Myeloid AMPK signaling restricts fibrosis but is not required for metformin improvements during CDAHFD-induced NASH in mice

Metabolic programming underpins inflammatory processes of immune cells. In the context of chronic liver disease, liver macrophage activation and response to hepatocellular damage is dependent on profound metabolic changes. Here, we sought to identify the role of an important metabolic regulator, AMP-activated protein kinase (AMPK), specifically within myeloid cells during the progression of non-alcoholic steatohepatitis (NASH) and whether treatment with metformin, a first line therapy for diabetes and activator of AMPK could stem disease progression. Male and female Prkaa1fl/fl/Prkaa2fl/fl (Flox) control and Flox-LysM-Cre+ (MacKO) mice were fed a low-fat control or a choline-deficient, amino acid defined 45% Kcal high fat diet (CDAHFD) for 8 weeks, where metformin was introduced in the drinking water (50 or 250 mg/kg/day) for the last 4 weeks. Hepatic steatosis and fibrosis were dramatically increased in response to CDAHFD-feeding compared to low-fat control. While myeloid AMPK signaling had no effect on markers of hepatic steatosis or circulating markers, fibrosis as measured by total liver collagen was significantly elevated in livers from MacKO mice, independent of sex. Although treatment with 50 mg/kg/day metformin had no effect on any parameter, intervention with 250 mg/kg/day metformin completely ameliorated hepatic steatosis and fibrosis in both male and female mice. While the protective effect of metformin was associated with lower final body weight, decrease expression of lipogenic and Col1a1 transcripts, it was independent of myeloid AMPK signaling. These results suggest that endogenous AMPK signaling in myeloid cells, both liver-resident and infiltrating, acts to restrict fibrogenesis during CDAHFD-induced NASH progression but is not the mechanism by which metformin improves markers of NASH.

The aryl hydrocarbon receptor in β-cells mediates the effects of TCDD on glucose homeostasis in mice

OBJECTIVE

Chronic exposure to persistent organic pollutants (POPs) is associated with increased incidence of type 2 diabetes, hyperglycemia, and poor insulin secretion in humans. Dioxins and dioxin-like compounds are a broad class of POPs that exert cellular toxicity through activation of the aryl hydrocarbon receptor (AhR). We previously showed that a single high-dose injection of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, aka dioxin; 20 μg/kg) in vivo reduced fasted and glucose-stimulated plasma insulin levels for up to 6 weeks in male and female mice. TCDD-exposed male mice were also modestly hypoglycemic and had increased insulin sensitivity, whereas TCDD-exposed females were transiently glucose intolerant. Whether these effects are driven by AhR activation in β-cells requires investigation.

METHODS

We exposed female and male β-cell specific Ahr knockout (βAhrKO) mice and littermate Ins1-Cre genotype controls (βAhrWT) to a single high dose of 20 μg/kg TCDD and tracked the mice for 6 weeks.

RESULTS

Under baseline conditions, deleting AhR from β-cells caused hypoglycemia in female mice, increased insulin secretion ex vivo in female mouse islets, and promoted modest weight gain in male mice. Importantly, high-dose TCDD exposure impaired glucose homeostasis and β-cell function in βAhrWT mice, but these phenotypes were largely abolished in TCDD-exposed βAhrKO mice.

CONCLUSION

Our study demonstrates that AhR signaling in β-cells is important for regulating baseline β-cell function in female mice and energy homeostasis in male mice. We also show that β-cell AhR signaling largely mediates the effects of TCDD on glucose homeostasis in both sexes, suggesting that the effects of TCDD on β-cell function and health are driving metabolic phenotypes in peripheral tissues.

Macrophage AMPK β1 activation by PF-06409577 reduces the inflammatory response, cholesterol synthesis, and atherosclerosis in mice

Atherosclerotic cardiovascular disease is characterized by both chronic low-grade inflammation and dyslipidemia. The AMP-activated protein kinase (AMPK) inhibits cholesterol synthesis and dampens inflammation but whether pharmacological activation reduces atherosclerosis is equivocal. In the current study, we found that the orally bioavailable and highly selective activator of AMPKβ1 complexes, PF-06409577, reduced atherosclerosis in two mouse models in a myeloid-derived AMPKβ1 dependent manner, suggesting a critical role for macrophages. In bone marrow-derived macrophages (BMDMs), PF-06409577 dose dependently activated AMPK as indicated by increased phosphorylation of downstream substrates ULK1 and acetyl-CoA carboxylase (ACC), which are important for autophagy and fatty acid oxidation/de novo lipogenesis, respectively. Treatment of BMDMs with PF-06409577 suppressed fatty acid and cholesterol synthesis and transcripts related to the inflammatory response while increasing transcripts important for autophagy through AMPKβ1. These data indicate that pharmacologically targeting macrophage AMPKβ1 may be a promising strategy for reducing atherosclerosis.

Choline metabolism underpins macrophage IL-4 polarization and RELMα up-regulation in helminth infection

Type 2 cytokines like IL-4 are hallmarks of helminth infection and activate macrophages to limit immunopathology and mediate helminth clearance. In addition to cytokines, nutrients and metabolites critically influence macrophage polarization. Choline is an essential nutrient known to support normal macrophage responses to lipopolysaccharide; however, its function in macrophages polarized by type 2 cytokines is unknown. Using murine IL-4-polarized macrophages, targeted lipidomics revealed significantly elevated levels of phosphatidylcholine, with select changes to other choline-containing lipid species. These changes were supported by the coordinated up-regulation of choline transport compared to naïve macrophages. Pharmacological inhibition of choline metabolism significantly suppressed several mitochondrial transcripts and dramatically inhibited select IL-4-responsive transcripts, most notably, Retnla. We further confirmed that blocking choline metabolism diminished IL-4-induced RELMα (encoded by Retnla) protein content and secretion and caused a dramatic reprogramming toward glycolytic metabolism. To better understand the physiological implications of these observations, naïve or mice infected with the intestinal helminth Heligmosomoides polygyrus were treated with the choline kinase α inhibitor, RSM-932A, to limit choline metabolism in vivo. Pharmacological inhibition of choline metabolism lowered RELMα expression across cell-types and tissues and led to the disappearance of peritoneal macrophages and B-1 lymphocytes and an influx of infiltrating monocytes. The impaired macrophage activation was associated with some loss in optimal immunity to H. polygyrus, with increased egg burden. Together, these data demonstrate that choline metabolism is required for macrophage RELMα induction, metabolic programming, and peritoneal immune homeostasis, which could have important implications in the context of other models of infection or cancer immunity.

Pancreas-derived DPP4 is not essential for glucose homeostasis under metabolic stress

Mice systemically lacking dipeptidyl peptidase-4 (DPP4) have improved islet health, glucoregulation, and reduced obesity with high-fat diet (HFD) feeding compared to wild-type mice. Some, but not all, of this improvement can be linked to the loss of DPP4 in endothelial cells (ECs), pointing to the contribution of non-EC types. The importance of intra-islet signaling mediated by α to β cell communication is becoming increasingly clear; thus, our objective was to determine if β cell DPP4 regulates insulin secretion and glucose tolerance in HFD-fed mice by regulating the local concentrations of insulinotropic peptides. Using β cell double incretin receptor knockout mice, β cell- and pancreas-specific Dpp4^-/- mice, we reveal that β cell incretin receptors are necessary for DPP4 inhibitor effects. However, although β cell DPP4 modestly contributes to high glucose (16.7 mM)-stimulated insulin secretion in isolated islets, it does not regulate whole-body glucose homeostasis.

Thermoneutral housing does not accelerate metabolic dysfunction-associated fatty liver disease in male or female C57Bl/6J mice fed a Western diet

Metabolic dysfunction-associated fatty liver disease (MAFLD) represents a growing cause of mortality and morbidity and encompasses a spectrum of liver pathologies. While dozens of preclinical models have been developed to recapitulate stages of MAFLD, few achieve fibrosis using an experimental design that mimics human pathogenesis. We sought to clarify whether the combination of thermoneutral (T_N) housing and consumption of a classical Western diet (WD) would accelerate the onset and progression of MAFLD. Male and female C57Bl/6J mice were fed a nutrient-matched low-fat control or Western diet (WD) for 16 weeks. Mice were housed with littermates at either standard temperature (T_S; 22°C) or thermoneutral-like conditions (T_N; ~29°C). Male but not female mice housed at T_N and fed a WD were significantly heavier than T_S -housed control animals. WD-fed mice housed under T_N conditions had lower levels of circulating glucose compared to T_S mice; however, there were select but minimal differences in other circulating markers. While WD-fed T_N males had higher liver enzyme and higher liver triglyceride levels, no differences in markers of liver injury or hepatic lipid accumulation were observed in females. Housing temperature had little effect on histopathological scoring of MAFLD progression in males; however, while female mice retained a level of protection, WD-T_N conditions trended toward a worsened hepatic phenotype, which was associated with higher macrophage transcript expression and content. Our results indicate that interventions coupling T_N housing and WD-induced MAFLD should be longer than 16 weeks to accelerate hepatic steatosis and increase inflammation in both sexes of mice.

Hepatocyte-derived DPP4 regulates portal GLP-1 bioactivity, modulates glucose production, and when absent influences NAFLD progression

Elevated circulating dipeptidyl peptidase-4 (DPP4) is a biomarker for liver disease, but its involvement in gluconeogenesis and metabolic associated fatty liver disease progression remains unclear. Here, we identified that DPP4 in hepatocytes but not TEK receptor tyrosine kinase-positive endothelial cells regulates the local bioactivity of incretin hormones and gluconeogenesis. However, the complete absence of DPP4 (Dpp4-/-) in aged mice with metabolic syndrome accelerates liver fibrosis without altering dyslipidemia and steatosis. Analysis of transcripts from the livers of Dpp4-/- mice displayed enrichment for inflammasome, p53, and senescence programs compared with littermate controls. High-fat, high-cholesterol feeding decreased Dpp4 expression in F4/80+ cells, with only minor changes in immune signaling. Moreover, in a lean mouse model of severe nonalcoholic fatty liver disease, phosphatidylethanolamine N-methyltransferase mice, we observed a 4-fold increase in circulating DPP4, in contrast with previous findings connecting DPP4 release and obesity. Last, we evaluated DPP4 levels in patients with hepatitis C infection with dysglycemia (Homeostatic Model Assessment of Insulin Resistance > 2) who underwent direct antiviral treatment (with/without ribavirin). DPP4 protein levels decreased with viral clearance; DPP4 activity levels were reduced at long-term follow-up in ribavirin-treated patients; but metabolic factors did not improve. These data suggest elevations in DPP4 during hepatitis C infection are not primarily regulated by metabolic disturbances.

Exercise training enhances muscle mitochondrial metabolism in diet-resistant obesity

Background

Current paradigms for predicting weight loss in response to energy restriction have general validity but a subset of individuals fail to respond adequately despite documented diet adherence. Patients in the bottom 20% for rate of weight loss following a hypocaloric diet (diet-resistant) have been found to have less type I muscle fibres and lower skeletal muscle mitochondrial function, leading to the hypothesis that physical exercise may be an effective treatment when diet alone is inadequate. In this study, we aimed to assess the efficacy of exercise training on mitochondrial function in women with obesity with a documented history of minimal diet-induced weight loss.

Methods

From over 5000 patient records, 228 files were reviewed to identify baseline characteristics of weight loss response from women with obesity who were previously classified in the top or bottom 20% quintiles based on rate of weight loss in the first 6 weeks during which a 900 kcal/day meal replacement was consumed. A subset of 20 women with obesity were identified based on diet-resistance (n=10) and diet sensitivity (n=10) to undergo a 6-week supervised, progressive, combined aerobic and resistance exercise intervention.

Findings

Diet-sensitive women had lower baseline adiposity, higher fasting insulin and triglycerides, and a greater number of ATP-III criteria for metabolic syndrome. Conversely in diet-resistant women, the exercise intervention improved body composition, skeletal muscle mitochondrial content and metabolism, with minimal effects in diet-sensitive women. In-depth analyses of muscle metabolomes revealed distinct group- and intervention- differences, including lower serine-associated sphingolipid synthesis in diet-resistant women following exercise training.

Interpretation

Exercise preferentially enhances skeletal muscle metabolism and improves body composition in women with a history of minimal diet-induced weight loss. These clinical and metabolic mechanism insights move the field towards better personalised approaches for the treatment of distinct obesity phenotypes.

Funding

Canadian Institutes of Health Research (CIHR-INMD and FDN-143278; CAN-163902; CIHR PJT-148634).

Fine tuning Acetyl-CoA Carboxylase 1 activity through localization: Functional genomics reveal a role for the lysine acetyltransferase NuA4 and sphingolipid metabolism in regulating Acc1 activity and localization

Acetyl-CoA Carboxylase 1 catalyzes the conversion of acetyl-CoA to malonyl-CoA, the committed step of de novo fatty acid synthesis. As a master regulator of lipid synthesis, acetyl-CoA carboxylase 1 has been proposed to be a therapeutic target for numerous metabolic diseases. We have shown that acetyl-CoA carboxylase 1 activity is reduced in the absence of the lysine acetyltransferase NuA4 in Saccharomyces cerevisiae. This change in acetyl-CoA carboxylase 1 activity is correlated with a change in localization. In wild-type cells, acetyl-CoA carboxylase 1 is localized throughout the cytoplasm in small punctate and rod-like structures. However, in NuA4 mutants, acetyl-CoA carboxylase 1 localization becomes diffuse. To uncover mechanisms regulating acetyl-CoA carboxylase 1 localization, we performed a microscopy screen to identify other deletion mutants that impact acetyl-CoA carboxylase 1 localization and then measured acetyl-CoA carboxylase 1 activity in these mutants through chemical genetics and biochemical assays. Three phenotypes were identified. Mutants with hyper-active acetyl-CoA carboxylase 1 form 1 or 2 rod-like structures centrally within the cytoplasm, mutants with mid-low acetyl-CoA carboxylase 1 activity displayed diffuse acetyl-CoA carboxylase 1, while the mutants with the lowest acetyl-CoA carboxylase 1 activity (hypomorphs) formed thick rod-like acetyl-CoA carboxylase 1 structures at the periphery of the cell. All the acetyl-CoA carboxylase 1 hypomorphic mutants were implicated in sphingolipid metabolism or very long-chain fatty acid elongation and in common, their deletion causes an accumulation of palmitoyl-CoA. Through exogenous lipid treatments, enzyme inhibitors, and genetics, we determined that increasing palmitoyl-CoA levels inhibits acetyl-CoA carboxylase 1 activity and remodels acetyl-CoA carboxylase 1 localization. Together this study suggests yeast cells have developed a dynamic feed-back mechanism in which downstream products of acetyl-CoA carboxylase 1 can fine-tune the rate of fatty acid synthesis.

Defective AMPK regulation of cholesterol metabolism accelerates atherosclerosis by promoting HSPC mobilization and myelopoiesis

Objectives

Dysregulation of cholesterol metabolism in the liver and hematopoietic stem and progenitor cells (HSPCs) promotes atherosclerosis development. Previously, it has been shown that HMG-CoA-Reductase (HMGCR), the rate-limiting enzyme in the mevalonate pathway, can be phosphorylated and inactivated by the metabolic stress sensor AMP-activated protein kinase (AMPK). However, the physiological significance of AMPK regulation of HMGCR to atherogenesis has yet to be elucidated. The aim of this study was to determine the role of AMPK/HMGCR axis in the development of atherosclerosis.

Methods

We have generated a novel atherosclerotic-prone mouse model with defects in the AMPK regulation of HMGCR (Apoe^−/−/Hmgcr KI mice). Atherosclerotic lesion size, plaque composition, immune cell and lipid profiles were assessed in Apoe^−/− and Apoe^−/−/Hmgcr KI mice.

Results

In this study, we showed that both male and female atherosclerotic-prone mice with a disruption of HMGCR regulation by AMPK (Apoe^−/−/Hmgcr KI mice) display increased aortic lesion size concomitant with an increase in plaque-associated macrophages and lipid accumulation. Consistent with this, Apoe^−/−/Hmgcr KI mice exhibited an increase in total circulating cholesterol and atherogenic monocytes, Ly6-C^hi subset. Mechanistically, increased circulating atherogenic monocytes in Apoe^−/−/Hmgcr KI mice was associated with enhanced egress of bone marrow HSPCs and extramedullary myelopoiesis, driven by a combination of elevated circulating 27-hydroxycholesterol and intracellular cholesterol in HSPCs.

Conclusions

Our results uncovered a novel signalling pathway involving AMPK-HMGCR axis in the regulation of cholesterol homeostasis in HSPCs, and that inhibition of this regulatory mechanism accelerates the development and progression of atherosclerosis. These findings provide a molecular basis to support the use of AMPK activators that currently undergoing Phase II clinical trial such as O–3O4 and PXL 770 for reducing atherosclerotic cardiovascular disease risks.

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AMPK regulation of HMGCR is critical for the control of endogenous cholesterol synthesis in HSPCs.

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AMPK-HMGCR signaling regulates HSPCs mobilization and myelopoiesis.

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Perturbation of AMPK regulation of HMGCR accelerates the development and progression of atherosclerosis.

Choline metabolism underpins macrophage IL-4 polarization in vitro and in vivo

Choline homeostasis in macrophage biology is important for LPS-polarized inflammation. In macrophages, choline mainly supports phosphatidylcholine (PC) synthesis, which is crucial for membrane production and cytokine secretion. Here, we examined choline metabolism in IL-4 polarized macrophages in vitro and in vivo. Like LPS, IL-4 increased choline transporter-like protein 1 (CTL1) expression, choline uptake and the incorporation of choline into PC. Targeted lipidomics analysis revealed increased PC content in IL-4-polarized macrophages, with an enrichment in low-saturated species. Pharmacological inhibition of choline uptake/choline kinase with hemicholinium-3 or RSM-932a showed no effect on certain hallmark IL-4-induced macrophage genes (Chil3, Mrc1, Arg1) but significantly reduced the transcript and protein expression of RELMα. Consistent with the function of RELMα in wound healing, 3T3-L1 cells healed more slowly in a scratch wound assay with conditioned media from IL-4 polarized macrophages in which choline metabolism was inhibited compared to vehicle-treated conditioned media. In addition, inhibiting choline metabolism completely prevented PD-L2 upregulation and increased PD-L1 expression on IL-4-polarized macrophages, together with suppressed cellular respiration and increased glycolysis. Furthermore, in vivo administration of RSM-932a (3 mg/kg i.p.) in C57BL/6J mice lowered RELMα in macrophages, decreased PD-L2 and increased PD-L1, and resulted in a loss of resident peritoneal F4/80hi macrophages. Choline represents an underappreciated regulator of macrophage immunometabolism and strategies to target macrophage choline uptake or choline metabolism may be therapeutically valuable.

Choline metabolism promotes M2 macrophage polarization in intestinal infection with helminth Heligmosomoides polygyrus

Choline is an essential nutrient functioning as a precursor for membrane phospholipid. In previous studies, LPS polarization augmented both the expression of the choline transporter CTL1 and choline uptake, while specific inhibition of choline transport led to increased TNFα and IL-6. However, whether choline metabolism regulates M2 macrophage polarization or Th2 cytokine inflammation in vivo is unclear. In vitro, CTL1 and phospholipid synthesis was induced in IL-4 polarized M2 macrophages. To determine the function of choline metabolism in M2 polarization in vivo, mice were infected with Heligmosomoides polygyrus, intraperitoneally injected with choline kinase α inhibitor, RSM-932A or vehicle, and sacrificed at day 17 post-infection. RSM-932A treatment impaired weight gain and peritoneal exudate cell numbers in both naïve and infected mice. Within the peritoneal cavity of infected mice, macrophages and B-1 lymphocytes were depleted by RSM-932A treatment, while monocytes and neutrophils were increased. Flow cytometric and intestinal immunofluorescence staining revealed that RSM-932A treatment prevented M2 macrophage polarization in H.polygyrus-infected mice with a significant reduction in expression of PD-L2 and CD206, and conversely increased expression of CD86 and PD-L1. Additionally, RELMα protein in the serum and peritoneal fluid was downregulated by RSM-932A treatment. The impaired M2 polarization was associated with some loss in optimal immunity to H.polygyrus with increased parasite egg burden but no differences in intestinal worm count. This study indicates that choline metabolism is required for M2 macrophage polarization and an optimal immune response against intestinal helminth infection.

Salsalate reduces atherosclerosis through AMPKβ1 in mice

Objective

Salsalate is a prodrug of salicylate that lowers blood glucose in people with type 2 diabetes. AMP-activated protein kinase (AMPK) is an αβγ heterotrimer which inhibits macrophage inflammation and the synthesis of fatty acids and cholesterol in the liver through phosphorylation of acetyl-CoA carboxylase (ACC) and HMG-CoA reductase (HMGCR), respectively. Salicylate binds to and activates AMPKβ1-containing heterotrimers that are highly expressed in both macrophages and liver, but the potential importance of AMPK and ability of salsalate to reduce atherosclerosis have not been evaluated.

Methods

ApoE^−/− and LDLr^−/− mice with or without (−/−) germline or bone marrow AMPKβ1, respectively, were treated with salsalate, and atherosclerotic plaque size was evaluated in serial sections of the aortic root. Studies examining the effects of salicylate on markers of inflammation, fatty acid and cholesterol synthesis and proliferation were conducted in bone marrow–derived macrophages (BMDMs) from wild-type mice or mice lacking AMPKβ1 or the key AMPK-inhibitory phosphorylation sites on ACC (ACC knock-in (KI)-ACC KI) or HMGCR (HMGCR-KI).

Results

Salsalate reduced atherosclerotic plaques in the aortic roots of ApoE^−/− mice, but not ApoE^−/− AMPKβ1^−/− mice. Similarly, salsalate reduced atherosclerosis in LDLr^−/− mice receiving wild-type but not AMPKβ1^−/− bone marrow. Reductions in atherosclerosis by salsalate were associated with reduced macrophage proliferation, reduced plaque lipid content and reduced serum cholesterol. In BMDMs, this suppression of proliferation by salicylate required phosphorylation of HMGCR and the suppression of cholesterol synthesis.

Conclusions

These data indicate that salsalate suppresses macrophage proliferation and atherosclerosis through an AMPKβ1-dependent pathway, which may involve HMGCR phosphorylation and cholesterol synthesis. Since rapidly-proliferating macrophages are a hallmark of atherosclerosis, these data indicate further evaluation of salsalate as a potential therapeutic agent for treating atherosclerotic cardiovascular disease.

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Salsalate (a dimer of salicylate) activates AMPK in macrophages and reduces atherosclerosis.

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Salicylate-induced reductions in atherosclerosis are associated with reduced macrophage proliferation and serum cholesterol.

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AMPK phosphorylation of HMG-CoA reductase is required for suppressing cholesterol synthesis and macrophage proliferation.

Ebola virus triggers receptor tyrosine kinase-dependent signaling to promote the delivery of viral particles to entry-conducive intracellular compartments

Filoviruses, such as the Ebola virus (EBOV) and Marburg virus (MARV), are causative agents of sporadic outbreaks of hemorrhagic fevers in humans. To infect cells, filoviruses are internalized via macropinocytosis and traffic through the endosomal pathway where host cathepsin-dependent cleavage of the viral glycoproteins occurs. Subsequently, the cleaved viral glycoprotein interacts with the late endosome/lysosome resident host protein, Niemann-Pick C1 (NPC1). This interaction is hypothesized to trigger viral and host membrane fusion, which results in the delivery of the viral genome into the cytoplasm and subsequent initiation of replication. Some studies suggest that EBOV viral particles activate signaling cascades and host-trafficking factors to promote their localization with host factors that are essential for entry. However, the mechanism through which these activating signals are initiated remains unknown. By screening a kinase inhibitor library, we found that receptor tyrosine kinase inhibitors potently block EBOV and MARV GP-dependent viral entry. Inhibitors of epidermal growth factor receptor (EGFR), tyrosine protein kinase Met (c-Met), and the insulin receptor (InsR)/insulin like growth factor 1 receptor (IGF1R) blocked filoviral GP-mediated entry and prevented growth of replicative EBOV in Vero cells. Furthermore, inhibitors of c-Met and InsR/IGF1R also blocked viral entry in macrophages, the primary targets of EBOV infection. Interestingly, while the c-Met and InsR/IGF1R inhibitors interfered with EBOV trafficking to NPC1, virus delivery to the receptor was not impaired in the presence of the EGFR inhibitor. Instead, we observed that the NPC1 positive compartments were phenotypically altered and rendered incompetent to permit viral entry. Despite their different mechanisms of action, all three RTK inhibitors tested inhibited virus-induced Akt activation, providing a possible explanation for how EBOV may activate signaling pathways during entry. In sum, these studies strongly suggest that receptor tyrosine kinases initiate signaling cascades essential for efficient post-internalization entry steps.

Ebola virus (EBOV) and Marburg virus (MARV) are zoonotic pathogens that can cause severe hemorrhagic fevers in humans and non-human primates. They are members of the growing Filoviridae family that also includes three other species of Ebolaviruses known to be highly pathogenic in humans. While vaccines for EBOV are being deployed and showed high efficacy, pan-filoviral treatment is still lacking. To infect cells, EBOV requires the endosomal/lysosomal resident protein Niemann-Pick C1 (NPC1). Accordingly, viral particles require extensive trafficking within endosomal pathways for entry and delivery of the viral genome into the host cell cytoplasm. Here, we used chemical biology to reveal that EBOV triggers receptor tyrosine kinase (RTK)-dependent signaling to traffic to intracellular vesicles that contain the receptor and are conducive to entry. The characterization of host trafficking factors and signaling pathways that are potentially triggered by the virus are important as these could be targeted for antiviral therapies. In our study, we identified several RTK inhibitors, some of which are FDA-approved drugs, that potently block EBOV infection. Since all filoviruses known to date, even Měnglà virus that was recently discovered in bats in China, use NPC1 as their entry receptor, these inhibitors have the potential to be effective pan-filovirus antivirals.

Salicylates Ameliorate Intestinal Inflammation by Activating Macrophage AMPK

BACKGROUND

Inflammatory bowel diseases are the most common chronic intestinal inflammatory conditions, and their incidence has shown a dramatic increase in recent decades. Limited efficacy and questionable safety profiles with existing therapies suggest the need for better targeting of therapeutic strategies. Adenosine monophosphate-activated protein kinase (AMPK) is a key regulator of cellular metabolism and has been implicated in intestinal inflammation. Macrophages execute an important role in the generation of intestinal inflammation. Impaired AMPK in macrophages has been shown to be associated with higher production of proinflammatory cytokines; however, the role of macrophage AMPK in intestinal inflammation and the mechanism by which it regulates inflammation remain to be determined. In this study, we investigated the role of AMPK with a specific focus on macrophages in the pathogenesis of intestinal inflammation.

METHODS

A dextran sodium sulfate-induced colitis model was used to assess the disease activity index, histological scores, macroscopic scores, and myeloperoxidase level. Proinflammatory cytokines such as tumor necrosis factor-α, interleukin-6, and interleukin-1β were measured by enzyme-linked immunosorbent assay. Transient transfection of AMPKβ1 and LC3-II siRNA in RAW 264.7 cells was performed to elucidate the regulation of autophagy by AMPK. The expression of p-AMPK, AMPK, and autophagy markers (eg, LC3-II, p62, Beclin-1, and Atg-12) was analyzed by Western blot.

RESULTS

Genetic deletion of AMPKβ1 in macrophages upregulated the production of proinflammatory cytokines, aggravated the severity of dextran sodium sulfate-induced colitis in mice, which was associated with an increased nuclear translocation of nuclear factor-κB, and impaired autophagy both in vitro and in vivo. Notably, the commonly used anti-inflammatory 5-aminosalicylic acid (ie, mesalazine) and sodium salicylate ameliorated dextran sodium sulfate-induced colitis through the activation of macrophage AMPK targeting the β1 subunit.

CONCLUSIONS

Together, these data suggest that the development of therapeutic agents targeting AMPKβ1 may be effective in the treatment of intestinal inflammatory conditions including inflammatory bowel disease.

Myeloid deletion and therapeutic activation of AMPK do not alter atherosclerosis in male or female mice

The dysregulation of myeloid-derived cell metabolism can drive atherosclerosis. AMP-activated protein kinase (AMPK) controls various aspects of macrophage dynamics and lipid homeostasis, which are important during atherogenesis. Using LysM-Cre to drive the deletion of both the α1 and α2 catalytic subunits (MacKO), we aimed to clarify the role of myeloid-specific AMPK signaling in male and female mice made acutely atherosclerotic by injection of AAV vector encoding a gain-of-function mutant PCSK9 (PCSK9-AAV) and WD feeding. After 6 weeks of WD feeding, mice received a daily injection of either the AMPK activator A-769662 or a vehicle control for an additional 6 weeks. Following this (12 weeks total), we assessed myeloid cell populations and differences between genotype or sex were not observed. Similarly, aortic sinus plaque size, lipid staining, and necrotic area did not differ in male and female MacKO mice compared with their littermate floxed controls. Moreover, therapeutic intervention with A-769662 showed no treatment effect. There were also no observable differences in the amount of circulating total cholesterol or triglyceride, and only minor differences in the levels of inflammatory cytokines between groups. Finally, CD68+ area and markers of autophagy showed no effect of either lacking AMPK signaling or AMPK activation. Our data suggest that while defined roles for each catalytic AMPK subunit have been identified, complete deletion of myeloid AMPK signaling does not significantly impact atherosclerosis. Additionally, these findings suggest that intervention with the first-generation AMPK activator A-769662 is not able to stem the progression of atherosclerosis.

Adipose Tissue Inflammation Is Directly Linked to Obesity-Induced Insulin Resistance, while Gut Dysbiosis and Mitochondrial Dysfunction Are Not Required

Obesity is associated with adipose tissue hypertrophy, systemic inflammation, mitochondrial dysfunction, and intestinal dysbiosis. Rodent models of high-fat diet (HFD)-feeding or genetic deletion of multifunctional proteins involved in immunity and metabolism are often used to probe the etiology of obesity; however, these models make it difficult to divorce the effects of obesity, diet composition, or immunity on endocrine regulation of blood glucose. We, therefore, investigated the importance of adipose inflammation, mitochondrial dysfunction, and gut dysbiosis for obesity-induced insulin resistance using a spontaneously obese mouse model. We examined metabolic changes in skeletal muscle, adipose tissue, liver, the intestinal microbiome, and whole-body glucose control in spontaneously hyperphagic C57Bl/6J mice compared to lean littermates. A separate subset of lean and obese mice was subject to 8 weeks of obesogenic HFD feeding, or to pair feeding of a standard rodent diet. Hyperphagia, obesity, adipose inflammation, and insulin resistance were present in obese mice despite consuming a standard rodent diet, and these effects were blunted with caloric restriction. However, hyperphagic obese mice had normal mitochondrial respiratory function in all tissues tested and no discernable intestinal dysbiosis relative to lean littermates. In contrast, feeding mice an obesogenic HFD altered the composition of the gut microbiome, impaired skeletal muscle mitochondrial bioenergetics, and promoted poor glucose control. These data show that adipose inflammation and redox stress occurred in all models of obesity, but gut dysbiosis and mitochondrial respiratory dysfunction are not always required for obesity-induced insulin resistance. Rather, changes in the intestinal microbiome and mitochondrial bioenergetics may reflect physiological consequences of HFD feeding.

Hepatic Choline Transport Is Inhibited During Fatty Acid-Induced Lipotoxicity and Obesity

Choline is an essential nutrient and a critical component of the membrane phospholipid phosphatidylcholine (PC), the neurotransmitter acetylcholine, while also contributing to the methylation pathway. In the liver specifically, PC is the major membrane constituent and can be synthesized by the cytidine diphosphate-choline or the phosphatidylethanolamine N-methyltransferase pathway. With the continuing global rise in the rates of obesity and nonalcoholic fatty liver disease, we sought to explore how excess fatty acids on primary hepatocytes and diet-induced obesity affect choline uptake and metabolism. Our results demonstrate that hepatocytes chronically treated with palmitate, but not oleate or a mixture, had decreased choline uptake, which was associated with lower choline incorporation into PC and lower expression of choline transport proteins. Interestingly, a reduction in the rate of degradation spared PC levels in response to palmitate when compared with control. The effects of palmitate treatment were independent of endoplasmic reticulum stress, which counterintuitively augmented choline transport and transporter expression. In a model of obesity-induced hepatic steatosis, male mice fed a 60% high-fat diet for 10 weeks had significantly diminished hepatic choline uptake compared with lean mice fed a control diet. Although the transcript and protein expression of various choline metabolic enzymes fluctuated slightly, we observed reduced protein expression of choline transporter-like 1 (CTL1) in the liver of mice fed a high-fat diet. Polysome profile analyses revealed that in livers of obese mice, the CTL1 transcript, despite being more abundant, was translated to a lesser extent compared with lean controls. Finally, human liver cells demonstrated a similar response to palmitate treatment. Conclusion: Our results suggest that the altered fatty acid milieu seen in obesity-induced fatty liver disease progression may adversely affect choline metabolism, potentially through CTL1, but that compensatory mechanisms work to maintain phospholipid homeostasis.

AMPK Promotes Xenophagy through Priming of Autophagic Kinases upon Detection of Bacterial Outer Membrane Vesicles

The autophagy pathway is an essential facet of the innate immune response, capable of rapidly targeting intracellular bacteria. However, the initial signaling regulating autophagy induction in response to pathogens remains largely unclear. Here, we report that AMPK, an upstream activator of the autophagy pathway, is stimulated upon detection of pathogenic bacteria, before bacterial invasion. Bacterial recognition occurs through the detection of outer membrane vesicles. We found that AMPK signaling relieves mTORC1-mediated repression of the autophagy pathway in response to infection, positioning the cell for a rapid induction of autophagy. Moreover, activation of AMPK and inhibition of mTORC1 in response to bacteria is not accompanied by an induction of bulk autophagy. However, AMPK signaling is required for the selective targeting of bacteria-containing vesicles by the autophagy pathway through the activation of pro-autophagic kinase complexes. These results demonstrate a key role for AMPK signaling in coordinating the rapid autophagic response to bacteria.

The citrus flavonoid nobiletin confers protection from metabolic dysregulation in high-fat-fed mice independent of AMPK

Obesity, dyslipidemia, and insulin resistance, the increasingly common metabolic syndrome, are risk factors for CVD and type 2 diabetes that warrant novel therapeutic interventions. The flavonoid nobiletin displays potent lipid-lowering and insulin-sensitizing properties in mice with metabolic dysfunction. However, the mechanisms by which nobiletin mediates metabolic protection are not clearly established. The central role of AMP-activated protein kinase (AMPK) as an energy sensor suggests that AMPK is a target of nobiletin. We tested the hypothesis that metabolic protection by nobiletin required phosphorylation of AMPK and acetyl-CoA carboxylase (ACC) in mouse hepatocytes, in mice deficient in hepatic AMPK (Ampkβ1 ^-/-), in mice incapable of inhibitory phosphorylation of ACC (AccDKI), and in mice with adipocyte-specific AMPK deficiency (iβ1β2AKO). We fed mice a high-fat/high-cholesterol diet with or without nobiletin. Nobiletin increased phosphorylation of AMPK and ACC in primary mouse hepatocytes, which was associated with increased FA oxidation and attenuated FA synthesis. Despite loss of ACC phosphorylation in Ampkβ1 ^-/- hepatocytes, nobiletin suppressed FA synthesis and enhanced FA oxidation. Acute injection of nobiletin into mice did not increase phosphorylation of either AMPK or ACC in liver. In mice fed a high-fat diet, nobiletin robustly prevented obesity, hepatic steatosis, dyslipidemia, and insulin resistance, and it improved energy expenditure in Ampkβ1 ^-/-, AccDKI, and iβ1β2AKO mice to the same extent as in WT controls. Thus, the beneficial metabolic effects of nobiletin in vivo are conferred independently of hepatic or adipocyte AMPK activation. These studies further underscore the therapeutic potential of nobiletin and begin to clarify possible mechanisms.

In Vitro Hepatitis C Virus Infection and Hepatic Choline Metabolism

Choline is an essential nutrient required for normal neuronal and muscular development, as well as homeostatic regulation of hepatic metabolism. In the liver, choline is incorporated into the main eukaryotic phospholipid, phosphatidylcholine (PC), and can enter one-carbon metabolism via mitochondrial oxidation. Hepatitis C virus (HCV) is a hepatotropic positive-strand RNA virus that similar to other positive-strand RNA viruses and can impact phospholipid metabolism. In the current study we sought to interrogate if HCV modulates markers of choline metabolism following in vitro infection, while subsequently assessing if the inhibition of choline uptake and metabolism upon concurrent HCV infection alters viral replication and infectivity. Additionally, we assessed whether these parameters were consistent between cells cultured in fetal bovine serum (FBS) or human serum (HS), conditions known to differentially affect in vitro HCV infection. We observed that choline transport in FBS- and HS-cultured Huh7.5 cells is facilitated by the intermediate affinity transporter, choline transporter-like family (CTL). HCV infection in FBS, but not HS-cultured cells diminished CTL1 transcript and protein expression at 24 h post-infection, which was associated with lower choline uptake and lower incorporation of choline into PC. No changes in other transporters were observed and at 96 h post-infection, all differences were normalized. Reciprocally, limiting the availability of choline for PC synthesis by use of a choline uptake inhibitor resulted in increased HCV replication at this early stage (24 h post-infection) in both FBS- and HS-cultured cells. Finally, in chronic infection (96 h post-infection), inhibiting choline uptake and metabolism significantly impaired the production of infectious virions. These results suggest that in addition to a known role of choline kinase, the transport of choline, potentially via CTL1, might also represent an important and regulated process during HCV infection.

Interleukin-18 up-regulates amino acid transporters and facilitates amino acid-induced mTORC1 activation in natural killer cells

Upon inflammation, natural killer (NK) cells undergo metabolic changes to support their high energy demand for effector function and proliferation. The metabolic changes are usually accompanied by an increase in the expression of nutrient transporters, leading to increased nutrient uptake. Among various cytokines inducing NK cell proliferation, the mechanisms underlying the effect of interleukin (IL)-18 in promoting NK cell proliferation are not completely understood. Here, we demonstrate that IL-18 is a potent cytokine that can enhance the expression of the nutrient transporter CD98/LAT1 for amino acids independently of the mTORC1 pathway and thereby induce a dramatic metabolic change associated with increased proliferation of NK cells. Notably, treatment of IL-18-stimulated NK cells with leucine activates the metabolic sensor mTORC1, indicating that the high expression of amino acid transporters induces amino acid-driven mTORC1 activation. Inhibition of the amino acid transporter CD98/LAT1 abrogated the leucine-driven mTORC1 activation and reduced NK cell effector function. Taken together, our study identified a novel role of IL-18 in up-regulating nutrient transporters on NK cells and thereby inducing metabolic changes, including the mTORC1 activation by amino acids.

A role for phosphatidylcholine and phosphatidylethanolamine in hepatic insulin signaling

Phosphatidylethanolamine N-methyltransferase (PEMT) is an important enzyme in hepatic phosphatidylcholine (PC) biosynthesis. Pemt^-/- mice fed a high-fat diet are protected from obesity and whole-body insulin resistance. However, Pemt^-/- mice develop severe nonalcoholic steatohepatitis (NASH). Because NASH is often associated with hepatic insulin resistance, we investigated whether the increased insulin sensitivity in Pemt^-/- mice was restricted to nonhepatic tissues or whether the liver was also insulin sensitive. Strikingly, the livers of Pemt^-/- mice compared with those of Pemt^+/+ mice were not insulin resistant, despite elevated levels of hepatic triacylglycerols and diacylglycerols, as well as increased hepatic inflammation and fibrosis. Endogenous glucose production was lower in Pemt^-/- mice under both basal and hyperinsulinemic conditions. Experiments in primary hepatocytes and hepatoma cells revealed improved insulin signaling in the absence of PEMT, which was not due to changes in diacylglycerols, ceramides, or gangliosides. On the other hand, the phospholipid composition in hepatocytes seems critically important for insulin signaling such that lowering the PC:phosphatidylethanolamine (PE) ratio improves insulin signaling. Thus, treatments to reduce the PC:PE ratio in liver may protect against the development of hepatic insulin resistance.-Van der Veen, J. N., Lingrell, S., McCloskey, N., LeBlond, N. D., Galleguillos, D., Zhao, Y. Y., Curtis, J. M., Sipione, S., Fullerton, M. D., Vance, D. E., Jacobs, R. L. A role for phosphatidylcholine and phosphatidylethanolamine in hepatic insulin signaling.

Inhibition of Adenosine Monophosphate-Activated Protein Kinase-3-Hydroxy-3-Methylglutaryl Coenzyme A Reductase Signaling Leads to Hypercholesterolemia and Promotes Hepatic Steatosis and Insulin Resistance

Adenosine monophosphate-activated protein kinase (AMPK) regulates multiple signaling pathways involved in glucose and lipid metabolism in response to changes in hormonal and nutrient status. Cell culture studies have shown that AMPK phosphorylation and inhibition of the rate-limiting enzyme in the mevalonate pathway 3-hydroxy-3-methylglutaryl (HMG) coenzyme A (CoA) reductase (HMGCR) at serine-871 (Ser871; human HMGCR Ser872) suppresses cholesterol synthesis. In order to evaluate the role of AMPK-HMGCR signaling in vivo, we generated mice with a Ser871-alanine (Ala) knock-in mutation (HMGCR KI). Cholesterol synthesis was significantly suppressed in wild-type (WT) but not in HMGCR KI hepatocytes in response to AMPK activators. Liver cholesterol synthesis and cholesterol levels were significantly up-regulated in HMGCR KI mice. When fed a high-carbohydrate diet, HMGCR KI mice had enhanced triglyceride synthesis and liver steatosis, resulting in impaired glucose homeostasis. Conclusion: AMPK-HMGCR signaling alone is sufficient to regulate both cholesterol and triglyceride synthesis under conditions of a high-carbohydrate diet. Our findings highlight the tight coupling between the mevalonate and fatty acid synthesis pathways as well as revealing a role of AMPK in suppressing the deleterious effects of a high-carbohydrate diet.

Does prenylation predict progression in NAFLD?

Non-alcoholic fatty liver disease (NAFLD) often develops in concert with related metabolic diseases, such as obesity, dyslipidemia and insulin resistance. Prolonged lipid accumulation and inflammation can progress to non-alcoholic steatohepatitis (NASH). Although factors associated with the development of NAFLD are known, triggers for the progression of NAFLD to NASH are poorly understood. Recent findings published in The Journal of Pathology reveal the possible regulation of NASH progression by metabolites of the mevalonate pathway. Mevalonate can be converted into the isoprenoids farnesyldiphosphate (FPP) and geranylgeranyl diphosphate (GGPP). GGPP synthase (GGPPS), the enzyme that converts FPP to GGPP, is dysregulated in humans and mice during NASH. Both FPP and GGPP can be conjugated to proteins through prenylation, modifying protein function and localization. Deletion or knockdown of GGPPS favors FPP prenylation (farnesylation) and augments the function of liver kinase B1, an upstream kinase of AMP-activated protein kinase (AMPK). Despite increased AMPK activation, livers in Ggpps-deficient mice on a high-fat diet poorly oxidize lipids due to mitochondrial dysfunction. Although work from Liu et al provides evidence as to the potential importance of the prenylation portion of the mevalonate pathway during NAFLD, future studies are necessary to fully grasp any therapeutic or diagnostic potential. Copyright © 2018 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Choline transport links macrophage phospholipid metabolism and inflammation

Choline is an essential nutrient that is required for synthesis of the main eukaryote phospholipid, phosphatidylcholine. Macrophages are innate immune cells that survey and respond to danger and damage signals. Although it is well-known that energy metabolism can dictate macrophage function, little is known as to the importance of choline homeostasis in macrophage biology. We hypothesized that the uptake and metabolism of choline are important for macrophage inflammation. Polarization of primary bone marrow macrophages with lipopolysaccharide (LPS) resulted in an increased rate of choline uptake and higher levels of PC synthesis. This was attributed to a substantial increase in the transcript and protein expression of the choline transporter-like protein-1 (CTL1) in polarized cells. We next sought to determine the importance of choline uptake and CTL1 for macrophage immune responsiveness. Chronic pharmacological or CTL1 antibody-mediated inhibition of choline uptake resulted in altered cytokine secretion in response to LPS, which was associated with increased levels of diacylglycerol and activation of protein kinase C. These experiments establish a previously unappreciated link between choline phospholipid metabolism and macrophage immune responsiveness, highlighting a critical and regulatory role for macrophage choline uptake via the CTL1 transporter.

Methods to Evaluate AMPK Regulation of Macrophage Cholesterol Homeostasis

Macrophages are a driving force in the development and progression of atherosclerosis, a chronic condition that can lead to cardiovascular disease. In this chapter we describe methods that monitor macrophage cholesterol homeostasis such as cholesterol synthesis, uptake, and efflux, all with the use of AMPK activators and potential genetic models that could help shed light on the role of this metabolic regulator in atherosclerosis and other chronic diseases.

The apolipoprotein C-III (Gln38Lys) variant associated with human hypertriglyceridemia is a gain-of-function mutation

Recent cell culture and animal studies have suggested that expression of human apo C-III in the liver has a profound impact on the triacylglycerol (TAG)-rich VLDL₁ production under lipid-rich conditions. The apoC-III Gln38Lys variant was identified in subjects of Mexican origin with moderate hypertriglyceridemia. We postulated that Gln38Lys (C3^QK), being a gain-of-function mutation, promotes hepatic VLDL₁ assembly/secretion. To test this hypothesis, we expressed C3^QK in McA-RH7777 cells and apoc3-null mice to contrast its effect with WT apoC-III (C3^WT). In both model systems, C3^QK expression increased the secretion of VLDL₁-TAG (by 230%) under lipid-rich conditions. Metabolic labeling experiments with C3^QK cells showed an increase in de novo lipogenesis (DNL). Fasting plasma concentration of TAG, cholesterol, cholesteryl ester, and FA were increased in C3^QK mice as compared with C3^WT mice. Liver of C3^QK mice also displayed an increase in DNL and expression of lipogenic genes as compared with that in C3^WT mice. These results suggest that C3^QK variant is a gain-of-function mutation that can stimulate VLDL₁ production, through enhanced DNL.

Muramyl Dipeptide-Based Postbiotics Mitigate Obesity-Induced Insulin Resistance via IRF4

Intestinal dysbiosis contributes to obesity and insulin resistance, but intervening with antibiotics, prebiotics, or probiotics can be limited by specificity or sustained changes in microbial composition. Postbiotics include bacterial components such as lipopolysaccharides, which have been shown to promote insulin resistance during metabolic endotoxemia. We found that bacterial cell wall-derived muramyl dipeptide (MDP) is an insulin-sensitizing postbiotic that requires NOD2. Injecting MDP lowered adipose inflammation and reduced glucose intolerance in obese mice without causing weight loss or altering the composition of the microbiome. MDP reduced hepatic insulin resistance during obesity and low-level endotoxemia. NOD1-activating muropeptides worsened glucose tolerance. IRF4 distinguished opposing glycemic responses to different types of peptidoglycan and was required for MDP/NOD2-induced insulin sensitization and lower metabolic tissue inflammation during obesity and endotoxemia. IRF4 was dispensable for exacerbated glucose intolerance via NOD1. Mifamurtide, an MDP-based drug with orphan drug status, was an insulin sensitizer at clinically relevant doses in obese mice.

AMP-activated protein kinase and its multifaceted regulation of hepatic metabolism

PURPOSE OF REVIEW

The current review summarizes recent advancements in our mechanistic and physiological understanding of the energy sensing AMP-activated protein kinase (AMPK) and its regulation of select aspects of hepatic metabolism.

RECENT FINDINGS

A highly conserved serine/threonine kinase, AMPK governs a multitude of cellular process to activate catabolic and inhibit anabolic pathways. Recent work has provided clarity as to the importance and contribution of the AMPK signaling cascade to various aspects of cellular metabolism, including lipid homeostasis, hepatic glucose production, mitochondrial metabolism, and autophagy.

SUMMARY

With more than 60 confirmed substrates, the physiological significance of AMPK signaling has been difficult to ascertain. The generation of targeted knock-in mutations on key AMPK substrates has begun to shed light on this complex system. Future studies are needed to further decipher the complexity, significance, and potential therapeutic targeting of hepatic AMPK signaling.

Defective NOD2 peptidoglycan sensing promotes diet-induced inflammation, dysbiosis, and insulin resistance

Pattern recognition receptors link metabolite and bacteria-derived inflammation to insulin resistance during obesity. We demonstrate that NOD2 detection of bacterial cell wall peptidoglycan (PGN) regulates metabolic inflammation and insulin sensitivity. An obesity-promoting high-fat diet (HFD) increased NOD2 in hepatocytes and adipocytes, and NOD2(-/-) mice have increased adipose tissue and liver inflammation and exacerbated insulin resistance during a HFD. This effect is independent of altered adiposity or NOD2 in hematopoietic-derived immune cells. Instead, increased metabolic inflammation and insulin resistance in NOD2(-/-) mice is associated with increased commensal bacterial translocation from the gut into adipose tissue and liver. An intact PGN-NOD2 sensing system regulated gut mucosal bacterial colonization and a metabolic tissue dysbiosis that is a potential trigger for increased metabolic inflammation and insulin resistance. Gut dysbiosis in HFD-fed NOD2(-/-) mice is an independent and transmissible factor that contributes to metabolic inflammation and insulin resistance when transferred to WT, germ-free mice. These findings warrant scrutiny of bacterial component detection, dysbiosis, and protective immune responses in the links between inflammatory gut and metabolic diseases, including diabetes.

MicroRNA-33-dependent regulation of macrophage metabolism directs immune cell polarization in atherosclerosis

Cellular metabolism is increasingly recognized as a controller of immune cell fate and function. MicroRNA-33 (miR-33) regulates cellular lipid metabolism and represses genes involved in cholesterol efflux, HDL biogenesis, and fatty acid oxidation. Here, we determined that miR-33-mediated disruption of the balance of aerobic glycolysis and mitochondrial oxidative phosphorylation instructs macrophage inflammatory polarization and shapes innate and adaptive immune responses. Macrophage-specific Mir33 deletion increased oxidative respiration, enhanced spare respiratory capacity, and induced an M2 macrophage polarization-associated gene profile. Furthermore, miR-33-mediated M2 polarization required miR-33 targeting of the energy sensor AMP-activated protein kinase (AMPK), but not cholesterol efflux. Notably, miR-33 inhibition increased macrophage expression of the retinoic acid-producing enzyme aldehyde dehydrogenase family 1, subfamily A2 (ALDH1A2) and retinal dehydrogenase activity both in vitro and in a mouse model. Consistent with the ability of retinoic acid to foster inducible Tregs, miR-33-depleted macrophages had an enhanced capacity to induce forkhead box P3 (FOXP3) expression in naive CD4(+) T cells. Finally, treatment of hypercholesterolemic mice with miR-33 inhibitors for 8 weeks resulted in accumulation of inflammation-suppressing M2 macrophages and FOXP3(+) Tregs in plaques and reduced atherosclerosis progression. Collectively, these results reveal that miR-33 regulates macrophage inflammation and demonstrate that miR-33 antagonism is atheroprotective, in part, by reducing plaque inflammation by promoting M2 macrophage polarization and Treg induction.

High intensity interval training improves liver and adipose tissue insulin sensitivity

Objective

Endurance exercise training reduces insulin resistance, adipose tissue inflammation and non-alcoholic fatty liver disease (NAFLD), an effect often associated with modest weight loss. Recent studies have indicated that high-intensity interval training (HIIT) lowers blood glucose in individuals with type 2 diabetes independently of weight loss; however, the organs affected and mechanisms mediating the glucose lowering effects are not known. Intense exercise increases phosphorylation and inhibition of acetyl-CoA carboxylase (ACC) by AMP-activated protein kinase (AMPK) in muscle, adipose tissue and liver. AMPK and ACC are key enzymes regulating fatty acid metabolism, liver fat content, adipose tissue inflammation and insulin sensitivity but the importance of this pathway in regulating insulin sensitivity with HIIT is unknown.

Methods

In the current study, the effects of 6 weeks of HIIT were examined using obese mice with serine–alanine knock-in mutations on the AMPK phosphorylation sites of ACC1 and ACC2 (AccDKI) or wild-type (WT) controls.

Results

HIIT lowered blood glucose and increased exercise capacity, food intake, basal activity levels, carbohydrate oxidation and liver and adipose tissue insulin sensitivity in HFD-fed WT and AccDKI mice. These changes occurred independently of weight loss or reductions in adiposity, inflammation and liver lipid content.

Conclusions

These data indicate that HIIT lowers blood glucose levels by improving adipose and liver insulin sensitivity independently of changes in adiposity, adipose tissue inflammation, liver lipid content or AMPK phosphorylation of ACC.

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High-intensity interval training (HIIT) improves exercise capacity and whole-body glucose homeostasis.

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HIIT enhances liver and adipose tissue insulin sensitivity independent of body weight and adiposity.

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HIIT does not change adipose tissue cell size, macrophage infiltration, inflammation and liver lipid content.

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HIIT exercise training improves insulin sensitivity independently of the AMPK-ACC signaling pathway.

Abstract 533: Salicylate Restores Macrophage Cholesterol Homeostasis via Activation of AMP-Activated Protein Kinase

Objectives: Atherosclerosis stems from imbalances in lipid metabolism and leads to maladaptive inflammatory responses. AMP-activated protein kinase (AMPK) is a highly conserved serine/threonine kinase that regulates many aspects of lipid and energy metabolism, although its specific role in controlling macrophage foam cell cholesterol homeostasis remains unclear.

Methods: We sought to address this question by testing the effects of AMPK-specific activators in primary bone marrow-derived macrophages from AMPK β1-deficient (β1-/-) mice.

Results: Macrophages from AMPK β1-/- mice had enhanced lipogenic potential and diminished cholesterol efflux, although cholesterol uptake was unaffected. Specific activation of Ampk β1 via salicylate (the unacetylated form of aspirin) or A-769662 (a small molecule activator), decreased the synthesis of both fatty acids and sterols in WT but not AMPK β1-/- macrophages. In lipid-laden macrophage foam cells, salicylate and A-769662 decreased cholesterol uptake and increased cholesterol efflux to HDL and apoA-I, effects that occurred in an AMPK β1-dependent manner. Increased cholesterol efflux was also associated with increased gene and protein expression of the ATP binding cassette transporters, ABCG1 and ABCA1. Moreover, in vivo reverse cholesterol transport was significantly suppressed in mice that received AMPK β1-/- macrophages compared to WT control.

Conclusion: Our data highlight the therapeutic potential of targeting macrophage AMPK with new or existing drugs for the restoration of cholesterol homeostasis during the early stages of atherosclerosis.

Salicylate improves macrophage cholesterol homeostasis via activation of Ampk

Atherosclerosis stems from imbalances in lipid metabolism and leads to maladaptive inflammatory responses. The AMP-activated protein kinase (Ampk) is a highly conserved serine/threonine kinase that regulates many aspects of lipid and energy metabolism, although its specific role in controlling macrophage cholesterol homeostasis remains unclear. We sought to address this question by testing the effects of direct Ampk activators in primary bone marrow-derived macrophages from Ampk β1-deficient (β1^−/−) mice. Macrophages from Ampk β1^−/− mice had enhanced lipogenic capacity and diminished cholesterol efflux, although cholesterol uptake was unaffected. Direct activation of Ampk β1 via salicylate (the unacetylated form of aspirin) or A-769662 (a small molecule activator), decreased the synthesis of FAs and sterols in WT but not Ampk β1^−/− macrophages. In lipid-laden macrophages, Ampk activation decreased cholesterol content (foam cell formation) and increased cholesterol efflux to HDL and apoA-I, effects that occurred in an Ampk β1-dependent manner. Increased cholesterol efflux was also associated with increased gene expression of the ATP binding cassette transporters, Abcg1 and Abca1. Moreover, in vivo reverse cholesterol transport was suppressed in mice that received Ampk β1^−/− macrophages compared with the WT control. Our data highlight the therapeutic potential of targeting macrophage Ampk with new or existing drugs for the possible reduction in foam cell formation during the early stages of atherosclerosis.

Fluvastatin causes NLRP3 inflammasome-mediated adipose insulin resistance

Statins reduce lipid levels and are widely prescribed. Statins have been associated with an increased incidence of type 2 diabetes, but the mechanisms are unclear. Activation of the NOD-like receptor family, pyrin domain containing 3 (NLRP3)/caspase-1 inflammasome, promotes insulin resistance, a precursor of type 2 diabetes. We showed that four different statins increased interleukin-1β (IL-1β) secretion from macrophages, which is characteristic of NLRP3 inflammasome activation. This effect was dose dependent, absent in NLRP3(-/-) mice, and prevented by caspase-1 inhibition or the diabetes drug glyburide. Long-term fluvastatin treatment of obese mice impaired insulin-stimulated glucose uptake in adipose tissue. Fluvastatin-induced activation of the NLRP3/caspase-1 pathway was required for the development of insulin resistance in adipose tissue explants, an effect also prevented by glyburide. Fluvastatin impaired insulin signaling in lipopolysaccharide-primed 3T3-L1 adipocytes, an effect associated with increased caspase-1 activity, but not IL-1β secretion. Our results define an NLRP3/caspase-1-mediated mechanism of statin-induced insulin resistance in adipose tissue and adipocytes, which may be a contributing factor to statin-induced development of type 2 diabetes. These results warrant scrutiny of insulin sensitivity during statin use and suggest that combination therapies with glyburide, or other inhibitors of the NLRP3 inflammasome, may be effective in preventing the adverse effects of statins.

AMPK phosphorylation of ACC2 is required for skeletal muscle fatty acid oxidation and insulin sensitivity in mice

AIMS/HYPOTHESIS

Obesity is characterised by lipid accumulation in skeletal muscle, which increases the risk of developing insulin resistance and type 2 diabetes. AMP-activated protein kinase (AMPK) is a sensor of cellular energy status and is activated in skeletal muscle by exercise, hormones (leptin, adiponectin, IL-6) and pharmacological agents (5-amino-4-imidazolecarboxamide ribonucleoside [AICAR] and metformin). Phosphorylation of acetyl-CoA carboxylase 2 (ACC2) at S221 (S212 in mice) by AMPK reduces ACC activity and malonyl-CoA content but the importance of the AMPK-ACC2-malonyl-CoA pathway in controlling fatty acid metabolism and insulin sensitivity is not understood; therefore, we characterised Acc2 S212A knock-in (ACC2 KI) mice.

METHODS

Whole-body and skeletal muscle fatty acid oxidation and insulin sensitivity were assessed in ACC2 KI mice and wild-type littermates.

RESULTS

ACC2 KI mice were resistant to increases in skeletal muscle fatty acid oxidation elicited by AICAR. These mice had normal adiposity and liver lipids but elevated contents of triacylglycerol and ceramide in skeletal muscle, which were associated with hyperinsulinaemia, glucose intolerance and skeletal muscle insulin resistance.

CONCLUSIONS/INTERPRETATION

These findings indicate that the phosphorylation of ACC2 S212 is required for the maintenance of skeletal muscle lipid and glucose homeostasis.

Reduced skeletal muscle AMPK and mitochondrial markers do not promote age-induced insulin resistance

In both rodents and humans, aging-associated reductions in skeletal muscle AMP-activated protein kinase (AMPK) activity and mitochondrial function have been linked to the development of skeletal muscle insulin resistance. However, whether reductions in skeletal muscle AMPK and mitochondrial capacity actually precipitate the development of aging-induced insulin resistance is not known. Mice lacking both isoforms of the AMPK β-subunit in skeletal muscle (AMPK-MKO) have no detectable AMPK activity and are characterized by large reductions in exercise capacity, mitochondrial content, and contraction-stimulated glucose uptake making them an ideal model to determine whether reductions in AMPK and mitochondrial content promote the development of aging-induced insulin resistance. In the current study we find that a lack of skeletal muscle AMPK results in a life-long reduction in mitochondrial activity but does not affect body mass, body composition, glucose tolerance, or insulin sensitivity as measured by hyperinsulinemic-euglycemic clamp in mice of old age (18 mo). These data demonstrate that reductions in skeletal muscle AMPK and mitochondrial activity do not cause the development of age-induced insulin resistance.

PPARδ activation attenuates hepatic steatosis in Ldlr-/- mice by enhanced fat oxidation, reduced lipogenesis, and improved insulin sensitivity

PPARδ regulates systemic lipid homeostasis and inflammation, but its role in hepatic lipid metabolism remains unclear. Here, we examine whether intervening with a selective PPARδ agonist corrects hepatic steatosis induced by a high-fat, cholesterol-containing (HFHC) diet. Ldlr(-/-) mice were fed a chow or HFHC diet (42% fat, 0.2% cholesterol) for 4 weeks. For an additional 8 weeks, the HFHC group was fed HFHC or HFHC plus GW1516 (3 mg/kg/day). GW1516-intervention significantly attenuated liver TG accumulation by induction of FA β-oxidation and attenuation of FA synthesis. In primary mouse hepatocytes, GW1516 treatment stimulated AMP-activated protein kinase (AMPK) and acetyl-CoA carboxylase (ACC) phosphorylation in WT hepatocytes, but not AMPKβ1(-/-) hepatocytes. However, FA oxidation was only partially reduced in AMPKβ1(-/-) hepatocytes, suggesting an AMPK-independent contribution to the GW1516 effect. Similarly, PPARδ-mediated attenuation of FA synthesis was partially due to AMPK activation, as GW1516 reduced lipogenesis in WT hepatocytes but not AMPKβ1(-/-) hepatocytes. HFHC-fed animals were hyperinsulinemic and exhibited selective hepatic insulin resistance, which contributed to elevated fasting FA synthesis and hyperglycemia. GW1516 intervention normalized fasting hyperinsulinemia and selective hepatic insulin resistance and attenuated fasting FA synthesis and hyperglycemia. The HFHC diet polarized the liver toward a proinflammatory M1 state, which was reversed by GW1516 intervention. Thus, PPARδ agonist treatment inhibits the progression of preestablished hepatic steatosis.

Endurance interval training in obese mice reduces muscle inflammation and macrophage content independently of weight loss

Obesity is associated with chronic low‐grade inflammation that involves infiltration of macrophages into metabolic organs such as skeletal muscle. Exercise enhances skeletal muscle insulin sensitivity independently of weight loss; but its role in regulating muscle inflammation is not fully understood. We hypothesized that exercise training would inhibit skeletal muscle inflammation and alter macrophage infiltration into muscle independently of weight loss. Wild type C57BL/6 male mice were fed a chow diet or a high‐fat diet (HFD, 45% calories fat) for 6 weeks. Then, mice maintained on the HFD either remained sedentary (HFD Sed) or exercised (HFD Ex) on a treadmill for another 6 weeks. The exercise training protocol involved conducting intervals of 2 min in duration followed by 2 min of rest for 60 min thrice weekly. Chow‐fed control mice remained sedentary for the entire 12 weeks. Muscle cytokine and macrophage gene expression analysis were conducted using qRT‐PCR, and muscle macrophage content was also measured using immunohistochemistry. Muscle cytokine protein content was quantified using a cytokine array. The HFD increased adiposity and weight gain compared to chow‐fed controls. HFD Sed and HFD Ex mice had similar body mass as well as total and visceral adiposity. However, despite similar adiposity, exercise reduced inflammation and muscle macrophage infiltration. We conclude that Endurance exercise training modulates the immune‐metabolic crosstalk in obesity independently of weight loss, and may have potential benefits in reducing obesity‐related muscle inflammation.

e12012

Obesity is associated with chronic low‐grade inflammation that involves infiltration of macrophages into metabolic organs such as skeletal muscle. Exercise enhances skeletal muscle insulin sensitivity independently of weight loss; but its role in regulating muscle inflammation is not fully understood. In this article, we show that endurance interval training inhibited skeletal muscle inflammation and reduced macrophage infiltration into muscle independently of weight loss in mice.

Single phosphorylation sites in Acc1 and Acc2 regulate lipid homeostasis and the insulin-sensitizing effects of metformin

The obesity epidemic has led to an increased incidence of nonalcoholic fatty liver disease (NAFLD) and type 2 diabetes. AMP-activated protein kinase (Ampk) regulates energy homeostasis and is activated by cellular stress, hormones and the widely prescribed type 2 diabetes drug metformin. Ampk phosphorylates mouse acetyl-CoA carboxylase 1 (Acc1; refs. 3,4) at Ser79 and Acc2 at Ser212, inhibiting the conversion of acetyl-CoA to malonyl-CoA. The latter metabolite is a precursor in fatty acid synthesis and an allosteric inhibitor of fatty acid transport into mitochondria for oxidation. To test the physiological impact of these phosphorylation events, we generated mice with alanine knock-in mutations in both Acc1 (at Ser79) and Acc2 (at Ser212) (Acc double knock-in, AccDKI). Compared to wild-type mice, these mice have elevated lipogenesis and lower fatty acid oxidation, which contribute to the progression of insulin resistance, glucose intolerance and NAFLD, but not obesity. Notably, AccDKI mice made obese by high-fat feeding are refractory to the lipid-lowering and insulin-sensitizing effects of metformin. These findings establish that inhibitory phosphorylation of Acc by Ampk is essential for the control of lipid metabolism and, in the setting of obesity, for metformin-induced improvements in insulin action.

Diacylglycerol kinase delta promotes lipogenesis

We have studied the relationship between diacylglycerol kinase delta (DGKδ) and lipogenesis. There is a marked increase in the expression of DGKδ during the differentiation of 3T3-L1 cells to adipocytes, as well as in the synthesis of neutral and polar lipids. When 3T3-L1 undifferentiated fibroblasts are transfected to express DGKδ, there is increased triglyceride synthesis without differentiation to adipocytes. Hence, expression of DGKδ promotes lipogenesis. Lipid synthesis is decreased in DGKδ knockout mouse embryo fibroblasts, especially for lipids with shorter acyl chains and limited unsaturation. This reduction occurs for both neutral and polar lipids. These findings suggest reduced de novo lipid synthesis. This is confirmed by measuring the incorporation of glycerol into polar and neutral lipids, which is higher in the wild type cells than in the DGKδ knockouts. In comparison, there was no change in lipid synthesis in DGKε knockout mouse embryo fibroblasts. We also demonstrate that the DGKδ knockout cells had a lower expression of acetyl-CoA carboxylase and fatty acid synthase as well as a lower degree of activation by phosphorylation of ATP citrate lyase. These three enzymes are involved in the synthesis of long chain fatty acids. Our results demonstrate that DGKδ markedly increases lipid synthesis, at least in part as a result of promoting the de novo synthesis of fatty acids.

Immunometabolism of AMPK in insulin resistance and atherosclerosis

Obesity leads to insulin resistance and atherosclerosis, which precede Type 2 diabetes and cardiovascular disease. Immunometabolism addresses how metabolic and inflammatory pathways converge to maintain health and a contemporary problem is determining how obesity-induced inflammation precipitates chronic diseases such as insulin resistance and atherosclerosis. AMP-activated protein kinase (AMPK) is an important serine/threonine kinase well known for regulating metabolic processes and maintaining energy homeostasis. However, both metabolic and immunological AMPK-mediated effects play a role in disease. Pro-inflammatory mediators suppress AMPK activity and hinder lipid oxidation. In addition, AMPK activation curbs inflammation by directly inhibiting pro-inflammatory signaling pathways and limiting the build-up of specific lipid intermediates that elicit immune responses. In the context of obesity and chronic disease, these reciprocal responses involve both immune and metabolic cells. Therefore, the immunometabolism of AMPK-mediated processes and therapeutics should be considered in atherosclerosis and insulin resistance.

Deletion of skeletal muscle SOCS3 prevents insulin resistance in obesity

Obesity is associated with chronic low-grade inflammation that contributes to defects in energy metabolism and insulin resistance. Suppressor of cytokine signaling (SOCS)-3 expression is increased in skeletal muscle of obese humans. SOCS3 inhibits leptin signaling in the hypothalamus and insulin signal transduction in adipose tissue and the liver. Skeletal muscle is an important tissue for controlling energy expenditure and whole-body insulin sensitivity; however, the physiological importance of SOCS3 in this tissue has not been examined. Therefore, we generated mice that had SOCS3 specifically deleted in skeletal muscle (SOCS MKO). The SOCS3 MKO mice had normal muscle development, body mass, adiposity, appetite, and energy expenditure compared with wild-type (WT) littermates. Despite similar degrees of obesity when fed a high-fat diet, SOCS3 MKO mice were protected against the development of hyperinsulinemia and insulin resistance because of enhanced skeletal muscle insulin receptor substrate 1 (IRS1) and Akt phosphorylation that resulted in increased skeletal muscle glucose uptake. These data indicate that skeletal muscle SOCS3 does not play a critical role in regulating muscle development or energy expenditure, but it is an important contributing factor for inhibiting insulin sensitivity in obesity. Therapies aimed at inhibiting SOCS3 in skeletal muscle may be effective in reversing obesity-related glucose intolerance and insulin resistance.

Loss of TDAG51 results in mature-onset obesity, hepatic steatosis, and insulin resistance by regulating lipogenesis

Regulation of energy metabolism is critical for the prevention of obesity, diabetes, and hepatic steatosis. Here, we report an important role for the pleckstrin homology-related domain family member, T-cell death-associated gene 51 (TDAG51), in the regulation of energy metabolism. TDAG51 expression was examined during adipocyte differentiation. Adipogenic potential of preadipocytes with knockdown or absence of TDAG51 was assessed. Weight gain, insulin sensitivity, metabolic rate, and liver lipid content were also compared between TDAG51-deficient (TDAG51(-/-)) and wild-type mice. In addition to its relatively high expression in liver, TDAG51 was also present in white adipose tissue (WAT). TDAG51 was downregulated during adipogenesis, and TDAG51(-/-) preadipocytes exhibited greater lipogenic potential. TDAG51(-/-) mice fed a chow diet exhibited greater body and WAT mass, had reduced energy expenditure, displayed mature-onset insulin resistance (IR), and were predisposed to hepatic steatosis. TDAG51(-/-) mice had increased hepatic triglycerides and SREBP-1 target gene expression. Furthermore, TDAG51 expression was inversely correlated with fatty liver in multiple mouse models of hepatic steatosis. Taken together, our findings suggest that TDAG51 is involved in energy homeostasis at least in part by regulating lipogenesis in liver and WAT, and hence, may constitute a novel therapeutic target for the treatment of obesity and IR.

Reduced Socs3 expression in adipose tissue protects female mice against obesity-induced insulin resistance

AIMS/HYPOTHESIS

Inflammation in obesity increases the levels of the suppressor of cytokine signalling-3 (SOCS3) protein in adipose tissue, but the physiological importance of this protein in regulating whole-body insulin sensitivity in obesity is not known.

METHODS

We generated Socs3 floxed (wild-type, WT) and Socs3 aP2 (also known as Fabp4)-Cre null (Socs3 AKO) mice. Mice were maintained on either a regular chow or a high-fat diet (HFD) for 16 weeks during which time body mass, adiposity, glucose homeostasis and insulin sensitivity were assessed.

RESULTS

The HFD increased SOCS3 levels in adipose tissue of WT but not Socs3 AKO mice. WT and Socs3 AKO mice had similar body mass and adiposity, assessed using computed tomography (CT) imaging, irrespective of diet or sex. On a control chow diet there were no differences in insulin sensitivity or glucose tolerance. When fed a HFD, female but not male Socs3 AKO mice had improved glucose tolerance as well as lower fasting glucose and insulin levels compared with WT littermates. Hyperinsulinaemic-euglycaemic clamps and positron emission tomography (PET) imaging demonstrated that improved insulin sensitivity was due to elevated adipose tissue glucose uptake. Increased insulin-stimulated glucose uptake in adipose tissue was associated with enhanced levels and activating phosphorylation of insulin receptor substrate-1 (IRS1).

CONCLUSIONS/INTERPRETATION

These data demonstrate that inhibiting SOCS3 production in adipose tissue of female mice is effective for improving whole-body insulin sensitivity in obesity.

Mechanism of hypertriglyceridemia in CTP:phosphoethanolamine cytidylyltransferase-deficient mice

Phosphatidylethanolamine is an important inner-leaflet phospholipid, and CTP:phosphoethanolamine cytidylyltransferase-Pcyt2 acts as the main regulator of the de novo phosphatidylethanolamine synthesis from ethanolamine and diacylglycerol. Complete deletion of the mouse Pcyt2 gene is embryonic lethal, and the single-allele deficiency leads to development of the metabolic syndrome phenotype, including liver steatosis, hypertriglyceridemia, obesity, and insulin resistance. This study aimed to specifically elucidate the mechanisms of hypertriglyceridemia in Pcyt2 heterozygous mice (Pcyt2(+/-)). Evidence here shows that unlike 8 week-old mice, 32 week- and 42 week-old Pcyt2(+/-) mice experience increased VLDL secretion and liver microsomal triglyceride transfer protein activity. Older Pcyt2(+/-) mice also demonstrate increased levels of postprandial plasma TAGs, increased stimulation of genes responsible for intestinal lipid absorption, transport and chylomicron secretion, and dramatically elevated plasma Angptl4, apoB-100, and apoB-48 content. In addition, plasma HL and LPL activities and TAG clearance following a lipid challenge were significantly reduced in Pcyt2(+/-) mice relative to control littermates. Collectively, these results establish that the hypertriglyceridemia that accompanies Pcyt2 deficiency is the result of multiple metabolic adaptations, including elevated hepatic and intestinal lipoprotein secretion and stimulated expression and/or activity of genes involved in lipid absorption and transport and lipoprotein assembly, together with reduced plasma TAG clearance and utilization with peripheral tissues.