Selective modulators of PPAR activity as new therapeutic tools in metabolic diseases

Peroxisome Proliferator Activated Receptors (PPARs) are regulators of metabolic pathways mainly of lipid metabolism and energy balance. Their medical importance is given by the fact that they have been implicated in development of insulin resistance, obesity and atherosclerosis. In recent years, major progress has been made in understanding the molecular basis of the function of these receptors. As a result of structural studies and identification of putative natural as well as synthetic ligands and activators of PPARs a new concept emerged and new drugs are on their ways to the clinic. The concept of Selective PPAR Modulators (SPPARM) was suggested by analogy to Selective Estrogen Receptor Modulators (SERM). SPPARMs activate the receptors in distinct ways leading to differential gene expression and biological response. The key features in understanding their action is most likely at the molecular details of ligand binding and the subsequently induced conformational changes as well as cofactor binding. A key aspect of this is that unlike classical steroid hormone receptors such as the retinoic acid receptor, the PPAR receptors have a rather large ligand-binding pocket which is not filled with the ligand entirely and the ligand also stabilizes the receptor's structure. The liganded receptor can have distinct conformations and this leads to different binding affinities for the various cofactors (coactivators and corepressors). In this review, we will introduce this concept, review the literature that supports it and present an overview of the receptor selective ligands including data about their mechanism of action and biological effects.