The transcription factor NF-B is one of the key regulators of genes involved in the immune and inflammatory response (for review, see Ref. 2). In mammalian cells, NF-B is composed of a homo-or heterodimer of various DNA-binding subunits. Five different DNA-binding subunits share a N-terminal homology domain, which confers DNA binding, dimerization, nuclear translocation, and interaction with the inhibitory IB proteins (for review, see Refs. 3 and 4). In most cell types these proteins sequester NF-B, which is frequently a heterodimer of the p50 and p65 (RelA) subunits, in the cytoplasm by masking their nuclear localization sequence. Constitutive NF-B activity in the cell nucleus can only be detected in certain neurons, some cells of the monocyte/macrophage lineage and B cells (for review, see Refs. 5 and 6

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To test whether the behaviour of transcription factor NF-kB as a promoter or antagonist of apoptosis depends on the apoptotic stimulus, we determined the influence of NF-kB on cell killing elicited by a variety of inducers within a given cell type. Inhibition of NF-kB by genetic and pharmacological approaches rendered HeLa cells more susceptible to TNF-a-induced cell killing, but protected them almost completely from H 2 O 2 -and pervanadate-induced apoptosis. TNF-a was unable to protect HeLa from H 2 O 2 -and pervanadate-induced apoptosis and further enhanced the cytotoxicity induced by these two adverse agents. Supernatants from HeLa cells stably overexpressing a transdominant negative form of IkB-a selectively increased the cytotoxicity of TNF-a for HeLa cells, suggesting that the enhanced susceptibility of these cells can be attributed to one or more secretable factors. Supershift experiments showed that the various apoptotic stimuli induced the same subset of DNA-binding subunits. Therefore, the nature of the signals elicited by the respective death inducers determines whether NF-kB induction leads to apoptosis or survival, suggesting that the manipulation of NF-kB activity may provide a new approach to adjuvant therapy in cancer treatment.Keywords: apoptosis; NF-kB; cancer therapy; TNF-a.In most cell types, the DNA-binding dimer of nuclear factor kB (NF-kB) is retained in the cytoplasm by interaction with an inhibitory protein, called inhibitor of NF-kB (IkB) [1]. A wide variety of stimuli including tumor necrosis factor a (TNF-a), interleukin-1 (IL-1) and T-cell activation lead to phosphorylation of the two major forms of IkB proteins, termed IkB-a and IkB-b. The inducible phosphorylation at serines 32 and 36 of IkB-a is a signal for subsequent ubiquitinylation and degradation by the 26S subunit of the proteasome [2,3]. There is recent evidence for the existence of alternative NF-kB activation pathways. UV radiation leads to the phosphorylation-independent degradation of 5], and reoxygenation-induced tyrosine phosphorylation of IkB-a allows its association with the regulatory subunit of phosphatidylinositol 3 H -kinase, thereby sequestering the inhibitory subunit from NF-kB [6]. Besides its well-established role for the immune response, there is growing evidence for an important function of NF-kB in cell proliferation [7,8] and apoptosis [9]. Anti-apoptotic activities of NF-kB are observed following some apoptotic stimuli, including the cytokine TNF-a, ionizing radiation and the cancer chemotherapeutic agent daunorubicine [10±13]. This protective effect of NF-kB on TNF-a-induced apoptosis is also seen in tumor-bearing mice [14]. In addition, p53-independent apoptosis induced by oncogenic Ras is suppressed by NF-kB activation [15]. On the other hand, there is ample evidence for apoptosispromoting functions of NF-kB. CD4 1 /CD8 1 thymocytes from mice overexpressing a transdominant negative form of IkB-a are resistant to activation-induced cell death [16]. The execution of NF-kB-dependent apoptosis, however,...

Infection with high-risk human papillomaviruses (HPV) can lead to the development of cervical carcinomas. This process critically depends on the virus-encoded E6 and E7 oncoproteins, which stimulate proliferation by manipulating the function of a variety of host key regulatory proteins. Here we show that both viral proteins dose-dependently interfere with the transcriptional activity of NF-B. A variety of experimental approaches revealed that a fraction of the E7 proteins is found in association with the IB kinase complex and attenuates induced kinase activity of IB kinase ␣ (IKK␣) and IKK␤, thus resulting in impaired IB␣ phosphorylation and degradation. Indirect immunofluorescence shows that E7 impairs TNF␣-induced nuclear translocation of NF-B, thus preventing NF-B from binding to its cognate DNA. While E7 obviates IKK activation in the cytoplasm, the E6 protein reduces NF-B p65-dependent transcriptional activity within the nucleus. We suggest that HPV oncogene-mediated suppression of NF-B activity contributes to HPV escape from the immune system. HPVs1 are small DNA viruses, and specific high-risk types such as the HPV type 16 (HPV16) or HPV18 are causative agents of some forms of anogenital and oral cancers (1). HPV16 encodes six early proteins including the major oncoproteins E6 and E7. Both proteins play a central role in the induction of benign proliferation and malignant transformation (2), and at least the persistence of E7 is necessary to maintain the transformed phenotype (3). These two oncoproteins are selectively and continuously expressed in HPV-induced tumors and manipulate cell proliferation upon physical and functional interaction with several master cell cycle regulators (4). E6 binds to p53 (5) and causes its ubiquitin-dependent degradation (6), thereby interfering with p53 functions in cell cycle control and apoptosis. In addition, the E6 protein binds to the protein kinase PKN (7) and other regulators including interferon regulatory factor 3 (8) and the proapoptotic Bak protein (9). The E7 protein interacts with so-called "pocket proteins" such as the retinoblastoma protein pRb, p107, and p130 (10), resulting in their enhanced phosphorylation and degradation (11). pRb destruction results in the release of E2F family transcription factors and subsequent activation of genes promoting cell proliferation (12). But the stimulatory effects of E7 on cell proliferation depends not only on its association with pRb (13, 14), because E7 targets the function of a plethora of regulators including cyclin E (15), acid alpha-glucosidase (16), and M2 pyruvate kinase (17). E7 also interferes with the activity of a variety of transcription factors such as AP-1 (18), interferon regulatory factor-1 (19), fork head domain transcription factor MPP2 (20), and TATA-box-binding protein (21). This multiplicity of interaction partners and additional levels of functional E7 regulation by phosphorylations (22), protein stability (23), and the oligomerization state (24) allow a highly complex and sophisticated manipulatio...