METTL14 is essential for β-cell survival and insulin secretion

Defects in the development, maintenance or expansion of β-cell mass can result in impaired glucose metabolism and diabetes. N⁶-methyladenosine affects mRNA stability and translation efficiency, and impacts cell differentiation and stress response. To determine if there is a role for m⁶A in β-cells, we investigated the effect of Mettl14, a key component of the m⁶A methyltransferase complex, on β-cell survival and function using rat insulin-2 promoter-Cre-mediated deletion of Mettl14 mouse line (βKO). We found that βKO mice with normal chow exhibited glucose intolerance, lower levels of glucose-stimulated insulin secretion, increased β-cell death and decreased β-cell mass. In addition, HFD-fed βKO mice developed glucose intolerance, decreased β-cell mass and proliferation, exhibited lower body weight, increased adipose tissue mass, and enhanced insulin sensitivity due to enhanced AKT signaling and decreased gluconeogenesis in the liver. HFD-fed βKO mice also showed a decrease in de novo lipogenesis, and an increase in lipolysis in the liver. RNA sequencing in islets revealed that Mettl14 deficiency in β-cells altered mRNA expression levels of some genes related to cell death and inflammation. Together, we showed that Mettl14 in β-cells plays a key role in β-cell survival, insulin secretion and glucose homeostasis.     

Inhibition of high-fat diet–induced obesity via reduction of ER-resident protein Nogo occurs through multiple mechanisms

Obesity is a risk factor for insulin resistance, type 2 diabetes, and cardiovascular diseases. Reticulon-4 (Nogo) is an endoplasmic reticulum–resident protein with unclear functions in obesity. Herein, we investigated the effect of Nogo on obesity and associated metabolic disorders. Human serum samples were collected to explore the relationship between circulating Nogo-B and body mass index value. Nogo-deficient and WT littermate control mice were fed normal chow or high-fat diet (HFD) for 14 weeks, and HFD-induced obese C57BL/6J mice were injected scrambled or Nogo siRNA for 2 weeks. We found that in human and mouse serum, Nogo-B was positively correlated to body mass index/bodyweight and lipid profiles. Reduced Nogo (by genetic deletion or siRNA transfection) protected mice against HFD-induced obesity and related metabolic disorders. We demonstrate that Nogo deficiency reversed HFD-induced whitening of brown adipose tissue, thereby increasing thermogenesis. It also ameliorated lipid accumulation in tissues by activating the adiponectin–adiponectin receptor 1–AMP-activated kinase α signaling axis. Finally, Nogo deficiency potently reduced HFD-induced serum proinflammatory cytokines and infiltration of macrophages into metabolic organs, which is related to enhanced NF-κB p65 degradation via the lysosome pathway. Collectively, our study suggests that reduced levels of Nogo protect mice against HFD-induced obesity by increasing thermogenesis and energy metabolism while inhibiting NF-κB-mediated inflammation. Our results indicate that inhibition of Nogo may be a potential strategy for obesity treatment.     

The METTL5-TRMT112 N6-methyladenosine methyltransferase complex regulates mRNA translation via 18S rRNA methylation

Ribosomal RNAs (rRNAs) have long been known to carry chemical modifications, including 2′O-methylation, pseudouridylation, N⁶-methyladenosine (m⁶A), and N^6,6-dimethyladenosine. While the functions of many of these modifications are unclear, some are highly conserved and occur in regions of the ribosome critical for mRNA decoding. Both 28S rRNA and 18S rRNA carry single m⁶A sites, and while the methyltransferase ZCCHC4 has been identified as the enzyme responsible for the 28S rRNA m⁶A modification, the methyltransferase responsible for the 18S rRNA m⁶A modification has remained unclear. Here, we show that the METTL5-TRMT112 methyltransferase complex installs the m⁶A modification at position 1832 of human 18S rRNA. Our work supports findings that TRMT112 is required for METTL5 stability and reveals that human METTL5 mutations associated with microcephaly and intellectual disability disrupt this interaction. We show that loss of METTL5 in human cancer cell lines and in mice regulates gene expression at the translational level; additionally, Mettl5 knockout mice display reduced body size and evidence of metabolic defects. While recent work has focused heavily on m⁶A modifications in mRNA and their roles in mRNA processing and translation, we demonstrate here that deorphanizing putative methyltransferase enzymes can reveal previously unappreciated regulatory roles for m⁶A in noncoding RNAs.     

Deep and accurate detection of m6A RNA modifications using miCLIP2 and m6Aboost machine learning

N6-methyladenosine (m⁶A) is the most abundant internal RNA modification in eukaryotic mRNAs and influences many aspects of RNA processing. miCLIP (m⁶A individual-nucleotide resolution UV crosslinking and immunoprecipitation) is an antibody-based approach to map m⁶A sites with single-nucleotide resolution. However, due to broad antibody reactivity, reliable identification of m⁶A sites from miCLIP data remains challenging. Here, we present miCLIP2 in combination with machine learning to significantly improve m⁶A detection. The optimized miCLIP2 results in high-complexity libraries from less input material. Importantly, we established a robust computational pipeline to tackle the inherent issue of false positives in antibody-based m⁶A detection. The analyses were calibrated with Mettl3 knockout cells to learn the characteristics of m⁶A deposition, including m⁶A sites outside of DRACH motifs. To make our results universally applicable, we trained a machine learning model, m6Aboost, based on the experimental and RNA sequence features. Importantly, m6Aboost allows prediction of genuine m⁶A sites in miCLIP2 data without filtering for DRACH motifs or the need for Mettl3 depletion. Using m6Aboost, we identify thousands of high-confidence m⁶A sites in different murine and human cell lines, which provide a rich resource for future analysis. Collectively, our combined experimental and computational methodology greatly improves m⁶A identification.     

Faecal microbiota transplantation-mediated jejunal microbiota changes halt high-fat diet-induced obesity in mice via retarding intestinal fat absorption

Faecal Microbiota Transplantation (FMT) is considered as a promising technology to fight against obesity. Wild boar has leanermuscle and less fat in comparison to the domestic pig, which were thought to be related with microbiota. To investigate the function and mechanism of the wild boar microbiota on obesity, we first analysed the wild boar microbiota composition via 16S rDNA sequencing, which showed that Firmicutes and Proteobacteria were the dominant bacteria. Then, we established a high-fat diet (HFD)-induced obesity model, and transfer low and high concentrations of wild boar faecal suspension in mice for 9 weeks. The results showed that FMT prevented HFD-induced obesity and lipid metabolism disorders, and altered the jejunal microbiota composition especially increasing the abundance of the Lactobacillus and Romboutsia, which were negatively correlated with obesity-related indicators. Moreover, we found that the anti-obesity effect of wild boar faecal suspension was associated with jejunal N6-methyladenosine (m⁶A) levels. Overall, these results suggest that FMT has a mitigating effect on HFD-induced obesity, which may be due to the impressive effects of FMT on the microbial composition and structure of the jejunum. These changes further alter intestinal lipid metabolism and m⁶A levels to achieve resistance to obesity.     

5-(4-Hydroxyphenyl)-3H-1,2-dithiole-3-thione-based fibrates as potential hypolipidemic and hepatoprotective agents

Hypolipidemic effects of the newly synthesized 5-(4-hydroxyphenyl)-3H-1,2-dithiole-3-thione-based fibrates were evaluated in Triton WR-1339 and high-fat diet (HFD)-induced hyperlipidemic mice. Preliminary screening of all the synthesized compounds was done by using an acute model (Triton WR-1339 model), in which compound 6 shown more significant antidyslipidemic activity than fenofibrate (FF). The compound 6 was also found to reduce serum triglyceride (TG), total cholesterol (TC) and low density lipoprotein cholesterin (LDL) in HFD-induced hyperlipidemic mice. Moreover, compound 6 displayed hepatoprotective effect, a significant amelioration in hepatic indices (AST and ALT) toxicity was observed and the histological examination showed that compound 6 inhibited the development of hepatic lipid accumulation and ameliorated the damage in hepatic tissue compared to model mice. Additional effects such as the potent antioxidant and anti-inflammatory action confirmed and reinforced the efficacy of compound 6 as a new agent of dual-effect hypolipidemic and hepatoprotective activities.     

Adipocyte inducible 6-phosphofructo-2-kinase suppresses adipose tissue inflammation and promotes macrophage anti-inflammatory activation

Obesity-associated inflammation in white adipose tissue (WAT) is a causal factor of systemic insulin resistance. To better understand how adipocytes regulate WAT inflammation, the present study generated chimeric mice in which inducible 6-phosphofructo-2-kinase was low, normal, or high in WAT while the expression of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 3 (Pfkfb3) was normal in hematopoietic cells, and analyzed changes in high-fat diet (HFD)-induced WAT inflammation and systemic insulin resistance in the mice. Indicated by proinflammatory signaling and cytokine expression, the severity of HFD-induced WAT inflammation in WT → Pfkfb3^+/– mice, whose Pfkfb3 was disrupted in WAT adipocytes but not hematopoietic cells, was comparable with that in WT → WT mice, whose Pfkfb3 was normal in all cells. In contrast, the severity of HFD-induced WAT inflammation in WT → Adi-Tg mice, whose Pfkfb3 was over-expressed in WAT adipocytes but not hematopoietic cells, remained much lower than that in WT → WT mice. Additionally, HFD-induced insulin resistance was correlated with the status of WAT inflammation and comparable between WT → Pfkfb3^+/– mice and WT → WT mice, but was significantly lower in WT → Adi-Tg mice than in WT → WT mice. In vitro, palmitoleate decreased macrophage phosphorylation states of Jnk p46 and Nfkb p65 and potentiated the effect of interleukin 4 on suppressing macrophage proinflammatory activation. Taken together, these results suggest that the Pfkfb3 in adipocytes functions to suppress WAT inflammation. Moreover, the role played by adipocyte Pfkfb3 is attributable to, at least in part, palmitoleate promotion of macrophage anti-inflammatory activation.     

Therapeutic Role of Ursolic Acid on Ameliorating Hepatic Steatosis and Improving Metabolic Disorders in High-Fat Diet-Induced Non-Alcoholic Fatty Liver Disease Rats

Background

Non-alcoholic fatty liver disease (NAFLD) is one of the most prevalent liver diseases around the world, and is closely associated with obesity, diabetes, and insulin resistance. Ursolic acid (UA), an ubiquitous triterpenoid with multifold biological roles, is distributed in various plants. This study was conducted to investigate the therapeutic effect and potential mechanisms of UA against hepatic steatosis in a high-fat diet (HFD)-induced obese non-alcoholic fatty liver disease (NAFLD) rat model.

Methodology/Principal Findings

Obese NAFLD model was established in Sprague-Dawley rats by 8-week HFD feeding. Therapeutic role of UA was evaluated using 0.125%, 0.25%, 0.5% UA-supplemented diet for another 6 weeks. The results from both morphologic and histological detections indicated that UA significantly reversed HFD-induced hepatic steatosis and liver injury. Besides, hepatic peroxisome proliferator-activated receptor (PPAR)-α was markedly up-regulated at both mRNA and protein levels by UA. Knocking down PPAR-α significantly inhibited the anti-steatosis role of UA in vitro. HFD-induced adverse changes in the key genes, which participated in hepatic lipid metabolism, were also alleviated by UA treatment. Furthermore, UA significantly ameliorated HFD-induced metabolic disorders, including insulin resistance, inflammation and oxidative stress.

Conclusions/Significance

These results demonstrated that UA effectively ameliorated HFD-induced hepatic steatosis through a PPAR-α involved pathway, via improving key enzymes in the controlling of lipids metabolism. The metabolic disorders were accordingly improved with the decrease of hepatic steatosis. Thereby, UA could be a promising candidate for the treatment of NAFLD.     

Role of Muscle c-Jun NH2-Terminal Kinase 1 in Obesity-Induced Insulin Resistance

Obesity caused by feeding of a high-fat diet (HFD) is associated with an increased activation of c-Jun NH₂-terminal kinase 1 (JNK1). Activated JNK1 is implicated in the mechanism of obesity-induced insulin resistance and the development of metabolic syndrome and type 2 diabetes. Significantly, Jnk1^−/− mice are protected against HFD-induced obesity and insulin resistance. Here we show that an ablation of the Jnk1 gene in skeletal muscle does not influence HFD-induced obesity. However, muscle-specific JNK1-deficient (M^KO) mice exhibit improved insulin sensitivity compared with control wild-type (M^WT) mice. Thus, insulin-stimulated AKT activation is suppressed in muscle, liver, and adipose tissue of HFD-fed M^WT mice but is suppressed only in the liver and adipose tissue of M^KO mice. These data demonstrate that JNK1 in muscle contributes to peripheral insulin resistance in response to diet-induced obesity.Obesity is a major risk factor for the development of insulin resistance, hyperglycemia, and metabolic syndrome that can lead to β-cell dysfunction and type 2 diabetes (8). The prevalence of human obesity represents a serious health problem in the United States. It is therefore important that we obtain a detailed understanding of the molecular mechanism that accounts for obesity-induced insulin resistance. Recent progress has led to the identification of signal transduction pathways that may mediate the effects of obesity on insulin resistance (14, 23).c-Jun NH₂-terminal kinase 1 (JNK1) represents one signaling pathway that has been implicated in the pathogenesis of metabolic syndrome and type 2 diabetes (21). JNK1 is activated when mice are fed a high-fat diet (HFD) (7). Moreover, Jnk1^−/− mice are protected against HFD-induced insulin resistance (7). The mechanism of protection is mediated, in part, by the failure of Jnk1^−/− mice to develop HFD-induced obesity (7). However, JNK1 can regulate insulin resistance independently of obesity. Thus, mice with an adipose tissue-specific JNK1 deficiency develop normal diet-induced obesity but exhibit selective protection against HFD-induced insulin resistance in both the liver and adipose tissue (16). These data indicate that adipose tissue JNK1 plays a critical role during the development of HFD-induced insulin resistance.The liver plays a key role in the insulin-stimulated disposal of blood glucose during the postprandial state because of reduced gluconeogenesis and increased glycogen synthesis (17). However, glucose uptake by skeletal muscle also makes a major contribution to insulin-stimulated glucose disposal (17). Muscle may therefore be an important target of obesity-induced JNK1 signaling and the regulation of glucose homeostasis.The purpose of this study was to test the role of JNK1 in muscle. Our approach was to examine the effect of a muscle-specific ablation of the Jnk1 gene in mice. We found that HFD-fed control wild-type (M^WT) mice and muscle-specific JNK1-deficient (M^KO) mice became similarly obese. However, M^KO mice were selectively protected against HFD-induced insulin resistance. This analysis demonstrates that muscle JNK1 contributes to the effects of obesity on insulin resistance.     

METTL3-mediated mRNA N6-methyladenosine is required for oocyte and follicle development in mice

Proper follicle development is very important for the production of mature oocytes, which is essential for the maintenance of female fertility. This complex biological process requires precise gene regulation. The most abundant modification of mRNA, N⁶-methyladenosine (m⁶A), is involved in many RNA metabolism processes, including RNA splicing, translation, stability, and degradation. Here, we report that m⁶A plays essential roles during oocyte and follicle development. Oocyte-specific inactivation of the key m⁶A methyltransferase Mettl3 with Gdf9-Cre caused DNA damage accumulation in oocytes, defective follicle development, and abnormal ovulation. Mechanistically, combined RNA-seq and m⁶A methylated RNA immunoprecipitation sequencing (MeRIP-seq) data from oocytes revealed, that we found METTL3 targets Itsn2 for m⁶A modification and then enhances its stability to influence the oocytes meiosis. Taken together, our findings highlight the crucial roles of mRNA m⁶A modification in follicle development and coordination of RNA stabilization during oocyte growth.Subject terms: Oogenesis, Infertility     

A high-fat diet catalyzes progression to hyperglycemia in mice with selective impairment of insulin action in Glut4-expressing tissues

Insulin resistance impairs postprandial glucose uptake through glucose transporter type 4 (GLUT4) and is the primary defect preceding type 2 diabetes. We previously generated an insulin-resistant mouse model with human GLUT4 promoter-driven insulin receptor knockout (GIRKO) in the muscle, adipose, and neuronal subpopulations. However, the rate of diabetes in GIRKO mice remained low prior to 6 months of age on normal chow diet (NCD), suggesting that additional factors/mechanisms are responsible for adverse metabolic effects driving the ultimate progression of overt diabetes. In this study, we characterized the metabolic phenotypes of the adult GIRKO mice acutely switched to high-fat diet (HFD) feeding in order to identify additional metabolic challenges required for disease progression. Distinct from other diet-induced obesity (DIO) and genetic models (e.g., db/db mice), GIRKO mice remained leaner on HFD feeding, but developed other cardinal features of insulin resistance syndrome. GIRKO mice rapidly developed hyperglycemia despite compensatory increases in β-cell mass and hyperinsulinemia. Furthermore, GIRKO mice also had impaired oral glucose tolerance and a limited glucose-lowering benefit from exendin-4, suggesting that the blunted incretin effect contributed to hyperglycemia. Secondly, GIRKO mice manifested severe dyslipidemia while on HFD due to elevated hepatic lipid secretion, serum triglyceride concentration, and lipid droplet accumulation in hepatocytes. Thirdly, GIRKO mice on HFD had increased inflammatory cues in the gut, which were associated with the HFD-induced microbiome alterations and increased serum lipopolysaccharide (LPS). In conclusion, our studies identified important gene/diet interactions contributing to diabetes progression, which might be leveraged to develop more efficacious therapies.     

MiR‐103‐3p targets the m6A methyltransferase METTL14 to inhibit osteoblastic bone formation

Impaired osteoblast function is involved in osteoporosis, and microRNA (miRNA) dysregulation may cause abnormal osteoblast osteogenic activity. However, the influence of miRNA on osteoblast activity and the underlying mechanisms remain elusive. In this study, miR‐103‐3p was found to be negatively correlated with bone formation in bone specimens from elderly women with fractures and ovariectomized (OVX) mice. Additionally, miR‐103‐3p directly targeted Mettl14 to inhibit osteoblast activity, and METTL14‐dependent N⁶‐methyladenosine (m⁶A) methylation inhibited miR‐103‐3p processing by the microprocessor protein DGCR8 and promoted osteoblast activity. Moreover, miR‐103‐3p inhibited bone formation in vivo, and therapeutic inhibition of miR‐103‐3p counteracted the decreased bone formation in OVX mice. Further, METTL14 was negatively correlated with miR‐103‐3p but positively correlated with bone formation in bone specimens from elderly women with fractures and OVX mice. Collectively, our results highlight the critical roles of the miR‐103‐3p/METTL14/m⁶A signaling axis in osteoblast activity, identifying this axis as a potential target for ameliorating osteoporosis.     

NF-κB p65 represses β-catenin-activated transcription of cyclin D1

We have previously reported that obesity-induced diabetes developed in high-fat diet (HFD)-fed BDF1 mice. This is caused by insufficient insulin response to an excess glucose load. In this study, we have shown that the enhanced expression of retinaldehyde dehydrogenase 3 (Raldh3) causes functional disorders of pancreatic islets in diabetic mouse models. In the pancreatic islets of HFD-induced diabetic BDF1 mice and spontaneously diabetic C57BL/KsJ^db/db mice, gene expression analysis with oligonucleotide microarray revealed a significant increase in Raldh3 expression. Exposure to a culture medium containing a higher glucose concentration (25 mM) significantly increased Raldh3 expression in murine MIN6 and alphaTC1 clone 9 cells, which derived from the α and β-cells of pancreatic islets, respectively. Overexpression of Raldh3 reduced the insulin secretion in MIN6 cells, and surprisingly, increased the glucagon secretion in alphaTC1 clone 9 cells. Furthermore, the knockdown of Raldh3 expression with siRNA decreased the glucagon secretion in alphaTC1 clone 9 cells. Raldh3 catalyzes the conversion of 13-cis retinal to 13-cis retinoic acid and we revealed that 13-cis retinoic acid significantly reduces cell viability in MIN6 and alphaTC1 clone 9 cells, but not in cells of H4IIEC3, 3T3-L1, and COS-1 cell lines. These findings suggest that an increasing expression of Raldh3 deregulates the balanced mechanisms of insulin and glucagon secretion in the pancreatic islets and may induce β-cell dysfunction leading to the development of type 2 diabetes.     

Methionine restriction alleviates high-fat diet-induced obesity: Involvement of diurnal metabolism of lipids and bile acids

Circadian misalignment induced by a high-fat diet (HFD) increases the risk of metabolic diseases. Methionine restriction (MR) is known to have the potential of alleviating obesity by improving insulin sensitivity. However, the role of the circadian clock in mediating the effects of MR on obesity-related metabolic disorders remains unclear. Ten-week-old male C57BL/6 J mice were fed with a low-fat diet (LFD) or a HFD for 4 wk., followed with a full diet (0.86% methionine, w/w) or a methionine-restricted diet (0.17% methionine, w/w) for 8 wk. Our results showed that MR attenuated insulin resistance triggered by HFD, especially at ZT12. Moreover, MR led to a time-specific enhancement of the expression of FGF21 and activated the AMPK/PGC-1α signaling. Notably, MR upregulated the cyclical levels of cholic acid (CA) and chenodeoxycholic acid (CDCA), and downregulated the cyclical level of deoxycholic acid (DCA) in the dark phase. MR restored the HFD-disrupted cyclical fluctuations of lipidolysis genes and BAs synthetic genes and improved the circulating lipid profile. Also, MR improved the expressions of clock-controlled genes (CCGs) in the liver and the brown adipose tissue throughout one day. In conclusion, MR exhibited the lipid-lowering effects on HFD-induced obesity and restored the diurnal metabolism of lipids and BAs, which could be partly explained by improving the expression of CCGs. These findings suggested that MR could be a potential nutritional intervention for attenuating obesity-induced metabolic misalignment.     

Magnolia Bioactive Constituent 4-O-Methylhonokiol Prevents the Impairment of Cardiac Insulin Signaling and the Cardiac Pathogenesis in High-Fat Diet-Induced Obese Mice

In obesity, cardiac insulin resistance is a putative cause of cardiac hypertrophy and dysfunction. In our previous study, we observed that Magnolia extract BL153 attenuated high-fat-diet (HFD)-induced cardiac pathogenic changes. In this study, we further investigated the protective effects of the BL153 bioactive constituent, 4-O-methylhonokiol (MH), against HFD-induced cardiac pathogenesis and its possible mechanisms. C57BL/6J mice were fed a normal diet or a HFD with gavage administration of vehicle, BL153, or MH (low or high dose) daily for 24 weeks. Treatment with MH attenuated HFD-induced obesity, as evidenced by body weight gain, and cardiac pathogenesis, as assessed by the heart weight and echocardiography. Mechanistically, MH treatment significantly reduced HFD-induced impairment of cardiac insulin signaling by preferentially augmenting Akt2 signaling. MH also inhibited cardiac expression of the inflammatory factors tumor necrosis factor-α and plasminogen activator inhibitor-1 and increased the phosphorylation of nuclear factor erythroid-derived 2-like 2 (Nrf2) as well as the expression of a Nrf2 downstream target gene heme oxygenase-1. The increased Nrf2 signaling was associated with decreased oxidative stress and damage, as reflected by lowered malondialdehyde and 3-nitrotyrosine levels. Furthermore, MH reduced HFD-induced cardiac lipid accumulation along with lowering expression of cardiac fatty acid translocase/CD36 protein. These results suggest that MH, a bioactive constituent of Magnolia, prevents HFD-induced cardiac pathogenesis by attenuating the impairment of cardiac insulin signaling, perhaps via activation of Nrf2 and Akt2 signaling to attenuate CD36-mediated lipid accumulation and lipotoxicity.     

Markedly Delayed Insulin Secretion and a High Rate of Undetected Overt Diabetes Characterize Glucose Metabolism in Adult Patients with Cystic Fibrosis After Lung Transplantation

Objective: Cystic fibrosis–related diabetes (CFRD) is associated with adverse clinical outcomes and should be screened for by an annual oral glucose tolerance test (OGTT). Since pathophysiologic studies have mainly been performed in a pediatric/adolescent, nontransplanted collective, we aimed to assess parameters of insulin secretion and sensitivity in adult cystic fibrosis (CF) patients after lung transplantation (LT).Methods: Twelve adult CF patients after LT without known diabetes (33.3 ± 11.5 years; body mass index [BMI] 21.5 ± 3.3 kg/m²) and 8 control subjects matched by age (36.0 ± 6.6 years; P>.05), BMI (22.3 ± 1.5 kg/m²; P>.05), and gender (CON group) underwent a 3-hour OGTT with glucose, insulin, and C-peptide measurements. Parameters of insulin secretion and sensitivity as well as lipid profiles were assessed.Results: In the CF group, 4 patients were diagnosed with overt diabetes (CFRD) compared to CF patients without diabetes (CF-noDM), of whom 6 had indeterminate glycemia with 1-h glucose values >200 mg/dL. The insulin peak after glucose load occurred after 30 minutes in CON, after 90 minutes in CF-noDM, and was missing in CFRD. Insulin sensitivity was comparable between the groups. Beta-cell glucose sensitivity was markedly reduced in CFRD (10.7 ± 5.8 pmol/min*m²*mM), higher in CF-noDM (39.9 ± 23.4 pmol/min*m²*mM), but still significantly lower compared to CON (108.3 ± 53.9 pmol/min*m²*mM; P = .0008). CFRD patients exhibited increased triglyceride levels and decreased high-density lipoprotein levels.Conclusion: Adult CF patients after LT have profound disturbances in glucose metabolism, with a high rate of undetected diabetes and markedly delayed insulin secretion. Curbed beta-cell glucose sensitivity rather than insulin resistance explains postprandial hyperglycemia and is accompanied by abnormalities in lipid metabolism.Abbreviations: AUC = area under the curve; BMI = body mass index; CF = cystic fibrosis; CFRD = cystic fibrosis–related diabetes; CFTR = cystic fibrosis transmembrane-conductance regulator; CF-TX = cystic fibrosis patients who underwent lung transplantation; CGM = continuous glucose monitoring; HbA1c = glycated hemoglobin; HDL = high-density lipoprotein; INDET = indeterminate glycemia; LDL = low-density lipoprotein; LT = lung transplantation; OGIS = oral glucose sensitivity index; OGTT = oral glucose tolerance test; QUICKI = quantitative insulin sensitivity check index     

Sphk2−/− mice are protected from obesity and insulin resistance

Sphingosine kinases phosphorylate sphingosine to sphingosine 1?phosphate (S1P), which functions as a signaling molecule. We have previously shown that sphingosine kinase 2 (Sphk2) is important for insulin secretion. To obtain a better understanding of the role of Sphk2 in glucose and lipid metabolism, we have characterized 20- and 52-week old Sphk2^?/? mice using glucose and insulin tolerance tests and by analyzing metabolic gene expression in adipose tissue. A detailed metabolic characterization of these mice revealed that aging Sphk2^?/? mice are protected from metabolic decline and obesity compared to WT mice. Specifically, we found that 52-week old male Sphk2^?/? mice had decreased weight and fat mass, and increased glucose tolerance and insulin sensitivity compared to control mice. Indirect calorimetry studies demonstrated an increased energy expenditure and food intake in 52-week old male Sphk2^?/? versus control mice. Furthermore, expression of adiponectin gene in adipose tissue was increased and the plasma levels of adiponectin elevated in aged Sphk2^?/? mice compared to WT. Analysis of lipid metabolic gene expression in adipose tissue showed increased expression of the Atgl gene, which was associated with increased Atgl protein levels. Atgl encodes for the adipocyte triglyceride lipase, which catalyzes the rate-limiting step of lipolysis. In summary, these data suggest that mice lacking the Sphk2 gene are protected from obesity and insulin resistance during aging. The beneficial metabolic effects observed in aged Sphk2^?/? mice may be in part due to enhanced lipolysis by Atgl and increased levels of adiponectin, which has lipid- and glucose-lowering effects.