Investigating the role of osteocyte steatosis as underlying mechanisms of the impaired osteocyte network due to bone metastasis

骨细胞是骨中最常见的细胞类型，通过骨陷窝-骨管网络(LCN)高度相互连接。通过这种交流工具，他们协调骨骼对机械刺激的适应，并通过与成骨细胞和破骨细胞交流来调节骨转换。最近的研究表明，骨细胞和肿瘤细胞之间通过骨转移过程相互作用，但其作用机制以及这如何影响对机械负荷的反应尚不清楚。在第一个资助期，我们已经确定了在乳腺癌和前列腺癌骨转移模型的同基因小鼠模型中，针对骨硬化性和溶骨性病变的骨细胞形态特征。这些改变，包括生存能力和连接性的降低，表明机械感觉能力受损。此外，在体外，我们鉴定了成骨细胞样的MLO-Y4细胞在RM1前列腺癌细胞上清液处理后发生了显著的代谢变化，并获得了以脂肪蓄积为特征的成脂表型。为此，我们现在提出了一种假说，即由于骨细胞中的脂质积累，即骨细胞脂肪变性，在骨转移的情况下，通过其LCN进行的机械感觉和与骨细胞的沟通受到了损害。为了进一步研究这一点，我们定义了三个特定目标：(I)研究RM1上清液处理后导致骨细胞脂肪变性的机制，并确定其对体外培养的骨细胞功能的影响(即机械传感以及与成骨细胞和破骨细胞的通讯)；(Ii)在前列腺癌骨转移过程中显示骨细胞及其LCN中的脂滴和其他与脂肪代谢相关的细胞器(过氧化酶体和线粒体)，并评估肿瘤细胞在体内对骨细胞机械负荷反应的影响，以及iii)验证前列腺癌患者骨转移活检组织中的关键发现。该项目将为骨细胞以前所未有的分辨率接触肿瘤细胞所经历的代谢变化提供新的见解，并可能提供新的分子机制，可用于干扰骨中央调节因子骨细胞和肿瘤细胞之间的恶性串扰。

Osteocytes are the most frequent cell type in bone and are highly interconnected via their lacunar-canalicular network (LCN). Via this communication tool, they orchestrate skeletal adaptation to mechanical stimuli and regulate bone turnover by communicating with osteoblasts and osteoclasts. Recent studies indicate a reciprocal interaction between osteocytes and tumor cells through the bone metastatic process, but little is known about the mechanisms of action and how this affects the response to mechanical loading. In the first funding period, we have identified osteocyte morphological signatures that are specific for osteosclerotic and osteolytic lesions in syngeneic mouse models of breast and prostate cancer bone metastasis models. These alterations, including reduced viability and connectivity, suggest an impaired ability for mechanosensation. Moreover, in vitro, we identified that the osteocyte-like MLO-Y4 cells undergo marked metabolic changes and acquire an adipogenic phenotype characterized by lipid accumulation after conditioning with RM1 prostate cancer cell supernatants. To that end, we have now formulated the hypothesis that due to lipid accumulation in osteocytes, i.e. osteocyte steatosis, mechanosensation via their LCN and communication with bone cells is impaired during bone metastasis conditions. To investigate this further, we defined three specific aims: (I) investigate the mechanism that leads to osteocyte steatosis after treatment with RM1 supernatants and characterize its impact on osteocyte function (i.e. mechanosensing and communication to osteoblasts and osteoclasts) in vitro, (II) visualize lipid droplets and other organelles related to fat metabolism (peroxisomes and mitochondria) in osteocytes and their LCN during prostate cancer bone metastasis and assess the impact of tumor cells on osteocyte response to mechanical loading in vivo, and III) validate key findings in biopsies from bone metastasis from patients with prostate cancer. This project will provide novel insights into the metabolic changes that osteocytes undergo upon tumor cell contact at unprecedented resolution and may deliver new molecular mechanisms that can be harnessed to interfere with the malignant crosstalk between the bone-central regulator osteocytes and tumor cells.