How does proteinuria cause progressive renal damage?

Proteinuria and hyperglycemia induce endoplasmic reticulum stress

The endoplasmic reticulum (ER) is an important site for protein folding and becomes "stressed" when its capacity to fold proteins is overwhelmed. In response, "unfolded protein response" (UPR) genes are induced, increasing the capacity to fold proteins; if the response is insufficient, then apoptosis ensues. For investigation of whether proteinuria and hyperglycemia induce ER stress in renal epithelial cells, microarray data from biopsies of established diabetic nephropathy (DN) were analyzed. Expression of UPR genes was significantly different in these biopsies than in control kidneys or biopsies of patients with mild DN, suggesting an association between the degree of DN and UPR gene expression. Expression of the transcription factor XBP1 and the ER chaperones HSPA5 and HYOU1 were increased, but the proapoptotic gene DDIT3 was unchanged. These findings were replicated in an independent cohort of patients with established DN by real-time reverse transcriptase-PCR. Immunofluorescence of renal biopsies from patients with DN confirmed the upregulation for HSPA5 and HYOU1 proteins in tubular epithelia. In biopsies of minimal-change disease, the mRNA levels of some ER stress molecules were also induced, but protein expression of HSPA5 and HYOU1 remained significantly lower than that observed in DN. Exposure of renal tubular epithelial cells to albumin and high glucose in vitro enhanced expression of genes involved in ER stress. These observations suggest that in proteinuric diseases, tubular epithelial cells undergo ER stress, which induces an adaptive, protective UPR. Although this may protect the cells from ER stress, persistence of hyperglycemia and proteinuria may eventually lead to apoptosis.

Megalin contributes to the early injury of proximal tubule cells during nonselective proteinuria

Megalin, a member of the LDL receptor family, is expressed on the apical membrane of proximal tubules and serves as an endocytic scavenger of filtered proteins and hence might contribute to the tubule injury as a consequence of glomerular disease. To study its role, we crossed megalin knockout mosaic mice (lacking megalin expression in 60% of proximal tubule cells) with NEP25 mice (a transgenic line expressing human CD25 in the podocyte). Treatment of this transgenic mouse with the immunotoxin causes nephrotic syndrome, focal segmental glomerulosclerosis and tubule-interstitial injury. Following this treatment, the double transgenic mice had massive non-selective proteinuria and mild glomerular and tubular injury. Comparison of megalin-containing to megalin-deficient proximal tubule cells within each kidney showed that albumin, immunoglobulin light chain, IgA and IgG were preferentially accumulated in proximal tubule cells expressing megalin. Tubule injury markers such as heme-oxygenase-1, monocyte chemoattractant protein-1 and cellular apoptosis were also preferentially found in these megalin-expressing cells. These results show that megalin plays a pivotal role in the reabsorption of small to large molecular size proteins and provides direct in vivo evidence that reabsorption of filtered proteins triggers events leading to tubule injury.

EMT and proteinuria as progression factors

Tubulointerstitial fibrosis is an integral part of the structural changes of the kidney in chronic progressive renal failure. The accumulation of the extracellular matrix in the tubulointerstitial space is mediated mainly by myofibroblasts. These are derived from resident interstitial fibroblasts, tubular epithelial cells, periadventitial cells, and possibly also mesenchymal stem cells and endothelial cells. Fibrosis is usually preceded by tubulointerstitial infiltration of mononuclear inflammatory cells. Proteinuria is one of several mechanisms of primary glomerular or vascular disease to transmit the disease process to the interstitial space. Increased protein filtration may have direct toxic effects on tubular epithelial cells, induce chemokine and cytokine secretion and result in increased expression of adhesion molecules, all contributing to the influx of mononuclear cells. Inflammatory cells in return secrete cytokines, which stimulate resident fibroblasts and tubular epithelial cells to differentiate into matrix-producing cells. The phenotypic conversion of primary epithelial cells into mesenchymal cells, termed epithelial-mesenchymal transition (EMT), has been studied in great detail in recent years. Several signal transduction pathways of this process have been clarified and may eventually result in novel therapeutic approaches. The severity of proteinuria and the extent of EMT have both been associated with the decline in renal function in clinical studies. Limiting proteinuria results in a slower decline of renal function deterioration, whereas reducing EMT has had beneficial effects in a number of animal studies, including those indicating reversal of fibrotic lesions. However, the association between proteinuria and EMT and vice versa is far from clear and has not been carefully studied.

Pioglitazone enhances the antihypertensive and renoprotective effects of candesartan in Zucker obese rats fed a high-protein diet

The metabolic syndrome is a risk factor for the development of chronic kidney disease. Angiotensin II type 1 receptor blockers (ARBs) and thiazolidinediones (TZDs) provide renovascular protection, probably in the metabolic syndrome. However, the effect of both agents administered together in patients with metabolic syndrome remains to be determined. The aim of this study was to assess the effects of ARB plus TZD combination therapy in Zucker obese rats fed a high-protein diet, an animal model of metabolic syndrome and renal injury. Zucker obese rats were fed a high-protein diet (OHP; n=6), a high-protein diet containing candesartan, an ARB (OHP+C; n=6), or a high-protein diet containing both candesartan and pioglitazone (OHP+CP; n=6) for 12 weeks. Systolic blood pressure and urinary protein excretion were measured throughout the study, and renal histology and immunohistochemistry were assessed at 12 weeks. OHP rats developed hypertension (157+/-4 mmHg) and proteinuria (178+/-44 mg/d), and these conditions were significantly ameliorated by candesartan (to 143+/-3 mmHg and 84+/-25 mg/d, respectively). Pioglitazone enhanced the antihypertensive and anti-proteinuric effects of candesartan (121+/-3 mmHg, 16+/-8 mg/d, respectively). Histologically, candesartan ameliorated glomerulosclerosis, podocyte injury, interstitial fibrosis and monocyte/macrophage infiltration into the tubulointerstitium in the kidneys of OHP rats. Pioglitazone abrogated residual interstitial fibrosis in the kidneys of OHP+C rats. Our results suggested that pioglitazone augmented the antihypertensive, anti-proteinuric and possibly renal anti-fibrotic actions of candesartan in Zucker obese rats fed a high-protein diet. The combination therapy of ARB and TZD may protect against renal injury in patients with metabolic syndrome.

Podocyte-specific overexpression of the antioxidant metallothionein reduces diabetic nephropathy

Podocytes are critical components of the selective filtration barrier of the glomerulus and are susceptible to oxidative damage. For investigation of the role of oxidative stress and podocyte damage in diabetic nephropathy, transgenic mice that overexpress the antioxidant protein metallothionein (MT) specifically in podocytes (Nmt mice) were produced. MT expression was increased six- and 18-fold in glomeruli of two independent lines of Nmt mice, and podocyte-specific overexpression was confirmed. Glomerular morphology and urinary albumin excretion were normal in Nmt mice. OVE26 transgenic mice, a previously reported model of diabetic nephropathy, were crossed with Nmt mice to determine whether an antioxidant transgene targeted to podocytes could reduce diabetic nephropathy. Double-transgenic OVE26Nmt mice developed diabetes similar to OVE26 mice, but MT overexpression reduced podocyte damage, indicated by more podocytes, less glomerular cell death, and higher density of podocyte foot processes. In addition, expansion of glomerular and mesangial volume were significantly less in OVE26Nmt mice compared with OVE26 mice. Four-month-old OVE26Nmt mice had a 70 to 90% reduction in 24-h albumin excretion, but this protection does not seem to be permanent. These results provide evidence for the role of oxidative damage to the podocyte in diabetic mice and show that protection of the podocyte can reduce or delay primary features of diabetic nephropathy.

Selective loss of podoplanin protein expression accompanies proteinuria and precedes alterations in podocyte morphology in a spontaneous proteinuric rat model

To evaluate changes during the development of proteinuria, podocyte morphology and protein expression were evaluated in spontaneously proteinuric, Dahl salt-sensitive (Dahl SS) rats. Dahl SS rats on a low-salt diet were compared with spontaneously hypertensive rats (SHR) at age 2, 4, 6, 8, and 10 weeks. Blood pressure, urinary protein excretion, urinary albumin excretion, and podocyte morphology were evaluated. In addition, the expression of 11 podocyte-related proteins was determined by analyzing protein and mRNA levels. In Dahl SS rats, proteinuria became evident around week 5, increasing thereafter. SHR rats remained non-proteinuric. Dahl SS rats showed widespread foot process effacement at 10 weeks. At < or =8 weeks, expression and distribution of the podocyte proteins was similar between the two strains, except for the protein podoplanin. At 4 weeks, podoplanin began decreasing in the glomeruli of Dahl SS rats in a focal and segmental fashion. Podoplanin loss increased progressively and correlated with albuminuria (r = 0.8, P < 0.001). Double labeling experiments revealed increased expression of the podocyte stress marker desmin in glomerular areas where podoplanin was lost. Dahl SS rats did not show podoplanin gene mutations or decreased mRNA expression. Thus, podocyte morphology and the expression and distribution of most podocyte-specific proteins were normal in young Dahl SS rats, despite marked proteinuria. Our study suggests that decreased expression of podoplanin plays a role in the decrease of glomerular permselectivity.

Relationship between albuminuria and total proteinuria in systemic lupus erythematosus nephritis: diagnostic and therapeutic implications

BACKGROUND AND OBJECTIVES

Albuminuria is regarded a sensitive measure of progression of glomerular disease. This study was undertaken in patients who had systemic lupus erythematosus glomerulonephritis (n = 57) and were followed in the Ohio SLE Study to determine whether measuring albuminuria offered clinical advantages over that of total proteinuria.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Twenty-four-hour urine collections (n = 127) were obtained at baseline and annually for measurement of microalbumin, total protein, and creatinine.

RESULTS

There was a strong linear relationship between microalbumin-creatinine and protein-creatinine ratios over the entire range of protein-creatinine ratios; however, in the protein-creatinine ratio range 0.0 to 0.3, as the protein-creatinine ratio increased, the microalbumin-protein ratio increased much more than the protein-creatinine ratio. Also, the greater the protein-creatinine ratio, the greater was the evidence for nonselective proteinuria (protein-creatinine ratio--microalbumin-creatinine ratio).

CONCLUSIONS

For the diagnosis of proteinuria renal flare, measuring albuminuria offers no advantage over measuring total proteinuria because changes in protein-creatinine and microalbumin-creatinine ratios are highly correlated over the designated ranges for systemic lupus erythematosus glomerulonephritis proteinuric flares. In those with normal-range proteinuria, subsequent changes in microalbumin-protein ratio might be a better forecaster of renal flare than changes in protein-creatinine or microalbumin-creatinine ratio. High protein-creatinine ratios are associated with evidence of nonselective proteinuria, which may increase the nephrotoxicity of proteinuria. Thus, using high-threshold criteria for systemic lupus erythematosus flare (allowing greater proteinuria increase before flare is declared) may expose the kidney to greater nephrotoxicity than using the low-threshold criteria for systemic lupus erythematosus flare.

Proteinuria in diabetic kidney disease: a mechanistic viewpoint

Proteinuria is the hallmark of diabetic kidney disease (DKD) and is an independent risk factor for both renal disease progression, and cardiovascular disease. Although the characteristic pathological changes in DKD include thickening of the glomerular basement membrane and mesangial expansion, these changes per se do not readily explain how patients develop proteinuria. Recent advances in podocyte and glomerular endothelial cell biology have shifted our focus to also include these cells of the glomerular filtration barrier in the development of proteinuria in DKD. This review describes the pathophysiological mechanisms at a cellular level which explain why patients with DKD develop proteinuria.

AT1 blockade during lactation as a model of chronic nephropathy: mechanisms of renal injury

Suppression of the renin-angiotensin system during lactation causes irreversible renal structural changes. In this study we investigated 1) the time course and the mechanisms underlying the chronic kidney disease caused by administration of the AT(1) receptor blocker losartan during lactation, and 2) whether this untoward effect can be used to engender a new model of chronic kidney disease. Male Munich-Wistar pups were divided into two groups: C, whose mothers were untreated, and L(Lact), whose mothers received oral losartan (250 mg.kg(-1).day(-1)) during the first 20 days after delivery. At 3 mo of life, both nephron number and the glomerular filtration rate were reduced in L(Lact) rats, whereas glomerular pressure was elevated. Unselective proteinuria and decreased expression of the zonula occludens-1 protein were also observed, along with modest glomerulosclerosis, significant interstitial expansion and inflammation, and wide glomerular volume variation, with a stable subpopulation of exceedingly small glomeruli. In addition, the urine osmolality was persistently lower in L(Lact) rats. At 10 mo of age, L(Lact) rats exhibited systemic hypertension, heavy albuminuria, substantial glomerulosclerosis, severe renal interstitial expansion and inflammation, and creatinine retention. Conclusions are that 1) oral losartan during lactation can be used as a simple and easily reproducible model of chronic kidney disease in adult life, associated with low mortality and no arterial hypertension until advanced stages; and 2) the mechanisms involved in the progression of renal injury in this model include glomerular hypertension, glomerular hypertrophy, podocyte injury, and interstitial inflammation.

Neutrophil gelatinase-associated lipocalin (NGAL) as a marker of kidney damage

Neutrophil gelatinase-associated lipocalin (NGAL) is a protein belonging to the lipocalin superfamily initially found in activated neutrophils, in accordance with its role as an innate antibacterial factor. However, it subsequently was shown that many other types of cells, including in the kidney tubule, may produce NGAL in response to various injuries. The increase in NGAL production and release from tubular cells after harmful stimuli of various kinds may have self-defensive intent based on the activation of specific iron-dependent pathways, which in all probability also represent the mechanism through which NGAL promotes kidney growth and differentiation. NGAL levels predict the future appearance of acute kidney injury after treatments potentially detrimental to the kidney and even the acute worsening of unstable nephropathies. Furthermore, recent evidence also suggests that NGAL somehow may be involved in the pathophysiological process of chronic renal diseases, such as polycystic kidney disease and glomerulonephritis. NGAL levels clearly correlate with severity of renal impairment, probably expressing the degree of active damage underlying the chronic condition. For all these reasons, NGAL may become one of the most promising next-generation biomarkers in clinical nephrology and beyond.

Complement-mediated dysfunction of glomerular filtration barrier accelerates progressive renal injury

Intrarenal complement activation leads to chronic tubulointerstitial injury in animal models of proteinuric nephropathies, making this process a potential target for therapy. This study investigated whether a C3-mediated pathway promotes renal injury in the protein overload model and whether the abnormal exposure of proximal tubular cells to filtered complement could trigger the resulting inflammatory response. Mice with C3 deficiency were protected to a significant degree against the protein overload-induced interstitial inflammatory response and tissue damage, and they had less severe podocyte injury and less proteinuria. When the same injury was induced in wild-type (WT) mice, antiproteinuric treatment with the angiotensin-converting enzyme inhibitor lisinopril reduced the amount of plasma protein filtered, decreased the accumulation of C3 by proximal tubular cells, and protected against interstitial inflammation and damage. For determination of the injurious role of plasma-derived C3, as opposed to tubular cell-derived C3, C3-deficient kidneys were transplanted into WT mice. Protein overload led to the development of glomerular injury, accumulation of C3 in podocytes and proximal tubules, and tubulointerstitial changes. Conversely, when WT kidneys were transplanted into C3-deficient mice, protein overload led to a more mild disease and abnormal C3 deposition was not observed. These data suggest that the presence of C3 increases the glomerular filtration barrier's susceptibility to injury, ultrafiltered C3 contributes more to tubulointerstitial damage induced by protein overload than locally synthesized C3, and local C3 synthesis is irrelevant to the development of proteinuria. It is speculated that therapies targeting complement combined with interventions to minimize proteinuria would more effectively prevent the progression of renal disease.

Mapping quantitative trait loci for proteinuria-induced renal collagen deposition

The progression of chronic kidney disease is a complex process influenced by genetic factors. Proteinuria is a predictor of functional deterioration and an accelerator of disease progression through renal parenchymal damage and interstitial fibrosis. To determine genetic components that might mediate renal fibrosis due to proteinuria, we mapped loci influencing the phenotype of two mouse strains differing in proteinuria-induced renal type I collagen (COLI) deposition. Collagen I deposition in 129S1/svImJ and C57BL/6J mice differs significantly among tested strains. We backcrossed 120 hemi-nephrectomized (129S1/svImJ x C57BL/6J) F1 x 129S1/svImJ backcrossed mice loaded with bovine serum albumin giving rise to proteinuria and renal COLI deposition. Quantitative trait loci (QTL) mapping was performed and our analysis identified one suggestive linkage for renal COLI deposition peaking at 87 cM near D2Mit224 (logarithm of odds: 2.41) on Chr 2. In silico analysis uncovered nine candidate genes. Hence, although more studies are needed, these QTL provide an initial cue to subsequent gene discovery, which might help unravel the genetics of renal fibrosis.

Urinary excretion of monocyte chemoattractant protein-1: a biomarker of active tubulointerstitial damage in patients with glomerulopathies

BACKGROUND/AIMS

The urinary concentration of the monocyte chemoattractant protein-1 (uMCP-1) chemokine is increased in several proteinuric and/or inflammatory renal diseases. In the present study, we evaluated the association between uMCP-1 and renal function, proteinuria, glomerular and interstitial macrophage infiltration, and renal fibrosis in patients with primary and secondary glomerulopathies diagnosed by renal biopsy.

METHODS

Thirty-seven patients aged 32.6 +/- 7.7 years were studied. uMCP-1 was determined by ELISA. Renal macrophage expression (CD68 positive cells) is reported as number of macrophages/10(4) microm2 of the cortical tubulointerstitial (TI) area or of glomerular capillary tuft area. Cortical interstitial fibrosis was quantitated by PicroSirius red staining under polarized light by a computerized manner.

RESULTS

The uMCP-1 ratio (pg/ml/urinary creatinine mg/ml) was positively correlated (Spearman coefficient) with proteinuria (r = 0.4629; p < 0.005) and number of macrophages in the cortical TI area (r = 0.64; p = 0.0005), and negatively correlated with creatinine clearance (r = -0.4877; p < 0.001). The uMCP-1 ratio was not significantly correlated with number of macrophages/glomerular capillary tuft area (r = 0.27; p = 0.19) or with percent cortical interstitial fibrosis (r = 0.08; p = 0.62).

CONCLUSIONS

The uMCP-1 excretion is a biomarker of the inflammatory activity of the TI area, and does not reflect chronic interstitial damage.

Diabetic nephropathy and proximal tubule ROS: challenging our glomerulocentricity

Diabetic nephropathy is currently viewed as a predominantly glomerular process with glomerular injury driving secondary tubular loss. Brezniceanu and colleagues apply transgenic methods to support a prominent role for reactive oxygen species as mediators and for the proximal tubule as a major site of early disease activity in diabetes. Results support evidence for early tubular apoptosis and atrophy in human diabetic nephropathy.

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.

Albuminuria: a target for treatment of type 2 diabetic nephropathy

Both renal and cardiovascular morbidity and mortality is increased markedly in patients with type 2 diabetes. Besides the classic risk factors and markers such as glucose, blood pressure, blood lipid profile, and lifestyle (smoking, overweight), novel risk markers are identified, among them urine albumin excretion. Levels of urinary albumin excretion greater than normal are observed frequently in patients with type 2 diabetes. Moderately increased levels of albuminuria, so-called microalbuminuria, are predictive both for progressive renal function loss to diabetic nephropathy, and for cardiovascular morbidity and mortality: the higher the albuminuria level, the more chance of renal and cardiovascular complications. More advanced levels of albuminuria (overt albuminuria) are observed in patients in the diabetic nephropathy state. In this condition, renal and cardiovascular risk are extremely high, and again one may observe that the level of albumin excretion is predictive of renal and cardiovascular outcome. Several drug strategies decrease the level of urinary albumin excretion in type 2 diabetic patients. Data on using drugs that intervene in the renin-angiotensin-aldosterone-system (RAAS) are the most extensive and conclusive. RAAS intervention is a very effective strategy to decrease the amount of albumin in the urine, independent from the blood pressure decreasing characteristics of the treatment. RAAS intervention is associated with long-term renal and cardiovascular protection. Importantly, the degree of short-term albuminuria decrease is associated with the degree of renal and cardiovascular protection: the more albuminuria reduction, the more protection. The protective predictive power of the albuminuria effect of RAAS intervention is not related to (or dissociated from) the blood pressure decreasing effect of these drugs. The protective effect of RAAS intervention is present at normoalbuminuric, microalbuminuric, and overt albuminuria levels. This makes albuminuria a target for therapy in type 2 diabetes. New drug strategies that decrease or prevent albuminuria without affecting other risk factors currently are being tested, and not only will add to underscoring the need to treat albuminuria as a separate target, but also will assist in reducing the enormous residual risk burden of individual diabetic patients.

Opioids induce renal abnormalities in tumor-bearing mice

BACKGROUND/AIMS

The etiology of renal dysfunction in cancer patients is likely to be multifactorial. A large proportion of these patients receive opioid analgesics, but whether opioids contribute to renal dysfunction remains uncertain. In a murine cancer model, we examined the effects of chronic opioid administration on renal function and pathology, and the molecular mechanisms involved.

METHODS

C3H/HeJ mice implanted with 2472 tumor cells were treated with either morphine or hydromorphone in clinically relevant doses, or PBS (controls). Renal function was assessed by blood and urine chemistry as well as by measuring mean arterial pressure (MAP) and kidney perfusion. Pathological changes in the kidneys were examined by routine histology. Molecular changes were examined by assessing eNOS, iNOS, HO-1 and COX-2 expression in whole-kidney lysates by Western immunoblotting, and cellular colocalization of these enzymes was determined using immunofluorescence microscopy.

RESULTS

Three weeks of opioid treatment resulted in increased kidney weight, elevated BUN and proteinuria, and decreased MAP. This was accompanied by histological abnormalities including glomerular enlargement, hypercellularity, peritubular congestion, vasodilatation and tubular casts. The vasoregulatory molecules iNOS, eNOS, HO-1 and COX-2 were upregulated in the kidneys. The NOS inhibitor L-NAME prevented the morphine-induced increase in perfusion and kidney weight.

CONCLUSIONS

The chronic use of clinically relevant doses of opioidsleads to structural kidney abnormalities, upregulates NOS, COX-2 and HO-1, and results in renal dysfunction in a murine model of cancer.

Mechanisms of progression of chronic kidney disease

Chronic kidney disease (CKD) occurs in all age groups, including children. Regardless of the underlying cause, CKD is characterized by progressive scarring that ultimately affects all structures of the kidney. The relentless progression of CKD is postulated to result from a self-perpetuating vicious cycle of fibrosis activated after initial injury. We will review possible mechanisms of progressive renal damage, including systemic and glomerular hypertension, various cytokines and growth factors, with special emphasis on the renin-angiotensin-aldosterone system (RAAS), podocyte loss, dyslipidemia and proteinuria. We will also discuss possible specific mechanisms of tubulointerstitial fibrosis that are not dependent on glomerulosclerosis, and possible underlying predispositions for CKD, such as genetic factors and low nephron number.