Mechanisms by which IkappaB proteins control NF-kappaB activity

Regulation of the NF-kappaB activation pathway by isolated domains of FIP3/IKKgamma, a component of the IkappaB-alpha kinase complex

FIP3, isolated as a type 2 adenovirus E3-14.7-kDa interacting protein, is an essential component of the multimeric IkappaB-alpha kinase (IKK) complex and has been shown to interact with various components (Fas receptor-interacting protein, NF-kappaB-inducing kinase, IKKbeta) of the NF-kappaB activation pathway. FIP3 has also been shown to repress basal and tumor necrosis factor (TNF) alpha-induced NF-kappaB activity as well as to induce cell death when overexpressed. The adenovirus E3-14.7-kDa protein (E3-14.7K) is an inhibitor of TNFalpha-induced cell death. In the current study, we generated deletion mutants to map the domains of FIP3, which are responsible for its various functions. The NF-kappaB inhibitory activity and the E3-14.7K binding domains were mapped at the carboxyl half of the FIP3 protein. We also found that the carboxyl-terminal half of FIP3 blocked TNFalpha-induced IkappaB-alpha phosphorylation and subsequent degradation, which suggests that the stabilization of the cytoplasmic inhibitor of NF-kappaB underlies the FIP3 inhibition of NF-kappaB activity. The amino-terminal 119 amino acids were responsible for the FIP3-IKKbeta and FIP3-IKKalpha interaction, and the middle of the protein (amino acids 201-300) appeared to be both the FIP3 self-association domain as well as the FIP3-Fas receptor-interacting protein interaction domain. Thus, FIP3 might serve as a scaffold protein to organize the various components of the IkappaB-alpha kinase complex. Whereas the full-length protein is required for efficient cell death, the amino-terminal 200 amino acids are sufficient to cause rounding and detachment of the cells from the monolayer.

Blocking and suppressing mechanisms of chemoprevention by dietary constituents

Many dietary constituents are chemopreventive in animal models, and experiments with cultured cells are revealing various potential mechanisms of action. Compounds classified as blocking agents can prevent, or greatly reduce, initiation of carcinogenesis, while suppressing agents affect later stages of the process by reducing cell proliferation. Many compounds have both types of activity. Blocking mechanisms include alteration of drug metabolising activities and scavenging of reactive oxygen species. Mechanisms which suppress tumorigenesis often involve modulation of signal transduction pathways, leading to altered gene expression, cell cycle arrest or apoptosis. As our knowledge of how these dietary components affect cell biochemistry improves, so the likelihood of success in chemoprevention trials and in provision of dietary advice to the general population to optimise the chances of preventing disease is increased.

The I kappa B/NF-kappa B system: a key determinant of mucosalinflammation and protection

The ubiquitous transcription factor NF-kappa B is a central regulator of the transcriptional activation of a number of genes involved in cell adhesion, immune and proinflammatory responses, apoptosis, differentiation, and growth. Induction of these genes in intestinal epithelial cells (IECs) by activated NF-kappa B profoundly influences mucosal inflammation and repair. NF-kappa B activation requires the removal of I kappa B from NF-kappa B by inducible proteolysis, which liberates this transcription factor for migration to the nucleus, where it binds to kappa B-regulatory elements and induces transcription. I kappa B alpha degradation is incomplete and delayed in IECs, resulting in buffered responses to luminal stimuli. The stimulatory environment partially determines whether the effect of NF-kappa B is protective or deleterious for the host. kappa B-dependent proinflammatory gene expression, particularly chemokines, major histocompatibility complex class II antigens, and adhesion molecules may be extremely important in early protective responses to mucosal pathogens but, when dysregulated, could lead to the development of chronic inflammation, as seen in inflammatory bowel diseases. The key role of NF-kappa B in regulating expression of a number of proinflammatory genes makes this protein an attractive target for selective therapeutic intervention.

N-terminal determinants of I kappa B alpha necessary for the cytoplasmic regulation of c-Rel

I kappa B alpha is a dual regulator of Rel/NF-kappa B transcription factors. I kappa B alpha retains inactive NF-kappa B dimers in the cytoplasm, and inhibits their DNA-binding and transcriptional activities in the nucleus. Our previous studies identified discrete functional domains in I kappa B alpha responsible for the cytoplasmic and nuclear regulation of c-Rel. Determinants necessary for regulating c-Rel in the nucleus mapped to the central ankyrin domain of I kappa B alpha and a few negatively-charged amino acids that follow in the C-terminal PEST region. In contrast, sequences involved in the cytoplasmic regulation of c-Rel reside in the N-terminal and central ankyrin domains of I kappa B alpha. Here, we present a refined mapping of the N-terminal determinants of I kappa B alpha necessary for the cytoplasmic regulation of c-Rel homodimers. We demonstrate that amino acids 48 - 58 in p40/I kappa B alpha are essential to block the nuclear localization of c-Rel dimers. These data define a region of I kappa B alpha that may be required for optimal masking of the c-Rel NLS, or for the nuclear export of c-Rel/I kappa B alpha complexes. These findings highlight a novel function for the N-terminus of I kappa B alpha in the control of the subcellular localization of Rel/NF-kappa B dimers. Given the implication of deregulated NF-kappa B activity in hematopoietic and solid tumors, our findings predict that certain alterations in this domain of I kappa B alpha may have severe biological repercussions.

Common pathway for the ubiquitination of IkappaBalpha, IkappaBbeta, and IkappaBepsilon mediated by the F-box protein FWD1

FWD1 (the mouse homolog of Drosophila Slimb and Xenopus betaTrCP, a member of the F-box- and WD40 repeat-containing family of proteins, and a component of the SCF ubiquitin ligase complex) was recently shown to interact with IkappaBalpha and thereby to promote its ubiquitination and degradation. This protein has now been shown also to bind to IkappaBbeta and IkappaBepsilon as well as to induce their ubiquitination and proteolysis. FWD1 was shown to recognize the conserved DSGPsiXS motif (where Psi represents the hydrophobic residue) present in the NH(2)-terminal regions of these three IkappaB proteins only when the component serine residues are phosphorylated. However, in contrast to IkappaBalpha and IkappaBbeta, the recognition site in IkappaBepsilon for FWD1 is not restricted to the DSGPsiXS motif; FWD1 also interacts with other sites in the NH(2)-terminal region of IkappaBepsilon. Substitution of the critical serine residues in the NH(2)-terminal regions of IkappaBalpha, IkappaBbeta, and IkappaBepsilon with alanines also markedly reduced the extent of FWD1-mediated ubiquitination of these proteins and increased their stability. These data indicate that the three IkappaB proteins, despite their substantial structural and functional differences, all undergo ubiquitination mediated by the SCF(FWD1) complex. FWD1 may thus play an important role in NF-kappaB signal transduction through regulation of the stability of multiple IkappaB proteins.

The ankyrin repeat: a diversity of interactions on a common structural framework

The ankyrin repeat is one of the most common protein sequence motifs. Recent X-ray and NMR structures of ankyrin-repeat proteins and their complexes have provided invaluable insights into the molecular basis of the extraordinary variety of biological activities of these molecules. In particular, they have begun to reveal how a large family of structurally related proteins can interact specifically with such a diverse array of macromolecular targets.

E1A sensitizes cells to tumor necrosis factor-induced apoptosis through inhibition of IkappaB kinases and nuclear factor kappaB activities

The adenovirus E1A protein has been implicated in increasing cellular susceptibility to apoptosis induced by tumor necrosis factor (TNF); however, its mechanism of action is still unknown. Since activation of nuclear factor kappaB (NF-kappaB) has been shown to play an anti-apoptotic role in TNF-induced apoptosis, we examined apoptotic susceptibility and NF-kappaB activation induced by TNF in the E1A transfectants and their parental cells. Here, we reported that E1A inhibited activation of NF-kappaB and rendered cells more sensitive to TNF-induced apoptosis. We further showed that this inhibition was through suppression of IkappaB kinase (IKK) activity and IkappaB phosphorylation. Moreover, deletion of the p300 and Rb binding domains of E1A abolished its function in blocking IKK activity and IkappaB phosphorylation, suggesting that these domains are essential for the E1A function in down-regulating IKK activity and NF-kappaB signaling. However, the role of E1A in inhibiting IKK activity might be indirect. Nevertheless, our results suggest that inhibition of IKK activity by E1A is an important mechanism for the E1A-mediated sensitization of TNF-induced apoptosis.

Negative regulation of transactivation function but not DNA binding of NF-kappaB and AP-1 by IkappaBbeta1 in breast cancer cells

The transcription factor NF-kappaB regulates the expression of genes involved in cancer cell invasion, metastasis, angiogenesis, and resistance to chemotherapy. In normal cells NF-kappaB is maintained in the cytoplasm by protein-protein interaction with inhibitor IkappaBs. In contrast, in cancer cells a substantial amount of NF-kappaB is in the nucleus and constitutively activates target genes. To understand the mechanisms of constitutive NF-kappaB activation, we have analyzed the function of IkappaBalpha and IkappaBbeta in breast cancer cells. In most cases, constitutive NF-kappaB DNA binding correlated with reduced levels of either IkappaBalpha or IkappaBbeta isoforms. Overexpression of IkappaBalpha but not IkappaBbeta1 resulted in reduced constitutive DNA binding of NF-kappaB in MDA-MB-231 cells. Unexpectedly, IkappaBbeta1 overexpression moderately increased 12-O-tetradecanoylphorbol-13-acetate- and interleukin-1-inducible NF-kappaB DNA binding. 12-O-Tetradecanoylphorbol-13-acetate- and interleukin-1-induced transactivation by NF-kappaB, however, was lower in IkappaBbeta1-overexpressing cells. Mutants of IkappaBbeta1 lacking the C-terminal casein kinase II phosphorylation sites, which form a stable complex with DNA bound NF-kappaB without inhibiting its transactivation in other cell types, repressed the transactivation by NF-kappaB in MDA-MB-231 cells. Consistent with the results of transient transfections, the expression of urokinase plasminogen activator, an NF-kappaB target gene, was reduced in IkappaBbeta1-overexpressing cells. These results suggest that depending on the cell type, IkappaBbeta1 represses the expression of NF-kappaB-regulated genes by inhibiting either DNA binding or transactivation function of NF-kappaB.