Pituitary adenylate cyclase-activating polypeptide enhances Ca(2+)-dependent neurotransmitter release from PC12 cells and cultured cerebellar granule cells without affecting intracellular Ca(2+) mobilization

Peroxisome proliferator-activated receptor gamma (PPAR gamma) belongs to a nuclear receptor super family that functions as a master regulator of adipocyte differentiation. PPAR gamma binds its DNA response element together with a partner, retinoid X receptor (RXR), in fat cells. Five RXR ligands (HX600, HX630, DA022, DA124, LGD1069, referred to as retinoid synergists) by themselves exhibit weak transactivation activity on the PPAR gamma response element. However, addition of PPAR gamma-specific ligand in this assay gave rise to a 5- to 13-fold increase, indicating a strong synergy between these ligands. LGD1069 was the most effective activator of the RXR/PPAR gamma heterodimer on the transactivation of the reporter gene. But, in contrast to the other four RXR ligands, LGD1069 did not show synergistic induction of ST 13 preadipocytes to adipocytes. This apparent contradiction may result from the ligand-binding property of LGD1069. In this article we discuss the fact that retinoid synergists also act as PPAR gamma synergists.     

In vivo activation of PPAR target genes by RXR homodimers

The ability of a retinoid X receptor (RXR) to heterodimerize with many nuclear receptors, including LXR, PPAR, NGF1B and RAR, underscores its pivotal role within the nuclear receptor superfamily. Among these heterodimers, PPAR:RXR is considered an important signalling mediator of both PPAR ligands, such as fatty acids, and 9-cis retinoic acid (9-cis RA), an RXR ligand. In contrast, the existence of an RXR/9-cis RA signalling pathway independent of PPAR or any other dimerization partner remains disputed. Using in vivo chromatin immunoprecipitation, we now show that RXR homodimers can selectively bind to functional PPREs and induce transactivation. At the molecular level, this pathway requires stabilization of the homodimer-DNA complexes through ligand-dependent interaction with the coactivator SRC1 or TIF2. This pathway operates both in the absence and in the presence of PPAR, as assessed in cells carrying inactivating mutations in PPAR genes and in wild-type cells. In addition, this signalling pathway via PPREs is fully functional and can rescue the severe hypothermia phenotype observed in fasted PPARalpha-/- mice. These observations have important pharmacological implications for the development of new rexinoid-based treatments.     

Genome-wide profiling of liver X receptor, retinoid X receptor, and peroxisome proliferator-activated receptor α in mouse liver reveals extensive sharing of binding sites

The liver X receptors (LXRs) are nuclear receptors that form permissive heterodimers with retinoid X receptor (RXR) and are important regulators of lipid metabolism in the liver. We have recently shown that RXR agonist-induced hypertriglyceridemia and hepatic steatosis in mice are dependent on LXRs and correlate with an LXR-dependent hepatic induction of lipogenic genes. To further investigate the roles of RXR and LXR in the regulation of hepatic gene expression, we have mapped the ligand-regulated genome-wide binding of these factors in mouse liver. We find that the RXR agonist bexarotene primarily increases the genomic binding of RXR, whereas the LXR agonist T0901317 greatly increases both LXR and RXR binding. Functional annotation of putative direct LXR target genes revealed a significant association with classical LXR-regulated pathways as well as peroxisome proliferator-activated receptor (PPAR) signaling pathways, and subsequent chromatin immunoprecipitation-sequencing (ChIP-seq) mapping of PPARα binding demonstrated binding of PPARα to 71 to 88% of the identified LXR-RXR binding sites. The combination of sequence analysis of shared binding regions and sequential ChIP on selected sites indicate that LXR-RXR and PPARα-RXR bind to degenerate response elements in a mutually exclusive manner. Together, our findings suggest extensive and unexpected cross talk between hepatic LXR and PPARα at the level of binding to shared genomic sites.     

肿瘤鞘脂代谢异常与肿瘤细胞对维甲酸类药物耐药性

维甲酸类药物对多种癌症有效,其作用包括诱导凋亡、抑制生长、促进分化等,这主要通过调节维甲酸受体包括维甲酸受体(retinoic acid receptor,RAR)和维甲酸X受体(rexinoid X receptor,RXR)的表达实现。目前发现,一些患者癌细胞的RAR、RXR或RAR/RXR表达缺乏,可能导致癌细胞对维甲酸产生耐药性。鞘脂代谢异常和维甲酸类受体表达缺失密切相关,在癌细胞对维甲酸产生耐药性中发挥着重要作用。本文就鞘脂代谢异常与维甲酸受体表达异常及维甲酸类药物耐药的相关性做一简要综述。     

Regulation of adiponectin receptor 1 in human hepatocytes by agonists of nuclear receptors

The adiponectin receptors AdipoR1 and AdipoR2 have been identified to mediate the insulin-sensitizing effects of adiponectin. Although AdipoR2 was suggested to be the main receptor for this adipokine in hepatocytes, AdipoR1 protein is highly abundant in primary human hepatocytes and hepatocytic cell lines. Nuclear receptors are main regulators of lipid metabolism and activation of peroxisome proliferator-activated receptor alpha and gamma, retinoid X receptor (RXR), and liver X receptor (LXR) by specific ligands may influence AdipoR1 abundance. AdipoR1 protein is neither altered by RXR or LXR agonists nor by pioglitazone. In contrast, fenofibric acid reduces AdipoR1 whereas hepatotoxic troglitazone upregulates AdipoR1 protein in HepG2 cells. Taken together this work shows for the first time that AdipoR1 protein is expressed in human hepatocytes but that it is not a direct target gene of nuclear receptors. Elevated AdipoR1 induced by hepatotoxic troglitazone may indicate a role of this receptor in adiponectin-mediated beneficial effects in liver damage.     

P450 gene induction by structurally diverse xenochemicals: central role of nuclear receptors CAR, PXR, and PPAR.

The biochemistry of foreign compound metabolism and the roles played by individual cytochrome P450 (CYP) enzymes in drug metabolism and in the toxification and detoxification of xenochemicals prevalent in the environment are important areas of molecular pharmacology and toxicology that have been widely studied over the past decade. Important advances in our understanding of the mechanisms through which foreign chemicals impact on these P450-dependent metabolic processes have been made during the past 2 years with several key discoveries relating to the mechanisms through which xenochemicals induce the expression of hepatic P450 enzymes. Roles for three "orphan" nuclear receptor superfamily members, designated CAR, PXR, and PPAR, in respectively mediating the induction of hepatic P450s belonging to families CYP2, CYP3, and CYP4 in response to the prototypical inducers phenobarbital (CAR), pregnenolone 16alpha-carbonitrile and rifampicin (PXR), and clofibric acid (PPAR) have now been established. Two other nuclear receptors, designated LXR and FXR, which are respectively activated by oxysterols and bile acids, also play a role in liver P450 expression, in this case regulation of P450 cholesterol 7alpha-hydroxylase, a key enzyme of bile acid biosynthesis. All five P450-regulatory nuclear receptors belong to the same nuclear receptor gene family (family NR1), share a common heterodimerization partner, retinoid X-receptor (RXR), and are subject to cross-talk interactions with other nuclear receptors and with a broad range of other intracellular signaling pathways, including those activated by certain cytokines and growth factors. Endogenous ligands of each of those nuclear receptors have been identified and physiological receptor functions are emerging, leading to the proposal that these receptors may primarily serve to modulate hepatic P450 activity in response to endogenous dietary or hormonal stimuli. Accordingly, P450 induction by xenobiotics may in some cases lead to a perturbation of endogenous regulatory circuits with associated pathophysiological consequences.     

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.