Isolation and culture of rodent osteoprogenitor cells

Extracellular Vesicles Secreted by Tumor Cells Promote the Generation of Suppressive Monocytes

Monocytes are among the first cells to infiltrate the tumor microenvironment. The conversion of monocytes to suppressor cells in the tumor microenvironment is crucial in evasion of the immune response and tumor maintenance. Tumor cells may secrete products that promote the conversion of monocytes to suppressor cells. Cells secrete extracellular vesicles (EVs) containing cargos of genetic materials and proteins as a way to communicate with neighboring cells. During pathologic conditions like cancers, tumor cells increase their EVs production containing microRNA, RNA, and proteins that may affect the immune cell response, contributing to the immunosuppressive microenvironment. Our studies show that EVs secreted by a wide range of murine tumor cells, including osteosarcoma, glioma, colon carcinoma, sarcoma, and melanoma, can be taken up by bone marrow-derived monocytes. The monocytes that took up the EVs secreted by tumor cells matured toward an immune-suppressive phenotype by upregulating the expression of suppressive cytokines and effector molecules. The monocytes also downregulated MHC class II and costimulatory molecules while increasing the expression of PD-L1 on their surface after taking up EVs from tumor cells. Most importantly, monocytes exposed to EVs secreted by tumor cells suppressed activated Ag-specific CD4+ T cells. These results show that tumor cells from several different tumor types secrete EVs which promote the conversion of monocytes into suppressor cells, thus promoting immune evasion. These studies suggest that EVs secreted by tumors are potentially a new target for future cancer therapy.

Effects of inhibiting PDK‑1 expression in bone marrow mesenchymal stem cells on osteoblast differentiation in vitro

Osteoblasts are the main functional cells in bone formation, which are responsible for the synthesis, secretion and mineralization of bone matrix. The PI3K/AKT signaling pathway is strongly associated with the differentiation and survival of osteoblasts. The 3-phosphoinositide-dependent protein kinase-1 (PDK-1) protein is considered the master upstream lipid kinase of the PI3K/AKT cascade. The present study aimed to investigate the role of PDK-1 in the process of mouse osteoblast differentiation in vitro. In the BX-912 group, BX-912, a specific inhibitor of PDK-1, was added to osteoblast induction medium (OBM) to treat bone marrow mesenchymal stem cells (BMSCs), whereas the control group was treated with OBM alone. Homozygote PDK1^flox/flox mice were designed and generated, and were used to obtain BMSCs^{PDK1flox/flox}. Subsequently, an adenovirus containing Cre recombinase enzyme (pHBAd-cre-EGFP) was used to disrupt the PDK-1 gene in BMSCs^{PDK1flox/flox}; this served as the pHBAd-cre-EGFP group and the efficiency of the disruption was verified. Western blot analysis demonstrated that the protein expression levels of phosphorylated (p)-PDK1 and p-AKT were gradually increased during the osteoblast differentiation process. Notably, BX-912 treatment and disruption of the PDK-1 gene with pHBAd-cre-EGFP effectively reduced the number of alkaline phosphatase (ALP)-positive cells and the optical density value of ALP activity, as well as the formation of cell mineralization. The mRNA expression levels of PDK-1 in the pHBAd-cre-EGFP group were significantly downregulated compared with those in the empty vector virus group on days 3–7. The mRNA expression levels of the osteoblast-related genes RUNX2, osteocalcin and collagen I were significantly decreased in the BX-912 and pHBAd-cre-EGFP groups on days 7 and 21 compared with those in the control and empty vector virus groups. Overall, the results indicated that BX-912 and disruption of the PDK-1 gene in vitro significantly inhibited the differentiation and maturation of osteoblasts. These experimental results provided an experimental and theoretical basis for the role of PDK-1 in osteoblasts.

Circadian transcription factor Dbp promotes rat calvarial osteoprogenitors osteogenic differentiation through Kiss1/GnRH/E2 signaling pathway loop

To determine the mechanism by which D-site-binding protein (Dbp) regulates rat calvarial osteoprogenitors (OPCs) osteogenic differentiation. α-Smooth muscle actin (α-SMA) + rat calvarial OPCs were extracted and purified using immunomagnetic beads. Cells were transduced with Dbp-lentivirus and divided into Dbp knockdown, Dbp overexpression and vehicle groups. After osteogenic induction for 21 days, Alizarin red staining and alkaline phosphatase (ALP) activity were examined. Expression levels of Runx2, Ocn, Osterix, Bmp4, Kiss1, and GnRH were determined using a quantitative real-time polymerase chain reaction. The observed changes in Kisspeptin, GnRH, ERα, and Runx2 were further validated via Western blot analysis. Furthermore, E2 and GnRH secretion levels were detected via an enzyme-linked immunosorbent assay (ELISA). Chromatin immunoprecipitation (ChIP) and luciferase assay were used to assess the effects of Dbp on the Kiss1 gene promoter. The coexpression of Dbp and Kisspeptin or GnRH was also evaluated via immunofluorescence. Following osteogenic induction, Dbp overexpression significantly increased calcium nodule formation and ALP activity, as well as Runx2, Ocn, Osterix, Bmp4, Kiss1, and GnRH messenger RNA expression, while Dbp knockdown presented the opposite results. Western blot analysis and ELISA results showed that Dbp significantly promotes Runx2, E2/ERα, Kisspeptin, and GnRH expression. These findings were confirmed by the ChIP assay, which indicated that the estrogen receptor promotes Kisspeptin expression after binding to the Kiss1 gene promoter, which is regulated by Dbp. Immunofluorescence assay showed that Dbp coexpression with Kisspeptin or GnRH varied depending on Dbp expression levels. Collectively, the circadian transcription factor Dbp promotes α-SMA + rat calvarial OPCs osteoblastic differentiation through Kiss1/GnRH/E2 signaling pathway loop.

KMN-159, a novel EP4 receptor selective agonist, stimulates osteoblastic differentiation in cultured whole rat bone marrow

KMN-159 is the lead compound from a series of novel difluorolactam prostanoid EP₄ receptor agonists aimed at inducing local bone formation while avoiding the inherent side effects of systemic EP₄ activation. KMN-159 is a potent, selective small molecule possessing pharmacokinetic properties amenable to local administration. Unfractionated rat bone marrow cells (BMCs) were treated once at plating with escalating doses of KMN-159 (1 pM to 10 μM). The resulting elevated alkaline phosphatase (ALP) levels measured 9 days post-dose are consistent with increased osteoblastic differentiation and exposure to KMN-159 at low nanomolar concentrations for as little as 30 min was sufficient to induce complete osteoblast differentiation of the BMCs from both sexes and regardless of age. ALP induction was blocked by an EP₄ receptor antagonist but not by EP₁ or EP₂ receptor antagonists and was not induced by EP₂ or EP₃ receptor agonists. Addition of BMCs to plates coated with KMN-159 24 days earlier resulted in ALP activation, highlighting the chemical stability of the compound. The expression of phenotype markers such as ALP, type I collagen, and osteocalcin was significantly elevated throughout the osteoblastic differentiation timecourse initiated by KMN-159 stimulation. An increased number of tartrate-resistant acid phosphatase-positive cells was observed KMN-159 or PGE₂ treated BMCs but only in the presence of exogenous receptor activator of nuclear factor kappa-Β ligand (RANKL). No change in the number of adipocytes was observed. KMN-159 also increased bone healing in a rat calvarial defect model with a healing rate equivalent to recombinant human bone morphogenetic protein-2. Our studies show that KMN-159 is able to stimulate osteoblastic differentiation with a very short time of exposure, supporting its potential as a therapeutic candidate for augmenting bone mass.

GGCX and VKORC1 inhibit osteocalcin endocrine functions

Cell-specific gene inactivation experiments delineate the functions of the enzymes required for osteocalcin modification and demonstrate that it is its uncarboxylated form that acts as a hormone.

Osteocalcin (OCN) is an osteoblast-derived hormone favoring glucose homeostasis, energy expenditure, male fertility, brain development, and cognition. Before being secreted by osteoblasts in the bone extracellular matrix, OCN is γ-carboxylated by the γ-carboxylase (GGCX) on three glutamic acid residues, a cellular process requiring reduction of vitamin K (VK) by a second enzyme, a reductase called VKORC1. Although circumstantial evidence suggests that γ-carboxylation may inhibit OCN endocrine functions, genetic evidence that it is the case is still lacking. Here we show using cell-specific gene inactivation models that γ-carboxylation of OCN by GGCX inhibits its endocrine function. We further show that VKORC1 is required for OCN γ-carboxylation in osteoblasts, whereas its paralogue, VKORC1L1, is dispensable for this function and cannot compensate for the absence of VKORC1 in osteoblasts. This study genetically and biochemically delineates the functions of the enzymes required for OCN modification and demonstrates that it is the uncarboxylated form of OCN that acts as a hormone.

Comparative genomics identifies the mouse Bmp3 promoter and an upstream evolutionary conserved region (ECR) in mammals

The Bone Morphogenetic Protein (BMP) pathway is a multi-member signaling cascade whose basic components are found in all animals. One member, BMP3, which arose more recently in evolution and is found only in deuterostomes, serves a unique role as an antagonist to both the canonical BMP and Activin pathways. However, the mechanisms that control BMP3 expression, and the cis-regulatory regions mediating this regulation, remain poorly defined. With this in mind, we sought to identify the Bmp3 promoter in mouse (M. musculus) through functional and comparative genomic analyses. We found that the minimal promoter required for expression in resides within 0.8 kb upstream of Bmp3 in a region that is highly conserved with rat (R. norvegicus). We also found that an upstream region abutting the minimal promoter acts as a repressor of the minimal promoter in HEK293T cells and osteoblasts. Strikingly, a portion of this region is conserved among all available eutherian mammal genomes (47/47), but not in any non-eutherian animal (0/136). We also identified multiple conserved transcription factor binding sites in the Bmp3 upstream ECR, suggesting that this region may preserve common cis-regulatory elements that govern Bmp3 expression across eutherian mammals. Since dysregulation of BMP signaling appears to play a role in human health and disease, our findings may have application in the development of novel therapeutics aimed at modulating BMP signaling in humans.

Cell-derived matrix enhances osteogenic properties of hydroxyapatite

The study aimed to evaluate osteogenic properties of hydroxyapatite (HA) scaffold combined with extracellular matrix (ECM) derived in vitro from rat primary calvarial osteoblasts or dermal fibroblasts. The cellular viability, and the ECM deposited onto synthetic HA microparticles were assessed by MTT, Glycosaminoglycan, and Hydroxyproline assays as well as immunohistochemistry and scanning electron microscopy after 21 days of culture. The decellularized HA-ECM constructs were implanted in critical-sized calvarial defects of Sprague-Dawley rats, followed by bone repair and local inflammatory response assessments by histomorphometry and immunohistochemistry at 12 weeks postoperatively. We demonstrated that HA supported cellular adhesion, growth, and ECM production in vitro, and the HA-ECM constructs significantly enhanced calvarial bone repair (p<0.05, Mann-Whitney U-test), compared with HA alone, despite the significantly increased number of CD68+ macrophages, and foreign body giant cells (p<0.05, Mann-Whitney U-test). Selective accumulation of bone sialoprotein, osteopontin, and periostin was observed at the tissue-HA interfaces. In conclusion, in vitro-derived ECM mimics the native bone matrix, enhances the osteogenic properties of the HA microparticles, and might modulate the local inflammatory response in a bone repair-favorable way. Our findings highlight the ability to produce functional HA-ECM constructs for bone tissue engineering applications.

Insulin signaling in osteoblasts integrates bone remodeling and energy metabolism

The broad expression of the insulin receptor suggests that the spectrum of insulin function has not been fully described. A cell type expressing this receptor is the osteoblast, a bone-specific cell favoring glucose metabolism through a hormone, osteocalcin, that becomes active once uncarboxylated. We show here that insulin signaling in osteoblasts is necessary for whole-body glucose homeostasis because it increases osteocalcin activity. To achieve this function insulin signaling in osteoblasts takes advantage of the regulation of osteoclastic bone resorption exerted by osteoblasts. Indeed, since bone resorption occurs at a pH acidic enough to decarboxylate proteins, osteoclasts determine the carboxylation status and function of osteocalcin. Accordingly, increasing or decreasing insulin signaling in osteoblasts promotes or hampers glucose metabolism in a bone resorption-dependent manner in mice and humans. Hence, in a feed-forward loop, insulin signals in osteoblasts activate a hormone, osteocalcin, that promotes glucose metabolism.