American journal of physiology. Endocrinology and metabolism最新文献

Objective: Hyperglucagonemia is a hallmark of metabolic diseases including type 2 diabetes and metabolic dysfunction-associated steatotic liver disease (MASLD), yet the underlying mechanisms are unclear. This study aimed to characterize the liver's role in glucagon clearance and to elucidate whether enzymatic degradation or receptor-mediated uptake is the dominant clearance mechanism-particularly in the context of obesity-associated hepatic steatosis. Methods: Using an in situ perfused single-pass mouse liver model, hepatic glucagon clearance/ disappearance was quantified directly in lean and diet-induced obese (DIO) male C57BL/6JRj mice. Glucagon disappearance across the liver was measured via immunoassays and mass spectrometry. To dissect the mechanism of clearance, glucagon was co-infused with either enzyme inhibitors (for dipeptidyl peptidase 4(DPP-4) and neprilysin) or a glucagon receptor antagonist. Results: In lean mice, the liver extracted 20% of inflowing portal venous glucagon (at levels corresponding to postprandial conditions), an effect abolished by glucagon receptor antagonism but unaffected by enzyme inhibition. Mass spectrometry confirmed that nearly all glucagon remained intact, supporting receptor-mediated internalization as the primary clearance mechanism. In DIO mice with hepatic steatosis determined as elevated hepatic triglyceride content, glucagon clearance was markedly reduced. The glucagon receptor was downregulated in livers of DIO mice. Conclusion: These findings identify the liver as an active site of glucagon clearance, potentially through receptor-mediated mechanisms. Obesity-related hepatic steatosis disrupts this process, and may contribute to hyperglucagonemia in metabolic disease. Targeting hepatic glucagon clearance may offer a novel approach to normalize glucagon levels and improve metabolic control.

Older people are prone to involuntary fasting, but systematic assessment of their mobilization and oxidation of the three macronutrients during fasting has not been previously reported. Because of changes in body composition and metabolic regulatory pathways with aging, older people might have different kinetics of utilization of macronutrient stores during fasting than younger people. We measured body composition by dual-energy X-ray absorptiometry and studied the effects of a 36-hour fast on protein mobilization, lipolysis, glucose output, and protein, fat, and carbohydrate oxidation in 10 older (60-81 y) and 10 younger (18-35 y) healthy adults. A 36-hour fast induced a pronounced shift toward fat oxidation: in the postabsorptive state fat oxidation averaged 52-55% of resting energy expenditure (REE) and increased to 63-68% after fasting, with corresponding decreases in carbohydrate oxidation. Protein oxidation decreased with fasting but still contributed 16-18% of REE in older participants. For several metabolic variables (respiratory exchange ratio, glucose Ra, glycerol Ra, urea Ra, urea excretion, and percent of REE from carbohydrate and fat), we observed significant age×fasting interactions (P < 0.01). Younger adults exhibited larger decreases in glucose Ra and greater increases in markers of protein turnover (leucine Ra, urea production) and lipolysis, whereas older adults showed blunted glucose and leucine responses but maintained substantial protein oxidation. These age-related differences suggest that older adults rely relatively more on ongoing protein catabolism during short-term fasting, which may contribute to vulnerability to muscle loss during illness or prolonged inadequate intake.

Hyperaminoacidemia is an early hallmark of insulin resistance, with aromatic and branched chain amino acids particularly associated with insulin resistance and type 2 diabetes. We previously showed that healthy adults with obesity exposed to acute hyperglycemia have lower brain glucose levels measured by magnetic resonance spectroscopy (MRS) than lean controls, suggesting that a blunted brain response to hyperglycemia may be an early marker of insulin resistance. Here, in a secondary analysis of our prior study, we used targeted mass spectrometry-based metabolomics to measure plasma amino acids in participants with and without obesity to determine if changes in peripheral metabolites associated with early insulin resistance such as amino acids were associated with changes in brain glucose levels during hyperglycemia. There were few differences in baseline amino acids between groups, but acute hyperglycemia unveiled higher plasma concentrations of amino acids including cysteine, cystine, glutamic acid, glutamine, methionine, and aromatic amino acids in obesity. Plasma glucagon levels were also higher in obesity during acute hyperglycemia. Higher plasma concentrations of aromatic amino acids and glucagon were significantly correlated with lower brain glucose levels, illustrating parallel development of central and peripheral metabolic changes in obesity.

Gestational diabetes mellitus (GDM) is a form of glucose intolerance that develops during pregnancy, affecting approximately 14% of pregnancies worldwide. GDM leads to an excess supply of nutrients to the fetus, influencing fetal metabolism and potentially stimulating metabolic inflammation (metaflammation), which may impact liver function. In fact, increased fetal liver size has been reported in GDM pregnancies. This study investigated the impact of GDM on fetal metaflammation and its potential association with the liver function parameters γ-glutamyl transferase (GGT), alanine transaminase (ALT), and bilirubin, in umbilical cord plasma (UCP). UCP samples were collected following healthy (n=57) and GDM pregnancies (n=25). Gluco-metabolic and lipid profiles, inflammatory markers, and liver function parameters were analyzed and the impact of GDM and the interrelation of liver biomarkers with maternal and neonatal parameters was assessed. UCP from GDM pregnancies exhibited increased levels of metaflammation-associated markers, including triglycerides, interleukin-6 and C- reactive protein (CRP). Among liver parameters, the detoxification and antioxidant defense enzyme GGT was elevated, whereas ALT and bilirubin species remained unchanged. Correlation analysis revealed that although GGT was increased in GDM, it was associated with improved metabolic characteristics, including reduced insulin resistance, insulin, C-peptide, and leptin, and increased high-density lipoprotein. Overall, these findings indicate that GDM environment shifts fetal metabolism toward metaflammation and elevates GGT, an enzyme involved in detoxification and protection against oxidative stress. The inverse association of GGT with insulin resistance suggests a compensatory role of GGT, potentially mitigating fetal metabolic and inflammatory disturbances in GDM through its antioxidant activity.

Aerobic exercise training is a potent intervention for the treatment and prevention of age-related metabolic disease, which is characterized by both insulin and glucagon resistance. Although insulin resistance is a key driver of metabolic disease in aging, glucagon signaling is equally critical in maintaining both glucose and lipid homeostasis, particularly during exercise. Previous studies have established the glucagon-sensitizing effects of exercise training in younger animals. Most studies in rodents use rigorous and carefully dosed forced exercise protocols. This forced exercise is a stressful paradigm. We implemented a voluntary wheel running (VWR) intervention to assess the effects of aging and exercise training on glucagon sensitivity. We initiated 12-wk of VWR in young adult (6-mo-old) and middle-aged (12-mo-old) C57BL/6NCrl male mice. Glucagon sensitivity, as assessed by glucagon-stimulated hyperglycemia, was decreased in middle-aged compared with young adult sedentary mice (P = 0.046). Although VWR did not affect glucose disposal, circulating insulin, glucagon, or insulin sensitivity, regardless of age, VWR improved glucagon responsivity only in middle-aged mice (P = 0.031). VWR increased hepatic glycogen content and increased glucagon-stimulated glycogen depletion, regardless of age (P < 0.01). Results from these studies suggest that exercise training can enhance liver glucagon action in aging mice without otherwise altering glucose homeostasis.NEW & NOTEWORTHY Few studies have examined the impact of aging on glucagon sensitivity. Here we show that glucagon sensitivity declines from young adulthood to middle age. Yet, 12 wk of voluntary wheel running, an exercise intervention without the stress of forced treadmill running or swimming, restores glucagon sensitivity in middle-aged male mice without otherwise altering glucose homeostasis.

The enzyme selenocysteine (Sec) lyase (SCLY) decomposes Sec, releasing selenide for the synthesis of selenoproteins, which contain Sec in their primary structure and participate in strong redox reactions, maintaining redox balance. We previously showed that global disruption of the Scly gene (Scly KO) in mice leads to obesity. Targeted deletion of Scly in agouti-related peptide neurons enhances energy expenditure and brown adipose tissue (BAT) activation, augmenting leanness. We hypothesized that Scly KO mice develop obesity due to failure of BAT-controlled mechanisms of energy expenditure due to redirection of Sec to an alternative pathway. We analyzed BAT from male Scly KO mice on selenium (Se)-adequate [0.25 parts per million (ppm)] and Se-deficient (0.08 ppm) diets for morphology, Se content, selenoprotein expression, thyroid hormones, and additional Sec-using pathways. We found that the BAT of Scly KO mice was enlarged, with lower Se levels, and substantial whitening on a Se-adequate diet. This phenotype worsened on low Se and coincided with a mild impairment in adapting to cold exposure. BAT whitening coincided with an increase in triglycerides and reduced 3-hydroxy-3-methylglutaryl coenzyme A and cholesterol. BAT selenoproteins regulating energy metabolism, type 2 iodothyronine deiodinase (DIO2), glutathione peroxidase 1 (GPX1), and glutathione peroxidase 1 (GPX4), were significantly decreased. DIO2 reduction corresponded with an increase in thyroxine and thyroid-stimulating hormone and a reduction in heat-producing uncoupling protein 1. Downregulation of GPX4 did not affect ferroptosis in the BAT. Therefore, the whitened BAT of the Scly KO mouse is a multifactorial process involving the disruption of BAT function through changes to selenoproteins involved in energy metabolism.NEW & NOTEWORTHY Global loss of the selenocysteine-decomposing enzyme selenocysteine lyase in mice leads to lipid accumulation and whitening of the brown adipose tissue, with consequent obesity development. Selenocysteine lyase modulates selenium levels and selenoprotein expression, specifically GPX1, GPX4, and DIO2, in brown adipocytes. Selenocysteine metabolic fate hinges on the actions of selenocysteine lyase.

Hepatic steatosis is the initial stage of metabolic dysfunction-associated steatotic liver disease (MASLD) and is highly prevalent among middle-aged men. Low testosterone levels and dietary fructose intake are independent risk factors for MASLD, although these can occur simultaneously. This study investigated the combined effects of testosterone deficiency and fructose intake on hepatic steatosis, focusing on the involvement of gut microbiota. Male mice were castrated or sham-operated at 8 wk of age and administered fructose water with or without antibiotics for 8 wk after being divided into six groups: Sham/Control, Sham/Fructose, Sham/Fructose + Antibiotics, Castration/Control, Castration/Fructose, and Castration/Fructose + Antibiotics. The castrated groups had lower body weights than the sham-operated groups, whereas castration did not affect portal and circulating fructose concentrations. Although castration alone did not affect hepatic lipid accumulation, it synergistically promoted fructose-induced triglyceride accumulation, which was alleviated by antibiotic treatment. The expression of lipogenesis-related genes (Srebp-1c), fatty acid transporters (Cd36), and fructose metabolism-related genes (Aldob, Khk-A, and Khk-C) was upregulated by the combination of castration and fructose intake, but antibiotic administration did not suppress this effect. Castration, fructose intake, and their combination influenced β-diversity, but not α-diversity of gut microbiota composition. Cecal pyruvate concentrations were increased by the combination of castration and fructose intake and were suppressed by antibiotics. PICRUSt2 and MaAsLin2 analyses supported pyruvate accumulation mediated by alterations in the gut microbiota. Furthermore, pyruvate promoted triglyceride accumulation in primary hepatocytes in the presence of fructose. Our results indicated that testosterone deficiency synergistically exacerbates fructose-induced hepatic steatosis, which is partly mediated by gut-derived pyruvate.NEW & NOTEWORTHY The combined effects of the factors that cause liver dysfunction are unclear. Here, we show that fructose intake and testosterone deficiency synergistically induce intestinal pyruvate accumulation and hepatic steatosis by altering gut microbiota and hepatic gene expression. Our findings provide a notion that pyruvate plays a novel role in hepatic steatosis. Moreover, our results suggest that risk factors for hepatic steatosis can exert a greater impact on disease development through complex mechanisms under certain conditions.

Sarcopenia in alcohol-related liver disease (ALD) is of high clinical significance, but there are no effective treatments. Hyaluronan 35 kDa (HA35), a glycosaminoglycan polymer, modulates responses to Toll-like receptor 4 (TLR4), a lipopolysaccharide receptor, to improve hepatic and macrophage function in ALD. We evaluated skeletal muscle responses to HA35 in preclinical models of ALD. Responses to ethanol, lipopolysaccharide (LPS), and HA35 were studied in differentiated murine C2C12/human-induced pluripotent stem cell (hiPSC)-derived myotubes, and wild-type/HA receptor CD44 knockout (CD44^-/-) mouse models of ALD (mALD). Signaling molecules and measures of protein homeostasis (proteostasis) were quantified by immunoblots, mitochondrial oxidative function was determined by high-sensitivity respirofluorometry, membrane elasticity by atomic force microscopy, and muscle contractile responses to electrical stimulation ex vivo were quantified. Multiomics, weighted gene co-expression network, and image analyses were also performed. Ethanol caused a sarcopenic phenotype in myotubes and mALD (lower myotube/muscle fiber diameter, muscle mass), less protein synthesis, impaired mTORC1 signaling, and higher autophagy markers and unaltered membrane elasticity. Expressions of LPS (TLR2, TLR4) and HA (CD44) receptors were upregulated by ethanol. In myotubes, LPS treatment caused a sarcopenic phenotype at lower concentrations in ethanol-treated myotubes. HA35 reversed ethanol/LPS-induced lower protein synthesis, impaired mTORC1 signaling, and mitochondrial complex I function in myotubes/muscle tissue. Muscle concentrations of HA fragments were lower in mALD, but beneficial responses to HA35 occurred without restoring HA concentrations, and HA35 was not beneficial in CD44^-/- mALD. Sarcopenia, signaling perturbations, and mitochondrial oxidative dysfunction in mALD are reversed by HA35, allowing for rapid clinical translation (ongoing clinicaltrials.gov, NCT05018481).NEW & NOTEWORTHY Sarcopenia in alcohol-related liver disease (ALD) is frequent and contributes to adverse clinical outcomes with limited treatment options. Hyaluronans (glycosaminoglycan polymers) of specific fragment sizes (30-40 kDa) have anti-inflammatory and tissue-protective properties. In an array of preclinical models of ALD, we demonstrate that HA35 reverses signaling, mitochondrial oxidative function, and phenotypic perturbations associated with sarcopenia in ALD. These effects are mediated without restoring the low muscle tissue concentrations of HA in ALD.

The Nr4a family of transcription factors controls the expression of genes essential for fuel metabolism and cellular proliferation. The loss of Nr4a1 in the 832/13 INS-1 β-cell line diminishes mitochondrial respiration, decreases ATP production, and impairs glucose-stimulated insulin secretion. Nr4a1 overexpression increases β-cell proliferation, and full-body Nr4a1 knockout mice have decreased β-cell area. Thus, we hypothesize that β-cell Nr4a1 expression plays a critical role in diabetes disease progression. Here we report the effects of β-cell specific Nr4a1 deletion in mice beginning at 3-mo of age. Although Nr4a1 β-cell specific deletion has no deleterious effect on male mice fed a standard or high-fat diet, or on female mice fed a standard chow diet, female mice fed a high-fat diet have decreased glucose tolerance, impaired insulin secretion, impaired expression of key glycolytic genes, and decreased pancreatic β-cell area. We demonstrate that estrogen is sufficient to induce β-cell Nr4a1 expression. Our data suggest that Nr4a1 is critical for maintaining functional β-cell mass in females as a response to the stress of increased adiposity.NEW & NOTEWORTHY We report the effects of β-cell specific Nr4a1 deletion in mice (Nr4a1β^-/-). We determined that Nr4a1β^-/- impairs glucose tolerance in female mice fed a high-fat diet. We found decreases in β-cell mass and glucose-stimulated insulin secretion. Nr4a1β^-/- decreased mRNA and protein levels of key genes involved in glucose utilization and cell cycle progression. Estrogen treatment induces mRNA and protein expression of Nr4a1 in cell lines as well as primary mouse islets.