Transcriptional induction of Smurf2 ubiquitin ligase by TGF-β

Emerging pharmacological options in the treatment of idiopathic pulmonary fibrosis (IPF)

INTRODUCTION

Idiopathic pulmonary fibrosis (IPF) is a progressive-fibrosing lung disease with a median survival of less than 5 years. Currently, two agents, pirfenidone and nintedanib are approved for this disease, and both have been shown to reduce the rate of decline in lung function in patients with IPF. However, both have significant adverse effects and neither completely arrest the decline in lung function.

AREAS COVERED

Thirty experimental agents with unique mechanisms of action that are being evaluated for the treatment of IPF are discussed. These agents work through various mechanisms of action, these include inhibition of transcription nuclear factor k-B on fibroblasts, reduced expression of metalloproteinase 7, the generation of more lysophosphatidic acids, blocking the effects of transforming growth factor ß, and reducing reactive oxygen species as examples of some unique mechanisms of action of these agents.

EXPERT OPINION

New drug development has the potential to expand the treatment options available in the treatment of IPF patients. It is expected that the adverse drug effect profiles will be more favorable than current agents. It is further anticipated that these new agents or combinations of agents will arrest the fibrosis, not just slow the fibrotic process.

Citronellal improves endothelial dysfunction by affecting the stability of the GCH1 protein

Endothelial dysfunction (ED) serves as the pathological basis for various cardiovascular diseases. Guanosine triphosphate cyclopyrrolone 1 (GCH1) emerges as a pivotal protein in sustaining nitric oxide (NO) production within endothelial cells, yet it undergoes degradation under oxidative stress, contributing to endothelial cell dysfunction. Citronellal (CT), a monoterpenoid, has been shown to ameliorate endothelial dysfunction induced by in atherosclerosis rats. However, whether CT can inhibit the degradation of GCH1 protein is not clear. It has been reported that ubiquitination may play a crucial role in regulating GCH1 protein levels and activities. However, the specific E3 ligase for GCH1 and the molecular mechanism of GCH1 ubiquitination remain unclear. Using data-base exploration analysis, we find that the levels of the E3 ligase Smad-ubiquitination regulatory factor 2 (Smurf2) negatively correlate with those of GCH1 in vascular tissues and HUVECs. We observe that Smurf2 interacts with GCH1 and promotes its degradation via the proteasome pathway. Interestingly, ectopic Smurf2 expression not only decreases GCH1 levels but also reduces cell proliferation and reactive oxygen species (ROS) levels, mostly because of increased GCH1 accumulation. Furthermore, we identify BH 4/eNOS as downstream of GCH1. Taken together, our results indicate that CT can obviously improve vascular endothelial injury in Type 1 diabetes mellitus (T1DM) rats and reverse the expressions of GCH1 and Smurf2 proteins in aorta of T1DM rats. Smurf2 promotes ubiquitination and degradation of GCH1 through proteasome pathway in HUVECs. We conclude that the Smurf2-GCH1 interaction might represent a potential target for improving endothelial injury.

LPA-induced expression of CCN2 in muscular fibro/adipogenic progenitors (FAPs): Unraveling cellular communication networks

Cellular Communication Network Factor 2, CCN2, is a profibrotic cytokine implicated in physiological and pathological processes in mammals. The expression of CCN2 is markedly increased in dystrophic muscles. Interestingly, diminishing CCN2 genetically or inhibiting its function improves the phenotypes of chronic muscular fibrosis in rodent models. Elucidating the cell-specific mechanisms behind the induction of CCN2 is a fundamental step in understanding its relevance in muscular dystrophies. Here, we show that the small lipids LPA and 2S-OMPT induce CCN2 expression in fibro/adipogenic progenitors (FAPs) through the activation of the LPA1 receptor and, to a lower extent, by also the LPA6 receptor. These cells show a stronger induction than myoblasts or myotubes. We show that the LPA/LPARs axis requires ROCK kinase activity and organized actin cytoskeleton upstream of YAP/TAZ signaling effectors to upregulate CCN2 levels, suggesting that mechanical signals are part of the mechanism behind this process. In conclusion, we explored the role of the LPA/LPAR axis on CCN2 expression, showing a strong cytoskeletal-dependent response in muscular FAPs.

CBGA ameliorates inflammation and fibrosis in nephropathy

Cannabidiol (CBD) is thought to have multiple biological effects, including the ability to attenuate inflammatory processes. Cannabigerols (CBGA and its decarboxylated CBG molecule) have pharmacological profiles similar to CBD. The endocannabinoid system has recently emerged to contribute to kidney disease, however, the therapeutic properties of cannabinoids in kidney disease remain largely unknown. In this study, we determined whether CBD and CBGA can attenuate kidney damage in an acute kidney disease model induced by the chemotherapeutic cisplatin. In addition, we evaluated the anti-fibrosis effects of these cannabinoids in a chronic kidney disease model induced by unilateral ureteral obstruction (UUO). We find that CBGA, but not CBD, protects the kidney from cisplatin-induced nephrotoxicity. CBGA also strongly suppressed mRNA of inflammatory cytokines in cisplatin-induced nephropathy, whereas CBD treatment was only partially effective. Furthermore, both CBGA and CBD treatment significantly reduced apoptosis through inhibition of caspase-3 activity. In UUO kidneys, both CBGA and CBD strongly reduced renal fibrosis. Finally, we find that CBGA, but not CBD, has a potent inhibitory effect on the channel-kinase TRPM7. We conclude that CBGA and CBD possess reno-protective properties, with CBGA having a higher efficacy, likely due to its dual anti-inflammatory and anti-fibrotic effects paired with TRPM7 inhibition.

To Ub or not to Ub: a regulatory question in TGF-β signaling

The transforming growth factor β (TGF-β) superfamily controls a wide spectrum of biological processes in metazoans, including cell proliferation, apoptosis, differentiation, cell-fate determination, and embryonic development. Deregulation of TGF-β-Smad signaling contributes to developmental anomalies and a variety of disorders and diseases such as tumorigenesis, fibrotic disorders, and immune diseases. In cancer, TGF-β has dual effects through its antiproliferative and prometastatic actions. At the cellular level, TGF-β functions mainly through the canonical Smad-dependent pathway in a cell type-specific and context-dependent manner. Accumulating evidence has demonstrated that ubiquitination plays a vital role in regulating TGF-β-Smad signaling. We summarize current progress on ubiquitination (Ub) and the ubiquitin ligases that regulate TGF-β-Smad signaling.

TFPI2 suppresses the interaction of TGF-β2 pathway regulators to promote endothelial-mesenchymal transition in diabetic nephropathy

Endothelial–mesenchymal transition (EndMT) is an important source of myofibroblasts, but also contributes to the progression of diabetic nephropathy (DN). By several differential gene expression analyses from the Gene Expression Omnibus (GEO) database, the tissue factor pathway inhibitor 2 (TFPI2) gene, known as a tumor suppressor, was shown to be dysregulated in DN; however, the potential role and regulatory mechanism of TFPI2 in DN are unclear. Here, we found abnormal upregulation of TFPI2 in the renal cortex of diabetic mice, accompanied by impaired renal function. We also injected a single dose of adeno-associated virus (AAV)2 carrying shRNA targeting TFPI2 intravenously into these mice and found that knockdown of TFPI2 improved renal function and reduced renal fibrosis and cell apoptosis in experimental DN. Furthermore, hyperglycemia-induced EndMT was inhibited in the absence of TFPI2, as evidenced by increased expression of endothelial markers (VE-cadherin and CD31) and decreased expression of mesenchymal markers (α-SMA, desmin, and FSP-1). To further explore the mechanism in vitro, human renal glomerular endothelial cells (hRGECs) were incubated in the presence of high glucose or transforming growth factor beta (TGF-β)2. TFPI2 deficiency inhibited high glucose-induced cell apoptosis and TGF-β2-induced EndMT in hRGECs, while overexpression of TFPI2 had the opposite effects. Importantly, TGF-β2 is a crucial driver of EndMT, and we found that TFPI2 promoted TGF-β2/Smad signaling activation by interferring the interaction of TGF-β pathway regulators (SMURF2 with SMAD7). Our results show that TFPI2 regulates EndMT and the TGF-β2 signaling pathway and is a potential promoter of DN pathogenesis.

SMURF2 phosphorylation at Thr249 modifies glioma stemness and tumorigenicity by regulating TGF-β receptor stability

Glioma stem cells (GSCs) contribute to the pathogenesis of glioblastoma, the most malignant form of glioma. The implication and underlying mechanisms of SMAD specific E3 ubiquitin protein ligase 2 (SMURF2) on the GSC phenotypes remain unknown. We previously demonstrated that SMURF2 phosphorylation at Thr²⁴⁹ (SMURF2^Thr249) activates its E3 ubiquitin ligase activity. Here, we demonstrate that SMURF2^Thr249 phosphorylation plays an essential role in maintaining GSC stemness and tumorigenicity. SMURF2 silencing augmented the self-renewal potential and tumorigenicity of patient-derived GSCs. The SMURF2^Thr249 phosphorylation level was low in human glioblastoma pathology specimens. Introduction of the SMURF2^T249A mutant resulted in increased stemness and tumorigenicity of GSCs, recapitulating the SMURF2 silencing. Moreover, the inactivation of SMURF2^Thr249 phosphorylation increases TGF-β receptor (TGFBR) protein stability. Indeed, TGFBR1 knockdown markedly counteracted the GSC phenotypes by SMURF2^T249A mutant. These findings highlight the importance of SMURF2^Thr249 phosphorylation in maintaining GSC phenotypes, thereby demonstrating a potential target for GSC-directed therapy.

Hiraiwa et al. show that phosphorylation of SMAD specific E3 ubiquitin protein ligase 2 (SMURF2) at Thr249 mediates ubiquitylation and degradation of the TGF-β receptor TGBR1 leading to loss of glioblastoma stem cell tumorigenic capacity. Their data elucidates a mechanism of regulation of the TGF-β signaling pathway that controls the stem cell status in glioblastoma.

Molecular Pathogenesis of Pulmonary Fibrosis, with Focus on Pathways Related to TGF-β and the Ubiquitin-Proteasome Pathway

Idiopathic pulmonary fibrosis (IPF) is a progressive and fatal interstitial lung disease. During the past decade, novel pathogenic mechanisms of IPF have been elucidated that have shifted the concept of IPF from an inflammatory-driven to an epithelial-driven disease. Dysregulated repair responses induced by recurrent epithelial cell damage and excessive extracellular matrix accumulation result in pulmonary fibrosis. Although there is currently no curative therapy for IPF, two medications, pirfenidone and nintedanib, have been introduced based on understanding the pathogenesis of the disease. In this review, we discuss advances in understanding IPF pathogenesis, highlighting epithelial-mesenchymal transition (EMT), the ubiquitin-proteasome system, and endothelial cells. TGF-β is a central regulator involved in EMT and pulmonary fibrosis. HECT-, RING finger-, and U-box-type E3 ubiquitin ligases regulate TGF-β-Smad pathway-mediated EMT via the ubiquitin-proteasome pathway. p27 degradation mediated by the SCF-type E3 ligase, Skp2, contributes to the progression of pulmonary fibrosis by promotion of either mesenchymal fibroblast proliferation, EMT, or both. In addition to fibroblasts as key effector cells in myofibroblast differentiation and extracellular matrix deposition, endothelial cells also play a role in the processes of IPF. Endothelial cells can transform into myofibroblasts; therefore, endothelial-mesenchymal transition can be another source of myofibroblasts.

Expression and function of Smad7 in autoimmune and inflammatory diseases

Transforming growth factor-β (TGF-β) plays a critical role in the pathological processes of various diseases. However, the signaling mechanism of TGF-β in the pathological response remains largely unclear. In this review, we discuss advances in research of Smad7, a member of the I-Smads family and a negative regulator of TGF-β signaling, and mainly review the expression and its function in diseases. Smad7 inhibits the activation of the NF-κB and TGF-β signaling pathways and plays a pivotal role in the prevention and treatment of various diseases. Specifically, Smad7 can not only attenuate growth inhibition, fibrosis, apoptosis, inflammation, and inflammatory T cell differentiation, but also promotes epithelial cells migration or disease development. In this review, we aim to summarize the various biological functions of Smad7 in autoimmune diseases, inflammatory diseases, cancers, and kidney diseases, focusing on the molecular mechanisms of the transcriptional and posttranscriptional regulation of Smad7.

The Post-translational Modifications of Smurf2 in TGF-β Signaling

Smad ubiquitin regulatory factor 2 (Smurf2), an essential negative regulator of TGF-β signaling, ubiquitinates TGF-β receptors (TβRs) and Smad proteins, inducing their proteasomal degradation. Smurf2 plays crucial roles in regulating TGF-β signaling and maintaining normal cellular functions and tissue homeostasis; dysfunction of Smurf2 triggers abnormal TGF-β signaling in pathological states. Smurf2 has been reported as a potentially strong candidate for targeting therapies for related diseases. Recent work has begun to focus on the regulation of Smurf2 itself, and emerging evidence indicates that Smurf2 is regulated by post-translational modifications (PTMs) mechanisms. These mechanisms predominantly regulate the expression level and E3 ligase activity of Smurf2, strongly suggesting that this protein contributes to complicated roles under multiple pathophysiological conditions. In this review, we cover some significant and novel mechanisms of the PTMs that potentially control Smurf2 participation in TGF-β signaling, including ubiquitylation, SUMOylation, neddylation, phosphorylation, and methylation in order to provide a broad view of the depth and sophistication of Smurf2 function in TGF-β regulation, as well as perspectives for future therapeutic directions for its associated diseases.

The testis protein ZNF165 is a SMAD3 cofactor that coordinates oncogenic TGFβ signaling in triple-negative breast cancer

Cancer/testis (CT) antigens are proteins whose expression is normally restricted to germ cells yet aberrantly activated in tumors, where their functions remain relatively cryptic. Here we report that ZNF165, a CT antigen frequently expressed in triple-negative breast cancer (TNBC), associates with SMAD3 to modulate transcription of transforming growth factor β (TGFβ)-dependent genes and thereby promote growth and survival of human TNBC cells. In addition, we identify the KRAB zinc finger protein, ZNF446, and its associated tripartite motif protein, TRIM27, as obligate components of the ZNF165-SMAD3 complex that also support tumor cell viability. Importantly, we find that TRIM27 alone is necessary for ZNF165 transcriptional activity and is required for TNBC tumor growth in vivo using an orthotopic xenograft model in immunocompromised mice. Our findings indicate that aberrant expression of a testis-specific transcription factor is sufficient to co-opt somatic transcriptional machinery to drive a pro-tumorigenic gene expression program in TNBC.

The functions and regulation of Smurfs in cancers

Smad ubiquitination regulatory factor 1 (Smurf1) and Smurf2 are HECT-type E3 ubiquitin ligases, and both Smurfs were initially identified to regulate Smad protein stability in the TGF-β/BMP signaling pathway. In recent years, Smurfs have exhibited E3 ligase-dependent and -independent activities in various kinds of cells. Smurfs act as either potent tumor promoters or tumor suppressors in different tumors by regulating biological processes, including metastasis, apoptosis, cell cycle, senescence and genomic stability. The regulation of Smurfs activity and expression has therefore emerged as a hot spot in tumor biology research. Further, the Smurf1- or Smurf2-deficient mice provide more in vivo clues for the functional study of Smurfs in tumorigenesis and development. In this review, we summarize these milestone findings and, in turn, reveal new avenues for the prevention and treatment of cancer by regulating Smurfs.

Activated Wnt/β-Catenin signaling contributes to E3 ubiquitin ligase EDD-conferred docetaxel resistance in prostate cancer

Docetaxel is commonly used to treat hormone-refractory prostate cancer (HRPC), but its clinical efficacy is limited by drug resistance, with the molecular mechanisms remaining elusive. The E3 ubiquitin ligase EDD modifies substrate proteins through ubiquitination and is involved in the regulation of cell proliferation and tumorigenesis. However, its role in docetaxel resistance of prostate cancer is unknown. Here, we show that EDD is upregulated in docetaxel-resistant HRPC cells, as well as in human HRPC treated with docetaxel chemotherapy. Functionally, EDD knockdown resensitizes HRPC cells to docetaxel in vitro and in vivo, and in reverse, EDD overexpression promotes docetaxel resistance. We further show that the Wnt/β-Catenin signaling is activated in docetaxel-resistant HRPC cells, which can be promoted by EDD. Finally, inhibiting Wnt signaling through β-Catenin knockdown remarkably attenuates EDD-mediated docetaxel resistance, suggesting that the activated Wnt/β-Catenin signaling is a key contributor to EDD-conferred docetaxel resistance in HRPC cells. Altogether, our study uncovers a positive role of EDD in docetaxel resistance in prostate cancer, and further links it with the regulation of Wnt/β-Catenin signaling.

The ubiquitination ligase SMURF2 reduces aerobic glycolysis and colorectal cancer cell proliferation by promoting ChREBP ubiquitination and degradation

The glucose-responsive transcription factor carbohydrate response element-binding protein (ChREBP) critically promotes aerobic glycolysis and cell proliferation in colorectal cancer cells. It has been reported that ubiquitination may be important in the regulation of ChREBP protein levels and activities. However, the ChREBP-specific E3 ligase and molecular mechanism of ChREBP ubiquitination remains unclear. Using database exploration and expression analysis, we found here that levels of the E3 ligase SMURF2 (Smad-ubiquitination regulatory factor 2) negatively correlate with those of ChREBP in cancer tissues and cell lines. We observed that SMURF2 interacts with ChREBP and promotes ChREBP ubiquitination and degradation via the proteasome pathway. Interestingly, ectopic SMURF2 expression not only decreased ChREBP levels but also reduced aerobic glycolysis, increased oxygen consumption, and decreased cell proliferation in colorectal cancer cells. Moreover, SMURF2 knockdown increased aerobic glycolysis, decreased oxygen consumption, and enhanced cell proliferation in these cells, mostly because of increased ChREBP accumulation. Furthermore, we identified Ser/Thr kinase AKT as an upstream suppressor of SMURF2 that protects ChREBP from ubiquitin-mediated degradation. Taken together, our results indicate that SMURF2 reduces aerobic glycolysis and cell proliferation by promoting ChREBP ubiquitination and degradation via the proteasome pathway in colorectal cancer cells. We conclude that the SMURF2-ChREBP interaction might represent a potential target for managing colorectal cancer.

USF2 inhibits the transcriptional activity of Smurf1 and Smurf2 to promote breast cancer tumorigenesis

Smurf1 (Smad ubiquitylation regulatory factor 1) and Smurf2 are negative regulators of the TGF-β (transforming growth factor-β) pathway. The protein stability and ubiquitin E3 activity regulation of Smurfs have been well studied. However, the mechanism of Smurfs expression at the transcriptional level remains uncharacterized. Here, we reported that USF2 (upstream stimulatory factor 2), a basic helix-loop-helix-leucine-zip transcription factor, is necessary for the transcriptional activity of Smurf1 and Smurf2. The 5'-flanking sequences of the Smurfs gene have more than one E-box motifs, and USF2 bounds the Smurfs promoter in vitro and in vivo. Over-expression USF2 inhibited the transcriptional activity of the Smurfs, and Smurfs mRNA was markedly decreased. Therefore, the activity of TGF-β was distinctly enhanced. Furthermore, in human breast cancers, USF2 was abnormally high expressed and correlated with cancer progression. USF2 was specifically inversely correlated with Smurfs in Luminal A subtype breast cancer patients. These findings suggest the mechanism regulation of Smurfs transcriptional activity, and shed new light on the cancer-promoting role of USF2.

The Cytokine TGF-β Promotes the Development and Homeostasis of Alveolar Macrophages

Alveolar macrophages (AMs) derive from fetal liver monocytes, which colonize the lung during embryonic development and give rise to fully mature AMs perinatally. AM differentiation requires granulocyte macrophage colony-stimulating factor (GM-CSF), but whether additional factors are involved in AM regulation is not known. Here we report that AMs, in contrast to most other tissue macrophages, were also dependent on transforming growth factor-β receptor (TGF-βR) signaling. Conditional deletion of TGF-βR in mice at different time points halted the development and differentiation of AMs. In adult mice, TGF-β was also critical for AM homeostasis. The source of TGF-β was AMs themselves, indicative of an autocrine loop that promotes AM self-maintenance. Mechanistically, TGF-βR signaling resulted in upregulation of PPAR-γ, a signature transcription factor essential for the development of AMs. These findings reveal an additional layer of complexity regarding the guidance cues, which govern the genesis, maturation, and survival of AMs.

SMURF2 regulates bone homeostasis by disrupting SMAD3 interaction with vitamin D receptor in osteoblasts

Coordination between osteoblasts and osteoclasts is required for bone health and homeostasis. Here we show that mice deficient in SMURF2 have severe osteoporosis in vivo. This low bone mass phenotype is accompanied by a pronounced increase in osteoclast numbers, although Smurf2-deficient osteoclasts have no intrinsic alterations in activity. Smurf2-deficient osteoblasts display increased expression of RANKL, the central osteoclastogenic cytokine. Mechanistically, SMURF2 regulates RANKL expression by disrupting the interaction between SMAD3 and vitamin D receptor by altering SMAD3 ubiquitination. Selective deletion of Smurf2 in the osteoblast lineage recapitulates the phenotype of germline Smurf2-deficient mice, indicating that SMURF2 regulates osteoblast-dependent osteoclast activity rather than directly affecting the osteoclast. Our results reveal SMURF2 as an important regulator of the critical communication between osteoblasts and osteoclasts. Furthermore, the bone mass phenotype in Smurf2- and Smurf1-deficient mice is opposite, indicating that SMURF2 has a non-overlapping and, in some respects, opposite function to SMURF1.

The balance between osteoclast and osteoblast-mediated bone turnover is essential for bone health and homeostasis. Here the authors show that both germline and osteoblast-specific Smurf2-deficient mice have osteoporosis as a result of increased osteoblast RANKL production and excess osteoclastogenesis.

The downregulation of SnoN expression in human renal proximal tubule epithelial cells under high-glucose conditions is mediated by an increase in Smurf2 expression through TGF-β1 signaling

Transforming growth factor (TGF)-β1 is a profibrotic cytokine that plays a critical role in the progression of diabetic nephropathy (DN). Previous studies have demonstrated that the Smad transcriptional co-repressor, Ski-related novel protein N (SnoN), an antagonizer of TGF-β1/Smad signaling, is downregulated in the kidneys of diabetic rats; however, the underlying molecular mechanisms remain elusive. In the present study, we demonstrated that the upregulation of Smad ubiquitination regulatory factor-2 (Smurf2), through TGF-β1/Smad signaling, contributes to the downregulation of SnoN under high-glucose conditions in primary human renal proximal tubule epithelial cells (hRPTECs). The hRPTECs were cultured in high-glucose (30 mmol/l D-glucose) medium in the presence or absence of either the proteasome inhibitor, MG132, or the TGF-β type I receptor kinase inhibitor, SB-431542. Small interfering RNA (siRNA) was used to silence Smurf2. The expression levels of SnoN, Smurf2, Smad2 and phosphorylated (p-)Smad2 were measured by western blot analysis and RT-qPCR. The protein levels of SnoN were markedly downregulated, while its mRNA levels were increased in the hRPTECs cultured under high-glucose conditions. The protein and mRNA levels of Smurf2 were significantly increased under high-glucose conditions. The knockdown of Smurf2 increased SnoN expression in the hRPTECs cultured in high-glucose medium. Moreover, MG132 partially inhibited SnoN degradation in the hRPTECs under high-glucose conditions and SB-431542 decreased the phosphorylation of Smad2 and the expression of Smurf2 induced under high-glucose conditions. Taken together, the findings of this study demonstrate that the downregulation of SnoN expression in hRPTECs under high-glucose conditions is mediated by the increased expression of Smurf2 through the TGF-β1/Smad signaling pathway.

Expression of Smad7 and Smad ubiquitin regulatory factor 2 in a rat model of chronic pancreatitis

OBJECTIVE

To quantify the expressions of Smad7 and Smad ubiquitin regulatory factor 2 (Smurf2) in the pancreas in rats with chronic pancreatitis (CP).

METHODS

A total of 16 male Wistar rats were randomly divided into the control group and the CP group, with 8 rats in each group. CP was induced in vivo with dibutyltin dichloride (DBTC). Four weeks after DBTC administration, histological assessment and the measurement of hydroxyproline content in the pancreatic tissues were performed to assess the inflammation and fibrosis of the pancreas. Immunohistochemistry and reverse transcription polymerase chain reaction (RT-PCR) for transforming growth factor (TGF)-β1 and α-smooth muscle actin (α-SMA) were applied to assess activated pancreatic stellate cells (PSC) and TGF-β1 expression. Smad7 and Smurf2 expressions in the pancreas were measured using Western blot and RT-PCR.

RESULTS

Typical histopathological characteristics of DBTC-induced CP in the rats with extensively activated PSC. Compared with the control group, the expressions of TGF-β1, α-SMA and hydroxyproline content in the pancreatic tissues in the CP group were significantly increased. Meanwhile, the mRNA and protein expressions of Smad7 and Smurf2 were significant increased in the fibrotic pancreas, in which the expressions of Smad7 proteins showed an obvious reduction compared with controls.

CONCLUSION

The dysregulation of Smad7 and Smurf2 may be associated with the pathogenesis of pancreatic fibrosis through the TGF-β signaling pathway.

AKT regulation of mesothelial-to-mesenchymal transition in peritoneal dialysis is modulated by Smurf2 and deubiquitinating enzyme USP4

Background

Transforming growth factor-β1 (TGF-β1) plays a key role in mesothelial-to-mesenchymal transition (MMT) during peritoneal dialysis (PD). However, the role of Akt in MMT transformation in PD is not clear.

Results

In this study, we observed that the phosphorylated form of protein kinase B (Akt), termed as pAkt, was up-regulated in the peritoneum of mice undergoing PD. It was associated with thickening of the peritoneum and up-regulation of TGF-β1. Upregulation of pAkt paralleled with the increased expression of Smad ubiquitination regulatory factor 2 (Smurf2), Vimentin and fibronectin (FN), and decreased expression of mothers against decapentaplegic homolog 7 (Smad7) and Zonula Occludens protein 1(ZO-1) in mice undergoing PD treatment and in TGF-β1 induced human peritoneal mesothelial cells (HPMCs). These changes were reversed with the treatment of a PI3K/Akt inhibitor LY294002 in vivo or in cells transfected with Akt dominant-negative (Akt-DN) plasmids in vitro. Increased Smurf2 expression in HPMCs, induced by TGF-β1 was accompanied with altered expression of Transforming growth factor receptor I (TβR-I), Smad7, ZO-1, Vimentin and FN via Akt modulation. In addition, inhibition of Ubiquitin carboxyl-terminal hydrolase 4 (USP4) decreased TGF- β1-induced expression of TβR-I and reversed the altered expression of Smad7, Smurf2, ZO-1 and Vimentin. Moreover, TGF-β1 accentuated the interactions between Smurf2 and Smad7, while reduced the association between TβR-I and Smurf2. These interactions were reversed by the treatment of Akt-DN and USP4 siRNA, respectively.

Conclusions

These data implied that Akt mediated MMT in PD via Smurf2 modulation/and or Smad7 degradation while conceivably maintaining the TβRI stability, most likely by the USP4.

Regulation of the transforming growth factor β pathway by reversible ubiquitylation

The transforming growth factor β (TGFβ) signalling pathway plays a central role during embryonic development and in adult tissue homeostasis. It regulates gene transcription through a signalling cascade from cell surface receptors to intracellular SMAD transcription factors and their nuclear cofactors. The extent, duration and potency of signalling in response to TGFβ cytokines are intricately regulated by complex biochemical processes. The corruption of these regulatory processes results in aberrant TGFβ signalling and leads to numerous human diseases, including cancer. Reversible ubiquitylation of pathway components is a key regulatory process that plays a critical role in ensuring a balanced response to TGFβ signals. Many studies have investigated the mechanisms by which various E3 ubiquitin ligases regulate the turnover and activity of TGFβ pathway components by ubiquitylation. Moreover, recent studies have shed new light into their regulation by deubiquitylating enzymes. In this report, we provide an overview of current understanding of the regulation of TGFβ signalling by E3 ubiquitin ligases and deubiquitylases.

TGF-β signalling and reactive oxygen species drive fibrosis and matrix remodelling in myxomatous mitral valves

AIMS

Myxomatous mitral valve disease (MMVD) is associated with leaflet thickening, fibrosis, matrix remodelling, and leaflet prolapse. Molecular mechanisms contributing to MMVD, however, remain poorly understood. We tested the hypothesis that increased transforming growth factor-β (TGF-β) signalling and reactive oxygen species (ROS) are major contributors to pro-fibrotic gene expression in human and mouse mitral valves.

METHODS AND RESULTS

Using qRT-PCR, we found that increased expression of TGF-β1 in mitral valves from humans with MMVD (n = 24) was associated with increased expression of connective tissue growth factor (CTGF) and matrix metalloproteinase 2 (MMP2). Increased levels of phospho-SMAD2/3 (western blotting) and expression of SMAD-specific E3 ubiquitin-protein ligases (SMURF) 1 and 2 (qRT-PCR) suggested that TGF-β1 signalling occurred through canonical signalling cascades. Oxidative stress (dihydroethidium staining) was increased in human MMVD tissue and associated with increases in NAD(P)H oxidase catalytic subunits (Nox) 2 and 4, occurring despite increases in superoxide dismutase 1 (SOD1). In mitral valves from SOD1-deficient mice, expression of CTGF, MMP2, Nox2, and Nox4 was significantly increased, suggesting that ROS can independently activate pro-fibrotic and matrix remodelling gene expression patterns. Furthermore, treatment of mouse mitral valve interstitial cells with cell permeable antioxidants attenuated TGF-β1-induced pro-fibrotic and matrix remodelling gene expression in vitro.

CONCLUSION

Activation of canonical TGF-β signalling is a major contributor to fibrosis and matrix remodelling in MMVD, and is amplified by increases in oxidative stress. Treatments aimed at reducing TGF-β activation and oxidative stress in early MMVD may slow progression of MMVD.

Roles of the Skp2/p27 axis in the progression of chronic nephropathy

S-phase kinase-associated protein 2 (Skp2) is an F-box protein component of the Skp/Cullin/F-box-type E3 ubiquitin ligase that targets several cell cycle regulatory proteins for degradation through the ubiquitin-dependent pathway. Skp2-mediated degradation of p27, a cyclin-dependent kinase inhibitor, is involved in cell cycle regulation. Tubular epithelial cell proliferation is a characteristic feature of renal damage that is apparent in the early stages of nephropathy. The p27 level is associated with the progression of renal injury, and increased Skp2 expression in progressive nephropathy is implicated in decreases of p27 expression. In Skp2^−/− mice, renal damage caused by unilateral ureteral obstruction (UUO) was ameliorated by p27 accumulation, mainly in tubular epithelial cells. However, the amelioration of UUO-induced renal injury in Skp2^−/− mice was prevented by p27 deficiency in Skp2^−/−/p27^−/− mice. These results suggest that the Skp2-mediated reduction in p27 is a pathogenic activity that occurs during the progression of nephropathy. Here, we discuss the roles of the Skp2/p27 axis and/or related signaling pathways/components in the progression of chronic nephropathy.

The role of the ubiquitin-proteasome system in kidney diseases

Proteins in mammalian cells are continually being degraded and synthesized. Protein degradation via the ubiquitin-proteasome system (UPS) is the major pathway for non-lysosomal proteolysis of intracellular proteins and plays important roles in a variety of fundamental cellular processes such as regulation of cell cycle progression, differentiation, apoptosis, sodium channel function, and modulation of inflammatory responses. The central element of this system is the covalent linkage of ubiquitins to targeted proteins, which are then recognized by the 26S proteasome composed of adenosine triphosphate-dependent, multi-catalytic proteases. Damaged or misfolded proteins, as well as regulatory proteins that control many critical cellular functions, are among the targets of this degradation process. Consequently, aberration of the system leads to dysregulation of cellular homeostasis and development of many diseases. Based on the findings, it is not surprising that abnormalities of the system are also associated with the pathogenesis of kidney diseases. In this review, I discuss (1) the basic mechanism of the UPS, and (2) the association between the pathogenesis of kidney diseases and the UPS. Diverse roles of the UPS are implicated in the development of kidney diseases, and further studies on this system may reveal new strategies for overcoming kidney diseases.

Overexpression of Smad ubiquitin regulatory factor 2 suppresses transforming growth factor-β mediated liver fibrosis

OBJECTIVE

To determine the function of Smad ubiquitin regulatory factor 2 (Smurf2) on the development of liver fibrosis and cirrhosis.

METHODS

In vivo Smurf2 expression in fibrotic and cirrhotic rat and human liver tissues were measured using reverse transcription-polymerase chain reaction, Western blot (WB) and immunohistochemistry. In vitro Smurf2 levels were determined in LX-2 cell line with or without transforming growth factor (TGF)-β1 treatment; I, III, IV collagen and laminin levels were determined by ELISA. The recombinant plasmid pcDNA3.1-Smurf2 was transfected into LX-2 cells, and WB and ELISA were utilized to analyze the expression of TGF-β receptor type I (TβRI), Smad7, collagens and laminin with or without proteasome inhibitor MG-132. Coimmunoprecipitation was utilized to characterize the interactions among these factors and the ubiquitination levels. pcDNA3.1-Smad7 vector was transfected and subsequent examinations were conducted just as Smurf2.

RESULTS

Smurf2 levels were elevated in the early period of fibrotic rat liver and TGF-β1-treated LX-2 cells but were reduced in the cirrhotic livers. Smurf2 overexpression in LX-2 cells reduced TβRI and Smad7 levels, which was accompanied by decreased collagen and laminin levels. Coimmunoprecipitation demonstrated that Smurf2 interacted with TβRI and Smad7, which increased TβRI and Smad7 ubiquitin levels. Smad7 overexpression reduced the TβRI level and was accompanied by decreased collagen and laminin levels. MG-132 could antagonize these effects.

CONCLUSION

Smurf2 interacts with Smad7 to suppress TGF-β-mediated liver fibrosis through the ubiquitin-dependent degradation of TβRI during the early period of liver fibrosis.

Transcriptional induction of salt-inducible kinase 1 by transforming growth factor β leads to negative regulation of type I receptor signaling in cooperation with the Smurf2 ubiquitin ligase

Transforming growth factor β (TGFβ) regulates many physiological processes and requires control mechanisms to safeguard proper and timely action. We have previously described how negative regulation of TGFβ signaling is controlled by the serine/threonine kinase salt-inducible kinase 1 (SIK1). SIK1 forms complexes with the TGFβ type I receptor and with the inhibitory Smad7 and down-regulates the type I receptor. We now demonstrate that TGFβ induces SIK1 levels via a direct transcriptional mechanism that implicates the Smad proteins, and we have mapped a putative enhancer element on the SIK1 gene. We provide evidence that the ubiquitin ligase Smurf2 forms complexes and functionally cooperates with SIK1. Both the kinase activity of SIK1 and the ubiquitin ligase activity of Smurf2 are important for proper type I receptor turnover. We also show that knockdown of endogenous SIK1 and Smurf2 enhances physiological signaling by TGFβ that leads to epithelial growth arrest. In conclusion, TGFβ induces expression of Smad7, Smurf2, and SIK1, the products of which physically and functionally interlink to control the activity of this pathway.

Lewis Y regulates signaling molecules of the transforming growth factor β pathway in ovarian carcinoma-derived RMG-I cells

LeY (Lewis Y) is a difucosylated oligosaccharide carried by glycoconjugates on the cell surface. Elevation of LeY is frequently observed in epithelial-derived cancers and is correlated to pathological staging and prognosis. To study the role of LeY on cancer cells, a stably LeY-overexpressing cell line, RMG-I-H, was developed previously by transfection of the α1,2-fucosyltransferase gene, a key enzyme that catalyzes the synthesis of LeY, into ovarian carcinoma-derived RMG-I cells. Our studies have shown that LeY is involved in the changes in biological behavior of RMG-I-H cells. However, the mechanism is still largely unknown. In this study, we determined the structural relationship and co-localization between LeY and TβRI/TβRII, respectively, and the potential cellular signaling mechanism was also investigated. We found that both TβRI and TβRII contain the LeY structure, and the level of LeY in TβRI and TβRII in RMG-I-H cells was significantly increased. Overexpression of LeY up-regulates the phosphorylation of ERK, Akt and down-regulates the phosphorylation of Smad2/3. In addition, the phosphorylation intensity was attenuated significantly by LeY monoantibody. These findings suggest that LeY is involved in the changes in biological behavior through TGF-β receptors via Smad, ERK/MAPK and PI3K/Akt signaling pathways. We suggest that LeY may be an important composition of growth factor receptors and could be an attractive candidate for cancer diagnosis and treatment.

Forty-eight hours of unloading and 24 h of reloading lead to changes in global gene expression patterns related to ubiquitination and oxidative stress in humans

Although short-term disuse does not result in measurable muscle atrophy, studies suggest that molecular changes associated with protein degradation may be initiated within days of the onset of a disuse stimulus. We examined the global gene expression patterns in sedentary men (n = 7, mean age ± SD = 22.1 ± 3.7 yr) following 48 h unloading (UL) via unilateral lower limb suspension and 24 h reloading (RL). Biopsy samples of the left vastus lateralis muscle were collected at baseline, 48 h UL, and 24 h RL. Expression changes were measured by microarray and gene clustering; identification of enriched functions and canonical pathways were performed using the Database for Annotation, Visualization and Integrated Discovery (DAVID) and Ingenuity Pathway Analysis (IPA). Four genes were validated with quantitative RT-PCR (qRT-PCR), and protein levels were measured with Western blot. Of the upregulated genes after UL, the most enriched functional group and highest ranked canonical pathway were related to protein ubiquitination. The oxidative stress response pathway was the second highest ranked canonical pathway. Of the downregulated genes, functions related to mitochondrial metabolism were the most highly enriched. In general, gene expression patterns following UL persisted following RL. qRT-PCR confirmed increases in mRNA for ubiquitin proteasome pathway-related E3 ligase Atrogin1 (but not accompanying increases in protein products) and stress response gene heme oxygenase-1 (HMOX, which showed a trend toward increases in protein products at 48 h UL) as well as extracellular matrix (ECM) component COL4A3. The gene expression patterns were not reversed on RL, suggesting that molecular responses to short-term periods of skeletal muscle inactivity may persist after activity resumes.

c-Ski, Smurf2, and Arkadia as regulators of TGF-β signaling: new targets for managing myofibroblast function and cardiac fibrosisThis article is one of a selection of papers published in a special issue celebrating the 125th anniversary of the Faculty of Medicine at the University of Manitoba

Recent studies demonstrate the critical role of the extracellular matrix in the organization of parenchymal cells in the heart. Thus, an understanding of the modes of regulation of matrix production by cardiac myofibroblasts is essential. Transforming growth factor β (TGF-β) signaling is transduced through the canonical Smad pathway, and the involvement of this pathway in matrix synthesis and other processes requires precise control. Inhibition of Smad signaling may be achieved at the receptor level through the targeting of the TGF-β type I receptors with an inhibitory Smad7 / Smurf2 complex, or at the transcriptional level through c-Ski / receptor-Smad / co-mediator Smad4 interactions. Conversely, Arkadia protein intensifies TGF-β-induced effects by marking c-Ski and inhibitory Smad7 for destruction. The study of these TGF-β mediators is essential for future treatment of fibrotic disease, and this review highlights recent relevant findings that may impact our understanding of cardiac fibrosis.

Degradation of activated protein kinases by ubiquitination

Protein kinases are important regulators of intracellular signal transduction pathways and play critical roles in diverse cellular functions. Once a protein kinase is activated, its activity is subsequently downregulated through a variety of mechanisms. Accumulating evidence indicates that the activation of protein kinases commonly initiates their downregulation via the ubiquitin/proteasome pathway. Failure to regulate protein kinase activity or expression levels can cause human diseases.

The role of NH4Cl and cysteine proteases in Human Papillomavirus type 16 infection

Background

The infectious pathway of the non-enveloped Human Papillomavirus Type 16 (HPV16) includes binding to the cell surface, clathrin-mediated endocytosis, and penetration into an endosome. HPV16 infection was shown to decrease in the presence of the lysosomotrophic neutralizing agent ammonium chloride (NH₄Cl). NH₄Cl neutralizes acidic endo-lysosome compartments, thus suggesting that pH was responsible for PV capsid conformational changes leading endosome escape.

Results

However, our data suggested that NH₄Cl blocked infection by preventing the movement of PV viral particles from the early endosome to the caveosome as was shown for JC virus [1,2]. We have confirmed that HPV 16 infection requires the trafficking of reporter-virions to the caveosome as is the case for BPV1 [3,4]. In this manuscript we propose that the observed decrease in infection of PV in the presence of NH₄Cl was due to a loss of movement of reporter-virions to caveosomes. We also demonstrate that cysteine proteases are involved in the infectious process, and that cathepsin B treatment of viral particles was shown to overcome the block of infection observed in the presence of furin inhibition. We confirmed the need for cathepsin B in HPV16 infection using cathepsin B null mouse embryonic fibroblasts.

Conclusion

We present data that suggest HPV16 infection is in part mediated by cysteine proteases, and that NH₄Cl blocks the intracellular trafficking of infectious viral particles. To our knowledge this is the first demonstration that cysteine proteases influence the infection of a non-enveloped virus.

Intrarenal RAS activity and urinary angiotensinogen excretion in anti-thymocyte serum nephritis rats

The differential roles of circulating and intrarenal renin-angiotensin system (RAS) in glomerulonephritis have not been elucidated. In this study, we investigated the levels of circulating and intrarenal RAS activity and urinary angiotensinogen (AGT) excretion in anti-thymocyte serum (ATS) nephritis induced by an ATS injection (ATS group). The effect of olmesartan, an angiotensin II (ANG II) type 1 receptor blocker (ARB), on the development of nephritis was also examined (ATS+ARB group). In addition, the rats received a saline injection instead of ATS (control group). Mesangial proliferation with transient proteinuria, which peaked at day 7, was significantly increased in the ATS group compared with the control group. The levels of glomerular AGT mRNA, intrarenal ANG II, and urinary AGT excretion in the ATS group were increased significantly at day 7 compared with the control group. Administration of olmesartan (ATS+ARB group) significantly decreased the levels of renal lesions, proteinuria, and intrarenal RAS activity compared with the ATS group. In addition, the levels of urinary AGT excretion correlated with the levels of glomerular damage, urinary protein excretion, and immunoreactivity for AGT and ANG II in kidney. On the other hand, plasma renin activity was significantly lower in the ATS group compared with the control group and significantly higher in the ATS+ARB group than in the ATS group. These data suggest that an increase in kidney-specific RAS activity, which parallels urinary AGT excretion, plays an important role in the development of ATS nephritis.

Regulation of embryonic endochondral ossification by Smurf2

Smurf2 is an E3 ubiquitin ligase that targets TGF-beta receptor activated Smad2 and Smad3 for the proteasome in primary articular chondrocytes, thus stimulating their hypertrophic differentiation. Comparatively, how Smurf2 functions in growth plate chondrocytes in a developing long bone is an open question. In this study, we measured the mRNA levels of endogenous Smurf2 and type X collagen in chick growth plate at different embryonic stages to monitor the correlation between the level of Smurf2 expression and chondrocyte maturational stage. We found that high levels of Smurf2 were associated with the differentiative and proliferative stages, while Smurf2 levels were thereafter decreased as the chondrocytes matured toward hypertrophy. In addition, we injected Smurf2-RCAS into chick wing buds at HH stage 20-23 and examined how the ectopic overexpression of Smurf2 in condensing chondrogenic mesenchyme affects the subsequent process of chondrocyte maturation and ossification during embryonic development. Histological analysis showed that overexpression of Smurf2 in a developing wing bud accelerated chondrocyte maturation and endochondral ossification, which may result from a decrease in TGF-beta signaling in the infected chondrocytes with Smurf2-RCAS.

The Nedd4-like family of E3 ubiquitin ligases and cancer

Accumulating evidence suggests that E3 ubiquitin ligases play important roles in cancer development. In this article, we provide a comprehensive summary of the roles of the Nedd4-like family of E3 ubiquitin ligases in human cancer. There are nine members of the Nedd4-like E3 family, all of which share a similar structure, including a C2 domain at the N-terminus, two to four WW domains in the middle of the protein, and a homologous to E6-AP COOH terminus domain at the C-terminus. The assertion that Nedd4-like E3s play a role in cancer is supported by the overexpression of Smurf2 in esophageal squamous cell carcinoma, WWP1 in prostate and breast cancer, Nedd4 in prostate and bladder cancer, and Smurf1 in pancreatic cancer. Because Nedd4-like E3s regulate ubiquitin-mediated trafficking, lysosomal or proteasomal degradation, and nuclear translocation of multiple proteins, they modulate important signaling pathways involved in tumorigenesis like TGFbeta, EGF, IGF, VEGF, SDF-1, and TNFalpha. Additionally, several Nedd4-like E3s directly regulate various cancer-related transcription factors from the Smad, p53, KLF, RUNX, and Jun families. Interestingly, multiple Nedd4-like E3s show ligase independent function. Furthermore, Nedd4-like E3s themselves are frequently regulated by phosphorylation, ubiquitination, translocation, and transcription in cancer cells. Because the regulation and biological output of these E3s is such a complex process, study of the role of these E3s in cancer development poses some challenges. However, understanding the oncogenic potential of these E3s may facilitate the identification and development of biomarkers and drug targets in human cancer.

A novel modulatory mechanism of transforming growth factor-beta signaling through decorin and LRP-1

Transforming growth factor-beta (TGF-beta) is a multifunctional cytokine that signals to the nucleus through cell surface transmembrane receptors with serine/threonine kinase activity and cytoplasmic effectors, including Smad proteins. Here we describe two novel modulators of this pathway, lipoprotein-receptor related protein (LRP-1) and decorin. Decorin null (Dcn null) myoblasts showed a diminished TGF-beta response that is restored by decorin re-expression. Importantly, this reactivation occurs without changes in the binding to TGF-beta receptors, Smad protein phosphorylation, or Smad-4 nuclear translocation. In wild type myoblasts, inhibition of decorin binding to LRP-1 and depletion of LRP-1 inhibited TGF-beta response to levels similar to those observed in Dcn null myoblasts. Re-expression of decorin in Dcn null myoblasts cannot restore TGF-beta response if the Smad pathway or phosphatidylinositol 3-kinase activity is inhibited, suggesting that this LRP-1-decorin modulatory pathway requires activation of the Smad pathway by TGF-beta and involves phosphatidylinositol 3-kinase activity. This work unveils a new regulatory mechanism for TGF-beta signaling by decorin and LRP-1.

Endoglin increases eNOS expression by modulating Smad2 protein levels and Smad2-dependent TGF-beta signaling

The endothelial nitric oxide synthase (eNOS) is a critical regulator of cardiovascular homeostasis, whose dysregulation leads to different vascular pathologies. Endoglin is a component of the transforming growth factor beta (TGF-beta) receptor complex present in endothelial cells that is involved in angiogenesis, cardiovascular development, and vascular homeostasis. Haploinsufficient expression of endoglin has been shown to downregulate endothelium-derived nitric oxide in endoglin(+/-) (Eng(+/-)) mice and cultured endothelial cells. Here, we find that TGF-beta1 leads to an increased vasodilatation in Eng(+/+) mice that is severely impaired in Eng(+/-) mice, suggesting the involvement of endoglin in the TGF-beta regulated vascular homeostasis. The endoglin-dependent induction of eNOS occurs at the transcriptional level and is mediated by the type I TGF-beta receptor ALK5 and its downstream substrate Smad2. In addition, Smad2-specific signaling is upregulated in endoglin-induced endothelial cells, whereas it is downregulated upon endoglin gene suppression with small interference RNA (siRNA). The endoglin-dependent upregulation of Smad2 was confirmed using eNOS and pARE promoters, whose activities are known to be Smad2 dependent, as well as with the interference of Smad2 with siRNA, Smurf2, or a dominant negative form of Smad2. Furthermore, increased expression of endoglin in endoglin-inducible endothelial cells or in transfectants resulted in increased levels of Smad2 protein without affecting the levels of Smad2 mRNA. The increased levels of Smad2 appear to be due to a decreased ubiquitination and proteasome-dependent degradation leading to stabilization of Smad2. These results suggest that endoglin enhances Smad2 protein levels potentiating TGF-beta signaling, and leading to an increased eNOS expression in endothelial cells.

Ubiquitin-dependent regulation of TGFbeta signaling in cancer

The transforming growth factorbeta (TGFbeta) superfamily regulates a broad spectrum of biological responses throughout embryonic development and adult life, including cell proliferation and differentiation, epithelial-to-mesenchymal transition, apoptosis, and angiogenesis. TGFbeta members initiate signaling by bringing together a complex of serine/threonine kinase receptors that transmit signals through intracellular Smad proteins. Genetic alterations in numerous components of the TGFbeta signaling pathway have been associated with several human cancers. In addition, tight regulation of TGFbeta signaling is pivotal to the maintenance of homeostasis and the prevention of carcinogenesis. The ubiquitin/proteosome system is one mechanism by which cells regulate the expression and activity of effectors of the TGFbeta signaling cascade. Mounting evidence also suggests that disruption of the ubiquitin-dependent degradation of components of the TGFbeta pathway leads to the development and progression of cancer. Therefore, understanding how these two pathways intertwine will contribute to the advancement of our knowledge of cancer development.

Downregulation of SnoN expression in obstructive nephropathy is mediated by an enhanced ubiquitin-dependent degradation

Smad transcriptional co-repressor SnoN acts as an antagonist that tightly controls the trans-activation of TGF-beta/Smad target genes. SnoN protein is reduced progressively in the fibrotic kidney after obstructive injury, suggesting that the loss of Smad antagonist is a critical event that leads to an uncontrolled fibrogenic signaling. However, the mechanism underlying SnoN downregulation remains unknown. This study investigated the regulation and mechanism of renal SnoN expression in vivo. Whereas SnoN protein was markedly diminished, its mRNA levels remained relatively constant in the obstructed kidney after ureteral ligation. An increased ubiquitination and proteasome-dependent degradation of SnoN was found in obstructed kidney, compared with sham controls. Smad ubiquitination regulatory factor-2, an E3 ubiquitin ligase, was induced and formed a complex with SnoN in vivo. In vitro, TGF-beta1 promoted SnoN protein degradation, which was mediated by ubiquitination and a proteasome-dependent mechanism. SnoN constitutively interacted with another Smad co-repressor, Ski, and they formed ternary complex with Smad2/3 upon TGF-beta1 stimulation. However, ectopic expression of Ski did not alter the degradation rate of SnoN. Blockage of SnoN degradation by proteasome inhibitor abolished TGF-beta1-mediated alpha-smooth muscle actin and fibronectin induction, suggesting that SnoN degradation could be necessary for TGF-beta1 to exert its fibrogenic action. Furthermore, knockdown of Smad ubiquitination regulatory factor-2 expression by small interfering RNA strategy led to an increase in SnoN abundance and inhibited the TGF-beta1-mediated gene transcription. These results indicate that downregulation of SnoN expression in the obstructed kidney is mediated by an enhanced ubiquitin-dependent degradation. Preservation of SnoN by inhibiting its degradation may be a novel strategy for targeting hyperactive Smad signaling in renal fibrotic diseases.

Ubiquitin-dependent degradation of SnoN and Ski is increased in renal fibrosis induced by obstructive injury

Transforming growth factor-beta (TGF-beta) plays a critical role in the progression of renal fibrosis. The activity of TGF-beta is tightly controlled by various mechanisms, among which antagonizing Smad-mediated gene transcription by co-repressors represents one of the important components. We investigated the expression, degradation, and ubiquitination of Smad transcriptional co-repressors SnoN (ski-related novel gene N) and Ski (Sloan-Kettering Institute proto-oncogene) in renal fibrogenesis. We also studied the involvement of Smad-ubiquitination regulatory factor 2 (Smurf2) in ubiquitination of SnoN protein. The kidneys of mice with unilateral ureteral obstruction (UUO) and those of sham-operated mice were used. Renal lesions and the expression of TGF-beta1, type I collagen, SnoN, Ski, and Smurf2 were examined by immunohistochemistry, Western blot, and/or real-time reverse transcriptase-polymerase chain reaction. Degradation and ubiquitination of SnoN/Ski proteins were also investigated. The obstructed kidneys of UUO mice showed progressive tubulointerstitial fibrosis, high expression levels of TGF-beta1, type I collagen, SnoN and Ski mRNAs, and low levels of SnoN and Ski proteins. Both degradation and ubiquitination of SnoN/Ski proteins were markedly increased in the obstructed kidneys, in which Smurf2 expression was increased. Smurf2 immunodepletion in extracts of obstructed kidneys resulted in reduced ubiquitination of SnoN. Our results suggest that the reduction of SnoN/Ski proteins resulting from increased ubiquitin-dependent degradation is involved in the progression of tubulointerstitial fibrosis.