INSERM, U1016, Institut Cochin, Paris, France.
Diabetes. 2011 May;60(5):1399-413. doi: 10.2337/db10-0452. Epub 2011 Apr 6.
Carbohydrate-responsive element-binding protein (ChREBP) is a key transcription factor that mediates the effects of glucose on glycolytic and lipogenic genes in the liver. We have previously reported that liver-specific inhibition of ChREBP prevents hepatic steatosis in ob/ob mice by specifically decreasing lipogenic rates in vivo. To better understand the regulation of ChREBP activity in the liver, we investigated the implication of O-linked β-N-acetylglucosamine (O-GlcNAc or O-GlcNAcylation), an important glucose-dependent posttranslational modification playing multiple roles in transcription, protein stabilization, nuclear localization, and signal transduction.
O-GlcNAcylation is highly dynamic through the action of two enzymes: the O-GlcNAc transferase (OGT), which transfers the monosaccharide to serine/threonine residues on a target protein, and the O-GlcNAcase (OGA), which hydrolyses the sugar. To modulate ChREBP(OG) in vitro and in vivo, the OGT and OGA enzymes were overexpressed or inhibited via adenoviral approaches in mouse hepatocytes and in the liver of C57BL/6J or obese db/db mice.
Our study shows that ChREBP interacts with OGT and is subjected to O-GlcNAcylation in liver cells. O-GlcNAcylation stabilizes the ChREBP protein and increases its transcriptional activity toward its target glycolytic (L-PK) and lipogenic genes (ACC, FAS, and SCD1) when combined with an active glucose flux in vivo. Indeed, OGT overexpression significantly increased ChREBP(OG) in liver nuclear extracts from fed C57BL/6J mice, leading in turn to enhanced lipogenic gene expression and to excessive hepatic triglyceride deposition. In the livers of hyperglycemic obese db/db mice, ChREBP(OG) levels were elevated compared with controls. Interestingly, reducing ChREBP(OG) levels via OGA overexpression decreased lipogenic protein content (ACC, FAS), prevented hepatic steatosis, and improved the lipidic profile of OGA-treated db/db mice.
Taken together, our results reveal that O-GlcNAcylation represents an important novel regulation of ChREBP activity in the liver under both physiological and pathophysiological conditions.
碳水化合物反应元件结合蛋白（ChREBP）是一种关键的转录因子，它介导葡萄糖对肝脏中糖酵解和脂肪生成基因的作用。我们之前曾报道过，特异性抑制肝脏 ChREBP 可通过特异性降低体内脂肪生成率来预防 ob/ob 小鼠的肝脂肪变性。为了更好地理解肝脏中 ChREBP 活性的调节，我们研究了 O-连接 β-N-乙酰氨基葡萄糖（O-GlcNAc 或 O-GlcNAc 化）的影响，O-GlcNAc 化是一种重要的葡萄糖依赖性翻译后修饰，在转录、蛋白质稳定、核定位和信号转导中发挥多种作用。
通过两种酶的作用，O-GlcNAc 化具有高度动态性：O-GlcNAc 转移酶（OGT）将单糖转移到靶蛋白的丝氨酸/苏氨酸残基上，O-GlcNAcase（OGA）将糖水解。为了在体外和体内调节 ChREBP（OG），我们通过腺病毒方法在小鼠肝细胞和 C57BL/6J 或肥胖 db/db 小鼠的肝脏中过度表达或抑制 OGT 和 OGA 酶。
我们的研究表明，ChREBP 与 OGT 相互作用，并在肝细胞中受到 O-GlcNAc 化的影响。当与体内活跃的葡萄糖通量结合时，O-GlcNAc 化稳定 ChREBP 蛋白并增加其对靶糖酵解（L-PK）和脂肪生成基因（ACC、FAS 和 SCD1）的转录活性。事实上，OGT 过表达显著增加了 fed C57BL/6J 小鼠肝核提取物中的 ChREBP（OG），进而导致脂肪生成基因表达增强和肝甘油三酯过度沉积。与对照组相比，高血糖肥胖 db/db 小鼠肝脏中的 ChREBP（OG）水平升高。有趣的是，通过 OGA 过表达降低 ChREBP（OG）水平可降低脂肪生成蛋白含量（ACC、FAS），防止肝脂肪变性，并改善 OGA 处理的 db/db 小鼠的脂质谱。
总之，我们的结果表明，在生理和病理生理条件下，O-GlcNAc 化是 ChREBP 活性在肝脏中的一个重要的新调节因素。
