Hada Manila, Subramanian Chitra, Andrews Phillip C, Kwok Roland P S
Department of Biological Chemistry, University of Michigan Medical School, Ann Arbor, MI, USA.
General Surgery, University of Michigan Medical School, Ann Arbor, MI, USA.
Tumour Biol. 2016 Oct;37(10):13903-13914. doi: 10.1007/s13277-016-5202-z. Epub 2016 Aug 3.
The first known function of Ku70 is as a DNA repair factor in the nucleus. Using neuronal neuroblastoma cells as a model, we have established that cytosolic Ku70 binds to the pro-apoptotic protein Bax in the cytosol and blocks Bax's cell death activity. Ku70-Bax binding is regulated by Ku70 acetylation in that when Ku70 is acetylated Bax dissociates from Ku70, triggering cell death. We propose that Ku70 may act as a survival factor in these cells such that Ku70 depletion triggers Bax-dependent cell death. Here, we addressed two fundamental questions about this model: (1) Does all Bax, which is a cytosolic protein, bind to all cytosolic Ku70? and (2) Is Ku70 a survival factor in cells types other than neuronal neuroblastoma cells? We show here that, in neuronal neuroblastoma cells, only a small fraction of Ku70 binds to a small fraction of Bax; most Bax is monomeric. Interestingly, there is no free or monomeric Ku70 in the cytosol; most cytosolic Ku70 is in complex with other factors forming several high molecular weight complexes. A fraction of cytosolic Ku70 also binds to cytosolic Ku80, Ku70's binding partner in the nucleus. Ku70 may not be a survival factor in some cell types (Ku70-depletion less sensitive) because Ku70 depletion does not affect survival of these cells. These results indicate that, in addition to Ku70 acetylation, other factors may be involved in regulating Ku70-Bax binding in the Ku70-depletion less sensitive cells because Ku70 acetylation in these cells is not sufficient to dissociate Bax from Ku70 or to activate Bax.
已知Ku70的首个功能是作为细胞核中的一种DNA修复因子。我们以神经母细胞瘤细胞为模型，证实胞质中的Ku70在胞质溶胶中与促凋亡蛋白Bax结合，并阻断Bax的细胞死亡活性。Ku70 - Bax的结合受Ku70乙酰化调控，即当Ku70乙酰化时，Bax从Ku70上解离，触发细胞死亡。我们提出，Ku70可能在这些细胞中作为一种生存因子，使得Ku70的缺失触发依赖Bax的细胞死亡。在此，我们针对该模型解决了两个基本问题：（1）作为胞质蛋白的所有Bax是否都与所有胞质中的Ku70结合？（2）Ku70在神经母细胞瘤细胞以外的其他细胞类型中是否为生存因子？我们在此表明，在神经母细胞瘤细胞中，只有一小部分Ku70与一小部分Bax结合；大多数Bax是单体形式。有趣的是，胞质溶胶中不存在游离或单体形式的Ku70；大多数胞质中的Ku70与其他因子形成复合物，构成几种高分子量复合物。一部分胞质中的Ku70还与胞质中的Ku80结合，Ku80是Ku70在细胞核中的结合伴侣。在某些细胞类型中（Ku70缺失敏感性较低），Ku70可能不是生存因子，因为Ku70的缺失并不影响这些细胞的存活。这些结果表明，除了Ku70乙酰化外，其他因素可能参与调控Ku70缺失敏感性较低的细胞中Ku70 - Bax的结合，因为这些细胞中的Ku70乙酰化不足以使Bax从Ku70上解离或激活Bax。
