Mechanism of action of a peroxisome proliferator-activated receptor (PPAR)-delta agonist on lipoprotein metabolism in dyslipidemic subjects with central obesity

New frontiers in the treatment of diabetic dyslipidemia

Dyslipidemia is a major risk factor for cardiovascular complications in people with diabetes. Lowering low-density lipoprotein cholesterol (LDL-C) levels is effective in the primary and secondary prevention of diabetic vascular complications. However, LDL-C levels do not reflect all aspects of diabetic dyslipidemia, which is characterized by hypertriglyceridemia and low high-density lipoprotein cholesterol (HDL-C). Statins, nicotinic acid, and fibrates play a role in treating diabetic dyslipidemia. Atherosclerosis is a major disorder of the blood vessel wall in patients with diabetes. A number of antihyperlipidemic agents may be beneficial and exhibit effects at the actual site of vascular disease and not only on plasma lipoprotein concentrations. Several novel therapeutic compounds are currently being developed. These include additional therapeutics for LDL-C, triglycerides, HDL-C, and modulators of inflammation that can be used as possible synergic agents for the treatment of atherosclerosis and irregularities in plasma lipoprotein concentrations.

The effects of PPARδ agonist and zinc on ovariectomized rats' vagina

OBJECTIVES

This study aimed to measure the effects of peroxisome proliferator-activated receptor-δ (PPARδ) agonist GW501516 (GW) and zinc sulfate (ZS) on ovariectomized rats' vaginal histomorphology and collagen expression.

METHODS

Two weeks after ovariectomy, rats received daily treatment with vaginal suppositories containing placebo, ZS, GW, ZS with GW, or estradiol for 2 weeks. Macroscopic measurements were taken and the midsection of the vagina was used for histology. Immunofluorescence was performed with antibodies against collagen I, III, and anti-actin or collagen I and V and anti-actin. Gene expression analysis of 3 collagen genes was performed by qRT-PCR.

RESULTS

Macroscopic measurements revealed that the genital hiatus was narrower in the ZS and ZS with GW groups, and the vagina was significantly longer in the animals treated with GW, ZS with GW, and estradiol compared to the placebo group. Microscopic measurements of the vaginal layers showed that the lamina propria and the vaginal muscularis were significantly thicker in the ZS and ZS with GW group compared to the placebo.The ratio of vaginal Col1a1/Col3a1 mRNA expression was significantly up-regulated by ZS with GW compared to placebo, whereas the ratio of vaginal Col1a1/Col5a1 expression was significantly up-regulated by ZS, GW, and ZS with GW. The ratio of vaginal collagen I/III protein expression was significantly up-regulated by ZS and ZS with GW, whereas the ratio of vaginal collagen I/V expression was significantly up-regulated by estradiol, ZS, and ZS with GW compared to control.

CONCLUSIONS

Vaginal suppositories containing zinc and PPARδ agonist significantly altered the vagina of ovariectomized rats.

Peroxisome proliferator-activated receptor δ: a multifaceted metabolic player

PURPOSE OF REVIEW

Therapeutic strategies to alleviate the growing epidemic of insulin-resistant syndromes (obesity and type 2 diabetes) as well as the conferred cardiovascular disease risk remain sparse. The peroxisome proliferator-activated receptor δ (PPARδ) has emerged as a versatile regulator of lipid homeostasis and inflammatory signaling, making it an attractive therapeutic target for the treatment and prevention of type 2 diabetes and atherosclerosis.

RECENT FINDINGS

PPARδ activation regulates lipid homeostasis and inflammatory signaling in a variety of cell types, conferring protection from metabolic disease and atherosclerosis. Specifically, PPARδ activation in the liver stimulates glucose utilization and inhibits gluconeogenesis, which improves insulin resistance and hyperglycemia. In macrophages, PPARδ-specific activation with synthetic agonists inhibits VLDL-induced triglyceride accumulation and inflammation. In mice, PPARδ agonists halt the progression of atherosclerosis and stabilize existing lesions by promoting an anti-inflammatory milieu within the diseased macrovasculature. In humans, PPARδ activation improves insulin sensitivity and reduces atherogenic dyslipidemia via a mechanism complementary to statin monotherapy.

SUMMARY

Recent advances in the understanding of PPARδ reveal that activation of this receptor represents a multifaceted therapeutic strategy for the prevention and treatment of insulin-resistant syndromes and atherosclerosis.

Altered expression of genes involved in lipid metabolism in obese subjects with unfavourable phenotype

Obesity (BMI ≥30 kg/m(2)) increases the risk of developing lifestyle-related diseases. A subgroup of obese individuals has been described as "metabolically healthy, but obese" (MHO). In contrast to at-risk obese (ARO), the MHO phenotype is defined by a favourable lipid profile and a normal or only slightly affected insulin sensitivity, despite the same amount of body fat. The objective was to characterize the metabolic phenotype of MHO subjects. We screened a variety of genes involved in lipid metabolism and inflammation in peripheral blood mononuclear cells (PBMC). Obese subjects (men and women; 18-70 years) with BMI ≥30 kg/m(2) were characterized as MHO (n = 9) or as ARO (n = 10). In addition, eleven healthy, normal weight subjects characterized as healthy by the same criteria as described for the MHO subjects were included. We found that with similar weight, total fat mass and fat mass distribution, the ARO subjects have increased plasma levels of gamma-glutamyl transpeptidase and free fatty acids. This group also has altered expression levels of a number of genes linked to lipid metabolism in PBMC with reduced gene expression levels of uncoupling protein 2, hormone-sensitive lipase and peroxisome proliferator-activated receptor δ compared with MHO subjects. The present metabolic differences between subgroups of obese subjects may contribute to explain some of the underlying mechanisms causing the increased risk of disease among ARO subjects compared with MHO subjects.

Misregulation of PPAR Functioning and Its Pathogenic Consequences Associated with Nonalcoholic Fatty Liver Disease in Human Obesity

Nonalcoholic fatty liver disease in human obesity is characterized by the multifactorial nature of the underlying pathogenic mechanisms, which include misregulation of PPARs signaling. Liver PPAR-α downregulation with parallel PPAR-γ and SREBP-1c up-regulation may trigger major metabolic disturbances between de novo lipogenesis and fatty acid oxidation favouring the former, in association with the onset of steatosis in obesity-induced oxidative stress and related long-chain polyunsaturated fatty acid n-3 (LCPUFA n-3) depletion, insulin resistance, hypoadiponectinemia, and endoplasmic reticulum stress. Considering that antisteatotic strategies targeting PPAR-α revealed that fibrates have poor effectiveness, thiazolidinediones have weight gain limitations, and dual PPAR-α/γ agonists have safety concerns, supplementation with LCPUFA n-3 appears as a promising alternative, which achieves both significant reduction in liver steatosis scores and a positive anti-inflammatory outcome. This latter aspect is of importance as PPAR-α downregulation associated with LCPUFA n-3 depletion may play a role in increasing the DNA binding capacity of proinflammatory factors, NF-κB and AP-1, thus constituting one of the major mechanisms for the progression of steatosis to steatohepatitis.

Orphan nuclear receptors and the regulation of nutrient metabolism: understanding obesity

Nuclear hormone receptors (NRs) are a superfamily of eukaryotic ligand-dependent transcription factors that translate endocrine, metabolic, nutritional, developmental, and pathophysiological signals into gene regulation. Members of the NR superfamily (on the basis of sequence homology) that lack identified natural and/or synthetic ligands are/were classified as "orphan" NRs. These members of the NR superfamily are abundantly expressed in tissues associated with major metabolic activity, such as skeletal muscle, adipose, and liver. Subsequently, in vivo genetic studies on these orphan NRs and exploitation of novel natural and synthetic agonists has revealed that orphan NRs regulate 1) carbohydrate, lipid, and energy homeostasis in a tissue-specific manner, and 2) the pathophysiology of dyslipidemia, obesity, Type 2 diabetes, and cardiovascular disease. This review discusses key studies that have implicated the orphan NRs as organ-specific regulators of metabolism and mediators of adverse pathophysiological effects. The emerging discovery of novel endogenous orphan NR ligands and synthetic agonists has provided the foundation for therapeutic exploitation of the orphans in the treatment of metabolic disease.

Reactive oxygen species signaling facilitates FOXO-3a/FBXO-dependent vascular BK channel β1 subunit degradation in diabetic mice

Activity of the vascular large conductance Ca(2+)-activated K(+) (BK) channel is tightly regulated by its accessory β(1) subunit (BK-β(1)). Downregulation of BK-β(1) expression in diabetic vessels is associated with upregulation of the forkhead box O subfamily transcription factor-3a (FOXO-3a)-dependent F-box-only protein (FBXO) expression. However, the upstream signaling regulating this process is unclear. Overproduction of reactive oxygen species (ROS) is a common finding in diabetic vasculopathy. We hypothesized that ROS signaling cascade facilitates the FOXO-3a/FBXO-mediated BK-β(1) degradation and leads to diabetic BK channel dysfunction. Using cellular biology, patch clamp, and videomicroscopy techniques, we found that reduced BK-β(1) expression in streptozotocin (STZ)-induced diabetic mouse arteries and in human coronary smooth muscle cells (SMCs) cultured with high glucose was attributable to an increase in protein kinase C (PKC)-β and NADPH oxidase expressions and accompanied by attenuation of Akt phosphorylation and augmentation of atrogin-1 expression. Treatment with ruboxistaurin (a PKCβ inhibitor) or with GW501516 (a peroxisome proliferator-activated receptor δ activator) reduced atrogin-1 expression and restored BK channel-mediated coronary vasodilation in diabetic mice. Our results suggested that oxidative stress inhibited Akt signaling and facilitated the FOXO-3a/FBXO-dependent BK-β(1) degradation in diabetic vessels. Suppression of the FOXO-3a/FBXO pathway prevented vascular BK-β(1) degradation and protected coronary function in diabetes.

Vascular effects of prostacyclin: does activation of PPARδ play a role?

Prostacyclin (PGI(2)) is a potent vasodilator that exerts multiple vasoprotective effects in the cardiovascular system. The effects of PGI(2) are mediated by activation of the cell membrane G-protein-coupled PGI(2) receptor (IP receptor). More recently, however, it has been suggested that PGI(2) might also serve as an endogenous ligand and activator of nuclear peroxisome proliferator-activated receptorδ (PPARδ). Consistent with this concept, studies designed to define pharmacological properties of stable PGI(2) analogs revealed that beneficial effects of these compounds appear to be mediated, in part, by activation of PPARδ. This review discusses emerging evidence regarding the contribution of PPARδ activation to vasoprotective and regenerative functions of PGI(2) and stable analogs of PGI(2).

New lipid-lowering drugs: an update

Lipid lowering is established as a proven intervention to reduce atherosclerosis and its complications. Statins form the basis of care but are not able to treat all aspects of dyslipidaemia. Many novel therapeutic compounds are being developed. These include additional therapeutics for low-density lipoprotein cholesterol, for example, thyroid mimetics (thyroid receptor beta-agonists), antisense oligonucleotides or microsomal transfer protein inhibitors (MTPI); triglycerides, for example, novel peroxosimal proliferator activating receptors agonists, MTPIs, diacylglycerol acyl transferase-1 inhibitors and high-density lipoprotein cholesterol (HDL-C), for example, mimetic peptides; HDL delipidation strategies and cholesterol ester transfer protein inhibitors and modulators of inflammation, for example, phospholipase inhibitors. Gene therapy for specific rare disorders, for example, lipoprotein lipase deficiency using alipogene tiparvovec is also in clinical trials. Lipid-lowering drugs are likely to prove a fast-developing area for novel treatments as possible synergies exist between new and established compounds for the treatment of atherosclerosis.

The renoprotective actions of peroxisome proliferator-activated receptors agonists in diabetes

Pharmaceutical agonists of peroxisome proliferator-activated receptors (PPARs) are widely used in the management of type 2 diabetes, chiefly as lipid-lowering agents and oral hypoglycaemic agents. Although most of the focus has been placed on their cardiovascular effects, both positive and negative, these agents also have significant renoprotective actions in the diabetic kidney. Over and above action on metabolic control and effects on blood pressure, PPAR agonists also appear to have independent effects on a number of critical pathways that are implicated in the development and progression of diabetic kidney disease, including oxidative stress, inflammation, hypertrophy, and podocyte function. This review will examine these direct and indirect actions of PPAR agonists in the diabetic kidney and explore recent findings of clinical trials of PPAR agonists in patients with diabetes.