胆汁酸核受体FXR在脂代谢-生物钟关系中的"纽带"作用研究

A large body of evidence from both human and animal studies suggests a relationship between circadian disorders and altered lipid metabolism. However, the underlying mechanisms for the coordinated control of the circadian clock and lipid metabolic pathways remain elusive. Our preliminary data show that normal mice have circadian oscillations in hepatic expression of clock genes and FXR (farnesoid X receptor).FXR shuttles between nucleus and cytoplasm with an approximate 24-h periodicity. However, such circadian rhythms are significantly blunted with altered zeitgeber day/night ratio in the liver from mice lacking FXR or mice fed with high fat diet. Mice lacking FXR also show abnormal diurnal rhythms of wheel-running activity, suggesting a change in locomotor activity. Taken together, we hypothesize that FXR is an important link that integrates the mammalian clock and lipid metabolism. The aim of this proposal is to explore the molecular mechanisms involved in the nucleocytoplasmic shuttling of FXR under physiological and pathological conditions with the mice models of sleep reduction and fatty liver. Furthermore, we plan to characterize the underlying signaling pathways that FXR deficiency induces circadian misalignment. The understanding of the molecular links between the core molecular clock and FXR signaling will be necessary to develop novel therapies targeting lipid metabolic diseases and sleep disorder.

研究表明，脂代谢与生物钟可以相互影响，但具体相互作用及其机制仍不明了。我们的前期研究发现，肝细胞的法尼脂衍生物X受体（farnesoid X receptor，FXR）存在明显的昼夜节律性细胞核-细胞浆穿梭（核-浆穿梭）活动，且昼夜节律紊乱和高脂饮食都会改变FXR的核-浆穿梭活性；FXR基因敲除小鼠的跑轮昼夜节律紊乱，每日跑轮运动时间明显延长。考虑到FXR是调节脂代谢的重要因子，同时受昼夜节律的调节，本课题提出如下科学假设： FXR可能是联系脂代谢与昼夜节律的重要纽带。本研究拟通过建立小鼠睡眠减少模型、脂肪肝模型及FXR基因敲除小鼠模型，从整体-细胞-分子三个层面，探索在生理和病理情况下生物钟调控FXR核-浆穿梭的分子机制及FXR表达降低抑制睡眠和延长运动周期的可能信号途径，从新的视角阐明脂代谢紊乱与生物钟相互作用的可能机制，为临床预防和综合治疗代谢性疾病及睡眠节律紊乱提供新思路。

研究表明，脂代谢与生物钟可以相互影响，但具体相互作用及其机制仍不明了。我们的研究发现：（1）肝细胞的法尼脂衍生物X受体（farnesoid X receptor，FXR）存在明显的昼夜节律性细胞核-细胞浆穿梭（核-浆穿梭）活动，用缩短24小时光照时间来模拟小鼠睡眠减少，发现在此模型中FXR蛋白大量聚集在细胞浆中，不能入核发挥生物功能，引起胆酸代谢异常。（2）发现光照缩短引起胆酸代谢异常的调控通路：肝脏内血红素加氧酶2（heme oxygenase2， Hmox2，HO-2）的表达存在明显的昼夜节律，光照可诱导Hmox2高表达。在睡眠减少模型中，光照时间缩短，Hmox2表达明显下调，致使血红素表达显著升高，进一步引起靶蛋白Rev-erbα表达升高和NPAS2、bmal1表达下调，Rev-erbα可促进胆酸合成，引起胆酸淤积，肝脏炎性损伤。（3）发现外周血胆酸升高可引起中枢非结合胆酸升高，激活SCN内胆酸膜受体（Takeda G-protein-coupled receptor）TGR5， 缩短小鼠睡眠-觉醒周期，从而扰乱小鼠正常24小时节律，同时还严重干扰血糖昼夜节律，降低糖耐量。因此本课题发现节律紊乱可引起肝脏胆酸代谢异常；同时外周胆酸异常升高同样会扰乱中枢SCN的正常节律振荡，因此胆酸是联系外周糖脂代谢与昼夜节律的重要纽带，除了FXR外，TGR5也是起到纽带作用的重要蛋白。该研究从新的视角阐明糖脂代谢紊乱与生物钟相互作用的可能机制，为临床预防和综合治疗代谢性疾病及睡眠节律紊乱提供新思路。