Elevated TGFbeta-Smad signalling in experimental Pkd1 models and human patients with polycystic kidney disease

Loss of Zeb2 in mesenchyme-derived nephrons causes primary glomerulocystic disease

Primary glomerulocystic kidney disease is a special form of renal cystic disorder characterized by Bowman's space dilatation in the absence of tubular cysts. ZEB2 is a SMAD-interacting transcription factor involved in Mowat-Wilson syndrome, a congenital disorder with an increased risk for kidney anomalies. Here we show that deletion of Zeb2 in mesenchyme-derived nephrons with either Pax2-cre or Six2-cre causes primary glomerulocystic kidney disease without tubular cysts in mice. Glomerulotubular junction analysis revealed many atubular glomeruli in the kidneys of Zeb2 knockout mice, which explains the presence of glomerular cysts in the absence of tubular dilatation. Gene expression analysis showed decreased expression of early proximal tubular markers in the kidneys of Zeb2 knockout mice preceding glomerular cyst formation, suggesting that defects in proximal tubule development during early nephrogenesis contribute to the formation of congenital atubular glomeruli. At the molecular level, Zeb2 deletion caused aberrant expression of Pkd1, Hnf1β, and Glis3, three genes causing glomerular cysts. Thus, Zeb2 regulates the morphogenesis of mesenchyme-derived nephrons and is required for proximal tubule development and glomerulotubular junction formation. Our findings also suggest that ZEB2 might be a novel disease gene in patients with primary glomerular cystic disease.

Cardiac dysfunction in Pkd1-deficient mice with phenotype rescue by galectin-3 knockout

Alterations in myocardial wall texture stand out among ADPKD cardiovascular manifestations in hypertensive and normotensive patients. To elucidate their pathogenesis, we analyzed the cardiac phenotype in Pkd1(cond/cond)Nestin(cre) (CYG+) cystic mice exposed to increased blood pressure, at 5 to 6 and 20 to 24 weeks of age, and Pkd1(+/-) (HTG+) noncystic mice at 5-6 and 10-13 weeks. Echocardiographic analyses revealed decreased myocardial deformation and systolic function in CYG+ and HTG+ mice, as well as diastolic dysfunction in older CYG+ mice, compared to their Pkd1(cond/cond) and Pkd1(+/+) controls. Hearts from CYG+ and HTG+ mice presented reduced polycystin-1 expression, increased apoptosis, and mild fibrosis. Since galectin-3 has been associated with heart dysfunction, we studied it as a potential modifier of the ADPKD cardiac phenotype. Double-mutant Pkd1(cond/cond):Nestin(cre);Lgals3(-/-) (CYG-) and Pkd1(+/-);Lgals3(-/-) (HTG-) mice displayed improved cardiac deformability and systolic parameters compared to single-mutants, not differing from the controls. CYG- and HTG- showed decreased apoptosis and fibrosis. Analysis of a severe cystic model (Pkd1(V/V); VVG+) showed that Pkd1(V/V);Lgals3(-/-) (VVG-) mice have longer survival, decreased cardiac apoptosis and improved heart function compared to VVG+. CYG- and VVG- animals showed no difference in renal cystic burden compared to CYG+ and VVG+ mice. Thus, myocardial dysfunction occurs in different Pkd1-deficient models and suppression of galectin-3 expression rescues this phenotype.

Recent Trends in ADPKD Research

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is one of the most common inherited disorders. It is the fourth leading cause of renal replacement and renal failure worldwide. Mutations in PKD1 or PKD2 cause ADPKD. Patients with ADPKD show progressive growth of renal cysts filled with cystic fluid, leading to end-stage renal disease (ESRD) and renal failure by their sixth decade of life. Currently, there are no curative treatments for ADPKD. Therefore, patients require dialysis or kidney transplantation. To date, researchers have elucidated many of the mechanisms that cause ADPKD and developed many methods to diagnose the disease. ADPKD is related to growth factors, signaling pathways, cell proliferation, apoptosis, inflammation, the immune system, structural abnormalities, epigenetic mechanisms, microRNAs, and so on. Various therapies have been reported to slow the progression of ADPKD and alleviate its symptoms.

Exploitation of the Polymeric Immunoglobulin Receptor for Antibody Targeting to Renal Cyst Lumens in Polycystic Kidney Disease

Autosomal-dominant polycystic kidney disease (ADPKD) is a common life-threatening genetic disease that leads to renal failure. No treatment is available yet to effectively slow disease progression. Renal cyst growth is, at least in part, driven by the presence of growth factors in the lumens of renal cysts, which are enclosed spaces lacking connections to the tubular system. We have shown previously shown that IL13 in cyst fluid leads to aberrant activation of STAT6 via the IL4/13 receptor. Although antagonistic antibodies against many of the growth factors implicated in ADPKD are already available, they are IgG isotype antibodies that are not expected to gain access to renal cyst lumens. Here we demonstrate that targeting antibodies to renal cyst lumens is possible with the use of dimeric IgA (dIgA) antibodies. Using human ADPKD tissues and polycystic kidney disease mouse models, we show that the polymeric immunoglobulin receptor (pIgR) is highly expressed by renal cyst-lining cells. pIgR expression is, in part, driven by aberrant STAT6 pathway activation. pIgR actively transports dIgA from the circulation across the cyst epithelium and releases it into the cyst lumen as secretory IgA. dIgA administered by intraperitoneal injection is preferentially targeted to polycystic kidneys whereas injected IgG is not. Our results suggest that pIgR-mediated transcytosis of antagonistic antibodies in dIgA format can be exploited for targeted therapy in ADPKD.

Antisense-mediated angiotensinogen inhibition slows polycystic kidney disease in mice with a targeted mutation in Pkd2

Renal cyst enlargement is associated with the activation of both the circulating and intrarenal renin-angiotensin systems. Angiotensinogen (AGT) is the substrate for renin. The aim of the present study was to determine the effect of AGT inhibition on renal cyst enlargement. An AGT antisense oligonucleotide (ASO) that selectively inhibits AGT mRNA was injected once weekly in PKD2WS25 mice [an orthologous model of human autosmal dominant polycystic kidney disease (PKD) involving mutation of the Pkd2 gene] from 4 to 16 wk of age. The AGT ASO resulted in a 40% decrease in AGT RNA in the kidney, a 60% decrease in AGT RNA in the liver, and a significant decrease in AGT protein in the kidney and serum. The AGT ASO resulted in a significant decrease in kidney size, cyst volume density, and blood urea nitrogen. The AGT ASO resulted in a significant decrease in transforming growth factor-β and interstitial fibrosis in the kidney. Mice treated with the AGT ASO had a significant decrease in proinflammatory cytokines [chemokine (C-X-C motif) ligand (CXCL)1 and IL-12] in the kidney. Cluster of differentiation (CD)36 is a scavenger receptor found on tubular cells that can activate the renin-angiotensin system. Administration of a CD36 ASO had no effect on PKD and kidney function, suggesting that the effect of the AGT ASO is independent of CD36. In summary, AGT inhibition resulted in significant decreases in kidney size and cyst volume and an improvement in kidney function in PKD mice. The AGT ASO resulted in a decrease in transforming growth factor-β, interstitial fibrosis, and the proinflammatory cytokines CXCL1 and IL-12 in the kidney.

Polycystic liver diseases: advanced insights into the molecular mechanisms

Polycystic liver diseases are genetic disorders characterized by progressive bile duct dilatation and/or cyst development. The large volume of hepatic cysts causes different symptoms and complications such as abdominal distension, local pressure with back pain, hypertension, gastro-oesophageal reflux and dyspnea as well as bleeding, infection and rupture of the cysts. Current therapeutic strategies are based on surgical procedures and pharmacological management, which partially prevent or ameliorate the disease. However, as these treatments only show short-term and/or modest beneficial effects, liver transplantation is the only definitive therapy. Therefore, interest in understanding the molecular mechanisms involved in disease pathogenesis is increasing so that new targets for therapy can be identified. In this Review, the genetic mechanisms underlying polycystic liver diseases and the most relevant molecular pathways of hepatic cystogenesis are discussed. Moreover, the main clinical and preclinical studies are highlighted and future directions in basic as well as clinical research are indicated.

Regulation of STATs by polycystin-1 and their role in polycystic kidney disease

Autosomal-dominant polycystic kidney disease (ADPKD) is a common genetic disease caused by mutations in the gene coding for polycystin-1 (PC1). PC1 can regulate STAT transcription factors by a novel, dual mechanism. STAT3 and STAT6 are aberrantly activated in renal cysts. Genetic and pharmacological approaches to inhibit STAT3 or STAT6 have led to promising results in ADPKD mouse models. Here, we review current findings that lead to a model of PC1 as a key regulator of STAT signaling in renal tubule cells. We discuss how PC1 may orchestrate appropriate epithelial responses to renal injury, and how this system may lead to aberrant STAT activation in ADPKD thereby causing inappropriate activation of tissue repair programs that culminate in renal cyst growth and fibrosis.

Dual role of acetaminophen in promoting hepatoma cell apoptosis and kidney fibroblast proliferation

Acetaminophen (APAP), is a safe analgesic and antipyretic drug at therapeutic dose, and is widely used in the clinic. However, high doses of APAP can induce hepatotoxicity and nephrotoxicity. Most studies have focused on high‑dose APAP‑induced acute liver and kidney injury. So far, few studies have investigated the effects of the therapeutic dose (1/10 of the high dose) or of the low dose (1/100 of the high dose) of APAP on the cells. The aim of this study was to investigate the cellular effects of therapeutic- or low‑dose APAP treatment on hepatoma cells and kidney fibroblasts. As expected, high‑dose APAP treatment inhibited while therapeutic and low‑dose treatment did not inhibit cell survival of kidney tubular epithelial cells. In addition, therapeutic-dose treatment induced an increase in the H2O2 level, activated the caspase‑9/‑3 cascade, and induced cell apoptosis of hepatoma cells. Notably, APAP promoted fibroblast proliferation, even at low doses. This study demonstrates that different cellular effects are exerted upon treatment with different APAP concentrations. Our results indicate that treatment with the therapeutic dose of APAP may exert an antitumor activity on hepatoma, while low‑dose treatment may be harmful for patients with fibrosis, since it may cause proliferation of fibroblasts.

Gpr48 deficiency induces polycystic kidney lesions and renal fibrosis in mice by activating Wnt signal pathway

G protein-coupled receptor 48 (Gpr48/Lgr4) is essential to regulate the development of multiple tissues in mice. The notion that Gpr48 functions in renal development prompted us to investigate the relation between Gpr48 and renal diseases. Using a Gpr48 knockout mice model, we observed that 66.7% Gpr48 null mice developed polycystic lesions in the kidney, while no cysts were observed in the kidneys of wild-type mice. Polycystic kidney disease 1 (PKD1) and PKD2 expressions were also markedly decreased in the Gpr48 knockout mice. Abnormal expressions of exra-cellular matrix protein lead to the progression of polycystic kidney disease and the formation of renal fibrosis in the Gpr48 null mice. The expressions of several Wnt molecules and its receptors were increased and marked β-catenin nuclear accumulation was observed in the Gpr48 null mice. The inhibitors of Wnt/β-catenin signal pathway such as GSK3β and axin2 were loss of function. The Wnt/PCP signaling pathway is also activated in Gpr48 null mice. However, TGF-β expression and phosphorylated Smad2/3 levels were not altered. Collectively, our results showed that Gpr48 null mice are at a greater risk of suffering from polycystic lesions and renal fibrosis. Moreover, the formation of polycystic lesions and renal fibrosis induced by Gpr48 deficiency involves the activation of Wnt signaling pathway but not the TGF-β/Smad pathway.

Identification of a small molecule preventing BMSC senescence in vitro by improving intracellular homeostasis via ANXA7 and Hmbox1

ABO was discovered to be a novel anti-aging chemical in cultured BMSCs by improving intracellular homeostasis.

A Pkd1-Fbn1 genetic interaction implicates TGF-β signaling in the pathogenesis of vascular complications in autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is a common cause of renal failure that is due to mutations in two genes, PKD1 and PKD2. Vascular complications, including aneurysms, are a well recognized feature of ADPKD, and a subgroup of families exhibits traits reminiscent of Marfan syndrome (MFS). MFS is caused by mutations in fibrillin-1 (FBN1), which encodes an extracellular matrix protein with homology to latent TGF-β binding proteins. It was recently demonstrated that fibrillin-1 deficiency is associated with upregulation of TGF-β signaling. We investigated the overlap between ADPKD and MFS by breeding mice with targeted mutations in Pkd1 and Fbn1. Double heterozygotes displayed an exacerbation of the typical Fbn1 heterozygous aortic phenotype. We show that the basis of this genetic interaction results from further upregulation of TGF-β signaling caused by Pkd1 haploinsufficiency. In addition, we demonstrate that loss of PKD1 alone is sufficient to induce a heightened responsiveness to TGF-β. Our data link the interaction of two important diseases to a fundamental signaling pathway.

Genetics of thoracic aortic aneurysm: at the crossroad of transforming growth factor-β signaling and vascular smooth muscle cell contractility

Aortic aneurysm, including both abdominal aortic aneurysm and thoracic aortic aneurysm, is the cause of death of 1% to 2% of the Western population. This review focuses only on thoracic aortic aneurysms and dissections. During the past decade, the genetic contribution to the pathogenesis of thoracic aortic aneurysms and dissections has revealed perturbed extracellular matrix signaling cascade interactions and deficient intracellular components of the smooth muscle contractile apparatus as the key mechanisms. Based on the study of different Marfan mouse models and the discovery of several novel thoracic aortic aneurysm genes, the involvement of the transforming growth factor-β signaling pathway has opened unexpected new avenues. Overall, these discoveries have 3 important consequences. First, the pathogenesis of thoracic aortic aneurysms and dissections is better understood, although some controversy still exists. Second, the management strategies for the medical and surgical treatment of thoracic aortic aneurysms and dissections are becoming increasingly gene-tailored. Third, the pathogenetic insights have delivered new treatment options that are currently being investigated in large clinical trials.

Dissection of Iliac Artery in a Patient With Autosomal Dominant Polycystic Kidney Disease: A Case Report

Autosomal dominant polycystic kidney disease (ADPKD) is a risk factor for several cardiovascular disorders such as intracranial aneurysm or aortic dissection, preferentially occurring at the thoracic or abdominal level. A 47-year-old man suffering from ADPKD had renal transplantation. Sixteen hours after surgery, he presented with left leg pain. Clinical and ultrasound examination revealed thrombosis of the external left iliac artery. Therefore, we decided to perform intra-arterial angiography to evaluate the possibility of an endovascular treatment. Aorto-femorography showed an obstruction of the external left iliac artery that was found during emergency surgery, consecutive to a dissection, which occurred following the surgery for kidney transplantation. The resected segment of the dissected vessel was analyzed by histology. Collagen fibers organization and density in the adventitia and smooth muscle cells density in the media were similar in the dissected and a normal artery from a healthy donor. By contrast, an almost complete disappearance and fragmentation of elastic lamellae were observed in the media of the dissected artery, most likely responsible for the weakening of the arterial wall and its dissection. Association between ADPKD and single dissection of the iliac artery has been rarely reported. Relationship between inactivation of polycystin/PKD genes and elastic fibers degradation through elevated TGFβ signaling and matrix metalloproteinase 2 (MMP2) elastolytic activity, as recently reported in ADPKD, would be worth investigating.

Cyst expansion and regression in a mouse model of polycystic kidney disease

Autosomal-dominant polycystic kidney disease is characterized by progressive cyst formation and fibrosis in the kidneys. Here we describe an orthologous Pkd1(nl,nl) mouse model, with reduced expression of the normal Pkd1 transcript, on a fixed genetic background of equal parts C57Bl/6 and 129Ola/Hsd mice (B6Ola-Pkd1(nl,nl)). In these mice, the first cysts develop from mature proximal tubules around birth. Subsequently, larger cysts become visible at day 7, followed by distal tubule and collecting duct cyst formation, and progressive cystic enlargement to develop into large cystic kidneys within 4 weeks. Interestingly, cyst expansion was followed by renal volume regression due to cyst collapse. This was accompanied by focal formation of fibrotic areas, an increased expression of genes involved in matrix remodeling and subsequently an increase in infiltrating immune cells. After an initial increase in blood urea within the first 4 weeks, renal function remained stable over time and the mice were able to survive up to a year. Also, in kidneys of ADPKD patients collapsed cysts were observed, in addition to massive fibrosis and immune infiltrates. Thus, B6Ola-Pkd1(nl,nl) mice show regression of cysts and renal volume that is not accompanied by a reduction in blood urea levels.

Tg737 signaling is required for hypoxia-enhanced invasion and migration of hepatoma cells

BACKGROUND

Although hypoxia is known to promote hepatoma cell invasion and migration, little is known regarding the molecular mechanisms of this process. Our previous research showed that loss of Tg737 is associated with hepatoma cell invasion and migration; therefore, we hypothesized that the Tg737 signal might be required for hypoxia-enhanced invasion and migration.

METHODS

We established in vitro normoxic or hypoxic models to investigate the role of Tg737 in the hypoxia-enhanced invasion and migration of hepatoma cells. The hepatoma cell lines HepG2 and MHCC97-H were subjected to normoxic or hypoxic conditions, and the cell adhesion, invasion, and migration capabilities were tested. The expression of Tg737 under normoxia or hypoxia was detected using western blot assays; cell viability was determined using flow cytometry. Furthermore, we created HepG2 and MHCC97-H cells that over expressed Tg737 prior to incubation under hypoxia and investigated their metastatic characteristics. Finally, we analyzed the involvement of critical molecular events known to regulate invasion and migration.

RESULTS

In this study, Tg737 expression was significantly inhibited in HepG2 and MHCC97-H cells following exposure to hypoxia. The down regulation of Tg737 expression corresponded to significantly decreased adhesion and increased invasion and migration. Hypoxia also decreased the expression/secretion of polycystin-1, increased the secretion of interleukin-8 (IL-8), and increased the levels of active and total transforming growth factor β 1 (TGF-β1), critical regulators of cell invasion and migration. Moreover, the decrease in adhesiveness and the increase in the invasive and migratory capacities of hypoxia-treated hepatoma cells were attenuated by pcDNA3.1-Tg737 transfection prior to hypoxia. Finally, following the up regulation of Tg737, the expression/secretion of polycystin-1 increased, and the secretion of IL-8 and the levels of active and total TGF-β1 decreased correspondingly.

CONCLUSIONS

These data provide evidence that Tg737 contributes to hypoxia-induced invasion and migration, partially through the polycystin-1, IL-8, and TGF-β1 pathway. Taken together, this work suggests that Tg737 is involved in the invasion and migration of hepatoma cells under hypoxia, with the involvement of the polycystin-1, IL-8, and TGF-β1 signaling pathway. Tg737 is a potential therapeutic target for inhibiting the high invasion and migration potential of hepatoma cells in hypoxic regions.

Mycophenolate mofetil inhibits macrophage infiltration and kidney fibrosis in long-term ischemia-reperfusion injury

Immunosuppressants have been widely used in renal transplantation, in which ischemia-reperfusion injury is inevitable. Mycophenolate mofetil (MMF) is a relative novel immunosuppressant and also attenuates ischemia-reperfusion injury in the acute phase, but its long-term effects are still obscure. Unilateral renal ischemia-reperfusion injury model was established in Sprague-Dawley rats and 30 mg/kg/day MMF or natural saline was administered a day before the surgery. Renal function was monitored, and histological changes and fibrosis in the kidney were evaluated in both short and long terms. TGF-β1 secretion and MCP-1 expression were determined by immunohistochemistry and real-time PCR respectively. The infiltration of macrophages in renal tissues was also assessed by fluorescence activated cell sorting (FACS). MMF treatment significantly improved renal function in ischemia-reperfusion injury rats in the short and long-term and also effectively prevented interstitial fibrosis. TGF-β1 secretion and MCP-1 expression in the renal tissue of MMF-treated rats were much lower than those in natural saline-treated rats, with much less macrophage infiltration as well. MMF treatment effectively prevented the deterioration of renal function and interstitial fibrosis in ischemia-reperfusion injury rats, which may be associated with decreased TGF-β1, MCP-1 and macrophages. These results provide evidence for the choice of MMF in the renal transplant patients not only for acute renal injury but also for long-term survival of renal allograft.

Dose-dependent effects of sirolimus on mTOR signaling and polycystic kidney disease

Inhibition of the mammalian target of rapamycin (mTOR) shows beneficial effects in animal models of polycystic kidney disease (PKD); however, two clinical trials in patients with autosomal dominant PKD failed to demonstrate a short-term benefit in either the early or progressive stages of disease. The stage of disease during treatment and the dose of mTOR inhibitors may account for these differing results. Here, we studied the effects of a conventional low dose and a higher dose of sirolimus (blood levels of 3 ng/ml and 30-60 ng/ml, respectively) on mTOR activity and renal cystic disease in two Pkd1-mutant mouse models at different stages of the disease. When initiated at early but not late stages of disease, high-dose treatment strongly reduced mTOR signaling in renal tissues, inhibited cystogenesis, accelerated cyst regression, and abrogated fibrosis and the infiltration of immune cells. In contrast, low-dose treatment did not significantly reduce renal cystic disease. Levels of p-S6Rp(Ser240/244), which marks mTOR activity, varied between kidneys; severity of the renal cystic phenotype correlated with the level of mTOR activity. Taken together, these data suggest that long-term treatment with conventional doses of sirolimus is insufficient to inhibit mTOR activity in renal cystic tissue. Mechanisms to increase bioavailability or to target mTOR inhibitors more specifically to kidneys, alone or in combination with other compounds, may improve the potential for these therapies in PKD.

Emerging roles for renal primary cilia in epithelial repair

Primary cilia are microscopic sensory antennae that cells in many vertebrate tissues use to gather information about their environment. In the kidney, primary cilia sense urine flow and are essential for the maintenance of epithelial architecture. Defects of this organelle cause the cystic kidney disease characterized by epithelial abnormalities. These findings link primary cilia to the regulation of epithelial differentiation and proliferation, processes that must be precisely controlled during epithelial repair in the kidney. Here, we consider likely roles for primary cilium-based signaling during responses to renal injury and ensuing epithelial repair processes.

Rosiglitazone inhibits transforming growth factor-β1 mediated fibrogenesis in ADPKD cyst-lining epithelial cells

Background

Interstitial fibrosis plays an important role in progressive renal dysfunction in autosomal dominant polycystic kidney disease (ADPKD). In our previous studies, we confirmed that PPAR-γ agonist, rosiglitazone could protect renal function and prolong the survival of a slowly progressive ADPKD animal model by reducing renal fibrosis. However, the mechanism remains unknown.

Methods

Primary culture epithelial cells pretreated with TGF-β1 were incubated with rosiglitazone. Extracellular matrix proteins were detected using real-time PCR and Western blotting. MAPK and Smad2 phosphorylation were measured with western blot. ERK1/2 pathway and P38 pathway were inhibited with the specific inhibitors PD98059 and SB203580. The Smad2 pathway was blocked with the siRNA. To address whether PPAR-γ agonist-mediated inhibition of TGF-β1–induced collagen type I expression was mediated through a PPAR-γ dependent mechanism, genetic and pharmaceutical approaches were used to block the activity of endogenous PPARγ.

Results

TGF-β1-stimulated collagen type I and fibronectin expression of ADPKD cyst-lining epithelia were inhibited by rosiglitazone in a dosage-dependent manner. Smad2, ERK1/2 and P38 pathways were activated in response to TGF-β1; however, TGF-β1 had little effect on JNK pathway. Rosiglitazone suppressed TGF-β1 induced Smad2 activation, while ERK1/2 and P38MAPK signals remained unaffected. Rosiglitazone could also attenuate TGF-β1-stimulated collagen type I and fibronectin expression in primary renal tubular epithelial cells, but had no effect on TGF-β1–induced activation of Smad2, ERK1/2 and P38 pathways. There was no crosstalk between the Smad2 and MAPK pathways in ADPKD cyst-lining epithelial cells. These inhibitory effects of rosiglitazone were reversed by the PPARγ specific antagonist GW9662 and PPARγ siRNA.

Conclusion

ADPKD cyst-lining epithelial cells participate in TGF-β1 mediated fibrogenesis. Rosiglitazone could suppress TGF-β1–induced collagen type I and fibronectin expression in ADPKD cyst-lining epithelia through modulation of the Smad2 pathway. Our study may provide therapeutic basis for clinical applications of rosiglitazone in retarding the progression of ADPKD.

Fibrosis and progression of autosomal dominant polycystic kidney disease (ADPKD)

The age on onset of decline in renal function and end-stage renal disease (ESRD) in autosomal polycystic kidney disease (ADPKD) is highly variable and there are currently no prognostic tools to identify patients who will progress rapidly to ESRD. In ADPKD, expansion of cysts and loss of renal function are associated with progressive fibrosis. Similar to the correlation between tubulointerstitial fibrosis and progression of chronic kidney disease (CKD), in ADPKD, fibrosis has been identified as the most significant manifestation associated with an increased rate of progression to ESRD. Fibrosis in CKD has been studied extensively. In contrast, little is known about the mechanisms underlying progressive scarring in ADPKD although some commonality may be anticipated. Current data suggest that fibrosis associated with ADPKD shares at least some of the "classical" features of fibrosis in CKD (increased interstitial collagens, changes in matrix metalloproteinases (MMPs), over-expression of tissue inhibitor of metalloproteinase-1 (TIMP-1), over-expression of plasminogen activator inhibitor-1 (PAI-1) and increased transforming growth factor beta (TGFβ) but that there are also some unique and stage-specific features. Epithelial changes appear to precede and to drive interstitial changes leading to the proposal that development of fibrosis in ADPKD is biphasic with alterations in cystic epithelia precipitating changes in interstitial fibroblasts and that reciprocal interactions between these cell types drives progressive accumulation of extracellular matrix (ECM). Since fibrosis is a major component of ADPKD it follows that preventing or slowing fibrosis should retard disease progression with obvious therapeutic benefits. The development of effective anti-fibrotic strategies in ADPKD is dependent on understanding the precise mechanisms underlying initiation and progression of fibrosis in ADPKD and the role of the intrinsic genetic defect in these processes. This article is part of a Special Issue entitled: Polycystic Kidney Disease.

Why kidneys fail in autosomal dominant polycystic kidney disease

The weight of evidence gathered from studies in humans with hereditary polycystic kidney disease (PKD)1 and PKD2 disorders, as well as from experimental animal models, indicates that cysts are primarily responsible for the decline in glomerular filtration rate that occurs fairly late in the course of the disease. The processes underlying this decline include anatomic disruption of glomerular filtration and urinary concentration mechanisms on a massive scale, coupled with compression and obstruction by cysts of adjacent nephrons in the cortex, medulla and papilla. Cysts prevent the drainage of urine from upstream tributaries, which leads to tubule atrophy and loss of functioning kidney parenchyma by mechanisms similar to those found in ureteral obstruction. Cyst-derived chemokines, cytokines and growth factors result in a progression to fibrosis that is comparable with the development of other progressive end-stage renal diseases. Treatment of renal cystic disorders early enough to prevent or reduce cyst formation or slow cyst growth, before the secondary changes become widespread, is a reasonable strategy to prolong the useful function of kidneys in patients with autosomal dominant polycystic kidney disease.

Exploring anti-TGF-β therapies in cancer and fibrosis

Transforming growth factor-β (TGF-β) is a multifunctional cytokine, with important roles in maintaining tissue homeostasis. TGF-β signals via transmembrane serine/threonine kinase receptors and intracellular Smad transcriptional regulators. Perturbed TGF-β signaling has been implicated in a large variety of pathological conditions. Increased TGF-β levels have been found in patients with cancer, fibrosis, and systemic sclerosis, and were correlated with disease severity. In cancer, TGF-β mediates tumor invasion and metastasis by affecting both tumor cells and the tumor microenvironment including fibroblast activation and immune suppression. Furthermore, TGF-β is a strong stimulator of extracellular matrix deposition. On the basis of these observations, small molecule inhibitors of the TGF-β receptor kinases, neutralizing antibodies that interfere with ligand?receptor interactions, antisense oligonucleotides reducing TGF-β expression, and soluble receptor ectodomains that sequester TGF-β have been developed to intervene with excessive TGF-β signaling activity in the aforementioned disorders. Here, we review the current state of anti-TGF-β therapy in clinical trials.

The Raf kinase inhibitor PLX5568 slows cyst proliferation in rat polycystic kidney disease but promotes renal and hepatic fibrosis

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD) is a common cause of renal failure. Aberrant epithelial cell proliferation is a major cause of progressive cyst enlargement in ADPKD. Since activation of the Ras/Raf signaling system has been detected in cyst-lining epithelia, inhibition of Raf kinase has been proposed as an approach to retard the progression of ADPKD. Methods and results. PLX5568, a novel selective small molecule inhibitor of Raf kinases, attenuated proliferation of human ADPKD cyst epithelial cells. It reduced in vitro cyst growth of Madin-Darby Canine Kidney cells and of human ADPKD cells within a collagen gel. In male cy/+ rats with polycystic kidneys, PLX5568 inhibited renal cyst growth along with a significant reduction in the number of proliferating cell nuclear antigen- and phosphorylated extracellular signal-regulated kinase-positive cyst-lining epithelial cells. Furthermore, treated animals showed increased capacity to concentrate urine. However, PLX5568 did not lead to a consistent improvement of renal function. Moreover, although relative cyst volume was decreased, total kidney-to-body weight ratio was not significantly reduced by PLX5568. Further analyses revealed a 2-fold increase of renal and hepatic fibrosis in animals treated with PLX5568.

CONCLUSIONS

PLX5568 attenuated cyst enlargement in vitro and in a rat model of ADPKD without improving kidney function, presumably due to increased renal fibrosis. These data suggest that effective therapies for the treatment of ADPKD will need to target fibrosis as well as the growth of cysts.

Intermediate volume on computed tomography imaging defines a fibrotic compartment that predicts glomerular filtration rate decline in autosomal dominant polycystic kidney disease patients

Total kidney and cyst volumes have been used to quantify disease progression in autosomal dominant polycystic kidney disease (ADPKD), but a causal relationship with progression to renal failure has not been demonstrated. Advanced image processing recently allowed to quantify extracystic tissue, and to identify an additional tissue component named "intermediate," appearing hypoenhanced on contrast-enhanced computed tomography (CT). The aim of this study is to provide a histological characterization of intermediate volume, investigate its relation with renal function, and provide preliminary evidence of its role in long-term prediction of functional loss. Three ADPKD patients underwent contrast-enhanced CT scans before nephrectomy. Histological samples of intermediate volume were drawn from the excised kidneys, and stained with hematoxylin and eosin and with saturated picrosirius solution for histological analysis. Intermediate volume showed major structural changes, characterized by tubular dilation and atrophy, microcysts, inflammatory cell infiltrate, vascular sclerosis, and extended peritubular interstitial fibrosis. A significant correlation (r = -0.69, P < 0.001) between relative intermediate volume and baseline renal function was found in 21 ADPKD patients. Long-term prediction of renal functional loss was investigated in an independent cohort of 13 ADPKD patients, followed for 3 to 8 years. Intermediate volume, but not total kidney or cyst volume, significantly correlated with glomerular filtration rate decline (r = -0.79, P < 0.005). These findings suggest that intermediate volume may represent a suitable surrogate marker of ADPKD progression and a novel therapeutic target.

Polycystic kidney disease: the complexity of planar cell polarity and signaling during tissue regeneration and cyst formation

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is an inherited systemic disease with intrarenal cystogenesis as its primary characteristic. A variety of mouse models provided information on the requirement of loss of balanced polycystin levels for initiation of cyst formation, the role of proliferation in cystogenesis and the signaling pathways involved in cyst growth and expansion. Here we will review the involvement of different signaling pathways during renal development, renal epithelial regeneration and cyst formation in ADPKD, focusing on planar cell polarity (PCP) and oriented cell division (OCD). This will be discussed in context of the hypothesis that aberrant PCP signaling causes cyst formation. In addition, the role of the Hippo pathway, which was recently found to be involved in cyst growth and tissue regeneration, and well-known for regulating organ size control, will be reviewed. The fact that Hippo signaling is linked to PCP signaling makes the Hippo pathway a novel cascade in cystogenesis. The newly gained understanding of the complex signaling network involved in cystogenesis and disease progression, not only necessitates refining of the current hypothesis regarding initiation of cystogenesis, but also has implications for therapeutic intervention strategies. This article is part of a Special Issue entitled: Polycystic Kidney Disease.

Molecular and cellular pathogenesis of autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is one of the most common human life-threatening monogenic disorders. The disease is characterized by bilateral, progressive renal cystogenesis and cyst and kidney enlargement, often leading to end-stage renal disease, and may include extrarenal manifestations. ADPKD is caused by mutation in one of two genes, PKD1 and PKD2, which encode polycystin-1 (PC1) and polycystin-2 (PC2), respectively. PC2 is a non-selective cation channel permeable to Ca(2+), while PC1 is thought to function as a membrane receptor. The cyst cell phenotype includes increased proliferation and apoptosis, dedifferentiation, defective planar polarity, and a secretory pattern associated with extracellular matrix remodeling. The two-hit model for cyst formation has been recently extended by the demonstration that early gene inactivation leads to rapid and diffuse development of renal cysts, while inactivation in adult life is followed by focal and late cyst formation. Renal ischemia/reperfusion, however, can function as a third hit, triggering rapid cyst development in kidneys with Pkd1 inactivation induced in adult life. The PC1-PC2 complex behaves as a sensor in the primary cilium, mediating signal transduction via Ca(2+) signaling. The intracellular Ca(2+) homeostasis is impaired in ADPKD, being apparently responsible for the cAMP accumulation and abnormal cell proliferative response to cAMP. Activated mammalian target for rapamycin (mTOR) and cell cycle dysregulation are also significant features of PKD. Based on the identification of pathways altered in PKD, a large number of preclinical studies have been performed and are underway, providing a basis for clinical trials in ADPKD and helping the design of future trials.

Transforming Growth Factor-β1 as a Common Target Molecule for Development of Cardiovascular Diseases, Renal Insufficiency and Metabolic Syndrome

Transforming growth factor-β1 (TGF-β1) is a polypeptide member of the transforming growth factor β superfamily of cytokines. It is a secreted protein that performs many cellular functions including control of cell growth, cell proliferation, cell differentiation and apoptosis. In the cardiovascular system, TGF-β1 plays pivotal roles in the pathogenesis of hypertension, restenosis after percutaneous coronary intervention, atherosclerosis, cardiac hypertrophy and heart failure. In addition, TGF-β1 has been shown to be increased in adipose tissue of obese subjects with insulin resistance. Furthermore, TGF-β1 is a potent initiator of proliferation of renal mesangial cells leading to chronic kidney disease. Some currently available agents can manipulate TGF-β1 expression leading to amelioration of cardiovascular diseases. Thus, an understanding of interactions between chronic kidney disease and metabolic syndrome and the development of cardiovascular diseases is an important issue, and attention should be given to TGF-β1 as a crucial factor for regulation and modulation of those pathological conditions.

In the ring with polycystic kidney disease--avoiding the knockout punch

Autosomal dominant polycystic kidney disease (PKD) is an inherited disease that results from mutations in either polycystin (PKD1) or polycystin 2 (PKD2), both of which are large, complex, and multifunctional proteins whose loss results in the development of numerous fluid-filled cysts and fibrosis that compromise renal function. A number of spontaneous and engineered mouse models of PKD have provided some understanding of many aspects of cyst development, modifier genes, and mechanistic pathways, but fall short of reproducing the human disease accurately. Two recent papers in The Journal of Pathology set out new models using miRNA, or inducible and targeted recombination, that achieve partial or timed suppression of Pkd1. Instead of knocking out Pkd1 immediately, or completely, these more subtle approaches may help deliver more faithful models of this significant human renal disease.