Mechanical strain- and high glucose-induced alterations in mesangial cell collagen metabolism: role of TGF-beta

Pathophysiology of mesangial expansion in diabetic nephropathy: mesangial structure, glomerular biomechanics, and biochemical signaling and regulation

Diabetic nephropathy, a kidney complication arising from diabetes, is the leading cause of death in diabetic patients. Unabated, the growing epidemic of diabetes is increasing instances of diabetic nephropathy. Although the main causes of diabetic nephropathy have been determined, the mechanisms of their combined effects on cellular and tissue function are not fully established. One of many damages of diabetic nephropathy is the development of fibrosis within the kidneys, termed mesangial expansion. Mesangial expansion is an important structural lesion that is characterized by the aberrant proliferation of mesangial cells and excess production of matrix proteins. Mesangial expansion is involved in the progression of kidney failure in diabetic nephropathy, yet its causes and mechanism of impact on kidney function are not well defined. Here, we review the literature on the causes of mesangial expansion and its impacts on cell and tissue function. We highlight the gaps that still remain and the potential areas where bioengineering studies can bring insight to mesangial expansion in diabetic nephropathy.

Role of G protein-coupled estrogen receptor 1 in modulating transforming growth factor-β stimulated mesangial cell extracellular matrix synthesis and migration

Estrogen has been demonstrated to exert beneficial effects on kidney; however, the role of G protein-coupled estrogen receptor 1 (GPER) is still uncertain. In the present study, we investigated the effect of 17β-estradiol and GPER agonist Fulvestrant on extracellular matrix production under transforming growth factor-β1 (TGF-β1) stimulation in human and rat mesangial cells. As a result, 17β-estradiol and Fulvestrant inhibit TGF-β1-induced type IV collagen and fibronectin expression in a dose-dependent manner, by suppressing acute Smad2/3 phosphorylation and Smad4 complex formation. Furthermore, estrogen and Fulvestrant also down-regulate Smad signaling by promoting ubiquitin/proteasome-dependent Smad2 degradation. These effects could be abrogated by receptor antagonist G-15 or GPER gene knockdown. GPER is also required for estrogen and Fulvestrant to regulate mesangial cell migration in response to TGF-β1. To conclude, GPER is crucial in modulating glomerular mesangial cell function including extracellular matrix production and migration.

Dehydroepiandrosterone induces ovarian and uterine hyperfibrosis in female rats

STUDY QUESTION

Do dehydroepiandrosterone (DHEA)-treated rats with polycystic ovary syndrome (PCOS) demonstrate a high level of fibrosis in ovarian and uterine tissues?

SUMMARY ANSWER

DHEA induces ovarian and uterine hyperfibrosis in rats, probably involving a transforming growth factor-β (TGF-β)-dependent mechanism.

WHAT IS KNOWN ALREADY

Chronic inflammation is the typical cause of fibrosis and is involved in the pathophysiological process of PCOS. Patients with PCOS are reported to have a higher serum level of TGF-β, a well-characterized key pro-fibrotic factor. Fibrillin-3, a protein capable of interacting with TGF-β, has been reported to be partially responsible for the fetal origin of PCOS.

STUDY DESIGN, SIZE, DURATION

Female Sprague-Dawley rats were treated with a vehicle control or DHEA for 35 days, with subsequent analyses of changes in morphology and gene expression in ovarian and uterine tissues. Rescue groups treated with metformin or simvastatin and their corresponding controls were also analyzed. A total of 80 rats were included.

PARTICIPANTS/MATERIALS, SETTING, METHODS

The PCOS model was induced by daily administration of DHEA s.c. to 3-week-old female rats, and the rescue groups were injected daily with either metformin or simvastatin in addition to DHEA. Serum steroid hormone levels were measured by enzyme-linked immunosorbent assay. Samples were stained with hematoxylin and eosin for histological morphology, and Sirius Red and immunohistochemistry for revealing collagens. The expression of fibrosis-related genes was analyzed both at mRNA (real-time RT-PCR) and protein (western blot) levels.

MAIN RESULTS AND THE ROLE OF CHANCE

DHEA-induced rats with PCOS exhibited significantly higher levels of fibrosis (collagen IV) in both ovarian and uterine tissues. In ovarian tissue, the expression of connective tissue growth factor (CTGF) increased following DHEA treatment at both mRNA and protein levels (P < 0.05, P < 0.001 versus controls, respectively). Similar results versus controls were obtained at a protein level for TGF-β (P < 0.01) and mRNA level for fibronectin (P < 0.05) and angiotensin-II (P < 0.05). Likewise, in uterine tissue, the protein levels of both CTGF and TGF-β were higher than controls following DHEA treatment (P < 0.05). Treatment with either metformin or simvastatin attenuated the fibrosis progression induced by DHEA exposure, as evidenced by a reduction of TGF-β, plus CTGF or not, in both ovarian and uterine tissues.

LIMITATIONS, REASONS FOR CAUTION

The particular mechanism involved in the DHEA-induced fibrosis was not fully revealed.

WIDER IMPLICATIONS OF THE FINDINGS

Ovarian and uterine hyperfibrosis may occur in patients with PCOS and result in anovulation or other PCOS-related phenotypes. Anti-fibrotic therapy, for example metformin treatment, may be beneficial for patients with PCOS.

STUDY FUNDING/COMPETING INTEREST(S)

This study was supported by the National Natural Science Foundation of China (81170541) and the Natural Basic Research Program of China (973 program 2010CB945103). The authors declare no conflicts of interest.

TGFβ receptor I transactivation mediates stretch-induced Pak1 activation and CTGF upregulation in mesangial cells

Increased intraglomerular pressure is an important pathogenic determinant of kidney fibrosis in the progression of chronic kidney disease, and can be modeled by exposing glomerular mesangial cells (MC) to mechanical stretch. MC produce extracellular matrix and profibrotic cytokines, including connective tissue growth factor (CTGF) when stretched. We show that p21-activated kinase 1 (Pak1) is activated by stretch in MC in culture and in vivo in a process marked by elevated intraglomerular pressures. Its activation is essential for CTGF upregulation. Rac1 is an upstream regulator of Pak1 activation. Stretch induces transactivation of the type I transforming growth factor β1 receptor (TβRI) independently of ligand binding. TβRI transactivation is required not only for Rac1/Pak1 activation, but also for activation of the canonical TGFβ signaling intermediate Smad3. We show that Smad3 activation is an essential requirement for CTGF upregulation in MC under mechanical stress. Pak1 regulates Smad3 C-terminal phosphorylation and transcriptional activation. However, a second signaling pathway, that of RhoA/Rho-kinase and downstream Erk activation, is also required for stretch-induced CTGF upregulation in MC. Importantly, this is also regulated by Pak1. Thus, Pak1 serves as a novel central mediator in the stretch-induced upregulation of CTGF in MC.

Elevated transcriptional co-activator p102 mediates angiotensin II type 1 receptor up-regulation and extracellular matrix overproduction in the high glucose-treated rat glomerular mesangial cells and isolated glomeruli

P102 is a multifunctional transcriptional co-activator. This experiment is designed to investigate the role of p102 in the activation of renin-angiotensin system (RAS) and sequentially extracellular matrix (ECM) over synthesis in diabetic nephropathy. Rat glomerular mesangial cells (MCs) or isolated glomeruli were cultured in normal glucose (NG, 5.5mM) or high glucose (HG, 25 mM) DMEM. The generation of reactive oxygen species was measured by 2',7'-dichlorodihydrofluorescein diacetate (DCFH-DA) fluorescent probe assay. The protein levels were analyzed by Western blot and the mRNA levels were evaluated by real-time PCR. HG treatment induced an increase in reactive oxygen species production. Culturing the cells in HG for 48 h, p102 mRNA and protein, angiotensin II type 1 receptor (AT1 receptor) mRNA, transforming growth factor-β1 (TGF-β1) and fibronectin proteins were significantly increased. NADPH oxidase inhibitor DPI blocked the HG-induced p102, TGF-β1 and fibronetcin elevations. Knockdown on p102 expression by siRNA depressed the HG-induced AT1 receptor up-regulation as well as the increases in TGF-β1 and fibronectin. In contrast, AT1 receptor antagonist candesartan did not influence p102 levels under either NG or HG condition, but blocked the HG-induced TGF-β1 and fibronectin increases. The results from isolated glomeruli were consistent with that of MCs, which showed that HG exposure stimulated the expression of p102. These results suggest that the overproduction of reactive oxygen species at the early stage of HG incubation stimulates p102 synthesis, which in turn up-regulates AT1 receptor expression. The activation of RAS stimulates TGF-β1 and fibronectin production, which further results in ECM accumulation.

Glibenclamide induces collagen IV catabolism in high glucose-stimulated mesangial cells

We have shown the full prevention of mesangial expansion in insulin-deficient diabetic rats by treatment with clinically-relevant dosages of glibenclamide (Glib). Studies in mesangial cells (MCs) also demonstrated reduction in the high glucose (HG)-induced accumulation of collagens, proposing that this was due to increased catabolism. In the present study, we investigated the signaling pathways that may be implicated in Glib action. Rat primary MCs were exposed to HG for 8 weeks with or without Glib in therapeutic (0.01 μM) or supratherapeutic (1.0 μM) concentrations. We found that HG increased collagen IV protein accumulation and PAI-1 mRNA and protein expression, in association with decreased cAMP generating capacity and decreased PKA activity. Low Glib increased collagen IV mRNA but fully prevented collagen IV protein accumulation and PAI-1 overexpression while enhancing cAMP formation and PKA activity. MMP2 mRNA, protein expression and gelatinolytic activity were also enhanced. High Glib was, overall, ineffective. In conclusion, low dosage/concentration Glib prevents HG-induced collagen accumulation in MC by enhancing collagen catabolism in a cAMP-PKA-mediated PAI-1 inhibition.

Uric acid as a mediator of diabetic nephropathy

Despite advances in the management of patients with diabetes, diabetic nephropathy (DN) remains the most common cause of end-stage renal disease in the United States and worldwide. Inflammation and endothelial dysfunction appear to play a central role in the onset and the progression of DN. Recent evidence has emerged in the past decade to suggest uric acid is an inflammatory factor and may play a role in endothelial dysfunction. This has lead our group and others to explore the role of uric acid in the onset and progression of DN. In this review, we highlight some of the animal and human studies that implicate uric acid in DN. Based on the evidence we review, we conclude the need for properly planned randomized controlled studies to decrease uric acid levels and assess the impact of such therapy on diabetic kidney disease.

CCN3/CCN2 regulation and the fibrosis of diabetic renal disease

Prior work in the CCN field, including our own, suggested to us that there might be co-regulatory activity and function as part of the actions of this family of cysteine rich cytokines. CCN2 is now regarded as a major pro-fibrotic molecule acting both down-stream and independent of TGF-beta1, and appears causal in the disease afflicting multiple organs. Since diabetic renal fibrosis is a common complication of diabetes, and a major cause of end stage renal disease (ESRD), we examined the possibility that CCN3 (NOV), might act as an endogenous negative regulator of CCN2 with the capacity to limit the overproduction of extracellular matrix (ECM), and thus prevent, or ameliorate fibrosis. We demonstrate, using an in vitro model of diabetic renal fibrosis, that both exogenous treatment with CCN3 and transfection with the over-expression of the CCN3 gene in mesangial cells markedly down-regulates CCN2 activity and blocks ECM over-accumulation stimulated by TGF-beta1. Conversely, TGF-beta1 treatment reduces endogenous CCN3 expression and increases CCN2 activity and matrix accumulation, indicating an important, novel yin/yang effect. Using the db/db mouse model of diabetic nephropathy, we confirm the expression of CCN3 in the kidney, with temporal localization that supports these in vitro findings. In summary, the results corroborate our hypothesis that one function of CCN3 is to regulate CCN2 activity and at the concentrations and conditions used down-regulates the effects of TGF-beta1, acting to limit ECM turnover and fibrosis in vivo. The findings suggest opportunities for novel endogenous-based therapy either by the administration, or the upregulation of CCN3.

CCN3 (NOV) is a negative regulator of CCN2 (CTGF) and a novel endogenous inhibitor of the fibrotic pathway in an in vitro model of renal disease

Fibrosis is a major cause of end-stage renal disease, and although initiation factors have been elucidated, uncertainty concerning the downstream pathways has hampered the development of anti-fibrotic therapies. CCN2 (CTGF) functions downstream of transforming growth factor (TGF)-beta, driving increased extracellular matrix (ECM) accumulation and fibrosis. We examined the possibility that CCN3 (NOV), another CCN family member with reported biological activities that differ from CCN2, might act as an endogenous negative regulator of ECM and fibrosis. We show that cultured rat mesangial cells express CCN3 mRNA and protein, and that TGF-beta treatment reduced CCN3 expression levels while increasing CCN2 and collagen type I activities. Conversely, either the addition of CCN3 or CCN3 overexpression produced a marked down-regulation of CCN2 followed by virtual blockade of both collagen type I transcription and its accumulation. This finding occurred in both growth-arrested and CCN3-transfected cells under normal growth conditions after TGF-beta treatment. These effects were not attributable to altered cellular proliferation as determined by cell cycle analysis, nor were they attributable to interference of Smad signaling as shown by analysis of phosphorylated Smad3 levels. In conclusion, both CCN2 and CCN3 appear to act in a yin/yang manner to regulate ECM metabolism. CCN3, acting downstream of TGF-beta to block CCN2 and the up-regulation of ECM, may therefore serve to naturally limit fibrosis in vivo and provide opportunities for novel, endogenous-based therapeutic treatments.

Fibrosis in diabetes complications: pathogenic mechanisms and circulating and urinary markers

Diabetes mellitus is characterized by a lack of insulin causing elevated blood glucose, often with associated insulin resistance. Over time, especially in genetically susceptible individuals, such chronic hyperglycemia can cause tissue injury. One pathological response to tissue injury is the development of fibrosis, which involves predominant extracellular matrix (ECM) accumulation. The main factors that regulate ECM in diabetes are thought to be pro-sclerotic cytokines and protease/anti-protease systems. This review will examine the key markers and regulators of tissue fibrosis in diabetes and whether their levels in biological fluids may have clinical utility.

The effect of bioreactor induced vibrational stimulation on extracellular matrix production from human derived fibroblasts

To study the affect of mechanical stimuli on human laryngeal fibroblasts, we developed bioreactors capable of vibrating cell seeded substrates at frequencies and displacements comparable to measured phonation values in human subjects. In addition, we developed a means of harvesting the secreted matrix as a bulk biomaterial by removing the polymer foam using an organic solvent. Using the system human derived laryngeal fibroblasts were subjected to vibrational stimuli (100 Hz) for 1-21 days. Following mechanical conditioning, extracellular matrix and matrix related gene expression, cytokine production, matrix protein accumulation, and construct material properties were assessed with DNA microarray, enzyme linked immunosorbent, indirect immunofluorescent, and uni-axial tensile assays respectively. The results show that vocal fold-like vibrational stimuli is sufficient to influence the expression of several key matrix and matrix related genes, enhance the secretion of the profibrotic cytokine TGFbeta1, increase the accumulation of the extracellular matrix proteins, fibronectin and collagen type 1, as well as enhance construct stiffness compared to non-stimulated controls. Our results demonstrate that high frequency substrate vibration, like cyclic strain, can accelerate matrix deposition from human derived laryngeal fibroblasts. The study supports the notion that preconditioning regimens using human cells may be useful for producing cell derived biomaterials for therapeutic application.

Variation of cyclic strain parameters regulates development of elastic modulus in fibroblast/substrate constructs

Dynamic mechanical culture systems are a widely studied approach for improving the functional mechanical properties of tissue engineering constructs intended for loading-bearing orthopedic applications such as tendon/ligament reconstruction. The design of effective mechanical stimulation regimes requires a fundamental understanding of the effects of cyclic strain parameters on the resulting construct properties. Toward this end, these studies employed a modular cyclic strain bioreactor system and fibroblast-seeded, porous polyurethane substrates to systematically investigate the effect of varying cyclic strain amplitude, rate, frequency, and daily cycle number on construct mechanical properties. Significant differences were observed in response to variation of all four loading parameters tested. In general, the highest values of elastic modulus within each experimental group were observed at low to intermediate values of the experimental variables tested, corresponding to the low to subphysiological range (2.5% strain amplitude, 25%/s strain rate, 0.1-0.5 Hz frequency, and 7,200-28,800 cycles/day). These studies demonstrate that fibroblasts are sensitive and responsive to multiple characteristics of their mechanical environment, and suggest that systematic optimization of dynamic culture conditions may be useful for the acceleration of construct maturation and mechanical function.

Mast cells and inflammatory kidney disease

Inflammatory kidney disease involves a complex network of interactions between resident kidney and infiltrating hematopoietic cells. Mast cells (MCs) are constitutively found in kidneys in small numbers but increase considerably in various renal diseases. While this increase is usually interpreted as a sign of pathological involvement, recent data using MC-deficient animals show their ability to restore kidney homeostasis. In anti-glomerular basement membrane antibody-induced glomerulonephritis, MCs are protective by initiating repair and remodeling functions counteracting the devastating effects of glomerular injury. Protection may also include immunoregulatory capacities to limit autoreactive T-cell responses. MCs also control tubulointerstitial fibrosis by activating tissue remodeling and neutralizing fibrotic factors. Release of mediators by MCs during inflammation, however, could also promote unwanted responses that ultimately lead to destruction of kidney structure, as exemplified by data showing either protection or aggravation in related renal disease models. Similarly, while the action of proteases may initially be beneficial, the generation of fibrosis-promoting angiotensin II by chymase also shows the limits of adaptive responses to achieve homeostasis. Thus, it is likely the physiological context involving the interaction with other cells and inflammatory mediators that determines the final action of MCs in the development of kidney disease.

Cyclic strain increases fibroblast proliferation, matrix accumulation, and elastic modulus of fibroblast-seeded polyurethane constructs

Rapid induction of matrix production and mechanical strengthening is essential to the development of bio-artificial constructs for repair and replacement of load-bearing connective tissues. Toward this end, we describe the development of a mechanical bioreactor and its application to investigate the influence of cyclic strain on fibroblast proliferation, matrix accumulation, and the mechanical properties of fibroblast-seeded polyurethane constructs (FSPC). Human fibroblasts were cultured in 10% serum-containing conditions within three-dimensional, porous elastomeric substrates under static conditions and a model regime of cyclic strain (10% strain, 0.25 Hz, 8 h/day), with and without ascorbic acid supplementation. After one week, the combination of cyclic strain and ascorbic acid resulted in significantly increased construct elastic modulus (>110%) relative to either condition alone. In contrast, cyclic strain alone was sufficient to stimulate significant increases in fibroblast proliferation. Mechanical strengthening of FSPCs was accompanied by increased type I collagen and fibronectin matrix accumulation and distribution, and significantly increased gene expression for type I collagen, TGFbeta-1, and CTGF. These results suggest that strain-induced conditioning in vitro leads to mechanical strengthening of fibroblast/material constructs, most likely resulting from increased collagen matrix deposition, secondary to strain-induced increases in cytokine production.

Adjuvant strategies for prevention of glomerulosclerosis

The glomerulosclerosis which frequently complicates diabetes and severe hypertension is mediated primarily by increased mesangial production and activation of transforming growth factor-beta (TGF-beta), which acts on mesangial cells to boost their production of matrix proteins while suppressing extracellular proteolytic activity. Hyperglycemia and glomerular hypertension work in various complementary ways to stimulate superoxide production via NADPH oxidase in mesangial cells; the resulting oxidant stress results in the induction and activation of TFG-beta. Nitric oxide, generated by glomerular capillaries and by mesangial cells themselves, functions physiologically to oppose mesangial TGF-beta overproduction; however, NO bioactivity is compromised by oxidant stress. In addition to low-protein diets and drugs that suppress angiotensin II activity, a variety of other agents and measures may have potential for impeding the process of glomerulosclerosis. These include vitamin E, which blunts the rise in mesangial diacylglycerol levels induced by hyperglycemia; statins and (possibly) policosanol, which down-regulate NADPH oxidase activity by diminishing isoprenylation of Rac1; lipoic acid, whose potent antioxidant activity antagonizes the impact of oxidant stress on TGF-beta expression; pyridoxamine, which inhibits production of advanced glycation endproducts; arginine, high-dose folate, vitamin C, and salt restriction, which may support glomerular production of nitric oxide; and estrogen and soy isoflavones, which may induce nitric oxide synthase in glomerular capillaries while also interfering with TGF-beta signaling. Further research along these lines may enable the development of complex nutraceuticals which have important clinical utility for controlling and preventing glomerulosclerosis and renal failure. Most of these measures may likewise reduce risk for left ventricular hypertrophy in hypertensives, inasmuch as the signaling mechanisms which mediate this disorder appear similar to those involved in glomerulosclerosis.

Erythromycin ameliorates renal injury via anti-inflammatory effects in experimental diabetic rats

AIMS/HYPOTHESIS

Recent studies have shown that the inflammatory process is involved in the pathogenesis of diabetic nephropathy. Fourteen-membered ring macrolides, including erythromycin, have anti-inflammatory, as well as antibacterial effects. The aim of this study was to investigate the renoprotective effects of erythromycin in streptozotocin (STZ)-induced diabetic rats.

METHODS

STZ-induced diabetic rats were treated orally with erythromycin (5 mg/kg body weight) or vehicle every day for 8 weeks. To evaluate the effect of erythromycin treatment, we measured urinary albumin excretion, and examined the following in the kidney: histological changes, the expression of intercellular adhesion molecule-1 (ICAM-1), macrophage infiltration, and nuclear factor-kappa B (NF-kappaB) activity.

RESULTS

Erythromycin significantly reduced urinary albumin excretion without affecting blood glucose levels and blood pressure. Erythromycin also attenuated glomerular hypertrophy, mesangial expansion, macrophage infiltration and ICAM-1 expression in renal tissues. The expression of the gene encoding TGFB1 (also known as TGF-beta1), type IV collagen protein production and NF-kappaB activity in renal tissues were increased in diabetic rats and reduced by erythromycin treatment.

CONCLUSIONS/INTERPRETATION

Erythromycin prevented renal injuries without changes of blood glucose levels and blood pressure in experimental diabetic rats. These results suggest that the renoprotective effects of erythromycin are based on its anti-inflammatory effect via suppression of NF-kappaB activation. Modulation of microinflammation with erythromycin may provide a new approach for diabetic nephropathy.

Effects of sulfonylureas, α-endosulfine counterparts, on glomerulosclerosis in type 1 and type 2 models of diabetes

BACKGROUND

Previously, we showed the expression of a unique sulfonylurea receptor (SUR) and its putative endogenous ligand, alpha-endosulfine, in mesangial cells and isolated glomeruli. Further, this ligand was up-regulated by high glucose concentration. To investigate the possible role of alpha-endosulfine up-regulation in diabetes, we administered sulfonylureas, the exogenous ligands of SUR, to diabetic animals.

METHODS

In streptozotocin-induced, insulin-deficient, diabetic rats, glomerulosclerosis, albuminuria, glomerular expression of fibronectin mRNA, and glomerular filtration rate (GFR) were studied for various periods up to 36 weeks. Several rat groups received either glibenclamide or tolazamide during the entire study period. Also, glomerulosclerosis and albuminuria were determined in insulin-resistant db/db mice, at 26 weeks of treatment with tolazamide.

RESULTS

Sulfonylureas did not improve hyperglycemia or reduce glycosylated hemoglobin levels. In insulin-deficient diabetic rats, sulfonylureas significantly decreased the degree of glomerulosclerosis and completely reversed the enhanced albumin excretion. Also, glibenclamide reduced diabetes-induced glomerular overexpression of fibronectin mRNA. Because glibenclamide may improve the afferent arteriolar dilatation of diabetes, thereby reducing glomerular hyperfiltration, its effect on GFR was determined. Glibenclamide did not alter glomerular hyperfiltration or renal hypertrophy, regardless of the intensity of hyperglycemia. Finally, in insulin-resistant mice, tolazamide did not alter the extent of diabetic glomerulosclerosis or increased albuminuria.

CONCLUSION

Long-term treatment with sulfonylureas completely prevents glomerular injury in insulin-deficient diabetes in rats. However, this protective effect is not demonstrable in an insulin-resistant model of the disease. We postulate that mesangial alpha-endosulfine up-regulation in the hyperglycemic milieu of insulin-deficient diabetes may retard glomerular extracellular matrix formation and mesangial expansion.

Glomerular expression of connective tissue growth factor mRNA in various renal diseases

Connective tissue growth factor (CTGF) is a cysteine-rich member of a new family of growth regulators. It is an important factor in the pathogenesis of mesangial matrix accumulation and progressive glomerulosclerosis. The present study was designed to elucidate the role of CTGF in diabetic nephropathy (DN), immunoglobulin A nephropathy (IgA-N), membranous nephropathy (MN), and minimal change nephrotic syndrome (MCNS). We evaluated the expression and localization of CTGF mRNA in surgically excised renal tissue samples from 10 patients with DN, 10 with IgA-N, 10 with MN, 10 with MCNS, and 10 normal human kidney (NHK) tissue samples, by using high-resolution in situ hybridization with digoxigenin-labelled oligonucleotide. To quantify CTGF mRNA expression, we counted all nuclei, and nuclei surrounded by CTGF-positive cytoplasm, in at least 10 randomly selected cross-sections of non-sclerotic glomeruli, and expressed the results as a percentage of total glomerular cells. In all glomeruli, CTGF mRNA was expressed mainly in glomerular intrinsic cells, including glomerular mesangial and epithelial cells and some cells of Bowman's capsule. The percentage of cells positive for CTGF mRNA was significantly higher in DN and IgA-N than in MN, MCNS and NHK. However, there was no significant difference in the percentage of CTGF mRNA-positive cells between DN and IgA-N. Our study indicates that CTGF may play an important role in the development and progression of glomerulosclerosis in DN and IgA-N, which are both accompanied by mesangial matrix expansion and comprise two major causes of end-stage renal failure.

Long-chain polyunsaturated fatty acids interact with nitric oxide, superoxide anion, and transforming growth factor-beta to prevent human essential hypertension

Patients with uncontrolled essential hypertension have elevated concentrations of superoxide anion (O(2)(-*)), hydrogen peroxide (H(2)O(2)), lipid peroxides, endothelin, and transforming growth factor-beta (TGF-beta) with a simultaneous decrease in endothelial nitric oxide (eNO), superoxide dismutase (SOD), vitamin E, and long-chain polyunsaturated fatty acids (LCPUFAs). Physiological concentrations of angiotensin II activate NAD(P)H oxidase and trigger free radical generation (especially that of O(2)(-*)). Normally, angiotensin II-induced oxidative stress is abrogated by adequate production and release of eNO, which quenches O(2)(-*) to restore normotension. Angiotensin II also stimulates the production of endothelin and TGF-beta. TGF-beta enhances NO generation, which in turn suppresses TGF-beta production. Thus, NO has a regulatory role on TGF-beta production and is also a physiological antagonist of endothelin. Antihypertensive drugs suppress the production of O(2)(-*) and TGF-beta and enhance eNO synthesis to bring about their beneficial actions. LCPUFAs suppress angiotensin-converting enzyme (ACE) activity, reduce angiotensin II formation, enhance eNO generation, and suppress TGF-beta expression. Perinatal supplementation of LCPUFAs decreases insulin resistance and prevents the development of hypertension in adult life, whereas deficiency of LCPUFAs in the perinatal period results in raised blood pressure later in life. Patients with essential hypertension have low concentrations of various LCPUFAs in their plasma phospholipid fraction. Based on this, it is proposed that LCPUFAs serve as endogenous regulators of ACE activity, O(2)(-*), eNO generation, and TGF-beta expression. Further, LCPUFAs have actions similar to statins, inhibit (especially omega-3 fatty acids) cyclooxygenase activity and suppress the synthesis of proinflammatory cytokines, and activate the parasympathetic nervous system, all actions that reduce the risk of major vascular events. Hence, it is proposed that availability of adequate amounts of LCPUFAs during the critical periods of growth prevents the development of hypertension in adulthood.

Activation of a local tissue angiotensin system in podocytes by mechanical strain11See Editorial by Kriz, p. 333

BACKGROUND

Glomerular capillary hypertension, a common denominator in various forms of progressive glomerular disease, results in mechanical distention of the capillary tuft, and subsequent injury of the overlying podocyte layer. The mechanisms by which elevated intraglomerular pressure is translated into a maladaptive podocyte response remain poorly understood. Angiotensin II plays a central role in the pathogenesis of chronic renal injury, largely through its actions on the subtype 1 receptor. Accordingly, we have tested the hypothesis that mechanical strain up-regulates local angiotensin II in podocytes, thereby resulting in a progressive reduction in podocyte number.

METHODS

Conditionally immortalized mouse podocytes were subjected to cyclical stretch of 10% amplitude. Nonstretched podocytes served as controls. Angiotensin II levels were measured in whole cell lysate by competitive enzyme-linked immunosorbent assay (ELISA). Expression of angiotensin II receptors (AT1R, AT2R) was measured by quantitative polymerase chain reaction (PCR) and Western blot analysis. Apoptosis was measured by Hoechst staining. Immunostaining for AT1R was performed in tissue sections from rats with 5/6 remnant kidney disease, a model of glomerular hypertension.

RESULTS

Mechanical strain increased angiotensin II production in podocytes at 24, 48, and 72 hours (P < 0.05 vs. nonstretched controls). Stretching podocytes resulted in a fivefold increase in AT1R mRNA expression at 24 hours and a twofold increase in protein levels vs. controls (P < 0.05), and also an increase in transforming growth hormone-beta (TGF-beta) mRNA expression. AT1R staining was increased in a podocyte distribution in the 5/6 remnant kidney, consistent with our in vitro findings. Mechanical strain resulted in a 2.5-fold increase in apoptosis (P < 0.001 vs. nonstretched controls) in an angiotensin II-dependent fashion.

CONCLUSION

Mechanical strain leads to up-regulation of the AT1R and increased angiotensin II production in conditionally immortalized podocytes. The resulting activation of a local tissue angiotensin system leads to an increase in podocyte apoptosis, mainly in an AT1R-mediated fashion.

Acousto-optical Characterization of the Viscoelastic Nature of a Nuchal Elastin Tissue Scaffold

A nondestructive, acousto-optical method for characterizing the mechanical loss factor of biological tissues and tissue scaffolds is presented and applied to the characterization of an elastin tissue scaffold derived from bovine nuchal ligament. The method relies on launching guided surface acoustic waves into the tissue scaffold with a small speaker and simultaneously illuminating a small region of the scaffold distant from the speaker with a low-power HeNe laser. The phase lag between the driving acoustic wave and the shift in the backscattered laser speckle pattern is determined as a measure of the mechanical loss factor of the scaffold, tan delta. Measurements of tan delta and elastic modulus were also made by traditional dynamic mechanical loading techniques. Through the central portion of the loading cycle, the elastic modulus of the elastin scaffold was 1.2 x 10(6) +/- 1 x 10(5) N x m(-2) (parallel to fiber orientation). The estimated value of tan delta in the direction parallel to the elastin fibers was 0.03 +/- 0.017 by traditional methods and 0.029 +/- 0.03 when using the acousto-optical method. In the direction perpendicular to fiber orientation, tan delta was measured as 0.14 +/- 0.056 by the acousto-optical method. Because of a lack of mechanical integrity, it was not possible to measure tan delta in the direction perpendicular to fiber orientation by traditional methods. The acousto-optical method may prove to be useful in the mechanical characterization of developing engineered tissues.

Urinary CCN2 (CTGF) as a possible predictor of diabetic nephropathy: preliminary report

BACKGROUND

It is currently impossible to reliably predict which diabetic patients will develop nephropathy and progress to kidney failure. Microalbuminuria, often regarded as a predictor of overt diabetic renal disease is, in fact, an indicator of established glomerular damage. We have shown that glomerular expression of the prosclerotic cytokine CCN2 (CTGF) is greatly up-regulated early in experimental and in human diabetes and mesangial cell exposure to CCN2 increases its production of extracellular matrix (ECM) molecules responsible for glomerulosclerosis. As an early marker, we therefore investigated the presence of CCN2 in urine and the relationship to diabetes and/or renal disease in an experimental model of diabetes and in a limited patient population.

METHODS

Urine samples from (1) healthy rats, (2) rats made diabetic by streptozotocin (STZ), (3) healthy human volunteers, (4) diabetic patients with renal disease, and (5) diabetic patients without renal disease were examined by Western blotting and/or enzyme-linked immunosorbent assay (ELISA) for qualitative and quantitative analysis of the of CCN2.

RESULTS

Low levels of urinary CCN2 were present in healthy, control rats, but were increased approximately sevenfold overall in STZ-diabetic animals. CCN2 levels were the highest at week 3 of diabetes, then decreased with time, but remained significantly elevated over controls even after 32 weeks. Consistently low levels of urinary CCN2 were also detected in healthy volunteers (mean value, 7.1 CCN2/mg creatinine). However, levels were elevated approximately sixfold in the majority of diabetic patients with nephropathy. A small number of the diabetic patients not yet exhibiting evidence of renal involvement demonstrated CCN2 urinary levels that were ninefold greater than controls. The remaining normoalbuminuric diabetic patients demonstrated CCN2 levels indistinguishable from those of healthy volunteers. Analysis by Western blotting confirmed the identity of the urinary CCN2. A molecular species equivalent to full-length CCN2 (37/39 kD doublet) was present in healthy controls. In contrast, the nephropathic group demonstrated multiple CCN2 bands.

CONCLUSION

These findings support our hypothesis that CCN2 is up-regulated early in the evolution of glomerulosclerosis, including that of diabetes. We contend that urinary CCN2 may both stage nephropathy and predict those patients who are destined for progressive glomerulosclerosis and end-stage renal disease (ESRD). Cross-sectional and prospective studies of larger, well-defined diabetic patients groups will be required to prove this hypothesis, and are ongoing.

GLUT-1 overexpression: Link between hemodynamic and metabolic factors in glomerular injury?

Mesangial matrix deposition is the hallmark of hypertensive and diabetic glomerulopathy. At similar levels of systemic hypertension, Dahl salt-sensitive but not spontaneously hypertensive rats (SHR) develop glomerular hypertension, which is accompanied by upregulation of transforming growth factor beta1 (TGF-beta1), mesangial matrix expansion, and sclerosis. GLUT-1 is ubiquitously expressed and is the predominant glucose transporter in mesangial cells. In mesangial cells in vitro, GLUT-1 overexpression increases basal glucose transport, resulting in excess fibronectin and collagen production. TGF-beta1 has been shown to upregulate GLUT-1 expression. We demonstrated that in hypertensive Dahl salt-sensitive (S) rats fed 4% NaCl (systolic blood pressure [SBP]: 236+/-9 mm Hg), but not in similarly hypertensive SHR (SBP: 230+/-10 mm Hg) or their normotensive counterparts (Dahl S fed 0.5% NaCl, SBP: 145+/-5 mm Hg; and Wistar-Kyoto, SBP: 137+/-3 mm Hg), there was an 80% upregulation of glomerular GLUT-1 protein expression (P< or =0.03). This was accompanied by a 2.7-fold upregulation of TGF-beta1 protein expression in glomeruli of DSH compared with DSN rats (P=0.02). TGF-beta1 expression was not upregulated and did not differ in the glomeruli of Wistar-Kyoto and SHR rats. As an in vitro surrogate of the in vivo hemodynamic stress imposed by glomerular hypertension, we used mechanical stretching of human and rat mesangial cells. We found that after 33 hours of stretching, mesangial cells overexpressed GLUT-1 (40%) and showed an increase in basal glucose transport of similar magnitude (both P< or =0.01), which could be blocked with an anti TGF-beta1-neutralizing antibody. These studies suggest a novel link between hemodynamic and metabolic factors that may cooperate in inducing progressive glomerular injury in conditions characterized by glomerular hypertension.

Hypertension accelerates diabetic nephropathy in Wistar fatty rats, a model of type 2 diabetes mellitus, via mitogen-activated protein kinase cascades and transforming growth factor-beta1

Although it is known that diabetic nephropathy is accelerated by hypertension, the mechanisms involved in this process are not clear. In this study we aimed to clarify these mechanisms using male Wistar fatty rats (WFR) as a type 2 diabetic model and male Wistar lean rats (WLR) as a control. Each group was fed a normal or high sodium diet from the age of 6 to 14 weeks. We determined the blood pressure and urinary albumin excretion (UAE). At the end of the study, the expressions of mitogen-activated protein kinases (MAPK) and transforming growth factor-beta1 (TGF-beta1) were examined in the isolated glomeruli by Western blot analysis, and the number of glomerular lesions was determined by conventional histology. High sodium load caused hypertension and a marked increase in UAE in the WFR but not in the WLR. Glomerular volume was increased in the hypertensive WFR. There was no difference among the four groups in the expression of c-Jun-NH2-terminal kinase (JNK). In contrast, the expressions of extracellular signal-regulated kinase 1/2 (ERK1/2) and its upstream regulator, MAPK/ERK kinase 1 (MEK1), were augmented in the hypertensive WFR. Expression of p38 MAPK was increased in the normotensive WFR, and further enhanced in the hypertensive WFR. Moreover, administration of high sodium load to WFR augmented the expression of TGF-beta1. In conclusion, systemic hypertension in WFR accelerates the diabetic nephropathy in type 2 diabetes via MEK-ERK and p38 MAPK cascades. TGF-beta1 is also involved in this mechanism.

Renal Dysfunction in HIV-1-infected Patients

Improved therapy directed against opportunistic infection and HIV-1 itself has resulted in greatly enhanced patient survival in the past decade among patients infected with HIV-1. Since patients are living longer, HIV-1 infection is associated with a rising burden of kidney disease. Approximately 14% of black patients and 6% of white patients dying with HIV-1 infection in 1999 in the United States had renal disease. Overall, 10% of patients dying with HIV-1 infection had renal failure. The most common glomerular diseases are focal segmental glomerulosclerosis and immune complex glomerulonephritis. Appropriate therapy for focal segmental glomerulosclerosis includes effective antiretroviral therapy and angiotensin antagonist medication. Drug toxicity is also common, often manifesting as electrolyte abnormalities, acute renal failure, interstitial nephritis, or nephrolithiasis. In particular, indinavir is associated with crystalluria, nephrolithiasis, interstitial nephritis, and lower urinary tract inflammation. Appropriate screening for renal disease and appropriate intervention will likely reduce the morbidity and mortality associated with progressive renal disease.

Haemodynamics in microvascular complications in type 1 diabetes

Type 1 diabetes is commonly associated with microvascular complications. Most of the microvascular blood vessels are involved but those in the kidney, retina and large nerves exhibit the more significant pathology. Haemodynamic and metabolic factors both alone and through the activation of a common pathway contribute to the characteristic dysfunction observed in diabetic vasculopathy. The haemodynamic abnormalities in type 1 diabetes are characterized by increased systemic blood pressure and altered blood flow with subsequent activation of various vasoactive factors, which can contribute to the maintenance of the haemodynamic alterations and to the development and progression of the microvascular complications. These vasoactive factors include vasoconstrictors such as angiotensin II, and endothelin, as well as vasodilators such as nitric oxide (NO). Systemic hypertension and vasoactive factors independently and in interaction with the metabolic pathway activate intracellular second messengers, nuclear transcription factors and various growth factors which lead to the typical functional and structural alterations of diabetic microvascular complications. Therapeutic strategies involved in the management and prevention of diabetic complications currently include antihypertensive agents, particularly those that interrupt the renin-angiotensin system. Further understanding of the interactions among the vasoactive factors, the intracellular second messengers and the growth factors may help to identify novel strategies to influence the action of the vasoactive factors. These novel therapies, together with specific inhibitors of the metabolic pathway or the common pathway, may provide the possibility of preventing or even reversing the progression of diabetic microvascular complications.

Mesangial cell gelatinase A synthesis is attenuated by oscillating hyperbaric pressure

Glomerular hypertension has been established as a major factor contributing to glomerular scarring. Underlying cellular mechanisms leading to matrix accumulation are largely unknown. The isolated effect of oscillating hyperbaric pressure [OP; P(max) 50 mmHg (1 mmHg=0.133 kPa), P(mean) 24 mmHg, with a fixed oscillation of 60/min] on matrix-degrading protease secretion by rat mesangial cells (MCs) was analysed using a pressure chamber model described previously [Mertens, Espenkott, Venjakob, Heintz, Handt and Sieberth (1998) Hypertension 32, 945-952]. MCs were grown under atmospheric pressure (AP) or a controlled OP, and protease synthesis and gene transcription were analysed. A distinct biphasic cellular response to OP with stimulated gelatinase A protein expression and enzyme activity during the initial 24 h, and subsequent inhibition, was apparent, as shown by gelatin zymography. Gelatinase B activity remained unchanged. The abundance of gelatinase A transcripts, determined by reverse transcriptase-PCR, indicated a concordant regulation of gene transcription. To elucidate underlying regulatory events, reporter constructs were transfected. In these experiments, a recently identified response element, RE-1, conferred a significant stimulatory effect within the initial 4 h of OP. Nuclear protein/RE-1 binding studies revealed additional complexes from 5 min up to 3 h after OP exposure, with intensities dependent on P(max). STAT3 was identified as a component of these novel complexes. Down-regulation of cis-activity after 48 h of OP exposure was not transferred via the proximal 1686 bp of the gelatinase A regulatory sequence. In conclusion, hyperbaric OP elicits time-dependent changes in rat MC gelatinase A gene transcription.

Add-on angiotensin II receptor blockade lowers urinary transforming growth factor-beta levels

Progression of renal failure, despite renoprotection with angiotensin-converting enzyme (ACE) inhibitors in patients with proteinuric nephropathies, may be caused by persistent renal production of transforming growth factor-beta1 (TGF-beta1) through the angiotensin II subtype 1 (AT1) receptors. We tested the hypothesis that AT1-receptor blocker therapy added to a background of chronic maximal ACE inhibitor therapy will result in a reduction in urinary TGF-beta1 levels in such patients. Sixteen patients completed a two-period, crossover, randomized, controlled trial, details of which have been previously reported. All patients were administered lisinopril, 40 mg/d, with either losartan, 50 mg/d, or placebo. Blood pressure (BP) was measured using a 24-hour ambulatory BP monitor. Overnight specimens of urine were analyzed for urine TGF-beta1, protein, and creatinine concentrations. Mean age of the study population was 53 +/- 9 (SD) years; body mass index, 38 +/- 5.7 kg/m2; seated BP, 156 +/- 18/88 +/- 12 mm Hg; and urine protein excretion, 3.6 +/- 0.71 g/g of creatinine. Twelve patients had diabetic nephropathy, and the remainder had chronic glomerulonephritis. At baseline, urinary TGF-beta1 levels were significantly increased in the study population compared with healthy controls (13.2 +/- 1.2 versus 1.7 +/- 1.1 ng/g creatinine; P < 0.001). There was a strong correlation between baseline urine protein excretion and urinary TGF-beta1 level (r2 = 0.53; P = 0.001), as well as systolic BP and urinary TGF-beta1 level (r2 = 0.57; P < 0.001). After 4 weeks of add-on losartan therapy, there was a 38% (95% confidence interval [CI], 16% to 55%) decline in urinary TGF-beta1 levels (13.3 [95% CI, 11.4 to 15.5] to 8.2 pg/mg creatinine [95% CI, 6.2 to 10.7]). The reduction in urinary TGF-beta1 levels occurred independent of changes in mean urinary protein excretion or BP. Thus, proteinuric patients with renal failure, despite maximal ACE inhibition, had increased urinary levels of TGF-beta1 that improved over 1 month of add-on therapy with losartan. We speculate that dual blockade with losartan and an ACE inhibitor may provide additional renoprotection by decreasing renal production of TGF-beta1.

Modulation of membrane traffic by mechanical stimuli

All cells experience and respond to mechanical stimuli, such as changes in plasma membrane tension, shear stress, hydrostatic pressure, and compression. This review is an examination of the changes in membrane traffic that occur in response to mechanical forces. The plasma membrane has an associated tension that modulates both exocytosis and endocytosis. As membrane tension increases, exocytosis is stimulated, which acts to decrease membrane tension. In contrast, increased membrane tension slows endocytosis, whereas decreased tension stimulates internalization. In most cases, secretion is stimulated by external mechanical stimuli. However, in some cells mechanical forces block secretion. External stimuli also enhance membrane and fluid endocytosis in several cell types. Transduction of mechanical stimuli into changes in exocytosis/endocytosis may involve the cytoskeleton, stretch-activated channels, integrins, phospholipases, tyrosine kinases, and cAMP.

Potentiation of glucose-mediated glomerular injury by mechanical strain

1. The glomerular injury of diabetes is characterized by the progressive accumulation of extracellular matrix in the mesangial regions, ultimately resulting in glomerulosclerosis. 2. The excessive glomerular extracellular matrix formation associated with the haemodynamic alteration of diabetes is the result of mesangial mechanical strain. 3. The increased synthesis and deposition of extracellular matrix is augmented by the presence of high glucose concentrations. 4. Both mechanical strain and high glucose share many of the mechanisms mediating their metabolic effects, including the stimulation of prosclerotic growth factors. 5. Little is known about factors that may influence the long-term effects of mechanical strain, but the preservation of the F-actin cytoskeleton is likely an important modulator of the resulting injury.

Pathogenesis of diabetic nephropathy: focus on transforming growth factor-beta and connective tissue growth factor

Although considerable improvement in the prognosis of diabetic nephropathy has been achieved in recent years due to intensive insulin and angiotensin-converting enzyme inhibitor treatment, these approaches do not provide complete protection against progression of diabetic nephropathy. An urgent need for additional novel therapies to prevent or further slow the progression of diabetic nephropathy motivated us to provide an up-to-date review with particular emphasis on the potential role of two growth factors--transforming growth factor-beta and connective tissue growth factor--in the pathogenesis of diabetic nephropathy. The most intensively studied to date, transforming growth factor-beta appears to play a central role in the pathogenesis of diabetic nephropathy. Recently, attention has focused on connective tissue growth factor, which mimics the biological activity of transforming growth factor-beta in profibrotic tissue formation. Thus, acting as a downstream mediator of the profibrotic activity of transforming growth factor-beta, connective tissue growth factor may constitute a more specific target for future antifibrotic therapies.

Renal fibrosis in diabetic and aortic-constricted hypertensive rats

To assess if the renal damage observed in rats with diabetes and hypertension is due to hemodynamic or metabolic changes, a progressive aortic constriction between the two renal arteries has been done in streptozotocin-induced diabetic rats (constriction + diabetes group) and in nondiabetic rats (constriction group). This model allows us to study two kidneys subjected to different perfusion pressure (PP) in the same metabolic environment. One-month-old rats (100-120 g body wt) were subjected to the aortic constriction procedure. Three months after constriction, glomerular filtration rate and renal plasma flow were similar in both kidneys of the two groups. PP was greater in the kidney placed over the ligature [constriction high-pressure kidney (CH) or constriction + diabetic high-pressure kidney (DH)] than in the one placed below the ligature [constriction low pressure (CL) or constriction + diabetic low pressure (DL)]. Proteinuria was higher in the CH than in the CL kidneys (512 +/- 61 vs. 361 +/- 38 microg/30 min, respectively) and much higher in the DH kidney (770 +/- 106 microg/30 min). Renal fibrosis was measured in tissue sections stained with Syrius red using a computer-assisted image analysis system. DH and DL kidneys showed higher corpuscular cross-sectional and capillary tuft areas than the CH and CL ones. The DH kidney showed slight mesangial expansion and thickening of the capillary walls, which were more pronounced in the former. Most renal corpuscles from CH and DH groups were nearly normal in morphology appearance, and only in some instances a slight increment in mesangium was observed. Transforming growth factor-beta1 (TGF-beta1) immunostaining revealed that DH kidneys showed the highest glomerular expression. We concluded that 1) diabetic animals develop glomerular but not interstitial fibrosis to a greater extent than nondiabetic animals and that this lesion principally occurs in the hypertensive kidney (DH), and 2) increased TGF-beta expression is associated with diabetic renal damage.

Connective tissue growth factor and its regulation: a new element in diabetic glomerulosclerosis

Connective tissue growth factor (CTGF), a member of the closely related CCN family of cytokines appears to be fibrotic in skin. To determine whether CTGF is implicated in diabetic glomerulosclerosis we studied cultured rat mesangial cells (MC) as well as kidney cortex and microdissected glomeruli from obese, diabetic db/db mice and their normal counterparts. Exposure of MC to rhCTGF significantly increased fibronectin and collagen type I secretion. Further, unstimulated MC expressed low levels of CTGF message and secreted minimal amounts of CTGF protein (36-38 kDa). However, exposure to TGF-beta, increased glucose concentrations, or cyclic mechanical strain, all causal factors in glomerulosclerosis, markedly induced the expression of CTGF transcripts. With all but mechanical strain there was a concomitant stimulation of CTGF protein secretion. TGF-beta also induced abundant quantities of a small molecular weight form of CTGF (18 kDa). The induction of CTGF protein by a high glucose concentration was mediated by TGF-beta, since a TGF-beta neutralizing antibody blocked this stimulation. In vivo studies using quantitative RT-PCR demonstrated that while CTGF transcripts were low in the glomeruli of control mice, expression was increased 27-fold after approximately 3.5 months of diabetes. These changes occurred early in diabetic nephropathy when mesangial expansion was mild, and interstitial disease and proteinuria were absent. A substantially reduced elevation of CTGF mRNA (2-fold) observed in whole kidney cortices indicted that the primary alteration of CTGF expression was in the glomerulus. These results suggest that CTGF upregulation is an important factor in the pathogenesis of mesangial matrix accumulation in both diabetic and non-diabetic glomerulosclerosis, acting downstream of TGF-beta.

Selenium-deficient diet induces renal oxidative stress and injury via TGF-beta1 in normal and diabetic rats

BACKGROUND

Oxidative stress has been implicated in the pathogenesis of diabetic nephropathy. Although glucose itself can initiate oxidative stress, deficiency of essential trace elements such as selenium (Se) may exacerbate this oxidative stress in diabetic rats. The mechanism by which Se deficiency causes oxidative stress and renal injury is not completely understood. This study tested the hypothesis that Se deficiency induces renal oxidative stress and renal injury via transforming growth factor-beta1 (TGF-beta1).

METHODS

Fifty-four male Wistar rats were used. Diabetes was induced in 27 rats by streptozotocin, and the other 27 rats received buffer only. Ten weeks after induction of diabetes, both normal and diabetic rats were killed, their kidneys removed, and glomeruli were isolated. Glomeruli from normal and diabetic rats were incubated in the presence of TGF-beta1 alone or its neutralizing antibody. Antioxidant enzyme (Cu-Zn) superoxide dismutase (Cu-Zn SOD), catalase, and glutathione peroxidase (GSH-Px) activities; total glutathione; and lipid peroxidation were determined. For Se studies, 15 normal and 15 diabetic rats were divided into groups of five each and fed either a regular, Se-deficient, or Se-supplemented diet one week after induction of diabetes. Ten weeks after feeding these diets, rats were killed and glomeruli were isolated. Oxidative stress was examined by determining the mRNA expressions for antioxidant enzymes and also for TGF-beta1. Plasma glucose and albuminuria were determined. Histology of the kidney and interlobular artery was evaluated by light microscopy.

RESULTS

In vitro studies showed that TGF-beta1 significantly reduced glomerular catalase and GSH-Px activities as well as total glutathione levels with an increase in lipid peroxidation in both normal and diabetic rats. Antibody to TGF-beta abrogated these changes. There was no effect of TGF-beta1 on Cu-Zn SOD. Like TGF-beta1, a Se-deficient diet caused a significant decrease in glomerular mRNA expression for Cu-Zn SOD, catalase, and GSH-Px, but a significant increase in TGF-beta1 mRNA expression. Also, a Se-deficient diet caused an increase in albuminuria, glomerular sclerosis, and plasma glucose levels in both normal and diabetic rats. The deficient diet caused a decrease in the lumen size of the interlobular artery. Se supplementation to diabetic rats up-regulated mRNA expression for antioxidant enzymes, and significantly reduced but did not normalize that of TGF-beta1. Glomerular sclerosis was normalized and the interlobular artery lumen size was greatly enlarged in diabetic rats by Se supplementation. Also, the tubulointerstitium was preserved by Se supplementation in diabetic rats.

CONCLUSIONS

The data show that TGF-beta1 is a pro-oxidant and Se deficiency increases oxidative stress via this growth factor. In addition, Se deficiency may simulate hyperglycemic conditions. Se supplementation to diabetic rats prevents not only oxidative stress but renal structural injury, as well.

Cyclic stretching of mesangial cells up-regulates intercellular adhesion molecule-1 and leukocyte adherence: a possible new mechanism for glomerulosclerosis

Intraglomerular hypertension is a primary causal factor in the progressive glomerulosclerosis that characterizes diabetic nephropathy or severe renal ablation. However, inflammation of the glomerular mesangium also participates in at least the early phase of these diseases. In glomerulonephritis, where inflammation is thought to be the predominant causal factor, intraglomerular hypertension is also often present. Mesangial cells (MCs) are critical in orchestrating key functions of the glomerulus including extracellular matrix metabolism, cytokine production, and interaction with leukocytes. Because MCs are subject to increased stretching when intraglomerular hypertension is present, and in glomerulonephritis MC/leukocyte interactions seem to be mediated primarily via the up-regulation of intercellular adhesion molecule-1 (ICAM-1), we examine the possibility that cyclic stretching is a stimulus for increased MC ICAM-1 activity. We demonstrate that the normal low levels of MC ICAM-1 mRNA and protein are dramatically up-regulated by even short intervals of cyclic stretch. This effect is dose- and time-dependent, and requires little amplitude and a brief period of elongation for significant induction. Stretch-induced MC ICAM-1 also leads to a marked elevation in phagocytic leukocyte adherence. This stimulated adherence is equal or greater than that induced by the inflammatory cytokine tumor necrosis factor-alpha, whereas an additive effect occurs when both are applied in combination. Our results indicate that stretch-induced ICAM-1 may provide a direct link between hypertension and inflammation in the progression of injury and glomerulosclerosis in diabetes, renal ablation, and other forms of glomerulonephritis.

Antibody to transforming growth factor-beta ameliorates tubular apoptosis in unilateral ureteral obstruction

BACKGROUND

Unilateral ureteral obstruction (UUO) is characterized by progressive renal atrophy, renal interstitial fibrosis, an increase in renal transforming growth factor-beta (TGF-beta), and renal tubular apoptosis. The present study was undertaken to determine the effect of a monoclonal antibody to TGF-beta (1D11) in UUO.

METHODS

Mechanical stretch was applied to tubular epithelial cells (NRK-52E) by a computer-assisted system. Three doses of 1D11 (either 0.5, 2, or 4 mg/rat) were administered to rats one day prior to UUO and every two days thereafter, and kidneys were harvested at day 13. Fibrosis was assessed by measuring tissue hydroxyproline and mRNA for collagen and fibronectin. Apoptosis was assessed with the terminal deoxy transferase uridine triphosphate nick end-labeling assay. TGF-beta levels were determined by bioassay. Western blot and immunostaining were used to identify proliferating cell nuclear antigen (PCNA), p53, bcl-2, and inducible nitric oxide synthase (iNOS).

RESULTS

Stretch significantly induced apoptosis in NRK-52E cells, which was accompanied by an increased release of TGF-beta; 1D11 (10 microg/mL) totally inhibited stretch-induced apoptosis. Control obstructed kidney contained 20-fold higher TGF-beta as compared with its unobstructed kidney; 1D11 neutralized tissue TGF-beta of the obstructed kidney. Control obstructed kidney exhibited significantly more fibrosis and tubular apoptosis than its unobstructed counterpart, which was blunted by 1D11. In contrast, 1D11 significantly increased tubular proliferation. p53 immunostaining was localized to renal tubular nuclei of control obstructed kidney and was diminished by 1D11. In contrast, bcl-2 was up-regulated in the 1D11-treated obstructed kidney. Total NOS activity and iNOS activity of the obstructed kidney were increased by 1D11 treatment.

CONCLUSION

The present study strongly suggests that an antibody to TGF-beta is a promising agent to prevent renal tubular fibrosis and apoptosis in UUO.

F-actin fiber distribution in glomerular cells: structural and functional implications

BACKGROUND

Glomerular distention is associated with cellular mechanical strain. In addition, glomerular distention/contraction is assumed to influence the filtration rate through changes in filtration surface area. A contractile cytoskeleton in podocytes and mesangial cells, formed by F-actin-containing stress fibers, maintains structural integrity and modulates glomerular expansion. In this study, the glomerular cell distribution of F-actin and vimentin filaments was studied in normal control and nine-month streptozotocin-diabetic rats. Results in situ were compared with observations in tissue culture.

METHODS

Microdissected rat glomeruli were perfused to obtain a physiological 25% glomerular expansion over the basal value. Fixation was done without change in glomerular volume. Dual fluorescent labeling of F-actin and vimentin was carried out, and samples were examined by confocal laser scanning microscopy.

RESULTS

The podocyte cell bodies and their cytoplasmic projections, including the foot processes, contained bundles of vimentin-containing fibers. Except for a thin layer at the base of foot processes, they did not demonstrate F-actin. While mesangial cells in culture had F-actin as long stress fibers resembling tense cables, mesangial cells stretched in situ contained a maze of short tortuous F-actin fibers organized in bundles that often encircled vascular spaces. This fibrillar organization was disrupted in diabetic glomeruli.

CONCLUSION

Mesangial cells, but not podocytes, contain a cytoskeleton capable of contraction that is disorganized in long-term diabetes. Together with previous observations, the distribution of this cytoskeleton suggests that mesangial cell contraction may be involved in the redistribution of glomerular capillary blood flow, but not substantially in the modulation of glomerular distention. Disorganization of stress fibers may be a cause of hyperfiltration in diabetes.

Expression of LOX-1, an oxidized low-density lipoprotein receptor, in experimental hypertensive glomerulosclerosis

Oxidized low-density lipoprotein (OxLDL) has been implicated in atherosclerosis and glomerulosclerosis. LOX-1 is a recently identified OxLDL receptor that is abundantly expressed in vascular endothelial cells. The aim of the present study was to investigate LOX-1 expression in the kidneys of hypertensive rats. Dahl salt-sensitive (DS) and salt-resistant (DR) rats were fed a 0.3% or 8% NaCl diet. Some DS 8% rats were treated with manidipine or hydralazine. LOX-1 gene expression was markedly elevated in the kidneys and glomeruli of hypertensive DS 8% rats compared with those of normotensive DR and DS 0.3% rats. Prolonged salt loading further increased the renal LOX-1 expression in DS rats. The LOX-1 upregulation in DS 8% rats was accompanied by renal overexpression of transforming growth factor-beta 1 and type I collagen, impaired renal function, and histologic glomerulosclerotic changes, all of which were ameliorated by antihypertensive treatment. LOX-1 was indeed expressed in the glomeruli in vivo and in cultured glomerular cells in vitro. However, LOX-1 expression was elevated in the aorta but not the kidneys of spontaneously hypertensive rats, which exhibited hypertension but minor glomerulosclerotic changes. In conclusion, the LOX-1 upregulation in the kidney of DS 8% rats was parallel to glomerulosclerotic changes and renal dysfunction, suggesting a possible pathogenetic role for renal LOX-1 in the progression to hypertensive glomerulosclerosis.

Cell cycle regulation in diabetic nephropathy

Cell cycle regulation in diabetic nephropathy. Renal hypertrophy is one of the earliest abnormalities of diabetic nephropathy. Although selected cell populations. such as tubulointerstitial fibroblasts, may undergo sustained proliferation in the diabetic environment, most renal cells such as mesangial cells are arrested in the G1-phase of the cell cycle after actively leaving G0-phase and some self-limited early proliferation. High glucose, transforming growth factor-beta (TGF-beta), angiotensin II, and probably other factors induce inhibitors of cyclin-dependent kinases (CDK) including p21Cip1 and p27KiP1. These CDK-inhibitors bind to and inactivate G1-phase cyclin/CDK complexes. The consequence is a lack in kinase activity, underphosphorylation of the retinoblastoma gene protein, and a failure to initiate the G1-S-phase transit. The half-life of CDK-inhibitors may also be increased by serine phosphorylation mediated through activated MAP kinases. Treatment of diabetic rats with angiotensin-converting enzyme inhibitors attenuates glomerular hypertrophy and abolishes the glomerular expression of the CDK-inhibitors p16INK4 and p27KiP1, thus indicating that the cell cycle arrest can be therapeutically influenced. Cell cycle proteins may also be involved in these molecular events, leading to a limited degree of tubular apoptosis, which is a feature of diabetic nephropathy. Although not definitively proven, accumulating evidence suggests that early hypertrophy of renal cells may act as pacemaker for subsequent irreversible structural changes, such as glomerulosclerosis and tubulointerstitial fibrosis. Therefore, a better understanding of altered processes of cell cycle regulation is necessary to develop novel therapeutic strategies to prevent diabetic nephropathy. The recent observation that glomerular hypertrophy and proteinuria do not develop in diabetic p21CiP1 knockout mice indicates that this approach is feasible.

Cellular and molecular mediators in common pathway mechanisms of chronic renal disease progression

Injury mechanisms activated by the hemodynamic adaptations to nephron loss are considered to represent a final common pathway that underlies the progressive nature of chronic renal disease. In this article, we review experimental evidence that the induction of cell adhesion molecule, cytokine and profibrotic growth factor gene expression and the resultant renal infiltration by inflammatory cells, especially macrophages, are important components of these common pathway mechanisms. Interventions aimed at inhibiting these mechanisms may offer new treatments for slowing or arresting the progression of chronic renal disease.

Modelling the effects of vascular stress in mesangial cells

It has recently been shown that mesangial cells are subjected to multiple forms of mechanical strain (fluid shear, hydrostatic pressure, and triaxial stretch) as a result of forces exerted by the vasculature. Nevertheless, the exact nature and the relative response to these stimuli have not been clarified. Although it is now well established that cyclic stretching of mesangial cells in culture results in the overproduction of extracellular matrix, indicating how intraglomerular hypertension may lead to glomerular scar formation, the contribution of different intracellular signalling mechanisms and extracellular mediators of the response are only now being identified. Recent studies point to a role for high glucose concentrations, transforming growth factor beta and its receptors, vascular endothelial growth factor, and connective tissue growth factor as important mediators, or modifiers of the response to mechanical strain. Although evidence exists for a role for protein kinase C, recent studies also implicate the mitogen-activated protein kinases along with enhanced DNA-binding activity of AP-1 as part of the signalling cascade altering matrix synthesis and cell proliferation in response to stretch. Finally, recent studies examining the effects of oscillating hyperbaric pressure demonstrate similarities, as well as differences, in comparison to those of cyclic stretch.

Regulation of connective tissue growth factor activity in cultured rat mesangial cells and its expression in experimental diabetic glomerulosclerosis

Connective tissue growth factor (CTGF) is a peptide secreted by cultured endothelial cells and fibroblasts when stimulated by transforming growth factor-beta (TGF-beta), and is overexpressed during fibrotic processes in coronary arteries and in skin. To determine whether CTGF is implicated in the pathogenesis of diabetic glomerulosclerosis, cultured rat mesangial cells (MC) as well as kidney cortex and microdissected glomeruli were examined from obese, diabetic db/db mice and their normal counterparts. Exposure of MC to recombinant human CTGF significantly increased fibronectin and collagen type I production. Furthermore, unstimulated MC expressed low levels of CTGF message and secreted minimal amounts of CTGF protein (36 to 38 kD) into the media. However, sodium heparin treatment resulted in a greater than fourfold increase in media-associated CTGF, suggesting that the majority of CTGF produced was cell- or matrix-bound. Exposure of MC to TGF-beta, increased glucose concentrations, or cyclic mechanical strain, all causal factors in diabetic glomerulosclerosis, markedly induced the expression of CTGF transcripts, while recombinant human CTGF was able to autoinduce its own expression. TGF-, and high glucose, but not mechanical strain, stimulated the concomitant secretion of CTGF protein, the former also inducing abundant quantities of a small molecular weight form of CTGF (18 kD) containing the heparin-binding domain. The induction of CTGF protein by a high glucose concentration was mediated by TGF-beta, since a TGF-beta-neutralizing antibody blocked this stimulation. In vivo studies using quantitative reverse transcription-PCR demonstrated that although CTGF transcripts were low in the glomeruli of control mice, expression was increased 28-fold after approximately 3.5 mo of diabetes. This change occurred early in the course of diabetic nephropathy when mesangial expansion was mild, and interstitial disease and proteinuria were absent. A substantially reduced elevation of CTGF mRNA (twofold) observed in whole kidney cortices indicated that the primary alteration of CTGF expression was in the glomerulus. These results suggest that CTGF upregulation is an important factor in the pathogenesis of mesangial matrix accumulation and progressive glomerulosclerosis, acting downstream of TGF-beta.

TGF-beta and fibrosis

Transforming growth factor-beta (TGF-beta) isoforms are multifunctional cytokines that play a central role in wound healing and in tissue repair. TGF-beta is found in all tissues, but is particularly abundant in bone, lung, kidney and placental tissue. TGF-beta is produced by many but not all parenchymal cell types, and is also produced or released by infiltrating cells such as lymphocytes, monocytes/macrophages, and platelets. Following wounding or inflammation, all these cells are potential sources of TGF-beta. In general, the release and activation of TGF-beta stimulates the production of various extracellular matrix proteins and inhibits the degradation of these matrix proteins, although exceptions to these principles abound. These actions of TGF-beta contribute to tissue repair, which under ideal circumstances leads to the restoration of normal tissue architecture and may involve a component of tissue fibrosis. In many diseases, excessive TGF-beta contributes to a pathologic excess of tissue fibrosis that compromises normal organ function, a topic that has been the subject of numerous reviews [1-3]. In the following chapter, we will discuss the role of TGF-beta in tissue fibrosis, with particular emphasis on renal fibrosis.

Mesangial cell signaling cascades in response to mechanical strain and glucose

BACKGROUND

Elevated glucose levels and glomerular hypertension (Pgc) are considered to contribute to the elaboration of matrix protein by mesangial cells (MCs) in diabetic glomeruli. MCs grown in 30 mM of glucose produce excessive matrix protein, as do MCs exposed to cyclic strain, and the combination of the two exacerbates this. Tight glucose control or reduction in Pgc clinically delays progression of diabetic nephropathy. MC c-fos is induced in response to either application of strain or high ambient glucose, inducing increases in activated protein-1 transactivational activity and extracellular matrix production. Stimuli that lead to c-fos induction pass through the three mitogen-activated protein (MAP) kinase pathways: p44/42, SAPK/JNK, and p38/HOG. We studied MAP kinase activation in MCs exposed to mechanical strain and a high glucose.

METHODS

MCs (passage 5 through 10) cultured for 96 hours on type 1 collagen-coated flexible-bottom plates in either 5.6 or 30 mM glucose were exposed to 5, 10, or 30 minutes of cyclic strain (60 cycles per min) by computer-driven generation of vacuums of -14 kPa, inducing 20% elongation in the diameter of the surface. Control MCs were grown on both coated rigid and flexible-bottom plates. Protein levels (by Western blot) and activity assays for all three kinase cascades were performed at baseline and after 5, 10, and 30 minutes. All experiments were performed in triplicate.

RESULTS

MAP kinase signaling was seen in response to stretch, and high ambient glucose affected the pattern of activation. Both p44/42 and p38HOG kinase activities showed small increases to a maximum of 2.5- to 3.5-fold greater than static MCs at 10 minutes. Activity in both kinase cascades was slightly suppressed by 30 mM glucose. In contrast, SAPK/JNK activity was present at a very low level in static MCs and increased markedly by 10 minutes of stretch. Thirty micromolars of glucose augmented this effect by a factor of six over MCs cultured in 5.6 mM glucose after 10 minutes of stretch. Neither glucose concentration nor mechanical strain had any effect on the protein expression of any of the kinases by Western blot.

CONCLUSIONS

MAP kinase cascade signaling is seen when physical force is applied to MCs, and glucose affects the pattern of activity. Thirty micromolars of glucose markedly increase the level of SAPK/JNK activation. This may have implications in diabetic signal transduction and matrix protein production.

Antihypertensive therapy and progression of chronic renal disease

This paper reviews findings on the relationship between blood pressure control and the progression of renal disease. Experimental studies demonstrate a correlation between systemic blood pressure and histologic glomerular injury and the delay in progression of renal disease with antihypertensive therapy, particularly with angiotensin-converting enzyme inhibitors. Recent clinical findings are reviewed, including epidemiologic data linking hypertension to subsequent renal disease, and clinical studies showing a beneficial effect on progression of renal disease with lower than usual blood pressure targets.