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Nomenclature
Three types of PPARs have been identified: alpha, gamma and delta (beta).
History
What happened to PPARβ (beta)?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 to the exclusion of PPARβ. Physiological functionAll PPARs dimerize 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 response elements). The DNA consensus sequence is AGGTCAXAGGTCA with X being a random nucleotide. Generally, this sequence occurs in the promotor region of a gene, and when the PPAR binds its ligand, transcription of targets genes are increased or decreased, depending on the gene. The RXR also forms a heterodimer with a number of other receptors: the vitamin D receptor and the thyroid hormone receptor. The function of PPARs is modified by the exact shape of their ligand-binding domain (see below) and by a number of co-activators and co-repressors, the presence of which can stimulate or inhibit receptor function.
GeneticsThe three main forms are transcribed from different genes:
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. StructureImage:PPARg.png PPAR gamma All PPARs have a basic structure of functional domains. The most important ones are the DBD (DNA binding domain) and the LBD (ligand binding domain). The DBD contains two zinc finger patterns which bind to the regulator region of DNA when the receptor is activated. The LBD has an extensive secondary structure of several alpha helices (13) and a beta sheet. Natural and synthetic ligands bind to the LBD, activating the receptor. Pharmacology and PPAR modulatorsPPARα and PPARγ are the targets of a number of known medications and are under continuing research for other forms of pharmacological modulation. Muraglitazar and tesaglitazar, both experimental compounds, binds to both PPAR-alpha and PPAR-gamma. PPAR-alpha modulatorsPPAR-alpha is the main target of fibrate drugs, a class of amphipathic carboxylic acids (clofibrate, gemfibrozil, ciprofibrate, bezafibrate and fenofibrate). They are used in cholesterol disorders (generally as an adjunctive to statins) and disorders that feature high triglycerides. PPAR-gamma modulatorsPPAR-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. Known inhibitors include the experimental agent GW-9662. See alsoSources
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