Inhibition of Tumor Necrosis Factor-induced necrotic cell death by the zinc finger protein A20

Tumor Necrosis Factor (TNF) is a cytokine that induces necrotic and apoptot ic forms of cell death. The TNF-induced signalling mechanisms leading to ne crosis or apoptosis are partially distinct, and are therefore likely to be regulated in a different way. The zinc finger protein A20 is a TNF-induced primary response gene that has been shown to inhibit TNF-induced apoptosis. However, its ability to inhibit the necrotic route of cell death as well a s the underlying mechanism remains unknown. Here we show that stable expres sion of A20 or a fusion protein consisting of Green Fluorescent Protein (GF P) and A20 protects the TNF-sensitive fibroblast cell line L929 partially f rom TNF-induced necrotic cell death. TNF-induced necrosis has been shown to involve the activation of several phospholipases, as well as an increased production of reactive oxygen radicals. The reduced TNF-sensitivity of A20- expressing L929 cells was correlated with a decrease of TNF-induced phospho lipase A(2) (PLA(2)), phospholipase C (PLC) and phospholipase D (PLD) activ ation. Furthermore, production of mitochondrial reactive oxygen intermediat es was retarded by overexpression of A20. These results demonstrate that A2 0 not only inhibits TNF-induced apoptosis but also TNF-induced necrosis, su ggesting that it interferes with an early step in TNF signalling which is r equired for both types of cell death.