HDL functionality is dependent on hepatocyte stress defense factors Nrf1 and Nrf2.

High density lipoproteins (HDL) promote homeostasis and counteract stressful tissue damage that underlie cardiovascular and other diseases by mediating reverse cholesterol transport, reducing inflammation, and abrogating oxidative damage. However, metabolically stressful conditions associated with atherosclerosis can impair these effects. Hepatocytes play a major role in the genesis and maturation of circulating HDL, and liver stress elicits marked regulatory changes to circulating HDL abundance and composition, which affect its functionality. The mechanisms linking liver stress to HDL function are incompletely understood. In this study, we sought to determine whether stress defending transcription factors nuclear factor erythroid 2 related factor-1 (Nrf1) and -2 (Nrf2) promote hepatocyte production of functional HDL. Using genetically engineered mice briefly fed a mild metabolically stressful diet, we investigated the effect of hepatocyte-specific deletion of Nrf1, Nrf2, or both on circulating HDL cholesterol, protein composition, and function. Combined deletion, but not single gene deletion, reduced HDL cholesterol and apolipoprotein A1 levels as well as the capacity of HDL to accept cholesterol undergoing efflux from cultured macrophages and to counteract tumor necrosis factor α-induced inflammatory effect on cultured endothelial cells. This coincided with substantial alteration to the HDL proteome, which correlated with liver gene expression profiles of corresponding proteins. Thus, our findings show complementary actions by hepatocyte Nrf1 and Nrf2 play a role in shaping HDL abundance and composition to promote production of functionally viable HDL. Consequently, our study illuminates the possibility that enhancing stress defense programming in the liver may improve atheroprotective and perhaps other health promoting actions of HDL.

Complementary gene regulation by NRF1 and NRF2 protects against hepatic cholesterol overload.

Hepatic cholesterol overload promotes steatohepatitis. Insufficient understanding of liver stress defense impedes therapy development. Here, we elucidate the role of stress defense transcription factors, nuclear factor erythroid 2 related factor-1 (NRF1) and -2 (NRF2), in counteracting cholesterol-linked liver stress. Using a diet that increases liver cholesterol storage, expression profiles and phenotypes of liver from mice with hepatocyte deficiency of NRF1, NRF2, or both are compared with controls, and chromatin immunoprecipitation sequencing is undertaken to identify target genes. Results show NRF1 and NRF2 co-regulate genes that eliminate cholesterol and mitigate inflammation and oxidative damage. Combined deficiency, but not deficiency of either alone, results in severe steatohepatitis, hepatic cholesterol overload and crystallization, altered bile acid metabolism, and decreased biliary cholesterol. Moreover, therapeutic effects of NRF2-activating drug bardoxolone require NRF1 and are supplemented by NRF1 overexpression. Thus, we discover complementary gene programming by NRF1 and NRF2 that counteract cholesterol-associated fatty liver disease progression.

Absence of CD36 alters systemic vitamin A homeostasis.

Fatty acid translocase (CD36) is a scavenger receptor with multiple ligands and diverse physiological actions. We recently reported that alcohol-induced hepatic retinoid mobilization is impaired in Cd36-/- mice, leading us to hypothesize that CD36 has a novel role in hepatic vitamin A mobilization. Given the central role of the liver in systemic vitamin A homeostasis we also postulated that absence of CD36 would affect whole-body vitamin A homeostasis. We tested this hypothesis in aging wild type and Cd36-/- mice, as well as mice fed a vitamin A-deficient diet. In agreement with our hypothesis, Cd36-/- mice accumulated hepatic retinyl ester stores with age to a greater extent than wild type mice. However, contrary to expectations, Cd36-/- mice consuming a vitamin A-deficient diet mobilized hepatic retinoid similar to wild type mice. Interestingly, we observed that Cd36-/- mice had significantly reduced white adipose tissue retinoid levels compared to wild type mice. In conclusion, we demonstrate that the absence of CD36 alters whole-body vitamin A homeostasis and suggest that this phenotype is secondary to the impaired chylomicron metabolism previously reported in these mice.

The role of adipose triglyceride lipase in lipid and glucose homeostasis: lessons from transgenic mice.

The ability of mammals to store and draw on fat reserves has been a driving force throughout evolution in an environment with intermittent nutrient availability. The discovery of adipose triglyceride lipase (ATGL) as a triglyceride lipase provided a heightened understanding of the mechanisms governing mobilization of fat reserves from adipose tissue. ATGL catalyses the initial step in adipose triglyceride lipolysis, working in concert with other enzymes to mobilize triglyceride for energy production. In addition to the role of ATGL in adipose tissue triglyceride mobilization, ATGL plays crucial roles in regulating lipid homeostasis in other tissues. These roles have been characterized primarily using transgenic mice with tissue-specific ATGL ablation. For example, the global ATGL knockout induces a severe cardiac defect that results in premature mortality that is mimicked by inducible cardiomyocyte-specific ATGL knockout. Global- and adipose-specific ATGL ablation induces a whole-body shift from lipid metabolism to glucose metabolism to satisfy metabolic demand primarily facilitated by an increase in glucose uptake by skeletal muscle. Generation of liver-specific ATGL knockouts has implicated hepatic lipolysis as a critical component of normal liver function. Analysis of ß-cell ATGL knockouts implicates the necessity of pancreatic ATGL in insulin secretion. The objective of this review is to discuss the contributions of ATGL to systemic lipid- and glucose-homeostasis discovered through the study of transgenic mice.

Dietary Macronutrient Composition Determines the Contribution of DGAT1 to Alcoholic Steatosis.

BACKGROUND: The first stage of alcoholic liver disease is hepatic steatosis. While alcohol is known to profoundly impact hepatic lipid metabolism, gaps in our knowledge remain regarding the mechanisms leading to alcohol-induced hepatic triglyceride (TG) accumulation. As the sole enzymes catalyzing the final step in TG synthesis, diacylglycerol O-acyltransferase (DGAT) 1 and 2 are potentially important contributors to alcoholic steatosis. Our goal was to study the effects of dietary fat content on alcohol-induced hepatic TG accumulation, and the relative contribution of DGAT1 and DGAT2 to alcoholic steatosis. METHODS: These studies were carried out in wild-type (WT) mice fed alcohol-containing high-fat or low-fat formulations of Lieber-DeCarli liquid diets, as well as follow-up studies in Dgat1-/- mice. RESULTS: A direct comparison of the low-fat and high-fat liquid diet in WT mice revealed surprisingly similar levels of alcoholic steatosis, although there were underlying differences in the pattern of hepatic lipid accumulation and expression of genes involved in hepatic lipid metabolism. Follow-up studies in Dgat1-/- mice revealed that these animals are protected from alcoholic steatosis when consumed as part of a high-fat diet, but not a low-fat diet. CONCLUSIONS: Dietary macronutrient composition influences the relative contribution of DGAT1 and DGAT2 to alcoholic steatosis, such that in the context of alcohol and a high-fat diet, DGAT1 predominates.

Application of imaging flow cytometry for characterization of acute inflammation in non-classical animal model systems.

Phagocytes display marked heterogeneity in their capacity to induce and control acute inflammation. This has a significant impact on the effectiveness of antimicrobial immune responses at different tissue sites as well as their predisposition for inflammation-associated pathology. Imaging flow cytometry provides novel opportunities for characterization of these phagocyte populations through high spatial resolution, statistical robustness, and a broad range of quantitative morphometric cell analysis tools. This study highlights an integrative approach that brings together new tools in imaging flow cytometry with conventional methodologies for characterization of phagocyte responses during acute inflammation. We focus on a comparative avian in vivo challenge model to showcase the added depth gained through these novel quantitative multiparametric approaches even in the absence of antibody-based cellular markers. Our characterization of acute inflammation in this model shows significant conservation of phagocytic capacity among avian phagocytes compared to other animal models. However, it also highlights evolutionary divergence with regards to phagocyte inflammation control mechanisms based on the internalization of apoptotic cells.