The inhibitors of apoptosis (IAPs) suppress apoptosis through the inhibition of the caspase cascade and thus are key proteins in the control of cell death. Here we have isolated the protein XIAP-associated factor 1 (XAF1) on the basis of its ability to bind XIAP, a member of the IAP family. XIAP suppresses caspase activation and cell death in vitro, and XAF1 antagonizes these XIAP activities. Expression of XAF1 triggers a redistribution of XIAP from the cytosol to the nucleus. XAF1 is ubiquitously expressed in normal tissues, but is present at low or undetectable levels in many different cancer cell lines. Loss of control over apoptotic signalling is now recognized as a critical event in the development of cancer. Our results indicate that XAF1 may be important in mediating the apoptosis resistance of cancer cells.

The cellular inhibitor of apoptosis 2 (cIAP2/HIAP1) is a potent inhibitor of apoptotic death. In contrast to the other members of the IAP family, cIAP2 is transcriptionally inducible by nuclear factor-B in response to multiple triggers. We demonstrate here that cIAP2 ؊/؊ mice exhibit profound resistance to lipopolysaccharide (LPS)-induced sepsis, specifically because of an attenuated inflammatory response. We show that LPS potently upregulates cIAP2 in macrophages and that cIAP2 ؊/؊ macrophages are highly susceptible to apoptosis in a LPS-induced proinflammatory environment. Hence, cIAP2 is critical in the maintenance of a normal innate immune inflammatory response.

The estrogen receptor (ER) is a ligand-activated transcription factor whose DNA-binding domain (ERDBD) has eight cysteines, which coordinate two zinc atoms, forming two zinc finger-like structures. We demonstrate the capability of iron to replace zinc in zinc finger (hereby referred to as iron finger) both in vivo (using Escherichia coli BL21 (DE3)) and in vitro. Iron has the ability to substitute for zinc in the ERDBD as demonstrated by mobility shift and methylation interference assays of iron finger, which show specific recognition of the estrogen response element. The DNA binding constants for both in vivo and in vitro iron-replaced zinc fingers were similar to that of the native finger. Atomic absorption analysis revealed a ratio of 2:1 iron atoms/mol of ERDBD protein, as found for zinc in the crystal structure of native ERDBD. More importantly, we demonstrate that iron finger in the presence of H2O2 and ascorbate generates highly reactive free radicals, causing a reproducible cleavage pattern to the proximate DNA, the estrogen response element. The deoxyribose method, used to detect free radical species generated, and the resultant cleaved DNA ends, caused by iron finger, suggest that the free radicals generated are hydroxyl radicals. Due to the close proximity of the zinc finger to DNA, we postulate that iron-substituted zinc finger may generate free radicals while bound to genetic regulatory response elements, leading to adverse consequences such as iron-induced toxicity and/or carcinogenesis.