Localization of Smad6 and Smad7 in the rat kidney and their regulated expression in the anti-Thy-1 nephritis

A comprehensive transcriptomic view on the role of SMAD4 gene by RNAi-mediated knockdown in porcine follicular granulosa cells

As a key mediator of the transforming growth factor-beta (TGF-β) signaling pathway, which plays a pivotal role in regulating mammalian reproductive performance, Sma- and Mad-related protein 4 (SMAD4) is closely associated with the development of ovarian follicular. However, current knowledge of the genome-wide view on the role of SMAD4 gene in mammalian follicular granulosa cells (GCs) is still largely unknown. In the present study, RNA-Seq was performed to investigate the effects of SMAD4 knockdown by RNA interference (SMAD4-siRNA) in porcine follicular GCs. A total of 1025 differentially expressed genes (DEGs), including 530 upregulated genes and 495 downregulated genes, were identified in SMAD4-siRNA treated GCs compared with that treated with NC-siRNA. Furthermore, functional enrichment analysis indicated that upregulated DEGs in SMAD4-siRNA treated cells were mainly enriched in cell-cycle related processes, interferon signaling pathway, and immune system process, while downregulated DEGs in SMAD4-siRNA treated cells were mainly involved in extracellular matrix organization/disassembly, pathogenesis, and cell adhesion. In particular, cell cycle and TGF-β signaling pathway were discovered as the canonical pathways changed under SMAD4-silencing. Taken together, our data reveals SMAD4 knockdown alters the expression of numerous genes involved in key biological processes of the development of follicular GCs and provides a novel global clue of the role of SMAD4 gene in porcine follicular GCs, thus improving our understanding of regulatory mechanisms of SMAD4 gene in follicular development.

Extracellular matrix-induced Hic-5 expression in glomerular mesangial cells leads to a prosclerotic phenotype independent of TGF-β

Chronic fibroproliferative diseases account for approximately 45% of all deaths in the developed world. In the kidney, glomerulosclerosis is the underlying pathology in approximately half of patients with renal failure receiving dialysis. Mesangial cell expression of the LIM protein hydrogen peroxide-induced clone-5 (Hic-5) is important in its pathogenesis. Hic-5 expression increases following mesangial cell attachment to collagen I, associated with increased collagen I expression and increased susceptibility to apoptosis both in vitro and in experimental glomerulosclerosis. TGF-β has an established role in many fibrotic diseases, including glomerulosclerosis, where it increases collagen I deposition in vivo and promotes mesangial cell apoptosis in vitro. In other cell types, TGF-β induces Hic-5 expression. We investigated whether Hic-5-induced changes in mesangial cell phenotype were TGF-β-dependent. Adding exogenous TGF-β to mesangial cell cultures failed to increase Hic-5 expression; blocking TGF-β signaling did not reduce Hic-5 expression. However, inducing Hic-5 expression in mesangial cells by adhesion to collagen I led to TGF-β expression, which was abolished by small interfering RNA (siRNA) Hic-5 knockdown. Mesangial cells expressing Hic-5 showed altered latent TGF-β-binding protein expression and Smad signaling, with enhanced susceptibility to TGF-β-induced apoptosis. Mesangial cell attachment to collagen I led to increased Hic-5 expression within 2-4 h and increased procollagen I transcription within 12 h, whereas adding TGF-β to siRNA Hic-5 knockdown mesangial cells increased procollagen I transcription to a lesser degree after 48 h. Mesangial cell Hic-5 expression was associated with increased α-smooth muscle actin and plasminogen activator inhibitor-1 expression. Taken together, these data indicate that there is a prosclerotic feedback loop in mesangial cells dependent on matrix-derived signals in which Hic-5 is a pivotal signaling protein. This feedback loop is TGF-β-independent. The role of TGF-β-dependent and -independent sclerotic pathways merit further investigation.

TGF-β/Smad signaling in kidney disease

Chronic progressive kidney diseases typically are characterized by active renal fibrosis and inflammation. Transforming growth factor-β1 (TGF-β1) is a key mediator in the development of renal fibrosis and inflammation. TGF-β1 exerts its biological effects by activating Smad2 and Smad3, which is regulated negatively by an inhibitory Smad7. In the context of fibrosis, although Smad3 is pathogenic, Smad2 and Smad7 are protective. Under disease conditions, Smads also interact with other signaling pathways, such as the mitogen-activated protein kinase and nuclear factor-κB pathways. In contrast to the pathogenic role of active TGF-β1, latent TGF-β1 plays a protective role in renal fibrosis and inflammation. Furthermore, recent studies have shown that TGF-β/Smad signaling plays a regulating role in microRNA-mediated renal injury. Thus, targeting TGF-β signaling by gene transfer of either Smad7 or microRNAs into diseased kidneys has been shown to retard progressive renal injury in a number of experimental models. In conclusion, TGF-β/Smad signaling plays a critical role in renal fibrosis and inflammation. Advances in understanding of the mechanisms of TGF-β/Smad signaling in renal fibrosis and inflammation during chronic kidney diseases should provide a better therapeutic strategy to combat kidney diseases.

Unfolded protein response causes a phenotypic shift of inflamed glomerular cells toward redifferentiation through dual blockade of Akt and Smad signaling pathways

During recovery from acute glomerulonephritis, cell proliferation, matrix expansion, and expression of the dedifferentiation marker α-smooth muscle actin (α-SMA) subside spontaneously. However, the molecular mechanisms underlying this recovery process remain elusive. In mesangioproliferative glomerulonephritis, the unfolded protein response (UPR) is induced in activated, dedifferentiated mesangial cells. We investigated the role of the UPR in mesangial cell deactivation and redifferentiation and found that, during experimental glomerulonephritis in rats, reinforcement of the UPR significantly attenuated mesangial cell proliferation, matrix expansion, and expression of α-SMA. Consistent with this in vivo result, induction of the UPR suppressed cell proliferation and transcriptional expression of type IV collagen (ColIV) and α-SMA in activated mesangial cells. The UPR reduced phosphorylation of Akt in vitro and in vivo, and it was responsible for attenuation of cell proliferation. The UPR also preferentially depressed levels of total and phosphorylated Smads without affecting transcriptional levels, and it was responsible for suppression of ColIV and α-SMA. Translational suppression via the eIF2α pathway, but not proteasome-mediated protein degradation, was responsible for the down-regulation of Smads. These results suggest the novel potential of the UPR to facilitate a phenotypic shift of activated glomerular cells toward deactivation and redifferentiation. The UPR may serve as endogenous machinery that supports recovery of glomeruli from acute inflammation.

Allogenic fetal membrane-derived mesenchymal stem cells contribute to renal repair in experimental glomerulonephritis

Mesenchymal stem cells (MSC) have been reported to be an attractive therapeutic cell source for the treatment of renal diseases. Recently, we reported that transplantation of allogenic fetal membrane-derived MSC (FM-MSC), which are available noninvasively in large amounts, had a therapeutic effect on a hindlimb ischemia model (Ishikane S, Ohnishi S, Yamahara K, Sada M, Harada K, Mishima K, Iwasaki K, Fujiwara M, Kitamura S, Nagaya N, Ikeda T. Stem Cells 26: 2625-2633, 2008). Here, we investigated whether allogenic FM-MSC administration could ameliorate renal injury in experimental glomerulonephritis. Lewis rats with anti-Thy1 nephritis intravenously received FM-MSC obtained from major histocompatibility complex-mismatched ACI rats (FM-MSC group) or a PBS (PBS group). Nephritic rats exhibited an increased urinary protein excretion in the PBS group, whereas the FM-MSC group rats had a significantly lower level of increase (P < 0.05 vs. PBS group). FM-MSC transplantation significantly reduced activated mesangial cell (MC) proliferation, glomerular monocyte/macrophage infiltration, mesangial matrix accumulation, as well as the glomerular expression of inflammatory or extracellular matrix-related genes including TNF-α, monocyte chemoattractant protein 1 (MCP-1), type I collagen, TGF-β, type 1 plasminogen activator inhibitor (PAI-1) (P < 0.05 vs. PBS group). In vitro, FM-MSC-derived conditioned medium significantly attenuated the expression of TNF-α and MCP-1 in rat MC through a prostaglandin E(2)-dependent mechanism. These data suggest that transplanted FM-MSC contributed to the healing process in injured kidney tissue by producing paracrine factors. Our results indicate that allogenic FM-MSC transplantation is a potent therapeutic strategy for the treatment of acute glomerulonephritis.

Renal inflammation is modulated by potassium in chronic kidney disease: possible role of Smad7

High-potassium diets have been shown to be beneficial in cardiovascular disease partly because of a blood pressure-lowering effect. The effect of potassium on inflammation has not been studied. We investigated the influence of potassium supplementation on the degree of renal inflammation and the intracellular signaling mechanisms that could mediate inflammation in chronic kidney disease (CKD). CKD was created in male Sprague-Dawley rats by subtotal nephrectomy. Two groups of CKD rats were pair fed with diets containing 2.1% potassium (potassium-supplemented diet) or 0.4% potassium (basal diet). Body weight, blood pressure, and blood and urine electrolytes were measured biweekly. The animals were euthanized at week 8, and the remnant kidneys were analyzed by histology, immunohistochemistry, Western blotting, and real-time quantitative PCR. In the CKD pair-fed groups, blood potassium concentration did not differ significantly, but blood pressure was lower in the potassium-supplemented group. Compared with the basal diet, potassium supplementation decreased renal tubulointerstitial injury and suppressed renal inflammation as evidenced by decreased macrophage infiltration, lower expression of inflammatory cytokines, and decreased NF-kappaB activation. These renoprotective effects were associated with downregulation of renal transforming growth facto-beta, upregulation of renal Smad7, and lower blood pressure. Our results show that potassium supplementation can reduce renal inflammation and hence, could modulate the progression of kidney injury in CKD.

Localization of TGF-beta signaling intermediates Smad2, 3, 4, and 7 in developing and mature human and mouse kidney

Smad proteins are signaling intermediates of the TGF-beta superfamily and are involved in a range of biological activities including development and immune responses. We studied the expression of TGF-beta-receptor activated Smads (Smad2 and Smad3), the common partner Smad (Smad4), an inhibitory Smad (Smad7), and the activated (phosphorylated) Smad2 (pSmad2) in developing and adult kidneys of humans and mice. These studies demonstrate associated expression of these Smads in multiple renal cell types in all developmental stages and in mature non-diseased kidneys. Smad expression is in general most widespread at the earliest stages of nephron development and diminishes as components of the nephrons become more differentiated. Paucity of Smad expression in mesangial cells in contrast to widespread expression of these Smads in glomerular visceral epithelial cells in both developing and mature kidneys was remarkable. Divergent and less extensive expression of Smad4, compared with other Smad proteins, was also demonstrated in tubules of human kidneys. Based on the observed expression patterns, these findings demonstrate, for the first time, expression of the TGF-beta-receptor-activated Smad2 and Smad3, the common mediator Smad4, and the inhibitory Smad7 in the developing human fetal kidney, extending observations previously made in rodent systems to humans.

Adrenomedullin impairs the profibrotic effects of transforming growth factor-beta1 through recruiting Smad6 protein in human renal tubular cells

Adrenomedullin (AM) was originally identified as a vasodilator peptide, and has recently been shown to be an antiproliferative factor in renal mesangial cells, suggesting that adrenomedullin may impair the progression of glomerulosclerosis. This study was to investigate the effect of adrenomedullin on transforming growth factor-beta1 (TGF-beta1)-stimulated cell growth, synthesis of extracellular matrix (ECM) components and the related molecular mechanism in a human tubular epithelial cell line HK-2. TGF-beta1 inhibited cell proliferation induced by fetal bovine serum, but neither AM itself affectted cell proliferation, nor did AM influence TGF-beta1-caused cell growth arrest. However, AM beginning at 10(-8) M alleviated the action of TGF-beta1-stimulated cellular collagen synthesis and secretion of fibronectin into cell culture supernatant. Activation of Smad proteins is known to be the key signaling pathway of the profibrotic effect of TGF-beta1, AM at 10(-8) M exerted no effect on TGF-beta1-induced Smad2 phosphorylation, but prevented the suppression of the inhibitory Smad6 protein by TGF-beta1 and restored Smad2-Samd6 complex formation. Our results suggest that AM can attenuate TGF-beta1-mediated renal tubulointerstitial ECM turnover via an antagonistic mechanism of inhibitory Smad in TGF-beta1-elicited signaling.

Transforming growth factor-beta and Smad signalling in kidney diseases

Extensive studies have demonstrated that transforming growth factor-beta (TGF-beta) plays an important role in the progression of renal diseases. TGF-beta exerts its biological functions mainly through its downstream signalling molecules, Smad2 and Smad3. It is now clear that Smad3 is critical for TGF-beta's pro-fibrotic effect, whereas the functions of Smad2 in fibrosis in response to TGF-beta still need to be determined. Our recent studies have demonstrated that Smad signalling is also a critical pathway for renal fibrosis induced by other pro-fibrotic factors, such as angiotensin II and advanced glycation end products (AGE). These pro-fibrotic factors can activate Smads directly and independently of TGF-beta. They can also cause renal fibrosis via the ERK/p38 MAP kinase-Smad signalling cross-talk pathway. In contrast, blockade of Smad2/3 activation by overexpression of an inhibitory Smad7 prevents collagen matrix production induced by TGF-beta, angiotensin II, high glucose and AGE and attenuates renal fibrosis in various animal models including rat obstructive kidney, remnant kidney and diabetic kidney diseases. Results from these studies indicate that Smad signalling is a key and final common pathway of renal fibrosis. In addition, TGF-beta has anti-inflammatory and immune-regulatory properties. Our most recent studies demonstrated that TGF-beta transgenic mice are protected against renal inflammation in mouse obstructive and diabetic models. Upregulation of renal Smad7, thereby blocking NF.kappaB activation via induction of IkappaBalpha, is a central mechanism by which TGF-beta inhibits renal inflammation. In conclusion, TGF-beta signals through Smad2/3 to mediate renal fibrosis, whereas induction of Smad7 inhibits renal fibrosis and inflammation. Thus, targeting Smad signalling by overexpression of Smad7 may have great therapeutic potential for kidney diseases.

Modulation of the TGFbeta/Smad signaling pathway in mesangial cells by CTGF/CCN2

Transforming growth factor-beta (TGFbeta) drives fibrosis in diseases such as diabetic nephropathy (DN). Connective tissue growth factor (CTGF; CCN2) has also been implicated in this, but the molecular mechanism is unknown. We show that CTGF enhances the TGFbeta/Smad signaling pathway by transcriptional suppression of Smad 7 following rapid and sustained induction of the transcription factor TIEG-1. Smad 7 is a known antagonist of TGFbeta signaling and TIEG-1 is a known repressor of Smad 7 transcription. CTGF enhanced TGFbeta-induced phosphorylation and nuclear translocation of Smad 2 and Smad 3 in mesangial cells. Antisense oligonucleotides directed against TIEG-1 prevented CTGF-induced downregulation of Smad 7. CTGF enhanced TGFbeta-stimulated transcription of the SBE4-Luc reporter gene and this was markedly reduced by TIEG-1 antisense oligonucleotides. Expression of the TGFbeta-responsive genes PAI-1 and Col III over 48 h was maximally stimulated by TGFbeta+CTGF compared to TGFbeta alone, while CTGF alone had no significant effect. TGFbeta-stimulated expression of these genes was markedly reduced by both CTGF and TIEG-1 antisense oligonucleotides, consistent with the endogenous induction of CTGF by TGFbeta. We propose that under pathological conditions, where CTGF expression is elevated, CTGF blocks the negative feedback loop provided by Smad 7, allowing continued activation of the TGFbeta signaling pathway.

Changes in Smad expression and subcellular localization in bleomycin-induced pulmonary fibrosis

Administration of bleomycin (BM) produces inflammation and fibrosis of the lung in humans and experimental animals. The molecular defects by which BM induces these pathological effects have not been studied in detail. We studied the expression of Smad family proteins, key molecules involved in mediating transforming growth factor (TGF)-β signaling from the cell membrane to the nucleus, during the early and late phases of BM-induced fibrogenesis. Pulmonary fibrosis was induced in male Sprague-Dawley rats by a single intratracheal injection (1.5 units) of BM. Control rats received saline. Rats were killed at 3, 5, 7, 14, and 28 days after BM, cytosolic and nuclear proteins were extracted and isolated from lung tissues, and Smad proteins were probed with specific antibodies. In BM-exposed lung tissue, compared with control, Smad3 decreased persistently in the cytosol and increased transiently in the nucleus. There was a persistent increase in phosphorylation and nuclear accumulation of Smad2/3. Smad4 was increased transiently in both the cytosol and nucleus. A significant and progressive decrease in the expression of Smad7, the endogenous inhibitor of TGF-β/Smad signaling, was observed after BM instillation. Collectively, our results indicate that an imbalance between agonistic Smads2–4 and antagonistic Smad7 may result in the unchecked activation of an autocrine TGF-β loop, which contributes to the pathogenesis of BM-induced pulmonary fibrosis.

Down-regulation of Smad7 expression by ubiquitin-dependent degradation contributes to renal fibrosis in obstructive nephropathy in mice

Overexpression of transforming growth factor beta (TGF-beta) has been shown to play pathogenic roles in progression of renal fibrosis, and the severity of tubulointerstitial fibrosis correlates better with renal function than the severity of glomerulosclerosis. Smad proteins are signaling transducers downstream from TGF-beta receptors. Three families of Smad proteins have been identified: receptor-regulated Smad2 and Smad3, common partner Smad4, and inhibitory Smad7 (part of a negative-feedback loop). We investigated Smad-mediated TGF-beta signaling pathway and regulatory mechanisms of inhibitory Smad7 in unilateral ureteral obstruction (UUO) kidneys in mice, a model of progressive tubulointerstitial fibrosis. Compared with sham-operated kidneys, the level of Smad7 protein, but not mRNA, decreased progressively in UUO kidneys, whereas immunoreactivity for nuclear phosphorylated Smad2 and Smad3 and renal fibrosis were inversely increased. Furthermore, we demonstrated that both the degradation and ubiquitination activity of Smad7 protein were increased markedly in UUO kidneys compared with sham-operated ones. We also found that both Smurf1 and Smurf2 (Smad ubiquitination regulatory factors), which are E3 ubiquitin ligases for Smad7, were increased and that they interacted with Smad7 in UUO kidneys. Our results suggest that the reduction of Smad7 protein resulting from enhanced ubiquitin-dependent degradation plays a pathogenic role in progression of tubulointerstitial fibrosis.

Smad expression during kidney development

Signaling by the transforming growth factor (TGF)-beta superfamily is important during kidney development. Here, we describe the spatial and temporal expression patterns of the Smads, the transcription factors that translate TGF- signals into gene expression. RT-PCR data and in situ hybridization analysis showed that the receptor-regulated (R) Smads (Smad1, -2, -3, -5, and -8), the common partner Smad (Smad4), and the inhibitory (I) Smads (Smad6 and -7) were all expressed during mouse kidney development from embryonic day 12 until the end of nephrogenesis at postnatal day 15. Each Smad had a distinct spatial distribution. All were expressed by mesenchymal cells in the nephrogenic zone and were downregulated once these cells began to epithelialize. The common partner Smad, Smad4, was present in uninduced mesenchymal cells and at ureteric bud tips. The bone morphogenetic-responsive R-Smads, Smad1, -5, and -8, were mainly expressed in the nephrogenic zone, whereas the TGF-- responsive R-Smads were predominantly noted in the medullary interstitium. Expression of the I-Smad Smad7 was also seen in mesenchymal cells in the interstitium. Based on the observed patterns of expression, we speculate that individual or combinations of Smads may play specific roles in cell-fate determination during kidney development.

Ubiquitin-dependent degradation of Smad2 is increased in the glomeruli of rats with anti-thymocyte serum nephritis

The overexpression of transforming growth factor (TGF)-beta and Smad-mediated intracellular TGF-beta signaling in the kidney underlies the development of renal scarring from pathological matrix accumulation. However, changes in the Smad proteins during the progression of kidney disease are unclear. In this study, we investigated the regulation of Smad proteins in the glomeruli of rats with anti-thymocyte serum nephritis. We found that Smad2 protein decreased markedly in nephritic glomeruli, whereas no significant changes were observed in the levels of Smad3 and Smad4 proteins. In contrast, the level of Smad2 mRNA in nephritic glomeruli did not differ significantly from that in control glomeruli. Based on recent reports of the ubiquitin-mediated degradation of Smad2, we investigated the degradation and ubiquitination activity directed against Smad2 in glomerular extracts. Both the degradation and ubiquitination of Smad2 were markedly increased in glomerular extracts from rats with nephritis. We also found that Smurf2, a ubiquitin ligase for Smad2, was increased in the nephritic glomerular extracts. These data suggest that the decrease in Smad2 resulted from enhanced ubiquitin-dependent degradation of Smad2 mediated by Smurf2, and is involved in the regulation of Smad2-mediated TGF-beta signaling in nephritic glomeruli.

Sustained activation of fibroblast transforming growth factor-beta/Smad signaling in a murine model of scleroderma

Transforming growth factor-beta is responsible for triggering a cascade of events leading to fibrosis in scleroderma. The Smads are intracellular signal transducers recently shown to mediate fibroblast activation and other profibrotic responses elicited by transforming growth factor-betain vitro. To understand better the involvement of Smads in the pathogenesis of fibrosis, we examined Smad expression and activation in situ in a murine model of scleroderma. Bleomycin injections induced striking dermal infiltration with macrophages by 3 d, and progressive fibrosis by 2 wk. Infiltrating macrophages and resident fibroblasts expressed Smad3, the positive mediator for transforming growth factor-beta responses. Importantly, in bleomycin-injected skin, fibroblasts showed predominantly nuclear localization of Smad3 and intense staining for phospho-Smad2/3. Furthermore, phosphorylated Smad2/3 in fibroblasts was detected even after the resolution of inflammation. Expression of Smad7, the endogenous inhibitor of transforming growth factor-beta/Smad signaling, was strongly induced in dermal cells by transforming growth factor-beta, but not by bleomycin injections. Collectively, these results indicate that bleomycin-induced murine scleroderma is associated with rapid and sustained induction of transforming growth factor-beta/Smad signaling in resident dermal fibroblasts. Despite apparent activation of the intracellular transforming growth factor-beta signaling pathway in the lesional dermis, the expression of transforming growth factor-beta-inducible Smad7 was not upregulated. In light of the critical function of Smad7 as an endogenous inhibitor of Smad signaling that restricts the duration and magnitude of transforming growth factor-beta responses, and as a mediator of apoptosis, relative Smad7 deficiency observed in the present studies may account for sustained activation of transforming growth factor-beta/Smad signaling in lesional tissues. These findings raise the possibility that Smads plays an important part in the pathogenesis of fibrosis, and may therefore represent targets for selective anti-fibrotic interventions.

Expression and regulation of intracellular SMAD signaling in scleroderma skin fibroblasts

OBJECTIVE

Scleroderma is characterized by excessive synthesis and accumulation of matrix proteins in lesional tissues. Transforming growth factor beta (TGFbeta) plays a central role in the pathogenesis of fibrosis by inducing and sustaining activation of fibroblasts; however, the underlying mechanisms are poorly understood. We undertook this study to examine the expression and function of SMADs, recently characterized intracellular effectors of TGFbeta signaling, in scleroderma fibroblasts.

METHODS

Primary dermal fibroblasts obtained from 14 patients with scleroderma and from 4 healthy adult volunteers were studied. Northern analysis was used to determine the expression of endogenous SMAD messenger RNA (mRNA), and Western analysis was used to determine SMAD protein expression. Intracellular compartmentalization of cellular SMAD proteins in the presence and absence of TGFbeta was studied by antibody-mediated immunofluorescence confocal microscopy. The effect of TGFbeta blockade on SMAD subcellular distribution was determined using anti-TGFbeta antibodies as well as a dominant-negative TGFbeta receptor type II (TGFbetaRII) vector to disrupt TGFbeta responses. SMAD-regulated luciferase reporter expression was examined to investigate the potential functional significance of activation and nuclear accumulation of endogenous SMADs in scleroderma fibroblasts.

RESULTS

Protein and mRNA levels of SMAD3, but not of SMAD4 or SMAD7, were variably elevated in scleroderma fibroblasts compared with those from healthy controls. In sharp contrast to control fibroblasts, which displayed predominantly cytoplasmic localization of SMAD3/4 in the absence of exogenous TGFbeta, in scleroderma fibroblasts SMAD3 and SMAD4 consistently showed elevated nuclear localization. Furthermore, phosphorylated SMAD2/3 levels were elevated and nuclear localization of phosphorylated SMAD2/3 was increased, suggesting activation of the SMAD pathway in scleroderma fibroblasts. Blockade of autocrine TGFbeta signaling with antibodies or by expression of dominant-negative TGFbetaRII failed to normalize SMAD subcellular distribution, suggesting that elevated nuclear SMAD import was due to alterations downstream of the TGFbeta receptors. The activity of a SMAD-responsive minimal promoter-reporter construct was enhanced in transiently transfected scleroderma fibroblasts.

CONCLUSION

This study is the first to demonstrate apparently ligand-independent constitutive activation of the intracellular TGFbeta/SMAD signaling axis in scleroderma fibroblasts. SMAD signaling may be a mechanism contributing to the characteristic phenotype of scleroderma fibroblasts and playing a role in the pathogenesis of fibrosis.

TGF-beta signal transduction and mesangial cell fibrogenesis

Transforming growth factor-beta (TGF-beta) is closely associated with progressive renal fibrosis. Significant progress has been accomplished in determining the cellular signaling pathways that are activated by TGF-beta. This knowledge is being applied to glomerular mesangial cell models of extracellular matrix (ECM) accumulation. A central component of TGF-beta-stimulated mesangial cell fibrogenesis is the TGF-beta family-specific Smad signal transduction pathway. However, while Smads play an important role in collagen accumulation, recent findings indicate that cross talk among a variety of pathways is necessary for maximal stimulation of collagen expression. Further investigation of these multiple interactions will provide insight into possible ways to interrupt cellular mechanisms of glomerular fibrogenesis.

Transforming growth factor-beta signal transduction and progressive renal disease

Transforming growth factor-beta (TGF-beta) superfamily members are multifunctional growth factors that play pivotal roles in development and tissue homeostasis. Recent studies have underscored the importance of TGF-beta in regulation of cell proliferation and extracellular matrix synthesis and deposition. TGF-beta signaling is initiated by ligand binding to a membrane-associated receptor complex that has serine/threonine kinase activity. This receptor complex phosphorylates specific Smad proteins, which then transduce the ligand-activated signal to the nucleus. Smad complexes regulate target gene transcription either by directly binding DNA sequences, or by complexing with other transcription factors or co-activators. There is extensive crosstalk between the TGF-beta signaling pathway and other signaling systems, including the mitogen-activated protein kinase pathways. The importance of TGF-beta in regulation of cell growth has been emphasized by recent observations that mutations of critical elements of the TGF-beta signaling system are associated with tumor progression in patients with many different types of epithelial neoplasms. TGF-beta has emerged as a predominant mediator of extracellular matrix production and deposition in progressive renal disease and in other forms of chronic tissue injury. In this overview, recent advances in our understanding of TGF-beta signaling, cell cycle regulation by TGF-beta, and the role of TGF-beta in progressive renal injury are highlighted.

The effects of platelet-derived growth factor antagonism in experimental glomerulonephritis are independent of the transforming growth factor-beta system

Platelet-derived growth factor B-chain (PDGF-B)- and transforming growth factor beta (TGF-beta)-mediated accumulation of extracellular matrix proteins contributes to many progressive renal diseases. In vivo, specific antagonism of either PDGF-B or TGF-beta in experimental mesangioproliferative glomerulonephritis resulted in an almost complete inhibition of matrix protein accumulation, which suggests an interaction between signaling pathways of these two growth factors. Because nothing is known on the nature of this possible interaction, PDGF-B was antagonized in the rat anti-Thy 1.1 model of glomerulonephritis by use of specific aptamers and its effects on the TGF-beta system were investigated. Antagonism of PDGF-B led to a significant reduction of glomerular matrix accumulation compared with scrambled aptamer-treated nephritic controls. PDGF-B antagonism had no effect on the overexpression of glomerular TGF-beta mRNA, TGF-beta protein, or the expression of TGF-beta receptor type I and II mRNA. By immunohistology, it was possible to detect overexpression of the cytoplasmic TGF-beta signaling molecules Smad2 (agonistic) and Smad7 (antagonistic) in glomeruli of nephritic control rats which peaked on day 7 after disease induction, i.e., the peak of mesangial cell proliferation in this model. However, immunohistology and Western blot analysis again revealed no difference in the glomerular expression of both Smad proteins between PDGF-B antagonized and nonantagonized nephritic animals. In addition, no difference in the glomerular expression of phosphorylated Smad2 (P-Smad2) was detected between the differently treated nephritic groups. These observations suggest that the effects of PDGF-B antagonism are independent of TGF-beta in mesangioproliferative glomerulonephritides.