Latchoumanin Olivier, Mynard Vanessa, Devin-Leclerc Jocelyne, Dugué Marie-Annick, Bertagna Xavier, Catelli Maria Grazia
Département d'Endocrinologie, Métabolisme et Cancer, Institut Cochin, F-75014 Paris, France.
Endocrinology. 2007 Jan;148(1):422-32. doi: 10.1210/en.2006-0460. Epub 2006 Oct 12.
We previously have described molecular mechanisms converging at the Nur response element-signal transducer and activator of transcription (STAT) composite site responsible for synergistic activation of the proopiomelanocortin (POMC) gene promoter by leukemia inhibitory factor (LIF) and CRH. In this study, we asked how glucocorticoids (GC), the physiological negative regulators of POMC gene expression, modulate this synergism. In the corticotroph cell line AtT-20, the response of the wild-type promoter to LIF+CRH was barely inhibited by GC, whereas a distal promoter subregion (-414/-293) encompassing the Nur response element-STAT site and devoid of the negative GC-responsive element located in the proximal domain, displayed a cooperative response to LIF+dexamethasone (DEX) and LIF+CRH+DEX treatments. LIF+CRH-stimulated ACTH secretion was also inefficiently inhibited by DEX in the same cell line. This study was focused thereafter on LIF+DEX cooperativity, which may be responsible, on the wild-type promoter, for lack of negative regulation by DEX of the LIF+CRH synergy. The STAT1-3 low-affinity site, in the context of the (-414/-293) subregion of the POMC promoter, was found necessary and sufficient for transcriptional synergism between activated GC receptor (GR) and STAT1-3. Moreover the activities of reporters specific for STAT1-3 or GR were reciprocally enhanced by DEX or LIF. Single and sequential chromatin immunoprecipitations revealed 1) a STAT-dependent corecruitment of coactivators after LIF and LIF+DEX stimulation and 2) a more lasting recruitment of both STAT3 and GR in the same enhanceosome on the endogenous POMC promoter after LIF+DEX joint stimulation than after the single one. Such events may be responsible for a lack of repressive property of GR unmasked on the whole POMC promoter during LIF+CRH stimulation and may contribute to the tonicity of the hypothalamic-pituitary-adrenal axis during inflammatory-infectious diseases.
我们之前已经描述了在Nur反应元件 - 信号转导和转录激活因子（STAT）复合位点汇聚的分子机制，该位点负责白血病抑制因子（LIF）和促肾上腺皮质激素释放激素（CRH）对阿黑皮素原（POMC）基因启动子的协同激活。在本研究中，我们探究了作为POMC基因表达的生理性负调节因子的糖皮质激素（GC）如何调节这种协同作用。在促肾上腺皮质激素细胞系AtT - 20中，野生型启动子对LIF + CRH的反应几乎未被GC抑制，而包含Nur反应元件 - STAT位点且缺乏位于近端结构域的负性GC反应元件的远端启动子亚区域（-414 / -293），对LIF + 地塞米松（DEX）以及LIF + CRH + DEX处理表现出协同反应。在同一细胞系中，LIF + CRH刺激的促肾上腺皮质激素（ACTH）分泌也未被DEX有效抑制。此后，本研究聚焦于LIF + DEX的协同作用，这可能是野生型启动子上DEX对LIF + CRH协同作用缺乏负调节的原因。在POMC启动子的（-414 / -293）亚区域背景下，发现STAT1 - 3低亲和力位点对于活化的糖皮质激素受体（GR）和STAT1 - 3之间的转录协同作用是必要且充分的。此外，DEX或LIF相互增强了针对STAT1 - 3或GR的报告基因的活性。单次和连续染色质免疫沉淀显示：1）LIF和LIF + DEX刺激后，共激活因子的STAT依赖性共募集；2）与单次刺激相比，LIF + DEX联合刺激后，内源性POMC启动子上同一增强体中STAT3和GR的募集更持久。这些事件可能是LIF + CRH刺激期间GR在整个POMC启动子上未表现出抑制特性的原因，并且可能有助于炎症 - 感染性疾病期间下丘脑 - 垂体 - 肾上腺轴的张力维持。
