The chemotherapeutic drug doxorubicin does not exacerbate p16Ink4a-positive senescent cell accumulation and cardiometabolic disease development in young adult female LDLR-deficient mice.

Cancer survivors who received chemotherapy, such as the anthracycline doxorubicin (DOX), have an increased risk of developing complications later in life, including the development of chronic metabolic diseases. Although the etiology of this increased risk for late metabolic complications in cancer survivors is poorly understood, a causal role of therapy-induced senescent cells has been suggested. To study the role of cellular senescence in chemotherapy-induced metabolic complications, young adult female low-density lipoprotein receptor-deficient (Ldlr-/-)-p16-3MR mice, in which p16Ink4a-positive (p16Ink4a+) senescent cells can be genetically eliminated, were treated with four weekly injections of DOX (2.5 mg/kg) followed by a high-fat high-cholesterol diet for 12 weeks. While DOX treatment induced known short-term effects, such as reduction in body weight, gonadal fat mass, and adipose tissue inflammation, it was not associated with significant long-term effects on glucose homeostasis, hepatic steatosis, or atherosclerosis. We further found no evidence of DOX-induced accumulation of p16Ink4a+-senescent cells at 1 or 12 weeks after DOX treatment. Neither did we observe an effect of elimination of p16Ink4a+-senescent cells on the development of diet-induced cardiometabolic complications in DOX-treated mice. Other markers for senescence were generally also not affected except for an increase in p21 and Cxcl10 in gonadal white adipose tissue long-term after DOX treatment. Together, our study does not support a significant role for p16Ink4a+-senescent cells in the development of diet-induced cardiometabolic disease in young adult DOX-treated female Ldlr-/- mice. These findings illustrate the need of further studies to understand the link between cancer therapy and cardiometabolic disease development in cancer survivors.

Autocrine FGF1 signaling promotes glucose uptake in adipocytes.

Fibroblast growth factor 1 (FGF1) is an autocrine growth factor released from adipose tissue during over-nutrition or fasting to feeding transition. While local actions underlie the majority of FGF1's anti-diabetic functions, the molecular mechanisms downstream of adipose FGF receptor signaling are unclear. We investigated the effects of FGF1 on glucose uptake and its underlying mechanism in murine 3T3-L1 adipocytes and in ex vivo adipose explants from mice. FGF1 increased glucose uptake in 3T3-L1 adipocytes and epididymal WAT (eWAT) and inguinal WAT (iWAT). Conversely, glucose uptake was reduced in eWAT and iWAT of FGF1 knockout mice. We show that FGF1 acutely increased adipocyte glucose uptake via activation of the insulin-sensitive glucose transporter GLUT4, involving dynamic crosstalk between the MEK1/2 and Akt signaling proteins. Prolonged exposure to FGF1 stimulated adipocyte glucose uptake by MEK1/2-dependent transcription of the basal glucose transporter GLUT1. We have thus identified an alternative pathway to stimulate glucose uptake in adipocytes, independent from insulin, which could open new avenues for treating patients with type 2 diabetes.

Identification of the fructose transporter GLUT5 (SLC2A5) as a novel target of nuclear receptor LXR.

Fructose has become a major constituent of our modern diet and is implicated as an underlying cause in the development of metabolic diseases. The fructose transporter GLUT5 (SLC2A5) is required for intestinal fructose absorption. GLUT5 expression is induced in the intestine and skeletal muscle of type 2 diabetes (T2D) patients and in certain cancers that are dependent on fructose metabolism, indicating that modulation of GLUT5 levels could have potential in the treatment of these diseases. Using an unbiased screen for transcriptional control of the human GLUT5 promoter we identified a strong and specific regulation by liver X receptor α (LXRα, NR1H3). Using promoter truncations and site-directed mutagenesis we identified a functional LXR response element (LXRE) in the human GLUT5 promoter, located at -385 bp relative to the transcriptional start site (TSS). Finally, mice treated with LXR agonist T0901317 showed an increase in Glut5 mRNA and protein levels in duodenum and adipose tissue, underscoring the in vivo relevance of its regulation by LXR. Together, our findings show that LXRα regulates GLUT5 in mice and humans. As a ligand-activated transcription factor, LXRα might provide novel pharmacologic strategies for the selective modulation of GLUT5 activity in the treatment of metabolic disease as well as cancer.

AAV8-mediated gene transfer of microRNA-132 improves beta cell function in mice fed a high-fat diet.

MicroRNAs have emerged as essential regulators of beta cell function and beta cell proliferation. One of these microRNAs, miR-132, is highly induced in several obesity models and increased expression of miR-132 in vitro modulates glucose-stimulated insulin secretion. The aim of this study was to investigate the therapeutic benefits of miR-132 overexpression on beta cell function in vivo. To overexpress miR-132 specifically in beta cells, we employed adeno-associated virus (AAV8)-mediated gene transfer using the rat insulin promoter in a double-stranded, self-complementary AAV vector to overexpress miR-132. Treatment of mice with dsAAV8-RIP-mir132 increased miR-132 expression in beta cells without impacting expression of miR-212 or miR-375. Surprisingly, overexpression of miR-132 did not impact glucose homeostasis in chow-fed animals. Overexpression of miR-132 did improve insulin secretion and hence glucose homeostasis in high-fat diet-fed mice. Furthermore, miR-132 overexpression increased beta cell proliferation in mice fed a high-fat diet. In conclusion, our data show that AAV8-mediated gene transfer of miR-132 to beta cells improves beta cell function in mice in response to a high-fat diet. This suggests that increased miR-132 expression is beneficial for beta cell function during hyperglycemia and obesity.

NF-κB p65 serine 467 phosphorylation sensitizes mice to weight gain and TNFα-or diet-induced inflammation.

The NF-κB family of transcription factors is essential for an effective immune response, but also controls cell metabolism, proliferation and apoptosis. Its broad relevance and the high connectivity to diverse signaling pathways require a tight control of NF-κB activity. To investigate the control of NF-κB activity by phosphorylation of the NF-κB p65 subunit, we generated a knock-in mouse model in which serine 467 (the mouse homolog of human p65 serine 468) was replaced with a non-phosphorylatable alanine (S467A). This substitution caused reduced p65 protein synthesis and diminished TNFα-induced expression of a selected group of NF-κB-dependent genes. Intriguingly, high-fat fed S467A mice displayed increased locomotor activity and energy expenditure, which coincided with a reduced body weight gain. Although glucose metabolism or insulin sensitivity was not improved, diet-induced liver inflammation was diminished in S467A mice. Altogether, this study demonstrates that phosphorylation of p65 serine 467 augment NF-κB activity and exacerbates various deleterious effects of overnutrition in mice.

Carriers of loss-of-function mutations in EXT display impaired pancreatic beta-cell reserve due to smaller pancreas volume.

Exotosin (EXT) proteins are involved in the chain elongation step of heparan sulfate (HS) biosynthesis, which is intricately involved in organ development. Loss of function mutations (LOF) in EXT1 and EXT2 result in hereditary exostoses (HME). Interestingly, HS plays a role in pancreas development and beta-cell function, and genetic variations in EXT2 are associated with an increased risk for type 2 diabetes mellitus. We hypothesized that loss of function of EXT1 or EXT2 in subjects with hereditary multiple exostoses (HME) affects pancreatic insulin secretion capacity and development. We performed an oral glucose tolerance test (OGTT) followed by hyperglycemic clamps to investigate first-phase glucose-stimulated insulin secretion (GSIS) in HME patients and age and gender matched non-affected relatives. Pancreas volume was assessed with magnetic resonance imaging (MRI). OGTT did not reveal significant differences in glucose disposal, but there was a markedly lower GSIS in HME subjects during hyperglycemic clamp (iAUC HME: 0.72 [0.46-1.16] vs. controls 1.53 [0.69-3.36] nmol·l-1·min-1, p<0.05). Maximal insulin response following arginine challenge was also significantly attenuated (iAUC HME: 7.14 [4.22-10.5] vs. controls 10.2 [7.91-12.70] nmol·l-1·min-1 p<0.05), indicative of an impaired beta-cell reserve. MRI revealed a significantly smaller pancreatic volume in HME subjects (HME: 72.0±15.8 vs. controls 96.5±26.0 cm3 p = 0.04). In conclusion, loss of function of EXT proteins may affect beta-cell mass and insulin secretion capacity in humans, and render subjects at a higher risk of developing type 2 diabetes when exposed to environmental risk factors.

ABCA1 influences neuroinflammation and neuronal death.

ATP-binding cassette transporter A1 (ABCA1) mediates cellular cholesterol efflux in the brain and influences whole brain cholesterol homeostasis. Activation of liver X receptors (LXRs), transcription factors that increase the expression of cholesterol transport genes including ABCA1, reduces neuroinflammation and pathology in neurodegenerative animal models suggesting that in addition to its involvement in cholesterol transport, ABCA1 may play a role in modulating the inflammatory response in the brain. We investigated the cell-type specific role of ABCA1 in neuroinflammation in vivo using mice specifically lacking brain ABCA1 (ABCA1(-B/-B)) as well as mice lacking neuronal (ABCA1(-N/-N)) and astrocytic (ABCA1(-Ast/-Ast)) ABCA1. ABCA1(-B/-B) mice exhibit cortical astrogliosis, increased inflammatory gene expression as well as activation of mitogen-activated protein kinases (MAPKs) following acute lipopolysaccharide (LPS) administration. Microglia cultured from ABCA1(-B/-B) mice exhibit augmented LPS-induced secretion of tumor necrosis factor α (TNFα) and decreased phagocytic activity, indicating an increase in a pro-inflammatory response. ABCA1(-N/-N) mice develop astrogliosis but show no change in inflammatory gene expression. Intriguingly, ABCA1(-Ast/-Ast) mice show neither astrogliosis nor elevated expression of inflammatory markers. Cortical apolipoprotein E (apoE) levels are reduced in ABCA1(-Ast/-Ast) but not in ABCA1(-N/-N) mice, providing in vivo evidence for the specific role of astrocyte ABCA1 in regulating brain apoE levels. Interestingly, cortical neuronal death is increased in 17month-old ABCA1(-B/-B) mice but not in ABCA1(-N/-N) or ABCA1(-Ast/-Ast) mice. Our findings suggest that coordinated ABCA1 activity across neurons and glial cells influences neuroinflammation and neurodegeneration.

Loss of both ABCA1 and ABCG1 results in increased disturbances in islet sterol homeostasis, inflammation, and impaired ß-cell function.

Cellular cholesterol homeostasis is important for normal ß-cell function. Disruption of cholesterol transport by decreased function of the ATP-binding cassette (ABC) transporter ABCA1 results in impaired insulin secretion. Mice lacking ß-cell ABCA1 have increased islet expression of ABCG1, another cholesterol transporter implicated in ß-cell function. To determine whether ABCA1 and ABCG1 have complementary roles in ß-cells, mice lacking ABCG1 and ß-cell ABCA1 were generated and glucose tolerance, islet sterol levels, and ß-cell function were assessed. Lack of both ABCG1 and ß-cell ABCA1 resulted in increased fasting glucose levels and a greater impairment in glucose tolerance compared with either ABCG1 deletion or loss of ABCA1 in ß-cells alone. In addition, glucose-stimulated insulin secretion was decreased and sterol accumulation increased in islets lacking both transporters compared with those isolated from knockout mice with each gene alone. Combined deficiency of ABCA1 and ABCG1 also resulted in significant islet inflammation as indicated by increased expression of interleukin-1ß and macrophage infiltration. Thus, lack of both ABCA1 and ABCG1 induces greater defects in ß-cell function than deficiency of either transporter individually. These data suggest that ABCA1 and ABCG1 each make complimentary and important contributions to ß-cell function by maintaining islet cholesterol homeostasis in vivo.

Islet cholesterol accumulation due to loss of ABCA1 leads to impaired exocytosis of insulin granules.

OBJECTIVE: The ATP-binding cassette transporter A1 (ABCA1) is essential for normal insulin secretion from ß-cells. The aim of this study was to elucidate the mechanisms underlying the impaired insulin secretion in islets lacking ß-cell ABCA1. RESEARCH DESIGN AND METHODS: Calcium imaging, patch clamp, and membrane capacitance were used to assess the effect of ABCA1 deficiency on calcium flux, ion channel function, and exocytosis in islet cells. Electron microscopy was used to analyze ß-cell ultrastructure. The quantity and distribution of proteins involved in insulin-granule exocytosis were also investigated. RESULTS: We show that a lack of ß-cell ABCA1 results in impaired depolarization-induced exocytotic fusion of insulin granules. We observed disturbances in membrane microdomain organization and Golgi and insulin granule morphology in ß-cells as well as elevated fasting plasma proinsulin levels in mice in the absence of ß-cell ABCA1. Acute cholesterol depletion rescued the exocytotic defect in ß-cells lacking ABCA1, indicating that elevated islet cholesterol accumulation directly impairs granule fusion and insulin secretion. CONCLUSIONS: Our data highlight a crucial role of ABCA1 and cellular cholesterol in ß-cells that is necessary for regulated insulin granule fusion events. These data suggest that abnormalities of cholesterol metabolism may contribute to the impaired ß-cell function in diabetes.

Plant sterols cause macrothrombocytopenia in a mouse model of sitosterolemia.

Mutations in either ABCG5 or ABCG8 cause sitosterolemia, an inborn error of metabolism characterized by high plasma plant sterol concentrations. Recently, macrothrombocytopenia was described in a number of sitosterolemia patients, linking hematological dysfunction to disturbed sterol metabolism. Here, we demonstrate that macrothrombocytopenia is an intrinsic feature of murine sitosterolemia. Abcg5-deficient (Abcg5(-/-)) mice showed a 68% reduction in platelet count, and platelets were enlarged compared with wild-type controls. Macrothrombocytopenia was not due to decreased numbers of megakaryocytes or their progenitors, but defective megakaryocyte development with deterioration of the demarcation membrane system was evident. Lethally irradiated wild-type mice transplanted with bone marrow from Abcg5(-/-) mice displayed normal platelets, whereas Abcg5(-/-) mice transplanted with wild-type bone marrow still showed macrothrombocytopenia. Treatment with the sterol absorption inhibitor ezetimibe rapidly reversed macrothrombocytopenia in Abcg5(-/-) mice concomitant with a strong decrease in plasma plant sterols. Thus, accumulation of plant sterols is responsible for development of macrothrombocytopenia in sitosterolemia, and blocking intestinal plant sterol absorption provides an effective means of treatment.

Reduced cholesterol absorption upon PPARdelta activation coincides with decreased intestinal expression of NPC1L1.

Peroxisome proliferator-activated receptors (PPARs) control the transcription of genes involved in lipid metabolism. Activation of PPARdelta may have antiatherogenic effects through the increase of plasma HDL, theoretically promoting reverse cholesterol transport from peripheral tissues toward the liver for removal via bile and feces. Effects of PPARdelta activation by GW610742 were evaluated in wild-type and Abca1-deficient (Abca1(-/-)) mice that lack HDL. Treatment with GW610742 resulted in an approximately 50% increase of plasma HDL-cholesterol in wild-type mice, whereas plasma cholesterol levels remained extremely low in Abca1(-/-) mice. Yet, biliary cholesterol secretion rates were similar in untreated wild-type and Abca1(-/-) mice and unaltered upon treatment. Unexpectedly, PPARdelta activation led to enhanced fecal neutral sterol loss in both groups without any changes in intestinal Abca1, Abcg5, Abcg8, and 3-hydroxy-3-methylglutaryl-coenzyme A reductase expression. Moreover, GW610742 treatment resulted in a 43% reduction of fractional cholesterol absorption in wild-type mice, coinciding with a significantly reduced expression of the cholesterol absorption protein Niemann-Pick C1-like 1 (Npc1l1) in the intestine. PPARdelta activation is associated with increased plasma HDL and reduced intestinal cholesterol absorption efficiency that may be related to decreased intestinal Npc1l1 expression. Thus, PPARdelta is a promising target for drugs aimed to treat or prevent atherosclerosis.