Activation of specific MEK-ERK cascade is necessary for TGFbeta signaling and crosstalk with PKA and PKC pathways in cultured rat articular chondrocytes

JunD Is Required for Proliferation of Prostate Cancer Cells and Plays a Role in Transforming Growth Factor-β (TGF-β)-induced Inhibition of Cell Proliferation

TGF-β inhibits proliferation of prostate epithelial cells. However, prostate cancer cells in advanced stages become resistant to inhibitory effects of TGF-β. The intracellular signaling mechanisms involved in differential effects of TGF-β during different stages are largely unknown. Using cell line models, we have shown that TGF-β inhibits proliferation in normal (RWPE-1) and prostate cancer (DU145) cells but does not have any effect on proliferation of prostate cancer (PC3) cells. We have investigated the role of Jun family proteins (c-Jun, JunB, and JunD) in TGF-β effects on cell proliferation. Jun family members were expressed at different levels and responded differentially to TGF-β treatment. TGF-β effects on JunD protein levels, but not mRNA levels, correlated with its effects on cell proliferation. TGF-β induced significant reduction in JunD protein in RWPE-1 and DU145 cells but not in PC3 cells. Selective knockdown of JunD expression using siRNA in DU145 and PC3 cells resulted in significant reduction in cell proliferation, and forced overexpression of JunD increased the proliferation rate. On the other hand, knockdown of c-Jun or JunB had little, if any, effect on cell proliferation; overexpression of c-Jun and JunB decreased the proliferation rate in DU145 cells. Further studies showed that down-regulation of JunD in response to TGF-β treatment is mediated via the proteasomal degradation pathway. In conclusion, we show that specific Jun family members exert differential effects on proliferation in prostate cancer cells in response to TGF-β, and inhibition of cell proliferation by TGF-β requires degradation of JunD protein.

Notoginsenoside Rb1 inhibits activation of ERK and p38 MAPK pathways induced by hypoxia and hypercapnia

The aim of the present study was to investigate the effect of notoginsenoside Rb1 (Rb1) on the ERK and p38 MAPK pathways in primary cultured pulmonary arterial smooth muscle cells (PASMCs) exposed to hypoxia and hypercapnia, in order to elucidate the mechanism underlying the effect of Rb1 on hypoxia and hypercapnia-induced pulmonary vasoconstriction (HHPV). PASMCs were isolated from Sprague-Dawley rats. The cells were divided into five groups: Normal (N), hypoxia and hypercapnia (H), Rb_L, Rb_M and Rb_H groups. N group cells were cultured under 5% CO₂ and 21% O₂. H, Rb_L, Rb_M and Rb_H groups were cultured under 6% CO₂ and 1% O₂. Prior to the hypoxia and hypercapnia exposure, Rb_L, Rb_M and Rb_H groups were treated with 8, 40 and 100 mg/ml Rb1 for 30 min, respectively. Phosphorylated extracellular signal-regulated kinase (P-ERK) and P-p38 protein, and ERK1/2 and p38 mRNA expression levels were detected using western blot and semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) analyses, respectively. The correlations between P-ERK protein and ERK1/2 mRNA, and between P-p38 protein and p38 mRNA were evaluated. Results of western blot and RT-PCR showed hypoxia and hypercapnia increased P-ERK and P-p38 protein, and ERK1/2 mRNA, respectively (P<0.05). Rb1 suppressed the increased P-ERK and P-p38 protein, and ERK1/2 and p38 mRNA by hypoxia and hypercapnia (P<0.05). P-ERK protein was positively correlated with ERK1 (r=0.5, P<0.01) and ERK2 mRNA (r=0.977, P<0.01). P-p38 protein was positively correlated with p38 mRNA (r=0.884, P<0.01). Thus, the present results indicate that Rb1 may ameliorate HHPV by suppressing ERK and p38 pathways. The study provides an experimental basis for investigating the clinical use of Rb1 in the management of HHPV-related disorders.

MEK/ERK pathway is positively involved in hypoxia-induced vasculogenic mimicry formation in hepatocellular carcinoma which is regulated negatively by protein kinase A

The aim of present investigation is to explore the molecular mechanisms of vasculogenic mimicry (VM) induced by hypoxia. Hepatocellular carcinoma cell lines were treated with CoCl2, and the VM-related parameters were assayed by real-time qPCR, Western blotting and immunofluorescence. Matrigel tube structure was also detected. We demonstrated that the expression of pMEK, MEK, pERK1/2 and ERK1/2 had a positive correlation with VM induced by hypoxia in MHCC97H while HepG2 signified VM under normoxia condition. PD98059 was negatively while epidermal growth factor positively participated in the increased tubes and area of VM. At the meaning time, the increased VM-related genes VE-cadherin, MMP2, MMP9, EphA2 and LAMC2 in hypoxia group were down-regulated by PD98059 in a dose-dependent manner. Furthermore, we elucidated that PKA, but not PKC, mediated the MEK/ERK pathway in a negative manner in VM. In conclusion, MEK/ERK pathway is positively involved in VM in hepatocellular carcinoma cell line, which was mediated by PKA negatively.

Treatment of cartilage defects by subchondral drilling combined with covering with atelocollagen membrane induces osteogenesis in a rat model

BACKGROUND

The coverage of the atelocollagen membrane at the chondral defect after subchondral drilling might improve the beneficial effects for cartilage repair because of the prevention of scattering and accumulation of cells and growth factors from bone marrow within the chondral defect. On the other hand, it might block cells and factors derived from the synovium or cause high pressure in the chondral defect, resulting in prevention of cells and growth factors gushing out from the bone marrow, which leads to disadvantages for cartilage repair.

METHOD

We tested this hypothesis in a 2-mm-diameter chondral defect created in the articular cartilage of the patellar groove in a rat models. Defects were left untreated, or were drilled or drilled and covered with an atelocollagen membrane; healing was evaluated by histology and gene expression analysis using real-time polymerase chain reaction and immunohistochemistry.

RESULTS

Membrane coverage induced bone tissue ingrowth into the punched chondral defect. At 1 week, expression of TGFβ, Sox9, Runx2, osteocalcin, Col1a1, and Col2a1 in the drilling group was significantly higher than in the covering group. At 4 weeks, expressions of TGFβ, Runx2, and Col1a1 were all significantly higher in the drilling group, while Sox9, osteocalcin, and Col2a1 were significantly higher in the covering group. Immunohistochemistry demonstrated Sox9, osteocalcin, and type II collagen on the bony reparative tissue in the covering group.

CONCLUSIONS

These results suggest that the atelocollagen membrane coverage resulted in inhibition of cartilage repair.

Role of insulin-like growth factors (IGFs), their receptors and genetic regulation in the chondrogenesis and growth of the mandibular condylar cartilage

Growth of the mandibular condylar cartilage (MCC) is reviewed as a function of genetic and epigenetic factors. The growth centers around the differential spatial concentration of the chondrocytes, influence of growth factors like TGF-β and heterogeneity in the number of IGF receptors, control the action of IGF. Besides these factors, growth of the mandibular condyle is influenced by differential response of chondrocytes as a function of their source/ageing, which in turn is regulated by TGF-β, BMPs and IGFs. While IGF-1 promotes proteoglycan synthesis and survival of the chondrocytes to maintain cartilage homeostasis, TGF-β synergistically catalysed the effect of IGF-1, while BMPs catalysed proteolysis as and when physiologically needed. To understand these processes, role of IGF-1 and its six receptors is at the center to a number of physiological processes being regulated by its mode of application for the growth and differentiation. Probing deeper, biological functions of IGFs seemed to depend on their level of free status rather than bound status to respective IGF-binding proteins (IGF-BPs), considered prerequisite to modulate their biological functions. Genetic regulation of their secretion has thrown light on their insulin-like structural homology, level and response in osteo-arthritis (OA), rheumatic arthritis (RA) and diabetes type-II. Biochemistry and spatial distribution of IGF receptors in different domains exerts control on IGF-1 activities. In ultimate analysis, IGF-axis conserved during the evolution to regulate cell growth and proliferation affect nearly every organ in the body as judged from the techniques determining skeletal maturity and decision making dependent on it for orthodontic, orthognathic/orthopedic and dental implant applications.

An update on transforming growth factor-β (TGF-β): sources, types, functions and clinical applicability for cartilage/bone healing

Transforming growth factor-β (TGF-β) has been reviewed for its sources, types of isoforms, biochemical effects on cartilage formation/repair, and its possible clinical applications. Purification of three isoforms (TGF-β-1, β-2 and β-3) and their biochemical characterization revealed mainly their homo-dimer nature, with heterodimers in traces, each monomer comprised of 112 amino acids and MW. of 12 500 Da. While histo-chemical staining by a variety of dyes has revealed precise localization of TGF-β in tissues, immune-blot technique has thrown light on their expression as a function of age (neonatal vs. adult), as also on its quantum in an active and latent state. X-ray crystallographic studies and nuclear magnetic resonance (NMR) analysis have unraveled mysteries of their three-dimensional structures, essential for understanding their functions. Their similarities have led to interchangeability in assays, while differences have led to their specialized clinical applicability. For this purpose, their latent (inactive) form is changed to an active form through enzymatic processes of phosphorylation/glycosylation/transamination/proteolytic degradation. Their functions encompass differentiation and de-differentiation of chondrocytes, synthesis of collagen and proteoglycans (PGs) and thereby maintain homeostasis of cartilage in several degenerative diseases and repair through cell cycle signaling and physiological control. While several factors affecting their performance are already identified, their interplay and chronology of sequences of functions is yet to be understood. For its success in clinical applications, challenges in judicious dealing with the factors and their interplay need to be understood.

Effect of interleukin-1beta on osteogenic protein 1-induced signaling in adult human articular chondrocytes

OBJECTIVE

Two major receptor-activated Smad (R-Smad) signaling pathways, bone morphogenetic protein (BMP) and MAPK, were examined in a model of interleukin-1beta (IL-1beta)-induced cartilage degeneration to investigate the effect of IL-1beta on osteogenic protein 1 (OP-1) signaling in adult human articular chondrocytes.

METHODS

Chondrocytes from the ankles of 26 normal human donors were cultured in high-density monolayers in serum-free medium. The effect of IL-1beta on BMP receptors was studied by reverse transcription-polymerase chain reaction and flow cytometry. Phosphorylation of R-Smads was tested in cells treated with IL-1beta (10 ng/ml), OP-1 (100 ng/ml), or the combination of IL-1beta and OP-1. Cell lysates were analyzed by Western blotting with polyclonal antibodies against 2 R-Smad phosphorylation sites (BMP- and MAPK-mediated) or with total, nonphosphorylated R-Smad as a control. To identify which MAPKs play a role in IL-1beta activation of the linker region, chondrocytes were preincubated with specific MAPK inhibitors (PD98059 for MAP/ERK, SP600125 for JNK, and SB203580 for p38).

RESULTS

IL-1beta reduced the number of activin receptor-like kinase 2 (ALK-2) and ALK-3 receptors, inhibited expression of Smad1 and Smad6, delayed and prematurely terminated the onset of OP-1-mediated R-Smad phosphorylation, and affected nuclear translocation of R-Smad/Smad4 complexes. The alternative phosphorylation of R-Smad in the linker region via the MAPK pathway (primarily p38 and JNK) was observed to be a possible mechanism through which IL-1beta offsets OP-1 signaling and the response to OP-1. Conversely, OP-1 was found to directly inhibit phosphorylation of p38.

CONCLUSION

These findings describe new mechanisms of the crosstalk between OP-1 and IL-1beta in chondrocytes. The study also identifies potential targets for therapeutic interventions in the treatment of cartilage-degenerative processes.

Synergistic role of c-Myc and ERK1/2 in the mitogenic response to TGF beta-1 in cultured rat nucleus pulposus cells

Introduction

Although transforming growth factor β1 (TGFβ1) is known to be a potent inhibitor of proliferation in most cell types, it accelerates proliferation in certain mesenchymal cells, such as articular chondrocytes and nucleus pulposus cells. The low ability for self-renewal of nucleus pulposus cells is one obstacle in developing new therapeutic options for intervertebral disc diseases, and utilizing cytokines is one of the strategies to regulate nucleus pulposus cell proliferation. However, the precise cell cycle progression and molecular mechanisms by which TGFβ1 stimulates cell growth remain unclear. The aim of this study was to elucidate a mechanism that enables cell proliferation with TGFβ1 stimulation.

Methods

We tested cultured rat nucleus pulposus cells for proliferation and cell cycle distribution under exogenous TGFβ1 stimulation with and without putative pharmaceutical inhibitors. To understand the molecular mechanism, we evaluated the expression levels of key regulatory G₁ phase proteins, c-Myc and the cyclin-dependent kinase inhibitors.

Results

We found that TGFβ1 promoted proliferation and cell cycle progression while reducing expression of the cyclin-dependent kinase inhibitors p21 and p27, which are downregulators of the cell cycle. Robust c-Myc expression for 2 h and immediate phosphorylation of extra cellular signal regulated kinase (ERK1/2) were detected in cultures when TGFβ1 was added. However, pretreatment with 10058-F4 (an inhibitor of c-Myc transcriptional activity) or PD98059 (an inhibitor of ERK1/2) suppressed c-Myc expression and ERK1/2 phosphorylation, and inhibited cell cycle promotion by TGFβ1.

Conclusions

Our experimental results indicate that TGFβ1 promotes cell proliferation and cell cycle progression in rat nucleus pulposus cells and that c-Myc and phosphorylated ERK1/2 play important roles in this mechanism. While the difference between rat and human disc tissues requires future studies using different species, investigation of distinct response in the rat model provides fundamental information to elucidate a specific regulatory pathway of TGFβ1.

TGF-beta coordinately activates TAK1/MEK/AKT/NFkB and SMAD pathways to promote osteoclast survival

To better understand the roles of TGF-beta in bone metabolism, we investigated osteoclast survival in response TGF-beta and found that TGF-beta inhibited apoptosis. We examined the receptors involved in promotion of osteoclast survival and found that the canonical TGF-beta receptor complex is involved in the survival response. The upstream MEK kinase TAK1 was rapidly activated following TGF-beta treatment. Since osteoclast survival involves MEK, AKT, and NFkappaB activation, we examined TGF-beta effects on activation of these pathways and observed rapid phosphorylation of MEK, AKT, IKK, IkappaB, and NFkappaB. The timing of activation coincided with SMAD activation and dominant negative SMAD expression did not inhibit NFkappaB activation, indicating that kinase pathway activation is independent of SMAD signaling. Inhibition of TAK1, MEK, AKT, NIK, IKK, or NFkappaB repressed TGF-beta-mediated osteoclast survival. Adenoviral-mediated TAK1 or MEK inhibition eliminated TGF-beta-mediated kinase pathway activation and constitutively active AKT expression overcame apoptosis induction following MEK inhibition. TAK1/MEK activation induces pro-survival BclX(L) expression and TAK1/MEK and SMAD pathway activation induces pro-survival Mcl-1 expression. These data show that TGF-beta-induced NFkappaB activation is through TAK1/MEK-mediated AKT activation, which is essential for TGF-beta to support of osteoclast survival.

Agonists of peroxisome proliferators-activated receptors (PPAR) alpha, beta/delta or gamma reduce transforming growth factor (TGF)-beta-induced proteoglycans' production in chondrocytes

OBJECTIVE

To investigate the potency of selective agonists of peroxisome proliferators-activated receptors' (PPAR) isotypes (alpha, beta/delta or gamma) to modulate the stimulating effect of transforming growth factor-beta1 (TGF-beta1) on proteoglycans' (PGs) synthesis in chondrocytes.

METHOD

Rat chondrocytes embedded in alginate beads and cultured under low serum conditions were exposed to TGF-beta1 (10 ng/ml), alone or in combination with the following agonists: Wy14643 for PPARalpha, GW501516 for PPARbeta/delta, rosiglitazone (ROSI) for PPARgamma, in the presence or absence of PPAR antagonists (GW6471 for PPARalpha, GW9662 for PPARgamma). PGs' synthesis was evaluated by radiolabelled sulphate incorporation and glycosaminoglycans' (GAGs) content by Alcian blue staining of beads and colorimetric 1.9 dimethyl-methylene blue assay after beads' solubilization. Phosphorylation of Extracellular Signal-related Kinase1/2 (ERK1/2), Smad2/3 and p38-MAPK was assessed by Western Blot and production of prostaglandin E2 (PGE2) by Enzyme immuno-assay (EIA). Levels of mRNA for PPAR target genes [acyl-CoA oxidase (ACO) for PPARalpha; mitochondrial carnitin palmitoyl transferase-1 (CPT-1) for PPARbeta/delta and adiponectin for PPARgamma], aggrecan, TGF-beta1 and genes controlling GAGs' side chains' synthesis were quantified by real time polymerase chain reaction and normalized over RP29 housekeeping gene.

RESULTS

ACO was selectively up-regulated by 100 microM of Wy14643, CPT-1 by 100 nM of GW501516 and adiponectin by 10 microM of ROSI without cell toxicity. TGF-beta1 increased PGs' synthesis by four-fold, GAGs' content and deposition by 3.5-fold and six-fold, respectively, while inducing aggrecan expression around 10-fold without modifying mRNA levels of GAGs' controlling enzymes. PPAR agonists inhibited the stimulating effect of TGF-beta1 by 24-44% on PGs' synthesis and over 75% on aggrecan, GAGs' content and deposition with the following rank order of potency: ROSI>GW501516> or =Wy14643. TGF-beta1-induced phosphorylation of Smad2/3 and ERK1/2 was reduced by ROSI over GW501516 but not by Wy14643 whereas stimulated PGE2 production was inhibited by Wy14643 over GW501516 but not by ROSI. The effect of PPAR agonists on PPAR target genes and TGF-beta1-induced aggrecan expression was reversed selectively by PPAR antagonists.

CONCLUSION

In chondrocytes' beads, PPAR agonists reduced the stimulating effect of TGF-beta1 on PGs by inhibiting TGF-beta1-induced aggrecan expression in an isotype-selective manner. Thus, PPAR agonists could be deleterious in situation of cartilage repair although being protective in situation of cartilage degradation.

Dynamic compression regulates the expression and synthesis of chondrocyte-specific matrix molecules in bone marrow stromal cells

The overall objective of the present study was to investigate the mechanotransduction of bovine bone marrow stromal cells (BMSCs) through the interactions between transforming growth factor beta1 (TGF-beta1), dexamethasone, and dynamic compressive loading. Overall, the addition of TGF-beta1 increased cell viability, extracellular matrix (ECM) gene expression, matrix synthesis, and sulfated glycosaminoglycan content over basal construct medium. The addition of dexamethasone further enhanced extracellular matrix gene expression and protein synthesis. There was little stimulation of ECM gene expression or matrix synthesis in any medium group by mechanical loading introduced on day 8. In contrast, there was significant stimulation of ECM gene expression and matrix synthesis in chondrogenic media by dynamic loading introduced on day 16. The level of stimulation was also dependent on the medium supplements, with the samples treated with basal medium being the least responsive and the samples treated with TGF-beta1 and dexamethasone being the most responsive at day 16. Both collagen I and collagen II gene expressions were more responsive to dynamic loading than aggrecan gene expression. Dynamic compression upregulated Smad2/3 phosphorylation in samples treated with basal and TGF-beta1 media. These findings suggest that interactions between mechanical stimuli and TGF-beta signaling may be an important mechanotransduction pathway for BMSCs, and they indicate that mechanosensitivity may vary during the process of chondrogenesis.

Transforming growth factor beta2-induced myofibroblastic differentiation of human retinal pigment epithelial cells: regulation by extracellular matrix proteins and hepatocyte growth factor

Retinal pigment epithelial (RPE) cells possess the potential to transdifferentiate into myofibroblasts after stimulation with transforming growth factor beta (TGFbeta) and are implicated in the pathogenesis of proliferative vitreoretinopathy. In this study we evaluated how TGFbeta2 and various extracellular matrix (ECM) proteins modulate the transdifferentiation of human fetal retinal pigment epithelial cells (RPE) cells into myofibroblast-like cells. Furthermore, we investigated whether hepatocyte growth factor (HGF) can suppress this transdifferentiation. RPE cells were cultured on ECM coated or uncoated surfaces in the presence or absence of TGFbeta2. HGF was added to certain cultures only once or on a daily basis during the treatment. Transdifferentiation of RPE cells into myofibroblasts was assessed by the quantitation of alpha-smooth muscle actin (alpha-SMA) using immunocytochemistry, flow cytometry, real-time PCR and Western blotting. TGFbeta2 induced a significant increase of alpha-SMA expression in a dose-dependent manner. Compared with growth on uncoated surfaces, RPE cultured on fibronectin (FN)-coated surfaces and stimulated with TGFbeta2 showed a significantly higher alpha-SMA expression than untreated cells. This upregulation of alpha-SMA could be markedly reduced by daily treatment with HGF; however, a single HGF administration did not significantly reduce alpha-SMA. These findings are important for further understanding the interaction of cytokines, RPE cells and their environment in mesenchymal transformation as well as its possible modulation. Continuous or long-term treatment with HGF should be further investigated for its potential to prevent mesenchymal transdifferentiation of RPE cells, and ultimately, PVR in vivo.

Anti-fibrotic effects of theophylline on lung fibroblasts

Theophylline has been used in the management of bronchial asthma and chronic obstructive pulmonary disease for over 50 years. It has not only a bronchodilating effect, but also an anti-inflammatory one conducive to the inhibition of airway remodeling, including subepithelial fibrosis. To date however, whether theophylline has a direct inhibitory effect on airway fibrosis has not been established. To clarify this question, we examined whether theophylline affected the function of lung fibroblasts. Theophylline suppressed TGF-beta-induced type I collagen (COL1) mRNA expression in lung fibroblasts and also inhibited fibroblast proliferation stimulated by FBS and TGF-beta-induced alpha-SMA protein. A cAMP analog also inhibited TGF-beta-induced COL1 mRNA expression in lung fibroblasts. A PKA inhibitor reduced the inhibitory effect of theophylline on TGF-beta-induced COL1 mRNA expression. These results indicate that theophylline exerts anti-fibrotic effects, at least partly, through the cAMP-PKA pathway.

PD-0200347, an alpha2delta ligand of the voltage gated calcium channel, inhibits in vivo activation of the Erk1/2 pathway in osteoarthritic chondrocytes: a PKCalpha dependent effect

OBJECTIVE

To explore the in vivo effects of PD-0200347, an alpha(2)delta ligand of voltage gated Ca(2+) channels, on cell signalling in osteoarthritic (OA) chondrocytes from an experimental dog model, and examine the effect of PD-0200347 on the major signalling pathways involved in OA cartilage degradation.

METHODS

OA was surgically induced in dogs by sectioning the anterior cruciate ligament. OA dogs were divided into three groups and treated orally with (a) placebo; (b) 15 mg/kg/day PD-0200347, or (c) 90 mg/kg/day PD-0200347. The animals were killed 12 weeks after surgery. Cartilage specimens from femoral condyles and tibial plateaus were processed for immunohistochemistry. Specific antibodies against the phosphorylated form of PKCalpha, Ras, c-Raf, the MAP kinases Erk1/2, p38, JNK, and the transcription factors, CREB and Elk-1, were used.

RESULTS

Levels of all the tested signalling mediators were increased in the placebo treated (OA) group compared with the normal group. PD-0200347 treatment significantly reduced the levels of the active forms of PKCalpha, c-Raf, Erk1/2, and Elk-1; however, the levels of the active forms of Ras, p38, JNK, and CREB were not affected by the PD-0200347 treatment.

CONCLUSION

The action of PD-0200347 on OA chondrocytes is probably mediated through the inhibition of Erk1/2 activation via a Ras independent mechanism. This effect is associated with reduction of the activation of transcription factors such as Elk-1, which leads to the inhibition of the induction of the major catabolic factors involved in the degradation process of OA cartilage.

Retinoid signaling regulates CTGF expression in hypertrophic chondrocytes with differential involvement of MAP kinases

Retinoids are important for growth plate chondrocyte maturation, but their downstream effectors remain unclear. Recently, CTGF (CCN2) was found to regulate chondrocyte function, particularly in the hypertrophic zone. The goal of the study was to determine whether CTGF is a retinoid signaling effector molecule, how it is regulated, and how it acts.

INTRODUCTION

Using a combination of in vivo and in vitro approaches, we carried out a series of studies at the cellular, biochemical, and molecular level to determine whether and how retinoid signaling is related to expression and function of connective tissue growth factor (CTGF) in chondrocyte maturation and endochondral ossification.

MATERIALS AND METHODS

Limbs of chick embryos in ovo were implanted with retinoic pan-antagonist RO 41-5253-filled beads, and phenotypic changes were assessed by in situ hybridization. CTGF gene expression and roles were tested in primary cultures of immature and hypertrophic chondrocytes. Cross-talk between retinoid signaling and other pathways was tested by determining endogenous levels of active ERK1/2 and p38 MAP kinases and phenotypic modulations exerted by specific antagonists of mitogen-activated protein (MAP) kinases and BMP signaling (Noggin).

RESULTS

Interference with retinoid signaling blocked expression of CTGF and other posthypertrophic markers in long bone anlagen in vivo and hypertrophic chondrocyte cultures, whereas all-trans-retinoic acid (RA) boosted CTGF expression and even induced it in immature proliferating cultures. Exogenous recombinant CTGF stimulated chondrocyte maturation, but failed to do so in presence of retinoid antagonists. Immunoblots showed that hypertrophic chondrocytes contained sizable levels of phosphorylated ERK1/2 and p38 MAP kinases that were dose- and time-dependently increased by RA treatment. Experimental ERK1/2 inhibition led to a severe drop in baseline and RA-stimulated CTGF expression, whereas p38 inhibition increased it markedly. These responses were gene-specific, because the opposite was seen with other hypertrophic chondrocyte genes such as collagen X and RA receptor gamma (RARgamma). Tests with Noggin showed that RA induction of CTGF expression was negatively influenced by BMP signaling, whereas induction of collagen X expression was BMP-dependent.

CONCLUSIONS

Retinoids appear to have a preeminent role in controlling expression and function of CTGF in hypertrophic and posthypertrophic chondrocytes and do so with differential cooperation and intervention of MAP kinases and BMP signaling.

TGF-beta signaling in chondrocytes

Transforming growth factor-beta (TGF-beta) regulates a large variety of cellular activities. Binding of TGF- beta to its cell surface receptor triggers several signaling cascades, among which the TGF- beta -Smad pathway is the most extensively studied. TGF- beta also activates protein kinases, including MAPK, PKA and PKC, and modulates gene expression via its delicate interaction with other signaling pathways. During endochondral bone formation, TGF- beta acts as a potent inhibitor of the terminal differentiation of epiphyseal growth plate chondrocytes. This effect appears to be primarily mediated by Smad molecules, although MAPK-ATF2 signaling is also involved. The rate of chondrocyte maturation is tightly regulated through the interactions of Smad-mediated signaling, the Wnt signaling pathway, and the transcription factor Runx2. Improving our understanding of the exact mechanisms underlying TGF- beta -mediated signaling pathways and their effects may greatly impact the diagnosis and treatment of many common orthopaedic diseases.

Protein kinases in chondrocyte signaling and osteoarthritis

Protein kinases, particularly mitogen-activated protein kinases and receptor-tyrosine kinases play crucial roles in mammalian cellular metabolism by regulating intracellular signaling pathways that control proliferation, differentiation, cytokine gene induction and cytokine responsiveness, matrix metalloproteinase gene expression, mechanical transduction, as well as programmed cell death (apoptosis). Many of these pathways are also important components of cartilage homeostasis because alterations in intracellular signaling pathways appear to play a prominent role in chondrocyte dysfunction that is part of osteoarthritis pathogenesis and disease progression. Several mitogen-activated protein kinases and receptor-tyrosine kinases have been characterized as participating in chondrocyte signaling pathways. They are c-Jun-amino-terminal protein kinase, p38 kinase, extracellular signal-regulated protein kinase, and Ror2. Janus kinases and signal transducers and activators of transcription factors (Janus kinase/signal transducers and activators of transcription pathway) are also implicated in modulating the chondrogenic phenotype. Mitogen-activated protein kinase activation is required for their role as phosphorylating enzymes. Activation results from mitogen-activated protein kinase phosphorylation carried out by at least seven upstream kinases known as mitogen-activated protein kinase kinases. Additional upstream kinases (for example, MKKKKs and MKKKs) often require low molecular weight GTP-binding proteins to mediate the mitogen-activated protein-kinase kinases cascade. Identifying the functions of mitogen-activated protein kinases in normal and aging cartilage and the extent to which mitogen-activated protein kinases may be altered in osteoarthritis cartilage and synovium will be critical for developing novel therapies for osteoarthritis management.

The MEK-ERK Signaling Pathway Is a Negative Regulator of Cartilage-specific Gene Expression in Embryonic Limb Mesenchyme

The extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase pathway, also known as the MEK-ERK kinase cascade, has recently been implicated in the regulation of embryonic cartilage differentiation. However, its precise role in this complex process remains controversial. To more thoroughly examine the role of the MEK-ERK kinase cascade in chondrogenesis, we analyzed the effects of two structurally different pharmacological inhibitors of MEK, the upstream kinase activator of ERK, on chondrocyte differentiation in micromass cultures of embryonic chick limb mesenchyme cells. We found that the MEK inhibitors, U0126 and PD98059, promote increased accumulation of cartilage-characteristic mRNA transcripts for type II collagen, aggrecan, and the transcription factor, Sox9. PD98059 treatment stimulated increased deposition of sulfated glycosaminoglycan into both Alcian blue-stainable cartilage matrix and the surrounding culture medium, whereas U0126 elevated glycosaminoglycan secretion into the medium fraction alone. Both MEK inhibitors increased total type II collagen protein accumulation in micromass culture and elevated the activity of a transfected type II collagen enhancer-luciferase reporter gene. Thus, pharmacological MEK inhibition induced increased expression of multiple chondrocyte differentiation markers. Conversely, transfection of limb mesenchyme cells with a constitutively active MEK1 plasmid resulted in a prominent decrease in the activity of a co-transfected type II collagen enhancer-luciferase reporter gene. Collectively, these findings support the hypothesis that signaling through the MEK-ERK kinase cascade may function as an important inhibitory regulator of embryonic cartilage differentiation.

Microenvironment regulation of extracellular signal-regulated kinase activity in chondrocytes: effects of culture configuration, interleukin-1, and compressive stress

OBJECTIVE

To compare extracellular signal-regulated kinase (ERK) activity in response to interleukin-1 (IL-1) in chondrocytes under various culture configurations designed for the study of cartilage biology and repair, and also in response to dynamic load for chondrocytes in cartilage.

METHODS

Isolated bovine articular chondrocytes were maintained in serum-supplemented medium under 4 culture configurations: high-density monolayer, attached to a cut surface of cartilage, within tissue-engineered constructs, or within intact cartilage explants. Samples were subjected to a change of medium with or without IL-1. Cartilage explants were also subjected to dynamic compression.

RESULTS

In chondrocyte monolayers, both basal and IL-1-stimulated ERK activities were similarly elevated at 0.5 hours after medium change, diminishing by 74% after 16 hours. In contrast, chondrocytes in other culture configurations exhibited lower basal levels of ERK activity and a moderate activation of ERK in response to IL-1 that was sustained over the 16-hour treatment time. The dynamic component of loading of cartilage explants led to a 5-fold activation of ERK, compared with free-swelling controls, that was indistinguishable from the effects of IL-1.

CONCLUSION

ERK signaling in response to IL-1 in chondrocyte monolayers exhibited a pattern that was distinct from that in other culture systems, suggesting that the extracellular matrix plays an important regulatory role in modulating the response to extracellular stimuli. Since IL-1 and dynamic loading have distinct effects on chondrocyte biosynthesis, signaling pathways other than ERK participate in the chondrocyte responses to these stimuli.

Functional domains of paired-like homeoprotein Cart1 and the relationship between dimerization and transcription activity

BACKGROUND

Cart1 encodes the paired-like homeodomain in the central portion of the gene, and plays a crucial role in the developmental lineage of bone and cartilage, especially in head formation. However, its transactivation mechanism is still poorly understood, including the target gene. Here, we report biochemical dissections of Cart1 functional domains and a relationship between dimerization and transcription activity.

RESULTS

Deletion studies of GAL4-fused Cart1 indicated that the transactivation domain is located in the middle portion of the C-terminal domain, but the N-terminal is also required for a full activation of the consensus palindromic binding site (TAATNNNATTA). Analysis of the basic amino acid residues at both ends of the homeodomain revealed that both sides act as nuclear localization signals, and are necessary for the cooperative binding to the palindromic sequence. In this study, two additional Cart1 isoforms that behave as dominant negatives were identified from rat chondrosarcoma cells. These isoforms suppressed the transcription activity of the wild-type, despite loss of DNA binding ability, and could interact with the wild-type in yeast. Finally, we demonstrated that wild-type Cart1 forms a DNA-independent homodimer in in vivo conditions, and that the transactivation of wild-type Cart1 was suppressed by the N- or C-terminal domain which was expressed in the nucleus.

CONCLUSION

These results revealed that homodimerization through direct interaction is necessary for the potent transcription activity of Cart1.

Subtype specific roles of beta-adrenergic receptors in apoptosis of adult rat ventricular myocytes

We have previously reported that beta-adrenergic receptor (beta-AR) stimulation promotes apoptosis in adult ventricular myocytes through PKCepsilon-mediated suppression of ERK. In this study, we investigated differential effects of beta-AR subtypes on this signal pathway. The apoptosis induced by the non-specific beta-AR agonist isoproterenol was largely blocked by the beta(1)-selective antagonist CGP 20712A, but not by the beta(2)-selective antagonist ICI 118551. A pro-apoptotic effect of beta(1)-AR was also blocked by the PKA inhibitor H89, while the protein kinase A (PKA) activators forskolin and dibutyryl-cAMP both induced apoptosis. These results indicate that beta(1)-AR-mediated PKA activation is largely responsible for the apoptosis induced by beta-AR in adult rat cardiac myocytes. This conclusion was also supported by the finding that PKA was preferentially activated by beta(1)-AR over beta(2)-AR. beta(2)-AR selectively induced anti-apoptotic ERK activation in the presence of PKCepsilon suppression, and this ERK activation was sensitive to pertussis toxin. PKCepsilon itself as well as Akt, the other anti-apoptotic factor were activated by both beta-AR subtypes. Thus, beta(1)-AR induces pro-apoptotic signals mainly through PKA activation. In contrast, beta(2)-AR is linked to Gi-mediated ERK activation, which is involved in the anti-apoptotic pathway, and is regulated by PKCepsilon. Therefore, our findings suggest a rather complex role for beta-AR subtypes in the regulation of apoptosis in adult ventricular myocytes.

Transforming growth factor-beta1 regulation of growth zone chondrocytes is mediated by multiple interacting pathways

Transforming growth factor beta 1 (TGF-beta1) affects growth plate chondrocytes through Smad-mediated mechanisms and has been shown to increase protein kinase C (PKC). This study determined if PKC mediates the physiological response of rat costochondral growth zone (GC) chondrocytes to TGF-beta1; if the physiological response occurs via type II or type III TGF-beta receptors, and, if so, which receptor mediates the increase in PKC; and the signal transduction pathways involved. Treatment of confluent GC cells with TGF-beta1 stimulated [(3)H]thymidine and [(35)S]sulfate incorporation as well as alkaline phosphatase (ALPase) and PKC specific activities. Inhibition of PKC with chelerythrine, staurosporine, or H-7 caused a dose-dependent decrease in these parameters, indicating that PKC signaling was involved. TGF-beta1-dependent PKC and the physiological response of GC cells to TGF-beta1 was reversed by anti-type II TGF-beta receptor antibody and soluble type II TGF-beta receptor, showing that TGF-beta1 mediates these effects through the type II receptor. The increase in [3H]thymidine incorporation and ALPase specific activity were also regulated by protein kinase A (PKA) signaling, since the effects of TGF-beta1 were partially blocked by the PKA inhibitor H-8. The mechanism of TGF-beta1 activation of PKC is through phospholipase A(2) (PLA(2)) and not through phospholipase C (PLC). Arachidonic acid increased PKC in control cultures and was additive with TGF-beta1. Prostanoids are required, as indomethacin blocked the effect of TGF-beta1, and Cox-1, but not Cox-2, is involved. TGF-beta1 stimulates prostaglandin E(2) (PGE(2)) production and exogenous PGE(2) stimulates PKC, but not as much as TGF-beta1, suggesting that PGE(2) is not sufficient for all of the prostaglandin effect. In contrast, TGF-beta1 was not regulated by diacylglycerol; neither dioctanoylglycerol (DOG) nor inhibition of diacylglycerol kinase with R59022 had an effect. G-proteins mediate TGF-beta1 signaling at different levels in the cascade. TGF-beta1-dependent increases in PGE(2) levels and PKC were augmented by the G protein activator GTP gamma S, whereas inhibition of G-protein activity via GDP beta S, pertussis toxin, or cholera toxin blocked stimulation of PKC by TGF-beta1, indicating that both G(i) and G(s) are involved. Inhibition of PKA with H-8 partially blocked TGF-beta1-dependent PKC, suggesting that PKA inhibition on the physiological response was via PKA regulation of PKC signaling. This indicates that multiple interacting signaling pathways are involved: TGF-beta1 stimulates PLA(2) and prostaglandin release via the action of Cox-1 on arachidonic acid. PGE(2) activates the EP2 receptor, leading to G-protein-dependent activation of PKA. PKA signaling results in increased PKC activity and PKC signaling regulates proliferation, differentiation, and matrix synthesis.

Recent advances in TGF-beta effects on chondrocyte metabolism. Potential therapeutic roles of TGF-beta in cartilage disorders

Novel approaches to treat osteoarthritis are required and progress in understanding the biology of cartilage disorders has led to the use of genes whose products stimulate cartilage repair or inhibit breakdown of the cartilaginous matrix. Among them, transforming growth factor-beta (TGF-beta) plays a significant role in promoting chondrocyte anabolism in vitro (enhancing matrix production, cell proliferation, osteochondrogenic differentiation) and in vivo (short-term intra-articular injections lead to increased bone formation and subsequent cartilage formation, beneficial effects on osteochondrogenesis). In vivo induction of the expression of TGF-beta and the use of gene transfer may provide a new approach for treatment of osteoarthritic lesions.

Role of tyrosine kinase and p44/42 MAPK in D(2)-like receptor-mediated stimulation of Na(+), K(+)-ATPase in kidney

Our laboratory has shown that dopamine D(2)-like receptor activation causes stimulation of Na(+), K(+)-ATPase (NKA) activity in the proximal tubules of the rat kidney. The present study was designed to investigate the cellular signaling mechanisms mediating this response to D(2)-like receptor activation. We measured the stimulation of NKA activity by bromocriptine (D(2)-like receptor agonist) in the absence and presence of PD-98059 [p44/42 mitogen-activated protein kinase (MAPK) kinase inhibitor] and genistein (tyrosine kinase inhibitor) in renal proximal tubules. Both agents inhibited bromocriptine-mediated stimulation of NKA, suggesting the involvement of p44/42 MAPK and tyrosine kinase in this response. Additionally, we found that bromocriptine increased the phosphorylation of p44/42 MAPK in the proximal tubules, which was blocked by PD-98059 and genistein. These results show that D(2)-like receptor activation causes stimulation of NKA activity by means of a tyrosine kinase-p44/42 MAPK pathway in the proximal tubules of the kidney.

Suppressor and oncogenic roles of transforming growth factor-beta and its signaling pathways in tumorigenesis

Transforming growth factor-beta (TGF-beta) has been implicated in oncogenesis since the time of its discovery almost 20 years ago. The complex, multifunctional activities of TGF-beta endow it with both tumor suppressor and tumor promoting activities, depending on the stage of carcinogenesis and the responsivity of the tumor cell. Dysregulation or alteration of TGF-beta signaling in tumorigenesis can occur at many different levels, including activation of the ligand, mutation or transcriptional suppression of the receptors, or alteration of downstream signal transduction pathways resulting from mutation or changes in expression patterns of signaling intermediates or from changes in expression of other proteins which modulate signaling. New insights into signaling from the TGF-beta receptors, including the identification of Smad signaling pathways and their interaction with mitogen-activated protein (MAP) kinase pathways, are providing an understanding of the changes involved in the change from tumor suppressor to tumor promoting activities of TGF-beta. It is now appreciated that loss of sensitivity to inhibition of growth by TGF-beta by most tumor cells is not synonymous with complete loss of TGF-beta signaling but rather suggests that tumor cells gain advantage by selective inactivation of the tumor suppressor activities of TGF-beta with retention of its tumor promoting activities, especially those dependent on cross talk with MAP kinase pathways and AP-1.

CTGF/Hcs24 induces chondrocyte differentiation through a p38 mitogen-activated protein kinase (p38MAPK), and proliferation through a p44/42 MAPK/extracellular-signal regulated kinase (ERK)

Connective tissue growth factor/hypertrophic chondrocyte specific gene product 24 (CTGF/Hcs24) promotes proliferation and differentiation of chondrocytes in culture. We investigated the roles of two major types of mitogen activated protein kinase (MAPK) in the promotion of proliferation and differentiation by CTGF/Hcs24. Here we report the effects of the MAPKK/MEK 1/2 inhibitor, PD098059, and p38 MAPK inhibitor, SB203580, in a human chondrosarcoma-derived chondrocytic cell line (HCS-2/8) and rabbit growth cartilage (RGC) cells treated with CTGF/Hcs24. In the proliferation phase, CTGF/Hcs24 induced a approximately fivefold increase in the phosphorylation of p44/42 MAPK/ERK and a approximately twofold increase in that of p38 MAPK in an in vivo kinase assay. These inhibitors of MAPKK and MAPK suppressed phosphorylation of ets-like gene-1 (Elk-1) and nuclear activating transcription factor-2 (Atf-2) induced by CTGF/Hcs24 in a dose-dependent manner, respectively. Western blot analysis showed that phosphorylation of ERK was induced from 30 to 60 min and phosphorylation of p38 MAPK from 10 to 15 min after the addition of CTGF/Hcs24 in confluence HCS-2/8 cells. PD098059 suppressed the DNA synthesis of HCS-2/8 cells and RGC cells, while SB203580 did not. On the other hand, the p38 MAPK inhibitor, SB203580, completely inhibited the CTGF/Hcs24-induced synthesis of proteoglycans in HCS-2/8 cells and RGC cells but the MEK1/2 inhibitor, PD098059, did not. These results suggest that ERK mediates the CTGF/Hcs24-induced proliferation of chondrocytes, and that p38 MAPK mediates the CTGF/Hcs24-induced differentiation of chondrocytes.

Expression profile of protein tyrosine kinase genes in human osteoarthritis chondrocytes

OBJECTIVE

To determine the expression profile of protein kinase (PK) and protein tyrosine kinase (PTK) genes in human primary osteoarthritis (OA) chondrocytes and to compare it with that of immortalized human chondrocytes T/C 28a4 with a view to learning whether T/C 28a4 cells can be used for elucidating signal transduction pathways in human chondrocytes.

DESIGN

We used the Atlas Human cDNA Array and a method based on PCR with degenerate primers to analyse the expression profile of protein kinase genes in primary human OA chondrocytes and compared it with that of immortalized human chondrocyte cell line T/C 28a4 using RT-PCR and Western blotting.

RESULTS

A total of 21 PTK genes were identified and several of these have never been shown to be expressed in human OA chondrocytes. Comparative expression analysis of some selected kinase genes showed that the mRNA expression pattern of many protein kinase genes in OA chondrocytes was identical to that of T/C 28a4 cells. However, there were differences in the level of protein expression of selected protein kinases in these cells. For example, mRNA expression of the novel kinase HCK was detected in OA chondrocytes and in the cell lines analysed but by Western blotting HCK protein was not detected in OA chondrocytes. In these studies, we also identified a novel mutant form of the discoidin domain receptor 2 (DDR2) transcript from chondrocyte-like cell line HTB-94.

CONCLUSIONS

Our results provide novel information about protein kinase gene expression in OA chondrocytes and indicate that the transformed chondrocyte cell line T/C 28a4 may be suitable for elucidating signal transduction pathways in chondrocytes and to investigate how they regulate chondrocyte function in inflammatory and degenerative joint diseases.

Dopamine D(2) receptor activation causes mitogenesis via p44/42 mitogen-activated protein kinase in opossum kidney cells

This study was conducted to determine the expression of dopamine D(2)-like receptors in opossum kidney (OK) cells and to examine the potential role of these receptors in mitogenesis. First, the presence of D(2)-like receptor binding sites in OK cell membranes was demonstrated by radioligand binding, using [(3)H]spiperone. The D(2)-like receptor subtypes expressed in OK cells were subsequently demonstrated, by Western blotting, to be D(2), D(3), and D(4) receptors. OK cells were stimulated with bromocriptine, (+/-)-2-(N-phenylethyl-N-propyl)amino-5-hydroxytetralin hydrochloride, (R)-(+)-2-dipropylamino-7-hydroxy-1,2,3,4-tetrahydronaphthalene hydrobromide, or PD 168,077 maleate (D(2)-like, D(2), D(3), and D(4) receptor agonists, respectively), and mitogenesis was measured as a function of [(3)H]thymidine incorporation. It was observed that, whereas bromocriptine and (+/-)-2-(N-phenylethyl-N-propyl)amino-5-hydroxytetralin hydrochloride produced increases in [(3)H]thymidine incorporation, (R)-(+)-2-dipropylamino-7-hydroxy-1,2,3,4-tetrahydronaphthalene hydrobromide and PD 168,077 maleate did not produce such an effect, indicating the involvement of D(2) receptors in the mitogenic response. Pertussis toxin and PD 98059 blocked the mitogenesis caused by bromocriptine, suggesting a role for G(i) or G(o) proteins and p44/42 mitogen-activated protein kinase (MAPK), respectively. Furthermore, it was observed that bromocriptine produced a time-dependent increase in the phosphorylation (activation) of p44/42 MAPK, which was blocked by domperidone, pertussis toxin, or PD 98059. Therefore, this study demonstrates that, although OK cells express D(2), D(3), and D(4) receptors, activation of only D(2) receptors causes mitogenesis via phosphorylation of p44/42 MAPK. Furthermore, the cellular mechanisms contributing to D(2) receptor-mediated phosphorylation of p44/42 MAPK seem to involve the tyrosine kinase, phosphatidylinositol-3-kinase, and protein kinase C pathways. It is likely that bromocriptine and other preferential D(2) receptor agonists might provide protection against ischemic reperfusion injury in renal proximal tubular cells, by increasing the survival rates for ischemic cells.