支链氨基酸代谢紊乱影响糖代谢的机制与转化医学研究

Diabetes is a major public health threat in China and worldwide. Lipid and carbohydrates have been recognized as major players in diabetes and thus studied extensively. However, little is known about the role of amino acids in this disease process. Recently, several clinical studies reported that high plasma level of Branched-Chain Amino Acid (BCAA) was tightly associated with type 2 diabetes and predictive to future insulin resistance. On the other hand, in animals, some studies showed that high level BCAA promoted diabetes while the others showed the opposite effect. The underlying mechanism of this discrepancy remains unclear while the adiposity has been indicated to play a role. Branched chain amino acid (BCAA), including leucine, isoleucine, and valine, are essential amino acids. BCAA’s abundance is tightly controlled by a degradation system consisting of dozens of enzymes. We have identified a protein phosphatase, named PP2Cm, as a key regulator of BCAA catabolism. Genetically ablation of PP2Cm in mice led to defect in BCAA disposal, resulting in elevated level of BCAA and their metabolites branched-chain keto acids (BCKA). Therefore, PP2Cm deficient animal offers us a unique opportunity to investigate the specific role of BCAA/BCKA in insulin resistance. In lean mice, our preliminary study demonstrated an improved glucose tolerance and higher glucose utilization in PP2Cm knockout mice. High BCAA diet also promoted glucose tolerance in lean wildtype mice. Insulin signaling was enhanced in PP2Cm knockout mice with a lower basal mTOR activity which likely released the feedback regulation on insulin signaling. Interestingly, we found that high level BCKA treatment suppressed the respiration in isolated mitochondria but enhanced the glucose utilization in cultured cell. BCKA also could suppress mTOR activity. These data suggested a unique role of BCKA in the metabolic regulation. The hypothesis of this project is that BCAA exerts opposite effect on glucose metabolism in lean and obese animals. The main objective is to elucidate the functional impact of BCAA/BCKA on glucose tolerance and utilization in lean and obese mice and to unravel the underlying mechanism of BCAA/BCKA mediated signaling and impact on energy metabolism. Finally, the potential approaches to treat diabetes by targeting BCAA will be explored. This proposal may clarify certain confusion in this field and uncover a novel and potentially important role for BCAA/BCKA in metabolic regulation. Understanding the mechanisms will help to identify novel therapeutic targets for this major disease.

近几年临床研究发现支链氨基酸代谢紊乱与糖尿病的发生发展紧密相关，血清支链氨基酸水平甚至可以预测糖尿病的发生及治疗手段的预后。基础研究则报道了高支链氨基酸具有促进或抑制糖代谢的两种截然不同的效果，原因尚不清楚，据推测与研究对象的肥胖程度有关，对该问题的探索是现在相关领域的热点之一。我们的前期工作显示高支链氨基酸在正常体重小鼠中增强了糖耐受、糖酵解及胰岛素敏感度，并且发现支链氨基酸代谢产物BCKA在这一过程中行使特殊功能。本课题拟在这些研究的基础上，充分利用我们独特的代谢紊乱小鼠模型，探索支链氨基酸在不同营养状态下（肥胖与正常体重）对糖代谢的影响，深入探讨支链氨基酸代谢异常在糖尿病发生发展中的功能及机制，并探索通过改变支链氨基酸摄入和代谢影响糖尿病发生发展的可行性。这些研究不但会丰富支链氨基酸及糖脂代谢的生物学知识，阐明代谢调控的新机制，并且有望为临床预防和治疗糖尿病提供新的理论基础及应用手段

传统对代谢病的研究更多地关注糖和脂，对三大营养物质之一的蛋白质/氨基酸的关注相对甚少。最近几年，国际代谢研究领域开始关注氨基酸在代谢紊乱性疾病的中功能，本项目在这一热点和前沿领域内取得了创新性很强的发现。.PP2Cm基因敲除导致支链氨基酸代谢缺陷。相对于野生型小鼠，正常饲料喂养的PP2Cm基因敲除小鼠体重偏低，摄食量不受影响，对碳水化合物的利用增加，糖耐受和胰岛素耐受更强。进一步对不同组织代谢组学检测显示PP2Cm基因敲除小鼠和野生型小鼠的代谢谱显著不同，其中支链氨基酸和糖代谢明显变化是主要的贡献者，糖代谢最显著的变化发生在肝脏中。在PP2Cm基因敲除小鼠肝脏中糖原代谢更活跃，中间产物丰度上升，糖原代谢关键酶表达上升，并且受支链酮酸调控，糖酵解上游中间产物浓度上升，三羧酸循环部分中间产物浓度明显下降，受支链酮酸调控。这些结果明确显示支链氨基酸代谢对糖代谢的重要影响。本课题研究还发现支链氨基酸分解代谢缺陷在促进肥胖相关胰岛素抵抗中发挥了重要作用。在肥胖ob/ob小鼠中，支链氨基酸分解代谢基因被抑制，导致支链氨基酸和支链酮酸累积。用BCKDK的小分子抑制剂纠正支链氨基酸分解代谢，有效改善ob/ob小鼠的胰岛素抵抗。限制蛋白质的摄入（同时减少了支链氨基酸摄入）能够获得类似的结果，相反的，增加支链氨基酸摄入促进小鼠胰岛素抵抗发生。机制研究发现支链酮酸和支链氨基酸一样，也能通过激活mTORC1抑制胰岛素信号通路。在这些研究的基础之上，我们还探索了支链氨基酸调控的糖脂代谢在干细胞、肿瘤、心脏相关疾病中的多种新功能。.本研究发现了支链氨基酸分解代谢在正常体重以及肥胖情况下对糖代谢的关键调控功能，揭示了全新的支链氨基酸对糖脂代谢的调控机制，拓展了代谢研究的新领域。这些研究不仅丰富了支链氨基酸及糖脂代谢的生物学知识，阐明代谢调控的新机制，而且为临床预防和治疗糖尿病提供新的理论基础及应用手段。