2.1. Intracellular Receptor of PPARγ
PPARs are members of the nuclear hormone receptor superfamily, many of which function as lipid-activated transcription factors [1]. There are three PPAR isoforms that include PPARα, β/δ, and γ that differ in ligand specificity, tissue distribution, and developmental expression [19]. PPARγ, the most extensively studied among the three PPAR subtypes, plays an important role in regulating lipid metabolism, glucose homeostasis, cell differentiation, and motility [10, 20]. There are 2 PPARγ isoforms, PPARγ1 and PPARγ2. PPARγ2 has 30 additional amino acids at the N-terminus in human caused by differential promoter usage and alternative splicing [21]. Genetic deletion of PPARγ1 causes embryonic mortality [9]. In contrast, deletion of PPARγ2 causes minimal alterations in lipid metabolism [22]. PPARγ1 is expressed in almost all tissues, whereas PPARγ2 is highly expressed in only the adipose tissue [21]. PPARγ is comprised of four functional parts: the N-terminal A/B region bears a ligand-independent transcription-activating motif AF-1; C region binds response elements; D region binds to various transcription cofactors; and E/F region has an interface for dimerizing with retinoid X receptor α (RXRα), an AF-2 ligand-dependent transcription-activating motif, and a ligand binding domain (LBD) [23]. PPARγ heterodimerizes with the retinoid X receptor α (RXRα), and it is the ligand binding domain (LBD) of PPARγ that interacts with its agonists, including LPA [3]. The PPARγ-RXRα heterodimer binds to the peroxisome proliferator response element (PPRE) in the promoter region of the target genes. In the absence of ligands, the corepressors, nuclear receptor corepressor (NCoR) and silencing mediator of retinoid (SMRT) and thyroid hormone, bind to the heterodimer to suppress the target gene activation [24]. Upon ligand binding, PPARγ undergoes a conformational change that facilitates the dissociation of the corepressors and recruits coactivators. According to their mechanism of action, coactivators can be divided into two large families: the former includes steroid receptor coactivator (SRC-1) and CBP/p300, that act in part as molecular scaffolds and in the other part by acetylating divers substrates. The latter, including peroxisome proliferator-activated receptor 1α (PGC-1α), does not act by remodeling chromatin [25]. It has been reported that DNA methylation and histone modification serve as epigenetic markers for active or inactive chromatin [26]. A variety of putative physiological PPARγ agonists have been identified [5, 27]. Since then, we and other authors have reported that selected forms of lysophospholipids, such as unsaturated LPA and alkyl glycerophosphate (AGP, 1-alkyl-2-hydroxy-sn-glycerol-3-phosphate), are physiological agonists of PPARγ [3, 4]. The different molecular species of LPA contain either saturated or unsaturated fatty acids. Saturated LPA species including palmitoyl (16 : 0) and stearoyl (18 : 0) LPA are inactive. Among these ligands, AGP stands out with an equilibrium binding constant of 60 nM [4] that is similar to that of thiazolidinedione (TZD) class of synthetic agonists. Interestingly, some of the residues required for PPARγ activation by AGP are different from those required by TZD drug. H323 and 449 within the LBD of PPARγ are required for the binding and activation by rosiglitazone but are not required by AGP. R288 is an important residue for the binding of the AGP but not the rosiglitazone. Y273 is required for activation by both agonists [4]. AGP is unique in that its potency far exceeds that of LPA in activating PPARγ [4]. The reason why AGP and unsaturated acyl-LPA species are the best activators of PPARγ may reflect the differential delivery of these LPA analogs to PPARγ versus saturated LPA species, which are inactive. Together, these data help to explain why PPARγ binds the unsaturated LPA and AGP but not saturated LPA. On the other hand, we showed that cPA negatively regulates PPARγ functions by stabilizing the SMRT-PPARγ complex [15] and blocks TZD-stimulated adipogenesis and lipid accumulation. This ligand-dependent corepressor exchange results in transcriptional repression of genes involved in the control of insulin action as well as a diverse range of other functions.