Expression of c-fos gene inhibits proteoglycan synthesis in transfected chondrocyte

The c-Fos/AP-1 inhibitor inhibits sulfur mustard-induced chondrogenesis impairment in zebrafish larvae

Sulfur mustard (SM, dichlorodiethyl sulfide) is a potent erosive chemical poison that can cause pulmonary lung, skin and eye disease complications in humans. Currently, there is no designated remedy for SM, and its operation's toxicological process remains unidentified. This work employed zebrafish as a model organism to investigate the toxic manifestations and mechanisms of exposure to SM, aiming to offer novel insights for preventing and treating this condition. The results showed that SM caused a decrease in the survival rate of the zebrafish larvae (LC50 = 2.47 mg/L), a reduction in the hatching rate, an increase in the pericardial area, and small head syndrome. However, T-5224 (a selective inhibitor of c-Fos/activator protein) attenuated the reduction in mortality (LC50 = 2.79 mg/L), the reduction in hatching rate, and the worsening of morphological changes. We discovered that SM causes cartilage developmental disorders in zebrafish larvae. The reverse transcription-quantitative polymerase chain reaction found that SM increased the expression of inflammation-related genes (IL-1β, IL-6, and TNF-α) and significantly increased cartilage development-related gene expression (fosab, mmp9, and atf3). However, the expression of sox9a, sox9b, and Col2a1a was reduced. The protein level detection also found an increase in c-fos protein expression and a significant decrease in COL2A1 expression. However, T-5224,also and mitigated the changes in gene expression, and protein levels caused by SM exposure. The results of this study indicate that SM-induced cartilage development disorders are closely related to the c-Fos/AP-1 pathway in zebrafish.

C-Fos regulation by the MAPK and PKC pathways in intervertebral disc cells

BACKGROUND

The gene encoding c-fos is an important factor in the pathogenesis of joint disease in patients with osteoarthritis. However, it is unknown whether the signal mechanism of c-fos acts in intervertebral disc (IVD) cells. We investigated whether c-fos is activated in relation to mitogen-activated protein kinases (MAPKs) and the protein kinase C (PKC) pathway in nucleus pulposus (NP) cells.

METHODOLOGY/RESULTS

Reverse transcription-polymerase chain reaction and western blotting analyses were used to measure the expression of c-fos in rat IVD cells. Transfections were performed to determine the effects of c-fos on target gene activity. The effect of c-fos protein expression was examined in transfection experiments and in a 3- (4,5-dimethylthiazol-2-yl) -2,5-diphenyltetrazolium bromide cell viability assay. Phorbol 12-myristate 13-acetate (PMA), the most commonly used phorbol ester, binds to and activates protein kinase C (PKC), causing a wide range of effects in cells and tissues. PMA induced the expression of c-fos gene transcription and protein expression, and led to activation of the MAPK pathways in NP cells. The c-fos promoter was suppressed completely in the presence of the MAPK inhibitor PD98059, an inhibitor of the MEK/ERK kinase cascade, but not in the presence of SKF86002, SB202190, or SP600125. The effects of the PKC pathway on the transcriptional activity of the c-fos were evaluated. PKCγ and PKCδ suppressed the promoter activity of c-fos. Treatment with c-fos inhibited aggrecan and Col2 promoter activities and the expression of these genes in NP cells.

CONCLUSIONS

This study demonstrated, for the first time, that the MAPK and PKC pathways had opposing effects on the regulation of c-fos in NP cells. Thus, the expression of c-fos can be suppressed in the extracellular matrix of NP cells.

Oxidative stress inhibits insulin-like growth factor-I induction of chondrocyte proteoglycan synthesis through differential regulation of phosphatidylinositol 3-Kinase-Akt and MEK-ERK MAPK signaling pathways

The ability of insulin-like growth factor I (IGF-I) to stimulate cartilage matrix synthesis is reduced in aged and osteoarthritic cartilage. Aging and osteoarthritis are associated with an increase in reactive oxygen species, which we hypothesized would interfere with normal IGF-I signaling. We compared IGF-I signaling in normal and osteoarthritic human articular chondrocytes and investigated the effects of oxidative stress induced by tert-butylhydroperoxide (tBHP). In normal human chondrocytes, IGF-I initiated a strong and sustained phosphorylation of IRS-1 (Tyr-612) and Akt (Ser-473) and transient ERK phosphorylation. In contrast, in osteoarthritic chondrocytes, which possessed elevated basal IRS-1 (Ser-312) and ERK phosphorylation, IGF-I failed to stimulate IRS-1 (Tyr-612) or Akt phosphorylation. In normal human chondrocytes, tBHP triggered strong IRS-1 (Ser-312 and Ser-616) and ERK phosphorylation and inhibited IGF-I-induced IRS-1 (Tyr-612) and Akt phosphorylation. Lentivirus-mediated overexpression of constitutively active (CA) Akt significantly enhanced proteoglycan synthesis, whereas both dominant negative Akt and CA MEK inhibited proteoglycan synthesis. CA Akt also promoted type II collagen and Sox9 expression, whereas tBHP treatment and CA MEK inhibited aggrecan, collagen II, and Sox9 mRNA expression. In osteoarthritic chondrocytes, the antioxidants Mn(III) tetrakis(4-benzoic acid)porphyrin and N-acetylcysteine increased the ratio of Akt to ERK phosphorylation and promoted IGF-I-mediated proteoglycan synthesis. Chemical inhibition of ERK significantly enhanced IGF-I phosphorylation of Akt and alleviated tBHP inhibition of Akt phosphorylation. These results demonstrate opposing roles for phosphatidylinositol 3-kinase-Akt and MEK-ERK in cartilage matrix synthesis and suggest that elevated levels of reactive oxygen species cause chondrocyte IGF-I resistance by altering the balance of Akt to ERK activity.

Sphingomyelinase decreases type II collagen expression in bovine articular cartilage chondrocytes via the ERK signaling pathway

OBJECTIVE

Ceramide, a mediator of proinflammatory cytokine signaling, induces cartilage degradation and reduces type II collagen synthesis in articular cartilage. The accumulation of ceramide is associated with arthritis in Farber's disease. The aim of this study was to investigate the mechanism of ceramide-induced down-regulation of type II collagen.

METHODS

Bovine articular chondrocytes were stimulated with sphingomyelinase (SMase) to increase levels of endogenous ceramide. Components of the ERK pathway were inhibited by Raf-1 kinase inhibitor and the MEK inhibitor, PD98059. Cell extracts were analyzed by Western blotting for ERK-1/2, SOX9, c-Fos, and type II collagen, and the level of c-fos messenger RNA (mRNA) was analyzed by quantitative polymerase chain reaction. Localization of ERK-1/2, SOX9, and c-Fos was assessed by immunocytochemistry and confocal microscopy.

RESULTS

SMase treatment of chondrocytes caused sustained phosphorylation of ERK-1/2 throughout the cytoplasm and nucleus that was reduced by inhibitors of Raf-1 kinase and MEK-1/2. SMase treatment of chondrocytes also induced translocation of c-Fos to the nucleus and phospho-SOX9 to the cytoplasm and increased expression of c-fos mRNA. Type II collagen expression, which was down-regulated by SMase treatment, was restored by the MEK-1/2 inhibitor, PD98059.

CONCLUSION

SMase down-regulates type II collagen in articular chondrocytes via activation of the ERK signaling cascade, redistribution of SOX9, and recruitment of c-Fos. This new mechanism for cartilage degradation provides potential targets for future treatment of arthritic disease.

Aggrecan expression is substantially and abnormally upregulated in Hutchinson–Gilford Progeria Syndrome dermal fibroblasts

Hutchinson-Gilford Progeria syndrome (HGPS) is a rare genetic disorder that displays features of segmental aging. It is manifested predominantly in connective tissue, with most prominent histological changes occurring in the skin, cartilage, bone and cardiovascular tissues. Detailed quantitative real time reverse-transcription polymerase chain reaction studies confirmed the previous observation that platelet-derived growth factor A-chain transcripts are consistently elevated 11+/-2- to 13+/-2-fold in two HGPS dermal fibroblast lines compared with age-matched controls. Furthermore, we identified two additional genes with substantially altered transcript levels. Nucleotide pyrophosphatase transcription was virtually shut down with decreased expression of 13+/-3- to 59+/-3-fold in HGPS, whereas aggrecan mRNA was elevated to 24+/-5 times to 41+/-4 times that of chronologically age-matched controls. Aggrecan, normally a component of cartilage and not always detectable in normal fibroblasts cultures, was secreted by HGPS fibroblast lines and was produced as a proteoglycan. This demonstrates that elevated aggrecan expression and its secretion are aberrant features of HGPS. We conclude that HGPS cells can display massively altered transcript levels leading to the secretion of inappropriate protein species.

Comparative effects of IL-1beta and hydrogen peroxide (H2O2) on catabolic and anabolic gene expression in juvenile bovine chondrocytes

OBJECTIVE

To compare the effects of hydrogen peroxide (H(2)O(2)) to those of interleukin-1beta (IL-1beta) on gene expression in juvenile bovine articular chondrocytes (BAC). The study analyses the activation of nuclear factor-kappa B (NF-kappaB) and activator protein-1 (AP-1) transcription factors, and the mRNA steady-state levels of the type II collagen, aggrecan core protein matrix, metalloproteinases (MMP-1, -3), and transforming growth factor-beta1 (TGF-beta1) genes.

METHODS

Confluent BAC cultures were treated for 3 and 24h with IL-1beta and/or different concentrations of H(2)O(2) (Protocol 1). Following initial treatment, a part of the cells was further subjected to another 24h with medium, in the presence of IL-1beta, to determine the effect of the cytokine on H(2)O(2) pre-treated cells (Protocol 2). Total RNA and nuclear protein extractions were performed to study mRNA steady-state levels (real-time polymerase chain reaction) and AP-1/NF-kappaB DNA binding (Electrophoretic Mobility Shift Assays), respectively.

RESULTS

IL-1beta enhanced both AP-1 and NF-kappaB binding, whereas H(2)O(2) only activated AP-1. H(2)O(2) pre-treatment decreased the IL-1beta activation of NF-kappaB. Both H(2)O(2) and IL-1beta down-regulated type II collagen and aggrecan expression and increased that of MMP-1 and -3. When cells were pre-treated with H(2)O(2), followed by IL-1beta, the effects were the same as those observed with H(2)O(2) alone. However, although H(2)O(2) and IL-1beta were capable of increasing TGF-beta1 expression separately, subsequent incubation with both factors led to a partial or total abolition of TGF-beta1 up-regulation.

CONCLUSION

The different regulation of NF-kappaB and AP-1 by H(2)O(2) and IL-1beta underlines the distinct roles played by the two transcription factors in the regulation of gene expression. H(2)O(2) and IL-1beta exert similar effects on matrix, MMPs and TGF-beta1 gene expression. However, the association of H(2)O(2) and IL-1beta does not cause synergic effect, and rather leads, in some cases, to an opposite effect. These data provide further insights into the respective roles of reactive oxygen species and cytokine in the pathophysiology of joint diseases.

c-Jun/activator protein-1 mediates interleukin-1beta-induced dedifferentiation but not cyclooxygenase-2 expression in articular chondrocytes

Interleukin (IL)-1beta is a major catabolic pro-inflammatory cytokine involved in cartilage destruction-associated processes, such as loss of the differentiated chondrocyte phenotype (dedifferentiation) and inflammation. Here, we investigated the role of c-Jun and activator protein-1 (AP-1) in IL-1beta-induced dedifferentiation and cyclooxygenase (COX)-2 expression in primary cultured chondrocytes. IL-1beta induced expression and transient phosphorylation of c-Jun in primary cultured chondrocytes. Ectopic expression of c-Jun was sufficient to cause dedifferentiation, whereas expression of dominant negative c-Jun blocked IL-1beta-induced dedifferentiation. Interestingly, modulation of c-Jun expression did not affect IL-1beta-induced COX-2 expression. Further experiments revealed that c-Jun phosphorylation was mediated by c-Jun N-terminal kinase and was required for IL-1beta-induced dedifferentiation but not COX-2 expression. Consistent with its ability to induce phosphorylation of c-Jun, IL-1beta caused transient activation of AP-1, which is necessary for IL-1beta-induced dedifferentiation. IL-1beta treatment suppressed expression of Sox-9, a major transcription factor that regulates type II collagen expression. Inhibition of c-Jun N-terminal kinase or AP-1 reversed IL-1beta-induced suppression of Sox-9, and ectopic expression of c-Jun was sufficient to cause suppression of Sox-9. Our results collectively suggest that IL-1beta suppresses type II collagen expression in articular chondrocytes by inducing expression and phosphorylation of c-Jun, AP-1 activation, and subsequent suppression of Sox-9.

Molecular phenotyping of HCS-2/8 cells as an in vitro model of human chondrocytes

OBJECTIVE

Cultures of primary articular chondrocytes for studying chondrocyte biology are notoriously difficult to handle. One alternative is the use of chondrocytic cell lines. Because the HCS-2/8 cells are the most widely used cell line in cartilage research, we investigated the molecular phenotype of these cells by mRNA-expression profiling.

DESIGN

Monolayers of HCS-2/8 cells were cultured to sub-confluence, confluence and over-confluence; primary human chondrocytes were grown in monolayer culture and alginate-bead cultures and several other chondrocytic cell lines were cultured as monolayers. RNA was isolated and analyzed by cDNA array profiling using Affymetrix GeneChips (U95A/U95Av2) and quantitative PCR.

RESULTS

Important similarities, but also remarkable differences between the HCS-2/8 cells and adult human articular chondrocytes were detected: Aggrecan and several cartilage typical collagens as well as SOX9 transcripts were strongly expressed in HCS-2/8 cells, whereas HCS-2/8 cells expressed hardly any chondrocyte-typical cartilage matrix degrading enzymes. Of all culturing conditions, clustering analysis showed that HCS-2/8 cultured at confluence are most closely related to primary chondrocytes.

CONCLUSION

Our study confirms how careful one needs to be in choosing in vitro model systems for investigating effects of interest. The major issue of chondrocyte cell lines appears to be that they mainly proliferate and show less expression of genes of matrix synthesis and turnover. A successful approach will have to select suitable chondrocyte cell lines and to validate findings obtained using primary chondrocytes. This allows to establish a reproducible in vitro model showing the property of interest and subsequently to relate back the obtained results to the physiologic situation.

Mechanical Compression of Cartilage Explants Induces Multiple Time-dependent Gene Expression Patterns and Involves Intracellular Calcium and Cyclic AMP

Chondrocytes are influenced by mechanical forces to remodel cartilage extracellular matrix. Previous studies have demonstrated the effects of mechanical forces on changes in biosynthesis and mRNA levels of particular extracellular matrix molecules, and have identified certain signaling pathways that may be involved. However, the broad extent and kinetics of mechano-regulation of gene transcription has not been studied in depth. We applied static compressive strains to bovine cartilage explants for periods between 1 and 24 h and measured the response of 28 genes using real time PCR. Compression time courses were also performed in the presence of an intracellular calcium chelator or an inhibitor of cyclic AMP-activated protein kinase A. Cluster analysis of the data revealed four main expression patterns: two groups containing either transiently up-regulated or duration-enhanced expression profiles could each be subdivided into genes that did or did not require intracellular calcium release and cyclic AMP-activated protein kinase A for their mechano-regulation. Transcription levels for aggrecan, type II collagen, and link protein were up-regulated approximately 2-3-fold during the first 8 h of 50% compression and subsequently down-regulated to levels below that of free-swelling controls by 24 h. Transcription levels of matrix metalloproteinases-3, -9, and -13, aggrecanase-1, and the matrix protease regulator cyclooxygenase-2 increased with the duration of 50% compression 2-16-fold by 24 h. Thus, transcription of proteins involved in matrix remodeling and catabolism dominated over anabolic matrix proteins as the duration of static compression increased. Immediate early genes c-fos and c-jun were dramatically up-regulated 6-30-fold, respectively, during the first 8 h of 50% compression and remained up-regulated after 24 h.

Therapeutic mechanism of ginkgo biloba exocarp polysaccharides on gastric cancer

AIM: To study the therapeutic mechanism of Ginkgo biloba exocarp polysaccharides (GBEP) on gastric cancer.

METHODS: Thirty patients with gastric cancer were treated with oral GBEP capsules. The area of tumors was measured by electron gastroscope before and after treatment, then the inhibitory and effective rates were calculated. The ultrastructures of tumor cells were examined by transmissional electron microscope. Cell culture, MTT, flow cytometry were performed to observe proliferation, apoptosis and changes of relevant gene expression of human gastric cancer SGC-7901 cells.

RESULTS: Compared with the statement before treatment, GBEP capsules could reduce the area of tumors, and the effective rate was 73.4%. Ultrastructural changes of the cells indicated that GBEP could induce apoptosis and differentiation in tumor cells of patients with gastric cancer. GBEP could inhibit the growth of human gastric cancer SGC-7901 cells following 24-72 h treatment in vitro at 10-320 mg/L, which was dose- and time-dependent. GBEP was able to elevate the apoptosis rate and expression of c-fos gene, but reduce the expression of c-myc and bcl-2 genes also in a dose-dependent manner.

CONCLUSION: The therapeutic mechanism of GBEP on human gastric cancer may relate to its effects on the expression of c-myc, bcl-2 and c-fos genes, which can inhibit proliferation and induce apoptosis and differentiation of tumor cells.

CTGF/Hcs24, hypertrophic chondrocyte-specific gene product, interacts with perlecan in regulating the proliferation and differentiation of chondrocytes

Connective tissue growth factor/hypertrophic chondrocyte-specific gene product 24 (CTGF/Hcs24) plays important roles in the control of the proliferation and differentiation of chondrocytes in vitro. To clarify the mechanisms of regulation by CTGF/Hcs24 with respect to cartilage metabolism, we investigated the interaction between CTGF/Hcs24 and heparan sulfate proteoglycan perlecan. An immunofluorescence study showed that CTGF/Hcs24 was colocalized with heparan sulfate and perlecan in human chondrosarcoma-derived chondrocytic cell line HCS-2/8 in vitro. Northern blot analysis showed that perlecan, syndecan-1, -2, and -4 transcripts were detected in HCS-2/8 cells. Particularly, expression of the perlecan gene increased markedly in HCS-2/8 cells by recombinant CTGF/Hcs24 (rCTGF/Hcs24) treatment. We also found that CTGF/Hcs24 interacted with perlecan from HCS-2/8 cells in vitro. Furthermore, CTGF/Hcs24-stimulated gene expression of the aggrecan gene, as well as DNA/proteoglycan synthesis, was diminished when HCS-2/8 cells were pretreated with heparinase, indicating that the effects of CTGF/Hcs24 on chondrocytes occurred through the interaction between CTGF/Hcs24 and heparan sulfate on the cells. An in vivo study using mouse growth plate revealed that CTGF/Hcs24 produced by hypertrophic chondrocytes was localized from the proliferative to the hypertrophic zone, whereas perlecan was predominantly localized in the prehyphertrophic zone. Consistent with such findings in vivo, the binding of (125)I-rCTGF/Hcs24 to maturing chondrocytes was at higher levels than that to chondrocytes in hypertrophic stages. These findings suggest that CTGF/Hcs24 produced in the hypertrophic region may act on chondrocytes in the proliferative and maturative zone via some heparan sulfate proteoglycan, such as perlecan.

Differentiation-dependent activation of the extracellular fatty acid binding protein (Ex-FABP) gene during chondrogenesis

Chicken hypertrophic chondrocytes secrete the extracellular fatty acid binding protein (Ex-FABP), a lipocalin not expressed by their undifferentiated precursors. Genomic clones coding for the full protein are here structurally and functionally analyzed. We first determined that the promoter sequence markedly differs from that reported for the homologous p20K, and we confirmed by genomic DNA Southern analysis the exactness of our sequence. This is of relevance since we have identified another lipocalin gene within the region of discrepancy, indicating thereby the existence of a lipocalin cluster within the same chromosomal locus. By transient transfections with 5'-deletions and the chloramphenicol acetyl transferase (CAT) reporter gene, the region between nt -926 and nt -629 was shown to be strongly active, specifically in hypertrophic chondrocytes and not in dedifferentiated cells. Responsive elements for several potential transcription factors lay within this sequence. Among those, activating protein-1 (AP-1) was shown to be involved in the regulation of the Ex-FABP gene during chondrocyte differentiation, as indicated by electrophoretic mobility shift assay, AP-1 site mutagenesis and functional interference assays.

Participation of Cbfa1 in regulation of chondrocyte maturation

OBJECTIVE

Cbfa1 is a transcription factor, which is classified into the runt family. The mice lacking this gene display complete loss of bone formation, indicating that Cbfa1 is an essential factor for osteoblast differentiation. The Cbfa1-deficient mice also show an abnormality in cartilage development. Although cartilage anlagens are well formed in these mice, endochondral ossification is blocked, and most of chondrocytes fail to differentiate into their maturation form as characterized by the absence of type X collagen and low levels of alkaline phosphatase activity. It is suggested that Cbfa1 may participate in chondrocyte differentiation. In this study, we have investigated the role of Cbfa1 in chondrocytes during their cytodifferentiation in vitro.

DESIGN

To investigate the role of Cbfa1 in regulation of chondrocyte differentiation, we over-expressed Cbfa1 or its dominant negative form in cultured chick chondrocytes using a retrovirus (RCAS)system and examined changes in chondrocyte behaviour induced by the introduced genes.

RESULTS

Mature chondrocytes isolated form the cephalic portion of sterna seemed to express Cbfa1 more prominently than immature chondrocytes isolated from the one-third caudal portion of sterna. Over-expression of Cbfa1 in immature chondrocytes strongly stimulated alkaline phosphatase activity and matrix calcification. In contrast, expression of a dominant negative form of Cbfa1, which lacks the C-terminal PST domain, severely inhibited alkaline phosphatase activity and matrix calcification in mature chondrocytes.

CONCLUSION

Taken together with the observation that Cbfa1 transcripts dominantly localized in hypertrophic chondrocytes as well as in osteoblasts, it is suggested that Cbfa1 plays an important role in the progression of chondrocyte maturation.

The possible role of c-fos expression in rheumatoid joint destruction

Abstract At present, although the etiology of rheumatoid arthritis (RA) remains unknown, most investigators believe that it is primarily an inflammatory disease of the synovial membrane of the joints. However, we have recently focused on the pathology of the early changes in articular cartilage and subchondral bone in RA patients, and have shown that RA involves articular cartilage and subchondral bone, not synovia. This research direction may lead to the development of a new specific treatment for the disease.

Compressive compared with tensile loading of medial collateral ligament scar in vitro uniquely influences mRNA levels for aggrecan, collagen type II, and collagenase

To test the hypothesis that loading conditions can be used to engineer early ligament scar behaviors, we used an in vitro system to examine the effect that cyclic hydrostatic compression and cyclic tension applied to 6-week rabbit medial collateral ligament scars had on mRNA levels for matrix molecules, collagenase, and the proto-oncogenes c-fos and c-jun. Our specific hypothesis was that tensile stress would promote more normal mRNA expression in ligament whereas compression would lead to higher levels of mRNA for cartilage-like molecules. Femur (injured medial collateral ligament)-tibia complexes were subjected to a hydrostatic pressure of 1 MPa or a tensile stress of 1 MPa of 0.5 Hz for 1 minute followed by 14 minutes of rest. On the basis of a preliminary optimization experiment, this 15-minute testing cycle was repeated for 4 hours. Semiquantitative reverse transcription-polymerase chain reaction analysis was performed for mechanically treated medial collateral ligament scars with use of rabbit specific primer sets for types I, II, and III collagen, decorin, biglycan, fibromodulin, versican, aggrecan, collagenase, c-fos, c-jun, and a housekeeping gene, glyceraldehyde-3-phosphate dehydrogenase. Cyclic hydrostatic compression resulted in a statistically significant increase in mRNA levels of type-II collagen (171% of nonloaded values) and aggrecan (313% of nonloaded values) but statistically significant decreases in collagenase mRNA levels (35% of nonloaded values). Cyclic tension also resulted in a statistically significant decrease in collagenase mRNA levels (66% of nonloaded values) and an increase in aggrecan mRNA levels (458% of nonloaded values) but no significant change in the mRNA levels for the other molecules. The results show that it is possible to alter mRNA levels for a subset of genes in scar tissue by supplying unique mechanical stimuli in vitro and thus that further investigation of scar engineering for potential reimplantation appears feasible.

Inhibition of chondrocyte differentiation in vitro by constitutive and inducible overexpression of the c-fos proto-oncogene

We have investigated the role of c-Fos in chondrocyte differentiation in vitro using both constitutive and inducible overexpression approaches in ATDC5 chondrogenic cells, which undergo a well-defined sequence of differentiation from chondroprogenitors to fully differentiated hypertrophic chondrocytes. Initially, we constitutively overexpressed exogenous c-fos in ATDC5 cells. Several stable clones expressing high levels of exogenous c-fos were isolated and those also expressing the cartilage marker type II collagen showed a marked decrease in cartilage nodule formation. To investigate further whether c-Fos directly regulates cartilage differentiation independently of potential clonal variation, we generated additional clones in which exogenous c-fos expression was tightly controlled by a tetracycline-regulatable promoter. Two clones, DT7.1 and DT12.4 were capable of nodule formation in the absence of c-fos. However, upon induction of exogenous c-fos, differentiation was markedly reduced in DT7.1 cells and was virtually abolished in clone DT12.4. Pulse experiments indicated that induction of c-fos only at early stages of proliferation/differentiation inhibited nodule formation, and limiting dilution studies suggested that overexpression of c-fos decreased the frequency of chondroprogenitor cells within the clonal population. Interestingly, rates of proliferation and apoptosis were unaffected by c-fos overexpression under standard conditions, suggesting that these processes do not contribute to the observed inhibition of differentiation. Finally, gene expression analyses demonstrated that the expression of the cartilage markers type II collagen and PTH/PTHrP receptor were down-regulated in the presence of exogenous c-Fos and correlated well with the differentiation status. Moreover, induction of c-fos resulted in the concomitant increase in the expression of fra-1 and c-jun, further highlighting the importance of AP-1 transcription factors in chondrocyte differentiation. These data demonstrate that c-fos overexpression directly inhibits chondrocyte differentiation in vitro, and therefore these cell lines provide very useful tools for identifying novel c-Fos-responsive genes that regulate the differentiation and activity of chondrocytes.

Detection of specific antibodies against human cultured chondrosarcoma (HCS-2/8) and osteosarcoma (Saos-2) cells in the serum of patients with osteoarthritis of the temporomandibular joint

To find specific humoral antibodies in sera from patients with temporomandibular joint (TMJ) osteoarthritis (OA), an immortal human chondrocyte (HCS-2/8) and osteoblast (Saos-2) cell line derived from a chondrosarcoma and an osteosarcoma, respectively, were used as source proteins of human antigens. Patients with chronically painful TMJ OA (n = 18) but no other joints symptoms were selected from a consecutive series of patients with temperomandibular disorders and sex-matched asymptomatic controls (n = 8) were also recruited. Cellular proteins of the HCS-2/8 and Saos-2 cells were subjected to Western blotting with the OA and control sera as probes. Band-recognition frequency and the peak optical density of the band were compared between groups by chi2 and t-tests. OA sera recognized various bands for the chondrocytes, and one of these (47-kDa) was specific for the OA sera. In two OA patients whose treatment outcome was less favorable, the reactivity against the 47-kDa protein was relatively high. In addition, the OA sera clearly cross-reacted with recombinant HSP47. Based on these findings, an autoimmune reaction against chondrocytes could be one of the exaggerating and/or perpetuating mechanisms in the pathophysiology of osteoarthritic TMJs, and the humoral antibody titre against the HSP47-like protein derived from the chondrocytes could be one of the possible markers for the prognosis of the joint pathology.

Transcriptional and posttranscriptional regulation of proteoglycan gene expression

Proteoglycans are among the most complex and sophisticated molecules of mammalian systems in terms of their protein and carbohydrate moieties. These macromolecules are in a continuous interplay with each other and the cell surface signal-transducing pathways, some of which are beginning to be elucidated. Because of their domain structure, catalytic potential, and diversity, these molecules appear to be designed for integrating numerous signaling events. For example, some proteoglycans interact with hyaluronan and lectins, thereby linking cell surfaces and distant matrix molecules. Some interact with collagen during the complex process of fibrillogenesis and regulate this biological process fundamental to animal life. Others interact with growth factors and serve as depot available during growth or tissue remodeling. In this review, we center on the most recent developments of proteoglycan biology, focusing primarily on genomic organization and transcriptional and posttranscriptional control. We discuss only those proteoglycans whose gene and promoter elements have been characterized and proved to be functional. When possible, we correlate the effects of growth factors and cytokines on proteoglycan gene expression with the topology of cis-acting elements in their genomic control regions. The analysis leads to a comprehensive critical appraisal of the principles that underlie the regulation of proteoglycan gene expression and to the delineation of common regulatory mechanisms.

Role of protein kinase A in collagenase-1 gene regulation by prostaglandin E1: studies in a rabbit synoviocyte cell line, HIG-82

Gene expression of the matrix-degrading enzyme collagenase-1 in rabbit synoviocytes and human fibroblasts is down-regulated by prostaglandin E1 (PGE1) through a cyclic adenosine monophosphate (cAMP)-dependent pathway. In the current study, we examined the role of protein kinase A (PKA) in the PGE1-mediated effect on collagenase-1 gene expression. Collagenase-1 gene expression was rapidly induced several-fold above control both by a phorbol ester, 12-o-tetradecanoyl phorbol 13 acetate, and interleukin-1 beta (IL-1 beta) in HIG-82 synoviocytes. Treatment with PGE1 and forskolin increased PKA activity in the HIG-82 cells within 15 minutes of adding the stimulating agents. Two inhibitors of PKA, the isoquinoline-sulfonamide derivative, H-89 and a cAMP analog, RpcAMP, blocked the ability of PGE1 to down-regulate collagenase-1 gene expression. However, if PGE1 was added from 6 h to 30 minutes before the PKA inhibitor H-89, collagenase-1 gene expression was inhibited. Constitutive PKA activity was increased in HIG-82 synoviocytes stably transfected with an expression vector pCMV.C alpha that caused the HIG-82 cells to overexpress an active catalytic subunit of PKA. Cells stably transfected with an inactive, mutated C-alpha-variant showed no change in PKA activity. Collagenase-1 mRNA levels in TPA-stimulated cells were reduced to baseline levels in the pCMV.C alpha but not in the mutated C-alpha-transfected cells. These data show the importance of PKA in regulating collagenase-1 gene expression in a synoviocyte cell line.

The effects of IL-1 on mitogen-activated protein kinases in rabbit articular chondrocytes

IL-1-activated chondrocytes express a large number of genes which contribute to cartilage degradation. The signaling pathways activated in response to IL-1 in these cells are not well-defined. We examined the effects of IL-1 and other stimuli on the mitogen activated protein kinase (MAPK) pathways in rabbit articular chondrocytes. We demonstrate that IL-1 activates three MAPKs, ERK, JNK and p38, in a time and dose-dependent manner. Activation is maximal by 15 minutes and returns to baseline levels by 1 hour. Maximal activation of ERK and p38 occurs with 1 ng/ml IL-1 whereas activation of JNK requires 10-fold higher levels. In contrast to IL-1, the PKC activator, PDBu preferentially activates ERK while TNF alpha preferentially activates JNK. LPS and TGF beta fail to stimulate any of the kinases examined. These results suggest that activation of the various MAPK pathways is important in the response of chondrocytes to IL-1, cytokines and growth factors.