Induction of nodular sclerosis by insulin in rat mesangial cells in vitro: studies of collagen

Insulin Prevents Hyperfiltration and Proteinuria but Not Glomerular Hypertrophy and Increases Mesangial Matrix Expansion in Diabetic Rats

OBJECTIVE

The aim of this work was to study the effect of 7 days of strict glycemic control with insulin on glomerular function and structure in streptozotocin (STZ)-diabetic rats.

MATERIALS AND METHODS

Three groups of adult male Fischer rats were studied: controls (n = 15), diabetics (n = 15), and insulin-treated diabetics (n = 15). Diabetes was induced by treating the rats with STZ (55 mg/kg i.p.). One week after the induction of diabetes, blood glucose, protein excretion rate (PER), glomerular filtration rate (GFR), and renal plasma flow (RPF) were estimated in each group. Furthermore, morphometric analysis was performed to estimate the tuft volume and changes in mesangial matrix area. The results are expressed as the mean ± SEM.

RESULTS

STZ diabetes caused significant increases in GFR (0.89 ± 0.1 to 1.21 ± 0.1 mL/min/100 g; p < 0.01) and RPF (1.78 ± 0.37 to 3.32 ± 0.6 mL/min/100 g; p < 0.05). Furthermore, the diabetic rats had higher glomerular volumes but mesangial matrix areas similar to controls. Insulin treatment prevented the increases in blood glucose (4.5 ± 0.2 mM), PER (66.1 ± 7.8 mg/day), GFR (0.6 ± 0.07 mL/min/100 g), and RPF (1.72 ± 0.36 mL/min/100 g), but did not prevent glomerular hypertrophy (21.7% increase), but induced mesangial matrix expansion (25% increase).

CONCLUSIONS

Insulin prevented the diabetes-induced hyperfiltration and proteinuria, but did not prevent glomerular growth, and induced mesangial expansion. Hyperglycemic episodes could be partly responsible for persistent glomerular growth and accelerated mesangial growth.

RNA-binding protein IGF2BP2/IMP2 is required for laminin-β2 mRNA translation and is modulated by glucose concentration

Laminin-β2 (LAMB2) is a critical component of the glomerular basement membrane as content of LAMB2 in part determines glomerular barrier permeability. Previously, we reported that high concentrations of glucose reduce expression of this laminin subunit at the translational level. The present studies were undertaken to further define systems that control Lamb2 translation and the effect of high glucose on those systems. Complementary studies were performed using in vitro differentiation of cultured podocytes and mesangial cells exposed to normal and elevated concentrations of glucose, and tissues from control and diabetic rats. Together, these studies provide evidence for regulation of Lamb2 translation by IMP2, an RNA binding protein that targets Lamb2 mRNA to the actin cytoskeleton. Expression of Imp2 itself is regulated by the transcription factor HMGA2, which in turn is regulated by the microRNA let-7b. Elevated concentrations of glucose increase let-7b, which reduces HMGA2 expression, in turn reducing IMP2 and LAMB2. Correlative changes in kidney tissues from control and streptozotocin-induced diabetic rats suggest these control mechanisms are operative in vivo and may contribute to proteinuria in diabetic nephropathy. To our knowledge, this is the first time that translation of Lamb2 mRNA has been linked to the actin cytoskeleton, as well as to specific RNA-binding proteins. These translational control points may provide new targets for therapy in proteinuric disorders such as diabetic nephropathy where LAMB2 levels are reduced.

Insulin-like growth factor-binding protein-5-induced laminin gamma1 transcription requires filamin A

Insulin-like growth factor-binding protein-5 (IGFBP-5) has IGF-1-independent intranuclear effects that are poorly defined. Treatment of cells with IGFBP-5 induces migration, prevents apoptosis, and leads to increased laminin subunit transcription. Similarly, filamin A (FLNa), an actin-binding protein that participates in cell attachment, plays important additional roles in signal transduction and modulation of transcriptional responses. In this report, we show that IGFBP-5 leads to dephosphorylation of FLNa with subsequent FLNa cleavage. Following cleavage, there is enhanced recruitment of Smad3/4 to a C-terminal FLNa fragment with nuclear translocation and subsequent binding to the promoter region of the laminin gamma1 (lamc1) gene. FLNa knockdown prevents IGFBP-5-mediated increases in lamc1 transcription. These data indicate that IGFBP-5 induces formation of a FLNa-based nuclear shuttle that recruits transcription factors and regulates transcription of IGFBP-5 target genes. These studies provide new insights into the mechanisms whereby IGFBP-5 and FLNa exert intranuclear effects.

Laminin alpha4-null mutant mice develop chronic kidney disease with persistent overexpression of platelet-derived growth factor

Each extracellular matrix compartment in the kidney has a unique composition, with regional specificity in the expression of various laminin isoforms. Although null mutations in the majority of laminin chains lead to specific developmental abnormalities in the kidney, Lama4-/- mice have progressive glomerular and tubulointerstitial fibrosis. These mice have a significant increase in expression of platelet-derived growth factor (PDGF)-BB, PDGF-DD, and PDGF receptor beta in association with immature glomerular and peritubular capillaries. In addition, mesangial cell exposure to alpha4-containing laminins, but not other isoforms, results in down-regulation of PDGF receptor mRNA and protein, suggesting a direct effect of LN411/LN421 on vessel maturation. Given the known role of overexpression of PDGF-BB and PDGF-DD on glomerular and tubulointerstitial fibrosis, these data suggest that failure of laminin alpha4-mediated down-regulation of PDGF activity contributes to the progressive renal lesions in this animal model. Given the recent demonstration that individuals with laminin alpha4 mutations develop cardiomyopathy, these findings may be relevant to kidney disease in humans.

Reductions in laminin beta2 mRNA translation are responsible for impaired IGFBP-5-mediated mesangial cell migration in the presence of high glucose

Insulin-like growth factor binding protein-5 (IGFBP-5) mediates mesangial cell migration through activation of cdc42, and laminin421 binding to alpha(6)beta(1)-integrin (Berfield AK, Hansen KM, Abrass CK. Am J Physiol Cell Physiol 291: C589-C599, 2006). Because glomerular expression of laminin beta(2) is reduced in diabetic rats (Abrass CK, Spicer D, Berfield AK, St. John PL, Abrahamson DR. Am J Pathol 151: 1131-1140, 1997), we directly examined the effect of hyperglycemia on mesangial cell migration and laminin beta2 expression. Migration mediated by IGFBP-5 is impaired in the presence of 25 mM glucose. This reduction in migration was found to result from a loss in mesangial cell synthesis of laminin421, and IGFBP-5-induced migration could be restored by replacing laminin421. Additional studies showed that there was selective reduction in mRNA translation of laminin beta2 in the presence of high glucose. Preserved synthesis of laminin beta1 indicates that not all proteins are reduced by high glucose and confirms prior data showing that laminin411 cannot substitute for laminin421 in IGFBP-5-mediated migration. Given the importance of mesangial migration in the reparative response to diabetes-associated mesangiolysis, these findings provide new insights into abnormalities associated with diabetic nephropathy and the potential importance of differential control of protein translation in determination of alterations of protein expression.

Impact of obesity on renal structure and function in the presence and absence of hypertension: evidence from melanocortin-4 receptor-deficient mice

The purpose of this study was to determine the long-term impact of obesity and related metabolic abnormalities in the absence and presence of hypertension on renal injury and salt-sensitivity of blood pressure. Markers of renal injury and blood pressure salt sensitivity were assessed in 52- to 55-wk-old normotensive melanocortin-4 receptor-deficient (MC4R-/-) mice and lean C57BL/6J wild-type (WT) mice and in 22-wk-old MC4R-/- and WT mice made hypertensive by N(G)-nitro-L-arginine methyl ester (L-NAME) in the drinking water for 8 wk. Old MC4R-/- mice were 60% heavier, hyperinsulinemic, and hyperleptinemic but had similar mean arterial pressure (MAP) as WT mice (115 +/- 2 and 117 +/- 2 mmHg) on normal salt diet (0.4% NaCl). A high-salt diet (4.0% NaCl) for 12 days did not raise MAP in obese or lean mice [DeltaMAP: MC4R (-/-) 4 +/- 2 mmHg; WT, 2 +/- 1 mmHg]. Obese MC4R-/- mice had 23% greater glomerular tuft area and moderately increased GFR compared with WT mice. Bowman's space, total glomerular area, mesangial matrix, urinary albumin excretion (UAE), renal TGF-beta and collagen expression were not significantly different between old MC4R-/- and WT mice. Renal lipid content was greater but renal macrophage count was markedly lower in MC4R-/- than WT mice. Mild increases in MAP during L-NAME treatment (approximately 16 mmHg) caused small, but greater, elevations in UAE, renal TGF-beta content, and macrophage infiltration in MC4R-/- compared with WT mice without significant changes in glomerular structure. Thus despite long-term obesity and multiple metabolic abnormalities, MC4R-/- mice have no evidence of renal injury or salt-sensitivity of blood pressure. These observations suggest that elevations in blood pressure may be necessary for obesity and related metabolic abnormalities to cause major renal injury or that MC4R-/- mice are protected from renal injury by mechanisms that are still unclear.

High ambient glucose augments angiotensin II-induced proinflammatory gene mRNA expression in human mesangial cells: effects of valsartan and simvastatin

BACKGROUND

Hyperglycemia may potentiate the adverse renal effects of angiotensin II (AII). In the kidney, the major target of AII action is the glomerular mesangial cell, where its hemodynamic and proinflammatory action contributes to renal injury. AII action is mediated by several types of cell receptors. Among those, the AT1 receptor has been best studied using specific AII receptor blockers (ARBs). These agents have emerged as major new modalities in the prevention and amelioration of renal disease where the ARB renoprotective anti-inflammatory properties could be more important than previously appreciated. Like the ARBs, statins may also modulate inflammatory responses that are renoprotective and complement their cholesterol-lowering effects.

AIM

The aim of this project was to (i) identify a repertoire of proinflammatory mesangial cell AII-inducible mRNAs; (ii) determine if the AII-induced proinflammatory mRNA responses depend on ambient glucose, and (iii) test the anti-inflammatory effectiveness of an ARB, valsartan, either alone or in combination with a statin, simvastatin.

RESULTS/CONCLUSIONS

Using high-density microarrays and real-time PCR we identified several AII-inducible proinflammatory mesangial genes that exhibited augmented mRNA responses in high-glucose milieu. Valsartan blocked the AII-induced mRNA expression of proinflammatory genes (i.e. MCP-1, LIF and COX-2) maintained in normal and high glucose. These observations add to the mounting evidence that ARBs have anti-inflammatory effects in the kidney, a beneficial effect that may be more important in protecting renal function in diabetic patients. While simvastatin inhibited expression of some mRNAs encoding chemokines/cytokines, it enhanced expression of mRNA encoding COX-2, a key mediator of inflammation. Thus, the non-cholesterol effects of statins on inflammatory responses appear complex.

Thiazolidinediones provide better renoprotection than insulin in an obese, hypertensive type II diabetic rat model

Hyperinsulinemia has been implicated in the development of diabetic nephropathy. In the present study we compared the renoprotective effects of the thiazolidinedione, pioglitazone (PGZ), to that of insulin in a hypertensive, obese, type II diabetic rat model. PGZ aggravated obesity and gave less glycemic control than insulin. However, renoprotection was markedly better with PZG compared to insulin as shown by lower proteinuria, improved renal function, and less histological evidence of diabetic glomerular and tubulointerstitial lesions. PZG and insulin both reduced renal accumulation of pentosidine and oxidative stress to a similar extent. In contrast, PGZ but not insulin suppressed enhanced transforming growth factor-beta (TGF-beta) expression. We further confirmed in cultured rat proximal tubular cells that insulin enhanced TGF-beta mRNA expression and protein production. Our results identify hyperinsulinemia and the attendant increase of TGF-beta expression as potential therapeutic targets in diabetes independent of glycemic control. This confirms prior clinical evidence that PZG provides renoprotection in obese, diabetic patients with nephropathy.

Albuminuria and chronic kidney disease in association with the metabolic syndrome

Chronic kidney disease is a worldwide public health problem because it is an important risk factor for cardiovascular disease and premature death. The metabolic syndrome, which is characterized by abdominal obesity, high blood pressure, impaired glucose tolerance, and dyslipidemia, is also an increasingly common disorder and a major risk factor for diabetes and cardiovascular disease. A close association has been found between the metabolic syndrome and the risk for developing renal impairment, clinically expressed in the form of microalbuminuria or chronic kidney disease. Several potential mechanisms, including insulin resistance, renal atherosclerosis, and inflammation, induce the deterioration of renal function. Despite the close association between the metabolic syndrome and renal impairment, it is still unclear whether and to what extent treating the metabolic syndrome will prevent renal impairment. A clinical trial is needed to clarify whether the effect of preventing and treating metabolic syndrome components will result in improved renal prognosis.

In vitro models of TGF-β-induced fibrosis suitable for high-throughput screening of antifibrotic agents

Progressive fibrosis is a cause of progressive organ dysfunction. Lack of quantitative in vitro models of fibrosis accounts, at least partially, for the slow progress in developing effective antifibrotic drugs. Here, we report two complementary in vitro models of fibrosis suitable for high-throughput screening. We found that, in mesangial cells and renal fibroblasts grown in eight-well chamber slides, transforming growth factor-β1 (TGF-β1) disrupted the cell monolayer and induced cell migration into nodules in a dose-, time- and Smad3-dependent manner. The nodules contained increased interstitial collagens and showed an increased collagen I:IV ratio. Nodules are likely a biological consequence of TGF-β1-induced matrix overexpression since they were mimicked by addition of collagen I to the cell culture medium. TGF-β1-induced nodule formation was inhibited by vacuum ionized gas treatment of the plate surface. This blockage was further enhanced by precoating plates with matrix proteins but was prevented, at least in part, by poly-l-lysine (PLL). We have established two cell-based models of TGF-β-induced fibrogenesis, using mesangial cells or fibroblasts cultured in matrix protein or PLL-coated 96-well plates, on which TGF-β1-induced two-dimensional matrix accumulation, three-dimensional nodule formation, and monolayer disruption can be quantitated either spectrophotometrically or by using a colony counter, respectively. As a proof of principle, chemical inhibitors of Alk5 and the antifibrotic compound tranilast were shown to have inhibitory activities in both assays.

Renal disease in obesity: the need for greater attention

Excess body weight may be associated with various functional/structural lesions of the kidney. The spectrum ranges from glomerulomegaly with or without focal or segmental glomerulosclerosis, to diabetic nephropathy, to carcinoma of the kidney and nephrolithiasis. The first sign of renal injury is microalbuminuria or frank proteinuria, in particular in the presence of hypertension. The occurrence of microalbuminuria and/or chronic kidney insufficiency (glomerular filtration rate < 60 mL/min/1.73 m2) is related to the increasing number of components of the metabolic syndrome, ie, central obesity, elevated fasting blood glucose level, hypertriglycerides, low high-density lipoprotein cholesterol, and hypertension. In the long run, end-stage renal failure may develop. An increased body mass index is particularly harmful in patients with reduced renal functional mass (unilateral renal agenesis or nephrectomy) and other renal diseases (immunoglobulin A nephritis and chronic graft dysfunction after kidney transplantation). In the pathogenesis of obesity-associated glomerulopathy, hyperfiltration is of fundamental importance. The factors involved are energy intake (high protein and salt), hyperinsulinemia, and enhanced tubuloglomerular feedback because of increased sodium reabsorption. The adrenergic and renin-angiotensin-aldosterone systems as well as glucocorticoids are stimulated. In addition, several active proteins generated in the central adipose tissue, such as leptin, proinflammatory cytokines, plasminogen activator inhibitor-1, angiotensinogen, and growth factors (transforming growth factor-beta1), as well as low levels of the protective adiponectin, may contribute to renal injury. Of greatest importance is the development of hypertension and of diabetes, which are directly related to the severity of central obesity. Obesity-associated renal disease should be prevented or retarded by weight reduction following lifestyle modification (salt restriction, hypocaloric diet, aerobic exercise), or eventually by antiobesity medication or bariatric surgery. In the presence of glomerulopathy and/or hypertension, angiotensin converting enzyme inhibitors or angiotensin II type I receptor blockers are the drugs of choice to improve glomerular hyperfiltration.

Insulin resistance, hyperinsulinemia, and renal injury: mechanisms and implications

Most of the basic components of the metabolic syndrome, namely type 2 diabetes mellitus, hypertension, obesity, or low high-density lipoprotein cholesterol levels, apart from being major risk factors for cardiovascular disease have been also associated with an increased risk of chronic kidney disease. However, several epidemiologic studies conducted over the past years suggest that the central component of the syndrome, insulin resistance, as well as compensatory hyperinsulinemia are independently associated with an increased prevalence of chronic kidney disease. In addition, background studies support the existence of several pathways linking insulin resistance and hyperinsulinemia with kidney damage. Insulin per se promotes the proliferation of renal cells and stimulates the production of other important growth factors such as insulin-like growth factor-1 and transforming growth factor beta. Insulin also upregulates the expression of angiotensin II type 1 receptor in mesangial cells, thus enhancing the deleterious effects of angiotensin II in the kidney, and stimulates production and renal action of endothelin-1. Moreover, insulin resistance and hyperinsulinemia are associated with decreased endothelial production of nitric oxide and increased oxidative stress which have been also implicated in the progression of diabetic nephropathy. This review analyzes the above and other potential mechanisms, through which insulin resistance and hyperinsulinemia can contribute to renal injury.

Rat glomerular mesangial cells require laminin-9 to migrate in response to insulin-like growth factor binding protein-5

Temporal and spatial differences in extracellular matrix play critical roles in cell proliferation, differentiation and migration. Different migratory stimuli use different substrates and receptors to achieve cell migration. To understand the mechanism of insulin-like growth factor binding protein-5 (IGFBP-5)-induced migration in mesangial cells, the roles of integrins and substrates were examined. IGFBP-5 induced an increase in mRNA expression for laminin (LN) chains lama4, lamb2, and lamc1, suggesting that LN-9 might be required for migration. Antibodies to the LNalpha(4) and LNbeta(2) chains, but not LNbeta(1), blocked IGFBP-5-induced migration. Anti-sense morpholino oligonucleotide inhibition of expression of LNalpha(4) substantially reduced expression of LN-8/9 (alpha(4)beta(1)gamma(1)/alpha(4)beta(2)gamma(1), 411/421) and prevented IGFBP-5-induced migration. Anti-sense inhibition of lamb2 reduced expression of LN-9. Absence of LN-9 prevented IGFBP-5-induced migration, which was not preserved by continued expression of LN-8. The requirement for LN-9 was further supported by studies of T98G cells, which express predominantly LN-8. IGFBP-5 had little effect on migration in these cells, but increased migration when T98G cells were plated on LN-8/9. IGFBP-5-mediated mesangial cell migration was inhibited by antibodies that block attachment to alpha(6)beta(1)-integrins but was unaffected by antibodies and disintegrins that block binding to other integrins. Furthermore, in cells with anti-sense inhibited expression of LN-9, integrin alpha(6)beta(1) was no longer detected on the cell surface. These studies suggest the specificity of mechanisms of migration induced by specific stimuli and for the first time demonstrate a unique function for LN-9 in mediating IGFBP-5-induced migration.

One-year post-transplant weight gain is a risk factor for graft loss

Metabolic syndrome (MS) and obesity participate in the pathogenesis of kidney disease. We explored the impact of MS and post-transplant weight gain on graft survival. Two hundred ninety-two renal transplant recipients (RTRs) were included in the study. Various parameters (e.g. anthropometric, biological) were measured at the time of transplantation as well as 1 year post-transplant. The proportion of patients with overweight or obesity significantly increased during the first year post-transplant (p = 0.04). Mean weight gain was 2.7 +/- 5.8 kg. Thirty patients (10.3%) lost their graft during follow-up. In multivariate analysis, patients with an increase in body mass index (BMI) of more than 5% at 1 year post-transplant had an increased risk of graft loss with (HR: 2.82 [95% CI: 1.11-7.44], p = 0.015) or without death censoring (HR: 2.31 [95% CI: 1.06-5.04], p = 0.035). Low creatinine clearance (HR: 4.72 [95% CI: 1.63-13.69], p = 0.004), high urinary protein excretion (HR: 3.21 [95% CI: 1.27-8.18], p = 0.014) and delayed graft function (DGF) (HR: 2.621 [95% CI: 1.07-6.39], p = 0.036) were also independent risk factors for graft loss. MS did not independently predict graft loss, partly due to significant interactions with low-grade inflammation. We conclude that post-transplant weight gain significantly reduces graft survival.

IGF-1 induces rat glomerular mesangial cells to accumulate triglyceride

Rat glomerular mesangial cells (MC) become lipid-laden foam cells when they are exposed to IGF-1. IGF-1 increased accumulation of triglyceride (TG) 2.5-fold in MC after 7 days. TG accumulation resulted from enhanced macropinocytosis and decreased efflux secondary to a 40-50% reduction in peroxisome proliferator-activated receptor (PPAR)-delta (PPARdelta). There was no evidence of primary or secondary changes in cholesterol or TG synthesis, increased uptake by LDL or scavenger receptors, or reduced efflux via ATP-binding cassette A-1. Although the lipid moiety taken up can be influenced by the concentration of cholesterol or TG in the medium, in standard medium MC preferentially accumulate TG. TG-rich MC foam cells fail to contract in response to angiotensin II (Berfield AK, Andress DL, and Abrass CK. Kidney Int 62: 1229-1237, 2002); however, their migratory response to IGF binding protein-5 is unaffected. This differs from cholesterol loading, which impairs both phagocytosis and migration. These findings have important implications for understanding the mechanisms that contribute to lipid accumulation in MC and the functional consequences of different forms of foam cells. These observations are relevant to understanding vascular disease and progressive renal diseases that are accelerated by abnormalities in lipid metabolism.

Glomerular structural and functional changes in a high-fat diet mouse model of early-stage Type 2 diabetes

AIMS/HYPOTHESIS

Type 2 diabetes often results in diabetic nephropathy, which is preceded by an elevated glomerular filtration rate (GFR). This study was designed to develop a mouse model of Type 2 diabetes and to elucidate the glomerular events in the early stages of diabetic nephropathy.

METHODS

Four-week-old mice were fed a normal or high-fat (42% of total calories from fat) diet, and body weight, blood glucose, insulin, leptin, lipids and GFR were monitored from 9 to 21 weeks or longer after the feeding programme. Mesangial cell dedifferentiation was accessed by alpha-smooth muscle actin staining. Glomerular hypertrophy was determined using image analysis with haematoxylin-eosin staining. Matrix deposition was determined by type IV collagen staining.

RESULTS

After 9 weeks, mice fed a high-fat diet weighed more than mice fed a normal diet (30.5+/-1.2 vs 22.3+/-0.5 g, p<0.05), and mice fed a high-fat diet were hyperinsulinaemic (283.9+/-69.7 vs 102.9+/-36.4 pmol/l, p<0.05), hyperglycaemic (8.0+/-0.6 vs 6.5+/-0.2 mmol/l, p<0.05) and their leptin levels were increased six-fold (1.48+/-0.45 vs 0.25+/-0.03 ng/ml, p<0.05). After 13 weeks, mice fed a high-fat diet showed hyperfiltration (GFR; 440+/-60 vs 210+/-10 microl/min, p<0.05). During the early stages of diabetic nephropathy, mesangial cell dedifferentiation was evident, shown by increased expression of alpha-smooth muscle actin in the glomeruli. After 9 weeks, mice fed a high-fat diet already demonstrated increased type IV collagen deposition. After 13 weeks, they developed enlarged glomerular tufts compared with those of their age-matched controls.

CONCLUSIONS/INTERPRETATION

The results of this study suggest that collagen IV deposition precedes the hyperfiltration and enlargement of glomeruli in early-stage diabetic nephropathy. Dedifferentiation of mesangial cells may be associated with collagen IV deposition.

Dietary flaxseed meal reduces proteinuria and ameliorates nephropathy in an animal model of type II diabetes mellitus

BACKGROUND

Evidence is emerging that varying the type or source of dietary protein intake can have beneficial effects on chronic renal disease. Consumption of soybean and soy-based food products, as the source of plant protein, can retard the development and progression of chronic renal disease. We studied the obese spontaneously hypertensive/NIH-corpulent (SHR/N-cp) rat, a model of obesity and type II diabetes mellitus that consistently develops nephropathy resembling diabetic nephropathy. We specifically sought to determine whether changing the source of protein intake from animal protein, casein, to plant protein in the form of either soy protein concentrate or flaxseed protein in the diet has a different impact on renal function and nephropathy in this model.

METHODS

Male obese SHR/N-cp rats were randomly assigned to one of three diets containing either 20% casein, 20% soy protein concentrate, or 20% flaxseed meal. Except for the protein source, all three diets were identical and contained similar amounts of protein, fat, carbohydrates, minerals, and vitamins. All animals were maintained on these diets for 6 months. At the end of the study, blood sampling and 24-hour urine collections were performed for renal functional measurements, and the kidneys were harvested and examined for histologic evaluation.

RESULTS

All three groups had similar amounts of food intake and body weight gain and exhibited fasting hyperglycemia and hyperinsulinemia. Plasma glucose levels did not differ among the three groups, but plasma insulin concentration was significantly lower in rats fed flaxseed meal than those fed either casein or soy protein concentrate. Mean plasma creatinine, creatinine clearance, and urinary urea excretion also did not differ significantly between the three groups. By contrast, urinary protein excretion was significantly lower (P < 0.01) in rats fed flaxseed than in rats fed either casein or soy protein concentrate. Morphologic analysis of renal structural lesions showed that the percentage of abnormal glomeruli with mesangial expansion and the tubulointerstitial score (an index of severity of tubulointerstitial damage) were significantly reduced in rats fed flaxmeal compared to those fed casein or soy protein concentrate.

CONCLUSION

We conclude that dietary protein substitution with flaxseed meal reduces proteinuria and glomerular and tubulointerstitial lesions in obese SHR/N-cp rats and that flaxseed meal is more effective than soy protein in reducing proteinuria and renal histologic abnormalities in this model. The reduction in proteinuria and renal injury was independent of the amount of protein intake and glycemic control. Which dietary component(s) present in flaxseed meal is (are) responsible for the renal protective effect remains to be determined.

Laminin-8/9 is synthesized by rat glomerular mesangial cells and is required for PDGF-induced mesangial cell migration

BACKGROUND

Laminin (LM), the major glycoprotein component of basement membranes is expressed as multiple isoforms in a developmentally regulated and tissue-specific manner. LM alpha4 has a limited tissue distribution and is highly expressed in the developing glomerulus. In the present study, we investigate the in vivo and in vitro expression and function of LM alpha4 in the glomerulus.

METHODS

LM alpha4 expression was examined by Northern blot, reverse transcription polymerase chain reaction (RT-PCR), Western blot, and immunofluorescence microscopy. Mesangial cells (MC) were plated on purified LM-1, LM-2, and LM-8/9. Immunofluorescence microscopy was performed to examine the cellular phenotypes induced by LM-1 and LM-8/9. A modified Boyden chamber method was used to assess laminin participation in platelet-derived growth factor (PDGF)-stimulated migration.

RESULTS

mRNA for LMalpha4 is expressed in cultured rat MC, and isolated rat and mouse glomeruli, but not in cultured rat glomerular epithelial cells or glomerular endothelial cells. Using antibodies specific for LM alpha4, a 240 kD band was detected in MC extract and a slightly smaller band was identified in extracted rat glomeruli. Purified LM-8/9 had MC adhesive activity comparable to LM-1 and LM-2. MC attached to LM-8/9 exhibited a unique phenotype. In contrast to LM-1, attachment of MC to LM-8/9 produced a highly arborized cell morphology with significantly reduced formation of focal contacts or stress fibers. LM alpha4 is utilized by MC during PDGF-stimulated migration.

CONCLUSION

LM alpha4 is synthesized by MC and persists in the mature glomerulus. LM-8/9 stimulates a unique cellular morphology, and they are utilized in PDGF-induced migration. These factors suggest that LM alpha4 plays an important role in MC differentiation and in the maintenance of MC phenotype.

Glomerular hypertrophy is associated with hyperinsulinemia and precedes overt diabetes in aging rhesus monkeys

BACKGROUND

Rhesus monkeys have a high prevalence of obesity and spontaneous type 2 diabetes mellitus after the age of 10 years. These monkeys go through a defined, sequential set of metabolic phases, including fasting hyperinsulinemia, impaired glucose tolerance, and fasting hyperglycemia. Using these monkeys, we addressed the hypothesis that renal structural features characteristic of diabetic nephropathy might precede the appearance of overt diabetes.

METHODS

We carried out a quantitative analysis of renal tissue, using light microscopy and electron microscopy, from 6 metabolically normal young monkeys, 7 metabolically normal aged monkeys, 7 hyperinsulinemic monkeys, and 18 diabetic monkeys.

RESULTS

Glomerular volume was increased significantly in hyperinsulinemic monkeys and diabetic monkeys compared with aged normal monkeys. In the normal monkey, glomerular basement membrane (GBM) width rises with age but reaches a plateau at about 20 years of age; the presence of diabetes was associated with markedly increased GBM width. Glomerular tuft volume and GBM width were correlated most closely with age and with glucose tolerance.

CONCLUSION

Diabetic monkey kidneys are characterized by glomerular enlargement, glomerulosclerosis, and thickening of the GBM. Glomerular hypertrophy begins in the prediabetic hyperinsulinemic phase. This finding suggests that early intervention may be required in human patients to preserve normal glomerular structure.

IGF-1-induced lipid accumulation impairs mesangial cell migration and contractile function

BACKGROUND

Chronic treatment of mesangial cells with insulin-like growth factor-1 (IGF-1) results in intracellular lipid accumulation. These mesangial cells resemble foam cells.

METHODS

To determine whether this phenotype affects cell function, lipid-laden mesangial cells were tested for their ability to migrate in response to IGF-binding protein-5 (IGFBP-5) and to contract in response to angiotensin II (Ang II). IGFBP-5 binding and subsequent activation of the signal transduction cascade for migration were examined. To confirm that lipid accumulation was responsible for impaired contractility, lipid was removed from lipid-laden mesangial cells and the cells were re-tested for contractile response to Ang II.

RESULTS

In comparison to control mesangial cells, lipid-laden cells failed to migrate in response to IGFBP-5. Although cellular binding of IGFBP-5 was not altered by lipid accumulation, IGFBP-5 failed to activate cdc42, a Rho GTPase required for IGFBP-5-mediated mesangial cell migration. These data indicate that lipid accumulation within the mesangial cell interferes with the signal transduction response to IGFBP-5. In addition, mesangial cells treated with IGF-1 had reduced contraction to Ang II. When lipid accumulation was exaggerated by adding cholesteryl esters to the culture medium, mesangial cells failed to contract in response to Ang II. Following removal of excess lipid from these mesangial cells, the contractile response to Ang II was restored.

CONCLUSIONS

IGF-1 induces lipid accumulation in mesangial cells, which impairs their ability to respond to specific migratory and contractile stimuli. These observations are relevant to understanding functional abnormalities in diseases where mesangial foam cells occur, such as focal and segmental glomerulosclerosis and diabetic nephropathy.

Prevention of obesity-linked renal disease: age-dependent effects of dietary food restriction

BACKGROUND

Hyperphagic obese Zucker rats develop glomerular injury and die of renal disease, an outcome prevented by food restriction at an early age. We examined the effects of food restriction imposed at different ages on systemic, renal hemodynamic, and hormonal changes to gain insight into the mechanisms of obesity-linked glomerular injury.

METHODS

At 6 weeks of age obese Zucker rats were either fed ad libitum or were restricted in food intake at various ages (6, 12, 26, or 50 weeks) to that consumed by lean Zucker rats (14 g/day). Every four weeks 24-hour urine collections, blood pressure, and venous blood samples were obtained until the end of study (60 weeks).

RESULTS

Food restriction at 6 or 12 weeks of age prevented glomerular injury and hypertrophy and delayed the development of hypertension, hypercholesterolemia, and hyperinsulinemia. Food restriction at 26 weeks of age reduced proteinuria, while restriction at 50 weeks prevented further increases in proteinuria without altering pre-existing hypercholesterolemia, hypertension, or hyperinsulinemia. Hypertriglyceridemia and glomerular hyperfiltration in the obese animals were reversed at any age by food restriction. Plasma leptin levels were elevated in all obese groups.

CONCLUSIONS

(1) Early food restriction provided the greatest metabolic and renal benefits; (2) glomerular injury correlated with hyperphagia-induced hyperfiltration and hypertriglyceridemia and both were prevented by food restriction; (3) hypercholesterolemia was due to an increase in LDL and/or VLDL cholesterol; and (4) leptin does not directly contribute to glomerular injury in the obese Zucker rat.

bcn-1 Element-dependent activation of the laminin gamma 1 chain gene by the cooperative action of transcription factor E3 (TFE3) and Smad proteins

Laminin is a major component of the extracellular matrix. The laminin gamma1 chain is the least variant component of the laminin heterotrimeric assembly. The laminin gamma1 chain gene (LAMC1) expression is induced by several factors, including transforming growth factor-beta (TGF-beta). LAMC1 promoter contains a highly conserved transcriptional element, bcn-1. We screened cDNA libraries with the yeast one-hybrid system to identify transcriptional factors that are recognized by the bcn-1 motif. Using this strategy we isolated the basic helix-loop-helix/leucine zipper (bHLHzip) E-box-binding transcription factor, TFE3. Until now, the E-box was the only element known to recruit the bHLHzip transcription factors. Although the bcn-1 element only remotely resembles the E-box sequence, we show that TFE3 binds and activates the bcn-1 element. TFE3 cooperates with Smad proteins in the activation of the LAMC1 promoter in cells, an effect that is critically dependent not only on the bcn-1 element but also on one of the Smad-binding elements (SBE). The cooperative induction of the LAMC1 promoter and the endogenous LAMC1 gene by TFE3 and Smad3 is augmented by the TGF-beta signaling pathway. Thus, the bcn-1 is a novel TFE3-dependent TGF-beta target element that regulates LAMC1 gene expression.

IGF-1 induces foam cell formation in rat glomerular mesangial cells

When rat glomerular mesangial cells (MCs) are cultured with IGF-1 they accumulate intracellular lipid and take on foam cell morphology. These changes were characterized by electron microscopy and Nile red staining. To define the mechanism responsible for IGF-1-mediated lipid uptake, MCs were evaluated for endocytosis, scavenger receptor activity, and receptor-mediated uptake by the LDL receptor. Lipid accumulation was markedly increased when MCs were cultured with IGF. The primary route of uptake was through enhanced endocytosis. Lipid-laden MCs have decreased phagocytic capacity and disrupted cytoskeletons. These data show that IGF-1 induces MC to take on a foam cell morphology and that lipid-laden MCs have impaired phagocytic function.

Inflammation is probably not a prerequisite for renal interstitial fibrosis in normoglycemic obese rats

We examined the role of inflammation in the development of renal interstitial fibrosis in Zucker obese rats, which rapidly present kidney lesions in the absence of hypertension and hyperglycemia. Type I and III collagens were quantified using a polarized light and computer-assisted image analyzer. The expression of mRNA encoding matrix components, adhesion molecules, chemokines, and growth factors was followed by RT-PCR. The presence of synthesized proteins as well as lymphocytes and macrophages was determined by immunohistochemistry. Interstitial fibrosis developed in two phases. The first phase occurred as early as 3 mo and resulted from a neosynthesis of type III collagen and fibronectin and a reduction of extracellular matrix catabolism, in parallel with an overexpression of transforming growth factor-beta(1) and in the absence of any lymphocyte or macrophage infiltration. After 6 mo, interstitial fibrosis worsened with a large accumulation of type I collagen, concomitantly with a large macrophage infiltration. Thus inflammation cannot explain the onset of interstitial fibrosis that developed in young, insulinoresistant, normoglycemic, obese Zucker rats but aggravated this process afterward.

Insulin-transferrin-selenous acid in growth medium alters the expression of PKC isoforms in mesangial cells

Glomerular mesangial cells (MCs) have been used as an in vitro model for glomerular disease. The culture conditions used for these cells vary and include the use of insulin or insulin-transferrin-selenous acid (ITS) in the growth medium. We studied the effect of ITS in the growth medium containing either normal or high glucose on the expression of protein kinase C (PKC) isoforms in MCs. In the presence of ITS in the medium, MCs expressed lower levels of both PKC isoforms in their cytosol in comparison to MCs grown in medium without ITS. Upon stimulation with PMA, both isoforms were translocated to the particulate (nucleus/cytoskeleton) compartment in MCs grown in presence of ITS. However, in the absence of ITS in the growth medium, both PKC isoforms were primarily translocated to the membrane compartment upon PMA stimulation. These results indicate that insulin in the growth medium may activate MCs resulting in translocation of PKC from the cytosol to other subcellular compartments. This effect is even evident in MCs grown in normal glucose concentration. Our data indicate that the use of ITS in growth medium and eventual interpretation from such experiments involving primary mesangial cells grown in culture needs careful evaluation.

IGFBP-5(201-218) stimulates Cdc42GAP aggregation and filopodia formationin migrating mesangial cells

BACKGROUND

We have previously shown that insulin-like growth factor-I (IGF-I) and IGF binding protein-5 (IGFBP-5) induce mesangial cell migration using separate stimulatory and effector pathways. The IGFBP-5 stimulatory pathway is mediated by the serine/threonine kinase IGFBP-5 receptor, which is activated by the carboxy-terminal peptide IGFBP-5201-218. In this study, we examined the direct effects of IGFBP-5201-218 on stimulatory and effector pathways that lead to a change in mesangial cell (MC) phenotype.

METHODS

Rapid actin reorganization, formation of filopodia, and characterization of novel substratum attachment structures that develop during IGFBP-5-mediating migration were examined by light, immunofluorescence, and electron microscopy. Using a wounding assay, migration was measured after the addition of stimulants and inhibitors.

RESULTS

Stimulation of MCs with IGFBP-5201-218 induces rapid actin reorganization and loss of peripheral focal adhesions. The MCs develop long cellular extensions where f-actin and beta-actin terminate in unique substratum attachments. Fluorescence microscopy of stimulated cells shows that Cdc42GAP aggregates within minutes following treatment with IGFBP-5201-218. In contrast, IGF-I increases staining for Rac-1, but not Cdc42GAP, in association with the formation of prominent leading lamellae without filopodia. Staurosporin inhibits cell migration and Cdc42GAP aggregation only when added within the first hour, suggesting that it inhibits the stimulatory effect of IGFBP-5201-218 by blocking the IGFBP-5 receptor serine/threonine kinase activity.

CONCLUSIONS

These data demonstrate that IGFBP-5201-218 preferentially activates Cdc42 and induces the formation of long filopodia with unique substratum attachments that produce a novel mode of locomotion.

SPARC regulates the expression of collagen type I and transforming growth factor-beta1 in mesangial cells

The matricellular protein SPARC is expressed at high levels in cells that participate in tissue remodeling and is thought to regulate mesangial cell proliferation and extracellular matrix production in the kidney glomerulus in a rat model of glomerulonephritis (Pichler, R. H., Bassuk, J. A., Hugo, C., Reed, M. J., Eng, E., Gordon, K. L., Pippin, J., Alpers, C. E., Couser, W. G., Sage, E. H., and Johnson, R. J. (1997) Am. J. Pathol. 148, 1153-1167). A potential mechanism by which SPARC controls both cell cycle and matrix production has been attributed to its regulation of a pleiotropic growth factor. In this study we used primary mesangial cell cultures from wild-type mice and from mice with a targeted disruption of the SPARC gene. SPARC-null cells displayed diminished expression of collagen type I mRNA and protein, relative to wild-type cells, by the criteria of immunocytochemistry, immunoblotting, and the reverse transcription-polymerase chain reaction. The SPARC-null cells also showed significantly decreased steady-state levels of transforming growth factor-beta1 (TGF-beta1) mRNA and secreted TGF-beta1 protein. Addition of recombinant SPARC to SPARC-null cells restored the expression of collagen type I mRNA to 70% and TGF-beta1 mRNA to 100% of wild-type levels. We conclude that SPARC regulates the expression of collagen type I and TGF-beta1 in kidney mesangial cells. Since increased mitosis and matrix deposition by mesangial cells are characteristics of glomerulopathies, we propose that SPARC is one of the factors that maintains the balance between cell proliferation and matrix production in the glomerulus.

Effect of high glucose on mesangial cell protein kinase C-delta and -epsilon is polyol pathway-dependent

In diabetes mellitus, enhanced activity of mesangial cell protein kinase C (PKC) may contribute to nephropathy. The purpose of this study was to determine whether high glucose alters mesangial cell diacylglycerol-sensitive PKC-alpha, -beta2, -delta, and -epsilon content, cellular distribution, and activity through polyol pathway activation. Primary cultured rat mesangial cells (passage 10) were growth-arrested in 0.5% fetal bovine serum and cultured in 5.6 mM glucose (NG) or 30 mM glucose (HG) for 48 h, with or without the aldose reductase inhibitor tolrestat or ARI-509. PKC isoform content in total cell lysates, or cytosol, membrane (Triton X-soluble), and particulate (sodium dodecyl sulfate-soluble) fractions was analyzed by immunoblotting, and band density in HG was expressed as a percentage of corresponding NG values. In HG at 48 h, increased total PKC-alpha (222 +/- 17% of NG, P < 0.001), -beta2 (209 +/- 12%, P < 0.001), and -epsilon (195 +/- 19%, P < 0.001) were observed. L-Glucose had no effect on total PKC isoform content. HG caused increased membrane- and particulate-associated PKC-alpha (257 +/- 87 and 327 +/- 66%, respectively, P < 0.05), membrane-associated PKC-delta (143 +/- 10%, P < 0.05), and membrane-associated PKC-epsilon (186 +/- 11%, P < 0.001), with no change in cytosol contents. The HG effects were not mimicked by L-glucose. In NG or HG, PKC-beta2 was not detected in the cytosol fraction, and membrane and particulate association were unchanged with phorbol ester stimulation. Confocal immunofluorescence imaging revealed that in HG, PKC-alpha, -delta, and -epsilon translocate to the nucleus and plasma membrane. Total PKC activity measured by in situ 32P-phosphorylation of the epidermal growth factor receptor substrate increased from 18 +/- 1 pmol/min per mg cell protein in NG to 33 +/- 3 pmol/min per mg cell protein in HG (P < 0.002 versus NG). In NG, tolrestat and ARI-509 exposure caused increased PKC activity, enhanced accumulation of total PKC-alpha and -beta2, with no change in total or fractional recovery of PKC-delta or -epsilon. In HG, tolrestat and ARI-509 prevented the increase in total PKC-epsilon and membrane-associated PKC-delta and -epsilon. It is concluded that within 48 h of HG, enhanced mesangial cell PKC activity is associated with accumulation and cellular redistribution of diacylglycerol-sensitive PKC isoforms, and that increased PKC-epsilon content and membrane-associated PKC-delta and -epsilon are dependent on polyol pathway activation.

Rat mesangial cells express two unique isoforms of laminin which modulate mesangial cell phenotype

Rat mesangial cells express two unique isoforms of laminin which can be modulated by culture medium composition. To define further the nature of laminin expressed by cultured rat mesangial cells, synthesis of individual laminin chains, as well as their trimeric association, was examined. Based on data from Northern analysis of mRNA expression, immunoblots, immunofluorescence staining and radioimmunoprecipitation of biosynthetically labeled proteins, mesangial cells express laminin beta1, beta2, and gamma1 chains. Mesangial cells do not express laminin alpha1 or alpha2. MC produce a unique alpha chain, designated alpha'm. These laminin chains assemble into two major isoforms. One contains alpha'mbeta1gamma1, co-precipitates with entactin and is assembled into the fibrillar extracellular matrix. The second isoform contains alpha'mbeta2 and a presumed gamma chain that migrates in gel slightly ahead of gamma1. The beta2-containing isoform is concentrated in punctate sites on the cell surface. In addition, mesangial cells display different phenotypes when plated on laminin-1 (alpha1beta1gamma1), as compared to purified beta2. An LRE-containing peptide of laminin beta2 serves as an attachment site for mesangial cells and is sufficient to induce the phenotype observed with intact beta2. These data suggest that laminin isoform expression plays an important role in mesangial cell phenotype and function.

Reduction of insulin and triglycerides delays glomerulosclerosis in obese Zucker rats

To evaluate the effect of insulin and/or triglycerides on the pathogenesis of glomerulosclerosis, acarbose (BAYg5421), an inhibitor of intestinal alpha-glucosidases, was administered as a dietary admix (40 mg/100 g chow) to Zucker obese rats (ZOA), from 1.5 months until sacrifice at 1.5, 5, 8, 10 and 15 months. Obese (ZO) and lean (ZL) rats served as controls. Despite a similar food intake, ZOA weighed less than ZO at all ages. Acarbose reduced serum triglycerides at all ages, and insulin until 10 months. Glycemia remained normal in all groups. Proteinuria developed with age and to a greater degree in ZO than in ZOA rats. In ZL, a faint proteinuria appeared only in the oldest animals. Glomerulosclerosis, tubular and interstitial lesions rapidly affected ZO kidneys. These lesions were reduced in ZOA until 10 months. Acarbose did not modify the hypertrophy of the glomeruli that developed after three months, but slowed down the expansion of the mesangial domain seen in ZO. Thus, by reducing the amount of ingested glucose, acarbose yielded a normal glycemia with a lesser production of insulin and reduced renal impairment. Therefore, insulin could be a key factor involved in the pathogenesis of glomerulosclerosis, either directly or through a control of triglyceride concentrations.

Heparin binding domain of insulin-like growth factor binding protein-5 stimulates mesangial cell migration

Insulin-like growth factor I (IGF-I) binding protein-5 (IGFBP-5) is produced by mesangial cells (MCs) and likely functions to modulate glomerular IGF-I activity. Although IGFBP-5 may be inhibitory for IGF-stimulated MC activity, preliminary studies suggested that IGFBP-5 acts directly on MCs. To investigate this further, we evaluated the effects of IGFBP-5 on rat MC migration. We found that the carboxytruncated fragment, IGFBP-5-(1-169), inhibited IGF-I-stimulated migration, but intact IGFBP-5 simulated migration when IGF-I was not present. Demonstration that 125I-labeled IGFBP-5 directly binds to MCs further supports an independent role for IGFBP-5. Because heparin inhibited MC binding of 125I-IGFBP-5, we tested the heparin binding peptide, IGFBP-5-(201-218), for stimulatory activity. IGFBP-5-(201-218) stimulated MC migration, and this effect was inhibited by heparin. Because the disintegrin, kistrin, blocked IGF-I-induced migration but not migration induced by IGFBP-5-(201-218), the migratory induction mechanism for the two peptides is different. These data indicate that separate, specific regions of IGFBP-5 are responsible for interactive effects with IGF-I as well as direct effects on MC activity.

Insulin-like growth factor I (IGF-I) induces unique effects in the cytoskeleton of cultured rat glomerular mesangial cells

Resident glomerular mesangial cells (MCs) have complex cytoskeletal organizations that maintain functional and structural integrity. The ability of cells to replicate, coordinate movement, change shape, and interact with contiguous cells or extracellular matrix depends on cytoskeletal organization. MCs synthesize insulin-like growth factor (IGF-I), express IGF-I receptors, and respond to IGF-I with increased proliferation. We noted that IGF-I treatment of mesangial cells was associated with a change in morphology. Therefore, these studies were undertaken to define specific IGF-I-mediated changes in cytoskeletal protein organization. Rat MCs were propagated from birth in culture without supplemental insulin. Quiescent, subconfluent cultures were treated with IGF-I (100 nM) for 1 hr. Rearrangements in f-actin, alpha-smooth muscle actin, beta-actin, vimentin, and vinculin were seen by fluorescence microscopy. As the cytoskeleton rearranged, alpha-smooth muscle actin dissociated from the f-actin bundles and beta-actin became polymerized under the leading lamellar edge. Ultrastructural changes were consistent with increased membrane turnover and metabolic activity. The normally sessile mesangial cell was induced by IGF-I to express a wound-healing phenotype characterized by movement and increased pinocytosis. These changes are different from those induced by insulin and have important implications for mesangial cell function.