Gold M R, Chan V W, Turck C W, DeFranco A L
George Williams Hooper Foundation, University of California, San Francisco 94143.
J Immunol. 1992 Apr 1;148(7):2012-22.
Cross-linking of the B cell AgR results in activation of mature B cells and tolerization of immature B cells. The initial signaling events stimulated by membrane immunoglobulin (mIg) cross-linking are tyrosine phosphorylation of a number of proteins. Among the targets of mIg-induced tyrosine phosphorylation are the tyrosine kinases encoded by the lyn, blk, fyn, and syk genes, the mIg-associated proteins MB-1 and Ig-beta, phospholipase C-gamma 1 and -gamma 2, as well as many unidentified proteins. In this report we show that mIg cross-linking also regulates phosphatidylinositol 3-kinase (PtdIns 3-kinase), an enzyme that phosphorylates inositol phospholipids and plays a key role in mediating the effects of tyrosine kinases on growth control in fibroblasts. Cross-linking mIg on B lymphocytes greatly increased the amount of PtdIns 3-kinase activity which could be immunoprecipitated with anti-phosphotyrosine (anti-tyr(P) antibodies. This response was observed after mIg cross-linking in mIgM- and mIgG-bearing B cell lines and after cross-linking either mIgM or mIgD in murine splenic B cells. Thus, regulation of PtdIns 3-kinase is a common feature of signaling by several different isotypes of mIg. This response was rapid and peaked 2 to 3 min after the addition of anti-Ig antibodies. The anti-Ig-stimulated increase in PtdIns 3-kinase activity associated with anti-Tyr(P) immunoprecipitates could reflect increased tyrosine phosphorylation of PtdIns 3-kinase, increased activity of the enzyme, or both. In favor of the first possibility, the tyrosine kinase inhibitor herbimycin A blocked the increase in ant-Tyr(P)-immunoprecipitated PtdIns 3-kinase activity as well as the anti-Ig-induced tyrosine phosphorylation. Moreover, this response was not secondary to phospholipase C activation but rather seemed to be a direct consequence of mIg-induced tyrosine phosphorylation. Activation of the phosphoinositide pathway by a transfected M1 muscarinic acetylcholine receptor expressed in WEHI-231 B lymphoma cells did not increase the amount of PtdIns 3-kinase activity which could be precipitated with anti-Tyr(P) antibodies. Similarly, inhibition of the phosphoinositide pathway did not abrogate the ability of mIg cross-linking to stimulate this response. Thus, mIg-induced tyrosine phosphorylation regulates PtdIns 3-kinase, an important mediator of growth control in fibroblasts and potentially an important regulatory component in B cells as well.
B细胞抗原受体的交联导致成熟B细胞活化和未成熟B细胞耐受。膜免疫球蛋白（mIg）交联刺激的初始信号事件是多种蛋白质的酪氨酸磷酸化。mIg诱导的酪氨酸磷酸化的靶标包括由lyn、blk、fyn和syk基因编码的酪氨酸激酶、mIg相关蛋白MB-1和Ig-β、磷脂酶C-γ1和-γ2，以及许多未鉴定的蛋白质。在本报告中，我们表明mIg交联还调节磷脂酰肌醇3-激酶（PtdIns 3-激酶），该酶使肌醇磷脂磷酸化，并在介导酪氨酸激酶对成纤维细胞生长控制的作用中起关键作用。在B淋巴细胞上交联mIg大大增加了可用抗磷酸酪氨酸（抗-Tyr(P)）抗体免疫沉淀的PtdIns 3-激酶活性的量。在携带mIgM和mIgG的B细胞系中进行mIg交联后，以及在小鼠脾B细胞中交联mIgM或mIgD后，均观察到这种反应。因此，PtdIns 3-激酶的调节是几种不同同种型mIg信号传导的共同特征。这种反应迅速，在加入抗Ig抗体后2至3分钟达到峰值。与抗-Tyr(P)免疫沉淀物相关的抗Ig刺激的PtdIns 3-激酶活性增加可能反映了PtdIns 3-激酶酪氨酸磷酸化增加、酶活性增加或两者兼有。支持第一种可能性的是，酪氨酸激酶抑制剂赫伯霉素A阻断了抗-Tyr(P)免疫沉淀的PtdIns 3-激酶活性的增加以及抗Ig诱导的酪氨酸磷酸化。此外，这种反应不是磷脂酶C活化的继发结果，而似乎是mIg诱导的酪氨酸磷酸化的直接后果。在WEHI-231 B淋巴瘤细胞中表达的转染M1毒蕈碱型乙酰胆碱受体激活磷酸肌醇途径并没有增加可用抗-Tyr(P)抗体沉淀的PtdIns 3-激酶活性的量。同样，磷酸肌醇途径的抑制并没有消除mIg交联刺激这种反应的能力。因此，mIg诱导的酪氨酸磷酸化调节PtdIns 3-激酶，它是成纤维细胞生长控制的重要介质，也可能是B细胞中的重要调节成分。
