Xenobiotic receptors

代谢性疾病包括肥胖、2型糖尿病(胰岛素抵抗)和非酒精性脂肪性肝病(NAFLD)。这些疾病与心血管疾病的风险增加有关，如动脉粥样硬化、非酒精性脂肪性肝炎(NASH)和癌症。根据癌症类型的不同，与肥胖和胰岛素抵抗相关的癌症风险显著增加35%以上。非酒精性脂肪肝和非酒精性脂肪肝与显著增加患肝癌的风险有关。能量摄入和能量消耗之间的长期失衡会导致肥胖，目前还没有安全有效的药物治疗方法。胆汁酸包括鹅去氧胆酸(CDCA)、去氧胆酸(DCA)和石胆酸(LCA)，是法尼类X受体(FXR)的内源性激活剂。此外，受肠道微生物区系调控的结合胆汁酸(T-β-MCA)在小鼠体内被发现是一种天然的FXR拮抗剂，肠道T-β-MCA水平的增加通过抑制肠道FXR信号通路改善HFD诱导的肥胖、糖耐量异常和NAFLD。然而，肠道FXR恢复HFD破坏的血糖稳态的分子机制尚不清楚，因此是本研究的主题。虽然T-β-MCA是一种在肝脏产生的天然FXR拮抗剂，但它在肠道中被胆盐水解酶(BSH)迅速水解为β-MCA，导致肠道中的水平太低，无法抑制FXR信号转导。因此，抑制细菌BSH可能通过增加肠道T-β-MCA来阻断肠道FXR信号转导。非处方药咖啡酸苯乙酯(CAPE)是一种非处方药，也是细菌BSH的抑制剂，它增加了肠道T-β-MCA的水平，选择性地抑制肠道FXR信号转导。肠FXR抑制通过抑制新的FXR靶基因Smpd3和Sptlc2在回肠上皮细胞中的表达而减少神经酰胺。较低的血清神经酰胺介导了肝脏线粒体乙酰辅酶A水平和丙酮酸羧基酶活性的降低，并减弱了肝脏的糖异生，这与体内体重变化和肝脏胰岛素信号无关；这可被神经酰胺或FXR激动剂GW4064处理的小鼠逆转。神经酰胺主要通过诱导内质网应激来显著减弱线粒体柠檬酸合成酶的活性，内质网应激触发肝脏线粒体乙酰辅酶A水平和丙酮酸羧基酶活性的增加。这些结果揭示了膳食补充咖啡酸苯乙酯和肠道FXR调节肝脏糖异生的机制，提示抑制肠道FXR是治疗高血糖的策略。另一项研究发现，FXR影响化学诱导的肝毒性。对接触工业化学品和药物的人来说，肝脏毒性是一个主要问题。破坏胆汁酸(BA)代谢的主要调节因子法尼醇X受体(FXR)可增强小鼠对毒物暴露后肝损伤的敏感性，但确切机制尚不清楚。在这项研究中，利用代谢组学、FXR缺失的小鼠和肝细胞和腺病毒，研究了BA代谢、FXR和化学诱导的肝毒性之间的相互联系。在FXR基因缺失的小鼠中，一次小剂量注射四氯化碳(CCl4)可导致更严重的肝细胞损伤和更高的促炎介质诱导，如趋化因子(C-C基序)配体2(CCL2)。血清代谢组学分析显示，这些小鼠循环中的牛磺胆酸盐(TCA)T-β-MCA显著增加，FXR基因缺失小鼠通过腺病毒强制表达胆盐输出蛋白(BSEP)可减轻CCl4诱导的肝损伤。TCA处理FXR缺失的肝细胞后，c-Jun-N末端激酶(JNK)活性和CCL2mRNA水平显著升高，CCL2mRNA表达上调可被JNK抑制剂SP600125抑制，表明TCA直接增强FXR缺乏时主要由JNK介导的肝细胞炎症信号。此外，预先应用SP600125或用重组腺病毒恢复FXR在肝脏中的表达，均可减轻CCl4所致的肝损伤。总之，这些结果表明，TCA-JNK轴可能与FXR缺失小鼠对CCl4诱导的急性肝损伤的易感性增加有关，并为FXR及其下游基因靶点(如BSEP)保护化学诱导的肝毒性的机制提供了线索。合成MCA的酶学尚不清楚。小鼠和大鼠可将鹅去氧胆酸(CDCA)在6β位羟基化形成α-MCA，将熊去氧胆酸(UDCA)羟基化形成β-MCA。然而，MCA在人类中没有形成到任何明显的程度，这种物种差异的机制尚不清楚。对几个Cyp缺失小鼠品系的比较表明，在Cyp2c簇缺失(Cyp2c-nul)小鼠的肝脏样本中没有检测到α-MCA和β-MCA。全局胆汁酸分析进一步表明，Cyp2c缺失的小鼠盲肠和粪便中不存在MCA及其偶联衍生物，而CDCA、UDCA含量较高。Cyp2c70氧化CDCA和UDCA生成α-MCA和β-MCA。CYP2C9人源化小鼠与Cyp2c缺失小鼠有相似的胆汁酸代谢产物，这表明人CYP2C9不氧化CDCA和UDCA，从而解释了MCA产生的物种差异。由于人类不产生MCA，他们缺乏T-β-MCA，这是小鼠体内的天然FXR拮抗剂，如上所述调节肥胖、胰岛素抵抗和NAFLD

Metabolic diseases include obesity type 2 diabetes mellitus (insulin resistance) and non-alcoholic fatty liver disease (NAFLD). These disorders are associated with increased risk for cardiovascular diseases such as atherosclerosis and non-alcoholic steatohepatitis (NASH), and cancer. There is a marked increase in cancer risk of over 35% depending on the cancer type, associated with obesity, and insulin resistance. NAFLD and NASH are associated with markedly increased risk for liver cancer. A chronic imbalance between energy intake and energy expenditure causes obesity for which there is no safe and effective drug therapy. Bile acids including chenodeoxycholic acid (CDCA), deoxycholic acid (DCA), and lithocholic acid (LCA) are endogenous activators of the farnesoid X receptor (FXR). In addition, the conjugated bile acid tauro-beta-muricholic acid (T-beta-MCA), that is modulated by the gut microbiota was found to be a natural FXR antagonist in mice, and increased intestinal T-beta-MCA levels ameliorate HFD-induced obesity, glucose intolerance and NAFLD via inhibition of intestinal FXR signaling However, the molecular mechanism by which intestinal FXR restores HFD-disrupted glucose homeostasis is poorly understood and the subject of the current study. While T-beta-MCA is a natural FXR antagonist produced in liver, it is rapidly hydrolyzed into beta-MCA by bile salt hydrolase (BSH) in the gut, resulting in levels that are too low in the intestine to inhibit FXR signaling. Therefore, inhibition of bacterial BSH could block intestinal FXR signaling by increasing intestinal T-beta-MCA. Caffeic acid phenethyl ester (CAPE), an over-the-counter dietary supplement and an inhibitor of bacterial BSH, increased levels of intestinal T-beta-MCA, which selectively suppresses intestinal FXR signaling. Intestinal FXR inhibition decreased ceramides by suppressing expression of the novel FXR target genes Smpd3 and Sptlc2 in intestinal ileum epithelial cells. The lower serum ceramides mediated decreased hepatic mitochondrial acetyl-CoA levels and pyruvate carboxylase activities, and attenuated hepatic gluconeogenesis, independent of body weight change and hepatic insulin signaling in vivo; this was reversed by treatment of mice with ceramides or the FXR agonist GW4064. Ceramides substantially attenuated mitochondrial citrate synthase activities primarily through induction of endoplasmic reticulum stress, which triggers increased hepatic mitochondrial acetyl-CoA levels and pyruvate carboxylase activities. These results reveal a mechanism by which the dietary supplement caffeic acid phenethyl ester and intestinal FXR regulates hepatic gluconeogenesis, and suggest that inhibiting intestinal FXR is a strategy for treating hyperglycemia. Another study found that FXR influences chemically-induced liver toxicity. Hepatotoxicity is of major concern for humans exposed to industrial chemicals and drugs. Disruption of farnesoid X receptor (FXR), a master regulator of bile acid (BA) metabolism, enhanced the sensitivity to liver injury in mice after toxicant exposure, but the precise mechanism remains unclear. In this study, the interconnection between BA metabolism, FXR, and chemically-induced hepatotoxicity was investigated using metabolomics, Fxr-null mice and hepatocytes, and adenovirus. A single low-dose intraperitoneal injection of carbon tetrachloride (CCl4), an inducer of acute hepatitis in mice, resulted in more severe hepatocyte damage and higher induction of pro-inflammatory mediators, such as chemokine (C-C motif) ligand 2 (Ccl2), in Fxr-null mice. Serum metabolomics analysis revealed marked increases in circulating taurocholate (TCA) T-beta-MCA in these mice, and forced expression of bile salt export protein (BSEP) by adenovirus in Fxr-null mice ameliorated CCl4-induced liver damage. Treatment of Fxr-null hepatocytes with TCA, but not T-beta-MCA, significantly increased c-Jun-N-terminal kinase (JNK) activation and Ccl2 mRNA levels, and up-regulation of Ccl2 mRNA was attenuated by co-treatment with a JNK inhibitor SP600125, indicating that TCA directly amplifies hepatocyte inflammatory signaling mainly mediated by JNK under FXR-deficiency. Additionally, pre-treatment with SP600125 or restoration of FXR expression in liver by use of recombinant adenovirus, attenuated CCl4-induced liver injury. Collectively, these results suggest that the TCA-JNK axis is likely associated with increased susceptibility to CCl4-induced acute liver injury in Fxr-null mice, and provide clues to the mechanism by which FXR and its downstream gene targets, such as BSEP, protects against chemically-induced hepatotoxicity. The enzymology by which MCA is synthesized was not known. Mice and rats can hydroxylate chenodeoxycholic acid (CDCA) at the 6beta-position to form alpha- MCA, and ursodeoxycholic acid (UDCA) to form beta-MCA). However, MCA is not formed in humans to any appreciable degree and the mechanism for this species difference is not known. Comparison of several Cyp-null mouse lines revealed that alpha-MCA and beta-MCA were not detected in the liver samples from Cyp2c-cluster null (Cyp2c-null) mice. Global bile acids analysis further revealed the absence of MCA and their conjugated-derivatives, and high concentration of CDCA, UDCA in Cyp2c-null mouse cecum and feces. Analysis of recombinant CYPs revealed that alpha-MCA and beta-MCA were produced by oxidation of CDCA and UDCA by Cyp2c70. CYP2C9-humanized mice have similar bile acid metabolites as the Cyp2c-null mice, indicating that human CYP2C9 does not oxidize CDCA and UDCA thus explaining the species differences in production of MCA. Since humans do not produce MCA, they lack T-beta-MCA, the natural FXR antagonists in mouse, that modulates obesity, insulin resistance and NAFLD as described above