Hepatic BMAL1 and HIF1a regulate a time-dependent hypoxic response and their absence leads to Hepatopulmonary Syndrome

The transcriptional response to hypoxia is temporally regulated yet the molecular underpinnings and physiological implications are unknown. We examined herein the roles of hepatic Bmal1 and Hif1a in the circadian response to hypoxia in mice. We found that the majority of the transcriptional response to hypoxia is dependent on Bmal1 or Hif1a, through shared and distinct roles that are daytime determined. We further show that HIF1a accumulation upon hypoxia is temporally regulated and Bmal1-dependent. Unexpectedly, mice lacking both hepatic Bmal1 and Hif1a are hypoxemic and exhibit increased mortality upon hypoxic exposure in a daytime-dependent manner. These mice display mild liver dysfunction with pulmonary vasodilation likely due to ERK activation, endothelial nitric oxide synthase and nitric oxide accumulation in lungs, suggestive of hepatopulmonary syndrome. Our findings indicate that hepatic BMAL1 and HIF1a are key time-dependent regulators of the response to hypoxia and provide molecular insight on the pathophysiology of hepatopulmonary syndrome. RNA-sequencing on lungs of AlbCRE or BHLKO mice at CT16. Lights were turned off at the transition of dark to light regiment on the day of the experiment, following regular LD regimen.机体对缺氧的转录应答呈现时序调控特征，但其分子基础与生理意义尚未明确。本研究以小鼠为模型，探讨了肝脏Bmal1与Hif1a在缺氧应答的昼夜节律调控中的作用。研究发现，绝大多数缺氧相关转录应答依赖于Bmal1或Hif1a，二者通过共有与特异性的调控途径发挥作用，且其功能受昼夜时段调控。我们进一步证实，缺氧诱导的HIF1α蛋白积累同样受时序调控，且该过程依赖于Bmal1。令人意外的是，同时缺失肝脏Bmal1与Hif1a的小鼠会出现低氧血症，且在缺氧暴露时的死亡率呈昼夜时段依赖性升高。该类小鼠表现出轻度肝功能异常，并伴随肺血管舒张，这一现象可能与肺组织中ERK激活、内皮型一氧化氮合酶表达及一氧化氮蓄积相关，提示存在肝肺综合征。本研究结果表明，肝脏BMAL1与HIF1α是缺氧应答的关键时序调控因子，为肝肺综合征的病理生理学机制提供了分子层面的新见解。本实验对AlbCRE小鼠与BHLKO小鼠的肺组织开展了RNA测序；实验遵循标准光暗（LD）周期，当日于暗转亮的过渡时段关闭照明，采样时间为昼夜节律时间CT16。