PPAR-gamma

Introduction

Peroxisome proliferator-activated receptor gamma (γ) is a protein in the nuclear receptors subfamily. It is one of three isotypes (-α, -β/ δ, and -γ) [1] of PPAR receptors and has two protein isoforms governed by splice variations, which result in differences in the length of the amino (N)-terminal region (PPARγ1 and PPARγ2) [2]. PPARγ is involved in transcriptional regulation of glucose and lipid homeostasis [1], and helps regulate adipocyte differentiation [3]. It has a , which allows it to interact with a wide array of ligands. typically triggers a conformational change of PPARγ, notably in the activation function-2 , which aids in the recruitment of co-regulatory factors to regulate gene transcription. PPARγ can form a with retinoic X receptor alpha (RXRα), a process necessary for most PPARγ-DNA interactions [4]. PPARγ is a molecular target for antidiabetic drugs such as thiazolidinediones (TZDs), which makes the protein a target for Type II Diabetes (T2D) drug research. Due to its involvement in metabolic and inflammatory processes, PPARγ also holds potential for treatments of many metabolic and chronic-inflammatory diseases, such as metabolic syndrome and inflammatory bowel disease, respectively. Errors in PPARγ-related regulation have also been implicated in atherosclerosis and various cancers, like colorectal, breast, and prostate cancers.

See also Intracellular receptors

Overall Structure and Ligand Binding

PPARγ is composed of the ligand-independent activation domain (AF-1 region and A/B-domain), a DNA-binding domain (DBD) (C-domain), a hinge region (D-domain), and a ligand-dependent ligand-binding domain (LBD) (E/F-domain and AF-2 region) [5]. The two PPARγ isoforms, PPARγ1 and PPARγ2, differ by only 30 amino acids at the N-terminal end. These added amino acids on PPARγ2 result in increased potency and adipose-selectivity, which makes this protein a key player of adipocyte differentiation [3]. The is composed of 13 α helices and 4 short β strands [1]. It has a T-shaped binding pocket with a volume of ~1440 Å3 [1, 6], which is larger than that of most nuclear receptors [7], allowing for interactions with a variety of ligands [8]. The PPARγ LBD is folded into a helical sandwich to provide a binding site for ligands. It is located at the C-terminal end of PPARγ and is composed of about 250 amino acids [5]. Activation by full agonists occurs through hydrogen bond interactions between the S289, H323, Y473, and H449 residues of the PPARγ-LBD [7] and polar functional groups on the ligand which are typically carbonyl or carboxyl oxygen atoms. Agonist binding results in a conformational change of the LBD AF-2 region, which is necessary for coactivator recruitment. This change can either be dramatic or subtle [1], which leads to stabilization of a charge clamp between helices H3 and H12 [9] to aid in associations with the LXXLL (L, leucine; X, any amino acid) motif of the coactivator [1, 10]. Ligand binding of PPARγ is regulated by communication between the N-terminal A/B domain, which is adjacent to the DBD, and the carboxyl-terminal LBD [11].

Ligand Activity

PPARγ ligands, fall into one of three categories: full agonist, partial agonist, or antagonist [5]. Full agonists have higher efficacy for activating PPARγ and higher potency [7], and their binding leads to the more dramatic conformational change [1]. Binding of partial agonists leads to the more subtle change [1] and results in lower efficacy and potency [7]. Antagonists do not activate PPARγ, so there is either no conformational change to exclude coactivators or a minor conformational change to accommodate corepressors [9,5]. Natural ligands of PPARγ include fatty acids, eicosanoids, and prostaglandins [4-8,11-13].

Coactivators/Corepressors

The of PPARγ is a groove created by hydrophobic residues of the H3, H3’, H4, and H12 helices [1]. Stabilization of the AF-2 domain is important for coactivator interactions, and is achieved through ligand binding [1]. Upon agonist binding, coactivators and other chromatin-remodeling cofactors, like histone deacetylases, are recruited and transcription is activated [14]. Coactivators can be regulated at the transcriptional and post-transcriptional levels, as well as by protein-kinase cascades [3]. PPARγ can actively silence genes it is bound to by recruiting a corepressor in the absence of a ligand. Once this occurs, an antagonist binds to stabilize the AF-2 region, preventing interactions with coactivators and activation of transcription [9]. Corepressor binding creates a three-turn α-helix corepressor motif important for preventing the AF-2 domain from assuming an active conformation [9]. Common coactivators of PPARγ include CBP/p300, the SRC family, and TRAP220 [3]. Common corepressors include SMART, NCoR, and RIP140 [3].

PPARγ/RXRα

PPARγ shows preferential heterodimerization with RXRα [3]. The asymmetry of PPARγ/RXRα packs positively and negatively charged regions together, and is needed for PPARγ binding to DNA [1]. The LBD and DBD of PPARγ are located close together, whereas the RXRα LBD and DBD are positioned farther apart. This difference in region proximity plays a role in heterodimerization [5]. The PPARγ/RXRα complex associates with PPAR response elements (PPREs) in promoter regions of targeted genes [8]. Each ligand-bound PPAR/RXRα complex will bind to a specific PPRE based on the recruited cofactor [8].

Functions

PPARγ is a key regulator of glucose and lipid homeostasis [1]. PPARγ mediates adipocyte differentiation and alters insulin sensitivity, inflammatory processes, and cell proliferation [11]. Ligand-dependent mechanisms include inhibition of inflammatory cytokine production and macrophage activation [8]. In addition to adipocytes, PPARγ is also found in the retina, cells of the immune system, and colonic epithelial cells [5]. PPARγ controls the expression of various genes, particularly those involved in fatty acid metabolism [5]. Regulated genes include adipocyte fatty acid binding protein (aP2), lipoprotein lipase (LPL), and acyl-CoA oxidase [4]. Tissue-specific deletions of PPARγ lead to insulin resistance, low viability of mature adipocytes, and lipodystrophy [3].

Potential of PPARγ in Disease Treatment

Synthetic TZDs were the first class of PPARγ ligands identified [8]. The most potent and selective agonist of PPARγ from this class is the insulin sensitizer rosiglitazone [8]. T2D is linked to higher levels of free fatty acids (FFAs) and triglycerides in the blood [8], which is a contributing factor for insulin resistance. Treatment with rosiglitazone and other TZDs reduces levels of FFAs and triglycerides, helping to restore insulin sensitivity [8]. TZDs also work by increasing levels of GLUT-4 and decreasing levels of pro-inflammatory cytokines [5]. PPARγ is found in high levels in colonic epithelial cells. The role of PPARγ in these cells may be related to regulation of immune response and colon inflammation [12]. The onset of Inflammatory Bowel Disease is thought to be caused by inflammatory cytokines present in the colon [12]. In patients with ulcerative colitis, colonic epithelial cells displayed impaired expression of PPARγ, an important mediator of aminosalicylate activities in Inflammatory Bowel Diseases [13]. TZD ligands could be implemented to reduce colonic inflammation [12]. Agonists have also been used in the treatment of colitis and psoriasis by inhibiting the inflammatory response of the epithelium and reducing cytokine production [8]. PPARγ inhibits activity of nuclear factor NFκB, which is higher in active ulcerative colitis patients [15]. PPARγ could also be implemented in the treatment of other chronic inflammation-related diseases. Immunomodulatory effects have been found with PPARγ agonists [16]. Rosiglitazone alongside adiponectin reduces renal disease, atherosclerosis, and production of autoantibodies, all of which are characteristic of the inflammatory autoimmune disease Systemic Lupus Erythematosus (SLE) [16]. PPARγ ligands hold potential as cancer treatments [11] due to their ability to inhibit angiogenesis, the process required for the growth and metastasis of solid tumors [8]. PPARγ activators have pro-differentiation and anti-proliferation effects [3]. TZDs have also been shown to inhibit proliferation of human breast, prostate, and colon cancer cells [8].