Chondrocyte metabolism as affected by vitamin A

AMPK Signaling in Energy Control, Cartilage Biology, and Osteoarthritis

The adenosine monophosphate (AMP)-activated protein kinase (AMPK) was initially identified as an enzyme acting as an "energy sensor" in maintaining energy homeostasis via serine/threonine phosphorylation when low cellular adenosine triphosphate (ATP) level was sensed. AMPK participates in catabolic and anabolic processes at the molecular and cellular levels and is involved in appetite-regulating circuit in the hypothalamus. AMPK signaling also modulates energy metabolism in organs such as adipose tissue, brain, muscle, and heart, which are highly dependent on energy consumption via adjusting the AMP/ADP:ATP ratio. In clinics, biguanides and thiazolidinediones are prescribed to patients with metabolic disorders through activating AMPK signaling and inhibiting complex I in the mitochondria, leading to a reduction in mitochondrial respiration and elevated ATP production. The role of AMPK in mediating skeletal development and related diseases remains obscure. In this review, in addition to discuss the emerging advances of AMPK studies in energy control, we will also illustrate current discoveries of AMPK in chondrocyte homeostasis, osteoarthritis (OA) development, and the signaling interaction of AMPK with other pathways, such as mTOR (mechanistic target of rapamycin), Wnt, and NF-κB (nuclear factor κB) under OA condition.

Distinct patterns of gene expression in the superficial, middle and deep zones of bovine articular cartilage

Hyaline articular cartilage will not heal spontaneously, and lesions in hyaline articular cartilage often result in degenerative joint disease. Considerable progress has been made with respect to the responsive stem cells, inductive signals and extracellular scaffolding required for the optimal regeneration of cartilage. However, many challenges remain, such as topographic differences in the functional zones of articular cartilage. We hypothesized that a distinct set of differentially expressed genes define the surface, middle and deep zones of hyaline articular cartilage. Microarray analysis of bovine articular cartilage from the superficial and middle zones revealed 52 genes differentially expressed ≥ 10-fold and 114 additional genes differentially expressed ≥ five-fold. However, no genes were identified with a ≥ five-fold difference in expression when comparing articular cartilage from the middle and deep zones. There are distinct, differential gene expression patterns in the superficial and middle zones of hyaline articular cartilage that highlight the functional differences between these zones. This investigation has implications for the tissue engineering and regeneration of hyaline articular cartilage.

Milk growth factor (MGF) induces transformation into ATDC5 cells, prechondrocytes, and cooperates with retinoic acid to transform the cells into new forms

The effects of milk growth factor (MGF) showed the transformation of ATDC5 prechondrocytes and differed from that of retinoic acid (RA) as follows. MGF (200 ng/ml) did not suppress the proliferation of ATDC5 cells, though RA (1 x 10(-7) M) suppressed the cell proliferation. However, MGF showed the result as RA, which was verified to suppress the production of proteoglycan. The synthesis of vimentin in ATDC5 cells was slightly induced by RA, but its withdrawal induced the large-scale induction and the fibril formation of vimentin, which may indicate that the cells became fibroblastic cells, namely dedifferentiation. MGF, which hardly induced the vimentin synthesis in ATDC5 cells, induced its synthesis under control by the withdrawal. MGF suppressed the synthesis of alpha-smooth muscle actin (alpha-SM-actin), which was apt to reverse in its withdrawal. However, RA did not affect this synthesis of ATDC5 cells. The combination of MGF and RA enlarged the cells and enhanced the synthesis of vimentin due to RA under control, however, almost terminated alpha-SM-actin-synthesis in the cells. And its effect is almost irreversible. Furthermore, the combination of MGF and RA prevented the induction of fibroblasts due to RA in the cells. And the withdrawal of the mixture transformed prechondrocytes into hypertrophic cells. Then, MGF contributes to bone metabolism in prechondrocyte.

Do changes in the mechanical properties of articular cartilage promote catabolic destruction of cartilage and osteoarthritis?

Osteoarthritis (OA) is a joint disease characterized by cartilage degeneration, a thickening of subchondral bone, and formation of marginal osteophytes. Previous mechanical characterization of cartilage in our laboratory suggests that energy storage and dissipation is reduced in osteoarthritis as the extent of fibrillation and fissure formation increases. It is not clear whether the loss of energy storage and dissipation characteristics is a result of biochemical and/or biophysical changes that occur to hyaline cartilage in joints. The purpose of this study is to present data, on the strain rate dependence of the elastic and viscous behaviors of cartilage, in order to further characterize changes that occur in the mechanical properties that are associated with OA. We have previously hypothesized that the changes seen in the mechanical properties of cartilage may be due to altered mechanochemical transduction by chondrocytes. Results of incremental tensile stress-strain tests at strain rates between 100%/min and 10,000%/min conducted on OA cartilage indicate that the slope of the elastic stress-strain curve increases with increasing strain rate, unlike the reported behavior of skin and self-assembled collagen fibers. It is suggested that the strain-rate dependence of the elastic stress-strain curve is due to the presence of large quantities of proteoglycans (PGs), which protect articular cartilage by increasing the apparent stiffness. The increased apparent stiffness of articular cartilage at high strain rates may limit the stresses borne and prolong the onset of OA. It is further hypothesized that increased compressive loading of chondrocytes in the intermediate zone of articular cartilage occurs as a result of normal wear to the superficial zone or from excessive impact loading. Once the superficial zone of articular cartilage is worn away, the tension is decreased throughout all cartilage zones leading to increased chondrocyte compressive loading and up-regulation of mechanochemical transduction processes that elaborate catabolic enzymes.

Expression of chicken 75-kDa gelatinase B-like enzyme in perivascular chondrocytes suggests its role in vascularization of the growth plate

A newly cloned avian 75-kDa gelatinase B-like enzyme is expressed by the cells surrounding the blood vessels of the growth plate and upregulated by angiogenic substances in cultured chondrocytes. Despite its low homology to mammalian gelatinase-B, the avian 75-kDa seems to function similarly in the context of endochondral bone formation.

INTRODUCTION

Gelatinase B/metalloproteinase (MMP)-9, a zinc-dependent protease of the MMP family, is a key regulator in the final step of endochondral ossification. Recently an avian 75-kDa gelatinase B-like enzyme that shows low sequence similarity to the mammalian enzyme (59% on the protein level) was cloned and characterized. However, its expression pattern in the chicken growth plate and its role in bone formation have not, so far, been examined.

RESULTS

Based on the published sequence, we cloned a 700-bp fragment from cDNA of the chicken growth plate and studied its expression pattern in primary chondrocytes. Because the basal expression level of gelatinase B was almost undetectable, we induced its expression by different culturing conditions, the most dramatic induction achieved by treatment with retinoic acid, which is known as an inducer of vascular invasion in the epiphyseal plates. The gelatinolitic activity, checked by zymography, detected bands corresponding to the gelatinase A and B as well as a new high-molecular weight band of approximately 200 kDa. We further studied the expression pattern of gelatinase B by in situ hybridization. The gelatinase B was expressed by the cells surrounding the blood vessels penetrating the growth plate and by chondrocytes located in the front of these vascular invasions in the borders between the bone and the cartilage, resembling the expression of mouse gelatinase B in the growth plate. The induction of rickets by a vitamin D-deficient diet reduced the expression levels of gelatinase B in the growth plate of 12-day-old chickens but did not affect the expression of gelatinase A mRNA.

CONCLUSION

The chicken growth plate has a distinctly different structure from the mammalian one: it is much wider, it contains more cells in each zone, and the blood vessels penetrate deeper into the hypertrophic zone. Nevertheless, the upregulation of the avian 75-kDa gelatinase B-like enzyme by vitamins A and D, coupled with its perivascular expression pattern in the growth plate, implies a similar role for the mammalian and avian genes in bone formation.

Discovery of sonic hedgehog expression in postnatal growth plate chondrocytes: differential regulation of sonic and Indian hedgehog by retinoic acid

Sonic hedgehog (Shh) is a key signal protein in early embryological patterning of limb bud development. Its analog, Indian hedgehog (Ihh), primarily expressed during early cartilage development in prehypertrophic chondrocytes, regulates proliferation and suppresses terminal differentiation of postnatal growth plate (GP) chondrocytes. We report here for the first time that both Shh and Ihh mRNA are expressed in the GP of rapidly growing 6-week-old broiler-strain chickens. They are also expressed in other tissues such as articular chondrocytes, kidney, and bone. In situ hybridization and RT-PCR analyses reveal Shh in all zones of the GP, with peak expression in late hypertrophy. Using primary cultures of GP chondrocytes in serum-containing medium, we followed the patterns of Shh and Ihh mRNA expression as the cultures matured and mineralized. We find a cyclical expression of both hedgehog genes during the early period of culture development between day 10 and 14; when one is elevated, the other tended to be suppressed, suggesting that the two hedgehogs may play complementary roles during GP development. Retinoic acid (RA), a powerful modulator of gene expression in cell differentiation, stimulates GP chondrocytes toward terminal differentiation, enhancing mineral formation. We find that RA strongly suppresses Ihh, but enhances expression of Shh in this system. While Ihh suppresses maturation of GP chondrocytes to hypertrophy, we hypothesize that Shh acts to push these cells toward hypertrophy.

Direct inhibition of Indian hedgehog expression by parathyroid hormone (PTH)/PTH-related peptide and up-regulation by retinoic acid in growth plate chondrocyte cultures

Indian hedgehog (Ihh) is highly expressed in prehypertrophic chondrocytes in vivo and has been proposed to regulate the proliferation and maturation of chondrocytes and bone collar formation in the growth plate. In high-density cultures of rabbit growth-plate chondrocytes, Ihh mRNA was also expressed at the highest level in the prehypertrophic stage. To explore endogenous factors that regulate Ihh expression in chondrocytes, we examined the effects of various growth factors on Ihh mRNA expression in this system. Retinoic acid (RA) and bone morphogenetic protein-2 enhanced Ihh mRNA expression, whereas PTH/PTH-related peptide (PTHrP) markedly suppressed Ihh expression. RA at more than 10(-8) M induced the expression of Ihh and Patched 1 (Ptc1) within 3 h, before it increased the type X collagen mRNA level at 6-24 h. Cycloheximide blocked the up-regulation of Ihh by RA, indicating the requirement of de novo protein synthesis for this stimulation. These findings suggest that RA is involved in the up-regulation of Ihh during endochondral bone formation. In contrast to RA, PTH (1-84) at 10(-7) M abolished the mRNA expression of Ihh and Ptc1 within 2-4 h, before it suppressed the expression of type X collagen at 12-24 h. The inhibition of Ihh expression by PTH (1-84) did not require de novo protein synthesis. PTH (1-34), PTHrP (1-34), and (Bu)(2)cAMP also suppressed Ihh expression. On the other hand, Ihh has been reported to induce PTHrP synthesis in the perichondrium. Consequently, the direct inhibitory action of PTH/PTHrP on Ihh appears to be a negative feedback mechanism that prevents excess PTHrP accumulation in cartilage.

Spatio-temporal distribution of chondromodulin-I mRNA in the chicken embryo: expression during cartilage development and formation of the heart and eye

To define genes specifically expressed in cartilage and during chondrogenesis, we compared by differential display-polymerase chain reaction (DD-PCR) the mRNA populations of differentiated sternal chondrocytes from chicken embryos with mRNA species modulated in vitro by retinoic acid (RA). Chondrocyte-specific gene expression is downregulated by RA, and PCR-amplified cDNAs from both untreated and RA-modulated cells were differentially displayed. Amplification products only from RNA of untreated chondrocytes were further analyzed, and a cDNA-fragment of the chondromodulin-I (ChM-I) mRNA was isolated. After obtaining full length cDNA clones, we have analyzed the mRNA expression patterns at different developmental stages by RNase protection assay and in situ hybridization. Analysis of different tissues and cartilage from 17-day-old chicken embryos showed ChM-I mRNA only in chondrocytes. During somitogenesis of the chicken embryo, ChM-I transcripts were detected in the notochord, the floor and the roof plate of the neural tube, and in cartilage precursor tissues such as the sclerotomes of the somites, the developing limbs, the pharyngeal arches, the otic vesicle, and the sclera. ChM-I continued to be expressed in differentiated cartilages derived from these tissues and also in noncartilaginous domains of the developing heart and retina. Thus, in the chicken, the expression of ChM-I is not restricted to mature cartilage but is already present during early development in precartilaginous tissues as well as in heart and eye.

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.

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).

Chondrocyte differentiation

Data obtained while investigating growth plate chondrocyte differentiation during endochondral bone formation both in vivo and in vitro indicate that initial chondrogenesis depends on positional signaling mediated by selected homeobox-containing genes and soluble mediators. Continuation of the process strongly relies on interactions of the differentiating cells with the microenvironment, that is, other cells and extracellular matrix. Production of and response to different hormones and growth factors are observed at all times and autocrine and paracrine cell stimulations are key elements of the process. Particularly relevant is the role of the TGF-beta superfamily, and more specifically of the BMP subfamily. Other factors include retinoids, FGFs, GH, and IGFs, and perhaps transferrin. The influence of local microenvironment might also offer an acceptable settlement to the debate about whether hypertrophic chondrocytes convert to bone cells and live, or remain chondrocytes and die. We suggest that the ultimate fate of hypertrophic chondrocytes may be different at different microanatomical sites.

Retinoic acid is a major regulator of chondrocyte maturation and matrix mineralization

During the process of endochondral bone formation, chondrocytes undergo a series of complex maturational changes. Our recent studies indicate that this maturational process is influenced by the vitamin A derivative retinoic acid (RA). To learn how this agent regulates chondrocyte development, we characterized matrix gene expression during maturation of cartilage cells in chick sternum. RNAs were isolated from the cephalic portion of day 13, 14, 16, 18, and 20 chick embryo sternum and analyzed via northern blots. Type II collagen RNA levels remained fairly constant during this developmental period. In contrast, expression of type X collagen and alkaline phosphatase (APase) genes was first detected at day 16, followed by that of osteonectin (ON) and osteopontin (OP). To explore the mechanisms triggering these changes, chondrocytes were isolated from the cephalic portion of day 17-18 sternum (US cells) and grown in monolayer in standard serum-containing medium. After 3 weeks in culture, most of the cells enlarged and became type X collagen-positive, but they exhibited low APase activity and contained only trace amounts of ON and OP mRNAs. Treatment of parallel 3-week-old cultures with RA (10-100 nM) rapidly increased expression of the APase, ON, and OP genes severalfold. In concert with a significant increase in APase activity, there was abundant calcium accumulation in the RA-treated cultures. Electron microscopy confirmed the formation of large matrix-associated mineral crystals and the presence of numerous matrix vesicles. The effects of RA were also studied in cultures of immature chondrocytes isolated from the caudal portion of sternum (LS cells).(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of expression of the chondrocytic phenotype in a skeletal cell line (CFK2) in vitro

We have examined in vitro the spontaneous and regulated expression of phenotypic characteristics associated with differentiated chondrocytes in an established skeletal cell line (CFK2) derived from fetal rat calvariae. Extended culture of CFK2 cells resulted in the appearance of glycosaminoglycans and type II collagen in the cell layer in association with the formation of focal nodes of cells. In addition, induction of mRNA-encoding link protein, cartilage-specific proteoglycan core protein, and thrombospondin was observed in the differentiated population (dCFK2 cells). The expression of these mRNAs was present for at least two passages after subculturing the dCFK2 cells. The dCFK2 cells also demonstrated enhanced parathyroid hormone (PTH)-stimulated adenylate cyclase activity. Proliferation of CFK2 cells was stimulated by the peptide regulatory factors EGF and PTH and inhibited by the steroidal agents dexamethasone and retinoic acid. EGF and retinoic acid inhibited the formation of cell foci and glycosaminoglycan deposition and the expression of mRNA-encoding link protein. In contrast, PTH and dexamethasone enhanced the formation of focal cellular nodes and augmented matrix deposition and link protein mRNA expression. These studies therefore show that the CFK2 cell line can serve as a nontransformed model of rat chondrocytic cells in which both induction and regulation of the expression of cartilaginous matrix components can be observed. This line thereby provides a unique renewable source of chondrocytic precursor cells and an excellent in vitro model for evaluating temporal and environmental control of chondrocyte differentiation and cartilage matrix production.

Differential and stage dependent effects of retinoic acid on chondrogenesis and synthesis of extracellular matrix macromolecules in chick craniofacial mesenchyme in vitro

Retinoic acid (RA) is well known to be a potent teratogen and induces a variety of facial defects in vivo, but at concentration levels lower than those that cause facial defects, RA seems to play an important role in normal facial development. In a previous study, we demonstrated the ability of RA to stimulate chondrogenesis in vitro in HH stage 23/24 chick mandibular (MND) but not frontonasal (FNP) mesenchyme cultured in a serum-free medium. The present study furthers these results by examining the effects of RA on chondrogenesis of chick facial mesenchyme at earlier embryonic stages and the effects on cell proliferation and synthesis of specific extracellular matrix macromolecules at stage 23/24. MND and FNP cells were cultured as micromasses for 4 days in defined media. As described previously, chondrogenesis in stage 23/24 MND cells was significantly enhanced by concentrations of RA of 0.1-1 ng/ml; however, at all earlier stages examined (18 to 22) RA at these concentrations had no significant effect. Higher concentrations of the retinoid inhibited chondrogenesis in MND cultures from all stages tested. Cells of the FNP from all stages displayed no significant change in chondrogenesis below 1 ng/ml RA and a dose dependent inhibition at higher concentrations. Thus RA's promotional effects in the face are not only tissue specific (MND), but also stage-dependent (HH 23/24). The specific effects of RA on matrix production and cell proliferation of stage 23/24 MND and FNP cells was examined by analysis of 35S sulfate, 3H thymidine and 3H proline incorporation. Analysis of 35S sulfate incorporation into sulfated proteoglycans confirmed that concentrations of RA of 0.1-1 ng/ml stimulated cartilage matrix production in MND but not FNP cultures. Above this level of RA, 35S sulfate incorporation was reduced in both. Likewise, 3H proline incorporation into collagenous protein, and to a lesser extent non-collagenous proteins, was stimulated by low levels of RA in MND, but not FNP cultures. Higher concentrations of the retinoid in either MND or FNP cultures did not lower collagen production, undoubtedly due to stimulation of non-chondrogenic cells within the population. This indicates that levels of RA as high as 100 ng/ml cause phenotypic change rather than cell death. This last point is corroborated by the analysis of 3H thymidine uptake in the cultures which was only transiently modified in most. The data indicate that cell proliferation occurred even in the presence of high RA levels.

Hypertrophic chondrocytes undergo further differentiation in culture

Conditions have been defined for promoting growth and differentiation of hypertrophic chondrocytes obtained in culture starting from chick embryo tibiae. Hypertrophic chondrocytes, grown in suspension culture as described (Castagnola P., G. Moro, F. Descalzi Cancedda, and R. Cancedda. 1986. J. Cell Biol. 102:2310-2317), when they reached the stage of single cells, were transferred to substrate-dependent culture conditions in the presence of ascorbic acid. Cells showed a change in morphology, became more elongated and flattened, expressed alkaline phosphatase, and eventually mineralized. Type II and X collagen synthesis was halted and replaced by type I collagen synthesis. In addition the cells started to produce and to secrete in large amount a protein with an apparent molecular mass of 82 KD in reducing conditions and 63 KD in unreducing conditions. This protein is soluble in acidic solutions, does not contain collagenous domains, and is glycosylated. The Ch21 protein, a marker of hypertrophic chondrocytes and bone cells, was synthesized throughout the culture. We have defined this additional differentiation stage as an osteoblast-like stage. Calcium deposition in the extracellular matrix occurred regardless of the addition of beta glycerophosphate to the culture medium. Comparable results were obtained both when the cells were plated at low density and when they were already at confluence and maintained in culture without passaging up to 50 d. When retinoic acid was added to the hypertrophic chondrocyte culture between day 1 and day 5 the maturation of the cells to the osteoblast-like stage was highly accelerated. The switch in the collagen secretion was already observed after 2 d and the production of the 63-kD protein after 3 d. Mineralization was observed after 15-20 d.

An established rat cell line expressing chondrocyte properties

Chondrocytes express a well-characterized set of marker proteins making these cells useful for studies on differentiation and regulation of gene expression. Because of the inherent instability of primary rat chondrocytes in culture, and because several rat chondrocyte genes have been cloned and characterized (including the collagen II promoter and enhancer), a rat chondrocyte cell line would be especially useful. To obtain this line we infected primary fetal rat costal chondrocytes with a recombinant retrovirus (NIH/J-2) carrying the myc and raf oncogenes, which have been shown to have an "immortalizing" function. Following infection, a rapidly proliferating clonal line was isolated that maintained a stable phenotype through 45 passages (11/2 year in culture). This line, termed IRC, grows in suspension culture as multicellular aggregates and in monolayer culture as polygonal cells which accumulate an alcian blue-stainable matrix. IRC cells synthesize high levels of cartilage proteoglycan core protein, and link protein, but show reduced collagen II expression. In addition, the cells express virally derived myc mRNA and protein, but do not express v-raf. Retinoic acid, which is a known modulator of chondrocyte phenotype, down-regulates expression of chondrocyte marker proteins, while stimulating v-myc expression by IRC cells. These data suggest that v-myc expression by chondrocytes results in rapid cell division and maintenance of many aspects of the differentiated phenotype. These "immortalized" cells, however, remain responsive to agents such as retinoic acid which modulate cell phenotype. The potential exists for development of chondrocyte cell lines from diseased cartilage, as well as from human cartilage.

Transplacental effects of vitamin A on fetal bones in mice--follow-up studies on postnatal recovery

Pregnant mice were injected with pharmacological doses of vitamin A during days 11-19 of gestation with the purpose of studying the long bones of offspring up to the age of 1 week. Tibiae were collected for routine light microscopic examination and tranmission electron microscopic examination. In addition, biochemical studies were conducted to determine the calcium, phosphorus, and magnesium content as well as the hydroxyproline and protein content of the bones. Treatment with vitamin A resulted in reduced weight and length of the long bones, as well as the presence of excessive calcification throughout the hypertrophic zone of the cartilaginous epiphyses. Matrix vesicles, many of them containing hydroxyapatite crystals, were observed and found to be distributed within the cartilaginous epiphyses in a similar pattern as in untreated control mice offspring, but mineral crystals were also observed unassociated with the matrix vesicles. The calcium, phosphate, magnesium, and hydroxyproline content was reduced in the vitamin A offspring. However, the percentage of these minerals expressed per dry weight bone was higher than in controls, verifying the morphological findings that although vitamin A inhibits bone growth, it enhances calcification in the growth plate.

Teratogenic effects of retinoic acid and dimethylsulfoxide on embryonic chick wing and somite

In order to document the stage(s) at which the embryonic chick wing bud is sensitive to vitamin A teratogenesis and the kinds of defects produced by vitamin A insult to the embryonic chick wing, 1-microgram doses of retinoic acid (1 microliter RA in 90% DMSO at a concentration of 1 microgram/microliter) were locally applied to the right wing bud of chick embryos at stages 17-23 (Hamburger and Hamilton: J. Morphol., 88:49-92, '51), and the resulting limb skeleton anatomy was observed at 10 days of incubation. Local application of RA at stages 17-20 resulted in distal wing skeleton defects. There were significantly more wing skeleton defects among embryos treated at these stages with RA solution than among solvent (DMSO)-treated control embryos and than among untreated control embryos. Wings of embryos treated with RA at stages 21-23 were always normal. Scapular and vertebral defects were seen at 10 days of incubation among embryos which had been treated prior to stage 21 with both the RA solution and the solvent control. Statistical analysis and histological data suggest that scapular and vertebral defects were caused by DMSO-induced damage to somites.

Comparative effects of retinoic acid and jervine on chondrocyte differentiation

Jervine and retinoic acid are both teratogenic to structures which are initially modelled in cartilage. Differences in periods of maximal sensitivity, as well as in certain aspects of the morphological manifestations of exposure, indicate that these two teratogens act via different molecular mechanisms. Here we compare the effects of jervine and retinoic acid in three culture systems which represent sequential stages of the chondrocyte lineage. Proliferation of pluripotent C3H 10T 1/2 cells was decreased by exposure to jervine but was not affected by retinoic acid. Differentiation of high-density "spot" cultures of embryonic limb bud mesenchyme were sensitive to both compounds. Mature chondrocytes were resistant to jervine but "dedifferentiated" after 48-hour exposure to retinoic acid. We conclude that jervine compromises rapidly dividing chondrogenic precursors, whereas retinoic acid has little effect prior to the expression of cartilage-specific proteins.

The effect of retinoic acid on proteoglycan turnover in bovine articular cartilage cultures

This paper describes proteoglycan catabolism by adult bovine articular cartilage treated with retinoic acid as a means of stimulating the loss of this macromolecule from the extracellular matrix of cartilage. Addition of retinoic acid (10(-12)-10(-6) M) to adult bovine articular cartilage which had been labeled with [35S]sulfate for 6 h after 5 days in culture, resulted in a dose-dependent increase in the rate of loss of 35S-labeled proteoglycans from the matrix of the tissue. Concomitant with this loss was a decrease in the proteoglycan content of the tissue. Incubation of cultures treated with 1 microM retinoic acid, at 4 degrees C, or with 0.5 mM cycloheximide, resulted in a significant decrease in the rate of retinoic acid-induced loss of proteoglycans and demonstrated cellular involvement in this process. Analysis of the 35S-labeled proteoglycans remaining in the matrix showed that the percentage of radioactivity associated with the small proteoglycan species extracted from the matrix of articular cartilage explants labeled with [35S]sulfate after 5 days in culture was 15% and this increased to 22% in tissue maintained in medium alone. In tissue treated with 1 microM retinoic acid for 6 days, the percentage of radioactivity associated with the small proteoglycan was 58%. Approximately 93% of the 35S-labeled proteoglycans released into the medium of control and retinoic acid-treated cultures was recovered in high density fractions after CsCl gradient centrifugation and eluted on Sepharose CL-2B as a broad peak with a Kav of 0.30-0.37. Less than 17% of these proteoglycans was capable of aggregating with hyaluronate. These results indicate that in both control and retinoic acid-treated cultures the larger proteoglycan species is lost to the medium at a greater rate than the small proteoglycan species. The effect of retinoic acid on proteoglycan turnover was shown to be reversible. Cartilage cultures maintained with retinoic acid for 1 day then switched to medium with 20% (v/v) fetal calf serum for the remainder of the culture period exhibited decreased rates of loss of 35S-labeled proteoglycans from the matrix and increased tissue hexuronate contents to levels near those observed in tissue maintained in medium with 20% (v/v) fetal calf serum throughout. Furthermore, following switching to 20% (v/v) fetal calf serum, the relative proportions of the 35S-labeled proteoglycan species remaining in the matrix of these cultures were similar to those of control cultures.

Retinoic acid rapidly reduces cartilage matrix synthesis by altering gene transcription in chondrocytes

Retinoic acid can alter the differentiation of a variety of cell types, including chondrocytes. This action may explain the high incidence of craniofacial and limb defects resulting from exposure to retinoic acid during development, and may be the basis for the compound's inhibition of a chondrosarcoma tumor in vivo. In order to understand the mechanism of action of retinoic acid, we studied the expression of chondrocyte-specific proteins as well as other proteins that indicate a shift in the differentiated phenotype of the cell following exposure to retinoic acid. After 48 hr of exposure to retinoic acid chondrocytes stopped synthesizing the chondrocyte-specific pro alpha 1 (II) chain of collagen II and a 370-kDa precursor protein of a cartilage-specific proteoglycan. Instead, the cells synthesized increased amounts of fibronectin and the pro alpha 1 chain of collagen III. These changes could be detected as early as 12 hr after treatment. In addition, the steady-state levels of specific mRNA transcripts coding for these differentiation markers correlated with their protein synthesis levels. Also, nuclear runoff experiments indicated that retinoic acid down regulated transcription of the collagen II gene, while stimulating collagen III gene transcription. These observations suggest that retinoic acid may alter the expression of the chondrocyte phenotype by selectively changing the normal pattern of gene expression.

Effect of vitamin A on glycoconjugate: synthesis in the trabecular meshwork. A preliminary report

We studied the effect of vitamin A on glycoconjugate synthesis in trabecular meshwork (TM) cells in vivo. The aqueous flow system of two cats and one monkey was exposed in vivo to retinoic acid dissolved in dimethyl sulfoxide (DMSO). One hour prior to enucleation, [3H]glucosamine was injected into all eyes. The tissue was subsequently processed for autoradiography. The density of grains (grains micron-2) in the vitamin A-treated eyes of Cats I and II was 77% and 30% less, respectively, than the density of grains in the control eyes. In the monkey the grain density was 56% less in the experimental eye compared with the control eye. These data suggest that exogenous vitamin A suppresses glycoconjugate synthesis in TM cells in vivo.

The effect of retinoic acid on proteoglycan biosynthesis in bovine articular cartilage cultures

The addition of retinoic acid to adult bovine articular cartilage cultures produces a concentration-dependent decrease in both proteoglycan synthesis and the proteoglycan content of the tissue. Total protein synthesis was not affected by the presence of retinoic acid, indicating that the inhibition of proteoglycan synthesis was not due to cytotoxicity. The proteoglycans synthesized in the presence of retinoic acid were similar in hydrodynamic size, ability to form aggregates with hyaluronate, and glycosaminoglycan composition to those of control cultures. However, the presence of larger glycosaminoglycan chains suggests that the core protein was substituted with fewer but longer glycosaminoglycan chains. In cultures maintained with retinoic acid, a decreased ratio of the large proteoglycan was synthesized relative to the small proteoglycan compared to that measured in control cultures. In cultures maintained with retinoic acid for 1 day and then switched to medium with 20% (v/v) fetal calf serum, the rate of proteoglycan synthesis and hexuronate contents increased within 5 days to levels near those of control cultures. Within 2 days of switching to medium with 20% (v/v) fetal calf serum, the relative proportions of the proteoglycan species were similar to those produced in cultures maintained in medium with 20% (v/v) fetal calf serum throughout. The rate of proteoglycan synthesis by bovine articular cartilage cultures exhibited an exponential decay following exposure to retinoic acid, with estimated half-lives of 11.5 and 5.3 h for tissue previously maintained in medium alone or containing 20% (v/v) fetal calf serum, respectively. The addition of 1 mM benzyl beta-D-xyloside only partially reversed the retinoic acid-mediated inhibition of proteoglycan synthesis. This indicates that the inhibition of proteoglycan synthesis by retinoic acid was due to both a decreased availability of xylosylated core protein and a decreased capacity of the chondrocytes to synthesize chondroitin sulfate chains.

Independence of cell shape and loss of cartilage matrix production during retinoic acid treatment of cultured chondrocytes

Retinoic acid has been shown to cause chondrocytes in culture to flatten and to inhibit the synthesis of cartilage specific components. Since the biochemical expression of chondrocytes is considered to be dependent on cell shape, it has been proposed that retinoic acid acts on these cells primarily by causing a change in cell morphology. This hypothesis was tested by culturing chick sternal chondrocytes suspended in methyl cellulose, which prevents cell flattening. Cultures were labeled with [35S]methionine and differentiation was assessed by polyacrylamide gel electrophoresis. The results showed that retinoic acid-treated chondrocytes in suspension remained rounded but synthesized proteins characteristic of flattened or dedifferentiated chondrocytes. Chondrocytes exposed to retinoic acid in suspension became fibroblastic when placed in monolayer culture in the absence of retinoic acid. This effect was irreversible after 2 weeks of culture. These results suggest that retinoic acid has a direct molecular or biochemical effect on the chondrocyte and that the cell shape change is secondary.

Influence of retinoic acid on the ultrastructure and hyaluronic acid synthesis of adult human epidermis in whole skin organ culture

Normal human skin was maintained in organ culture under chemically defined conditions. All-trans retinoic acid was added to the culture medium at the final concentration of 5 mumol/l. After 5 days in culture samples were either harvested for electron microscopy or labeled with 3H-glucosamine for 24 h. After labeling, epidermis was separated from dermis and both tissue compartments were analyzed for the content of 3H-labeled glycosaminoglycans (GAGs) using CPC-precipitation and thin layer chromatography after enzymatic degradation into specific disaccharides. Retinoic acid caused a marked change in the epidermal tissue architecture. The epidermal cells were flattened and contained fewer desmosomes and tonofilaments than control explants. Retinoic acid induced accumulation of fine granular material in the intercellular spaces in the upper, and less dense, flocculent material in the lower epidermis. The analysis of 3H-glycosaminoglycans showed that in the epidermis retinoic acid elevated the amount of labeled hyaluronate by 70%, whereas sulfated GAGs were not significantly increased. In dermis the incorporation of 3H-glucosamine into neither hyaluronate nor sulfated GAGs was stimulated by the retinoic acid. It is concluded that retinoic acid significantly modifies the differentiation of normal adult human epidermis by decreasing cytoskeleton components and by inducing the synthesis of new intercellular material, at least a part of which is hyaluronic acid. As a consequence, the cohesion between the epidermal cells was apparently weakened.

Vitamin A and bone formation. Effect of an excess of retinol on bone collagen synthesis in vitro

The influence of an excess of retinol on bone formation was studied by using cultures of embryonic-chick calvaria. Retinol decreased collagen synthesis in a dose-dependent manner, non-collagenous protein synthesis being relatively unaffected. Collagen synthesis was significantly inhibited after 24 h of culture with retinol and was progressively decreased, compared with control cultures containing no retinol, as the period of culture was increased. The effect of retinol on collagen synthesis could be reversed by incubation of calvaria for further periods in retinol-free medium. Incorporation of [3H]thymidine and [3H]uridine into DNA and RNA respectively was not altered by culturing calvaria with retinol for 22 h. These latter findings, and the selectivity for collagen synthesis, all suggested that the effect observed was not a cell-toxicity phenomenon. The effect of retinol on collagen synthesis by chick calvarial osteoblasts was probably direct and not mediated by osteoclasts, since a negligible number of the latter cells is present in chick calvaria. In cultures of neonatal murine calvaria, which contain many osteoclasts, retinol similarly inhibited synthesis of collagen, but not of non-collagenous protein; the concentrations of retinol necessary to produce the response were similar to those required to stimulate bone resorption in vitro.

Retinoids inhibit the differentiation of embryonic-mouse mesenchymal cells in vitro

The influence of all-trans retinoic acid, 13-cis retinoid acid and two aromatic retinoids (Ro 10-1670, Ro 11-1430) on the chondrogenic differentiation of mesenchymal cells in vitro was studied electron-microscopically. Organ cultures of limb buds from mouse embryos (Day 11) and high-density cultures of embryonic-mouse mesenchymal cells (Day 12) were used as experimental models. After a 6-day culture in the control medium, the development of hyaline cartilage was observed in both systems. In cultures which were treated with retinoids from Day 1 through Day 3 and then incubated in the control medium for 3 more days, the mesenchymal cells still maintained the morphological features of the blastema stage; cartilage synthesis was reduced (low retinoid concentrations) or completely absent (high retinoid concentrations). These findings indicate that the treatment of embryonic-mouse mesenchymal cells with retinoids induces a persistent and dose-dependent inhibition of chondrogenic differentiation, which in quantitative terms, is variably expressed during treatment with different retinoids. These inhibitory effects of retinoids on chondrogenesis are probably implicated in the pathogenetic mechanisms of their teratogenic action in vivo.

Effect of retinoids on collagen production by chondrocytes in culture

The effect of vitamin A and of some of its derivatives on chondrocytes in culture has been studied. In the presence of retinoids the proliferation of the cells decreased and they lost their characteristic polygonal shape and assumed a fibroblast-like morphology. All retinoids also caused dedifferentiation of chondrocytes as indicated by the induction of types I and III collagen. 13-cis retinoic acid (= isotretinoin) was the most active derivative in this aspect. Since appropriate control of the synthesis of extracellular matrix proteins is a prerequisite for their normal physiological function, alterations such as those observed here may be involved in the pathogenesis of side effects which are observed during the treatment of dermatological disorders with retinoic acid derivatives.

Prostaglandin stimulation of adenosine 3',5'-monophosphate accumulation in cultured chondrocytes in the presence or absence of parathyroid hormone

The effect of prostaglandin analogues on the cyclic AMP level in cultured chondrocytes were examined. Prostaglandin E1 at 0.4 to 30 microM, increased the intracellular concentration of cyclic AMP in chondrocytes. Its effect was rapid, being evident within 1 min and reaching a maximum in 10 to 20 min. The maximum level was sustained until 30 min after its addition and then decreased gradually. Prostaglandin D2 and E2 also increased the cyclic AMP level in chondrocytes, but they had less effect than prostaglandin E1. Prostaglandin A1 had no effect on the nucleotide level in chondrocytes, although they markedly increased the level in fibroblasts. The time course of stimulation of cyclic AMP accumulation in chondrocytes by prostaglandin E1, D2 or E2 was quite different from that by parathyroid hormone (PTH): the effect of prostaglandin was slower and more sustained than that of PTH. PTH potentiated the effect of prostaglandin E1, E2, or D2 on the cyclic AMP level in chondrocytes and that the combined effects of prostaglandin and PTH were more than additive. Addition of an inhibitor of cyclic nucleotide phosphodiesterase with prostaglandin, PTH or both produced a synergistic effect on the accumulation of cyclic AMP in the chondrocytes. These findings suggest that prostaglandin E1, E2, and D2 increase the synthesis of cyclic AMP and that the combined effect of the prostaglandins and PTH on the cyclic AMP level in chondrocytes is partly attributed to the synergistic synthesis of cyclic AMP in the cells.

Restoration by cyclic AMP of the differentiated phenotype of chondrocytes from de-differentiated cells pretreated with retinoids

Parathyroid hormone (PTH) increases the cyclic AMP level in rabbit costal chondrocytes in culture. PTH, dibutyryl cyclic AMP (DBcAMP), and 8-bromo cyclic AMP (8-Br cAMP)induce ornithine decarboxylase (ODC) and expression of the differentiated phenotype of chondrocytes in this cell system. On the other hand, retinoids inhibit expression of the differentiated phenotype of chondrocytes. In the present study, the effects of PTH, DBcAMP, and 8-Br cAMP on rabbit costal chondrocytes pretreated with retinoids were examined. PTH did not increase the cellular cyclic AMP level in de-differentiated cells that had been pretreated with retinyl acetate or retinoic acid for three days, but it did increase the cyclic AMP level four days after removal of retinoids. PTH did not stimulate ODC activity or expression of the differentiated phenotype of chondrocytes in the de-differentiated state. On the other hand, DBcAMP or 8-Br cAMP stimulated expression of the differentiated phenotype of chondrocytes even in de-differentiated cells, as judged by morphological and histological changes of the cells and increase in glycosaminoglycan synthesis. Cyclic AMP analogues also induced ODC in these cells.

Estimation of creatine phosphokinase and hydroxyproline in the developing limb: its use in evaluating the effect of teratogens on myogenesis and chondrogenesis

Forelimbs of mouse fetuses were examined for tissue-specific, drug-induced alterations in their biochemical composition. The activity of the enzyme creatine phospholinase (CPK; to estimate myogenesis) and the content of hydroxyproline (HP; to estimate chondrogenesis) were compared in homogenates of control and treated mouse-fetus forelimbs on day 14 of gestation. In addition, the content of DNA, RNA, and protein was also measured. Injection of 6-aminonicotinamide (6-AN) (15 mg/kg) on day 10 resulted in an overall growth retardation in day 14 fetuses and all biochemical parameters tested were reduced. The ratio of PH:CPK was unaffected by 6-AN treatment. Retinoic acid (vitamin A acid; 100 mg/kg), administered to pregnant female mice on day 10, produced severe forelimb defects and resulted in a signific 10 resulted in an overall growth retardation in day 14 fetuses and all biochemical parameters tested were reduced. The ratio of PH:CPK was unaffected by 6-AN treatment. Retinoic acid (vitamin A acid; 100 mg/kg), administered to pregnant female mice on day 10, produced severe forelimb defects and resulted in a signific 10 resulted in an overall growth retardation in day 14 fetuses and all biochemical parameters tested were reduced. The ratio of PH:CPK was unaffected by 6-AN treatment. Retinoic acid (vitamin A acid; 100 mg/kg), administered to pregnant female mice on day 10, produced severe forelimb defects and resulted in a significant reduction in day 14 forelimb HP and RNA content, without altering CPK, DNA, or protein; thus, the HP:CPK ratio was decreased. These results indicated that 1) 6-AN nonspecifically retards growth and cyto-differentiation in limbs; 2) retinoic acid inhibits synthesis of collagen and RNA; 3) retinoic acid has a differential effect upon chondrogenic and myogenic tissues of the limb, and 4) the comparison of HP content and CPK activity in tissue homogenates is an acceptable method of evaluating teratogenic compounds for selective effects.

Polyamine and differentiation: induction of ornithine decarboxylase by parathyroid hormone is a good marker of differentiated chondrocytes

The activity of ornithine decarboxylase (OD-Case:L-ornithine carboxy-lyase, EC 4.1.1.17) in rabbit costal chondrocytes in culture increased markedly after addition of parathyroid hormone (PTH), reaching a maximum 4 to 5 hr after PTH addition. The increase in ODCase activity was followed by increase in the intracellular concentrations of polyamines, especially putrescine, which increased in 6 hr to about 3-fold that of untreated cultures. The induction of ODCase by PTH was not observed in L, 3T3, HeLa, buffalo rat liver, or BHK cells. Retinyl acetate and retinoic acid both inhibited expression of the differentiated phenotype of chondrocytes by rabbit costal chondrocytes in culture within 3 days after their addition, as judged by morphological change and decrease in sulfate incorporation into glycosaminoglycans but did not inhibit cell proliferation. PTH could not induce an increase in ODCase in de-differentiated cells that had been pretreated with retinyl acetate or retinoic acid for 3 days. but 4 days after removal of the retinoids, these de-differentiated cells regained the ability to synthesize ODCase in response to PTH. These facts suggest that the induction of ODCase and the formation of putrescine by PTH are good markers of the differentiated phenotype of cultured chondrocytes.

Phosphorylation of proteoglycans in human articular cartilage

A study of human articular cartilage indicated that componenet proteoglycans can be phosphorylated. Phosphorylation, also found in a specimen of human epiphysial cartilage, occurred when [gamma-32P]-ATP or 32Pi was included in the in vitro incubation medium. Treatment of the phosphorylated proteoglycans with chondroitinase and chondrosulfatases effectively removed the chondroitin sulfate without dephosphorylating the remaining molecule. Since phosphorylation could be effected in a totally chemically defined medium, it appears that the necessary enzyme systems for this reaction are contained entirely within chondrocytes.

Effect of retinyl acetate on sulfated glycosaminoglycan biosynthesis in dermal and epidermal cells in vitro

The effects of retinyl acetate on the biosynthesis of sulfated glycosaminoglycans in dermal and epidermal cells isolated from newborn mice was investigated. Three compartments were analysed for [35S]-glycosaminoglycans; the culture medium, the cellular matrix, and the cells. The individual levels of chondroitin 4-sulfate, dermatan sulfate and heparin and/or heparan sulfate in these compartments as a function of retinyl acetate was also analysed. The addition of retinyl acetate resulted in a dose dependent increase 35SO4 incorporation in the cellular and matrix compartments of the dermis in vitro. At the optimum concentration of 1.8 X 10(-6) M, this increase was 50%. The levels of35SO4 incorporated into medium glycosaminoglycans were relatively constant. There were also changes in the levels of the individual sulfated glycosaminoglycans. The glycosaminoglycan profile was modified differently in each of the three compartments analysed. In the epidermal cells, retinyl acetate at an optimum concentration of 1.8 X 10(-6) M resulted in a dose dependent increase in 35SO4 incorporation in the cellular, matrix and medium compartments. There was no apparent change in the glycosaminoglycan profile, with heparin and/or heparan sulfate being the major sulfated glycosaminoglycan.