Effects of the PPAR-δ agonist MBX-8025 on atherogenic dyslipidemia

Potentiation of colon cancer susceptibility in mice by colonic epithelial PPAR-δ/β overexpression

BACKGROUND

The nuclear receptor peroxisome proliferator-activated receptor-δ/β (PPAR-d) is upregulated in human colorectal cancers, but its role in colonic tumorigenesis remains controversial.

METHODS

We generated a novel mouse model of intestinally targeted PPAR-d overexpression to simulate PPAR-d upregulation in human colon carcinogenesis. Colon-specific PPAR-d overexpression was confirmed by real-time reverse transcription polymerase chain reaction, immunoblotting, and activity assays. Mice with and without targeted PPAR-d overexpression were tested for azoxymethane (AOM)-induced colonic tumorigenesis. Mouse whole-genome transcriptome microarray analyses were performed to identify PPAR-d target genes to promote tumorigenesis. We used linear models to test for PPAR-d overexpression trend effects on tumor multiplicity. All statistical tests were two-sided.

RESULTS

Targeted PPAR-d overexpression markedly increased colonic tumor incidence (from 0 of 10 wild-type [WT] littermate mice to 9 of 10 mice [P < .001] in 2 FVB/N background mouse lines [villin-PPAR-d-1 and villin-PPAR-d-2] at a 5-mg/kg AOM dose) and multiplicity (number of tumors per mouse per mg/kg dose of AOM increased from 0.47 [95% confidence interval [CI] = 0.22 to 0.72] for the WT littermates to 2.15 [95% CI = 1.90 to 2.40] [P < .001] for the villin-PPAR-d-1 mice and from 0.44 [95% CI = 0.09 to 0.79] for the WT littermates to 1.91 [95% CI = 1.57 to 2.25] [P < .001] for the villin-PPAR-d-2 mice). PPAR-d overexpression reversed resistance to AOM-induced colonic tumorigenesis in C57BL/6 mice. PPAR-d overexpression modulated expression of several novel PPAR-d target genes in normal-appearing colonic epithelial cells of mice with PPAR-d overexpression in a pattern that matched the changes in colonic tumors.

CONCLUSIONS

Our finding that PPAR-d upregulation profoundly enhances susceptibility to colonic tumorigenesis should impact the development of strategies of molecularly targeting PPAR-d in cancer and noncancerous diseases.

New peroxisome proliferator-activated receptor agonists: potential treatments for atherogenic dyslipidemia and non-alcoholic fatty liver disease

INTRODUCTION

Novel peroxisome proliferator-activated receptor (PPAR) modulators (selective PPAR modulators [SPPARMs]) and dual PPAR agonists may have an important role in the treatment of cardiometabolic disorders owing to lipid-modifying, insulin-sensitizing and anti-inflammatory effects.

AREAS COVERED

This review summarizes the efficacy of new PPAR agonists and SPPARMs that are under development for the treatment of atherogenic dyslipidemia and non-alcoholic fatty liver disease (NAFLD).

EXPERT OPINION

ABT-335 is a new formulation of fenofibrate that has been approved for concomitant use with statins. K-877, a SPPARM-α with encouraging preliminary results in modulating atherogenic dyslipidemia, and INT131, a SPPARM-γ with predominantly insulin-sensitizing actions, may also have favorable lipid-modifying effects. Although the development of dual PPAR-α/γ agonists (glitazars) and the SPPARM-δ GW501516 has been abandoned because of safety issues, another SPPARM-δ (MBX-8025) and a dual PPAR-α/δ agonist (GFT-505) have shown promising efficacy in decreasing plasma triglyceride and increasing high-density lipoprotein cholesterol concentrations, as well as improving insulin sensitivity and liver function. The beneficial effects of GFT-505 are complemented by preclinical findings that indicate reduction of hepatic fat accumulation, inflammation and fibrosis, making it a promising candidate for the treatment of NAFLD/nonalcoholic steatohepatitis (NASH). Long-term trials are required to test the efficacy and safety of these new PPAR agonists in reducing cardiovascular outcomes and treating NAFLD/NASH.

Pleiotropic effects of peroxisome proliferator-activated receptor γ and δ in vascular diseases

Peroxisome proliferator-activated receptors gamma (PPARγ) and delta (PPARδ) are nuclear receptors that have significant physiological effects on glucose and lipid metabolism. Experimental studies in animal models of metabolic disease have demonstrated their effects on improving lipid profile, insulin sensitivity, and reducing inflammatory responses. PPARγ and -δ are also expressed in the vasculature and their beneficial effects have been examined in various cardiovascular disease models such as atherosclerosis, hypertension, diabetic vascular complications, etc. using pharmacological ligands or genetic tools including viral vectors and transgenic mice. These studies suggest that PPARγ and δ are antiinflammatory, antiatherogenic, antioxidant, and antifibrotic against vascular diseases. Several signaling pathways, effector molecules, as well as coactivators/repressors have been identified as responsible for the protective effects of PPARγ and -δ in the vasculature. We discuss the pleiotropic effect of PPARγ and δ in vascular dysfunction, including atherosclerosis, hypertension, vascular remodeling, vascular injury, and diabetic vasculopathy, in various animal models, and the major underlying mechanisms. We also compare the phenotypes of several endothelial cell/vascular smooth muscle-specific PPARγ and -δ knockout and overexpressing transgenic mice in various disease models, and the implications underlying the functional importance of vascular PPARγ and δ in regulating whole-body homeostasis.

Peroxisome proliferator-activated receptor delta and cardiovascular disease

Recent reports have shown that peroxisome proliferator-activated receptor delta (PPARD) plays an important role in different vascular processes suggesting that PPARD is a significant modulator of cardiovascular disease. This review will focus on PPARD in relation to cardiovascular risk factors based on cell, animal and human data. Mouse studies suggest that Ppard is an important metabolic modulator that may have implications for cardiovascular disease (CVD). Specific human PPARD gene variants show no clear association with CVD but interactions between variants and lifestyle factors might influence disease risk. During recent years, development of specific and potent PPARD agonists has also made it possible to study the effects of PPARD activation in humans. PPARD agonists seem to exert beneficial effects on dyslipidemia and insulin-resistant syndromes but safety issues have been raised due to the role that PPARD plays in cell proliferation. Thus, large long term outcome as well as detailed safety and tolerability studies are needed to evaluate whether PPARD agonists could be used to treat CVD in humans.

Novel benefits of peroxisome proliferator-activated receptors on cardiovascular risk

PURPOSE OF REVIEW

This review provides an overview of newly described mechanisms by which peroxisome proliferator-activated receptors (PPARs) (α, γ, and δ) regulate several factors associated with cardiovascular risk.

RECENT FINDINGS

PPAR agonists have known effects on plasma lipoprotein levels, inflammation, and insulin resistance all of which influence the risk of cardiovascular disease. Recent studies provide more detail regarding the mechanisms behind these changes. PPAR-α activation in the enterocyte on HDL and chylomicron formation. PPAR-γ agonists reduce inflammation, in part, through direct effects on adipocytes and regulatory T cells within visceral adipose. PPAR-δ also has a relatively high expression in the macrophage. Incubation of macrophages with PPAR-δ agonists was shown to inhibit foam cell formation induced excessive levels of VLDL remnants.

SUMMARY

Treatments that activate PPAR-α, PPAR-γ, and PPAR-δ alone or in combination have the potential to reduce cardiovascular risk although multiple independent mechanisms. Treatment with PPAR agonists can reduce the burden of atherogenic postprandial lipoproteins and improve vascular function, reduce inflammation and inhibit foam cell formation. All of these would be expected to have favorable effects on cardiovascular risk. The challenge remains to develop compounds that maximize these potential cardiovascular benefits while minimizing undesirable effects of these compounds.

Peroxisome proliferator-activated receptor targets for the treatment of metabolic diseases

Metabolic syndrome is estimated to affect more than one in five adults, and its prevalence is growing in the adult and pediatric populations. The most widely recognized metabolic risk factors are atherogenic dyslipidemia, elevated blood pressure, and elevated plasma glucose. Individuals with these characteristics commonly manifest a prothrombotic state and a proinflammatory state as well. Peroxisome proliferator-activated receptors (PPARs) may serve as potential therapeutic targets for treating the metabolic syndrome and its related risk factors. The PPARs are transcriptional factors belonging to the ligand-activated nuclear receptor superfamily. So far, three isoforms of PPARs have been identified, namely, PPAR- α, PPAR-β/δ, and PPAR-γ. Various endogenous and exogenous ligands of PPARs have been identified. PPAR- α and PPAR- γ are mainly involved in regulating lipid metabolism, insulin sensitivity, and glucose homeostasis, and their agonists are used in the treatment of hyperlipidemia and T2DM. Whereas PPAR- β / δ function is to regulate lipid metabolism, glucose homeostasis, anti-inflammation, and fatty acid oxidation and its agonists are used in the treatment of metabolic syndrome and cardiovascular diseases. This review mainly focuses on the biological role of PPARs in gene regulation and metabolic diseases, with particular focus on the therapeutic potential of PPAR modulators in the treatment of thrombosis.

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.

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.