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Accumulating evidence indicates that the immune and skeletal systems interact with each other through various regulators during the osteoclastogenic process. Among these regulators, the bioactive lipid sphingosine-1-phosphate (S1P), which is synthesized by sphingosine kinase 1/2 (SPHK1/2), has recently been recognized to play a role in immunity and bone remodeling through its receptor sphingosine-1-phosphate receptor 1 (S1PR1). However, little is known regarding the potential role of S1PR1 signaling in inflammatory bone loss. We observed that SPHK1 and S1PR1 were upregulated in human apical periodontitis, accompanied by macrophage infiltration and enhanced expression of receptor activator of NF-κB ligand (RANKL, an indispensable factor in osteoclastogenesis and bone resorption) and increased numbers of S1PR1-RANKL double-positive cells in lesion tissues. Using an in vitro co-culture model of macrophages and bone marrow stromal cells (BMSCs), it was revealed that in the presence of lipopolysaccharide (LPS) stimulation, macrophages could significantly induce SPHK1 activity, which resulted in activated S1PR1 in BMSCs. The activated S1P-S1PR1 signaling was responsible for the increased RANKL production in BMSCs, as S1PR1-blockage abolished this effect. Applying a potent S1P-S1PR1 signaling modulator, Fingolimod (FTY720), in a Wistar rat apical periodontitis model effectively prevented bone lesions in vivo via downregulation of RANKL production, osteoclastogenesis, and bone resorption. Our data unveiled the regulatory role of SPHK1-S1PR1-RANKL axis in inflammatory bone lesions and proposed a potential therapeutic intervention by targeting this cell-signaling pathway to prevent bone loss. © 2018 American Society for Bone and Mineral Research.

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Angiotensin‐converting enzyme 2 (ACE2) has been identified as the functional receptor of severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) and a target for disease prevention. However, the relationship between ACE2 expression and its clinical implications in SARS‐CoV‐2 pathogenesis remain unknown. Here, we explored the location and expression of ACE2, and its correlation with gender, age and cigarette smoke (CS), in a CS‐exposed mouse model and 224 non‐malignant lung tissues (125 non‐smokers, 81 current smokers and 18 ex‐smokers) by immunohistochemistry. Moreover, the correlations of ACE2 with CS‐induced oxidative stress‐related markers, hypoxia inducible factor‐1α (HIF‐1α), inducible nitric oxide synthase (iNOS), and 4‐hydroxynonenal (4‐HNE) were investigated. Chromatin immunoprecipitation and luciferase reporter assays identified the cause of ACE2 overexpression in human primary lung epithelial cells. We demonstrated that ACE2 was predominantly overexpressed on the apical surface of bronchial epithelium, while reduced in alveolar epithelium, owing to the dramatically decreased abundance of alveolar type II pneumocytes in CS‐exposed mouse lungs. Consistent with this, ACE2 was primarily significantly overexpressed in human bronchial and alveolar epithelial cells in smokers regardless of age or gender. Decreased ACE2 expression was observed in bronchial epithelial cells from ex‐smokers compared with current‐smokers, especially in those who had ceased smoking for more than 10 years. Moreover, ACE2 expression was positively correlated with the levels of HIF‐1α, iNOS, and 4‐HNE in both mouse and human bronchioles. The results were further validated using a public available dataset from The Cancer Genome Atlas (TCGA) and our previous integrated data from Affymetrix U133 Plus2.0 microarray (AE‐meta). Finally, our results showed that HIF‐1α transcriptionally upregulates ACE2 expression. Our results indicate that smoking‐induced ACE2 overexpression in the apical surface of bronchial epithelial cells provide a route by which SARS‐CoV‐2 enters host cells, which supports clinical relevance in attenuating the potential transmission risk of COVID‐19 in smoking populations by smoking cessation. This article is protected by copyright. All rights reserved.