TNF-alpha activates at least two apoptotic signaling cascades.

Apoptosis, the process whereby cells activate an intrinsic death program, can be induced in HeLa cells by TNF-alpha treatment. The aims of the present study were (i) to examine the precise role and the origin of Reactive Oxygen Species (ROS) in the TNF-alpha-induced programmed cell death, (ii) to characterize and order the morphological and mitochondrial changes associated with this process and (iii) to link these events with the activation of caspases. Analyses were performed on TNF-alpha-treated cells in the presence of an anti-oxidant, or of a general caspase inhibitor. To assess the role of mitochondria in the cell death signal transduction, these studies were also realized on HeLa-variant cell lines lacking functional mitochondrial respiratory chain. We show that at least two separate signaling cascades, both mediated by Z-VAD-sensitive caspase(s), contribute to the TNF-alpha-induced apoptosis of HeLa cells. One signaling pathway involves an early mitochondria-dependent ROS production, the other being ROS-independent.

Bcl-2 and Hsp27 act at different levels to suppress programmed cell death.

Apoptosis and necrosis, two morphologically distinct forms of cell death, can be induced by common stimuli depending on the doses and the cell type. This study compares the protective effect of oncoprotein Bcl-2 and of the small stress protein Hsp27 on these two types of cell death. We use rat embryo fibroblasts conditionally immortalized by the tsA58 mutant of SV40 large T antigen as parental cells to develop cell lines carrying inducible bcl-2 or hsp27 genes. Two apoptotic stimuli were used: shift to the restrictive temperature that induced p53-mediated apoptosis and treatment with low doses of hydrogen peroxide. Necrosis was induced by high doses of hydrogen peroxide. Although Bcl-2 and Hsp27 protect these cells from necrotic death, only Bcl-2 appears capable of preventing apoptotic death. Bcl-2 protection is not mediated by a negative effect on the induction of the p53 responsive genes bax or waf1 but it slows down at least two stages of apoptosis: decrease of mitochondrial membrane potential and subsequent morphological changes. In contrast, although Hsp27 has been recently shown to inhibit apoptosis induced by various stimuli, its overexpression has no effect on apoptosis in this cell system. It should be also noticed that the apoptotic stimuli (temperature shift or hydrogen peroxide treatment) induce Hsp27, but not Bcl-2 accumulation suggesting that, in parental cells, Hsp27 might already provide some protection. However, taken together these results suggest that Hsp27, as well as Bcl-2, acts at several levels to inhibit cell death, but that their protective functions only partially overlap.