Loss of hepatic Nmnat1 has no impact on diet-induced fatty liver disease.

Nicotinamide adenine dinucleotide (NAD+), a biological molecule integral to redox reactions involved in multiple cellular processes, has the potential to treat nonalcoholic fatty liver diseases (NAFLDs) and nonalcoholic steatohepatitis (NASH). Nicotinamide mononucleotide adenylyltransferase (Nmnat1), one of the NAD+ biosynthesizing enzymes, plays a central role in all NAD+ metabolic pathways and it is vital to embryonic development. However, the function of Nmnat1 in metabolic pathology and, specifically, in the development and progression of NAFLD and NASH remains unexplored. First, we generated hepatic Nmnat1 knockout (H-Nmnat1-/-) mice to investigate the physiological function of Nmnat1 and found that NAD+ levels were significantly lower in H-Nmnat1-/- mice than control mice. However, H-Nmnat1-/- mice appeared normal with comparable metabolic activity. Next, we used three different diet-induced NASH models to assess the pathophysiological role of Nmant1 in metabolic disorders and discovered that hepatic loos of Nmnat1 decreased 35%-40% of total NAD+ in an obese state. Nevertheless, our analysis of phenotypic variations found comparable body composition, gene expression, and liver histology in all NASH models in H-Nmnat1-/- mice. We also found that aged H-Nmnat1-/- mice exhibited comparable liver phenotypes with control mice. These findings suggest that Nmnat1 has a redundancy to the pathophysiology of obesity-induced hepatic disorders.

Crystal structure of GCN5 PCAF N-terminal domain reveals atypical ubiquitin ligase structure.

General control nonderepressible 5 (GCN5, also known as Kat2a) and p300/CBP-associated factor (PCAF, also known as Kat2b) are two homologous acetyltransferases. Both proteins share similar domain architecture consisting of a PCAF N-terminal (PCAF_N) domain, acetyltransferase domain, and a bromodomain. PCAF also acts as a ubiquitin E3 ligase whose activity is attributable to the PCAF_N domain, but its structural aspects are largely unknown. Here, we demonstrated that GCN5 exhibited ubiquitination activity in a similar manner to PCAF and its activity was supported by the ubiquitin-conjugating enzyme UbcH5. Moreover, we determined the crystal structure of the PCAF_N domain at 1.8 Å resolution and found that PCAF_N domain folds into a helical structure with a characteristic binuclear zinc region, which was not predicted from sequence analyses. The zinc region is distinct from known E3 ligase structures, suggesting this region may form a new class of E3 ligase. Our biochemical and structural study provides new insight into not only the functional significance of GCN5 but also into ubiquitin biology.

EP4 receptor-associated protein regulates gluconeogenesis in the liver and is associated with hyperglycemia in diabetic mice.

Prostaglandin E2 receptor 4-associated protein (EPRAP) is a key molecule in suppressing inflammatory responses in macrophages. EPRAP is expressed not only in macrophages but also in hepatocytes; however, the role of EPRAP in hepatocytes has not yet been defined. To examine the physiological role of hepatic EPRAP in mice, we performed the glucose tolerance test and the hyperinsulinemic-euglycemic clamp in high-fat sucrose diet (HFSD)-fed wild-type (WT) and Eprap null mice. We evaluated the contribution of EPRAP to gluconeogenesis by pyruvate tolerance test and primary hepatocyte experiments. Furthermore, lentivirus-expressing Eprap-specific small-hairpin RNA was injected in db/ db mice. HFSD-fed Eprap null mice had significantly lower blood glucose levels than HFSD-fed WT mice. Eprap null mice also had low glucose levels after fasting or pyruvic acid injection. Moreover, primary hepatocytes from Eprap-deficient mice showed decreased glucose production and lower expression of the Phosphoenol pyruvate carboxykinase and Glucose 6-phosphatase genes. Lentivirus-mediated hepatic Eprap suppression decreased glucose levels and the expression of gluconeogenic genes in db/ db mice. We conclude that EPRAP regulates gluconeogenesis in hepatocytes and is associated with hyperglycemia in diabetic mice. Our data suggest that suppression of EPRAP could be a novel strategy for the treatment of diabetes.

Paternal allelic mutation at the Kcnq1 locus reduces pancreatic ß-cell mass by epigenetic modification of Cdkn1c.

Genetic factors are important determinants of the onset and progression of diabetes mellitus. Numerous susceptibility genes for type 2 diabetes, including potassium voltage-gated channel, KQT-like subfamily Q, member1 (KCNQ1), have been identified in humans by genome-wide analyses and other studies. Experiments with genetically modified mice have also implicated various genes in the pathogenesis of diabetes. However, the possible effects of the parent of origin for diabetes susceptibility alleles on disease onset have remained unclear. Here, we show that a mutation at the Kcnq1 locus reduces pancreatic ß-cell mass in mice by epigenetic modulation only when it is inherited from the father. The noncoding RNA KCNQ1 overlapping transcript1 (Kcnq1ot1) is expressed from the Kcnq1 locus and regulates the expression of neighboring genes on the paternal allele. We found that disruption of Kcnq1 results in reduced Kcnq1ot1 expression as well as the increased expression of cyclin-dependent kinase inhibitor 1C (Cdkn1c), an imprinted gene that encodes a cell cycle inhibitor, only when the mutation is on the paternal allele. Furthermore, histone modification at the Cdkn1c promoter region in pancreatic islets was found to contribute to this phenomenon. Our observations suggest that the Kcnq1 genomic region directly regulates pancreatic ß-cell mass and that genomic imprinting may be a determinant of the onset of diabetes mellitus.

Dietary Mung Bean Protein Reduces Hepatic Steatosis, Fibrosis, and Inflammation in Male Mice with Diet-Induced, Nonalcoholic Fatty Liver Disease.

BACKGROUND: As the prevalence of nonalcoholic fatty liver disease (NAFLD), including steatosis and nonalcoholic steatohepatitis, is increasing, novel dietary approaches are required for the prevention and treatment of NAFLD. OBJECTIVE: We evaluated the potential of mung bean protein isolate (MuPI) to prevent NAFLD progression. METHODS: In Expts. 1 and 2, the hepatic triglyceride (TG) concentration was compared between 8-wk-old male mice fed a high-fat diet (61% of energy from fat) containing casein, MuPI, and soy protein isolate and an MuPI-constituent amino acid mixture as a source of amino acids (18% of energy) for 4 wk. In Expt. 3, hepatic fatty acid synthase (Fasn) expression was evaluated in 8-wk-old male Fasn-promoter-reporter mice fed a casein- or MuPI-containing high-fat diet for 20 wk. In Expt. 4, hepatic fibrosis was examined in 8-wk-old male mice fed an atherogenic diet (61% of energy from fat, containing 1.3 g cholesterol/100 g diet) containing casein or MuPI (18% of energy) as a protein source for 20 wk. RESULTS: In the high fat-diet mice, the hepatic TG concentration in the MuPI group decreased by 66% and 47% in Expt. 1 compared with the casein group (P < 0.001) and the soy protein isolate group (P = 0.001), respectively, and decreased by 56% in Expt. 2 compared with the casein group (P = 0.011). However, there was no difference between the MuPI-constituent amino acid mixture and casein groups in Expt. 2. In Expt. 3, Fasn-promoter-reporter activity and hepatic TG concentration were lower in the MuPI group than in those fed casein (P < 0.05). In Expt. 4, in mice fed an atherogenic diet, hepatic fibrosis was not induced in the MuPI group, whereas it developed overtly in the casein group. CONCLUSION: MuPI potently reduced hepatic lipid accumulation in mice and may be a potential foodstuff to prevent NAFLD onset and progression.

Hepatocyte ß-Klotho regulates lipid homeostasis but not body weight in mice.

ß-Klotho (ß-Kl), a transmembrane protein expressed in the liver, pancreas, adipose tissues, and brain, is essential for feedback suppression of hepatic bile acid synthesis. Because bile acid is a key regulator of lipid and energy metabolism, we hypothesized potential and tissue-specific roles of ß-Kl in regulating plasma lipid levels and body weight. By crossing ß-kl(-/-) mice with newly developed hepatocyte-specific ß-kl transgenic (Tg) mice, we generated mice expressing ß-kl solely in hepatocytes (ß-kl(-/-)/Tg). Gene expression, metabolomic, and in vivo flux analyses consistently revealed that plasma level of cholesterol, which is over-excreted into feces as bile acids in ß-kl(-/-), is maintained in ß-kl(-/-) mice by enhanced de novo cholesterogenesis. No compensatory increase in lipogenesis was observed, despite markedly decreased plasma triglyceride. Along with enhanced bile acid synthesis, these lipid dysregulations in ß-kl(-/-) were completely reversed in ß-kl(-/-)/Tg mice. In contrast, reduced body weight and resistance to diet-induced obesity in ß-kl(-/-) mice were not reversed by hepatocyte-specific restoration of ß-Kl expression. We conclude that ß-Kl in hepatocytes is necessary and sufficient for lipid homeostasis, whereas nonhepatic ß-Kl regulates energy metabolism. We further demonstrate that in a condition with excessive cholesterol disposal, a robust compensatory mechanism maintains cholesterol levels but not triglyceride levels in mice.

Growth arrest and DNA damage-inducible 34 regulates liver regeneration in hepatic steatosis in mice.

UNLABELLED: The liver has robust regenerative potential in response to damage, but hepatic steatosis (HS) weakens this potential. We found that the enhanced integrated stress response (ISR) mediated by phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF2α) impairs regeneration in HS and that growth arrest and DNA damage-inducible 34 (Gadd34)-dependent suppression of ISR plays a crucial role in fatty liver regeneration. Although mice fed a high-fat diet for 2 weeks developed moderate fatty liver with no increase in eIF2α phosphorylation before 70% hepatectomy, they showed impaired liver regeneration as a result of reduced proliferation and increased death of hepatocytes with increased phosphorylation of eIF2α and ISR. An increased ISR through Gadd34 knockdown induced C/EBP homologous protein (CHOP)-dependent apoptosis and receptor-interacting protein kinase 3-dependent necrosis, resulting in increased hepatocyte death during fatty liver regeneration. Furthermore, Gadd34 knockdown and increased phosphorylation of eIF2α decreased cyclin D1 protein and reduced hepatocyte proliferation. In contrast, enhancement of Gadd34 suppressed phosphorylation of eIF2α and reduced CHOP expression and hepatocyte apoptosis without affecting hepatocyte proliferation, clearly improving fatty liver regeneration. In more severe fatty liver of leptin receptor-deficient db/db mice, forced expression of hepatic Gadd34 also promoted hepatic regeneration after hepatectomy. CONCLUSION: Gadd34-mediated regulation of ISR acts as a physiological defense mechanism against impaired liver regeneration resulting from steatosis and is thus a possible therapeutic target for impaired regeneration in HS.

The transcription factor ATF3 switches cell death from apoptosis to necroptosis in hepatic steatosis in male mice.

Hepatocellular death increases with hepatic steatosis aggravation, although its regulation remains unclear. Here we show that hepatic steatosis aggravation shifts the hepatocellular death mode from apoptosis to necroptosis, causing increased hepatocellular death. Our results reveal that the transcription factor ATF3 acts as a master regulator in this shift by inducing expression of RIPK3, a regulator of necroptosis. In severe hepatic steatosis, after partial hepatectomy, hepatic ATF3-deficient or -overexpressing mice display decreased or increased RIPK3 expression and necroptosis, respectively. In cultured hepatocytes, ATF3 changes TNFα-dependent cell death mode from apoptosis to necroptosis, as revealed by live-cell imaging. In non-alcoholic steatohepatitis (NASH) mice, hepatic ATF3 deficiency suppresses RIPK3 expression and hepatocellular death. In human NASH, hepatocellular damage is correlated with the frequency of hepatocytes expressing ATF3 or RIPK3, which overlap frequently. ATF3-dependent RIPK3 induction, causing a modal shift of hepatocellular death, can be a therapeutic target for steatosis-induced liver damage, including NASH.

Hepatic FASN deficiency differentially affects nonalcoholic fatty liver disease and diabetes in mouse obesity models.

Nonalcoholic fatty liver disease (NAFLD) and type 2 diabetes are interacting comorbidities of obesity, and increased hepatic de novo lipogenesis (DNL), driven by hyperinsulinemia and carbohydrate overload, contributes to their pathogenesis. Fatty acid synthase (FASN), a key enzyme of hepatic DNL, is upregulated in association with insulin resistance. However, the therapeutic potential of targeting FASN in hepatocytes for obesity-associated metabolic diseases is unknown. Here, we show that hepatic FASN deficiency differentially affects NAFLD and diabetes depending on the etiology of obesity. Hepatocyte-specific ablation of FASN ameliorated NAFLD and diabetes in melanocortin 4 receptor-deficient mice but not in mice with diet-induced obesity. In leptin-deficient mice, FASN ablation alleviated hepatic steatosis and improved glucose tolerance but exacerbated fed hyperglycemia and liver dysfunction. The beneficial effects of hepatic FASN deficiency on NAFLD and glucose metabolism were associated with suppression of DNL and attenuation of gluconeogenesis and fatty acid oxidation, respectively. The exacerbation of fed hyperglycemia by FASN ablation in leptin-deficient mice appeared attributable to impairment of hepatic glucose uptake triggered by glycogen accumulation and citrate-mediated inhibition of glycolysis. Further investigation of the therapeutic potential of hepatic FASN inhibition for NAFLD and diabetes in humans should thus consider the etiology of obesity.

Impaired regulation of hepatic glucose production in mice lacking the forkhead transcription factor Foxo1 in liver.

The hallmark of type 2 diabetes is excessive hepatic glucose production. Several transcription factors and coactivators regulate this process in cultured cells. But gene ablation experiments have yielded few clues as to the physiologic mediators of this process in vivo. We show that inactivation of the gene encoding forkhead protein Foxo1 in mouse liver results in 40% reduction of glucose levels at birth and 30% reduction in adult mice after a 48 hr fast. Gene expression and glucose clamp studies demonstrate that Foxo1 ablation impairs fasting- and cAMP-induced glycogenolysis and gluconeogenesis. Pgc1alpha is unable to induce gluconeogenesis in Foxo1-deficient hepatocytes, while the cAMP response is significantly blunted. Conversely, Foxo1 deletion in liver curtails excessive glucose production caused by generalized ablation of insulin receptors and prevents neonatal diabetes and hepatosteatosis in insulin receptor knockout mice. The data provide a unifying mechanism for regulation of hepatic glucose production by cAMP and insulin.

Overexpression of KLF15 transcription factor in adipocytes of mice results in down-regulation of SCD1 protein expression in adipocytes and consequent enhancement of glucose-induced insulin secretion.

Krüppel-like factor 15 (KLF15), a member of the Krüppel-like factor family of transcription factors, has been found to play diverse roles in adipocytes in vitro. However, little is known of the function of KLF15 in adipocytes in vivo. We have now found that the expression of KLF15 in adipose tissue is down-regulated in obese mice, and we therefore generated adipose tissue-specific KLF15 transgenic (aP2-KLF15 Tg) mice to investigate the possible contribution of KLF15 to various pathological conditions associated with obesity in vivo. The aP2-KLF15 Tg mice manifest insulin resistance and are resistant to the development of obesity induced by maintenance on a high fat diet. However, they also exhibit improved glucose tolerance as a result of enhanced insulin secretion. Furthermore, this enhancement of insulin secretion was shown to result from down-regulation of the expression of stearoyl-CoA desaturase 1 (SCD1) in white adipose tissue and a consequent reduced level of oxidative stress. This is supported by the findings that restoration of SCD1 expression in white adipose tissue of aP2-KLF15 Tg mice exhibited increased oxidative stress in white adipose tissue and reduced insulin secretion with hyperglycemia. Our data thus provide an example of cross-talk between white adipose tissue and pancreatic ß cells mediated through modulation of oxidative stress.

Living and working environments are important determinants of glycemic control in patients with diabetes during the COVID-19 pandemic: A retrospective observational study.

AIM: To investigate (1) the association of lifestyle changes and living and working conditions with glycemic control and (2) whether treatment was intensified appropriately in patients with diabetes under the first COVID-19 state of emergency in Japan. MATERIALS AND METHODS: A total of 321 participants were included. Participants completed a questionnaire regarding lifestyle changes, including diet, physical activity, and living and working conditions during the COVID-19 pandemic. The change in hemoglobin A1c (HbA1c) levels was estimated before (June 1, 2019 to August 31, 2019) and during (June 1, 2020 to August 31, 2020) the pandemic. Factors associated with changes in HbA1c levels were examined by multiple linear regression analysis. The proportion of patients who received treatment intensification for diabetes was compared between before and during the pandemic. RESULTS: There was no significant change in HbA1c levels before the pandemic and during the pandemic (7.13 ± 0.98% vs 7.18 ± 1.01%, P = 0.186). Teleworking (estimate 0.206, P = 0.004) and living with a dog (estimate -0.149, P = 0.038) were significantly associated with changes in HbA1c levels after adjusting for covariates. There was no significant difference in the proportion of patients who received treatment intensification for diabetes during the pandemic and before the pandemic in either the elderly or non-elderly patients. CONCLUSIONS: Overall glycemic control did not worsen during the pandemic. Nonetheless, environmental factors, including telework, were found to influence glycemic control in patients with diabetes. Further studies are needed to clarify whether the COVID-19 pandemic could affect treatment intensification for diabetes.

Role of hepatic STAT3 in brain-insulin action on hepatic glucose production.

STAT3 regulates glucose homeostasis by suppressing the expression of gluconeogenic genes in the liver. The mechanism by which hepatic STAT3 is regulated by nutritional or hormonal status has remained unknown, however. Here, we show that an increase in the plasma insulin concentration, achieved either by glucose administration or by intravenous insulin infusion, stimulates tyrosine phosphorylation of STAT3 in the liver. This effect of insulin was mediated by the hormone's effects in the brain, and the increase in hepatic IL-6 induced by the brain-insulin action is essential for the activation of STAT3. The inhibition of hepatic glucose production and of expression of gluconeogenic genes induced by intracerebral ventricular insulin infusion was impaired in mice with liver-specific STAT3 deficiency or in mice with IL-6 deficiency. These results thus indicate that IL-6-STAT3 signaling in the liver contributes to insulin action in the brain, leading to the suppression of hepatic glucose production.

Role of STAT-3 in regulation of hepatic gluconeogenic genes and carbohydrate metabolism in vivo.

The transcription factor, signal transducer and activator of transcription-3 (STAT-3) contributes to various physiological processes. Here we show that mice with liver-specific deficiency in STAT-3, achieved using the Cre-loxP system, show insulin resistance associated with increased hepatic expression of gluconeogenic genes. Restoration of hepatic STAT-3 expression in these mice, using adenovirus-mediated gene transfer, corrected the metabolic abnormalities and the alterations in hepatic expression of gluconeogenic genes. Overexpression of STAT-3 in cultured hepatocytes inhibited gluconeogenic gene expression independently of peroxisome proliferator-activated receptor-gamma coactivator-1 alpha (PGC-1 alpha), an upstream regulator of gluconeogenic genes. Liver-specific expression of a constitutively active form of STAT-3, achieved by infection with an adenovirus vector, markedly reduced blood glucose, plasma insulin concentrations and hepatic gluconeogenic gene expression in diabetic mice. Hepatic STAT-3 signaling is thus essential for normal glucose homeostasis and may provide new therapeutic targets for diabetes mellitus.

Diet intake control is indispensable for the gluconeogenic response to sodium-glucose cotransporter 2 inhibition in male mice.

AIMS/INTRODUCTION: Sodium-glucose cotransporter 2 inhibitor (SGLT2i) lowers blood glucose and causes a whole-body energy deficit by boosting renal glucose excretion, thus affecting glucose and energy metabolism. This energy deficit not only decreases bodyweight, but also increases food intake. This food intake increase offsets the SGLT2i-induced bodyweight decrease, but the effect of the food intake increase on the SGLT2i regulation of glucose metabolism remains unclear. MATERIALS AND METHODS: We administered SGLT2i (luseogliflozin) for 4 weeks to hepatic gluconeogenic enzyme gene G6pc reporter mice with/without obesity, which were either fed freely or under a 3-hourly dietary regimen. The effect of feeding condition on the gluconeogenic response to SGLT2i was evaluated by plasma Gaussia luciferase activity, an index of the hepatic gluconeogenic response, in G6pc reporter mice. Energy expenditure was measured by indirect calorimetry. RESULTS: In the lean mice under controlled feeding, SGLT2i decreased bodyweight and plasma glucose, and increased the hepatic gluconeogenic response while decreasing blood insulin. SGLT2i also increased oxygen consumption under controlled feeding. However, free feeding negated all of these effects of SGLT2i. In the obese mice, SGLT2i decreased bodyweight, blood glucose and plasma insulin, ameliorated the upregulated hepatic gluconeogenic response, and increased oxygen consumption under controlled feeding. Under free feeding, although blood glucose was decreased and plasma insulin tended to decrease, the effects of SGLT2i - decreased bodyweight, alleviation of the hepatic gluconeogenic response and increased oxygen consumption - were absent. CONCLUSIONS: Food intake management is crucial for SGLT2i to affect glucose and energy metabolism during type 2 diabetes treatment.

All roads lead to FoxO.

Stress-activated kinases control metabolism by antagonizing the early steps of insulin signal transduction. Two papers now demonstrate that Jnk, the prototypical stress-activated kinase, controls life span in Drosophila and C. elegans by promoting phosphorylation of the forkhead protein FoxO (Oh et al., 2005; Wang et al., 2005). The findings provide yet another mechanism by which metabolic and stress responses are integrated via phosphorylation of FoxO proteins.

Podocyte-specific deletion of tubular sclerosis complex 2 promotes focal segmental glomerulosclerosis and progressive renal failure.

Obesity can initiate and accelerate the progression of kidney diseases. However, it remains unclear how obesity affects renal dysfunction. Here, we show that a newly generated podocyte-specific tubular sclerosis complex 2 (Tsc2) knockout mouse model (Tsc2Δpodocyte) develops proteinuria and dies due to end-stage renal dysfunction by 10 weeks of age. Tsc2Δpodocyte mice exhibit an increased glomerular size and focal segmental glomerulosclerosis, including podocyte foot process effacement, mesangial sclerosis and proteinaceous casts. Podocytes isolated from Tsc2Δpodocyte mice show nuclear factor, erythroid derived 2, like 2-mediated increased oxidative stress response on microarray analysis and their autophagic activity is lowered through the mammalian target of rapamycin (mTOR)-unc-51-like kinase 1 pathway. Rapamycin attenuated podocyte dysfunction and extends survival in Tsc2Δpodocyte mice. Additionally, mTOR complex 1 (mTORC1) activity is increased in podocytes of renal biopsy specimens obtained from obese patients with chronic kidney disease. Our work shows that mTORC1 hyperactivation in podocytes leads to severe renal dysfunction and that inhibition of mTORC1 activity in podocytes could be a key therapeutic target for obesity-related kidney diseases.

Stepwise cell fate decision pathways during osteoclastogenesis at single-cell resolution.

Osteoclasts are the exclusive bone-resorbing cells, playing a central role in bone metabolism, as well as the bone damage that occurs under pathological conditions1,2. In postnatal life, haematopoietic stem-cell-derived precursors give rise to osteoclasts in response to stimulation with macrophage colony-stimulating factor and receptor activator of nuclear factor-κB ligand, both of which are produced by osteoclastogenesis-supporting cells such as osteoblasts and osteocytes1-3. However, the precise mechanisms underlying cell fate specification during osteoclast differentiation remain unclear. Here, we report the transcriptional profiling of 7,228 murine cells undergoing in vitro osteoclastogenesis, describing the stepwise events that take place during the osteoclast fate decision process. Based on our single-cell transcriptomic dataset, we find that osteoclast precursor cells transiently express CD11c, and deletion of receptor activator of nuclear factor-κB specifically in CD11c-expressing cells inhibited osteoclast formation in vivo and in vitro. Furthermore, we identify Cbp/p300-interacting transactivator with Glu/Asp-rich carboxy-terminal domain 2 (Cited2) as the molecular switch triggering terminal differentiation of osteoclasts, and deletion of Cited2 in osteoclast precursors in vivo resulted in a failure to commit to osteoclast fate. Together, the results of this study provide a detailed molecular road map of the osteoclast differentiation process, refining and expanding our understanding of the molecular mechanisms underlying osteoclastogenesis.

GCN2 regulates pancreatic ß cell mass by sensing intracellular amino acid levels.

EIF2AK4, which encodes the amino acid deficiency-sensing protein GCN2, has been implicated as a susceptibility gene for type 2 diabetes in the Japanese population. However, the mechanism by which GCN2 affects glucose homeostasis is unclear. Here, we show that insulin secretion is reduced in individuals harboring the risk allele of EIF2AK4 and that maintenance of GCN2-deficient mice on a high-fat diet results in a loss of pancreatic ß cell mass. Our data suggest that GCN2 senses amino acid deficiency in ß cells and limits signaling by mechanistic target of rapamycin complex 1 to prevent ß cell failure during the consumption of a high-fat diet.

Dual role of transcription factor FoxO1 in controlling hepatic insulin sensitivity and lipid metabolism.

Hepatic insulin resistance affects both carbohydrate and lipid metabolism. It has been proposed that insulin controls these 2 metabolic branches through distinct signaling pathways. FoxO transcription factors are considered effectors of the pathway regulating hepatic glucose production. Here we show that adenoviral delivery of constitutively nuclear forkhead box O1 (FoxO1) to mouse liver results in steatosis arising from increased triglyceride accumulation and decreased fatty acid oxidation. FoxO1 gain of function paradoxically increased insulin sensitivity by promoting Akt phosphorylation, while FoxO1 inhibition via siRNA decreased it. We show that FoxO1 regulation of Akt phosphorylation does not require DNA binding and is associated with repression of the pseudokinase tribble 3 (Trb3), a modulator of Akt activity. This unexpected dual role of FoxO1 in promoting insulin sensitivity and lipid synthesis in addition to glucose production has the potential to explain the peculiar admixture of insulin resistance and sensitivity that is commonly observed in the metabolic syndrome.