Contribution of Src-FAK signaling to the induction of connective tissue growth factor in renal fibroblasts

Conjunction junction, what's the function? CCN proteins as targets in fibrosis and cancers

Cellular communication network (CCN) proteins are matricellular proteins that coordinate signaling among extracellular matrix, secreted proteins, and cell surface receptors. Their specific in vivo function is context-dependent, but they play profound roles in pathological conditions, such as fibrosis and cancers. Anti-CCN therapies are in clinical consideration. Only recently, however, has the function of these complex molecules begun to emerge. This review summarizes and interprets our current knowledge regarding these fascinating molecules and provides experimental evidence for their utility as therapeutic targets.

Mechanical Forces in Cutaneous Wound Healing: Emerging Therapies to Minimize Scar Formation

Significance: Excessive scarring is major clinical and financial burden in the United States. Improved therapies are necessary to reduce scarring, especially in patients affected by hypertrophic and keloid scars. Recent Advances: Advances in our understanding of mechanical forces in the wound environment enable us to target mechanical forces to minimize scar formation. Fetal wounds experience much lower resting stress when compared with adult wounds, and they heal without scars. Therapies that modulate mechanical forces in the wound environment are able to reduce scar size. Critical Issues: Increased mechanical stresses in the wound environment induce hypertrophic scarring via activation of mechanotransduction pathways. Mechanical stimulation modulates integrin, Wingless-type, protein kinase B, and focal adhesion kinase, resulting in cell proliferation and, ultimately, fibrosis. Therefore, the development of therapies that reduce mechanical forces in the wound environment would decrease the risk of developing excessive scars. Future Directions: The development of novel mechanotherapies is necessary to minimize scar formation and advance adult wound healing toward the scarless ideal. Mechanotransduction pathways are potential targets to reduce excessive scar formation, and thus, continued studies on therapies that utilize mechanical offloading and mechanomodulation are needed.

Amniotic membrane promotes focal adhesion remodeling to stimulate cell migration

During wound healing, the migration of keratinocytes onto newly restored extracellular matrix aims to reestablish continuity of the epidermis. The application of amniotic membrane (AM) to chronic, deep traumatic, non-healing wounds has proven successful at stimulating re-epithelialization. When applied on epithelial cell cultures, AM activates extracellular signal-regulated kinases 1/2 (ERK1/2) and c-Jun N-terminal kinases 1/2 (JNK1/2), with the overexpression and phosphorylation of c-Jun along the wound edge. The effect of AM on the migration of cells was investigated by studying critical proteins involved in the focal adhesions turn-over: Focal Adhesion Kinase (FAK), Paxillin and Vinculin. In Mv1Lu and HaCaT cells, validated models for cell migration and wound healing, AM affected the expression and activation of Paxillin, but did not affect Vinculin expression, both factors which integrate into focal adhesions. Moreover, AM regulation also affected FAK activity through phosphorylation. Finally, we have determined that AM regulation of focal adhesions involves both JNK and MEK MAP kinase signaling pathways. This data provides a molecular background to understand how AM regulates critical cell and molecular aspects of cell migration, organizing and directing the movement of cells by the continuous formation, maturation, and turnover of focal adhesion structures at the migration leading edge.

Cell surface receptors for CCN proteins

The CCN family (CYR61; CTGF; NOV; CCN1-6; WISP1-3) of matricellular proteins in mammals is comprised of six homologous members that play important roles in development, inflammation, tissue repair, and a broad range of pathological processes including fibrosis and cancer. Despite considerable effort to search for a high affinity CCN-specific receptor akin to growth factor receptors, no such receptor has been found. Rather, CCNs bind several groups of multi-ligand receptors as characteristic of other matricellular proteins. The most extensively documented among CCN-binding receptors are integrins, including αvβ3, αvβ5, α5β1, α6β1, αIIbβ3, αMβ2, and αDβ2, which mediate diverse CCN functions in various cell types. CCNs also bind cell surface heparan sulfate proteoglycans (HSPGs), low density liproprotein receptor-related proteins (LRPs), and the cation-independent mannose-6-phosphate (M6P) receptor, which are endocytic receptors that may also serve as co-receptors in cooperation with other cell surface receptors. CCNs have also been reported to bind FGFR-2, Notch, RANK, and TrkA, potentially altering the affinities of these receptors for their ligands. The ability of CCNs to bind a multitude of receptors in various cell types may account for the remarkable versatility of their functions, and underscore the diverse signaling pathways that mediate their activities.

New molecular insights in diabetic nephropathy

Diabetes mellitus represents one of the major causes of functional kidney impairment. The review highlights the most significant steps made over the last decades in understanding the molecular basis of diabetic nephropathy (DN), which may provide reliable biomarkers for early diagnosis and prognosis, along with new molecular targets for personalized medicine. There is an increased interest in developing new therapeutic strategies to slow DN progression for improving patients' quality of life and reducing all-cause morbidity and disease-associated mortality. It is highly important to have a science-based medical attitude when facing diabetic patients with associated comorbidities and risk of rapid evolution toward end-stage renal disease. The data discussed herein were mainly from MEDLINE and PubMed articles published in English from 1990 to 2015 and from up-to-date. The search term was "diabetic nephropathy and oxidative stress".

Dual inhibition of Src family kinases and Aurora kinases by SU6656 modulates CTGF (connective tissue growth factor) expression in an ERK-dependent manner

Src kinases are regulators of the expression of connective tissue growth factor (CTGF/CCN2), which plays a role in fibrotic injuries. The aim of the present study was to evaluate the potential of SU6656, a dual inhibitor of Src family and Aurora kinases, to interfere with the synthesis of this pro-fibrotic factor. SU6656 impaired TGF-β-mediated upregulation of CTGF mRNA and protein in proximal epithelial HKC-8 cells, and also reduced CTGF expression in cells exposed to autocrine growth factors. In association with the inhibition of Src family kinases and diminished focal adhesion kinase activity, adherence of the cells was reduced. Furthermore, SU6656 interfered with Aurora kinase activity resulting in inhibition of cell division and formation multilobular nuclei after 24h. Comparable alterations were observed in primary tubular cells. When cell division was inhibited by SU6656 or ZM447439, a specific inhibitor of Aurora kinases, CTGF levels were back to control or even increased after 48h. The activity of RhoA-Rho kinase and ERK signaling was analyzed to delineate the signaling pathways responsible for the biphasic regulation of CTGF. While Rho kinase was not significantly altered by SU6656, ERK activity was inhibited in the early phase and increased after 24-48h. ERK activity correlated with secreted CTGF. As ZM447439 increased ERK activity only after 48h, cellular reorganization is likely responsible for triggering the ERK-dependent upregulation of CTGF. Taken together, in non-transformed epithelial cells, SU6656 modulates the expression of the pro-fibrotic factor CTGF in a time-dependent manner by inhibition of Src kinases and Aurora kinases.

Thrombin-induced CCN2 expression in human lung fibroblasts requires the c-Src/JAK2/STAT3 pathway

Thrombin might activate c-Src to induce JAK2 activation, which causes STAT3 activation, inducing CCN2 expression in human lung fibroblasts.

Thrombin is a multifunctional serine protease and an important fibrotic mediator that induces CCN2 expression. We previously showed that thrombin induces CCN2 expression via an ASK1-dependent JNK/AP-1 pathway in human lung fibroblasts. In this study, we further investigated the roles of c-Src, JAK2, and STAT3 in thrombin-induced CCN2 expression. Thrombin-induced CCN2 expression and CCN2-Luc activity were attenuated by a JAK inhibitor (AG490) and JAK2DN, STAT3DN, and the STAT decoy ODN. Moreover, transfection of cells with a CCN2-mtSTAT-Luc construct inhibited thrombin-induced CCN2-Luc activity. Treatment of cells with thrombin caused JAK2 phosphorylation at Tyr1007/1008 and STAT3 phosphorylation at Tyr705 in time-dependent manners. Thrombin-induced STAT3 phosphorylation was inhibited by AG490 and JAK2DN. Thrombin-induced STAT3 binding to the CCN2 promoter was analyzed by a DNA-binding affinity pull-down assay. In addition, thrombin-induced CCN2 expression and CCN2-Luc activity were inhibited by c-SrcDN and PP2 (an Src inhibitor). Transfection of cells with c-SrcDN also inhibited thrombin-induced JAK2 and STAT3 phosphorylation. Taken together, these results indicate that thrombin might activate c-Src to induce JAK2 activation, which in turn, causes STAT3 activation, and finally induces CCN2 expression in human lung fibroblasts.

Cytoskeletal architecture regulates cyclooxygenase-2 in human endothelial cells: autocrine modulation by prostacyclin

Endothelium is a highly dynamic tissue that controls vascular homeostasis. This requires constant rearrangements of the shape or function of endothelial cells that cannot set aside the role of the cytoskeleton. The aim of this study was to determine the mechanisms by means of which cytoskeletal alterations induce cyclooxygenase-2 (Cox-2) expression in human endothelial cells using compounds that interfere with microtubule or actin architecture. Microtubule disruption by nocodazole markedly increased Cox-2 expression and activity, and provoked paracellular gap formation, a cardinal feature of endothelial barrier dysfunction. The Cox-2 metabolite prostacyclin down-regulated Cox-2 through an autocrine receptor-mediated mechanism, and partially prevented the disassembly of endothelial monolayers. There was also an interaction between microtubules and actin filaments in nocodazole-induced Cox-2 expression. Nocodazole provoked the dissolution of the F-actin cortical ring and stress fiber formation, increased actin glutathionylation, and concomitantly lowered intracellular levels of reduced glutathione. The restoration of glutathione levels by N-acetylcysteine opposed Cox-2 expression and preserved the integrity of endothelial monolayers. Among the signaling pathways connecting microtubule disruption with Cox-2 up-regulation, crucial roles are played by Src family kinase activation, serine/threonine phosphatase 2A inhibition, and the phosphorylation of mitogen activated protein kinase p38. Our findings provide a mechanistic insight into the observation that Cox-2 is induced in endothelial cells under cytoskeleton-perturbing conditions such as those occurring in the presence of atherogenic/inflammatory stimuli and oxidative stress. In this scenario, Cox-2 up-regulation by endothelia exposed to noxious conditions can be considered protective of the vasodilatory and anti-thrombotic properties of the vessel wall.

Soft tissue mechanotransduction in wound healing and fibrosis

Recent evidence suggests that mechanical forces can significantly impact the biologic response to injury. Integrated mechanical and chemical signaling networks have been discovered that enable physical cues to regulate disease processes such as pathologic scar formation. Distinct molecular mechanisms control how tensional forces influence wound healing and fibrosis. Conceptual frameworks to understand cutaneous repair have expanded beyond traditional cell-cytokine models to include dynamic interactions driven by mechanical force and the extracellular matrix. Strategies to manipulate these biomechanical signaling networks have tremendous therapeutic potential to reduce scar formation and promote skin regeneration.

Role of sphingosine 1-phosphate and lysophosphatidic acid in fibrosis

This review highlights an emerging role for sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA) in many different types of fibrosis. Indeed, both LPA and S1P are involved in the multi-process pathogenesis of fibrosis, being implicated in promoting the well-established process of differentiation of fibroblasts to myofibroblasts and the more controversial epithelial-mesenchymal transition and homing of fibrocytes to fibrotic lesions. Therefore, targeting the production of these bioactive lysolipids or blocking their sites/mechanisms of action has therapeutic potential. Indeed, LPA receptor 1 (LPA(1)) selective antagonists are currently being developed for the treatment of fibrosis of the lung as well as a neutralising anti-S1P antibody that is currently in Phase 1 clinical trials for treatment of age related macular degeneration. Thus, LPA- and S1P-directed therapeutics may not be too far from the clinic. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.

Formation of cysts by principal-like MDCK cells depends on the synergy of cAMP- and ATP-mediated fluid secretion

It has been suggested that more than 70% of the renal cysts in patients with autosomal dominant polycystic kidney disease (ADPKD) arise from the collecting duct and that within this segment cysts originate almost exclusively from principal rather than intercalated cells. The mechanisms for this predisposition of principal cells have so far remained elusive. We, therefore, used Madin-Darby canine kidney (MDCK) subclones resembling principal cells and alpha-intercalated cells in a three-dimensional in vitro model to determine differences in cystogenesis and cyst growth, including the response to cyclic adenosine monophosphate (cAMP) elevation and the dependence on ATP signaling. We found that in vitro cysts developed only from principal-like but not from intercalated-like MDCK cell clones. This specificity could be verified in mixed MDCK cultures enriched for principal- or intercalated-like cells. In vitro cyst growth upon elevation of intracellular cAMP was mainly driven by fluid secretion, rather than increased cell proliferation. The cAMP-dependent fluid secretion was found to depend on extracellular adenosine-5'-triphosphate (ATP) and to act synergistically with purinergic signaling, as the use of the ATP scavenger apyrase, as well as the P2 receptor inhibitor suramin, reduced cAMP-driven fluid secretion, while increasing extracellular ATP potentiated cAMP-mediated cyst growth. In conclusion, we provide in vitro evidence for the ability of principal rather than intercalated cells to form cysts, based on a synergism of cAMP and ATP signaling in enhancing apical fluid secretion.

The contribution of adhesion signaling to lactogenesis

The mammary gland undergoes hormonally controlled cycles of pubertal maturation, pregnancy, lactation, and involution, and these processes rely on complex signaling mechanisms, many of which are controlled by cell-cell and cell-matrix adhesion. The adhesion of epithelial cells to the extracellular matrix initiates signaling mechanisms that have an impact on cell proliferation, survival, and differentiation throughout lactation. The control of integrin expression on the mammary epithelial cells, the composition of the extracellular matrix and the presence of secreted matricellular proteins all contribute to essential adhesion signaling during lactogenesis. In vitro and in vivo studies, including the results from genetically engineered mice, have shed light on the regulation of these processes at the cell and tissue level and have led to increased understanding of the essential signaling components that are regulated in temporal and cell specific manner during lactogenesis. Recent studies suggest that a secreted matricellular protein, CTGF/CCN2, may play a role in lactogenic differentiation through binding to β1 integrin complexes, enhancing the production of extracellular matrix components and contributions to cell adhesion signaling.

Src is a major signaling component for CTGF induction by TGF-beta1 in osteoblasts

Connective tissue growth factor (CTGF/CCN2) is induced by transforming growth factor beta1 (TGF-beta1) where it acts as a downstream mediator of TGF-beta1 induced matrix production in osteoblasts. We have shown the requirement of Src, Erk, and Smad signaling for CTGF induction by TGF-beta1 in osteoblasts; however, the potential interaction among these signaling pathways remains undetermined. In this study we demonstrate that TGF-beta1 activates Src kinase in ROS17/2.8 cells and that treatment with the Src family kinase inhibitor PP2 prevents Src activation and CTGF induction by TGF-beta1. Additionally, inhibiting Src activation prevented Erk activation, Smads 2 and 3 activation and nuclear translocation by TGF-beta1, demonstrating that Src is an essential upstream signaling partner of both Erk and Smads in osteoblasts. MAPKs such as Erk can modulate the Smad pathway directly by mediating the phosphorylation of Smads or indirectly through activation/inactivation of required nuclear co-activators that mediate Smad DNA binding. When we treated cells with the Erk inhibitor, PD98059, it inhibited TGF-beta1-induced CTGF protein expression but had no effect on Src activation, Smad activation or Smad nuclear translocation. However PD98059 impaired transcriptional complex formation on the Smad binding element (SBE) of the CTGF promoter, demonstrating that Erk activation was required for SBE transactivation. These data demonstrate that Src is an essential upstream signaling transducer of Erk and Smad signaling with respect to TGF-beta1 in osteoblasts and that Smads and Erk function independently but are both essential for forming a transcriptionally active complex on the CTGF promoter in osteoblasts.

Phase 1 study of anti-CTGF monoclonal antibody in patients with diabetes and microalbuminuria

BACKGROUND AND OBJECTIVES

This report summarizes the first phase 1 trial treating patients with microalbuminuric diabetic kidney disease (DKD) using FG-3019, a human monoclonal antibody to connective tissue growth factor (CTGF). CTGF is critically involved in processes of progressive fibrosis, including DKD. This phase 1, open-label, dose-escalation trial evaluated safety, pharmacokinetics, and possible therapeutic effects of FG-3019 on albuminuria, proteinuria, and tubular proteins.

DESIGN, SETTING, PARTICIPANTS, AND MEASUREMENTS

Microalbuminuric subjects (n = 24) with type 2 (79%) or type 1 (21%) diabetes received 3 or 10 mg/kg FG-3019 dosed intravenously every 14 days for four doses. Albuminuria and safety follow-up were to days 62 and 365, respectively.

RESULTS

No infusion was interrupted for symptoms, although 5 of 24 subjects had mild infusion-day adverse events thought to be possibly drug-related. No subject developed anti-FG-3019 antibodies. FG-3019 clearance was lower at 10 mg/kg than at 3 mg/kg, suggesting a saturable elimination pathway. Although this study was not designed for efficacy testing, it was notable that urinary albumin/creatinine ratio (ACR) decreased significantly from mean pretreatment ACR of 48 mg/g to mean post-treatment (day 56) ACR of 20 mg/g (P = 0.027) without evidence for a dose-response relationship.

CONCLUSIONS

Treatment of microalbuminuric DKD subjects using FG-3019 was well tolerated and associated with a decrease in albuminuria. The data demonstrate a saturable pathway for drug elimination, minimal infusion adverse events, and no significant drug-attributable adverse effects over the year of follow-up. Changes in albuminuria were promising but require validation in a prospective, randomized, blinded study.

Linking cell structure to gene regulation: signaling events and expression controls on the model genes PAI-1 and CTGF

The microtubule and microfilament cytoskeletal systems as well as cell-to-cell contacts and cell-matrix interactions are critical regulators of cell structure and function. Alterations in cell shape profoundly influence signaling events and gene expression programs that impact a spectrum of biological responses including cell growth, migration and apoptosis. These same pathways also contribute to the progression of several important pathologic conditions (e.g., arteriosclerosis, vascular fibrosis, and endothelial dysfunction). Indeed, hemodynamic forces in the vascular compartment are established modifiers of endothelial and smooth muscle cell cytoarchitecture and orchestrate complex genetic and biological responses in concert with contributions from the extracellular matrix (ECM), growth factors (e.g., EGF, and TGF-beta) and cell adhesion receptors (e.g., integrins, and cadherins). The profibrotic matricellular proteins plasminogen activator inhibitor-1 (PAI-1) and connective tissue growth factor (CTGF) are prominent members of a subset of genes the expression of which is highly responsive to cell shape-altering stimuli (i.e., disruption of the actin-based and microtubule networks, shear strain and cyclic stretch). Since both PAI-1 and CTGF are major mediators of cardiovascular fibrotic disease, understanding cell structure-linked signaling cascades provides potential avenues for focused therapy. It is increasingly evident that growth factor receptors (EGFR) are activated by changes in cytoarchitecture and that the "repressive state" of certain signaling proteins (e.g., SMAD, and Rho-GEFs) is maintained by sequestration on cell structural networks. Functional repression can be relieved by cytoskeletal perturbations (e.g., in response to treatment with network-specific drugs) resulting in activation of signaling cascades (e.g., Rho, and MAPK) with associated changes in gene reprogramming. Recent studies document a complex network of both similar and unique signaling control elements leading to the induction of PAI-1 and CTGF in response to modifications in cell shape. The purpose of this review is to highlight our current understanding of "cell deformation"-responsive signaling cascades focusing on the potential value of targeting such pathways, and their model response genes (e.g., PAI-1, and CTGF), as a therapeutic option for the treatment of fibrotic diseases.

Downregulation of connective tissue growth factor by three-dimensional matrix enhances ovarian carcinoma cell invasion

Epithelial ovarian carcinoma (EOC) is a leading cause of death from gynecologic malignancies, due mainly to the prevalence of undetected metastatic disease. The process of cell invasion during intraperitoneal anchoring of metastatic lesions requires concerted regulation of many processes, including modulation of adhesion to the extracellular matrix and localized invasion. Exploratory cDNA microarray analysis of early response genes (altered after 4 hr of 3D collagen culture) coupled with confirmatory real-time reverse-transcriptase polymerase chain reaction, multiple 3D cell culture matrices, Western blot, immunostaining, adhesion, migration and invasion assays were used to identify modulators of adhesion pertinent to EOC progression and metastasis. cDNA microarray analysis indicated a dramatic downregulation of connective tissue growth factor (CTGF) in EOC cells placed in invasion- mimicking conditions (3D Type I collagen). Examination of human EOC specimens revealed that CTGF expression was absent in 46% of the tested samples (n = 41), but was present in 100% of normal ovarian epithelium samples (n = 7). Reduced CTGF expression occurs in many types of cells and may be a general phenomenon displayed by cells encountering a 3D environment. CTGF levels were inversely correlated with invasion such that downregulation of CTGF increased, while its upregulation reduced collagen invasion. Cells adhered preferentially to a surface comprised of both collagen I and CTGF relative to either component alone using alpha6beta1 and alpha3beta1 integrins. Together these data suggest that downregulation of CTGF in EOC cells may be important for cell invasion through modulation of cell-matrix adhesion.

Cytoskeletal reorganisation, 1alpha,25-dihydroxy vitamin D3 and human MG63 osteoblast maturation

Bone tissue is especially receptive to physical stimulation and agents with the capacity to mimic the signalling incurred via mechanical loading on osteoblasts may find an application in a bone regenerative setting. Recently this laboratory revealed that the major serum lipid, lysophosphatidic acid (LPA), co-operated with 1alpha,25-dihydroxy vitamin D3 (D3) in stimulating human osteoblast maturation. Actin stress fiber accrual in LPA treated osteoblasts would have generated peripheral tension which in turn may have heightened the maturation response of these cells to D3. To test this hypothesis we examined if other agents known to trigger stress fiber accumulation co-operated with D3 in stimulating human osteoblast maturation. Colchicine, nocodazole and LPA all co-operated with D3 to promote MG63 maturation in a MEK dependent manner. In contrast, calpeptin, a direct activator of Rho kinase and stress fiber accumulation did not act with D3 to secure MG63 differentiation. Herein we describe how the signalling elicited via microtubule disruption cooperates with D3 in the development of mature osteoblasts.

Cell type-specific regulation of CCN2 protein expression by PI3K-AKT-FoxO signaling

The biological activity of connective tissue growth factor (CTGF, CCN2) is regulated at the level of intracellular signaling leading to gene expression, and by its extracellular interaction partners which determine the functional outcome of CCN2 action. In this overview, we summarize the data which provide evidence that one of the major signaling pathways, phosphatidylinositol-3 kinase (PI3K)-AKT signaling, shows a remarkable cell type-dependence in terms of regulation of CCN2 expression. In smooth muscle cells, fibroblasts, and epithelial cells, inhibition of this pathway either reduced CCN2 expression or was not involved in CCN2 gene expression depending on the stimulus used. In microvascular endothelial cells by contrast, activation of PI3K-AKT signaling was inversely related to CCN2 expression. Upregulation of CCN2 upon inhibition of PI3K-AKT was also observed in primary cultures of human endothelial cells (HUVEC) exposed to laminar flow in an in vitro flow-through system. In different types of endothelial cells, FoxO transcription factors, which are negatively regulated by AKT, were identified as potent activators of CCN2 gene expression. In HUVEC, we observed a correlation between enhanced nuclear localization of FoxO1 and increased synthesis of CCN2 protein in areas of non-uniform shear stress. These data indicate that FoxO proteins are key regulators of CCN2 gene expression which determine the effect of PI3K-AKT activation in terms of CCN2 regulation. Short summary Phosphatidylinositol-3 kinase (PI3K)-AKT signaling shows a remarkable cell type-dependence in terms of regulation of CCN2 expression. In endothelial cells activation of PI3K - AKT signaling was inversely related to CCN2 expression. FoxO transcription factors, which are negatively regulated by AKT, were identified as potent activators of CCN2 gene expression.

Connective tissue growth factor and cardiac fibrosis

Cardiac fibrosis is a major pathogenic factor in a variety of cardiovascular diseases and refers to an excessive deposition of extracellular matrix components in the heart, which leads to cardiac dysfunction and eventually overt heart failure. Evidence is accumulating for a crucial role of connective tissue growth factor (CTGF) in fibrotic processes in several tissues including the heart. CTGF orchestrates the actions of important local factors evoking cardiac fibrosis. The central role of CTGF as a matricellular protein modulating the fibrotic process in cardiac remodelling makes it a possible biomarker for cardiac fibrosis and a potential candidate for therapeutic intervention to mitigate fibrosis in the heart.

Differential requirement for MEK/ERK and SMAD signaling in PAI-1 and CTGF expression in response to microtubule disruption

Colchicine and nocodazole, both established microtubule disruptors, are useful tools to investigate cytoskeletal-dependent signaling cascades and the associated downstream transcriptional targets. Since cytoskeletal events impact pathophysiologic consequences in the vascular system, the signaling requirements underlying colchicine-stimulated expression of PAI-1 and CTGF, two prominent cell deformation-sensitive fibrosis-initiating proteins, were evaluated in vascular smooth muscle cells. Microtubule disruption rapidly induced EGFR transactivation (at the src kinase-sensitive EGFR(Y845) site) in a ROS-dependent manner. Genetic deficiency of EGFR, inhibition of EGFR signaling with AG1478 or introduction of a kinase-deficient EGFR construct effectively blocked colchicine-stimulated PAI-1 and CTGF expression. MEK/ERK involvement downstream of ROS generation was critical for PAI-1, but not CTGF, expression following cytoskeletal perturbation suggesting bifurcation of signaling pathways downstream of EGFR activation. Colchicine also stimulated SMAD2/3 phosphorylation by a Rho/ROCK-dependent mechanism independent of TGF-beta1 release or receptor activity. Rho/ROCK signaling initiated by tubulin network collapse was required for both CTGF and PAI-1 induction. Colchicine-initiated SMAD3 phosphorylation, however, was essential for PAI-1, but not CTGF, expression further highlighting divergence of signaling events downstream of Rho/ROCK that mediate microtubule deformation-associated changes in profibrotic gene transcription.

Up-regulation of connective tissue growth factor in endothelial cells by the microtubule-destabilizing agent combretastatin A-4

Incubation of microvascular endothelial cells with combretastatin A-4 phosphate (CA-4P), a microtubule-destabilizing compound that preferentially targets tumor vessels, altered cell morphology and induced scattering of Golgi stacks. Concomitantly, CA-4P up-regulated connective tissue growth factor (CTGF/CCN2), a pleiotropic factor with antiangiogenic properties. In contrast to the effects of other microtubule-targeting agents such as colchicine or nocodazole, up-regulation of CTGF was only detectable in sparse cells, which were not embedded in a cell monolayer. Furthermore, CA-4P induced CTGF expression in endothelial cells, forming tube-like structures on basement membrane gels. Up-regulation of CTGF by CA-4P was dependent on Rho kinase signaling and was increased when p42/44 mitogen-activated protein kinase was inhibited. Additionally, FoxO transcription factors were identified as potent regulators of CTGF expression in endothelial cells. Activation of FoxO transcription factors by inhibition of phosphatidylinositol 3-kinase/AKT signaling resulted in a synergistic increase in CA-4P-mediated CTGF induction. CA-4P-mediated expression of CTGF was thus potentiated by the inhibition of kinase pathways, which are targets of novel antineoplastic drugs. Up-regulation of CTGF by low concentrations of CA-4P may thus occur in newly formed tumor vessels and contribute to the microvessel destabilization and antiangiogenic effects of CA-4P observed in vivo.

Tendon healing in vivo: gene expression and production of multiple growth factors in early tendon healing period

PURPOSE

The actions of growth factors during healing of injured flexor tendons are not well characterized, although information pertinent to some individual growth factors is available. We studied gene expression and protein production of a number of growth factors at several time points during the early healing period in a chicken model.

METHODS

Seventy-four long toes of 37 white Leghorn chickens were used. The flexor digitorum profundus tendons of 60 toes were surgically repaired after complete transection and were harvested for analysis 3, 5, 7, 9, 14, and 21 days after surgery. The expression of 6 growth factors was studied at 4 time points after surgery with real-time quantitative polymerase chain reactions, and production and distribution of 3 growth factors at all 6 time points were studied by immunohistochemical staining with antibodies. Fourteen tendons that had no surgery served as day 0 controls. Tendon healing status was also assessed histologically.

RESULTS

Throughout the early tendon healing period, connective tissue growth factor (CTGF) and transforming growth factor beta (TGF-beta) showed high levels of gene expression. Levels of gene expression of vascular endothelial growth factor (VEGF) and insulin-like growth factor 1 (IGF-1) were high or moderately high. Expression of the TGF-beta gene was upregulated after injury, whereas the basic fibroblast growth factor (bFGF) gene was downregulated at all postsurgical time points and expressed at the lowest levels among 6 growth factor genes 2 to 3 weeks after surgery. The platelet-derived growth factor B (PDGF-B) gene was also minimally expressed. Findings of immunohistochemistry corresponded to TGF-beta, bFGF, and IGF-1 gene expression.

CONCLUSIONS

In this model, up to 3 weeks after surgery, gene expression and production of TGF-beta are high and are upregulated in this healing period. However, expression of the bFGF gene and protein is low and decreases in the healing tendon. The CTGF, VEGF, and IGF-1 genes are expressed at high or moderately high levels, but PDGF-B is minimally expressed.

The small GTPase Rac-1 is a regulator of mesangial cell morphology and thrombospondin-1 expression

Thrombospondin-1 (TSP-1), which is synthesized by mesangial cells, is known for its anti-angiogenic activity and its ability to activate latent TGF-beta. TSP-1 is upregulated in renal diseases associated with tissue remodeling. Therefore, we hypothesized that the expression of TSP-1 might be modulated by changes in cell morphology involving proteins of the Rho family. Spreading of mesangial cells after detachment and reseeding was characterized by the formation of lamellipodia and focal adhesions, pointing toward a Rac-1-mediated rearrangement of actin structures. Clustering of focal adhesion proteins was also observed in a model system of nocodazole-induced disruption of microtubules. These morphological alterations were impeded by pharmacological inhibition of Src family kinases, of the small GTPase Rac-1, or by downregulation of Rac-1 by siRNA. Upon cell spreading, TSP-1 was upregulated in the absence and much more prominently in the presence of serum, but also after nocodazole treatment. TSP-1 upregulation was controlled by activation of Src family kinases, ERK 1/2 and Rac-1, whereas activation of RhoA-ROCK signaling was not linked to TSP-1 induction. We thus provide evidence that TSP-1 expression is induced by common signaling pathways, which are activated by morphological alterations of renal mesangial cells or by soluble factors as contained in serum, and these pathways include Src family kinases, ERK 1/2 and Rac-1. Our data suggest that tissue remodeling activates gene expression of pathophysiologically relevant proteins such as TSP-1.

The Contributions of Integrin Affinity and Integrin-Cytoskeletal Engagement in Endothelial and Smooth Muscle Cell Adhesion to Vitronectin

The serine proteinase inhibitor, plasminogen activator inhibitor type-1 (PAI-1), binds to the adhesion protein vitronectin with high affinity at a site that is located directly adjacent to the vitronectin RGD integrin binding sequence. The binding of PAI-1 to vitronectin sterically blocks integrin access to this site and completely inhibits the binding of purified integrins to vitronectin; however, its inhibition of endothelial and smooth muscle cell adhesion to vitronectin is at most 50-75%. Because PAI-1 binds vitronectin with approximately 10-100-fold higher affinity than purified integrins, we have analyzed the mechanism whereby these cells are able to overcome this obstacle. Our studies exclude proteolytic removal of PAI-1 from vitronectin as the mechanism, and show instead that cell adhesion in the presence of PAI-1 is dependent on integrin-cytoskeleton engagement. Disrupting endothelial or smooth muscle cell actin polymerization and/or focal adhesion assembly reduces cell adhesion to vitronectin in the presence of PAI-1 to levels similar to that observed for the binding of purified integrins to vitronectin. Furthermore, endothelial cell, but not smooth muscle cell adhesion to vitronectin in the presence of PAI-1 requires both polymerized microtubules and actin, further demonstrating the importance of the cytoskeleton for integrin-mediated adhesion. Finally, we show that cell adhesion in the presence of PAI-1 leads to colocalization of PAI-1 with the integrins alphavbeta3 and alphavbeta5 at the cell-matrix interface.

Pharmacological inhibition of RhoA signaling prevents connective tissue growth factor induction in endothelial cells exposed to non-uniform shear stress

Shear stress changes play an important role in atheroma formation. This study focussed on atherogenic protein expression under non-uniform shear stress and the pharmacological modulation of shear-related endothelial dysfunction. Bifurcating flow-through cell culture slides were used to expose HUVECs to steady laminar or non-uniform shear stress for 18 h at 10 dyn/cm(2). Protein expression was determined by immunofluorescence, and quantified using MetaVue software. Laminar shear stress resulted in cell alignment, reduced F-actin fibers, and significant induction of endothelial nitric oxide synthase expression. Under non-uniform shear stress at bifurcations, minor upregulation of adhesion molecules was observed. Connective tissue growth factor (CTGF) was significantly downregulated by laminar shear stress and induced in cells exposed to non-uniform shear stress. CTGF upregulation by non-uniform shear stress was RhoA-dependent, because it was almost completely inhibited in cells transfected with dominant negative RhoA-N19, and when cells were treated with 1 micromol/L simvastatin during flow. Pre-incubation of HUVECs with inhibitors of Rho-associated kinase before exposure to flow significantly suppressed the CTGF induction in regions of non-uniform shear stress. In conclusion, non-uniform shear stress-dependent CTGF expression requires active RhoA and can be prevented pharmacologically. Interference with shear stress-induced protein expression may inhibit endothelial dysfunction in atheroprone vessel regions.

Mechanical regulation of the Cyr61/CCN1 and CTGF/CCN2 proteins

Cells in various anatomical locations are constantly exposed to mechanical forces from shear, tensile and compressional forces. These forces are significantly exaggerated in a number of pathological conditions arising from various etiologies e.g., hypertension, obstruction and hemodynamic overload. Increasingly persuasive evidence suggests that altered mechanical signals induce local production of soluble factors that interfere with the physiologic properties of tissues and compromise normal functioning of organ systems. Two immediate early gene-encoded members of the family of the Cyr61/CTGF/Nov proteins referred to as cysteine-rich protein 61 (Cyr61/CCN1) and connective tissue growth factor (CTGF/CCN2), are highly expressed in several mechanical stress-related pathologies, which result from either increased externally applied or internally generated forces by the actin cytoskeleton. Both Cyr61 and CTGF are structurally related but functionally distinct multimodular proteins that are expressed in many organs and tissues only during specific developmental or pathological events. In vitro assessment of their biological activities revealed that Cyr61 expression induces a genetic reprogramming of angiogenic, adhesive and structural proteins while CTGF promotes distinctively extracellular matrix accumulation (i.e., type I collagen) which is the principal hallmark of fibrotic diseases. At the molecular level, expression of the Cyr61 and CTGF genes is regulated by alteration of cytoskeletal actin dynamics orchestrated by various components of the signaling machinery, i.e., small Rho GTPases, mitogen-activated protein kinases, and actin binding proteins. This review discusses the mechanical regulation of the Cyr61 and CTGF in various tissues and cell culture models with a special attention to the cytoskeletally based mechanisms involved in such regulation.

STAT3-independent inhibition of lysophosphatidic acid-mediated upregulation of connective tissue growth factor (CTGF) by cucurbitacin I

Cucurbitacins are recognised as anti-tumour agents because of their interference with STAT3 signalling, but may also affect the integrity of the actin cytoskeleton. In the present study the effect of cucurbitacin I was investigated in fibroblasts. In these cells, cucurbitacin I interfered with lysophosphatidic acid (LPA) signalling. It inhibited tyrosine phosphorylation of focal adhesion proteins and induction of connective tissue growth factor (CTGF), a potent profibrotic protein. Inhibition of Src family kinases with PP2, but not the inactive analogue PP3, also interfered with LPA-mediated tyrosine phosphorylation and induction of CTGF. Jak2-STAT3 signalling seemed to be the connecting link, because CTGF induction was sensitive to AG490, an inhibitor of Jak2, and cucurbitacin I, an inhibitor of Jak2 and STAT3. However, LPA did not activate tyrosine phosphorylation of STAT3. Furthermore, cucurbitacin I was as effective in STAT3 knock out cells as in control cells. Therefore, the inhibitory effect of cucurbitacin I was not related to inhibition of STAT3. Immunocytochemical analysis of cucurbitacin I-treated cells revealed disassembly of F-actin fibres, reorganisation into F-actin patches and resolution of focal adhesions. The phenotypic changes resembled changes observed after treatment of the cells with cytochalasin D, which has been shown to interfere with CTGF induction. Concentrations of cucurbitacin I, which have been shown to target Jak2-STAT3 signalling, thus, profoundly affect the actin cytoskeleton and may therefore modulate cell morphology, migration, adherence and gene expression also in non-tumour cells.