Lipopolysaccharide (LPS) directly suppresses growth hormone receptor (GHR) expression through MyD88-dependent and -independent Toll-like receptor-4/MD2 complex signaling pathways
Lipopolysaccharide (LPS) directly suppresses growth hormone receptor (GHR) expression through MyD88-dependent and -independent Toll-like receptor-4/MD2 complex signaling pathways
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DOI：
10.1016/j.mce.2007.05.013
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发表时间：
2007-08-15
影响因子：
4.1
通讯作者：
Menon, Ram K.
中科院分区：
文献类型：
--
作者：
Dejkhamron, Prapai;Thimmarayappa, Jamuna;Menon, Ram K.
败血症与生长激素（GH）不敏感有关，在完整的动物中，细菌细胞壁的主要表面成分脂多糖（LPS）抑制生长激素受体（GHR）基因表达。目前对脂多糖诱导的GHR启动子效应的普遍解释是，这种效应是通过细胞因子的产生间接产生的。我们最近的研究表明，饱和游离脂肪酸（FFAs）抑制小鼠GHR启动子的活性。饱和脂肪酸是脂多糖生物活性所必需的脂质A部分的重要组成部分。假设：LPS通过与toll样受体(TLR)/MD2复合物的相互作用和同源信号通路的激活，直接调节GHR显性启动子的活性。结果：在表达内源性TLR4和MD2的RAW 264.7细胞瞬时转染实验中，LPS处理抑制了GHR启动子活性。共同转染显性阴性TLR4消除了这种对GHR启动子活性的影响。在缺乏内源性TLR4或MD2的HEK 293T细胞中，TLR4和MD2的异位表达导致lps诱导的GHR启动子活性抑制。对GHR启动子活性的抑制作用在暴露于LPS后5-6小时就可以证明，并持续到24小时。无脂肪酸的LPS未能对GHR启动子产生类似的作用，LPS的作用被多粘菌素b所消除。通过异位表达野生型和突变型MD2的实验，确定了辅助因子MD2对LPS对GHR启动子作用的重要作用。在这些细胞中共转染cd14未能改变LPS对GHR启动子活性的影响。细胞培养上清的分析排除了LPS的作用继发于转染细胞释放细胞因子的可能性。在F442A前脂肪细胞中证实了LPS对内源性GHR启动子活性和蛋白表达的影响。在HEK 293T细胞中，突变体MyD88或突变体TRIF的异位表达消除了LPS对GHR启动子的影响，表明LPS对GHR启动子的影响既通过MyD88依赖性途径，也通过MyD88非依赖性途径。结论：LPS通过myd88依赖性和非依赖性TLR4信号通路直接抑制GHR基因表达。我们的研究结果建立了一种新的不依赖细胞因子的机制来降低细菌败血症中GHR的表达。2007爱思唯尔爱尔兰有限公司版权所有。
Introduction: Sepsis is associated with growth hormone (GH) insensitivity and in the intact animal the major surface component of the bacterial cell wall, lipopolysaccharide (LPS), inhibits GH receptor (GHR) gene expression. The prevailing explanation for LPS-induced effects on the GHR promoter is that this effect is indirect via generation of cytokines. Our recent studies demonstrate that saturated free fatty acids (FFAs) inhibit the activity of the murine GHR promoter. Saturated FFAs are an essential component of the lipid A moiety of LPS required for biological activity of LPS.Hypothesis: LPS directly modulates the activity of the dominant GHR promoter via interaction with Toll-like receptor(s) (TLR)/MD2 complex and activation of cognate signaling pathway(s).Results: In transient transfection experiments with RAW 264.7 cells which express endogenous TLR4 and MD2, LPS treatment inhibited GHR promoter activity. Co-transfection of dominant negative TLR4 abrogated this effect on GHR promoter activity. In HEK 293T cells, which are devoid of endogenous TLR4 or MD2, ectopic expression of TLR4 and MD2 resulted in LPS-induced inhibition of the GHR promoter activity. The inhibition of GHR promoter activity was demonstrable by 5-6 h after exposure to LPS and persisted at 24 h. Fatty-acid free LPS failed to elicit a similar effect on the GHR promoter and the effect of LPS was abrogated by Polymyxin B. The essential role of the cofactor MD2 on the effect of LPS on the GHR promoter was established in experiments using ectopic expression of wild type and mutant MD2. Cotransfection of CD 14 in these cells failed to alter the effect of LPS on the activity of the GHR promoter. Analysis of cell culture supernatant excluded the possibility that the effect of LPS was secondary to release of cytokines from the transfected cells. The effect of LPS on the endogenous GHR promoter activity and protein expression was confirmed in F442A preadipocyte cells. In HEK 293T cells, ectopic expression of mutant MyD88 or mutant TRIF abrogated the effect of LPS on the GHR promoter, suggesting that the effect of LPS on the GHR promoter was via both MyD88-dependent and -independent pathways.Conclusions: LPS acts through both MyD88-dependent and -independent TLR4 signaling pathways to directly inhibit GHR gene expression. Our results establish a novel cytokine-independent mechanism for decrease in GHR expression in bacterial sepsis. (c) 2007 Elsevier Ireland Ltd. All rights reserved.