Recent advances in the TR2 and TR4 orphan receptors of the nuclear receptor superfamily

The human testicular receptor 2 (TR2) and TR4 orphan receptors are two evolutionarily related proteins belonging to the nuclear receptor superfamily. Numerous TR2 and TR4 variants and homologs have been identified from different species, including vertebrates (e.g. human, murine, rabbit, fish, and amphibian) and invertebrates (e.g. Drosophila, sea urchin, and nematode) since TR2 was initially isolated over a decade ago. Specific tissue distribution, genomic organization, and chromosomal assignment of both orphan receptors have been investigated. In order to reveal the physiological functions played by both TR2 and TR4, upstream modulators of TR2 and TR4 gene expression, their downstream target gene regulation, feedback mechanisms, and differential modulation mediated by the recruitment of other nuclear receptors and coregulators have been investigated. Studies summarized in the present report have provided unexpected insights into the TR2 and TR4 functions in a variety of biological processes. The essential and difficult tasks of identifying orphan receptor ligands, agonist/antagonist assignment, their physiological functions, and mechanisms of action will continue to challenge nuclear receptor researchers in the future.     

Negative regulation of parathyroid hormone-related protein expression by steroid hormones

Elevated parathyroid hormone-related protein (PTHrP) is responsible for humoral hypercalcemia of malignancy (HHM), which is of clinical significance in treatment of terminal patients with malignancies. Steroid hormones were known to cause suppression of PTHrP expression. However, detailed studies linking multiple steroid hormones to PTHrP expression are lacking. Here we studied PTHrP expression in response to steroid hormones in four cell lines with excessive PTHrP production. Our study established that steroid hormones negatively regulate PTHrP expression. Vitamin D receptor, estrogen receptor α, glucocorticoid receptor, and progesterone receptor, were required for repression of PTHrP expression by the cognate ligands. A notable exception was the androgen receptor, which was dispensable for suppression of PTHrP expression in androgen-treated cells. We propose a pathway(s) involving nuclear receptors to suppress PTHrP expression.     

Three amino acids specify coactivator choice by retinoid X receptors

Binding of agonists to nuclear receptors results in a conformational change in receptor structure that promotes interaction between activated receptors and coactivators. Receptor-coactivator interactions are mediated by the agonist-dependent formation of a hydrophobic pocket on the part of receptors, and short leucine-rich sequences termed LxxLL motifs or nuclear receptor boxes present in coactivators. RXR-PPARgamma (retinoid X receptor-peroxisome proliferator-activated receptor-gamma) heterodimers play important roles in adipocyte and macrophage differentiation and have been implicated as therapeutic targets in diabetes, atherosclerosis, and cancer. Analysis of interactions between RXR-PPARgamma heterodimers and coactivator nuclear receptor boxes suggests that RXR and PPARgamma can distinguish among coactivators by recognizing distinct structural features of nuclear receptor boxes. The results also indicate that coactivator choice by RXR is mediated by three nonconserved amino acids of the nuclear receptor box. The ability of an optimized seven-amino acid nuclear receptor box to specifically interact with RXR and function as a selective inhibitor suggests the coactivator-binding pocket may serve as a new target for drug discovery.     

Steroid hormone biotransformation and xenobiotic induction of hepatic steroid metabolizing enzymes

Normal reproductive development depends on the interplay of steroid hormones with their receptors at specific tissue sites. The concentrations of hormone ligands in the circulation and at target sites are maintained through coordinated regulation on steroid biosynthesis and degradation. Changed bioavailability of steroids, through alteration of steroidogenesis or biotransformation rates, leads to changes in endocrine function. Steroid hormones lose their receptor reactivity in most cases when they are bound to binding proteins, while metabolic conversion can result in either active or inactive metabolites. Hydroxylation by cytochrome P450 (CYP) enzymes and conjugation with glucuronide and sulfate are among the major hepatic pathways of steroid inactivation. The expression of these biotransformation enzymes can be induced by many xenobiotics. The barbiturate phenobarbital and the environmental toxicant 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene (DDE) are among the well characterized inducers for the CYP 2B and 3A enzymes and selected conjugation enzymes. The induction of the steroid biotransformation enzymes is partly mediated through the activation of a group of nuclear receptors including the glucocorticoid receptor, the constitutive androstane receptor (CAR), the pregnane X receptor (PXR), and the peroxisome proliferator activated receptors (PPAR). Drug or chemical-induced increases in hepatic enzyme activities are often a basis for drug-drug interactions that lead to enhanced elimination and reduced therapeutic efficacy of steroidal drugs. The effects of enzyme induction on endogenous steroid clearance, along with its possible consequence, are less well understood. While enzyme induction by xenobiotics may increase clearance of the endogenous steroid, regulatory mechanisms for steroid homeostasis may adapt and compensate for altered clearance.     

肌钙蛋白I2辅助活化多种核受体的反式作用

为深入研究乳腺癌中孤儿受体ERRα1参与基因表达调控的详细机理,特别是核受体辅激活蛋白在其中的作用,以ERRα1的LBD为诱饵,用酵母双杂交系统筛选人乳腺组织cDNA文库得到了与其有明显相互作用的快速骨骼肌型肌钙蛋白I(TNNI2).应用酵母双杂交技术研究表明,TNNI2与多种核受体存在相互作用,且这种作用依赖于功能性的核受体AF2结构域.在哺乳细胞瞬时共转染实验中,TNNI2显示了对多种核受体反式激活功能的辅助活化作用.研究证明,TNNI2与许多辅激活蛋白类似,以配体依赖(对类固醇激素受体而言)或非依赖(对孤儿受体而言)的方式与核受体功能性AF2结构域相互作用,并增强多种核受体介导的反式作用.