TGF-beta stimulates biglycan synthesis via p38 and ERK phosphorylation of the linker region of Smad2

The cytoprotective effect of biglycan core protein involves Toll-like receptor 4 signaling in cardiomyocytes

AIMS

Exogenously administered biglycan (core protein with high-molecular weight glycosaminoglycan chains) has been shown to protect neonatal cardiomyocytes against simulated ischemia/reperfusion injury (SI/R), however, the mechanism of action is not clear. In this study we aimed to investigate, which structural component of biglycan is responsible for its cardiocytoprotective effect and to further explore the molecular mechanisms involved in the cytoprotection.

METHODS AND RESULTS

A pilot study was conducted to demonstrate that both native (glycanated) and deglycanated biglycan can attenuate cell death induced by SI/R in a dose-dependent manner in primary neonatal cardiomyocytes isolated from Wistar rats. In separate experiments, we have shown that similarly to glycanated biglycan, recombinant human biglycan core protein (rhBGNc) protects cardiomyocytes against SI/R injury. In contrast, the glycosaminoglycan component dermatan sulfate had no significant effect on cell viability, while chondroitin sulfate further enhanced cell death induced by SI/R. Treatment of cardiomyocytes with rhBGNc reverses the effect of SI/R upon markers of necrosis, apoptosis, mitochondrial membrane potential, and autophagy. We have also shown that pharmacological blockade of Toll-like receptor 4 (TLR4) signaling or its downstream mediators (IRAK1/4, ERK, JNK and p38 MAP kinases) abolished the cytoprotective effect of rhBGNc against SI/R injury. Pretreatment of cardiomyocytes with rhBGNc for 20h resulted in increased Akt phosphorylation and NO production without having significant effect on phosphorylation of ERK1/2, STAT3, and on the production of superoxide. Treatment over 10min and 1h with rhBGNc increased ERK1 phosphorylation, while the SI/R-induced increase in superoxide production was attenuated by rhBGNc. Blockade of NO synthesis also prevented the cardiocytoprotective effect of rhBGNc.

CONCLUSIONS

The core protein of exogenous biglycan protects myocardial cells from SI/R injury via TLR4-mediated mechanisms involving activation of ERK, JNK and p38 MAP kinases and increased NO production. The cytoprotective effect of rhBGNc is due to modulation of SI/R-induced changes in necrosis, apoptosis and autophagy.

Cellular and Molecular Pathology of Age-Related Macular Degeneration: Potential Role for Proteoglycans

Age-related macular degeneration (AMD) is a retinal disease evident after the age of 50 that damages the macula in the centre of retina. It leads to a loss of central vision with retained peripheral vision but eventual blindness occurs in many cases. The initiation site of AMD development is Bruch's membrane (BM) where multiple changes occur including the deposition of plasma derived lipids, accumulation of extracellular debris, changes in cell morphology, and viability and the formation of drusen. AMD manifests as early and late stage; the latter involves cell proliferation and neovascularization in wet AMD. Current therapies target the later hyperproliferative and invasive wet stage whilst none target early developmental stages of AMD. In the lipid deposition disease atherosclerosis modified proteoglycans bind and retain apolipoproteins in the artery wall. Chemically modified trapped lipids are immunogenic and can initiate a chronic inflammatory process manifesting as atherosclerotic plaques and subsequent artery blockages, heart attacks, or strokes. As plasma derived lipoprotein deposits are found in BM in early AMD, it is possible that they arise by a similar process within the macula. In this review we consider aspects of the pathological processes underlying AMD with a focus on the potential role of modifications to secreted proteoglycans being a cause and therefore a target for the treatment of early AMD.

Multiple Growth Factors, But Not VEGF, Stimulate Glycosaminoglycan Hyperelongation in Retinal Choroidal Endothelial Cells

A major feature of early age-related macular degeneration (AMD) is the thickening of Bruch's membrane in the retina and an alteration in its composition with increased lipid deposition. In certain pathological conditions proteoglycans are responsible for lipid retention in tissues. Growth factors are known to increase the length of glycosaminoglycan chains and this can lead to a large increase in the interaction between proteoglycans and lipids. Using choroidal endothelial cells, we investigated the effects of a number of AMD relevant growth factors TGFβ, thrombin, PDGF, IGF and VEGF on proteoglycan synthesis. Cells were characterized as of endothelial origin using the specific cell markers endothelial nitric oxide synthesis and von Willebrand factor and imaged using confocal microscopy. Cells were treated with growth factors in the presence and absence of the appropriate inhibitors and were radiolabeled with [35S]-SO4. Proteoglycans were isolated by ion exchange chromatography and sized using SDS-PAGE. Radiosulfate incorporation was determined by the cetylpyridinium chloride (CPC) precipitation technique. To measure cellular glycosaminoglycan synthesizing capacity we added xyloside and assessed the xyloside-GAGs by SDS-PAGE. TGFβ, thrombin, PDGF & IGF dose-dependently stimulated radiosulfate incorporation and GAG elongation as well as xyloside-GAG synthesis, however VEGF treatment did not stimulate any changes in proteoglycan synthesis. VEGF did not increase pAKT but caused a large increase in pERK relative to the response to PDGF. Thus, AMD relevant agonists cause glycosaminoglycan hyperelongation of proteoglycans synthesised and secreted by retinal choroidal endothelial cells. The absence of a response to VEGF is intriguing and identifies proteoglycans as a novel potential target in AMD. Future studies will examine the relevance of these changes to enhanced lipid binding and the development of AMD.

Tumour endothelial cells in high metastatic tumours promote metastasis via epigenetic dysregulation of biglycan

Tumour blood vessels are gateways for distant metastasis. Recent studies have revealed that tumour endothelial cells (TECs) demonstrate distinct phenotypes from their normal counterparts. We have demonstrated that features of TECs are different depending on tumour malignancy, suggesting that TECs communicate with surrounding tumour cells. However, the contribution of TECs to metastasis has not been elucidated. Here, we show that TECs actively promote tumour metastasis through a bidirectional interaction between tumour cells and TECs. Co-implantation of TECs isolated from highly metastatic tumours accelerated lung metastases of low metastatic tumours. Biglycan, a small leucine-rich repeat proteoglycan secreted from TECs, activated tumour cell migration via nuclear factor-κB and extracellular signal–regulated kinase 1/2. Biglycan expression was upregulated by DNA demethylation in TECs. Collectively, our results demonstrate that TECs are altered in their microenvironment and, in turn, instigate tumour cells to metastasize, which is a novel mechanism for tumour metastasis.

The role of specific Smad linker region phosphorylation in TGF-β mediated expression of glycosaminoglycan synthesizing enzymes in vascular smooth muscle

Hyperelongation of glycosaminoglycan chains on proteoglycans facilitates increased lipoprotein binding in the blood vessel wall and the development of atherosclerosis. Increased mRNA expression of glycosaminoglycan chain synthesizing enzymes in vivo is associated with the development of atherosclerosis. In human vascular smooth muscle, transforming growth factor-β (TGF-β) regulates glycosaminoglycan chain hyperelongation via ERK and p38 as well as Smad2 linker region (Smad2L) phosphorylation. In this study, we identified the involvement of TGF-β receptor, intracellular serine/threonine kinases and specific residues on transcription factor Smad2L that regulate glycosaminoglycan synthesizing enzymes. Of six glycosaminoglycan synthesizing enzymes, xylosyltransferase-1, chondroitin sulfate synthase-1, and chondroitin sulfotransferase-1 were regulated by TGF-β. In addition ERK, p38, PI3K and CDK were found to differentially regulate mRNA expression of each enzyme. Four individual residues in the TGF-β receptor mediator Smad2L can be phosphorylated by these kinases and in turn regulate the synthesis and activity of glycosaminoglycan synthesizing enzymes. Smad2L Thr220 was phosphorylated by CDKs and Smad2L Ser250 by ERK. p38 selectively signalled via Smad2L Ser245. Phosphorylation of Smad2L serine residues induced glycosaminoglycan synthesizing enzymes associated with glycosaminoglycan chain elongation. Phosphorylation of Smad2L Thr220 was associated with XT-1 enzyme regulation, a critical enzyme in chain initiation. These findings provide a deeper understanding of the complex signalling pathways that contribute to glycosaminoglycan chain modification that could be targeted using pharmacological agents to inhibit the development of atherosclerosis.

aPKC alters the TGFβ response in NSCLC cells through both Smad-dependent and Smad-independent pathways

Transforming growth factor b (TGFb) signaling controls many cellular responses including proliferation, epithelial to mesenchymal transition and apoptosis, through the activation of canonical (Smad) as well as non-canonical (e.g., Par6) pathways. Previous studies from our lab have demonstrated that aPKC inhibition regulates TGFb receptor trafficking and signaling. Here, we report that downstream TGFb-dependent transcriptional responses in aPKC-silenced NSCLC cells were reduced compared with those of control cells, despite a temporal extension of Smad2 phosphorylation. We assessed SARA–Smad2–Smad4 association and observed that knockdown of aPKC increased SARA (also known as ZFYVE9) levels and SARA–Smad2 complex formation, increased cytoplasmic retention of Smad2 and reduced Smad2–Smad4 complex formation, which correlated with reduced Smad2 nuclear translocation. Interestingly, we also detected an increase in p38 MAPK phosphorylation and apoptosis in aPKC-silenced cells, which were found to be TRAF6-dependent. Taken together, our results suggest that aPKC isoforms regulate Smad and non-Smad TGFb pathways and that aPKC inhibition sensitizes NSCLC cells to undergo TGFb dependent apoptosis.

Evaluation of the potential synergism of imatinib-related poly kinase inhibitors using growth factor stimulated proteoglycan synthesis as a model response

INTRODUCTION

Tyrosine kinase inhibitors were the first class of smart drugs being specifically designed to inhibit a disease causing target. There is a very important but unresolved question as whether or not the overall therapeutic role of an individual tinib results from an action at its primary target, a single most likely, tyrosine kinase, or from the combined or aggregate action at the multiple targets which each tinib addresses.

METHODS

We selected a series of ten tinibs (gefitinib, sunitinib, lapatinib, erlotinib, imatinib, sorafenib, axitinib, vanitinib, bosutinib, dasatinib) with various known targets and investigated their activities in the inhibition of proteoglycan synthesis and GAG hyperelongation stimulated by a tyrosine kinase receptor agonist, platelet derived growth factor (PDGF) and for contrast, a serine/threonine kinase receptor agonist, TGF β and some downstream signalling pathways.

RESULTS

The inhibitory activity varied from little to total inhibition. The actions of the tinibs were directed more towards inhibition of the tyrosine kinase, PDGF receptor signalling pathway compared to the TGF β.

CONCLUSION

There was no suggestion of any synergistic effect arising from inhibition of multiple kinases as the most potent compound, dasatinib, is known to inhibit the broadest spectrum of kinases.

RETRACTED: Effects of geniposide on hepatocytes undergoing epithelial-mesenchymal transition in hepatic fibrosis by targeting TGFβ/Smad and ERK-MAPK signaling pathways

This article has been retracted: please see Elsevier Policy on Article Withdrawal (https://www.elsevier.com/about/our-business/policies/article-withdrawal). This article has been retracted at the request of the Editor-in-Chief. The corresponding author, Dr Byoungduck Park, requested publication of a corrigendum to correct figure 2B which reused control data from a different publication (doi: 10.1016/j.intimp.2015.02.014). Upon further inspection, the Biochimie editorial team noticed that: Comparison of Fig 2B with Fig 4C of a previous publication in International Immunopharmacology by two co-authors (doi: 10.1016/j.intimp.2015.02.014) reveals that western blot β-actin control data from the earlier paper were re-used in a different experiment shown in Figure 2B of the article in Biochimie, after adjustment of the brightness/contrast. Furthermore, the same bands, after more image manipulation were presented as Smad3 data in Figure 4C of the Biochimie article. Here the image manipulation involved notably the rotation of the set of bands by 180° and some adjustment of the height/width ratio. The authors apologise for any confusion that may have arisen from their article.

The expansion of GPCR transactivation-dependent signalling to include serine/threonine kinase receptors represents a new cell signalling frontier

G protein-coupled receptor (GPCR) signalling is mediated through transactivation-independent signalling pathways or the transactivation of protein tyrosine kinase receptors and the recently reported activation of the serine/threonine kinase receptors, most notably the transforming growth factor-β receptor family. Since the original observation of GPCR transactivation of protein tyrosine kinase receptors, there has been considerable work on the mechanism of transactivation and several pathways are prominent. These pathways include the "triple membrane bypass" pathway and the generation of reactive oxygen species. The recent recognition of GPCR transactivation of serine/threonine kinase receptors enormously broadens the GPCR signalling paradigm. It may be predicted that the transactivation of serine/threonine kinase receptors would have mechanistic similarities with transactivation of tyrosine kinase pathways; however, initial studies suggest that these two transactivation pathways are mechanistically distinct. Important questions are the relative importance of tyrosine and serine/threonine transactivation pathways, the contribution of transactivation to overall GPCR signalling, mechanisms of transactivation and the range of cell types in which this phenomenon occurs. The ultimate significance of transactivation-dependent signalling remains to be defined but it appears to be prominent and if so will represent a new cell signalling frontier.

Transforming growth factor β-mediated site-specific Smad linker region phosphorylation in vascular endothelial cells

Objectives

Transforming growth factor (TGF)-β regulates the function of vascular endothelial cells and may be involved in endothelial dysfunction. The canonical TGF-β pathway involves TGF-β receptor-mediated carboxy-terminal phosphorylation of Smad2; however, TGF-β signalling also activates numerous serine/threonine kinases that phosphorylate Smad2 in its linker region. The expression of phosphorylated Smad linker proteins were determined following TGF-β stimulation in the absence and presence of different serine/threonine kinase inhibitors in vascular endothelial cells.

Methods

Proteins were quantified by Western blotting using specific antibodies to individual phosphorylated Smad2 linker region residues.

Key findings

TGF-β mediated the phosphorylation of all four Smad2 linker region residues of interest. Erk and Jnk specifically phosphorylate Ser245 while all mitogen-activated protein kinases phosphorylate Ser250 and Ser255. Thr220 and Ser245 are phosphorylated by phosphoinositide 3 kinase (PI3K), while Ser255 was phosphorylated by the PI3K/Akt pathway. CDK and GSK-3 were shown to phosphorylate Thr220 and Ser245. TGF-β also mediated plasminogen activator inhibitor-1 gene expression that was attenuated by p38 and CDK inhibitors.

Conclusions

TGF-β-mediated phosphorylation of individual serine/threonine sites in the linker region of Smad2 occurs in a highly specific manner by kinases. These phosphorylations provide an opportunity to further understand a therapeutically targeted and very specific signalling pathway in vascular endothelial cells.

Araf kinase antagonizes Nodal-Smad2 activity in mesendoderm development by directly phosphorylating the Smad2 linker region

Smad2/3-mediated transforming growth factor β signalling and the Ras-Raf-Mek-Erk cascade have important roles in stem cell and development and tissue homeostasis. However, it remains unknown whether Raf kinases directly crosstalk with Smad2/3 signalling and how this would regulate embryonic development. Here we show that Araf antagonizes mesendoderm induction and patterning activity of Nodal/Smad2 signals in vertebrate embryos by directly inhibiting Smad2 signalling. Knockdown of araf in zebrafish embryos leads to an increase of activated Smad2 with a decrease in linker phosphorylation; consequently, the embryos have excess mesendoderm precursors and are dorsalized. Mechanistically, Araf physically binds to and phosphorylates Smad2 in the linker region with S253 being indispensable in a Mek/Erk-independent manner, thereby attenuating Smad2 signalling by accelerating degradation of activated Smad2. Our findings open avenues for investigating the potential significance of Raf regulation of transforming growth factor β signalling in versatile biological and pathological processes in the future.

Smad phospho-isoforms direct context-dependent TGF-β signaling

Better understanding of TGF-β signaling has deepened our appreciation of normal epithelial cell homeostasis and its dysfunction in such human disorders as cancer and fibrosis. Smad proteins, which convey signals from TGF-β receptors to the nucleus, possess intermediate linker regions connecting Mad homology domains. Membrane-bound, cytoplasmic, and nuclear protein kinases differentially phosphorylate Smad2 and Smad3 to create C-tail (C), the linker (L), or dually (L/C) phosphorylated (p, phospho-) isoforms. According to domain-specific phosphorylation, distinct transcriptional responses, and selective metabolism, Smad phospho-isoform pathways can be grouped into 4 types: cytostatic pSmad3C signaling, mitogenic pSmad3L (Ser-213) signaling, invasive/fibrogenic pSmad2L (Ser-245/250/255)/C or pSmad3L (Ser-204)/C signaling, and mitogenic/migratory pSmad2/3L (Thr-220/179)/C signaling. We outline how responses to TGF-β change through the multiple Smad phospho-isoforms as normal epithelial cells mature from stem cells through progenitors to differentiated cells, and further reflect upon how constitutive Ras-activating mutants favor the Smad phospho-isoform pathway promoting tumor progression. Finally, clinical analyses of reversible Smad phospho-isoform signaling during human carcinogenesis could assess effectiveness of interventions aimed at reducing human cancer risk. Spatiotemporally separate, functionally different Smad phospho-isoforms have been identified in specific cells and tissues, answering long-standing questions about context-dependent TGF-β signaling.

Transforming growth factor-β signalling: role and consequences of Smad linker region phosphorylation

Transforming growth factor-β (TGF-β) is a secreted homodimeric protein that plays an important role in regulating various cellular responses including cell proliferation and differentiation, extracellular matrix production, embryonic development and apoptosis. Disruption of the TGF-β signalling pathway is associated with diverse disease states including cancer, renal and cardiac fibrosis and atherosclerosis. At the cell surface TGF-β complex consists of two type I and two type II transmembrane receptors (TβRI and TβRII respectively) which have serine/threonine kinase activity. Upon TGF-β engagement TβRII phosphorylates TβRI which in turn phosphorylates Smad2/3 on two serine residues at their C-terminus which enables binding to Smad4 to form heteromeric Smad complexes that enter the nucleus to initiate gene transcription including for extracellular matrix proteins. TGF-β signalling is also known to activate other serine/threonine kinase signalling that results in the phosphorylation of the linker region of Smad2. The Smad linker region is defined as the domain which lies between the MH1 and MH2 domains of a Smad protein. Serine/threonine kinases that are known to phosphorylate the Smad linker region include mitogen-activated protein kinases, extracellular-signal regulated kinase, Jun N-terminal kinase and p38 kinase, the tyrosine kinase Src, phosphatidylinositol 3'-kinase, cyclin-dependent kinases, rho-associated protein kinase, calcium calmodulin-dependent kinase and glycogen synthase kinase-3. This review will cover the role of Smad linker region phosphorylation downstream of TGF-β signalling in vascular cells. Key factors including the identification of the kinases that phosphorylate individual Smad residues, the upstream agents that activate these kinases, the cellular location of the phosphorylation event and the importance of the linker region in regulation and expression of genes induced by TGF-β are covered.

(S)-[6]-Gingerol inhibits TGF-β-stimulated biglycan synthesis but not glycosaminoglycan hyperelongation in human vascular smooth muscle cells

Objectives

(S)-[6]-Gingerol is under investigation for a variety of therapeutic uses. Transforming growth factor (TGF)-β stimulates proteoglycan synthesis, leading to increased binding of low-density lipoproteins, which is the initiating step in atherosclerosis. We evaluated the effects of (S)-[6]-gingerol on these TGF-β-mediated proteoglycan changes to explore its potential as an anti-atherosclerotic agent.

Methods

Purified (S)-[6]-gingerol was assessed for its effects on proteoglycan synthesis by [35S]-sulfate incorporation into glycosaminoglycan chains and [35S]-Met/Cys incorporation into proteoglycans and total proteins in human vascular smooth muscle cells. Biglycan level was assessed by real-time quantitative polymerase chain reactions and the effects of (S)-[6]-gingerol on TGF-β signalling by assessment of the phosphorylation of Smads and Akt by western blotting.

Key findings

(S)-[6]-Gingerol concentration-dependently inhibited TGF-β-stimulated proteoglycan core protein synthesis, and this was not secondary to inhibition of total protein synthesis. (S)-[6]-Gingerol inhibited biglycan mRNA expression. (S)-[6]-Gingerol did not inhibit TGF-β-stimulated glycosaminoglycan hyperelongation or phosphorylation of Smad 2, in either the carboxy terminal or linker region, or Akt phosphorylation.

Conclusions

The activity of (S)-[6]-gingerol to inhibit TGF-β-stimulated biglycan synthesis suggests a potential role for ginger in the prevention of atherosclerosis or other lipid-binding diseases. The signalling studies indicate a novel site of action of (S)-[6]-gingerol in inhibiting TGF-β responses.

Smad2-dependent glycosaminoglycan elongation in aortic valve interstitial cells enhances binding of LDL to proteoglycans

OBJECTIVE

Calcific aortic valve disease is a progressive condition that shares some common pathogenic features with atherosclerosis. Transforming growth factor-β1 is a recognized mediator of atherosclerosis and is expressed in aortic valve lesions. Transforming growth factorβ1 stimulates glycosaminoglycan elongation of proteoglycans that is associated with increased lipid binding. We investigated the presence of transforming growth factor-β1 and downstream signaling intermediates in diseased human aortic valves and the effects of activated transforming growth factor-β1 receptor signaling on aortic valve interstitial cell proteoglycan synthesis and lipid binding as a possible mechanism for the initiation of the early lesion of calcific aortic valve disease.

METHODS AND RESULTS

Diseased human aortic valve leaflets demonstrated strong immunohistochemical staining for transforming growth factor-β1 and phosphorylated Smad2/3. In primary porcine aortic valve interstitial cells, Western blots showed that transforming growth factor-β1 stimulated phosphorylation in both the carboxy and linker regions of Smad2/3, which was inhibited by the transforming growth factor-β1 receptor inhibitor SB431542. Gel electrophoresis and size exclusion chromatography demonstrated that SB431542 decreased transforming growth factor-β1-mediated [(35)S]-sulfate incorporation into proteoglycans in a dose-dependent manner. Further, in proteoglycans derived from transforming growth factor-β1-treated valve interstitial cells, gel mobility shift assays demonstrated that inhibition of transforming growth factor-β1 receptor signaling resulted in decreased lipid binding.

CONCLUSIONS

Classic transforming growth factor-β1 signaling is present in human aortic valves in vivo and contributes to the modification of proteoglycans expressed by valve interstitial cells in vitro. These findings suggest that transforming growth factor-β1 may promote increased low-density lipoprotein binding in the early phases of calcific aortic valve disease.

Interindividual transcriptional regulation of the human biglycan gene involves three common molecular haplotypes

OBJECTIVE

The extracellular matrix proteoglycan biglycan (BGN) is involved in cardiovascular disease pathophysiology, as it mediates the subendothelial retention of atherogenic apolipoprotein B-containing lipoproteins, affects adaptive remodeling after myocardial infarction, and exerts proinflammatory effects in macrophages. In a cardiovascular disease-related setting of vascular endothelial cells and human monocytes, we examined the molecular mechanisms of common molecular haplotypes affecting human BGN transcriptional regulation.

APPROACH AND RESULTS

After the molecular characterization of the BGN promoter, we determined the prevalence of BGN promoter variants (1199 base pair portion) in 87 individuals of European ancestry, and identified 3 molecular haplotypes by subcloning and sequencing of subjects' single DNA strands: MolHap1 [G(-578)-G(-151)-G(+94)] MolHap2 [G(-578)-A(-151)-T(+94)] and MolHap3 [A(-578)-G(-151)-G(+94)]. By 5' rapid amplification of cDNA-ends, we detected 1 additional upstream transcription start site at position -46 in EA.hy926 endothelial cells. Reporter gene assays located the BGN core promoter to the region spanning positions -39 and +162. Strongest promoter activity was mapped to the region between -1231 and -935. The introduction of MolHap2 and MolHap3 into the active BGN promoter led to a significant loss of transcriptional activity (all probability values <0.05), compared with MolHap1. By use of electrophoretic mobility shift assays, chromatin immunoprecipitation assays, and cotransfection of transcription factors, we identified specificity protein 1, v-ets erythroblastosis virus E26 oncogene homolog (ETS) family members, and an activator protein-1 complex to interact differentially with the BGN promoter in the context of each individual MolHap.

CONCLUSIONS

Our results indicate that molecular haplotypes within the BGN promoter may contribute to the molecular basis of interindividually different transcriptional BGN regulation, possibly modulating the predisposition to cardiovascular disease-related phenotypes.

Therapeutic implications of endothelin and thrombin G-protein-coupled receptor transactivation of tyrosine and serine/threonine kinase cell surface receptors

Objectives

This review discusses the latest developments in G protein coupled receptor (GPCR) signalling related to the transactivation of cell surface protein kinase receptors and the therapeutic implications.

Key findings

Multiple GPCRs have been known to transactivate protein tyrosine kinase receptors for almost two decades. More recently it has been discovered that GPCRs can also transactivate protein serine/threonine kinase receptors such as that for transforming growth factor (TGF)-β. Using the model of proteoglycan synthesis and glycosaminoglycan elongation in human vascular smooth muscle cells which is a component of an in vitro model of atherosclerosis, the dual tyrosine and serine/threonine kinase receptor transactivation pathways appear to account for all of the response to the agonists, endothelin and thrombin.

Summary

The broadening of the paradigm of GPCR receptor transactivation explains the broad range of activities of these receptors and also the efficacy of GPCR antagonists in cardiovascular therapeutics. Deciphering the mechanisms of transactivation with the aim of identifying a common therapeutic target remains the next challenge.

Tgf-beta induced Erk phosphorylation of smad linker region regulates smad signaling

The Transforming Growth Factor-Beta (TGF-β) family is involved in regulating a variety of cellular processes such as apoptosis, differentiation, and proliferation. TGF-β binding to a Serine/Threonine kinase receptor complex causes the recruitment and subsequent activation of transcription factors known as smad2 and smad3. These proteins subsequently translocate into the nucleus to negatively or positively regulate gene expression. In this study, we define a second signaling pathway leading to TGF-β receptor activation of Extracellular Signal Regulated Kinase (Erk) in a cell-type dependent manner. TGF-β induced Erk activation was found in phenotypically normal mesenchymal cells, but not normal epithelial cells. By activating phosphotidylinositol 3-kinase (PI3K), TGF-β stimulates p21-activated kinase2 (Pak2) to phosphorylate c-Raf, ultimately resulting in Erk activation. Activation of Erk was necessary for TGF-β induced fibroblast replication. In addition, Erk phosphorylated the linker region of nuclear localized smads, resulting in increased half-life of C-terminal phospho-smad 2 and 3 and increased duration of smad target gene transcription. Together, these data show that in mesenchymal cell types the TGF-β/PI3K/Pak2/Raf/MEK/Erk pathway regulates smad signaling, is critical for TGF-β-induced growth and is part of an integrated signaling web containing multiple interacting pathways rather than discrete smad/non-smad pathways.

Transforming growth factor-β increases vascular smooth muscle cell proliferation through the Smad3 and extracellular signal-regulated kinase mitogen-activated protein kinases pathways

INTRODUCTION

We have previously demonstrated that transforming growth factor-β (TGF-β) in the presence of elevated levels of Smad3, its primary signaling protein, stimulates rat vascular smooth muscle cell (VSMC) proliferation and intimal hyperplasia. The mechanism is partly through the nuclear exportation of phosphorylated cyclin-dependent kinase inhibitor p27. The objective of this study is to clarify the downstream pathways through which Smad3 produces its proliferative effect. Specifically, we evaluated the role of extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase (MAPK) in TGF-β-induced VSMC proliferation.

METHODS

Cultured rat aortic VSMCs were incubated with TGF-β at varying concentrations and times, and phosphorylated ERK was measured by Western blotting. Smad3 was enhanced in VSMCs using an adenovirus expressing Smad3 or inhibited with small interfering RNA (siRNA). For in vivo experiments, male Sprague-Dawley rats underwent carotid balloon injury, followed by intraluminal infection with an adenovirus expressing Smad3. Arteries were harvested at 3 days and subjected to immunohistochemistry for Smad3, phospho-ERK MAPK, and proliferating cell nuclear antigen.

RESULTS

In cultured VSMCs, TGF-β induced activation and phosphorylation of ERK MAPK in a time-dependent and concentration-dependent manner. Overexpression of the signaling protein Smad3 enhanced TGF-β-induced activation of ERK MAPK, whereas inhibition of Smad3 with a siRNA blocked ERK MAPK phosphorylation in response to TGF-β. These data suggest that Smad3 acts as a signaling intermediate between TGF-β and ERK MAPK. Inhibition of ERK MAPK activation with PD98059 completely blocked the ability of TGF-β/Smad3 to stimulate VSMC proliferation, demonstrating the importance of ERK MAPK in this pathway. Immunoprecipitation of phospho-ERK MAPK and blotting with Smad3 revealed a physical association, suggesting that activation of ERK MAPK by Smad3 requires a direct interaction. In an in vivo rat carotid injury model, overexpression of Smad3 resulted in an increase in phosphorylated ERK MAPK as well as increased VSMC proliferation as measured by proliferating cell nuclear antigen.

CONCLUSIONS

Our findings demonstrate a mechanism through which TGF-β stimulates VSMC proliferation. Although TGF-β has been traditionally identified as an inhibitor of proliferation, our data suggest that TGF-β enhances VSMC proliferation through a Smad3/ERK MAPK signaling pathway. These findings at least partly explain the mechanism by which TGF-β enhances intimal hyperplasia. Knowledge of this pathway provides potential novel targets that may be used to prevent restenosis.

Mnk Kinases in Cytokine Signaling and Regulation of Cytokine Responses

The kinases Mnk1 and Mnk2 are activated downstream of the p38 MAPK and MEK/ERK signaling pathways. Extensive work over the years has shown that these kinases control phosphorylation of the eukaryotic initiation factor 4E (eIF4E) and regulate engagement of other effector elements, including hnRNPA1 and PSF. Mnk kinases are ubiquitously expressed and play critical roles in signaling for various cytokine receptors, while there is emerging evidence that they have important functions as mediators of pro-inflammatory cytokine production. In this review the mechanisms of activation of MNK pathways by cytokine receptors are addressed and their roles in diverse cytokine-dependent biological processes are reviewed. The clinical-translational implications of such work and the relevance of future development of specific MNK inhibitors for the treatment of malignancies and auto-immune disorders are discussed.

G protein coupled receptor transactivation: extending the paradigm to include serine/threonine kinase receptors

The current paradigm of G protein coupled receptor signaling involves a classical pathway being the activation of phospholipase C and the generation of 1,4,5-inositol trisphosphate, signaling through β-arrestin scaffold molecules and the transactivation of tyrosine kinase growth factor receptors. Transactivation greatly expands the range of signaling pathways and responses attributable to the receptor. Recently it has been revealed that G protein coupled receptor agonists can also transactivate the serine/threonine kinase cell surface receptor for transforming growth factor-β (Alk5). This leads to the generation of carboxyl terminal phosphorylated Smad2 which is the immediate downstream product of the activated Alk5. Thus, the current paradigm of G protein coupled signaling can be expanded to include the transactivation of the serine kinase receptor Alk5. These insights expand the possibilities for outcomes of therapeutically targeting GPCRs where more substantive and prolonged actions such as the synthesis of extracellular matrix may be affected.

The Roles of Mitogen-Activated Protein Kinase Pathways in TGF-β-Induced Epithelial-Mesenchymal Transition

The mitogen-activated protein kinase (MAPK) pathway allows cells to interpret external signals and respond appropriately, especially during the epithelial-mesenchymal transition (EMT). EMT is an important process during embryonic development, fibrosis, and tumor progression in which epithelial cells acquire mesenchymal, fibroblast-like properties and show reduced intercellular adhesion and increased motility. TGF-β signaling is the first pathway to be described as an inducer of EMT, and its relationship with the Smad family is already well characterized. Studies of four members of the MAPK family in different biological systems have shown that the MAPK and TGF-β signaling pathways interact with each other and have a synergistic effect on the secretion of additional growth factors and cytokines that in turn promote EMT. In this paper, we present background on the regulation and function of MAPKs and their cascades, highlight the mechanisms of MAPK crosstalk with TGF-β signaling, and discuss the roles of MAPKs in EMT.

Retention of atherogenic lipoproteins in the artery wall and its role in atherogenesis

AIMS

In this review, we discuss the mechanisms behind the binding of low-density lipoproteins (LDL) to the arterial wall and how this interaction might be targeted to prevent atherosclerosis.

DATA SYNTHESIS

An increasing body of evidence shows that accumulation of LDL in the vessel wall is a critical step in the development of atherosclerosis. The retained lipoproteins subsequently provoke an inflammatory response that ultimately leads to atherosclerosis. In the arterial wall, LDL binds ionically to proteoglycans in the extracellular matrix. In particular, proteoglycans with elongated glycosaminoglycan (GAG) chains seem to play a crucial role in this process.

CONCLUSIONS

The LDL-proteoglycan interaction is a highly regulated process that might provide new therapeutic targets against cardiovascular disease.

Mnk Kinases in Cytokine Signaling and Regulation of Cytokine Responses

The kinases Mnk1 and Mnk2 are activated downstream of the p38 MAPK and MEK/ERK signaling pathways. Extensive work over the years has shown that these kinases control phosphorylation of the eukaryotic initiation factor 4E (eIF4E) and regulate engagement of other effector elements, including hnRNPA1 and PSF. Mnk kinases are ubiquitously expressed and play critical roles in signaling for various cytokine receptors, while there is emerging evidence that they have important functions as mediators of pro-inflammatory cytokine production. In this review the mechanisms of activation of MNK pathways by cytokine receptors are addressed and their roles in diverse cytokine-dependent biological processes are reviewed. The clinical-translational implications of such work and the relevance of future development of specific MNK inhibitors for the treatment of malignancies and auto-immune disorders are discussed.

TGF-β stimulates biglycan core protein synthesis but not glycosaminoglycan chain elongation via Akt phosphorylation in vascular smooth muscle

Transforming growth factor-β (TGF-β) can mediate proteoglycan synthesis via Smad and non-Smad signalling pathways in vascular smooth muscle (VSM). We investigated whether TGF-β-mediated proteoglycan synthesis is via PI3K/Akt. TGF-β induced a rapid phosphorylation of Akt that continued upto 4 h. Akt phosphorylation was blocked by Akt1/2 inhibitor SN30978; however, it did not block Smad2 phosphorylation at either the carboxy or linker regions indicating that TGF-β-mediated Akt phosphorylation is independent of Smad2 signalling. The role of Akt in TGF-β-mediated proteoglycan synthesis was investigated. Treatment with SN30978 showed a concentration-dependent decrease in TGF-β-mediated [(35)S]-sulphate and [(35)S]-Met/Cys incorporation into secreted proteoglycans; however, SDS-PAGE showed no change in biglycan size. In TGF-β-treated cells, biglycan mRNA levels increased by 40-100% in 24 h and was significantly blocked by SN30978. Our findings demonstrate that Akt is a downstream signalling component of TGF-β-mediated biglycan core protein synthesis but not glycosaminoglycan chain hyper-elongation in VSM.

Smad phosphoisoform signaling specificity: the right place at the right time

Transforming growth factor (TGF)-β antagonizes mitogenic Ras signaling during epithelial regeneration, but TGF-β and Ras act synergistically in driving tumor progression. Insights into these apparently contradictory effects have come from recent detailed analyses of the TGF-β signaling process. Here, we summarize the different modes of TGF-β/Ras signaling in normal epithelium and neoplasms and show how perturbation of TGF-β signaling by Ras may contribute to a shift from tumor-suppressive to protumorigenic TGF-β activity during tumor progression. Smad proteins, which convey signals from TGF-β receptors to the nucleus, have intermediate linker regions between conserved Mad homology (MH) 1 and MH2 domains. TGF-β Type I receptor and Ras-associated kinases differentially phosphorylate Smad2 and Smad3 to create C-terminally (C), linker (L) or dually (L/C) phosphorylated (p) isoforms. In epithelial homeostasis, TGF-β-mediated pSmad3C signaling opposes proliferative responses induced by mitogenic signals. During carcinogenesis, activation of cytoplasmic Ras-associated kinases including mitogen-activated protein kinase confers a selective advantage on benign tumors by shifting Smad3 signaling from a tumor-suppressive pSmad3C to an oncogenic pSmad3L pathway, leading to carcinoma in situ. Finally, at the edges of advanced carcinomas invading adjacent tissues, nuclear Ras-associated kinases such as cyclin-dependent kinases, together with cytoplasmic kinases, alter TGF-β signals to more invasive and proliferative pSmad2L/C and pSmad3L/C signaling. Taken together, TGF-β signaling specificity arises from spatiotemporal dynamics of Smad phosphoisoforms. Based on these findings, we have reason to hope that pharmacologic inhibition of linker phosphorylation might suppress progression to human advanced carcinomas by switching from protumorigenic to tumor-suppressive TGF-β signaling.

Smad phosphoisoform signals in acute and chronic liver injury: similarities and differences between epithelial and mesenchymal cells

Hepatocellular carcinoma (HCC) usually arises from hepatic fibrosis caused by chronic inflammation. In chronic liver damage, hepatic stellate cells undergo progressive activation to myofibroblasts (MFB), which are important extracellular-matrix-producing mesenchymal cells. Concomitantly, perturbation of transforming growth factor (TGF)-β signaling by pro-inflammatory cytokines in the epithelial cells of the liver (hepatocytes) promotes both fibrogenesis and carcinogenesis (fibro-carcinogenesis). Insights into fibro-carcinogenic effects on chronically damaged hepatocytes have come from recent detailed analyses of the TGF-β signaling process. Smad proteins, which convey signals from TGF-β receptors to the nucleus, have intermediate linker regions between conserved Mad homology (MH) 1 and MH2 domains. TGF-β type I receptor and pro-inflammatory cytokine-activated kinases differentially phosphorylate Smad2 and Smad3 to create phosphoisoforms phosphorylated at the COOH-terminal, linker, or both (L/C) regions. After acute liver injury, TGF-β-mediated pSmad3C signaling terminates hepatocytic proliferation induced by the pro-inflammatory cytokine-mediated mitogenic pSmad3L pathway; TGF-β and pro-inflammatory cytokines synergistically enhance collagen synthesis by activated hepatic stellate cells via pSmad2L/C and pSmad3L/C pathways. During chronic liver disease progression, pre-neoplastic hepatocytes persistently affected by TGF-β together with pro-inflammatory cytokines come to exhibit the same carcinogenic (mitogenic) pSmad3L and fibrogenic pSmad2L/C signaling as do MFB, thereby accelerating liver fibrosis while increasing risk of HCC. This review of Smad phosphoisoform-mediated signals examines similarities and differences between epithelial and mesenchymal cells in acute and chronic liver injuries and considers Smad linker phosphorylation as a potential target for the chemoprevention of fibro-carcinogenesis.

Smad linker region phosphorylation in the regulation of extracellular matrix synthesis

The canonical TGF-β signalling pathway involves Smad transcription factors through direct serine phosphorylation of the carboxy termini, nuclear translocation and regulation of transcription by receptor-regulated (R)-Smad complexes. Smads can also be phosphorylated in the linker region most prominently by the action of mitogen-activated protein (MAP) kinases, which in turn have been activated by TGF-β or a multitude of other growth factors and hormones. Linker region phosphorylation can prevent nuclear translocation of Smads and inhibit TGF-β signalling, potentially leading to oncogenesis. However, some evidence has revealed that linker region phosphorylated Smads can be translocated to the nucleus where they regulate transcription particularly of the synthesis of extracellular matrix molecules. Matrix molecules such as collagen and proteoglycans are involved in diseases such a fibrosis and atherosclerosis, respectively, and the involvement of linker region phosphorylation may represent a new therapeutic target.

Transforming growth factor-β regulation of proteoglycan synthesis in vascular smooth muscle: contribution to lipid binding and accelerated atherosclerosis in diabetes

Atherosclerosis is accelerated in the setting of diabetes, but the factors driving this phenomenon remain elusive. Hyperglycemia leads to elevated levels of transforming growth factor (TGF)-β and TGF-β has been implicated as a factor in atherosclerosis. Given the established association between hyperglycemia and elevated TGF-β, it is plausible that elevated TGF-β levels in diabetes play a pathogenic role in the development of accelerated atherosclerosis. TGF-β is a potent regulator of extracellular matrix synthesis, including many actions on proteoglycan synthesis that lead to increased binding to low-density lipoprotein and therefore potentially increased lipid retention in the vessel wall and accelerated atherosclerosis. TGF-β signals through the canonical TGF-β receptor I-mediated phosphorylation of Smad transcription factors and TGF-β signaling is also known to involve, positively and negatively, interactions with the mitogen-activated protein kinase pathways. The focus of the present review is on the effects of TGF-β on proteoglycan synthesis in vascular smooth muscle and particularly the signaling pathways through which TGF-β exerts its effects, because those pathways may be therapeutic targets for the prevention of pathological modifications in the proteoglycan component of the vessel wall in the vascular diseases of diabetes.