骨钙素通过GPRC6A调控肝脏脂质代谢的分子机制

As an important component of bone extracellular matrix, osteocalcin has been found closely associated with energy, glucose, and lipid metabolism. However, little has been known about the relationship between osteocalcin and liver lipid metabolism. Our pilot experiments suggested that treatment with exogenous osteocalcin could improve liver glucose and lipid metabolism, alleviate liver fat deposition in mouse on high-fat diets; G protein coupled receptor, family C group 6 subtype A (GPRC6A), an osteocalcin-specific receptor, was expressed in mouse liver cells. Thus, it is warrant to hypothesize that osteocalcin may affect the oxidative stress, apoptosis and autophagy by binding with GPRC6A, leading to an improved liver lipid metabolism and reduced liver fat deposition. To verify this hypothesis, we aim to investigate the regulatory mechanism of osteocalcin and its receptor GPRC6A on liver lipid metabolic homeostasis by means of real-time PCR, Western blot, RNA interference, and immunohistochemistry from different aspects of animals, tissues, cells, and molecules. Osteocalcin gene knockout mice, GPRC6A gene liver conditional knockout mice, high fat diet induced mouse model of liver lipidosis, mice primary hepatocytes and human liver tissue will be involved. This study may provide new insights into the influence of osteocalcin on liver lipid metabolic homeostasis from a new perspective of osteocalcin and its receptor-GPRC6A.

骨钙素是骨细胞外基质的重要组分，研究发现其与能量及糖脂代谢关系密切。但骨钙素调控肝脏脂质代谢的机制仍知之甚少。我们预实验结果提示外源性骨钙素干预可改善高脂饮食喂养小鼠肝脏糖脂代谢及脂质沉积；骨钙素特异性受体—G蛋白偶联受体C家族6组成员A（GPRC6A）在小鼠肝细胞表达。为此，我们提出假说：骨钙素可能通过与肝细胞GPRC6A结合后，减轻氧化应激、凋亡及自噬，从而改善肝脏脂质代谢并减轻肝脏脂质沉积。为验证这一假说，我们将通过骨钙素基因敲除小鼠、GPRC6A基因肝脏条件性敲除小鼠、高脂饮食诱导的肝脏脂质沉积小鼠模型，小鼠原代肝细胞、人体肝脏组织，采用real-time PCR、Western blot、RNA干扰、免疫组化等方法，从整体、组织、细胞及分子等层面明确骨钙素及GPRC6A在维持肝脏脂质代谢稳态及改善肝脏脂质沉积中的作用及其调控机制，为寻找骨钙素在维持脂代谢稳态中的作用提供新的思路。

骨钙素是一种骨来源的激素，它在骨骼-能量代谢的对话中扮演重要角色。既往研究已经发现对小鼠进行非羧化骨钙素处理可以预防高脂饮食诱导的非酒精性脂肪性肝病（NAFLD），然而潜在的作用机制仍不明确。尽管GPRC6A是骨钙素的推定受体，目前尚无直接证据证明骨钙素是通过肝脏的GPRC6A来发挥缓解NAFLD的效果。本研究旨在通过两种基因编辑小鼠（骨钙素全身性敲除小鼠和GPRC6A肝脏条件性敲除小鼠）来研究骨钙素通过肝脏GPRC6A影响糖脂代谢的机制。我们的研究结果发现，在高脂饮食条件下，骨钙素对于维持正常的糖脂代谢是至关重要的。同时我们发现骨钙素部分通过肝脏的GPRC6A发挥缓解高脂饮食诱导的NAFLD的效果。高脂饮食条件下骨钙素通过肝脏的GPRC6A促进脂质分解，抑制脂质合成，从而减少肝脏的脂质沉积。在普通饮食条件下，无论骨钙素全身性敲除还是GPRC6A条件性敲除，对小鼠全身的糖脂代谢影响不明显。另外，本研究的人群研究见到：①血清骨钙素是中心型肥胖和低度白蛋白尿的负性影响因素；②在单纯性糖耐量异常人群中血清骨钙素 与相对骨骼肌指数呈正相关性；③在中老年男性人群中，血清骨钙素可加强10年动脉粥样硬化性心血管疾病风险指数对颈动脉内中膜增厚的识别；④在男性冠心病人群中，基线低血清骨钙素水平与未来主要不良心血管事件的发生呈负相关，且能够识别左室收缩功能不全及心源性死亡。本研究的结果为骨钙素缓解NAFLD、心血管代谢性疾病的的机制进行了重要的补充，肝脏GPRC6A可能作为潜在的治疗NAFLD的药物靶点。