目的:肝脏是能量代谢的中心调节因子,通过葡萄糖和脂肪酸等底物的内在加工以及分泌内分泌因子来发挥其影响。被称为肝细胞因子,影响外周组织的新陈代谢。人类全基因组关联研究表明,抑制素βE基因(INHBE)中预测的功能丧失变体,编码推定的肝细胞因子激活素E,与腹部脂肪量减少和心脏代谢疾病风险相关。然而,肝激活素E的调节以及激活素E对肥胖和代谢疾病的影响尚不清楚。这里,我们研究了肝脏激活素E和脂肪代谢之间的关系,测试激活素E作为肝脏脂肪的一部分的假设,响应于血清脂肪酸和肝脏脂肪酸暴露升高,器官间反馈回路抑制脂肪组织脂解。
方法:使用经禁食的CL316,243处理的小鼠体内和使用经脂肪酸处理的Huh7肝细胞体外评估从脂肪脂解释放的肝激活素E和非酯化脂肪酸(NEFA)之间的关系。使用Inhbe基因敲除小鼠的组合检查了激活素E对脂肪脂解的影响,肝细胞特异性过表达激活素E的小鼠模型,和用富含激活素E的培养基处理的小鼠棕色脂肪细胞。
结果:通过禁食或CL316,243治疗诱导脂肪分解,增加体内肝细胞NEFA暴露,增加肝脏Inhbe表达。同样,用脂肪酸孵育Huh7人肝细胞增加INHBE的表达。Inhbe在小鼠中的遗传消融增加了空腹循环NEFA和肝脏甘油三酯积累。用活化素E条件培养基处理小鼠棕色脂肪细胞和过表达活化素E抑制小鼠脂肪分解和降低血清FFA水平,分别。激活素E对脂肪分解的抑制作用在CRISPR介导的ALK7缺陷细胞和ALK7激酶缺陷小鼠中丧失。InhbeKO小鼠活化素E-ALK7信号轴的破坏降低了HFD喂养后的肥胖,而是导致肝脏脂肪变性和胰岛素抵抗.
结论:综合来看,我们的数据表明,激活素E作为肝脏-脂肪反馈回路的一部分,因此,作为对血清游离脂肪酸增加和肝脏甘油三酯升高的反应,激活素E从肝细胞中释放,并通过ALK7在脂肪中发出信号以抑制脂解,从而减少游离脂肪酸向肝脏的流出和防止过度的肝脏脂质积累。我们发现,通过在小鼠中消融Inhbe破坏这种激活素E-ALK7器官间通信网络会增加脂解并减少肥胖,但导致肝脏甘油三酯升高和胰岛素敏感性受损。这些结果突出了肝脏脂肪,激活素E-ALK7信号轴作为代谢稳态的关键调节因子。
OBJECTIVE: The liver is a central regulator of energy metabolism exerting its influence both through intrinsic processing of substrates such as glucose and fatty acid as well as by secreting endocrine factors, known as hepatokines, which influence metabolism in peripheral tissues. Human genome wide association studies indicate that a predicted loss-of-function variant in the Inhibin βE gene (INHBE), encoding the putative hepatokine Activin E, is associated with reduced abdominal fat mass and cardiometabolic disease risk. However, the regulation of hepatic Activin E and the influence of Activin E on adiposity and metabolic disease are not well understood. Here, we examine the relationship between hepatic Activin E and adipose metabolism, testing the hypothesis that Activin E functions as part of a liver-adipose, inter-organ feedback loop to suppress adipose tissue lipolysis in response to elevated serum fatty acids and hepatic fatty acid exposure.
METHODS: The relationship between hepatic Activin E and non-esterified fatty acids (NEFA) released from adipose lipolysis was assessed in vivo using fasted CL 316,243 treated mice and in vitro using Huh7 hepatocytes treated with fatty acids. The influence of Activin E on adipose lipolysis was examined using a combination of Inhbe knockout mice, a mouse model of hepatocyte-specific overexpression of Activin E, and mouse brown adipocytes treated with Activin E enriched media.
RESULTS: Increasing hepatocyte NEFA exposure in vivo by inducing adipose lipolysis through fasting or CL 316,243 treatment increased hepatic Inhbe expression. Similarly, incubation of Huh7 human hepatocytes with fatty acids increased expression of INHBE. Genetic ablation of Inhbe in mice increased fasting circulating NEFA and hepatic triglyceride accumulation. Treatment of mouse brown adipocytes with Activin E conditioned media and overexpression of Activin E in mice suppressed adipose lipolysis and reduced serum FFA levels, respectively. The suppressive effects of Activin E on lipolysis were lost in CRISPR-mediated ALK7 deficient cells and ALK7 kinase deficient mice. Disruption of the Activin E-ALK7 signaling axis in Inhbe KO mice reduced adiposity upon HFD feeding, but caused hepatic steatosis and insulin resistance.
CONCLUSIONS: Taken together, our data suggest that Activin E functions as part of a liver-adipose feedback loop, such that in response to increased serum free fatty acids and elevated hepatic triglyceride, Activin E is released from hepatocytes and signals in adipose through ALK7 to suppress lipolysis, thereby reducing free fatty acid efflux to the liver and preventing excessive hepatic lipid accumulation. We find that disrupting this Activin E-ALK7 inter-organ communication network by ablation of Inhbe in mice increases lipolysis and reduces adiposity, but results in elevated hepatic triglyceride and impaired insulin sensitivity. These results highlight the liver-adipose, Activin E-ALK7 signaling axis as a critical regulator of metabolic homeostasis.