Serotonergic 5-HT(2B) receptor controls tissue-nonspecific alkaline phosphatase activity in osteoblasts via eicosanoids and phosphatidylinositol-specific phospholipase C

Antipsychotic-induced bone loss: the role of dopamine, serotonin and adrenergic receptor signalling

Antipsychotics are commonly used in treating psychiatric disorders. These medications primarily target dopamine the serotonin receptors, they have some affinity to adrenergic, histamine, glutamate and muscarinic receptors. There is clinical evidence that antipsychotic use decreases BMD and increases fracture risk, with dopamine, serotonin and adrenergic receptor-signalling becoming an increasing area of focus where the presence of these receptors in osteoclasts and osteoblasts have been demonstrated. Osteoclasts and osteoblasts are the most important cells in the bone remodelling and the bone regeneration process where the activity of these cells determine the bone resorption and formation process in order to maintain healthy bone. However, an imbalance in osteoclast and osteoblast activity can lead to decreased BMD and increased fracture risk, which is also believed to be exacerbated by antipsychotics use. Therefore, the aim of this review is to provide an overview of the mechanisms of action of first, second and third generation antipsychotics and the expression profiles of dopamine, serotonin and adrenergic receptors during osteoclastogenesis and osteoblastogenesis.

Calcium-Dependent Cytosolic Phospholipase A2α as Key Factor in Calcification of Subdermally Implanted Aortic Valve Leaflets

Calcium-dependent cytosolic phospholipase A2α (cPLA2α) had been previously found to be overexpressed by aortic valve interstitial cells (AVICs) subjected to in vitro calcific induction. Here, cPLA2α expression was immunohistochemically assayed in porcine aortic valve leaflets (iAVLs) that had undergone accelerated calcification subsequent to 2- to 28-day-long implantation in rat subcutis. A time-dependent increase in cPLA2α-positive AVICs paralleled mineralization progression depending on dramatic cell membrane degeneration with the release of hydroxyapatite-nucleating acidic lipid material, as revealed by immunogold particles decorating organelle membranes in 2d-iAVLs, as well as membrane-derived lipid byproducts in 7d- to 28d-iAVLs. Additional positivity was detected for (i) pro-inflammatory IL-6, mostly exhibited by rat peri-implant cells surrounding 14d- and 28d-iAVLs; (ii) calcium-binding osteopontin, with time-dependent increase and no ossification occurrence; (iii) anti-calcific fetuin-A, mostly restricted to blood plasma within vessels irrorating the connective envelopes of 28d-iAVLs; (iv) early apoptosis marker annexin-V, limited to sporadic AVICs in all iAVLs. No positivity was found for either apoptosis executioner cleaved caspase-3 or autophagy marker MAP1. In conclusion, cPLA2α appears to be a factor characterizing AVL calcification concurrently with a distinct still uncoded cell death form also in an animal model, as well as a putative target for the prevention and treatment of calcific valve diseases.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Serotonin receptor type 2B activation augments TNF-α-induced matrix mineralization in murine valvular interstitial cells

Calcification, fibrosis, and chronic inflammation are the predominant features of calcific aortic valve disease, a life-threatening condition. Drugs that induce serotonin (5-hydroxytryptamine [5-HT]) are known to damage valves, and activated platelets, which carry peripheral serotonin, are known to promote calcific aortic valve stenosis. However, the role of 5-HT in valve leaflet pathology is not known. We tested whether serotonin mediates inflammation-induced matrix mineralization in valve cells. Real-time reverse transcription-polymerase chain reaction analysis showed that murine aortic valve interstitial cells (VICs) expressed both serotonin receptor types 2A and 2B (Htr2a and Htr2b). Although Htr2a expression was greater at baseline, Htr2b expression was induced several-fold more than Htr2a in response to the pro-calcific tumor necrosis factor-α (TNF-α) treatment. 5-HT also augmented TNF-α-induced osteoblastic differentiation and matrix mineralization of VIC, but 5-HT alone had no effects. Inhibition of serotonin receptor type 2B, using specific inhibitors or lentiviral knockdown in VIC, attenuated 5-HT effects on TNF-α-induced osteoblastic differentiation and mineralization. 5-HT treatment also augmented TNF-α-induced matrix metalloproteinase-3 expression, which was also attenuated by Htr2b knockdown. Htr2b expression in aortic roots and serum levels of peripheral 5-HT were also greater in the hyperlipidemic Apoe^-/- mice than in control normolipemic mice. These findings suggest a new role for serotonin signaling in inflammation-induced calcific valvulopathy.

Critical Involvement of Calcium-Dependent Cytosolic Phospholipase A2α in Aortic Valve Interstitial Cell Calcification

The involvement of calcium-dependent cytosolic phospholipase A2α (cPLA2α) in aortic valve calcification is not exhaustively elucidated. Here, cPLA2α expression in aortic valve interstitial cell (AVIC) pro-calcific cultures simulating either metastatic or dystrophic calcification was estimated by qPCR, Western blotting, and counting of cPLA2α-immunoreactive cells, with parallel ultrastructural examination of AVIC calcific degeneration. These evaluations also involved pro-calcific AVIC cultures treated with cPLA2α inhibitor dexamethasone. cPLA2α over-expression resulted for both types of pro-calcific AVIC cultures. Compared to controls, enzyme content was found to increase by up to 300% and 186% in metastatic and dystrophic calcification-like cultures, respectively. Increases in mRNA amounts were also observed, although they were not as striking as those in enzyme content. Moreover, cPLA2α increases were time-dependent and strictly associated with mineralization progression. Conversely, drastically lower levels of enzyme content resulted for the pro-calcific AVIC cultures supplemented with dexamethasone. In particular, cPLA2α amounts were found to decrease by almost 88% and 48% in metastatic and dystrophic calcification-like cultures, respectively, with mRNA amounts showing a similar trend. Interestingly, these drastic decreases in cPLA2α amounts were paralleled by drastic decreases in mineralization degrees, as revealed ultrastructurally. In conclusion, cPLA2α may be regarded as a crucial co-factor contributing to AVIC mineralization in vitro, thus being an attractive potential target for designing novel therapeutic strategies aimed to counteract onset or progression of calcific aortic valve diseases.

Modulation of osteoblast differentiation and bone mass by 5-HT2A receptor signaling in mice

Recent studies reported that serotonin (5-hydroxytryptamine, 5-HT) may be an endogenous paracrine and/or autocrine factor that is used for intercellular communication in bone cells and between multiple organs regulating bone homeostasis. In the present study, we showed that the administration of MDL11939, a selective 5-HT2A receptor antagonist, reduced bone mass in mice. The loss of bone mass in MDL11939-treated mice was associated with impaired bone formation in vivo, as demonstrated by the lower expression of osterix (Osx) and osteocalcin than that in vehicle-treated mice. On the other hand, no significant differences were observed in osteoclast numbers between MDL11939- and vehicle-treated mice. The pharmacological blockade of 5-HT2A receptor signaling significantly decreased alkaline phosphatase activity in osteoblastic cells. In addition, the knockdown of the 5-HT2A receptor by a siRNA treatment decreased Osx, but not Runx2 gene expression in MC3T3-E1 cells. These results suggest that 5-HT2A receptor signaling mediated bone mass by regulating osteoblast differentiation.

Tissue-nonspecific alkaline phosphatase as a target of sFRP2 in cardiac fibroblasts

Recent studies of myocardial infarction in secreted Frizzled-related protein 2 (sFRP2) knockout mice and our hamster heart failure therapy based on sFRP2 blockade have established sFRP2 as a key profibrotic cytokine in the heart. The failing hamster heart is marked by prominent fibrosis and calcification with elevated expression of sFRP2. Noting the involvement of tissue-nonspecific alkaline phosphatase (TNAP) in bone mineralization and vascular calcification, we determined whether sFRP2 might be an upstream regulator of TNAP. Biochemical assays revealed an approximately twofold increase in the activity of TNAP and elevated levels of inorganic phosphate (Pi) in the failing heart compared with the normal heart. Neither was this change detected in the liver or hamstring muscle nor was it associated with systemic hyperphosphatemia. TNAP was readily cloned from the hamster heart and upon overexpression increased the level of extracellular but not intracellular Pi, which is consistent with the cell surface location of the ectoenzyme. In line with the previous demonstration that sFRP2 blockade attenuated fibrosis, we show here that the therapy downregulated TNAP. This in vivo finding is corroborated by the in vitro study showing that cultured cardiac fibroblasts treated with recombinant sFRP2 protein exhibited progressive increase in the expression and activity of TNAP, which was completely abrogated by cycloheximide or tunicamycin. Induction of TNAP by sFRP2 is restricted to cardiac fibroblasts among the multiple cell types examined, and was not observed with sFRP4. The current work indicates that sFRP2 may promote cardiac fibrocalcification through coordinate activation of tolloid-like metalloproteinases and TNAP.

Comparative studies between mice molars and incisors are required to draw an overview of enamel structural complexity

In the field of dentistry, the murine incisor has long been considered as an outstanding model to study amelogenesis. However, it clearly appears that enamel from wild type mouse incisors and molars presents several structural differences. In incisor, exclusively radial enamel is observed. In molars, enamel displays a high level of complexity since the inner part is lamellar whereas the outer enamel shows radial and tangential structures. Recently, the serotonin 2B receptor (5-HT_2BR) was shown to be involved in ameloblast function and enamel mineralization. The incisors from 5HT_2BR knockout (KO) mice exhibit mineralization defects mostly in the outer maturation zone and porous matrix network in the inner zone. In the molars, the mutation affects both secretory and maturation stages of amelogenesis since pronounced alterations concern overall enamel structures. Molars from 5HT_2BR KO mice display reduction in enamel thickness, alterations of inner enamel architecture including defects in Hunter-Schreger Bands arrangements, and altered maturation of the outer radial enamel. Differences of enamel structure were also observed between incisor and molar from other KO mice depleted for genes encoding enamel extracellular matrix proteins. Thus, upon mutation, enamel analysis based exclusively on incisor defects would be biased. In view of the functional relationship between enamel structure and tooth morphogenesis, identification of molecular actors involved in amelogenesis requires comparative studies between mice molars and incisors.

The expression of groups IIE and V phospholipase A2 is associated with an increased expression of osteogenic molecules in human calcified aortic valves

AIM

Eicosanoids play various pathogenic roles in aortic valve calcification. Eicosanoids are derived from the arachidonic acid generated by phospholipase A2 (PLA2). We therefore sought to determine whether PLA2s are expressed in human aortic valves and, if so, whether the expression of PLA2s is related to the expression of osteogenic molecules in these tissues.

METHODS

Histological and gene expression analyses of 38 non-rheumatic aortic valves obtained at the time of cardiac valve replacement surgery were conducted. Moreover, gene expression analyses were performed using valve interstitial cells (VICs) obtained from human aortic valves.

RESULTS

Among the PLA2s examined, the degree of immunoreactivity for PLA2s-IIE and -V was found to significantly correlate with the grade of calcification in the aortic valves. The degree of immunoreactivity and gene expression levels of PLA2s-IIE and -V significantly correlated with those of bone morphogenetic protein (BMP)-2, osteopontin and alkaline phosphatase (ALP). In addition, immunoreactivity for cyclooxygenase (COX)-1, COX-2 and 5-lipoxygenase, downstream enzymes of PLA2 in the arachidonic acid cascade, was co-localized with that for PLA2s-IIE and -V in cells expressing α-smooth muscle actin and macrophages expressing CD68. Furthermore, in the in vitro experiments using cultured VICs, the mRNA expression levels of BMP-2, osteopontin and ALP were suppressed by the inhibition of the expression of PLA2s-IIE or -V with specific siRNAs.

CONCLUSIONS

The expression of PLA2s-IIE and -V correlates with the development of calcification as well as the expression of pro-osteogenic molecules in human aortic valves, and inhibiting the expression of PLA2s-IIE and -V suppresses the induction of osteogenic molecules in cultured cells. Therefore, PLA2s-IIE and -V may play a role in the pathogenesis of valve calcification.

Serotonin regulates osteoblast proliferation and function in vitro

The monoamine serotonin (5-hydroxytryptamine, 5-HT), a well-known neurotransmitter, also has important functions outside the central nervous system. The objective of this study was to investigate the role of 5-HT in the proliferation, differentiation, and function of osteoblasts in vitro. We treated rat primary calvarial osteoblasts with various concentrations of 5-HT (1 nM to 10 µM) and assessed the rate of osteoblast proliferation, expression levels of osteoblast-specific proteins and genes, and the ability to form mineralized nodules. Next, we detected which 5-HT receptor subtypes were expressed in rat osteoblasts at different stages of osteoblast differentiation. We found that 5-HT could inhibit osteoblast proliferation, differentiation, and mineralization at low concentrations, but this inhibitory effect was mitigated at relatively high concentrations. Six of the 5-HT receptor subtypes (5-HT_1A, 5-HT_1B, 5-HT_1D, 5-HT_2A, 5-HT_2B, and 5-HT_2C) were found to exist in rat osteoblasts. Of these, 5-HT_2A and 5-HT_1B receptors had the highest expression levels, at both early and late stages of differentiation. Our results indicated that 5-HT can regulate osteoblast proliferation and function in vitro.

Antagonism of serotonin receptor 1B decreases viability and promotes apoptosis in the COS canine osteosarcoma cell line

Serotonin receptor 1B (5HTR1B) traditionally exhibits anti-proliferative activity in osteoblasts. We examined the expression and function of 5HTR1B in the COS canine osteosarcoma cell line and normal canine osteoblasts. Equal levels of 5HTR1B gene and protein expression were found between normal and malignant osteoblasts. Treatment with serotonin enhanced viability of osteosarcoma cells but not normal osteoblasts. Challenge with the 5HTR1B agonist anpirtoline caused no change in cell viability. Rather incubation with the specific receptor antagonist SB224289 caused reduction in osteoblast viability, with this effect more substantial in osteosarcoma cells. Investigation of this inhibitory activity showed 5HTR1B antagonism induces apoptosis in malignant cells. Evaluation of phosphorylated levels of CREB and ERK, transcriptional regulators associated with serotonin receptor signalling in osteoblasts, revealed aberrant 5HTR1B signalling in COS. Our results confirm the presence of 5HTR1B in a canine osteosarcoma cell line and highlight this receptor as a possible novel therapeutic target.

Deletion of serotonin 2B receptor provokes structural alterations of mouse dental tissues

Rampant caries and periodontal diseases occur in patients treated with antidepressants such as serotonin reuptake inhibitors (SRIs; e.g., Prozac) which target the serotonin transporter (SERT). As the serotonin 2B receptor (5HT2BR) regulates SERT functionality and capacity to recognize SRIs, we investigated the potential role of 5HT2BR on dental tissues by exploiting 5HT2BR knockout (KO) mice. Compared to wild-type (WT) mice, several structural differences were identified in the teeth of KO mice. In the molar of KO mice, rod curvatures and twisting were altered compared to WT mice, suggesting involvement of 5HT2BR at early stages of enamel formation. The volume of the KO enamel layer was also reduced, and larger porosities were observed in the prismatic enamel, with smaller crystallite thickness. Crystallite pattern disorganization and occlusal abrasion were enhanced in female KO mice, indicating a sexual dimorphism. In the incisor, no difference was detected in the width of the enamel layer between KO and WT mice; however, enamel maturation differed in absence of 5HT2BR. Specifically, the outer aprismatic enamel border was 1.5- to 2-fold larger in KO compared to WT mice, together with a decreased etching pattern. Finally, although no noticeable difference was observed in dentin, the micro-CT three-dimensional pulp reconstruction evidenced a decrease in both length and width of dentin formation in the root canals of the KO versus WT mice. These data provide evidence that 5HT2BR-mediated signaling pathways are involved in enamel formation and dentinogenesis.

Serotonin modulation of macrophage polarization: inflammation and beyond

Macrophages display a ample plethora of effector functions whose acquisition is promoted by the surrounding cytokine and cellular environment. Depending on the stimulus, macrophages become specialized ("polarized") for either pathogen elimination, tissue repair and wound healing or immunosuppression. This "polarization" versatility allows macrophages to critically contribute to tissue homeostasis, as they promote initiation and resolution of inflammatory responses. As a consequence, deregulation of the tissue macrophage polarization balance is an etiological agent of chronic inflammation, autoimmune diseases, cancer and even obesity and insulin resistance. In the present review we describe current concepts on the molecular basis and the patho-physiological implications of macrophage polarization, and describe its modulation by serotonin (5-HT), a neurotransmitter that regulates inflammation and tissue repair via a large set of receptors (5-HTR1-7). 5-HT modulates the phenotypic and functional polarization of macrophages, and contributes to the maintenance of an anti-inflammatory state mainly via 5-HTR2B and 5-HTR7, whose activation has a great impact on macrophage gene expression profile. The identification of 5-HTR2B and 5-HTR7 as functionally-relevant polarization markers suggests their therapeutic value in inflammatory pathologies as well as their potential involvement in linking the immune and nervous systems.

The expression and role of serotonin receptor 5HTR2A in canine osteoblasts and an osteosarcoma cell line

Background

The significance of the serotonergic system in bone physiology and, more specifically, the importance of the five hydroxytryptamine receptor 2A (5HTR2A) in normal osteoblast proliferation have been previously described; however the role of serotonin in osteosarcoma remains unclear. Particularly, the expression and function of 5HTR2A in canine osteosarcoma has not yet been studied, thus we sought to determine if this indoleamine modulates cellular proliferation in vitro. Using real time quantitative reverse transcription PCR and immunoblot analyses, we explored receptor expression and signaling differences between non-neoplastic canine osteoblasts (CnOb) and an osteosarcoma cell line (COS). To elucidate specific serotonergic signaling pathways triggered by 5HTR2A, we performed immunoblots for ERK and CREB. Finally, we compared cell viability and the induction of apoptosis in the presence 5HTR2A agonists and antagonists.

Results

5HTR2A was overexpressed in the malignant cell line in comparison to normal cells. In CnOb cells, ERK phosphorylation (ERK-P) decreased in response to both serotonin and a specific 5HTR2A antagonist, ritanserin. In contrast, ERK-P abundance increased in COS cells following either treatment. While endogenous CREB was undetectable in CnOb, CREB was observed constitutively in COS, with expression and exhibited increased CREB phosphorylation following escalating concentrations of ritanserin. To determine the influence of 5HTR2A signaling on cell viability we challenged cells with ritanserin and serotonin. Our findings confirmed that serotonin treatment promoted cell viability in malignant cells but not in normal osteoblasts. Conversely, ritanserin reduced cell viability in both the normal and osteosarcoma cells. Further, ritanserin induced apoptosis in COS at the same concentrations associated with decreased cell viability.

Conclusions

These findings confirm the existence of a functional 5HTR2A in a canine osteosarcoma cell line. Results indicate that intracellular second messenger signal coupling of 5HTR2A is different between normal and malignant cells, warranting further research to investigate its potential as a novel therapeutic target for canine osteosarcoma.

Possible mechanisms for the skeletal effects of antipsychotics in children and adolescents

The increasing use of antipsychotics (APs) to treat pediatric psychiatric conditions has led to concerns over the long-term tolerability of these drugs. While the risk of cardiometabolic abnormalities has received most of the attention, preclinical and clinical studies provide preliminary evidence that APs can adversely impact bone metabolism. This would be most concerning in children and adolescents as suboptimal bone accrual during development may lead to increased fracture risk later in life. However, the potential mechanisms of action through which APs may impact bone turnover and, consequently, bone mineral content are not clear. Emerging data suggest that the skeletal effects of APs are complex, with APs directly and indirectly impacting bone cells through modulation of multiple signaling pathways, including those involving dopamine D2, serotonin, adrenergic, and prolactin receptors, as well as by affecting gonadotropins. Determining the action of APs on skeletal development is further complicated by polypharmacy. In children and adolescents, APs are frequently coprescribed with psychostimulants and selective serotonin reuptake inhibitors, which have also been linked to changes in bone metabolism. This review discusses the mechanisms by which APs may influence bone metabolism. Also covered are preclinical and pediatric findings concerning the impact of APs on bone turnover. However, the dearth of clinical information despite the potential public health significance of this issue underscores the need for further studies. The review ends with a call for clinicians to be vigilant about promoting optimal overall health in chronically ill youth with psychopathology, particularly when pharmacotherapy is unavoidable.

Phospholipases of mineralization competent cells and matrix vesicles: roles in physiological and pathological mineralizations

The present review aims to systematically and critically analyze the current knowledge on phospholipases and their role in physiological and pathological mineralization undertaken by mineralization competent cells. Cellular lipid metabolism plays an important role in biological mineralization. The physiological mechanisms of mineralization are likely to take place in tissues other than in bones and teeth under specific pathological conditions. For instance, vascular calcification in arteries of patients with renal failure, diabetes mellitus or atherosclerosis recapitulates the mechanisms of bone formation. Osteoporosis—a bone resorbing disease—and rheumatoid arthritis originating from the inflammation in the synovium are also affected by cellular lipid metabolism. The focus is on the lipid metabolism due to the effects of dietary lipids on bone health. These and other phenomena indicate that phospholipases may participate in bone remodelling as evidenced by their expression in smooth muscle cells, in bone forming osteoblasts, chondrocytes and in bone resorbing osteoclasts. Among various enzymes involved, phospholipases A₁ or A₂, phospholipase C, phospholipase D, autotaxin and sphingomyelinase are engaged in membrane lipid remodelling during early stages of mineralization and cell maturation in mineralization-competent cells. Numerous experimental evidences suggested that phospholipases exert their action at various stages of mineralization by affecting intracellular signaling and cell differentiation. The lipid metabolites—such as arachidonic acid, lysophospholipids, and sphingosine-1-phosphate are involved in cell signaling and inflammation reactions. Phospholipases are also important members of the cellular machinery engaged in matrix vesicle (MV) biogenesis and exocytosis. They may favour mineral formation inside MVs, may catalyse MV membrane breakdown necessary for the release of mineral deposits into extracellular matrix (ECM), or participate in hydrolysis of ECM. The biological functions of phospholipases are discussed from the perspective of animal and cellular knockout models, as well as disease implications, development of potent inhibitors and therapeutic interventions.

FOXO1 orchestrates the bone-suppressing function of gut-derived serotonin

Serotonin is a critical regulator of bone mass, fulfilling different functions depending on its site of synthesis. Brain-derived serotonin promotes osteoblast proliferation, whereas duodenal-derived serotonin suppresses it. To understand the molecular mechanisms of duodenal-derived serotonin action on osteoblasts, we explored its transcriptional mediation in mice. We found that the transcription factor FOXO1 is a crucial determinant of the effects of duodenum-derived serotonin on bone formation We identified two key FOXO1 complexes in osteoblasts, one with the transcription factor cAMP-responsive element-binding protein 1 (CREB) and another with activating transcription factor 4 (ATF4). Under normal levels of circulating serotonin, the proliferative activity of FOXO1 was promoted by a balance between its interaction with CREB and ATF4. However, high circulating serotonin levels prevented the association of FOXO1 with CREB, resulting in suppressed osteoblast proliferation. These observations identify FOXO1 as the molecular node of an intricate transcriptional machinery that confers the signal of duodenal-derived serotonin to inhibit bone formation.

Enamel alterations in serotonin 2B receptor knockout mice

The role of the serotonin 2B receptor (5-HT(2B) R) in enamel formation and mineralization was explored in adult 5HT(2B) R knockout (KO) mice compared with wild-type (WT) mice. In the molar, quantitative data obtained by micro-computed tomography imaging showed that the overall volume of the enamel layer was firmly reduced in KO mice. Defective mineralization was ascertained by energy-dispersive X-ray microanalysis. We also observed, using scanning electron microscopy, that parazones in the KO mice included two or three helicoidally twisted rods within Hunter-Schreger bands, instead of a single rod, as found in the WT mice. Minor disturbances were also detected in the incisors of KO mice. Structural modifications, thinner enamel crystallites, and porosities observed in KO mice indicate that the 5-HT(2B) R-mediated signaling pathways as part of the enamel formation process. These data provide a basis for evaluating the role of 5-HT(2B) R in ameloblast functions. Defects observed in the mineralization and structure of enamel in KO mice highlight that the 5-HT(2B) R interferes with the mechanisms directing amelogenesis.

Cannabinoid receptor 1 regulates ERK and GSK-3β-dependent glucocorticoid inhibition of osteoblast differentiation in murine MC3T3-E1 cells

Supraphysiological glucocorticoid administration accelerates loss of survival and differentiation in osteoblastic cells, thereby increasing the risks of osteopenic or osteonecrotic disorders. Neuroendocrine component type 1 cannabinoid receptor (CB1) is found to regulate bone mass. This study characterized the biological role of CB1 in glucocorticoid-induced suppression of osteoblast differentiation. Murine MC3T3-E1 osteoblasts were incubated under osteogenic conditions in the presence or absence of 1 μM glucocorticoid, RNA interference, CB1 antagonist AM251, and agonist WIN55212-2. Cell survival was detected by formazan synthesis and TUNEL staining. Osteoblast differentiation was quantified by mineralized matrix accumulation and expression of the osteogenic factors Runx2 and osteocalcin. Expression of signaling molecules was assessed by immunoblotting. Glucocorticoid increased CB1 expression in association with decreased osteocalcin expression and mineralized nodule deposition. CB1 RNA interference and AM251 attenuated the deleterious actions of glucocorticoid treatment on survival and osteogenic activities, whereas activating CB1 by WIN55212-2 impaired osteoblast differentiation. CB1 signaling regulated JNK, ERK, GSK-3β, and Akt activation as well as Runx2 and IGF-I expression. Inhibition of GSK-3β by the kinase-inactive GSK-3β mutant or activation of ERK by the active MEK-1 mutant abrogated glucocorticoid-induced inhibition of osteoblast differentiation. Glucocorticoid-induced CB1 expression occurred via glucocorticoid receptor-dependent transcriptional and translational regulation. Gain of Runx2 function and loss of MKP-1 action attenuated glucocorticoid-induced enhancement of CB1 expression. Taken together, CB1 regulation of ERK and GSK-3β-dependent pathways participates in glucocorticoid inhibition of Runx2 signaling and osteoblast differentiation. Runx2 reciprocally regulates glucocorticoid-induced promotion of CB1 signaling. Our findings provide new insights into the role of the neuroendocrine component CB1 in glucocorticoid-induced osteoblast dysfunction.