Albumin induces cellular fibrosis by upregulating transforming growth factor-beta ligand and its receptors in renal distal tubule cells

Immunohistochemical Expression of TGF-β1 in Kidneys of Cats with Chronic Kidney Disease

Transforming growth factor-beta 1 (TGF-β1) plays a central role in the progression of chronic kidney disease (CKD). However, in feline CKD, renal expression of TGF-β1 and how it changes as the disease progresses have not been fully studied. In the present study, we immunohistochemically assessed the renal expression levels of TGF-β1 in cats with CKD and statistically analyzed its correlation with CKD severity. Clear immunosignals were detected in the glomerular mesangial cells, Bowman’s capsules, proximal tubules, distal nephrons, platelets, and vascular smooth muscles in the kidneys of cats with CKD. Statistically, luminal signals in the distal nephrons showed positive correlations with plasma creatinine levels and glomerulosclerosis, while those in the proximal tubules and platelets showed negative correlations with plasma urea and/or creatinine levels. Therefore, it was suggested that the changes in the renal expression of TGF-β1 could be associated with progression of feline CKD.

Up-regulation of the kinin B2 receptor pathway modulates the TGF-β/Smad signaling cascade to reduce renal fibrosis induced by albumin

The presence of high protein levels in the glomerular filtrate plays an important role in renal fibrosis, a disorder that justifies the use of animal models of experimental proteinuria. Such models have proved useful as tools in the study of the pathogenesis of chronic, progressive renal disease. Since bradykinin and the kinin B2 receptor (B2R) belong to a renoprotective system with mechanisms still unclarified, we investigated its anti-fibrotic role in the in vivo rat model of overload proteinuria. Upon up-regulating the kinin system by a high potassium diet we observed reduction of tubulointerstitial fibrosis, decreased renal expression of α-smooth muscle actin (α-SMA) and vimentin, reduced Smad3 phosphorylation and increase of Smad7. These cellular and molecular effects were reversed by HOE-140, a specific B2R antagonist. In vitro experiments, performed on a cell line of proximal tubular epithelial cells, showed that high concentrations of albumin induced expression of mesenchymal biomarkers, in concomitance with increases in TGF-β1 mRNA and its functionally active peptide, TGF-β1. Stimulation of the tubule cells by bradykinin inhibited the albumin-induced changes, namely α-SMA and vimentin were reduced, and cytokeratin recovered together with increase in Smad7 levels and decrease in type II TGF-β1 receptor, TGF-β1 mRNA and its active fragment. The protective changes produced by bradykinin in vitro were blocked by HOE-140. The development of stable bradykinin analogues and/or up-regulation of the B2R signaling pathway may prove value in the management of chronic renal fibrosis in progressive proteinuric renal diseases.

In vitro investigation of renal epithelial injury suggests that primary cilium length is regulated by hypoxia-inducible mechanisms

Primary cilia are non-motile sensory organelles that project from cells in many tissues. The role of renal primary cilium-based signalling in regulating epithelial cell proliferation and differentiation is highlighted by studies showing that defects of the cilium lead to epithelial de-differentiation, over proliferation and polycystic kidney disease. Recent studies show that renal primary cilia may also play a role in controlling epithelial differentiation during renal repair. After injury, renal cilium length increases dramatically and then undergoes a normalization that coincides with structural and functional repair in both human patients and mouse models of renal injury. These changes in cilium length are likely to modulate cilium-based signalling, but the injury-related factors that influence renal primary cilium length have yet to be determined. Here, we investigated the effect of three factors commonly associated with renal injury on renal cilium length in an in vitro setting. MDCK (Madin Darby canine kidney) cell cultures bearing primary cilia were treated with BSA to simulate albuminuria, cobalt chloride to simulate hypoxia and the inflammation-related cytokine tumour necrosis factor α. Primary cilium length was only increased in cultures treated with cobalt chloride. Our results suggest a role for hypoxia and the induction of HIF-1α (hypoxia-inducible factor 1α) in increasing renal primary cilium length following renal injury.

CUBN is a gene locus for albuminuria

Identification of genetic risk factors for albuminuria may alter strategies for early prevention of CKD progression, particularly among patients with diabetes. Little is known about the influence of common genetic variants on albuminuria in both general and diabetic populations. We performed a meta-analysis of data from 63,153 individuals of European ancestry with genotype information from genome-wide association studies (CKDGen Consortium) and from a large candidate gene study (CARe Consortium) to identify susceptibility loci for the quantitative trait urinary albumin-to-creatinine ratio (UACR) and the clinical diagnosis microalbuminuria. We identified an association between a missense variant (I2984V) in the CUBN gene, which encodes cubilin, and both UACR (P = 1.1 × 10(-11)) and microalbuminuria (P = 0.001). We observed similar associations among 6981 African Americans in the CARe Consortium. The associations between this variant and both UACR and microalbuminuria were significant in individuals of European ancestry regardless of diabetes status. Finally, this variant associated with a 41% increased risk for the development of persistent microalbuminuria during 20 years of follow-up among 1304 participants with type 1 diabetes in the prospective DCCT/EDIC Study. In summary, we identified a missense CUBN variant that associates with levels of albuminuria in both the general population and in individuals with diabetes.

Local overexpression of interleukin-1 family, member 6 relates to the development of tubulointerstitial lesions

Identification of factors that exacerbate a disease is important for the development of biomarkers. In this study, we discovered ectopic overexpression of interleukin-1 family, member-6 (IL-1F6) in several murine renal diseases. IL-1F6 participates in cytokine/chemokine production in the epithelium. In PCR array analysis for inflammatory mediators, Il1f6 showed the highest expression in the kidney of the B6.MRLc1 glomerulonephritis model. IL-1F6 was localized in the epithelium from the DCTs to CCDs, which showed tubular dilations or epithelial deciduations. Ultrastructual examination of the epithelial cells revealed that IL-1F6 was localized on the cytoplasmic ribosome, vesicles, and nucleus. In and around these tubules, we found infiltrations of CD3-positive T-cells and nestin- or alpha-smooth-muscle actin-positive mesenchymal cells. Expression of the IL-1F6 protein and Il1f6 mRNA in the kidney was increased by the development of TILs in the B6.MRLc1 model and in lupus (BXSB, NZB/WF1, and MRL/lpr), nephrotic syndrome (ICGN), and streptozotocin-induced diabetic models. IL-1F6 was also detected in the epithelia having squamous or deciduous contours in other organs such as the skin, esophagus, thymus, or uterus. In vitro analysis using M-1 cells from the murine collecting duct revealed that Il1f6 mRNA induction was related to the upregulation of IL-6, TGF-beta receptor-1, and mesenchymal markers and to the downregulation of epithelial markers and changes in the squamous cells of the epithelium. Interestingly, urine Il1f6 mRNA expression was detected earlier than renal dysfunctions in these mouse models. Ectopic overexpression of IL-1F6 in kidneys is associated with TILs and especially with cell infiltrations and changes in epithelial morphology. We propose that local overexpression of IL-1F6 is related to the development of TILs.

Bone morphogenetic protein-2 antagonizes renal interstitial fibrosis by promoting catabolism of type I transforming growth factor-beta receptors

TGF-beta is a therapeutic target for renal fibrosis. Scientists have long sought ways to antagonize TGF-beta to ameliorate diabetic nephropathy. Bone morphogenetic protein (BMP-2) is a member of the TGF-beta superfamily and is highly regulated in the kidney. Thus, the role of BMP-2 was investigated in NRK-49F cells (rat fibroblasts). We showed that TGF-beta1 induces an increase in fibronectin. Treatment with exogenous BMP-2 or pCMV-BMP-2 significantly reversed the TGF-beta1-induced increase in fibronectin concomitant with a significant decrease in type I TGF-beta receptors (TGF-beta RI). Moreover, BMP-2 significantly shortened the half-life of TGF-beta RI. These results are related to proteosomal activation because MG132, a proteasome inhibitor, abolished BMP-2-mediated degradation of TGF-beta RI. This was confirmed because BMP-2 time course dependently enhanced the ubiquitination level of TGF-beta RI. In addition, Smads would seem to be involved in the interaction of BMP-2 and TGF-beta. We demonstrated that BMP-2 significantly reversed the TGF-beta1-induced increase in pSmad2/3 and reversed the TGF-beta1-induced decrease in inhibitory Smad7. Most importantly, Smad7 small interfering RNA abolished the BMP-2-induced decrease in TGF-beta RI. We evaluated the clinical efficacy of BMP-2 using unilateral ureteral obstruction rats. BMP-2 was administered ip for 7 d. In the unilateral ureteral obstruction kidneys, interstitial fibrosis was prominent. However, treatment with BMP-2 dramatically reduced Masson's trichrome staining (collagen) in the interstitial and tubular areas of the kidneys concomitantly with a reduction in TGF-beta RI. These results suggest that BMP-2 acts as a novel fibrosis antagonizing cytokine partly by down-regulating TGF-beta RI and Smads.

Safflower extract: a novel renal fibrosis antagonist that functions by suppressing autocrine TGF-beta

Progressive renal disease is characterized by the accumulation of extracellular matrix proteins in the renal interstitium. Hence, developing agents that antagonize fibrogenic signals is a critical issue facing researchers. The present study investigated the blood-circulation-promoting Chinese herb, safflower, on fibrosis status in NRK-49F cells, a normal rat kidney interstitial fibroblast, to evaluate the underlying signal transduction mechanism of transforming growth factor-beta (TGF-beta), a potent fibrogenic growth factor. Safflower was characterized and extracted using water. Renal fibrosis model was established both in vitro with fibroblast cells treated with beta-hydroxybutyrate and in vivo using rats undergone unilateral ureteral obstruction (UUO). Western blotting was used to examine protein expression in TGF-beta-related signal proteins such as type I and type II TGF-beta receptor, Smads2/3, pSmad2/3, Smads4, and Smads7. ELISA was used to analyze bioactive TGF-beta1 and fibronectin levels in the culture media. Safflower extract (SE) significantly inhibited beta-HB-induced fibrosis in NRK cells concomitantly with dose-dependent inhibition of the type I TGF-beta1 receptor and its down-stream signals (i.e., Smad). Moreover, SE dose-dependently enhanced inhibitory Smad7. Thus, SE can suppress renal cellular fibrosis by inhibiting the TGF-beta autocrine loop. Moreover, remarkably lower levels of tissue collagen were noted in the nephron and serum TGF-beta1 of UUO rats receiving oral SE (0.15 g/3 ml/0.25 kg/day) compared with the untreated controls. Hence, SE is a potential inhibitor of renal fibrosis. We suggest that safflower is a novel renal fibrosis antagonist that functions by down-regulating TGF-beta signals.

Albumin transport and processing by the proximal tubule: physiology and pathophysiology

PURPOSE OF REVIEW

Significant epidemiological and clinical trial evidence supports the association between increased urinary albumin excretion, cardiovascular events and renal failure. An increase in albumin excretion has traditionally been considered to reflect a 'glomerular' leak of protein; however, it is now recognized that significant tubular reabsorption of albumin occurs under physiological conditions that may be modified by genetic determinants, systemic disease and drug therapies.

RECENT FINDINGS

The endocytosis of albumin by the proximal tubule is a highly regulated process depending on protein-protein interactions between several membrane proteins and scaffolding and regulatory molecules. The elucidation of these interactions is an ongoing research focus. There is also mounting evidence for a transcytotic pathway for retrieval of albumin from the tubular filtrate. The molecular basis for the role of albuminuria in both interstitial renal disease and cardiovascular pathology continues to be defined. The clinical implications of albuminuria due to a glomerular leak vs. reduced tubular reabsorption of albumin are, however, now under consideration. In particular, the prognostic implication of microalbuminuria induced by the more potent 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors is under study.

SUMMARY

The currently defined mechanisms underpinning the tubular reabsorption of albumin, how these are modified by pathology and pharmacology, and the clinical implications are the subject of this review.

CD36 is a novel and potential anti-fibrogenic target in albumin-induced renal proximal tubule fibrosis

Albumin is not only a risk factor for diabetic nephropathy (DN), but also a therapeutic target. Hence, scientists have long sought ways to elucidate the interactions between albumin and diabetic renal tubule fibrosis. CD36, a surface receptor for thrombospondin-1, has been reported to interact with latent transforming growth factor-beta1 (TGF-beta1) and activate its fibrogenic bioactivity. This study elucidates the interactions between CD36 and renal tubule fibrosis. LLC-PK1 cells were applied to represent renal proximal tubule cells. The expression of CD36 was evaluated by flow cytometry. Fibronectin was assayed by Western blot and enzyme-linked immunosorbent assay (ELISA). Bioactive TGF-beta1 was assayed by ELISA. We demonstrated that albumin was shown significantly to inhibit cell growth without affecting hypertrophy status since protein content and cell size remained unaffected under albumin treatment. Moreover, albumin dose-dependently (0, 1, or 10 mg/ml) enhanced the secretion of bioactive TGF-beta1 and fibronectin with the upregulation of CD36. Intriguingly, CD36 siRNA, a potent silencer for CD36 effectively suppressed the albumin-induced increase in CD36, TGF-beta1, and even fibronectin level. Accordingly, albumin is a pro-fibrogenic factor for proximal tubule cells since albumin per se markedly upregulated the expression of TGF-beta1 and fibronectin. Most importantly, CD36 may mediate albumin-induced cellular fibrosis since CD36 siRNA appeared to have anti-fibrosis effects. This work suggests that CD36 is a novel and potential therapeutic target for diabetic renal tubule fibrosis.

Therapeutic potential of TGF-beta inhibition in chronic renal failure

Chronic kidney diseases are emerging as a worldwide public health problem. The progression of kidney diseases closely correlates with the accumulation of extracellular matrix leading to glomerulosclerosis and tubulointerstitial injury. Transforming growth factor (TGF)-beta has been identified as a key mediator of kidney matrix accumulation. Overexpression of TGF-beta isoforms and their receptors was observed in a variety of renal diseases in both animals and humans. Given its crucial role in fibrotic kidney disease, TGF-beta has been recently considered as a possible target in the management of chronic renal diseases. This review discusses the role of TGF-beta in renal fibrosis and provides an overview of the strategies that, when interfering with TGF-beta expression and signalling, could be employed as new renoprotective treatments.