Substitution of a conserved amino acid residue alters the ligand binding properties of peroxisome proliferator activated receptors

Cannabinoid activation of PPAR alpha; a novel neuroprotective mechanism

BACKGROUND AND PURPOSE

Although CB(1) receptor activation evokes neuroprotection in response to cannabinoids, some cannabinoids have been reported to be peroxisome proliferator activated receptor (PPAR) ligands, offering an alternative protective mechanism. We have, therefore, investigated the ability of a range of cannabinoids to activate PPAR alpha and for N-oleoylethanolamine (OEA), an endogenous cannabinoid-like compound (ECL), to evoke neuroprotection.

EXPERIMENTAL APPROACH

Assays of PPAR alpha occupancy and gene transactivation potential were conducted in cell-free and transfected HeLa cell preparations, respectively. In vivo estimates of PPAR alpha activation through fat mobilization and gene transcription were conducted in mice. Neuroprotection in vivo was investigated in wild-type and PPAR alpha gene-disrupted mice.

KEY RESULTS

The ECLs OEA, anandamide, noladin ether and virodhamine were found to bind to the purified PPAR alpha ligand binding domain and to increase PPAR alpha-driven transcriptional activity. The high affinity synthetic CB(1/2) cannabinoid agonist WIN 55212-2 bound to PPAR alpha equipotently with the PPARalpha agonist fenofibrate, and stimulated PPARalpha-mediated gene transcription. The phytocannabinoid delta 9 tetrahydrocannabinol was without effect. OEA and WIN 55212-2 induced lipolysis in vivo, while OEA pre-treatment reduced infarct volume from middle cerebral artery occlusion in wild-type, but not in PPAR alpha-null mice. OEA treatment also led to increased expression of the NFkappa B-inhibitory protein, Ikappa B, in mouse cerebral cortex, while expression of the NFkappa B-regulated protein COX-2 was inhibited.

CONCLUSIONS AND IMPLICATIONS

These data demonstrate the potential for a range of cannabinoid compounds, of diverse structures, to activate PPAR alpha and suggest that at least some of the neuroprotective properties of these agents could be mediated by nuclear receptor activation.

Ligand-binding regulation of LXR/RXR and LXR/PPAR heterodimerizations: SPR technology-based kinetic analysis correlated with molecular dynamics simulation

Liver X receptor (LXR) and peroxisome proliferator-activated receptor (PPAR) are two members of nuclear receptors involved in the nutrient metabolisms of dietary fatty acid and cholesterol. They are found to be of cross-talk function in that LXR regulates fatty acid synthesis and PPAR controls fatty acid degradation. LXRs (LXRalpha and LXRbeta) function by forming obligate heterodimers with the retinoid X receptor (RXR), and subsequently binding to specific DNA response elements within the regulatory regions of their target genes. In this work, the kinetic features concerning LXR/RXR and LXR/PPAR interactions have been fully investigated using surface plasmon resonance (SPR) technology. It is found that LXRs could bind to all the three PPAR subtypes, PPARalpha, PPARgamma and PPARdelta with different binding affinities, and such receptor/receptor interactions could be regulated by ligand binding. Moreover, molecular dynamics (MD) simulations were performed on six typical complex models. The results revealed that ligands may increase the interaction energies between the receptor interfaces of the simulated receptor/receptor complexes. The MD results are in agreement with the SPR data. Further analyses on the MD results indicated that the ligand binding might increase the hydrogen bonds between the interfaces of the receptor/receptor complex.

Ertiprotafib improves glycemic control and lowers lipids via multiple mechanisms

Ertiprotafib belongs to a novel class of insulin sensitizers developed for treatment of type 2 diabetes. In insulin-resistant rodent models, ertiprotafib and a close analog lowered both fasting blood glucose and insulin levels and improved glycemic excursion during an oral glucose tolerance test. In addition, treatment of rodents improved lipid profiles, with significantly lowered triglyceride and free fatty acid levels. These results suggested that this therapeutic activity might involve mechanisms in addition to PTP1b inhibition. In this study, we demonstrate that ertiprotafib activates peroxisome proliferator-activated receptor (PPAR)alpha and PPARgamma at concentrations comparable with those of known agonists of these regulators. Furthermore, it is able to drive adipocyte differentiation of C3H10T(1/2) cells, a hallmark of PPARgamma activation. Livers from ertiprotafib-treated animals showed significant induction of acyl-CoA oxidase activity, probably caused by PPARalpha engagement in these animals. We also show that ertiprotafib inhibits PTP1b in vitro with nonclassic kinetics at concentrations above its EC(50) for PPAR agonism. Thus, the complete mechanism of action for ertiprotafib and related compounds in vivo may involve multiple independent mechanisms, including (but not necessarily limited to) PTP1b inhibition and dual PPARalpha/PPARgamma agonism. Ertiprotafib pharmacology and interpretation of clinical results must be seen in light of this complexity.

Molecular modelling of the peroxisome proliferator-activated receptor alpha (PPAR alpha) from human, rat and mouse, based on homology with the human PPAR gamma crystal structure

The generation of homology models of human, rat and mouse peroxisome proliferator-activated receptor alpha (PPAR alpha) are reported, based on the recently published crystal structure of the human PPAR gamma ligand-binding domain (LBD) with bound ligand, rosiglitazone. It is found that a template of peroxisome proliferating fibrate drugs and related compounds can fit within the putative ligand-binding site of rat PPAR alpha, via contacts with amino acid residues which are consistent with their biological potency for peroxisome proliferation, site-directed mutagenesis experiments and with quantitative structure-activity relationship (QSAR) analysis studies. The experimental binding affinity of leukotriene B(4) (LTB(4)) for the mouse PPAR alpha agrees closely with the calculated value based on the modelled interactions, whereas selective PPAR alpha ligands such as clofibric acid are able to fit the human PPAR alpha binding site in agreement with reported site-directed mutagenesis information.

Identification of peroxisome proliferator-responsive human genes by elevated expression of the peroxisome proliferator-activated receptor alpha in HepG2 cells

In mice and other sensitive species, PPARalpha mediates the induction of mitochondrial, microsomal, and peroxisomal fatty acid oxidation, peroxisome proliferation, liver enlargement, and tumors by peroxisome proliferators. In order to identify PPARalpha-responsive human genes, HepG2 cells were engineered to express PPARalpha at concentrations similar to mouse liver. This resulted in the dramatic induction of mRNAs encoding the mitochondrial HMG-CoA synthase and increases in fatty acyl-CoA synthetase (3-8-fold) and carnitine palmitoyl-CoA transferase IA (2-4-fold) mRNAs that were dependent on PPARalpha expression and enhanced by exposure to the PPARalpha agonist Wy14643. A PPAR response element was identified in the proximal promoter of the human HMG-CoA synthase gene that is functional in its native context. These data suggest that humans retain a capacity for PPARalpha regulation of mitochondrial fatty acid oxidation and ketogenesis. Human liver is refractory to peroxisome proliferation, and increased expression of mRNAs for the peroxisomal fatty acyl-CoA oxidase, bifunctional enzyme, or thiolase, which accompanies peroxisome proliferation in responsive species, was not evident following Wy14643 treatment of cells expressing elevated levels of PPARalpha. Additionally, no significant differences were seen for the expression of apolipoprotein AI, AII, or CIII; medium chain acyl-CoA dehydrogenase; or stearoyl-CoA desaturase mRNAs.