Noxa and ceramide :crosstalk between Bcl-2 family and p53-dependent ceramide accumulation in mediating intrinsic apoptosis in Molt-4 human T-cell leukemia and HCT116 colon cancer cell lines -

Abstract

The imbalance between cell proliferation and apoptosis disturbs health and homeostasis of living systems and as a result might favor tumor development and accelerated cell growth or degeneration and cell loss. This equilibrium depends on the coordinated action of various tumor suppressors specifically p53 protein, the sphingolipid ceramide, and Bcl-2 family proteins. In fact, several tumors were shown to possess defects in ceramide biosynthesis such as overexpression of glucosylceramide synthase in metastatic breast cancer, stage III. Hence, ceramide functions as a tumor suppressor lipid by regulating apoptosis, cell cycle arrest and senescence. One of the known ceramide regulators is p53 which once activated will trigger ceramide accumulation and subsequently mitochondrial outer membrane permeabilization (MOMP) followed by apoptosis. Likewise, Noxa, a BH3-only pro-apoptotic protein, which is in turn activated by p53, regulates downstream proteins that initiate the apoptotic machinery. Here we identify a novel link between Noxa and ceramide in the apoptotic pathway that, to our knowledge, were previously thought to work independently. Specifically, we show that ceramide accumulation was triggered by γ-irradiation in the presence of both p53 and Noxa, whereas the blocking of either molecule has inhibited this increase in ceramide levels. In fact, the blocking of Noxa protein did not only affect ceramide generation but also prevented caspase activation and decreased cellular death, thus underscoring the role of Noxa in the apoptotic pathway triggered by γ-irradiation. Furthermore, mitochondrial ceramide accumulation and cytochrome c release occurred simultaneously, suggesting that ceramide’s role, at least in this context, is intricately related to the execution phase of apoptosis. Although, in our model, both extrinsic and intrinsic pathways were triggered, Noxa and ceramide were exclusively functioning in intrinsic apoptosis. The data presented indicate that ceramide-induced apoptosis is depen