Effect of retinoic acid on protein synthesis by foetal bovine chondrocytes in high-density culture: down-regulation of the glucose-regulated protein, GRP-78, and type II collagen

SHetA2 Attack on Mortalin and Colleagues in Cancer Therapy and Prevention

Heat Shock Proteins of the 70-kDa family (HSP70s) do not cause cancer by themselves, but instead protect cells as they transform into cancer. These molecular chaperones bind numerous client proteins and utilize ATP hydrolysis to facilitate proper protein folding, formation of functional complexes and cellular localizations, or degradation of irreparably damaged proteins. Their transient upregulation by stressful situations avoids induction of programmed cell death. Continued upregulation of the mortalin, heat shock cognate (hsc70) and glucose regulated protein 78 (Grp78) support cancer development and progression by supporting pro-proliferative and metabolic functions and repressing pro-death functions of oncoproteins and tumor suppressor proteins. This review describes the discovery and development of a lead anti-cancer compound, sulfur heteroarotinoid A2 (SHetA2, NSC726189), which was originally developed to bind retinoic acid receptors, but was subsequently found to work independently of these receptors. The discovery and validation of mortalin, hsc70 and Grp78 as SHetA2 target proteins is summarized. The documented and hypothesized roles of these HSP70 proteins and their clients in the mechanism of SHetA2 inhibition of cancer without toxicity are discussed. Use of this mechanistic data to evaluate drug action in a cancer clinical trial and develop synergistic drug combinations is explained. Knowledge needed to optimize SHetA2 analogs for use in cancer therapy and prevention is proposed as future directions.

Ribosome and Translational Control in Stem Cells

Embryonic stem cells (ESCs) and adult stem cells (ASCs) possess the remarkable capacity to self-renew while remaining poised to differentiate into multiple progenies in the context of a rapidly developing embryo or in steady-state tissues, respectively. This ability is controlled by complex genetic programs, which are dynamically orchestrated at different steps of gene expression, including chromatin remodeling, mRNA transcription, processing, and stability. In addition to maintaining stem cell homeostasis, these molecular processes need to be rapidly rewired to coordinate complex physiological modifications required to redirect cell fate in response to environmental clues, such as differentiation signals or tissue injuries. Although chromatin remodeling and mRNA expression have been extensively studied in stem cells, accumulating evidence suggests that stem cell transcriptomes and proteomes are poorly correlated and that stem cell properties require finely tuned protein synthesis. In addition, many studies have shown that the biogenesis of the translation machinery, the ribosome, is decisive for sustaining ESC and ASC properties. Therefore, these observations emphasize the importance of translational control in stem cell homeostasis and fate decisions. In this review, we will provide the most recent literature describing how ribosome biogenesis and translational control regulate stem cell functions and are crucial for accommodating proteome remodeling in response to changes in stem cell fate.

Immunohistochemical analysis of cartilage-derived retinoic acid-sensitive protein (CD-RAP)/melanoma inhibitory activity (MIA) in murine, canine, bovine and equine cerebrospinal tissues

Cartilage-derived retinoic acid-sensitive protein (CD-RAP)/melanoma inhibitory activity (MIA), which appears abundantly in hypertrophic cartilage at the stage of endochondral ossification, is also detected in cerebrospinal fluid (CSF) following spinal cord injury. In this study, the localization of the CD-RAP/MIA molecule in normal tissues of the spine and brain obtained from mice, rats, dogs, cattle and horses was examined using immunohistochemistry with a specific antibody. The positive signals of CD-RAP/MIA were found at nerve cells in the spinal cords of all species and were especially strong at cerebellar Purkinje cells. The results suggested that CD-RAP/MIA included in normal cerebrospinal tissues could be a biomarker associated with tissue injuries, as the molecules might flow into the CSF.

Identification of phosphoproteins as possible differentiation markers in all-trans-retinoic acid-treated neuroblastoma cells

Background

Neuroblastic tumors account for 9–10% of pediatric tumors and neuroblastoma (NB) is the first cause of death in pre-school age children. NB is classified in four stages, depending on the extent of spreading. A fifth type of NB, so-called stage 4S (S for special), includes patients with metastatic tumors but with an overall survival that approximates 75% at five years. In most of these cases, the tumor regresses spontaneously and regression is probably associated with delayed neuroblast cell differentiation.

Methodology/Principal Findings

In order to identify new early markers to follow and predict this process for diagnostic and therapeutics intents, we mimicked the differentiation process treating NB cell line SJ-NK-P with all-trans-retinoic acid (ATRA) at different times; therefore the cell proteomic pattern by mass spectrometry and the phosphoproteomic pattern by a 2-DE approach coupled with anti-phosphoserine and anti-phosphotyrosine western blotting were studied.

Conclusions/Significance

Proteomic analysis identified only two proteins whose expression was significantly different in treated cells versus control cells: nucleoside diphosphate kinase A (NDKA) and reticulocalbin-1 (RCN1), which were both downregulated after 9 days of ATRA treatment. However, phosphoproteomic analysis identified 8 proteins that were differentially serine-phosphorylated and 3 that were differentially tyrosine-phosphorylated after ATRA treatment. All proteins were significantly regulated (at least 0.5-fold down-regulated). Our results suggest that differentially phosphorylated proteins could be considered as more promising markers of differentiation for NB than differentially expressed proteins.

Proteomic and redox-proteomic evaluation of homogentisic acid and ascorbic acid effects on human articular chondrocytes

Alkaptonuria (AKU) is a rare genetic disease associated with the accumulation of homogentisic acid (HGA) and its oxidized/polymerized products in connective tissues up to the deposition of melanin-like pigments (ochronosis). Since little is known on the effects of HGA and its metabolites on articular cells, we carried out a proteomic and redox-proteomic analysis to investigate how HGA and ascorbic acid (ASC) affect the human chondrocytic protein repertoire. We settled up an in vitro model using a human chondrocytic cell line to evaluate the effects of 0.33 mM HGA, alone or combined with ASC. We found that HGA and ASC significantly affect the levels of proteins with specific functions in protein folding, cell organization and, notably, stress response and cell defense. Increased protein carbonyls levels were found either in HGA or ASC treated cells, and evidences produced in this paper support the hypothesis that HGA-induced stress might be mediated by protein oxidation. Our finding can lay the basis towards the settling up of more sophisticated models to study AKU and ochronosis.

Differential liver protein expression during schistosomiasis

The arrival of eggs in the liver during Schistosoma mansoni infection initiates a protective granulomatous response; however, as the infection progresses, this response results in chronic liver fibrosis. To better understand the impact of schistosomiasis on liver function, we used a proteomic approach to identify proteins whose expression was significantly altered in schistosome-infected mice 8 weeks postinfection. Identification of differentially expressed proteins by mass fingerprinting revealed that schistosome infection markedly reduced the abundance of proteins associated with several normal liver functions (i.e., citric acid cycle, fatty acid cycle, and urea cycle), while proteins associated with stress responses, acute phase reactants, and structural components were all significantly more abundant. The expression patterns of several immunity-related proteins (peroxiredoxin 1, arginase 1, and galectin 1) suggested that different protein forms are associated with schistosome infection. These findings indicate that acute schistosomiasis has a significant impact on specific liver functions and, moreover, that the alterations in specific protein isoforms and upregulation of unique proteins may be valuable as new markers of disease.

Change in the synthesis rates of ocular retinoic acid and scleral glycosaminoglycan during experimentally altered eye growth in marmosets

PURPOSE

The purpose of this study was to examine the possibility that all-trans-retinoic acid (RA) in the eye is a signal related to changes in scleral extracellular matrix in a primate model of postnatal eye growth.

METHODS

Juvenile marmosets (Callithrix jacchus) were divided into two experimental groups based on their response to monocular deprivation with diffusers: group 1, treated eyes becoming longer than fellow control eyes (n = 8), and group 2, treated eyes becoming shorter than control eyes (n = 7). Eyes were enucleated, dissected, and assayed for changes in the rates of scleral glycosaminoglycan (GAG) synthesis and ocular RA synthesis. The rate of incorporation of (35)SO4 into CPC-precipitable GAG in scleras was taken as a measure of the rate of synthesis of proteoglycans. In the same eyes the rate of RA synthesis in vivo was measured separately in the retina and the choroid/RPE (choroid with RPE attached) by HPLC. The effect of RA on the rate of scleral GAG synthesis was also examined in tissue-cultured pieces of sclera from additional marmosets.

RESULTS

Induced changes in vitreous chamber length in diffuser-treated eyes correlated inversely with the rate of scleral GAG synthesis (P < 0.05) and directly correlated with the rate of RA synthesis measured separately in the retina (P < 0.05) and the choroid/RPE (P < 0.05). In group 1, the rate of scleral GAG synthesis was significantly lower (P < 0.01) in the treated eyes relative to control eyes, and the rate of RA synthesis in both the retina and the choroid/RPE was significantly higher (P < 0.01). In group 2, the rates of scleral GAG synthesis and RA synthesis in either the retina or choroid/RPE were not found to change significantly in the treated eyes compared with the control eyes. RA partially reduces the rate of scleral GAG synthesis in tissue-cultured primate sclera in a dose-dependent manner after several days.

CONCLUSIONS

RA may play a role in the visual control of postnatal eye growth in primates, possibly by inducing changes in scleral extracellular matrix associated with increasing eye size. Decreasing growth rate below control levels may involve other mechanisms.

Effect of matrix depleting agents on the expression of chondrocyte metabolism by equine chondrocytes

This study was carried out to investigate the effect of two enzymes (collagenase and chondroitinase) and two cytokines/metabolites (interleukin-1beta and retinoic acid) of known catabolic activity on the expression of cartilage metabolism/phenotype in equine articular cartilage. Articular cartilage explants from 11 horses (5-13 years old) were treated for 48 h and assayed for total sulphated glycosaminoglycan (GAG), the incorporation of 35S-sulphate, collagen degradation and mRNA expression of the proteoglycans collagen II, collagen IIA, collagen III, collagen IX, collagen X, collagen XI and glyceraldehyde-3-phosphate (GAPDH). Purified collagenase and retinoic acid were responsible for increased GAG loss from the tissues. Chondroitinase, responsible for catalysing the elimination of glucuronate residues from chondroitin A, B and C (Chondroitinase ABC) and retinoic acid treatment induced an inhibition of proteoglycan synthesis, whereas collagenase treatment did not. Collagenase activity was correlated with increased appearance of the CB11B epitope and type II collagen denaturation. By RT-PCR there was evidence of expression of altered collagen type IIA in purified collagenase treated tissues.

Characterization and chondrocyte differentiation stage-specific expression of KRAB zinc-finger protein gene ZNF470

As part of a study to identify novel transcriptional regulators of chondrogenesis-related gene expression, we have cloned and characterized cDNA for zinc-finger protein 470 (ZNF470), the human ortholog of which encodes a 717 amino acid residue protein containing 17 Cys(2)His(2) zinc-finger domains, as well as KRAB-A and KRAB-B motifs. The cDNA library used to isolate the initial ZNF470 clone was prepared from human bone marrow-derived mesenchymal progenitor cells at an intermediate stage of chondrogenic differentiation. We have determined the intron-exon structure of the human ZNF470 gene, which has been mapped to a zinc-finger cluster in a known imprinted region of human chromosome 19q13.4. ZNF470 is expressed at high levels in human testis and is expressed at low or undetectible levels in other adult tissues. Human ZNF470 expressed in mammalian cells as an EGFP fusion protein localizes predominantly to the nucleus, consistent with a role in transcriptional regulation. ZNF470, analyzed by quantitative real time PCR, was transiently expressed before the maximal expression of COL2A1 during chondrogenic differentiation in vitro. We have also characterized the bovine ortholog of human ZNF470, which encodes a 508 amino acid residue protein having 10 zinc-finger domains. A bovine ZNF470 cDNA clone was used to examine expression of ZNF470 in bovine articular chondrocytes treated with retinoic acid to stimulate dedifferentiation. Bovine ZNF470 expression was undetectable in freshly isolated bovine articular chondrocytes, but was dramatically upregulated in dedifferentiated retinoic acid-treated chondrocytes. These results, in two model systems, suggest a possible role for ZNF470 in the regulation of chondrogenesis-specific gene expression.

Calreticulin, PDI, Grp94 and BiP chaperone proteins are associated with retained COMP in pseudoachondroplasia chondrocytes

Cartilage oligomeric matrix protein (COMP), a large pentameric glycoprotein and member of the thrombospondin (TSP) group of extracellular proteins, is found in the territorial matrix surrounding chondrocytes. More than 50 unique COMP mutations have been identified as causing two skeletal dysplasias: pseudoachondroplasia (PSACH); and multiple epiphyseal dysplasia (EDM1). Recent studies suggest that calcium-binding and calcium-induced protein folding differ between wild type and mutant proteins, and abnormal processing of the mutant COMP protein contributes to the characteristic enlarged lamellar appearing rER cisternae in PSACH and EDMI chondrocytes in vivo and in vitro. Towards the goal of delineating the pathogenesis of PSACH and EDM1, in-vivo PSACH growth plate and in-vitro PSACH chondrocytes cultured in alginate beads were examined to identify and localize the chaperone proteins participating in the processing of the retained extracellular matrix proteins in the PSACH rER. Aggrecan was localized to both the rER cisternae and matrix while COMP and type IX collagen were only found in the rER. Type II collagen was solely found in the ECM suggesting that it is processed and transported differently from other retained ECM proteins. Five chaperone proteins: BiP (Grp78); calreticulin (CRT); protein disulfide (PDI); ERp72; and Grp94, demonstrated immunoreactivity in the enlarged PSACH cisternae and the short rER channels of chondrocytes from both in-vivo and in-vitro samples. The chaperone proteins cluster around the electron dense material within the enlarged rER cisternae. CRT, PDI and GRP94 AB-gold particles appear to be closely associated with COMP. Immunoprecipitation and Western blot, and Fluorescence Resonance Energy Transfer (FRET) analyses indicate that CRT, PDI and GRP94 are in close proximity to normal and mutant COMP and BiP to mutant COMP. These results suggest that these proteins play a role in the processing and transport of wild type COMP in normal chondrocytes and in the retention of mutant COMP in PSACH chondrocytes.

Matrix metalloproteinase-1, -3, -13 and aggrecanase-1 and -2 are differentially expressed in experimental osteoarthritis

The aim of this study was to characterize the cellular phenotypes of articular cartilage and meniscus in rabbits with experimentally induced osteoarthritis (OA), by histological and molecular biological techniques. OA was induced by severing the anterior cruciate ligament of the knee and rabbits were killed 2, 4 or 9 weeks following surgery. Our histological observations show a progressive destruction of extracellular matrix in both tissues. To determine whether these morphological changes could be related to alterations in the regulation of gene expression for a subset of relevant molecules, levels of mRNA for proteinases and one inhibitor (MMP-1, -3 and -13, aggrecanase-1 and -2 and TIMP-1), matrix molecules and one chaperone (type II and X collagens, aggrecan, osteonectin, betaig-h3 and BiP) were assessed by reverse transcription-polymerase chain reaction. Our results indicate that for most markers expression profiles were similar in both tissues. In particular, matrix protein gene expression remained stable or varied little during progression of OA, suggesting a poor repair capacity of the tissues. MMP gene expression increased rapidly whereas aggrecanase gene expression remained stable. These findings suggest that differential regulation of mRNA levels of MMP-1, -3 and -13 on the one hand and aggrecanase-1 and -2 on the other, occurs during OA.

Drug-induced rheumatic disorders: incidence, prevention and management

The purpose of this article is to review the causes, the clinical manifestations and the management of the more frequent drug-induced rheumatic disorders. These include: (i) articular and periarticular manifestations induced by fluoroquinolones, nonsteroidal anti-inflammatory drugs, injections of corticosteroids, and retinoids; (ii) multisystemic manifestations such as drug-induced lupus and arthritis induced by vaccination, Bacillus Calmette-Guerin therapy and cytokines; (iii) drug-induced disorders of bone metabolism (corticosteroid-induced osteoporosis, drug-induced osteomalacia and osteonecrosis); and (iv) iatrogenic complex regional pain syndromes. Disorders caused by nonpharmacological and rarely used treatments have been deliberately excluded. Knowledge of these drug-induced clinical symptoms or syndromes allows an earlier diagnosis and treatment, and earlier drug withdrawal if necessary. With the introduction of new medications such as the recombinant cytokines and antiretroviral treatments, the number of drug-induced rheumatic disorders is likely to increase.

Lineage-dependent collagen expression and assembly during osteogenic or chondrogenic differentiation of a mesoblastic cell line

The mesoblastic clone, C1, behaves as a tripotential progenitor able to self-renew and to differentiate toward osteogenesis, chondrogenesis, or adipogenesis in response to specific inducers. In this study, expression and deposition by the C1 cells of essential components of the extracellular matrix, collagens type I, II, III, V, XI, VI, IX, and X were followed along the osteogenic and chondrogenic pathways, through biochemical, immunochemical, and electron microscopy analyses. Implementation of each program involves profiles of collagen synthesis and matrix assembly close to those documented in vivo. Depending on the applied inducers, cells adopt a defined identity and, controls acting at transcriptional and posttranslational levels adapt the set of deposited collagens to one particular cell fate. Osteogenic C1 cells selectively build a type I collagen matrix also containing type III, V, and XI collagens but selectively exclude type II collagen. Chondrogenic C1 cells first elaborate a type II collagen network and then acquire hypertrophic chondrocyte properties while assembling a type X collagen matrix as in the growth plate. This study provides an example of how a mesoblastic cell line can develop, in vitro, each of its genetic programs up to terminal differentiation. Intrinsic factors and time-dependent cell-matrix interactions might, as in vivo, underline the implementation of an entire differentiation program.

Regulation of growth, protein synthesis, and maturation of fetal bovine epiphyseal chondrocytes grown in high-density culture in the presence of ascorbic acid, retinoic acid, and dihydrocytochalasin B

Phenotypic expression of chondrocytes can be modulated in vitro by changing the culture technique and by agents such vitamins and growth factors. We studied the effects of ascorbic acid, retinoic acid (0.5 and 10 microM), and dihydrocytochalasin B (3, 10, 20 microM DHCB), separately or in combination (ascorbic acid + retinoic acid or ascorbic acid + DHCB), on the induction of maturation of fetal bovine epiphyseal chondrocytes grown for up to 4 weeks at high density in medium containing 10% fetal calf serum and the various agents. In the absence of any agent or with retinoic acid or DHCB alone, the metabolic activity of the cells remained very low after day 6, with no induction of type I or X collagen synthesis nor increase in alkaline phosphatase activity. Chondrocytes treated with fresh ascorbic acid showed active protein synthesis associated with expression of types I and X after 6 and 13 days, respectively. This maturation was not accompanied by obvious hypertrophy of the cells or high alkaline phosphatase activity. Addition of retinoic acid to the ascorbic acid-treated cultures decreased the level of type II collagen synthesis and delayed the induction of types I and X collagen, which were present only after 30 days. A striking increase in alkaline phosphatase activity (15-20-fold) was observed in the presence of both ascorbic acid and the highest dose of retinoic acid (10 microM). DHCB was also a potent inhibitor of the maturation induced by treatment with ascorbic acid, as the chondrocytes maintained their rounded shape and synthesized type II collagen without induction of type I or X collagen. The pattern of protein secretion was compared under all culture conditions by two-dimensional gel electrophoresis. The different regulations of chondrocyte differentiation by ascorbic acid, retinoic acid, and DHCB were confirmed by the important qualitative and quantitative changes in the pattern of secreted proteins observed by two-dimensional gel electrophoresis along the study.

Immunohistochemical observations on the initial disorders of the epiphyseal growth plate in rats induced by high dose of vitamin A

The initial disorders of the epiphyseal growth plate cartilage were immunohistochemically examined in the proximal tibia of rats administered a high dose of vitamin A. Male Wistar rats were given 100,000 IU/100 g body weight/day of vitamin A for administration periods of 1 to 5 days (Day 1 to 5) from 4 weeks after birth or were given deionized water and used as control. They were sacrificed after 5-bromo-2'-deoxyuridine (BrdU) injection on Day 1 to Day 5 to remove the tibiae. The tibiae were processed for immunohistochemical examinations using antibodies against type I, II, X collagens and BrdU. BrdU-incorporated chondrocytes and type X collagen-negative area were reduced since Day 2 and type X collagen-positive area was reduced since Day 4. The cartilage matrix partially lost type II collagen and deposited type I collagen in the epiphyseal growth plate near the periosteum on Day 5. These findings suggest that a high dose of vitamin A initially disturbed the differentiation from resting to proliferating chondrocytes, subsequently inhibited the differentiation from proliferating to hypertrophic chondrocytes, caused the chondrocytes to deviate from the process of normal differentiation, and finally resulted in the deformation of the epiphyseal growth plate.

Retinoic acid-induced type II collagen degradation does not correlate with matrix metalloproteinase activity in cartilage explant cultures

OBJECTIVE

To determine the role of matrix metalloproteinases (MMPs) in retinoic acid (RetA)-induced degradation of type II collagen in cartilage.

METHODS

Bovine nasal cartilage explants were cultured with 1 microM RetA or in 3 nM interleukin-1alpha (IL-1alpha). Release of proteoglycan and type II collagen into the medium was measured by colorimetric assay and immunoassay, respectively. MMP activity in the medium was determined using a quenched fluorescent substrate assay, while specific collagenases were identified by Western immunoblotting. In some cases the effects of low molecular mass synthetic MMP inhibitors and serum on collagen degradation were studied.

RESULTS

RetA promoted maximal breakdown of type II collagen after 4 or 5 weeks in culture, compared with 3 weeks in culture with IL-1alpha. In IL-1alpha-stimulated cultures, collagen degradation was coincident with a large increase in MMP activity in the culture medium, whereas in RetA-stimulated cultures, there was only a small increase. In Western immunoblots of culture media containing RetA, prointerstitial collagenase and active collagenase 3 were sometimes detected, but not in all experiments. In IL-1alpha cultures, active interstitial collagenase was always detected, and active collagenase 3 was detectable in some experiments. Neutrophil collagenase was not detected in any cultures. IL-1alpha-stimulated collagen degradation was effectively inhibited by a potent, broad-spectrum inhibitor of MMPs, whereas it was poorly inhibited by a weak MMP inhibitor. The same 2 compounds were both only weak inhibitors of RetA-induced collagen degradation. When fetal calf serum was included in cartilage cultures, MMP activity in the culture medium was reduced to low levels. This resulted in a marked inhibition of IL-1alpha-induced type II collagen degradation, whereas there was no inhibition of RetA-induced collagen degradation.

CONCLUSION

Unlike IL-1alpha, RetA induces degradation of type II collagen in cartilage explants by a mechanism that is mainly independent of those MMPs that can be detected in the culture medium.

Retinoids and their receptors in skeletal development

The embryonic vertebrate limb serves as an excellent experimental model system in which to study mechanisms that regulate morphogenesis of the skeleton. The appendicular skeleton arises through the process of endochondral ossification, whereby a cartilage template is initially formed and subsequently replaced by bone. One molecule that has a dramatic effect on these processes is the vitamin-A metabolite, retinoic acid (RA). RA functions through a class of nuclear hormone receptors, the retinoic acid receptors (RARs) and retinoid-X-receptors (RXRs), to regulate gene transcription. Experimental evidence from RA teratogenesis suggests that the presence of ligand-activated RARs and/or inappropriate expression of RARs inhibits chondrogenesis. Conversely, genetic analysis has shown that the absence of the receptors can lead to deficiencies in cartilage formation while also promoting chondrogenesis at ectopic sites. Taken together, these studies suggest that the RARs play a fundamental role in the early stages of skeletal development, specifically those involved in the formation of prechondrogenic condensations and their subsequent differentiation into chondroblasts.

Analysis of types I, II, III, IX and XI collagens synthesized by fetal bovine chondrocytes in high-density culture

OBJECTIVE

This study was undertaken in order to determine phenotypic modulation of the chondrocytes more closely in high-density culture conditions and to clarify the role of ascorbate. Levels of five collagen types were analyzed qualitatively and quantitatively, and their distribution was observed in the cell layer and the culture medium.

DESIGN

Types I, II, III, IX and XI collagens, synthesized by fetal bovine chondrocytes in high-density culture, were analyzed qualitatively and quantitatively by direct measurement of radiolabeled collagens separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and by specific radioimmunoassays.

RESULTS

Under the experimental conditions used in this study (0.6 x 10(6) cells/cm2), chondrocytes did not proliferate in the absence of ascorbate, whereas a twofold increase in cell number was observed in the presence of ascorbate at day 14. Cartilage-specific collagens (types II, IX and XI) were synthesized throughout the culture period (up to 47 days), as was type III collagen, which appeared as early as day 1 and was essentially present in the culture medium. Partial dedifferentiation of chondrocytes was demonstrated by the synthesis of type I collagen, which was detected by day 2 in culture medium containing ascorbate, and by day 6 without ascorbate. After 33 days of culture, a threefold increase in type I collagen synthesis was observed in culture medium with ascorbate, reaching 66% of the type II collagen content of the cell layer. One month of culture marked the onset of a progressive decrease in the synthesis of all collagen types.

CONCLUSIONS

Under these high-density culture conditions, fetal bovine chondrocytes undergo a time and ascorbate-dependent program of partial dedifferentiation. This system provides a simple model for studying the initial mechanisms of chondrocytes dedifferentiation.

Retinoic acid stimulates matrix calcification and initiates type I collagen synthesis in primary cultures of avian weight-bearing growth plate chondrocytes

The effect of retinoic acid (RA) on primary cultures of growth plate chondrocytes obtained from weight-bearing joints was examined, Chondrocytes were isolated from the tibial epiphysis of 6- to 8-week-old broiler-strain chickens and cultured in either serum-containing or serum-free media. RA was administered at low levels either transiently or continuously after the cells had become established in culture. Effects of RA on cellular protein levels, alkaline phosphatase (AP) activity, synthesis of proteoglycan (PG), matrix calcification, cellular morphology, synthesis of tissue-specific types of collagen, and level of matrix metalloproteinase (MMP) activity were explored. RA treatment generally increased AP activity and stimulated mineral deposition, especially if present continuously. RA also caused a shift in cell morphology from spherical/polygonal to spindle-like. This occurred in conjunction with a change in the type of collagen synthesized: type X and II collagens were decreased, while synthesis of type I collagen was increased. There was also a marked increase in the activity of MMP. Contrasting effects of continuous RA treatment on cellular protein levels were seen: they were enhanced in serum-containing media, but decreased in serum-free HL-1 media. Levels of RA as low as 10 nM significantly inhibited PG synthesis and caused depletion in the levels of PG in the medium and cell-matrix layer. Thus, in these appendicular chondrocytes, RA suppressed chondrocytic (PG, cartilage-specific collagens) and enhanced osteoblastic phenotype (cell morphology, type I collagen, alkaline phosphatase, and mineralization).

Cloning of a retinoic acid-sensitive mRNA expressed in cartilage and during chondrogenesis

Retinoic acid (RA) is known to play a role in various aspects of skeletal development in vivo, including morphogenesis, growth plate maturation, and apoptosis. In cell culture, RA treatment of chondrocytes suppresses the differentiated phenotype characterized by production of type II collagen and aggrecan. In an effort to discover molecules involved in regulation of the chondrocyte phenotype or related to developmental processes such as chondrogenesis, mRNAs from bovine chondrocytes cultured with and without RA were amplified by reverse transcription-polymerase chain reaction (PCR) and compared by differential display. PCR products whose expression was inhibited by RA treatment were cloned. One cDNA encodes a molecule we call cartilage-derived retinoic acid-sensitive protein (CD-RAP), and its properties are described here. The full-length bovine CD-RAP mRNA was cloned after amplification by the rapid amplification of cDNA ends procedure, and a part of the rat CD-RAP mRNA was amplified by reverse transcription-PCR using sequence-specific primers. The bovine CD-RAP mRNA contains an open reading frame of 130 amino acids. CD-RAP mRNA expression, as determined by Northern blot analysis and in situ hybridization, was present only in cartilage primordia and cartilage. The inhibition of CD-RAP mRNA expression by RA in vitro was time- and dose-dependent and was tested over concentrations from 10(-8) to 10(-6) M. Southern blot analysis of genomic DNA indicated that CD-RAP was encoded by a single copy gene and that no other genes were closely related. What appears to be the human homologue of CD-RAP was recently isolated and cloned from a melanoma cell line and shown to function as a growth inhibitory protein (Blesch, A., Boberhoff, A.-K., Apfel, R., Behl, C., Hessdoerfer, B., Schmitt, A., Jachimcza, P., Lottspeich, F., Buettner, R., and Bogdahn, U. (1994) Cancer Res. 54, 5695-5701). Neither CD-RAP nor this protein showed any homology to known proteins. We speculate that, in vivo, CD-RAP functions during cartilage development and maintenance.

Two-dimensional electrophoresis of intracellular and secreted protein synthesized by fetal bovine chondrocytes in high-density culture

In order to study the mechanisms involved in the differentiation/dedifferentiation of chondrocytes, fetal bovine chondrocytes in high-density cultures were treated with retinoic acid, an agent known to modify the chondrocyte phenotype (10 mumol/L between day 2 to day 5 of culture). The synthesis of intracellular and secreted proteins was studied by two-dimensional electrophoresis in cell lysates and culture media after labeling with [35S]methionine for the last 14 h of culture. The proteins expressed in control and retinoic acid-treated cells were identified by microsequencing after "in-gel" tryptic digestion of the spot or by immunodetection with specific antibodies after two-dimensional gel blotting. Intracellular protein modifications included one of 56.9 kDa and with an isoelectric point (pI) of 5.8 whose synthesis was previously reported to be up-regulated by 75%. Microsequencing of two internal peptides did not reveal a known protein. Changes to the chondrocyte phenotype were also recorded in the culture medium, as a decrease in type II collagen synthesis and expression of the small proteoglycan, decorin. Several new spots were also observed after treatment with retinoic acid, including a large, diffuse spot, not yet characterized, with a mean molecular mass of 39 kDa and a pI of 4.5-5.0. Under our experimental conditions, retinoic acid induces morphological changes of the chondrocytes and dramatic changes in the synthesis of several intracellular and secreted proteins that predate the synthesis of collagen type I (the classical marker of chondrocyte dedifferentiation).