Murthy S, Mathur S N, Field F J
Department of Internal Medicine and Veterans Affairs, University of Iowa, Iowa City, Iowa 52242, USA.
J Biol Chem. 2000 Mar 31;275(13):9222-9. doi: 10.1074/jbc.275.13.9222.
In inflammatory conditions of the gut, cytokines are released into the mucosa and submucosa propagating and sustaining the inflammatory response. In CaCo-2 cells, we have shown that various inflammatory cytokines interfere with the secretion of lipids, an effect that is likely caused by the release of a ligand to the epidermal growth factor (EGF) receptor. In the present study, the role of the EGF receptor signaling pathway and the effects of the cytokines tumor necrosis factor-alpha (TNF-alpha) and and interleukin 1beta (IL-1beta) on triacylglycerol-rich lipoprotein secretion were investigated. CaCo-2 cells were incubated with oleic acid to enhance triacylglycerol-rich lipoprotein secretion. TNF-alpha and IL-1beta significantly decreased the basolateral secretion of apolipoprotein B (apoB) mass, with IL-1beta being more potent. Tyrphostin, an inhibitor of the EGF receptor intrinsic tryosine kinase, prevented or markedly attenuated the decrease in apoB secretion by TNF-alpha or IL-1beta. Both cytokines increased the phosphorylation of the EGF receptor by 30 min. Moreover, phosphotyrosine immunoblots of the EGF receptor demonstrated an increase in tyrosine residues phosphorylated by 0.5 and 6.5 h. At both these time points, TNF-alpha and IL-1beta also decreased the binding of EGF to its cell surface receptor. At 6.5 h, activation of the EGF receptor was sustained. In contrast, the early activation of the receptor was only transient as receptor phosphorylation and binding of EGF to its receptor returned to basal levels by 2 h. Preventing ligand binding to the EGF receptor by a receptor-blocking antibody attenuated receptor activation observed after 6.5 h. This did not occur at 0.5 h, suggesting that early activation of the EGF receptor was non-ligand-mediated. Similarly, apoB secretion was inhibited by an early non-ligand-mediated process; whereas at the later time, inhibition of apoB secretion was ligand-mediated. Thus, the inflammatory cytokines TNF-alpha and IL-1beta interfere with the secretion of triacylglycerol-rich lipoproteins by both early and delayed signaling events mediated by the EGF receptor signaling pathway.
在肠道炎症状态下，细胞因子释放到黏膜和黏膜下层，从而传播并维持炎症反应。在Caco-2细胞中，我们已经表明，多种炎症细胞因子会干扰脂质分泌，这种效应可能是由表皮生长因子（EGF）受体配体的释放所引起的。在本研究中，我们调查了EGF受体信号通路的作用以及细胞因子肿瘤坏死因子-α（TNF-α）和白细胞介素1β（IL-1β）对富含三酰甘油脂蛋白分泌的影响。用油酸孵育Caco-2细胞以增强富含三酰甘油脂蛋白的分泌。TNF-α和IL-1β显著降低了载脂蛋白B（apoB）质量的基底外侧分泌，其中IL-1β的作用更强。酪氨酸磷酸化酶抑制剂 tyrphostin可预防或显著减轻TNF-α或IL-1β引起的apoB分泌减少。两种细胞因子均在30分钟时增加了EGF受体的磷酸化。此外，EGF受体的磷酸酪氨酸免疫印迹显示，在0.5小时和6.5小时时，磷酸化的酪氨酸残基有所增加。在这两个时间点，TNF-α和IL-1β也降低了EGF与其细胞表面受体的结合。在6.5小时时，EGF受体的激活持续存在。相比之下，受体的早期激活只是短暂的，因为受体磷酸化以及EGF与其受体的结合在2小时时恢复到基础水平。用受体阻断抗体阻止配体与EGF受体结合，可减弱6.5小时后观察到的受体激活。在0.5小时时未出现这种情况，这表明EGF受体的早期激活是非配体介导的。同样，apoB分泌在早期也是由非配体介导的过程所抑制；而在后期，apoB分泌的抑制是由配体介导的。因此，炎症细胞因子TNF-α和IL-1β通过EGF受体信号通路介导的早期和延迟信号事件，干扰了富含三酰甘油脂蛋白的分泌。
