Examination of nuclear receptor expression in osteoblasts reveals Rorβ as an important regulator of osteogenesis

Consequences of Aging on Bone

With the aging of the global population, the incidence of musculoskeletal diseases has been increasing, seriously affecting people's health. As people age, the microenvironment within skeleton favors bone resorption and inhibits bone formation, accompanied by bone marrow fat accumulation and multiple cellular senescence. Specifically, skeletal stem/stromal cells (SSCs) during aging tend to undergo adipogenesis rather than osteogenesis. Meanwhile, osteoblasts, as well as osteocytes, showed increased apoptosis, decreased quantity, and multiple functional limitations including impaired mechanical sensing, intercellular modulation, and exosome secretion. Also, the bone resorption function of macrophage-lineage cells (including osteoclasts and preosteoclasts) was significantly enhanced, as well as impaired vascularization and innervation. In this study, we systematically reviewed the effect of aging on bone and the within microenvironment (including skeletal cells as well as their intracellular structure variations, vascular structures, innervation, marrow fat distribution, and lymphatic system) caused by aging, and mechanisms of osteoimmune regulation of the bone environment in the aging state, and the causal relationship with multiple musculoskeletal diseases in addition with their potential therapeutic strategy.

The role of retinoic acid receptor-related orphan receptors in skeletal diseases

Bone homeostasis, depending on the balance between bone formation and bone resorption, is responsible for maintaining the proper structure and function of the skeletal system. As an important group of transcription factors, retinoic acid receptor-related orphan receptors (RORs) have been reported to play important roles in bone homeostasis by regulating the transcription of target genes in skeletal cells. On the other hand, the dysregulation of RORs often leads to various skeletal diseases such as osteoporosis, rheumatoid arthritis (RA), and osteoarthritis (OA). Herein, we summarized the roles and mechanisms of RORs in skeletal diseases, aiming to provide evidence for potential therapeutic strategies.

Estrogen-regulated miRs in bone enhance osteoblast differentiation and matrix mineralization

Estrogen signaling is critical for the development and maintenance of healthy bone, and age-related decline in estrogen levels contributes to the development of post-menopausal osteoporosis. Most bones consist of a dense cortical shell and an internal mesh-like network of trabecular bone that respond differently to internal and external cues such as hormonal signaling. To date, no study has assessed the transcriptomic differences that occur specifically in cortical and trabecular bone compartments in response to hormonal changes. To investigate this, we employed a mouse model of post-menopausal osteoporosis (ovariectomy, OVX) and estrogen replacement therapy (ERT). mRNA and miR sequencing revealed distinct transcriptomic profiles between cortical and trabecular bone in the setting of OVX and ERT. Seven miRs were identified as likely contributors to the observed estrogen-mediated mRNA expression changes. Of these, four miRs were prioritized for further study and decreased predicted target gene expression in bone cells, enhanced the expression of osteoblast differentiation markers, and altered the mineralization capacity of primary osteoblasts. As such, candidate miRs and miR mimics may have therapeutic relevance for bone loss resulting from estrogen depletion without the unwanted side effects of hormone replacement therapy and therefore represent novel therapeutic approaches to combat diseases of bone loss.

Utilizing Variants Identified with Multiple Genome-Wide Association Study Methods Optimizes Genomic Selection for Growth Traits in Pigs

Improving the prediction accuracies of economically important traits in genomic selection (GS) is a main objective for researchers and breeders in the livestock industry. This study aims at utilizing potentially functional SNPs and QTLs identified with various genome-wide association study (GWAS) models in GS of pig growth traits. We used three well-established GWAS methods, including the mixed linear model, Bayesian model and meta-analysis, as well as 60K SNP-chip and whole genome sequence (WGS) data from 1734 Yorkshire and 1123 Landrace pigs to detect SNPs related to four growth traits: average daily gain, backfat thickness, body weight and birth weight. A total of 1485 significant loci and 24 candidate genes which are involved in skeletal muscle development, fatty deposition, lipid metabolism and insulin resistance were identified. Compared with using all SNP-chip data, GS with the pre-selected functional SNPs in the standard genomic best linear unbiased prediction (GBLUP), and a two-kernel based GBLUP model yielded average gains in accuracy by 4 to 46% (from 0.19 ± 0.07 to 0.56 ± 0.07) and 5 to 27% (from 0.16 ± 0.06 to 0.57 ± 0.05) for the four traits, respectively, suggesting that the prioritization of preselected functional markers in GS models had the potential to improve prediction accuracies for certain traits in livestock breeding.

Pregnane X receptor (PXR) represses osteoblast differentiation through repression of the Hedgehog signaling pathway

The pregnane X receptor (PXR, NR1I2) belongs to the nuclear receptor family and functions as a xenobiotic and endobiotic sensor by binding to various molecules through its relatively flexible ligand-binding domain. In addition to these well-known canonical roles, we previously reported that PXR represses osteoblast differentiation. However, the mechanisms underlying the PXR-mediated repression of osteoblast differentiation remains unknown. In this study, we analyzed the changes in global gene expression profiles induced by PXR in calvarial osteoblasts cultured in standard fetal bovine serum (in which PXR induces repression of differentiation), and in those cultured in charcoal-stripped fetal bovine serum (in which PXR does not induce repression of differentiation). The comparison revealed that PXR attenuated the Hedgehog-mediated signaling in culture conditions that induced PXR-mediated repression of differentiation. Real-time PCR analysis showed that PXR repressed the Hedgehog signaling-induced genes such as Gli1 and Hhip, and conversely induced the Hedgehog signaling-repressed genes such as Cdon, Boc, and Gas1. Activation of Smo-mediated signaling in osteoblasts following treatment with a Smo agonist (SAG) significantly restored Gli-mediated transcriptional activity and osteoblast differentiation. Our results demonstrate the osteoblast-autonomous effects of PXR and identify a novel regulation of Hedgehog signaling by nuclear receptors.

Rora Regulates Neutrophil Migration and Activation in Zebrafish

Neutrophil migration and activation are essential for defense against pathogens. However, this process may also lead to collateral tissue injury. We used microRNA overexpression as a platform and discovered protein-coding genes that regulate neutrophil migration. Here we show that miR-99 decreased the chemotaxis of zebrafish neutrophils and human neutrophil-like cells. In zebrafish neutrophils, miR-99 directly targets the transcriptional factor RAR-related orphan receptor alpha (roraa). Inhibiting RORα, but not the closely related RORγ, reduced chemotaxis of zebrafish and primary human neutrophils without causing cell death, and increased susceptibility of zebrafish to bacterial infection. Expressing a dominant-negative form of Rorα or disrupting the roraa locus specifically in zebrafish neutrophils reduced cell migration. At the transcriptional level, RORα regulates transmembrane signaling receptor activity and protein phosphorylation pathways. Our results, therefore, reveal previously unknown functions of miR-99 and RORα in regulating neutrophil migration and anti-microbial defense.

Prostaglandin E2 Enhances Aged Hematopoietic Stem Cell Function

Aging of hematopoiesis is associated with increased frequency and clonality of hematopoietic stem cells (HSCs), along with functional compromise and myeloid bias, with donor age being a significant variable in survival after HSC transplantation. No clinical methods currently exist to enhance aged HSC function, and little is known regarding how aging affects molecular responses of HSCs to biological stimuli. Exposure of HSCs from young fish, mice, nonhuman primates, and humans to 16,16-dimethyl prostaglandin E₂ (dmPGE₂) enhances transplantation, but the effect of dmPGE₂ on aged HSCs is unknown. Here we show that ex vivo pulse of bone marrow cells from young adult (3 mo) and aged (25 mo) mice with dmPGE₂ prior to serial competitive transplantation significantly enhanced long-term repopulation from aged grafts in primary and secondary transplantation (27 % increase in chimerism) to a similar degree as young grafts (21 % increase in chimerism; both p < 0.05). RNA sequencing of phenotypically-isolated HSCs indicated that the molecular responses to dmPGE₂ are similar in young and old, including CREB1 activation and increased cell survival and homeostasis. Common genes within these pathways identified likely key mediators of HSC enhancement by dmPGE₂ and age-related signaling differences. HSC expression of the PGE₂ receptor EP4, implicated in HSC function, increased with age in both mRNA and surface protein. This work suggests that aging does not alter the major dmPGE₂ response pathways in HSCs which mediate enhancement of both young and old HSC function, with significant implications for expanding the therapeutic potential of elderly HSC transplantation.

Natural products as modulators of retinoic acid receptor-related orphan receptors (RORs)

Covering: 1994 to 2020 Retinoic acid receptor-related orphan receptors (RORs) belong to a subfamily of the nuclear receptor superfamily and possess prominent roles in circadian rhythm, metabolism, inflammation, and cancer. They have been subject of research for over two decades and represent attractive but challenging drug targets. Natural products were among the first identified ligands of RORs and continue to be of interest to this day. This review focuses on ligands and indirect modulators of RORs from natural sources and explores their roles in a therapeutic context.

Senescent cells exacerbate chronic inflammation and contribute to periodontal disease progression in old mice

BACKGROUND

Coinciding with other chronic comorbidities, the prevalence of periodontal disease increases with aging. Mounting evidence has established that senescent cells accumulate at sites of age-related pathologies, where they promote "non-microbial" inflammation. We hypothesized that alveolar bone osteocytes develop senescence characteristics in old age.

METHODS

Alveolar bone samples were obtained from young (6 months) and old (20 to 22 months) mice to evaluate the expression of senescence biomarkers by immunofluorescent staining. Osteocyte-enriched fractions were used to characterize the age-related senescence-associated secretory phenotype (SASP) gene expression profile. Primary alveolar bone cells were exposed to the SASP via in vitro senescent conditioned media (SCM) administration. A multiplex assay confirmed protein levels of specific cytokines. Interactions with bacterial components were evaluated by stimulating cells with lipopolysaccharide (LPS).

RESULTS

Increased senescence-associated distension of satellites (SADS) and p16^Ink4a mRNA expression were identified in alveolar bone osteocytes with aging. These findings were associated with increased levels of DNA damage, and activated p38 MAPK, both inducers of senescence. Furthermore, interleukin-6 (IL6), IL17, IGFBP4, and MMP13 were significantly upregulated with aging in osteocyte-enriched samples. Interestingly, SCM potentiated the LPS-induced expression of IL1α, IL1β, and IL6. Cell migration and differentiation were also impeded by SCM. These in vitro effects were ameliorated by the p38 MAPK inhibitor SB202190.

CONCLUSIONS

Accumulation of senescent osteocytes contributes to deterioration of the periodontal environment by exacerbating chronic inflammation and reducing regeneration in old age. Cellular senescence is a cell-intrinsic response to DNA damage, and a host-related mechanism associated with aging that could potentiate inflammation induced by bacterial components.

Differential processing and localization of human Nocturnin controls metabolism of mRNA and nicotinamide adenine dinucleotide cofactors

Nocturnin (NOCT) is a eukaryotic enzyme that belongs to a superfamily of exoribonucleases, endonucleases, and phosphatases. In this study, we analyze the expression, processing, localization, and cellular functions of human NOCT. We find that NOCT protein is differentially expressed and processed in a cell and tissue type-specific manner to control its localization to the cytoplasm or mitochondrial exterior or interior. The N terminus of NOCT is necessary and sufficient to confer import and processing in the mitochondria. We measured the impact of cytoplasmic NOCT on the transcriptome and observed that it affects mRNA levels of hundreds of genes that are significantly enriched in osteoblast, neuronal, and mitochondrial functions. Recent biochemical data indicate that NOCT dephosphorylates NADP(H) metabolites, and thus we measured the effect of NOCT on these cofactors in cells. We find that NOCT increases NAD(H) and decreases NADP(H) levels in a manner dependent on its intracellular localization. Collectively, our data indicate that NOCT can regulate levels of both mRNAs and NADP(H) cofactors in a manner specified by its location in cells.

Estradiol-induced RORα expression positively regulates osteoblast differentiation

The retinoic acid receptor-related orphan receptor alpha (RORα) is a member of the nuclear hormone receptor superfamily. Several studies show that estradiol is related to RORα expression. However, the link between estradiol and RORα in osteoblast differentiation remains unknown. Here, we showed that estradiol induces RORα expression in C3H10T1/2 and MC3T3-E1 cells. RORα overexpression increased the expression of osteogenic genes including bone morphogenetic protein 2 (BMP2), distal-less homeobox 5, inhibitor of DNA binding, runt-related transcription factor 2 (Runx2), and osteocalcin. In addition, RORα increased phosphorylation of smad1/5/9. Furthermore, RORα knockdown suppressed estradiol-induced BMP2 and Runx2 protein level. Also, we confirmed that estradiol-induced ALP staining and matrix mineralization was attenuated in RORα knockdown. Summarily, these results suggest that estradiol-induced RORα promotes osteoblast differentiation.

Retinoic acid disrupts osteogenesis in pre-osteoblasts by down-regulating WNT signaling

The skull bones are formed by osteoblasts by intramembranous ossification. WNT signaling is a regulator of bone formation. Retinoic Acid (RA) act as a teratogen affecting craniofacial development. We evaluated the effects of RA on the differentiation and mineralization of MC-3T3 cells, and on the expression of WNT components. MC-3T3 were cultured with or without 0.5 μM RA in osteogenic medium and mineralization was assessed by alizarin red staining. The expression of osteogenic marker genes and WNT genes was evaluated at several time points up to 28 days. RA significantly inhibited MC-3T3 mineralization (p < 0.01), without affecting ALP activity or Alp gene expression. Both parameters gradually increased in time. During culture, RA stimulated Runx2 expression at 14 and 28 days compared to the respective controls (p < 0.05). Also, RA significantly reduced Sp7 expression at days 14 and 21 (p < 0.05). Simultaneously, RA significantly reduced the expression of the WNT genes cMyc, Lef1, Lrp5, Lrp6 and Wnt11 compared to the controls (p < 0.05). In contrast, RA increased the expression of the WNT inhibitors Dkk1 at day 21 and Dkk2 at days 14 and 21 (p < 0.01). Our data indicate that RA disrupts osteogenic differentiation and mineralization by inhibiting WNT signaling.

RORB and RORC associate with human islet dysfunction and inhibit insulin secretion in INS-1 cells

Little is known about the expression and function of Retinoic acid-related orphan receptors (RORA, B, and C) in pancreatic β cells. Here in, we utilized cDNA microarray and RNA sequencing approaches to investigate the expression pattern of ROR receptors in normal and diabetic human pancreatic islets. Possible correlations between RORs expression and HbA_1c levels as well as insulin secretory capacity in isolated human islets were evaluated. The impact of RORB and RORC expression on insulin secretion in INS-1 (832/13) cells was validated as well. While RORA was the highest expressed gene among the three RORs in human islet cells, RORC was the highest expressed in INS-1 cells (832/13) and while RORB was the lowest expressed gene in human islet cells, RORA was the highest expressed in INS-1 cells (832/13). The expression of RORB and RORC was significantly lower in diabetic/hyperglycemic donors as compared with non-diabetic counterparts. Furthermore, while the expression of RORB correlated positively with insulin secretion and negatively with HbA_1c, that of RORC correlated negatively with HbA_1c. The expression pattern of RORA did not correlate with either of the two parameters. siRNA silencing of RORB or RORC in INS-1 (832/13) cells resulted in a significant downregulation of insulin mRNA expression and insulin secretion. These findings suggest that RORB and RORC are part of the molecular cascade that regulates insulin secretion in pancreatic β cells; and insight that provides for further work on the potential therapeutic utility of RORB and RORC genes in β cell dysfunction in type 2 diabetes.

miR-219a-5p Regulates Rorβ During Osteoblast Differentiation and in Age-related Bone Loss

Developing novel approaches to treat skeletal disorders requires an understanding of how critical molecular factors regulate osteoblast differentiation and bone remodeling. We have reported that (1) retinoic acid receptor-related orphan receptor beta (Rorβ) is upregulated in bone samples isolated from aged mice and humans in vivo; (2) Rorβ expression is inhibited during osteoblastic differentiation in vitro; and (3) genetic deletion of Rorβ in mice results in preservation of bone mass during aging. These data establish that Rorβ inhibits osteogenesis and that strict control of Rorβ expression is essential for bone homeostasis. Because microRNAs (miRNAs) are known to play important roles in the regulation of gene expression in bone, we explored whether a predicted subset of nine miRNAs regulates Rorβ expression during both osteoblast differentiation and aging. Mouse osteoblastic cells were differentiated in vitro and assayed for Rorβ and miRNA expression. As Rorβ levels declined with differentiation, the expression of many of these miRNAs, including miR-219a-5p, was increased. We further demonstrated that miR-219a-5p was decreased in bone samples from old (24-month) mice, as compared with young (6-month) mice, concomitant with increased Rorβ expression. Importantly, we also found that miR-219a-5p expression was decreased in aged human bone biopsies compared with young controls, demonstrating that this phenomenon also occurs in aging bone in humans. Inhibition of miR-219a-5p in mouse calvarial osteoblasts led to increased Rorβ expression and decreased alkaline phosphatase expression and activity, whereas a miR-219a-5p mimic decreased Rorβ expression and increased osteogenic activity. Finally, we demonstrated that miR-219a-5p physically interacts with Rorβ mRNA in osteoblasts, defining Rorβ as a true molecular target of miR-219a-5p. Overall, our findings demonstrate that miR-219a-5p is involved in the regulation of Rorβ in both mouse and human bone. © 2018 American Society for Bone and Mineral Research.

Identification of potent RORβ modulators: Scaffold variation

We sought to develop RORβ-selective probe molecules in order to investigate the function of the receptor in vitro and in vivo and its role in the pathophysiology of disease. To accomplish this, we modified a potent dual RORβ/RORγ inverse agonist from the primary literature with the goal of improving selectivity for RORβ vs RORγ. Truncation of the Western portion of the molecule ablated activity at RORγ and led to a potent series of RORβ modulators. Continued exploration of this series investigated alternate replacement cores for the aminothiazole ring. Numerous suitable replacements were found during the course of our SAR investigations and are reported herein.

Insights into the Role of Circadian Rhythms in Bone Metabolism: A Promising Intervention Target?

Numerous physiological processes of mammals, including bone metabolism, are regulated by the circadian clock system, which consists of a central regulator, the suprachiasmatic nucleus (SCN), and the peripheral oscillators of the BMAL1/CLOCK-PERs/CRYs system. Various bone turnover markers and bone metabolism-regulating hormones such as melatonin and parathyroid hormone (PTH) display diurnal rhythmicity. According to previous research, disruption of the circadian clock due to shift work, sleep restriction, or clock gene knockout is associated with osteoporosis or other abnormal bone metabolism, showing the importance of the circadian clock system for maintaining homeostasis of bone metabolism. Moreover, common causes of osteoporosis, including postmenopausal status and aging, are associated with changes in the circadian clock. In our previous research, we found that agonism of the circadian regulators REV-ERBs inhibits osteoclast differentiation and ameliorates ovariectomy-induced bone loss in mice, suggesting that clock genes may be promising intervention targets for abnormal bone metabolism. Moreover, osteoporosis interventions at different time points can provide varying degrees of bone protection, showing the importance of accounting for circadian rhythms for optimal curative effects in clinical treatment of osteoporosis. In this review, we summarize current knowledge about circadian rhythms and bone metabolism.

Osteoprotection Through the Deletion of the Transcription Factor Rorβ in Mice

There is a clinical need to identify new molecular targets for the treatment of osteoporosis, particularly those that simultaneously inhibit bone resorption while stimulating bone formation. We have previously shown in overexpression studies that retinoic acid receptor-related orphan receptor β (Rorβ) suppresses in vitro osteoblast differentiation. In addition, the expression of Rorβ is markedly increased in bone marrow-derived mesenchymal stromal cells with aging in both mice and humans. Here we establish a critical role for Rorβ in regulating bone metabolism using a combination of in vitro and in vivo studies. We used Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/Cas9 gene editing to demonstrate that loss of Rorβ in osteoblasts enhances Wnt signaling, specifically through increased recruitment of β-catenin to T-cell factor/lymphoid enhancer factor (Tcf/Lef) DNA binding sites in the promoters of the Wnt target genes Tcf7 and Opg. This resulted in increased osteogenic gene expression and suppressed osteoclast formation through increased osteoprotegerin (OPG) secretion in Rorβ-deficient cells. Consistent with our in vitro data, genetic deletion of Rorβ in both female and male mice resulted in preserved bone mass and microarchitecture with advancing age due to increased bone formation with a concomitant decrease in resorption. The improved skeletal phenotype in the Rorβ^-/- mice was also associated with increased bone protein levels of TCF7 and OPG. These data demonstrate that loss of Rorβ has beneficial skeletal effects by increasing bone formation and decreasing bone resorption, at least in part through β-catenin-dependent activation of the Wnt pathway. Thus, inhibition of Rorβ represents a novel approach to potentially prevent or reverse osteoporosis. © 2017 American Society for Bone and Mineral Research.

Identification of an aminothiazole series of RORβ modulators

Crystallography has identified stearic acid, ALRT 1550 and ATRA as ligands that bind RORβ, however, none of these molecules represent good starting points to develop optimized small molecule modulators. Recently, Compound 1 was identified as a potent dual RORβ and RORγ inverse agonist with no activity towards RORα (Fig. 1). To our knowledge, this is one of only two small molecule RORβ inverse agonists identified in the primary literature from a tractable chemical series and represents an ideal starting point from which to design RORβ-selective modulators. Herein we describe our SAR optimization efforts that led to a series of potent neutral antagonists of RORβ.

Multiple faces of fibroblast growth factor-23

PURPOSE OF REVIEW

This review examines the role of fibroblast growth factor-23 (FGF-23) in mineral metabolism, innate immunity and adverse cardiovascular outcomes.

RECENT FINDINGS

FGF-23, produced by osteocytes in bone, activates FGFR/α-Klotho (α-Kl) complexes in the kidney. The resulting bone-kidney axis coordinates renal phosphate reabsorption with bone mineralization, and creates a counter-regulatory feedback loop to prevent vitamin D toxicity. FGF-23 acts to counter-regulate the effects of vitamin D on innate immunity and cardiovascular responses. FGF-23 is ectopically expressed along with α-Kl in activated macrophages, creating a proinflammatory paracrine signaling pathway that counters the antiinflammatory actions of vitamin D. FGF-23 also inhibits angiotensin-converting enzyme 2 expression and increases sodium reabsorption in the kidney, leading to hypertension and left ventricular hypertrophy. Finally, FGF-23 is purported to cause adverse cardiac and impair neutrophil responses through activation of FGFRs in the absence of α-Kl. Although secreted forms of α-Kl have FGF-23 independent effects, the possibility of α-Kl independent effects of FGF-23 is controversial and requires additional experimental validation.

SUMMARY

FGF-23 participates in a bone-kidney axis regulating mineral homeostasis, proinflammatory paracrine macrophage signaling pathways, and in a bone-cardio-renal axis regulating hemodynamics that counteract the effects of vitamin D.

The multi-faceted role of retinoid X receptor in bone remodeling

Retinoid X receptors (RXRs) form a unique subclass within the nuclear receptor (NR) superfamily of ligand-dependent transcription factors. RXRs are obligatory partners for a number of other NRs, placing RXRs in a coordinating role at the crossroads of multiple signaling pathways. In addition, RXRs can function as self-sufficient homodimers. Recent advances have revealed RXRs as novel regulators of osteoclastogenesis and bone remodeling. This review outlines the versatility of RXR action in the control of transcription of bone-forming osteoblasts and bone-resorbing osteoclasts, both through heterodimerization with other NRs and through RXR homodimerization. RXR signaling is currently a major therapeutic target and, therefore, knowledge of how RXR signaling affects bone remodeling creates enormous potential for the translation of basic research findings into successful clinical therapies to increase bone mass and improve bone quality.

Retinoid-Related Orphan Receptor β and Transcriptional Control of Neuronal Differentiation

The ability to generate neuronal diversity is central to the function of the nervous system. Here we discuss the key neurodevelopmental roles of retinoid-related orphan receptor β (RORβ) encoded by the Rorb (Nr1f2) gene. Recent studies have reported loss of function of the human RORB gene in cases of familial epilepsy and intellectual disability. Principal sites of expression of the Rorb gene in model species include sensory organs, the spinal cord, and brain regions that process sensory and circadian information. Genetic analyses in mice have indicated functions in circadian behavior, vision, and, at the cellular level, the differentiation of specific neuronal cell types. Studies in the retina and sensory areas of the cerebral cortex suggest that this orphan nuclear receptor acts at decisive steps in transcriptional hierarchies that determine neuronal diversity.

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.

Retinoic acid-related orphan receptor RORβ, circadian rhythm abnormalities and tumorigenesis (Review)

Nuclear receptors are a superfamily of transcription factors including the steroid hormone receptors, non-steroid hormone receptors and the orphan nuclear receptor family. Retinoic acid-related orphan receptor (ROR)β, as a member of the orphan nuclear receptor family, plays an important regulatory role in the maintenance of a variety of physiological and pathological processes. RORβ has been determined to act as an osteogenic repressor in regulating bone formation, and is involved in regulating circadian rhythm. The findings of recent studies concerning the association between tumorigenesis and circadian rhythm have shown that an aberrant circadian rhythm may promote tumorigenesis and tumor progression. The mechanisms discussed in this review demonstrate how aberrant RORβ-induced circadian rhythm may become a new direction for future studies on tumorigenesis and strategy design for cancer prevention.

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.

ERRγ is not required for skeletal development but is a RUNX2-dependent negative regulator of postnatal bone formation in male mice

To assess the effects of the orphan nuclear Estrogen receptor-related receptor gamma (ERRγ) deficiency on skeletal development and bone turnover, we utilized an ERRγ global knockout mouse line. While we observed no gross morphological anomalies or difference in skeletal length in newborn mice, by 8 weeks of age ERRγ +/− males but not females exhibited increased trabecular bone, which was further increased by 14 weeks. The increase in trabecular bone was due to an increase in active osteoblasts on the bone surface, without detectable alterations in osteoclast number or activity. Consistent with the histomorphometric results, we observed an increase in gene expression of the bone formation markers alkaline phosphatase (Alp) and bone sialoprotein (Bsp) in bone and increase in serum ALP, but no change in the osteoclast regulators receptor activator of NF-κB ligand (RANKL) and osteoprotegerin (OPG) or the resorption marker carboxy-terminal collagen crosslinks (CTX). More colony forming units-alkaline phosphatase and -osteoblast (CFU-ALP, CFU-O respectively) but not CFU-fibroblast (CFU-F) formed in ERRγ +/− versus ERRγ +/+ stromal cell cultures, suggesting that ERRγ negatively regulates osteoblast differentiation and matrix mineralization but not mesenchymal precursor number. By co-immunoprecipitation experiments, we found that ERRγ and RUNX2 interact in an ERRγ DNA binding domain (DBD)-dependent manner. Treatment of post-confluent differentiating bone marrow stromal cell cultures with Runx2 antisense oligonucleotides resulted in a reduction of CFU-ALP/CFU-O in ERRγ +/− but not ERRγ +/+ mice compared to their corresponding sense controls. Our data indicate that ERRγ is not required for skeletal development but is a sex-dependent negative regulator of postnatal bone formation, acting in a RUNX2- and apparently differentiation stage-dependent manner.

Vitamin a is a negative regulator of osteoblast mineralization

An excessive intake of vitamin A has been associated with an increased risk of fractures in humans. In animals, a high vitamin A intake leads to a reduction of long bone diameter and spontaneous fractures. Studies in rodents indicate that the bone thinning is due to increased periosteal bone resorption and reduced radial growth. Whether the latter is a consequence of direct effects on bone or indirect effects on appetite and general growth is unknown. In this study we therefore used pair-feeding and dynamic histomorphometry to investigate the direct effect of a high intake of vitamin A on bone formation in rats. Although there were no differences in body weight or femur length compared to controls, there was an approximately halved bone formation and mineral apposition rate at the femur diaphysis of rats fed vitamin A. To try to clarify the mechanism(s) behind this reduction, we treated primary human osteoblasts and a murine preosteoblastic cell line (MC3T3-E1) with the active metabolite of vitamin A; retinoic acid (RA), a retinoic acid receptor (RAR) antagonist (AGN194310), and a Cyp26 inhibitor (R115866) which blocks endogenous RA catabolism. We found that RA, via RARs, suppressed in vitro mineralization. This was independent of a negative effect on osteoblast proliferation. Alkaline phosphatase and bone gamma carboxyglutamate protein (Bglap, Osteocalcin) were drastically reduced in RA treated cells and RA also reduced the protein levels of Runx2 and Osterix, key transcription factors for progression to a mature osteoblast. Normal osteoblast differentiation involved up regulation of Cyp26b1, the major enzyme responsible for RA degradation, suggesting that a drop in RA signaling is required for osteogenesis analogous to what has been found for chondrogenesis. In addition, RA decreased Phex, an osteoblast/osteocyte protein necessary for mineralization. Taken together, our data indicate that vitamin A is a negative regulator of osteoblast mineralization.

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.

Sclerostin is expressed in osteoclasts from aged mice and reduces osteoclast-mediated stimulation of mineralization

Osteoclast-mediated bone resorption precedes osteoblast-mediated bone formation through early adulthood, but formation fails to keep pace with resorption during aging. We previously identified several factors produced by osteoclasts that promote bone formation. In this study, we determined if osteoclast-produced factors contribute to the impaired bone formation with aging. We previously found that mice between the ages of 18 and 22 months develop age-related bone loss. Bone marrow-derived pre-osteoclasts were isolated from 6-week, 12-month, and 18- to 24-month-old mice and differentiated into osteoclasts in vitro. Conditioned media were collected and compared for osteoblast mineralization support. Conditioned medium from osteoclasts from all ages was able to support mineralization of bone marrow stromal cells. Concentrating the conditioned medium from 6-week-old and 12-month-old mouse marrow cells-derived osteoclasts enhanced mineralization support whereas concentrated conditioned medium from 18- to 24-month-old mouse marrow-derived osteoclasts repressed mineralization compared to base medium. This observation suggests that an inhibitor of mineralization was secreted by aged murine osteoclasts. Gene and protein analysis revealed that the Wnt antagonist sclerostin was significantly elevated in the conditioned media from 24-month-old mouse cells compared to 6-week-old mouse cells. Antibodies directed to sclerostin neutralized the influences of the aged mouse cell concentrated conditioned media on mineralization. Sclerostin is primarily produced by osteocytes in young animals. This study demonstrates that osteoclasts from aged mice also produce sclerostin in quantities that may contribute to the age-related impairment in bone formation.

Identification of Rorβ targets in cultured osteoblasts and in human bone

Control of osteoblastic bone formation involves the cumulative action of numerous transcription factors, including both activating and repressive functions that are important during specific stages of differentiation. The nuclear receptor retinoic acid receptor-related orphan receptor β (Rorβ) has been recently shown to suppress the osteogenic phenotype in cultured osteoblasts, and is highly upregulated in bone marrow-derived osteogenic precursors isolated from aged osteoporotic mice, suggesting Rorβ is an important regulator of osteoblast function. However the specific gene expression patterns elicited by Rorβ are unknown. Using microarray analysis, we identified 281 genes regulated by Rorβ in an MC3T3-E1 mouse osteoblast cell model (MC3T3-Rorβ-GFP). Pathway analysis revealed alterations in genes involved in MAPK signaling, genes involved in extracellular matrix (ECM) regulation, and cytokine-receptor interactions. Whereas the identified Rorβ-regulated ECM genes normally decline during osteoblastic differentiation, they were highly upregulated in this non-mineralizing MC3T3-Rorβ-GFP model system, suggesting that Rorβ may exert its anti-osteogenic effects through ECM disruption. Consistent with these in vitro findings, the expression of both RORβ and a subset of RORβ-regulated genes were increased in bone biopsies from postmenopausal women (73±7 years old) compared to premenopausal women (30±5 years old), suggesting a role for RORβ in human age-related bone loss. Collectively, these data demonstrate that Rorβ regulates known osteogenic pathways, and may represent a novel therapeutic target for age-associated bone loss.