Department of Medicine, University of California San Diego, La Jolla, California, United States of America.
PLoS One. 2012;7(2):e32378. doi: 10.1371/journal.pone.0032378. Epub 2012 Feb 22.
Oxidative modification of low-density lipoprotein (LDL) turns it into an endogenous ligand recognized by pattern-recognition receptors. We have demonstrated that minimally oxidized LDL (mmLDL) binds to CD14 and mediates TLR4/MD-2-dependent responses in macrophages, many of which are MyD88-independent. We have also demonstrated that the mmLDL activation leads to recruitment of spleen tyrosine kinase (Syk) to TLR4 and TLR4 and Syk phosphorylation. In this study, we produced a macrophage-specific Syk knockout mouse and used primary Syk(-/-) macrophages in our studies. We demonstrated that Syk mediated phosphorylation of ERK1/2 and JNK, which in turn phosphorylated c-Fos and c-Jun, respectively, as assessed by an in vitro kinase assay. c-Jun phosphorylation was also mediated by IKKε. c-Jun and c-Fos bound to consensus DNA sites and thereby completed an AP-1 transcriptional complex and induced expression of CXCL2 and IL-6. These results suggest that Syk plays a key role in TLR4-mediated macrophage responses to host-generated ligands, like mmLDL, with subsequent activation of an AP-1 transcription program.
氧化修饰的低密度脂蛋白（LDL）使其变成一种被模式识别受体识别的内源性配体。我们已经证明，最小氧化的 LDL（mmLDL）与 CD14 结合，并在巨噬细胞中介导 TLR4/MD-2 依赖性反应，其中许多反应是 MyD88 非依赖性的。我们还证明，mmLDL 的激活导致脾酪氨酸激酶（Syk）向 TLR4 和 TLR4 的募集和 Syk 的磷酸化。在这项研究中，我们产生了一种巨噬细胞特异性的 Syk 敲除小鼠，并在我们的研究中使用原代 Syk(-/-)巨噬细胞。我们证明，Syk 介导 ERK1/2 和 JNK 的磷酸化，这反过来分别磷酸化 c-Fos 和 c-Jun，如体外激酶测定所示。c-Jun 的磷酸化也由 IKKε 介导。c-Jun 和 c-Fos 与共识 DNA 位点结合，从而完成 AP-1 转录复合物，并诱导 CXCL2 和 IL-6 的表达。这些结果表明，Syk 在 TLR4 介导的巨噬细胞对宿主产生的配体（如 mmLDL）的反应中发挥关键作用，随后激活 AP-1 转录程序。
