Cambier J C, Fisher C L, Pickles H, Morrison D C
Department of Pediatrics, National Jewish Center for Immunology and Respiratory Medicine, Denver, CO.
J Immunol. 1990 Jul 1;145(1):13-9.
We have previously shown that ligation of murine B cell membrane IgM or IgD can lead to inactivation of the signal transducing ability of unligated Ag receptors. We describe further studies of the molecular basis of this desensitization. Consistent with the possibility that ligand induced desensitization is mediated by protein kinase C (PKC) are findings that demonstrate that both Ig binding ligands and PKC activators (DIC8 or PMA) induce desensitization in virtually all resting B cells. However, ligand-induced desensitization is longer lived than PMA- or DIC8-induced desensitization and insensitive to the PKC inhibitor staurosporine. Further, biochemical studies indicate that insufficient PKC activation is induced by ligation of membrane Ig to mediate the observed desensitization. Thus data indicate that PKC must play only a minor role in ligand-induced membrane Ig desensitization. Further studies explored the molecular source and target of effectors that mediate ligand-induced desensitization. Data indicate that phosphoinositide hydrolysis is neither necessary nor sufficient for ligand induction of desensitization. Finally, ligand-induced desensitization appears to be mediated by uncoupling of membrane Ig from G proteins that regulate phospholipase C because ligand desensitized cells are hyperresponsive to agents including ALF4- and mastoparan which activate G proteins leading to mobilization of Ca2+. Thus, the function of G proteins and further downstream elements that mediate Ca2+ mobilization is intact. Taken together, these data are most consistent with ligand-induced membrane Ig desensitization being mediated by a non-PKC, non phosphatidylinositol 4,5-bisphosphate hydrolysis involving mechanism that has as its target a structure that is very proximal to the receptor, such as the receptor itself or a transducer complex analogous to CD3.
我们之前已经表明，连接小鼠B细胞膜IgM或IgD可导致未连接的抗原受体信号转导能力失活。我们描述了对这种脱敏作用分子基础的进一步研究。与配体诱导的脱敏作用由蛋白激酶C（PKC）介导的可能性一致的是，有研究结果表明，Ig结合配体和PKC激活剂（DIC8或佛波酯）实际上可在所有静息B细胞中诱导脱敏作用。然而，配体诱导的脱敏作用比佛波酯或DIC8诱导的脱敏作用持续时间更长，且对PKC抑制剂星形孢菌素不敏感。此外，生化研究表明，膜Ig连接诱导的PKC激活不足，无法介导观察到的脱敏作用。因此，数据表明PKC在配体诱导的膜Ig脱敏作用中仅起次要作用。进一步的研究探索了介导配体诱导脱敏作用的效应器的分子来源和靶点。数据表明，磷酸肌醇水解对于配体诱导的脱敏作用既非必要条件也非充分条件。最后，配体诱导的脱敏作用似乎是通过膜Ig与调节磷脂酶C的G蛋白解偶联来介导的，因为配体脱敏的细胞对包括ALF4和mastoparan在内的激活G蛋白导致Ca2+动员的试剂反应过度。因此，介导Ca2+动员的G蛋白及更下游元件的功能是完整的。综上所述，这些数据最符合配体诱导的膜Ig脱敏作用是由一种非PKC、不涉及磷脂酰肌醇4,5-二磷酸水解的机制介导的，该机制的靶点是与受体非常接近的结构，如受体本身或类似于CD3的转导复合物。
