The role of tubulointerstitial injury in chronic renal failure

Epithelial-mesenchymal transition of tubular epithelial cells in human renal biopsies

BACKGROUND

In recent studies performed on cultured cells and experimental nephropathies, it has been hypothesized that tubular epithelial cells (TEC), via epithelial-mesenchymal transformation (EMT), can become collagen-producing cells. According to this theory, they should proceed through several activating steps, such as proliferation and phenotype changes, to eventually synthesize extracellular matrix (ECM).

METHODS

To evaluate whether EMT operates in human TECs, 133 renal biopsies of different renal diseases were studied, analyzing by immunohistochemistry and in situ hybridization the possible expression of markers of proliferation (PCNA, Mib-1), cellular phenotype (vimentin, alpha-SMA, cytokeratin, ZO-1) and ECM production (prolyl 4-hydroxylase, HSP47, interstitial collagens).

RESULTS

Independently of histological diagnosis, variable degrees of TEC positivity for PCNA (2.7 +/- 2.4 cells/field) and Mib-1 (1.9 +/- 2.3) were present. TECs expressing vimentin (1.4 +/- 4.7) and alpha-smooth muscle actin (alpha-SMA; 0.04 +/- 0.4) also were detected. It was possible to observe loss of epithelial antigens from 8 to 10% of the tubular cross sections. Moreover, TECs were stained by prolyl 4-hydroxylase (3.6 +/- 4.3), heat shock protein-47 (HSP47; 2.9 +/- 5.4), collagen type I (0.2 +/- 2.7) and type III (0.3 +/- 2.0). Collagen types I and III mRNAs were found in 0.8 to 1.4 cells/field. The number of TEC with EMT features were associated with serum creatinine and the degree of interstitial damage (P< or = 0.03), and even considering the 45 cases with mild interstitial lesions, the tubular expression of all markers remained strictly associated with renal function (P< or = 0.01).

CONCLUSIONS

Our results suggest that, via transition to a mesenchymal phenotype, TEC can produce ECM proteins in human disease and directly intervene in the fibrotic processes. Moreover, the association of EMT features with serum creatinine supports the value of these markers in the assessment of disease severity.

Role of connective tissue growth factor in fibronectin expression and tubulointerstitial fibrosis

Connective tissue growth factor (CTGF) is one of the candidate factors mediating downstream events of transforming growth factor-beta (TGF-beta), but its role in fibrogenic properties of TGF-beta and in tubulointerstitial fibrosis has not yet been clarified. Using unilateral ureteral obstruction (UUO) in rats, we analyzed gene expression of TGF-beta1, CTGF, and fibronectin. We further investigated the effect of blockade of endogenous CTGF on TGF-beta-induced fibronectin expression in cultured rat renal fibroblasts by antisense oligodeoxynucleotide (ODN) treatment. After UUO, CTGF mRNA expression in the obstructed kidney was significantly upregulated subsequent to TGF-beta1, followed by marked induction of fibronectin mRNA. By in situ hybridization, CTGF mRNA was detected mainly in the interstitial fibrotic areas and tubular epithelial cells as well as in parietal glomerular epithelial cells in the obstructed kidney. The interstitial cells expressing CTGF mRNA were also positive for alpha-smooth muscle actin. CTGF antisense ODN transfected into cultured renal fibroblasts significantly attenuated TGF-beta-stimulated upregulation of fibronectin mRNA and protein compared with control ODN transfection, together with inhibited synthesis of type I collagen. With the use of a reporter assay, rat fibronectin promoter activity was increased by 2.5-fold with stimulation by TGF-beta1, and this increase was abolished with antisense CTGF treatment. Thus CTGF plays a crucial role in fibronectin synthesis induced by TGF-beta, suggesting that CTGF blockade could be a possible therapeutic target against tubulointerstitial fibrosis.

A role for Wnt-4 in renal fibrosis

Wnt-4 is a secreted glycoprotein that is critical for genitourinary development but found only in the most distal collecting duct epithelium in the normal murine adult kidney. Wnt4 expression is induced throughout the collecting ducts in four murine models of renal injury that produce tubulointerstitial fibrosis: folic acid-induced nephropathy, unilateral ureteral obstruction, renal needle puncture, and genetic polycystic kidney disease. Wnt4 activation induced by injury is limited to collecting ducts, with initial activation in the collecting duct epithelium followed by activation in fibrotic lesions surrounding the collecting ducts. The highest cellular Wnt4 expression is in interstitial fibroblasts in the fibrotic lesions that also express high levels of collagen-alpha1(I) mRNA and alpha-smooth muscle actin. In support of a functional role for Wnt-4 in these activated myofibroblasts, Wnt-4 induces stabilization of cytosolic beta-catenin in a cultured myofibroblast cell line. Furthermore, Wnt-4-producing fibroblasts placed under the renal capsule of adult mice induce lesions with tubular epithelial destruction. These observations suggest a role for Wnt-4 in the pathogenesis of renal fibrosis.

Pathophysiology of renal disease associated with liver disorders: implications for liver transplantation. Part I

Renal and hepatic function are often intertwined through both the existence of associated primary organ diseases and hemodynamic interrelationships. This connection occasionally results in the chronic failure of both organs, necessitating combined liver-kidney transplantation (LKT). Since 1988, more than 850 patients in the United States have received such transplants, with patient survival somewhat less than that for patients receiving either organ alone. Patients with renal failure caused by acute injury or hepatorenal syndrome have classically not been included as candidates for combined transplantation because of the reversibility of renal dysfunction after liver transplantation. However, the rate and duration of renal failure before liver transplantation is increasing in association with prolonged waiting list times. Thus, the issue of acquired permanent renal damage in the setting of hepatic failure continues to confront the transplant community. The following article and its sequel (Part II, to be published in vol 8, no 3 of this journal) attempt to review the problem of primary and secondary renal disease in patients with end-stage liver disease, elements involved in renal disease progression and recovery, the impact of renal disease on liver transplant outcome, and results of combined LKT; outline the steps in the pretransplantation renal evaluation; and provide the beginnings of an algorithm for making the decision for combined LKT.

A chemokine receptor CCR-1 antagonist reduces renal fibrosis after unilateral ureter ligation

The expression of chemokines and their receptors is thought to contribute to leukocyte infiltration and progressive renal fibrosis after unilateral ureter obstruction (UUO). We hypothesized that blocking the chemokine receptor CCR1 using the nonpeptide antagonist BX471 could reduce leukocyte infiltration and renal fibrosis after UUO. UUO kidneys from BX471-treated mice (day 0-10 and day 6-10) revealed a 40-60% reduction of interstitial macrophage and lymphocyte infiltrate compared with controls. Treated mice also showed a marked reduction of CCR1 and CCR5 mRNA levels, and FACS analysis showed a comparable reduction of CD8+/CCR5+ T cells. Markers of renal fibrosis, such as interstitial fibroblasts, interstitial volume, mRNA and protein expression for collagen I, were all significantly reduced by BX471-treatment compared with vehicle controls. By contrast treatment was ineffective when the drug was supplied only from days 0 to 5. In summary, blockade of CCR1 substantially reduces cell accumulation and renal fibrosis after UUO. Most interestingly, late onset of treatment is also effective. We therefore conclude that CCR1 blockade may represent a new therapeutic strategy for reducing cellular infiltration and renal fibrosis as major factors in the progression to end-stage renal failure.

Renal tubulointerstitial damage caused by persistent proteinuria is attenuated in AT1-deficient mice: role of endothelin-1

Using angiotensin II (AngII) type 1A receptor-deficient mice [AT1(-/-)], in which we induced protein overload nephropathy, we explored the potential implication of AngII and endothelin-1 (ET-1) in the tubulointerstitial damage because of persistent proteinuria. At day 7, AT1(-/-) showed marked proteinuria to a similar extent to that of wild-type mice (WT). However, at day14, AT1(-/-) had significantly less proteinuria, renal damage, transforming growth factor-beta, and matrix mRNA expression and mortality. AT1(-/-) also showed a significant diminution in the activation of the transcriptional factors nuclear factor-kappaB and AP-1. Unexpectedly, AT1(-/-) had a higher interstitial infiltration than WT. The administration of the angiotensin-converting enzyme inhibitor quinapril to WT caused a marked improvement in proteinuria and renal lesions, resembling that seen in untreated AT1(-/-). However, the interstitial infiltration persisted in AT1(-/-) when treated with quinapril. Because ET-1 may participate in the recruitment of mononuclear cells, we also studied the implication of this peptide. AT1(-/-) had a significantly higher ET-1 expression in tubular epithelial cells than WT. The administration of the dual ETA/ETB antagonist bosentan to AT1(-/-) considerably reduced the interstitial infiltrates. Bosentan also exerted a beneficial effect on proteinuria, renal lesions, and mortality in WT. These data show that in overload nephropathy, proteinuria and renal lesions are, to a large extent, AngII-dependent. The up-regulation of ET-1 in tubular epithelial cells in AT1(-/-), associated with interstitial infiltrates, suggests that the combination of drugs interfering with both vasopeptides may be of therapeutic interest in renal diseases with severe proteinuria and tubulointerstitial damage.

Renal fibrosis: collagen composition and assembly regulates epithelial-mesenchymal transdifferentiation

Type IV collagen is a major component of basement membranes and it provides structural and functional support to various cell types. Type IV collagen exists in a highly complex suprastructure form and recent studies implicate that protomer (the trimeric building unit of type IV collagen) assembly is mediated by the NC1 domain present in the C-terminus of each collagen alpha-chain polypeptide. Here we show that type IV collagen contributes to the maintenance of the epithelial phenotype of proximal tubular epithelial cells, whereas type I collagen promotes epithelial-to-mesenchymal transdifferentiation (EMT). In addition, the recombinant human alpha1NC1 domain inhibits assembly of type IV collagen NC1 hexamers and potentially disrupts the deposition of type IV collagen, facilitating EMT in vitro. Inhibition of type IV collagen assembly by the alpha1NC1 domain up-regulates the production of transforming growth factor-beta1 in proximal tubular epithelial cells, an inducer of EMT. These results strongly suggest that basement membrane architecture is pivotal for the maintenance of epithelial phenotype and that changes in basement membrane architecture potentially lead to up-regulation of transforming growth factor-beta1, which contributes to EMT during renal fibrosis.

Cytokine cross-talk between tubular epithelial cells and interstitial immunocompetent cells

Immunostaining of renal biopsies of patients with interstitial rejection of allografts or other forms of interstitial inflammation has demonstrated the presence of activated T cells and monocytes/macrophages in the tubulointerstitial area. Cytokines that are produced by infiltrating cells are capable of activating tubular epithelial cells. In turn tubular epithelial cells can produce a wide variety of inflammatory mediators, including chemokines, which further regulate cellular influx. Interfering in this cross-talk between tubular epithelial cells and infiltrating cells might provide new options for therapeutic intervention.