Peroxisome proliferator-activated receptor ( PPARs )
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, gamma, and delta (beta):
PPAR α (alpha) – expressed in liver, kidney, heart, muscle, adipose tissue, and others
PPAR β/δ (beta/delta) – expressed in many tissues but markedly in brain, adipose tissue, and skin
PPAR γ (gamma) – although transcribed by the same gene, this PPAR through alternative splicing is expressed in three forms:
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 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.
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.
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.
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 characterized by the body being unable to efficiently deal with glucose resulting in insulin resistance and sometimes diabetes).
Dual and pan PPAR agonists
A fourth class of dual PPAR agonists, so-called glitazars, which bind to both the α and γ PPAR isoforms, are currently under active investigation for treatment of a larger subset of the symptoms of the metabolic syndrome. These include the experimental compounds aleglitazar, muraglitazar and tesaglitazar. In June 2013, saroglitazar was the first glitazar to be approved for clinical use.
In addition, there is continuing research and development of new dual α/δ and γ/δ PPAR agonists for additional therapeutic indications, as well as “pan” agonists acting on all three isoforms.
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.
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 a number of 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.
The three main forms are transcribed from different genes:
PPARα – chromosome 22q12-13.1 (OMIM 170998)
PPARβ/δ – chromosome 6p21.2-21.1 (OMIM 600409)
PPARγ – chromosome 3p25 (OMIM 601487).
Hereditary disorders of all PPARs have been described, generally leading to a loss in function and concomitant lipodystrophy, insulin resistance, and/or acanthosis nigricans. Of PPARγ, a gain-of-function mutation has been described and studied (Pro12Ala) which decreased the risk of insulin resistance; it is quite prevalent (allele frequency 0.03 – 0.12 in some populations). In contrast, pro115gln is associated with obesity. Some other polymorphisms have high incidence in populations with elevated body mass indexes.