Inserm, U1016, Paris, France.
J Hepatol. 2012 Jan;56(1):199-209. doi: 10.1016/j.jhep.2011.07.019. Epub 2011 Aug 9.
BACKGROUND & AIMS: In liver, the glucose-responsive transcription factor ChREBP plays a critical role in converting excess carbohydrates into triglycerides through de novo lipogenesis. Although the importance of ChREBP in glucose sensing and hepatic energy utilization is strongly supported, the mechanism driving its activation in response to glucose in the liver is not fully understood. Indeed, the current model of ChREBP activation, which depends on Serine 196 and Threonine 666 dephosphorylation, phosphatase 2A (PP2A) activity, and xylulose 5-phosphate (X5P) as a signaling metabolite, has been challenged.
We inhibited PP2A activity in HepG2 cells through the overexpression of SV40 small t antigen and addressed the importance of ChREBP dephosphorylation on Ser-196 using a phospho-specific antibody. To identify the exact nature of the metabolite signal required for ChREBP activity in liver, we focused on the importance of G6P synthesis in liver cells, through the modulation of glucose 6-phosphate dehydrogenase (G6PDH) activity, the rate-limiting enzyme of the pentose phosphate pathway in hepatocytes, and in HepG2 cells using both adenoviral and siRNA approaches.
In contrast to the current proposed model, our study reports that PP2A activity is dispensable for ChREBP activation in response to glucose and that dephosphorylation on Ser-196 is not sufficient to promote ChREBP nuclear translocation in the absence of a rise in glucose metabolism. By deciphering the respective roles of G6P and X5P as signaling metabolites, our study reveals that G6P produced by GK, but not X5P, is essential for both ChREBP nuclear translocation and transcriptional activity in response to glucose in liver cells.
Altogether, our study, by reporting that G6P is the glucose-signaling metabolite, challenges the PP2A/X5P-dependent model currently described for ChREBP activation in response to glucose in liver.
在肝脏中，葡萄糖反应转录因子 ChREBP 通过从头合成脂肪来将多余的碳水化合物转化为甘油三酯，从而在将多余的碳水化合物转化为甘油三酯方面发挥关键作用。尽管 ChREBP 在葡萄糖感应和肝能量利用中的重要性得到了强有力的支持，但葡萄糖在肝脏中激活它的机制尚未完全了解。事实上，目前的 ChREBP 激活模型依赖于丝氨酸 196 和苏氨酸 666 的去磷酸化、磷酸酶 2A（PP2A）活性和木酮糖 5-磷酸（X5P）作为信号代谢物，这一模型受到了挑战。
我们通过过表达 SV40 小 t 抗原来抑制 HepG2 细胞中的 PP2A 活性，并使用磷酸特异性抗体解决 ChREBP 在丝氨酸 196 上的去磷酸化对其活性的重要性。为了确定肝脏中 ChREBP 活性所需的代谢物信号的确切性质，我们通过调节葡萄糖 6-磷酸脱氢酶（G6PDH）活性，即肝细胞中戊糖磷酸途径的限速酶，重点研究了 G6P 在肝细胞中的重要性，以及在 HepG2 细胞中通过腺病毒和 siRNA 方法。
与目前提出的模型相反，我们的研究报告称，PP2A 活性对于葡萄糖刺激下 ChREBP 的激活是可有可无的，而且在没有葡萄糖代谢升高的情况下，丝氨酸 196 的去磷酸化不足以促进 ChREBP 核易位。通过破译 G6P 和 X5P 作为信号代谢物的各自作用，我们的研究表明，由 GK 产生的 G6P，但不是 X5P，对于葡萄糖刺激下肝细胞中 ChREBP 的核易位和转录活性都是必不可少的。
总的来说，我们的研究报告称 G6P 是葡萄糖信号代谢物，这挑战了目前描述的葡萄糖刺激下肝脏中 ChREBP 激活的 PP2A/X5P 依赖性模型。
