Peroxisome proliferator-activated receptor gamma and metabolic disease

Reducing glycosphingolipid content in adipose tissue of obese mice restores insulin sensitivity, adipogenesis and reduces inflammation

Adipose tissue is a critical mediator in obesity-induced insulin resistance. Previously we have demonstrated that pharmacological lowering of glycosphingolipids and subsequently GM3 by using the iminosugar AMP-DNM, strikingly improves glycemic control. Here we studied the effects of AMP-DNM on adipose tissue function and inflammation in detail to provide an explanation for the observed improved glucose homeostasis. Leptin-deficient obese (Lep^Ob) mice were fed AMP-DNM and its effects on insulin signalling, adipogenesis and inflammation were monitored in fat tissue. We show that reduction of glycosphingolipid biosynthesis in adipose tissue of Lep^Ob mice restores insulin signalling in isolated ex vivo insulin-stimulated adipocytes. We observed improved adipogenesis as the number of larger adipocytes was reduced and expression of genes like peroxisome proliferator-activated receptor (PPAR) γ, insulin responsive glucose transporter (GLUT)-4 and adipsin increased. In addition, we found that adiponectin gene expression and protein were increased by AMP-DNM. As a consequence of this improved function of fat tissue we observed less inflammation, which was characterized by reduced numbers of adipose tissue macrophages (crown-like structures) and reduced levels of the macrophage chemo attractants monocyte-chemoattractant protein-1 (Mcp-1/Ccl2) and osteopontin (OPN). In conclusion, pharmacological lowering of glycosphingolipids by inhibition of glucosylceramide biosynthesis improves adipocyte function and as a consequence reduces inflammation in adipose tissue of obese animals.

Effects of the PPARgamma agonist troglitazone on endothelial cells in vivo and in vitro: differences between human and mouse

Peroxisome proliferator-activated receptor gamma (PPARgamma) agonists and PPARgamma/alpha dual agonists have been or are being developed for clinical use in the treatment of type 2 diabetes mellitus and hyperlipidemias. A common tumor finding in rodent carcinogenicity studies for these agonists is hemangioma/hemangiosarcoma in mice but not in rats. We hypothesized that increased endothelial cell proliferation may be involved in the mechanism of PPAR agonist-induced vascular tumors in mice, and we investigated the effects on endothelial cells utilizing troglitazone, the first clinically used PPARgamma agonist, in vivo and in vitro. Troglitazone (400 and 800 mg/kg/day) induced hemangiosarcomas in mice in a 2-year bioassay. We showed that troglitazone increased endothelial cell proliferation in brown and white adipose tissue and liver in mice at sarcomagenic doses after 4 weeks of treatment. Troglitazone was cytotoxic both to human dermal microvascular endothelial cells (HMEC1) and mouse mammary fat pad microvascular endothelial cells (MFP MVEC) at high concentrations. However, MFP MVEC were more resistant to the cytotoxic effects of troglitazone based on the much lower LC(50) in HMEC1 (17.4 muM) compared to MFP MVEC (92.2 muM). Troglitazone increased the proliferation and survival of MFP MVEC but not HMEC1 in growth factor reduced conditions. Our data demonstrate that troglitazone may induce hemangiosarcomas in mice, at least in part, through enhancement of survival and proliferation of microvascular endothelial cells. Such an effect does not occur with human cells, suggesting that human may react differently to exposure to PPAR agonists compared with mice.

Peroxisome proliferator-activated receptors in diabetic nephropathy

Diabetic nephropathy is a leading cause of end-stage renal disease, which is increasing in incidence worldwide, despite intensive treatment approaches such as glycemic and blood pressure control in patients with diabetes mellitus. New therapeutic strategies are needed to prevent the onset of diabetic nephropathy. Peroxisome proliferator-activated receptors (PPARs) are ligand-activated nuclear transcription factors that play important roles in lipid and glucose homeostases. These agents might prevent the progression of diabetic nephropathy, since PPAR agonists improve dyslipidemia and insulin resistance. Furthermore, data from murine models suggest that PPAR agonists also have independent renoprotective effects by suppressing inflammation, oxidative stress, lipotoxicity, and activation of the renin-angiotensin system. This review summarizes data from clinical and experimental studies regarding the relationship between PPARs and diabetic nephropathy. The therapeutic potential of PPAR agonists in the treatment of diabetic nephropathy is also discussed.

Cellular and physiological effects of C-peptide

In recent years, accumulating evidence indicates a biological function for proinsulin C-peptide. These results challenge the traditional view that C-peptide is essentially inert and only useful as a surrogate marker of insulin release. Accordingly, it is now clear that C-peptide binds with high affinity to cell membranes, probably to a pertussis-toxin-sensitive G-protein-coupled receptor. Subsequently, multiple signalling pathways are potently and dose-dependently activated in multiple cell types by C-peptide with the resulting activation of gene transcription and altered cell phenotype. In diabetic animals and Type 1 diabetic patients, short-term studies indicate that C-peptide also enhances glucose disposal and metabolic control. Furthermore, results derived from animal models and clinical studies in Type 1 diabetic patients suggest a salutary effect of C-peptide in the prevention and amelioration of diabetic nephropathy and neuropathy. Therefore a picture of Type 1 diabetes as a dual-hormone-deficiency disease is developing, suggesting that the replacement of C-peptide alongside insulin should be considered in its management.

The peroxisome proliferator-activated receptor gamma agonist rosiglitazone ameliorates murine lupus by induction of adiponectin

Systemic lupus erythematosus (SLE) is an inflammatory autoimmune disease for which current therapy is suboptimal. SLE is characterized by autoantibody production, with renal disease and premature atherosclerosis being common and severe manifestations causing appreciable morbidity and mortality. Peroxisome proliferator-activated receptor gamma (PPARgamma) agonists are widely used in the treatment of diabetes mellitus for their insulin-sensitizing properties, but also have immunomodulatory effects. In this report, we show that the PPARgamma agonist rosiglitazone reduces autoantibody production, renal disease, and atherosclerosis in mouse models of SLE. The beneficial effect of rosiglitazone on SLE manifestations depends on the induction of adiponectin, because rosiglitazone has no effect on autoantibody production or renal disease in lupus mice that lack adiponectin. In addition, lupus mice that lack adiponectin develop more severe disease than adiponectin-sufficient lupus mice, indicating that endogenous adiponectin is involved in regulating disease activity. Furthermore, administration of exogenous adiponectin ameliorates disease. These experiments suggest that PPARgamma agonists may be useful agents for the treatment of SLE. They also demonstrate that induction of adiponectin is a major mechanism underlying the immunomodulatory effects of PPARgamma agonists.

Regulation of Wnt/beta-catenin pathway by cPLA2alpha and PPARdelta

Cytosolic phospholipase A(2)alpha (cPLA(2)alpha) is a rate-limiting key enzyme that releases arachidonic acid (AA) from membrane phospholipid for the production of biologically active lipid mediators including prostaglandins, leukotrienes and platelet-activating factor. cPLA(2)alpha is translocated to nuclear envelope in response to intracellular calcium increase and the enzyme is also present inside the cell nucleus; however, the biological function of cPLA(2)alpha in the nucleus remains unknown. Here we show a novel role of cPLA(2)alpha for activation of peroxisome proliferator-activated receptor-delta (PPARdelta) and beta-catenin in the nuclei. Overexpression of cPLA(2)alpha in human cholangiocarcinoma cells induced the binding of PPARdelta to beta-catenin and increased their association with the TCF/LEF response element. These effects are inhibited by the cPLA(2)alpha siRNA and inhibitors as well as by siRNA knockdown of PPARdelta. Overexpression of PPARdelta or treatment with the selective PPARdelta ligand, GW501516, also increased beta-catenin binding to TCF/LEF response element and increased its reporter activity. Addition of AA and GW501516 to nuclear extracts induced a comparable degree of beta-catenin binding to TCF/LEF response element. Furthermore, cPLA(2)alpha protein is present in the PPARdelta and beta-catenin binding complex. Thus the close proximity between cPLA(2)alpha and PPARdelta provides a unique advantage for their efficient functional coupling in the nucleus, where AA produced by cPLA(2)alpha becomes immediately available for PPARdelta binding and subsequent beta-catenin activation. These results depict a novel interaction linking cPLA(2)alpha, PPARdelta and Wnt/beta-catenin signaling pathways and provide insight for further understanding the roles of these key molecules in human cells and diseases.

Thiazolidinediones are partial agonists for the glucocorticoid receptor

Although thiazolidinediones were designed as specific peroxisome proliferator-activated receptor (PPAR)-gamma-ligands, there is evidence for some off-target effects mediated by a non-PPARgamma mechanism. Previously we have shown that rosiglitazone has antiinflammatory actions not explicable by activation of PPARgamma,but possibly by the glucocorticoid receptor (GR). Rosiglitazone induces nuclear translocation both of GR-green fluorescent protein, and endogenous GR in HeLa and U20S cells but with slower kinetics than dexamethasone. Rosiglitazone also induces GR phosphorylation (Ser211), a GR ligand-binding-specific effect. Rosiglitazone drives luciferase expression from a simple glucocorticoid-response element containing reporter gene in a GR-dependent manner (EC50 4 microm), with a similar amplitude response to the partial GR agonist RU486. Rosiglitazone also inhibits dexamethasone-driven reporter gene activity (IC50 2.9 microm) in a similar fashion to RU486, suggesting partial agonist activity. Importantly we demonstrate a similar effect in PPARgamma-null cells, suggesting both GR dependence and PPARgamma independence. Rosiglitazone also activates a GAL4-GR chimera, driving a upstream activating sequence promoter, demonstrating DNA template sequence independence and furthermore enhanced steroid receptor coactivator-1-GR interaction, measured by a mammalian two-hybrid assay. Both ciglitazone and pioglitazone, structurally related to rosiglitazone, show similar effects on the GR. The antiproliferative effect of rosiglitazone is increased in U20S cells that overexpress GR, suggesting a biologically important GR-dependent component of rosiglitazone action. Rosiglitazone is a partial GR agonist, affecting GR activation and trafficking to influence engagement of target genes and affect cell function. This novel mode of action may explain some off-target effects observed in vivo. Additionally, antagonism of glucocorticoid action may contribute to the antidiabetic actions of rosiglitazone.

High-throughput screening for analysis of in vitro toxicity

The influence of combinatorial chemistry and high-throughput screening (HTS) technologies in the pharmaceutical industry during the last 10 years has been enormous. However, the attrition rate of drugs in the clinic due to toxicity during this period still remained 40-50%. The need for reduced toxicity failure led to the development of early toxicity screening assays. This chapter describes the state of the art for assays in the area of genotoxicity, cytotoxicity, carcinogenicity, induction of specific enzymes from phase I and II metabolism, competition assays for enzymes of phase I and II metabolism, embryotoxicity as well as endocrine disruption and reprotoxicity. With respect to genotoxicity, the full Ames, Ames II, Vitotox, GreenScreen GC, RadarScreen, and non-genotoxic carcinogenicity assays are discussed. For cytotoxicity, cellular proliferation, calcein uptake, oxygen consumption, mitochondrial activity, radical formation, glutathione depletion as well as apoptosis are described. For high-content screening (HCS), the possibilities for analysis of cytotoxicity, micronuclei, centrosome formation and phospholipidosis are examined. For embryotoxicity, endocrine disruption and reprotoxicity alternative assays are reviewed for fast track analysis by means of nuclear receptors and membrane receptors. Moreover, solutions for analyzing enzyme induction by activation of nuclear receptors, like AhR, CAR, PXR, PPAR, FXR, LXR, TR and RAR are given.

PPAR and Pain

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factor belonging to a nuclear hormone receptor superfamily, containing three isoforms (alpha, beta/delta, and gamma). PPARs play a critical physiological role as a primary lipid sensor and regulator of lipid metabolism. Thus, its ligands are clinically used for treatment of type 2 diabetes and hyperlipidemia. On the other hand, PPAR ligands exert the antineuroinflammatory activity through preventing upregulation of inflammatory mediators in animal models for neurodegenerative disease and autoimmune disease. Neuropathic pain and inflammatory pain, clinically important one, are chronically progressed and underlain by neuroinflammation. In a few years, some studies using experimental models emerge that administration of PPAR ligands reduces inflammatory pain and neuropathic pain. PPAR ligands repress expression of genes for inflammatory mediators involved in both pains, such as proinflammatory cytokines, by a molecular mechanism termed ligand-dependent direct transrepression. Alternative mechanism is independent of transcriptional regulation of target genes, such as inhibition of activity of ion channels involved in the development of inflammatory pain and neuropathic pain, and therefore the analgesic effect occurs with rapid onset. The effects of PPAR ligands on neuroinflammation in animal models suggest their possible use for treating human inflammatory pain and neuropathic pain.

Atomic structure of mutant PPARgamma LBD complexed with 15d-PGJ2: novel modulation mechanism of PPARgamma/RXRalpha function by covalently bound ligands

15-deoxy-Delta(12,14)-prostaglandin J(2) (15d-PGJ(2)) activates a nuclear receptor heterodimer, peroxisome proliferators-activated receptor gamma (PPARgamma)/ retinoid X receptor (RXRalpha) through covalent binding to Cys285 in PPARgamma ligand-binding domain (LBD). Here, we present the 1.9A crystal structure of C285S mutant LBD complexed with 15d-PGJ(2), corresponding to the non-covalently bound state. The ligand lies adjacent to a hydrogen-bond network around the helix H2 and the nearby beta-sheet. Comparisons with previous structures clarified the relationships between PPARgamma function and conformational alterations of LBD during the process of covalently binding ligands, such as 15d-PGJ(2), and thus suggested a mechanism, by which these ligands modulate PPARgamma/RXRalpha function through conformational changes of the loop following helix H2' and the beta-sheet.

PPAR-gamma-independent inhibitory effect of rosiglitazone on glucose synthesis in primary cultured rabbit kidney-cortex tubules

Therapeutic effect of rosiglitazone has been reported to result from an improvement of insulin sensitivity and inhibition of glucose synthesis. As the latter process occurs in both liver and kidney cortex the aim of this study was to elucidate the rosiglitazone action on glucose formation in both tissues. Primary cultured cells of both liver and kidney cortex grown in defined medium were use throughout. To identify the mechanism responsible for drug-induced changes, intracellular gluconeogenic intermediates and enzyme activities were determined. In contrast to hepatocytes, the administration of a 10 micromol/L concentration of rosiglitazone to renal tubules resulted in about a 70% decrease in the rate of gluconeogenesis, accompanied by an approximately 75% decrease in alanine utilization and a 35% increase in lactate synthesis. The effect of rosiglitazone was not abolished by GW9662, the PPAR-gamma irreversible antagonist, indicating that this action is not dependent on PPAR-gamma activation. In view of rosiglitazone-induced changes in gluconeogenic intermediates and a diminished incorporation of 14CO2 into pyruvate, it is likely that the drug causes a decline in flux through pyruvate carboxylase and (or) phosphoenolpyruvate carboxykinase. It is likely that the hypoglycemic action of rosiglitazone is PPAR-gamma independent and results mainly from its inhibitory effects on renal gluconeogenesis.

Integration of metabolism and inflammation by lipid-activated nuclear receptors

The nuclear receptors known as PPARs and LXRs are lipid-activated transcription factors that have emerged as key regulators of lipid metabolism and inflammation. PPARs and LXRs are activated by non-esterified fatty acids and cholesterol metabolites, respectively, and both exert positive and negative control over the expression of a range of metabolic and inflammatory genes. The ability of these nuclear receptors to integrate metabolic and inflammatory signalling makes them attractive targets for intervention in human metabolic diseases, such as atherosclerosis and type 2 diabetes, as well as for the modulation of inflammation and immune responses.

Peroxisome proliferator-activated receptor-gamma and growth inhibition by its ligands in prostate cancer

BACKGROUND

Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) is expressed in certain human cancers. Ligand-induced PPAR-gamma activation can result in growth inhibition and differentiation in these cancer cells; however, the precise mechanism for the anti-proliferative effect of PPAR-gamma ligands is not clear.

METHODS

In this study, we examined the expression of PPAR-gamma in human prostate cancer and the effect of two PPAR-gamma ligands, 15 deoxy-Delta(12,14)-prostaglandin J2 (15d-PGJ2) and troglitazone, on prostate cancer cell growth.

RESULTS

PPAR-gamma is frequently over-expressed in androgen independent prostate cancer cell lines and human prostate cancer tissues (22 of 47; 47%). Both 15d-PGJ2 and troglitazone inhibited proliferation and DNA synthesis of prostate cancer cell lines in a dose-dependent manner, and slightly increased the proportion of cells with S-phase DNA content. Prostate specific antigen (PSA) promoter reporter assays showed that troglitazone and 15d-PGJ2 down-regulated androgen stimulated reporter gene activity in prostate cancer cell lines LNCaP. Interestingly, LNCaP with troglitazone dramatically suppressed PSA protein expression without suppressing AR expression.

CONCLUSIONS

Taken together, these results suggest that PPAR-gamma ligands may be a useful therapeutic agent for the treatment of prostate cancer.

Coordination of inflammation and metabolism by PPAR and LXR nuclear receptors

Biological systems are integrated networks constantly responding to internal and external stimulators. Understanding the intrinsic response to an imbalanced system provides the opportunity to develop therapeutic approaches to reinstate the natural balanced state. Increasing evidence suggests that members of the nuclear receptor superfamily integrate both inflammatory and metabolic signals to maintain homeostasis in immune cells such as macrophages and lymphocytes. PPAR and LXR are nuclear receptors activated by fatty acid and cholesterol derivatives respectively that control the expression of an array of genes involved in lipid metabolism and inflammation. Recent studies have uncovered distinct mechanisms for transcriptional regulation of metabolic and inflammatory target genes by PPAR and LXR and have expanded the biology of these receptors to include roles in alternative macrophage activation and adaptive immunity.

Coordination of inflammation and metabolism by PPAR and LXR nuclear receptors

Biological systems are integrated networks constantly responding to internal and external stimulators. Understanding the intrinsic response to an imbalanced system provides the opportunity to develop therapeutic approaches to reinstate the natural balanced state. Increasing evidence suggests that members of the nuclear receptor superfamily integrate both inflammatory and metabolic signals to maintain homeostasis in immune cells such as macrophages and lymphocytes. PPAR and LXR are nuclear receptors activated by fatty acid and cholesterol derivatives respectively that control the expression of an array of genes involved in lipid metabolism and inflammation. Recent studies have uncovered distinct mechanisms for transcriptional regulation of metabolic and inflammatory target genes by PPAR and LXR and have expanded the biology of these receptors to include roles in alternative macrophage activation and adaptive immunity.

Fat and beyond: the diverse biology of PPARgamma

The nuclear receptor PPARgamma is a ligand-activated transcription factor that plays an important role in the control of gene expression linked to a variety of physiological processes. PPARgamma was initially characterized as the master regulator for the development of adipose cells. Ligands for PPARgamma include naturally occurring fatty acids and the thiazolidinedione (TZD) class of antidiabetic drugs. Activation of PPARgamma improves insulin sensitivity in rodents and humans through a combination of metabolic actions, including partitioning of lipid stores and the regulation of metabolic and inflammatory mediators termed adipokines. PPARgamma signaling has also been implicated in the control of cell proliferation, atherosclerosis, macrophage function, and immunity. Here, we review recent advances in our understanding of the diverse biological actions of PPARgamma with an eye toward the expanding therapeutic potential of PPARgamma agonist drugs.

The Pal3 promoter sequence is critical for the regulation of human renin gene transcription by peroxisome proliferator-activated receptor-gamma

We recently reported that human renin gene transcription is stimulated by the nuclear receptor peroxisome proliferator-activated receptor (PPAR)-gamma in the renin-producing cell line Calu-6. The effect of PPARgamma was mapped to two sequences in the renin promoter: a direct repeat hormone response element (HRE), which is related to the classical PPAR response element (PPRE) and a nonconsensus palindromic element with a 3-bp spacer (Pal3). We now find that PPARgamma binds to the renin HRE. Neither the human renin HRE nor the consensus PPRE was sufficient to attain the maximal stimulation of renin promoter activity by the PPARgamma agonist rosiglitazone. In contrast, the human renin Pal3 element mediates both the full PPARgamma-dependent activation of transcription and the PPARgamma-driven basal renin gene transcription. The human renin Pal3 sequence was found to selectively bind PPARgamma and the retinoid X receptor-alpha from Calu-6 nuclear extracts. This is in contrast to the consensus PPRE, which can bind other nuclear proteins. PPARgamma knockdown paradoxically did not attenuate the stimulation of the endogenous renin gene expression by rosiglitazone. Similarly, a deficiency of PPARgamma did not attenuate the activation of the minimal human renin promoter, which contains the endogenous Pal3 motif. However, when the human renin Pal3 site was replaced by the consensus PPRE sequence, PPARgamma knockdown abrogated the effect of rosiglitazone on renin promoter activity. Thus, the human renin Pal3 site appears to be critical for the PPARgamma-dependent regulation of gene expression by mediating maximal transcription activation, particularly at the low cellular level of PPARgamma.

Nuclear receptor signalling in dendritic cells connects lipids, the genome and immune function

Dendritic cells (DCs) are sentinels of the immune system and represent a heterogeneous cell population. The existence of distinct DC subsets is due to their inherent plasticity and to the changing microenvironment modulating their immunological properties. Numerous signalling pathways have impacts on DCs. It appears that besides cytokines/chemokines, lipid mediators also have profound effects on the immunogenicity of DCs. Some of these lipid mediators exert an effect through nuclear hormone receptors. Interestingly, more recent findings suggest that DCs are able to convert precursors to active hormones, ligands for nuclear receptors. Some of these DC-derived lipids, in particular retinoic acid (RA), have a central function in shaping T-cell development and effector functions. In this review, we summarize and highlight the function of a set of nuclear receptors (PPARγ, RA receptor, vitamin D receptor and glucocorticoid receptor) in DC biology. Defining the contribution of nuclear hormone receptor signalling in DCs can help one to understand the regulatory logic of lipid signalling and allow the exploitation of their potential for therapeutic intervention in various immunological diseases.

PPARgamma agonists as therapeutics for the treatment of Alzheimer's disease

Alzheimer's disease (AD) is characterized by the deposition of beta-amyloid within the brain parenchyma and is accompanied by the impairment of neuronal metabolism and function, leading to extensive neuronal loss. The disease involves the perturbation of synaptic function, energy, and lipid metabolism. The development of amyloid plaques results in the induction of a microglial-mediated inflammatory response. The nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma) is a ligand-activated transcription factor whose biological actions are to regulate glucose and lipid metabolism and suppress inflammatory gene expression. Thus, agonists of this receptor represent an attractive therapeutic target for AD. There is now an extensive body of evidence that has demonstrated the efficacy of PPARgamma agonists in ameliorating disease-related pathology and improved learning and memory in animal models of AD. Recent clinical trials of the PPARgamma agonist rosiglitazone have shown significant improvement in memory and cognition in AD patients. Thus, PPARgamma represents an important new therapeutic target in treating AD.

Apolipoprotein C-III: understanding an emerging cardiovascular risk factor

The concurrence of visceral obesity, insulin resistance and dyslipidaemia comprises the concept of the metabolic syndrome. The metabolic syndrome is an escalating problem in developed and developing societies that tracks with the obesity epidemic. Dyslipidaemia in the metabolic syndrome is potently atherogenic and, hence, is a major risk factor for CVD (cardiovascular disease) in these subjects. It is globally characterized by hypertriglyceridaemia, near normal LDL (low-density lipoprotein)-cholesterol and low plasma HDL (high-density lipoprotein)-cholesterol. ApoC-III (apolipoprotein C-III), an important regulator of lipoprotein metabolism, is strongly associated with hypertriglyceridaemia and the progression of CVD. ApoC-III impairs the lipolysis of TRLs [triacylglycerol (triglyceride)-rich lipoproteins] by inhibiting lipoprotein lipase and the hepatic uptake of TRLs by remnant receptors. In the circulation, apoC-III is associated with TRLs and HDL, and freely exchanges among these lipoprotein particle systems. However, to fully understand the complex physiology and pathophysiology requires the application of tracer methodology and mathematical modelling. In addition, experimental evidence shows that apoC-III may also have a direct role in atherosclerosis. In the metabolic syndrome, increased apoC-III concentration, resulting from hepatic overproduction of VLDL (very-LDL) apoC-III, is strongly associated with delayed catabolism of triacylglycerols and TRLs. Several therapies pertinent to the metabolic syndrome, such as PPAR (peroxisome-proliferator-activated receptor) agonists and statins, can regulate apoC-III transport in the metabolic syndrome. Regulating apoC-III metabolism may be an important new therapeutic approach to managing dyslipidaemia and CVD risk in the metabolic syndrome.

Intracellular signalling by C-peptide

C-peptide, a cleavage product of the proinsulin molecule, has long been regarded as biologically inert, serving merely as a surrogate marker for insulin release. Recent findings demonstrate both a physiological and protective role of C-peptide when administered to individuals with type I diabetes. Data indicate that C-peptide appears to bind in nanomolar concentrations to a cell surface receptor which is most likely to be G-protein coupled. Binding of C-peptide initiates multiple cellular effects, evoking a rise in intracellular calcium, increased PI-3-kinase activity, stimulation of the Na(+)/K(+) ATPase, increased eNOS transcription, and activation of the MAPK signalling pathway. These cell signalling effects have been studied in multiple cell types from multiple tissues. Overall these observations raise the possibility that C-peptide may serve as a potential therapeutic agent for the treatment or prevention of long-term complications associated with diabetes.

Pioglitazone attenuates diabetic nephropathy through an anti-inflammatory mechanism in type 2 diabetic rats

BACKGROUND

Peroxisome proliferator-activated receptors (PPARs) are nuclear transcription factors that play a role in insulin sensitivity, lipid metabolism and inflammation. However, the effects of PPARgamma agonist on renal inflammation have not been fully examined in type 2 diabetic nephropathy.

METHODS

In the present study, we investigated the effect and molecular mechanism of the PPARgamma agonist, pioglitazone, on the progression of diabetic nephropathy in type 2 diabetic rats. Inflammatory markers including NF-kappaB, MCP-1 and pro-fibrotic cytokines were determined by RT-PCR, western blot, immunohistochemical staining and EMSA. In addition, to evaluate the direct anti-inflammatory effect of PPARgamma agonist, we performed an in vitro study using mesangial cells.

RESULTS

Treatment of OLETF rats with pioglitazone improved insulin sensitivity and kidney/body weight, but had a little effect on blood pressure. Pioglitazone treatment markedly reduced urinary albumin and MCP-1 excretion, and ameliorated glomerulosclerosis. In cDNA microarray analysis using renal cortical tissues, several inflammatory and profibrotic genes were significantly down-regulated by pioglitazone including NF-kappaB, CCL2, TGFbeta1, PAI-1 and VEGF. In renal tissues, pioglitazone treatment significantly reduced macrophage infiltration and NF-kappaB activation in association with a decrease in type IV collagen, PAI-1, and TGFbeta1 expression. In cultured mesangial cells, pioglitazone-activated endogenous PPARgamma transcriptional activity and abolished high glucose-induced collagen production. In addition, pioglitazone treatment also markedly suppressed high glucose-induced MCP-1 synthesis and NF-kappaB activation.

CONCLUSIONS

These data suggest that pioglitazone not only improves insulin resistance, glycaemic control and lipid profile, but also ameliorates renal injury through an anti-inflammatory mechanism in type 2 diabetic rats.

Management of type 2 diabetes mellitus: is it time for a paradigm shift?

Type 2 diabetes mellitus is a multi-organ disease that results from the combination of insulin resistance and a beta-cell secretory defect. The worldwide prevalence of type 2 diabetes has increased substantially during the past decade, and patients with this disease continue to experience a high incidence of morbidity and mortality. Because the complications associated with this disease affect multiple organ systems and have a dramatic impact on daily life, the importance of lowering glycosylated hemoglobin (HbA(1c)) levels to within the normal range cannot be overemphasized. The introduction in the past decade of several new classes of pharmacological agents to treat patients with type 2 diabetes now provides the opportunity to focus therapy on treating the underlying disease process instead of just reacting to the blood glucose levels. The thiazolidinediones are unique in their ability to modulate free fatty acid metabolism and to improve insulin sensitivity. These agents also exert numerous nonglycemic effects on the vasculature and lipid metabolism and may improve many of the risk factors associated with metabolic syndrome. Data from the United Kingdom Prospective Diabetes Study (UKPDS) group showed that conventional methods of managing type 2 diabetes, including the use of sulfonylureas or biguanides, do not provide long-term glycemic control. Consequently, new treatment paradigms stressing the earlier use of thiazolidinediones, either alone or in combination with metformin, may lead to more durable glycemic control, thus facilitating the reduction of complications in patients with type 2 diabetes.

Effects of thiazolidinediones on blood pressure

Peroxisome-proliferator-activated receptor-gamma (PPAR-gamma) agonists (known as thiazolidinediones; TDZs) activate nuclear receptors that regulate gene expression; they were developed as insulin-sensitizing drugs to treat type 2 diabetes mellitus. Although the prototypic TZD troglitazone was withdrawn from the market due to hepatic toxicity, rosiglitazone and pioglitazone are mainstays in managing type 2 diabetes mellitus. TZDs exert their hypoglycemic effect by reducing insulin resistance, hence improving insulin sensitivity. However, TZDs also exhibit a broad range of cardiovascular actions, with the clinical consequence of reduction in blood pressure (BP), observed in animal models and human diabetic subjects. The magnitude of reduction appears to be about 4 to 5 mm Hg in systolic and 2 to 4 mm Hg in diastolic BP--sufficient to significantly reduce subsequent cardiovascular event rates. But these BP-reducing properties, which are not present with metformin or sulfonylureas, are particularly important when viewed in conjunction with hypoglycemic effects. A significant proportion of patients with type 2 diabetes mellitus and BP mildly above target range might be successfully treated for both processes with a single drug.

Therapeutic potential of peroxisome proliferators--activated receptor-alpha/gamma dual agonist with alleviation of endoplasmic reticulum stress for the treatment of diabetes

OBJECTIVE

Peroxisome proliferator-activated receptor (PPAR) alpha/gamma dual agonists have the potential to be used as therapeutic agents for the treatment of type 2 diabetes. This study evaluated the function of macelignan, a natural compound isolated from Myristica fragrans, as a dual agonist for PPARalpha/gamma and investigated its antidiabetes effects in animal models.

RESEARCH DESIGN AND METHODS

GAL4/PPAR chimera transactivation was performed and the expression of PPARalpha/gamma target genes was monitored to examine the ability of macelignan to activate PPARalpha/gamma. Additionally, macelignan was administrated to obese diabetic (db/db) mice to investigate antidiabetes effects and elucidate its molecular mechanisms.

RESULTS

Macelignan reduced serum glucose, insulin, triglycerides, free fatty acid levels, and triglycerides levels in the skeletal muscle and liver of db/db mice. Furthermore, macelignan significantly improved glucose and insulin tolerance in these mice, and without altering food intake, their body weights were slightly reduced while weights of troglitazone-treated mice increased. Macelignan increased adiponectin expression in adipose tissue and serum, whereas the expression and serum levels of tumor necrosis factor-alpha and interleukin-6 decreased. Macelignan downregulated inflammatory gene expression in the liver and increased AMP-activated protein kinase activation in the skeletal muscle of db/db mice. Strikingly, macelignan reduced endoplasmic reticulum (ER) stress and c-Jun NH(2)-terminal kinase activation in the liver and adipose tissue of db/db mice and subsequently increased insulin signaling.

CONCLUSIONS

Macelignan enhanced insulin sensitivity and improved lipid metabolic disorders by activating PPARalpha/gamma and attenuating ER stress, suggesting that it has potential as an antidiabetes agent for the treatment of type 2 diabetes.

Peroxisome Proliferator-Activated Receptor-γ–Mediated Effects in the Vasculature

Peroxisome proliferator-activated receptor (PPAR)-γ is a nuclear receptor and transcription factor in the steroid superfamily. PPAR-γ agonists, the thiazolidinediones, are clinically used to treat type 2 diabetes. In addition to its function in adipogenesis and increasing insulin sensitivity, PPAR-γ also plays critical roles in the vasculature. In vascular endothelial cells, PPAR-γ activation inhibits endothelial inflammation by suppressing inflammatory gene expression and therefore improves endothelial dysfunction. In vascular smooth muscle cells, PPAR-γ activation inhibits proliferation and migration and promotes apoptosis. In macrophages, PPAR-γ activation suppresses inflammation by regulating gene expression and increases cholesterol uptake and efflux. A recurring theme in many cell types is the modulation of the innate immunity system particularly through altering the activity of the nuclear factor κB. This system is likely to be even more prominent in modulating disease in vascular cells. The effects of PPAR-γ in the vascular cells translate into the beneficial function of this transcription factor in vascular disorders, including hypertension and atherosclerosis. Both human genetic studies and animal studies using transgenic mice have demonstrated the importance of PPAR-γ in these disorders. However, recent clinical studies have raised significant concerns about the cardiovascular side effects of thiazolidinediones, particularly rosiglitazone. Weighing the potential benefit and harm of PPAR-γ activation and exploring the functional mechanisms may provide a balanced view on the clinical use of these compounds and new approaches to the future therapeutics of vascular disorders associated with diabetes.

1,1-bis(3'-indolyl)-1-(p-substitutedphenyl)methanes induce apoptosis and inhibit renal cell carcinoma growth

PURPOSE

1,1-Bis(3'-indolyl)-1-(p-substitutedphenyl)methanes [methylene-substituted diindolylmethanes (C-DIM)] containing p-trifluoromethyl, p-t-butyl, and p-phenyl substituents activate peroxisome proliferator-activated receptor gamma (PPARgamma) and inhibit growth of several different cancer cell lines through receptor-dependent and receptor-independent pathways. The purpose of this study is to investigate the anticancer activity of these compounds in renal cell carcinoma.

EXPERIMENTAL DESIGN

The anticancer activity of the p-t-butyl-substituted C-DIM compound (DIM-C-pPhtBu) was investigated in ACHN and 786-0 renal cell carcinoma cell lines and in an orthotopic model for renal carcinogenesis using ACHN cells injected directly into the kidney.

RESULTS

PPARgamma is overexpressed in ACHN cells and barely detectable in 786-0 cells, and treatment with DIM-C-pPhtBu induces proteasome-dependent degradation of cyclin D1 and variable effects on p21 and p27 expression in both cell lines. DIM-C-pPhtBu also induced several common proapoptotic responses in ACHN and 786-0 cells, including increased expression of nonsteroidal anti-inflammatory drug-activated gene-1 and endoplasmic reticulum stress, which activates death receptor 5 and the extrinsic pathway of apoptosis. Activation of these responses was PPARgamma independent. In addition, DIM-C-pPhtBu (40 mg/kg/d) also inhibited tumor growth in an orthotopic mouse model for renal carcinogenesis, and this was accompanied by induction of apoptosis in renal tumors treated with DIM-C-pPhtBu but not in tumors treated with the corn oil vehicle (control).

CONCLUSIONS

DIM-C-pPhtBu and related compounds are cytotoxic to renal cancer cells and activate multiple proapoptotic and growth-inhibitory pathways. The results coupled with in vivo anticancer activity show the potential of DIM-C-pPhtBu and related C-DIMs for clinical treatment of renal adenocarcinoma.

Signalling pathways controlling fatty acid desaturation

Microorganisms, plants and animals regulate the synthesis of unsaturated fatty acids (UFAs) during changing environmental conditions as well as in response to nutrients. Unsaturation of fatty acid chains has important structural roles in cell membranes: a proper ratio of saturated to UFAs contributes to membrane fluidity. Alterations in this ratio have been implicated in various disease states including cardiovascular diseases, immune disorders, cancer and obesity. They are also the major components of triglycerides and intermediates in the synthesis of biologically active molecules such as eicosanoids, which mediates fever, inflammation and neurotransmission. UFAs homeostasis in many organisms is achieved by feedback regulation of fatty acid desaturases gene transcription. Here, we review recently discovered components and mechanisms of the regulatory machinery governing the transcription of fatty acid desaturases in bacteria, yeast and animals.

Thiazolidinediones as anti-inflammatory and anti-atherogenic agents

In the last few years, there has been increasing focus on the impact of interventions on cardiovascular outcomes in patients with type 2 diabetes. Insulin resistance and hyperglycaemia often co-exist with a cluster of risk factors for coronary artery disease, but the underlying mechanisms leading to the development of such vascular complications are complex. The over-production of free radicals in patients suffering from diabetes results in a state of oxidative stress, which leads to endothelial dysfunction and a greater risk of atherosclerosis. Moreover, inflammatory factors which play a critical role in atherothrombosis and plaque rupture are often found to be at elevated levels in this patient population. Thiazolidinediones (TZDs) are now routinely used to manage glucose levels, and have been suggested to influence other cardiovascular risk factors and therefore the pathways leading to macrovascular events. Consequently, recent studies have investigated the anti-inflammatory and anti-atherogenic properties of TZDs. The data available up to the present time, in the context of the emerging cardiovascular outcome profiles of rosiglitazone and pioglitazone, will be discussed here.

Use of insulin sensitizers in NASH

This article briefly discusses nonalcoholic fatty liver disease (NAFLD) and its association with the metabolic syndrome, its pathogenesis and natural history. It then presents a detailed discussion on the efficacy and safety of different insulin sensitizers in patients who have NASH.

Peroxisome Proliferator-Activated Receptor-γ Is Involved in the Control of Renin Gene Expression

Based on the presence of a functional retinoic acid receptor/retinoid X receptor transcription factor binding sequence (hormone-responsive element) in the renin gene enhancer and on the fact that the peroxisome proliferator-activated receptors (PPARs) bind to DNA as heterodimers with retinoid X receptors, we speculated that PPARs are involved in the regulation of renin gene expression. To test this hypothesis, we used the human renin-producing cell line CaLu-6. Endogenous or pharmacological PPARγ agonists (unsaturated fatty acids and thiazolidinediones, respectively) stimulated renin gene expression. Surprisingly, we found that PPARγ targets a palindromic repeat with a 3-bp spacer (Pal3) in the proximal human renin promoter. Thus, renin is the first gene described with a functional Pal3 sequence. PPARγ agonists also stimulated renin gene expression in cultured native juxtaglomerular cells, which are the main source of renin in vivo. In summary, PPARγ was identified as a novel intracellular mediator involved in the upregulation of renin transcription.

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.

Human immunodeficiency virus (HIV)-1 viral protein R suppresses transcriptional activity of peroxisome proliferator-activated receptor {gamma} and inhibits adipocyte differentiation: implications for HIV-associated lipodystrophy

HIV-1-infected patients may develop lipodystrophy and insulin resistance. We investigated the effect of the HIV-1 accessory protein viral protein R (Vpr) on the activity of the peroxisome proliferator-activating receptor-gamma (PPARgamma), a key regulator of adipocyte differentiation and tissue insulin sensitivity. We studied expression of PPARgamma-responsive reporter genes in 3T3-L1 mouse adipocytes. We investigated Vpr interaction with the PPAR/retinoid X receptor (RXR)-binding site of the c-Cbl-associating protein (CAP) gene using the chromatin immunoprecipitation assay as well as the interaction of Vpr and PPARgamma using coimmunoprecipitation. Finally, we studied the ability of exogenous Vpr protein to enter cultured adipocytes and retard differentiation. We found that Vpr suppressed PPARgamma-induced transactivation in both undifferentiated and differentiated 3T3-L1 cells. Transcriptional suppression by Vpr required an intact LXXLL coactivator motif. Vpr suppressed mRNA expression of PPARgamma-responsive genes in undifferentiated 3T3-L1 cells and associated with the PPAR/RXR-binding site located in the promoter region of the CAP gene. Vpr interacted with the ligand-binding domain of PPARgamma in an agonist-dependent fashion in vitro. Vpr delivered either by an expression plasmid or as protein added to media suppressed PPARgamma agonist-induced adipocyte differentiation, assessed as lipid accumulation and mRNA expression of the adipocyte differentiation marker adipocyte P2 in 3T3-L1 cells. In conclusion, circulating Vpr or, alternatively, Vpr produced as a consequence of direct infection of adipocytes could suppress in vivo differentiation of preadipocytes by acting as a corepressor of PPARgamma-mediated gene transcription. Vpr may alter sensitivity to insulin and thereby contribute to the development of lipodystrophy and insulin resistance observed in HIV-1-infected patients.

Thiazolidinediones: effects on insulin resistance and the cardiovascular system

Thiazolidinediones (TZDs) have been used for the treatment of hyperglycaemia in type 2 diabetes for the past 10 years. They may delay the development of type 2 diabetes in individuals at high risk of developing the condition, and have been shown to have potentially beneficial effects on cardiovascular risk factors. TZDs act as agonists of peroxisome proliferator-activated receptor-gamma (PPAR-gamma) primarily in adipose tissue. PPAR-gamma receptor activation by TZDs improves insulin sensitivity by promoting fatty acid uptake into adipose tissue, increasing production of adiponectin and reducing levels of inflammatory mediators such as tumour necrosis factor-alpha (TNF-alpha), plasminogen activator inhibitor-1(PAI-1) and interleukin-6 (IL-6). Clinically, TZDs have been shown to reduce measures of atherosclerosis such as carotid intima-media thickness (CIMT). However, in spite of beneficial effects on markers of cardiovascular risk, TZDs have not been definitively shown to reduce cardiovascular events in patients, and the safety of rosiglitazone in this respect has recently been called into question. Dual PPAR-alpha/gamma agonists may offer superior treatment of insulin resistance and cardioprotection, but their safety has not yet been assured.

Drug insight: effects mediated by peroxisome proliferator-activated receptor-gamma in CNS disorders

The finding that activation of peroxisome proliferator-activated receptor-gamma (PPARgamma) suppresses inflammation in peripheral macrophages and in models of human autoimmune disease instigated the evaluation of this salutary action for the treatment of CNS disorders with an inflammatory component. The fact that NSAIDs delay the onset of and reduce the risk of developing Alzheimer's disease (AD), while also binding to and activating PPARgamma, led to the hypothesis that one dimension of NSAID protection in AD is mediated by PPARgamma. Several lines of evidence from experiments using AD-related transgenic cellular and animal models have supported this hypothesis. The capacity of PPARgamma agonists to elicit anti-inflammatory, anti-amyloidogenic and insulin-sensitizing effects might account for their observed protective effects. Several clinical trials employing PPARgamma agonists have yielded promising results, and further trials are in preparation. Positive outcomes following PPARgamma administration have been obtained in animal models of other neurodegenerative diseases, including Parkinson's disease and amyotrophic lateral sclerosis, both of which are associated with a considerable degree of neuroinflammation. Finally, activation of PPARgamma has been found to be protective in several models of multiple sclerosis. The verification of these findings in human cells prompted the initiation of clinical studies evaluating PPARgamma activation in patients with multiple sclerosis.

Current and novel therapies for the treatment of nonalcoholic steatohepatitis

The worldwide epidemic of obesity and the metabolic syndrome has made nonalcoholic fatty liver disease (NAFLD), one of the most important liver diseases of our time. NAFLD is now the commonest cause of abnormal liver test results in industrialized countries and its incidence is rising. The current treatment of nonalcoholic steatohepatitis (NASH) has focused on lifestyle modification to achieve weight loss and modification of risk factors, such as insulin resistance, dyslipidemia, and hyperglycemia associated with the metabolic syndrome. With our increasing understanding of the pathogenesis of NASH, have come a plethora of new pharmacologic options with great potential to modify the natural history of NAFLD and NASH. This article focuses on a number of novel molecular targets for the treatment of NASH as well as the evidence for currently available therapy. It should be noted, however, that in part because of the long natural history of NASH and NAFLD, no therapy to date has been shown to unequivocally alter liver-related morbidity and mortality in these patients.