An Early Oxygen-Dependent Step Is Required for Dexamethasone-Induced Apoptosis of Immature Mouse Thymocytes

Abstract

The roles of oxygen and reactive oxygen intermediates in apoptosis are unclear at present. Although oxygen and reactive oxygen intermediates are not required for the execution of apoptosis, oxygen may be involved in at least some forms of apoptosis. In this study we show that dexamethasone (Dex)-induced apoptosis of immature mouse thymocytes is completely inhibited by hypoxic culture. In contrast, anti-CD95 thymocyte apoptosis is unaffected by hypoxia, indicating the existence of two forms of thymocyte apoptosis: an oxygen-dependent pathway (Dex induced) and an oxygen-independent pathway (anti-CD95 induced). Furthermore, hypoxia inhibited mitochondrial permeability transition (PT) in Dex-treated, but not in anti-CD95-treated, thymocytes, suggesting that the oxygen-sensitive step is upstream of mitochondria. Both Dex- and anti-CD95-induced PT and apoptosis were dependent on activation of IL-converting enzyme-like protease, as PT and apoptosis were inhibited by preincubation with Cbz-Val-Ala-Asp-fluoromethyl ketone, an irreversible inhibitor of IL-converting enzyme-like proteases. In addition, hypoxia inhibited the activation by Dex of caspase-3 (CPP32)-like proteases. Our data show that the private signaling pathways of Dex (oxygen dependent) and anti-CD95 (oxygen independent) both converge upstream of mitochondrial changes. The oxygen-dependent step in Dex-induced apoptosis lies upstream of caspase-3-like protease activation. Our observations support a model of apoptosis signaling in which independent pathways (oxygen dependent and oxygen independent) particular to each stimuli converge at a central point in the apoptotic cascade.

Hypoxia in the thymus: role of oxygen tension in thymocyte survival

“…Previous studies have shown that dexamethasone-induced apoptosis of rat thymocytes is inhibited in hypoxic atmospheres (0.5-5% O 2 ) and increased in high oxygen atmospheres (95% O 2 ) (37). The dexamethasone-induced apoptosis of murine thymocytes was also shown to be inhibited by hypoxia (24,38). Our studies further confirm the role of oxygen in regulating spontaneous thymocyte apoptosis and also document new methods for demonstrating hypoxia in thymocytes both in vitro and in vivo.…”

Thioredoxin-interacting protein (txnip) is a glucocorticoid-regulated primary response gene involved in mediating glucocorticoid-induced apoptosis

“…Interestingly, decreased thioredoxin activity has been suspected of playing a role in Dex-induced apoptosis, since Dex treatment decreases thioredoxin levels and increases oxidative damage in primary thymocytes and lymphoma cell lines (Tome et al, 2001). Furthermore, overexpression of thioredox in WEHI7.2 lymphoma cells inhibits Dexinduced apoptosis (Baker et al, 1997) and treatment of thymocytes with antioxidants or culturing thymocytes under hypoxic conditions protects against Dex-induced apoptosis (Torres-Roca et al, 2000;Tonomura et al, 2003). What has been missing is a link between glucocorticoid receptor-mediated gene transcription during the initiation stage of Dex-induced apoptosis and downstream effects on thioredoxin levels and oxygen radical formation.…”

Role of caspases in the regulation of apoptotic pancreatic islet beta‐cells death

“…Mitochondria undergoing PT and MTP release mitochondrial proteins (e.g., cytochrome-c and apoptosisinducing factor (AIF) into the cytoplasm (Kristal and Brown, 1999;Torres-Roca et al, 2000). Cytochrome-c can then bind to Apaf-1 (a mammalian protein with homology to Ced-4) which, in the presence of dATP, is capable of activating pro-caspase-9.…”