Ascorbic acid regulates multiple metabolic activities of cartilage cells

Effectiveness of Clinical-Grade Chondroitin Sulfate and Ascorbic Acid in Mitigating Cryoprotectant Toxicity in Porcine Articular Cartilage

High concentrations of cryoprotective agents (CPAs) are required to achieve successful vitrification of articular cartilage; however, CPA cytotoxicity causes chondrocyte death. To reduce CPA toxicity, supplementation with research-grade additives, in particular chondroitin sulfate (CS) and ascorbic acid (AA), have previously been shown to improve chondrocyte recovery and metabolic function after exposure to CPAs at hypothermic conditions. However, it is necessary to evaluate the pharmaceutical equivalent clinical grade of these additives to facilitate the supplementation of additives into future vitrification protocols, which will be designed for vitrifying human articular cartilage in tissue banks. We sought to investigate the effectiveness of clinical-grade CS, AA, and N-acetylcysteine (NAC) in mitigating toxicity to chondrocytes during CPA exposure and removal, and determine whether a combination of two additives would further improve chondrocyte viability. We hypothesized that clinical-grade additives would exert chondroprotective effects comparable to those of research-grade additives, and that this protective effect would be enhanced if two additives were combined when compared with a single additive. The results indicated that both clinical-grade and research-grade additives significantly improved cell viability (p < 0.10) compared with the negative control (CPA with no additives). CS, AA, and NAC+AA increased cell viability significantly (p < 0.10) compared with the negative control. However, NAC, NAC+CS, and CS+AA did not improve cell viability when compared with the negative control (p > 0.10). We demonstrated that supplementation with clinical-grade CS or AA significantly improved chondrocyte viability in porcine cartilage subjected to high CPA concentrations, whereas supplementation with clinical-grade NAC did not benefit chondrocyte viability. Supplementation with clinical-grade additives in CPA solutions can mitigate CPA toxicity, which will be important in translating previously developed effective protocols for the vitrification of articular cartilage to human tissue banks.

In ovo feeding of vitamin C regulates splenic development through purine nucleotide metabolism and induction of apoptosis in broiler chickens

Nutrition in early life has a long-term influence on later health. In order to the explore effects of in ovo feeding (IOF) of vitamin C on splenic development, splenic metabolism and apoptosis were detected in embryo, adult chickens and in vitro. A total of 360 fertile eggs were selected and randomly assigned to control (CON) and vitamin C (VC) groups which were injected with saline and vitamin C on embryonic day 11, respectively. Functional enrichment of differentially expressed genes by transcriptome on embryonic day 19 suggested that purine nucleotide metabolism might be a potential pathway for the IOF of vitamin C to regulate spleen development. Additionally, the IOF of vitamin C significantly increased splenic vitamin C content on post-hatch day 21. Meanwhile, the splenic expression of adenosine deaminase, serine/threonine kinase 1 and proliferating cell nuclear antigen was down-regulated, whereas the expression of cysteinyl aspartate specific proteinase 9 was up-regulated in the VC group. On post-hatch day 42, the IOF of vitamin C significantly down-regulated the splenic expression of B-cell lymphoma 2 and increased the mRNA level of cysteinyl aspartate specific proteinase 9. The IOF of vitamin C could regulate the expression of genes related to adenylate metabolism and increased the apoptosis rate in vitro, which is consistent with the result in vivo. In conclusion, the IOF of vitamin C regulated splenic development and maturation by affecting purine nucleotide metabolism pathway and promoting apoptosis.

Small molecule compounds promote the proliferation of chondrocytes and chondrogenic differentiation of stem cells in cartilage tissue engineering

The application of tissue engineering to generate cartilage is limited because of low proliferative ability and unstable phenotype of chondrocytes. The sources of cartilage seed cells are mainly chondrocytes and stem cells. A variety of methods have been used to obtain large numbers of chondrocytes, including increasing chondrocyte proliferation and stem cell chondrogenic differentiation via cytokines, genes, and proteins. Natural or synthetic small molecule compounds can provide a simple and effective method to promote chondrocyte proliferation, maintain a stable chondrocyte phenotype, and promote stem cell chondrogenic differentiation. Therefore, the study of small molecule compounds is of great importance for cartilage tissue engineering. Herein, we review a series of small molecule compounds and their mechanisms that can promote chondrocyte proliferation, maintain chondrocyte phenotype, or induce stem cell chondrogenesis. The studies in this field represent significant contributions to the research in cartilage tissue engineering and regenerative medicine.

Metabolomics-Driven Elucidation of Cellular Nitrate Tolerance Reveals Ascorbic Acid Prevents Nitroglycerin-Induced Inactivation of Xanthine Oxidase

Glyceryl trinitrate (GTN) has found widespread use for the treatment of angina pectoris, a pathological condition manifested by chest pain resulting from insufficient blood supply to the heart. Metabolic conversion of GTN, a nitric oxide (NO) pro-drug, into NO induces vasodilation and improves blood flow. Patients develop tolerance to GTN after several weeks of continuous use, limiting the potential for long-term therapy. The mechanistic cause of nitrate tolerance is relatively unknown. We developed a cell culture model of nitrate tolerance that utilizes stable isotopes to measure metabolism of ¹⁵N₃-GTN into ¹⁵N-nitrite. We performed global metabolomics to identify the mechanism of GTN-induced nitrate tolerance and to elucidate the protective role of vitamin C (ascorbic acid). Metabolomics analyses revealed that GTN impaired purine metabolism and depleted intracellular ATP and GTP. GTN inactivated xanthine oxidase (XO), an enzyme that is critical for the metabolic bioactivation of GTN into NO. Ascorbic acid prevented inactivation of XO, resulting in increased NO production from GTN. Our studies suggest that ascorbic acid has the ability to prevent nitrate tolerance by protecting XO, but not aldehyde dehydrogenase (another GTN bioactivating enzyme), from GTN-induced inactivation. Our findings provide a mechanistic explanation for the previously observed beneficial effects of ascorbic acid in nitrate therapy.

Vitamin C down-regulate apo(a) expression via Tet2-dependent DNA demethylation in HepG2 cells

Lipoprotein(a)[Lp(a)] is a risk factor for coronary heart diseases. However, the metabolism of this protein remains poorly understood. Efficient and specific drugs that can decrease high plasma levels of Lp(a) have not been developed yet. Vitamin C is responsible for maintaining the catalytic activity of a group of iron and 2-oxoglutarate (2OG)-dependent dioxygenases and induces the generation of 5-hydroxymethylcytosine (5hmC) via Ten-eleven translocation (Tet) dioxygenases. In addition, It has been reported vitamin C deficiency induces atherosclerosis and increases Lp(a) and apo(a) plasma levels in Lp(a)+ mice. However, the mechanism is still unclear. In this study, we investigated the effects of vitamin C on apo(a) expression and the possible molecular mechanism of vitamin C that influences apolipoprotein(a) [apo(a)] biosynthesis in HepG2 cells. Results showed that vitamin C significantly inhibited the expression and secretion levels of apo(a). Vitamin C can also increase ELK1 expression and hydroxymethylation of ELK1 promoter and the globle DNA in HepG2 cells. In addition, the effects of vitamin C inhibiting the apo(a) expression were attenuated by ELK1siRNA and Tet2siRNA. These results suggested vitamin C down-regulate apo(a) expression via Tet2-dependent DNA demethylation in HepG2 cells.

Ascorbic acid prolongs the viability and stability of isolated perfused lungs: A mechanistic study using 31P and hyperpolarized 13C nuclear magnetic resonance

Ex vivo lung perfusion (EVLP) has recently shown promise as a means of more accurately gauging the health of lung grafts and improving graft performance post-transplant. However, reperfusion of ischemic lung promotes the depletion of high-energy compounds and a progressive loss of normal mitochondrial function, and it remains unclear how and to what extent the EVLP approach contributes to this metabolic decline. Although ascorbate has been used to mitigate the effects of ischemia-reperfusion injury, the nature of its effects during EVLP are also not clear. To address these uncertainties, this study monitored the energy status of lungs during EVLP and after the administration of ascorbate using (31)P and hyperpolarized (13)C NMR (nuclear magnetic resonance). Our experiments demonstrated that the oxidative phosphorylation capacity and pyruvate dehydrogenase flux of lungs decline during ex vivo perfusion. The addition of ascorbate to the perfusate prolonged lung viability by 80% and increased the hyperpolarized (13)C bicarbonate signal by a factor of 2.7. The effect of ascorbate is apparently due not to its antioxidant quality but rather to its ability to energize cellular respiration given that it increased the lung's energy charge significantly, whereas other antioxidants (glutathione and α-lipoic acid) did not alter energy metabolism. During ascorbate administration, inhibition of mitochondrial complex I with rotenone depressed energy charge and shifted the metabolic state of the lung toward glycolysis; reenergizing the electron transport chain with TMPD (N,N,N',N'-tetramethyl-p-phenylenediamine) recovered metabolic activity. This indicates that ascorbate slows the decline of the ex vivo perfused lung's mitochondrial activity through an independent interaction with the electron transport chain complexes.

PKCε signalling activates ERK1/2, and regulates aggrecan, ADAMTS5, and miR377 gene expression in human nucleus pulposus cells

The protein kinase C (PKC) signaling, a major regulator of chondrocytic differentiation, has been also implicated in pathological extracellular matrix remodeling, and here we investigate the mechanism of PKCε-dependent regulation of the chondrocytic phenotype in human nucleus pulposus (NP) cells derived from herniated disks. NP cells from each donor were successfully propagated for 25+ culture passages, with remarkable tolerance to repeated freeze-and-thaw cycles throughout long-term culturing. More specifically, after an initial downregulation of COL2A1, a stable chondrocytic phenotype was attested by the levels of mRNA expression for aggrecan, biglycan, fibromodulin, and lumican, while higher expression of SOX-trio and Patched-1 witnessed further differentiation potential. NP cells in culture also exhibited a stable molecular profile of PKC isoforms: throughout patient samples and passages, mRNAs for PKC α, δ, ε, ζ, η, ι, and µ were steadily detected, whereas β, γ, and θ were not. Focusing on the signalling of PKCε, an isoform that may confer protection against degeneration, we found that activation with the PKCε-specific activator small peptide ψεRACK led sequentially to a prolonged activation of ERK1/2, increased abundance of the early gene products ATF, CREB1, and Fos with concurrent silencing of transcription for Ki67, and increases in mRNA expression for aggrecan. More importantly, ψεRACK induced upregulation of hsa-miR-377 expression, coupled to decreases in ADAMTS5 and cleaved aggrecan. Therefore, PKCε activation in late passage NP cells may represent a molecular basis for aggrecan availability, as part of an PKCε/ERK/CREB/AP-1-dependent transcriptional program that includes upregulation of both chondrogenic genes and microRNAs. Moreover, this pathway should be considered as a target for understanding the molecular mechanism of IVD degeneration and for therapeutic restoration of degenerated disks.

Optimization of a chondrogenic medium through the use of factorial design of experiments

The standard culture system for in vitro cartilage research is based on cells in a three-dimensional micromass culture and a defined medium containing the chondrogenic key growth factor, transforming growth factor (TGF)-β1. The aim of this study was to optimize the medium for chondrocyte micromass culture. Human chondrocytes were cultured in different media formulations, designed with a factorial design of experiments (DoE) approach and based on the standard medium for redifferentiation. The significant factors for the redifferentiation of the chondrocytes were determined and optimized in a two-step process through the use of response surface methodology. TGF-β1, dexamethasone, and glucose were significant factors for differentiating the chondrocytes. Compared to the standard medium, TGF-β1 was increased 30%, dexamethasone reduced 50%, and glucose increased 22%. The potency of the optimized medium was validated in a comparative study against the standard medium. The optimized medium resulted in micromass cultures with increased expression of genes important for the articular chondrocyte phenotype and in cultures with increased glycosaminoglycan/DNA content. Optimizing the standard medium with the efficient DoE method, a new medium that gave better redifferentiation for articular chondrocytes was determined.

Vitamin C deficiency activates the purine nucleotide cycle in zebrafish

Vitamin C (ascorbic acid, AA) is a cofactor for many important enzymatic reactions and a powerful antioxidant. AA provides protection against oxidative stress by acting as a scavenger of reactive oxygen species, either directly or indirectly by recycling of the lipid-soluble antioxidant, α-tocopherol (vitamin E). Only a few species, including humans, guinea pigs, and zebrafish, cannot synthesize AA. Using an untargeted metabolomics approach, we examined the effects of α-tocopherol and AA deficiency on the metabolic profiles of adult zebrafish. We found that AA deficiency, compared with subsequent AA repletion, led to oxidative stress (using malondialdehyde production as an index) and to major increases in the metabolites of the purine nucleotide cycle (PNC): IMP, adenylosuccinate, and AMP. The PNC acts as a temporary purine nucleotide reservoir to keep AMP levels low during times of high ATP utilization or impaired oxidative phosphorylation. The PNC promotes ATP regeneration by converting excess AMP into IMP, thereby driving forward the myokinase reaction (2ADP → AMP + ATP). On the basis of this finding, we investigated the activity of AMP deaminase, the enzyme that irreversibly deaminates AMP to form IMP. We found a 47% increase in AMP deaminase activity in the AA-deficient zebrafish, complementary to the 44-fold increase in IMP concentration. These results suggest that vitamin C is crucial for the maintenance of cellular energy metabolism.

Characterization of ex vivo-generated bovine and human cartilage by immunohistochemical, biochemical, and magnetic resonance imaging analyses

Osteoarthritis (OA) is a prevalent age-associated disease involving altered chondrocyte homeostasis and cartilage degeneration. The avascular nature of cartilage and the altered chondrocyte phenotype characteristic of OA severely limit the capacity for in vivo tissue regeneration. Cell- and tissue-based repair has the potential to revolutionize treatment of OA, but those approaches have exhibited limited clinical success to date. In this study, we test the hypothesis that bovine and human chondrocytes in a collagen type I scaffold will form hyaline cartilage ex vivo with immunohistochemical, biochemical, and magnetic resonance (MR) endpoints similar to the original native cartilage. Chondrocytes were isolated from 1- to 3-week-old calf knee cartilage or from cartilage obtained from human total knee arthroplasties, suspended in 2.7 mg/mL collagen I, and plated as 300 microL spot cultures with 5 x 10(6) each. Medium formulations were varied, including the amount of serum, the presence or absence of ascorbate, and treatments with cytokines. Bovine chondrocytes generated metachromatic territorial and interstitial matrix and accumulated type II collagen over time. Type VI collagen was confined primarily to the pericellular region. The ex vivo-formed bovine cartilage contained more chondroitin sulfate per dry weight than native cartilage. Human chondrocytes remained viable and generated metachromatic territorial matrix, but were unable to support interstitial matrix accumulation. MR analysis of ex vivo-formed bovine cartilage revealed evidence of progressively maturing matrix, but MR-derived indices of tissue quality did not reach those of native cartilage. We conclude that the collagen-spot culture model supports formation and maturation of three-dimensional hyaline cartilage from active bovine chondrocytes. Future studies will focus on determining the capacity of human chondrocytes to show comparable tissue formation.

Differentiation potential of human muscle-derived cells towards chondrogenic phenotype in alginate beads culture

OBJECTIVE

The aim of this study was to evaluate the differentiation potential of two populations of muscle-derived cells (CD56- and CD56+) towards chondrogenic phenotype in alginate beads culture and to compare the effect of transforming growth factor beta 1 (TGFbeta1) on the differentiation process in these populations.

METHODS

Muscle CD56- and CD56+ cells were cultured in alginate beads, in a chondrogenic medium, containing or not TGFbeta1 (10 ng/ml). Cultures were maintained for 3, 7, 14 or 21 days in a humidified culture incubator. At harvest, one culture of each set was fixed for alcian blue staining and aggrecan detection. The steady-state level of matrix macromolecules mRNA was assessed by real-time polymerase chain reaction (PCR). Protein detection was performed by western-blot analysis. The binding activity of nuclear extracts to Cbfa1 DNA sequence was also evaluated by electrophoretic mobility shift assays (EMSA).

RESULTS

Chondrogenic differentiation of both CD56+ and CD56- muscle-derived cells was improved in alginate scaffold, even without growth factor, as suggested by increased chondrogenesis markers expression during the culture. Furthermore, TGFbeta1 enhanced the differentiation process and allowed to maintain a high expression of markers of mature chondrocytes. Of importance, the combination of alginate and TGFbeta1 treatment resulted in a further down-regulation of collagen type I and type X, as well as Cbfa1 both expression and binding activity.

CONCLUSIONS

Thus, alginate scaffold and chondrogenic medium are sufficient to lead both populations CD56+ and CD56- towards chondrogenic differentiation. Moreover, TGFbeta1 enhances this process and allows to maintain the chondrogenic phenotype by inhibiting terminal differentiation, particularly for CD56- cells.

Matrix development in self-assembly of articular cartilage

Background

Articular cartilage is a highly functional tissue which covers the ends of long bones and serves to ensure proper joint movement. A tissue engineering approach that recapitulates the developmental characteristics of articular cartilage can be used to examine the maturation and degeneration of cartilage and produce fully functional neotissue replacements for diseased tissue.

Methodology/Principal Findings

This study examined the development of articular cartilage neotissue within a self-assembling process in two phases. In the first phase, articular cartilage constructs were examined at 1, 4, 7, 10, 14, 28, 42, and 56 days immunohistochemically, histologically, and through biochemical analysis for total collagen and glycosaminoglycan (GAG) content. Based on statistical changes in GAG and collagen levels, four time points from the first phase (7, 14, 28, and 56 days) were chosen to carry into the second phase, where the constructs were studied in terms of their mechanical characteristics, relative amounts of collagen types II and VI, and specific GAG types (chondroitin 4-sulfate, chondroitin 6-sulfate, dermatan sulfate, and hyaluronan). Collagen type VI was present in initial abundance and then localized to a pericellular distribution at 4 wks. N-cadherin activity also spiked at early stages of neotissue development, suggesting that self-assembly is mediated through a minimization of free energy. The percentage of collagen type II to total collagen significantly increased over time, while the proportion of collagen type VI to total collagen decreased between 1 and 2 wks. The chondroitin 6- to 4- sulfate ratio decreased steadily during construct maturation. In addition, the compressive properties reached a plateau and tensile characteristics peaked at 4 wks.

Conclusions/Significance

The indices of cartilage formation examined in this study suggest that tissue maturation in self-assembled articular cartilage mirrors known developmental processes for native tissue. In terms of tissue engineering, it is suggested that exogenous stimulation may be necessary after 4 wks to further augment the functionality of developing constructs.

Steroid hormones strongly support bovine articular cartilage integration in the absence of interleukin-1β

OBJECTIVE

Posttraumatic integration of articular cartilage at fracture sites is essential for mechanical stability of cartilage, and ruptured cartilage is a prerequisite for early osteoarthritis. This study was undertaken to investigate effects on articular cartilage integration mediated by steroid hormones, interleukin-1beta (IL-1beta), and combinations thereof.

METHODS

Articular cartilage blocks were cultured in partial apposition for 2 weeks with ascorbic acid, testosterone, 17beta-estradiol, and dehydroepiandrosterone (DHEA), with or without IL-1beta. Mechanical integration was measured as adhesive strength, i.e., the maximum force at rupture of integrated cartilage blocks divided by the overlap area. Glycosaminoglycan content was used to study synthesized extracellular matrix.

RESULTS

Culture in medium without supplements did not lead to integration (adhesive strength 0 kPa). With administration of ascorbic acid (100 microg/ml), the median adhesive strength was 49 kPa. In comparison with ascorbic acid alone, all steroid hormones induced a strong, concentration-dependent stimulation of integration (with maximum values observed with DHEA at 3 x 10(-5)M, testosterone at 10(-8)M, and 17beta-estradiol at 10(-11)M). For testosterone and 17beta-estradiol, this was also reflected by an increase of glycosaminoglycan content. Adhesive strength was increased with IL-1beta at 10 pg/ml, but not at 1 pg/ml or 100 pg/ml. In the presence of both IL-1beta and sex hormones, integration of articular cartilage was reduced.

CONCLUSION

This is the first study to demonstrate that steroid hormones such as 17beta-estradiol, DHEA, and testosterone stimulate articular cartilage integration. This effect is abrogated by low concentrations of IL-1beta. In the absence of IL-1beta or after neutralization of IL-1beta, steroid hormones might be favorable adjuvant compounds to optimize cartilage integration.

Effects of transforming growth factor beta1 and dexamethasone on the growth and chondrogenic differentiation of adipose-derived stromal cells

The effects of soluble mediators and medium supplements commonly used to induce chondrogenic differentiation in different cell culture systems were investigated to define their dose-response profiles and potentially synergistic effects on the chondrogenic differentiation of adipose-derived adult stromal (ADAS) cells. Human ADAS cells were suspended within alginate beads and cultured in basal medium with insulin, transferrin, and selenious acid (ITS+) or fetal bovine serum (FBS) and treated with different doses and combinations of TGF-beta1 (0, 1, and 10 ng/mL) and dexamethasone (0, 10, and 100 nM). Cell growth and chondrogenic differentiation were assessed by measuring DNA content, protein and proteoglycan synthesis rates, and proteoglycan accumulation. The combination of ITS+ and TGF-beta1 significantly increased cell proliferation. Protein synthesis rates were increased by TGF-beta1 and dexamethasone in the presence of ITS+ or FBS. While TGF-beta1 significantly increased proteoglycan synthesis and accumulation by 1.5- to 2-fold in the presence of FBS, such effects were suppressed by dexamethasone. In summary, the combination of TGF-beta1 and ITS+ stimulated cell growth and synthesis of proteins and proteoglycans by human ADAS cells. The addition of dexamethasone appeared to amplify protein synthesis but had suppressive effects on proteoglycan synthesis and accumulation.

Intervertebral disc tissue engineering II: cultures of nucleus pulposus cells

The main objective of the current investigation was to regenerate cells of the nucleus pulposus without loss of phenotype. Nucleus pulposus cells were isolated from intervertebral discs from adult rabbits, grown in monolayer culture, and then maintained as a micromass pellet in tube culture. The specimens were evaluated by transmission and light microscopy, reverse transcriptase polymerase chain reaction, and immunohistochemistry. Nucleus pulposus cells proliferated in monolayer culture. When almost confluent, the cells were transferred to a tube and sedimented to form a pellet. The cells reverted to a rounded configuration and formed cell nests surrounded by extensive extracellular matrix, similar to that seen in vivo. These cells did not proliferate. Similar to that observed in situ, cells in pellet culture also expressed aggrecan, CD44, collagen Type II, and collagen Type I, but not collagen Type X, and had low alkaline phosphatase activity. The results of the investigation indicated that nucleus pulposus cells grown in monolayer culture might revert to their original characteristics when transferred to an environment that allows three-dimensional growth, such as upon implantation, a one-step approach. The results also indicated that the two-stage culture procedure might provide an expedient technique to regenerate nucleus pulposus tissue for disc repair.

AG-041R stimulates cartilage matrix synthesis without promoting terminal differentiation in rat articular chondrocytes

OBJECTIVE

AG-041R, a novel indolin-2-one derivative, has recently been demonstrated to induce systemic hyaline cartilage hyperplasia in rats. The aim of this study was to characterize its anabolic actions on chondrocytes.

DESIGN

Chondrocytes were isolated from knee joints of 5-week-old SD rats. Effects of AG-041R on cartilage matrix synthesis were examined by measuring [(35)S]sulfate incorporation into proteoglycans, Alcian blue staining, and Northern blotting of cartilage matrix genes. ALP activity, mineral deposition and the expression of markers for hypertrophic chondrocytes, were assessed for terminal differentiation of chondrocytes. Roles of endogenous TGF-beta/BMPs and MEK1/Erk signaling in the action of AG-041R were investigated using the neutralizing soluble receptors and the MEK1 inhibitor.

RESULTS

AG-041R accelerated proteoglycan synthesis assessed by both [(35)S]sulfate incorporation and Alcian blue stainable extracellular matrix accumulation. It also up-regulated the gene expression of type II collagen and aggrecan, as well as tenascin, a marker for articular cartilage. In contrast, AG-041R suppressed ALP activity, mineralization, and the gene expression of type X collagen and Cbfa1, indicating that AG-041R prevents chondrocyte terminal differentiation. AG-041R increased in BMP-2 mRNA, and the neutralizing soluble receptor for BMPs reversed the stimulatory effects of AG-041R on cartilage matrix synthesis. Moreover, AG-041R activated MEK1/Erk pathway, which was revealed to prevent chondrocyte terminal differentiation.

CONCLUSION

AG-041R stimulates cartilage matrix synthesis without promoting terminal differentiation in rat articular chondrocytes, which is mediated at least in part by endogenous BMPs and Erk. The data demonstrates that AG-041R has a potential to be a useful therapeutic agent for articular cartilage disorders.

Collagen IV promotes repair of renal cell physiological functions after toxicant injury

Collagen IV is found in the renal proximal tubular cell (RPTC) basement membrane and is a mediator of renal development and function. Pharmacological concentrations of L-ascorbic acid phosphate (AscP) promote the repair of physiological functions in RPTC sublethally injured by S-(1,2-dichlorovinyl)-L-cysteine (DCVC). We hypothesized that AscP promotes RPTC repair by stimulating collagen IV synthesis and/or deposition. RPTC exhibited increased synthesis but decreased deposition of collagen IV after DCVC exposure. In contrast, RPTC cultured in pharmacological concentrations of AscP maintained collagen IV deposition. The activity of prolyl hydroxylase was decreased in RPTC after DCVC injury, an effect that was partially attenuated in injured RPTC cultured in pharmacological concentrations of AscP. The addition of exogenous collagen IV to the culture media of DCVC-injured RPTC promoted the repair of mitochondrial function and Na(+)-K(+)-ATPase activity. However, neither collagen I, laminin, nor fibronectin promoted cell repair. These data demonstrate an association between AscP-stimulated deposition of collagen IV and exogenous collagen IV and repair of physiological functions, suggesting that collagen IV plays a specific role in RPTC repair after sublethal injury.

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.

Cloning differentially regulated genes from chondrocytes using agarose gel differential display

The technique of RNA differential display has been used extensively to clone differentially expressed genes from a wide variety of cells and tissues. Recently, a simplified method of cloning differential display products, separated on agarose gels, was described. Here we report an adaption of this method, using total RNA, to clone differentially expressed genes. The approach is simple and rapid, and requires only small quantities of total RNA. Utilising this approach, we have cloned three differentially regulated genes from chondrocytes stimulated to hypertrophy in vitro, and confirmed their pattern of expression by Northern blotting. These gene fragments were sequenced and found to correspond to known genes, although only one has previously been isolated from chondrocytes.

Modulation of chondrocyte proliferation by ascorbic acid and BMP-2

Chondrocytes show an unusual ability to thrive under serum-free conditions as long as insulin, thyroxine, and cysteine are present. Studies with sternal chondrocytes from chick embryos indicate that thymidine incorporation in chondrocytes cultured under serum-free conditions is 30-50% of that seen with fetal bovine serum (FBS). In contrast, skin fibroblast proliferation in serum-free culture is <5% of that seen with serum. Addition of 30-50 microM ascorbic acid to serum-free medium stimulates chondrocyte proliferation 4-5x, resulting in levels of thymidine incorporation higher than that seen with 10% serum. Three to five hours of ascorbate exposure is sufficient to stimulate proliferation, with maximal stimulation seen after 12-15 h. Bromo-deoxyuridine (BrdU) labelling indicated that approximately 25% of chondrocytes transit S phase during a 4-h period (16-20 h after ascorbate). Once maximal stimulation is reached, the proliferation rate remains fairly constant over at least 40 h. Ascorbate therefore increases the steady-state level of chondrocytes in the cycle. Because the stimulation of chondrocyte proliferation was greater than the net increase in cell numbers, we examined the level of apoptosis. Nuclear morphology, terminal uridine nucleotide end-labelling (TUNEL) assay, and 7-AAD/Hoechst dye FACS analyses all indicated that approximately 15% of the ascorbate-treated chondrocytes were undergoing apoptosis, while only 5% of the control chondrocytes were apoptotic. When prehypertrophic chondrocytes from the cephalic region of embryonic sternae were stimulated to undergo hypertrophy with rhBMP-2 + ascorbate, levels of apoptosis were similar to that seen with ascorbate alone. In contrast, treatment of caudal chondrocytes with BMP plus ascorbate does not induce hypertrophy, and the proportion of apoptotic cells was less than that seen with ascorbate alone. These results imply that in chondrocytes the transition to hypertrophy is associated with a decreased number of proliferating cells and a relatively high level of apoptosis.

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.

Cartilage matrix protein forms a type II collagen-independent filamentous network: analysis in primary cell cultures with a retrovirus expression system

Cartilage matrix protein (CMP) is expressed specifically in mature cartilage and consists of two von Willebrand factor A domains (CMP-A1 and CMP-A2) that are separated by an epidermal growth factor-like domain, and a coiled-coil tail domain at the carboxyl terminal end. We have shown previously that CMP interacts with type II collagen-containing fibrils in cartilage. In this study, we describe a type II collagen-independent CMP filament and we analyze the structural requirement for the formation of this type of filament. Recombinant wild-type CMP and two mutant forms were expressed in chick primary cell cultures using a retrovirus expression system. In chondrocytes, the wild-type virally encoded CMP is able to form disulfide bonded trimers and to assemble into filaments. Filaments also form with CMP whose Cys455 and Cys457 in the tail domain were mutagenized to prevent interchain disulfide bond formation. Therefore, intermolecular disulfide bonds are not necessary for the assembly of CMP into filaments. Both the wild-type and the double cysteine mutant also form filaments in fibroblasts, indicating that chondrocyte-specific factors are not required for filament formation. A truncated form of CMP that consists only of the CMP-A2 domain and the tail domain can form trimers but fails to form filaments, indicating that the deleted CMP-A1 domain and/or the epidermal growth factor domain are necessary for filament assembly but not for trimer formation. Furthermore, the expression of the virally encoded truncated CMP in chondrocyte culture disrupts endogenous CMP filament formation. Together these data suggest a role for CMP in cartilage matrix assembly by forming filamentous networks that require participation and coordination of individual domains of CMP.

Ascorbic acid deficiency in guinea pigs: contrasting effects of tissue ascorbic acid depletion and of associated inanition on status indices related to collagen and vitamin D

To investigate the sensitivity of guinea pig tissues to ascorbic acid depletion, as distinct from inanition, young male guinea pigs were maintained with either ascorbic acid restriction or total diet restriction for 8 weeks. One group (A) received no ascorbic acid for 3 weeks, then 0.5 mg/d for 5 weeks; one group (B) was weight-matched by restricted food intake to the first group; a third (marginally deficient) group (C) received 1 mg ascorbic acid/d throughout; a fourth was weight-matched to this group (D); and a fifth group received the control diet ad lib. (E). Both of the groups with restricted ascorbic acid intakes (A and C) developed very low tissue ascorbic acid contents, but only the first group (severely deficient group A) also exhibited a severely reduced growth rate. This group also exhibited reduced femur calcium and hydroxyproline contents and reduced skin hydroxyproline content. These changes were not seen in the corresponding weight-matched group (B). Neither plasma alkaline phosphatase (EC 3.1.3.1) activity, nor a variety of indices of vitamin D status exhibited changes which could be attributed specifically to reduced ascorbic acid intake and hence to lowered tissue ascorbic acid content. It is concluded that low tissue ascorbic acid levels in guinea pigs clearly alter the connective tissue composition of growing femur and skin, but do not necessarily produce a major, specific effect on vitamin D status. Moreover, the control of inanition is crucial to permit interpretation of the changes in metabolism that are caused by ascorbic acid deficiency.

Ascorbate on cell growth and differentiation

Ascorbate, an essential nutrient in humans, primates, and guinea pig, is involved in many cellular functions. Ascorbate also modulates cell growth and differentiation. Ascorbate can reduce or stimulate the growth of tumor cells, depending on the cell type. The inhibitory effect is not specific for the biological active isomer L-ascorbate, and isoascorbate and D-ascorbate are more effective in reducing cell growth than L-ascorbate. These results indicate that ascorbate has a cytotoxic effect by killing cells directly, rather a cytostatic one. However, only L-ascorbate is able to stimulate cell growth, but the mechanism of this stimulation is still unknown. L-Ascorbate stimulates the in vitro differentiation of several mesenchyme-derived cell types by altering the expression of multiple genes as the cell progresses through specific differentiation programs. Stimulation of collagen matrix at gene transcription, mRNA stabilization, hydroxylation, and secretion is a key role for L-ascorbate. L-Ascorbate also prevents cell transformation by stabilization of the differentiated state and cooperates with other agents to induce differentiation in a leukemia cell line.

Cell stage-dependent effects of ascorbic acid on cultured porcine bone cells

Pig bone cells were isolated from fetuses or young animals. In culture, they proliferated for 6 days, became confluent and began differentiating. The effects of ascorbic acid (AsA) on cell proliferation, alkaline phosphatase (ALP) activity, non-collagenous protein (NCP) and collagen synthesis, were studied by adding AsA to the medium at different times during culture. AsA affected fetal and post-natal cells similarly: ALP activity, NCP and collagen synthesis were markedly reduced in cells treated before confluence, but were strongly and dose-dependently stimulated in cells treated after confluence. AsA also stimulated cell proliferation. The cell stage-dependent action of AsA suggests that it may interfere with differentiation. The effects of AsA on ALP activity and DNA content were not coupled to its effect on collagen synthesis, raising the question of whether AsA action is matrix-mediated.