining water and electrolyte balance during chronic salt loading. Male Wistar rats were divided into a high salt (HS) group that received a diet containing 8% sodium chloride (NaCl) and a normal salt group that received a diet containing 0.4% NaCl. Plasma AM concentrations as well as expression of AM mRNA in the adrenal gland and kidney were then measured after 3, 7, 14, and 28 days. After 28 days, sodium and water excretion were significantly higher in HS rats than in control, although blood pressure and fluid volume were not significantly affected. Moreover, although plasma AM remained unchanged for up to 14 days, it was increased 2.5-fold in HS rats after 28 days on a high salt diet, and there were corresponding 3-fold and 1.5-fold increases in the levels of AM mRNA in the adrenal gland and kidney, respectively. At the same time, expression of calcitonin receptor-like receptor mRNA was significantly upregulated in both kidney and adrenal gland, as was expression of receptor activity-modify protein 1 (RAMP1) and RAMP2 mRNA in the adrenals and expression of RAMP3 in kidneys. Taken together, these results suggest that AM plays a role in the regulation of water and electrolyte balance in animals chronically ingesting high levels of salt.

Angiotensin II modulates gene expression of adrenomedullin receptor components in rat cardiomyocytes

Both adrenomedullin (AM) and angiotensin II (Ang II) are locally-acting hormones in the cardiac ventricles. Previously we reported that AM inhibits Ang II-induced hypertrophy of cultured rat neonatal cardiomyocytes. In this study, we examined whether Ang II affects the gene expression of the AM receptor components of calcitonin-receptor-like receptor (CRLR) and receptor-activity-modifying protein (RAMP) in rat cardiomyocytes. The mRNA levels of RAMP1 and RAMP3 were significantly elevated following 24-h treatment with Ang II without a change of those of RAMP2 and CRLR. AM increased the intracellular cAMP level and the cAMP accumulation by AM was significantly amplified by the 24-h preincubation with Ang II. The effects of Ang II on RAMP1 and RAMP3 expression were abolished by an Ang II type 1 (AT1) receptor antagonist, but not by an AT2 receptor antagonist. Thus, Ang II modulates gene expression of the AM receptor components via AT1 receptor, suggesting alteration of AM actions by Ang II in cultured rat cardiomyocytes.