Interaction of adrenomedullin and platelet-derived growth factor on rat mesangial cell production of endothelin

Adrenomedullin in rat follicles and corpora lutea: expression, functions and interaction with endothelin-1

BACKGROUND

Adrenomedullin (ADM), a novel vasorelaxant peptide, was found in human/rat ovaries. The present study investigated the interaction of ADM and endothelin-1 (ET-1) in follicles and newly formed corpora lutea (CL) and the actions of ADM on progesterone production in CL during pregnancy.

METHODS

The peptide and gene expression level of adrenomedullin in small antral follicles, large antral follicles and CL was studied by real-time RT-PCR and EIA. The effect of ADM treatment on oestradiol production in 5-day follicular culture and on progesterone production from CL of different pregnant stages was measured by EIA. The interaction of ADM and ET-1 in follicles and CL at their gene expression level was studied by real-time RT-PCR.

RESULTS

In the rat ovary, the gene expression of Adm increased during development from small antral follicles to large antral follicles and CL. In vitro treatment of preantral follicular culture for 5 days with ADM increased oestradiol production but did not affect follicular growth or ovulation rate. The regulation of progesterone production by ADM in CL in culture was pregnancy-stage dependent, inhibitory at early and late pregnancy but stimulatory at mid-pregnancy, which might contribute to the high progesterone production rate of the CL at mid-pregnancy. Moreover, the interaction between ADM and ET-1 at both the production and functional levels indicates that these two vasoactive peptides may form an important local, fine-tuning regulatory system together with LH and prolactin for progesterone production in rat CL.

CONCLUSIONS

As the CL is the major source of progesterone production even after the formation of placenta in rats, ADM may be an important regulator in progesterone production to meet the requirement of pregnancy.

Kidney is in trouble with mediators

The kidneys receive 20-25 % of cardiac output and play a main role in the control of cardiovascular homeostasis. It is an endocrine organ that regulates and produces many substances, scavenger particles and immune complexes. Cytokines, growth factors, reactive oxygen metabolites, bioactive lipids, proteases, vasoactive substances such as nitric oxide (NO), adrenomedullin (AM), urotensin-II (U-II), have been released in several diseases, and kidney is one of mostly affected organs in body. Some of these mediators act in a paracrine fashion while some act in autocrine. They play important roles in modulating the cardiovascular responses, renal hemodynamics, and probably in mediating the clinical and laboratory manifestations of several renal diseases. These mediators are like "a double edged sword". While small amounts of them mediate many physiological events, little excess may cause the damage to the healthy cells. Many investigators have searched the role(s) of mediators in several diseases. However, the findings are mostly like the model of "chicken and egg", and indistinguishable as to whether they are the causes of, or results of the diseases. We will discuss mainly the possible roles of NO, AM and U-II in children with several renal diseases and summarize what is known, and what must be known about these mediators.

Adrenomedullin gene expression and levels in the cardiovascular system after treatment with lipopolysaccharide

To study the effect of septicaemia, the temporal changes in tissue adrenomedullin (AM) and preproAM mRNA levels were studied in the heart and blood vessels after lipopolysaccharide (LPS) injection. Radioimmunoassay and solution hybridization-RNase protection assays were used to follow the changes in AM and its mRNA levels respectively after intraperitoneal injection of 10 mg/kg LPS in rats. The preproAM mRNA levels increased at 1 h in the right atrium after LPS injection, while the AM contents decreased at 1 h in the left atrium. The preproAM mRNA levels increased at 3 and 6 h in the left ventricle, whereas it increased at 6 h in the right ventricles after LPS injection. There was an increase in preproAM mRNA levels at 1 and 3 h in the mesenteric artery, while AM levels were increased at 1, 3 and 6 h. However, there were no such changes in the thoracic aorta. There were also increases in tumour necrosis factor-alpha (TNF-alpha), interleukin (IL)-1beta and IL-6 in the heart, and in the mesenteric artery (TNF-alpha and IL-1beta) and in thoracic aorta (IL-1beta and IL-6). The present results suggest that the biosynthesis and secretion of AM may be increased in cardiovascular tissues of rats injected with LPS, and that AM may play multiple roles in inflammation.

The clinical relevance of adrenomedullin: a promising profile?

Adrenomedullin (AM) is a peptide that possesses potentially beneficial properties. Since the initial discovery of the peptide by Kitamura et al. in 1993, the literature has been awash with reports describing its novel mechanisms of action and huge potential as a therapeutic target. Strong evidence now exists that AM is able to act as an autocrine, paracrine, or endocrine mediator in a number of biologically significant functions, including the endothelial regulation of blood pressure, protection against organ damage in sepsis or hypoxia, and the control of blood volume through the regulation of thirst. Its early promise as a potential mediator/modulator of disease was not, however, entirely as a result of the discovery of physiological functions but due more to the observation of increasing levels measured in plasma in direct correlation with disease progression. In health, AM circulates at low picomolar concentrations in plasma in 2 forms, a mature 52-amino acid peptide and an immature 53-amino acid peptide. Plasma levels of AM have now been shown to be increased in a number of pathological states, including congestive heart failure, sepsis, essential hypertension, acute myocardial infarction, and renal impairment. These earliest associations have been further supplemented with evidence of a role for AM in other pathologies including, most intriguingly, cancer. In this review, we offer a timely review of our current knowledge on AM and give a detailed account of the putative role of AM in those clinical areas in which the best therapeutic opportunities might exist.

Congenital unilateral ureteropelvic junction obstruction of the rat: a useful animal model for human ureteropelvic junction obstruction?

OBJECTIVES

To investigate the expression of endothelin-1 (ET-1) and adrenomedullin (ADM) in the renal pelvis, stenotic ureteropelvic junction, and ureter of 20 male Wistar rats with congenital unilateral ureteropelvic junction obstruction; the normal contralateral kidneys served as controls. The molecular pathophysiology of congenital ureteropelvic junction obstruction is still unclear. The implication of altered peptidergic innervation is under discussion. Our study group has recently been able to demonstrate a significant increase in ET-1 and a significant decrease in ADM in prestenotic and stenotic tissue, but not in the remainder of the ureter, compared with controls.

METHODS

Twenty animals were killed, and samples of the renal pelvis, ureteropelvic junction, upper ureter, middle part of the ureter, and lower ureter were immediately snap-frozen and stored in liquid nitrogen. Total RNA was extracted, and subsequently 1 microg of RNA was reversely transcribed. mRNA expression of ET-1 and ADM was determined semiquantitatively using on-line polymerase chain reaction. The expression of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was determined to relate the specific mRNA expression to the expression of a housekeeping gene.

RESULTS

We found a significant increase in the expression of ET-1 in the obstructed junctions related to GAPDH (P <0.001). The expression of ADM, however, revealed no statistically significant differences. No differences at all could be detected in the tissue samples from the rest of the ureter.

CONCLUSIONS

Alterations in the local production of peptidergic neurotransmitters, especially ET-1, may contribute to the molecular pathogenesis of ureteropelvic junction obstruction. Results previously obtained in the stenotic tissue from children were confirmed in the stenotic tissue from the rat model. We hypothesize that the alterations are disease-, but not age-specific.

Adrenomedullin and total nitrite levels in children with Henoch-Schönlein purpura

Nitric oxide (NO) is synthesized from endothelium and has an important role in the control of vascular tonus. Adrenomedullin (AM) is a potent vasodilator, and cytoprotective peptide is produced not only in adrenal medulla, but also in the vascular smooth muscle and endothelial cells. To investigate the endothelial synthesis of AM and NO, and endothelial injury in Henoch-Schönlein purpura (HSP), we measured their levels in 16 children with HSP, who were evaluated during the acute and remission phases, and compared with 12 healthy controls. Plasma AM levels (pmol/ml) were significantly higher in acute phase children (46.87+/-11.49) than in those in remission (35.59+/-12.39, p<0.01) and controls (30.70+/-9.12, p<0.001). Similarly, plasma total nitrite levels (mumol/l) were higher in acute phase patients (47.50+/-12.30) than in those in remission (35.94+/-10.08, p<0.005) and controls (34.56+/-11.51, p<0.05). Urinary excretion of AM (pmol/mg creatinine) was higher in acute phase patients (53.85+/-23.22) than in remission patients (29.97+/-9.33, p<0.01) and controls (37.43+/-15.78, p<0.05). Patients had increased urinary nitrite excretion (mumol/mg creatinine) in acute phase (2.39+/-1.18) compared to those in remission (1.53+/-0.90, p<0.05) and controls (1.05+/-0.61, p<0.005). There was no significant difference between remission phase and controls in AM and nitrite levels ( p>0.05). This study concluded that AM and NO may have a role in the immunoinflammatory process of HSP, especially in the active stage, although whether this perpetuates, or protects against, further vascular injury is not clear. Further studies are needed to clearly establish the roles of AM and NO in the pathogenesis of HSP.

Adrenomedullin and heart failure

Evidence suggests that adrenomedullin (AM) plays a role in the pathophysiology of heart failure. Circulating concentrations of AM are elevated in cardiovascular disease in proportion to the severity of cardiac and hemodynamic impairment. Raised plasma AM levels following acute cardiac injury and in heart failure provide prognostic information on adverse outcomes. In heart failure, elevated circulating AM also identifies patients likely to receive long-term benefit from inclusion of additional anti-failure therapy (carvedilol). Administration of AM in experimental and human heart failure induces reductions in arterial pressure and cardiac filling pressures, and improves cardiac output, in association with inhibition of plasma aldosterone (despite increased renin release) and sustained (or augmented) renal glomerular filtration and sodium excretion. Furthermore, AM in combination with other therapies (angiotensin-converting enzyme inhibition and augmentation of the natriuretic peptides) results in hemodynamic and renal benefits greater than those achieved by the agents separately. Manipulation of the AM system holds promise as a therapeutic strategy in cardiac disease.

Increased endothelin-1 and decreased adrenomedullin gene expression in the stenotic tissue of congenital pelvi-ureteric junction obstruction in children

OBJECTIVES

To test the hypothesis that the gene expression of endothelin-1 and adrenomedullin may be altered in stenotic tissues of patients with congenital hydronephrosis caused by pelvi-ureteric junction (PUJ) obstruction.

MATERIALS AND METHODS

Using real-time reverse transcription-polymerase chain reaction, mRNA of smooth muscle-constricting endothelin-1 and of smooth muscle-relaxing adrenomedullin was quantified in tissue specimens of 20 patients with PUJ obstruction (mean age 5.1 years, SD 7.0) and of 21 controls with normal PUJs (mean age 23.5 years, SD 24.2).

RESULTS

The amount of endothelin-1 mRNA in stenotic specimens was higher than in the controls, indicated by significantly lower threshold cycles (Ct values) in real-time PCR for the target gene in the obstructive tissue, with mean (SD) values of 24.9 (1.6) and 26.0 (2.1) (P < 0.05), respectively. The endothelin-1/CD31 ratio was significantly higher in the patients (P < 0.05) than in controls. In addition, adrenomedullin gene expression in the obstructed junctions was significantly lower than in normal junctions, with higher Ct values for the patient group of 26.7 (1.6) vs 25.2 (1.8) (P < 0.05) and lower adrenomedullin mRNA when standardized to glyceraldehyde-3-phosphate dehydrogenase (GAPDH) (P < 0.05), CD31 (P < 0.01) and smooth muscle alpha-actin mRNA (P < 0.01). The two groups showed no significant differences for GAPDH and CD31 mRNA content, whereas there was about twice as much alpha-actin mRNA in stenotic tissues than in unaffected PUJs, shown by the lower Ct values for the patient group of 16.9 (2.0) vs 17.9 (2.6) (P < 0.05). Furthermore, endothelin-1, adrenomedullin and alpha-actin mRNA amounts were independent of age.

CONCLUSION

Taken together these results provide evidence that the production of autocrine/paracrine acting endothelin-1 and adrenomedullin is altered in tissues of patients with genuine PUJ obstruction, and may be involved in the pathogenesis of congenital hydronephrosis.

Comparative actions of adrenomedullin and nitroprusside: interactions with ANG II and norepinephrine

The role of adrenomedullin (ADM) in volume and pressure homeostasis remains undefined. Accordingly, we compared the biological responses to infusions of ADM and nitroprusside (NP; matched for reduction of arterial pressure) and assessed their effects on responses to ANG II and norepinephrine in eight conscious sheep. During matched falls in arterial pressure (8-10 mmHg, both P < 0.001) ADM and NP induced similar increases in heart rate. ADM increased cardiac output (P < 0.001), and the fall in calculated peripheral resistance was greater with ADM than NP (P = 0.013). ADM infusions raised plasma ADM levels (P < 0.001), plasma renin activity (P = 0.001), and ANG II (P < 0.001) but tended to blunt any concurrent rise in aldosterone compared with NP (P = 0.056). ADM maintained both urine flow (P < 0.001) and sodium excretion (P = 0.01) compared with falls observed with NP. ADM attenuated the vasopressor actions of exogenous ANG II (P = 0.006) but not norepinephrine. In addition, ADM antagonized the ANG II-induced rise in plasma aldosterone (P < 0.001). In conclusion, ADM induces a different spectrum of hemodynamic, renal, and endocrine actions to NP. These results clarify mechanisms by which ADM might contribute to volume and pressure homeostasis.

Adrenomedullin and its receptor complexes in remnant kidneys of rats with renal mass ablation: decreased expression of calcitonin receptor-like receptor and receptor-activity modifying protein-3

Adrenomedullin (AM) has vasodilator and diuretic actions, similarly to natriuretic peptides. AM receptor complexes are composed of calcitonin receptor-like receptor (CRLR) and receptor-activity modifying protein-2 (RAMP2), or CRLR and RAMP3. We aimed to know whether gene expression of AM and AM receptor complexes are regulated in kidneys under pathophysiological conditions. Expression of AM, RAMP2, RAMP3 and CRLR mRNA was studied in the remnant kidney of rats with renal mass ablation using competitive quantitative RT-PCR techniques. Partial cloning was performed to determine the rat RAMP3 nucleotide sequence. In normal rat kidneys, expression levels of RAMP2, RAMP3, CRLR and AM mRNAs were 26.5 +/- 1.9 mmol/mole of GAPDH, 7.7 +/- 0.9 mmol/mole of GAPDH, 3.6 +/- 0.2 mmol/mole of GAPDH and 0.57 +/- 0.03 mmol/mole of GAPDH (mean +/- SE, n = 6), respectively. RAMP3 mRNA levels decreased significantly to about 50% and about 70% of control (sham-operated rats) 4 days and 14 days after 5/6 nephrectomy, respectively. CRLR mRNA levels also decreased significantly to about 30% and about 43% of control. Sodium intake restriction had no significant effects on the RAMP3 and CRLR gene expression. On the other hand, RAMP2 mRNA expression in the kidney was suppressed by sodium intake restriction regardless of nephrectomy, while RAMP2 levels in the remnant kidney were not significantly changed by 5/6 nephrectomy. Neither 5/6 nephrectomy or sodium intake restriction had any significant effects on the AM gene expression in the kidney. The present study showed that expression of mRNAs encoding AM, RAMP2, RAMP3 and CRLR were differentially regulated in remnant kidneys of rats with renal mass ablation.

Cellular and molecular actions of adrenomedullin in glomerular mesangial cells

Adrenomedullin (AM), a potent vasodilatory and hypotensive peptide produces several biological outcomes in glomerular mesangial cells. Mesangial cells are important in the pathogenesis of glomerulonephritis, and therefore the actions of AM on mesangial cells have important clinical and therapeutic implications. This minireview describes the various actions of AM on mesangial cell function and the signal transduction mechanisms involved. As in other systems, most actions of AM can be explained by increase in cAMP levels in the cell, although a few exceptions remain. The fact that most data obtained to date has been in culture, the physiological significance of the actions of AM in mesangial cells is discussed.

Role of adrenomedullin and its receptor system in renal pathophysiology

Adrenomedullin (AM), a potent vasorelaxing, natriuretic and cell growth-modulating peptide, is thought to act as an autocrine/paracrine regulator in renal glomeruli and tubules. AM receptors comprise the calcitonin receptor-like receptor (CRLR) and a family of receptor-activity-modifying proteins (RAMPs 1-3); however, the pathophysiological role of AM and its receptor system in the kidney remains to be clarified. We examined the regulation of their expression in a rat model of renal injury and found that RAMP1, RAMP2 and CRLR expressions were markedly upregulated upon induction of fibrosis during obstructive nephropathy. Since AM exerts potent antiproliferative effects in various cell types, upregulation of the AM receptor system may play important roles in modulating the progression of renal diseases.

Adrenomedullin and diabetes mellitus

Adrenomedullin (AM) is a novel 52 amino acid peptide hormone, originally isolated from human pheochromocytoma. AM acts as a local autocrine and/or paracrine vasoactive hormone and has vasodilator and blood pressure lowering properties. AM as a vasodilative molecule protects the vascular wall but its exact role is still uncertain. AM is considered to play an important endocrine role in various tissues in maintaining electrolyte and fluid homeostasis. Its plasma concentration in healthy conditions is low. In hypertension, chronic renal failure and congestive heart failure its plasma concentration increases in a parallel manner with the severity of the disease. It is assumed that this peptide plays an important role in physiological and pathological conditions compensating the effects of vasoconstrictive molecules. Investigations have proven that in diabetic angiopathies the levels and production of vasoconstrictive factors and AM are increased, while other relaxing substances such as nitric oxide (NO) are decreased. It is still uncertain whether the increased release of AM is a compensatory mechanism or a coincidental event. Although the precise role of AM in the pathogenesis of diabetic complications is still to be elucidated, the altered concentration of AM in diabetes could indicate a certain interaction between AM induction and vascular function. Hence, the induction of vascular AM can be a new target of therapeutic approach to diabetic complications.

Novel neurohumoral factors in congestive heart failure: adrenomedullin

Adrenomedullin is a 52-amino acid peptide that circulates in human plasma. The plasma concentrations of the peptide are increased in cardiovascular disease in proportion to the degree of hemodynamic impairment. Plasma adrenomedullin levels in heart failure, and in subjects with acute myocardial infarction, have been shown to convey independent prognostic information. Adrenomedullin has multiple biologic effects, but characteristically causes vasodilatation. The actions of adrenomedullin and the activation of this peptide in cardiovascular disease suggest it may have an important pathophysiologic role in heart failure. Manipulation of adrenomedullin or its receptor may have therapeutic potential.

Gene expression of neuronal nitric oxide synthase and adrenomedullin in human neuroblastoma using real-time PCR

The objective of our study was to assess the gene expression of the antiproliferative systems neuronal nitric oxide synthase (nNOS) and adrenomedullin (AM) in human neuroblastoma. A novel real-time PCR method was evaluated using neuropeptide Y (NPY) for validation. Glyceraldehyd-3-phospate dehydrogenase (GAPDH) and NPY gene expression in neuroblastomas of 50 patients were measured in parallel by competitive quantitative and TaqMan real-time RT-PCR. AM and nNOS mRNA were determined by real-time PCR. Our results showed a linear relationship between competitive quantitative and real-time RT-PCR measurements of NPY and GAPDH (r = 0.87 and r = 0.92, respectively). AM and nNOS mRNA was found in all tumor samples. AM/GAPDH mRNA increased with higher differentiation according to Shimada (p = 0.013). There was no relation between MYCN amplification nor with the tumor stage (p = 0.78 and p = 0.30, respectively). AM/GAPDH did not relate to recurrence or death in a 5-year follow-up period. Neuronal NOS/GAPDH expression did not relate to any biological or clinical parameter of prognosis or differentiation. Similar results were obtained when the neuronal marker protein gene product 9.5 (PGP9.5) was used to normalize mRNA concentration. In conclusion, TaqMan real-time PCR appears to be a reliable method to quantify gene expression in neuroblastomas. Adrenomedullin mRNA in neuroblastoma is linked to tumor differentiation but not to prognostic markers.

Adrenomedullin attenuates pressor response to angiotensin II in conscious sheep

Biologic actions attributed to adrenomedullin include reduction of arterial pressure and suppression of aldosterone secretion. To assess possible in vivo antiangiotensin II actions of adrenomedullin, we examined hemodynamic and adrenal responses to stepped angiotensin II infusions with or without co-infusions of adrenomedullin (33 ng/kg/min) in conscious sheep under controlled conditions of a low sodium intake. Plasma adrenomedullin levels rose during peptide infusions (p < 0.001) to plateau at approximately 15-18 pM. The dose-dependent pressor response (15-20 mm Hg) of angiotensin II was both delayed and markedly attenuated (p = 0.017) by adrenomedullin, which also stimulated heart rate (p < 0.001) and cardiac output (p < 0.001). Adrenomedullin prevented the angiotensin II-induced increase in peripheral resistance (p < 0.001). Plasma aldosterone responses to angiotensin II were variable and were not significantly altered by concomitant adrenomedullin infusion. In conclusion, low-dose infusion of adrenomedullin administered to conscious sheep on a low-salt diet clearly antagonized the vasopressor actions of administered angiotensin II while stimulating cardiac output and heart rate. The data suggest a possible role for adrenomedullin in cardiovascular homeostasis in part through antagonism of the vasopressor action of angiotensin II.

Adrenomedullin, a multifunctional regulatory peptide

Since the discovery of adrenomedullin in 1993 several hundred papers have been published regarding the regulation of its secretion and the multiplicity of its actions. It has been shown to be an almost ubiquitous peptide, with the number of tissues and cell types synthesizing adrenomedullin far exceeding those that do not. In Section II of this paper we give a comprehensive review both of tissues and cell lines secreting adrenomedullin and of the mechanisms regulating gene expression. The data on circulating adrenomedullin, obtained with the various assays available, are also reviewed, and the disease states in which plasma adrenomedullin is elevated are listed. In Section III the pharmacology and biochemistry of adrenomedullin binding sites, both specific sites and calcitonin gene-related peptide (CGRP) receptors, are discussed. In particular, the putative adrenomedullin receptor clones and signal transduction pathways are described. In Section IV the various actions of adrenomedullin are discussed: its actions on cellular growth, the cardiovascular system, the central nervous system, and the endocrine system are all considered. Finally, in Section V, we consider some unresolved issues and propose future areas for research.

Adrenomedullin: potential in physiology and pathophysiology

Adrenomedullin (ADM), a 52-amino acid ringed-structure peptide with C-terminal amidation, was originally isolated from human pheochromocytoma. ADM mediates vasodilatory and natriuretic properties through the second messenger cyclic adenosine 3',5'-monophosphate (cAMP), nitric oxide and the renal prostaglandin system. ADM immunoreactivity and its gene are widely distributed in cardiovascular, pulmonary, renal, gastrointestinal, cerebral and endocrine tissues. ADM is also synthesized and secreted from vascular endothelial and smooth muscle cells. When injected intravenously, ADM increases flow rates predominantly in organs in which the ADM gene is highly expressed, suggesting that ADM acts as a local autocrine and/or paracrine vasoactive hormone. In addition, ADM is a circulating hormone and its plasma concentration is increased in various cardiorenal diseases such as hypertension, chronic renal failure and congestive heart failure. Current evidence suggests that ADM plays an important role in fluid and electrolyte homeostasis and cardiorenal regulation, however further investigations are required to address the importance of ADM under various physiological and pathophysiological conditions.

Regulation of rat mesangial cell migration by platelet-derived growth factor, angiotensin II, and adrenomedullin

This study sought to determine whether platelet-derived growth factor (PDGF) and angiotensin II (AngII) stimulate migration of cultured rat glomerular mesangial cells. After finding that this was so, the effects of adrenomedullin (ADM) and cAMP-elevating agents on basal and stimulated mesangial cell migration were examined. Two isoforms of PDGF, AB and BB, stimulated migration in a concentration-dependent manner between 1 and 50 ng/ml, while the AA isoform lacked significant effect. AngII modestly but significantly stimulated migration in a concentration-dependent manner between 10(-7) and 10(-6) mol/L. Rat ADM significantly inhibited the PDGF BB- and AngII-stimulated migration in a concentration-dependent manner between 10(-8) and 10(-7) mol/L. Inhibition by rat ADM was accompanied by an increase in cellular cAMP. cAMP agonists or inducers such as 8-bromo cAMP, forskolin, and prostaglandin I2 also significantly reduced the stimulated migration. H 89, a protein kinase A (PKA) inhibitor, attenuated the inhibitory effect of ADM, and a calcitonin gene-related peptide (CGRP) receptor antagonist, human CGRP (8-37), abolished the inhibitory effects of rat ADM. These results suggest that PDGF AB and BB as well as AngII stimulate rat mesangial cell migration and that ADM can inhibit PDGF BB- and AngII-stimulated migration, at least in part through cAMP-dependent mechanisms likely to involve specific ADM receptors with which CGRP interacts. The adenylate cyclase/cAMP/PKA system may be involved in the migration-inhibitory effect of ADM in these cells.

Proadrenomedullin N-terminal 20 peptide (PAMP) immunoreactivity in vertebrate juxtaglomerular granular cells identified by both light and electron microscopy

The gene for adrenomedullin (AM), a multifunctional peptide hormone, is expressed in mammalian renal tissue and has been shown to stimulate renin release. The exact cell source of this peptide and its gene-related partner, proadrenomedullin N-terminal 20 peptide (PAMP), in kidney is still uncertain. In the present study we have identified PAMP-immunoreactive cells in the kidney of different mammalian species, including man, by light microscopy. In addition, these cells have been further studied in mouse kidney by both light and electron microscopic techniques. At the light microscopic level, PAMP immunolabeling is preferentially located in the subendothelial cells of the enlarged glomerular afferent arterioles, that is, in the juxtaglomerular cells. However, these cells do not show immunolabeling for AM. At the electron microscopic level, the immunostaining appears inside the renin-containing secretory granules of the juxtaglomerular cells. These results confirm the direct link between renin and the AM peptide family and provide a morphological basis for studying the potential modulatory function of AM and PAMP in the control of renin activity. In contrast, neither AM nor PAMP immunoreactivities were detected in the kidney of nonmammalian vertebrates, other than in blood vessels of particular species, providing a new phylogenetic difference in the juxtaglomerular apparatus between mammalian and nonmammalian vertebrates.

Adrenomedullin and the control of fluid and electrolyte homeostasis

Two potent hypotensive peptides, adrenomedullin (AM) and proadrenomedullin N-terminal 20 peptide (PAMP), are encoded by the adrenomedullin gene. AM stimulates nitric oxide production by endothelial cells, whereas PAMP acts presynaptically to inhibit adrenergic nerves that innervate blood vessels. Complementary, but mechanistically unique, actions also occur in the anterior pituitary gland where both peptides inhibit adrenocorticotropin release. In the adrenal gland both AM and PAMP inhibit potassium and angiotensin II-stimulated aldosterone secretion. Natriuretic and diuretic actions of AM reflect unique actions of the peptide on renal blood flow and tubular function. In the brain AM inhibits water intake and, in a physiologically relevant manner, salt appetite. Both AM and PAMP act in the brain to elevate sympathetic tone, effects that mirror the positive inotropic action of AM in the heart. Cardioprotective actions in the brain and heart may be important counter-regulatory actions that buffer the extreme hypotensive actions of the peptides when released in sepsis. Thus the biologic actions of the proadrenomedullin-derived peptides seem well coordinated to contribute to the physiologic regulation of volume and electrolyte homeostasis.

Proadrenomedullin-derived peptides

Posttranslational processing of the adrenomedullin gene product results in the formation of at least two biologically active peptides, adrenomedullin (AM) and proadrenomedullin N-20 terminal peptide (PAMP). Produced predominantly in the vasculature, both peptides are potent hypotensive agents, albeit via unique mechanisms of action. The gene is transcribed in a variety of other tissues including brain, pituitary, and kidney. Numerous actions have been reported most related to the physiologic control of fluid and electrolyte homeostasis. In the kidney, AM is diuretic and natriuretic, and both AM and PAMP inhibit aldosterone secretion by direct adrenal actions. In pituitary gland, both peptides at physiologically relevant doses inhibit basal ACTH secretion, again by apparently differing mechanisms. Additionally, AM antagonizes CRH-stimulated ACTH release. The peptides are produced in numerous brain sites, including hypothalamus and brainstem. Inhibition of AVP release has been reported and the physiologic significance of AM's ability to inhibit water drinking and salt appetite has been established. Thus the peptides appear to act in brain and pituitary gland to facilitate the loss of plasma volume, actions which complement their hypotensive effects in the blood vessel. Interestingly, direct cardiac effects (positive inotropism and chronotropism) and CNS actions (sympathostimulation) have been reported, leading to the hypothesis that these peptides also can exert important cardioprotective effects, helping to prevent vascular collapse during states of high AM secretion such as sepsis.

Gene transcription and synthesis of adrenomedullin by cultured human renal cells

Adrenomedullin (ADM) is a novel 52 amino acid peptide with a potent vasodilator effect. Gene expression of ADM is found in human kidney but the exact cell source in the kidney is uncertain. Its plasma level is raised in association with changes in sympathetic nervous activity and body fluid volume in hypertension and chronic renal failure. Herein, we examined the presence of mRNA encoding for ADM in cultured human glomerular cells. Adrenomedullin in cell culture supernatant was measured by a radio-immunoassay (with a detection level of 3.2 pmol/l). Adrenomedullin mRNA was found in cultured mesangial and glomerular epithelial cells as well as in vascular endothelial cells. Supernatant levels of ADM for cultured mesangial and glomerular epithelial cells were 21.2 and < 3.2 pmol/l respectively. Contrary to vascular smooth muscle cells, the gene expression for ADM in mesangial cells was up-regulated when incubated with increasing concentration of TNF-alpha or fetal bovine serum (FBS) but this effect was not observed with very high concentration. Parallel results were observed in adrenomedullin levels in supernatant from mesangial cell cultures. Forskolin, captopril, or TGF-beta had no effect on the transcription or synthesis of ADM in mesangial cells. The gene expression for ADM in glomerular epithelial cells was down-regulated when incubated with increasing concentration of TNF-alpha, forskolin or FBS. The ADM levels in all supernatant from resting glomerular epithelial cell cultures were < 3.2 pmol/l. Recent murine data show that ADM stimulates the release of cAMP but suppresses mitogenesis in cultured mesangial cells. Our results suggest ADM is synthesized by mesangial cells in an autocrine fashion and the peptide may potentially be involved in intra-renal blood pressure control.

Cytoprotective effects of adrenomedullin in glomerular cell injury: central role of cAMP signaling pathway

Activation of cAMP signaling pathway was shown to inhibit some pathobiologic processes in mesangial cells (MC). We investigated whether adrenomedullin (ADM), a potent agonist of adenylate cyclase, is synthesized in MC and whether it can, via cAMP, suppress the generation of reactive oxygen metabolites (ROM) and proliferation of cells in glomeruli. With the use of an immunohistologic technique ADM was detected in mesangial and microvascular areas of rat glomeruli. MC grown in primary culture synthesized ADM, and the synthesis was stimulated by TNF alpha and IL-1 beta but not by PDGF and EGF. ADM inhibited ROM generation in MC dose-dependently and caused in situ activation of protein kinase A (PKA). In macrophages (cell line J774) ROM generation was about four times higher than in MC and was inhibited by ADM in a similar way as in MC. The rate of MC proliferation, measured by [3H]-incorporation, and the activity of mitogen-activated protein kinase (MAPK) stimulated by PDGF and EGF were dose-dependently inhibited by ADM; the maximum inhibition (at 10 nM ADM) was about -80%. Mitogenesis of MC and MAPK activity when stimulated to a similar extent by endothelin (ET-1) was inhibited by ADM to a significantly (P < 0.01) lesser degree (-30%). Further, ADM inhibited PDF-stimulated mitogenesis and activation of MAPK in cultured vascular smooth muscle cells (VSMC). The inhibition of PDGF-activated MAPK by ADM in VSMC was reversed by the protein kinase A (PKA) inhibitor, H89. Taken together, results indicate the adrenomedullin (ADM) generated in mesangial cells (MC) can suppress, via activation of the cAMP-protein kinase A (PKA) signaling pathway, reactive oxygen metabolites (ROM) generation in MC and infiltrating macrophages as well as mitogen-activated protein kinase (MAPK)-mediated mitogenesis in MC and vascular smooth muscle cells (VSMC). We suggest that introglomerular ADM may serve as a cytoprotective autoacoid that suppresses pathobiologic processes evoked by immuno-inflammatory injury of glomeruli.