Dietary carbohydrates regulate hepatic lipogenesis by controlling the expression of critical enzymes in glycolytic and lipogenic pathways. Here we report that the transcription factor XBP1, a key regulator of the Unfolded Protein Response (UPR), is required for the unrelated function of normal fatty acid synthesis in the liver. XBP1 protein expression is induced in the liver by a high carbohydrate diet and directly controls the induction of critical genes involved in fatty acid synthesis. Inducible, selective deletion of XBP1 in liver resulted in marked hypocholesterolemia and hypotriglyceridemia, secondary to a decreased production of lipids from the liver. This phenotype was not accompanied by hepatic steatosis or significant compromise in protein secretory function. The discovery of XBP1 as a regulator of lipogenesis has important implications for human dyslipidemias.Hepatic lipid synthesis increases upon ingestion of carbohydrates, which may be converted into triglyceride (TG) in the liver and transported to adipose tissue for energy storage. Dysregulation of hepatic lipid metabolism is closely related to the development of metabolic syndrome, a condition characterized by the constellation of central obesity, dyslipidemia, elevated blood glucose and hypertension (1). In mammals, hepatic lipid metabolism is controlled by transcription factors, such as liver X receptor (LXR), sterol regulatory element-binding proteins (SREBPs) and ChREBP that regulate the expression of critical enzymes involved in glycolytic and lipogenic pathways (2).XBP1 is a key regulator of the mammalian unfolded protein response (UPR) or endoplasmic reticulum (ER) stress response (3). Upon ER stress, the proximal sensor and endoribonuclease IRE1α induces unconventional splicing of XBP1 mRNA to generate a mature mRNA encoding an active transcription factor, XBP1s, which directly binds to the promoter region of ER chaperone genes (4-6). Mice lacking XBP1 display severe abnormalities in the development and function of professional secretory cells, such as 4 Corresponding authors:

Proper coordination of cholesterol biosynthesis and trafficking is essential to human health. The sterol regulatory element binding proteins (SREBPs) are key transcription regulators of genes involved in cholesterol biosynthesis/uptake. We show here that microRNAs (miR-33a/b) embedded within introns of the SREBP genes target the ATP-binding cassette transporter A1 (ABCA1), an important regulator of high-density lipoprotein (HDL) synthesis and reverse cholesterol transport, for post-transcriptional repression. Antisense inhibition of miR-33 in cell lines causes upregulation of ABCA1 expression and increased cholesterol efflux, and injection of mice on a western-type diet with locked nucleic acid (LNA)-antisense oligonucleotides results in elevated plasma HDL. Collectively, our findings indicate that miR-33 acts in concert with the SREBP host genes to control cholesterol homeostasis, and suggest that miR-33 may represent a therapeutic target for ameliorating cardiometabolic diseases.

Background and Aims Coronavirus disease 2019 (COVID‐19), the illness caused by the SARS‐CoV‐2 virus, is rapidly spreading throughout the world. Hospitals and healthcare providers are preparing for the anticipated surge in critically ill patients, but few are wholly equipped to manage this new disease. The goals of this document are to provide data on what is currently known about COVID‐19, and how it may impact hepatologists and liver transplant providers and their patients. Our aim is to provide a template for the development of clinical recommendations and policies to mitigate the impact of the COVID‐19 pandemic on liver patients and healthcare providers. Approach and Results This article discusses what is known about COVID‐19 with a focus on its impact on hepatologists, liver transplant providers, patients with liver disease, and liver transplant recipients. We provide clinicians with guidance for how to minimize the impact of the COVID‐19 pandemic on their patients’ care. Conclusions The situation is evolving rapidly, and these recommendations will need to evolve as well. As we learn more about how the COVID‐19 pandemic impacts the care of patients with liver disease, we will update the online document available at https://www.aasld.org/about-aasld/covid-19-and-liver.

Nonalcoholic fatty liver disease (NAFLD) is characterized by hepatic lipid accumulation in the absence of excess alcohol intake. NAFLD is the most common chronic liver disease, and ongoing research efforts are focused on understanding the underlying pathobiology of hepatic steatosis with the anticipation that these efforts will identify novel therapeutic targets. Under physiological conditions, the low steady-state triglyceride concentrations in the liver are attributable to a precise balance between acquisition by uptake of non-esterified fatty acids from the plasma and by de novo lipogenesis, versus triglyceride disposal by fatty acid oxidation and by the secretion of triglyceride-rich lipoproteins. In NAFLD patients, insulin resistance leads to hepatic steatosis by multiple mechanisms. Greater uptake rates of plasma non-esterified fatty acids are attributable to increased release from an expanded mass of adipose tissue as a consequence of diminished insulin responsiveness. Hyperinsulinemia promotes the transcriptional upregulation of genes that promote de novo lipogenesis in the liver. Increased hepatic lipid accumulation is not offset by fatty acid oxidation or by increased secretion rates of triglyceride-rich lipoproteins. This review discusses the molecular mechanisms by which hepatic triglyceride homeostasis is achieved under normal conditions, as well as the metabolic alterations that occur in the setting of insulin resistance and contribute to the pathogenesis of NAFLD.

Triglyceride molecules represent the major form of storage and transport of fatty acids within cells and in the plasma. The liver is the central organ for fatty acid metabolism. Fatty acids accrue in liver by hepatocellular uptake from the plasma and by de novo biosynthesis. Fatty acids are eliminated by oxidation within the cell or by secretion into the plasma within triglyceride-rich very low-density lipoproteins. Notwithstanding high fluxes through these pathways, under normal circumstances the liver stores only small amounts of fatty acids as triglycerides. In the setting of overnutrition and obesity, hepatic fatty acid metabolism is altered, commonly leading to the accumulation of triglycerides within hepatocytes, and to a clinical condition known as nonalcoholic fatty liver disease (NAFLD). In this review, we describe the current understanding of fatty acid and triglyceride metabolism in the liver and its regulation in health and disease, identifying potential directions for future research. Advances in understanding the molecular mechanisms underlying the hepatic fat accumulation are critical to the development of targeted therapies for NAFLD. © 2018 American Physiological Society. Compr Physiol 8:1-22, 2018.

Non-alcoholic fatty liver disease (NAFLD) is a liver manifestation of metabolic syndrome. Overconsumption of high-fat diet (HFD) and increased intake of sugar sweetened beverages are major risk-factors for development of NAFLD. Today the most commonly consumed sugar is high fructose corn syrup. Hepatic lipids may be derived from dietary intake, esterification of plasma free fatty acids (FFA) or hepatic de novo lipogenesis (DNL). A central abnormality in NAFLD is enhanced de novo lipogenesis. Hepatic de novo lipogenesis is increased in individuals with NAFLD, while the contribution of dietary fat and plasma FFA to hepatic lipids is not significantly altered. The importance of DNL in NAFLD is further established in mouse studies with knockout of genes involved in this process. Dietary fructose increases levels of enzymes involved in DNL even more strongly than HFD. Several properties of fructose metabolism make it particularly lipogenic. Fructose is absorbed via portal vein and delivered to the liver in much higher concentrations as compared to other tissues. Fructose increases protein levels of all DNL enzymes during its conversion into triglycerides. Additionally, fructose supports lipogenesis in the setting of insulin resistance as fructose does not require insulin for its metabolism and it directly stimulates SREBP1c, a major transcriptional regulator of DNL. Fructose also leads to ATP depletion and suppression of mitochondrial fatty acid oxidation resulting in increased production of reactive oxygen species. Furthermore fructose promotes ER stress and uric acid formation, additional insulin independent pathways leading to DNL. In summary, fructose metabolism supports DNL more strongly than HFD and hepatic DNL is a central abnormality in NAFLD. Disrupting fructose metabolism in the liver may provide a new therapeutic option for the treatment of NAFLD.

Insulin resistance plays a central role in the development of the metabolic syndrome, but how it relates to cardiovascular disease remains controversial. Liver insulin receptor knockout (LIRKO) mice have pure hepatic insulin resistance. On a standard chow diet, LIRKO mice have a proatherogenic lipoprotein profile with reduced high-density lipoprotein (HDL) cholesterol and very low-density lipoprotein (VLDL) particles that are markedly enriched in cholesterol. This is due to increased secretion and decreased clearance of apolipoprotein B-containing lipoproteins, coupled with decreased triglyceride secretion secondary to increased expression of Pgc-1 beta (Ppargc-1b), which promotes VLDL secretion, but decreased expression of Srebp-1c (Srebf1), Srebp-2 (Srebf2), and their targets, the lipogenic enzymes and the LDL receptor. Within 12 weeks on an atherogenic diet, LIRKO mice show marked hypercholesterolemia, and 100% of LIRKO mice, but 0% of controls, develop severe atherosclerosis. Thus, insulin resistance at the level of the liver is sufficient to produce the dyslipidemia and increased risk of atherosclerosis associated with the metabolic syndrome.

Genome-wide association studies (GWASs) have linked genes to various pathological traits. However, the potential contribution of regulatory noncoding RNAs, such as microRNAs (miRNAs), to a genetic predisposition to pathological conditions has remained unclear. We leveraged GWAS meta-analysis data from >188,000 individuals to identify 69 miRNAs in physical proximity to single-nucleotide polymorphisms (SNPs) associated with abnormal levels of circulating lipids. Several of these miRNAs (miR-128-1, miR-148a, miR-130b, and miR-301b) control the expression of key proteins involved in cholesterol-lipoprotein trafficking, such as the low-density lipoprotein (LDL) receptor (LDLR) and the ATP-binding cassette A1 (ABCA1) cholesterol transporter. Consistent with human liver expression data and genetic links to abnormal blood lipid levels, overexpression and antisense targeting of miR-128-1 or miR-148a in high-fat diet–fed C57BL/6J and Apoe-null mice resulted in altered hepatic expression of proteins involved in lipid trafficking and metabolism, and in modulated levels of circulating lipoprotein-cholesterol and triglycerides. Taken together, these findings support the notion that altered expression of miRNAs may contribute to abnormal blood lipid levels, predisposing individuals to human cardiometabolic disorders.