Huang Y, Johnson K R, Norris J S, Fan W
Department of Pathology and Laboratory Medicine, Medical University of South Carolina, Charleston 29425, USA.
Cancer Res. 2000 Aug 15;60(16):4426-32.
Paclitaxel (Taxol), a naturally occurring antimitotic agent, has shown significant cell-killing activity in a variety of tumor cells through induction of apoptosis. The mechanism by which paclitaxel induces cell death is not entirely clear. Recent studies in our laboratory demonstrated that glucocorticoids selectively inhibited paclitaxel-induced apoptosis without affecting the ability of paclitaxel to induce microtubule bundling and mitotic arrest. This finding suggests that apoptotic cell death induced by paclitaxel may occur via a pathway independent of mitotic arrest. In the current study, through analyses of a number of apoptosis-associated genes or regulatory proteins, we discovered that paclitaxel significantly down-regulated IkappaB-alpha, the cytoplasmic inhibitor of transcription factor nuclear factor-kappaB (NF-kappaB), which in turn promoted the nuclear translocation of NF-kappaB and its DNA binding activity. In contrast, we found that glucocorticoids could antagonize paclitaxel-mediated NF-kappaB nuclear translocation and activation through induction of IkappaB-alpha protein synthesis. Northern blotting analyses demonstrated that the steady-state level of IkappaB-alpha mRNA was not affected by paclitaxel, which suggests that the down-regulation of IkappaB-alpha by paclitaxel is attributable to protein degradation rather than suppression of transcription. Furthermore, through transfection assays, we demonstrated that tumor cells stably transfected with antisense IkappaB-alpha expression vectors remarkably increased their sensitivity to paclitaxel-induced apoptosis. Finally, we found that a key subunit of IkappaB kinase (IKK) complex, IKKbeta, was up-regulated by paclitaxel, which implies that paclitaxel might down-regulate IkappaB-alpha through modulation of IKKbeta activity. All of these results suggest that the NF-kappaB/IkappaB-alpha signaling pathway may contribute to the mediation of paclitaxel-induced cell death in solid tumor cells.
紫杉醇（泰素）是一种天然存在的抗有丝分裂剂，通过诱导细胞凋亡在多种肿瘤细胞中显示出显著的细胞杀伤活性。紫杉醇诱导细胞死亡的机制尚不完全清楚。我们实验室最近的研究表明，糖皮质激素选择性地抑制紫杉醇诱导的细胞凋亡，而不影响紫杉醇诱导微管成束和有丝分裂停滞的能力。这一发现表明，紫杉醇诱导的凋亡性细胞死亡可能通过一条独立于有丝分裂停滞的途径发生。在本研究中，通过对多个凋亡相关基因或调节蛋白的分析，我们发现紫杉醇显著下调转录因子核因子κB（NF-κB）的细胞质抑制剂IκB-α，进而促进NF-κB的核转位及其DNA结合活性。相反，我们发现糖皮质激素可通过诱导IκB-α蛋白合成来拮抗紫杉醇介导的NF-κB核转位和激活。Northern印迹分析表明，IκB-α mRNA的稳态水平不受紫杉醇影响，这表明紫杉醇对IκB-α的下调归因于蛋白质降解而非转录抑制。此外，通过转染实验，我们证明稳定转染反义IκB-α表达载体的肿瘤细胞对紫杉醇诱导的细胞凋亡敏感性显著增加。最后，我们发现IκB激酶（IKK）复合物的一个关键亚基IKKβ被紫杉醇上调，这意味着紫杉醇可能通过调节IKKβ活性来下调IκB-α。所有这些结果表明，NF-κB/IκB-α信号通路可能参与介导实体瘤细胞中紫杉醇诱导的细胞死亡。
