Renga Barbara, Mencarelli Andrea, Cipriani Sabrina, D'Amore Claudio, Zampella Angela, Monti Maria Chiara, Distrutti Eleonora, Fiorucci Stefano
Dipartimento di Medicina Clinica e Sperimentale, Università degli Studi di Perugia, Italy.
Biochim Biophys Acta. 2011 Nov;1812(11):1522-31. doi: 10.1016/j.bbadis.2011.06.009. Epub 2011 Jul 1.
Hepatic transport and metabolism of glutamate and glutamine are regulated by intervention of several proteins. Glutamine is taken up by periportal hepatocytes and is the major source of ammonia for urea synthesis and glutamate for N-acetylglutamate (NAG) synthesis, which is catalyzed by the N-acetylglutamate synthase (NAGS). Glutamate is taken up by perivenous hepatocytes and is the main source for the synthesis of glutamine, catalyzed by glutamine synthase (GS). Accumulation of glutamate and ammonia is a common feature of chronic liver failure, but mechanism that leads to failure of the urea cycle in this setting is unknown. The Farnesoid X Receptor (FXR) is a bile acid sensor in hepatocytes. Here, we have investigated its role in the regulation of the metabolism of both glutamine and glutamate. In vitro studies in primary cultures of hepatocytes from wild type and FXR(-/-) mice and HepG2 cells, and in vivo studies, in FXR(-/-) mice as well as in a rodent model of hepatic liver failure induced by carbon tetrachloride (CCl(4)), demonstrate a role for FXR in regulating this metabolism. Further on, promoter analysis studies demonstrate that both human and mouse NAGS promoters contain a putative FXRE, an ER8 sequence. EMSA, ChIP and luciferase experiments carried out to investigate the functionality of this sequence demonstrate that FXR is essential to induce the expression of NAGS. In conclusion, FXR activation regulates glutamine and glutamate metabolism and FXR ligands might have utility in the treatment of hyperammonemia states.
谷氨酸和谷氨酰胺的肝脏转运及代谢受多种蛋白质的干预调节。门静脉周围的肝细胞摄取谷氨酰胺，它是尿素合成中氨的主要来源以及N - 乙酰谷氨酸（NAG）合成中谷氨酸的主要来源，NAG的合成由N - 乙酰谷氨酸合酶（NAGS）催化。肝静脉周围的肝细胞摄取谷氨酸，它是谷氨酰胺合成的主要来源，由谷氨酰胺合成酶（GS）催化。谷氨酸和氨的蓄积是慢性肝衰竭的常见特征，但在此情况下导致尿素循环功能障碍的机制尚不清楚。法尼酯X受体（FXR）是肝细胞中的胆汁酸感受器。在此，我们研究了其在调节谷氨酰胺和谷氨酸代谢中的作用。在野生型和FXR（ - / - ）小鼠的原代肝细胞培养物以及HepG2细胞中的体外研究，以及在FXR（ - / - ）小鼠以及四氯化碳（CCl₄）诱导的肝衰竭啮齿动物模型中的体内研究，均证明FXR在调节这种代谢中发挥作用。此外，启动子分析研究表明，人和小鼠的NAGS启动子均含有一个假定的FXRE，即ER8序列。为研究该序列的功能而进行的电泳迁移率变动分析（EMSA）、染色质免疫沉淀（ChIP）和荧光素酶实验表明，FXR对于诱导NAGS的表达至关重要。总之，FXR激活调节谷氨酰胺和谷氨酸代谢，FXR配体可能在治疗高氨血症状态中具有应用价值。
