Gibson S, Widmann C, Johnson G L
Program in Molecular Signal Transduction, Division of Basic Sciences, National Jewish Medical and Research Center, Denver, Colorado, 80206, USA.
J Biol Chem. 1999 Apr 16;274(16):10916-22. doi: 10.1074/jbc.274.16.10916.
MEK kinase 1 (MEKK1) is a 196-kDa enzyme that is involved in the regulation of the c-Jun N-terminal kinase (JNK) pathway and apoptosis. In cells exposed to genotoxic agents including etoposide and cytosine arabinoside, MEKK1 is cleaved at Asp874 by caspases. The cleaved kinase domain of MEKK1, itself, stimulates caspase activity leading to apoptosis. Kinase-inactive MEKK1 expressed in HEK293 cells effectively blocks genotoxin-induced apoptosis. Treatment of cells with taxol, a microtubule stabilizing agent, did not induce MEKK1 cleavage in cells, and kinase-inactive MEKK1 expression failed to block taxol-induced apoptosis. MEKK1 became activated in HEK293 cells exposed to taxol, but in contrast to etoposide-treatment, taxol failed to increase JNK activity. Taxol treatment of cells, therefore, dissociates MEKK1 activation from the regulation of the JNK pathway. Overexpression of anti-apoptotic Bcl2 blocked MEKK1 and taxol-induced apoptosis but did not block the caspase-dependent cleavage of MEKK1 in response to etoposide. This indicates Bcl2 inhibition of apoptosis is, therefore, downstream of caspase-dependent MEKK1 cleavage. The results define the involvement of MEKK1 in the induction of apoptosis by genotoxins but not microtubule altering drugs.
丝裂原活化蛋白激酶激酶1（MEKK1）是一种196千道尔顿的酶，参与c-Jun氨基末端激酶（JNK）信号通路的调控及细胞凋亡过程。在暴露于包括依托泊苷和阿糖胞苷等基因毒性药物的细胞中，MEKK1会在天冬氨酸874位点被半胱天冬酶切割。切割后的MEKK1激酶结构域本身会刺激半胱天冬酶活性，从而导致细胞凋亡。在人胚肾293（HEK293）细胞中表达的激酶失活型MEKK1能有效阻断基因毒素诱导的细胞凋亡。用紫杉醇（一种微管稳定剂）处理细胞，不会诱导细胞中MEKK1的切割，且激酶失活型MEKK1的表达也无法阻断紫杉醇诱导的细胞凋亡。在暴露于紫杉醇的HEK293细胞中，MEKK1被激活，但与依托泊苷处理不同的是，紫杉醇未能增加JNK活性。因此，紫杉醇处理细胞会使MEKK1的激活与JNK信号通路的调控相分离。抗凋亡蛋白Bcl2的过表达可阻断MEKK1和紫杉醇诱导的细胞凋亡，但不能阻断细胞对依托泊苷反应时MEKK1的半胱天冬酶依赖性切割。这表明Bcl2对细胞凋亡的抑制作用位于半胱天冬酶依赖性MEKK1切割的下游。这些结果明确了MEKK1参与基因毒素而非微管改变药物诱导的细胞凋亡过程。
