Conditional hepatocyte ablation of PDIA1 uncovers indispensable roles in both APOB and MTTP folding to support VLDL secretion.

OBJECTIVES: The assembly and secretion of hepatic very low-density lipoprotein (VLDL) plays pivotal roles in hepatic and plasma lipid homeostasis. Protein disulfide isomerase A1 (PDIA1/P4HB) is a molecular chaperone whose functions are essential for protein folding in the endoplasmic reticulum. Here we investigated the physiological requirement in vivo for PDIA1 in maintaining VLDL assembly and secretion. METHODS: Pdia1/P4hb was conditionally deleted in adult mouse hepatocytes and the phenotypes characterized. Mechanistic analyses in primary hepatocytes determined how PDIA1 ablation alters MTTP synthesis and degradation as well as altering synthesis and secretion of Apolipoprotein B (APOB), along with complementary expression of intact PDIA1 vs a catalytically inactivated PDIA1 mutant. RESULTS: Hepatocyte-specific deletion of Pdia1/P4hb inhibited hepatic MTTP expression and dramatically reduced VLDL production, leading to severe hepatic steatosis and hypolipidemia. Pdia1-deletion did not affect mRNA expression or protein stability of MTTP but rather prevented Mttp mRNA translation. We demonstrate an essential role for PDIA1 in MTTP synthesis and function and show that PDIA1 interacts with APOB in an MTTP-independent manner via its molecular chaperone function to support APOB folding and secretion. CONCLUSIONS: PDIA1 plays indispensable roles in APOB folding, MTTP synthesis and activity to support VLDL assembly. Thus, like APOB and MTTP, PDIA1 is an obligatory component of hepatic VLDL production.

Cellular stress and coagulation factor production: when more is not necessarily better.

Remarkably, it has been 40 years since the isolation of the 2 genes involved in hemophilia A (HA) and hemophilia B (HB), encoding clotting factor (F) VIII (FVIII) and FIX, respectively. Over the years, these advances led to the development of purified recombinant protein factors that are free of contaminating viruses from human pooled plasma for hemophilia treatments, reducing the morbidity and mortality previously associated with human plasma-derived clotting factors. These discoveries also paved the way for modified factors that have increased plasma half-lives. Importantly, more recent advances have led to the development and Food and Drug Administration approval of a hepatocyte-targeted, adeno-associated viral vector-mediated gene transfer approach for HA and HB. However, major concerns regarding the durability and safety of HA gene therapy remain to be resolved. Compared with FIX, FVIII is a much larger protein that is prone to misfolding and aggregation in the endoplasmic reticulum and is poorly secreted by the mammalian cells. Due to the constraint of the packaging capacity of adeno-associated viral vector, B-domain deleted FVIII rather than the full-length protein is used for HA gene therapy. Like full-length FVIII, B-domain deleted FVIII misfolds and is inefficiently secreted. Its expression in hepatocytes activates the cellular unfolded protein response, which is deleterious for hepatocyte function and survival and has the potential to drive hepatocellular carcinoma. This review is focused on our current understanding of factors limiting FVIII secretion and the potential pathophysiological consequences upon expression in hepatocytes.

Ectopic clotting factor VIII expression and misfolding in hepatocytes as a cause for hepatocellular carcinoma.

Hemophilia A gene therapy targets hepatocytes to express B domain deleted (BDD) clotting factor VIII (FVIII) to permit viral encapsidation. Since BDD is prone to misfolding in the endoplasmic reticulum (ER) and ER protein misfolding in hepatocytes followed by high-fat diet (HFD) can cause hepatocellular carcinoma (HCC), we studied how FVIII misfolding impacts HCC development using hepatocyte DNA delivery to express three proteins from the same parental vector: (1) well-folded cytosolic dihydrofolate reductase (DHFR); (2) BDD-FVIII, which is prone to misfolding in the ER; and (3) N6-FVIII, which folds more efficiently than BDD-FVIII. One week after DNA delivery, when FVIII expression was undetectable, mice were fed HFD for 65 weeks. Remarkably, all mice that received BDD-FVIII vector developed liver tumors, whereas only 58% of mice that received N6 and no mice that received DHFR vector developed liver tumors, suggesting that the degree of protein misfolding in the ER increases predisposition to HCC in the context of an HFD and in the absence of viral transduction. Our findings raise concerns of ectopic BDD-FVIII expression in hepatocytes in the clinic, which poses risks independent of viral vector integration. Limited expression per hepatocyte and/or use of proteins that avoid misfolding may enhance safety.

Chop/Ddit3 depletion in ß cells alleviates ER stress and corrects hepatic steatosis in mice.

Type 2 diabetes (T2D) is a metabolic disorder characterized by hyperglycemia, hyperinsulinemia, and insulin resistance (IR). During the early phase of T2D, insulin synthesis and secretion by pancreatic ß cells is enhanced, which can lead to proinsulin misfolding that aggravates endoplasmic reticulum (ER) protein homeostasis in ß cells. Moreover, increased circulating insulin may contribute to fatty liver disease. Medical interventions aimed at alleviating ER stress in ß cells while maintaining optimal insulin secretion are therefore an attractive therapeutic strategy for T2D. Previously, we demonstrated that germline Chop gene deletion preserved ß cells in high-fat diet (HFD)-fed mice and in leptin receptor-deficient db/db mice. In the current study, we further investigated whether targeting Chop/Ddit3 specifically in murine ß cells conferred therapeutic benefits. First, we showed that Chop deletion in ß cells alleviated ß cell ER stress and delayed glucose-stimulated insulin secretion (GSIS) in HFD-fed mice. Second, ß cell-specific Chop deletion prevented liver steatosis and hepatomegaly in aged HFD-fed mice without affecting basal glucose homeostasis. Third, we provide mechanistic evidence that Chop depletion reduces ER Ca2+ buffering capacity and modulates glucose-induced islet Ca2+ oscillations, leading to transcriptional changes of ER chaperone profile ("ER remodeling"). Last, we demonstrated that a GLP1-conjugated Chop antisense oligonucleotide strategy recapitulated the reduction in liver triglycerides and pancreatic insulin content. In summary, our results demonstrate that Chop depletion in ß cells provides a therapeutic strategy to alleviate dysregulated insulin secretion and consequent fatty liver disease in T2D.

Factor VIII exhibits chaperone-dependent and glucose-regulated reversible amyloid formation in the endoplasmic reticulum.

Hemophilia A, an X-linked bleeding disorder caused by deficiency of factor VIII (FVIII), is treated by protein replacement. Unfortunately, this regimen is costly due to the expense of producing recombinant FVIII as a consequence of its low-level secretion from mammalian host cells. FVIII expression activates the endoplasmic reticulum (ER) stress response, causes oxidative stress, and induces apoptosis. Importantly, little is known about the factors that cause protein misfolding and aggregation in metazoans. Here, we identified intrinsic and extrinsic factors that cause FVIII to form aggregates. We show that FVIII forms amyloid-like fibrils within the ER lumen upon increased FVIII synthesis or inhibition of glucose metabolism. Significantly, FVIII amyloids can be dissolved upon restoration of glucose metabolism to produce functional secreted FVIII. Two ER chaperone families and their cochaperones, immunoglobulin binding protein (BiP) and calnexin/calreticulin, promote FVIII solubility in the ER, where the former is also required for disaggregation. A short aggregation motif in the FVIII A1 domain (termed Aggron) is necessary and sufficient to seed ß-sheet polymerization, and BiP binding to this Aggron prevents amyloidogenesis. Our findings provide novel insight into mechanisms that limit FVIII secretion and ER protein aggregation in general and have implication for ongoing hemophilia A gene-therapy clinical trials.

PDIA1/P4HB is required for efficient proinsulin maturation and ß cell health in response to diet induced obesity.

Regulated proinsulin biosynthesis, disulfide bond formation and ER redox homeostasis are essential to prevent Type two diabetes. In ß cells, protein disulfide isomerase A1 (PDIA1/P4HB), the most abundant ER oxidoreductase of over 17 members, can interact with proinsulin to influence disulfide maturation. Here we find Pdia1 is required for optimal insulin production under metabolic stress in vivo. ß cell-specific Pdia1 deletion in young high-fat diet fed mice or aged mice exacerbated glucose intolerance with inadequate insulinemia and increased the proinsulin/insulin ratio in both serum and islets compared to wildtype mice. Ultrastructural abnormalities in Pdia1-null ß cells include diminished insulin granule content, ER vesiculation and distention, mitochondrial swelling and nuclear condensation. Furthermore, Pdia1 deletion increased accumulation of disulfide-linked high molecular weight proinsulin complexes and islet vulnerability to oxidative stress. These findings demonstrate that PDIA1 contributes to oxidative maturation of proinsulin in the ER to support insulin production and ß cell health.

Loss of lipin 1-mediated phosphatidic acid phosphohydrolase activity in muscle leads to skeletal myopathy in mice.

Lipin 1 regulates glycerolipid homeostasis by acting as a phosphatidic acid phosphohydrolase (PAP) enzyme in the triglyceride-synthesis pathway and by regulating transcription factor activity. Mutations in human lipin 1 are a common cause of recurrent rhabdomyolysis in children. Mice with constitutive whole-body lipin 1 deficiency have been used to examine mechanisms connecting lipin 1 deficiency to myocyte injury. However, that mouse model is confounded by lipodystrophy not phenocopied in people. Herein, 2 muscle-specific mouse models were studied: 1) Lpin1 exon 3 and 4 deletion, resulting in a hypomorphic protein without PAP activity, but which preserved transcriptional coregulatory function; and 2) Lpin1 exon 7 deletion, resulting in total protein loss. In both models, skeletal muscles exhibited a chronic myopathy with ongoing muscle fiber necrosis and regeneration and accumulation of phosphatidic acid and, paradoxically, diacylglycerol. Additionally, lipin 1-deficient mice had abundant, but abnormal, mitochondria likely because of impaired autophagy. Finally, these mice exhibited increased plasma creatine kinase following exhaustive exercise when unfed. These data suggest that mice lacking lipin 1-mediated PAP activity in skeletal muscle may serve as a model for determining the mechanisms by which lipin 1 deficiency leads to myocyte injury and for testing potential therapeutic approaches.-Schweitzer, G. G., Collier, S. L., Chen, Z., McCommis, K. S., Pittman, S. K., Yoshino, J., Matkovich, S. J., Hsu, F.-F., Chrast, R., Eaton, J. M., Harris, T. E., Weihl, C. C., Finck, B. N. Loss of lipin 1-mediated phosphatidic acid phosphohydrolase activity in muscle leads to skeletal myopathy in mice.

Trehalose inhibits solute carrier 2A (SLC2A) proteins to induce autophagy and prevent hepatic steatosis.

Trehalose is a naturally occurring disaccharide that has gained attention for its ability to induce cellular autophagy and mitigate diseases related to pathological protein aggregation. Despite decades of ubiquitous use as a nutraceutical, preservative, and humectant, its mechanism of action remains elusive. We showed that trehalose inhibited members of the SLC2A (also known as GLUT) family of glucose transporters. Trehalose-mediated inhibition of glucose transport induced AMPK (adenosine 5'-monophosphate-activated protein kinase)-dependent autophagy and regression of hepatic steatosis in vivo and a reduction in the accumulation of lipid droplets in primary murine hepatocyte cultures. Our data indicated that trehalose triggers beneficial cellular autophagy by inhibiting glucose transport.

Loss of Mitochondrial Pyruvate Carrier 2 in the Liver Leads to Defects in Gluconeogenesis and Compensation via Pyruvate-Alanine Cycling.

Pyruvate transport across the inner mitochondrial membrane is believed to be a prerequisite for gluconeogenesis in hepatocytes, which is important for the maintenance of normoglycemia during prolonged food deprivation but also contributes to hyperglycemia in diabetes. To determine the requirement for mitochondrial pyruvate import in gluconeogenesis, mice with liver-specific deletion of mitochondrial pyruvate carrier 2 (LS-Mpc2(-/-)) were generated. Loss of MPC2 impaired, but did not completely abolish, hepatocyte conversion of labeled pyruvate to TCA cycle intermediates and glucose. Unbiased metabolomic analyses of livers from fasted LS-Mpc2(-/-) mice suggested that alterations in amino acid metabolism, including pyruvate-alanine cycling, might compensate for the loss of MPC2. Indeed, inhibition of pyruvate-alanine transamination further reduced mitochondrial pyruvate metabolism and glucose production by LS-Mpc2(-/-) hepatocytes. These data demonstrate an important role for MPC2 in controlling hepatic gluconeogenesis and illuminate a compensatory mechanism for circumventing a block in mitochondrial pyruvate import.

A murine Niemann-Pick C1 I1061T knock-in model recapitulates the pathological features of the most prevalent human disease allele.

Niemann-Pick Type C1 (NPC1) disease is a rare neurovisceral, cholesterol-sphingolipid lysosomal storage disorder characterized by ataxia, motor impairment, progressive intellectual decline, and dementia. The most prevalent mutation, NPC1(I1061T), encodes a misfolded protein with a reduced half-life caused by ER-associated degradation. Therapies directed at stabilization of the mutant NPC1 protein reduce cholesterol storage in fibroblasts but have not been tested in vivo because of lack of a suitable animal model. Whereas the prominent features of human NPC1 disease are replicated in the null Npc1(-/-) mouse, this model is not amenable to examining proteostatic therapies. The objective of the present study was to develop an NPC1 I1061T knock-in mouse in which to test proteostatic therapies. Compared with the Npc1(-/-) mouse, this Npc1(tm(I1061T)Dso) model displays a less severe, delayed form of NPC1 disease with respect to weight loss, decreased motor coordination, Purkinje cell death, lipid storage, and premature death. The murine NPC1(I1061T) protein has a reduced half-life in vivo, consistent with protein misfolding and rapid ER-associated degradation, and can be stabilized by histone deacetylase inhibition. This novel mouse model faithfully recapitulates human NPC1 disease and provides a powerful tool for preclinical evaluation of therapies targeting NPC1 protein variants with compromised stability.

Rhabdomyolysis-Associated Mutations in Human LPIN1 Lead to Loss of Phosphatidic Acid Phosphohydrolase Activity.

Rhabdomyolysis is an acute syndrome due to extensive injury of skeletal muscle. Recurrent rhabdomyolysis is often caused by inborn errors in intermediary metabolism, and recent work has suggested that mutations in the human gene encoding lipin 1 (LPIN1) may be a common cause of recurrent rhabdomyolysis in children. Lipin 1 dephosphorylates phosphatidic acid to form diacylglycerol (phosphatidic acid phosphohydrolase; PAP) and acts as a transcriptional regulatory protein to control metabolic gene expression. Herein, a 3-year-old boy with severe recurrent rhabdomyolysis was determined to be a compound heterozygote for a novel c.1904T>C (p.Leu635Pro) substitution and a previously reported genomic deletion of exons 18-19 (E766-S838_del) in LPIN1. Western blotting with patient muscle biopsy lysates demonstrated a marked reduction in lipin 1 protein, while immunohistochemical staining for lipin 1 showed abnormal subcellular localization. We cloned cDNAs to express recombinant lipin 1 proteins harboring pathogenic mutations and showed that the E766-S838_del allele was not expressed at the RNA or protein level. Lipin 1 p.Leu635Pro was expressed, but the protein was less stable, was aggregated in the cytosol, and was targeted for proteosomal degradation. Another pathogenic single amino acid substitution, lipin 1 p.Arg725His, was well expressed and retained its transcriptional regulatory function. However, both p.Leu635Pro and p.Arg725His proteins were found to be deficient in PAP activity. Kinetic analyses demonstrated a loss of catalysis rather than diminished substrate binding. These data suggest that loss of lipin 1-mediated PAP activity may be involved in the pathogenesis of rhabdomyolysis in lipin 1 deficiency.

Liver-specific loss of lipin-1-mediated phosphatidic acid phosphatase activity does not mitigate intrahepatic TG accumulation in mice.

Lipin proteins (lipin 1, 2, and 3) regulate glycerolipid homeostasis by acting as phosphatidic acid phosphohydrolase (PAP) enzymes in the TG synthesis pathway and by regulating DNA-bound transcription factors to control gene transcription. Hepatic PAP activity could contribute to hepatic fat accumulation in response to physiological and pathophysiological stimuli. To examine the role of lipin 1 in regulating hepatic lipid metabolism, we generated mice that are deficient in lipin-1-encoded PAP activity in a liver-specific manner (Alb-Lpin1(-/-) mice). This allele of lipin 1 was still able to transcriptionally regulate the expression of its target genes encoding fatty acid oxidation enzymes, and the expression of these genes was not affected in Alb-Lpin1(-/-) mouse liver. Hepatic PAP activity was significantly reduced in mice with liver-specific lipin 1 deficiency. However, hepatocytes from Alb-Lpin1(-/-) mice had normal rates of TG synthesis, and steady-state hepatic TG levels were unaffected under fed and fasted conditions. Furthermore, Alb-Lpin1(-/-) mice were not protected from intrahepatic accumulation of diacylglycerol and TG after chronic feeding of a diet rich in fat and fructose. Collectively, these data demonstrate that marked deficits in hepatic PAP activity do not impair TG synthesis and accumulation under acute or chronic conditions of lipid overload.

Identification of protein disulfide isomerase 1 as a key isomerase for disulfide bond formation in apolipoprotein B100.

Apolipoprotein (apo) B is an obligatory component of very low density lipoprotein (VLDL), and its cotranslational and posttranslational modifications are important in VLDL synthesis, secretion, and hepatic lipid homeostasis. ApoB100 contains 25 cysteine residues and eight disulfide bonds. Although these disulfide bonds were suggested to be important in maintaining apoB100 function, neither the specific oxidoreductase involved nor the direct role of these disulfide bonds in apoB100-lipidation is known. Here we used RNA knockdown to evaluate both MTP-dependent and -independent roles of PDI1 in apoB100 synthesis and lipidation in McA-RH7777 cells. Pdi1 knockdown did not elicit any discernible detrimental effect under normal, unstressed conditions. However, it decreased apoB100 synthesis with attenuated MTP activity, delayed apoB100 oxidative folding, and reduced apoB100 lipidation, leading to defective VLDL secretion. The oxidative folding-impaired apoB100 was secreted mainly associated with LDL instead of VLDL particles from PDI1-deficient cells, a phenotype that was fully rescued by overexpression of wild-type but not a catalytically inactive PDI1 that fully restored MTP activity. Further, we demonstrate that PDI1 directly interacts with apoB100 via its redox-active CXXC motifs and assists in the oxidative folding of apoB100. Taken together, these findings reveal an unsuspected, yet key role for PDI1 in oxidative folding of apoB100 and VLDL assembly.

Inhibiting monoacylglycerol acyltransferase 1 ameliorates hepatic metabolic abnormalities but not inflammation and injury in mice.

Abnormalities in hepatic lipid metabolism and insulin action are believed to play a critical role in the etiology of nonalcoholic steatohepatitis. Monoacylglycerol acyltransferase (MGAT) enzymes convert monoacylglycerol to diacylglycerol, which is the penultimate step in one pathway for triacylglycerol synthesis. Hepatic expression of Mogat1, which encodes an MGAT enzyme, is increased in the livers of mice with hepatic steatosis, and knocking down Mogat1 improves glucose metabolism and hepatic insulin signaling, but whether increased MGAT activity plays a role in the etiology of nonalcoholic steatohepatitis is unclear. To examine this issue, mice were placed on a diet containing high levels of trans fatty acids, fructose, and cholesterol (HTF-C diet) or a low fat control diet for 4 weeks. Mice were injected with antisense oligonucleotides (ASOs) to knockdown Mogat1 or a scrambled ASO control for 12 weeks while remaining on diet. The HTF-C diet caused glucose intolerance, hepatic steatosis, and induced hepatic gene expression markers of inflammation, macrophage infiltration, and stellate cell activation. Mogat1 ASO treatment, which suppressed Mogat1 expression in liver and adipose tissue, attenuated weight gain, improved glucose tolerance, improved hepatic insulin signaling, and decreased hepatic triacylglycerol content compared with control ASO-treated mice on HTF-C chow. However, Mogat1 ASO treatment did not reduce hepatic diacylglycerol, cholesterol, or free fatty acid content; improve histologic measures of liver injury; or reduce expression of markers of stellate cell activation, liver inflammation, and injury. In conclusion, inhibition of hepatic Mogat1 in HTF-C diet-fed mice improves hepatic metabolic abnormalities without attenuating liver inflammation and injury.

Abrogating monoacylglycerol acyltransferase activity in liver improves glucose tolerance and hepatic insulin signaling in obese mice.

Monoacylglycerol acyltransferase (MGAT) enzymes convert monoacylglycerol to diacylglycerol (DAG), a lipid that has been linked to the development of hepatic insulin resistance through activation of protein kinase C (PKC). The expression of genes that encode MGAT enzymes is induced in the livers of insulin-resistant human subjects with nonalcoholic fatty liver disease, but whether MGAT activation is causal of hepatic steatosis or insulin resistance is unknown. We show that the expression of Mogat1, which encodes MGAT1, and MGAT activity are also increased in diet-induced obese (DIO) and ob/obmice. To probe the metabolic effects of MGAT1 in the livers of obese mice, we administered antisense oligonucleotides (ASOs) against Mogat1 to DIO and ob/ob mice for 3 weeks. Knockdown of Mogat1 in liver, which reduced hepatic MGAT activity, did not affect hepatic triacylglycerol content and unexpectedly increased total DAG content. Mogat1 inhibition also increased both membrane and cytosolic compartment DAG levels. However, Mogat1 ASO treatment significantly improved glucose tolerance and hepatic insulin signaling in obese mice. In summary, inactivation of hepatic MGAT activity, which is markedly increased in obese mice, improved glucose tolerance and hepatic insulin signaling independent of changes in body weight, intrahepatic DAG and TAG content, and PKC signaling.

Glucose transporter 8 (GLUT8) mediates fructose-induced de novo lipogenesis and macrosteatosis.

Non-alcoholic fatty liver disease (NAFLD) is the most common liver disease in the world, and it is thought to be the hepatic manifestation of the metabolic syndrome. Excess dietary fructose causes both metabolic syndrome and NAFLD in rodents and humans, but the pathogenic mechanisms of fructose-induced metabolic syndrome and NAFLD are poorly understood. GLUT8 (Slc2A8) is a facilitative glucose and fructose transporter that is highly expressed in liver, heart, and other oxidative tissues. We previously demonstrated that female mice lacking GLUT8 exhibit impaired first-pass hepatic fructose metabolism, suggesting that fructose transport into the hepatocyte, the primary site of fructose metabolism, is in part mediated by GLUT8. Here, we tested the hypothesis that GLUT8 is required for hepatocyte fructose uptake and for the development of fructose-induced NAFLD. We demonstrate that GLUT8 is a cell surface-localized transporter and that GLUT8 overexpression or GLUT8 shRNA-mediated gene silencing significantly induces and blocks radiolabeled fructose uptake in cultured hepatocytes. We further show diminished fructose uptake and de novo lipogenesis in fructose-challenged GLUT8-deficient hepatocytes. Finally, livers from long term high-fructose diet-fed GLUT8-deficient mice exhibited attenuated fructose-induced hepatic triglyceride and cholesterol accumulation without changes in hepatocyte insulin-stimulated Akt phosphorylation. GLUT8 is thus essential for hepatocyte fructose transport and fructose-induced macrosteatosis. Fructose delivery across the hepatocyte membrane is thus a proximal, modifiable disease mechanism that may be exploited to prevent NAFLD.

PGC-1ß and ChREBP partner to cooperatively regulate hepatic lipogenesis in a glucose concentration-dependent manner.

Peroxisome proliferator-activated receptorγ coactivators (PGC-1α and PGC-1ß) play important roles in the transcriptional regulation of intermediary metabolism. To evaluate the effects of overexpressing PGC-1α or PGC-1ß at physiologic levels in liver, we generated transgenic mice with inducible overexpression of PGC-1α or PGC-1ß. Gene expression array profiling revealed that whereas both PGC-1 family proteins induced mitochondrial oxidative enzymes, the expression of several genes involved in converting glucose to fatty acid was induced by PGC-1ß, but not PGC-1α. The increased expression of enzymes involved in carbohydrate utilization and de novo lipogenesis by PGC-1ß required carbohydrate response element binding protein (ChREBP). The interaction between PGC-1ß and ChREBP, as well as PGC-1ß occupancy of the liver-type pyruvate kinase promoter, was influenced by glucose concentration and liver-specific PGC-1ß(-/-) hepatocytes were refractory to the lipogenic response to high glucose conditions. These data suggest that PGC-1ß-mediated coactivation of ChREBP is involved in the lipogenic response to hyperglycemia.

Glucose transporter-8 (GLUT8) mediates glucose intolerance and dyslipidemia in high-fructose diet-fed male mice.

Members of the glucose transporter (GLUT) family of membrane-spanning hexose transporters are subjects of intensive investigation for their potential as modifiable targets to treat or prevent obesity, metabolic syndrome, and type 2 diabetes mellitus. Mounting evidence suggests that the ubiquitously expressed class III dual-specificity glucose and fructose transporter, GLUT8, has important metabolic homeostatic functions. We therefore tested the hypothesis that GLUT8 mediates the deleterious metabolic effects of chronic high-fructose diet exposure. Here we demonstrate resistance to high-fructose diet-induced glucose intolerance and dyslipidemia concomitant with enhanced oxygen consumption and thermogenesis in GLUT8-deficient male mice. Independent of diet, significantly lower systolic blood pressure both at baseline and after high-fructose diet feeding was also observed by tail-cuff plethysmography in GLUT8-deficient mice vs wild-type controls. Resistance to fructose-induced metabolic dysregulation occurred in the context of enhanced hepatic peroxisome proliferator antigen receptor-γ (PPARγ) protein abundance, whereas in vivo hepatic adenoviral GLUT8 overexpression suppressed hepatic PPARγ expression. Taken together, these findings suggest that GLUT8 blockade prevents fructose-induced metabolic dysregulation, potentially by enhancing hepatic fatty acid metabolism through PPARγ and its downstream targets. We thus establish GLUT8 as a promising target in the prevention of diet-induced obesity, metabolic syndrome, and type 2 diabetes mellitus in males.

Association between specific adipose tissue CD4+ T-cell populations and insulin resistance in obese individuals.

BACKGROUND & AIMS: An increased number of macrophages in adipose tissue is associated with insulin resistance and metabolic dysfunction in obese people. However, little is known about other immune cells in adipose tissue from obese people, and whether they contribute to insulin resistance. We investigated the characteristics of T cells in adipose tissue from metabolically abnormal insulin-resistant obese (MAO) subjects, metabolically normal insulin-sensitive obese (MNO) subjects, and lean subjects. Insulin sensitivity was determined by using the hyperinsulinemic euglycemic clamp procedure. METHODS: We assessed plasma cytokine concentrations and subcutaneous adipose tissue CD4(+) T-cell populations in 9 lean, 12 MNO, and 13 MAO subjects. Skeletal muscle and liver samples were collected from 19 additional obese patients undergoing bariatric surgery to determine the presence of selected cytokine receptors. RESULTS: Adipose tissue from MAO subjects had 3- to 10-fold increases in numbers of CD4(+) T cells that produce interleukin (IL)-22 and IL-17 (a T-helper [Th] 17 and Th22 phenotype) compared with MNO and lean subjects. MAO subjects also had increased plasma concentrations of IL-22 and IL-6. Receptors for IL-17 and IL-22 were expressed in human liver and skeletal muscle samples. IL-17 and IL-22 inhibited uptake of glucose in skeletal muscle isolated from rats and reduced insulin sensitivity in cultured human hepatocytes. CONCLUSIONS: Adipose tissue from MAO individuals contains increased numbers of Th17 and Th22 cells, which produce cytokines that cause metabolic dysfunction in liver and muscle in vitro. Additional studies are needed to determine whether these alterations in adipose tissue T cells contribute to the pathogenesis of insulin resistance in obese people.