Ionizing radiation induces apoptosis in human Molt-4 leukemia cells in a p53-dependent manner. The tumor suppressor p53 stimulates various downstream targets that presumably trigger, individually or in concert, de novo ceramide synthesis and intrinsic apoptosis via mitochondrial outer membrane permeabilization (MOMP). Among these targets, BH3-only protein Noxa was found to be promptly activated by p53 prior to ceramide accumulation and apoptosis in response to irradiation. To evaluate the relation between Noxa and ceramide in irradiation-induced apoptosis, Noxa was silenced in Molt-4 cells and apoptosis, p53 expression, and ceramide accumulation were assessed in response to irradiation. In the absence of Noxa, irradiation of Molt-4 cells still induced apoptosis in a p53-dependent manner however ceramide levels decreased significantly although they remained higher than untreated control. Upon irradiation, Noxa was found to translocate to the mitochondria where endogenous ceramide accumulation was observed. In contrast, overexpression of Bcl-2, another mitochondrial protein, in Molt-4 cells abolished the endogenous ceramide accumulation and apoptosis. In irradiation-induced, p53-dependent pathways of apoptosis, the pro-apoptotic Noxa represents one of several, yet to be identified, pathways simultaneously triggered by p53 to produce mitochondrial ceramide accumulation and apoptosis. In contrast, Bcl-2 functions as a broader inhibitor of both ceramide accumulation and apoptosis. Altogether, these results indicate that members of the Bcl-2 family differentially regulate ceramide accumulation and reveal the existence of crosstalk between Bcl-2 family members and ceramide in mediating p53-dependent apoptosis in Molt-4 human T-cell leukemia.

电离辐射以p53依赖性方式诱导人Molt-4白血病细胞凋亡。肿瘤抑制因子p53刺激各种下游靶标，推测它们可能通过线粒体外膜通透性（MOMP）单独或协同触发从头神经酰胺合成和固有凋亡。在这些靶标中，发现仅BH3蛋白Noxa在神经酰胺蓄积并响应辐射而凋亡之前被p53迅速激活。为了评估Noxa和神经酰胺在辐射诱导的细胞凋亡中的关系，将Noxa在Molt-4细胞中沉默，并评估辐射，凋亡，p53表达和神经酰胺蓄积。在没有Noxa的情况下，Molt-4细胞的照射仍以p53依赖性方式诱导凋亡，但是神经酰胺水平显着降低，尽管它们仍高于未处理的对照组。辐射后，发现Noxa易位至线粒体，在那里观察到内源性神经酰胺积聚。相反，Molt-4细胞中另一种线粒体蛋白Bcl-2的过表达消除了内源性神经酰胺的积累和凋亡。在辐射诱导的依赖于p53的细胞凋亡途径中，促凋亡Noxa代表由p53同时触发以产生线粒体神经酰胺蓄积和凋亡的几种途径之一（尚未确定）。相反，Bcl-2充当神经酰胺积累和凋亡的更广泛的抑制剂。共，