Xanthine oxidoreductase is a regulator of adipogenesis and PPARgamma activity

Differentiation of human adipose-derived stem cells into fat involves reactive oxygen species and Forkhead box O1 mediated upregulation of antioxidant enzymes

Both reactive oxygen species (ROS) and Forkhead box O (FOXO) family transcription factors are involved in the regulation of adipogenic differentiation of preadipocytes and stem cells. While FOXO has a pivotal role in maintaining cellular redox homeostasis, the interactions between ROS and FOXO during adipogenesis are not clear. Here we examined how ROS and FOXO regulate adipogenesis in human adipose-derived stem cells (hASC). The identity of isolated cells was confirmed by their surface marker expression pattern typical for human mesenchymal stem cells (positive for CD29, CD44, CD73, CD90, and CD105, negative for CD45 and CD31). Using a standard adipogenic cocktail consisting of insulin, dexamethasone, indomethacin, and 3-Isobutyl-1-methylanxthine (IDII), adipogenesis was induced in hASC, which was accompanied by ROS generation. Scavenging ROS production with N-acetyl-L-cysteine or EUK-8, a catalytic mimetic of superoxide dismutase (SOD) and catalase, inhibited IDII-induced adipogenesis. We then mimicked IDII-induced oxidative stress through a lentiviral overexpression of Nox4 and an exogenous application of hydrogen peroxide in hASC and both manipulations significantly enhanced adipogenesis without changing the adipogenic differentiation rate. These data suggest that ROS promoted lipid accumulation in hASC undergoing adipogenesis. Antioxidant enzymes, including SOD2, catalase, and glutathione peroxidase were upregulated by IDII during adipogenesis, and these effects were blunted by FOXO1 silencing, which also suppressed significantly IDII-induced adipogenesis. Our findings demonstrated a balance of ROS generation and endogenous antioxidants in cells undergoing adipogenesis. Approaches targeting ROS and/or FOXO1 in adipocytes may bring new strategies to prevent and treat obesity and metabolic syndrome.

Loss of coiled-coil domain containing 80 negatively modulates glucose homeostasis in diet-induced obese mice

Coiled-coil domain containing 80 (Ccdc80) is a secreted protein highly enriched in mouse and human white adipose tissue (WAT) that plays an important role during adipocyte differentiation in vitro. To investigate the physiological function of Ccdc80 in energy and glucose homeostasis, we generated mice in which the gene encoding Ccdc80 was disrupted. Mice lacking Ccdc80 showed increased sensitivity to diet-induced hyperglycemia and glucose intolerance while displaying reduced glucose-stimulated insulin secretion in vivo. Gene expression analysis by microarray revealed that only 10 transcripts were simultaneously altered in pancreas, skeletal muscle, and WAT from Ccdc80(-/-) mice, including some components of the circadian clock. Expression of the core clock member Arntl/Bmal1 was reduced whereas that of the oscillating transcription factors Dbp and Tef was increased in all tissues examined. Furthermore, knockdown of Ccdc80 in 3T3-L1 cells led to an increase of Dbp mRNA levels during adipocyte differentiation, suggesting that Ccdc80 might be involved in the regulation of this gene in a cell-autonomous manner. Importantly, transcriptional alterations in Ccdc80(-/-) mice were associated with changes in feeding behavior, increased caloric intake, decreased energy expenditure, and obesity. Taken together, our results suggest that Ccdc80 is a novel modulator of glucose and energy homeostasis during diet-induced obesity.

Contribution of uric acid to cancer risk, recurrence, and mortality

Two risk factors for the development and progression of cancers that are amenable to life style modification are chronic inflammation and the metabolic syndrome. This review proposes two new targets that may mechanistically integrate inflammation and metabolic syndrome, have been largely ignored, and are known to be druggable. Recent evidence has demonstrated that elevated serum uric acid (hyperuricemia) is associated with excess cancer risk, recurrence, and mortality. Although uric acid (UA) can function as a systemic antioxidant, its pro-inflammatory properties have been postulated to play an important role in the pathogenesis of cancer. Furthermore, obesity, Type 2 Diabetes Mellitus (T2DM), and the metabolic syndrome (MetS) are also associated with excess cancer, chronic inflammation, and with hyperuricemia, suggesting that UA may represent an important link between these disorders and the development of cancer. While pharmacological modulation of hyperuricemia could in principal augment anti-cancer therapeutic strategies, some cancer cells express low intracellular levels of the enzyme Xanthine Oxidoreductase (XOR) that are associated with increased cancer aggressiveness and poor clinical outcome. Thus, systemic pharmacological inhibition of XOR may worsen clinical outcome, and specific strategies that target serum uric acid (SUA) without inhibiting tumor cell XOR may create new therapeutic opportunities for cancer associated with hyperuricemia. This review will summarize the evidence that elevated SUA may be a true risk factor for cancer incidence and mortality, and mechanisms by which UA may contribute to cancer pathogenesis will be discussed in the hope that these will identify new opportunities for cancer management.

Untargeted plasma metabolite profiling reveals the broad systemic consequences of xanthine oxidoreductase inactivation in mice

A major challenge in systems biology is integration of molecular findings for individual enzyme activities into a cohesive high-level understanding of cellular metabolism and physiology/pathophysiology. However, meaningful prediction for how a perturbed enzyme activity will globally impact metabolism in a cell, tissue or intact organisms is precluded by multiple unknowns, including in vivo enzymatic rates, subcellular distribution and pathway interactions. To address this challenge, metabolomics offers the potential to simultaneously survey changes in thousands of structurally diverse metabolites within complex biological matrices. The present study assessed the capability of untargeted plasma metabolite profiling to discover systemic changes arising from inactivation of xanthine oxidoreductase (XOR), an enzyme that catalyzes the final steps in purine degradation. Using LC-MS coupled with a multivariate statistical data analysis platform, we confidently surveyed >3,700 plasma metabolites (50-1,000 Da) for differential expression in XOR wildtype vs. mice with inactivated XOR, arising from gene deletion or pharmacological inhibition. Results confirmed the predicted derangements in purine metabolism, but also revealed unanticipated perturbations in metabolism of pyrimidines, nicotinamides, tryptophan, phospholipids, Krebs and urea cycles, and revealed kidney dysfunction biomarkers. Histochemical studies confirmed and characterized kidney failure in xor-nullizygous mice. These findings provide new insight into XOR functions and demonstrate the power of untargeted metabolite profiling for systemic discovery of direct and indirect consequences of gene mutations and drug treatments.

Urate transporter gene SLC22A12 polymorphisms associated with obesity and metabolic syndrome in Caucasians with hypertension

BACKGROUND/AIMS

Hyperuricemia is associated with obesity and the metabolic syndrome. URAT1 is a urate transporter, and we tested the association of URAT1 transporter gene (SLC22A12) polymorphisms with obesity and the metabolic syndrome in hypertensive subjects.

METHODS

Patients with essential hypertension (n = 414) from a randomized controlled study were genotyped for SLC22A12 SNPs rs11602903, rs505802 and rs11231825.

RESULTS

In Caucasians, SLC22A12 SNPs were associated with the body mass index (BMI). rs11602903 was associated with BMI (p < 0.0001), waist circumference (p = 0.003), HDL cholesterol (p = 0.018) and the metabolic syndrome (p = 0.033), and accounted for 7% of the variation of BMI in Caucasians. In African Americans, SLC22A12 SNP rs11602903 was not associated with BMI, waist circumference, HDL cholesterol or triglycerides.

CONCLUSION

The URAT1 gene SLC22A12 polymorphism may play a role in obesity and the metabolic syndrome in Caucasian hypertensive subjects.

Cross-sectional and longitudinal associations between serum uric acid and metabolic syndrome

Research on the importance of serum uric acid (SUA) as a contributing metabolic factor to cardiovascular diseases has conducted to conflicting results, with most studies assuming a cross-sectional design. The aim of this study was to evaluate the association of SUA and metabolic syndrome (MetS) and its features. A representative sample of 2,485 individuals aged ≥18 years was randomly selected from the non-institutionalized resident population of Porto, Portugal. A total of 1,054 eligible subjects were included for the longitudinal analyses. Hyperuricemia was defined as SUA ≥70 mg/L in men and ≥60 mg/L in women. MetS was defined according the Joint Interim (2009) criteria. Associations were estimated using Poison regression and binomial models. In the cross-sectional analysis, subjects with hyperuricemia had a 2.10-fold increased risk of MetS as compared with normouricemic subjects (PR = 2.10, 95% CI: 1.68-2.63). Among MetS features, high triglycerides presented the strongest association with hyperuricemia (PR = 2.32, 95% CI: 1.84-2.91). The MetS crude incidence rate was 4.5/100 person-year (95% CI: 3.9-5.2) in normal uricemic and 13.0/100 person-year (95% CI: 8.5-20.0) in hyperuricemic participants. Using a multivariate longitudinal approach, hyperuricemia was positively associated with MetS incidence rate ratios (IRR = 1.73, 95% CI: 1.08-2.76). One standard deviation increase of SUA concentration was associated with a 1.22-fold increase in MetS risk (IRR = 1.22, 95% CI: 1.05-1.42). Elevated SUA presented the strongest association with high-triglycerides concentration (IRR = 1.44, 95%: 1.22-1.71) and waist circumference (IRR = 1.25, 95%: 1.05-1.49). The independent positive association between SUA and MetS suggested by this longitudinal study supports that SUA might be a risk factor for MetS.

Association between serum uric acid level and metabolic syndrome

OBJECTIVES

Serum uric acid levels have been reported to be associated with a variety of cardiovascular conditions. However, the direct association between uric acid levels and metabolic syndrome remains controversial. Thus, we evaluated the association of serum uric acid levels and metabolic syndrome in a community-based cohort study in Korea.

METHODS

We performed cross-sectional analysis of baseline data of 889 males and 1491 females (aged 38 to 87) who participated in baseline examinations of the Korean Genome and Epidemiology Study: Kanghwa study. Blood samples were collected after at least an 8 hour fast. Uric acid quartiles were defined as follows: <4.8, 4.8-<5.6, 5.6-<6.5, ≥6.5 mg/dL in males; and <3.8, 3.8-<4.3, 4.3-<5.1, ≥5.1 mg/dL in females. Metabolic syndrome was defined by the National Cholesterol Education Program Adult Treatment Panel III Criteria with adjusted waist circumference cutoffs (90 cm for males; 80 cm for females). The association between serum uric acid quartiles and metabolic syndrome was assessed using multivariate logistic regression.

RESULTS

The odds ratio for having metabolic syndrome in the highest versus lowest quartiles of serum uric acid levels was 2.67 (95% confidence interval [CI], 1.60 to 4.46) in males and 2.14 (95% CI, 1.50 to 3.05) in females after adjusting for age, smoking, alcohol intake, body mass index, total cholesterol, HbA1c, albumin, γ-glutamyltransferase, blood urea nitrogen, and log C-reactive protein. The number of metabolic abnormalities also increased gradually with increasing serum uric acid levels (adjusted p for trend < 0.001 in both sexes).

CONCLUSIONS

Higher serum uric acid levels are positively associated with the presence of metabolic syndrome in Korean males and females.

Mdm2 controls CREB-dependent transactivation and initiation of adipocyte differentiation

The role of the E3 ubiquitin ligase murine double minute 2 (Mdm2) in regulating the stability of the p53 tumor suppressor is well documented. By contrast, relatively little is known about p53-independent activities of Mdm2 and the role of Mdm2 in cellular differentiation. Here we report a novel role for Mdm2 in the initiation of adipocyte differentiation that is independent of its ability to regulate p53. We show that Mdm2 is required for cAMP-mediated induction of CCAAT/enhancer-binding protein δ (C/EBPδ) expression by facilitating recruitment of the cAMP regulatory element-binding protein (CREB) coactivator, CREB-regulated transcription coactivator (Crtc2)/TORC2, to the c/ebpδ promoter. Our findings reveal an unexpected role for Mdm2 in the regulation of CREB-dependent transactivation during the initiation of adipogenesis. As Mdm2 is able to promote adipogenesis in the myoblast cell line C2C12, it is conceivable that Mdm2 acts as a switch in cell fate determination.

Cidea is an essential transcriptional coactivator regulating mammary gland secretion of milk lipids

Adequate lipid secretion by mammary glands during lactation is essential for the survival of mammalian offspring. However, the mechanism governing this process is poorly understood. Here we show that Cidea is expressed at high levels in lactating mammary glands and its deficiency leads to premature pup death as a result of severely reduced milk lipids. Furthermore, the expression of xanthine oxidoreductase (XOR), an essential factor for milk lipid secretion, is markedly lower in Cidea-deficient mammary glands. Conversely, ectopic Cidea expression induces the expression of XOR and enhances lipid secretion in vivo. Unexpectedly, as Cidea has heretofore been thought of as a cytoplasmic protein, we detected it in the nucleus and found it to physically interact with transcription factor CCAAT/enhancer-binding protein β (C/EBPβ) in mammary epithelial cells. We also observed that Cidea induces XOR expression by promoting the association of C/EBPβ onto, and the dissociation of HDAC1 from, the promoter of the Xdh gene encoding XOR. Finally, we found that Fsp27, another CIDE family protein, is detected in the nucleus and interacts with C/EBPβ to regulate expression of a subset of C/EBPβ downstream genes in adipocytes. Thus, Cidea acts as a previously unknown transcriptional coactivator of C/EBPβ in mammary glands to control lipid secretion and pup survival.

The inflammasome puts obesity in the danger zone

Obesity-induced inflammation is an important contributor to the induction of insulin resistance. Recently, the cytokine interleukin-1β (IL-1β) has emerged as a prominent instigator of the proinflammatory response in obesity. Several studies over the last year have subsequently deciphered the molecular mechanisms responsible for IL-1β activation in adipose tissue, liver, and macrophages and demonstrated a central role of the processing enzyme caspase-1 and of the protein complex leading to its activation called the inflammasome. These data suggest that activation of the inflammasome represents a crucial step in the road from obesity to insulin resistance and type 2 diabetes.

Xanthine oxidoreductase is involved in macrophage foam cell formation and atherosclerosis development

OBJECTIVE

Hyperuricemia is common in patients with metabolic syndrome. We investigated the role of xanthine oxidoreductase (XOR) in atherosclerosis development, and the effects of the XOR inhibitor allopurinol on this process.

METHODS AND RESULTS

Oral administration of allopurinol to ApoE knockout mice markedly ameliorated lipid accumulation and calcification in the aorta and aortic root. In addition, allopurinol treatment or siRNA-mediated gene knockdown of XOR suppressed transformation of J774.1 murine macrophage cells, treated with acetylated LDL or very low density lipoprotein (VLDL) into foam cells. This inhibitory effect of allopurinol was also observed in primary cultured human macrophages. In contrast, overexpression of XOR promoted transformation of J774.1 cells into foam cells. Interestingly, SR-A1, SR-B1, SR-B II, and VLDL receptors in J774.1 cells were reduced by XOR knockdown, and increased by XOR overexpression. Conversely, expressions of ABCA1 and ABCG1 were increased by XOR knockdown and suppressed by XOR overexpression. Finally, productions of inflammatory cytokines accompanied by foam cell formation were also reduced by allopurinol administration.

CONCLUSIONS

These results strongly suggest XOR activity and/or its expression level to contribute to macrophage foam cell formation. Thus, XOR inhibitors may be useful for preventing atherosclerosis.

Xanthine oxidoreductase: a journey from purine metabolism to cardiovascular excitation-contraction coupling

Xanthine oxidoreductase (XOR) is a ubiquitous complex cytosolic molybdoflavoprotein which controls the rate limiting step of purine catabolism by converting xanthine to uric acid. It is known that optimum concentrations of uric acid (UA) and reactive oxygen species (ROS) are necessary for normal functioning of the body. The ability of XOR to perform detoxification reactions, and to synthesize UA and reactive oxygen species (ROS) makes it a versatile intra- and extra-cellular protective "housekeeping enzyme". It is also an important component of the innate immune system. The enzyme is a target of drugs against gout and hyperuricemia and the protein is of major interest as it is associated with ischemia reperfusion (I/R) injury, vascular disorders in diabetes, cardiovascular disorders, adipogenesis, metabolic syndrome, cancer, and many other disease conditions. Xanthine oxidoreductase in conjugation with antibodies has been shown to have an anti-tumor effect due to its ability to produce ROS, which in turn reduces the growth of cancer tissues. Apart from this, XOR in association with nitric oxide synthase also participates in myocardial excitation-contraction coupling. Although XOR was discovered over 100 years ago, its physiological and pathophysiological roles are still not clearly elucidated. In this review, various physiological and pathophysiological functional aspects of XOR and its association with various forms of cancer are discussed in detail.

Transcriptional regulatory program in wild-type and retinoblastoma gene-deficient mouse embryonic fibroblasts during adipocyte differentiation

Background

Although many molecular regulators of adipogenesis have been identified a comprehensive catalogue of components is still missing. Recent studies showed that the retinoblastoma protein (pRb) was expressed in the cell cycle and late cellular differentiation phase during adipogenesis. To investigate this dual role of pRb in the early and late stages of adipogenesis we used microarrays to perform a comprehensive systems-level analysis of the common transcriptional program of the classic 3T3-L1 preadipocyte cell line, wild-type mouse embryonic fibroblasts (MEFs), and retinoblastoma gene-deficient MEFs (Rb-/- MEFs).

Findings

Comparative analysis of the expression profiles of 3T3-L1 cells and wild-type MEFs revealed genes involved specifically in early regulation of the adipocyte differentiation as well as secreted factors and signaling molecules regulating the later phase of differentiation. In an attempt to identify transcription factors regulating adipogenesis, bioinformatics analysis of the promoters of coordinately and highly expressed genes was performed. We were able to identify a number of high-confidence target genes for follow-up experimental studies. Additionally, combination of experimental data and computational analyses pinpointed a feedback-loop between Pparg and Foxo1.

To analyze the effects of the retinoblastoma protein at the transcriptional level we chose a perturbated system (Rb-/- MEFs) for comparison to the transcriptional program of wild-type MEFs. Gene ontology analysis of 64 deregulated genes showed that the Rb-/- MEF model exhibits a brown(-like) adipocyte phenotype. Additionally, the analysis results indicate a different or additional role for pRb family member involvement in the lineage commitment.

Conclusion

In this study a number of commonly modulated genes during adipogenesis in 3T3-L1 cells and MEFs, potential transcriptional regulation mechanisms, and differentially regulated targets during adipocyte differentiation of Rb-/- MEFs could be identified. These data and the analysis provide a starting point for further experimental studies to identify target genes for pharmacological intervention and ultimately remodeling of white adipose tissue into brown adipose tissue.

Hyperuricemia as a mediator of the proinflammatory endocrine imbalance in the adipose tissue in a murine model of the metabolic syndrome

OBJECTIVE

Hyperuricemia is strongly associated with obesity and metabolic syndrome and can predict visceral obesity and insulin resistance. Previously, we showed that soluble uric acid directly stimulated the redox-dependent proinflammatory signaling in adipocytes. In this study we demonstrate the role of hyperuricemia in the production of key adipokines.

RESEARCH DESIGN AND METHODS

We used mouse 3T3-L1 adipocytes, human primary adipocytes, and a mouse model of metabolic syndrome and hyperuricemia.

RESULTS

Uric acid induced in vitro an increase in the production (mRNA and secreted protein) of monocyte chemotactic protein-1 (MCP-1), an adipokine playing an essential role in inducing the proinflammatory state in adipocytes in obesity. In addition, uric acid caused a decrease in the production of adiponectin, an adipocyte-specific insulin sensitizer and anti-inflammatory agent. Uric acid-induced increase in MCP-1 production was blocked by scavenging superoxide or by inhibiting NADPH oxidase and by stimulating peroxisome-proliferator-activated receptor-γ with rosiglitazone. Downregulation of the adiponectin production was prevented by rosiglitazone but not by antioxidants. In obese mice with metabolic syndrome, we observed hyperuricemia. Lowering uric acid in these mice by inhibiting xanthine oxidoreductase with allopurinol could improve the proinflammatory endocrine imbalance in the adipose tissue by reducing production of MCP-1 and increasing production of adiponectin. In addition, lowering uric acid in obese mice decreased macrophage infiltration in the adipose tissue and reduced insulin resistance.

CONCLUSIONS

Hyperuricemia might be partially responsible for the proinflammatory endocrine imbalance in the adipose tissue, which is an underlying mechanism of the low-grade inflammation and insulin resistance in subjects with the metabolic syndrome.

Monoalkylglycerol ether lipids promote adipogenesis

The molecular mechanisms that lead to the generation of adipose tissue (adipogenesis) are of basic and biomedical interest. Cellular models of adipogenesis have proven extremely valuable in defining biomolecules-primarily genes and proteins-that regulate adipogenesis. Here, the analysis of differentiating adipocytes using an untargeted metabolomics approach led to the discovery of the monoalkylglycerol ethers as a natural class of adipocyte differentiation factors.

Arxes: retrotransposed genes required for adipogenesis

Retrotransposed sequences arise from messenger RNAs (mRNAs) that have been reinserted into genomic DNA by reverse transcription. Usually, these sequences are embedded in dormant regions, collect missense mutations over time and constitute processed, nonfunctional pseudogenes. There are thousands of processed pseudogenes in the mouse and human genome. Here, we report evidence for two paralog genes (termed Arxes1 and Arxes2), which arose by retrotransposition of the signal peptidase Spcs3 followed by a segmental duplication event. They gained a functional promoter that we show to be transactivated by adipogenic transcription factors. We further show that the Arxes mRNAs are highly expressed in adipose tissue and strongly upregulated during adipogenesis in different cell models. Additionally, their expression is elevated by an anti-diabetic agent in vitro and in vivo. Importantly, we provide evidence that the Arxes genes are translated and that the proteins are located in the endoplasmic reticulum. Although the sequence similarity and subcellular location are reminiscent of their parental gene, our data suggest that the Arxes have developed a different function, since their expression is required for adipogenesis, whereas Spcs3 is dispensable. In summary, we report retrotransposed-duplicated genes that evolved from a parental gene to function in a tissue and adipogenesis-specific context.

Endogenous ligands for nuclear receptors: digging deeper

Nuclear receptors (NRs) are hormone-sensing transcription factors that translate dietary or endocrine signals into changes in gene expression. Therefore, the adoption of orphan NRs through the identification of their endogenous ligands is a key element for our understanding of their biology. In this minireview, we give an update on recent progress in regard to endogenous ligands for a cluster of NRs with high sequence homology, namely peroxisome proliferator-activated receptors α and γ, Rev-erbα, and related receptors. This knowledge about the nature and physiology of these ligands may create new opportunities for therapeutic drug development.

Increased frequency of metabolic syndrome and its individual metabolic abnormalities in Japanese patients with primary gout

BACKGROUND

Gout patients are frequently complicated with hypertension, obesity, dyslipidemia, and/or impaired glucose tolerance, which are components of the metabolic syndrome and risks for atherosclerotic diseases.

OBJECTIVES

To determine the relationship between metabolic syndrome and gout, as well as plasma concentrations of adipocytokines in gout patients.

SUBJECTS AND METHODS

The frequency of metabolic syndrome as well as its constituents were investigated in 258 male gout patients and 111 males who attended an annual check-up examination. In addition, plasma concentrations of adipocytokines were measured in 107 of the patients.

RESULTS

Gout patients had a higher prevalence of metabolic syndrome as compared with the controls (36.4% vs. 15.3%, P < 0.0001). In addition, frequencies of individual metabolic abnormalities, such as waist circumference >85 cm, hypertension, and hypertriglyceridemia, were significantly increased in the gout patients as compared with the controls. Furthermore, uric acid over-production gout had a significantly higher prevalence of metabolic syndrome as compared with uric acid under-excretion gout (48.6% vs. 32.4%, P < 0.001). The plasma concentrations of leptin and plasminogen activator inhibitor-1 were significantly higher in the patients (P < 0.05, respectively), while that of adiponectin and the adiponectin/leptin ratio were significantly decreased in the gout patients as compared with the controls (P < 0.05, respectively).

CONCLUSION

A higher prevalence of metabolic syndrome in gout patients may in part contribute to susceptibility to atherosclerotic diseases. Therefore, more attention should be paid to the presence of metabolic syndrome in gout patients to reduce their risk for cardiovascular disease complications.

Peroxisome proliferator-activated receptor-gamma increases adiponectin secretion via transcriptional repression of endoplasmic reticulum chaperone protein ERp44

Adiponectin, an adipocyte-derived hormone, is a versatile player involved in the regulation of energy homeostasis, cardiovascular disease, and diabetes. Within adipocytes, adiponectin is retained in the lumen of the endoplasmic reticulum (ER) by binding to the thiol protein ER resident protein 44 kDa (ERp44), which is apparently regulated by the activation of nuclear receptor peroxisome proliferator-activated receptor (PPAR)-gamma. However, the precise role of ERp44 in adiponectin secretion remains elusive. In the present study, we investigated the functional correlation between ERp44 and adiponectin in a pig model. The transcription of porcine ERp44 was regulated by PPARgamma, which was consistent with the finding of putative peroxisome proliferator response element sites within ERp44 promoter. Using chromatin immunoprecipitation and luciferase reporter assays, we demonstrated that the transcription of porcine ERp44 is repressed through binding of PPARgamma to a peroxisome proliferator response element site located between positions -981 and -1004 in its 5'-flanking region. In human embryonic kidney 293 cells stably transfected with cDNA encoding porcine adiponectin, the secretion of adiponectin was significantly up-regulated and the ERp44 mRNA was down-regulated observably, by either the treatment of PPARgamma agonist rosiglitazone or the overexpression of PPARgamma in these cells. Taken together, our results indicated that PPARgamma is an essential regulatory factor for the transcriptional activity of ERp44, which in turn controls the secretion of adiponectin.

Epidermis-type lipoxygenase 3 regulates adipocyte differentiation and peroxisome proliferator-activated receptor gamma activity

The nuclear receptor peroxisome proliferator-activated receptor gamma (PPAR gamma) is essential for adipogenesis. Although several fatty acids and their derivatives are known to bind and activate PPAR gamma, the nature of the endogenous ligand(s) promoting the early stages of adipocyte differentiation has remained enigmatic. Previously, we showed that lipoxygenase (LOX) activity is involved in activation of PPAR gamma during the early stages of adipocyte differentiation. Of the seven known murine LOXs, only the unconventional LOX epidermis-type lipoxygenase 3 (eLOX3) is expressed in 3T3-L1 preadipocytes. Here, we show that forced expression of eLOX3 or addition of eLOX3 products stimulated adipogenesis under conditions that normally require an exogenous PPAR gamma ligand for differentiation. Hepoxilins, a group of oxidized arachidonic acid derivatives produced by eLOX3, bound to and activated PPAR gamma. Production of hepoxilins was increased transiently during the initial stages of adipogenesis. Furthermore, small interfering RNA-mediated or retroviral short hairpin RNA-mediated knockdown of eLOX3 expression abolished differentiation of 3T3-L1 preadipocytes. Finally, we demonstrate that xanthine oxidoreductase (XOR) and eLOX3 synergistically enhanced PPAR gamma-mediated transactivation. Collectively, our results indicate that hepoxilins produced by the concerted action of XOR and eLOX3 may function as PPAR gamma activators capable of promoting the early PPAR gamma-dependent steps in the conversion of preadipocytes into adipocytes.

Transcriptional targets in adipocyte biology

The global burden of metabolic disease demands that we develop new therapeutic strategies. Many of these approaches may center on manipulating the behavior of adipocytes, which contribute directly and indirectly to a host of disease processes including obesity and type 2 diabetes. One way to achieve this goal will be to alter key transcriptional pathways in fat cells, such as those regulating glucose uptake, lipid handling, or adipokine secretion. In this review, we look at what is known about how adipocytes govern their physiology at the gene expression level, and discuss novel ways that we can accelerate our understanding of this area.

Xanthine oxidoreductase depletion induces renal interstitial fibrosis through aberrant lipid and purine accumulation in renal tubules

Xanthine oxidoreductase (XOR) is an enzyme responsible for purine degradation, reactive oxygen species production, and adipogenesis. XOR gene-disrupted (XOR(-/-)) mice demonstrate renal failure and early death within several months. The aim of this study was to elucidate the mechanism of renal damage in XOR(-/-) mice and to determine the physiological role of XOR in the kidney. Histological analysis revealed that renal tubular damage in XOR(-/-) mice was accompanied by deposition of crystals and lipid-rich substances. Triglyceride content in renal homogenates was significantly increased in XOR(-/-) mice. The level of lipogenesis-related gene expression was comparable in XOR(+/+) and XOR(-/-) mice, whereas the expression of adipogenesis-related gene expression was significantly elevated in XOR(-/-) mice. Urinary excretions of xanthine and hypoxanthine were markedly elevated in XOR(-/-) mice. Immunohistochemical analysis, Western blotting, and real time RT-PCR revealed that various markers of fibrosis, inflammation, ischemia, and oxidative stress were increased in XOR(-/-) mice. Finally, we demonstrate that primary renal epithelial cells from XOR(-/-) mice are more readily transformed to myofibroblasts, which is a marker of increased epithelial mesenchymal transition. These results suggest that XOR gene disruption induced the depletion of uric acid and the accumulation of triglyceride-rich substances, xanthine, and hypoxanthine in the renal tubules. We believe that these changes contribute to a complex cellular milieu characterized by inflammation, tissue hypoxia, and reactive oxygen species production, ultimately resulting in renal failure through increased renal interstitial fibrosis.

Inhibition of thymic adipogenesis by caloric restriction is coupled with reduction in age-related thymic involution

Aging of thymus is characterized by reduction in naive T cell output together with progressive replacement of lymphostromal thymic zones with adipocytes. Determining how calorie restriction (CR), a prolongevity metabolic intervention, regulates thymic aging may allow identification of relevant mechanisms to prevent immunosenescence. Using a mouse model of chronic CR, we found that a reduction in age-related thymic adipogenic mechanism is coupled with maintenance of thymic function. The CR increased cellular density in the thymic cortex and medulla and preserved the epithelial signatures. Interestingly, CR prevented the age-related increase in epithelial-mesenchymal transition (EMT) regulators, FoxC2, and fibroblast-specific protein-1 (FSP-1), together with reduction in lipid-laden thymic fibroblasts. Additionally, CR specifically blocked the age-related elevation of thymic proadipogenic master regulator, peroxisome proliferator activated receptor gamma (PPARgamma), and its upstream activator xanthine-oxidoreductase (XOR). Furthermore, we found that specific inhibition of PPARgamma in thymic stromal cells prevented their adipogenic transformation in an XOR-dependent mechanism. Activation of PPARgamma-driven adipogenesis in OP9-DL1 stromal cells compromised their ability to support T cell development. Conversely, CR-induced reduction in EMT and thymic adipogenesis were coupled with elevated thymic output. Compared with 26-mo-old ad libitum fed mice, the T cells derived from age-matched CR animals displayed greater proliferation and higher IL-2 expression. Furthermore, CR prevented the deterioration of the peripheral TCR repertoire diversity in older animals. Collectively, our findings demonstrate that reducing proadipogenic signaling in thymus via CR may promote thymopoiesis during aging.

Raised natriuretic peptides, big-endothelin-1 and improved beta-cell function in type 2 diabetic males with hyperuricaemia

Urate, a naturally-occurring antioxidant, is a marker/factor for cardiovascular disease. Hyperuricaemia is associated with IR, MetS and endothelial dysfunction. We characterised the associations between neurohormones, uricaemia, and glucose homeostasis in type 2 diabetes mellitus (T2DM) males. Cross-sectional; 705 T2DM males divided into two groups: uric acid < 7.0 mg/dl (normouricaemic; n=476) versus uric acid >or= 7.0 mg/dl (hyperuricaemic; n=229). HOMA beta-cell function (B), insulin sensitivity (S), hyperbolic product (BxS), and (BxS) loss rate were determined alongside neurohormones (Nt-proANP, BNP, Big ET-1 and UII). Mean age and diabetes duration were not different between groups. Hyperuricaemics had more macroangiopathy, total/central adiposity, IR, hypertension, dyslipidemia and MetS prevalence. Nt-proANP and BNP levels were more than twice as high in hyperuricaemics, whereas Big ET-1 and UII were higher by 46% and 14%, respectively. HOMA (BxS) was higher in hyperuricaemics: 31 (16)% vs. 26 (18)% (p=0.0004). BxS loss rate was faster in normouricaemics: 1.36 (0.54)% vs. 1.20 (0.43)%/year(-1) (p<0.0001 ). The proportion with HbA(1C) < 7.0% was 39% (normouricaemics) vs. 49% (hyperuricaemics; p=0.0091). In T2DM males, hyperuricaemia is associated with raised neurohormones together with better beta-cell indices. Urate's dual properties may translate into beneficial (glucose homeostasis) and detrimental (raised neurohormones) effects.

Reactive oxygen species facilitate adipocyte differentiation by accelerating mitotic clonal expansion

Growth-arrested 3T3-L1 preadipocytes rapidly express CCAAT/enhancer-binding protein-beta (C/EBPbeta) upon hormonal induction of differentiation. However, the DNA binding activity of C/EBPbeta is not activated until the cells synchronously reenter S phase during the mitotic clonal expansion (MCE) phase of differentiation. In this period, C/EBPbeta is sequentially phosphorylated by MAPK and glycogen synthase kinase-3beta, inducing C/EBPbeta DNA binding activity and transcription of its target genes. Because the DNA binding activity of C/EBPbeta is further enhanced by oxidation in vitro, we investigated how redox state affects C/EBPbeta DNA binding and MCE during adipogenesis. When 3T3-L1 cells were treated with H(2)O(2) and hormonal stimuli, differentiation was accelerated with increased expression of peroxisome proliferator-activated receptor gamma. Interestingly, cell cycle progression (S to G(2)/M phase) was markedly enhanced by H(2)O(2), whereas antioxidants caused an S phase arrest during the MCE. H(2)O(2) treatment resulted in the early appearance of a punctate pattern observed by immunofluorescent staining of C/EBPbeta, which is a hallmark for C/EBPbeta binding to regulatory elements, whereas a short antioxidant treatment rapidly dispersed the centromeric localization of C/EBPbeta. Consistently, reactive oxygen species production was increased during 3T3-L1 differentiation. Our results indicate that redox-induced C/EBPbeta DNA binding activity, along with the dual phosphorylation of C/EBPbeta, is required for the MCE and terminal differentiation of adipocytes.

Increased xanthine oxidase in the skin of preeclamptic women

Xanthine oxioreductase is the holoenzyme responsible for terminal purine catabolism. Under conditions of metabolic stress or heightened proinflammatory cytokine production, this enzyme is preferentially in its oxidized form, xanthine oxidase, with catalytic action that generates uric acid and the free radical superoxide. As preeclampsia is characterized by heightened inflammation, oxidative stress, and hyperuricemia, it has been proposed that xanthine oxidase plays a pivotal role in this hypertensive disorder of pregnancy. We sought to determine whether xanthine oxidase protein content was higher in maternal tissue of preeclamptic mothers, compared to healthy pregnant controls, using immunohistochemical analysis of skin biopsies. We further compared xanthine oxidase immunoreactivity in skin biopsies from preeclamptic women and patients with several inflammatory conditions. In preeclamptic women, intense xanthine oxidase immunoreactivity was present within the epidermis. By contrast, only very faint xanthine oxidase staining was observed in skin biopsies from healthy pregnant controls. Further, a role for inflammation in the increase of xanthine oxidase was suggested by similar findings of heightened xanthine oxidase immunoreactivity in the skin biopsies from nonpregnant individuals diagnosed with conditions of systemic inflammation. The finding of increased xanthine oxidase in maternal tissue, most likely as the result of heightened maternal inflammation, suggests maternal xanthine oxidase as a source of free radical and uric acid generation in preeclampsia.

Hypothesis: could excessive fructose intake and uric acid cause type 2 diabetes?

We propose that excessive fructose intake (>50 g/d) may be one of the underlying etiologies of metabolic syndrome and type 2 diabetes. The primary sources of fructose are sugar (sucrose) and high fructose corn syrup. First, fructose intake correlates closely with the rate of diabetes worldwide. Second, unlike other sugars, the ingestion of excessive fructose induces features of metabolic syndrome in both laboratory animals and humans. Third, fructose appears to mediate the metabolic syndrome in part by raising uric acid, and there are now extensive experimental and clinical data supporting uric acid in the pathogenesis of metabolic syndrome. Fourth, environmental and genetic considerations provide a potential explanation of why certain groups might be more susceptible to developing diabetes. Finally, we discuss the counterarguments associated with the hypothesis and a potential explanation for these findings. If diabetes might result from excessive intake of fructose, then simple public health measures could have a major impact on improving the overall health of our populace.

Bidirectional modulation of adipogenesis by the secreted protein Ccdc80/DRO1/URB

Adipocyte-secreted proteins play important roles in metabolic regulation through autocrine, paracrine, and endocrine mechanisms. Using transcriptional profiling, we identified coiled-coil domain containing 80 (Ccdc80; also known as DRO1 and URB) as a novel secreted protein highly expressed in white adipose tissue. In 3T3-L1 cells Ccdc80 is expressed and secreted in a biphasic manner with high levels in postconfluent preadipocytes and terminally differentiated adipocytes. To determine whether Ccdc80 regulates adipocyte differentiation, Ccdc80 expression was manipulated using both knockdown and overexpression approaches. Small hairpin RNA-mediated silencing of Ccdc80 in 3T3-L1 cells inhibits adipocyte differentiation. This phenotype was partially reversed by treating the knockdown cells with Ccdc80-containing conditioned medium from differentiated 3T3-L1 cells. Molecular studies indicate that Ccdc80 is required for the full inhibition of T-cell factor-mediated transcriptional activity, down-regulation of Wnt/beta-catenin target genes during clonal expansion, and the subsequent induction of C/EBPalpha and peroxisome proliferator-activated receptor gamma. Surprisingly, overexpression of Ccdc80 in 3T3-L1 cells also inhibits adipocyte differentiation without affecting the repression of the Wnt/beta-catenin signaling pathway. Taken together, these data suggest that Ccdc80 plays dual roles in adipogenesis by mechanisms that involve at least in part down-regulation of Wnt/beta-catenin signaling and induction of C/EBPalpha and peroxisome proliferator-activated receptor gamma.

Migratory activity of human breast cancer cells is modulated by differential expression of xanthine oxidoreductase

Xanthine oxidoreductase (XOR) may exert an important, but poorly defined, role in the pathogenesis of breast cancer (BC). Loss of XOR expression was linked to aggressive BC, and recent clinical observations have suggested that decreasing XOR may be functionally linked to BC aggressiveness. The goal of the present investigation was to determine whether the decreased XOR observed in clinically aggressive BC was an intrinsic property of highly invasive mammary epithelial cells (MEC). Expression of XOR was investigated using HC11 mouse MEC, HB4a and MCF-10A normal human MEC, and several human mammary tumor cells including MCF-7 and MDA-MB-231. Consistent with clinical observations, data shown here revealed high levels of XOR in normal HC11 and MCF-10A cells that was markedly reduced in highly invasive mammary tumor cells. The contribution of XOR to tumor cell migration in vitro was investigated using MDA-MB-231 and MCF-7 cells and clonally selected derivatives of HC11 that exhibit either weak or strong migration in vitro. We observed that over-expression of an XOR cDNA in MDA-MB-231 and in HC11-C24, both possessing weak XOR expression and high migratory capacity, inhibited their migration in vitro. Conversely, pharmacological inhibition of XOR in MCF-7 and HC11-C4, both possessing high XOR expression and weak migratory capacity, stimulated their migration in vitro. Further experiments suggested that XOR derived ROS mediated this effect and also modulated COX-2 and MMP levels and function. These data demonstrate a functional link between XOR expression and MEC migration and suggest a potential role for XOR in suppressing BC pathogenesis.

Retinol saturase promotes adipogenesis and is downregulated in obesity

Adipocyte differentiation is controlled by many transcription factors, but few known downstream targets of these factors are necessary for adipogenesis. Here we report that retinol saturase (RetSat), which is an enzyme implicated in the generation of dihydroretinoid metabolites, is induced during adipogenesis and is directly regulated by the transcription factor peroxisome proliferator activated receptor gamma (PPARgamma). Ablation of RetSat dramatically inhibited adipogenesis but, surprisingly, this block was not overcome by the putative product of RetSat enzymatic activity. On the other hand, ectopic RetSat with an intact, but not a mutated, FAD/NAD dinucleotide-binding motif increased endogenous PPARgamma transcriptional activity and promoted adipogenesis. Indeed, RetSat was not required for adipogenesis when cells were provided with exogenous PPARgamma ligands. In adipose tissue, RetSat is expressed in adipocytes but is unexpectedly downregulated in obesity, most likely owing to infiltration of macrophages that we demonstrate to repress RetSat expression. Thiazolidinedione treatment reversed low RetSat expression in adipose tissue of obese mice. Thus, RetSat plays an important role in the biology of adipocytes, where it favors normal differentiation, yet is reduced in the obese state. RetSat is thus a novel target for therapeutic intervention in metabolic disease.

Lessons from comparative physiology: could uric acid represent a physiologic alarm signal gone awry in western society?

Uric acid has historically been viewed as a purine metabolic waste product excreted by the kidney and gut that is relatively unimportant other than its penchant to crystallize in joints to cause the disease gout. In recent years, however, there has been the realization that uric acid is not biologically inert but may have a wide range of actions, including being both a pro- and anti-oxidant, a neurostimulant, and an inducer of inflammation and activator of the innate immune response. In this paper, we present the hypothesis that uric acid has a key role in the foraging response associated with starvation and fasting. We further suggest that there is a complex interplay between fructose, uric acid and vitamin C, with fructose and uric acid stimulating the foraging response and vitamin C countering this response. Finally, we suggest that the mutations in ascorbate synthesis and uricase that characterized early primate evolution were likely in response to the need to stimulate the foraging "survival" response and might have inadvertently had a role in accelerating the development of bipedal locomotion and intellectual development. Unfortunately, due to marked changes in the diet, resulting in dramatic increases in fructose- and purine-rich foods, these identical genotypic changes may be largely responsible for the epidemic of obesity, diabetes and cardiovascular disease in today's society.

Novel insights into adipogenesis from omics data

Obesity, the excess accumulation of adipose tissue, is one of the most pressing health problems in both the Western world and in developing countries. Adipose tissue growth results from two processes: the increase in number of adipocytes (hyperplasia) that develop from precursor cells, and the growth of individual fat cells (hypertrophy) due to incorporation of triglycerides. Adipogenesis, the process of fat cell development, has been extensively studied using various cell and animal models. While these studies pointed out a number of key factors involved in adipogenesis, the list of molecular components is far from complete. The advance of high-throughput technologies has sparked many experimental studies aimed at the identification of novel molecular components regulating adipogenesis. This paper examines the results of recent studies on adipogenesis using high-throughput technologies. Specifically, it provides an overview of studies employing microarrays for gene expression profiling and studies using gel based and non-gel based proteomics as well as a chromatin immunoprecipitation followed by microarray analysis (ChIP-chip) or sequencing (ChIP-seq). Due to the maturity of the technology, the bulk of the available data was generated using microarrays. Therefore these data sets were not only reviewed but also underwent meta analysis. The review also shows that large-scale omics technologies in conjunction with sophisticated bioinformatics analyses can provide not only a list of novel players, but also a global view on biological processes and molecular networks. Finally, developing technologies and computational challenges associated with the data analyses are highlighted, and an outlook on the questions not previously addressed is provided.

Role of xanthine oxidoreductase in cardiac nitroso-redox imbalance

Emerging evidence supports the importance of nitroso-redox balance in the cardiovascular system. Xanthine oxidoreductase (XOR) is a major oxidative enzyme and increased XOR activity, leading to both increased production of reactive oxygen species and uric acid, is implicated in heart failure. Within the heart, XOR activity stimulates cardiomyocyte hypertrophy, apoptosis, and impairs matrix structure. The underpinnings of these derangements can be linked not solely to oxidative stress, but may also involve the process of nitroso-redox imbalance. In this regard, XOR interacts with nitric oxide signaling at numerous levels, including a direct protein-protein interaction with neuronal nitric oxide synthase (NOS1) in the sarcoplasmic reticulum. Deficiency or translocation of NOS1 away from this microdomain leads to increased activity of XOR, which in turn impairs excitation-contraction coupling and myofilament calcium sensitivity. There is a mounting abundance of preclinical data supporting beneficial effects of inhibiting XOR, but translation to the clinic continues to be incomplete. A growing understanding of XOR and its role in nitroso-redox imbalance has great potential to lead to improved pathophysiologic insights and possibly therapeutic advances.

Identification of proteins binding to E-Box/Ku86 sites and function of the tumor suppressor SAFB1 in transcriptional regulation of the human xanthine oxidoreductase gene

The xanthine oxidoreductase gene (XOR) encodes an important source of reactive oxygen species and uric acid, and its expression is associated with various human diseases including several forms of cancer. We previously reported that basal human XOR (hXOR) expression is restricted or repressed by E-box and TATA-like elements and a cluster of transcriptional proteins, including AREB6-like proteins and DNA-dependent protein kinase (DNA-PK). We now demonstrate that the cluster contains the tumor suppressors SAFB1, BRG1, and SAF-A. We further demonstrate that SAFB1 silencing increases hXOR expression and that SAFB1 directly binds to the E-box. Multiple studies in vitro and in vivo including pulldown, immunoprecipitation and chromatin immunoprecipitation analyses indicate that SAFB1, Ku86, and BRG1 associate with each other. The results suggest that the SAFB1 complex binds to the hXOR promoter in a chromatin environment and plays a critical role in restricting hXOR expression via its direct interaction with the E-box, DNA-PK, and tumor suppressors. Moreover, we demonstrate that the cytokine, oncostatin M (OSM), induces the phosphorylation of SAFB1 and that the OSM-induced hXOR mRNA expression is significantly inhibited by silencing the DNA-PK catalytic subunit or SAFB1 expression. The present studies for the first time demonstrate that hXOR is a tumor suppressor-targeted gene and that the phosphorylation of SAFB1 is regulated by OSM, providing a molecular basis for understanding the role of SAFB1-regulated hXOR transcription in cytokine stimulation and tumorigenesis.

Methods that resolve different contributions of clonal expansion to adipogenesis in 3T3-L1 and C3H10T1/2 cells

The mouse embryo fibroblast cell lines 3T3-L1 and C3H10T1/2 differentiate to adipocytes that exhibit similar insulin regulation of lipogenesis. These cell lines, however, differ appreciably in the processes that produce the major regulator PPAR gamma. Each line is stimulated by a mixture of insulin, dexamethasone, and methylisobutylxanthine (IDM). In the first 24h, IDM activates each cell type to produce similar regulatory changes and cell contraction. However, the increase in PPARy is delayed by 24h in typical 3T3-L1 cells compared with C3H10T1/2 cells. This delay is caused by the need for one or two rounds of cell division (clonal expansion) for PPAR gamma synthesis in 3T3-L1 cells. This expansion also occurs in C3H10T1/2 cells, but is not needed for PPAR gamma synthesis and differentiation. Other 3T3-L1 sublines have been described that follow the C3H10T1/2 pattern of differentiation. Culture conditions and inhibitors are described here that remove clonal expansion in C3H10T1/2 cells. With these constraints the cells retain full commitment to differentiation. This distinction is significant because many agents suppress differentiation in 3T3-L1 cells through inhibition of clonal expansion. Other effects on differentiation may be seen in C3H10T1/2 cells that are obscured in 3T3-L1 cells due to this inhibition of proliferation. Human preadipocytes do not need clonal expansion for adipogenesis, thus paralleling C3H10T1/2 cells.

Insulin regulates leptin secretion from 3T3-L1 adipocytes by a PI 3 kinase independent mechanism

To better define the molecular mechanisms underlying leptin release from adipocytes, we developed a novel protocol that maximizes leptin production from 3T3-L1 adipocytes. The addition of a PPARgamma agonist to the Isobutylmethylxanthine/Dexamethasone/Insulin differentiation cocktail increased leptin mRNA levels by 5-fold, maintained insulin sensitivity, and yielded mature phenotype in cultured adipocytes. Under these conditions, acute insulin stimulation for 2 h induced a two-fold increase in leptin secretion, which was independent of new protein synthesis, and was not due to alterations in glucose metabolism. Stimulation with insulin for 15 min induced the same level of leptin release and was blocked by Brefeldin A. Inhibiting PI 3-kinase with wortmannin had no effect on insulin stimulation of leptin secretion. These studies show that insulin can stimulate leptin release via a PI3K independent mechanism and provide a cellular system for studying the effect of insulin and potentially other mediators on leptin secretion.

Identification of a domain within peroxisome proliferator-activated receptor gamma regulating expression of a group of genes containing fibroblast growth factor 21 that are selectively repressed by SIRT1 in adipocytes

Peroxisome proliferator-activated receptor gamma (PPARgamma) activity is regulated through association with ligands that include the thiazolidinedione class of antidiabetic drugs, as well as derivatives of polyunsaturated fatty acids. Induction of PPARgamma target gene expression involves ligand-dependent reconfiguration of the ligand-binding domain (LBD), followed by recruitment of specific transcriptional coactivators. In this study, we have identified an amino acid (F372) within helix 7 of the LBD that is required for the response of PPARgamma to endogenous ligands. Additionally, the data show that this amino acid is also required for expression of a novel subset of adipocyte genes (group 2), including fibroblast growth factor 21 (FGF21), and that the FGF21 gene is a direct target of PPARgamma. Expression of the group 2 genes is selectively repressed by the NAD-dependent deacetylase SIRT1 in mature 3T3-L1 adipocytes, since knockdown of SIRT1 through the constitutive expression of a corresponding RNA interference enhances their expression without affecting the expression of classic adipogenic genes, such as adiponectin and FABP4/aP2. It appears that many of the group 2 genes repressed by SIRT1 in mature adipocytes correspond to the same set of genes that are selectively activated by treatment of fat cells with the PPARgamma ligand, troglitazone. These data support a role for helix 7 of the LBD of PPARgamma in regulating adipocyte function and suggest that inhibition of SIRT1 in adipocytes induces the same insulin-sensitizing action as PPARgamma ligands.

Is the fructose index more relevant with regards to cardiovascular disease than the glycemic index?

The glycemic index (G.I.) is a means for categorizing carbohydrates based on their ability to raise blood glucose, subsequently this index has been popularized as a way for selecting foods to reduce the risk for obesity, diabetes, and cardiovascular disease. We suggest that the G.I. is better aimed at identifying foods that stimulate insulin secretion rather than foods that stimulate insulin resistance. In this regard, fructose has a low G.I. but may be causally linked with the obesity and cardiovascular disease epidemic. The reported association of high G.I. with cardiovascular disease may be due to the association of sugar intake which contains fructose, but which has a high G.I. due to its glucose content. We propose the use of a fructose index to categorize foods and propose studies to determine the effect of low fructose diets as a means to prevent obesity, diabetes, and cardiovascular disease in the population.