Hepatic Steatosis and ER Stress-Inducible Transcription Factor CREBH

描述（由申请人提供）：肝性脂肪变性或脂肪肝被认为是非酒精性脂肪性肝病（NAFLD）的关键代谢前体，非酒精性脂肪性肝病是美国肝脏相关疾病和死亡的主要原因。NAFLD的疾病进展目前被认为是由对肝脂肪变性（“第一次打击”）施加的急性损伤（“第二次打击”）引发的。然而，对“两次击中”触发从可逆性脂肪变性转变为NAFLD的分子基础的精确理解仍然是难以捉摸的。以前，我们揭示了一种新的肝脏特异性转录因子CREBH（环AMP反应元件结合蛋白H），它被内质网（ER）应激激活，介导肝脏中的急性期炎症反应。最近，我们已经积累了强有力的初步证据表明，CREBH在代谢应激条件下调节肝脏脂质稳态中起着至关重要的作用。饱和脂肪酸、炎症刺激或高脂喂养可在体外或体内诱导CREBH裂解，导致其活化。在急性内质网应激或致动脉粥样硬化的高脂饮食中，小鼠CREBH的缺失导致关键脂肪生成酶的表达降低，并减少肝脏脂质蓄积。高脂喂养6个月后，与对照组小鼠相比，CREBH基因敲除小鼠显示出显著更少的肝脏脂肪变性和炎症，但更高的胰岛素敏感性和葡萄糖耐量。此外，发现CREBH在代谢应激下激活肝细胞中关键脂肪生成调节剂的表达，包括CCAAT增强子结合蛋白β（C/EBP 2）和过氧化物酶体增殖物激活受体γ（PPAR 3）。这些观察结果导致了该提议的中心假设：由过量饱和脂肪酸或促炎细胞因子诱导的代谢应激激活CREBH;激活的CREBH然后作为脂肪生成转录调节因子起作用以传播肝脂肪变性和脂肪性肝炎。在本研究申请中，我们将阐明CREBH在调节肝脂肪变性和NAFLD发展中的病理生理作用和分子机制。为了实现我们的研究目标，我们将追求三个互补的具体目标：（1）阐明代谢因素，包括饱和脂肪酸和促炎细胞因子，激活CREBH的调节机制：（2）破译CREBH介导的应激信号调节肝脏脂质稳态的分子基础;（3）研究代谢应激下CREBH在脂肪肝向脂肪性肝炎转变中的作用。这项工作代表了一种新的途径，以阐明ER应激相关的机制，在肝脂肪变性和脂肪性肝炎，目前知之甚少。这些研究的完成不仅将确定一种新的应激诱导的转录因子调节肝脏脂质代谢的分子基础，而且对于设计预防和治疗人类NAFLD及其相关代谢综合征的新策略也具有重要意义。

DESCRIPTION (provided by applicant): Hepatic steatosis or fatty liver is considered the key metabolic precursor to non-alcoholic fatty liver disease (NAFLD), the major cause of liver-associated illness and death in the United States. Disease progression in NAFLD is currently thought to be triggered by an acute insult (the "second hit") that is imposed on hepatic steatosis (the "first hit"). However, a precise understanding of the molecular basis by which the "two hits" trigger the transition from reversible steatosis to NAFLD remains elusive. Previously, we revealed a novel liver- specific transcription factor CREBH (cyclic-AMP-response-element-binding protein H), which is activated by endoplasmic reticulum (ER) stress to mediate an acute-phase inflammatory response in the liver. Recently, we have accumulated strong preliminary evidence that CREBH plays a crucial role in regulating hepatic lipid homeostasis under metabolic stress conditions. Saturated fatty acids, inflammatory stimuli, or high-fat feeding can induce cleavage of CREBH in vitro or in vivo, leading to its activation. Deletion of CREBH in mice resulted in decreased expression of key lipogenic enzymes and reduced hepatic lipid accumulation in response to acute ER stress or atherogenic high-fat feeding. After the high-fat feeding for 6 months, CREBH null mice displayed significantly less hepatic steatosis and inflammation but greater insulin sensitivity and glucose tolerance, compared to the control mice. Furthermore, CREBH was found to activate expression of key lipogenic regulators, including CCAAT-enhancer-binding protein beta (C/EBP2) and peroxisome proliferator- activated receptor gamma (PPAR3), in liver hepatocytes under the metabolic stress. These observations lead to the central hypothesis of this proposal: metabolic stress, induced by excessive saturated fatty acids or pro- inflammatory cytokines, activates CREBH; activated CREBH then functions as a lipogenic transcriptional regulator to propagate hepatic steatosis and steatohepatitis. In this grant application, we will elucidate the pathophysiologic role and molecular mechanism of CREBH in regulating hepatic steatosis and the development of NAFLD. To achieve our research goal, we will pursue three complementary specific aims: (1) to delineate the regulatory mechanism by which metabolic factors, including saturated fatty acids and pro- inflammatory cytokines, activate CREBH; (2) to decipher the molecular basis of CREBH-mediated stress signaling in regulating hepatic lipid homeostasis; (3) to determine the role of CREBH in the transition of hepatic steatosis to steatohepatitis under the metabolic stress. This work represents a novel avenue to elucidate ER stress-associated mechanisms in hepatic steatosis and steatohepatitis that are currently poorly understood. Completion of the proposed studies will not only define the molecular basis by which a novel, stress-induced transcription factor regulates hepatic lipid metabolism, but will also be significant for designing new strategies for the prevention and treatment of human NAFLD and its associated metabolic syndromes.