Hepatocyte nuclear factor 4α (HNF4α) is an established transcriptional master regulator of differentiation, maintenance, and metabolism. Polymorphisms in HNF4α are linked to several diseases in humans including diabetes and nonalcoholic fatty liver disease (NAFLD). Identifying novel regulation of lipid metabolism by HNF4α would inform on NAFLD development and progression. We directly assessed HNF4α activity through chromatin immunoprecipitation (ChIP)-sequencing and integration of untargeted lipidomics. Direct regulation by HNF4α can be difficult to assess due to the role of HNF4α in liver homeostasis; to rapidly disrupt activity, mice were exposed to cold stress which induces hepatic steatosis in several hours. Cold exposure shifted HNF4α activity with differential genome occupancy of more than 50% of HNF4α binding sites. Focusing on HNF4α binding to promoter with active transcription determined that HNF4α directly regulates fatty acid desaturation, ether lipid synthesis, and peroxisomal biogenesis in response to cold exposure. Integration of lipidomics found that cold exposure increases the very long chain polyunsaturated fatty acid composition of the hepatic lipid pool, including ether lipids, in an HNF4α dependent manner. Because portions of ether lipid synthesis are in the peroxisome and peroxisomal biogenesis is directly HNF4α regulated, we analyzed peroxisomal abundance and found increases with cold exposure that are ablated with loss of HNF4α. This peroxisomal regulation was independent of PPARα— a known regulator of peroxisomes and lipid metabolism—since loss of HNF4α was not rescued by PPARα overexpression. These data determined that regulation of hepatic steatosis by HNF4α is more complex than triglyceride accumulation and includes acyl chain modifications, ether lipid synthesis, and peroxisomal oxidation.

HNF4α is a known master regulator of hepatocyte differentiation and is required for hepatocyte maintenance in part through regulation of metabolism. In humans, polymorphisms in HNF4α are known to cause maturity onset diabetes of the young, disrupt bile acid biosynthesis, and hypolipidemia that cause decreased exercise tolerance. We aimed to identify other hepatic lipid processing pathways that are HNF4α regulated and used cold exposure as a physiological stress to rapidly perturb HNF4α activity and lipid homeostasis. By utilizing cold exposure with three unbiased approaches of forward genetics, chromatin immunoprecipitation (ChIP)-sequencing, and lipidomics, we were able to identify novel lipid processing pathways that are HNF4α regulated. We found that HNF4α activity is regulated by cold exposure within six hours, with differential HNF4α occupancy in more than 13,000 genomic sites. These sites of occupancy include regulators of peroxisomal biogenesis and fatty modifications including: Acox1, Fads1, Ehhadh, Elovl5, Pex16, and Pex11a . Global lipidomics determined that cold exposure alters plasmalogens and very long chain polyunsaturated fatty acids and use of liver-specific HNF4α knockout mice confirmed this regulation was HNF4α dependent. Together these data demonstrate that HNF4α regulates hepatic and systemic lipid metabolism through transcriptional regulation of peroxisomal biogenesis and fatty acid modifications.

Low-protein diets promote metabolic health in humans and rodents. Despite evidence that sex and genetic background are key factors in the response to diet, most protein intake studies examine only a single strain and sex of mice. Using multiple strains and both sexes of mice, we find that improvements in metabolic health in response to reduced dietary protein strongly depend on sex and strain. While some phenotypes were conserved across strains and sexes, including increased glucose tolerance and energy expenditure, we observed high variability in adiposity, insulin sensitivity, and circulating hormones. Using a multi-omics approach, we identified mega-clusters of differentially expressed hepatic genes, metabolites, and lipids associated with each phenotype, providing molecular insight into the differential response to protein restriction. Our results highlight the importance of sex and genetic background in the response to dietary protein level, and the potential importance of a personalized medicine approach to dietary interventions.

Adipose tissue has a variety of diverse functions that maintain energy homeostasis. In conditions of excess energy availability, adipose tissue increases its lipid storage and communicates the nutritional abundance to various organs in the body. In conditions of energy depletion, such as fasting, cold exposure, or prolonged exercise, triglycerides stored in adipose tissue are released as free fatty acids to support the shift to catabolic metabolism. These diverse functions of storage, communication, and energy homeostasis are shared between numerous adipose depots including subcutaneous, visceral, brown, beige, intramuscular, marrow, and dermal adipose tissue. As organisms age, the cellular composition of these depots shifts to facilitate increased inflammatory cell infiltration, decreased vasculature, and increased adipocyte quantity and lipid droplet size. The purpose of this review is to give a comprehensive overview of the molecular and cellular changes that occur in various aged adipose depots and discuss their impact on physiology. The molecular signature of aged adipose leads to higher prevalence of metabolic disease in aged populations including type 2 diabetes, cardiovascular disease, Alzheimer?s disease, and certain types of cancer.

White adipocytes store excess energy in the form of triglycerides. During energy replete conditions, these triglycerides are broken down by lipolysis leading to the release of free fatty acids and glycerol. The free fatty acids are either released into the circulation where they are taken up by peripheral tissues or they are further processed in the adipocyte. The focus of this chapter will be on the signaling associated with the free fatty acid release, lipid species produced from these fatty acids in the adipocyte, and how these lipids are mobilized in the circulation. We will also discuss the contrast between acute stimulation of lipolysis in conditions such as fasting compared to the chronic stimulation of basal lipolysis observed in obesity.