The ubiquitin proteasome system (UPS) is important in maintaining protein homeostasis. of steatosis in the liver. Our results indicate that Nrf1 plays an integral role in the maintenance of proteasome function in hepatocytes and in the prevention of liver steatosis development. Moreover these results spotlight an association between proteasome dysfunction Saxagliptin ER stress and steatosis. Keywords: Nrf1 proteasome ER stress steatohepatitis transcriptional regulation Introduction The Ubiquitin-Proteasome System (UPS) is the major intracellular proteolytic pathway in the cell [1 2 The UPS plays a major role in the degradation of mutant proteins proteins that are terminally misfolded or damaged by oxidative stress [3 4 In addition the UPS controls the turnover of regulatory molecules involved in gene transcription cell cycle control and various transmission transduction pathways. It is crucial for cells to Saxagliptin maintain adequate proteasomal function as aberrations in the UPS have been shown to contribute to numerous pathological conditions in humans [5 6 In neurodegenerative disorders apoptosis of neurons is usually associated with the accumulation of mutant proteins and proteasome dysfunction [7 8 A number of liver diseases including non-alcoholic steatohepatitis [9] alcoholic cirrhosis [10] and hepatocellular carcinoma [11] show accumulation of ubiquitin-conjugated proteins suggesting that proteasome function is also compromised in these conditions [12]. Proteins destined for proteolysis by the proteasome are tagged by covalent attachment of polyubiquitin chains and subsequently recognized by the 26S proteasome for degradation [13]. The 26S proteasome is usually a multi-protein complex consisting Saxagliptin of a central proteolytic core (20S) particle with regulatory caps (19S) at either end. The core is usually arranged into two outer and inner rings each consisting of seven different alpha- and beta-subunits respectively. Each 19S particle is made of ATPase (Rpt 1-6) and non-ATPase (Rpn 1-14) subunits. The outer rings of the core regulate access of protein substrates to the inner chamber that contains the proteolytic sites. The 19S cap functions to bind unfold and regulate access of polyubiquitinated proteins into the 20S core particle where they are degraded into small peptides [14 15 Nuclear factor erythroid-derived 2-related factor 1 (Nrf1) is usually a member of the CNC subfamily of basic-leucine zipper transcription factors [16]. CNC factors form heterodimers MIF with small-Maf-proteins and regulate transcriptional activation through the antioxidant response element (ARE) located at the promoter region of various antioxidant genes [17 18 Antioxidant genes regulated by Nrf1 include those encoding NAD(P)H:quinone oxidoreductase 1 metallothioneins glutamate cysteine ligase catalytic and modifier subunits that are involved in glutathione biosynthesis and hemeoxygenase 1 [19-22]. Aside from antioxidant genes Nrf1 has been shown to regulate genes involved in development and other cellular functions [23]. Osterix a zinc finger transcription factor that plays an important role in the differentiation of osteoblast and bone formation has been shown to be regulated by Nrf1 [24]. Nrf1 has also been reported to function as a repressor of transcription. Nrf1 interacts with C/EBP-β to repress expression of the dentin sialophosphoprotein (DSPP) gene in undifferentiated odontoblast [25] and Nrf1 has also been implicated in the unfavorable regulation of Saxagliptin iNOS expression [26]. Recent findings show that Nrf1 is also involved in regulating proteasome gene expression. Inactivation of Nrf1 in neurons prospects to a coordinate down-regulation of Psma and Psmb genes encoding alpha- and beta-subunits of the 20S core as well as components of the 19S regulatory subcomplex and neurodegeneration [27]. While these findings show that Nrf1 modulates constitutive expression of proteasome genes in neurons studies in both human and mouse cells demonstrate that induction of proteasome subunit genes in response to proteasome inhibition is also Nrf1-dependent [28]. These Saxagliptin studies suggest a regulatory role for Nrf1 beyond oxidative stress response. However the function of Nrf1 in regulating proteasome activity in other tissue compartments remained to be decided. Previously we showed that inactivation of Nrf1 in mouse hepatocytes lead to the spontaneous.