Hepatic Steatosis and ER Stress-Inducible Transcription Factor CREBH

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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.