Renin and angiotensinogen expression during the evolution of diabetes

Urinary Angiotensinogen in Patients With Type 1 Diabetes With Microalbuminuria: Gender Differences and Effect of Intensive Insulin Therapy

Introduction

Angiotensinogen (AOG) is the precursor of peptides of the renin angiotensin system (RAS). Because insulin up-regulates transcriptional factors that normally repress kidney AOG synthesis, we evaluated urinary AOG (uAOG) in patients with type 1 diabetes (T1D) and microalbuminuria who are receiving either intensive or conventional insulin therapy.

Methods

Urine samples from participants of the Diabetes Control and Complications Trial (DCCT) were used for the following: (i) uAOG/creatinine measurements in 103 patients with microalbuminuria and 103 patients with normoalbuminuria, matched for age, gender, disease duration, and allocation to insulin therapy; and (ii) uAOG/creatinine measurements from patients with microalbuminuria allocated to intensive insulin therapy (n = 58) or conventional insulin therapy (n = 41) after 3 years on each modality.

Results

uAOG was higher in patients who started with microalbuminuria than in those with normoalbuminuria (6.65 vs. 4.0 ng/mg creatinine, P < 0.01). uAOG was higher in females than in males with microalbuminuria (11.7 vs. 5.4 ng/mg creatinine, P = 0.015). uAOG was lower in patients with microalbuminuria allocated to intensive insulin therapy than in conventional insulin therapy (3.98 vs. 7.42 ng/mg creatinine, P < 0.01). These differences in uAOG were observed though albumin excretion rate (AER) was not significantly different.

Conclusion

In patients with T1D and microalbuminuria, uAOG is increased and varies with gender and the type of insulin therapy independently of AER. This suggests that AOG production is increased in females and it is decreased by intensive insulin therapy. The reduction in uAOG with intensive insulin therapy, by kidney RAS downregulation, may contribute to the known renoprotective action associated with intensive insulin and improved glycemic control.

Intrarenal renin angiotensin system revisited: role of megalin-dependent endocytosis along the proximal nephron

The existence of a local renin angiotensin system (RAS) of the kidney has been established. Angiotensinogen (AGT), renin, angiotensin-converting enzyme (ACE), angiotensin receptors, and high concentrations of luminal angiotensin II have been found in the proximal tubule. Although functional data have documented the relevance of a local RAS, the dualism between biosynthesis and endocytotic uptake of its components and their cellular processing has been incompletely understood. To resolve this, we have selectively analyzed their distribution, endocytosis, transcytosis, and biosynthesis in the proximal tubule. The presence of immunoreactive AGT, restricted to the early proximal tubule, was due to its retrieval from the ultrafiltrate and storage in endosomal and lysosomal compartments. Cellular uptake was demonstrated by autoradiography of radiolabeled AGT and depended on intact endocytosis. AGT was identified as a ligand of the multiple ligand-binding repeats of megalin. AGT biosynthesis was restricted to the proximal straight tubule, revealing substantial AGT mRNA expression. Transgenic AGT overexpression under the control of an endogenous promoter was also restricted to the late proximal tubule. Proximal handling of renin largely followed the patterns of AGT, whereas its local biosynthesis was not significant. Transcytotic transport of AGT in a proximal cell line revealed a 5% recovery rate after 1 h. ACE was expressed along late proximal brush-border membrane, whereas ACE2 was present along the entire segment. Surface expression of ACE and ACE2 differed as a function of endocytosis. Our data on the localization and cellular processing of RAS components provide new aspects of the functional concept of a "self-contained" renal RAS.

The intrarenal renin-angiotensin system: from physiology to the pathobiology of hypertension and kidney disease

In recent years, the focus of interest on the role of the renin-angiotensin system (RAS) in the pathophysiology of hypertension and organ injury has changed to a major emphasis on the role of the local RAS in specific tissues. In the kidney, all of the RAS components are present and intrarenal angiotensin II (Ang II) is formed by independent multiple mechanisms. Proximal tubular angiotensinogen, collecting duct renin, and tubular angiotensin II type 1 (AT1) receptors are positively augmented by intrarenal Ang II. In addition to the classic RAS pathways, prorenin receptors and chymase are also involved in local Ang II formation in the kidney. Moreover, circulating Ang II is actively internalized into proximal tubular cells by AT1 receptor-dependent mechanisms. Consequently, Ang II is compartmentalized in the renal interstitial fluid and the proximal tubular compartments with much higher concentrations than those existing in the circulation. Recent evidence has also revealed that inappropriate activation of the intrarenal RAS is an important contributor to the pathogenesis of hypertension and renal injury. Thus, it is necessary to understand the mechanisms responsible for independent regulation of the intrarenal RAS. In this review, we will briefly summarize our current understanding of independent regulation of the intrarenal RAS and discuss how inappropriate activation of this system contributes to the development and maintenance of hypertension and renal injury. We will also discuss the impact of antihypertensive agents in preventing the progressive increases in the intrarenal RAS during the development of hypertension and renal injury.

Tissue gene expression of renin-angiotensin system in human type 2 diabetic nephropathy

OBJECTIVE

Recent studies have proved that blockade of the renin-angiotensin system (RAS) retards the progression of diabetic nephropathy, whereas hyporeninemia is known as a typical state in diabetic subjects. The purpose of this study is to determine whether expression levels of RAS differ between nondiabetic and diabetic renal tissues with accurate quantitative method.

RESEARCH DESIGN AND METHODS

Subjects were 66 nondiabetic and 8 diabetic patients with biopsy-proven renal diseases. The eight diabetic subjects suffered from type 2 diabetes with overt proteinuria. Renal histology revealed typical diffuse or nodular lesions with linear IgG deposit on immunofluorescent staining and thickened basement membrane on electronic microscopy. Total RNA from a small part of the renal cortical biopsy specimens was reverse-transcribed, and the resultant cDNA was amplified for new major components of RAS (i.e., renin, renin receptor, angiotensinogen, ACE, ACE2, angiotensin II type 1 receptor, and angiotensin II type 2 receptor) and measured.

RESULTS

Among these components, a significant upregulation was observed in the ACE gene in diabetic renal tissue.

CONCLUSIONS

The results suggest that renal tissue RAS might be activated in the respect that ACE gene expression is upregulated in spite of a tendency to low renin expression in type 2 diabetic nephropathy.

Inhibition of diabetic nephropathy by a decoy peptide corresponding to the "handle" region for nonproteolytic activation of prorenin

We found that when a site-specific binding protein interacts with the "handle" region of the prorenin prosegment, the prorenin molecule undergoes a conformational change to its enzymatically active state. This nonproteolytic activation is completely blocked by a decoy peptide with the handle region structure, which competitively binds to such a binding protein. Given increased plasma prorenin in diabetes, we examined the hypothesis that the nonproteolytic activation of prorenin plays a significant role in diabetic organ damage. Streptozotocin-induced diabetic rats were treated with subcutaneous administration of handle region peptide. Metabolic and renal histological changes and the renin-Ang system components in the plasma and kidneys were determined at 8, 16, and 24 weeks following streptozotocin treatment. Kidneys of diabetic rats contained increased Ang I and II without any changes in renin, Ang-converting enzyme, or angiotensinogen synthesis. Treatment with the handle region peptide decreased the renal content of Ang I and II, however, and completely inhibited the development of diabetic nephropathy without affecting hyperglycemia. We propose that the nonproteolytic activation of prorenin may be a significant mechanism of diabetic nephropathy. The mechanism and substances causing nonproteolytic activation of prorenin may serve as important therapeutic targets for the prevention of diabetic organ damage.

The tissue renin-angiotensin-aldosterone system in diabetes mellitus

Diabetes is associated with an inordinate burden of cardiovascular and renal disease, which is expected to accelerate during the next few decades. The relationship between the renin-angiotensin system (RAS) and diabetic macrovascular and microvascular disease is well established. The contribution of the tissue RAS in disease pathogenesis has recently been the focus of much interest, and has prompted investigators to explore the use of high-dose RAS inhibition with monotherapy or combination therapy to provide a more complete blockade of the actions of angiotensin II, beyond lowering blood pressure. There is now evidence to support this approach to maximize cardiovascular and renal protection. Optimal dosing strategies remain uncertain, but tissue specificity does not appear to be important if the doses of angiotensin-converting enzyme-I and angiotensin-receptor binders are high enough. The purpose of this review is to highlight our current understanding of the role of the tissue RAS in the pathogenesis of diabetic end-organ damage and ongoing strategies to interfere pharmacologically.

Embryonic origin and lineage of juxtaglomerular cells

To define the embryonic origin and lineage of the juxtaglomerular (JG) cell, transplantation of embryonic kidneys between genetically marked and wild-type mice; labeling studies for renin, smooth muscle, and endothelial cells at different developmental stages; and single cell RT-PCR for renin and other cell identity markers in prevascular kidneys were performed. From embryonic kidney day 12 to day 15 (E12 to E15), renin cells did not yet express smooth muscle or endothelial markers. At E16 renin cells acquired smooth muscle but not endothelial markers, indicating that these cells are not related to the endothelial lineage, and that the smooth muscle phenotype is a later event in the differentiation of the JG cell. Prevascular genetically labeled E12 mouse kidneys transplanted into the anterior chamber of the eye or under the kidney capsule of adult mice demonstrated that renin cell progenitors originating within the metanephric blastema differentiated in situ to JG cells. We conclude that JG cells originate from the metanephric mesenchyme rather than from an extrarenal source. We propose that renin cells are less differentiated than (and have the capability to give rise to) smooth muscle cells of the renal arterioles.

Early diabetes mellitus stimulates proximal tubule renin mRNA expression in the rat

BACKGROUND

Enhanced intrarenal angiotensin II (Ang II) activity may contribute to diabetic nephropathy. The proximal tubule is a proposed site of significant intrarenal Ang II production. We determined the effect of early diabetes on mRNA expression of components of the proximal tubule renin-angiotensin system.

METHODS

Three groups of male Sprague-Dawley rats were studied after two weeks: (1) control (C), (2) streptozotocin-induced diabetes (STZ), and (3) STZ-induced diabetes, with normoglycemia maintained by insulin implants (STZ-I). Competitive reverse transcription-polymerase chain reaction was used to assay mRNA for renin, angiotensinogen, and angiotensin-converting enzyme in suspensions of proximal tubules; plasma and kidney levels of Ang II were measured by radioimmunoassay, and Western analysis of Ang II subtype 1 (AT1) receptors was performed.

RESULTS

STZ rats tended to have increased plasma and intrarenal levels of Ang II compared with C and STZ-I rats. In proximal tubules, mRNA for renin was significantly increased in STZ rats, with reversal to control values in STZ-I rats (C, 2432 +/- 437 vs. STZ, 5688 +/- 890 fg/0.25 microg RNA, P < 0.05 vs. C, N = 9, vs. STZ-I, 1676 +/- 376 fg/0.25 microg RNA, P = NS vs. C). In STZ rats, the AT1 receptor antagonist losartan caused a further fivefold increase in proximal tubule renin mRNA, associated with proximal tubular renin immunostaining. STZ had no significant effect on mRNA expression for angiotensinogen or angiotensin-converting enzyme in proximal tubules. By Western blot analysis, cortical and proximal tubule AT1 receptor protein expression was significantly decreased in STZ rats.

CONCLUSIONS

These data suggest activation of the proximal tubule renin-angiotensin system in early STZ diabetes, mediated at least partly by enhanced expression of renin mRNA. Increased local production of Ang II could contribute to tubulointerstitial injury in this model.

Angiotensin II and its receptors in the diabetic kidney

Studies using either angiotensin-converting enzyme inhibitors or type 1 (AT(1)) angiotensin II (ANG II)-receptor blockers indicate that ANG II is a mediator of progressive injury in diabetic nephropathy. However, suppression of the systemic renin-angiotensin system (RAS) generally has been shown in diabetes mellitus. Evidence suggests that intrarenal RASs within glomeruli and proximal tubules may be activated with hyperglycemia, leading to stimulation of local ANG II production, which may exert feedback inhibition of systemic renin release. Once formed, intrarenal ANG II exerts most of its well-described effects through binding to AT(1) receptors that are abundantly present in cells of the glomeruli, tubules, vasculature, and interstitium. Thus, AT(1)-receptor activation increases vascular resistance, reduces renal blood flow, and stimulates production of extracellular matrix in the mesangium and tubulointerstitium. Recent studies suggest that the adult kidney also expresses type 2 (AT(2)) ANG II receptors in glomeruli, tubular segments, and vasculature. AT(2)-receptor activation is associated with increased intrarenal nitric oxide production, stimulation of natriuresis, and inhibition of cell growth and matrix synthesis, effects that oppose those of kidney AT(1) receptors. A number of studies have shown a reduction in kidney AT(1)-receptor expression in diabetic nephropathy, suggesting that the balance between AT(1)- and AT(2)-receptor-mediated cell-signaling events may be a determinant of progression rate in diabetic nephropathy and that unopposed stimulation of AT(2) receptors by ANG II with use of AT(1)-receptor blockers may contribute to the beneficial properties of these agents. Determination of the expression pattern of AT(2) receptors in diabetes and further definition of the role of AT(2) receptors in opposing the detrimental effects of AT(1) receptors may lead to more selective targeting of the RAS in diabetic nephropathy.

The paradox of the low-renin state in diabetic nephropathy

Although diabetic nephropathy is often a low renin state, the renin system appears to be implicated in its pathogenesis. In this study, it was hypothesized that the low plasma renin activity (PRA) is misleading, masking and perhaps reflecting an activated intrarenal renin system. PRA and renal vascular responses (inulin and para-aminohippurate clearance) to graded doses of an angiotensin II (AngII) antagonist, irbesartan, were assessed in eight healthy volunteers and 12 patients with type 2 diabetes mellitus and nephropathy on a 10 mmol Na intake, to activate the renin system. Basal PRA was suppressed in type 2 diabetes mellitus compared with the healthy subjects (0.58 +/- 0.14 versus 1.58 +/- 0.28 ng/L per s, mean +/- SEM; P < 0.01). Despite the low PRA, renal perfusion rose more in response to irbesartan in type 2 diabetes mellitus (714 +/- 83 to 931 +/- 116 ml/min; P = 0.002) than normal (624 +/- 29 to 772 +/- 49 ml/min; P = 0.008). The youngest patients were hyperfiltrating and showed the largest rise in renal plasma flow in response to irbesartan, whereas renal plasma flow rose less and GFR fell in patients with low basal GFR. PRA rose in response to irbesartan more gradually in the patients with type 2 diabetes mellitus, but ultimately matched the normal response. To account for the apparent paradox of a heightened renal hemodynamic response to an AngII antagonist in the face of a low PRA in type 2 diabetes mellitus, and the rise in PRA following the AngII antagonist, it is proposed that there is increased intrarenal AngII production in type 2 diabetes mellitus. This increase could account for suppressed circulating renin, the exaggerated renal vasodilator response to irbesartan, and the therapeutic effectiveness of interrupting the renin system in diabetic nephropathy.

Role of angiotensin-converting enzyme inhibition in glucose metabolism and renal injury in diabetes

The role of angiotensin-converting enzyme (ACE) inhibition in glucose metabolism and renal injury in diabetes has been extensively investigated in diabetic humans, as well as in animal models of diabetes. Accumulated data indicate that ACE inhibitors have either no adverse effect on glucose control or insulin sensitivity or may even improve them. ACE inhibitors also appear to have neutral or positive effects on lipid metabolism. The variability of results between studies may relate to differences in experimental design, the degree of glycemia or insulin resistance, potassium balance, and dose or duration of ACE inhibitor treatment, among others. In contrast, ACE inhibitors have proved effective in limiting proteinuria and retarding renal function loss in insulin-dependent diabetes mellitus (IDDM) or non-insulin-dependent diabetes mellitus (NIDDM) patients. In rats with experimental or spontaneous diabetes, ACE inhibitors also reduce proteinuria and limit glomerular as well as tubulointerstitial damage, independent of their effects on systemic arterial pressure. How ACE inhibitors limit renal injury in diabetes is not entirely clear, but hemodynamic and nonhemodynamic mechanisms may be involved. Increasing evidence suggests that the intrarenal renin-angiotensin system (RAS) may be altered or activated in the diabetic kidney. Such activation may be specifically inhibited by ACE inhibitors and may explain the superiority of this class of agents over other antihypertensive agents in reducing proteinuria and slowing the progression of diabetic nephropathy.

Effect of glucose on the expression of the angiotensinogen gene in opossum kidney cells

To investigate whether D(+)-glucose has a stimulatory effect on the expression of the angiotensinogen (Ang) gene in opossum kidney (OK) cells, we used OK cells with a fusion gene containing various lengths of the 5'-flanking regulatory sequence of the rat Ang gene fused with the human growth hormone (hGH) gene as a reporter, stably integrated into their genomes. The level of expression of the fusion gene was quantified by the amount of immunoreactive-human growth hormone (IR-hGH) secreted into the medium. The addition of D(+)-glucose stimulated the expression of pOGH (Ang N-1498/+18) in OK 27 cells in a dose-dependent manner (5 to 25 mM), whereas the addition of D-mannitol, L-glucose and 2-deoxy-D-glucose (25 mM) had no effect. The stimulatory effect of D(+)-glucose (25 mM) was blocked by the presence of staurosporine or H7 (an inhibitor of protein kinase C) or U73122 (an inhibitor of phospholipase C and A2) but not blocked by the presence of Rp-cAMP (an inhibitor of cAMP-dependent protein kinase A). The addition of D(+)-glucose (25 mM) also stimulated the expression of pOGH (Ang N-960/+18) and pOGH (Ang N-688/+18) in OK 960 and OK 688 cells, respectively. It had no stimulatory effect, however, on the expression of pOGH (Ang N-280/+18) and pOGH (Ang N-35/+18) in OK 280 and OK 35 cells, respectively. The addition of D(+)-glucose also had no effect on the expression of pTKGH in OK 13 cells, an OK cell line, into which had been stably integrated a fusion gene, pTKGH containing the promoter/enhancer DNA sequence of the viral thymidine-kinase (TK) gene fused with a human growth hormone gene as a reporter. These studies demonstrate that the stimulatory effect of high D(+)-glucose concentration (25 mM) on the expression of the angiotensinogen-growth hormone fusion genes in OK cells is mediated via the 5'-flanking region of the angiotensinogen gene and the protein kinase C signal transduction pathway. Our data indicate that a high glucose concentration may activate the renin-angiotensin system in the renal proximal tubular cells.

Hepatic and extrahepatic angiotensinogen gene expression in rats with acute nephrotic syndrome

Plasma concentration and urine excretion of the renin-angiotensin system proteins are altered in rats with nephrotic syndrome (NS). In this work the messenger ribonucleic acid (mRNA) levels of angiotensinogen (Ao) were analyzed with the slot-blot hybridization technique in liver and other extrahepatic tissues: kidney, heart, brain, and adrenal gland from control, nephrotic, and pair-fed (PF) rats. NS was induced by a single injection of puromycin amino-nucleoside (PAN). Although a great urinary excretion and half-normal plasma levels of Ao were observed on day 6 after PAN injection, when NS was clearly established, hepatic Ao mRNA levels did not change. Furthermore, the Ao mRNA levels did not change in any of the extrahepatic tissues studied on day 6, nor did its hepatic levels at days 1, 3, 5, or 7 after PAN injection. These data suggest that the hepatic and extrahepatic Ao mRNA levels are unaltered during the development of the acute NS induced by PAN.

The role of angiotensin II in diabetic nephropathy: emphasis on nonhemodynamic mechanisms

Several systemic or intrarenal networks of cytokines and growth factors can be modulated by the diabetic state. We summarize the status of the renin-angiotensin system in diabetes mellitus and review the evidence of its involvement in the pathogenesis of diabetic nephropathy. Particular emphasis is placed on the nonhemodynamic properties of this vasoactive agent as both a renal growth factor and a profibrogenic peptide. Antagonizing the effects of angiotensin II with converting enzyme inhibitors is an established protective strategy in the management of diabetic nephropathy even in the absence of systemic hypertension. This and other indirect evidence from experimental animal studies suggest that the intrarenal concentration of angiotensin II may be increased as a result of increased synthesis and despite enhanced breakdown, that this peptide participates in the progression of diabetic nephropathy. However, down-regulation of angiotensin type 1 (AT1)-receptors is one of the abnormalities of both tubules and glomeruli in diabetic renal disease. A heightened bioactivation of the intrarenal angiotensin II system is therefore likely but not certain. Studies in cultured proximal tubular and glomerular mesangial cells have disclosed striking similarities between the effects of high glucose-containing medium and of treatment with angiotensin II on the growth properties and the induction of cytokines in these cells. There may also exist additive effects of angiotensin II and high glucose on signal-transduction pathways, such as activation of protein kinase C, although the contractile response to angiotensin II may be blunted by high glucose in mesangial cells. An important downstream mediator of the effects of both angiotensin II and high glucose is the activation of transforming growth factor-beta that can mediate at least some of the hypertrophic and profibrotic effects of either angiotensin II or high glucose in the diabetic kidney.

Angiotensinase A gene expression and enzyme activity in isolated glomeruli of diabetic rats

One of the characteristics of early diabetic nephropathy is glomerular hyperfiltration and hyperperfusion. Many factors have been suggested to induce glomerular hyperperfusion among which are an increased production of vasodilatory prostanoids, an increased synthesis of nitric oxide, a reduced responsiveness of afferent glomerular arterioles to vasoconstrictor stimuli due to diabetic metabolic disturbances and a decreased receptor density for angiotensin II. It has been known for years that angiotensin II is formed locally due to the local activation of the renin angiotensin system. The local angiotensin II concentration, however, is not only regulated by the synthesis rate but also by the local degradation through activation of an aminopeptidase. The main finding of the present study was that the mRNA expression and activity of the angiotensin II degrading enzyme, angiotensinase A, was increased twofold in diabetic rats at 5 weeks and that the increase in mRNA expression was suppressed by insulin therapy and short-term treatment with the angiotensin II antagonist saralasin, whereas angiotensinase A enzyme activity was only reduced by saralasin and not by insulin. These results demonstrate that the angiotensin II degrading exopeptidase angiotensinase A is activated in diabetic glomeruli. This increased activity may be an additional mechanism to explain glomerular hyperfiltration and hyperperfusion in early diabetic nephropathy.

Developmental consequences of the renin-angiotensin system

Molecular, cellular, and physiological studies indicate that the renin-angiotensin system (RAS) is highly expressed during early kidney development. We propose that a major function of the RAS during early embryonic development is the modulation of growth processes that lead the primitive kidney into a properly differentiated and architecturally organized organ suited for independent extrauterine life. As development progresses, the RAS acquires new and overlapping functions such as the endocrine and paracrine regulation of blood pressure and renal hemodynamics. Disease states in adult mammals often result in expression of RAS genes and phenotypic changes resembling the embryonic pattern, emphasizing the importance of undertaking developmental studies. Because of their importance in health and disease, the immediate challenge is to identify the mechanisms that regulate the unique development of the RAS and its role(s) in normal and abnormal growth processes.

Ontogeny of renin and AT1 receptor in the rat

The enzyme renin and the angiotensin II (Ang II), subtype I receptor (ATI) are developmentally regulated in a tissue-specific manner. In early life, renin is expressed widely along the renal vasculature. As maturation progresses, there is a decrease in renin mRNA levels and a shift in the localization of renin close to the glomerulus. In addition, in the newborn rat, the number of renin-secreting cells is higher than in the adult rat. Exposure of neonatal and adult cells to Ang II results in a decrease of similar magnitude in the number of renin-secreting cells. These findings suggest that the high levels of renin observed in immature animals are due to increased renin synthesis and release rather than to a blunted response to Ang II. Expression of the ATI gene is also developmentally regulated in a tissue-specific manner. With maturation, ATI mRNA levels decrease in the kidney while they increase in the liver. The localization of ATI transcripts in precursor cells of the nephrogenic cortex suggests a role for this receptor in nephron growth and development. Inhibition of ATI with DUP753 results in delayed kidney and somatic growth and in increased renin mRNA levels and recruitment of renin-containing cells. These observations suggest that Ang II exerts a tonic negative feedback on renin gene expression via the ATI receptor subtype. Further studies are necessary to delineate the molecular and cellular signals mediating these developmental changes.