Concomitant recruitment of ERK1/2 and p38 MAPK signalling pathway is required for activation of cytoplasmic phospholipase A2 via ATP in articular chondrocytes

Distinct mono- and dinucleotide-specific P2Y receptors in A549 lung epithelial cells: different control of arachidonic acid release and nitric oxide synthase expression

P2Y nucleotide receptors activated by mono- and dinucleotides have already been found in lung tissue. Here, we compare effects of dinucleotides and mononucleotides on arachidonic acid release, intracellular calcium mobilization, and inducible nitric oxide synthase (iNOS) expression in the alveolar lung cell line A549. Both types of nucleotides were effective. Diadenosine polyphosphates (Ap(n)A, n=2 to 5) increased arachidonic acid release and raised intracellular calcium concentration ([Ca(2+)](i)), albeit with lower potency than mononucleotides (ATP, UTP, UDP). Among the dinucleotides only diadenosine tetraphosphate (Ap(4)A) was a potent agonist. Arachidonic acid release induced by Ap(4)A was almost completely abolished in the presence of the P2 receptor antagonists suramin and Reactive blue 2, whereas arachidonic acid release evoked by ATP, UTP or UDP was hardly reduced by these antagonists. Both, the mononucleotides ATP and UDP and the dinucleotide Ap(4)A induced the expression of iNOS in the cytoplasm around the nucleus, similar to the expression of iNOS evoked by lipopolysaccharide. iNOS is barely detectable in unstimulated cells. Suramin selectively blocked the capacity of Ap(4)A to induce iNOS, but not that of ATP or UDP. Thus, we find the same pharmacology for nucleotide-induced arachidonic acid release and iNOS expression. Therefore, we suggest that a distinct P2Y receptor subtype specifically activated by Ap(4)A exists in A549 cells, which is sensitive to the antagonist suramin, in contrast to other P2Y receptor subtypes activated by mononucleotides which are suramin-insensitive. Distinct P2Y receptors activated by mononucleotides or by Ap(4)A could play a role in inflammatory conditions by affecting the release of arachidonic acid and the expression of iNOS. Therefore, these receptors present a promising target in inflammatory diseases.

Integration of P2Y receptor-activated signal transduction pathways in G protein-dependent signalling networks

The role of nucleotides in intracellular energy provision and nucleic acid synthesis has been known for a long time. In the past decade, evidence has been presented that, in addition to these functions, nucleotides are also autocrine and paracrine messenger molecules that initiate and regulate a large number of biological processes. The actions of extracellular nucleotides are mediated by ionotropic P2X and metabotropic P2Y receptors, while hydrolysis by ecto-enzymes modulates the initial signal. An increasing number of studies have been performed to obtain information on the signal transduction pathways activated by nucleotide receptors. The development of specific and stable purinergic receptor agonists and antagonists with therapeutical potential largely contributed to the identification of receptors responsible for nucleotide-activated pathways. This article reviews the signal transduction pathways activated by P2Y receptors, the involved second messenger systems, GTPases and protein kinases, as well as recent findings concerning P2Y receptor signalling in C6 glioma cells. Besides vertical signal transduction, lateral cross-talks with pathways activated by other G protein-coupled receptors and growth factor receptors are discussed.

Innervation, inflammation, and hypermobility may characterize pathologic disc degeneration: review of animal model data

Animal models provide important clues to the pathomechanisms of human intervertebral disc degeneration. Previous reviews on this topic have highlighted the fact that loss of nuclear volume (and, consequently, pressure) is a common trigger for tissue-remodeling and anatomic change consistent with degeneration in humans. Unfortunately, a large gap still exists in the medical knowledge base that serves to distinguish symptomatic from asymptomatic degeneration. Because disc degeneration per se is not a basis for clinical intervention, identification of specific features underlying discogenic pain is of the utmost importance to advance the current level of care and identify novel therapeutic targets. This article presents animal-model evidence that pathologic, or painful, degeneration is characterized by ineffective injury-healing of peripheral tissue. Because the disc is only vascularized at the vertebral end plate and the outer part of the anulus, these are the likely sites for focal damage, inflammation, neoinnervation, and nociceptor sensitization. Consequently, while nuclear insufficiency is likely the root of degenerative change, the end plate and peripheral part of the anulus are more likely the source of patient discomfort.

Induction of cytosolic phospholipase A2 by lipopolysaccharide in canine tracheal smooth muscle cells: involvement of MAPKs and NF-kappaB pathways

Cytosolic phospholipase A2 (cPLA2) plays a pivotal role in mediating agonist-induced arachidonic acid (AA) release for prostaglandins (PG) synthesis induced by bacterial lipopolysaccharide (LPS) and cytokines. However, the intracellular signaling pathways mediating LPS-induced cPLA2 expression and PGE2 synthesis in canine tracheal smooth muscle cells (TSMCs) remains unknown. LPS-induced expression of cPLA2 and release of PGE2 was attenuated by inhibitors of tyrosine kinase (genistein), phosphatidylcholine-phospholipase C (D609), phosphatidylinositol-phospholipase C (U73122), PKC (GF109203X and staurosporine), removal of Ca2+ by BAPTA/AM plus EDTA, MEK1/2 (PD98059), p38 (SB202190), JNK (SP600125), and phosphatidylinositol 3-kinase (PI3-K; LY294002 and wortmannin). The involvement of MPAKs in LPS-induced responses was further confirmed by transfection of TSMCs with dominant negative mutants of ERK2 and p38. LPS-induced cPLA2 expression and PGE2 synthesis was inhibited by a selective NF-kappaB inhibitor (helenalin) and transfection with dominant negative mutants of NF-kappaB inducing kinase (NIK), IkappaB kinase (IKK)-alpha, and IKK-beta, consistent with that LPS-stimulated both IkappaB-alpha degradation and NF-kappaB translocation into nucleus in these cells. LPS-stimulated cPLA2 phosphorylation was inhibited by PD98059, GF109203X, and staurosporine, indicating the regulation by p42/p44 MAPK and PKC. Moreover, LPS-induced up-regulation of cPLA2 and COX-2 linked to PGE2 synthesis was inhibited by AACOCF3 (a selective cPLA2 inhibitor), implying the involvement of cPLA2 in these responses. These findings suggest that phosphorylation and expression of cPLA2 correlates with the release of PGE2 from LPS-challenged TSMCs, at least in part, mediated through MAPKs and NF-kappaB signaling pathways. LPS-mediated responses were modulated by PLC, Ca2+, PKC, tyrosine kinase, and PI3-K in TSMCs.

Red blood cells: reservoirs of cis- and trans-epoxyeicosatrienoic acids

Epoxyeicosatrienoic acids (EETs) are candidate endothelium-derived hyperpolarizing factors that demonstrate a wide range of biological effects. The presence of both cis- and trans-EETs in rat plasma was identified with HPLC-electrospray ionization tandem mass spectrometry in this study. The total EETs in plasma are 38.2 ng/ml with cis-EETs representing 21.4 +/- 0.4 ng/ml and trans-EETs 16.8 +/- 0.4 ng/ml. EETs in RBCs were estimated to be 20.2 ng/10(9) RBCs, which corresponds to 200 ng in RBCs contained in 1 ml blood. RBC incubation with 10 mM tert-butyl hydroperoxide resulted in 4.4-fold increase of total cis-EETs (from 9.2 to 40.2 ng/10(9) RBCs) and 5.5-fold increase of total trans-EETs (from 11.0 to 60.8 ng/10(9) RBCs). EETs were released (2 ng/ml) from RBCs after incubation at 37 degrees C for 10 min even after being washed 3 times, indicating that RBCs are reservoirs of plasma EETs. The identification of cis- and trans-EETs in RBCs and in plasma as well as their release from RBCs suggest a vasoregulatory role of RBCs in view of their potent vasoactivity.

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.

Signal transduction pathways: new targets for treating rheumatoid arthritis

Biotherapies and other new treatments introduced over the last few years have considerably enriched the therapeutic armamentarium for rheumatoid arthritis. Nevertheless, primary refractoriness or secondary escape phenomenon may occur, indicating a need for identifying new treatment targets. Promising candidates can be found among compounds involved in signal transduction pathways, most notably protein kinases (mitogen-activated protein kinase, MAPK and phosphatidylinositol-3 protein kinase, PI3) and transcription factors (nuclear factor kappa B, NF-kappaB; activating protein 1, AP-1; CCAAT/enhancer-binding protein, C/EBP and signal transducer and activator of transcription, STAT). Inhibition of signal transduction pathways may be achievable via three main strategies: pharmacological inhibitors, anti-sense or more specific inhibitors such as oligionucleotides or interfering mRNA, and induced overexpression of naturally occurring inhibitors. Clinical trials are under way to evaluate pharmacological inhibitors such as p38 MAPK. Although the preliminary results are promising, proof of safety has not yet been obtained. Signal transduction pathways are involved in normal processes, whose inhibition might produce untoward effects.

Extracellular nucleotides activate Runx2 in the osteoblast-like HOBIT cell line: a possible molecular link between mechanical stress and osteoblasts' response

Dynamic mechanical loading increases bone density and strength and promotes osteoblast proliferation, differentiation and matrix production, by acting at the gene expression level. Molecular mechanisms through which mechanical forces are conversed into biochemical signalling in bone are still poorly understood. A growing body of evidence point to extracellular nucleotides (i.e., ATP and UTP) as soluble factors released in response to mechanical stimulation in different cell systems. Runx2, a fundamental transcription factor involved in controlling osteoblasts differentiation, has been recently identified as a target of mechanical signals in osteoblastic cells. We tested the hypothesis that these extracellular nucleotides could be able to activate Runx2 in the human osteoblastic HOBIT cell line. We found that ATP and UTP treatments, as well as hypotonic stress, promote a significant stimulation of Runx2 DNA-binding activity via a mechanism involving PKC and distinct mitogen-activated protein kinase cascades. In fact, by using the specific inhibitors SB203580 (specific for p38 MAPK) and PD98059 (specific for ERK-1/2 MAPK), we found that ERK-1/2, but not p38, play a major role in Runx2 activation. On the contrary, another important transcription factor, i.e., Egr-1, that we previously demonstrated being activated by extracellular released nucleotides in this osteoblastic cell line, demonstrated to be susceptible to both ERK-1/2 and p38 kinases. These data suggest a possible differential involvement of these two transcription factors in response to extracellularly released nucleotides. The biological relevance of our data is strengthened by the finding that a target gene of Runx2, i.e., Galectin-3, is up-regulated by ATP stimulation of HOBIT cells with a comparable kinetic of that found for Runx2. Since it is known that osteocytes are the primary mechanosensory cells of the bone, we hypothesize that they may signal mechanical loading to osteoblasts through release of extracellular nucleotides. Altogether, these data suggest a molecular mechanism explaining the purinoreceptors-mediated activation of specific gene expression in osteoblasts and could be of help in setting up new pharmacological strategies for the intervention in bone loss pathologies.

Immature murine articular chondrocytes in primary culture: a new tool for investigating cartilage

OBJECTIVE

Many genetically modified animal models are providing new keys for unlocking the pathophysiology of cartilage degradation. To produce a tool for cellular and molecular studies in genetically engineered murine models, we defined the optimal culture conditions for primary cultures of articular chondrocytes from newborn mice (C57Bl/6).

METHODS

To determine whether the cultured cells exhibited the typical articular chondrocyte phenotype, we examined several morphological, biochemical, and functional features.

RESULTS

The cells had the typical chondrocyte morphology, with a rounded or polygonal shape. Immunolocalization studies showed high levels of type II collagen and aggrecan expression, together with sulfated glycosaminoglycan accumulation. Type II collagen and aggrecan expression decreased with passaging. In contrast, type I collagen expression was low in primary cultures and high after four passages, indicating a fibroblast phenotype. To evaluate the functional integrity of our cultured cells, we evaluated their ability to produce prostaglandin E2 (PGE2) and nitric oxide (NO) in response to the catabolic cytokine interleukin (IL)-1beta (10 ng/ml). Production of both PGE2 and NO increased significantly as compared to untreated controls. In addition, IL-1beta induced COX-2 expression by the cultured cells, as shown by Western blotting.

CONCLUSIONS

Since functional and molecular parameters can be measured readily in mice, the immature murine articular chondrocyte (iMAC) model described here should prove a powerful tool for research, particularly as many transgenic and knockout mouse strains are available, even if iMACs are not optimal substitutes for human chondrocytes.

Up-regulation of microsomal prostaglandin E synthase 1 in osteoarthritic human cartilage: critical roles of the ERK-1/2 and p38 signaling pathways

OBJECTIVE

Microsomal prostaglandin E synthase 1 (mPGES-1) is the final enzyme of the cascade that produces prostaglandin E(2) (PGE(2)), a key actor in arthritis. To study mPGES-1 synthesis in human cartilage and its regulation by interleukin-1beta (IL-1beta), we used human cartilage and an immortalized human chondrocyte cell line. Furthermore, we investigated the signaling pathways involved in mPGES-1 expression.

METHODS

We used real-time quantitative reverse transcription-polymerase chain reaction, Northern blotting, and Western blotting to measure mPGES-1 messenger RNA (mRNA) and protein expression in human chondrocytes. PGE(2) production was measured by enzyme-linked immunosorbent assay.

RESULTS

Cartilage specimens from osteoarthritis (OA) patients contained far greater amounts of mPGES-1 and cyclooxygenase 2 (COX-2) mRNA than did normal cartilage. Incubation with IL-1beta markedly increased mPGES-1 mRNA and protein in a dose-dependent and time-dependent manner, in parallel with an increase in PGE(2) levels. Both PD98059, an ERK pathway inhibitor, and SB203580, a p38alpha/beta MAPK inhibitor, abolished the increases in mPGES-1 mRNA and protein in response to IL-1beta. The specific p38alpha MAPK inhibitor SC906 suppressed IL-1beta-induced COX-2 expression but not IL-1beta-induced mPGES-1 expression, suggesting preferential involvement of p38beta MAPK in IL-1beta-induced mPGES-1 expression.

CONCLUSION

This study is the first to show that mPGES-1 is stimulated in human chondrocytes by the proinflammatory cytokine IL-1beta via activation of both ERK-1/2 and p38 MAPK in an isoform-specific manner. We postulate that mPGES-1 may be a novel target for OA therapy.

The regulation of chondrocyte function by proinflammatory mediators: prostaglandins and nitric oxide

Within the mature articular cartilage matrix, which has no blood or nerve supply, chondrocytes show little metabolic activity with low turnover of matrix components. Under conditions of stress because of biomechanical factors, however, chondrocytes are capable of producing mediators that are associated with inflammation, including cytokines such as interleukin-1 and tumor necrosis factor-alpha, which in turn stimulate the production of prostaglandins and nitric oxide. Chondrocytes also express receptors for these mediators, which accumulate at high local concentrations and can act in an autocrine-paracrine fashion to feedback-regulate chondrocyte responses. Prostaglandin E2 can exert catabolic or anabolic effects depending on the microenvironment. Nitric oxide can promote cellular injury and increase chondrocyte susceptibility to cytokine-induced apoptosis. Because cross-talk between these mediators produces complex modulation of catabolic and anabolic pathways, further studies in vitro and in vivo are required to elucidate their precise roles in osteoarthritis.

Signaling transduction: target in osteoarthritis

PURPOSE OF REVIEW

The pathophysiology of osteoarthritis is the result of an imbalance between anabolic and catabolic pathways. This imbalance is the result of the activation of joint cells by inflammatory mediators, matrix components, and mechanical stress. All these mediators act through specific receptors that transmit the signals to the nucleus to activate the transcription of matrix metalloproteinases and inflammatory genes. Targeting these signaling pathways in osteoarthritis is considered a novel approach to modulate this imbalance.

RECENT FINDINGS

Although many signaling pathways are necessary for physiologic cell life, it is now well established that a few are more specifically induced in an inflammatory environment. In osteoarthritis, the nuclear factor-kappaB and mitogen-activated protein kinase pathways have been shown to play a predominant role in the expression of metalloproteinases and inflammatory genes and proteins. Also involved in the activation of osteoarthritic cells are other molecules interacting with one or several signaling pathways, such as nitric oxide, peroxisome proliferator-activated receptor-gamma ligands, or C/EBP transcriptional factors. Based on this knowledge, specific inhibitors for some of these signaling pathways have been designed and include p38 mitogen-activated protein kinase or nuclear factor-kappaB inhibitors. Experimental studies evaluating cartilage degradation in arthritis models are promising, although fewer have been done specifically in osteoarthritis models.

SUMMARY

Targeting signaling pathways in osteoarthritis did not seem feasible a few years ago because of the complexity of the multiple intracellular pathways, mainly physiologic, defined by a high degree of redundancy and cross-talk. However, important advances in the knowledge of chondrocyte and synoviocyte signaling in osteoarthritis have been achieved in recent years and suggest that inhibitors of specific signaling pathways could shortly provide effective treatments for this disease.

Subtype specific internalization of P2Y1 and P2Y2 receptors induced by novel adenosine 5'-O-(1-boranotriphosphate) derivatives

P2Y-nucleotide receptors represent important targets for drug development. The lack of stable and receptor specific agonists, however, has prevented successful therapeutic applications. A novel series of P-boronated ATP derivatives (ATP-alpha-B) were synthesized by substitution of a nonbridging O at P(alpha) with a BH(3) group. This introduces a chiral center, thus resulting in diastereoisomers. In addition, at C2 of the adenine ring a further substitution was made (Cl- or methylthio-). The pairs of diastereoisomers were denoted here as A and B isomers. Here, we tested the receptor subtype specificity of these analogs on HEK 293 cells stably expressing rat P2Y(1) and rat P2Y(2) receptors, respectively, both attached to the fluorescent marker protein GFP (rP2Y(1)-GFP, rP2Y(2)-GFP). We investigated agonist-induced receptor endocytosis, [Ca(2+)](i) rise and arachidonic acid (AA) release. Agonist-induced endocytosis of rP2Y(1)-GFP was more pronounced for the A isomers than the corresponding B counterparts for all ATP-alpha-B analogs. Both 2-MeS-substituted diastereoisomers induced a greater degree of agonist-induced receptor endocytosis as compared to the 2-Cl-substituted derivatives. Endocytosis results are in accordance with the potency to induce Ca(2+) release by these compounds in HEK 293 cells stably transfected with rP2Y(1). In case of rP2Y(2)-GFP, the borano-nucleotides were very weak agonists in comparison to UTP and ATP in terms of Ca(2+) release, AA release and in inducing receptor endocytosis. The different ATP-alpha-B derivatives and also the diastereoisomers were equally ineffective. Thus, the new agonists may be considered as potent and highly specific agonist drug candidates for P2Y(1) receptors. The difference in activity of the ATP analogs at P2Y receptors could be used as a tool to investigate structural differences between P2Y receptor subtypes.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.