Prorenin activation and prohormone convertases in the mouse As4.1 cell line

Cell-cell contacts via N-cadherin induce a regulatory renin secretory phenotype in As4.1 cells

The lack of a clonal renin-secreting cell line has greatly hindered the investigation of the regulatory mechanisms of renin secretion at the cellular, biochemical, and molecular levels. In the present study, we investigated whether it was possible to induce phenotypic switching of the renin-expressing clonal cell line As4.1 from constitutive inactive renin secretion to regulated active renin secretion. When grown to postconfluence for at least two days in media containing fetal bovine serum or insulin-like growth factor-1, the formation of cell-cell contacts via N-cadherin triggered downstream cellular signaling cascades and activated smooth muscle-specific genes, culminating in phenotypic switching to a regulated active renin secretion phenotype, including responding to the key stimuli of active renin secretion. With the use of phenotype-switched As4.1 cells, we provide the first evidence that active renin secretion via exocytosis is regulated by phosphorylation/dephosphorylation of the 20 kDa myosin light chain. The molecular mechanism of phenotypic switching in As4.1 cells described here could serve as a working model for full phenotypic modulation of other secretory cell lines with incomplete phenotypes.

A new channel for the control of renin secretion in juxtaglomerular cells

The role of membrane channels in juxtaglomerular cell physiology is only partially understood. Pannexin 1 is a mechanosensitive, nonjunctional channel known for its role in adenosine triphosphate release. The study by DeLalio et al. documents involvement of pannexin 1 in renin secretion by studying mice deficient in pannexin 1 in renin-secreting cells and a prorenin-secreting cell line. Pannexin 1 is believed to suppress renin secretion and thereby modify blood pressure. The commentary addresses the broader physiological implication of these observations for the regulation of renin and blood pressure.

Transcriptome Analysis of Human Reninomas as an Approach to Understanding Juxtaglomerular Cell Biology

Renin, a key component in the regulation of blood pressure in mammals, is produced by the rare and highly specialized juxtaglomerular cells of the kidney. Chronic stimulation of renin release results in a recruitment of new juxtaglomerular cells by the apparent conversion of adjacent smooth muscle cells along the afferent arterioles. Because juxtaglomerular cells rapidly dedifferentiate when removed from the kidney, their developmental origin and the mechanism that explains their phenotypic plasticity remain unclear. To overcome this limitation, we have performed RNA expression analysis on 4 human renin-producing tumors. The most highly expressed genes that were common between the reninomas were subsequently used for in situ hybridization in kidneys of 5-day-old mice, adult mice, and adult mice treated with captopril. From the top 100 genes, 10 encoding for ligands were selected for further analysis. Medium of human embryonic kidney 293 cells transfected with the mouse cDNA encoding these ligands was applied to (pro)renin-synthesizing As4.1 cells. Among the ligands, only platelet-derived growth factor B reduced the medium and cellular (pro)renin levels, as well as As4.1 renin gene expression. In addition, platelet-derived growth factor B-exposed As4.1 cells displayed a more elongated and aligned shape with no alteration in viability. This was accompanied by a downregulated expression of α-smooth muscle actin and an upregulated expression of interleukin-6, suggesting a phenotypic shift from myoendocrine to inflammatory. Our results add 36 new genes to the list that characterize renin-producing cells and reveal a novel role for platelet-derived growth factor B as a regulator of renin-synthesizing cells.

General lysosomal hydrolysis can process prorenin accurately

Renin, an aspartyl protease that catalyzes the rate-limiting step of the renin-angiotensin system, is first synthesized as an inactive precursor, prorenin. Prorenin is activated by the proteolytic removal of an amino terminal prosegment in the dense granules of the juxtaglomerular (JG) cells of the kidney by one or more proteases whose identity is uncertain but commonly referred to as the prorenin-processing enzyme (PPE). Because several extrarenal tissues secrete only prorenin, we tested the hypothesis that the unique ability of JG cells to produce active renin might be explained by the existence of a PPE whose expression is restricted to JG cells. We found that inducing renin production by the mouse kidney by up to 20-fold was not associated with the concomitant induction of candidate PPEs. Because the renin-containing granules of JG cells also contain several lysosomal hydrolases, we engineered mouse Ren1 prorenin to be targeted to the classical vesicular lysosomes of cultured HEK-293 cells, where it was accurately processed and stored. Furthermore, we found that HEK cell lysosomes hydrolyzed any artificial extensions placed on the protein and that active renin was extraordinarily resistant to proteolytic degradation. Altogether, our results demonstrate that accurate processing of prorenin is not restricted to JG cells but can occur in classical vesicular lysosomes of heterologous cells. The implication is that renin production may not require a specific PPE but rather can be achieved by general hydrolysis in the lysosome-like granules of JG cells.

Cathepsin B is not the processing enzyme for mouse prorenin

Renin, an aspartyl protease that catalyzes the rate-limiting step in the renin-angiotensin system (RAS), is proteolytically activated by a second protease [referred to as the prorenin processing enzyme (PPE)] before its secretion from the juxtaglomerular cells of the kidney. Although several enzymes are capable of activating renin in vitro, the leading candidate for the PPE in the kidney is cathepsin B (CTSB) due to is colocalization with the renin precursor (prorenin) in juxtaglomerular cell granules and because of its site-selective activation of human prorenin both in vitro and in transfected tissue culture cell models. To verify the role of CTSB in prorenin processing in vivo, we tested the ability of CTSB-deficient (CTSB−/−) mice to generate active renin. CTSB−/− mice do not exhibit any overt symptoms (renal malformation, preweaning mortality) typical of an RAS deficiency and have normal levels of circulating active renin, which, like those in control animals, rise more than 15-fold in response to pharmacologic inhibition of the RAS. The mature renin enzyme detected in kidney lysates of CTSB−/− mice migrates at the same apparent molecular weight as that in control mice, and the processing to active renin is not affected by chloroquine treatment of the animals. Finally, the distribution and morphology of renin-producing cells in the kidney is normal in CTSB−/− mice. In conclusion, CTSB-deficient mice exhibit no differences compared with controls in their ability to generate active renin, and our results do not support CTSB as the PPE in mice.

Regulation of renin release by local and systemic factors

The renin-angiotensin system (RAS) is critically involved in the regulation of the salt and volume status of the body and blood pressure. The activity of the RAS is controlled by the protease renin, which is released from the renal juxtaglomerular epithelioid cells into the circulation. Renin release is regulated in negative feedback-loops by blood pressure, salt intake, and angiotensin II. Moreover, sympathetic nerves and renal autacoids such as prostaglandins and nitric oxide stimulate renin secretion. Despite numerous studies there remained substantial gaps in the understanding of the control of renin release at the organ or cellular level. Some of these gaps have been closed in the last years by means of gene-targeted mice and advanced imaging and electrophysiological methods. In our review, we discuss these recent advances together with the relevant previous literature on the regulation of renin release.

Genetic disruption of gamma-melanocyte-stimulating hormone signaling leads to salt-sensitive hypertension in the mouse

The gamma-melanocyte-stimulating hormone (gamma-MSH) is a natriuretic peptide derived from the N-terminal region of proopiomelanocortin (POMC). Evidence suggests that it may be part of the coordinated response to a low-sodium diet (LSD). We tested the effect of the HSD (8% NaCl) compared with LSD (0.07%) on mean arterial pressure (MAP) in mice with targeted disruption of the PC2 gene (PC2(-/-)), necessary for processing of POMC into gamma-MSH, or the melanocortin receptor 3 gene (Mc3r(-/-); the receptor for MSH). In wild-type mice, HSD for 1 week did not alter MAP versus LSD mice, but plasma gamma-MSH immunoreactivity was more than double the LSD value. In contrast, in PC2(-/-) mice, MAP on the LSD was not greater than in wild-type mice, but plasma gamma-MSH was reduced to one-seventh the wild-type value. On the HSD, MAP rose to a markedly hypertensive level while plasma gamma-MSH concentration remained severely depressed. Intravenous infusion of gamma-MSH (0.2 pmol/min) for 30 min to PC2(-/-) mice after 1 week of HSD lowered MAP from hypertensive levels to normal; infusion of alpha-MSH at the same rate had no effect. Injection of 60 fmol of gamma-MSH into the lateral cerebral ventricle of hypertensive mice also lowered MAP to normal. Administration of a stable analogue of gamma-MSH intra-abdominally by microosmotic pump to PC2(-/-) mice prevented the development of hypertension when ingesting the HSD. In mice with targeted disruption of the Mc3r gene, the HSD also led to marked hypertension accompanied by elevated plasma levels of gamma-MSH; infusion of exogenous gamma-MSH to these mice had no effect on MAP. These results strongly suggest that PC2-dependent processing of POMC into gamma-MSH is necessary for the normal response to the HSD. gamma-MSH deficiency results in marked salt-sensitive hypertension that is rapidly improved with exogenous gamma-MSH through a central site of action. alpha-MSH infused at the same rate had no effect on MAP, indicating that the hypertension is a specific consequence of impaired POMC processing into gamma-MSH. Absence of Mc3r produces gamma-MSH resistance and hypertension on the HSD. These findings demonstrate a novel pathway mediating salt-sensitivity of blood pressure.

Tumor necrosis factor-alpha inhibits renin gene expression

Renin, produced in renal juxtaglomerular (JG) cells, is a fundamental regulator of blood pressure. Accumulating evidence suggests that cytokines may directly influence renin production in the JG cells. TNF-alpha, which is one of the key mediators in immunity and inflammation, is known to participate in the control of vascular proliferation and contraction and hence in the pathogenesis of cardiovascular diseases. Thus TNF-alpha may exert its effects on the cardiovascular system through modulation of renal renin synthesis. Therefore we have tested the effect of TNF-alpha on renin transcription in As4.1 cells, which represent transformed mouse JG cells, and in native mouse JG cells in culture. Renin gene expression was also determined in mice lacking the gene for TNF-alpha (TNF-alpha knockout mice). TNF-alpha inhibited renin gene expression via an inhibition of the transcriptional activity, targeting the proximal 4.1 kb of the renin promoter in As4.1 cells. TNF-alpha also attenuated forskolin-stimulated renin gene expression in primary cultures of mouse JG cells. Mice lacking the TNF-alpha gene had almost threefold higher basal renal renin mRNA abundance relative to the control strain. The general physiological regulation of renin expression by salt was not disturbed in TNF-alpha knockout mice. Our data suggest that TNF-alpha inhibits renin gene transcription at the cellular level and thus may act as a modulator of renin synthesis in (physio)pathological situations.

Prorenins activation by an enzyme from rat plasma (PreR-Co)

The aim of the present research was to explore the capacity of PreR-Co to process prorenin purified from kidney and corpora lutea (CL) and to study its action on extrarenal tissues. The PreR-Co was obtained from plasma as a single electrophoretic band by (NH4)2SO4 precipitation, gel filtration, anti-rat albumin immunoaffinity, and ion-exchange chromatography. Prorenin free of renin was obtained after (NH4)2SO4 precipitation, gel filtration, and ion-exchange chromatography by a passage through an affinity gel of H-77 Sepharose. SDS-PAGE of supernatant and of acidic elution from gel, exhibited a single band of 43 kDa and 35 kDa, respectively; both recognized by the specific anti rat renin antibody. The isolated renin was not attacked by PreR-Co; on the contrary prorenin was completely activated. The product of PreR-Co-activated prorenin showed an analogous MW to that of renin and was recognized by the specific antibody. In addition to processing kidney prorenin, PreR-Co was able to cleave inactive renin from ovary, CL, uterus and adrenal gland homogenates. However, the amount of active renin generated from these tissues was lower than those produced by trypsin activation. PreR-Co is a good candidate for the role of the enzyme involved in tissues prorenin activation.

Neuronal cell lines expressing PC5, but not PC1 or PC2, process Pro-CCK into glycine-extended CCK 12 and 22

Endocrine tumor cells in culture and in vitro cleavage assays have shown that PC1 and PC2 are capable of processing pro-CCK into smaller, intermediate and final, bioactive forms. Similar studies have shown that PC5 has the ability to process a number of propeptides. Here, we use GT1-7 (mouse hypothalamic) and SK-N-MC and SK-N-SH (human neuroblastoma) tumor cell lines to study the ability of PC5 to process pro-CCK. RT-PCR and Western blot analysis showed that the cells express PC5 mRNA and protein, but not PC1 or PC2. They were engineered to stably overexpress CCK and cell media was analyzed for pro-CCK expression and cleavage of the prohormone. Radioimmunoassays showed that pro-CCK was expressed, but no amidated CCK was detected. Lack of production of amidated CCK may be due to the lack of the appropriate carboxypeptidase and amidating enzymes. Production of glycine-extended CCK processing products was evaluated by treatment of media with carboxypeptidase B followed by analysis with a CCK Gly RIA. Glycine-extended forms of the peptide were found in the media. The predominant forms co-eluted with CCK 12 Gly and CCK 22 Gly on gel filtration chromatography. The results demonstrate that these cell lines which express PC5 and not PC1 or PC2 have the ability to process pro-CCK into intermediate, glycine-extended forms more closely resembling pro-CCK products in intestine than in brain.

LXRalpha functions as a cAMP-responsive transcriptional regulator of gene expression

LXRalpha is a member of a nuclear receptor superfamily that regulates transcription. LXRalpha forms a heterodimer with RXRalpha, another member of this family, to regulate the expression of cholesterol 7alpha-hydroxylase by means of binding to the DR4-type cis-element. Here, we describe a function for LXRalpha as a cAMP-responsive regulator of renin and c-myc gene transcriptions by the interaction with a specific cis-acting DNA element, CNRE (an overlapping cAMP response element and a negative response element). Our previous studies showed that renin gene expression is regulated by cAMP, at least partly, through the CNRE sequence in its 5'-flanking region. This sequence is also found in c-myc and several other genes. Based on our cloning results using the yeast one-hybrid system, we discovered that the mouse homologue of human LXRalpha binds to the CNRE and demonstrated that it binds as a monomer. To define the function of LXRalpha on gene expression, we transfected the renin-producing renal As4.1 cells with LXRalpha expression plasmid. Overexpression of LXRalpha in As4.1 cells confers cAMP inducibility to reporter constructs containing the renin CNRE. After stable transfection of LXRalpha, As4.1 cells show a cAMP-inducible up-regulation of renin mRNA expression. In parallel experiments, we demonstrated that LXRalpha can also bind to the homologous CNRE in the c-myc promoter. cAMP promotes transcription through c-myc/CNRE:LXRalpha interaction in LXRalpha transiently transfected cells and increases c-myc mRNA expression in stably transfected cells. Identification of LXRalpha as a cAMP-responsive nuclear modulator of renin and c-myc expression not only has cardiovascular significance but may have generalized implication in the regulation of gene transcription.

Current topic: the uteroplacental renin-angiotensin system

The components of the renin-angiotensin system (RAS) are expressed in the uteroplacental unit. The expression varies between species, probably due to the marked species differences in placental architecture. The conditions for angiotensin (Ang) II formation exist and Ang II receptors are present throughout the human uteroplacental unit, indicating the presence of a functional local RAS. The uteroplacental RAS interacts with other regulatory systems and in this way modulates various aspects of tissue function. It is suggested that the uteroplacental RAS is important for the regeneration of the endometrium after shedding, and for decidualization, implantation and placentation. The RAS participates in the regulation of the uteroplacental blood flow, prostaglandin synthesis and oestradiol secretion. Disturbances of the uteroplacental RAS may lead to dysfunctional bleeding and to reduced uteroplacental blood flow in pregnancies complicated by pre-eclampsia and intrauterine growth retardation.

Hydrolysis by plasma kallikrein of fluorogenic peptides derived from prorenin processing site

Human plasma kallikrein (HPK) activates plasma prorenin to renin, and the physiological significance of this activation is still unknown. In this paper we investigated the efficiency and the cleavage pattern of the hydrolysis by HPK of the internally quenched fluorescent peptides (qf-peptides) derived from the amino acid sequence of human prorenin cleavage site. The peptide Abz-F-S-Q-P-M-K-R-L-T-L-G-N-T-T-Q-EDDnp (Abz=ortho-aminobenzoic acid, and EDDnp=N-[2,4-dinitrophenyl]-ethylene diamine), that corresponds to the amino acid sequence P(7) to P(7)' of human prorenin cleavage site, is hydrolyzed at the correct processing site (R-L bond) with k(cat)/K(m)=85 mM(-1) s(-1). Alanine was scanned in all positions from P(5) to P(5)' in order to investigate the substrate specificity requirements of HPK. The qf-peptides derived from the equivalent segment of rat prorenin, that has Lys-Lys as basic amino acid pair, and the peptide Abz-NVTSPVQ-EDDnp that contains the proposed cleavage site of rat prorenin have very low susceptibility to hydrolysis by rat plasma kallikrein. These data are according to the previously reported absence of rat plasma prorenin activation by rat plasma kallikrein (RPK), and with the view that prorenin activation in rat requires alternative enzymes and/or mechanism. All the obtained peptides described in this paper were also assayed with bovine trypsin that was taken as a reference protease because it is commonly used to activate prorenin.

Hydrolysis by cathepsin B of fluorescent peptides derived from human prorenin

Cathepsin B is a lysosomal thiolprotease that, because of its colocalization with renin and its ability to activate prorenin, has been proposed as a prorenin processing enzyme. To characterize the biochemical aspect of this potential cathepsin B activity in more detail, we synthesized and assayed with human cathepsin B the internally quenched fluorescent peptide Abz-FSQPMKRLTLGNTTQ-EDDnp (Abz, ortho-aminobenzoic acid fluorescent group and EDDnp, N-¿2, 4-dinitrophenyl-ethylenediamine quencher group) that contains 7 amino acids for each side of the R-L bond that is the processing site of human prorenin. Human cathepsin B hydrolyzed this peptide at the correct site (R-L bond), with k(cat)/K(m)=75 mmol/L(-1) s(-1). Analogues of this peptide obtained by Ala scanning at positions P(5) to P(5)' were also synthesized and assayed as substrates for human cathepsin B. The obtained specificity constant (k(cat)/K(m)) values have a significant parallel with the previous data of prorenin activation by AtT-20 cells and in vitro by cathepsin B. In addition, we demonstrated the presence of cathepsin B-like activity in rat mesangial cells and the ability of its whole soluble fraction lysates, as well as that of purified cloned rat cathepsin B, to hydrolyze Abz-IKKSSF-EDDnp at the K-S bond, which contains 6 amino acids of rat prorenin processing site. The specificity data of cathepsin B toward peptides derived from prorenin processing site support the view that human or rodent cathepsin B could be involved in the intracellular processing of prorenin that is locally synthesized or taken up from the extracellular compartment.

Immunohistochemical distribution of the prohormone convertase PC5-A in rat brain

Prohormone convertase 5 is an endoprotease of the kexin/subtilisin-like family, which has been postulated to play a role in the proteolytic maturation of a variety of pro-peptides in the mammalian brain. In order to gain insight into the functional role of prohormone convertase 5 in the central nervous system, the regional, cellular and subcellular distributions of the enzyme were investigated by immunohistochemistry in rat brain using an N-terminal-directed specific antibody shown previously to recognize both the mature and unprocessed forms of the enzyme. Throughout the brain, prohormone convertase 5 immunoreactivity was concentrated within nerve cell bodies and proximal dendrites. No prohormone convertase 5 immunoreactivity was associated with astrocytes, as confirmed by the absence of prohormone convertase 5 immunolabeling in cells immunopositive for the glial protein S-100alpha. Within neurons, prohormone convertase 5 immunoreactivity was concentrated within the Golgi apparatus, as revealed immunohistochemically within the same sections using antibodies against the medial cisternae protein MG-160. It was also present within small vesicular-like elements distributed throughout the cytoplasm of perikarya and dendrites, but not of axons, as confirmed by its lack of co-localization with the synaptic terminal marker Dynamin-1. These results suggest that prohormone convertase 5 is active within early compartments of the neuronal regulated secretory pathway and that it is unlikely to be released with its processed substrates. At the regional level, prohormone convertase 5-immunoreactive perikarya were distributed extensively throughout the forebrain. The most numerous and intensely labeled were detected in the olfactory bulb, cerebral cortex, globus pallidus, endopeduncular and subthalamic nuclei, septum, diagonal band of Broca, magnocellular and medial preoptic areas, supraoptic and arcuate nuclei of the hypothalamus, and anterodorsal, laterodorsal, paraventricular and reticular nuclei of the thalamus. Moderate to dense neuronal labeling was also evident in the olfactory tubercle, caudate-putamen, claustrum, bed nucleus of the stria terminalis, substantia innominata, hippocampus, amygdala, and remaining thalamic and hypothalamic nuclei. This widespread distribution suggests that prohormone convertase 5 is involved in the processing of a variety of neuropeptide and/or neurotrophin precursors in mammalian brain.

The renin-angiotensin system in hybrid NG108-15 cells. Renin gene is from mouse neuroblastoma, angiotensinogen and angiotensin-converting enzyme genes are of rat glioma origin

Angiotensin II (Ang II) increases the level of tyrosine phosphorylation of several proteins in nondifferentiated NG108-15 cells, a hybrid derived from the fusion of mouse neuroblastoma and rat glioma cells. Conversely, incubation of NG108-15 cells with an angiotensin-converting enzyme (ACE) inhibitor decreased the basal level of tyrosine phosphorylation of proteins, suggesting that locally secreted Ang II may act as an autocrine regulator. By RT-PCR, we found that nondifferentiated NG108-15 cells contained the mRNA transcript of the rat angiotensinogen, mouse renin and rat ACE genes, thus confirming that NG108-15 cells contain all the elements of a local renin-angiotensin system.