Exogenous sphingomyelinase increases collagen and sulphated glycosaminoglycan production by primary articular chondrocytes: an in vitro study
© Gilbert et al.; licensee BioMed Central Ltd. 2006
Received: 23 March 2006
Accepted: 20 April 2006
Published: 12 May 2006
We previously established a role for the second messenger ceramide in protein kinase R (PKR)-mediated articular cartilage degradation. Ceramide is known to play a dual role in collagen gene regulation, with the effect of ceramide on collagen promoter activity being dependent on its concentration. Treatment of cells with low doses of sphingomyelinase produces small increases in endogenous ceramide. We investigated whether ceramide influences articular chondrocyte matrix homeostasis and, if so, the role of PKR in this process. Bovine articular chondrocytes were stimulated for 7 days with sphingomyelinase to increase endogenous levels of ceramide. To inhibit PKR, 2-aminopurine was added to duplicate cultures. De novo sulphated glycosaminoglycan and collagen synthesis were measured by adding [35S]-sulphate and [3H]-proline to the media, respectively. Chondrocyte phenotype was investigated using RT-PCR and Western blot analysis. Over 7 days, sphingomyelinase increased the release of newly synthesized sulphated glycosaminoglycan and collagen into the media, whereas inhibition of PKR in sphingomyelinase-treated cells reduced the level of newly synthesized sulphated glycosaminoglycan and collagen. Sphingomyelinase treated chondrocytes expressed col2a1 mRNA, which is indicative of a normal chondrocyte phenotype; however, a significant reduction in type II collagen protein was detected. Therefore, small increments in endogenous ceramide in chondrocytes appear to push the homeostatic balance toward extracellular matrix synthesis but at the expense of the chondrocytic phenotype, which was, in part, mediated by PKR.
The signalling molecule ceramide belongs to a family of highly hydrophobic molecules containing a variable length fatty acid linked to sphingosine . As well as its established role in membrane structure, many studies have now shown that ceramide is a key second messenger, activating a number of intracellular signalling cascades that are implicated in a wide range of cellular functions such as proliferation, differentiation, necrosis and apoptosis [2–4]. Interestingly, Sabatini and coworkers [5, 6] recently implicated ceramide signalling in the regulation of proteoglycan degradation and mRNA expression of matrix metalloproteinases (MMPs) 1, 3 and 13 in rabbit articular chondrocytes. Furthermore, we demonstrated that application of exogenous ceramide induces articular cartilage degradation, which is, in part, mediated through protein kinase R (PKR) [7, 8]. Treatment of cartilage explants with the short chain, cell permeable ceramide analogue C2-ceramide resulted in PKR-mediated increases in chondrocyte death and release of proteoglycans and pro- and active MMP-2. In addition, ceramide has been shown to activate PKR in leukaemia cell lines, and at high concentrations it results in PKR-mediated inhibition of protein synthesis . Thus, ceramide signalling, via the PKR pathway, may play a pivotal role in articular cartilage metabolism.
Evidence suggests that there is a dual role for sphingolipids in collagen gene regulation, supporting the existence of a sphingolipid rheostat . Low concentrations of ceramide stimulate type I collagen promoter activity in fibroblasts, whereas high concentrations of ceramide potently inhibit collagen gene transcription and decrease collagen protein production in fibroblasts and hepatic stellate cells [17–19]. To our knowledge, no studies have been conducted to investigate the effect of ceramide accumulation on chondrocyte extracellular matrix (ECM) homeostasis. However, the research described above suggests that increases in endogenous ceramide may affect cartilage ECM protein transcription and translation, as well as activating degradative pathways that are involved in the pathogenesis of diseases such as osteoarthritis. The aims of the present study were therefore to investigate the effect of increasing the levels of endogenous ceramide on articular chondrocyte homeostasis and to determine whether any ceramide-induced changes in matrix metabolism are mediated via the PKR signalling pathway.
Materials and methods
All chemicals were obtained from Sigma (Poole, UK) unless otherwise stated and were of analytical grade or above. Culture medium consisted of Dulbecco's modified eagle's medium (DMEM; DMEM-Glutamax-I™, Invitrogen, Paisley, UK) supplemented with 100 U/ml penicillin, 100 μg/ml streptomycin, 50 μg/ml ascorbate-2-phosphate and 1× insulin-transferrin-sodium selenite (ITS). For radiolabelling experiments, DMEM-Glutamax-I™ was replaced with a 1:1 mixture of DMEM-Glutamax-I™ and Hams F12 media.
Primary articular chondrocyte culture
Articular cartilage was taken from the metacarpalphalangeal joint of 7-day-old calves within 12 hours of slaughter using a scalpel, and full-depth cartilage explants (20–70 mg) were cultured overnight at 37°C in a humidified atmosphere of 5% carbon dioxide and 95% air in 1 ml of DMEM-Glutamax-I™ supplemented with 10% foetal calf serum. DMEM-Glutamax-I™ containing foetal calf serum was removed and chondrocytes isolated as previously described . Following isolation, chondrocytes were cultured (1 × 106 cells/well of a 24-well plate) overnight at 37°C in serum-free DMEM-Glutamax-I™ supplemented with ITS in order to maintain their chondrocytic phenotype  and prevent serum withdrawal activation of signalling pathways . To increase endogenous levels of ceramide, chondrocytes were stimulated for up to 10 days with bacterial SMase (0.1–1.0 U/ml) . Media and treatments were refreshed at 7 days if cultures were extended to 10 days. To investigate the role of PKR in SMase-mediated responses, the PKR inhibitor 2-aminopurine (2AP; 1 mmol/l) was added to duplicate cultures 1 hour before and during the addition of treatments. This concentration inhibits activation of PKR in a number of cell types [4, 8, 23–26] and does not affect chondrocyte viability . Following treatment, media was removed and stored at -20°C and 200 μl ice-cold extract buffer (0.9% Triton X-100) containing protease inhibitors (1 μmol/l leupepstatin hemisulphate, 150 nmol/l aprotinin, 0.5 mmol/l EDTA disodium salt, 500 μmol/l AEBSF HCl, 1 μmol/l E64; Merck Biosciences, Nottingham, UK) and phosphatase inhibitors (phosphatase inhibitor cocktail set II, according to manufacturer's instructions; Merck Biosciences, Nottingham, UK) was added to the cells. Cell extracts were stored at -80°C for future analysis.
Cytotoxicity assay and total cell number
Cell death was assessed using the CytoTox 96® assay (Promega, Southampton, UK), which quantitatively measures lactate dehydrogenase (LDH) present in the culture media that has been released upon natural lysis of cells during the culture period [8, 27]. This assay measures both primary and secondary necrotic cell death. Differences in the release of LDH associated with culture treatment were expressed as absorbance units. The total cell number, at the end of each treatment, was also determined using the CytoTox 96® assay. This assay can be used to measure indirectly the LDH activity present in the cytoplasm of cells that are intact at the end of the culture period. Cell quantification, therefore, occurs following lysis of the cells by the addition of extract buffer. The number of cells present is directly proportional to the absorbance value, which represents LDH activity .
Analysis of proteoglycan release
The amount of sulphated glycosaminoglycan (sGAG) released into the medium of chondrocyte cultures was measured using the dimethylmethylene blue (DMMB) assay using chondroitin-4-sulphate-C from shark cartilage as a standard, as described previously . Differences in the release of sGAG associated with culture treatment were expressed as micrograms of GAG released per cell.
Determination of protein concentration
The protein concentration of cell extracts after 24 hours of treatments was determined using the BCA method, in accordance with the manufacturer's instructions (Perbio Science, Cramlington, UK).
Analysis of de novo matrix synthesis using [35S]-sulphate and [3H]-proline radiolabelling
To measure newly synthesized protein and sGAGs, chondrocytes (4 × 105 cells/well of a 48-well plate) were treated with sphingomyelinase (0.1 U/ml) in the presence of 20 μCi/ml of [3H]-proline and 10 μCi/ml [35S]-sulphate (GE Healthcare, Chalfont St Giles, UK). At the end of the treatment period, unincorporated radiolabel was removed from the media and cell extracts using Ultrafree®-MC centrifugal filter units, in accordance with the manufacturer's instructions (Millipore, Watford, UK). Incorporated [35S] radioactivity in the culture media and cell extracts was counted (Beckman Scintillation Counter; Beckman Coulter, High Wycombe, UK) as a measure of de novo sGAG synthesis.
De novo collagen synthesis was determined by digesting labelled protein in media and cell extracts with 8U bacterial collagenase (Worthington's Type 3 collagenase; Lorne Laboratories, Reading, UK) overnight at 37°C . Digested collagen fragments were removed using Ultrafree®-MC filter units and the remaining undigested [3H] counts taken as a measure of noncollagenous protein. Collagenous protein was calculated using the following equation: collagen (counts/min) = total protein (counts before digestion) – noncollagenous protein (counts after digestion).
RNA extraction, cDNA synthesis and PCR
Annealing temp (°C)
Product size (bp)
Reference/GenBank accession number
Valcourt and coworkers 
Type II collagen and aggrecan gene expression were measured by quantitative PCR (qPCR). cDNA was produced as detailed above and qPCR carried out using an ABI 7700 Sequence Detection System, in accordance with the manufacturer's instructions (Applied Biosystems, Warrington, UK) using 300 nmol/l forward and reverse primers and 200 nmol/l probe (5' 6-carboxyfluorescein and 3' 6-carboxytetramethylrhodamine). The GAPDH gene was used as an endogenous control to normalize for differences in the amount of total RNA present in each sample; GAPDH primers (forward: 5'-GGCATCGTGGAGGGACTTATGA-3'; reverse: 5'-CAGAAGACTGTGGATGGCCC-3') and probe (5'-CACTGTCCACGCCATCACTGC-3') were purchased from Applied Biosystems. Primers and probes to type II collagen and aggrecan were as previously described .
Western blot analysis of type II collagen
To further investigate the phenotype of bovine chondrocytes following culture in the presence and absence of SMase, Western blotting was performed, as described previously . Cell associated material and media samples (from equivalent cell numbers) were reduced (5% β-mercaptoethanol) and resolved on 7.5% (weight/vol) SDS-polyacrylamide gels and transferred subsequently to PVDF membrane (Immobilon; Millipore). Binding of our monclonal antibody to type II collagen (AVT6E3)  and horseradish peroxidase conjugated anti-mouse IgG was detected using enhanced chemiluminescence reagents (GE Healthcare) on Hyperfilm-ECL (GE Healthcare).
Data are representative of at least three independent experiments except for the radiation experiment, which was repeated twice. Data are presented, following normalization to cell number, as mean ± standard error (n ≥ 3), tested for normality and equal variances, and analyzed by Student's two-sample t test (Minitab Statistical Software; Minitab Ltd, Coventry, UK). Treatments were compared with untreated control cells and differences were considered significant at the 5% level (P < 0.05).
Effect of increasing doses of exogenous sphingomyelinase on chondrocyte function
A dose of 0.1 U/ml was chosen for further study because this caused a minimal level of cell death at 24 hours (control 0.17 ± 0.0003 versus SMase 0.2 ± 0.005). An identical experiment was thus performed and cells cultured for 1–7 days. Cell number, cell death and the amount of sGAG released into the media over this period were measured. Over 7 days, an equivalent level of cell death was observed in all cultures regardless of treatment and did not exceed 10–15% of the total cell number (data not shown). Despite this, over the same culture period, significantly fewer cells were found in SMase-treated cultures than in controls (P = 0.049), suggesting reduced proliferation (Figure 2d). In addition, SMase significantly increased the amount of sGAG released in to the media (P = 0.046; Figure 2e).
Effect of SMase on de novosGAG and collagen synthesis
Investigation of chondrocyte phenotype following culture with SMase
Effect of inhibiting activation of PKR on cartilage matrix homeostasis
Both acidic and neutral sphingomyelinases are expressed by articular chondrocytes
This study demonstrates for the first time that ECM homeostasis in articular cartilage chondrocytes can be profoundly altered by triggering the ceramide signalling pathway. Over 24 hours, raising endogenous levels of ceramide in articular cartilage chondrocytes by treatment with 0.1 U/ml bacterial SMase caused a dose-dependent increase in cell death with a concomitant decrease in cell number. This is in accordance with the known role for ceramide in initiating a cellular stress response resulting in cell death . It should be noted that the assay used to measure cell death in this study detects loss of membrane integrity and thus measures necrosis, either primary or secondary (cultured cells that are undergoing apoptosis in vitro eventually undergo secondary necrosis). Therefore, further studies are necessary to determine the extent of apoptotic cell death. Over the extended culture period, SMase treatment resulted in a further reduction in cell number compared with that in control cultures, with the majority of the decrease occurring in the early stages of the treatment; thereafter the rate of proliferation was similar to that in controls (Figure 1d). Because there was no concomitant increase in cell death, this suggests that SMase treatment also decreased chondrocyte proliferation. This in accordance with studies in human keratinocytes, which have shown that a rapid (15 minutes) but transient (returning to baseline by 1 hour) increase in endogenous ceramide occurs following treatment with 0.1 U/ml neutral SMase followed by reduced cellular proliferation over 6 days, the extent of which was equivalent to that seen in the present study .
Data obtained from the DMMB assay indicated that SMase increased the release of sGAG from articular chondrocytes. Because this assay does not discriminate between whole sGAG and degraded sGAG fragments, we used incorporation of [35S] to determine whether low concentrations (0.1 U/ml) of exogenous SMase affected sGAG synthesis or degradation. As well as increasing sGAG synthesis, SMase also significantly enhanced the level of de novo collagen and total protein in the media over seven days of culture, suggesting that SMase acts on chondrocytes to increase expression of ECM components. The hydrolysis of sphingomyelin by the action of SMases is the primary mechanism for rapidly increasing ceramide levels in the cell . As discussed above, at the concentration (0.1 U/ml) used, SMase induces a rapid but transient rise in endogenous ceramide in human keratinocytes . Our data correlate with recent studies in fibroblasts that showed that low doses of ceramide stimulate collagen production . This is contrast to the effect caused by high ceramide, which is thought to inhibit collagen production [15, 17, 18] because of its conversion to sphingosine-1-phosphate or other inhibitory intermediates, thus promoting anticeramide affects .
When chondrocytes are cultured as monolayers on plastic they rapidly de-differentiate, losing expression of type II collagen. More specifically they shift their expression from type IIB to type IIA procollagen . Our monolayer cultures supplemented with ITS retained expression of the normal chondrocyte markers Sox9, aggrecan and type IIB collagen. These were still expressed by SMase-treated chondrocytes with no detectable expression of type IIA mRNA. However, SMase reduced type II collagen protein expression (Western blot), despite increasing total collagen production (3[H]-proline incorporation) and maintaining col2a1 mRNA expression (qPCR). Therefore, although low levels of endogenous ceramide in chondrocytes appeared to push the homeostatic balance toward ECM synthesis, which is in accordance with studies in fibroblasts , this may have been at the expense of type II collagen expression.
Preliminary work within our laboratory suggests that the SMase-induced increase in total collagen production is not due to increases in type I or III collagen, but further investigation is clearly warranted. We propose that small increases in cellular ceramide, as mimicked here, may contribute to the increases in proteoglycan and collagen synthesis [38–40] that are observed in the 'biosynthetic phase' in early osteoarthritis . Given that excessive ceramide accumulation within cartilage is known to produce an osteoarthritis-like phenotype , we hypothesize that treatment of chondrocytes with high doses of SMase would result in an accumulation of endogenous ceramide levels within the cells and that it is this that signals downstream to promote cartilage degradative events. This idea that high levels of ceramide promote cartilage degeneration is supported by our earlier studies in which application of C2-ceramide increased MMP expression and activation and proteoglycan release from articular cartilage explants . Thus, further investigations to relate levels of ceramide, sphingosine and sphingosine-1-phosphate to chondrocyte ECM synthesis and degradation are clearly needed to determine how the current data fit into the notion of a 'sphingolipid rheostat' .
Because our previous studies showed that the protein kinase PKR plays a pivotal role in cartilage homeostasis , we inhibited PKR activity to determine whether PKR is involved in the observed changes in matrix synthesis. In control cells, inhibition of PKR caused a significant increase in de novo protein synthesis found within the cell and associated matrix but no change in the level released into the media. This is in keeping with the known role played by PKR as an inhibitor of translation . However, inhibition of PKR activity in SMase-treated chondrocytes significantly reduced the amount of newly synthesized sGAG and collagen detected in the media, suggesting a role for PKR in SMase-induced matrix synthesis. Because high levels of ceramide have previously been shown to result in PKR-mediated inhibition of protein synthesis in a leukaemia cell line , this would suggest that a complex interplay of signalling pathways are involved in SMase-mediated PKR signalling in chondrocytes, the exact nature of which remains to be elucidated.
Finally, we showed, for the first time, that articular chondrocytes can express both acidic and neutral SMases and so are able, given the appropriate external signal, to raise levels of endogenous ceramide. It has been shown that TNF-α can increase cellular ceramide levels via the de novo pathway as well as by binding to its membrane receptor (TNFR55), causing activation of neutral or acidic SMase [36, 42, 43]. Depending on which SMase is activated, an inflammatory (neutral SMase) or apoptotic (acidic SMase) response then occurs. Because TNF-α levels are elevated in arthritis and TNFR55 expression is increased in arthritic disease , our future studies will determine whether TNF-α-mediated activation of neutral SMase and ceramide generation plays a role in cartilage degradation.
In the present study we found that sphingomyelinase, at low concentration, stimulated ECM synthesis in articular chondrocytes, and this was in part mediated by PKR. Importantly, the increase in collagen production was not due to increases in type II collagen. Therefore, small increases in endogenous ceramide in chondrocytes appear to push the homeostatic balance toward ECM synthesis but at the expense of the chondrocytic phenotype. We therefore hypothesize that during the 'biosynthetic phase' in early osteoarthritis, the observed increases in proteoglycan and collagen synthesis may be due to a small increase in cellular ceramide triggered by circulating cytokines such as TNF-α via activation of PKR. Excessive ceramide accumulation may then play a role in the later stages of cartilage degradation.
= Dulbecco's modified eagle's medium
= dimethylmethylene blue
= extracellular matrix
= glyceraldehyde-3-phosphate dehydrogenase
= insulin-transferrin-sodium selenite
= lactate dehydrogenase
= matrix metalloproteinase
= protein kinase R
= reverse transcription polymerase chain reaction
= sulphated glycosaminoglycan
= tumour necrosis factor.
The authors should like to thank the Arthritis Research Campaign for funding this work (Grant numbers: SJG 16436 and M0650 and EJB 14874) and Dr Ilyas Khan for provision of the Sox9 primers.
- Kolesnick RN, Goni FM, Alonso A: Compartmentalization of ceramide signaling: physical foundations and biological effects. J Cell Physiol. 2000, 184: 285-300. 10.1002/1097-4652(200009)184:3<285::AID-JCP2>3.0.CO;2-3.View ArticlePubMedGoogle Scholar
- Goni FM, Alonso A: Sphingomyelinases: enzymology and membrane activity. FEBS Lett. 2002, 531: 38-46. 10.1016/S0014-5793(02)03482-8.View ArticlePubMedGoogle Scholar
- Ruvolo PP: Intracellular signal transduction pathways activated by ceramide and its metabolites. Pharmacol Res. 2003, 47: 383-392. 10.1016/S1043-6618(03)00050-1.View ArticlePubMedGoogle Scholar
- Ruvolo PP, Gao F, Blalock WL, Deng X, Stratford May W: Ceramide regulates protein synthesis by a novel mechanism involving the cellular PKR activator, RAX. J Biol Chem. 2001, 276: 11754-11758. 10.1074/jbc.M011400200.View ArticlePubMedGoogle Scholar
- Sabatini M, Rolland G, Leonce S, Thomas M, Lesur C, Perez V, de Nanteuil G, Bonnet J: Effects of ceramide on apoptosis, proteoglycan degradation, and matrix metalloproteinase expression in rabbit articular cartilage. Biochem Biophys Res Commun. 2000, 267: 438-444. 10.1006/bbrc.1999.1983.View ArticlePubMedGoogle Scholar
- Sabatini M, Thomas M, Deschamps C, Lesur C, Rolland G, de Nanteuil G, Bonnet J: Effects of ceramide on aggrecanase activity in rabbit articular cartilage. Biochem Biophys Res Commun. 2001, 283: 1105-1110. 10.1006/bbrc.2001.4920.View ArticlePubMedGoogle Scholar
- Gilbert SJ, Duance VC, Mason DJ: Tumour necrosis factor alpha up-regulates protein kinase R (PKR)-activating protein (PACT) and increases phosphorylation of PKR and eukaryotic initiation factor 2-alpha in articular chondrocytes. Biochem Soc Trans. 2002, 30: 886-889. 10.1042/BST0300886.View ArticlePubMedGoogle Scholar
- Gilbert SJ, Duance VC, Mason DJ: Does protein kinase R mediate TNF-alpha- and ceramide-induced increases in expression and activation of matrix metalloproteinases in articular cartilage by a novel mechanism?. Arthritis Res Ther. 2004, 6: R46-R55. 10.1186/ar1024.PubMed CentralView ArticlePubMedGoogle Scholar
- Hannun YA, Luberto C: Ceramide in the eukaryotic stress response. Trends Cell Biol. 2000, 10: 73-80. 10.1016/S0962-8924(99)01694-3.View ArticlePubMedGoogle Scholar
- Colell A, Coll O, Mari M, Fernandez-Checa JC, Garcia-Ruiz C: Divergent role of ceramide generated by exogenous sphingomyelinases on NF-kappa B activation and apoptosis in human colon HT-29 cells. FEBS Lett. 2002, 526: 15-20. 10.1016/S0014-5793(02)03106-X.View ArticlePubMedGoogle Scholar
- Mimeault M: New advances on structural and biological functions of ceramide in apoptotic/necrotic cell death and cancer. FEBS Lett. 2002, 530: 9-16. 10.1016/S0014-5793(02)03432-4.View ArticlePubMedGoogle Scholar
- Mathias S, Pena LA, Kolesnick RN: Signal transduction of stress via ceramide. Biochem J. 1998, 335: 465-480.PubMed CentralView ArticlePubMedGoogle Scholar
- Cuvillier O, Pirianov G, Kleuser B, Vanek PG, Coso OA, Gutkind S, Spiegel S: Suppression of ceramide-mediated programmed cell death by sphingosine-1-phosphate. Nature. 1996, 381: 800-803. 10.1038/381800a0.View ArticlePubMedGoogle Scholar
- Maceyka M, Payne SG, Milstien S, Spiegel S: Sphingosine kinase, sphingosine-1-phosphate, and apoptosis. Biochim Biophys Acta. 2002, 1585: 193-201.View ArticlePubMedGoogle Scholar
- Sato M, Markiewicz M, Yamanaka M, Bielawska A, Mao C, Obeid LM, Hannun YA, Trojanowska M: Modulation of transforming growth factor-beta (TGF-beta) signaling by endogenous sphingolipid mediators. J Biol Chem. 2003, 278: 9276-9282. 10.1074/jbc.M211529200.View ArticlePubMedGoogle Scholar
- Farina F, Cappello F, Todaro M, Bucchieri F, Peri G, Zummo G, Stassi G: Involvement of caspase-3 and GD3 ganglioside in ceramide-induced apoptosis in Farber disease. J Histochem Cytochem. 2000, 48: 57-62.PubMedGoogle Scholar
- Solis-Herruzo JA, Brenner DA, Chojkier M: Tumor necrosis factor alpha inhibits collagen gene transcription and collagen synthesis in cultured human fibroblasts. J Biol Chem. 1988, 263: 5841-5845.PubMedGoogle Scholar
- Hernandez-Munoz I, de la Torre P, Sanchez-Alcazar JA, Garcia I, Santiago E, Munoz-Yague MT, Solis-Herruzo JA: Tumor necrosis factor alpha inhibits collagen alpha 1(I) gene expression in rat hepatic stellate cells through a G protein. Gastroenterology. 1997, 113: 625-640. 10.1053/gast.1997.v113.pm9247485.View ArticlePubMedGoogle Scholar
- Reunanen N, Foschi M, Han J, Kahari VM: Activation of extracellular signal-regulated kinase 1/2 inhibits type I collagen expression by human skin fibroblasts. J Biol Chem. 2000, 275: 34634-34639. 10.1074/jbc.C000175200.View ArticlePubMedGoogle Scholar
- Vaughan-Thomas A, Young RD, Phillips AC, Duance VC: Characterization of type XI collagen-glycosaminoglycan interactions. J Biol Chem. 2001, 276: 5303-5309. 10.1074/jbc.M008764200.View ArticlePubMedGoogle Scholar
- Chua KH, Aminuddin BS, Fuzina NH, Ruszymah BH: Insulin-transferrin-selenium prevent human chondrocyte dedifferentiation and promote the formation of high quality tissue engineered human hyaline cartilage. Eur Cell Mater. 2005, 9: 58-67. discussion 67PubMedGoogle Scholar
- Patel CV, Handy I, Goldsmith T, Patel RC: PACT, a stress-modulated cellular activator of interferon-induced double-stranded RNA-activated protein kinase, PKR. J Biol Chem. 2000, 275: 37993-37998. 10.1074/jbc.M004762200.View ArticlePubMedGoogle Scholar
- Pataer A, Vorburger SA, Barber GN, Chada S, Mhashilkar AM, Zou-Yang H, Stewart AL, Balachandran S, Roth JA, Hunt KK, Swisher SG: Adenoviral transfer of the melanoma differentiation-associated gene 7 (mda7) induces apoptosis of lung cancer cells via up-regulation of the double-stranded RNA-dependent protein kinase (PKR). Cancer Res. 2002, 62: 2239-2243.PubMedGoogle Scholar
- Williams BR: Signal integration via PKR. Sci STKE. 2001, 2001: RE2-PubMedGoogle Scholar
- Cheshire JL, Williams BR, Baldwin AS: Involvement of double-stranded RNA-activated protein kinase in the synergistic activation of nuclear factor-kappaB by tumor necrosis factor-alpha and gamma-interferon in preneuronal cells. J Biol Chem. 1999, 274: 4801-4806. 10.1074/jbc.274.8.4801.View ArticlePubMedGoogle Scholar
- Osman F, Jarrous N, Ben-Asouli Y, Kaempfer R: A cis-acting element in the 3'-untranslated region of human TNF-alpha mRNA renders splicing dependent on the activation of protein kinase PKR. Genes Dev. 1999, 13: 3280-3293. 10.1101/gad.13.24.3280.PubMed CentralView ArticlePubMedGoogle Scholar
- Milner JM, Rowan AD, Elliott SF, Cawston TE: Inhibition of furin-like enzymes blocks interleukin-1alpha/oncostatin M-stimulated cartilage degradation. Arthritis Rheum. 2003, 48: 1057-1066. 10.1002/art.10873.View ArticlePubMedGoogle Scholar
- Little CB, Flannery CR, Hughes CE, Mort JS, Roughley PJ, Dent C, Caterson B: Aggrecanase versus matrix metalloproteinases in the catabolism of the interglobular domain of aggrecan in vitro. Biochem J. 1999, 344: 61-68. 10.1042/0264-6021:3440061.PubMed CentralPubMedGoogle Scholar
- Scott BJ, Bateman JE, Bradwell AR: The detection of tritium-labeled ligands and their carrier proteins using a multiwire proportional counter. Anal Biochem. 1982, 123: 1-10. 10.1016/0003-2697(82)90615-7.View ArticlePubMedGoogle Scholar
- Valcourt U, Gouttenoire J, Moustakas A, Herbage D, Mallein-Gerin F: Functions of transforming growth factor-beta family type I receptors and Smad proteins in the hypertrophic maturation and osteoblastic differentiation of chondrocytes. J Biol Chem. 2002, 277: 33545-33558. 10.1074/jbc.M202086200.View ArticlePubMedGoogle Scholar
- Darling EM, Athanasiou KA: Rapid phenotypic changes in passaged articular chondrocyte subpopulations. J Orthop Res. 2005, 23: 425-432. 10.1016/j.orthres.2004.08.008.View ArticlePubMedGoogle Scholar
- Vaughan-Thomas A, Gilbert SJ, Duance VC: Elevated levels of proteolytic enzymes in the aging human vitreous. Invest Ophthalmol Vis Sci. 2000, 41: 3299-3304.PubMedGoogle Scholar
- Young RD, Vaughan-Thomas A, Wardale RJ, Duance VC: Type II collagen deposition in cruciate ligament precedes osteoarthritis in the guinea pig knee. Osteoarthritis Cartilage. 2002, 10: 420-428. 10.1053/joca.2002.0530.View ArticlePubMedGoogle Scholar
- Aigner T, Zhu Y, Chansky HH, Matsen FA, Maloney WJ, Sandell LJ: Reexpression of type IIA procollagen by adult articular chondrocytes in osteoarthritic cartilage. Arthritis Rheum. 1999, 42: 1443-1450. 10.1002/1529-0131(199907)42:7<1443::AID-ANR18>3.0.CO;2-A.View ArticlePubMedGoogle Scholar
- Buisson-Legendre N, Bernard P, Bobichon H, Emonard H, Schneider C, Maquart FX, Haye B, Hornebeck W: Involvement of the 92-kDa gelatinase (matrix metalloproteinase-9) in the ceramide-mediated inhibition of human keratinocyte growth. Biochem Biophys Res Commun. 1999, 260: 634-640. 10.1006/bbrc.1999.0565.View ArticlePubMedGoogle Scholar
- Dbaibo GS, El-Assaad W, Krikorian A, Liu B, Diab K, Idriss NZ, El-Sabban M, Driscoll TA, Perry DK, Hannun YA: Ceramide generation by two distinct pathways in tumor necrosis factor alpha-induced cell death. FEBS Lett. 2001, 503: 7-12. 10.1016/S0014-5793(01)02625-4.View ArticlePubMedGoogle Scholar
- Valcourt U, Gouttenoire J, Aubert-Foucher E, Herbage D, Mallein-Gerin F: Alternative splicing of type II procollagen pre-mRNA in chondrocytes is oppositely regulated by BMP-2 and TGF-beta1. FEBS Lett. 2003, 545: 115-119. 10.1016/S0014-5793(03)00510-6.View ArticlePubMedGoogle Scholar
- Sandell LJ, Aigner T: Articular cartilage and changes in arthritis. An introduction: cell biology of osteoarthritis. Arthritis Res. 2001, 3: 107-113. 10.1186/ar148.PubMed CentralView ArticlePubMedGoogle Scholar
- Goldring MB: The role of the chondrocyte in osteoarthritis. Arthritis Rheum. 2000, 43: 1916-1926. 10.1002/1529-0131(200009)43:9<1916::AID-ANR2>3.0.CO;2-I.View ArticlePubMedGoogle Scholar
- van der Kraan PM, van den Berg WB: Anabolic and destructive mediators in osteoarthritis. Curr Opin Clin Nutr Metab Care. 2000, 3: 205-211. 10.1097/00075197-200005000-00007.View ArticlePubMedGoogle Scholar
- Clemens MJ, Bushell M, Jeffrey IW, Pain VM, Morley SJ: Translation initiation factor modifications and the regulation of protein synthesis in apoptotic cells. Cell Death Differ. 2000, 7: 603-615. 10.1038/sj.cdd.4400695.View ArticlePubMedGoogle Scholar
- Kolesnick RN, Kronke M: Regulation of ceramide production and apoptosis. Annu Rev Physiol. 1998, 60: 643-665. 10.1146/annurev.physiol.60.1.643.View ArticlePubMedGoogle Scholar
- Reunanen N, Westermarck J, Hakkinen L, Holmstrom TH, Elo I, Eriksson JE, Kahari VM: Enhancement of fibroblast collagenase (matrix metalloproteinase-1) gene expression by ceramide is mediated by extracellular signal-regulated and stress-activated protein kinase pathways. J Biol Chem. 1998, 273: 5137-5145. 10.1074/jbc.273.9.5137.View ArticlePubMedGoogle Scholar
- Westacott CI, Barakat AF, Wood L, Perry MJ, Neison P, Bisbinas I, Armstrong L, Millar AB, Elson CJ: Tumor necrosis factor alpha can contribute to focal loss of cartilage in osteoarthritis. Osteoarthritis Cartilage. 2000, 8: 213-221. 10.1053/joca.1999.0292.View ArticlePubMedGoogle Scholar
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