Sphingolipids in alcoholic liver disease

酒精性肝病(ALD)仍然是全球最常见的慢性肝病。尽管广泛 研究表明，由于对疾病的了解有限，没有FDA批准的疗法可用于ALD的任何阶段 发病机制。因此，尚未得到满足的需要是确定新的靶点，以开发有效的治疗方法 ALD很紧急。酒精不仅会改变肝脏脂肪和胆汁酸的代谢，还会破坏肠道微生物群 和肠道屏障功能，导致肠道渗漏和细菌移位，以及激活 全身和肝脏炎症。我们之前已经报道过，结合胆汁酸激活鞘氨醇- 1磷酸受体2(S1PR2)，进一步激活ERK1/2和AKT。S1P是研究最多的 鞘磷脂，由鞘氨醇通过鞘氨醇激酶1(SphK1)或SphK2合成。S1P可以 作为细胞内信号分子或五种配体调节各种基本的细胞反应 GPCRS，S1PR1-5。SphK2产生的核S1P是一种有效的组蛋白脱乙酰基酶的天然抑制物 (HDAC1/2)。干扰素基因刺激物(STING)的激活也被认为是关键信号 在ALD。最近的一项研究报道，SphK2介导的CD11b+巨噬细胞产生核S1P是一种 强烈的刺痛抑制作用，抑制肺泡巨噬细胞的炎症反应。我们有 报道称，在NIAAA ALD小鼠模型中，SphK2-/-小鼠出现了更严重的脂肪肝和肝损伤。 在ALD肝硬变患者和ALD小鼠模型中，肝脏SphK2的表达水平均显著降低。 酒精喂养显著增加SphK2-/-小鼠的肠道通透性和细菌易位。我们的 初步数据进一步表明，1)酒精喂养导致全球SphK2-/-更严重的肝损伤 小鼠肝细胞特异性SphK2基因敲除(SphK2Hep-/-)；2)肠上皮细胞特异性SphK2基因敲除 (SphK2IEC-/-)小鼠比SphK2fl/fl小鼠更容易发生酒精性肝损伤；3)RNAseq 分析表明，长期饮酒显著扰乱了肝脏鞘脂、脂肪酸和胆汁酸。 4)SphK2基因缺失抑制血管内皮细胞的生长 肠道器官。基于这些关键发现，我们假设SphK2/S1P介导的干扰 胃肠-肝轴的信号通路在酒精性肝损伤中起关键作用。两个具体目标是 提出用来检验这一假说。1)探讨SphK2在调节肝脏脂质中的作用及机制 急性和慢性酒精应激条件下的代谢和炎症反应。2)：至 明确SphK2在调节肠道屏障功能中的作用，并确定其细胞机制 SphK2调节肠道肝轴，以响应酒精诱导的压力。实现这些具体目标将 极大地提高了我们目前对SphK2/S1P介导的组织和细胞类型特异性作用的理解 ALD中的信号通路，这为开发新的治疗干预措施带来了巨大的希望，不仅对 ALD也适用于其他相关的代谢性疾病。

Alcoholic liver disease (ALD) remains the most common chronic liver disease worldwide. Despite extensive studies, no FDA-approved therapy is available for any stage of ALD due to the limited understanding of disease pathogenesis. Therefore, the unmet need to identify novel targets for developing effective therapeutics against ALD is urgent. Alcohol not only alters hepatic lipid and bile acid metabolism but also disrupts the gut microbiome and intestinal barrier function, which results in a leaky gut and bacterial translocation as well as activation of systemic and hepatic inflammation. We have previously reported that conjugated bile acids activate sphingosine- 1 phosphate receptor 2 (S1PR2), which further activates ERK1/2 and AKT. S1P is one of the most studied sphingolipids and is synthesized from sphingosine by either sphingosine kinase 1 (SphK1) or SphK2. S1P can regulate various fundamental cellular responses either as an intracellular signaling molecule or a ligand for five GPCRs, S1PR1-5. SphK2-generated nuclear S1P is a potent natural inhibitor of histone deacetylases (HDAC1/2). Activation of the stimulator of interferon genes (STING) also has been identified as critical signaling in ALD. A recent study reported that SphK2-mediated production of nuclear S1P in CD11b+ macrophages is a strong inhibitor of STING and suppresses the inflammatory response in alveolar macrophages. We have reported that SphK2-/- mice developed more severe fatty liver and hepatic injury in the NIAAA ALD mouse model. Hepatic SphK2 expression levels were markedly reduced in both ALD cirrhotic patients and ALD mouse models. Alcohol-feeding significantly increased intestinal permeability and bacterial translocation in SphK2-/- mice. Our preliminary data further showed that 1) alcohol-feeding induced more severe liver injury in the global SphK2-/- mice than hepatocyte-specific SphK2 knockout (SphK2Hep-/-); 2) intestinal epithelial cell-specific SphK2 knockout (SphK2IEC-/-) mice were more prone to alcohol-induced liver injury compared to SphK2fl/fl mice; 3) RNAseq analysis showed that chronic alcohol feeding significantly disrupted hepatic sphingolipid, fatty acid, and bile acid metabolism and activated inflammatory and fibrotic responses; 4) deletion of SphK2 inhibited the growth of intestinal organoids. Based on these key findings, we HYPOTHESIZE that disruption of SphK2/S1P-mediated signaling pathways in the gut-liver axis plays a critical role in alcohol-induced liver injury. Two specific aims are proposed to test the hypothesis. 1) To investigate the role and mechanisms of SphK2 in modulating hepatic lipid metabolism and inflammatory response under the conditions of acute and chronic alcohol-induced stress. 2): To define the role of SphK2 in modulating intestinal barrier function and to identify the cellular mechanisms by which SphK2 modulates the gut liver axis in response to alcohol-induced stress. Accomplishing these specific aims will significantly advance our current understanding of the tissue and cell-type-specific roles of SphK2/S1P-mediated signaling pathways in ALD, which holds great promise for developing novel therapeutic interventions not only for ALD but also for other related metabolic diseases.