Retinoic Acid Receptor γ Activity in Mesenchymal Stem Cells Regulates Endochondral Bone, Angiogenesis, and B Lymphopoiesis

International Union of Basic and Clinical Pharmacology CXIII: Nuclear Receptor Superfamily-Update 2023

The NR superfamily comprises 48 transcription factors in humans that control a plethora of gene network programs involved in a wide range of physiologic processes. This review will summarize and discuss recent progress in NR biology and drug development derived from integrating various approaches, including biophysical techniques, structural studies, and translational investigation. We also highlight how defective NR signaling results in various diseases and disorders and how NRs can be targeted for therapeutic intervention via modulation via binding to synthetic lipophilic ligands. Furthermore, we also review recent studies that improved our understanding of NR structure and signaling. SIGNIFICANCE STATEMENT: Nuclear receptors (NRs) are ligand-regulated transcription factors that are critical regulators of myriad physiological processes. NRs serve as receptors for an array of drugs, and in this review, we provide an update on recent research into the roles of these drug targets.

Resolving trabecular metaphyseal bone profiles downstream of the growth plate adds value to bone histomorphometry in mouse models

INTRODUCTION

Histomorphometry of rodent metaphyseal trabecular bone, by histology or microCT, is generally restricted to the mature secondary spongiosa, excluding the primary spongiosa nearest the growth plate by imposing an 'offset'. This analyses the bulk static properties of a defined segment of secondary spongiosa, usually regardless of proximity to the growth plate. Here we assess the value of trabecular morphometry that is spatially resolved according to the distance 'downstream' of-and thus time since formation at-the growth plate. Pursuant to this, we also investigate the validity of including mixed primary-secondary spongiosal trabecular bone, extending the analysed volume 'upstream' by reducing the offset. Both the addition of spatiotemporal resolution and the extension of the analysed volume have potential to enhance the sensitivity of detection of trabecular changes and to resolve changes occurring at different times and locations.

METHOD

Two experimental mouse studies of trabecular bone are used as examples of different factors influencing metaphyseal trabecular bone: (1) ovariectomy (OVX) and pharmacological prevention of osteopenia and (2) limb disuse induced by sciatic neurectomy (SN). In a third study into offset rescaling, we also examine the relationship between age, tibia length, and primary spongiosal thickness.

RESULTS

Bone changes induced by either OVX or SN that were early or weak and marginal were more pronounced in the mixed primary-secondary upstream spongiosal region than in the downstream secondary spongiosa. A spatially resolved evaluation of the entire trabecular region found that significant differences between experimental and control bones remained undiminished either right up to or to within 100 μm from the growth plate. Intriguingly, our data revealed a remarkably linear downstream profile for fractal dimension in trabecular bone, arguing for an underlying homogeneity of the (re)modelling process throughout the entire metaphysis and against strict anatomical categorization into primary and secondary spongiosal regions. Finally, we find that a correlation between tibia length and primary spongiosal depth is well conserved except in very early and late life.

CONCLUSIONS

These data indicate that the spatially resolved analysis of metaphyseal trabecular bone at different distances from the growth plate and/or times since formation adds a valuable dimension to histomorphometric analysis. They also question any rationale for rejecting primary spongiosal bone, in principle, from metaphyseal trabecular morphometry.

Single-cell spatiotemporal analysis reveals cell fates and functions of transplanted mesenchymal stromal cells during bone repair

Mesenchymal stromal cells (MSCs) transplantation could enhance bone repair. However, the cell fate of transplanted MSCs, in terms of their local distribution and spatial associations with other types of cells were poorly understood. Here, we developed a single-cell 3D spatial correlation (sc3DSC) method to track transplanted MSCs based on deep tissue microscopy of fluorescent nanoparticles (fNPs) and immunofluorescence of key proteins. Locally delivered fNP-labeled MSCs enhanced tibial defect repair, increased the number of stem cells and vascular maturity in mice. fNP-MSCs persisted in the defect throughout repair. But only a small portion of transplanted cells underwent osteogenic differentiation (OSX+); a significant portion has maintained their expression of mesenchymal stem cell and skeletal stem cell markers (SCA-1 and PRRX1). Our results contribute to the optimization of MSC-based therapies. The sc3DSC method may be useful in studying cell-based therapies for the regeneration of other tissue types or disease models.

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Transplanted marrow stromal cells associated with vessels during bone defect repair

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Small proportion of transplanted cells differentiated into osteogenic cells

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A proportion of transplanted cells maintained expressions of stem cell markers

Selective Retinoic Acid Receptor γ Antagonist 7C is a Potent Enhancer of BMP-Induced Ectopic Endochondral Bone Formation

Bone morphogenetic proteins (BMPs) have been clinically applied for induction of bone formation in musculoskeletal disorders such as critical-sized bone defects, nonunions, and spinal fusion surgeries. However, the use of supraphysiological doses of BMP caused adverse events, which were sometimes life-threatening. Therefore, safer treatment strategies for bone regeneration have been sought for decades. Systemic administration of a potent selective antagonist of retinoic acid nuclear receptor gamma (RARγ) (7C) stimulated BMP-induced ectopic bone formation. In this study, we developed 7C-loaded poly lactic nanoparticles (7C-NPs) and examined whether local application of 7C enhances BMP-induced bone regeneration. The collagen sponge discs that absorbed recombinant human (rh) BMP-2 were implanted into the dorsal fascia of young adult mice to induce ectopic bone. The combination of rhBMP-2 and 7C-NP markedly increased the total bone volume and thickness of the bone shell of the ectopic bone in a dose-dependent manner compared to those with rhBMP-2 only. 7C stimulated sulfated proteoglycan production, expression of chondrogenic marker genes, and Sox9 reporter activity in both chondrogenic cells and MSCs. The findings suggest that selective RARγ antagonist 7C or the related compounds potentiate the bone inductive ability of rhBMP-2, as well as support any future research to improve the BMP-2 based bone regeneration procedures in a safe and efficient manner.

Titanium with nanotopography attenuates the osteoclast-induced disruption of osteoblast differentiation by regulating histone methylation

The bone remodeling process is crucial for titanium (Ti) osseointegration and involves the crosstalk between osteoclasts and osteoblasts. Considering the high osteogenic potential of Ti with nanotopography (Ti Nano) and that osteoclasts inhibit osteoblast differentiation, we hypothesized that nanotopography attenuate the osteoclast-induced disruption of osteoblast differentiation. Osteoblasts were co-cultured with osteoclasts on Ti Nano and Ti Control and non-co-cultured osteoblasts were used as control. Gene expression analysis using RNAseq showed that osteoclasts downregulated the expression of osteoblast marker genes and upregulated genes related to histone modification and chromatin organization in osteoblasts grown on both Ti surfaces. Osteoclasts also inhibited the mRNA and protein expression of osteoblast markers, and such effect was attenuated by Ti Nano. Also, osteoclasts increased the protein expression of H3K9me2, H3K27me3 and EZH2 in osteoblasts grown on both Ti surfaces. ChIP assay revealed that osteoclasts increased accumulation of H3K27me3 that represses the promoter regions of Runx2 and Alpl in osteoblasts grown on Ti Control, which was reduced by Ti Nano. In conclusion, these data show that despite osteoclast inhibition of osteoblasts grown on both Ti Control and Ti Nano, the nanotopography attenuates the osteoclast-induced disruption of osteoblast differentiation by preventing the increase of H3K27me3 accumulation that represses the promoter regions of some key osteoblast marker genes. These findings highlight the epigenetic mechanisms triggered by nanotopography to protect osteoblasts from the deleterious effects of osteoclasts, which modulate the process of bone remodeling and may benefit the osseointegration of Ti implants.

Current Understanding of Osteoimmunology in Certain Osteoimmune Diseases

The skeletal system and immune system seem to be two independent systems. However, there in fact are extensive and multiple crosstalk between them. The concept of osteoimmunology was created to describe those interdisciplinary events, but it has been constantly updated over time. In this review, we summarize the interactions between the skeletal and immune systems in the co-development of the two systems and the progress of certain typical bone abnormalities and bone regeneration on the cellular and molecular levels according to the mainstream novel study. At the end of the review, we also highlighted the possibility of extending the research scope of osteoimmunology to other systemic diseases. In conclusion, we propose that osteoimmunology is a promising perspective to uncover the mechanism of related diseases; meanwhile, a study from the point of view of osteoimmunology may also provide innovative ideas and resolutions to achieve the balance of internal homeostasis.

Regulation of murine B lymphopoiesis by stromal cells

B lymphocytes are crucial for the body's humoral immune response, secreting antibodies generated against foreign antigens to fight infection. Adult murine B lymphopoiesis is initiated in the bone marrow and additional maturation occurs in the spleen. In both these organs, B lymphopoiesis involves interactions with numerous different non-hematopoietic cells, also known as stromal or microenvironment cells, which provide migratory, maturation, and survival signals. A variety of conditional knockout and transgenic mouse models have been used to identify the roles of distinct microenvironment cell types in the regulation of B lymphopoiesis. These studies have revealed that mesenchymal lineage cells and endothelial cells comprise the non-hematopoietic microenvironment cell types that support B lymphopoiesis in the bone marrow. In the spleen, various types of stromal cells and endothelial cells contribute to B lymphocyte maturation. More recently, comprehensive single cell RNA-seq studies have also been used to identify clusters of stromal cell types in the bone marrow and spleen, which will aid in further identifying key regulators of B lymphopoiesis. Here, we review the different types of microenvironment cells and key extrinsic regulators that are known to be involved in the regulation of murine B lymphopoiesis in the bone marrow and spleen.

The role of Serpina3n in the reversal effect of ATRA on dexamethasone-inhibited osteogenic differentiation in mesenchymal stem cells

Background

Glucocorticoid-induced osteoporosis (GIOP) is the most common secondary osteoporosis. Patients with GIOP are susceptible to fractures and the subsequent delayed bone union or nonunion. Thus, effective drugs and targets need to be explored. In this regard, the present study aims to reveal the possible mechanism of the anti-GIOP effect of all-trans retinoic acid (ATRA).

Methods

Bone morphogenetic protein 9 (BMP9)-transfected mesenchymal stem cells (MSCs) were used as an in vitro osteogenic model to deduce the relationship between ATRA and dexamethasone (DEX). The osteogenic markers runt-related transcription factor 2 (RUNX2), alkaline phosphatase (ALP), and osteopontin were detected using real-time quantitative polymerase chain reaction, Western blot, and immunofluorescent staining assay. ALP activities and matrix mineralization were evaluated using ALP staining and Alizarin Red S staining assay, respectively. The novel genes associated with ATRA and DEX were detected using RNA sequencing (RNA-seq). The binding of the protein–DNA complex was validated using chromatin immunoprecipitation (ChIP) assay. Rat GIOP models were constructed using intraperitoneal injection of dexamethasone at a dose of 1 mg/kg, while ATRA intragastric administration was applied to prevent and treat GIOP. These effects were evaluated based on the serum detection of the osteogenic markers osteocalcin and tartrate-resistant acid phosphatase 5b, histological staining, and micro-computed tomography analysis.

Results

ATRA enhanced BMP9-induced ALP, RUNX2 expressions, ALP activities, and matrix mineralization in mouse embryonic fibroblasts as well as C3H10T1/2 and C2C12 cells, while a high concentration of DEX attenuated these markers. When DEX was combined with ATRA, the latter reversed DEX-inhibited ALP activities and osteogenic markers. In vivo analysis showed that ATRA reversed DEX-inhibited bone volume, bone trabecular number, and thickness. During the reversal process of ATRA, the expression of retinoic acid receptor beta (RARβ) was elevated. RARβ inhibitor Le135 partly blocked the reversal effect of ATRA. Meanwhile, RNA-seq demonstrated that serine protease inhibitor, clade A, member 3N (Serpina3n) was remarkably upregulated by DEX but downregulated when combined with ATRA. Overexpression of Serpina3n attenuated ATRA-promoted osteogenic differentiation, whereas knockdown of Serpina3n blocked DEX-inhibited osteogenic differentiation. Furthermore, ChIP assay revealed that RARβ can regulate the expression of Serpina3n.

Conclusion

ATRA can reverse DEX-inhibited osteogenic differentiation both in vitro and in vivo, which may be closely related to the downregulation of DEX-promoted Serpina3n. Hence, ATRA may be viewed as a novel therapeutic agent, and Serpina3n may act as a new target for GIOP.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02347-0.

Activation of creER recombinase in the mouse calvaria induces local recombination without effects on distant skeletal segments

Conditional creER-mediated gene inactivation or gene induction has emerged as a robust tool for studying gene functions in mouse models of tissue development, homeostasis, and regeneration. Here, we present a method to conditionally induce cre recombination in the mouse calvarial bone while avoiding systemic recombination in distal bones. To test our method, we utilized Prx1creER-egfp;td-Tomato mice and delivered 4-hydroxytamoxifen (4-OHT) to the mouse calvaria, subperiosteally. First, we showed that two calvaria subperiosteal injections of 10 µg of 4-OHT (3.3 mg of 4-OHT/kg of body weight) can induce local recombination as efficiently as two intraperitoneal systemic injections of 200 μg of tamoxifen (70 mg of tamoxifen/kg of body weight). Then, we studied the recombination efficiency of various subperiosteal calvaria dosages and found that two subperiosteal injections of 5 µg 4-OHT (1.65 mg of 4-OHT/kg of body weight) uphold the same recombination efficiency observed with higher dosages. Importantly, the result indicated that the low dosage does not induce significant systemic recombination in remote skeletal tissues. With the proposed local low dosage protocol, the recombination efficiency at the injection site (calvarial bone) reached 94%, while the recombination efficiency at the mandible and the digits was as low as the efficiency measured in control animals.

The characterization of distinct populations of murine skeletal cells that have different roles in B lymphopoiesis

Hematopoiesis is extrinsically controlled by cells of the bone marrow microenvironment, including skeletal lineage cells. The identification and subsequent studies of distinct subpopulations of maturing skeletal cells is currently limited due to a lack of methods to isolate these cells. We found that murine Lineage-CD31-Sca-1-CD51+ cells can be divided into four subpopulations using flow cytometry, based on their expression of the platelet derived growth factor receptors ⍺ and β (PDGFR⍺ and PDGFRβ). The use of different skeletal lineage reporters confirmed the skeletal origin of the four populations. Multiplex immunohistochemistry studies revealed that all four populations were localized near the growth plate and trabecular bone and were rarely found near cortical bone regions or in central bone marrow. Functional studies revealed differences in their abundance, colony-forming unit-fibroblast capacity and potential to differentiate into mineralized osteoblasts or adipocytes in vitro. Furthermore, the four populations had distinct gene expression profiles and differential cell surface expression of leptin receptor (LEPR) and vascular cell adhesion molecule 1 (VCAM-1). Interestingly, we discovered that one of these four different skeletal populations showed the highest expression of genes involved in the extrinsic regulation of B lymphopoiesis. This cell population varied in abundance between distinct hematopoietically active skeletal sites, and significant differences in the proportions of B lymphocyte precursors were also observed in these distinct skeletal sites. It also supported pre-B lymphopoiesis in culture. Our method to isolate four distinct maturing skeletal populations will assist in elucidating the roles of distinct skeletal niche cells in regulating hematopoiesis and bone.

Adult mesenchymal stem cells and their exosomes: Sources, characteristics, and application in regenerative medicine

Human mesenchymal stem cells (MSCs) have become a hot topic in the development of cell therapies and bioengineering. All kinds of MSCs are genomic stable and have the self-renewal ability. Main sources of MSCs are bone marrow, adipose tissues, umbilical cord and placental tissues. MSCs can be cultured in many undifferentiated passages to grow into more specialized cells, produce secretory factors and also support trophic functions in the body. Exosomes, derived from MSCs also have great potential in regenerative medicine and tissue engineering. Exosomes are secreted by MSCs and have the same therapeutic potential as their parent cells. MSCs and their exosomes combined with biomaterials can also be more effective in promoting the regeneration of tissues and organs. However, for use of MSCs-exosomes as a clinical agent different MSCs-exosomes have been manufactured and their therapeutics effects demonstrated in clinical studies. But there are still many characteristics which are unknown and many barriers still need to be conquered. In this review, we not only highlighted the characteristics of human MSCs and their exosomes, but also provided their latest therapeutic strategies in regenerative medicine.

Effects of chemotherapy agents used to treat pediatric acute lymphoblastic leukemia patients on bone parameters and longitudinal growth of juvenile mice

Acute lymphoblastic leukemia (ALL) is the most common childhood cancer. Therapies for pediatric ALL have improved such that more than 80% of patients survive to 5 years post-therapy, and most survive to adulthood. These ALL patients experience long-term side effects that permanently affect their quality of life, with bone loss and reduced longitudinal growth being the most common skeletal complications. To determine the effects of the chemotherapeutic agents used in ALL induction therapy on bone density and longitudinal growth in mice, we treated juvenile mice with doxorubicin, dexamethasone, vincristine, l-asparaginase, or combination therapy. At adulthood, mice were culled and bones collected and scanned by micro-computed tomography (micro-CT). Mice that received doxorubicin and combination therapy exhibited reduced longitudinal growth and significant reductions in trabecular bone volume, trabecular thickness, and trabecular number, with increased trabecular separation. Mean cortical thickness, cortical area, marrow area, endocortical perimeter, and polar moment of inertia were significantly reduced by doxorubicin and combination therapy. Vincristine treatment significantly decreased trabecular bone volume, trabecular number, and increased trabecular separation but had no effects on cortical bone. Dexamethasone treatment increased trabecular bone separation, cortical marrow area, and cortical bone periosteal perimeter. Mice treated with l-asparaginase did not have any bone phenotypes. In conclusion, these data indicate that the majority of the chemotherapy agents used in induction therapy for pediatric ALL have long-term effects on bone in mice. A single dose of doxorubicin in juvenile mice was sufficient to cause the majority of the bone phenotypes, with combination therapy intensifying these effects.

The Antagonist of Retinoic Acid Receptor α, ER-50891 Antagonizes the Inhibitive Effect of All-Trans Retinoic Acid and Rescues Bone Morphogenetic Protein 2-Induced Osteoblastogenic Differentiation

Background

Hypervitaminosis A, alcoholism or medical treatment for acute promyelocytic leukaemia may cause unphysiologically high accumulation of all-trans retinoic acid (ATRA), which could inhibit osteoblastogenesis, thereby triggering osteoporosis. We have shown that bone morphogenetic protein-2 (BMP-2) can only partially antagonize the inhibitive effects of ATRA. In this study, we hypothesized that antagonists of retinoic acid receptors (RARs) could further antagonize the inhibitive effect of ATRA and rescue BMP2-induced osteoblastogenesis.

Materials and Methods

We first screened the dose-dependent effects of the specific antagonists of RAR α, β and γ and transforming growth factor-beta receptor (ER-50891, LE-135, MM11253, and SB-43142, respectively) on ATRA-induced inhibition of the total cell metabolic activity and proliferation of preosteoblasts. We selected ER-50891 and tested its effects on osteoblastogenesis with the presence or absence of 1 μM ATRA and/or 200 ng/mL BMP-2. We measured the following parameters: Alkaline phosphatase activity (ALP), osteocalcin (OCN) expression and extracellular matrix mineralization as well as the level of phosphorylated Smad1/5.

Results

ER-50891 but not LE-135, MM11253, or SB-431542 significantly antagonized the inhibition of ATRA and enhanced the total cell metabolic activity and proliferation of preosteoblasts. Dose-dependent assays show ER-50891 could also rescue ATRA inhibited OCN expression and mineralization with or without the induction of BMP. ER-50891 also suppressed the ALP activity that was synergistically enhanced by BMP and ATRA. Neither ATRA, nor ER-50891 or their combination significantly affected the level of BMP-induced phosphorylated Smad1/5.

Conclusion

The antagonist of RARα, ER-50891 could significantly attenuate ATRA’s inhibitive effects on BMP 2-induced osteoblastogenesis.

TGFβ Inhibition Stimulates Collagen Maturation to Enhance Bone Repair and Fracture Resistance in a Murine Myeloma Model

Multiple myeloma is a plasma cell malignancy that causes debilitating bone disease and fractures, in which TGFβ plays a central role. Current treatments do not repair existing damage and fractures remain a common occurrence. We developed a novel low tumor phase murine model mimicking the plateau phase in patients as we hypothesized this would be an ideal time to treat with a bone anabolic. Using in vivo μCT we show substantial and rapid bone lesion repair (and prevention) driven by SD-208 (TGFβ receptor I kinase inhibitor) and chemotherapy (bortezomib and lenalidomide) in mice with human U266-GFP-luc myeloma. We discovered that lesion repair occurred via an intramembranous fracture repair-like mechanism and that SD-208 enhanced collagen matrix maturation to significantly improve fracture resistance. Lesion healing was associated with VEGFA expression in woven bone, reduced osteocyte-derived PTHrP, increased osteoblasts, decreased osteoclasts, and lower serum tartrate-resistant acid phosphatase 5b (TRACP-5b). SD-208 also completely prevented bone lesion development in mice with aggressive JJN3 tumors, and was more effective than an anti-TGFβ neutralizing antibody (1D11). We also discovered that SD-208 promoted osteoblastic differentiation (and overcame the TGFβ-induced block in osteoblastogenesis) in myeloma patient bone marrow stromal cells in vitro, comparable to normal donors. The improved bone quality and fracture-resistance with SD-208 provides incentive for clinical translation to improve myeloma patient quality of life by reducing fracture risk and fatality. © 2019 American Society for Bone and Mineral Research.

RARG Gene Dysregulation in Acute Myeloid Leukemia

Retinoic acid receptor γ (RARγ) belongs to the nuclear receptor superfamily and shares 90% homology with retinoic acid receptor α (RARα) and retinoic acid receptor β (RARβ). RARA rearrangements are well-known to be involved in acute promyelocytic leukemia (APL), but RARG rearrangements can also resemble this kind of leukemia. In this review we trace the role of RARγ, considering both its physiological and oncogenic contribution; from 2011 to date, nine cases of patients harboring RARG fusions have been reported. These patients showed typical APL features, including the clinical presentation, coagulation abnormalities and morphological features of bone marrow (BM), but are not responsive to APL standard therapy. We stress the urgent need for a better comprehension of the critical role of RARG dysregulation in the leukemogenesis process, since optimum therapy strategies have not yet been established.

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.

The Pleiotropic Role of Retinoic Acid/Retinoic Acid Receptors Signaling: From Vitamin A Metabolism to Gene Rearrangements in Acute Promyelocytic Leukemia

The family of retinoic acid receptors (RARs: RARα, -β, and -γ) has remarkable pleiotropy characteristics, since the retinoic acid/RARs pathway is involved in numerous biological processes not only during embryonic development, but also in the postnatal phase and during adulthood. In this review, we trace the roles of RA/RARs signaling in the immune system (where this pathway has both an immunosuppressive role or is involved in the inflammatory response), in hematopoiesis (enhancing hematopoietic stem cell self-renewal, progenitor cells differentiation or maintaining the bone marrow microenvironment homeostasis), and in bone remodeling (where this pathway seems to have controversial effects on bone formation or osteoclast activation). Moreover, in this review is shown the involvement of RAR genes in multiple chromosomal rearrangements generating different fusion genes in hematological neoplasms, with a particular focus on acute promyelocytic leukemia and its variant subtypes. The effect of different RARs fusion proteins on leukemic transformation, on patients’ outcome, and on therapy response is also discussed.

Vitamin A decreases the anabolic bone response to mechanical loading by suppressing bone formation

Increased vitamin A consumption is associated with decreased cortical bone mass and increased fracture risk in humans. Rodent studies have demonstrated that hypervitaminosis A increases cortical bone resorption, whereas the importance of the effects on bone formation is less well defined. We used an experimental model of increased bone formation by loading of the tibiae to investigate the effect of vitamin A on bone formation. Control [retinol activity equivalents (RAE) 4.5 µg/g chow] or vitamin A (RAE 60 µg/g chow) diets were given to female C57BL/6N mice for 4 wk, after which the tibiae were subjected to axial loading on alternate days for 2 wk, while the diets were continued. Vitamin A inhibited the loading-induced increase in trabecular and cortical bone volume. This was attributed to inhibition of loading-induced increase in osteoblast number and activity, and expression of osteoblastic genes Sp7, Alpl, and Col1a1 in cortical bone. Vitamin A, loading, and combination thereof also resulted in site-specific effects on bone composition measured by Raman spectroscopy. In summary, a clinically relevant dose of vitamin A suppresses the loading-induced gain of bone mass by decreasing bone formation. These observations may have implications for regulation of bone mass caused by physical activity and the risk of osteoporosis in humans.-Lionikaite, V., Henning, P., Drevinge, C., Shah, F. A., Palmquist, A., Wikström, P., Windahl, S. H., Lerner, U. H. Vitamin A decreases the anabolic bone response to mechanical loading by suppressing bone formation.