Effect of thiazolidinediones on expression of UCP2 and adipocyte markers in human PAZ6 adipocytes

Impaired energy expenditure following exposure to either DDT or DDE in mice may be mediated by DNA methylation changes in brown adipose

The insecticide dichlorodiphenyltrichloroethane (DDT) and its persistent metabolite, dichlorodiphenyldichloroethylene (DDE), have been associated with increased adiposity and obesity in multiple generations of rodents and humans. These lipophilic pollutants accumulate in adipose tissue and appear to decrease energy expenditure through the impairment of thermogenesis in brown adipose tissue (BAT). We hypothesized that impaired thermogenesis is due to persistent epigenetic modifications of BAT. To address this, we exposed C57BL/6 J mice to DDT or DDE from gestational day (GD) 11.5 to postnatal day (PND) 5, evaluated longitudinal body temperature, and performed reduced representation bisulfite sequencing and RNA sequencing of BAT from infant and adult offspring. Exposure to DDT or DDE reduced core body temperature in adult mice, and differential methylation at the pathway and gene level was persistent from infancy to adulthood. Furthermore, thermogenesis and biological pathways essential for thermogenic function, such as oxidative phosphorylation and mechanistic target of rapamycin kinase (mTOR) signaling, were enriched with differential methylation and RNA transcription in adult mice exposed to DDT or DDE. PAZ6 human brown preadipocytes were differentiated in the presence of DDT or DDE to understand the brown adipocyte-autonomous effect of these pollutants. In vitro exposure led to limited changes in RNA expression; however, mitochondrial membrane potential was decreased in vitro with 0.1 µM and 1 µM doses of DDT or DDE. These results demonstrate that concentrations of DDT and DDE relevant to human exposure have a significant effect on thermogenesis, the transcriptome, and DNA methylome of mouse BAT and the mitochondrial function of human brown adipocytes.

Short O-GlcNAcase Is Targeted to the Mitochondria and Regulates Mitochondrial Reactive Oxygen Species Level

O-GlcNAcylation is a reversible post-translational modification involved in the regulation of cytosolic, nuclear, and mitochondrial proteins. Only two enzymes, OGT (O-GlcNAc transferase) and OGA (O-GlcNAcase), control the attachment and removal of O-GlcNAc on proteins, respectively. Whereas a variant OGT (mOGT) has been proposed as the main isoform that O-GlcNAcylates proteins in mitochondria, identification of a mitochondrial OGA has not been performed yet. Two splice variants of OGA (short and long isoforms) have been described previously. In this work, using cell fractionation experiments, we show that short-OGA is preferentially recovered in mitochondria-enriched fractions from HEK-293T cells and RAW 264.7 cells, as well as mouse embryonic fibroblasts. Moreover, fluorescent microscopy imaging confirmed that GFP-tagged short-OGA is addressed to mitochondria. In addition, using a Bioluminescence Resonance Energy Transfer (BRET)-based mitochondrial O-GlcNAcylation biosensor, we show that co-transfection of short-OGA markedly reduced O-GlcNAcylation of the biosensor, whereas long-OGA had no significant effect. Finally, using genetically encoded or chemical fluorescent mitochondrial probes, we show that short-OGA overexpression increases mitochondrial ROS levels, whereas long-OGA has no significant effect. Together, our work reveals that the short-OGA isoform is targeted to the mitochondria where it regulates ROS homoeostasis.

Uncoupling Protein 2 as a Pathogenic Determinant and Therapeutic Target in Cardiovascular and Metabolic Diseases

Uncoupling protein 2 (UCP2) is a mitochondrial protein that acts as an anion carrier. It is involved in the regulation of several processes, including mitochondrial membrane potential, generation of reactive oxygen species within the inner mitochondrial membrane and calcium homeostasis. UCP2 expression can be regulated at different levels: genetic (gene variants), transcriptional [by peroxisome proliferator-activated receptors (PPARs) and microRNAs], and post-translational. Experimental evidence indicates that activation of UCP2 expression through the AMPK/PPAR-α axis exerts a protective effect toward renal damage and stroke occurrence in an animal model of ischemic stroke (IS) associated with hypertension. UCP2 plays a key role in heart diseases (myocardial infarction and cardiac hypertrophy) and metabolic disorders (obesity and diabetes). In humans, UCP2 genetic variants (-866G/A and Ala55Val) associate with an increased risk of type 2 diabetes mellitus and IS development. Over the last few years, many agents that modulate UCP2 expression have been identified. Some of them are natural compounds of plant origin, such as Brassica oleracea, curcumin, berberine and resveratrol. Other molecules, currently used in clinical practice, include anti-diabetic (gliptin) and chemotherapeutic (doxorubicin and taxol) drugs. This evidence highlights the relevant role of UCP2 for the treatment of a wide range of diseases, which affect the national health systems of Western countries. We will review current knowledge on the physiological and pathological implications of UCP2 with particular regard to cardiovascular and metabolic disorders and will focus on the available therapeutic approaches affecting UCP2 level for the treatment of human diseases.

Lipopolysaccharide Induces GFAT2 Expression to Promote O-Linked β-N-Acetylglucosaminylation and Attenuate Inflammation in Macrophages

Glycosylation with O-linked β-N-acetylglucosamine (O-GlcNAcylation) is a reversible posttranslational modification that regulates the activity of intracellular proteins according to glucose availability and its metabolism through the hexosamine biosynthesis pathway. This modification has been involved in the regulation of various immune cell types, including macrophages. However, little is known concerning the mechanisms that regulate the protein O-GlcNAcylation level in these cells. In the present work, we demonstrate that LPS treatment induces a marked increase in protein O-GlcNAcylation in RAW264.7 cells, bone marrow-derived and peritoneal mouse macrophages, as well as human monocyte-derived macrophages. Targeted deletion of OGT in macrophages resulted in an increased effect of LPS on NOS2 expression and cytokine production, suggesting that O-GlcNAcylation may restrain inflammatory processes induced by LPS. The effect of LPS on protein O-GlcNAcylation in macrophages was associated with an increased expression and activity of glutamine fructose 6-phosphate amidotransferase (GFAT), the enzyme that catalyzes the rate-limiting step of the hexosamine biosynthesis pathway. More specifically, we observed that LPS potently stimulated GFAT2 isoform mRNA and protein expression. Genetic or pharmacological inhibition of FoxO1 impaired the LPS effect on GFAT2 expression, suggesting a FoxO1-dependent mechanism. We conclude that GFAT2 should be considered a new LPS-inducible gene involved in regulation of protein O-GlcNAcylation, which permits limited exacerbation of inflammation upon macrophage activation.

Identification of insulin-sensitizing molecules acting by disrupting the interaction between the Insulin Receptor and Grb14

Metabolic diseases are characterized by a decreased action of insulin. During the course of the disease, usual treatments frequently fail and patients are finally submitted to insulinotherapy. There is thus a need for innovative therapeutic strategies to improve insulin action. Growth factor receptor-bound protein 14 (Grb14) is a molecular adapter that specifically binds to the activated insulin receptor (IR) and inhibits its tyrosine kinase activity. Molecules disrupting Grb14-IR binding are therefore potential insulin-sensitizing agents. We used Structure-Based Virtual Ligand Screening to generate a list of 1000 molecules predicted to hinder Grb14-IR binding. Using an acellular bioluminescence resonance energy transfer (BRET) assay, we identified, out of these 1000 molecules, 3 compounds that inhibited Grb14-IR interaction. Their inhibitory effect on insulin-induced Grb14-IR interaction was confirmed in co-immunoprecipitation experiments. The more efficient molecule (C8) was further characterized. C8 increased downstream Ras-Raf and PI3-kinase insulin signaling, as shown by BRET experiments in living cells. Moreover, C8 regulated the expression of insulin target genes in mouse primary hepatocytes. These results indicate that C8, by reducing Grb14-IR interaction, increases insulin signalling. The use of C8 as a lead compound should allow for the development of new molecules of potential therapeutic interest for the treatment of diabetes.

Common and distinct regulation of human and mouse brown and beige adipose tissues: a promising therapeutic target for obesity

Obesity, which underlies various metabolic and cardiovascular diseases, is a growing public health challenge for which established therapies are inadequate. Given the current obesity epidemic, there is a pressing need for more novel therapeutic strategies that will help adult individuals to manage their weight. One promising therapeutic intervention for reducing obesity is to enhance energy expenditure. Investigations into human brown fat and the recently discovered beige/brite fat have galvanized intense research efforts during the past decade because of their pivotal roles in energy dissipation. In this review, we summarize the evolution of human brown adipose tissue (hBAT) research and discuss new in vivo methodologies for evaluating energy expenditure in patients. We highlight the differences between human and mouse BAT by integrating and comparing their cellular morphology, function, and gene expression profiles. Although great advances in hBAT biology have been achieved in the past decade, more cellular models are needed to acquire a better understanding of adipose-specific processes and molecular mechanisms. Thus, this review also describes the development of a human brown fat cell line, which could provide promising mechanistic insights into hBAT function, signal transduction, and development. Finally, we focus on the therapeutic potential and current limitations of hBAT as an anti-glycemic, anti-lipidemic, and weight loss-inducing ‘metabolic panacea’.

O-GlcNAcylation of FoxO1 in pancreatic β cells promotes Akt inhibition through an IGFBP1-mediated autocrine mechanism

O-GlcNAcylation on serine/threonine is a post-translational modification that controls the activity of nucleocytoplasmic proteins according to glucose availability. We previously showed that O-GlcNAcylation of FoxO1 in liver cells increases its transcriptional activity. In the present study, we evaluated the potential involvement of FoxO1 O-GlcNAcylation in the context of pancreatic β-cell glucotoxicity. FoxO1 was O-GlcNAcylated in INS-1 832/13 β cells and isolated rat pancreatic islets. O-GlcNAcylation of FoxO1 resulted in a 2-fold increase in its transcriptional activity toward a FoxO1 reporter gene and a 3-fold increase in the expression of the insulin-like growth factor-binding protein 1 (Igfbp1) gene at the mRNA level, resulting in IGFBP1 protein oversecretion by the cells. Of note, increased IGFBP1 in the culture medium inhibited the activity of the insulin-like growth factor 1 receptor (IGF1R)/phosphatidyl inositol 3 kinase (PI3K)/Akt pathway. We reveal in this report a novel mechanism by which O-GlcNAcylation inhibits Akt activity through an autocrine mechanism. However, although inhibition of IGFBP1 expression using siRNA restored the PI3 kinase/Akt pathway, it did not rescue INS-1 832/13 cells from high-glucose- or O-glcNAcylation-induced cell death. In contrast, FoxO1 down-regulation by siRNA led to 30 to 60% protection of INS-1 832/13 cells from death mediated by glucotoxic conditions. Therefore, whereas FoxO1 O-GlcNAcylation inhibits Akt through an IGFBP1-mediated autocrine pathway, the deleterious effects of FoxO1 O-GlcNAcylation on cell survival appeared to be independent of this pathway.

O-GlcNAcylation-inducing treatments inhibit estrogen receptor α expression and confer resistance to 4-OH-tamoxifen in human breast cancer-derived MCF-7 cells

O-GlcNAcylation (addition of N-acetyl-glucosamine on serine or threonine residues) is a post-translational modification that regulates stability, activity or localization of cytosolic and nuclear proteins. O-linked N-acetylgluocosmaine transferase (OGT) uses UDP-GlcNAc, produced in the hexosamine biosynthetic pathway to O-GlcNacylate proteins. Removal of O-GlcNAc from proteins is catalyzed by the β-N-Acetylglucosaminidase (OGA). Recent evidences suggest that O-GlcNAcylation may affect the growth of cancer cells. However, the consequences of O-GlcNAcylation on anti-cancer therapy have not been evaluated. In this work, we studied the effects of O-GlcNAcylation on tamoxifen-induced cell death in the breast cancer-derived MCF-7 cells. Treatments that increase O-GlcNAcylation (PUGNAc and/or glucosoamine) protected MCF-7 cells from death induced by tamoxifen. In contrast, inhibition of OGT expression by siRNA potentiated the effect of tamoxifen on cell death. Since the PI-3 kinase/Akt pathway is a major regulator of cell survival, we used BRET to evaluate the effect of PUGNAc+glucosamine on PIP₃ production. We observed that these treatments stimulated PIP₃ production in MCF-7 cells. This effect was associated with an increase in Akt phosphorylation. However, the PI-3 kinase inhibitor LY294002, which abolished the effect of PUGNAc+glucosamine on Akt phosphorylation, did not impair the protective effects of PUGNAc+glucosamine against tamoxifen-induced cell death. These results suggest that the protective effects of O-GlcNAcylation are independent of the PI-3 kinase/Akt pathway. As tamoxifen sensitivity depends on the estrogen receptor (ERα) expression level, we evaluated the effect of PUGNAc+glucosamine on the expression of this receptor. We observed that O-GlcNAcylation-inducing treatment significantly reduced the expression of ERα mRNA and protein, suggesting a potential mechanism for the decreased tamoxifen sensitivity induced by these treatments. Therefore, our results suggest that inhibition of O-GlcNAcylation may constitute an interesting approach to improve the sensitivity of breast cancer to anti-estrogen therapy.

Effect of insulin analogues on insulin/IGF1 hybrid receptors: increased activation by glargine but not by its metabolites M1 and M2

Background

In diabetic patients, the pharmacokinetics of injected human insulin does not permit optimal control of glycemia. Fast and slow acting insulin analogues have been developed, but they may have adverse properties, such as increased mitogenic or anti-apoptotic signaling. Insulin/IGF1 hybrid receptors (IR/IGF1R), present in most tissues, have been proposed to transmit biological effects close to those of IGF1R. However, the study of hybrid receptors is difficult because of the presence of IR and IGF1R homodimers. Our objective was to perform the first study on the pharmacological properties of the five marketed insulin analogues towards IR/IGF1R hybrids.

Methodology

To study the effect of insulin analogues on IR/IGF1R hybrids, we used our previously developed Bioluminescence Resonance Energy Transfer (BRET) assay that permits specific analysis of the pharmacological properties of hybrid receptors. Moreover, we have developed a new, highly sensitive BRET-based assay to monitor phophatidylinositol-3 phosphate (PIP₃) production in living cells. Using this assay, we performed a detailed pharmacological analysis of PIP₃ production induced by IGF1, insulin and insulin analogues in living breast cancer-derived MCF-7 and MDA-MB231 cells.

Results

Among the five insulin analogues tested, only glargine stimulated IR/IGF1R hybrids with an EC50 that was significantly lower than insulin and close to that of IGF1. Glargine more efficiently stimulated PIP₃ production in MCF-7 cells but not in MDA-MB231 cells as compared to insulin. In contrast, glargine metabolites M1 and M2 showed lower potency for hybrid receptors stimulation, PIP₃ production, Akt and Erk1/2 phosphorylation and DNA synthesis in MCF-7 cells, compared to insulin.

Conclusion

Glargine, possibly acting through IR/IGF1R hybrids, displays higher potency, whereas its metabolites M1 and M2 display lower potency than insulin for the stimulation of proliferative/anti-apoptotic pathways in MCF-7 cells.

PAZ6 cells constitute a representative model for human brown pre-adipocytes

The role of brown adipose tissue (BAT) in human metabolism and its potential as an anti-obesity target organ have recently received much renewed attention. Following radiological detection of substantial amounts of BAT in adults by several independent research groups, an increasing number of studies are now dedicated to uncover BAT's genetic, developmental, and environmental determinants. In contrast to murine BAT, human BAT is not present as a single major fat depot in a well-defined location. The distribution of BAT in several areas in the body significantly limits its availability to research. A human brown adipocyte cell line is therefore critical in broadening the options available to researchers in the field. The human BAT-cell line PAZ6 was created to address such a need and has been well characterized by several research groups around the world. In the present review, we discuss their findings and propose potential applications of the PAZ6 cells in addressing the relevant questions in the BAT field, namely for future use in therapeutic applications.

Endogenous peroxisome proliferator-activated receptor-gamma augments fatty acid uptake in oxidative muscle

In the setting of insulin resistance, agonists of peroxisome proliferator-activated receptor (PPAR)-gamma restore insulin action in muscle and promote lipid redistribution. Mice with muscle-specific knockout of PPARgamma (MuPPARgammaKO) develop excess adiposity, despite reduced food intake and normal glucose disposal in muscle. To understand the relation between muscle PPARgamma and lipid accumulation, we studied the fuel energetics of MuPPARgammaKO mice. Compared with controls, MuPPARgammaKO mice exhibited significantly increased ambulatory activity, muscle mitochondrial uncoupling, and respiratory quotient. Fitting with this latter finding, MuPPARgammaKO animals compared with control siblings exhibited a 25% reduction in the uptake of the fatty acid tracer 2-bromo-palmitate (P < 0.05) and a 13% increase in serum nonesterified fatty acids (P = 0.05). These abnormalities were associated with no change in AMP kinase (AMPK) phosphorylation, AMPK activity, or phosphorylation of acetyl-CoA carboxylase in muscle and occurred despite increased expression of fatty acid transport protein 1. Palmitate oxidation was not significantly altered in MuPPARgammaKO mice despite the increased expression of several genes promoting lipid oxidation. These data demonstrate that PPARgamma, even in the absence of exogenous activators, is required for normal rates of fatty acid uptake in oxidative skeletal muscle via mechanisms independent of AMPK and fatty acid transport protein 1. Thus, when PPARgamma activity in muscle is absent or reduced, there will be decreased fatty acid disposal leading to diminished energy utilization and ultimately adiposity.

Comparative expression analysis of isolated human adipocytes and the human adipose cell lines LiSa-2 and PAZ6

OBJECTIVE

To obtain insight in the extent to which the human cell lines LiSa-2 and PAZ6 resemble isolated primary human adipocytes.

DESIGN

A combination of cDNA subtraction (representative difference analysis; RDA) and cDNA microarray analysis was used to select adipose specific genes to compare isolated (pre-)adipocytes with (un)differentiated LiSa-2 and PAZ6 cells.

MEASUREMENTS

RDA was performed on adipose tissue against lung tissue. A total of 1400 isolated genes were sequenced and cDNA microarray technology was used for further adipose related gene selection. 30 genes that were found to be enriched in adipose tissue were used to compare isolated human adipocytes and LiSa-2 and PAZ6 cells in the differentiated and undifferentiated states.

RESULTS

RDA and microarray analysis resulted in the identification of adipose enriched genes, but not in adipose specific genes. Of the 30 most differentially expressed genes, as expected, most were related to lipid metabolism. The second category consisted of methyltransferases, DNMT1, DNMT3a, RNMT and SHMT2, of which the expression was differentiation dependent and higher in differentiated adipocytes. Using the 30 adipose expressed genes, it was found that isolated adipocytes on one hand, and PAZ6 and LiSa-2 adipocytes on the other, differ primarily in lipid metabolism. Furthermore, LiSa-2 cells seem to be more similar to isolated adipocytes than PAZ6 cells.

CONCLUSION

The LiSa-2 cell line is a good model for differentiated adipocytes, although one should keep in mind that the lipid metabolism in these cells deviates from the in vivo situation Furthermore, our results imply that methylation may have an important function in terminal adipocyte differentiation.

Regulation of uncoupling protein-2 mRNA in L6 myotubules: II: Thyroid hormone amplifies stimulation of uncoupling protein-2 gene by thiazolidinediones and other peroxisome proliferator-activated receptor ligands in L6 myotubules: evidence for a priming effect

The stimulation of the uncoupling protein-2 gene (ucp2) by thyroid hormone (triiodothyronine [T3]) in vivo is variable, suggesting complex interactions and even the possibility of indirect effects. We investigated the effect of T3 on ucp2 expression in L6 myotubules. Alone, T3 did not significantly stimulate ucp2 expression in L6 cells, but it amplified the stimulation by thiazolidinediones (TZDs). L6 cells expressed both alpha1 and beta1 thyroid hormone receptors and the data were consistent with the effect being mediated by these receptors. T3 also enhanced the stimulation of ucp2 by the nonselective peroxisome proliferator-activated receptor (PPAR) ligands bezafibrate and carbacyclin, but not that by oleic acid or norepinephrine. L6 cells expressed PPARbeta and PPARgamma, but not PPARalpha. As short as a 1-h preexposure of L6 cells to T3 was sufficient to amplify the effect of PPAR ligands. Neither transcription nor translation was needed for this effect of T3. T3 did not affect the t1/2 of UCP2 mRNA. The histone deacetylases inhibitor trichostatin A (TSA) stimulated the expression of ucp2 but did not add to the effect of T3 nor did this hormone enhance the effect of TSA. These results suggest that T3 selectively enhances the transcriptional stimulation of ucp2 by TZDs and nonselective PPAR ligands by priming the gene to a transactivating signal(s) generated by such ligands.

Modulation of lipid metabolism by energy status of adipocytes: implications for insulin sensitivity

It is becoming evident that insulin resistance of white adipose tissue is a major factor underlying the cardiovascular risk of obesity. Impaired fat storage rather than altered glucose metabolism in adipocytes probably contributes to development of insulin resistance in muscle and other tissues, in particular via increased delivery of nonesterified fatty acids into circulation. Lipid metabolism of adipose tissue is affected by the energy status of fat cells. In vitro experiments indicated the dependence of both lipogenesis and lipolysis on ATP levels in adipocytes. Thus, respiratory uncoupling in adipocytes that results in stimulation of energy dissipation and depression of ATP synthesis may contribute to the control of lipid metabolism, adiposity, and insulin sensitivity. This notion is supported by the expression of UCPs in adipocytes, for example, UCP2, UCP5, as well as some protonophoric anion transporters, and by induction of UCP1 and UCP3 in white fat by pharmacological treatments that reduce adiposity. A negative correlation between expression of UCPs in adipocytes and accumulation of white fat was also found. Expression of UCP1 from the adipose-specific promoter in the aP2-Ucp1 transgenic mice mitigated obesity induced by genetic or dietary factors. The obesity resistance, accompanied by respiratory uncoupling in adipocytes and increased energy expenditure, resulted from ectopic expression of UCP1 in white, but not brown fat. Probably due to depression of the ATP/ADP ratio, both fatty acid synthesis and lipolytic action of norepinephrine in adipocytes of transgenic mice were relatively low. Expression of regulatory G-proteins, which are essential for both catecholamine and insulin signaling in adipocytes, was also altered by ectopic UCP1. These results support the role of protonophoric proteins in adipocytes in the control of adiposity and insulin sensitivity. Antidiabetic effects of thiazolidinediones, fibrates, beta(3)-adrenoreceptor agonists, dietary n-3 PUFAs, and leptin may be explained at least partially by their effects on the energy and hence also the lipid metabolism of fat cells.

Mitochondrial Proliferation, Dna Depletion and Adipocyte Differentiation in Subcutaneous Adipose Tissue of HIV-Positive Haart Recipients

Objectives

To examine the in vivo effects of highly active antiretroviral therapy (HAART) regimens on adipose tissue mitochondrial DNA (mtDNA) depletion, mitochondrial organellar proliferation, and markers of adipocyte differentiation and phenotype.

Design and methods

DNA and mRNA quantification using real-time PCR methods was performed on adipose tissue samples from 31 HIV-infected individuals, of whom 11 were treatment-naive and 20 were receiving HAART. mtDNA depletion was measured as mtDNA copies/cell, and mitochondrial proliferation by quantification of mitochondrial protein mass. Regulation of mitochondrial biogenesis was assessed by NRF-1 and mtTFA mRNA. PPAR γ, UCP2 and UCP1 mRNA expression was used to assess adipocyte differentiation and phenotype.

Results

Stavudine-based HAART recipients ( n=10) displayed significant mtDNA depletion (12.8% of control, P<0.001), mildly increased mitochondrial protein mass (2.6-fold of control, P=0.032) and decreased expression of PPARγ (53.9% of control, P=0.021), UCP2 (62.2% of control, P=0.024) and UCP3 (51.8% of control, P=0.047) mRNA compared with controls. Zidovudine-based HAART recipients ( n=7) also displayed significant mtDNA depletion (34.45% of control, P=0.031), increased mitochondrial protein mass (5.7-fold of control, P=0.009), and markedly increased UCP1 (18-fold of control, P=0.009) mRNA. Elevated UCP1 mRNA expression was found to be associated with non-stavudine (zidovudine or abacavir), protease inhibitor (PI)-containing HAART (95-fold of non-stavudine, non-PI-containing HAART, P=0.006).

Conclusion

Differential effects of stavudine and zidovudine therapy on mtDNA depletion and expression of adipocyte differentiation markers PPARγ and UCP2 were observed, consistent with increased adipose tissue toxicity associated with stavudine therapy. Increased UCP1 mRNA, a marker of brown adipose tissue phenotype, was associated with non-stavudine, PI-containing HAART, and may represent an adaptive response to the increased fatty acid flux associated with PI therapy, and may contribute to the increased resting energy expenditure reported in such patients.

Functional expression of MT2 (Mel1b) melatonin receptors in human PAZ6 adipocytes

Several reports have demonstrated that the pineal hormone, melatonin, plays an important role in body mass regulation in mammals. To date, however, the target tissues and relevant biochemical mechanisms involved remain uncharacterized. As adipose tissue is the principal site of energy storage in the body, we investigated whether melatonin could also act on this tissue. Semiquantitative RT-PCR analysis revealed the expression of MT1 and MT2 melatonin receptor mRNAs in the human brown adipose cell line, PAZ6, as well as in human brown and white adipose tissue. Binding analysis with 2-[(125)I]iodomelatonin ((125)I-Mel) revealed the presence of a single, high affinity binding site in PAZ6 adipocytes with a binding capacity of 7.46 +/- 1.58 fmol/mg protein and a K(d) of 457 +/- 5 pM. Both melatonin and the MT2 receptor-selective antagonist, 4-phenyl-2-propionamidotetraline, competed with 2-[(125)I]iodomelatonin binding, with respective K(i) values of 3 x 10(-11) and 1.5 x 10(-11) M. Functional expression of melatonin receptors in PAZ6 adipocytes was indicated by the melatonin-induced, dose-dependent inhibition of forskolin-stimulated cAMP levels and basal cGMP levels with IC(50) values of 2 x 10(-9) and 3 x 10(-10) M, respectively. Modulation of the cGMP pathway by melatonin further supports functional expression of MT2 receptors, as this pathway was shown to be specific for that subtype in humans. In addition, long-term melatonin treatment of PAZ6 adipocytes was found to decrease the expression of the glucose transporter Glut4 and glucose uptake, an important parameter of adipocyte metabolism. These results suggest that melatonin may act directly at MT2 receptors on human brown adipocytes to regulate adipocyte physiology.

Fast decline of hematopoiesis and uncoupling protein 2 content in human liver after birth: location of the protein in Kupffer cells

Hepatic hematopoiesis is prominent during fetal life and ceases around birth. In rodent liver, the decline of the hepatic hematopoiesis starts abruptly at birth being accompanied by a decrease of mitochondrial uncoupling protein 2 (UCP2) expression in monocytes/macrophages, whereas hepatocytes may express UCP2 only under pathologic situations. The goals of this study were to characterize hepatic hematopoiesis in humans around birth, and to identify cells expressing UCP2. Hematopoiesis was evaluated histologically in the liver of 22 newborns (mostly very premature neonates), who died between 45 min and 140 d after birth, and one fetus. UCP2 expression was characterized by Northern blots, immunoblotting, immunohistochemistry, and by in situ hybridization. The number of hematopoietic cells started to decrease rapidly at birth, irrespectively of the gestational age (23-40 wk) of neonates. A similar decline was observed for UCP2 expression, which was relatively high in fetal liver. UCP2 was detected only in myeloid cells (mainly in Kupffer cells), but not in hepatocytes, although sepsis or other pathologies occurred in the critically ill newborns. Kupffer cells represent the major site of mitochondrial UCP2 expression in the human newborn. UCP2 may be essential for the differentiation and function of macrophages and serve as a marker for these cells in human liver during the perinatal period.

Mitochondrial uncoupling proteins in energy expenditure

Four recently discovered homologues of the brown adipose tissue-specific mitochondrial uncoupling protein (UCP1) vary from 29% to 58% in their similarity to UCP1. Although these homologues share important structural features with UCP1 and like UCP1 can reduce the mitochondrial membrane potential when expressed in yeast, there is no clear evidence that they can function thermogenically in vivo. On the other hand, evidence continues to accumulate indicating that the up-regulation of Ucp1 reduces excessive adiposity.

Nonhypoglycemic drug reactions of agents used to treat diabetes

The different classes of agents now available for the treatment of diabetes, each with its own unique mechanism of action, present the clinician with numerous choices to achieve glycemic control. Therapy can be tailored for each patient. If the primary problem is insulin resistance, a thiazolidinedione can be used. The clinician should remember the potential adverse reactions of the drugs and their potential interaction with other conditions a patient may have. After this careful consideration, the optimal therapeutic regimen for a patient can be developed.

Pioglitazone

Pioglitazone is an orally administered insulin sensitising thiazolidinedione agent that has been developed for the treatment of type 2 diabetes mellitus. Pioglitazone activates the nuclear peroxisome proliferator activated receptor-gamma (PPAR-gamma), which leads to the increased transcription of various proteins regulating glucose and lipid metabolism. These proteins amplify the post-receptor actions of insulin in the liver and peripheral tissues, which leads to improved glycaemic control with no increase in the endogenous secretion of insulin. In placebo-controlled clinical trials, monotherapy with pioglitazone 15 to 45 mg/day has been shown to decrease blood glycosylated haemoglobin (HbA1c) levels in patients with type 2 diabetes mellitus. The addition of pioglitazone 30 mg/day to preexisting therapy with metformin, or of pioglitazone 15 or 30 mg/day to sulphonylurea, insulin or voglibose therapy, has been shown to decrease HbA1c and fasting blood glucose levels significantly in patients with poorly controlled type 2 diabetes mellitus. Pioglitazone has also been associated with improvements in serum lipid profiles in randomised placebo-controlled clinical studies. The drug has been well tolerated by adult patients of all ages in clinical studies. Oedema has been reported with monotherapy, and pooled data have shown hypoglycaemia in 2 to 15% of patients after the addition of pioglitazone to sulphonylurea or insulin treatment. There have been no reports of hepatotoxicity.