Hepatic Steatosis and ER Stress-Inducible Transcription Factor CREBH

描述(申请人提供)：肝脏脂肪变性或脂肪肝被认为是非酒精性脂肪性肝病(NAFLD)的主要代谢前驱疾病，在美国，NAFLD是肝脏相关疾病和死亡的主要原因。目前，非酒精性脂肪肝的疾病进展被认为是由肝脏脂肪变性的急性侮辱(“第二次打击”)(“第一次打击”)引发的。然而，对两次打击触发从可逆性脂肪变性向非酒精性脂肪性肝病转变的分子基础的准确理解仍然难以捉摸。此前，我们发现了一种新的肝脏特异性转录因子CREBH(Cycle-AMP-Response-Element-Binding Protein H)，它可以被内质网(ER)应激激活，介导肝脏的急性期炎症反应。最近，我们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方面发挥着关键作用。饱和脂肪酸、炎症刺激或高脂喂养都能在体外或体内诱导CREBH的裂解，导致其激活。在小鼠中，CREBH基因的缺失导致关键的造脂酶的表达减少，并减少了对急性内质网应激或致动脉粥样硬化的高脂饮食所造成的肝脏脂肪堆积。高脂喂养6个月后，CREBH基因缺失小鼠的肝脏脂肪变性和炎症明显减轻，但胰岛素敏感性和糖耐量高于对照组小鼠。此外，CREBH还能激活代谢应激下肝细胞中关键的生脂调节因子，包括CCAAT增强子结合蛋白β(C/EBP2)和过氧化物酶体增殖物激活受体γ(PPAR3)的表达。这些观察结果导致了这一提议的中心假设：过度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随后作为造脂转录调节因子传播肝脏脂肪变性和脂肪性肝炎。在这项拨款申请中，我们将阐明CREBH在调节肝脏脂肪变性和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，我们将追求三个相辅相成的特定目标：(1)阐明包括饱和脂肪酸和促炎细胞因子在内的代谢因子激活CREBH的调控机制；(2)破译CREBH介导的应激信号在调节肝脏脂质稳态中的分子基础；(3)确定CREBH在代谢应激下肝脏脂肪变性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来阐明内质网应激在肝脏脂肪变性和脂肪性肝炎中的相关机制，这些机制目前尚不清楚。这些研究的完成不仅将确定一种新的应激诱导转录因子调节肝脏脂肪代谢的分子基础，而且将对设计预防和治疗人类NAFLD及其相关代谢综合征的新策略具有重要意义。

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.