VitaeGene PPARs

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PPARs = Peroxisome Proliferator-Activated Receptor

In the field of molecular biology, the peroxisome proliferator-activated receptors (PPARs) are a group of nuclear receptor proteins that function as transcription factors regulating the expression of genes. PPARs play essential roles in the regulation of cellular differentiation, development, and metabolism (carbohydrate, lipid, protein), and tumorigenesis of higher organisms.

Three types of PPARs have been identified: Alpha, Beta (Delta), and Gamma

Alpha

PPAR α (alpha)

expressed in the Liver, Kidney, Heart, Muscle, Adipose tissue, and others

Beta/Delta

PPAR β/δ (beta/delta)

expressed in many tissues but markedly in the Brain, Adipose tissue, and Skin

Gamma

PPAR γ (gamma)

although transcribed by gamma, this PPAR through alternative splicing is expressed in three forms

PPAR Gamma (γ)

γ1

expressed in virtually all tissues, including heart, muscle, colon, kidney, pancreas, and spleen

γ2

expressed mainly in adipose tissue (30 amino acids longer)

γ3

expressed in macrophages, large intestine, white adipose tissue

PPAR Agonists

PPAR agonists are drugs which act upon the peroxisome proliferator-activated receptor. They are .used for the treatment of symptoms of the metabolic syndrome, mainly for lowering triglycerides and blood sugar

PPAR-alpha agonists

PPARα (alpha) is the main target of fibrate drugs, a class of amphipathic carboxylic acids (clofibrate, gemfibrozil, ciprofibrate, bezafibrate, and fenofibrate). They were originally indicated for cholesterol disorders and more recently for disorders that feature high triglycerides.

PPAR-delta agonists

PPARδ (delta) is the main target of a research chemical named GW501516. It has been shown that agonism of PPARδ changes the body’s fuel preference from glucose to lipids, but ironically improves metabolic syndrome (which is characterised by the body being unable to efficiently deal with glucose resulting in insulin resistance and sometimes diabetes).

PPAR-gamma agonists

PPARγ (gamma) is the main target of the drug class of thiazolidinediones (TZDs), used in diabetes mellitus and other diseases that feature insulin resistance. It is also mildly activated by certain NSAIDs (such as ibuprofen) and indoles, as well as from a number of natural compounds. Known inhibitors include the experimental agent GW-9662. They are also used in treating hyperlipidaemia in atherosclerosis. Here they act by increasing the expression of ABCA1, which transports extra-hepatic cholesterol into HDL. Increased uptake and excretion from the liver therefore follows. Animal studies have shown their possible role in amelioration of pulmonary inflammation, especially in asthma.

History of PPARs

PPARs were originally identified in Xenopus frogs as receptors that induce the proliferation of peroxisomes in cells. The first PPAR (PPARα) was discovered during the search of a molecular target for a group of agents then referred to as peroxisome proliferators, as they increased peroxisomal numbers in rodent liver tissue, apart from improving insulin sensitivity. These agents, pharmacologically related to the fibrates were discovered in the early 1980s. When it turned out that PPARs played a much more versatile role in biology, the agents were in turn termed PPAR ligands. The best-known PPAR ligands are the thiazolidinediones;.

After PPARδ (delta) was identified in humans in 1992, it turned out to be closely related to the PPARβ (beta) previously described during the same year in other animals (Xenopus). The name PPARδ is generally used in the US, whereas the use of the PPARβ denomination has remained in Europe where this receptor was initially discovered in Xenopus.

Physiological Function

All PPARs heterodimerize with the retinoid X receptor (RXR) and bind to specific regions on the DNA of target genes. These DNA sequences are termed PPREs (peroxisome proliferator hormone response elements). The DNA consensus sequence is AGGTCANAGGTCA, with N being any nucleotide. In general, this sequence occurs in the promoter region of a gene, and, when the PPAR binds its ligand, transcription of target genes is increased or decreased, depending on the gene. The RXR also forms a heterodimer with several other receptors (e.g., vitamin D and thyroid hormone).

The function of PPARs is modified by the precise shape of their ligand-binding domain induced by ligand binding and by a number of coactivator and corepressor proteins, the presence of which can stimulate or inhibit receptor function, respectively.

Endogenous ligands for the PPARs include free fatty acids and eicosanoids. PPARγ is activated by PGJ2 (a prostaglandin) and certain members of the 5-HETE family of arachidonic acid metabolites including 5-oxo-15(S)-HETE and 5-oxo-ETE. In contrast, PPARα is activated by leukotriene B4. Certain members of the 15-Hydroxyicosatetraenoic acid family of arachidonic acid metabolites, including 15(S)-HETE, 15(R)-HETE, and 15-HpETE activate to varying degrees PPAR alpha, beta/delta, and gamma. PPARγ activation by agonist RS5444 may inhibit anaplastic thyroid cancer growth.