Liver X Receptors Regulate the Transcriptional Activity of the Glucocorticoid Receptor: Implications for the Carbohydrate Metabolism

GLUCOCORTICOIDS are steroid hormones that strongly influence intermediary carbohydrate metabolism by increasing the transcription rate of glucose-6-phosphatase (G6Pase), a key enzyme of gluconeogenesis, and suppress the immune system through the glucocorticoid receptor (GR). The liver X receptors (LXRs), on the other hand, bind to cholesterol metabolites, heterodimerize with the retinoid X receptor (RXR), and regulate the cholesterol turnover, the hepatic glucose metabolism by decreasing the expression of G6Pase, and repress a set of inflammatory genes in immune cells. Since the actions of these receptors overlap with each other, we evaluated the crosstalk between the GR- and LXR-mediated signaling systems. Transient transfection-based reporter assays and gene silencing methods using siRNAs for LXRs showed that overexpression/ligand (GW3965) activation of LXRs/RXRs repressed GR-stimulated transactivation of certain glucocorticoid response element (GRE)-driven promoters in a gene-specific fashion. Activation of LXRs by GW3965 attenuated dexamethasone-stimulated elevation of circulating glucose in rats. It also suppressed dexamethasone-induced mRNA expression of hepatic glucose-6-phosphatase (G6Pase) in rats, mice and human hepatoma HepG2 cells, whereas endogenous, unliganded LXRs were required for dexamethasone-induced mRNA expression of phosphoenolpyruvate carboxylase. In microarray transcriptomic analysis of rat liver, GW3965 differentially regulated glucocorticoid-induced transcriptional activity of about 15% of endogenous glucocorticoid-responsive genes. To examine the mechanism through which activated LXRs attenuated GR transcriptional activity, we examined LXRα/RXRα binding to GREs. Endogenous LXRα/RXRα bound GREs and inhibited GR binding to these DNA sequences both in in vitro and in vivo chromatin immunoprecipitation assays, while their recombinant proteins did so on classic or G6Pase GREs in gel mobility shift assays. We propose that administration of LXR agonists may be beneficial in glucocorticoid treatment- or stress-associated dysmetabolic states by directly and gene-specifically attenuating the transcriptional activity of the GR on glucose and/or lipid metabolism.

糖皮质激素（glucocorticoids）是一类类固醇激素，可通过提升糖异生关键酶葡萄糖-6-磷酸酶（glucose-6-phosphatase, G6Pase）的转录速率，强力调控碳水化合物中间代谢，并通过糖皮质激素受体（glucocorticoid receptor, GR）抑制免疫系统功能。与之相对，肝X受体（liver X receptors, LXRs）可结合胆固醇代谢物，与维甲酸X受体（retinoid X receptor, RXR）形成异二聚体，调控胆固醇代谢周转；其可通过降低G6Pase的表达调控肝脏糖代谢，并抑制免疫细胞内一组炎症基因的表达。鉴于这两类受体的作用存在重叠，本研究评估了GR介导与LXR介导的信号通路之间的串扰。基于瞬时转染的报告基因实验以及使用靶向LXRs的小干扰RNA（small interfering RNA, siRNAs）进行的基因沉默实验结果显示：LXRs/RXRs的过表达或配体（GW3965）激活，可通过基因特异性方式抑制部分糖皮质激素反应元件（glucocorticoid response element, GRE）驱动的启动子的GR诱导型反式激活。GW3965激活LXRs，可减弱大鼠体内地塞米松诱导的循环葡萄糖水平升高；该处理同时可抑制大鼠、小鼠及人肝癌HepG2细胞中地塞米松诱导的肝脏G6Pase的mRNA表达，而内源性未结合配体的LXRs则是地塞米松诱导磷酸烯醇式丙酮酸羧激酶mRNA表达所必需的。在大鼠肝脏的微阵列转录组分析中，GW3965对约15%的内源性糖皮质激素响应基因的糖皮质激素诱导转录活性存在差异性调控。为探究激活的LXRs减弱GR转录活性的机制，本研究检测了LXRα/RXRα与GRE的结合情况。体内外染色质免疫沉淀实验均证实，内源性LXRα/RXRα可结合GRE并抑制GR与这些DNA序列的结合；凝胶迁移实验则显示，二者的重组蛋白可结合经典GRE或G6Pase GRE序列。本研究提出，通过直接且基因特异性地减弱GR在糖代谢和/或脂质代谢中的转录活性，肝X受体激动剂的给药或可改善糖皮质激素治疗相关或应激相关的代谢紊乱状态。