Changes in RANKL/OPG/RANK gene expression in peripheral mononuclear cells following treatment with estrogen or raloxifene

The RANKL/OPG/RANK pathway is the key mediator of osteoclastogenesis. Mononuclear cells may be implicated in post-menopausal osteoporosis. The effect of estrogen or raloxifene on bone resorption and the expression of RANKL/OPG/RANK in peripheral blood mononuclear cells (PBMCs) was examined. Twenty-nine women with post-menopausal osteoporosis were treated with estrogen (HRT) or raloxifene for 12 months. Bone mineral density (BMD) was measured at baseline and at 12 months at the spine and hip. Serum C-terminal telopeptide (CTX) and OPG were measured at baseline and at 1, 3, 6 and 12 months. PBMCs were isolated from 17 women and changes in RANKL, OPG and RANK mRNA were determined. The effects of estrogen or raloxifene in PBMCs in vitro were also assessed. BMD increased following treatment (lumbar spine % change mean [S.E.M.]: 4.3% [0.9], p<0.001). Serum CTX decreased (6 months: -43.7% [6.0], p<0.0001). Serum OPG declined gradually (12 months: -26.4% [4.4], p<0.001). RANKL, OPG and RANK gene expression decreased (6 months: RANKL 50.0% [24.8] p<0.001, OPG: 21.7% [28] p<0.001, RANK: 76.6% [10.2] p=0.015). Changes in OPG mRNA correlated with changes in BMD (r=-0.53, p=0.027) and CTX (r=0.7, p=0.0044). Down-regulation in RANKL, OPG, RANK mRNA and reduction in bone resorption was also seen in vitro. These results suggest that the expression of RANKL/OPG/RANK in PBMCs are responsive to the slowing in bone turnover/remodeling associated with treatment with estrogen or raloxifene. Further confirmatory studies are needed.     

Understanding the local actions of lipids in bone physiology

The adult skeleton is a metabolically active organ system that undergoes continuous remodeling to remove old and/or stressed bone (resorption) and replace it with new bone (formation) in order to maintain a constant bone mass and preserve bone strength from micro-damage accumulation. In that remodeling process, cellular balances – adipocytogenesis/osteoblastogenesis and osteoblastogenesis/osteoclastogenesis – are critical and tightly controlled by many factors, including lipids as discussed in the present review.Interest in the bone lipid area has increased as a result of in vivo evidences indicating a reciprocal relationship between bone mass and marrow adiposity. Lipids in bones are usually assumed to be present only in the bone marrow. However, the mineralized bone tissue itself also contains small amounts of lipids which might play an important role in bone physiology. Fatty acids, cholesterol, phospholipids and several endogenous metabolites (i.e., prostaglandins, oxysterols) have been purported to act on bone cell survival and functions, the bone mineralization process, and critical signaling pathways. Thus, they can be regarded as regulatory molecules important in bone health. Recently, several specific lipids derived from membrane phospholipids (i.e., sphingosine-1-phosphate, lysophosphatidic acid and different fatty acid amides) have emerged as important mediators in bone physiology and the number of such molecules will probably increase in the near future. The present paper reviews the current knowledge about: (1°) bone lipid composition in both bone marrow and mineralized tissue compartments, and (2°) local actions of lipids on bone physiology in relation to their metabolism. Understanding the roles of lipids in bone is essential to knowing how an imbalance in their signaling pathways might contribute to bone pathologies, such as osteoporosis.     

OPG/RANK/RANKL系统与骨质疏松研究最新进展

骨质疏松是严重威胁中老年人健康的骨科常见病,OPG/RANK/RANKL是参与调节骨重建的最重要的分子系统之一,与骨疾病相关的骨质疏松有密切联系,并已成为药物设计的新靶点.因此,对该系统的深入研究将为骨生理、病理机制阐明及骨疾病防治带来积极影响.     

Inhibition effect of enteropeptidase on RANKL–RANK signalling by cleavage of RANK

Enteropeptidase can cleave trypsinogen on the sequence of Asp-Asp-Asp-Asp-Lys and plays an important role in food digestion. The RANKL–RANK signalling pathway plays a pivotal role in bone remodelling. In this study, we reported that enteropeptidase can inhibit the RANKL–RANK signalling pathway through the cleavage of RANK. A surrogate peptide blocking assay indicated that enteropeptidase could specifically cleave RANK on the sequence NEEDK. Osteoclast differentiation assay and NF-κB activity assay confirmed that enteropeptidase could inhibit osteoclastogenesis in vitro through the cleavage of RANK. This is the first study to prove that the RANKL–RANK signalling pathway can be inhibited by cleavage of RANK instead of targeting RANKL.     

Crystal structure of bacterial cell-surface alginate-binding protein with an M75 peptidase motif

Receptor activator of NF-κB (RANK) and RANK ligand (RANKL) are known to play an important role in the development and progression of breast cancer. However, the mechanisms by which stimuli regulate the expression of RANK and RANKL in breast cancer cells are largely unknown. In this study, we show that hypoxia, a common feature of malignant tumors, can enhance the expression of RANK and RANKL mRNA and protein in MDA-MB-231 and MCF-7 breast cancer cells. In addition, we found that hypoxia induced hypoxia-inducible factor-1 alpha (HIF-1α) and phosphorylation of Akt, resulting in upregulation of RANK and RANKL expression; HIF-1α-targeted siRNA and PI3K-Akt inhibitor abrogated this upregulation in MDA-MB-231 cells. Furthermore, we also observed that hypoxia accelerated RANKL-mediated cell migration, which was inhibited following HIF-1α knockdown and PI3K-Akt inhibition. Thus, we provide evidence that hypoxia upregulates RANK and RANKL expression and increases RANKL-induced cell migration via the PI3K/Akt-HIF-1α pathway.     

Early estrogen-induced gene 1, a novel RANK signaling component,is essential for osteoclastogenesis

The receptor activator of NF-κB (RANK) and immunoreceptor tyrosine-based activation motif (ITAM)-containing adaptors are essential factors involved in regulating osteoclast formation and bone remodeling. Here, we identify early estrogen-induced gene 1 (EEIG1) as a novel RANK ligand (RANKL)-inducible protein that physically interacts with RANK and further associates with Gab2, PLCγ2 and Tec/Btk kinases upon RANKL stimulation. EEIG1 positively regulates RANKL-induced osteoclast formation, likely due to its ability to facilitate RANKL-stimulated PLCγ2 phosphorylation and NFATc1 induction. In addition, an inhibitory peptide designed to block RANK-EEIG1 interaction inhibited RANKL-induced bone destruction by reducing osteoclast formation. Together, our results identify EEIG1 as a novel RANK signaling component controlling RANK-mediated osteoclast formation, and suggest that targeting EEIG1 might represent a new therapeutic strategy for the treatment of pathological bone resorption.