Downregulation of renin gene expression by interleukin-1

Beyond the Paradigm: Novel Functions of Renin-Producing Cells

The juxtaglomerular renin-producing cells (RPC) of the kidney are referred to as the major source of circulating renin. Renin is the limiting factor in renin-angiotensin system (RAS), which represents a proteolytic cascade in blood plasma that plays a central role in the regulation of blood pressure. Further cells disseminated in the entire organism express renin at a low level as part of tissue RASs, which are thought to locally modulate the effects of systemic RAS. In recent years, it became increasingly clear that the renal RPC are involved in developmental, physiological, and pathophysiological processes outside RAS. Based on recent experimental evidence, a novel concept emerges postulating that next to their traditional role, the RPC have non-canonical RAS-independent progenitor and renoprotective functions. Moreover, the RPC are part of a widespread renin lineage population, which may act as a global stem cell pool coordinating homeostatic, stress, and regenerative responses throughout the organism. This review focuses on the RAS-unrelated functions of RPC - a dynamic research area that increasingly attracts attention.

Control of renin [corrected] gene expression

Renin, as part of the renin-angiotensin system, plays a critical role in the regulation of blood pressure, electrolyte homeostasis, mammalian renal development, and progression of fibrotic/hypertrophic diseases. Renin gene transcription is subject to complex developmental and tissue-specific regulation. Initial studies using the mouse As4.1 cell line, which has many characteristics of the renin-expressing juxtaglomerular cells of the kidney, have identified a proximal promoter region (-197 to -50 bp) and an enhancer (-2,866 to -2,625 bp) upstream of the Ren-1(c) gene, which are critical for renin gene expression. The proximal promoter region contains several transcription factor binding sites including a binding site for the products of the developmental control genes Hox. The enhancer consists of at least 11 transcription factor binding sites and is responsive to various signal transduction pathways including cAMP, retinoic acid, endothelin-1, and cytokines, all of which are known to alter renin mRNA levels. Furthermore, in vivo models have validated several of these key components found within the proximal promoter region and the enhancer as well as other key sites necessary for renin gene transcription.

Physiology of Kidney Renin

The protease renin is the key enzyme of the renin-angiotensin-aldosterone cascade, which is relevant under both physiological and pathophysiological settings. The kidney is the only organ capable of releasing enzymatically active renin. Although the characteristic juxtaglomerular position is the best known site of renin generation, renin-producing cells in the kidney can vary in number and localization. (Pro)renin gene transcription in these cells is controlled by a number of transcription factors, among which CREB is the best characterized. Pro-renin is stored in vesicles, activated to renin, and then released upon demand. The release of renin is under the control of the cAMP (stimulatory) and Ca2+(inhibitory) signaling pathways. Meanwhile, a great number of intrarenally generated or systemically acting factors have been identified that control the renin secretion directly at the level of renin-producing cells, by activating either of the signaling pathways mentioned above. The broad spectrum of biological actions of (pro)renin is mediated by receptors for (pro)renin, angiotensin II and angiotensin-( 1 – 7 ).

Enhancer-dependent inhibition of mouse renin transcription by inflammatory cytokines

Inflammatory cytokines have been shown to inhibit renin gene expression in the kidney in vivo and the kidney tumor-derived As4.1 cell line. In this report, we show that cytokines oncostatin M (OSM), IL-6, and IL-1beta inhibit transcriptional activity associated with 4.1 kb of the mouse renin 5'-flanking sequence in As4.1 cells. The 242-bp enhancer (-2866 to -2625 bp) is sufficient to mediate the observed inhibitory effects. Sequences within the enhancer required for inhibition by each of these cytokines have been determined by deletional and mutational analysis. Results indicate that a 39-bp region (CEC) containing a cAMP-responsive element, an E-box, and a steroid receptor-binding site, previously identified as the most critical elements for enhancer activity, is sufficient for the inhibition induced by IL-1beta. However, mutation of each of the three component sites does not abolish the inhibition by IL-1beta, suggesting that the target(s) of cytokine action may not be the transcription factors binding directly to these sites. This CEC region is also critical, but not sufficient, for the inhibition mediated by OSM and IL-6. These data suggest that the direct target of the associated cytokines may be coactivators interacting with transcription factors binding at the enhancer. Finally, we show that OSM treatment caused a 17-fold increase in promoter activity when only 2,625 bp of the Ren-1(c) flanking sequence were tested, in which the enhancer is not present. Three regions including -2625 to -1217 bp, the HOX.PBX binding site at -60 bp, and -59 to +6 bp have been found to contribute to this induction.

Control of renin synthesis

Studies published recently have considerably enhanced our understanding of the mechanisms controlling renin production. With regard to the control of renin transcription, two enhancer regions have been identified that markedly augment renin synthesis in cell lines. In the absence of this enhancer activity, the basic promoter of the renin gene increases transcription only two- to threefold. The location of one (Jones CA, Sigmund CD, McGowan RA, Kane-Haas CM, and Gross KW. Mol Endocrinol 4: 375-383, 1990) transcription enhancer in the mouse gene is at about -2.7 kb and in humans at roughly -11 kb. A second important region has been identified in a chorionic cell line to be located approximately 5 kb upstream of the transcription start site in humans. Another potentially important regulatory region may lie within approximately 3.9 kb upstream of the -11 kb enhancer, as suggested by several conserved sequences among species in this region. In addition to the control of renin transcription, it seems that renin translation and the stability of renin mRNA are also effectively regulated. This occurs via the 3'-untranslated region, to which several proteins can bind. The binding proteins were identified as hnRNP K and E1, dynamin, nucleolin, MINT homologous protein, and Y-Box 1.

Intercellular communication between renin expressing As4.1 cells, endothelial cells and smooth muscle cells

Angiotensin II (AII) regulation of renin production by the juxtaglomerular (JG) cells of the kidney is commonly thought to occur through a direct feedback mechanism. However, recent evidence suggests that other cells in the vicinity may indirectly mediate AII's effect on renin production. Therefore we investigated whether an in vitro model of JG cells (As4.1) could have intercellular communication with endothelial or smooth muscle cells, which are in proximity to JG cells in vivo. 6-carboxyfluorescein was introduced to individual bovine aortic endothelial cells in co-culture with As4.1 cells. Coupling was observed 84% of the time at resting membrane potential and was attenuated by membrane depolarization or octanol (1 mM). Calcein green transfer between human aortic smooth muscle and As4.1 cells occurred 82% of the time and was inhibited by octanol. Expression of connexin 37, 40, 43, and 45 were detected in As4.1 cells using RT-PCR. Stimulation of As4.1 cells by AII failed to alter [Ca(2+)](i) or renin mRNA levels. These findings support the existence of gap junctions between renin producing cells and other cell types of the JG region. Moreover the lack of effect by AII suggest that feedback regulation of renin by AII may be due in part to intercellular communication with cells in proximity to JG cells.

Endothelin-1 increases calcium and attenuates renin gene expression in As4.1 cells

Endothelin-1 (ET-1) is a potent vasoconstrictor and blood pressure modulator. Renin secretion from juxtaglomerular (JG) cells is crucial for blood pressure and electrolyte homeostasis and has been shown to be modulated by ET-1; however, the cellular and molecular mechanism of this regulation is not clear. The purpose of this study was to gain a better understanding of the cellular and molecular pathways activated by ET-1 by using a renin-producing cell line As4.1. ET-1 caused an increase in As4.1 cell intracelluar Ca(2+) concentration ([Ca(2+)](i)) mediated by the ET(A) receptor as its antagonist, BQ-123, abolished the response. The nitric oxide donor nitroprusside, but not 8-bromo-cGMP, reduced the time necessary for successive ET-1 responses. Endothelin-3 had no effect on [Ca(2+)](i). ET-1 dose dependently increased total inositol phosphates with an EC(50) of 2.1 nM. ET-1 reduced renin mRNA by 68% independently of changes in message decay. With the use of a renin-luciferase reporter system in As4.1 cells, ET-1 reduced luciferase activity by 51%, suggesting that renin gene transcription is directly modified by ET-1.

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.

Critical roles of a cyclic AMP responsive element and an E-box in regulation of mouse renin gene expression

Mouse As4.1 cells, obtained after transgene-targeted oncogenesis to induce neoplasia in renal renin expressing cells, express high levels of renin mRNA from their endogenous Ren-1(c) gene. We have previously identified a 242-base pair enhancer (coordinates -2866 to -2625 relative to the CAP site) upstream of the mouse Ren-1(c) gene. This enhancer, in combination with the proximal promoter (-117 to +6), activates transcription nearly 2 orders of magnitude in an orientation independent fashion. To further delimit sequences necessary for transcriptional activation, renin promoter-luciferase reporter gene constructs containing selected regions of the Ren-1(c) enhancer were analyzed after transfection into As4.1 cells. These results demonstrate that several regions are required for full enhancer activity. Sequences from -2699 to -2672, which are critical for the enhancer activity, contain a cyclic AMP responsive element (CRE) and an E-box. Electrophoretic mobility shift assays demonstrated that transcription factors CREB/CREM and USF1/USF2 in As4.1 cell nuclear extracts bind to oligonucleotides containing the Ren-1(c) CRE and E-box, respectively. These two elements are capable of synergistically activating transcription from the Ren-1(c) promoter. Moreover, mutation of either the CRE or E-box results in almost complete loss of enhancer activity, suggesting the critical roles these two elements play in regulating mouse Ren-1(c) gene expression. Although the Ren-1(c) gene contains a CRE, its expression is not induced by cAMP in As4.1 cells. This appears to reflect constitutive activation of protein kinase A in As4.1 cells since treatment with the protein kinase A inhibitor, H-89, caused a significant reduction in Ren-1(c) gene expression and this reduction is mediated through the CRE at -2699 to -2688.

Myocardial hypertrophy in transgenic mice overexpressing human interleukin 1alpha

BACKGROUND

Interleukin (IL)-1 has profound effects on nonimmune cells and organs, including the heart. The effects of IL-1 on transgenic hearts have not yet been described.

METHODS AND RESULTS

We generated transgenic mice overexpressing the human IL-1 gene under control of the cytomegalovirus enhancer/chicken beta-actin promoter. Heart weight-body weight ratio increased 1.4- to 2.2-fold in transgenic mice compared with wild-type mice. Lung weight-body weight ratio also increased in transgenic mice, all of which died within 14 days of birth. Light microscopy revealed concentric hypertrophy with cardiomyocyte hypertrophy in all transgenic mice and pulmonary edema in some of them. Electron microscopy showed myofilament loss and an increased number of giant mitochondria, but no sarcomere disarray. Northern blotting showed that gene expression had been reprogrammed in the left ventricle of transgenic mice. Expression of fetal-type genes such as prepro-atrial natriuretic factor and beta-myosin heavy chain were increased, but voltage-dependent calcium channel messenger RNA expression was decreased in the left ventricle of transgenic mice compared with that of wild-type mice.

CONCLUSIONS

IL-1 may cause structural and functional alterations in cardiac myocytes.

Calcium-dependent activation of phospholipase C by mechanical distension in renin-expressing As4.1 cells

One of the major physiological regulators for the production and release of renin from the kidney is blood pressure. The juxtaglomerular (JG) cells, located primarily at the afferent arterioles leading to the glomerulus, are thought to be the baroreceptor of the kidney and adjust their ability to secrete renin in an inverse relationship to changes in pressure (mechanical force). The characteristics of JG cells that allow them to sense and respond to changes in mechanical force at the cellular level are not clear. By use of a renin-expressing clonal cell line (As4.1) as a model for JG cells, it was the purpose of this paper to identify cellular pathways that are activated by mechanical distension. Fura 2-labeled As4.1 cells were mechanically probed to observe changes of intracellular calcium concentration ([Ca(2+)](i)). Mechanical distension of As4.1 cells resulted in an influx of Ca(2+) to the cytosol, mediated by stretch-activated ion channels and dependent on the presence of extracellular Ca(2+). Furthermore, cyclic mechanical distension elevated total inositol phosphates (IP) in As4.1 cells. This response was also dependent on the presence of extracellular Ca(2+), and the addition of U-73122, a phospholipase C (PLC) antagonist, significantly attenuated the increase of IP. Taken together, these findings demonstrate the calcium-dependent activation of PLC and the subsequent increase of IP and [Ca(2+)](i) to be a potentially important pathway for the modality of pressure sensing by renin-expressing cells in response to mechanical stimulation.

Cyclic mechanical distension regulates renin gene transcription in As4.1 cells

The renin-producing and -secreting juxtaglomerular (JG) cells are thought to function as the baroreceptor of the kidney. The mechanism by which changes in pressure, or mechanical force, regulate renin at the molecular level has not been elucidated. The renin gene-expressing and -secreting clonal cell line As4.1 was derived from transgene-targeted oncogenesis in mice and was used as a cellular model for JG cells. As4.1 cells subjected to cyclic mechanical distension for a period of 24 h at various frequencies (0. 05 or 0.5 Hz) and magnitudes (12 or 24% elongation) were analyzed via Northern analysis for renin mRNA levels. Results indicate that renin gene expression is decreased by 50-85% and returns to basal levels after a 24-h recovery period. Renin gene expression was attenuated independently of elevated cell growth or changes in renin message decay, suggesting that renin gene transcription is directly modulated by mechanical distension. Transient transfection of As4.1 cells with renin 5' flanking sequence-luciferase reporter gene constructs confirmed the role of mechanical stimulation in regulating renin gene transcription. A 43% inhibition of luciferase activity, by stretch, was observed in cells transfected with a 4,000 base pair 5' flanking sequence to the renin proximal promoter. These results demonstrate for the first time that changes in mechanical force can result in the regulation of renin gene transcription and thus provide further insight into the baroreceptor properties of renin-expressing cells.

Concerted actions of IL-1beta inhibit Na+ absorption and stimulate anion secretion by IMCD cells

Increasing evidence indicates that factors other than adrenocorticoid hormones can influence long-term regulation of Na+ transport by inner medullary collecting duct (IMCD) cells. We now report that, of 14 interleukins tested, only interleukin-1alpha (IL-1alpha) and IL-1beta inhibited Na+ transport by primary cultures of rat IMCD. IL-1beta reduced both basal and mineralocorticoid (MC)-stimulated Na+ transport by 50-70%; its effect on glucocorticoid (GC)-stimulated Na+ transport was significantly less. IL-1beta continued to blunt MC stimulation of Na+ transport even after it had been removed from the medium for 24 h. The onset of action to inhibit Na+ transport was within 20 min. The acute effect from the basolateral surface was greater than that from the apical surface, but the effect from each surface was additive. In addition to its inhibitory effect on Na+ transport, chronic IL-1beta exposure increased both basal and cAMP-stimulated anion secretion rates. IL-1beta had no acute effect on anion secretion. Monolayers chronically treated with IL-1beta had an increased capacity to secrete fluid, as predicted from its effects on ion transport. Inhibitors of cyclooxygenase did not blunt the actions of IL-1beta. Furthermore, IL-1beta did not produce a rise in intracellular Ca2+. These results suggest novel signaling pathways induced by IL-1beta regulating Na+ and Cl- transport by the IMCD.

Regulation of human renin gene promoter activity: a new negative regulatory region determines the responsiveness to TNF alpha

BACKGROUND

The renin-angiotensin system has been known to regulate blood pressure and body fluid homeostasis. Several lines of evidence have shown that renin gene expression and release are up-regulated by beta-adrenergic stimulation, sodium depletion, and angiotensin converting enzyme inhibition, but down-regulated by cytokines. To further characterize the human renin gene (hREN) promoter structure, its regulation, and to identify an appropriate cell system for study, we examined five cell lines and investigated drug effects on the hREN promoter expression.

METHODS

Using the hREN-luciferase reporter gene constructs in the DNA transfection assays, approximately 5 kb of the hREN 5' flanking region was assessed for promoter activity in five different cell lines. Regulation of the hREN promoter activity was investigated using Y-1 adrenal cells that were transfected with the hREN-luciferase DNA and were treated with forskolin, calcium ionophore A23187, phorbol ester, angiotensin II (Ang II), or cytokines.

RESULTS

Transient transfection analysis showed that the 5 kb hREN 5' flanking DNA alone was able to confer significant promoter activity in Y-1 adrenal cells. In transfected Y-1 cells, luciferase reporter expression was induced by forskolin, suppressed by the calcium ionophore A23187, and phorbol ester in a dose-dependent manner, but was unaffected by angiotensin II (Ang II). However, when Y-1 reporter cells were transfected with human angiotensin II receptor type 1 (AT1) cDNA, hREN promoter activity was dose-dependently down-regulated by Ang II, which was blockable by losartan, an AT1-selective antagonist. Further studies also showed that hREN promoter activity in Y-1 cells was selectively down-regulated by TNF alpha. Deletion of the hREN promoter sequences between position -3916 and -2822 not only enhanced hREN promoter activity by approximately tenfold, but also caused a failure of down-regulation by TNF alpha. In contrast, neither interleukin (IL)-1 alpha, IL-1 beta, IL-2, nor IL-6 exerted any significant effect.

CONCLUSIONS

Together the results suggest that TNF alpha is a negative regulator of the hREN expression in the adrenal cells, and that the TNF alpha responsiveness may be controlled by elements located between the positions -3916 and -2822 of the hREN promoter. Moreover, the Y-1 cell line may provide a valuable model system for studying renin gene regulation.

The human placental renin-angiotensin system

The human placenta and related tissues are considered to be examples of the recently accepted local renin-angiotensin systems (RAS). The brain is another example of a system that is thought to be regulated independently of the kidney and the role of angiotensin within the CNS as a neural mediator has drawn considerable attention. It has been known for a long time that many of the neuroendocrine mediators and receptors are expressed in the placenta and it has been suggested that there are many parallels between the classical neuroendocrine system and the placental one. The present review summarizes information that components of the RAS are expressed in uteroplacental tissues, are regulated by endogenous substances, and have important biological functions within this reproductive system. A comparison of similarities and differences between the classical and the placental RAS may provide clues to functions in other endocrine and neuroendocrine systems. The major components of the placental RAS that are considered are renin, prorenin, angiotensin I, angiotensin II, angiotensin converting enzyme (ACE), angiotensin receptors, and angiotensinogen (renin substrate). The factors that regulate these components at the cellular and the nuclear level are described. It is concluded that prorenin via angiotensin-dependent and angiotensin-independent mechanisms influences functions within uteroplacental tissues. Some of these actions are direct and others are mediated by the release of different signalling molecules. These features are similar to many neuroendocrine systems and utilize some of the same messengers.