Mechanisms of immunoproteasome-mediated metabolic disorders

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ABSTRACT The immunoproteasome is an inducible type of proteasome whose expression is enhanced by inflammation and oxidative stress. Although the immunoproteasome was found about 30 years ago, the function of the immunoproteasome is poorly understood except its role in producing antigenic peptides. Recent studies have shown that the immunoproteasome regulates cell signaling, differentiation and cytokine secretion in non-immune cells. We found that 95% of proteasome is the immunoproteasome in the mouse liver when mTORC1 is constitutively active. Numerous mutations and polymorphisms have been identified in immunoproteasome-specific genes among human populations of metabolic, autoimmune, and rare diseases. The immunoproteasome level is highly increased in proportion to the degree of inflammation in liver biopsies from patients who have chronic active hepatitis or cirrhosis. Despite the relevance of the immunoproteasome in many human diseases, its functions remain poorly understood mainly due to the lack of knowledge on its target substrates. The objective of this proposed study is to determine the mechanism by which the immunoproteasome digests its preferential target proteins and determine how the selective digestion impairs hepatic and global energy metabolism. We developed mice where the immunoproteasome is depleted specifically in the liver. Interestingly, the depletion of hepatic immunoproteasome led to beneficial metabolic outcomes protecting the mice against obesity, insulin resistance and steatosis. As a possible mechanism, we hypothesize that the immunoproteasome induces metabolic reprogramming by selective digestion of proteins involved in metabolic homeostasis and anti-inflammation. We have three specific aims to address the hypothesis. First, we will define the immunoproteasome as a crucial mediator that impairs hepatic lipid metabolism using a genetic mouse model, primary hepatocytes, and immunoproteasome inhibitors. Second, we will determine the role of the immunoproteasome in oxidative stress-induced insulin resistance. Based on our preliminary study, we hypothesize that the immunoproteasome suppresses Akt-dependent metabolism in chronic oxidative stress or inflammation. Using genetic and chemical approaches, we will demonstrate that the immunoproteasome negatively regulates Akt signaling under oxidative stress and define the immunoproteasome as a promising therapeutic target for insulin resistance. Third, we will use quantitative proteomics and proximity-based labeling methods to comprehensively identify target proteins of the immunoproteasome. We will choose a handful identified proteins and validate them as targets of the immunoproteasome. The identified proteins will allow us to define novel metabolic pathways regulated by the immunoproteasome. The outcomes will innovate our understanding of the immunoproteasome by defining the immunoproteasome as a key mediator and as a promising therapeutic target for inflammatory metabolic diseases.