Distinct role of endocytosis for Smad and non-Smad TGF-β signaling regulation in hepatocytes

Cholesterol modulates type I/II TGF-β receptor complexes and alters the balance between Smad and Akt signaling in hepatocytes

Cholesterol mediates membrane compartmentalization, affecting signaling via differential distribution of receptors and signaling mediators. While excessive cholesterol and aberrant transforming growth factor-β (TGF-β) signaling characterize multiple liver diseases, their linkage to canonical vs. non-canonical TGF-β signaling remained unclear. Here, we subjected murine hepatocytes to cholesterol depletion (CD) or enrichment (CE), followed by biophysical studies on TGF-β receptor heterocomplex formation, and output to Smad2/3 vs. Akt pathways. Prior to ligand addition, raft-dependent preformed heteromeric receptor complexes were observed. Smad2/3 phosphorylation persisted following CD or CE. CD enhanced phospho-Akt (pAkt) formation by TGF-β or epidermal growth factor (EGF) at 5 min, while reducing it at later time points. Conversely, pAkt formation by TGF-β or EGF was inhibited by CE, suggesting a direct effect on the Akt pathway. The modulation of the balance between TGF-β signaling to Smad2/3 vs. pAkt (by TGF-β or EGF) has potential implications for hepatic diseases and malignancies.

The Interplay of TGF-β1 and Cholesterol Orchestrating Hepatocyte Cell Fate, EMT, and Signals for HSC Activation

BACKGROUND & AIMS

Transforming growth factor-β1 (TGF-β1) plays important roles in chronic liver diseases, including metabolic dysfunction-associated steatotic liver disease (MASLD). MASLD involves various biological processes including dysfunctional cholesterol metabolism and contributes to progression to metabolic dysfunction-associated steatohepatitis and hepatocellular carcinoma. However, the reciprocal regulation of TGF-β1 signaling and cholesterol metabolism in MASLD is yet unknown.

METHODS

Changes in transcription of genes associated with cholesterol metabolism were assessed by RNA sequencing of murine hepatocyte cell line (alpha mouse liver 12/AML12) and mouse primary hepatocytes treated with TGF-β1. Functional assays were performed on AML12 cells (untreated, TGF-β1 treated, or subjected to cholesterol enrichment [CE] or cholesterol depletion [CD]), and on mice injected with adenovirus-associated virus 8-control/TGF-β1.

RESULTS

TGF-β1 inhibited messenger RNA expression of several cholesterol metabolism regulatory genes, including rate-limiting enzymes of cholesterol biosynthesis in AML12 cells, mouse primary hepatocytes, and adenovirus-associated virus-TGF-β1-treated mice. Total cholesterol levels and lipid droplet accumulation in AML12 cells and liver tissue also were reduced upon TGF-β1 treatment. Smad2/3 phosphorylation after 2 hours of TGF-β1 treatment persisted after CE or CD and was mildly increased after CD, whereas TGF-β1-mediated AKT phosphorylation (30 min) was inhibited by CE. Furthermore, CE protected AML12 cells from several effects mediated by 72 hours of incubation with TGF-β1, including epithelial-mesenchymal transition, actin polymerization, and apoptosis. CD mimicked the outcome of long-term TGF-β1 administration, an effect that was blocked by an inhibitor of the type I TGF-β receptor. In addition, the supernatant of CE- or CD-treated AML12 cells inhibited or promoted, respectively, the activation of LX-2 hepatic stellate cells.

CONCLUSIONS

TGF-β1 inhibits cholesterol metabolism whereas cholesterol attenuates TGF-β1 downstream effects in hepatocytes.

circEIF3I facilitates the recruitment of SMAD3 to early endosomes to promote TGF-β signalling pathway-mediated activation of MMPs in pancreatic cancer

BACKGROUND

Among digestive tract tumours, pancreatic ductal adenocarcinoma (PDAC) shows the highest mortality trend. Moreover, although PDAC metastasis remains a leading cause of cancer-related deaths, the biological mechanism is poorly understood. Recent evidence demonstrates that circular RNAs (circRNAs) play important roles in PDAC progression.

METHODS

Differentially expressed circRNAs in normal and PDAC tissues were screened via bioinformatics analysis. Sanger sequencing, RNase R and actinomycin D assays were performed to confirm the loop structure of circEIF3I. In vitro and in vivo functional experiments were conducted to assess the role of circEIF3I in PDAC. MS2-tagged RNA affinity purification, mass spectrometry, RNA immunoprecipitation, RNA pull-down assay, fluorescence in situ hybridization, immunofluorescence and RNA-protein interaction simulation and analysis were performed to identify circEIF3I-interacting proteins. The effects of circEIF3I on the interactions of SMAD3 with TGFβRI or AP2A1 were measured through co-immunoprecipitation and western blotting.

RESULTS

A microarray data analysis showed that circEIF3I was highly expressed in PDAC cells and correlated with TNM stage and poor prognosis. Functional experiments in vitro and in vivo revealed that circEIF3I accelerated PDAC cells migration, invasion and metastasis by increasing MMPs expression and activity. Mechanistic research indicated that circEIF3I binds to the MH2 domain of SMAD3 and increases SMAD3 phosphorylation by strengthening the interactions between SMAD3 and TGFβRI on early endosomes. Moreover, AP2A1 binds with circEIF3I directly and promotes circEIF3I-bound SMAD3 recruitment to TGFβRI on early endosomes. Finally, we found that circEif3i exerts biological functions in mice similar to those of circEIF3I in humans PDAC.

CONCLUSIONS

Our study reveals that circEIF3I promotes pancreatic cancer progression. circEIF3I is a molecular scaffold that interacts with SMAD3 and AP2A1 to form a ternary complex, that facilitates the recruitment of SMAD3 to early endosomes and then activates the TGF-β signalling pathway. Hence, circEIF3I is a potential prognostic biomarker and therapeutic target in PDAC.

Dysregulation of the immune response in TGF-β signalopathies

The transforming growth factor-β (TGF-β) family of cytokines exerts pleiotropic functions during embryonic development, tissue homeostasis and repair as well as within the immune system. Single gene defects in individual component of this signaling machinery cause defined Mendelian diseases associated with aberrant activation of TGF-β signaling, ultimately leading to impaired development, immune responses or both. Gene defects that affect members of the TGF-β cytokine family result in more restricted phenotypes, while those affecting downstream components of the signaling machinery induce broader defects. These rare disorders, also known as TGF-β signalopathies, provide the unique opportunity to improve our understanding of the role and the relevance of the TGF-β signaling in the human immune system. Here, we summarize this elaborate signaling pathway, review the diverse clinical presentations and immunological phenotypes observed in these patients and discuss the phenotypic overlap between humans and mice genetically deficient for individual components of the TGF-β signaling cascade.

Transforming growth factor-β challenge alters the N-, O-, and glycosphingolipid glycomes in PaTu-S pancreatic adenocarcinoma cells

Pancreatic ductal adenocarcinoma (PDAC) is characterized by poor prognosis and high mortality. Transforming growth factor-β (TGF-β) plays a key role in PDAC tumor progression, which is often associated with aberrant glycosylation. However, how PDAC cells respond to TGF-β and the role of glycosylation therein is not well known. Here, we investigated the TGF-β-mediated response and glycosylation changes in the PaTu-8955S (PaTu-S) cell line deficient in SMA-related and MAD-related protein 4 (SMAD4), a signal transducer of the TGF-β signaling. PaTu-S cells responded to TGF-β by upregulating SMAD2 phosphorylation and target gene expression. We found that TGF-β induced expression of the mesenchymal marker N-cadherin but did not significantly affect epithelial marker E-cadherin expression. We also examined differences in N-glycans, O-glycans, and glycosphingolipid-linked glycans in PaTu-S cells upon TGF-β stimulation. TGF-β treatment primarily induced N-glycome aberrations involving elevated levels of branching, core fucosylation, and sialylation in PaTu-S cells, in agreement with TGF-β-induced changes in the expression of glycosylation-associated genes. In addition, we observed differences in O glycosylation and glycosphingolipid glycosylation profiles after TGF-β treatment, including lower levels of sialylated Tn antigen and neoexpression of globosides. Furthermore, the expression of transcription factor sex-determining region Y-related high-mobility group box 4 was upregulated upon TGF-β stimulation, and its depletion blocked TGF-β-induced N-glycomic changes. Thus, TGF-β-induced N-glycosylation changes can occur in a sex-determining region Y-related high-mobility group box 4–dependent and SMAD4-independent manner in the pancreatic PaTu-S cancer cell line. Our results open up avenues to study the relevance of glycosylation in TGF-β signaling in SMAD4-inactivated PDAC.

pVHL-mediated SMAD3 degradation suppresses TGF-β signaling

Transforming growth factor β (TGF-β) signaling plays a fundamental role in metazoan development and tissue homeostasis. However, the molecular mechanisms concerning the ubiquitin-related dynamic regulation of TGF-β signaling are not thoroughly understood. Using a combination of proteomics and an siRNA screen, we identify pVHL as an E3 ligase for SMAD3 ubiquitination. We show that pVHL directly interacts with conserved lysine and proline residues in the MH2 domain of SMAD3, triggering degradation. As a result, the level of pVHL expression negatively correlates with the expression and activity of SMAD3 in cells, Drosophila wing, and patient tissues. In Drosophila, loss of pVHL leads to the up-regulation of TGF-β targets visible in a downward wing blade phenotype, which is rescued by inhibition of SMAD activity. Drosophila pVHL expression exhibited ectopic veinlets and reduced wing growth in a similar manner as upon loss of TGF-β/SMAD signaling. Thus, our study demonstrates a conserved role of pVHL in the regulation of TGF-β/SMAD3 signaling in human cells and Drosophila wing development.

Targeting the mercapturic acid pathway for the treatment of melanoma

The treatment of metastatic melanoma is greatly hampered by the simultaneous dysregulation of several major signaling pathways that suppress apoptosis and promote its growth and invasion. The global resistance of melanomas to therapeutics is also supported by a highly active mercapturic acid pathway (MAP), which is responsible for the metabolism and excretion of numerous chemotherapy agents. The relative importance of the MAP in melanoma survival was not recognized until demonstrated that B16 melanoma undergoes dramatic apoptosis and regression upon the depletion or inhibition of the MAP transporter protein RLIP. RLIP is a multi-functional protein that couples ATP hydrolysis with the movement of substances. As the rate-limiting step of the MAP, the primary function of RLIP in the plasma membrane is to catalyze the ATP-dependent efflux of unmetabolized drugs and toxins, including glutathione (GSH) conjugates of electrophilic toxins (GS-Es), which are the precursors of mercapturic acids. Clathrin-dependent endocytosis (CDE) is an essential mechanism for internalizing ligand-receptor complexes that promote tumor cell proliferation through autocrine stimulation (Wnt5a, PDGF, βFGF, TNFα) or paracrine stimulation by hormones produced by fibroblasts (IGF1, HGF) or inflammatory cells (IL8). Aberrant functioning of these pathways appears critical for melanoma cell invasion, metastasis, and evasion of apoptosis. This review focuses on the selective depletion or inhibition of RLIP as a highly effective targeted therapy for melanoma that could cause the simultaneous disruption of the MAP and critical peptide hormone signaling that relies on CDE.

Injury-induced Cavl-expressing cells at lesion rostral side play major roles in spinal cord regeneration

The extent of cellular heterogeneity involved in neuronal regeneration after spinal cord injury (SCI) remains unclear. Therefore, we established stress-responsive transgenic zebrafish embryos with SCI. As a result, we found an SCI-induced cell population, termed SCI stress-responsive regenerating cells (SrRCs), essential for neuronal regeneration post-SCI. SrRCs were mostly composed of subtypes of radial glia (RGs-SrRCs) and neuron stem/progenitor cells (NSPCs-SrRCs) that are able to differentiate into neurons, and they formed a bridge across the lesion and connected with neighbouring undamaged motor neurons post-SCI. Compared to SrRCs at the caudal side of the SCI site (caudal-SrRCs), rostral-SrRCs participated more actively in neuronal regeneration. After RNA-seq analysis, we discovered that caveolin 1 (cav1) was significantly upregulated in rostral-SrRCs and that cav1 was responsible for the axonal regrowth and regenerative capability of rostral-SrRCs. Collectively, we define a specific SCI-induced cell population, SrRCs, involved in neuronal regeneration, demonstrate that rostral-SrRCs exhibit higher neuronal differentiation capability and prove that cav1 is predominantly expressed in rostral-SrRCs, playing a major role in neuronal regeneration after SCI.

TGF-β Signaling

Transforming growth factor-β (TGF-β) represents an evolutionarily conserved family of secreted polypeptide factors that regulate many aspects of physiological embryogenesis and adult tissue homeostasis. The TGF-β family members are also involved in pathophysiological mechanisms that underlie many diseases. Although the family comprises many factors, which exhibit cell type-specific and developmental stage-dependent biological actions, they all signal via conserved signaling pathways. The signaling mechanisms of the TGF-β family are controlled at the extracellular level, where ligand secretion, deposition to the extracellular matrix and activation prior to signaling play important roles. At the plasma membrane level, TGF-βs associate with receptor kinases that mediate phosphorylation-dependent signaling to downstream mediators, mainly the SMAD proteins, and mediate oligomerization-dependent signaling to ubiquitin ligases and intracellular protein kinases. The interplay between SMADs and other signaling proteins mediate regulatory signals that control expression of target genes, RNA processing at multiple levels, mRNA translation and nuclear or cytoplasmic protein regulation. This article emphasizes signaling mechanisms and the importance of biochemical control in executing biological functions by the prototype member of the family, TGF-β.

Hepatocyte caveolin-1 modulates metabolic gene profiles and functions in non-alcoholic fatty liver disease

Caveolin-1 (CAV1) is a crucial regulator of lipid accumulation and metabolism. Previous studies have shown that global Cav1 deficiency affects lipid metabolism and hepatic steatosis. We aimed to analyze the consequences of hepatocyte-specific Cav1 knockout under healthy conditions and upon non-alcoholic fatty liver disease (NAFLD) development. Male and female hepatocyte-specific Cav1 knockout (HepCAV1ko) mice were fed a methionine/choline (MCD) deficient diet for 4 weeks. MCD feeding caused severe hepatic steatosis and slight fibrosis. In addition, liver function parameters, i.e., ALT, AST, and GLDH, were elevated, while cholesterol and glucose level were reduced upon MCD feeding. These differences were not affected by hepatocyte-specific Cav1 knockout. Microarray analysis showed strong differences in gene expression profiles of livers from HepCAV1ko mice compared those of global Cav1 knockout animals. Pathway enrichment analysis identified that metabolic alterations were sex-dimorphically regulated by hepatocyte-specific CAV1. In male HepCAV1ko mice, metabolic pathways were suppressed in NAFLD, whereas in female knockout mice induced. Moreover, gender-specific transcription profiles were modulated in healthy animals. In conclusion, our results demonstrate that hepatocyte-specific Cav1 knockout significantly altered gene profiles, did not affect liver steatosis and fibrosis in NAFLD and that gender had severe impact on gene expression patterns in healthy and diseased hepatocyte-specific Cav1 knockout mice.

Micropeptide CIP2A-BP encoded by LINC00665 inhibits triple-negative breast cancer progression

TGF-β signaling pathway plays a key role in breast cancer metastasis. Recent studies suggest that TGF-β regulates tumor progression and invasion not only via transcriptional regulation, but also via translational regulation. Using both bioinformatics and experimental tools, we identified a micropeptide CIP2A-BP encoded by LINC00665, whose translation was downregulated by TGF-β in breast cancer cell lines. Using TNBC cell lines, we showed that TGF-β-activated Smad signaling pathway induced the expression of translation inhibitory protein 4E-BP1, which inhibited eukaryote translation initiation factor elF4E, leading to reduced translation of CIP2A-BP from LINC00665. CIP2A-BP directly binds tumor oncogene CIP2A to replace PP2A's B56γ subunit, thus releasing PP2A activity, which inhibits PI3K/AKT/NFκB pathway, resulting in decreased expression levels of MMP-2, MMP-9, and Snail. Downregulation of CIP2A-BP in TNBC patients was significantly associated with metastasis and poor overall survival. In the MMTV-PyMT model, either introducing CIP2A-BP gene or direct injection of CIP2A-BP micropeptide significantly reduced lung metastases and improved overall survival. In conclusion, we provide evidence that CIP2A-BP is both a prognostic marker and a novel therapeutic target for TNBC.

Clathrin switches transforming growth factor-β role to pro-tumorigenic in liver cancer

BACKGROUND & AIMS

Upon ligand binding, tyrosine kinase receptors, such as epidermal growth factor receptor (EGFR), are recruited into clathrin-coated pits for internalization by endocytosis, which is relevant for signalling and/or receptor degradation. In liver cells, transforming growth factor-β (TGF-β) induces both pro- and anti-apoptotic signals; the latter are mediated by the EGFR pathway. Since EGFR mainly traffics via clathrin-coated vesicles, we aimed to analyse the potential role of clathrin in TGF-β-induced signalling in liver cells and its relevance in liver cancer.

METHODS

Real-Time PCR and immunohistochemistry were used to analyse clathrin heavy-chain expression in human (CLTC) and mice (Cltc) liver tumours. Transient knockdown (siRNA) or overexpression of CLTC were used to analyse its role on TGF-β and EGFR signalling in vitro. Bioinformatic analysis was used to determine the effect of CLTC and TGFB1 expression on prognosis and overall survival in patients with hepatocellular carcinoma (HCC).

RESULTS

Clathrin expression increased during liver tumorigenesis in humans and mice. CLTC knockdown cells responded to TGF-β phosphorylating SMADs (canonical signalling) but showed impairment in the anti-apoptotic signals (EGFR transactivation). Experiments of loss or gain of function in HCC cells reveal an essential role for clathrin in inhibiting TGF-β-induced apoptosis and upregulation of its pro-apoptotic target NOX4. Autocrine TGF-β signalling in invasive HCC cells upregulates CLTC expression, switching its role to pro-tumorigenic. A positive correlation between TGFB1 and CLTC was found in HCC cells and patients. Patients expressing high levels of TGFB1 and CLTC had a worse prognosis and lower overall survival.

CONCLUSIONS

This work describes a novel role for clathrin in liver tumorigenesis, favouring non-canonical pro-tumorigenic TGF-β pathways. CLTC expression in human HCC samples could help select patients that would benefit from TGF-β-targeted therapy.

LAY SUMMARY

Clathrin heavy-chain expression increases during liver tumorigenesis in humans (CLTC) and mice (Cltc), altering the cellular response to TGF-β in favour of anti-apoptotic/pro-tumorigenic signals. A positive correlation between TGFB1 and CLTC was found in HCC cells and patients. Patients expressing high levels of TGFB1 and CLTC had a worse prognosis and lower overall survival. CLTC expression in HCC human samples could help select patients that would benefit from therapies targeting TGF-β.

TGF-β in Hepatic Stellate Cell Activation and Liver Fibrogenesis-Updated 2019

Liver fibrosis is an advanced liver disease condition, which could progress to cirrhosis and hepatocellular carcinoma. To date, there is no direct approved antifibrotic therapy, and current treatment is mainly the removal of the causative factor. Transforming growth factor (TGF)-β is a master profibrogenic cytokine and a promising target to treat fibrosis. However, TGF-β has broad biological functions and its inhibition induces non-desirable side effects, which override therapeutic benefits. Therefore, understanding the pleiotropic effects of TGF-β and its upstream and downstream regulatory mechanisms will help to design better TGF-β based therapeutics. Here, we summarize recent discoveries and milestones on the TGF-β signaling pathway related to liver fibrosis and hepatic stellate cell (HSC) activation, emphasizing research of the last five years. This comprises impact of TGF-β on liver fibrogenesis related biological processes, such as senescence, metabolism, reactive oxygen species generation, epigenetics, circadian rhythm, epithelial mesenchymal transition, and endothelial-mesenchymal transition. We also describe the influence of the microenvironment on the response of HSC to TGF-β. Finally, we discuss new approaches to target the TGF-β pathway, name current clinical trials, and explain promises and drawbacks that deserve to be adequately addressed.

Sec3 knockdown inhibits TGF-β induced epithelial-mesenchymal transition through the down-regulation of Akt phosphorylation in A549 cells

The exocyst, an evolutionarily conserved octomeric protein complex, has been demonstrated as an essential component for vesicle tethering during cell exocytosis, and participates in various physiological processes in the cell. Although subunits of the exocyst complex have been reported to be involved in the regulation of TGF-β induced cancer cell migration and epithelial-mesenchymal transition (EMT), the potential function of Sec3 in these regulated processes remains unclear. Here, we show that Sec3 knockdown abolishes TGF-β stimulated A549 lung cancer cell migration in vitro and causes defects in the regulated EMT process. In addition, we find that depletion of Sec3 significantly inhibits TGF-β stimulated Akt phosphorylation in A549 cells, whereas the increase of Smad2 phosphorylation is unaffected. Furthermore, replenishment of an RNAi-resistant form of Sec3 is shown to restore the defects of TGF-β induced cell migration, EMT and Akt signaling activation. In summary, our study provides evidence that Sec3 is involved in TGF-β induced cell migration and EMT processes, presumably through the regulation of PI3K/Akt signaling activation in A549 cancer cells.

Specificity, versatility, and control of TGF-β family signaling

Encoded in mammalian cells by 33 genes, the transforming growth factor-β (TGF-β) family of secreted, homodimeric and heterodimeric proteins controls the differentiation of most, if not all, cell lineages and many aspects of cell and tissue physiology in multicellular eukaryotes. Deregulation of TGF-β family signaling leads to developmental anomalies and disease, whereas enhanced TGF-β signaling contributes to cancer and fibrosis. Here, we review the fundamentals of the signaling mechanisms that are initiated upon TGF-β ligand binding to its cell surface receptors and the dependence of the signaling responses on input from and cooperation with other signaling pathways. We discuss how cells exquisitely control the functional presentation and activation of heteromeric receptor complexes of transmembrane, dual-specificity kinases and, thus, define their context-dependent responsiveness to ligands. We also introduce the mechanisms through which proteins called Smads act as intracellular effectors of ligand-induced gene expression responses and show that the specificity and impressive versatility of Smad signaling depend on cross-talk from other pathways. Last, we discuss how non-Smad signaling mechanisms, initiated by distinct ligand-activated receptor complexes, complement Smad signaling and thus contribute to cellular responses.

Effects of NLRP6 on the proliferation and activation of human hepatic stellate cells

Nod-like receptor pyrin domain-containing proteins (NLRPs) are known to take part in the pathogenesis of chronic liver diseases, including liver fibrosis. However, no known direct role of NLRP6, a member of NLRPs, has been reported in liver fibrosis. Here, we found that NLRP6 expression was decreased in fibrotic and cirrhotic livers. In a human hepatic stellate cell line, LX-2, overexpression of NLRP6 suppressed cell proliferation, hydroxyproline accumulation, as well as the expression of type I and type III collagens (Col-I and Col-III), α-smooth muscle actin (α-SMA) and matrix metalloproteinases (MMP2 and MMP9), whereas NLRP6 knockdown displayed reverse effects. Furthermore, NLRP6 significantly suppressed the phosphorylation of Smad2/3 (p-Smad2/3) and enhanced the expression of protein phosphatase magnesium dependent 1 A (PPM1A), the only phosphatase for Smad2/3. NLRP6 overexpression abrogated TGF-β1-stimulated hydroxyproline accumulation and p-Smad2/3. Co-immunoprecipitation assay demonstrated that NLRP6 was able to form a complex with PPM1A. NLRP6 overexpression did not change the level of p-Smad2/3 in LX-2 cells with PPM1A knockdown. These data indicated that PPM1A was required for the inhibitory effects of NLRP6 on TGF-β1/Smad2/3 signaling. In conclusion, our results suggest that NLRP6 exerts anti-fibrotic effects in LX-2 cells via regulating PPM1A/Smad2/3 and that NLRP6 may be an effective target in the treatment of liver fibrosis.

Blockage of AKAP12 accelerates angiotensin II (Ang II)-induced cardiac injury in mice by regulating the transforming growth factor β1 (TGF-β1) pathway

Hypertension is a multifactorial chronic inflammatory disease that leads to cardiac remodeling. A-kinase anchor protein 12 (AKAP12) is a scaffolding protein that has multiple functions in various biological events, including the regulation of vessel integrity and differentiation of neural barriers in blood. However, the role of AKAP12 in angiotensin II (Ang II)-induced cardiac injury remains unclear. In the present study, Ang II infusion reduced AKAP12 expressions in the hearts of wild-type (WT) mice, and AKAP12 knockout (KO) enhanced the infiltration of inflammatory cells. In addition, AKAP12 deletion accelerated Ang II-induced cardiac histologic alterations and dysfunction. Further, AKAP12^-/- aggravated heart failure by promoting the inflammation, oxidative stress, cellular apoptosis, and autophagy induced by Ang II. Furthermore, AKAP12 KO elevated Ang II-induced cardiac fibrosis, as indicated by the following: (1) Masson trichrome staining showed that Ang II infusion markedly increased fibrotic areas of the WT mouse heart, which was greatly accelerated in AKAP12^-/- mice; (2) immunohistochemistry analysis showed increased expression of transforming growth factor β1 (TGF-β1) and α-smooth muscle actin (α-SMA) in the AKAP12^-/- mouse heart; (3) reverse transcription-quantitative real-time polymerase chain reaction (RT-qPCR) analysis showed increased expression of fibrosis-related molecules in the AKAP12-deficient mouse heart; and (4) Western blot analysis indicated significantly higher upregulation of p-SMAD2/3 in the AKAP12^-/- mouse heart. In vitro, AKAP12 knockdown in HL-1 cells was responsible for TGF-β1-induced inflammation, the generation of reactive oxygen species (ROS), apoptosis, autophagy, and fibrosis. Furthermore, overexpression of AKAP12 reduced fibrosis triggered by TGF-β1 in cells. Overall, our study suggests that fibrosis induced by Ang II may be alleviated by AKAP12 expression through inactivation of the TGF-β1 pathway.

Ethanol sensitizes hepatocytes for TGF-β-triggered apoptosis

Alcohol abuse is a global health problem causing a substantial fraction of chronic liver diseases. Abundant TGF-β—a potent pro-fibrogenic cytokine—leads to disease progression. Our aim was to elucidate the crosstalk of TGF-β and alcohol on hepatocytes. Primary murine hepatocytes were challenged with ethanol and TGF-β and cell fate was determined. Fluidigm RNA analyses revealed transcriptional effects that regulate survival and apoptosis. Mechanistic insights were derived from enzyme/pathway inhibition experiments and modulation of oxidative stress levels. To substantiate findings, animal model specimens and human liver tissue cultures were investigated. Results: On its own, ethanol had no effect on hepatocyte apoptosis, whereas TGF-β increased cell death. Combined treatment led to massive hepatocyte apoptosis, which could also be recapitulated in human HCC liver tissue treated ex vivo. Alcohol boosted the TGF-β pro-apoptotic gene signature. The underlying mechanism of pathway crosstalk involves SMAD and non-SMAD/AKT signaling. Blunting CYP2E1 and ADH activities did not prevent this effect, implying that it was not a consequence of alcohol metabolism. In line with this, the ethanol metabolite acetaldehyde did not mimic the effect and glutathione supplementation did not prevent the super-induction of cell death. In contrast, blocking GSK-3β activity, a downstream mediator of AKT signaling, rescued the strong apoptotic response triggered by ethanol and TGF-β. This study provides novel information on the crosstalk between ethanol and TGF-β. We give evidence that ethanol directly leads to a boost of TGF-β’s pro-apoptotic function in hepatocytes, which may have implications for patients with chronic alcoholic liver disease.

Intracellular trafficking of transforming growth factor β receptors

Transforming growth factor β (TGFβ) family members signal via heterotetrameric complexes of type I (TβRI) and type II (TβRII) dual specificity kinase receptors. The availability of the receptors on the cell surface is controlled by several mechanisms. Newly synthesized TβRI and TβRII are delivered from the Golgi apparatus to the cell surface via separate routes. On the cell surface, TGFβ receptors are distributed between different microdomains of the plasma membrane and can be internalized via clathrin- and caveolae-mediated endocytic mechanisms. Although receptor endocytosis is not essential for TGFβ signaling, localization of the activated receptor complexes on the early endosomes promotes TGFβ-induced Smad activation. Caveolae-mediated endocytosis, which is widely regarded as a mechanism that facilitates the degradation of TGFβ receptors, has been shown to be required for TGFβ signaling via non-Smad pathways. The importance of proper control of TGFβ receptor intracellular trafficking is emphasized by clinical data, as mislocalization of receptors has been described in connection with several human diseases. Thus, control of intracellular trafficking of the TGFβ receptors together with the regulation of their expression, posttranslational modifications and down-regulation, ensure proper regulation of TGFβ signaling.

The level of caveolin-1 expression determines response to TGF-β as a tumour suppressor in hepatocellular carcinoma cells

Hepatocellular carcinoma (HCC) is a heterogeneous tumour associated with poor prognostic outcome. Caveolin-1 (CAV1), a membrane protein involved in the formation of caveolae, is frequently overexpressed in HCC. Transforming growth factor-beta (TGF-β) is a pleiotropic cytokine having a dual role in hepatocarcinogenesis: inducer of apoptosis at early phases, but pro-tumourigenic once cells acquire mechanisms to overcome its suppressor effects. Apoptosis induced by TGF-β is mediated by upregulation of the NADPH oxidase NOX4, but counteracted by transactivation of the epidermal growth factor receptor (EGFR) pathway. Previous data suggested that CAV1 is required for the anti-apoptotic signals triggered by TGF-β in hepatocytes. Whether this mechanism is relevant in hepatocarcinogenesis has not been explored yet. Here we analysed the TGF-β response in HCC cell lines that express different levels of CAV1. Accordingly, stable CAV1 knockdown or overexpressing cell lines were generated. We demonstrate that CAV1 is protecting HCC cells from TGF-β-induced apoptosis, which attenuates its suppressive effect on clonogenic growth and increases its effects on cell migration. Downregulation of CAV1 in HLE cells promotes TGF-β-mediated induction of the pro-apoptotic BMF, which correlates with upregulation of NOX4, whereas CAV1 overexpression in Huh7 cells shows the opposite effect. CAV1 silenced HLE cells show attenuation in TGF-β-induced EGFR transactivation and activation of the PI3K/AKT pathway. On the contrary, Huh7 cells, which do not respond to TGF-β activating the EGFR pathway, acquire the capacity to do so when CAV1 is overexpressed. Analyses in samples from HCC patients revealed that tumour tissues presented higher expression levels of CAV1 compared with surrounding non-tumoural areas. Furthermore, a significant positive correlation among the expression of CAV1 and TGFB1 was observed. We conclude that CAV1 has an essential role in switching the response to TGF-β from cytostatic to tumourigenic, which could have clinical meaning in patient stratification.

Dynamin-dependent endocytosis of Bone Morphogenetic Protein2 (BMP2) and its receptors is dispensable for the initiation of Smad signaling

Bone Morphogenetic Protein (BMP) signal transduction via the canonical Smad158 pathway has previously been linked to dynamin-dependent endocytosis, since the application of chemical inhibitors of clathrin or dynamin in functional cell culture based assays negatively affects initiation and propagation of the Smad response. More recent studies, however, demonstrated efficient Smad signaling by non-internalizable BMP2. The role of endocytosis in BMP signal transduction thus remained controversial. In our study we aimed to refine cell biological assays and to apply novel tools, including a new site-directed fluorescently labeled BMP2 ligand, to revisit key steps in BMP Smad signaling. We found that dynamin2 function was required for BMP2 uptake but was dispensable for C-terminal phosphorylation, nuclear translocation and transcriptional activity of BMP-dependent Smads. Furthermore, we demonstrated a role of dynamin2 in the regulation of steady-state and surface BMP receptor levels, as well as an impact on Smad1 protein level. Thus, dynamin2 allows for modulation of basal and ligand-dependent Smad signaling capacity. High levels of functional dynamin2 enhanced the myogenic differentiation of precursor cells. From our study we conclude that dynamin-dependent endocytosis serves as a regulatory mechanism to fine-tune Smad signaling, but it is not a prerequisite for signal initiation and propagation. Our findings contribute to the understanding of fundamental mechanisms of BMP signaling and thus provide important information for future consideration in the context of therapeutic applications of BMPs.

Caveolin-1-dependent activation of the metalloprotease TACE/ADAM17 by TGF-β in hepatocytes requires activation of Src and the NADPH oxidase NOX1

Transforming growth factor-β (TGF-β) plays a dual role in hepatocytes, inducing both pro- and anti-apoptotic responses, the balance between which decides cell fate. Survival signals are mediated by the epidermal growth factor receptor (EGFR) pathway, which is activated by TGF-β. We have previously shown that caveolin-1 (CAV1) is required for activation of the metalloprotease tumour necrosis factor (TNF)-α-converting enzyme/a disintegrin and metalloproteinase 17 (TACE/ADAM17), and hence transactivation of the EGFR pathway. The specific mechanism by which TACE/ADAM17 is activated has not yet been determined. Here we show that TGF-β induces phosphorylation of sarcoma kinase (Src) in hepatocytes, a process that is impaired in Cav1(-/-) hepatocytes, coincident with a decrease in phosphorylated Src in detergent-resistant membrane fractions. TGF-β-induced activation of TACE/ADAM17 and EGFR phosphorylation were blocked using the Src inhibitor PP2. Cav1(+/+) hepatocytes showed early production of reactive oxygen species (ROS) induced by TGF-β, which was not seen in Cav1(-/-) cells. Production of ROS was inhibited by both the NADPH oxidase 1 (NOX1) inhibitor STK301831 and NOX1 knock-down, which also impaired TACE/ADAM17 activation and thus EGFR phosphorylation. Finally, neither STK301831 nor NOX1 silencing impaired Src phosphorylation, but PP2 blocked early ROS production, showing that Src is involved in NOX1 activation. As expected, inhibition of Src or NOX1 increased TGF-β-induced cell death in Cav1(+/+) cells. In conclusion, CAV1 is required for TGF-β-mediated activation of TACE/ADAM17 through a mechanism that involves phosphorylation of Src and NOX1-mediated ROS production.

Endocytosis and trafficking of BMP receptors: Regulatory mechanisms for fine-tuning the signaling response in different cellular contexts

Signaling by bone morphogenetic protein (BMP) receptors is regulated at multiple levels in order to ensure proper interpretation of BMP stimuli in different cellular settings. As with other signaling receptors, regulation of the amount of exposed and signaling-competent BMP receptors at the plasma-membrane is predicted to be a key mechanism in governing their signaling output. Currently, the endocytosis of BMP receptors is thought to resemble that of the structurally related transforming growth factor-β (TGF-β) receptors, as BMP receptors are constitutively internalized (independently of ligand binding), with moderate kinetics, and mostly via clathrin-mediated endocytosis. Also similar to TGF-β receptors, BMP receptors are able to signal from the plasma membrane, while internalization to endosomes may have a signal modulating effect. When at the plasma membrane, BMP receptors localize to different membrane domains including cholesterol rich domains and caveolae, suggesting a complex interplay between membrane distribution and internalization. An additional layer of complexity stems from the putative regulatory influence on the signaling and trafficking of BMP receptors exerted by ligand traps and/or co-receptors. Furthermore, the trafficking and signaling of BMP receptors are subject to alterations in cellular context. For example, genetic diseases involving changes in the expression of auxiliary factors of endocytic pathways hamper retrograde BMP signals in neurons, and perturb the regulation of synapse formation. This review summarizes current understanding of the trafficking of BMP receptors and discusses the role of trafficking in regulation of BMP signals.

ShcA Protects against Epithelial-Mesenchymal Transition through Compartmentalized Inhibition of TGF-β-Induced Smad Activation

Epithelial–mesenchymal transition (EMT) is a normal cell differentiation event during development and contributes pathologically to carcinoma and fibrosis progression. EMT often associates with increased transforming growth factor-β (TGF-β) signaling, and TGF-β drives EMT, in part through Smad-mediated reprogramming of gene expression. TGF-β also activates the Erk MAPK pathway through recruitment and Tyr phosphorylation of the adaptor protein ShcA by the activated TGF-β type I receptor. We found that ShcA protects the epithelial integrity of nontransformed cells against EMT by repressing TGF-β-induced, Smad-mediated gene expression. p52ShcA competed with Smad3 for TGF-β receptor binding, and down-regulation of ShcA expression enhanced autocrine TGF-β/Smad signaling and target gene expression, whereas increased p52ShcA expression resulted in decreased Smad3 binding to the TGF-β receptor, decreased Smad3 activation, and increased Erk MAPK and Akt signaling. Furthermore, p52ShcA sequestered TGF-β receptor complexes to caveolin-associated membrane compartments, and reducing ShcA expression enhanced the receptor localization in clathrin-associated membrane compartments that enable Smad activation. Consequently, silencing ShcA expression induced EMT, with increased cell migration, invasion, and dissemination, and increased stem cell generation and mammosphere formation, dependent upon autocrine TGF-β signaling. These findings position ShcA as a determinant of the epithelial phenotype by repressing TGF-β-induced Smad activation through differential partitioning of receptor complexes at the cell surface.

The adaptor protein ShcA protects epithelial cells from transitioning toward a mesenchymal phenotype by controlling partitioning of the TGF-β receptor and repressing downstream Smad2/3 activation.

TGF-β family proteins control cell differentiation and various cell functions. Increased TGF-β signaling, acting through heteromeric receptor complexes, contributes to carcinoma progression and fibrosis. TGF-β drives epithelial–mesenchymal transdifferentiation (EMT), which enables cell migration and invasion. Upon TGF-β binding, “type I” receptors activate, through phosphorylation, Smad2 and Smad3 that control target gene transcription. In EMT, Smad complexes activate the expression of EMT “master” transcription factors and cooperate with these to repress the epithelial phenotype and activate mesenchymal gene expression. TGF-β receptors also activate Erk MAPK signaling, involving association of the adaptor protein ShcA and Tyr phosphorylation of ShcA by type I receptors. We now show that the predominant ShcA isoform, p52ShcA, competes with Smad2/3 for binding to type I TGF-β receptors, thus repressing Smad2/3 activation in response to TGF-β and localizing the receptors to caveolar compartments. Consequently, decreased ShcA expression enhanced TGF-β receptor localization in clathrin compartments and autocrine Smad2/3 signaling, repressed the epithelial phenotype, and promoted EMT. The changes following decreased ShcA expression resulted in increased cell migration and invasion, as well as increased stem cell generation, dependent upon autocrine TGF-β signaling. These findings position ShcA as a determinant of the epithelial phenotype by repressing TGF-β-induced Smad activation through differential partitioning of receptor complexes at the cell surface.

miR-30c and miR-193 are a part of the TGF-β-dependent regulatory network controlling extracellular matrix genes in liver fibrosis

OBJECTIVE

MicroRNAs (miRNAs) have recently emerged as novel regulators in liver fibrosis. miR-30c and miR-193 are involved in fibrotic remodeling processes and cancer development, respectively. This study aimed to explore the role of miR-30c and miR-193 in liver fibrosis.

METHODS

The regulation of miRNAs in carbon tetrachloride-induced liver fibrosis was analyzed by microarray. Expression patterns of miR-193 and miR-30c were further confirmed in fibrotic liver samples obtained from two murine models of hepatic fibrosis and human tissues. On a functional level, miRNA levels were analyzed in the context of transforming growth factor (TGF-β) mediated activation of hepatic stellate cells (HSCs). Finally, predicted targets were assessed for their roles in fibrosis by transfecting murine HSCs with miRNA mimics.

RESULTS

Microarray analysis in murine fibrotic livers revealed a panel of 44 dysregulated miRNAs. In addition to previously established miRNAs known to be regulated in liver fibrosis in a TGF-β-dependent manner (e.g., miR-29, miR-133), miR-193 and miR-30c were observed to be specifically downregulated not only in experimental hepatofibrogenesis but also in human liver fibrosis, while they showed a reciprocal expression pattern after recovery from liver fibrosis. Functional experiments confirmed the TGF-β-dependent downregulation of these respective new miRNAs in HSCs. Finally, we identified TGF-β2 and SNAIL1, important regulators of extracellular matrix, as potential target genes of miR-193 and miR-30 in liver fibrosis.

CONCLUSION

These results suggest that miR-30 and miR-193 are members of a network of miRNAs modifying the TGF-β-dependent regulation of extracellular matrix-related genes in HSCs in the manifestation and resolution of liver fibrosis.

Hepatocyte fate upon TGF-β challenge is determined by the matrix environment

Primary hepatocytes are a versatile tool to investigate all aspects of liver function, and frequently used in drug development and testing. Upon TGF-β challenge, hepatocytes either undergo an epithelial to mesenchymal transition (EMT) or apoptosis: culture on stiff collagen (monolayer) results in EMT whereas hepatocytes in a soft collagen matrix (sandwich) undergo programmed cell death. In this study, we analyzed the transcriptional programs triggered by TGF-β under different culture conditions. Our results indicate that TGF-β initiates an identical transcription profile in hepatocytes irrespective of the cellular environment. The fact that we nevertheless observe two vastly different phenotypes indicates that the matrix environment rather than the TGF-β induced expression signature is the major determinant of the hepatocellular response. To confirm the impact of the surrounding matrix environment on the hepatocytes׳ phenotype in response to TGF-β signaling, we studied the effect of Snail overexpression and knockout in both culture conditions. Neither genetic manipulation showed an impact on hepatocytes' fate upon TGF-β treatment, confirming the crucial role of the surrounding matrix. Our findings provide novel insights into the hepatocellular basis of the fate decision between EMT and apoptotic cell death, and might explain why liver cells can react in very different manners to identical stimuli when tissue remodeling has changed the matrix environment, as occurs in a fibrotic liver.

TGF-β Induces Degradation of PTHrP Through Ubiquitin-Proteasome System in Hepatocellular Carcinoma

Both transforming growth factor-β (TGF-β) and parathyroid hormone-related protein (PTHrP) regulate important cellular processes, such as apoptosis in the development of hepatocellular carcinoma. However, the mechanisms of regulation of PTHrP by TGF-β are largely unknown. We hypothesized that TGF-β regulates the expression of PTHrP protein through a post-translational mechanism. Using hepatocellular carcinoma cell lines as the in vitro model, we investigated the effects of TGF-β on protein expression and post-translational processing of PTHrP. We found that TGF-β treatment led to protein degradation of PTHrP through the ubiquitin-proteasome-dependent pathway. We also provided evidence to show that Smurf2 was the E3 ligase responsible for the ubiquitination of PTHrP. Furthermore, using immunohistochemistry on human hepatocellular carcinoma specimens and a tissue array, we found that the expression of PTHrP was predominantly in the cancer cells, whereas the expression of TGF-β was present in non-neoplastic liver tissue adjacent to hepatocellular carcinoma. Our findings reveal a novel mechanism whereby TGF-β may regulate PTHrP in hepatocellular carcinogenesis and lack of TGF-β in hepatocellular carcinoma may promote cancer progression. Promotion of PTHrP degradation provides a novel target of therapeutic intervention to sensitize hepatocellular carcinoma cells to cytostatic and/or pro-apoptotic signals.

Nitric oxide and carbon monoxide antagonize TGF-β through ligand-independent internalization of TβR1/ALK5

Transforming growth factor (TGF)-β plays a central role in vascular homeostasis and in the pathology of vascular disease. There is a growing appreciation for the role of nitric oxide (NO) and carbon monoxide (CO) as highly diffusible, bioactive signaling molecules in the vasculature. We hypothesized that both NO and CO increase endocytosis of TGF-β receptor type 1 (TβR1) in vascular smooth muscle cells (VSMCs) through activation of dynamin-2, shielding cells from the effects of circulating TGF-β. In this study, primary cultures of VSMCs from Sprague-Dawley rats were treated with NO-releasing molecule 3 (a NO chemical donor), CO-releasing molecule 2 (a CO chemical donor), or control. NO and CO stimulated dynamin-2 activation in VSMCs. NO and CO promoted time- and dose-dependent endocytosis of TβR1. By decreasing TβR1 surface expression through this dynamin-2-dependent process, NO and CO diminished the effects of TGF-β on VSMCs. These findings help explain an important mechanism by which NO and CO signal in the vasculature by decreasing surface expression of TβR1 and the cellular response to TGF-β.

Caveolin-1 is required for TGF-β-induced transactivation of the EGF receptor pathway in hepatocytes through the activation of the metalloprotease TACE/ADAM17

Transforming growth factor-beta (TGF-β) plays a dual role in hepatocytes, inducing both pro- and anti-apoptotic responses, whose balance decides cell fate. Survival signals are mediated by the epidermal growth factor receptor (EGFR) pathway, which is activated by TGF-β in these cells. Caveolin-1 (Cav1) is a structural protein of caveolae linked to TGF-β receptors trafficking and signaling. Previous results have indicated that in hepatocytes, Cav1 is required for TGF-β-induced anti-apoptotic signals, but the molecular mechanism is not fully understood yet. In this work, we show that immortalized Cav1(-/-) hepatocytes were more sensitive to the pro-apoptotic effects induced by TGF-β, showing a higher activation of caspase-3, higher decrease in cell viability and prolonged increase through time of intracellular reactive oxygen species (ROS). These results were coincident with attenuation of TGF-β-induced survival signals in Cav1(-/-) hepatocytes, such as AKT and ERK1/2 phosphorylation and NFκ-B activation. Transactivation of the EGFR pathway by TGF-β was impaired in Cav1(-/-) hepatocytes, which correlated with lack of activation of TACE/ADAM17, the metalloprotease responsible for the shedding of EGFR ligands. Reconstitution of Cav1 in Cav1(-/-) hepatocytes rescued wild-type phenotype features, both in terms of EGFR transactivation and TACE/ADAM17 activation. TACE/ADAM17 was localized in detergent-resistant membrane (DRM) fractions in Cav1(+/+) cells, which was not the case in Cav1(-/-) cells. Disorganization of lipid rafts after treatment with cholesterol-binding agents caused loss of TACE/ADAM17 activation after TGF-β treatment. In conclusion, in hepatocytes, Cav1 is required for TGF-β-mediated activation of the metalloprotease TACE/ADAM17 that is responsible for shedding of EGFR ligands and activation of the EGFR pathway, which counteracts the TGF-β pro-apoptotic effects. Therefore, Cav1 contributes to the pro-tumorigenic effects of TGF-β in liver cancer cells.

Significance of expression of Smad ubiquitination regulatory factors in liver fibrosis

AIM: To observe the expression of Smad ubiquitination regulatory factor (Smurf)1, Smurf2, Smad3 and Smad7 proteins in liver fibrosis and to evaluate their interactions.

METHODS: Immunohistochemistry was applied to detect the expression of Smurf1, Smurf2, Smad3 and Smad7 in 9 normal liver tissue samples and 38 chronic HBV infection tissue samples.

RESULTS: Smurf1, Smurf2, Smad3 and Smad7 showed widespread expression in the liver parenchymal cells and nonparenchymal cells. Compared with normal liver tissue, the positive rates of Smad3 and Smurf2 expression increased significantly (66.7% vs 100%, 66.7% vs 92.1%, P < 0.01 for both) and the positive rate of Smad7 expression decreased significantly in liver fibrosis (77.8% vs 39.5%, P < 0.01), although the positive rate of Smurf1 expression had no significant change (77.8% vs 63.2%, P > 0.05). There were significant positive correlations between Smad3 and Smurf2 expression and the degree of fibrosis (P < 0.01 for both). Smad7 expression was negatively correlated with the degree of fibrosis (P < 0.01). There was no significant correlation between Smurf1 expression and liver fibrosis (P > 0.05). Smurf2 expression was positively correlated with Smad3 expression (P < 0.01) and negatively with Smad7 expression (P < 0.01). There was no significant relationship between Smurf1 and Smurf2, Smad3, Smad7 expression (P > 0.05 for all). There was a negative correlation between expression of Smad3 and that of Smad7 (P < 0.01).

CONCLUSION: The increase of Smad3 signal and decrease of Smad7 signal may lead to the development of liver fibrosis, and Smurf2 may play a bidirectionally regulatory role in the progression of liver fibrosis.

Dab2 inhibits the cholesterol-dependent activation of JNK by TGF-β

TGF-β signals through Smad-dependent and non-Smad pathways, depending on cell context. In ovarian cancer cells, the clathrin adaptor Dab2 enhances internalization of the type I TGF-β receptor, restricts its lateral mobility, and inhibits TGF-β–mediated, cholesterol-dependent JNK activation.

Transforming growth factor-β (TGF-β) ligands activate Smad-mediated and noncanonical signaling pathways in a cell context–dependent manner. Localization of signaling receptors to distinct membrane domains is a potential source of signaling output diversity. The tumor suppressor/endocytic adaptor protein disabled-2 (Dab2) was proposed as a modulator of TGF-β signaling. However, the molecular mechanism(s) involved in the regulation of TGF-β signaling by Dab2 were not known. Here we investigate these issues by combining biophysical studies of the lateral mobility and endocytosis of the type I TGF-β receptor (TβRI) with TGF-β phosphoprotein signaling assays. Our findings demonstrate that Dab2 interacts with TβRI to restrict its lateral diffusion at the plasma membrane and enhance its clathrin-mediated endocytosis. Small interfering RNA–mediated knockdown of Dab2 or Dab2 overexpression shows that Dab2 negatively regulates TGF-β–induced c-Jun N-terminal kinase (JNK) activation, whereas activation of the Smad pathway is unaffected. Moreover, activation of JNK by TGF-β in the absence of Dab2 is disrupted by cholesterol depletion. These data support a model in which Dab2 regulates the domain localization of TβRI in the membrane, balancing TGF-β signaling via the Smad and JNK pathways.

A systems toxicology approach to the surface functionality control of graphene-cell interactions

The raised considerable concerns about the possible environmental health and safety impacts of graphene nanomaterials and their derivatives originated from their potential widespread applications. We performed a comprehensive study about biological interaction of grapheme nanomaterials, specifically in regard to its differential surface functionalization (oxidation status), by using OMICS in graphene oxide (GO) and reduced graphene oxide (rGO) treated HepG2 cells. Differential surface chemistry (particularly, oxidation - O/C ratio) modulates hydrophobicity/philicity of GO/rGO which in turn governs their biological interaction potentiality. Similar toxic responses (cytotoxicity, DNA damage, oxidative stress) with differential dose dependency were observed for both GO and rGO but they exhibited distinct mechanism, such as, hydrophilic GO showed cellular uptake, NADPH oxidase dependent ROS formation, high deregulation of antioxidant/DNA repair/apoptosis related genes, conversely, hydrophobic rGO was found to mostly adsorbed at cell surface without internalization, ROS generation by physical interaction, poor gene regulation etc. Global gene expression and pathway analysis displayed that TGFβ1 mediated signaling played the central role in GO induced biological/toxicological effect whereas rGO might elicited host-pathogen (viral) interaction and innate immune response through TLR4-NFkB pathway. In brief, the distinct biological and molecular mechanisms of GO/rGO were attributed to their differential surface oxidation status.

Comparative analysis of TGF-β/Smad signaling dependent cytostasis in human hepatocellular carcinoma cell lines

Hepatocellular carcinoma (HCC) is a major public health problem due to increased incidence, late diagnosis and limited treatment options. TGF-β is known to provide cytostatic signals during early stages of liver damage and regeneration, but exerts tumor promoting effects in onset and progression of liver cancer. To understand the mechanistic background of such a switch, we systematically correlated loss of cytostatic TGF-β effects with strength and dynamics of its downstream signaling in 10 HCC cell lines. We demonstrate that TGF-β inhibits proliferation and induces apoptosis in cell lines with low endogenous levels of TGF-β and Smad7 and strong transcriptional Smad3 activity (PLC/PRF/5, HepG2, Hep3B, HuH7), previously characterized to express early TGF-β signatures correlated with better outcome in HCC patients. TGF-β dependent cytostasis is blunted in another group of cell lines (HLE, HLF, FLC-4) expressing high amounts of TGF-β and Smad7 and showing significantly reduced Smad3 signaling. Of those, HLE and HLF exhibit late TGF-β signatures, which is associated with bad prognosis in HCC patients. RNAi with Smad3 blunted cytostatic effects in PLC/PRF/5, Hep3B and HuH7. HCC-M and HCC-T represent a third group of cell lines lacking cytostatic TGF-β signaling despite strong and prolonged Smad3 phosphorylation and low Smad7 and TGF-β expression. Inhibitory linker phosphorylation, as in HCC-T, may disrupt C-terminally phosphorylated Smad3 function. In summary, we assort 10 HCC cell lines in at least two clusters with respect to TGF-β sensitivity. Cell lines responsive to the TGF-β cytostatic program, which recapitulate early stage of liver carcinogenesis exhibit transcriptional Smad3 activity. Those with disturbed TGF-β/Smad3 signaling are insensitive to TGF-β dependent cytostasis and might represent late stage of the disease. Regulation of this switch remains complex and cell line specific. These features may be relevant to discriminate stage dependent TGF-β functions for the design of efficient TGF-β directed therapy in liver cancer.

The less-often-traveled surface of stem cells: caveolin-1 and caveolae in stem cells, tissue repair and regeneration

Stem cells are an important resource for tissue repair and regeneration. While a great deal of attention has focused on derivation and molecular regulation of stem cells, relatively little research has focused on how the subcellular structure and composition of the cell membrane influences stem cell activities such as proliferation, differentiation and homing. Caveolae are specialized membrane lipid rafts coated with caveolin scaffolding proteins, which can regulate cholesterol transport and the activity of cell signaling receptors and their downstream effectors. Caveolin-1 is involved in the regulation of many cellular processes, including growth, control of mitochondrial antioxidant levels, migration and senescence. These activities are of relevance to stem cell biology, and in this review evidence for caveolin-1 involvement in stem cell biology is summarized. Altered stem and progenitor cell populations in caveolin-1 null mice suggest that caveolin-1 can regulate stem cell proliferation, and in vitro studies with isolated stem cells suggest that caveolin-1 regulates stem cell differentiation. The available evidence leads us to hypothesize that caveolin-1 expression may stabilize the differentiated and undifferentiated stem cell phenotype, and transient downregulation of caveolin-1 expression may be required for transition between the two. Such regulation would probably be critical in regenerative applications of adult stem cells and during tissue regeneration. We also review here the temporal changes in caveolin-1 expression reported during tissue repair. Delayed muscle regeneration in transgenic mice overexpressing caveolin-1 as well as compromised cardiac, brain and liver tissue repair and delayed wound healing in caveolin-1 null mice suggest that caveolin-1 plays an important role in tissue repair, but that this role may be negative or positive depending on the tissue type and the nature of the repair process. Finally, we also discuss how caveolin-1 quiescence-inducing activities and effects on mitochondrial antioxidant levels may influence stem cell aging.

Transmembrane transport of the Gαq protein carboxyl terminus imitation polypeptide GCIP-27

The Gαq protein carboxyl terminus imitation polypeptide (GCIP)-27 has been shown to affect cardiac hypertrophy and vascular remodeling in various models both in vitro and in vivo. Transport across the plasma membrane is a critical step in regulating the action of this peptide drug. This study was designed to explore the mechanisms underlying the transmembrane transport of GCIP-27. The peptide drug was labeled with fluorescein isothiocyanate (FITC), and measured in a time- and concentration-dependent manner using laser confocal microscopy. Various transport inhibitors, including energy and endocytosis inhibitors, were used to identify the factors that regulate its transmembrane transport. GCIP-27 transport was examined in cardiomyocytes, cardiac fibroblasts, vascular endothelial cells, vascular smooth muscle cells (VSMCs) and hepatocytes. Atomic force microscopy and scanning electron microscopy were used to determine the ultrastructure of the cardiomyocyte membranes. The results showed that GCIP-27 was transported through the plasmalemma in a time- and concentration-dependent manner. The rate of uptake and the level of GCIP-27 in the cells decreased significantly after treatment with energy inhibitors, methyl-ß-cyclodextrin chlorpromazine or heparin. GCIP-27 levels in VSMCs and cardiomyocytes were significantly greater than the levels observed in hepatocytes, cardiac fibroblasts and vascular endothelial cells. Treatment with GCIP-27 led to a marked increase in the surface roughness of the cellular membrane. In conclusion, the transmembrane transport of GCIP-27 is mediated by endocytosis, which requires energy, and GCIP-27 preferentially enters myocardial cells and VSMCs.

Distinct dedifferentiation processes affect caveolin-1 expression in hepatocytes

BACKGROUND

Dedifferentiation and loss of hepatocyte polarity during primary culture of hepatocytes are major drawbacks for metabolic analyses. As a prominent profibrotic cytokine and potent inducer of epithelial mesenchymal transition (EMT), TGF-β contributes to these processes in liver epithelial cells. Yet, a distinction between culture dependent and TGF-β driven hepatocyte dedifferentiation has not been shown to date.

RESULTS

Here, we show that in both settings, mesenchymal markers are induced. However, upregulation of Snai1 and downregulation of E-Cadherin are restricted to TGF-β effects, neglecting a full EMT of culture dependent hepatocyte dedifferentiation. Mechanistically, the latter is mediated via FAK/Src/ERK/AKT pathways leading to the induction of the oncogene caveolin-1 (Cav1). Cav1 was recently proposed as a new EMT marker, but our results demonstrate Cav1 is not up-regulated in TGF-β mediated hepatocyte EMT, thus limiting validity of its use for this purpose. Importantly, marking differences on Cav1 expression exist in HCC cell lines. Whereas well differentiated HCC cell lines exhibit low and inducible Cav1 protein levels - by TGF-β in a FAK/Src dependent manner, poorly differentiated cell lines display high Cav1 expression levels which are not further modulated by TGF-β.

CONCLUSIONS

This study draws a detailed distinction between intrinsic and TGF-β mediated hepatocyte dedifferentiation and elucidates cellular pathways involved. Additionally, by evaluating the regulation of the oncogene Cav1, we provide evidence to argue against Cav1 as a reliable EMT marker.

Caveolin-1 abrogates TGF-β mediated hepatocyte apoptosis

Transforming growth factor (TGF)-β has a dual role in liver, providing cytostatic effects during liver damage and regeneration, as well as carcinogenic functions in malignant transformation and hepatocellular cancer. In cultured hepatocytes, TGF-β can trigger apoptosis and epithelial-mesenchymal transition (EMT). Caveolin-1 is associated with progression of hepatocellular cancer and has been linked to TGF-β signaling. This study aimed at elucidating whether Caveolin-1 regulates TGF-β mediated hepatocyte fate. Knockdown of Caveolin-1 strongly reduced TGF-β mediated AKT phosphorylation, thus sensitized primary murine hepatocytes for proapoptotic TGF-β signaling. Restoration of AKT activity in Caveolin-1 knockdown cells via expression of a constitutive active AKT mutant did not completely blunt the apoptotic response to TGF-β, indicating an additional mechanism how Caveolin-1 primes hepatocytes for resistance to TGF-β triggered apoptosis. On the molecular level, Caveolin-1 interfered with TGF-β initiated expression of the proapoptotic mediator BIM. Additionally, RNAi for Caveolin-1 reduced (and its overexpression increased) expression of antiapoptotic mediators BCL-2 and BCL-xl. Noteworthy, reduced Caveolin-1 protein levels had no effect on collagen 1α1, E- and N-cadherin expression upon TGF-β challenge and thus no effect on hepatocyte EMT. Hence, via affecting TGF-β mediated non-Smad AKT signaling and regulation of pro- and antiapoptotic factors, Caveolin-1 is a crucial hepatocyte fate determinant for TGF-β effects.

Elevated expression of cav-1 in a subset of SSc fibroblasts contributes to constitutive Alk1/Smad1 activation

Previous studies have shown that the transforming growth factor (TGF)β/Alk1/Smad1 signaling pathway is constitutively activated in a subset of systemic sclerosis (SSc) fibroblasts and this pathway is a critical regulator of CCN2 gene expression. Caveolin-1 (cav-1), an integral membrane protein and the main component of caveolae, has also been implicated in SSc pathogenesis. This study was undertaken to evaluate the role of caveolin-1 in Smad1 signaling and CCN2 expression in healthy and SSc dermal fibroblasts. We show that a significant subset of SSc dermal fibroblasts has up-regulated cav-1 expression in vitro, and that cav-1 up-regulation correlates with constitutive Smad1 phosphorylation. In addition, basal levels of phospho-Smad1 were down-regulated after inhibition of cav-1 in SSc dermal fibroblasts. Caveolin-1 formed a protein complex with Alk1 in dermal fibroblasts, and this association was enhanced by TGFβ. By using siRNA against cav-1 and adenoviral cav-1 overexpression we demonstrate that activation of Smad1 in response to TGFβ requires cav-1 and that cav-1 is sufficient for Smad-1 phosphorylation. We also show that cav-1 is a positive regulator of CCN2 gene expression, and that it is required for the basal and TGFβ-induced CCN2 levels. In conclusion, this study has revealed an important role of cav-1 in mediating TGFβ/Smad1 signaling and CCN2 gene expression in healthy and SSc dermal fibroblasts.

Comparative analysis of TGF-β/Smad signaling dependent cytostasis in human hepatocellular carcinoma cell lines

Hepatocellular carcinoma (HCC) is a major public health problem due to increased incidence, late diagnosis and limited treatment options. TGF-β is known to provide cytostatic signals during early stages of liver damage and regeneration, but exerts tumor promoting effects in onset and progression of liver cancer. To understand the mechanistic background of such a switch, we systematically correlated loss of cytostatic TGF-β effects with strength and dynamics of its downstream signaling in 10 HCC cell lines. We demonstrate that TGF-β inhibits proliferation and induces apoptosis in cell lines with low endogenous levels of TGF-β and Smad7 and strong transcriptional Smad3 activity (PLC/PRF/5, HepG2, Hep3B, HuH7), previously characterized to express early TGF-β signatures correlated with better outcome in HCC patients. TGF-β dependent cytostasis is blunted in another group of cell lines (HLE, HLF, FLC-4) expressing high amounts of TGF-β and Smad7 and showing significantly reduced Smad3 signaling. Of those, HLE and HLF exhibit late TGF-β signatures, which is associated with bad prognosis in HCC patients. RNAi with Smad3 blunted cytostatic effects in PLC/PRF/5, Hep3B and HuH7. HCC-M and HCC-T represent a third group of cell lines lacking cytostatic TGF-β signaling despite strong and prolonged Smad3 phosphorylation and low Smad7 and TGF-β expression. Inhibitory linker phosphorylation, as in HCC-T, may disrupt C-terminally phosphorylated Smad3 function. In summary, we assort 10 HCC cell lines in at least two clusters with respect to TGF-β sensitivity. Cell lines responsive to the TGF-β cytostatic program, which recapitulate early stage of liver carcinogenesis exhibit transcriptional Smad3 activity. Those with disturbed TGF-β/Smad3 signaling are insensitive to TGF-β dependent cytostasis and might represent late stage of the disease. Regulation of this switch remains complex and cell line specific. These features may be relevant to discriminate stage dependent TGF-β functions for the design of efficient TGF-β directed therapy in liver cancer.

A dual AP-1 and SMAD decoy ODN suppresses tissue fibrosis and scarring in mice

The transforming growth factor-β (TGF-β) signaling pathway promotes tissue fibrosis and scarring through SMAD (small mothers against decapentaplegic)-dependent and SMAD-independent mechanisms. However, inhibition of SMAD-mediated signal transduction alone induces an excessive inflammatory response that impairs the antifibrotic effects of TGF-β inhibitors. In this study, we designed and characterized a dual-functional transcription activator protein 1 (AP-1) and SMAD decoy oligodeoxynucleotide, antifibrosis oligodeoxynucleotide 4 (AFODN4) in vitro and in vivo. AFODN4 binds directly to recombinant AP-1 and SMAD with high affinity. AFODN4 significantly inhibited the DNA-binding and transcriptional activities of both AP-1 and SMAD, as well as the production of fibrotic mediators stimulated by TGF-β1 or TGF-β2 in L929 murine fibroblasts. Local administration of AFODN4 significantly inhibited fibrosis associated with acute dermal wounds in mice. Intriguingly, AFODN4 inhibited AP-1-mediated production of proinflammatory mediators, which can be caused by blockage of SMAD alone in vitro and in vivo. Collectively, these findings suggest that dual inhibition of SMAD and AP-1 signaling by AFODN4 is a useful strategy for the development of new antifibrotic agents.

TGF-β signaling in onset and progression of hepatocellular carcinoma

Transforming growth factor (TGF)-β is a central regulator in chronic liver disease, contributing to all stages of disease progression from initial liver injury through inflammation and fibrosis to cirrhosis and hepatocellular carcinoma. Liver damage-induced levels of active TGF-β enhance hepatocyte destruction and mediate hepatic stellate cell and fibroblast activation resulting in a wound-healing response, including myofibroblast generation and extracellular matrix deposition. Further evidence points to a decisive role of cytostatic and apoptotic functions mediated on hepatocytes, which is critical for the control of liver mass, with loss of TGF-β activities resulting in hyperproliferative disorders and cancer. This concept is based on studies that describe a bipartite role of TGF-β with tumor suppressor functions at early stages of liver damage and regeneration, whereas during cancer progression TGF-β may turn from a tumor suppressor into a tumor promoter that exacerbates invasive and metastatic behavior. We have delineated this molecular switch of the pathway from cytostatic to tumor promoting in further detail and identify activation of survival signaling pathways in hepatocytes as a most critical requirement. Targeting the TGF-β signaling pathway has been explored to inhibit liver disease progression. While interfering with TGF-β signaling in various short-term animal models has demonstrated promising results, liver disease progression in humans is a process of decades with different phases in which TGF-β or its targeting may have both beneficial and adverse outcomes. We emphasize that, in order to achieve therapeutic effects, targeting TGF-β signaling in the right cell type at the right time is required.

Differential regulation of Smad3 and of the type II transforming growth factor-β receptor in mitosis: implications for signaling

The response to transforming growth factor-β (TGF-β) depends on cellular context. This context is changed in mitosis through selective inhibition of vesicle trafficking, reduction in cell volume and the activation of mitotic kinases. We hypothesized that these alterations in cell context may induce a differential regulation of Smads and TGF-β receptors. We tested this hypothesis in mesenchymal-like ovarian cancer cells, arrested (or not) in mitosis with 2-methoxyestradiol (2ME2). In mitosis, without TGF-β stimulation, Smad3 was phosphorylated at the C-terminus and linker regions and localized to the mitotic spindle. Phosphorylated Smad3 interacted with the negative regulators of Smad signaling, Smurf2 and Ski, and failed to induce a transcriptional response. Moreover, in cells arrested in mitosis, Smad3 levels were progressively reduced. These phosphorylations and reduction in the levels of Smad3 depended on ERK activation and Mps1 kinase activity, and were abrogated by increasing the volume of cells arrested in mitosis with hypotonic medium. Furthermore, an Mps1-dependent phosphorylation of GFP-Smad3 was also observed upon its over-expression in interphase cells, suggesting a mechanism of negative regulation which counters increases in Smad3 concentration. Arrest in mitosis also induced a block in the clathrin-mediated endocytosis of the type II TGF-β receptor (TβRII). Moreover, following the stimulation of mitotic cells with TGF-β, the proteasome-mediated attenuation of TGF-β receptor activity, the degradation and clearance of TβRII from the plasma membrane, and the clearance of the TGF-β ligand from the medium were compromised, and the C-terminus phosphorylation of Smad3 was prolonged. We propose that the reduction in Smad3 levels, its linker phosphorylation, and its association with negative regulators (observed in mitosis prior to ligand stimulation) represent a signal attenuating mechanism. This mechanism is balanced by the retention of active TGF-β receptors at the plasma membrane. Together, both mechanisms allow for a regulated cellular response to TGF-β stimuli in mitosis.

Dynamics of TGF-β/Smad signaling

The physiological responses to TGF-β stimulation are diverse and vary amongst different cell types and environmental conditions. Even though the principal molecular components of the canonical and the non-canonical TGF-β signaling pathways have been largely identified, the mechanism that underlies the well-established context dependent physiological responses remains a mystery. Understanding how the components of TGF-β signaling function as a system and how this system functions in the context of the global cellular regulatory network requires a more quantitative and systematic approach. Here, we review the recent progress in understanding TGF-β biology using integration of mathematical modeling and quantitative experimental analysis. These studies reveal many interesting dynamics of TGF-β signaling and how cells quantitatively decode variable doses of TGF-β stimulation.

Coated pit-mediated endocytosis of the type I transforming growth factor-β (TGF-β) receptor depends on a di-leucine family signal and is not required for signaling

The roles of transforming growth factor-β (TGF-β) receptor endocytosis in signaling have been investigated in numerous studies, mainly through the use of endocytosis inhibitory treatments, yielding conflicting results. Two potential sources for these discrepancies were the pleiotropic effects of a general blockade of specific internalization pathways and the scarce information on the regulation of the endocytosis of the signal-transducing type I TGF-β receptor (TβRI). Here, we employed extracellularly tagged myc-TβRI (wild type, truncation mutants, and a series of endocytosis-defective and endocytosis-enhanced mutants) to directly investigate the relationship between TβRI endocytosis and signaling. Our findings indicate that TβRI is targeted for constitutive clathrin-mediated endocytosis via a di-leucine (Leu(180)-Ile(181)) signal and an acidic cluster motif. Using Smad-dependent transcriptional activation assays and following Smad2/3 nuclear translocation in response to TGF-β stimulation, we show that TβRI endocytosis is dispensable for TGF-β signaling and may play a role in signal termination. Alanine replacement of Leu(180)-Ile(181) led to partial constitutive activation of TβRI, resulting in part from its retention at the plasma membrane and in part from potential alterations of TβRI regulatory interactions in the vicinity of the mutated residues.

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.