TGFβ signaling and cardiovascular diseases

Transforming growth factor β (TGFβ) family members are involved in a wide range of diverse functions and play key roles in embryogenesis, development and tissue homeostasis. Perturbation of TGFβ signaling may lead to vascular and other diseases. In vitro studies have provided evidence that TGFβ family members have a wide range of diverse effects on vascular cells, which are highly dependent on cellular context. Consistent with these observations genetic studies in mice and humans showed that TGFβ family members have ambiguous effects on the function of the cardiovascular system. In this review we discuss the recent advances on TGFβ signaling in (cardio)vascular diseases, and describe the value of TGFβ signaling as both a disease marker and therapeutic target for (cardio)vascular diseases.     

IFN-α/β and autophagy

Hepatitis C virus (HCV) infects approximately 130 million people worldwide. The clinical sequelae of this chronic disease include cirrhosis, functional failure and carcinoma of the liver. HCV induces autophagy, a fundamental cellular process for maintaining homeostasis and mediating innate immune response, and also inhibits autophagic protein degradation and suppresses antiviral immunity. In addition to this ploy, the HCV serine protease composed of the viral non-structural proteins 3/4A (NS3/4A) can enzymatically digest two cellular proteins, mitochondria-associated anti-viral signaling protein (MAVS) and Toll/interleukin-1 receptor domain containing adaptor inducing IFN-β (TRIF). Since these two proteins are the adaptor molecules in the retinoic acid-inducible gene I (RIG-I) and TLR3 pathways, respectively, their cleavage has been suggested as a pivotal mechanism by which HCV blunts the IFN-α/β signaling and antiviral responses. Thus far, how HCV perturbs autophagy and copes with IFN-α/β in the liver remains unclear.     

RIG-I and dsRNA-Induced IFNβ Activation

Except for viruses that initiate RNA synthesis with a protein primer (e.g., picornaviruses), most RNA viruses initiate RNA synthesis with an NTP, and at least some of their viral _pppRNAs remain unblocked during the infection. Consistent with this, most viruses require RIG-I to mount an innate immune response, whereas picornaviruses require mda-5. We have examined a SeV infection whose ability to induce interferon depends on the generation of capped dsRNA (without free 5′ tri-phosphate ends), and found that this infection as well requires RIG-I and not mda-5. We also provide evidence that RIG-I interacts with poly-I/C in vivo, and that heteropolymeric dsRNA and poly-I/C interact directly with RIG-I in vitro, but in different ways; i.e., poly-I/C has the unique ability to stimulate the helicase ATPase of RIG-I variants which lack the C-terminal regulatory domain.     

Mitochondrial import and degradation of amyloid-β peptide

Mitochondrial dysfunctions associated with amyloid-β peptide (Aβ) accumulation in mitochondria have been observed in Alzheimer's disease (AD) patients' brains and in AD mice models. Aβ is produced by sequential action of β- and γ-secretases cleaving the amyloid precursor protein (APP). The γ-secretase complex was found in mitochondria-associated endoplasmic reticulum membranes (MAM) suggesting that this could be a potential site of Aβ production, from which Aβ is further transported into the mitochondria. In vitro, Aβ was shown to be imported into the mitochondria through the translocase of the outer membrane (TOM) complex. The mitochondrial presequence protease (PreP) is responsible for Aβ degradation reducing toxic effects of Aβ on mitochondrial functions. The proteolytic activity of PreP is, however, lower in AD brain temporal lobe mitochondria and in AD transgenic mice models, possibly due to an increased reactive oxygen species (ROS) production. Here, we review the intracellular mechanisms of Aβ production, its mitochondrial import and the intra-mitochondrial degradation. We also discuss the implications of a reduced efficiency of mitochondrial Aβ clearance for AD. Understanding the underlying mechanisms may provide new insights into mitochondria related pathogenesis of AD and development of drug therapy against AD. This article is part of a Special Issue entitled: 18th European Bioenergetic Conference.     

Structure and Dynamics of Amyloid-β Segmental Polymorphisms

It is believed that amyloid-beta (Aβ) aggregates play a role in the pathogenesis of Alzheimer's disease. Aβ molecules form β-sheet structures with multiple interaction sites. This polymorphism gives rise to differences in morphology, physico-chemical property and level of cellular toxicity. We have investigated the conformational stability of various segmental polymorphisms using molecular dynamics simulations and find that the segmental polymorphic models of Aβ retain a U-shaped architecture. Our results demonstrate the importance of inter-sheet side chain-side chain contacts, hydrophobic contacts among the strands (β1 and β2) and of salt bridges in stabilizing the aggregates. Residues in β-sheet regions have smaller fluctuation while those at the edge and loop region are more mobile. The inter-peptide salt bridges between Asp23 and Lys28 are strong compared to intra-chain salt bridge and there is an exchange of the inter-chain salt-bridge with intra-chain salt bridge. As our results suggest that Aβ exists under physiological conditions as an ensemble of distinct segmental polymorphs, it may be necessary to account in the development of therapeutics for Alzheimer's disease the differences in structural stability and aggregation behavior of the various Aβ polymorphic forms.     

TGF-β and BMP signaling in osteoblast differentiation and bone formation

Transforming growth factor-beta (TGF-β)/bone morphogenic protein (BMP) signaling is involved in a vast majority of cellular processes and is fundamentally important throughout life. TGF-β/BMPs have widely recognized roles in bone formation during mammalian development and exhibit versatile regulatory functions in the body. Signaling transduction by TGF-β/BMPs is specifically through both canonical Smad-dependent pathways (TGF-β/BMP ligands, receptors and Smads) and non-canonical Smad-independent signaling pathway (e.g. p38 mitogen-activated protein kinase pathway, MAPK). Following TGF-β/BMP induction, both the Smad and p38 MAPK pathways converge at the Runx2 gene to control mesenchymal precursor cell differentiation. The coordinated activity of Runx2 and TGF-β/BMP-activated Smads is critical for formation of the skeleton. Recent advances in molecular and genetic studies using gene targeting in mice enable a better understanding of TGF-β/BMP signaling in bone and in the signaling networks underlying osteoblast differentiation and bone formation. This review summarizes the recent advances in our understanding of TGF-β/BMP signaling in bone from studies of genetic mouse models and human diseases caused by the disruption of TGF-β/BMP signaling. This review also highlights the different modes of cross-talk between TGF-β/BMP signaling and the signaling pathways of MAPK, Wnt, Hedgehog, Notch, and FGF in osteoblast differentiation and bone formation.     

Structural and mechanistic commonalities of amyloid-β and the prion protein

Amyloid beta (Aβ) is a major causative agent of Alzheimer disease (AD). This neurotoxic peptide is generated as a result of the cleavage of the Amyloid-Precursor-Protein (APP) by the action of β-secretase and γ-secretase. The neurotoxicity was previously thought to be the result of aggregation. However, recent studies suggest that the interaction of Aβ with numerous cell surface receptors such as N-methyl-D-aspartate (NMDA), receptor for advanced glycosylation end products (RAGE), P75 neurotrophin receptor (P75^NTR) as well as cell surface proteins such as the cellular prion protein (PrP^c) and heparan sulfate proteoglycans (HSPG) strongly enhances Aβ induced apoptosis and thereby contributes to neurotoxicity. This review focuses on the molecular mechanism resulting in Aβ-shedding as well as Aβ-induced apoptotic processes, genetic risk factors for familial AD and interactions of Aβ with cell surface receptors and proteins, with particular emphasis on the cellular prion protein. Furthermore, comparisons are drawn between AD and prion disorders and the role of laminin, an extracellular matrix protein, glycosaminoglycans and the 37 kDa/67 kDa laminin receptor (LRP/LR) have been highlighted with regards to both neurodegenerative diseases.Key words: Alzheimer disease, amyloid β, apoptosis, 37 kDa/67 kDa laminin receptor, prion proteinsAlzheimer disease (AD), primarily defined by psychiatrist Alois Alzheimer in 1906, is a neurodegenerative disorder and currently exhibits a prevalence that “doubles approximately every five years from 0.5% at the common age of onset-65 years old.”¹ This disease is the most common form of dementia afflicting the elderly and at present affects in excess of 37 million people globally² and it is predicted that 100 million people will be living with the disease by 2050.³AD has received mounting scientific interest and has stimulated tireless research endeavours not only due to the complex mechanism by which it is caused; the multitude of contributing factors and contradictions which have arisen between hypotheses and acquired results, but also due to the rise in life expectancies⁴ owing to the advent of modern medicine, which has socio-economic implications particularly in terms of strain placed upon national health systems.     

Oligomerization and toxicity of Aβ fusion proteins

This study has found that the Maltose binding protein Aβ42 fusion protein (MBP-Aβ42) forms soluble oligomers while the shorter MBP-Aβ16 fusion and control MBP did not. MBP-Aβ42, but neither MBP-Aβ16 nor control MBP, was toxic in a dose-dependent manner in both yeast and primary cortical neuronal cells. This study demonstrates the potential utility of MBP-Aβ42 as a reagent for drug screening assays in yeast and neuronal cell cultures and as a candidate for further Aβ42 characterization.     

TGFβ and Retinoic Acid Intersect in Immune-Regulation

Transforming growth factor (TGFβ) prevents TH1 and TH2 differentiation and converts naïve CD4 cells into Foxp3-expressing T regulatory (Treg) cell1, 2. In sharp contrast, in the presence of pro-inflammatory cytokines, including IL-6, TGFβ not only inhibits Foxp3 expression but also promotes the differentiation of pro-inflammatory IL17-producing CD4 effector T (TH17) cells3-5. This reciprocal TGFβ-dependent differentiation imposes a critical dilemma between pro- and anti-inflammatory immunity and suggests that a sensitive regulatory mechanism must exist to control TGFβ-driven TH17 effector and Treg differentiation. A vitamin A metabolite, retinoic acid (RA), was recently identified as a key modulator of TGFβ-driven immune deviation capable of suppressing TH17 differentiation while promoting Foxp3+Treg generation 6-10.     

Transforming Growth Factor-β and Ischemic Brain Injury

1. Necrosis and apoptosis are the two fundamental hallmarks of neuronal death in stroke. Nevertheless, thrombolysis, by using the recombinant serine protease t-PA, remains until now the only approved treatment of stroke in man.2. Over the last years, the cytokine termed Transforming Growth Factor-1 (TGF-1) has been found to be strongly up-regulated in the central nervous system following ischemia-induced brain damage.3. Recent studies have shown a neuroprotective activity of TGF-1 against ischemia-induced neuronal death. In vitro, TGF-1 protects neurons against excitotoxicity by inhibiting the t-PA-potentiated NMDA-induced neuronal death through a mechanism involving the up-regulation of the type-1 plasminogen activator inhibitor (PAI-1) in astrocytes.4. In addition, TGF-1 has been recently characterized as an antiapoptotic factor in a model of staurosporine-induced neuronal death through a mechanism involving activation of the extracellular signal-regulated kinase 1/2 (Erk1/2) and a concomitant increase phosphorylation of the antiapoptotic protein Bad.5. Altogether, these observations suggest that either TGF- signaling or TGF-1-modulated genes could be good targets for the development of new therapeutic strategies for stroke in man.     

TGFβ and matrix-regulated epithelial to mesenchymal transition

Background

The progression of cancer through stages that guide a benign hyperplastic epithelial tissue towards a fully malignant and metastatic carcinoma, is driven by genetic and microenvironmental factors that remodel the tissue architecture. The concept of epithelial–mesenchymal transition (EMT) has evolved to emphasize the importance of plastic changes in tissue architecture, and the cross-communication of tumor cells with various cells in the stroma and with specific molecules in the extracellular matrix (ECM).

Scope of the review

Among the multitude of ECM-embedded cytokines and the regulatory potential of ECM molecules, this article focuses on the cytokine transforming growth factor β (TGFβ) and the glycosaminoglycan hyaluronan, and their roles in cancer biology and EMT. For brevity, we concentrate our effort on breast cancer.

Major conclusions

Both normal and abnormal TGFβ signaling can be detected in carcinoma and stromal cells, and TGFβ-induced EMT requires the expression of hyaluronan synthase 2 (HAS2). Correspondingly, hyaluronan is a major constituent of tumor ECM and aberrant levels of both hyaluronan and TGFβ are thought to promote a wounding reaction to the local tissue homeostasis. The link between EMT and metastasis also involves the mesenchymal–epithelial transition (MET). ECM components, signaling networks, regulatory non-coding RNAs and epigenetic mechanisms form the network of regulation during EMT-MET.

General significance

Understanding the mechanism that controls epithelial plasticity in the mammary gland promises the development of valuable biomarkers for the prognosis of breast cancer progression and even provides new ideas for a more integrative therapeutic approach against disease. This article is part of a Special Issue entitled Matrix-mediated cell behaviour and properties.     

Matrix-dependent perturbation of TGFβ signaling and disease

Transforming growth factor beta (TGFβ) is a multipotent cytokine that is sequestered in the extracellular matrix (ECM) through interactions with a number of ECM proteins. The ECM serves to concentrate latent TGFβ at sites of intended function, to influence the bioavailability and/or function of TGFβ activators, and perhaps to regulate the intrinsic performance of cell surface effectors of TGFβ signal propagation. The downstream consequences of TGFβ signaling cascades in turn provide feedback modulation of the ECM. This review covers recent examples of how genetic mutations in constituents of the ECM or TGFβ signaling cascade result in altered ECM homeostasis, cellular performance and ultimately disease, with an emphasis on emerging therapeutic strategies that seek to capitalize on this refined mechanistic understanding.     

Decorin-TGFβ axis in hepatic fibrosis and cirrhosis

Hepatic fibrosis and cirrhosis are worldwide health care problems, especially in regions with a high rate of hepatitis infection. As these diseases affect a major part of the human population, the search for antifibrotic therapies has a high priority in medical research. Transforming growth factor β1 (TGF-β1) is one of the most powerful profibrotic cytokines. Thus, blocking TGF-β1 activity by natural inhibitors represents a valid and logical strategy to combat hepatic fibrosis. One of the natural inhibitors of TGF-β1 is decorin, a small leucine-rich proteoglycan that binds with high affinity to this cytokine and prevents its interaction with pro-fibrotic receptors. Recent evidence has shown that decorin has a protective role in liver fibrogenesis insofar as its genetic ablation in mice leads to enhanced matrix deposition, impaired matrix degradation, and "activation" of hepatic stellate cells, the main producers of fibrotic tissue. Moreover, TGF-β1 exerts a stronger effect when functional decorin is absent. These data provide robust genetic evidence for a direct role of endogenous decorin in preventing and retarding hepatic fibrosis. Thus, boosting the endogenous production of decorin or systemic delivery of recombinant decorin could represent an additional therapeutic modality against hepatic fibrosis.