Hsueh Robert C, Hammill Adrienne M, Lee Jamie A, Uhr Jonathan W, Scheuermann Richard H
Laboratory of Molecular Pathology and Department of Pathology, University of Texas Southwestern Medical Center, Dallas, TX, USA.
BMC Immunol. 2002 Dec 6;3:16. doi: 10.1186/1471-2172-3-16.
Immature B lymphocytes and certain B cell lymphomas undergo apoptotic cell death following activation of the B cell antigen receptor (BCR) signal transduction pathway. Several biochemical changes occur in response to BCR engagement, including activation of the Syk tyrosine kinase. Although Syk activation appears to be necessary for some downstream biochemical and cellular responses, the signaling events that precede Syk activation remain ill defined. In addition, the requirements for complete activation of the Syk-dependent signaling step remain to be elucidated.
A mutant form of Syk carrying a combination of a K395A substitution in the kinase domain and substitutions of three phenylalanines (3F) for the three C-terminal tyrosines was expressed in a murine B cell lymphoma cell line, BCL1.3B3 to interfere with normal Syk regulation as a means to examine the Syk activation step in BCR signaling. Introduction of this kinase-inactive mutant led to the constitutive activation of the endogenous wildtype Syk enzyme in the absence of receptor engagement through a 'dominant-positive' effect. Under these conditions, Syk kinase activation occurred in the absence of phosphorylation on Syk tyrosine residues. Although Syk appears to be required for BCR-induced apoptosis in several systems, no increase in spontaneous cell death was observed in these cells. Surprisingly, although the endogenous Syk kinase was enzymatically active, no enhancement in the phosphorylation of cytoplasmic proteins, including phospholipase Cgamma2 (PLCgamma2), a direct Syk target, was observed.
These data indicate that activation of Syk kinase enzymatic activity is insufficient for Syk-dependent signal transduction. This observation suggests that other events are required for efficient signaling. We speculate that localization of the active enzyme to a receptor complex specifically assembled for signal transduction may be the missing event.
未成熟B淋巴细胞和某些B细胞淋巴瘤在B细胞抗原受体（BCR）信号转导途径激活后会发生凋亡性细胞死亡。BCR结合会引发多种生化变化，包括Syk酪氨酸激酶的激活。尽管Syk激活似乎是某些下游生化和细胞反应所必需的，但Syk激活之前的信号事件仍不清楚。此外，Syk依赖性信号转导步骤的完全激活所需条件仍有待阐明。
在小鼠B细胞淋巴瘤细胞系BCL1.3B3中表达了一种Syk突变体，其激酶结构域中有K395A替换，并且三个C末端酪氨酸被三个苯丙氨酸（3F）替换，以此干扰正常的Syk调节，作为研究BCR信号传导中Syk激活步骤的一种手段。引入这种激酶失活突变体通过“显性阳性”效应导致在没有受体结合的情况下内源性野生型Syk酶的组成性激活。在这些条件下，Syk激酶激活在Syk酪氨酸残基未磷酸化的情况下发生。尽管在几个系统中Syk似乎是BCR诱导凋亡所必需的，但在这些细胞中未观察到自发细胞死亡增加。令人惊讶的是，尽管内源性Syk激酶具有酶活性，但未观察到包括直接Syk靶点磷脂酶Cγ2（PLCγ2）在内的细胞质蛋白磷酸化增强。
这些数据表明Syk激酶酶活性的激活不足以进行Syk依赖性信号转导。这一观察结果表明有效信号传导还需要其他事件。我们推测活性酶定位于专门为信号转导组装的受体复合物可能是缺失的事件。
