Effect of retinoic acid on the resorptive activity of chick osteoclasts in vitro

Retinoic acid receptor signalling directly regulates osteoblast and adipocyte differentiation from mesenchymal progenitor cells

Low and high serum retinol levels are associated with increased fracture risk and poor bone health. We recently showed retinoic acid receptors (RARs) are negative regulators of osteoclastogenesis. Here we show RARs are also negative regulators of osteoblast and adipocyte differentiation. The pan-RAR agonist, all-trans retinoic acid (ATRA), directly inhibited differentiation and mineralisation of early osteoprogenitors and impaired the differentiation of more mature osteoblast populations. In contrast, the pan-RAR antagonist, IRX4310, accelerated differentiation of early osteoprogenitors. These effects predominantly occurred via RARγ and were further enhanced by an RARα agonist or antagonist, respectively. RAR agonists similarly impaired adipogenesis in osteogenic cultures. RAR agonist treatment resulted in significant upregulation of the Wnt antagonist, Sfrp4. This accompanied reduced nuclear and cytosolic β-catenin protein and reduced expression of the Wnt target gene Axin2, suggesting impaired Wnt/β-catenin signalling. To determine the effect of RAR inhibition in post-natal mice, IRX4310 was administered to male mice for 10 days and bones were assessed by µCT. No change to trabecular bone volume was observed, however, radial bone growth was impaired. These studies show RARs directly influence osteoblast and adipocyte formation from mesenchymal cells, and inhibition of RAR signalling in vivo impairs radial bone growth in post-natal mice.

The role of vitamin A and retinoic acid receptor signaling in post-natal maintenance of bone

Vitamin A and retinoid derivatives are recognized as morphogens that govern body patterning and skeletogenesis, producing profound defects when in excess. In post-natal bone, both high and low levels of vitamin A are associated with poor bone heath and elevated risk of fractures. Despite this, the precise mechanism of how retinoids induce post-natal bone changes remains elusive. Numerous studies have been performed to discover how retinoids induce these changes, revealing a complex morphogenic regulation of bone through interplay of different cell types. This review will discuss the direct and indirect effects of retinoids on mediators of bone turnover focusing on differentiation and activity of osteoblasts and osteoclasts and explains why some discrepancies in this field have arisen. Importantly, the overall effect of retinoids on the skeleton is highly site-specific, likely due to differential regulation of osteoblasts and osteoclasts at trabecular vs. cortical periosteal and endosteal bone surfaces. Further investigation is required to discover the direct gene targets of retinoic acid receptors (RARs) and molecular mechanisms through which these changes occur. A clear role for RARs in regulating bone is now accepted and the therapeutic potential of retinoids in treating bone diseases has been established.

Retinoid receptors in bone and their role in bone remodeling

Vitamin A (retinol) is a necessary and important constituent of the body which is provided by food intake of retinyl esters and carotenoids. Vitamin A is known best for being important for vision, but in addition to the eye, vitamin A is necessary in numerous other organs in the body, including the skeleton. Vitamin A is converted to an active compound, all-trans-retinoic acid (ATRA), which is responsible for most of its biological actions. ATRA binds to intracellular nuclear receptors called retinoic acid receptors (RARα, RARβ, RARγ). RARs and closely related retinoid X receptors (RXRα, RXRβ, RXRγ) form heterodimers which bind to DNA and function as ligand-activated transcription factors. It has been known for many years that hypervitaminosis A promotes skeleton fragility by increasing osteoclast formation and decreasing cortical bone mass. Some epidemiological studies have suggested that increased intake of vitamin A and increased serum levels of retinoids may decrease bone mineral density and increase fracture rate, but the literature on this is not conclusive. The current review summarizes how vitamin A is taken up by the intestine, metabolized, stored in the liver, and processed to ATRA. ATRA's effects on formation and activity of osteoclasts and osteoblasts are outlined, and a summary of clinical data pertaining to vitamin A and bone is presented.

Vitamin a metabolism, action, and role in skeletal homeostasis

Vitamin A (retinol) is ingested as either retinyl esters or carotenoids and metabolized to active compounds such as 11-cis-retinal, which is important for vision, and all-trans-retinoic acid, which is the primary mediator of biological actions of vitamin A. All-trans-retinoic acid binds to retinoic acid receptors (RARs), which heterodimerize with retinoid X receptors. RAR-retinoid X receptor heterodimers function as transcription factors, binding RAR-responsive elements in promoters of different genes. Numerous cellular functions, including bone cell functions, are mediated by vitamin A; however, it has long been recognized that increased levels of vitamin A can have deleterious effects on bone, resulting in increased skeletal fragility. Bone mass is dependent on the balance between bone resorption and bone formation. A decrease in bone mass may be caused by either an excess of resorption or decreased bone formation. Early studies indicated that the primary skeletal effect of vitamin A was to increase bone resorption, but later studies have shown that vitamin A can not only stimulate the formation of bone-resorbing osteoclasts but also inhibit their formation. Effects of vitamin A on bone formation have not been studied in as great a detail and are not as well characterized as effects on bone resorption. Several epidemiological studies have shown an association between vitamin A, decreased bone mass, and osteoporotic fractures, but the data are not conclusive because other studies have found no associations, and some studies have suggested that vitamin A primarily promotes skeletal health. In this presentation, we have summarized how vitamin A is absorbed and metabolized and how it functions intracellularly. Vitamin A deficiency and excess are introduced, and detailed descriptions of clinical and preclinical studies of the effects of vitamin A on the skeleton are presented.

Retinoic acid induces two osteocalcin isoforms and inhibits markers of osteoclast activity in Atlantic cod (Gadus morhua) ex vivo cultured craniofacial tissues

Nutritional status including vitamin A could explain some of the developmental deformities observed in cultivated teleosts, including Atlantic cod (Gadus morhua). In the present study we aimed to investigate the transcriptional effect of retinoic acid (RA) on bone related genes using Atlantic cod craniofacial explants tissue cultures. Two different osteoblast specific osteocalcin/bone gla protein isoforms were discovered in cod. Transcription of both isoforms was up-regulated following RA treatment of 65 dph cod lower jaw explants. In contrast, transcripts coding for genes related to bone resorption and osteoclast activity, matrix metalloproteinase 9 and cathepsin K were down-regulated following RA treatment. This could be linked to the decreased transcriptional ratio between receptor activator of nuclear factor kappa-B ligand rankl and osteoprotegerin observed in the same tissue samples. RA treatment of juvenile explants had no effect on runt-related transcription factor 2 and osterix mRNA levels. However, osterix was significantly down-regulated in 25 dph cod head explants following RA treatment. In situ hybridizations revealed differential spatial distribution of the two isoforms and the predominant expression of cathepsin K in bone surrounding tissues. The present study indicates that RA causes a shift in the balance between osteoclast activity and osteoblast activity in favor of the latter.

Retinoic acid increases proliferation of human osteoclast progenitors and inhibits RANKL-stimulated osteoclast differentiation by suppressing RANK

It has been shown that high vitamin A intake is associated with bone fragility and fractures in both animals and humans. However, the mechanism by which vitamin A affects bones is unclear. In the present study, the direct effects of retinoic acid (RA) on human and murine osteoclastogenesis were evaluated using cultured peripheral blood CD14⁺ monocytes and RAW264.7 cells. Both the activity of the osteoclast marker tartrate resistant acid phosphatase (TRAP) in culture supernatant and the expression of the genes involved in osteoclast differentiation together with bone resorption were measured. To our knowledge, this is the first time that the effects of RA on human osteoclast progenitors and mature osteoclasts have been studied in vitro. RA stimulated proliferation of osteoclast progenitors both from humans and mice. In contrast, RA inhibited differentiation of the receptor activator of nuclear factor κB ligand (RANKL)-induced osteoclastogenesis of human and murine osteoclast progenitors via retinoic acid receptors (RARs). We also show that the mRNA levels of receptor activator of nuclear factor κB (RANK), the key initiating factor and osteoclast associated receptor for RANKL, were potently suppressed by RA in osteoclast progenitors. More importantly, RA abolished the RANK protein in osteoclast progenitors. This inhibition could be partially reversed by a RAR pan-antagonist. Furthermore, RA treatment suppressed the expression of the transcription factor nuclear factor of activated T-cells cytoplasmic 1 (NFATc1) and increased the expression of interferon regulatory factor-8 (IRF-8) in osteoclast progenitors via RARs. Also, RA demonstrated differential effects depending on the material supporting the cell culture. RA did not affect TRAP activity in the culture supernatant in the bone slice culture system, but inhibited the release of TRAP activity if cells were cultured on plastic. In conclusion, our results suggest that retinoic acid increases proliferation of human osteoclast progenitors and that it inhibits RANK-stimulated osteoclast differentiation by suppressing RANK.

Retinoid-induced bone thinning is caused by subperiosteal osteoclast activity in adult rodents

Excess of Vitamin A (retinol) and related compounds (retinoids) induces bone fragility and is associated with increased hip fracture incidence in humans. Yet, their impact on the adult skeleton has been studied in relatively little detail. It is assumed that they induce generalized bone loss and decrease long-bone thickness due to reduction of radial bone growth. Here we characterized early skeletal responses of adult rodents to retinoid treatment, revealing novel aspects of retinoid action on the mature skeleton. The retinoid Ro 13-6298, given subcutaneously for 4 days, induced bone loss in the hind limbs of 12- and 56-week-old rats and of 15-week-old mice. In vivo monitoring of bone mass and geometry changes by peripheral quantitative computed tomography demonstrated that bone mass decline was due to subperiosteal cortical bone loss, which induced a shrinkage of bone diameter, whilst cancellous bone mass was preserved. We observed that the native retinoic acid isomer all-trans RA induced an identical pattern of bone loss. Histomorphometric evaluation revealed that increased subperiosteal osteoclastic bone resorption caused the cortical bone destruction. Interestingly, bone resorption was suppressed in cancellous bone, which was in agreement with reduced in vitro formation of osteoclasts from bone marrow cells that were derived from the proximity of cancellous bone. The retinoid-induced increase in subperiosteal bone resorption could be blocked by bisphosphonate as direct potent inhibitor of osteoclast action, but not by estradiol. Retinoid treatment induced a reduction of bone-forming surfaces at the subperiosteal site, but not in cancellous bone. In vitro osteoblast performance was also reduced or unchanged, depending on the cellular system used and assay type/duration. In conclusion, our studies revealed that the impact of retinoids on bone is highly bone-compartment-specific at early treatment phases. Furthermore, we showed that bone diameter shrinks in the adult skeleton after retinoid treatment due to subperiosteal osteoclastic bone resorption. Thus, retinoid-induced bone thinning is not only due to reduced radial bone growth as previously assumed. Our findings might explain why high intake of retinol is associated with increased hip fracture risk in the elderly and suggest a therapy to prevent such potential negative effects.

Differential regulation of the carbonic anhydrase II gene expression by hormonal nuclear receptors in monocytic cells: identification of the retinoic acid response element

The Carbonic Anhydrase II (CAII) gene that encodes an enzyme involved in proton production is expressed in several cell types including monocyte/macrophage-derived osteoclasts. We have analyzed the regulation of the chicken CAII promoter/reporter construct by nuclear hormone receptors of the VDR subfamily in HD11 avian macrophages. The CAII expression is stimulated by 1, 25-dihydroxyvitamin D(3) but not by 9-cis retinoic acid and repressed by VDR overexpression due to RXR squelching. It is also stimulated by all-trans retinoic acid only when RARalpha is overexpressed, and is dependent on a RARE located in the distal part of the promoter and bound by RARalpha homodimer. Finally, in macrophages, unlike in erythrocytes, the CAII promoter is unresponsive to thyroid hormone. Our results demonstrate the first retinoic acid response element in the CAII promoter and show that according to cell type, different nuclear receptors of the VDR subfamily can regulate the CAII gene.

The integrin alphavbeta5 is expressed on avian osteoclast precursors and regulated by retinoic acid

Osteoclasts arise by proliferation, differentiation, and subsequent fusion of marrow-derived precursors, all processes requiring attachment to matrix. Integrins are important mediators of cell-matrix recognition and bone is rich in proteins containing the Arg-Gly-Asp motif, recognized primarily by alphav integrins. Thus, we determined if avian osteoclast precursors express integrins capable of mediating initial attachment to matrix proteins. Early, marrow-derived osteoclast precursors, when first isolated, contain no detectable alphavbeta3, but express an alphav integrin with an 80 kDa associated beta subunit. Immunoprecipitation with an antibody raised against the conserved beta5 cytoplasmic tail sequence indicates the the alphav associated the integrin is alphavbeta5. Retinoic acid is a resorptive steroid, and its exposure to early osteoclast precursors prompts a time- and dose-dependent decrease in alphavbeta5 expression, while simultaneously stimulating alphavbeta3 expression. Northern analysis reveals that retinoic acid decreases beta5 steady-state mRNA, nontranscriptionally, without altering that of alphav. The finding alphavbeta5 expression decreases under the influence of retinoic acid, an osteoclastogenic steroid, while those of alphavbeta3 rise, suggests that these closely related integrins play separate and complementary roles during osteoclast differentiation.

Gingival fluid IL-1beta in postmenopausal females on supportive periodontal therapy. A longitudinal 2-year study

Posterior interproximal alveolar bone in 59 women, within 5 years after menopause, was assessed at baseline and after 2 years of supportive periodontal therapy (history of moderate/advanced periodontitis) using digitized image analysis. Baseline lumbar spine bone mineral density, smoking status, and yearly serum estradiol (E2) levels also were obtained to group subjects. An additional 16 non-periodontitis postmenopausal women were followed 2 years for clinical and estrogen status. 2-min GCF IL-1beta levels averaged from 2 baseline periodontal pockets (in periodontitis subjects) and 2 non-periodontitis sites (in non-periodontitis and periodontitis subjects) were determined with an enzyme immunoassay. A progressive and stable site were also monitored every 6 months for GCF IL-1beta in 15 patients. Results after 2 years indicated that 17 subjects had no posterior interproximal sites losing > or =0.4 mm of alveolar crest bone height, while 13 subjects had > or =3 such sites. Using analysis of variance, none of the above clinical groupings resulted in a significant difference in mean baseline or longitudinal GCF IL-1beta levels. However, when subjects who lost alveolar crest bone height were considered, E2-sufficient subjects had significantly depressed baseline GCF IL-1beta (in past-periodontitis sites) compared to E2-deficient patients (9.1+/-2.1 versus 31.7+/-10.2 pg/2-min sample, p<0.05), suggesting E2 influences gingival IL-1beta production in progressive periodontitis patients.

Retinoic acid stimulates expression of the functional osteoclast integrin alpha v beta 3: transcriptional activation of the beta 3 but not the alpha v gene

The capacity of osteoclasts to resorb bone depends, in part, on the surface expression of the integrin alpha v beta 3. We have investigated whether the steroid hormone retinoic acid, known to stimulate bone resorption, regulates the appearance of the alpha v beta 3 complex in avian osteoclast presursors. Using surface labeling, followed by immunoprecipitation with a alpha v beta 3-specific antibody, we show that retinoic acid increases surface expression of the heterodimer in a dose- and time-dependent manner. Northern analysis reveals that the high basal steady-state levels of alpha v mRNA do not change, while those for beta 3 rise significantly from their initially low levels. Nuclear run-on studies confirm that steroid treatment stimulates transcription of the beta 3, but not the alpha v gene. Osteoclast precursors treated with retinoic acid exhibit increased multinucleation and expression of the osteoclast marker enzyme tartrate-resistant acid phosphatase. However, the fused cells do not have an increased capacity to resorb bone. In summary, multinucleated cells generated in this study do not represent fully differentiated bone-resorbing polykaryons. These results suggest that treatment of osteoclast precursors with retinoic acid is necessary, but insufficient, for expression of the mature osteoclast phenotype.

Homeostatic control of plasma calcium concentration

Due to the importance of Ca2+ in the regulation of vital cellular and tissue functions, the concentration of Ca2+ in body fluids is closely guarded by an efficient feedback control system. This system includes Ca(2+)-transporting subsystems (bone, and kidney), Ca2+ sensing, possibly by a calcium-sensing receptor, and calcium-regulating hormones (parathyroid hormone [PTH], calcitonin [CT], and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]). In humans and birds, acute Ca2+ perturbations are handled mainly by modulation of kidney Ca2+ reabsorption and by bone Ca2+ flow under PTH and possibly CT regulation, respectively. Chronic perturbations are also handled by the more sluggish but economic regulatory action of 1,25(OH2)D3 on intestinal calcium absorption. Peptide hormone secretion is modulated by Ca2+ and several secretagogues. The hormones' signal is produced by interaction with their respective receptors, which evokes the cAMP and phospholipase C-IP3-Ca2+ signal transduction pathways. 1,25 (OH)2D3 operates through a cytoplasmic receptor in controlling transcription and through a membrane receptor that activates the Ca2+ and phospholipase C messenger system. The calciotropic hormones also influence processes not directly associated with Ca2+ regulation, such as cell differentiation, and may thus affect the calcium-regulating subsystems also indirectly.

Retinoic acid directly stimulates osteoclastic bone resorption and gene expression of cathepsin K/OC-2

Vitamin A metabolites such as all-trans-retinoic acid (all-trans-RA) affect several steps of metabolic processes in vertebrates. In the last few years, several studies have shown the effect of RA on bone formation and metabolism. However, mechanisms of its action still remain unclear, especially with respect to the regulation of bone cells. Therefore, this study was carried out to clarify how RA regulates the activity of osteoclasts. Using a pit assay involving unfractionated bone cells, including osteoclasts obtained from rabbits, we found that RA stimulated an increase in the bone-resorbing activity in a dose- and time-dependent manner. Furthermore, this effect occurred more rapidly than that of treatments with 1 alpha,25-dihydroxyvitamin D3. However, this effect of RA may be partly related to cross-talk between osteoclasts and other types of cells. Therefore we studied the effect of RA on isolated osteoclasts. We found that all-trans-RA regulates the gene expression of cathepsin K/OC-2, a dominant cysteine proteinase, at the transcriptional level in mature osteoclasts isolated from rabbits. Moreover, retinoic acid-receptor alpha mRNA and retinoid X-receptor beta mRNA were expressed in these mature osteoclasts. Our results indicate that osteoclasts are target cells for RA and that RA might regulate a part of bone formation and metabolism through osteoclasts.

The distribution of cellular retinoic acid-binding protein I (CRABPI) and cellular retinol-binding protein I (CRBPI) during molar tooth development and eruption in the rat

The distribution of cellular retinoic acid-binding protein (CRABPI) and cellular retinol binding protein (CRBPI) was studied in a series of prenatal and early postnatal rats, covering the main stages of development and eruption of the molar teeth. CRABPI positive cells were found in the mesenchymal cells of the dental follicle from the cap stage and in the dental papilla from the early bell stage. In the dental papilla, CRABPI positive cells were situated adjacent to the enamel organ in the cervical loop region and in the subodontoblastic region. Newly formed odontoblasts were CRABPI positive for a short period of time. The enamel organ was CRBPI and CRABPI negative, except for the presence of CRABPI positive cells in the internal enamel epithelium over the tip of cusps and in parts of the stratum intermedium. During root formation, CRABPI positive cells were found in the developing periodontal ligament, in the dental papilla adjacent to the epithelial root sheath and in the subodontoblastic zone. During crown formation, CRBPI positive cells were mainly localized to the mesenchymal cells of the dental papilla during the cap stage of crown development. The periosteum of the developing mandible contained CRABPI positive cells while some osteoclasts appeared to show a weak but positive reaction to CRBPI. The findings were considered in terms of the possible significance of retinoid-binding proteins during tooth and bone development.