Riches D W, Chan E D, Winston B W
Department of Pediatrics, National Jewish Center for Immunology and Respiratory Medicine, Denver, CO, USA.
Immunobiology. 1996 Oct;195(4-5):477-90. doi: 10.1016/s0171-2985(96)80017-9.
Tumor necrosis factor alpha (TNF-alpha) is a pleiotropic cytokine produced predominantly by macrophages. In addition, macrophages respond to TNF-alpha by differentiating to express different groups of gene products. Our laboratory recently showed that the context in which TNF-alpha is recognized by macrophages dramatically impacts the pattern of gene expression and hence investigating the mechanism of TNF-alpha signal transduction will be important in understanding how this molecule regulates macrophage differentiation. TNF-alpha is recognized by two cell surface receptors, CD120a (p55) and CD120b (p75) that belong to the TNF/NGF receptor family. Signalling is initiated by receptor multimerization in the plane of the plasma membrane. The initial signalling events activated by receptor cross-linking are unknown although activation of the mitogen-activated protein kinase (MAPK) cascade occurs shortly after ligand binding to CD120a (p55). We have investigated the upstream kinases that mediate the activation of p42mapk/erk2 following cross-linking of CD120a (p55) in mouse macrophages. Exposure of mouse macrophages to TNF-alpha stimulated a time-dependent increase in the activity of MEK1, that temporally preceded peak activation of p42mapk/erk2. MEKs, dual specificity T/Y kinases, act as a convergence point for several signalling pathways including Ras/Raf, MEKK and Mos. Incubation of macrophages with TNF-alpha was found to transiently stimulate an MEKK that peaked in activity within 30 sec of exposure and progressively declined towards basal levels by 5 min. By contrast, under these conditions, activation of either c-Raf-1 or Raf-B was not detected. These data suggest that the activation of the MAPK cascade in response to TNF-alpha is mediated by the sequential activation of an MEKK and MEK1 in a c-Raf-1 and Raf-B-independent fashion. The implications of these findings will be discussed in the context of the regulation of macrophage gene expression.
肿瘤坏死因子α（TNF-α）是一种主要由巨噬细胞产生的多效性细胞因子。此外，巨噬细胞通过分化以表达不同组的基因产物来对TNF-α作出反应。我们实验室最近表明，巨噬细胞识别TNF-α的背景会显著影响基因表达模式，因此研究TNF-α信号转导机制对于理解该分子如何调节巨噬细胞分化至关重要。TNF-α由属于TNF/NGF受体家族的两种细胞表面受体CD120a（p55）和CD120b（p75）识别。信号传导通过质膜平面中的受体多聚化启动。尽管在配体与CD120a（p55）结合后不久丝裂原活化蛋白激酶（MAPK）级联被激活，但受体交联激活的初始信号事件尚不清楚。我们研究了在小鼠巨噬细胞中CD120a（p55）交联后介导p42mapk/erk2激活的上游激酶。将小鼠巨噬细胞暴露于TNF-α会刺激MEK1活性随时间增加，这在时间上先于p42mapk/erk2的峰值激活。MEK是双特异性T/Y激酶，是包括Ras/Raf、MEKK和Mos在内的几种信号通路的汇聚点。发现用TNF-α孵育巨噬细胞会短暂刺激一种MEKK，其活性在暴露后30秒内达到峰值，并在5分钟时逐渐降至基础水平。相比之下，在这些条件下，未检测到c-Raf-1或Raf-B的激活。这些数据表明，响应TNF-α的MAPK级联激活是以c-Raf-1和Raf-B非依赖性方式通过MEKK和MEK1的顺序激活介导的。这些发现的意义将在巨噬细胞基因表达调控的背景下进行讨论。
