Hepatocyte growth factor induces branching tubulogenesis in MDCK cells by modulating the activin-follistatin system

The activin-follistatin system: Key regulator of kidney development, regeneration, inflammation, and fibrosis

Activins, multifunctional cytokines of the transforming growth factor-beta superfamily, play critical roles in the regulation of growth and differentiation in multiple biological systems. Activin activity is finely regulated by the endogenous antagonist follistatin. Early studies reported that activins are involved in renal organogenesis, but subsequent research demonstrated that activins also play a significant role in kidney regeneration following injury. The results of more recent studies suggest activins play roles in both inflammatory kidney diseases and renal fibrosis, conditions that often culminate in end-stage renal disease. Given these findings, the inhibition of activin activity represents a promising therapeutic approach for treating a range of kidney disorders. This review discusses the latest discoveries concerning the role of the activin-follistatin system in renal development and pathophysiology and explores the potential therapeutic implications of targeting this system in the management of kidney diseases.

The Reign of Follistatin in Tumors and Their Microenvironment: Implications for Drug Resistance

Follistatin (FST) is a potent neutralizer of the transforming growth factor-β superfamily and is associated with normal cellular programs and various hallmarks of cancer, such as proliferation, migration, angiogenesis, and immune evasion. The aberrant expression of FST by solid tumors is a well-documented observation, yet how FST influences tumor progression and therapy response remains unclear. The recent surge in omics data has revealed new insights into the molecular foundation underpinning tumor heterogeneity and its microenvironment, offering novel precision medicine-based opportunities to combat cancer. In this review, we discuss these recent FST-centric studies, thereby offering an updated perspective on the protean role of FST isoforms in shaping the complex cellular ecosystem of tumors and in mediating drug resistance.

Association of Serum Activin Levels with Allograft Outcomes in Patients with Kidney Transplant: Results from the KNOW-KT

INTRODUCTION

Serum activin A has been reported to contribute to vascular calcification and kidney fibrosis in chronic kidney disease. We aimed to investigate whether higher serum activin levels were associated with poor allograft outcomes in patients with kidney transplantation (KT).

METHODS

A total of 860 KT patients from KNOW-KT (Korean Cohort Study for Outcome in Patients with Kidney Transplantation) were analyzed. We measured serum activin levels pre-KT and 1 year after KT. The primary outcome was the composite of a ≥50% decline in estimated glomerular filtration rate and graft failure. Multivariable cause-specific hazard model was used to analyze association of 1-year activin levels with the primary outcome. The secondary outcome was coronary artery calcification score (CACS) at 5 years after KT.

RESULTS

During the median follow-up of 6.7 years, the primary outcome occurred in 109 (12.7%) patients. The serum activin levels at 1 year were significantly lower than those at pre-KT (488.2 ± 247.3 vs. 704.0 ± 349.6). When patients were grouped based on the median activin level at 1 year, the high-activin group had a 1.91-fold higher risk (95% CI, 1.25-2.91) for the primary outcome compared to the low-activin group. A one-standard deviation increase in activin levels as a continuous variable was associated with a 1.36-fold higher risk (95% CI, 1.16-1.60) for the primary outcome. Moreover, high activin levels were significantly associated with 1.56-fold higher CACS (95% CI, 1.12-2.18).

CONCLUSION

Post-transplant activin levels were independently associated with allograft functions as well as coronary artery calcification in KT patients.

Renal Abscess Caused by Crizotinib: A Rare Case Report

Crizotinib is a tyrosine kinase inhibitor that has been found to be effective in the treatment of c-ros oncogene 1-positive non-small cell lung cancer. Although this targeted agent for treating cancer has shown superiority to standard chemotherapy in some ways, this drug has adverse effects, such as the development of renal abscesses. Some associated renal damage may disappear with crizotinib withdrawal. Hence, we present the case of a 58-year-old man with non-small cell lung cancer on crizotinib therapy who developed bilateral renal abnormal space-occupying lesions, successively which were difficult to identify using various imaging methods; even PET-CT highly suspected the right renal masses as malignant. Finally, the right renal lesions were confirmed as renal abscesses by postoperative pathology. The left renal lesion was considered as renal cysts through the lesion disappearing after crizotinib withdrawal. There have been very few reports in this respect, especially proved by various methods and confirmed by postoperative pathology. It is important to recognize this drug-related complication in order to avoid incorrect diagnosis and inadequate therapy. It is necessary to monitor renal changes after taking crizotinib.

Urinary Activin A: A Novel Biomarker for Human Acute Kidney Injury

Activin is a multifunctional cytokine belonging to the transforming growth factor (TGF)-β superfamily that regulates the growth and differentiation of cells in various organs. We previously reported that activin A, which is absent in normal kidneys, was significantly increased in the ischemic kidney, and that the blockade of activin action by follistatin, an activin antagonist, significantly enhanced tubular regeneration after renal ischemia, suggesting that activin A acts as an endogenous inhibitor of tubular repair after kidney injury in rodents. However, the role of activin A in human acute kidney injury (AKI) remains unclear. In this analysis, we measured serum and urinary activin A in human AKI (n = 39) and tested if activin A might serve as a biomarker for AKI. Urinary activin A, which was undetectable in healthy controls, was significantly increased in AKI (0.0 ± 0.0 vs. 173.4 ± 58.8 pg/mL, p < 0.05). The urinary activin A level in patients with AKI stage 3, was significantly higher than that in patients with AKI stages 1 and 2. Patients who required renal replacement therapy (RRT) had a significantly higher urinary activin A level than patients who did not require RRT. Urinary activin A might be a useful non-invasive biomarker for the severity of AKI.

The emerging role of activins in renal disease

PURPOSE OF REVIEW

This review highlights recent discoveries and advances that have been made in understanding the role of the TGFβ superfamily members activins, and in particular, activin A (ActA), in renal disease.

RECENT FINDINGS

A deleterious role for ActA in renal disease and its complications has begun to emerge. We summarize data supporting an important contribution of ActA to kidney fibrosis and inflammation of varying causes, as well as its role in the development of a particular bone mineral disorder seen in chronic kidney disease (CKD) called mineral bone disorder (MBD), including vascular calcification. Finally, we discuss ActA in the context of anemia associated with chronic kidney disease and review potential approaches to treatment based on ActA blockade.

SUMMARY

ActA is an important contributor to the pathogenesis of acute and chronic kidney disease of varying causes. Preclinical studies show that ActA inhibition, through various approaches, is protective in rodent models of kidney disease. The potential adverse effects of some of these approaches can be attributed to their targeting of other TGFβ family ligands. Further preclinical and clinical investigations testing the therapeutic efficacy of more selective ActA inhibition on the progression of acute and chronic kidney disease and its impact on bone-mineral disorder would more definitively establish its role in renal disease.

Urinary Activin A is a novel biomarker reflecting renal inflammation and tubular damage in ANCA-associated vasculitis

Activin A, a member of the transforming growth factor-beta superfamily, is a critical modulator of inflammation and plays a key role in controlling the cytokine cascade that drives the inflammatory response. However, the role of activin A in inflammatory kidney diseases remains unknown. To address this issue, we examined here whether activin A can be detected in the kidney and/or urine from patients with antineutrophil cytoplasmic antibody (ANCA) -associated vasculitis (AAV). Fifty-one patients who had been diagnosed with AAV and were treated in our department between November 2011 to March 2018 were included in this study. Forty-one patients had renal complications (renal AAV). Serum and urinary activin A levels were measured by enzyme-linked immunosorbent assay. Correlation of urinary activin A concentration with clinical parameters was analyzed. Urinary activin A was undetectable in healthy volunteers. In contrast, urinary activin A concentration was significantly increased in patients with renal AAV but not in those with non-renal AAV. Urinary activin A concentration decreased rapidly after immunosuppressive treatment. There was a significant correlation of urinary activin A level with urinary protein, L-FABP, and NAG. Histologic evaluation revealed that urinary activin A levels were significantly higher in patients with cellular crescentic glomeruli than in those lacking this damage. In situ hybridization demonstrated that the mRNA encoding the activin A βA subunit was undetectable in normal kidneys but accumulated in the proximal tubules and crescentic glomeruli of the kidneys of patients with renal AAV. Immunostaining showed that activin A protein also was present in the proximal tubules, crescentic glomeruli, and macrophages infiltrating into the interstitium in the kidneys of patients with renal AAV. These data suggested that urinary activin A concentration reflects renal inflammation and tubular damage in AAV and may be a useful biomarker for monitoring renal AAV.

Enhancement of HGF-induced tubulogenesis by endothelial cell-derived GDNF

Tubulogenesis, the organization of epithelial cells into tubular structures, is an essential step during renal organogenesis as well as during the regeneration process of renal tubules after injury. In the present study, endothelial cell-derived factors that modulate tubule formation were examined using an in vitro human tubulogenesis system. When human renal proximal tubular epithelial cells (RPTECs) were cultured in gels, tubular structures with lumens were induced in the presence of hepatocyte growth factor (HGF). Aquaporin 1 was localized in the apical membrane of these tubular structures, suggesting that these structures are morphologically equivalent to renal tubules in vivo. HGF-induced tubule formation was significantly enhanced when co-cultured with human umbilical vein endothelial cells (HUVECs) or in the presence of HUVEC-conditioned medium (HUVEC-CM). Co-culture with HUVECs did not induce tubular structures in the absence of HGF. A phospho-receptor tyrosine kinase array revealed that HUVEC-CM markedly enhanced phosphorylation of Ret, glial cell-derived neurotrophic factor (GDNF) receptor, in HGF-induced tubular structures compared to those without HUVEC-CM. HUVECs produced GDNF, and RPTECs expressed both Ret and GDNF family receptor alpha1 (co-receptor). HGF-induced tubule formation was significantly enhanced by addition of GDNF. Interestingly, not only HGF but also GDNF significantly induced phosphorylation of the HGF receptor, Met. These data indicate that endothelial cell-derived GDNF potentiates the tubulogenic properties of HGF and may play a critical role in the epithelial-endothelial crosstalk during renal tubulogenesis as well as tubular regeneration after injury.

Activin A in Mammalian Physiology

Activins are dimeric glycoproteins belonging to the transforming growth factor beta superfamily and resulting from the assembly of two beta subunits, which may also be combined with alpha subunits to form inhibins. Activins were discovered in 1986 following the isolation of inhibins from porcine follicular fluid, and were characterized as ovarian hormones that stimulate follicle stimulating hormone (FSH) release by the pituitary gland. In particular, activin A was shown to be the isoform of greater physiological importance in humans. The current understanding of activin A surpasses the reproductive system and allows its classification as a hormone, a growth factor, and a cytokine. In more than 30 yr of intense research, activin A was localized in female and male reproductive organs but also in other organs and systems as diverse as the brain, liver, lung, bone, and gut. Moreover, its roles include embryonic differentiation, trophoblast invasion of the uterine wall in early pregnancy, and fetal/neonate brain protection in hypoxic conditions. It is now recognized that activin A overexpression may be either cytostatic or mitogenic, depending on the cell type, with important implications for tumor biology. Activin A also regulates bone formation and regeneration, enhances joint inflammation in rheumatoid arthritis, and triggers pathogenic mechanisms in the respiratory system. In this 30-yr review, we analyze the evidence for physiological roles of activin A and the potential use of activin agonists and antagonists as therapeutic agents.

Renal Injury during Long-Term Crizotinib Therapy

Crizotinib is highly effective against anaplastic lymphoma kinase-positive and c-ros oncogen1-positive non-small cell lung cancer. Renal dysfunction is associated with crizotinib therapy but the mechanism is unknown. Here, we report a case of anaplastic lymphoma kinase positive non-small cell lung cancer showing multiple cysts and dysfunction of the kidneys during crizotinib administration. We also present results demonstrating that long-term crizotinib treatment induces fibrosis and dysfunction of the kidneys by activating the tumor necrosis factor-α/nuclear factor-κB signaling pathway. In conclusion, this study shows the renal detrimental effects of crizotinib, suggesting the need of careful monitoring of renal function during crizotinib therapy.

Identification of Urinary Activin A as a Novel Biomarker Reflecting the Severity of Acute Kidney Injury

Acute kidney injury (AKI) is a common but complex condition that is associated with increased morbidity and mortality. In the present study, we examined whether urinary activin A, a member of the TGF-beta superfamily, is present in mice with ischemia-reperfusion injury and in humans with AKI, as well as its potential as a biomarker for AKI. Expression of activin A was markedly increased in ischemic mouse kidneys. In situ hybridization demonstrated that activin mRNA was expressed in tubular cells of ischemic kidneys but not of normal kidneys. Immunoreactive activin A, which was absent in normal kidneys, was detected in the cytoplasm of proximal tubular cells in ischemic kidneys. Activin A was undetectable in the urine of normal mice. In contrast, activin A was significantly increased in the urine of ischemic mice at 3 h after reperfusion. Urinary activin A levels increased according to the period of ischemia. In humans, urinary activin A was almost undetectable in healthy volunteers and in patients with pre-renal AKI, but was significantly increased in patients with renal AKI. There was no significant correlation between urinary activin A and serum activin A. Collectively, urinary activin A might be a useful biomarker reflecting the severity of AKI.

dNK cells facilitate the interaction between trophoblastic and endothelial cells via VEGF-C and HGF

Decidual NK (dNK) cells, identified as CD56^brightCD16^-CD3^-, account for ~70% of lymphocytes within the uterine wall during early pregnancy. Accumulating evidence suggests that tight interactions between placental trophoblasts and dNK cells are critical for trophoblast cell differentiation. However, the underlying mechanism remains to be explored in detail. In the present study, conditioned medium (CM) was collected from cultured primary human dNK cells. Primary cytotrophoblasts (CTBs) or the human trophoblast cell line HTR8/SVneo was treated with dNK-CM and co-cultured with human umbilical vein endothelial cells (HUVECs) in a three-dimensional Matrigel scaffold, and the formation of tube structures was dynamically monitored with live cell imaging. Trophoblast invasion was analyzed with a transwell invasion assay. The data demonstrated that the treatment of HTR8/SVneo cells or CTBs with dNK-CM remarkably promoted trophoblast invasion and tube formation in the presence of HUVECs. The epithelial marker E-cadherin was reduced, while the expression of endothelial markers NCAM, VE-cadherin and integrin β1 was significantly promoted in the HTR8/SVneo cells upon treatment with dNK-CM. Antibody blocking experiments revealed that the dNK cells promoted trophoblast invasion through the production of IL-8 and HGF, and they induced trophoblast differentiation toward endothelial phenotype by producing VEGF-C and HGF. These results provide new evidence to clarify the finely tuned interactions between trophoblasts and dNK cells at the maternal-fetal interface.

Involvement of infiltrating macrophage-derived activin A in the progression of renal damage in MRL-lpr mice

Lupus nephritis is a life-threatening complication of systemic lupus erythematosus (SLE). Various growth factors, cytokines, and chemokines are implicated in the development of SLE. However, the pathophysiological processes involved in the development of lupus nephritis still remain unclear. In this study, we examined the involvement of activin A, a member of the transforming growth factor β (TGF-β) superfamily, in the progression of renal damage in lupus-prone MRL- lpr mice. Activin A was not expressed in the kidneys of control MRL-MpJ mice but was detectable in perivascular infiltrating cluster of differentiation 68 (CD68)-positive cells in the kidneys of MRL- lpr mice. Urinary activin A, which was also absent in MRL-MpJ mice, was detectable in MRL- lpr mice from 16 wk onward. Urinary activin A levels were significantly correlated with the number of perivascular inflammatory cell layers, the number of crescentic glomeruli, and the percentage of Elastica van Gieson (EVG)-positive fibrotic areas, but not with urinary protein levels or serum activin A. When activin action was blocked in vivo by the intraperitoneal administration of an activin antagonist, follistatin, the number of crescentic glomeruli, percentage of EVG-positive fibrotic areas, CD68-positive cell infiltration, and proteinuria were significantly reduced in a dose-dependent manner. These data suggest that infiltrating macrophage-derived activin A is involved in the progression of renal damage in MRL- lpr mice.

Regenerative medicine for the kidney: renotropic factors, renal stem/progenitor cells, and stem cell therapy

The kidney has the capacity for regeneration and repair after a variety of insults. Over the past few decades, factors that promote repair of the injured kidney have been extensively investigated. By using kidney injury animal models, the role of intrinsic and extrinsic growth factors, transcription factors, and extracellular matrix in this process has been examined. The identification of renal stem cells in the adult kidney as well as in the embryonic kidney is an active area of research. Cell populations expressing putative stem cell markers or possessing stem cell properties have been found in the tubules, interstitium, and glomeruli of the normal kidney. Cell therapies with bone marrow-derived hematopoietic stem cells, mesenchymal stem cells, endothelial progenitor cells, and amniotic fluid-derived stem cells have been highly effective for the treatment of acute or chronic renal failure in animals. Embryonic stem cells and induced pluripotent stem cells are also utilized for the construction of artificial kidneys or renal components. In this review, we highlight the advances in regenerative medicine for the kidney from the perspective of renotropic factors, renal stem/progenitor cells, and stem cell therapies and discuss the issues to be solved to realize regenerative therapy for kidney diseases in humans.

Paracrine effects of stem cells in wound healing and cancer progression (Review)

Stem cells play an important role in tissue repair and cancer development. The capacity to self-renew and to differentiate to specialized cells allows tissue-specific stem cells to rebuild damaged tissue and cancer stem cells to initiate and promote cancer. Mesenchymal stem cells, attracted to wounds and cancer, facilitate wound healing and support cancer progression primarily by secreting bioactive factors. There is now growing evidence that, like mesenchymal stem cells, also tissue-specific and cancer stem cells manipulate their environment by paracrine actions. Soluble factors and microvesicles released by these stem cells have been shown to protect recipient cells from apoptosis and to stimulate neovascularization. These paracrine mechanisms may allow stem cells to orchestrate wound healing and cancer progression. Hence, understanding these stem cell-driven paracrine effects may help to improve tissue regeneration and cancer treatment.

Cell polarity as a regulator of cancer cell behavior plasticity

Cell polarization is an evolutionarily conserved process that facilitates asymmetric distribution of organelles and proteins and that is modified dynamically during physiological processes such as cell division, migration, and morphogenesis. The plasticity with which cells change their behavior and phenotype in response to cell intrinsic and extrinsic cues is an essential feature of normal physiology. In disease states such as cancer, cells lose their ability to behave normally in response to physiological cues. A molecular understanding of mechanisms that alter the behavior of cancer cells is limited. Cell polarity proteins are a recognized class of molecules that can receive and interpret both intrinsic and extrinsic signals to modulate cell behavior. In this review, we discuss how cell polarity proteins regulate a diverse array of biological processes and how they can contribute to alterations in the behavior of cancer cells.

Enhancement of in vitro human tubulogenesis by endothelial cell-derived factors: implications for in vivo tubular regeneration after injury

Renal proximal tubular epithelium can regenerate after various insults. To examine whether the tubular repair process is regulated by surrounding peritubular capillaries, we established an in vitro human tubulogenesis model that mimics in vivo tubular regeneration after injury. In this model, HGF, a potent renotropic factor, dose dependently induced tubular structures in human renal proximal tubular epithelial cells cultured in gels. Consistent with regenerating tubular cells after injury, HGF-induced tubular structures expressed a developmental gene, Pax-2, and a mesenchymal marker, vimentin, and formed a lumen with aquaporin-1 expression. Electron microscopic analysis showed the presence of microvilli on the apical site of the lumen, suggesting that these structures are morphologically equivalent to renal tubules in vivo. When cocultured with human umbilical vein endothelial cells (HUVEC), HGF-induced tubular formation was significantly enhanced. This could not be reproduced by the addition of VEGF, basic FGF, or PDGF. Protein array revealed that HUVEC produced various matrix metalloproteinases (MMPs). The stimulatory effects of coculture with HUVEC or HUVEC-derived conditional medium were almost completely abolished by addition of the tissue inhibitor of metalloproteinase (TIMP)-1 or TIMP-2. These data suggest that endothelial cell-derived factors including MMPs play a critical role in tubulogenesis and imply a potential role of peritubular capillary endothelium as a source of factor(s) required for tubular recovery after injury.

Hepatocyte growth factor/activin A/follistatin system activation during hemodialysis with different low molecular weight heparins

Hepatocyte growth factor (HGF), activin A (Act A), and follistatin (FS) compose an organotrophic system; interestingly it is modified by heparin. To understand if LMWHs (considered distinct drugs) have different clinical profiles regarding the above growth factors, we studied the effects of enoxaparin, nadroparin, and dalteparin on their plasma levels. Seventeen chronic HD patients completed this prospective, crossover trial. They were randomized into six groups: each patient was administered enoxaparin (effective dose of 0.75 mg/kg), nadroparin (70.4 IU/kg) and dalteparin (78.6 IU/kg) in three time periods of two months each. At the end of this period, the cytokine's plasma levels were measured by immunoassays at the start and at 10 min and 180 min of the HD procedure. At 10 min, we observed a striking increase in plasma HGF (32-fold), Act A (4-fold), and FS (53%), all p = 0.0003. The levels of HGF and Act A remained markedly elevated after 180 min (by 295% and 87%, respectively; both p < 0.002), while those of FS returned to baseline. There were no differences in cytokine profile comparing both their peak concentrations and the areas under the curve. Enoxaparin, dalteparin, and nadroparin are seemingly not different considering the release of HGF/Act A/FS during HD procedures; this may reflect their similar profile in other aspects. Moreover, the concentrations of HGF/Act A/FS are close to therapeutic ones, which may partly explain the mechanisms underlying some of the emerging extra-anticoagulant effects of LMWHs.

Activin A: autocrine regulator of kidney development and repair

The research described in this review suggests a novel and important role for activin A in the developmental and repair processes of the kidney (Table 1). The results obtained in these studies indicate that activin A is a negative regulator of kidney development and plays an essential part in kidney diseases, such as acute renal failure or renal fibrosis. It is also possible that activin A is a key player in the pathophysiological processes of other kidney diseases, such as congenital urogenital abnormalities, renal cystic disease and renal cell carcinoma. Activin A is thus a potential target for therapeutic interventions in kidney diseases. To address this issue, more detailed analysis on the regulation of activin production, modulation of activin activity and activin target genes is required.

Activin A as a critical mediator of capillary formation: interaction with the fibroblast growth factor action

The present study was conducted to elucidate the role of activin A in capillary formation. When bovine aortic endothelial cells (BAEC) were cultured in a collagen gel, basic fibroblast growth factor (FGF-2) induced tube formation. Activin A also induced tube formation and the addition of two factors together was more effective. BAEC produced both FGF-2 and activin A as autocrine factors. Exogenous FGF-2 did not affect the production of activin A but instead upregulated the type II activin receptor. On the other hand, activin A increased the expression of FGF-2 as well as the FGF receptor. Most importantly, when the action of endogenous activin A was blocked by adding follistatin, the tubulogenic action of FGF-2 was nearly completely inhibited. Activin-induced tubulogenesis was markedly inhibited by overexpression of Smad7, an inhibitory Smad. Similarly, an inhibitor of p44/42 mitogen-activated protein (MAP) kinase attenuated the activin-mediated tubulogenesis, whereas an inhibitor of p38 MAP kinase had no effect. These results indicate that FGF-2 and activin A enhance their signals each other in BAEC, and endogenous activin A is critical for FGF-2-induced capillary formation.

The developmental nephrome: systems biology in the developing kidney

PURPOSE OF REVIEW

A set of important genes and signaling pathways involved in kidney development is emerging from analyses of mutant mice, in-vitro models, and global gene expression patterns. Conversion of data into dynamic models or networks through the synthesis of information at multiple levels is crucial for a better understanding of kidney development.

RECENT FINDINGS

Genetic and in-vitro evidence is beginning to provide a limited sense of the network topology in stages of kidney development. Intriguing data from other fields suggest how, with the aid of large-scale gene expression studies, these stages might be represented as dynamic attractor states. It is also suggested how branching morphogenesis of the epithelial ureteric bud may be sustained by an autocatalytic set of proteins whose interactions lead to repeated rounds of tip and stalk generation. Accumulating data in lower organisms suggest network topologies may be quite flexible, and the implications of these results for varieties of tubulogenesis and renal regeneration after acute injury are discussed.

SUMMARY

Currently it may be feasible to build tentative dynamic multistage models of nephrogenesis that facilitate experimental thinking. As data accumulate, it may become possible to test their predictive value.

Inhibitory effect of c-Met mutants on the formation of branching tubules by a porcine aortic endothelial cell line

The association of hepatocyte growth factor (HGF) with its high-affinity receptor (c-Met) has been shown to induce mitogenesis, motogenesis and morphogenesis in a variety of cell types. Various point mutations in c-Met have been identified in hereditary and sporadic papillary renal carcinomas as well as in other carcinomas. In the present study, we examined the effects of c-Met point mutations on the morphology of a porcine aortic endothelial (PAE) cell line. When cultured in three-dimensional collagen gel, PAE cells formed branching tubule structures, and HGF treatment caused breakdown of the structures and induced a scattered morphology. The exogenous expression of c-Met point mutants inhibited the formation of tubules. HGF treatment induced the formation of tubules by PAE cells expressing some c-Met mutants, but it induced the scattering of PAE cells expressing other c-Met mutants. The presence of a low concentration of a mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEK) inhibitor cancelled the inhibitory effect of the c-Met point mutations on the formation of tubules. These results suggest that c-Met point mutations affect the extracellular signal-regulated kinase (ERK) signaling required for the formation of tubules by PAE cells, and HGF binding changes the conformation of c-Met mutants, leading to the different signals required for formation of tubules and cell scattering.

Different Effects of Growth Factors on Human Renal Early Distal Tubular Cells in vitro

BACKGROUND

In the kidney, recovery from tubular damage requires regenerative mechanisms leading to re-epithelialization of the injured tubules. Current evidence supports the para- or autocrine role of growth factors in repair and regeneration of ischemic or nephrotoxic experimental acute renal failure.

METHODS

We evaluated the effects of EGF, HGF, IGF-1, and bFGF on human renal thick ascending limb and distal convoluted cells (TALDC) in vitro. TALDC were isolated by immunomagnetic separation and cultured. Signal transduction of the growth factors was evaluated by Western blot of ERK1/2 MAP-K phosphorylation. Cell proliferation was measured by MTT assay and a fluorometric assay.

RESULTS

A significant, dose- and time-dependent phosphorylation of ERK1/2 could be detected exclusively after stimulation with EGF. No other growth factor induced a significant MAPK phosphorylation. In the same manner, proliferation assays showed a significant growth-promoting effect of EGF. Neither HGF, nor IGF-1 or bFGF showed a stimulative effect on TALDC proliferation.

CONCLUSION

The present study highlights the effects of growth factors on cultured TALDC and supports the hypothesis that in vivo EGF plays a para- or autocrine role during renal repair mechanisms.

Activin A is an endogenous inhibitor of ureteric bud outgrowth from the Wolffian duct

Development of metanephric kidney begins with ureteric bud outgrowth from the Wolffian duct (WD). GDNF is believed to be a crucial positive signal in the budding process, but the negative regulation of this process remains unclear. Here, we examined the role of activin A, a member of TGF-beta family, in bud formation using an in vitro WD culture system. When cultured with the surrounding mesonephros, WDs formed many ectopic buds in response to GDNF. While the activin signaling pathway is normally active along the non-budding WD (as measured by expression of activin A and phospho-Smad2/3), activin A was absent and phospho-Smad2/3 was undetectable in the ectopic buds induced by GDNF. To examine the role of activin A in bud formation, we attempted to inactivate activin action. Interestingly, the addition of neutralizing anti-activin A antibody potentiated GDNF action. To further clarify the role of activin A, we also tested the effect of activin blockade on the WD cultured in the absence of mesonephros. WDs without mesonephros did not form ectopic buds even in the presence of GDNF. In contrast, blockade of activin action with a variety of agents acting through different mechanisms (natural antagonist, neutralizing antibodies, siRNA) enabled GDNF to induce ectopic buds. Inhibition of GDNF-induced bud formation by activin A was accompanied by inhibition of cell proliferation, reduced expression of Pax-2, and decreased phosphorylation of PI3-kinase and MAP kinase in the WD. Our data suggest that activin A is an endogenous inhibitor of bud formation and that cancellation of activin A autocrine action may be critical for the initiation of this process.

Therapeutic potential of follistatin for colonic inflammation in mice

BACKGROUND AND AIMS

Activins belong to the transforming growth factor-beta superfamily. Recent studies have shown that activin and its natural antagonist, follistatin, are involved in tissue repair and inflammatory processes. The aim of this study was to determine whether neutralization of activins with follistatin would have an in vivo anti-inflammatory effect in several murine models of colitis.

METHODS

We assessed activin levels in the colitis induced by intracolonic administration of trinitrobenzene sulfonic acid (TNBS). We subsequently tested the effects of an intraperitoneal injection of follistatin before or after induction of TNBS colitis. We also examined the established colitis induced by oral dextran sulfate sodium (DSS) as well as the spontaneous colitis that develops in interleukin (IL)-10 gene-deficient (IL-10 -/- ) mice.

RESULTS

Levels of activin transcripts in the colon during the acute phase of TNBS colitis were up-regulated. Epithelial cells, infiltrating macrophages (Mvarphi), and endothelial cells produced excess activin betaA. Pretreatment with follistatin increased the survival rate of mice with TNBS colitis from 33% to 82% and decreased the plasma levels of IL-6 and amyloid A. Administration of follistatin also reduced the histologic score and tissue myeloperoxidase activity in established TNBS and DSS colitis and reduced the severity of the colitis in IL-10 -/- mice. Based on results obtained from 3 mouse models and from in vitro experiments, follistatin promoted the proliferation of colonic epithelial cells.

CONCLUSIONS

Neutralization of activins by follistatin promoted epithelial cell division and tissue repair, clearly suggesting a treatment modality for intestinal inflammation.

Crucial role of activin a in tubulogenesis of endothelial cells induced by vascular endothelial growth factor

The present study was conducted to elucidate the role of activin A in tubulogenesis of vascular endothelial cells. Activin A was produced in bovine aortic endothelial cells (BAEC). These cells also expressed the type I and type II activin receptors. When added to BAEC cultured in a collagen gel, activin A induced capillary formation. Activin A was as potent as vascular endothelial growth factor (VEGF) and markedly enhanced VEGF-induced tubulogenesis. To examine the role of endogenous activin A, we added follistatin, an inhibitor of activin A. Follistatin nearly completely blocked the VEGF-induced tubulogenesis, and the effect of follistatin was reproduced by transfection of the dominant-negative type II activin receptor gene. In BAEC, activin A increased the expression of VEGF and the VEGF receptors, Flt-1 and Flk-1. On the other hand, VEGF increased the production of activin A. Finally, addition of follistatin, which blocks the action of endogenous activin A, reduced the expression of Flt-1 and Flk-1. These results indicate that an autocrine factor activin A amplifies the effect of VEGF by up-regulating VEGF and its receptors. This effect of activin A is critical in the VEGF-induced tubulogenic morphogenesis in BAEC.

Gene expression alterations during HGF-induced dedifferentiation of a renal tubular epithelial cell line (MDCK) using a novel canine DNA microarray

Hepatocyte growth factor (HGF) elicits a broad spectrum of biological activities, including epithelial cell dedifferentiation. One of the most widely used and best-studied polarized epithelial cell lines is the Madin-Darby canine kidney (MDCK) cell line. Here, we describe and validate the early response of polarized monolayers of MDCK cells stimulated with recombinant HGF using a novel canine DNA microarray designed to query 12,473 gene sequences. In our survey, eight genes previously implicated in the HGF signaling pathway were differentially regulated, demonstrating that the system was responsive to HGF. Also identified were 117 genes not previously known to be involved in the HGF pathway. The results were confirmed by real-time PCR or Western blot analysis for 38 genes. Of particular interest were the large number of differentially regulated genes encoding small GTPases, proteins involved in endoplasmic reticulum translation, proteins involved in the cytoskeleton, the extracellular matrix, and the hematopoietic and prostaglandin systems.

Significance of branched chain amino acids as possible stimulators of hepatocyte growth factor

Amino acids can serve as regulatory molecules that modulate numerous cellular functions. Branched chain amino acids (BCAAs) are known to exert influences on cellular metabolism, amino acid transport, protein turn over, and gene expression. However, the mechanisms involved in the specific effect of BCAAs have not been clarified. BCAA supplementation therapy is a current treatment for patients with liver cirrhosis, therefore, specific BCAA activities should be examined. Hepatocyte growth factor (HGF) is considered to be a pleiotropic factor, and is reported to modulate gene expression and to stimulate the proliferation and functions of many cell types, including hepatocytes. A potential application of HGF for several types of diseases has been postulated. Here, we describe the potential of BCAAs as a therapeutic agent that acts through the induction of HGF production in the liver.

TGF-β superfamily members modulate growth, branching, shaping, and patterning of the ureteric bud

Protein-rich fractions inhibitory for isolated ureteric bud (UB) growth were separated from a conditioned medium secreted by cells derived from the metanephric mesenchyme (MM). Elution profiles and immunoblotting indicated the presence of members of the transforming growth factor-beta (TGF-beta) superfamily. Treatment of cultured whole embryonic kidney with BMP2, BMP4, activin, or TGF-beta1 leads to statistically significant differences in the overall size of the kidney, the number of UB branches, the length and angle of the branches, as well as in the thickness of the UB stalks. Thus, the pattern of the ureteric tree is altered. LIF, however, appeared to have only minimal effect on growth and development of the whole embryonic kidney in organ culture. The factors all directly inhibited, in a concentration-dependent fashion, the growth and branching of the isolated UB, albeit to different extents. Antagonists of some of these factors reduced their inhibitory effect. Detailed examination of TGF-beta1-treated UBs revealed only a slight increase in the amount of apoptosis in tips by TUNEL staining, but diminished proliferation throughout by Ki67 staining. These data suggest an important direct modulatory role for BMP2, BMP4, LIF, TGF-beta1, and activin (as well as their antagonists) on growth and branching of the UB, possibly in shaping the growing UB by playing a role in determining the number of branches, as well as where and how the branches occur. In support of this notion, UBs cultured in the presence of fibroblast growth factor 7 (FGF7), which induces the formation of globular structures with little distinction between the stalk and ampullae [Mech. Dev. 109 (2001) 123], and TGF-beta superfamily members lead to the formation of UBs with clear stalks and ampullae. This indicates that positive (i.e., growth and branch promoting) and negative (i.e., growth and branch inhibiting) modulators of UB morphogenesis can cooperate in the formation of slender arborized UB structures similar to those observed in the intact developing kidney or in whole embryonic kidney organ culture. Finally, purification data also indicate the presence of an as yet unidentified soluble non-heparin-binding activity modulating UB growth and branching. The data suggest how contributions of positive and negative growth factors can together (perhaps as local bipolar morphogenetic gradients existing within the mesenchyme) modulate the vectoral arborization pattern of the UB and shape branches as they develop, thereby regulating both nephron number and tubule/duct caliber. We suggest that TGF-beta-like molecules and other non-heparin-binding inhibitory factors can, in the appropriate matrix context, facilitate "braking" of the branching program as the UB shifts from a rapid branching stage (governed by a feed-forward mechanism) to a stage where branching slows down (negative feedback) and eventually stops.

Activation of hepatocyte growth factor/activin A/follistatin system during hemodialysis: Role of heparin

BACKGROUND

Hepatocyte growth factor (HGF), activin A, and follistatin compose an organotrophic system that may be modulated by heparin. We prospectively studied the effects of unfractionated heparin (UFH) versus low-molecular-weight heparin (LMWH) enoxaparin-anticoagulated hemodialysis on plasma levels of the cytokines.

METHODS

The factors were measured by immunoassays in 25 chronic hemodialysis patients at the start and at 10 and 180 minutes of the hemodialysis procedure anticoagulated with bolus enoxaparin. Then, the patients were randomized to either receive a bolus and infusion of UFH or to continue LMWH, and were reexamined after 12 weeks.

RESULTS

Predialysis HGF and follistatin were increased (both P < 0.0001), while activin A was normal in hemodialysis patients. Baseline HGF directly correlated with activin A in hemodialysis subjects (P=0.004). In healthy controls, it was positively associated with follistatin (P=0.001). Both HGF and activin A were markedly increased at each interval of enoxaparin-anticoagulated hemodialysis, and follistatin was increased at 10 minutes (all P < 0.0001). The early increments in HGF and follistatin directly depended on the dose of enoxaparin (both P < 0.030). Remarkably, the rise in activin A was inversely associated with the predialysis level of the cytokine (P < 0.0001). The actions of UFH resembled those of LMWH, although the releasing effects on the growth factors were not dose-dependent. The switch from LMWH to UFH resulted in a significant increase in over-dialysis HGF, a fall in follistatin, and no change in activin A.

CONCLUSION

HGF/activin A/follistatin system is activated and disturbed in chronic hemodialysis patients, including depletion of tissue stores of activin A. Type and dose of heparin used during hemodialysis procedures profoundly influence this pleiotropic system, and may thus modulate vital body functions and course of critical diseases.

The activin-follistatin system in the neonatal ovine uterus

Uterine gland development or adenogenesis in the neonatal ovine uterus involves budding and tubulogenesis followed by coiling and branching morphogenesis of the glandular epithelium (GE) from the luminal epithelium (LE) between birth (Postnatal Day [PND] 0) and PND 56. Activins, which are members of the transforming growth factor beta superfamily, and follistatin, an inhibitor of activins, regulate epithelial branching morphogenesis in other organs. The objective of the present study was to determine effects of postnatal age on expression of follistatin, inhibin alpha subunit, betaA subunit, betaB subunit, activin receptor (ActR) type IA, ActRIB, and ActRII in the developing ovine uterus. Ewes were ovariohysterectomized on PND 0, 7, 14, 21, 28, 35, 42, 49, or 56. The uterus was analyzed by in situ hybridization and immunohistochemistry. Neither inhibin alpha subunit mRNA or protein was detected in the neonatal uterus. Expression of betaA and betaB subunits was detected predominantly in the endometrial LE and GE and myometrium between PND 0 and PND 56. In all uterine cell types, ActRIA, ActRIB, and ActRII were expressed, with the highest levels observed in the endometrial LE and GE and myometrium. Between PND 0 and PND 14, follistatin was detected in all uterine cell types. However, between PND 21 and PND 56, follistatin was only detected in the stroma and myometrium and not in the developing GE. Collectively, the present results indicate that components of the activin-follistatin system are expressed in the developing neonatal ovine uterus and are potential regulators of endometrial gland morphogenesis.

Activin a produced by ureteric bud is a differentiation factor for metanephric mesenchyme

The present study was conducted to investigate the role of the activin-follistatin system in the development of metanephros. Organ culture system and cultured metanephric mesenchymal cells were used to address this issue. Activin A was localized in ureteric bud. Activin type II receptor was localized in ureteric bud as well as metanephric mesenchyme. In an organ culture system, exogenous activin A reduced the size of cultured metanephroi, delayed ureteric bud branching, and enlarged the tips of ureteric bud. Follistatin, an antagonist of activin A was used to clarify the role of endogenous activin A. Exogenous follistatin enlarged the size of cultured metanephroi, increased ureteric bud branching, and promoted cell growth in ureteric bud. Blockade of activin signaling by adenoviral transfection of dominantly negative activin mutant receptor mimics the effect of follistatin. In cultured metanephric mesenchymal cells, activin A promoted cell growth; conversely, follistatin induced apoptosis. Furthermore, activin A induced the expressions of epithelial differentiation markers in these cells. These results suggest that activin A produced by ureteric bud is not only an important regulator of ureteric bud branching, but also a differentiation factor for metanephric mesenchyme during kidney development.

The molecular control of renal branching morphogenesis: current knowledge and emerging insights

Mammalian kidney development requires the formation of a patterned, branched network of collecting ducts, a process termed renal branching morphogenesis. Disruption of renal branching morphogenesis during human kidney development results in renal dysplasia, the major cause of renal failure in young children. Genetic evidence, combined with in vitro data, have implicated transcription factors, secreted growth factors, and cell surface signaling peptides as critical regulators of renal branching morphogenesis. This review discusses the current knowledge regarding the regulation of renal branching morphogenesis in vivo provided by the analysis of genetic mutations in mice and humans which disrupt collecting duct system development. In addition, in vivo and in vitro evidence regarding the functions of several other gene families are considered, rendering new insight into emerging regulatory roles for these molecules in renal branching morphogenesis.

Activin A: an autocrine regulator of cell growth and differentiation in renal proximal tubular cells

BACKGROUND

Activin A is involved in tubular regeneration after ischemia/reperfusion injury. The present study was conducted to examine the role of activin A in cell growth, apoptosis and differentiation of tubular cells.

METHODS

We performed cell proliferation assays (MTT assay, [3H]-thymidine incorporation) and apoptosis detection assays (nuclear staining, DNA ladder formation, TUNEL staining) using LLC-PK1 cells. Expression of activin and activin receptor in LLC-PK1 cells also were examined by real-time polymerase chain reaction (PCR) and immunostaining. Stable cell lines expressing the truncated type II activin receptor were generated and the phenotype of these cells was analyzed.

RESULTS

Activin A inhibited DNA synthesis and cell growth in a dose-dependent manner and induced apoptosis in LLC-PK1 cells. The expression level of mRNA for the activin betaA subunit was markedly increased when the growth was stimulated. The expression of the type II activin receptor was observed in LLC-PK1 cells. The growth rate of cells expressing dominantly negative activin receptor was significantly faster than that of non-transfected cells. The expression level and pattern of cytokeratin and vimentin in these cells were quite different compared to non-transfected cells. When cultured in collagen gel, these cells formed multiple processes, which was not observed in non-transfected cells. Finally, the expression of Pax-2 was markedly elevated in these cells.

CONCLUSIONS

Activin A acts as an autocrine inhibitor of cell growth, an inducer of apoptosis, and an important modulator of differentiation in cultured proximal tubular cells.

Engraftment and differentiation of human metanephroi into functional mature nephrons after transplantation into mice is accompanied by a profile of gene expression similar to normal human kidney development

Metanephroi, the embryonic precursors of the adult kidney, can be induced in vivo to grow and develop. Despite their potential clinical utility for transplantation, the ability of human metanephroi to differentiate after transplantation into functional mature nephrons is mostly unknown. To address this, 70-d human metanephroi were transplanted into NOD/SCID mice; global gene expression patterns that underlie development of human metanephric transplants were analyzed and compared with normal human kidney development. In addition, functionality of the grafts was assessed by dimercaptosuccinic acid radioisotope scans at different times after transplantation. The results of hybridization to cDNA arrays when RNA was derived from normal human kidneys at 8, 12, 16, and 20 wk gestation demonstrated that a subset of 240 genes changed substantially with time. The induced genes can be classified as cell cycle regulators, transcription and growth factors, and signaling, transport, adhesion, and extracellular matrix molecules. Strikingly, clustering analysis of global gene expression at 2, 6, and 10 wk after metanephric transplantation revealed an expression profile that characterizes normal human kidney development. Moreover, maturation of the transplants was accompanied by an increased uptake of dimercaptosuccinic acid. Nevertheless, expression levels of specific genes were mostly found to be suppressed in the transplants compared with the normal kidneys. These data provide insights into human kidney development and support the possibility of the transplantability of human metanephroi. Understanding of the molecular regulation of the transplanted developing metanephroi might lead to the development of strategies aimed at increasing the levels of specific genes, nephron endowment, and graft function.

The role of the activin-follistatin system in the developmental and regeneration processes of the kidney

Regeneration processes in many tissues are modulated by various factors, which are involved in their organogenesis. Activin A, a member of the TGF-beta superfamily, inhibits branching tubulogenesis of the kidney in organ culture system as well as in in vitro tubulogenesis model. On the other hand, follistatin, an antagonist activin A, reverses the effect of activin A on kidney development, induces branching tubulogenesis, and also promotes tubular regeneration after ischemia/reperfusion injury by blocking the action of endogenous activin A. The activin-follistatin system is one of the important regulatory systems modulating developmental and regeneration processes of the kidneys.

Activins as regulators of branching morphogenesis

Development of glandular organs such as the kidney, lung, and prostate involves the process of branching morphogenesis. The developing organ begins as an epithelial bud that invades the surrounding mesenchyme, projecting dividing epithelial cords or tubes away from the site of initiation. This is a tightly regulated process that requires complex epithelial-mesenchymal interactions, resulting in a three-dimensional treelike structure. We propose that activins are key growth and differentiation factors during this process. The purpose of this review is to examine the direct, indirect, and correlative lines of evidence to support this hypothesis. The expression of activins is reviewed together with the effect of activins and follistatins in the development of branched organs. We demonstrate that activin has both negative and positive effects on cell growth during branching morphogenesis, highlighting the complex nature of activin in the regulation of proliferation and differentiation. We propose potential mechanisms for the way in which activins modify branching and address the issue of whether activin is a regulator of branching morphogenesis.

Involvement of the activin-follistatin system in tubular regeneration after renal ischemia in rats

This study was conducted to investigate the involvement of the activin-follistatin system in renal regeneration after ischemic injury. Expression of mRNA for the activin beta(A) subunit was not detected in normal kidneys but increased markedly after renal ischemia. Immunoreactive beta(A) subunit was detected in tubular cells of the outer medulla in ischemic but not normal kidneys. Expression of mRNA for follistatin, an antagonist of activin A, was abundant in tubular cells of the outer medulla in normal kidneys and decreased significantly after renal ischemia. For assessment of the role of the activin-follistatin system in renal regeneration after ischemic injury, recombinant follistatin was intravenously infused into rats with renal ischemia, at the time of reperfusion. Exogenous follistatin prevented the histologic changes induced by ischemic injury, reduced apoptosis in tubular cells, and accelerated tubular cell proliferation. Serum levels of creatinine and blood urea nitrogen were significantly lower in follistatin-treated rats. Conversely, intravenous administration of recombinant activin A inhibited tubular cell proliferation after ischemic injury. These results indicate that the activin-follistatin system participates in renal regeneration after ischemic injury. Follistatin administered intravenously accelerates renal regeneration after renal ischemia, presumably by blocking the actions of endogenous activin.

Role of the activin-follistatin system in the morphogenesis and regeneration of the renal tubules

Activin A inhibits branching tubulogenesis of the kidney during development. Activin A also inhibits branching tubulogenesis in MDCK cells, an in vitro tubulogenesis model. On the other hand, follistatin, an antagonist of activin A, reverses the effect of activin A and induces branching tubulogenesis. Follistatin also promotes tubular regeneration after ischemia/reperfusion injury. The activin/follistatin system is one of the important regulatory systems modulating developmental and regeneration processes of the kidney.

Evidence for activin A and follistatin involvement in the systemic inflammatory response

The inflammatory cascade is a multifactorial process regulated by interwoven cytokine and growth factor networks. This review summarizes the emerging evidence that implicate activin A and follistatin in inflammatory processes. Our recent studies have determined that activin A is released early in the cascade of circulatory cytokines during systemic inflammatory episodes, roughly coincident with tumour necrosis factor (TNF)-alpha and before interleukin (IL)-6 and follistatin. The source(s) of this activin A are not yet established, but prime candidates are monocytes/macrophages, other immune cell types or vascular endothelial cells. Clinical data are limited, but activin beta(A) subunit mRNA or activin A protein is elevated in inflammatory bowel diseases and inflammatory arthropathies, and circulating concentrations of follistatin are elevated in patients with sepsis. In more mechanistic approaches, in vitro studies show that activin A can have both pro- and anti-inflammatory actions on key inflammatory mediators such as TNFalpha, IL-1beta and IL-6. Furthermore, there is emerging understanding of how the intracellular signaling pathway for activin A, incorporating Smads, may interact with and be modulated by other key regulatory cytokines and growth factors.