waf1 . Furthermore, p21 waf1 expression and cell cycle arrest were found to depend predominantly on the canonical NF-B pathway, since it was reversed by RNA interference-mediated suppression of RelA. In contrast, suppression of the p100/p52 NF-B subunit had little effect on p21 waf1 . These data reveal that in ALCL cells, in contrast to other cell types, CD30 stimulation elicits p21 waf1 -mediated arrest through the canonical but not the alternative NF-B pathway.

Recent studies have shown that members of the inhibitor of apoptosis (IAP) protein family are highly expressed in several classes of cancer. The primary implication of these findings is that the elevated expression of IAPs is not coincidental but actually participates in oncogenesis by helping to allow the malignant cell to avoid apoptotic cell death. This concept, together with the discovery of several IAP-regulatory proteins that use a conserved mode of action, has stimulated a major effort by many research groups to devise IAP-targeting strategies as a means of developing novel antineoplastic drugs. In this Review, we consider the evidence both for and against the IAPs being valid therapeutic targets, and we describe the types of strategies being used to neutralize their functions.

Expression and signaling of CD30, a tumor necrosis factor (TNF) receptor family member, is upregulated in numerous lymphoid-derived neoplasias, most notably anaplastic large cell lymphoma (ALCL) and Hodgkin's lymphoma (HL). To gain insight into the mechanism of CD30 signaling, we used an affinity purification strategy that led to the identification of the aryl hydrocarbon receptor nuclear translocator (ARNT) as a CD30 interacting protein that modulated the activity of the RelB subunit of the transcription factor nuclear factor (NF)-κB. ALCL cells deficient in ARNT exhibited defects in RelB recruitment to NF-κB-responsive promoters whereas RelA recruitment to the same sites was potentiated, resulting in augmented expression of these NF-κB-responsive genes. These findings indicate that ARNT functions in concert with RelB in a CD30-induced negative feedback mechanism.

It has been suggested that the Drosophila Hid protein interacts with the baculovirus Op-IAP protein in a manner similar to that of human Smac binding to XIAP, based largely on amino acid sequence homology. However, there is little direct experimental evidence in support of this hypothesis; indeed, evidence exists from previous studies suggesting that the mode of binding is not similar. We have now precisely mapped the interaction between Hid and Op-IAP, and we show clearly for the first time that the biochemical interactions between the amino terminus of Hid and BIR2 of Op-IAP are highly similar to those found between the processed amino terminus of Smac and BIR3 of XIAP. Also similar to Smac, the amino terminus of Hid must be processed to bind Op-IAP. In addition, our data also suggest that a second interaction between Hid and Op-IAP exists that does not involve the amino terminus of Hid, which may explain some of the earlier contradictory results. The evolutionary conservation of this mechanism of binding underscores its importance in apoptotic regulation. Nevertheless, interaction with Hid is not sufficient for Op-IAP to inhibit apoptosis induced by Hid overexpression or by treatment with actinomycin D, indicating that additional sequence elements are required for the antiapoptotic function of Op-IAP.

SYNOPSIS Deregulated expression of members of the Inhibitor of Apoptosis (IAP) family has been found in a wide variety of neoplastic cells, and synthetic IAP antagonists represent a promising novel class of chemotherapeutic agents. Early work focused on the ability of these compounds to block the caspase inhibitory function of XIAP. However, recent studies have shown that IAP antagonists, although primarily designed to target XIAP, trigger a ubiquitin-mediated degradation of two related proteins, c-IAP1 and c-IAP2, and through this process potentiate the death of tumor cells via autocrine cellular signaling pathways. In this context, the relative contribution of XIAP as a target of this class of compounds is unclear. Here we examine the involvement of XIAP using a recently described synthetic IAP antagonist, AEG40730, and through the comparison of a human tumor cell line targeted for XIAP with its isogenic, wild type control line. Treatment with nanomolar concentrations of AEG40730 resulted in the loss of both XIAP and c-IAP1 proteins, albeit with different kinetics. While XIAP-deficient HCT116 cells retained some sensitivity to AEG40730 to external apoptotic stimuli, the data suggest that IAP antagonists such as AEG40730 exert their apoptotic enhancing effects through XIAP in addition to the c-IAPs. These data indicate that IAP antagonists can target multiple IAPs to augment distinct pro-apoptotic signaling pathways, thereby revealing the potential for these compounds in cancer therapy and underscoring the promise of IAP-targeted therapies.