The primary function of APOE (apolipoprotein E) is to mediate the transport of cholesterol- and lipid-containing lipoprotein particles into cells by receptor-mediated endocytosis. APOE also has pro- and antiinflammatory effects, which are both context and concentration dependent. For example, Apoe^−/− mice exhibit enhanced airway remodeling and hyperreactivity in experimental asthma, whereas increased APOE levels in lung epithelial lining fluid induce IL-1β secretion from human asthmatic alveolar macrophages. However, APOE-mediated airway epithelial cell inflammatory responses and signaling pathways have not been defined. Here, RNA sequencing of human asthmatic bronchial brushing cells stimulated with APOE identified increased expression of mRNA transcripts encoding multiple proinflammatory genes, including CXCL5 (C-X-C motif chemokine ligand 5), an epithelial-derived chemokine that promotes neutrophil activation and chemotaxis. We subsequently characterized the APOE signaling pathway that induces CXCL5 secretion by human asthmatic small airway epithelial cells (SAECs). Neutralizing antibodies directed against TLR4 (Toll-like receptor 4), but not TLR2, attenuated APOE-mediated CXCL5 secretion by human asthmatic SAECs. Inhibition of TAK1 (transforming growth factor-β–activated kinase 1), IκKβ (inhibitor of nuclear factor κ B kinase subunit β), TPL2 (tumor progression locus 2), and JNK (c-Jun N-terminal kinase), but not p38 MAPK (mitogen-activated protein kinase) or MEK1/2 (MAPK kinase 1/2), attenuated APOE-mediated CXCL5 secretion. The roles of TAK1, IκKβ, TPL2, and JNK in APOE-mediated CXCL5 secretion were verified by RNA interference. Furthermore, RNA interference showed that after APOE stimulation, both NF-κB p65 and TPL2 were downstream of TAK1 and IκKβ, whereas JNK was downstream of TPL2. In summary, elevated levels of APOE in the airway may activate a TLR4/TAK1/IκKβ/NF-κB/TPL2/JNK signaling pathway that induces CXCL5 secretion by human asthmatic SAECs. These findings identify new roles for TLR4 and TPL2 in APOE-mediated proinflammatory responses in asthma.

APOE（载脂蛋白 E）的主要功能是通过受体介导的内吞作用介导含胆固醇和脂质的脂蛋白颗粒转运到细胞中。APOE 还具有促炎和抗炎作用，这取决于环境和浓度。例如，Apoe ^-/-小鼠在实验性哮喘中表现出增强的气道重塑和高反应性，而肺上皮内衬液中增加的 APOE 水平诱导人哮喘肺泡巨噬细胞分泌 IL-1β。然而，APOE 介导的气道上皮细胞炎症反应和信号通路尚未确定。在这里，用 APOE 刺激的人类哮喘支气管刷细胞的 RNA 测序发现编码多种促炎基因的 mRNA 转录本的表达增加，包括 CXCL5（CXC 基序趋化因子配体 5），这是一种促进中性粒细胞活化和趋化性的上皮衍生趋化因子。我们随后表征了诱导人哮喘性小气道上皮细胞 (SAEC) 分泌 CXCL5 的 APOE 信号通路。针对 TLR4（Toll 样受体 4）的中和抗体，但不是 TLR2，减弱了 APOE 介导的人哮喘 SAEC 分泌的 CXCL5。抑制 TAK1（转化生长因子 β 活化激酶 1）、IκKβ（核因子 κ B 激酶亚基 β 抑制剂）、TPL2（肿瘤进展位点 2）和 JNK（c-Jun N 端激酶），但不抑制p38 MAPK（丝裂原活化蛋白激酶）或 MEK1/2（MAPK 激酶 1/2）减弱了 APOE 介导的 CXCL5 分泌。通过RNA干扰验证了TAK1、IκKβ、TPL2和JNK在APOE介导的CXCL5分泌中的作用。此外，RNA干扰显示APOE刺激后，NF-κB p65和TPL2均位于TAK1和IκKβ的下游，而JNK位于TPL2的下游。总之，气道中 APOE 水平升高可能会激活 TLR4/TAK1/IκKβ/NF-κB/TPL2/JNK 信号通路，该通路诱导人类哮喘性 SAEC 分泌 CXCL5。