Characterization of the PGE2 receptor subtype in bovine chondrocytes in culture

Role of G-proteins in the differentiation of epiphyseal chondrocytes

Herein, we review the regulation of differentiation of the growth plate chondrocytes by G-proteins. In connection with this, we summarize the current knowledge regarding each family of G-protein α subunit, specifically, Gα(s), Gα(q/11), Gα(12/13), and Gα(i/o). We discuss different mechanisms involved in chondrocyte differentiation downstream of G-proteins and different G-protein-coupled receptors (GPCRs) activating G-proteins in the epiphyseal chondrocytes. We conclude that among all G-proteins and GPCRs expressed by chondrocytes, Gα(s) has the most important role and prevents premature chondrocyte differentiation. Receptor for parathyroid hormone (PTHR1) appears to be the major activator of Gα(s) in chondrocytes and ablation of either one leads to accelerated chondrocyte differentiation, premature fusion of the postnatal growth plate, and ultimately short stature.

Induction of nerve growth factor expression and release by mechanical and inflammatory stimuli in chondrocytes: possible involvement in osteoarthritis pain

Introduction

Nerve growth factor (NGF) level is increased in osteoarthritis (OA) joints and is involved in pain associated with OA. Stimuli responsible for NGF stimulation in chondrocytes are unknown. We investigated whether mechanical stress and proinflammatory cytokines may influence NGF synthesis by chondrocytes.

Methods

Primary cultures of human OA chondrocytes, newborn mouse articular chondrocytes or cartilage explants were stimulated by increasing amounts of IL-1β, prostaglandin E₂ (PGE₂), visfatin/nicotinamide phosphoribosyltransferase (NAMPT) or by cyclic mechanical compression (0.5 Hz, 1 MPa). Before stimulation, chondrocytes were pretreated with indomethacin, Apo866, a specific inhibitor of NAMPT enzymatic activity, or transfected by siRNA targeting visfatin/NAMPT. mRNA NGF levels were assessed by real-time quantitative PCR and NGF released into media was determined by ELISA.

Results

Unstimulated human and mouse articular chondrocytes expressed low levels of NGF (19.2 ± 8.7 pg/mL, 13.5 ± 1.0 pg/mL and 4.4 ± 0.8 pg/mL/mg tissue for human and mouse articular chondrocytes and costal explants, respectively). Mechanical stress induced NGF release in conditioned media. When stimulated by IL-1β or visfatin/NAMPT, a proinflammatory adipokine produced by chondocytes in response to IL-1β, a dose-dependent increase in NGF mRNA expression and NGF release in both human and mouse chondrocyte conditioned media was observed. Visfatin/NAMPT is also an intracellular enzyme acting as the rate-limiting enzyme of the generation of NAD. The expression of NGF induced by visfatin/NAMPT was inhibited by Apo866, whereas IL-1β-mediated NGF expression was not modified by siRNA targeting visfatin/NAMPT. Interestingly, PGE₂, which is produced by chondrocytes in response to IL-1β and visfatin/NAMPT, did not stimulate NGF production. Consistently, indomethacin, a cyclooxygenase inhibitor, did not counteract IL-1β-induced NGF production.

Conclusions

These results show that mechanical stress, IL-1β and extracellular visfatin/NAMPT, all stimulated the expression and release of NGF by chondrocytes and thus suggest that the overexpression of visfatin/NAMPT and IL-1β in the OA joint and the increased mechanical loading of cartilage may mediate OA pain via the stimulation of NGF expression and release by chondrocytes.

IL-1beta stimulates the expression of prostaglandin receptor EP4 in human chondrocytes by increasing production of prostaglandin E2

Prostaglandin (PG) E(2), which exerts its actions via the PG receptors EP1-4, is produced from arachidonic acid by cyclooxygenase (COX)-1 and COX-2. The aim of this study was to investigate the mechanisms by which interleukin (IL)-1beta induces the expression of PG receptors in cultured human chondrocytes and to explore the role of PGE(2) in this process. The cells were cultured with 0, 10, or 100 U/mL IL-1beta with or without 1 muM celecoxib, a specific inhibitor of COX-2, for up to 28 days. Expression of the genes encoding COX-1, COX-2, and EP1-4 was quantified using real-time PCR, and expression of the corresponding proteins was examined using immunohistochemical staining. PGE(2) production was determined using ELISA. IL-1beta treatment caused a marked dose- and time-dependent increase in the levels of PGE(2), COX-2, and EP4 as compared with the untreated control. It did not affect the expression of COX-1, and it decreased the expression of EP1 and EP2. EP3 expression was not detected in either the absence or the presence of IL-1beta. When celecoxib was also present, IL-1beta failed to stimulate PGE(2) production and EP4 expression, but its stimulatory effect on COX-2 expression and its inhibitory effect on EP1 and EP2 expression were unchanged. IL-1beta increases the production of PGE(2), COX-2, and the PG receptor EP4 in cultured human chondrocytes. The increase in EP4 expression appears to be a result of the increased PGE(2) production.

Prostaglandin E2 receptors EP1 and EP4 are up-regulated in rabbit chondrocytes by IL-1β, but not by TNFα

Prostaglandin E2 (PGE2) exerts its actions through the binding of the high affinity EP receptors. We wanted to evaluate the regulation of EP1 and EP4, and the expression of cyclooxygenase (COX)-2, main enzyme responsible for PGE2 synthesis in inflammatory situations, in healthy rabbit chondrocytes stimulated with inflammatory mediators locally increased during osteoarthritis. Articular chondrocytes obtained from healthy rabbits were stimulated with interleukin (IL)-1beta (0.1-100 u/ml) or tumour necrosis factor (TNF)alpha (100 ng/ml). Where indicated, cells were preincubated with non-steroidal antiinflammatory drugs (NSAIDs) (10(-6) M) to inhibit PGE2 synthesis. IL-1beta induced a dose and time-dependent increase in EP1, EP4 and COX-2 expression. However, TNFalpha presence did not induce a significant modification in EP1, EP4 or COX-2 gene expression at any time of study. NSAID presence significantly inhibited PGE2 release but did not modify the EP receptors or COX-2 expression induced by IL-1beta. Our results indicate that EP1 and EP4 receptors, and COX-2 are up-regulated in IL-1beta-stimulated chondrocytes, while no significant modifications are observed in TNFalpha-stimulated cells. NSAIDs were unable to modify the expression of these mediators induced by IL-1beta. Therefore, the increase in PGE2 synthesis, induced by IL-1beta, does not seem to mediate the increase in EP receptor expression, in rabbit chondrocytes.

Cyclooxygenases and prostaglandin E2 receptors in growth plate chondrocytes in vitro and in situ--prostaglandin E2 dependent proliferation of growth plate chondrocytes

Prostaglandin E₂ (PGE₂) plays an important role in bone development and metabolism. To interfere therapeutically in the PGE₂ pathway, however, knowledge about the involved enzymes (cyclooxygenases) and receptors (PGE₂ receptors) is essential. We therefore examined the production of PGE₂ in cultured growth plate chondrocytes in vitro and the effects of exogenously added PGE₂ on cell proliferation. Furthermore, we analysed the expression and spatial distribution of cyclooxygenase (COX)-1 and COX-2 and PGE₂ receptor types EP1, EP2, EP3 and EP4 in the growth plate in situ and in vitro. PGE₂ synthesis was determined by mass spectrometry, cell proliferation by DNA [³H]-thymidine incorporation, mRNA expression of cyclooxygenases and EP receptors by RT-PCR on cultured cells and in homogenized growth plates. To determine cellular expression, frozen sections of rat tibial growth plate and primary chondrocyte cultures were stained using immunohistochemistry with polyclonal antibodies directed towards COX-1, COX-2, EP1, EP2, EP3, and EP4. Cultured growth plate chondrocytes transiently secreted PGE₂ into the culture medium. Although both enzymes were expressed in chondrocytes in vitro and in vivo, it appears that mainly COX-2 contributed to PGE₂-dependent proliferation. Exogenously added PGE₂ stimulated DNA synthesis in a dose-dependent fashion and gave a bell-shaped curve with a maximum at 10^-8 M. The EP1/EP3 specific agonist sulprostone and the EP1-selective agonist ONO-D1-004 increased DNA synthesis. The effect of PGE₂ was suppressed by ONO-8711. The expression of EP1, EP2, EP3, and EP4 receptors in situ and in vitro was observed; EP2 was homogenously expressed in all zones of the growth plate in situ, whereas EP1 expression was inhomogenous, with spared cells in the reserve zone. In cultured cells these four receptors were expressed in a subset of cells only. The most intense staining for the EP1 receptor was found in polygonal cells surrounded by matrix. Expression of receptor protein for EP3 and EP4 was observed also in rat growth plates. In cultured chrondrocytes, however, only weak expression of EP3 and EP4 receptor was detected. We suggest that in growth plate chondrocytes, COX-2 is responsible for PGE₂ release, which stimulates cell proliferation via the EP1 receptor.

Differential regulation of EP receptor isoforms during chondrogenesis and chondrocyte maturation

Regulation of chondrogenesis and chondrocyte maturation by prostaglandins has been a topic of interest during recent years. Particular focus on this area derives from the realization that inhibition of prostaglandin synthesis with non-steroidal anti-inflammatory drugs could impact these cartilage-related processes which are important in skeletal development and are recapitulated during bone healing either post-trauma or post-surgery. In addition to reviewing the relevant literature focused on prostaglandin synthesis and signaling through the G-protein coupled EP receptors, we present novel findings that establish the expression profile of EP receptors in chondroprogenitors and chondrocytes. Further, we begin to examine the signaling that may be involved with the transduction of PGE2 effects in these cells. Our findings suggest that EP2 and EP4 receptor activation of cAMP metabolism may represent a central axis of events that facilitate the impact of PGE2 on the processes of mesenchymal stem cell commitment to chondrogenesis and ultimate chondrocyte maturation.

Investigation of the agonist activity of prostacyclin analogues on prostanoid EP4 receptors using GW 627368 and taprostene: evidence for species differences

The possibility that the prostacyclin analogues AFP-07 and cicaprost relax saphenous vein preparations of pig, guinea-pig and rabbit by simultaneously activating prostanoid EP4 and IP (prostacyclin) receptors was investigated using the high-affinity EP4 antagonist GW 627368. The IP receptor system in each preparation was suppressed with the partial agonist taprostene. The results indicate that AFP-07 and cicaprost are moderately potent EP4 agonists on pig saphenous vein, but much weaker EP4 agonists on guinea-pig saphenous vein. GW 627368 did not antagonise AFP-07- and cicaprost-induced relaxation of rabbit saphenous vein, which contrasts with a previous study using the EP4 blocker AH 23848. However, treatment with taprostene was of less value due to poorer antagonism of the rabbit IP system; this may be related to the presence of a sensitive EP2 relaxant system. Relaxation of each preparation induced by the selective EP2 agonist ONO-AE1-259 was unaffected by GW 627368, with and without taprostene.

cAMP-mediated inhibition of DNA replication and S phase progression: involvement of Rb, p21Cip1, and PCNA

cAMP exerts an antiproliferative effect on a number of cell types including lymphocytes. This effect of cAMP is proposed to be mediated by its ability to inhibit G1/S transition. In this report, we provide evidence for a new mechanism whereby cAMP might inhibit cellular proliferation. We show that elevation of intracellular levels of cAMP inhibits DNA replication and arrests the cells in S phase. The cAMP-induced inhibition of DNA synthesis was associated with the increased binding of p21Cip1 to Cdk2-cyclin complexes, inhibition of Cdk2 kinase activity, dephosphorylation of Rb, and dissociation of PCNA from chromatin in S phase cells. The ability of cAMP to inhibit DNA replication and trigger release of PCNA from chromatin required Rb and p21Cip1 proteins, since both processes were only marginally affected by increased levels of cAMP in Rb-/- and p21Cip1-/- 3T3 fibroblasts. Importantly, the implications of cAMP-induced inhibition of DNA synthesis in cancer treatment was demonstrated by the ability of cAMP to reduce apoptosis induced by S phase-specific cytotoxic drugs. Taken together, these results demonstrate a novel role for cAMP in regulation of DNA synthesis and support a model in which activation of cAMP-dependent signaling protects cells from the effect of S phase-specific antitumor agents.

Implication of prostaglandin E(2) in TNF-alpha-induced release of m-calpain from HCS-2/8 chondrocytes. Inhibition of m-calpain release by NSAIDs

OBJECTIVE

Calpains are known as Ca(2+)-dependent intracellular neutral cysteine proteases. However, m-calpain is detected in synovial fluid of arthritic joints and is shown to possess the proteoglycanase activity in vitro. The mechanism of m-calpain release into the extracellular spaces during arthritis has not yet been well characterized. In the present study, we have analyzed m-calpain release from cultured chondrocytes stimulated by a proinflammatory cytokine, tumor necrosis factor-alpha (TNF-alpha). The effects of non-steroidal anti-inflammatory drugs (NSAIDs) on m-calpain release were also examined.

METHODS

Human chondrocytic HCS-2/8 cells were stimulated by TNF-alpha in the presence or absence of an NSAID. m-Calpain in the cells and culture medium was quantified by Western blot analysis using an anti-m-calpain antibody. Western blots were subjected to densitometric analysis and band intensities were determined.

RESULTS

TNF-alpha (10 ng/ml) stimulated m-calpain release with transient increase in cellular m-calpain in HCS-2/8 cells. NSAIDs examined (aspirin, loxoprofen-SRS, diclofenac sodium, indomethacin and NS398) inhibited m-calpain release and production of prostaglandin E(2) (PGE(2)) induced by 10 ng/ml TNF-alpha. Exogenously added PGE(2) accelerated the release of m-calpain in response to a lower concentration of TNF-alpha (1 ng/ml). AH6809, an EP1/2 antagonist, but not SC19220 (an EP1 antagonist), effectively inhibited TNF-alpha-induced m-calpain release. In contrast, butaprost, an EP2 agonist, accelerated release of m-calpain by 1 ng/ml TNF-alpha.

CONCLUSIONS

These results suggest that TNF-alpha stimulates upregulation and release of m-calpain in chondrocytic HCS-2/8 cells, and that stimulation of EP2-PGE(2) receptor by produced PGE(2) is deeply involved in this process.

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.

The role of prostaglandin E2 receptors in the pathogenesis of rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory disorder leading to bone and cartilage destruction. A substantial body of evidence suggests that prostaglandin E2 (PGE2) contributes to the pathogenesis of RA, and nonsteroidal anti-inflammatory drugs, inhibitors of the synthesis of PGE2 and other prostanoids, continue to be used in the treatment of this disease. To begin to understand the mechanism by which prostaglandins modulate the pathophysiology of this disease, we examined mice lacking each of the four known PGE2 (EP) receptors after generation of collagen antibody-induced arthritis, an animal model of RA. Homozygous deletion of the EP1, EP2, or EP3 receptors did not affect the development of arthritis, whereas EP4 receptor-deficient mice showed decreased incidence and severity of disease. These animals also showed reduced inflammation as assessed by circulating IL-6 and serum amyloid A levels. Joint histopathology of EP4(-/-) animals revealed reduced bone destruction, proteoglycan loss, and type II collagen breakdown in cartilage compared with EP4(+/+) mice. Furthermore, liver and macrophages isolated from EP4(-/-) animals produced significantly less IL-1 beta and IL-6 than control samples. Thus, PGE2 contributes to disease progression at least in part by binding to the EP4 receptor. Antagonists of this receptor might therefore provide novel agents for the treatment of RA.

Effects of polysulfated glycosaminoglycan and triamcinolone acetonid on the production of proteinases and their inhibitors by IL-1alpha treated articular chondrocytes

In this study we determined the in vitro effects of polysulfated glycosaminoglycan (PSGAG) and the glucocorticoid triamcinolone acetonid (TA) on the IL-1 altered expression and activity of matrix metalloproteinases (MMP-1, MMP-3), tissue inhibitor of metalloproteinases-1, the plasminogen activators tPA and uPA and plasminogen activator inhibitor 1 by articular chondrocytes. Bovine chondrocytes were cultured in alginate gel beads. Cells were treated with interleukin-1alpha (IL-1alpha) in the presence of vehicle or drugs at various concentrations. After 48hr mRNA expression of MMP-1, MMP-3, TIMP-1, uPA, tPA and PAI-1 was analyzed by RT-PCR-ELISA. The protein synthesis of TIMP-1 and MMP-3 was determined by immunoprecipitation, PAI-1 protein was quantitated by ELISA. The activity of enzymes and inhibitors was measured by functional assays. Treating chondrocytes with IL-1 induced the expression of MMPs and downregulated TIMP-1 but stimulated both the expression of PAs and PAI-1. Both drugs significantly reduced collagenase and proteoglycanase activities which was accompanied by inhibition of the expression of MMP-1 and MMP-3. The IL-1 decreased expression of TIMP-1 was further reduced by TA, which resulted in a significant loss of TIMP activity. No effects on TIMP activity or TIMP-1 biosynthesis were observed after treatment of chondrocytes with PSGAG. Both drugs inhibited the IL-1-induced mRNA expression of tPA, whereas expression of uPA was only mildly reduced by PSGAG, which also induced PAI-1 above IL-1 stimulated levels. As inhibition of collagenase activities and tPA expression by PSGAG occurred at physiological concentrations it might be of clinical relevance, indicating that PSGAG could help reducing cartilage degradation and has a strong anti-fibrinolytic potential. Due to their co-regulation of MMPs and TIMP(s) glucocorticoids should be carefully studied for their overall effect on extracellular matrix proteolysis.

Development of selective tolerance to interleukin-1beta by human chondrocytes in vitro

Interleukin-1 induces release of NO and PGE(2) and production of matrix degrading enzymes in chondrocytes. In osteoarthritis (OA), IL-1 continually, or episodically, acts on chondrocytes in a paracrine and autocrine manner. Human chondrocytes in chondron pellet culture were treated chronically (up to 14 days) with IL-1beta. Chondrons from OA articular cartilage were cultured for 3 weeks before treatment with IL-1beta (0.05-10 ng/ml) for an additional 2 weeks. Spontaneous release of NO and IL-1beta declined over the pretreatment period. In response to IL-1beta (0.1 ng/ml), NO and PGE(2) release was maximal on Day 2 or 3 and then declined to near basal level by Day 14. Synthesis was recovered by addition of 1 ng/ml IL-1beta on Day 11. Expression of inducible nitric oxide synthase (iNOS), detected by immunofluorescence, was elevated on Day 2 and declined through Day 14, which coordinated with the pattern of NO release. On the other hand, IL-1beta-induced MMP-13 synthesis was elevated on Day 3, declined on Day 5, and then increased again through Day 14. IL-1beta increased glucose consumption and lactate production throughout the treatment. IL-1beta stimulated proteoglycan degradation in the early days and inhibited proteoglycan synthesis through Day 14. Chondron pellet cultures from non-OA cartilage released the same amount of NO but produced less PGE(2) and MMP-13 in response to IL-1beta than OA cultures. Like the OA, IL-1beta-induced NO and PGE(2) release decreased over time. In conclusion, with prolonged exposure to IL-1beta, human chondrocytes develop selective tolerance involving NO and PGE(2) release but not MMP-13 production, metabolic activity, or matrix metabolism.

Evaluation of the influence of prostaglandin E2 on recombinant equine interleukin-1beta-stimulated matrix metalloproteinases 1, 3, and 13 and tissue inhibitor of matrix metalloproteinase 1 expression in equine chondrocyte cultures

OBJECTIVE

To determine the effects of prostaglandin E2 (PGE2) on recombinant equine interleukin (IL)-1beta-stimulated expression of matrix metalloproteinases (MMP 1, MMP 3, MMP 13) and tissue inhibitor of matrix metalloproteinase 1 (TIMP 1) in vitro.

SAMPLE POPULATION

Cultured equine chondrocytes.

PROCEDURE

Stationary monolayers of first-passage chondrocytes were exposed to graduated concentrations of PGE2 with or without a subsaturating dose (50 pg/ml) of recombinant equine IL-1beta (reIL-1beta) to induce expression of MMP 1, MMP 3, MMP 13, and TIMP 1, followed by RNA isolation and northern blotting. In subsequent experiments, gene expression was similarly quantified from mRNA isolated from cultures pretreated with phenylbutazone to quench endogenous PGE2 synthesis, followed by exposure to reIL-1beta and exogenous PGE2 (5 mg/ml) with appropriate controls.

RESULTS

Exogenous PGE2 (10 mg/ml) significantly reduced reIL-1beta-induced expression of MMP 1, MMP 3, MMP 13, and TIMP 1. Abrogation of cytokine induction with this dose of PGE2 was comparable to that for dexamethasone (10(-5) M) control. Similarly, pretreatment with phenylbutazone, followed by exposure to relL-1beta and PGE2 (5 mg/ml), was associated with a reduced expression of the genes of interest, an effect that was significant for MMP 1, MMP 13, and TIMP 1.

CONCLUSIONS AND CLINICAL RELEVANCE

The MMP and TIMP 1 are important mediators in the pathophysiologic events in osteoarthritis. The potential for physiologically relevant regulation of expression of these genes by PGE2 is a consideration in the use of drugs that inhibit prostanoid synthesis in the treatment of equine arthropathies.

Stimulation of cyclooxygenase-2-activity by nitric oxide-derived species in rat chondrocyte: lack of contribution to loss of cartilage anabolism

Cross-talk between inducible nitric oxide synthase (NOS II) and cyclooxygenase-2 (COX-2) was investigated in rat chondrocytes. In monolayers, interleukin-1beta (IL-1beta) induced COX-2 and NOS II expression in a dose- and time-dependent manner, to produce high prostaglandin E(2) (PGE(2)) and nitrite (NO(2)(-)) levels in an apparently coordinated fashion. COX-2 mRNA was induced earlier (30 min. versus 4 hr) and less markedly (4-fold versus 12-fold at 24 hr) than NOS II, and was poorly affected by the translational inhibitor cycloheximide (CHX). IL-1beta did not stabilize COX-2 mRNA in contrast to CHX. Indomethacin and NS-398 lacked any effect on NO(2)(-) levels whereas L-NMMA and SMT reduced PGE(2) levels at concentration inhibiting NO(2)(-) production from 50 to 90%, even when added at a time allowing a complete expression of both enzymes (8 hr). Basal COX activity was unaffected by NO donors. The SOD mimetic, CuDips inhibited COX-2 activity by more than 75% whereas catalase did not. Inhibition of COX-2 by CuDips was not sensitive to catalase, consistent with a superoxide-mediated effect. In tridimensional culture, IL-1beta inhibited radiolabelled sodium sulphate incorporation while stimulating COX-2 and NOS II activities. Cartilage injury was corrected by L-NMMA or CuDips but not by NSAIDs, consistent with a peroxynitrite-mediated effect. These results show that in chondrocytes: (i) COX2 and NOS II genes are induced sequentially and distinctly by IL-1beta; (ii) COX-1 and COX-2 activity are affected differently by NO-derived species; (iii) peroxynitrite accounts likely for stimulation of COX-2 activity and inhibition of proteoglycan synthesis induced by IL-1beta.

Permeability of endothelial monolayers to albumin is increased by bradykinin and inhibited by prostaglandins

Using monolayers of bovine aortic endothelial cells (BAEC) in modified Boyden chambers, we examined the role of prostaglandins (PGs) in the bradykinin (BK)-induced increase of albumin permeability. BK induced a concentration-dependent increase of the permeability of BAEC, which reached 49.9 +/- 1% at the concentration of 10(-8) M. Two inhibitors of the prostaglandin G/H synthase, indomethacin (2.88 microM) and ibuprofen (10 microM), potentiated BK-induced permeability 1.8- and 3.9-fold, respectively. Exogenously administered PGE2 and iloprost, a stable analog of prostacyclin, attenuated the effect of BK in a concentration-dependent manner. Butaprost equally reduced the effect of BK, suggesting the participation of the EP2 receptor in this phenomenon. However, the EP4-selective antagonist AH-23848 did not significantly inhibit the protective effect of PGE2. The inhibitory effect of PGE2 was reversed by the adenylate cyclase inhibitor MDL-12330A (10 microM). These results suggest that BK-induced increase of permeability of BAEC monolayer to (125)I-labeled albumin is negatively regulated by PGs. This postulated autocrine activity of PGs may involve an increase in the intracellular level of cAMP.

Prostaglandin E(2) increases cyclic AMP and inhibits endothelin-1 production/secretion by guinea-pig tracheal epithelial cells through EP(4) receptors

Prostaglandin E(2) (PGE(2)) increased adenosine 3' : 5'-cyclic monophosphate (cyclic AMP) formation in tracheal epithelial cells and concomitantly decreased the production/secretion of immunoreactive endothelin (irET). Naturally occurring prostanoids and selective and non-selective EP receptor agonists showed the following rank order of potency in stimulating cyclic AMP generation by epithelial cells: PGE(2) (EP-selective)>16,16-dimethyl PGE(2) (EP-selective)>11-deoxy PGE(2) (EP-selective)>>>iloprost (IP/EP(1)/EP(3)-selective), butaprost (EP(2)-selective), PGD(2) (DP-selective), PGF(2alpha) (FP-selective). The lack of responsiveness of the latter prostanoids indicated that the prostanoid receptor present in these cells is not of the DP, FP, IP, EP(1), EP(2) or EP(3) subtype. Pre-incubating the cells with the selective TP/EP(4)-receptor antagonists AH23848B and AH22921X antagonized the PGE(2)-evoked cyclic AMP generation. This suggested that EP(4) receptors mediate PGE(2) effects. However, in addition to any antagonistic effects at EP(4)-receptors, both compounds, to a different extent, modified cyclic AMP metabolism. The selective EP(1), DP and EP(2) receptor antagonist (AH6809) failed to inhibit PGE(2)-evoked cyclic AMP generation which confirmed that the EP(2) receptor subtype did not contribute to the change in cyclic AMP formation in these cells. The PGE(2)-induced inhibition of irET production by guinea-pig tracheal epithelial cells was due to cyclic AMP generation and activation of the cyclic AMP-dependent protein kinase since this effect was reverted by the cyclic AMP antagonist Rp-cAMPS. These results provide the first evidence supporting the existence of a functional prostaglandin E(2) receptor that shares the pharmacological features of the EP(4)-receptor subtype in guinea-pig tracheal epithelial cells. These receptors modulate cyclic AMP formation as well as ET-1 production/secretion in these cells.

Characterization of the prostaglandin receptors in human osteoblasts in culture

Prostaglandins have complex actions on bone metabolism that depend on interactions with different types and subtypes of receptors. Our objective was to characterize the prostaglandins receptors present in primary cultures of human osteoblasts. RT-PCR analysis revealed the presence of DP, EP(4), IP, FP and TP receptor mRNA in primary cultures of human osteoblasts. FP receptor mRNA was detected only after 3 weeks of confluency, all the others were detected at every culture time tested. To verify the functionality of these receptors we challenged the cells with the prostanoids and synthetic analogues and determined the intracellular levels of cAMP. All receptors found by RT-PCR were coupled to second messengers except for the DP subtype. These results clearly show the presence of functional EP(4), IP, FP and TP receptors in human osteoblasts in culture.

Functional pharmacological evidence for EP2 and EP4 prostanoid receptors in immortalized human trabecular meshwork and non-pigmented ciliary epithelial cells

The aim of these studies was to characterize the molecular pharmacology of the prostanoid receptors positively coupled to stimulation of adenylyl cyclase activity in immortalized human trabecular meshwork (TM-3) cells and to compare these results with that of the receptors in immortalized human nonpigmented epithelial (NPE) cells. In general, the TM-3 and NPE cells showed a similar profile with respect to their responses to various prostaglandin (PG) receptor agonists. The rank order of potency (EC50; means +/- SEM) for these compounds in the TM-3 cells was: PGE2 (124 +/- 21 nM) > 13,14-dihydro-PGE1 (430 +/- 110 nM) = PGE1 (522 +/- 345 nM) > 11-deoxy-PGE1 (1063 +/- 118 nM) = 16,16-dimethyl-PGE2 (1776 +/- 460 nM) = butaprost (1920 +/- 527 nM) >> PGD2 = PGI2 = PGF2alpha (n = 3 - 12). While the agonist profile indicated the presence of EP2 receptors, the effects of the EP4 receptor antagonists suggested the additional expression of EP4 receptors in both of these cells. Thus, the EP4 receptor antagonist, AH23848B, at a concentration of 30 microM, caused a dextral shift in the PGE2 concentration-response curves in both TM-3 and NPE cells coupled with a 20-28% decrease in the maximal response of PGE2, indicating apparent noncompetitive antagonism profiles. The antagonist potency of AH23848B in these cells was: Kb = 38.4 +/- 14.8 microM and 23.5 +/- 4.5 microM; -log Kb = 4.7. The other EP4 receptor antagonist, AH22921 (-log Kb = 4.1 - 4.7), was weaker than AH23848B. Taken together, these pharmacological studies have shown than TM-3 and NPE cells apparently contain functional EP2 and EP4 prostanoid receptors positively coupled to adenylyl cyclase.

Induction of apoptosis in bovine articular chondrocyte by prostaglandin E(2) through cAMP-dependent pathway

OBJECTIVE

Regulation of important biological processes such as proliferation and differentiation of articular chondrocytes is known to be mediated by prostaglandin E(2) (PGE(2)) in both normal and pathological states. Articular chondrocytes also undergo apoptosis, a biological phenomenon implicated in many physiological processes. Whether or not PGE(2) induces apoptosis in articular chondrocytes, however, is not known.

DESIGN

Bovine articular chondrocytes were cultured with or without PGE(2) for 24 h and amounts of fragmented DNA, which is a distinct characteristic of apoptosis, were measured by enzyme-linked immunosorbent assay. Also effect of cyclic AMP (cAMP), which is one of the intracellular downstream mediator of PGE(2), on chondrocyte apoptosis was investigated.

RESULTS

Administration of exogenous PGE(2) on bovine articular cartilage grown as a monolayer culture resulted in the induction of DNA fragmentation. This DNA fragmentation was accompanied with a marked dose-dependent increase in intracellular cAMP. Also cultured cells were treated with cAMP analogue, dibutyryl-cAMP or forskolin, a direct activator of adenylate cyclase, and the incidence of apoptosis in the chondrocytes was determined. As well as PGE(2), dibutyryl-cAMP and forskolin stimulated chondrocyte DNA fragmentation.

CONCLUSIONS

It is the first report that PGE(2) can induce articular chondrocyte apoptosis in vitro. It is also suggest that apoptosis of chondrocytes by PGE(2) is linked with cAMP-dependent pathway.

Endogenous EP4 prostaglandin receptors coupled positively to adenylyl cyclase in Chinese hamster ovary cells: pharmacological characterization

The purpose of these studies was to investigate the pharmacology of E-series and selected prostaglandins of other classes on adenylyl cyclase activity in Chinese hamster ovary (CHO) cells expressing an endogenous prostanoid receptor and to compare these responses with those from immortalized human non-pigmented ciliary epithelial (NPE) cells containing the EP2 receptor. 11-deoxy-PGE2 was the most potent of the 16 prostanoid agonists tested for stimulating cAMP formation with a potency (EC50) value of 26 +/- 6 nM in the CHO cells. The endogenous ligand, PGE2, exhibited potencies of 40 +/- 7 nM (n = 24) in the CHO cells and 67 +/- 9 nM (n = 46) in the NPE cells. The EP2 receptor agonist, butaprost, produced an EC50 value of 212 +/- 58 nM (n = 4) in the NPE cells while being inactive (EC50 > 10,000 nM, n = 6) in the CHO cells. The EP4 receptor selective antagonists, AH22921 and AH23848B, at a concentration of 30 microM, caused a 2.2 +/- 0.5 (n = 4) and 8.2 +/- 2.7 (n = 4) fold rightward shift in the PGE2 concentration-response curves in the CHO cells, yielding apparent pKb values of 4.6 +/- 0.6 and 5.3 +/- 0.2 (n = 4), respectively. AH22921 and AH23848B were non-competitive antagonists at the CHO cell EP4 receptor, but did not shift the PGE2 concentration-response curves in the NPE cells containing the EP2 receptor. These studies have characterized the functional prostaglandin receptors in CHO cells pharmacologically and shown them to be consistent with the EP4 subtype.

The anabolic effect of PGE2 in rat bone marrow cultures is mediated via the EP4 receptor subtype

Prostaglandin E2 (PGE2) is an anabolic agent in vivo that stimulates bone formation by recruiting osteoblasts from bone marrow precursors. To understand which of the known PGE2 receptors (EP1-4) is involved in this process, we tested the effect of PGE2 and various EP agonists and/or antagonists on osteoblastic differentiation in cultures of bone marrow cells by counting bone nodules and measuring alkaline phosphatase activity. PGE2 increased both parameters, peaking at 100 nM, an effect that was mimicked by forskolin and was abolished by 2',3'-dideoxyadenosine (an adenylate cyclase inhibitor) and was thus cAMP dependent, pointing to the involvement of EP2 or EP4. Consistently, 17-phenyl-omega-trinor PGE2 (EP1 agonist) and sulprostone (EP3/EP1 agonist) lacked any anabolic activity. Furthermore, butaprost (EP2 agonist) was inactive, 11-deoxy-PGE1 (EP4/EP2 agonist) was as effective as PGE2, and the PGE2 effect was abolished dose dependently by the selective EP4 antagonist AH-23848B, suggesting the involvement of EP4. We also found that PGE2 increased nodule formation and AP activity when added for the initial attachment period of 24 h only. Thus this study shows that PGE2 stimulates osteoblastic differentiation in bone marrow cultures, probably by activating the EP4 receptor, and that this effect may involve recruitment of noncommitted (nonadherent) osteogenic precursors, in agreement with its suggested mode of operation in vivo.