Coupling of a signal response domain in I kappa B alpha to multiple pathways for NF-kappa B activation

NF-kappaB inhibits transcription of the H(+)-K(+)-ATPase alpha(2)-subunit gene: role of histone deacetylases

The H(+)-K(+)-ATPase alpha(2) (HKalpha(2)) gene plays a central role in potassium homeostasis, yet little is known about its transcriptional control. We recently demonstrated that the proximal promoter confers basal transcriptional activity in mouse inner medullary collecting duct 3 cells. We sought to determine whether the kappaB DNA binding element at -104 to -94 influences basal HKalpha(2) gene transcription in these cells. Recombinant NF-kappaB p50 footprinted the region -116/-94 in vitro. Gel shift and supershift analysis revealed NF-kappaB p50- and p65-containing DNA-protein complexes in nuclear extracts of mouse inner medullary collecting duct 3 cells. A promoter-luciferase construct with a mutated -104/-94 NF-kappaB element exhibited higher activity than the wild-type promoter in transfection assays. Overexpression of NF-kappaB p50, p65, or their combination trans-repressed the HKalpha(2) promoter. The histone deacetylase (HDAC) inhibitor trichostatin A partially reversed NF-kappaB-mediated trans-repression of the HKalpha(2) promoter. HDAC6 overexpression inhibited HKalpha(2) promoter activity, and HDAC6 coimmunoprecipitated with NF-kappaB p50 and p65. These results suggest that HDAC6, recruited to the DNA protein complex, acts with NF-kappaB to suppress HKalpha(2) transcription and identify NF-kappaB p50 and p65 as novel binding partners for HDAC6.

Construction and characterization of an intracellular single-chain human antibody to hepatitis C virus non-structural 3 protein

BACKGROUND/AIMS

We developed a single-chain antibody fragment (scFv) to the non-structural 3 protein (NS3) of hepatitis C virus (HCV) and tested its ability to interfere with the HCV replication cycle in infected hepatocytes.

METHODS

The variable regions of the human monoclonal antibody CM3.B6 that recognizes a conformational epitope within the helicase domain of NS3 were introduced into adenoviral vectors for expression in mammalian hepatocytes. Expression and binding properties of the scFv were analyzed by immunological assays. Effects of intracellular expression of the scFv on HCV replication were assessed in primary hepatocytes isolated from explanted livers of patients with chronic HCV infection by reverse transcription-polymerase chain reaction.

RESULTS

Transduction of HepG2 cells by the recombinant adenoviruses resulted in stable, efficient expression of scFv in the cytoplasm that was non-toxic to the cells. The scFv specifically bound to its cognate antigen. Significantly, intracellular expression of scFv resulted in a decrease in HCV genomic RNA in HCV infected hepatocytes.

CONCLUSIONS

These results indicate that specific binding of a scFv to NS3 may inhibit one or more functions of this essential viral protein thus interfering with the HCV replication cycle.

IKK beta is required for Bcl-2-mediated NF-kappa B activation in ventricular myocytes

The transcription factor nuclear factor kappa B (NF-kappa B) is regulated by cytoplasmic inhibitor I kappa B alpha. An integral step in the activation of NF-kappa B involves the phosphorylation and degradation of I kappa B alpha. We have previously reported that I kappa B alpha activity is diminished in ventricular myocytes expressing Bcl-2 (de Moissac, D., Zheng, H., and Kirshenbaum, L. A. (1999) J. Biol. Chem. 274, 29505-29509). The underlying mechanism by which Bcl-2 activates NF-kappa B is undefined. In view of growing evidence that the I kappa B kinases (IKKs), notably IKK beta, are involved in signal induced phosphorylation of I kappa B alpha, we ascertained whether IKK beta is necessary and sufficient for Bcl-2 mediated NF-kappa B activation. Here we demonstrate that expression of Bcl-2 in ventricular myocytes resulted in an increase in NF-kappa B-dependent DNA binding, NF-kappa B gene transcription and reduced I kappa B alpha levels. An increase in the IKK beta kinase activity was observed in cells expressing full-length Bcl-2 but not in cells expressing the BH4 deletion mutant of Bcl-2 (Delta BH4; residues 10-30). Catalytically inactive mutants of IKK beta, but not IKK alpha, suppressed Bcl-2-mediated I kappa B alpha phosphorylation and NF-kappa B activation. Transfection of human embryonic 293 cells with a kinase-defective Raf-1 or a kinase-defective mitogen-activated protein kinase/extracellular signal-regulated kinase kinase-1 (MEKK-1) suppressed Bcl-2-mediated IKK beta activity and NF-kappa B activation. Further, Bcl-2-mediated NF-kappa B activity was impaired in nullizygous mouse embryonic fibroblasts deficient for IKK beta. In this report, we provide the first direct evidence that Bcl-2 activates NF-kappa B by a signaling mechanism that involves Raf-1/MEKK-1 mediated activation of IKK beta.

NF-kappaB activates fibronectin gene expression in rat hepatocytes

Fibronectin (FN) plays a role in various biological processes such as fibrosis and tumor metastasis. In this study, we investigated the regulation of FN gene expression by NF-kappaB transcription factor. Transient expression of NF-kappaB p65 increased FN promoter activity in rat hepatocytes. Deletion analysis of FN promoter localized the NF-kappaB-responsive region at the position between -1214 and -1126. Mutations in a putative NF-kappaB element (5(')-GAGAATTTCC-3(')) at -1180 blocked most of the p65-induced promoter activity. Electromobility shift assays showed that the expression of p65 induced the binding of the p65/p65 homodimer to the NF-kappaB site at -1180. Stably p65-expressing cells showed increase of promoter activity, FN protein, and its mRNA levels over control cells. Furthermore, treatment of cells with interleukin-1beta, a NF-kappaB-stimulating cytokine, also increased promoter activity, FN production, and mRNA levels. These results show that NF-kappaB activates FN gene expression by binding to the responsive element at -1180 as the p65/p65 homodimer in rat hepatocytes.

Regulation of human airway epithelial cell IL-8 expression by MAP kinases

Recent studies indicate that maximal IL-8 protein expression requires activation of NF-kappaB as well as activation of the MAP kinases ERK, JNK, and p38. However, the precise relationship between NF-kappaB transactivation and MAP kinase activation remains unclear. We examined the requirements of NF-kappaB, ERK, JNK, and p38 for TNF-alpha-induced transcription from the IL-8 promoter in a human bronchial epithelial cell line. Treatment with TNF-alpha induced activation of all three MAP kinases. Using a combination of chemical and dominant-negative inhibitors, we found that inhibition of NF-kappaB, ERK, and JNK, but not p38, each decreased TNF-alpha-induced transcription from the IL-8 promoter. Inhibition of JNK signaling also substantially reduced TNF-alpha-induced NF-kappaB transactivation, whereas inhibition of ERK and p38 had no effect. On the other hand, ERK was required and sufficient for TNF-alpha-induced activation of activator protein (AP)-1 promoter sequences, which together function as a basal level enhancer. JNK activation was also required for AP-1 transactivation. Finally, inhibition of p38 attenuated IL-8 protein abundance, suggesting that p38 regulates IL-8 expression in a posttranscriptional manner. We conclude that, in human airway epithelial cells, MAP kinases may regulate IL-8 promoter activity by NF-kappaB-dependent (in the case of JNK) and -independent (ERK) processes, as well as by posttranscriptional mechanisms (p38).

A novel protein overexpressed in hepatoma accelerates export of NF-kappa B from the nucleus and inhibits p53-dependent apoptosis

NF-kappa B is a transcription factor that can protect from or contribute to apoptosis. Here we report identification of HSCO that binds to NF-kappa B and inhibits apoptosis. HSCO mRNA was overexpressed in 20 of 30 hepatocellular carcinomas analyzed. Overexpression of HSCO inhibited caspase 9 activation and apoptosis induced by DNA damaging agents, while it augmented apoptosis induced by TNFalpha. Like I kappa B alpha, HSCO inhibited NF-kappa B activity and abrogated p53-induced apoptosis. However, the underlying mechanism was different. HSCO is a nuclear-cytoplasmic shuttling protein, bound to RelA NF-kappa B, and HSCO sequestered it in the cytoplasm by accelerating its export from the nucleus. These results suggest that overexpression of HSCO suppresses p53-induced apoptosis by preventing nuclear localization of NF-kappa B during signaling and thus contributes to hepatocarcinogenesis.

p38 MAPK enhances STAT1-dependent transcription independently of Ser-727 phosphorylation

The transcription factor signal transducer and activator of transcription 1 (STAT1) requires phosphorylation at both Tyr-701 and Ser-727 for full activation. IFN-gamma induces phosphorylation of both residues, whereas stress signals like UV or lipopolysaccharide stimulate phosphorylation of Ser-727 only. Using p38alpha mitogen-activated protein kinase (MAPK)-deficient cells, we show that the stress-induced phosphorylation of Ser-727 requires p38alpha MAPK activity, whereas IFN-gamma-stimulated Ser-727 phosphorylation occurs independently of the p38alpha pathway. Consistently, IFN-gamma stimulated expression of the STAT1 target gene IRF1 to a similar extent in both wild-type and p38alpha-deficient cells. However, stress-induced activation of the p38 MAPK pathway considerably enhanced the IFN-gamma-induced expression of both the endogenous IRF1 gene and a reporter driven by the IFN-gamma-activated sequence element of the IRF1 promoter. This enhancement occurred independently of increased phosphorylation of Ser-727 by the p38 pathway. Taken together, these results demonstrate an interaction between IFN-gamma signaling and the p38 pathway that leads to increased transcriptional activation by STAT1 independently of phosphorylation at Ser-727.

Radiosensitization by overexpression of the nonphosphorylation form of IkappaB-alpha in human glioma cells

To assess the role of NF-kappaB in cellular radiosensitivity, we constructed mutated IkappaB expression plasmids for SY-IkappaB (with mutations at residues of 32, 36 and 42) expression in human malignant glioma cells (radiosensitive MO54 and radioresistant T98 cells), giving respective cell types referred to as MO54-SY4 and T98-SY14. Both of the clones expressing SY-IkappaB became radiosensitive, compared with the parental MO54 and T98 cells. A treatment with herbimycin A or genistein did not change the radiosensitivity of cells expressing SY-IkappaB, but made both the MO54 and T98 parental cells more sensitive to ionizing radiation. A treatment with TNF-alpha induced DNA fragmentation and apoptosis in cells expressing SY-IkappaB, but not in MO54 and T98 cells. The survival after X-ray exposure of the parental MO54 cells was slightly increased by a TNF-alpha treatment, but that of the parental T98 cells did not change. The change in sensitivity to ultra-violet (UV) radiation and adriamycin in MO54-SY4 cells was very similar to that for X-ray sensitivity, but no change was observed in T98-SY14 cells. Significant sublethal damage repair was observed in T98 cells, whereas MO54 cells showed little repair activity. The expression of p53 was enhanced in the parental MO54 cells, while the p53 levels in the MO54-SY4, and in the parent and clonal T98 cells, did not change. Our data suggest that the serine and tyrosine phosphorylation of IkappaB-alpha may play a role in determining the radiosensitivity of malignant glioma cells.

Identification of polymorphisms of the IkappaBalpha gene associated with an increased risk of multiple myeloma

When NF-kappaB proteins are bound to IkappaBalpha, they remain in the cytosol, and are unable to act as transcription factors. Phosphorylation of IkappaBalpha at Serine32 and Serine36 has been shown to stimulate ubiquitination followed by proteasome-mediated degradation of IkappaBalpha, resulting in the release of active NF-kappaB. NF-kappaB activity is associated with bone loss and B cell growth as well as chemotherapy resistance. Because previous studies have shown abnormalities of the IkappaBalpha gene in patients with lymphoma, we determined whether alterations of this gene also occur in multiple myeloma (MM). We determined the DNA sequence of the IkappaBalpha gene from bone marrow mononuclear cells from 18 MM patients and 24 healthy subjects as well as two MM cell-lines. We identified eight polymorphisms. Statistically, the prevalence of three polymorphisms, one in exon 1 and two in exon 6, were significantly higher in MM patients (alpha>1) compared with samples from control subjects. Six of eight polymorphisms in myeloma samples have also been identified in previous studies of IkappaBalpha sequences derived from lymphoma samples. In addition, we detected two polymorphisms in the IkappaBalpha gene that have not been previously reported. Together, these results provide the basis for future evaluation the IkappaBalpha/NF-kappaB pathway in MM patients.

Regulation of airway smooth muscle cyclin D1 transcription by protein kinase C-delta

The precise mechanism by which protein kinase C-delta (PKCdelta) inhibits cell cycle progression is not known. We investigated the regulation of cyclin D1 transcription by PKCdelta in primary bovine airway smooth muscle cells. Overexpression of the active catalytic subunit of PKCdelta attenuated platelet-derived growth factor (PDGF)-mediated transcription from the cyclin D1 promoter, whereas overexpression of a dominant-negative PKCdelta increased promoter activity. A PKCdelta-specific pseudosubstrate increased cyclin D1 protein abundance. To determine the transcriptional mechanism by which PKCdelta negatively regulates cyclin D1 expression, we transiently transfected cells with cDNAs encoding cyclin D1 promoter 5' deletions and site mutations in the context of a -66 promoter fragment. We found that the -57 to -52 CRE/ATF2 site functions as a basal level and PDGF enhancer, whereas the -39 to -30 nuclear factor-kappaB site functions as a basal level suppressor. Further, PDGF and PKCdelta responsiveness of the cyclin D1 promoter was maintained following 5' deletion to the Ets-containing -22 minimal promoter. Finally, using electrophoretic mobility gel shift and reporter assays, we determined that PKCdelta inhibits CRE/ATF2 binding and transactivation, activates nuclear factor-kappaB binding and transactivation, and attenuates Ets transactivation. These data suggest that PKCdelta attenuates cyclin D1 promoter activity via the regulation of three distinct cis-acting regulatory elements.

Phosphorylation-dependent ubiquitylation and degradation of androgen receptor by Akt require Mdm2 E3 ligase

The androgen receptor (AR) controls several biological functions including prostate cell growth and apoptosis. However, the mechanism by which AR maintains its stability to function properly remains largely unknown. Here we show that Akt and Mdm2 form a complex with AR and promote phosphorylation-dependent AR ubiquitylation, resulting in AR degradation by the proteasome. The effect of Akt on AR ubiquitylation and degradation is markedly impaired in a Mdm2-null cell line compared with the wild-type cell line, suggesting that Mdm2 is involved in Akt-mediated AR ubiquitylation and degradation. Furthermore, we demonstrate that the E3 ligase activity of Mdm2 and phosphorylation of Mdm2 by Akt are essential for Mdm2 to affect AR ubiquitylation and degradation. These results suggest that phosphorylation-dependent AR ubiquitylation and degradation by Akt require the involvement of Mdm2 E3 ligase activity, a novel mechanism that provides insight into how AR is targeted for degradation.

Characterization of the nuclear import and export functions of Ikappa B(epsilon)

Control over the nuclear localization of nuclear factor kappaB/Rel proteins is accomplished in large part through association with members of the inhibitor of kappaB (IkappaB) protein family. For example, the well studied IkappaBalpha protein actively shuttles between the nucleus and the cytoplasm and both inhibits nuclear import and mediates nuclear export of NF-kappaB/Rel proteins. In contrast, the IkappaBbeta protein can inhibit nuclear import of NF-kappaB/Rel proteins but does not remove NF-kappaB/Rel proteins from the nucleus. To further understand how the IkappaB proteins control the nuclear-cytoplasmic distribution of NF-kappaB/Rel proteins, we have characterized the nuclear import and nuclear export functions of IkappaBepsilon. Our results indicate that the IkappaBepsilon protein, like the IkappaBalpha protein, actively shuttles between the nucleus and the cytoplasm. Similar to IkappaBalpha, nuclear import of IkappaBepsilon is mediated by its ankyrin repeat domain and is not blocked by the dominant-negative RanQ69L protein. However, the nuclear import function of the IkappaBepsilon ankyrin repeat domain is markedly less efficient than that of IkappaBalpha, with the result that nuclear shuttling of IkappaBepsilon between the nucleus and the cytoplasm is significantly slower than IkappaBalpha. Nuclear export of IkappaBepsilon is mediated by a short leucine-rich nuclear export sequence (NES)-like sequence ((343)VLLPFDDLKI(352)), located between amino acids 343 and 352. This NES-like sequence is required for RanGTP-dependent binding of IkappaBepsilon to CRM1. Nuclear accumulation of IkappaB(epsilon) is increased by either leptomycin B treatment or alanine substitutions within the IkappaBepsilon-derived NES. A functional NES is required for both efficient cytoplasmic retention and post-induction control of c-Rel by IkappaBepsilon, consistent with the notion that IkappaBepsilon-mediated nuclear export contributes to control over the nucleocytoplasmic distribution of NF-kappaB/Rel proteins.

Genetic evidence for the essential role of beta-transducin repeat-containing protein in the inducible processing of NF-kappa B2/p100

Processing of the nf kappa b2 gene product p100 to generate p52 is an important step in NF-kappa B regulation. This step is regulated by a nonclassical NF-kappa B signaling pathway involving the NF-kappa B-inducing kinase (NIK). NIK induces p100 processing by triggering phosphorylation of specific C-terminal serines of p100. However, the downstream molecular events leading to p100 processing remain unclear. Here we show that NIK induced the physical recruitment of beta-transducin repeat-containing protein (beta-TrCP), a component of the SCF ubiquitin ligase complex, to p100. This event required the phosphorylation sites as well as the death domain of p100. Using the RNA interference technique, we demonstrated that beta-TrCP is essential for NIK-induced p100 ubiquitination and processing. Interestingly the constitutive processing of p100 mutants was independent of beta-TrCP. These results suggest that beta-TrCP is an essential component of NIK-induced p100 processing.

NF-kappaB inhibition in EBV-transformed lymphoblastoid cell lines

Epstein-Barr virus (EBV) transforms B-lymphocytes into lymphoblastoid cell lines usurping multiple signaling pathways including NF-kappaB activation. To determine whether NF-kappaB activity is essential in the growth and survival of EBV-transformed lymphoblastoid cell lines, a non-degradable IkappaBa mutant was expressed under tetracycline regulation in IB4 cells. NF-kappaB inhibition caused caspase 3 and 8 activation, PARP cleavage, and DNA fragmentation indicative of apoptosis. Mitochondrial membrane potential was diminished without release of cytochrome c or apoptosis initiating factor. z-VAD.FMK, a general caspase inhibitor, failed to block apoptosis, indicating a distinct pathway contributes to cell death. Bfl-1 expression, an anti-apoptotic Bcl-2 family member, is diminished after NF-kappaB inhibition whereas Bcl-2 and Bcl-x/L expression is unaffected. These studies suggest that NF-kappaB itself, or NF-kappaB-regulated genes, will be successful molecular targets for the treatment of EBV-associated diseases.

Activation of cyclin D1 and D2 promoters by human T-cell leukemia virus type I tax protein is associated with IL-2-independent growth of T cells

Our aim was to examine the involvement of G(1) cell-cycle regulators in cell growth dysregulation induced by HTLV-I. Compared to uninfected cells, higher expression levels of cyclin D1 and D2 mRNA were detected in HTLV-I-infected T-cell lines, which were at least in part mediated by the viral transforming protein Tax since Tax activated both cyclin D1 and D2 promoters in the human T-cell line Jurkat. A Tax mutant that did not activate NF-kappaB failed to activate cyclin D1 and D2 promoters. Inhibitors of NF-kappaB (dominant negative IkappaBs mutants) suppressed Tax-dependent activation of cyclin D1 and D2 promoters, indicating that Tax-induced activation was mediated by NF-kappaB. Wild-type and mutant Tax capable of activating NF-kappaB, but not Tax mutant incapable of activating NF-kappaB, converted cell growth of a T-cell line from being IL-2-dependent to being IL-2-independent; and this conversion was associated with IL-2-independent induction of cyclins D1 and D2. Our data suggest that induction of cyclins D1 and D2 by Tax is involved in IL-2-independent cell-cycle progression as well as IL-2-independent transformation of primary human T cells by HTLV-I. High expression levels of cyclin D1 and D2 mRNAs were also detected in some patients with ATL. Our findings link HTLV-I infection to changes in cellular D-type cyclin gene expression, transformation of T cells and subsequent development of T-cell leukemia.

A novel in vitro assay for deubiquitination of I kappa B alpha

The ubiquitin-proteasome pathway (UPP) controls a wide range of signal transduction cascades by targeting key regulatory proteins for 26S proteasome-mediated degradation. Several observations suggest that protein deubiquitination may modulate this process; however, few experiments have been performed to test this idea. An excellent model system for studying the regulatory role of the UPP is signal transduction via the nuclear factor-kappa B (NF-kappa B) family of transcription factors. The principal inhibitor of NF-kappa B, I kappa B alpha, is polyubiquitinated and degraded in response to diverse stimuli. In this study, we sought to determine whether I kappa B alpha deubiquitination also occurs. We established an in vitro deubiquitination assay using polyubiquitinated I kappa B alpha as the substrate. Our data provide evidence of an I kappa B alpha-directed deubiquitinating activity present in lysates of several cell lines. This activity was inhibited by ubiquitin aldehyde, a specific inhibitor of deubiquitinating enzymes, as well as by alkylating reagents or heat, but was unaffected by the inhibition of several other classes of proteases. Cell lysates and the deubiquitinating enzyme, UCH-L3, hydrolyzed ubiquitin 7-amido-4-methylcoumarin, a model substrate for assaying deubiquitinating activities. However, UCH-L3 had no detectable activity toward ubiquitinated I kappa B alpha, thus suggesting a degree of enzymatic specificity in the deubiquitination of I kappa B alpha. This assay will be useful for the study of I kappa B alpha deubiquitination. Moreover, this assay can be adapted to monitor the deubiquitination of other proteins modified by ubiquitin conjugation.

Inhibition of transforming growth factor beta signaling and Smad-dependent activation of transcription by the Latent Membrane Protein 1 of Epstein-Barr virus

Inhibition of transforming growth factor beta (TGFbeta) signaling by the Epstein-Barr virus Latent Membrane Protein 1 (LMP1) may account, at least in part, for the oncogenic activity of LMP1. We found that LMP1 is a potent inhibitor of TGFbeta signaling and Smad-dependent activation of transcription in 293 epithelial cells and COS-7 fibroblasts. LMP1 strongly inhibited the uninduced and the Smad-inducible activity of the promoters of the human p21/WAF1/Cip1 gene and the mouse Smad7 gene. Inhibition of TGFbeta signaling and Smad-dependent activation of transcription by LMP1 was greatly reduced by deletion of both C-terminal activating regions 1 and 2 of LMP1 as well as by overexpression of a non-degradable form of IkappaB. In contrast, specific inhibitors of p38 kinase or MEK kinase did not reverse the inhibitory activity of LMP1. TGFbeta signaling was enhanced by overexpression of dominant negative forms of the LMP1 effectors TRAF2, NIK, and IKKbeta and was abolished by overexpression of p65/RelA or a p50/p65 fusion protein. Deletion of the transactivation domain of p65 abolished its inhibitory activity. Immunoblotting and immunofluorescence microscopy indicated that suppression of TGFbeta signaling and Smad transcriptional activity by LMP1 was not due to Smad degradation or cytoplasmic retention suggesting that LMP1 affects the nuclear function of Smad proteins. Our data are consistent with an essential role of NF-kappaB activation by LMP1 in the inhibition of TGFbeta signaling and Smad-mediated transcriptional responses.

Distinct roles of the Ikappa B kinase alpha and beta subunits in liberating nuclear factor kappa B (NF-kappa B) from Ikappa B and in phosphorylating the p65 subunit of NF-kappa B

Phosphatidylinositol 3'-kinase (PI3K) and the serine/threonine kinase AKT have critical roles in phosphorylating and transactivating the p65 subunit of nuclear factor kappaB (NF-kappaB) in response to the pro-inflammatory cytokines interleukin-1 (IL-1) and tumor necrosis factor (TNF). Mouse embryo fibroblasts (MEFs) lacking either the alpha or beta subunit of IkappaB kinase (IKK) were deficient in NF-kappaB-dependent transcription following treatment with IL-1 or TNF. However, in contrast to IKKbeta-null MEFs, IKKalpha-null MEFs were not substantially defective in the cytokine-stimulated degradation of Ikappabetaalpha or in the nuclear translocation of NF-kappaB. The IKK complexes from IKKalpha- or IKKbeta-null MEFs were both deficient in PI3K-mediated phosphorylation of the transactivation domain of the p65 subunit of NF-kappaB in response to IL-1 and TNF, and constitutively activated forms of PI3K or AKT did not potentiate cytokine-stimulated activation of NF-kappaB in either IKKalpha- or IKKbeta-null MEFs. Collectively, these data indicate that, in contrast to IKKbeta, which is required for both NF-kappaB liberation and p65 phosphorylation, IKKalpha is required solely for the cytokine-induced phosphorylation and activation of the p65 subunit of NF-kappaB that are mediated by the PI3K/AKT pathway.

Activation of NF-kappaB is essential for hepatocyte growth factor-mediated proliferation and tubulogenesis

Hepatocyte growth factor (HGF) and its receptor, Met, regulate a number of biological functions in epithelial and nonepithelial cells, such as survival, motility, proliferation, and tubular morphogenesis. The transcription factor NF-kappaB is activated in response to a wide variety of stimuli, including growth factors, and is involved in biological responses in part overlapping with those triggered by HGF. In this work we used the liver-derived MLP29 cell line to study the possible involvement of NF-kappaB in HGF/Met signaling. HGF stimulates NF-kappaB DNA binding and transcriptional activation via the canonical IkappaB phosphorylation-degradation cycle and via the extracellular signal-regulated kinase 1/2 and p38 mitogen-activated protein kinase cascades. Phosphatidylinositol 3-kinase is not involved in Met-mediated NF-kappaB activation. Blockage of NF-kappaB activation in MLP29 cells by forced expression of the NF-kappaB super-repressor IkappaB(alpha)2A does not interfere with HGF-induced scatter but inhibits proliferation and tubulogenesis. Surprisingly, in the same cells NF-kappaB appears to be dispensable for the antiapoptotic function of HGF.

Regulation of antigen receptor gene assembly in lymphocytes

Somatic alterations affecting the mammalian genome occur exclusively in B and T cells. Developing lymphocytes employ a series of DNA recombination events (V(D)J recombination) to assemble a diverse repertoire of immunoglobulin (Ig) and T cell receptor (TCR) variable regions from a large array of germline gene segments. V(D)J recombination is required not only for receptor diversification but also for lymphocyte development. At a molecular level, these recombination events are directed by conserved DNA sequences flanking all antigen receptor gene segments that function as recognition signals for a single recombinase activity. Despite these shared features, recombination events are controlled at the levels of stage- and tissue-specificity. Our primary research focus is to dissect the mechanisms that regulate assembly of antigen receptor loci by rendering certain gene segments accessible to a common V(D)J recombinase. This article discusses recent discoveries from the author's laboratory that address this long-standing issue. We have found that transcriptional promoters are critical cis-acting regulatory elements for targeting efficient recombination of chromosomal gene segments. We have also demonstrated that activation of NF-kappaB signaling in precursor B cells is required for global regulation of Ig light chain gene assembly. Together, these findings provide key insights into the genetic mechanisms that regulate antigen receptor diversity and the developmental pathways leading to the acquisition of lymphocyte effector function.

Molecular mechanisms in lymphocyte activation and growth

Transcription factor NF-kappaB is biochemically coupled to the T cell antigen receptor (TCR) and activated transiently during an adaptive immune response. The author's laboratory is investigating the signal-dependent regulation of NF-kappaB, its downstream gene targets, and its function in lymphocyte biology. Our studies have revealed novel enzymatic checkpoints in the NF-kappaB signaling pathway and constitutive repressors of NF-kappaB that might be clinically applicable for therapeutic control of the immune system. We have also found that the Tax transforming protein encoded by human T cell leukemia virus type 1 (HTLV1) binds to and persistently activates an inducible protein kinase in the TCR/NF-kappaB axis. This viral/host interaction appears to trigger the inappropriate expression of NF-kappaB and the development of HTLV1-associated disease.

Molecular mechanisms in lymphocyte activation and growth

Transcription factor NF-kappaB is biochemically coupled to the T cell antigen receptor (TCR) and activated transiently during an adaptive immune response. The author's laboratory is investigating the signal-dependent regulation of NF-kappaB, its downstream gene targets, and its function in lymphocyte biology. Our studies have revealed novel enzymatic checkpoints in the NF-kappaB signaling pathway and constitutive repressors of NF-kappaB that might be clinically applicable for therapeutic control of the immune system. We have also found that the Tax transforming protein encoded by human T cell leukemia virus type 1 (HTLV1) binds to and persistently activates an inducible protein kinase in the TCR/NF-kappaB axis. This viral/host interaction appears to trigger the inappropriate expression of NF-kappaB and the development of HTLV1-associated disease.

Contrasting effects of IG20 and its splice isoforms, MADD and DENN-SV, on tumor necrosis factor alpha-induced apoptosis and activation of caspase-8 and -3

We identified a novel cDNA (IG20) that is homologous to cDNAs encoding a protein differentially expressed in normal and neoplastic cells (DENN-SV) and human MADD (MAPK-activating death domain-containing protein). Furthermore, we show that the above variants most likely result from alternative splicing of a single gene. Functional analyses of these variants in permanently transfected HeLa cells revealed that IG20 and DENN-SV render them more susceptible or resistant to tumor necrosis factor alpha (TNF-alpha)-induced apoptosis, respectively. All variants tested could interact with TNF receptor 1 and activate ERK and nuclear factor kappaB. However, relative to control cells, only cells expressing IG20 showed enhanced TNF-alpha-induced activation of caspase-8 and -3, whereas cells expressing DENN-SV showed either reduced or no caspase activation. Transfection of these cells with a cDNA encoding CrmA maximally inhibited apoptosis in HeLa-IG20 cells. Our results show that IG20 can promote TNF-alpha-induced apoptosis and activation of caspase-8 and -3 and suggest that it may play a novel role in the regulation of the pleiotropic effects of TNF-alpha through alternative splicing.

Retroviral oncoprotein Tax induces processing of NF-kappaB2/p100 in T cells: evidence for the involvement of IKKalpha

IkappaB kinase (IKK) is a key mediator of NF-kappaB activation induced by various immunological signals. In T cells and most other cell types, the primary target of IKK is a labile inhibitor of NF-kappaB, IkappaBalpha, which is responsible for the canonical NF-kappaB activation. Here, we show that in T cells infected with the human T-cell leukemia virus (HTLV), IKKalpha is targeted to a novel signaling pathway that mediates processing of the nfkappab2 precursor protein p100, resulting in active production of the NF-kappaB subunit, p52. This pathogenic action is mediated by the HTLV-encoded oncoprotein Tax, which appears to act by physically recruiting IKKalpha to p100, triggering phosphorylation-dependent ubiquitylation and processing of p100. These findings suggest a novel mechanism by which Tax modulates the NF-kappaB signaling pathway.

Involvement of nuclear factor-kappaB in endothelin-A-receptor-induced proliferation and inhibition of apoptosis

Endothelins have been implicated in the regulation of cell proliferation, differentiation, and apoptosis, but the mechanisms of these complex events are not yet fully understood. Although the nuclear factor-kappaB (NF-kappaB) was shown to play a prominent role in the above processes, its participation in endothelin receptor A (ET(A)R) signaling has not been previously demonstrated. This study provides evidence that NF-kappaB is involved in ET(A)R-induced proliferation and inhibition of apoptosis. Endothelin (ET)-1, ET-3, and sarafotoxin b induce cell proliferation and prevent apoptosis induced by serum deprivation in a Chinese hamster lung (CCL39) cell line that stably expresses ET(A)R (CCL39ET(A)). Activation of ET(A)R resulted in enhanced DNA-binding activity of NF-kappaB and degradation of IkappaB-alpha. Expression of the dominant negative form of IkappaB-alpha (IkappaB deltaN) inhibited the proliferative activities mediated by ET(A)R as well as its anti-apoptotic activities. Treatment of the cells with prostaglandin A1, an inhibitor of IkappaB kinase-beta, reduced ET-1-induced proliferation and its anti-apoptotic effect. These findings indicate that the regulation of cell proliferation and apoptosis by ET(A)R is mediated by the ET(A)R-activated NF-kappaB.

Role of nuclear factor-kappa B (NF-kappa B) in inflammation, periodontitis, and atherogenesis

Atherosclerosis, the major cause of death and disability in the United States, is a chronic disease with inflammatory components. The first objective of this review is to explain how activation of NF-kappa B contributes to atherosclerosis. The second objective is to describe a potential link between inflammation, activation of NF-kappa B, and periodontitis. The nuclear transcription factor NF-kappa B controls the expression of many genes linked to atherogenesis including those involved with inflammation. We hypothesize that one unifying mechanism in this complex disease is the activation of NF-kappa B. The mechanism(s) that activates NF-kappa B in atherogenesis is unknown and the effect of inhibiting NF-kappa B activation on atherogenesis is untested. Periodontal disease has now been established as a risk factor for atherosclerosis and its thrombotic complications. It is unknown if periodontal disease contributes to the initiation or progression of atherosclerosis. We hypothesize that the chronic and intense inflammatory response accompanying periodontal disease produces an excess burden of circulating mediators of inflammation that initiate or exacerbate the inflammatory components of atherogenesis. Further understanding of the mechanisms involved in the activation of NF-kappa B in atherosclerosis could lead to important therapeutic applications especially as it relates to the impact of periodontitis.

A role for NF-kappa B in the induction of beta-R1 by interferon-beta

Previous experiments have suggested that induction of the beta-R1 gene by interferon (IFN)-beta required transcription factor ISGF-3 (IFN-stimulated gene factor-3) and an additional component. We now provide evidence that nuclear factor-kappaB (NF-kappaB) can serve as this component. Site-directed mutagenesis of an NF-kappaB binding site in the beta-R1 promoter or over-expression of an IkappaBalpha super-repressor abrogated IFN-beta-mediated induction of a beta-R1 promoter-reporter. IFN-beta treatment did not augment abundance of NF-kappaB but did lead to phosphorylation of the p65 NF-kappaB subunit. It is proposed that IFN-beta-mediated enhancement of the transactivation competence of NF-kappaB components is required for inducible transcription of the beta-R1 promoter. These results provide a novel insight into the role of NF-kappaB in the transcriptional response to IFN-beta.

Inefficient ZAP-70 phosphorylation and decreased thymic selection in vivo result from inhibition of NF-kappaB/Rel

Signaling from the TCR regulates T lymphoid survival, deletion by apoptosis, and selective clonal expansion. One set of signaling pathways activated during thymic selection leads to degradation of a cytosolic retention protein, the inhibitor of kappaB (IkappaB)alpha, followed by nuclear translocation of the NF-kappaB/Rel family of transcription factors. It has been found previously that NF-kappaB proteins mediate a pathway signaling the survival of mature T cells and protection of thymocytes against TNF-induced apoptosis. In contrast, we show in this study that a transgenic inhibitor of NF-kappaB/Rel signaling interferes with the negative selection of immature thymocytes by endogenous MHC ligands in vivo. Positive selection of the H-Y TCR also was diminished. This attenuation of thymic selection efficiency was associated with decreased ZAP-70 phosphorylation and TCR signaling of CD69 induction. These findings demonstrate that the NF-kappaB transcriptional pathway plays an important role in normal processes of clonal deletion and they indicate that the NF-kappaB/IkappaB axis can regulate the efficiency of TCR signaling.

IKKgamma /NEMO facilitates the recruitment of the IkappaB proteins into the IkappaB kinase complex

IKKgamma/NEMO is an essential regulatory component of the IkappaB kinase complex that is required for NF-kappaB activation in response to various stimuli including tumor necrosis factor-alpha and interleukin-1beta. To investigate the mechanism by which IKKgamma/NEMO regulates the IKK complex, we examined the ability of IKKgamma/NEMO to recruit the IkappaB proteins into this complex. IKKgamma/NEMO binding to wild-type, but not to a kinase-deficient IKKbeta protein, facilitated the association of IkappaBalpha and IkappaBbeta with the high molecular weight IKK complex. Following tumor necrosis factor-alpha treatment of HeLa cells, the majority of the phosphorylated form of endogenous IkappaBalpha was associated with the high molecular weight IKK complex in HeLa cells and parental mouse embryo fibroblasts but not in IKKgamma/NEMO-deficient cells. Finally, we demonstrate that IKKgamma/NEMO facilitates the association of the IkappaB proteins and IKKbeta and leads to increases in IKKbeta kinase activity. These results suggest that an important function of IKKgamma/NEMO is to facilitate the association of both IKKbeta and IkappaB in the high molecular weight IKK complex to increase IkappaB phosphorylation.

Salicylates inhibit flavivirus replication independently of blocking nuclear factor kappa B activation

Flaviviruses comprise a positive-sense RNA genome that replicates exclusively in the cytoplasm of infected cells. Whether flaviviruses require an activated nuclear factor(s) to complete their life cycle and trigger apoptosis in infected cells remains elusive. Flavivirus infections quickly activate nuclear factor kappa B (NF-kappaB), and salicylates have been shown to inhibit NF-kappaB activation. In this study, we investigated whether salicylates suppress flavivirus replication and virus-induced apoptosis in cultured cells. In a dose-dependent inhibition, we found salicylates within a range of 1 to 5 mM not only restricted flavivirus replication but also abrogated flavivirus-triggered apoptosis. However, flavivirus replication was not affected by a specific NF-kappaB peptide inhibitor, SN50, and a proteosome inhibitor, lactacystin. Flaviviruses also replicated and triggered apoptosis in cells stably expressing IkappaBalpha-DeltaN, a dominant-negative mutant that antagonizes NF-kappaB activation, as readily as in wild-type BHK-21 cells, suggesting that NF-kappaB activation is not essential for either flavivirus replication or flavivirus-induced apoptosis. Salicylates still diminished flavivirus replication and blocked apoptosis in the same IkappaBalpha-DeltaN cells. This inhibition of flaviviruses by salicylates could be partially reversed by a specific p38 mitogen-activated protein (MAP) kinase inhibitor, SB203580. Together, these results show that the mechanism by which salicylates suppress flavivirus infection may involve p38 MAP kinase activity but is independent of blocking the NF-kappaB pathway.

Analysis of the NF-kappa B and PI 3-kinase/Akt survival pathways in nerve growth factor-dependent neurons

Nerve growth factor (NGF) readdition to NGF-deprived neurons can halt Jun N-terminal kinase (JNK) activation, cytochrome c release, and cell death through mechanisms that may involve phosphatidylinositol (PI) 3-kinase, Akt, and nuclear factor kappa B (NF-kappaB). We found that expression of the NF-kappaB protein c-Rel in NGF-deprived neurons blocks cytochrome c release but does not inhibit c-Jun phosphorylation. Conversely, inhibition of NF-kappaB in NGF-maintained neurons promotes cytochrome c release and cell death. In contrast to c-Rel, activated PI 3-kinase and Akt inhibit c-Jun phosphorylation but have only a small effect on cytochrome c release. Finally, although c-Rel can protect neurons from death caused by inhibitors of PI 3-kinase or Akt, NF-kappaB function is not critical for Akt-promoted survival. These results suggest that the PI 3-kinase/Akt and NF-kappaB survival pathways target distinct cell death events in neurons.

Dual role of inflammatory stimuli in activation-induced cell death of mouse microglial cells. Initiation of two separate apoptotic pathways via induction of interferon regulatory factor-1 and caspase-11

We have previously shown that mouse microglial cells undergo apoptosis upon inflammatory activation and that nitric oxide (NO) is the major autocrine mediator in this process (Lee, P., Lee, J., Kim, S., Yagita, H., Lee, M. S., Kim, S. Y., Kim, H., and Suk, K. (2001) Brain Res. 892, 380-385). Here, we present evidence that interferon regulatory factor-1 (IRF-1) and caspase-11 are the essential molecules in activation-induced cell death of microglial cells. The apoptogenic action of inflammatory stimuli such as lipopolysaccharide (LPS) and interferon-gamma (IFNgamma) was mediated through the induction of IRF-1 and caspase-11 expression in two separate events. Although IRF-1 was required for NO synthesis, caspase-11 induction was necessary for NO-independent apoptotic pathway. Microglial cells from IRF-1-deficient mice showed markedly decreased NO production, and they were partially resistant to apoptosis in response to LPS/IFNgamma but were sensitive to NO donor exposure. LPS/IFNgamma treatment resulted in the induction of caspase-11 followed by activation of caspase-11, -1, and -3. Inactivation of caspase-11 by the transfection of dominant-negative mutant or treatment with the caspase inhibitors rendered microglial cells partially resistant to LPS/IFNgamma-induced apoptosis. Inhibition of both NO synthesis and caspase-11 completely blocked LPS/IFNgamma-induced cytotoxicity. These results indicated that LPS/IFNgamma not only induced the production of cytotoxic NO through IRF-1 but also initiated the NO-independent apoptotic pathway through the induction of caspase-11 expression.

Signaling pathways activated by daunorubicin

The anthracycline daunorubicin is widely used in the treatment of acute nonlymphocytic leukemia. The drug has, of course, been the object of intense basic research, as well as preclinical and clinical study. As reviewed in this article, evidence stemming from this research clearly demonstrates that cell response to daunorubicin is highly regulated by multiple signaling events, including a sphingomyelinase-initiated sphingomyelin-ceramide pathway, mitogen-activated kinase and stress-activated protein/c-Jun N-terminal kinase activation, transcription factors such as nuclear factor kappa B, as well as the Fas/Fas-ligand system. These pathways are themselves influenced by a number of lipid products (diacylglycerol, sphingosine-1 phosphate, and glucosyl ceramide), reactive oxygen species, oncogenes (such as the tumor suppressor gene p53), protein kinases (protein kinase C and phosphoinositide-3 kinase), and external stimuli (hematopoietic growth factors and the extracellular matrix). In light of the complexity and diversity of these observations, a comprehensive review has been attempted toward the understanding of their individual implication (and regulation) in daunorubicin-induced signaling. (Blood. 2001;98:913-924)

NF-kappa B-mediated chemoresistance in breast cancer cells

BACKGROUND

Nuclear factor-kappa B (NF-kappa B) is a known survival pathway, and it may explain differential sensitivity to tumor necrosis factor-alpha (TNF-alpha) and chemotherapeutic-induced apoptosis in apoptotically sensitive (APO+) and apoptotically resistant (APO-) Michigan Cancer Foundation-7 breast cancer cells.

METHODS

Crystal violet viability and luciferase reporter gene assays were used to determine the inhibitory concentration of viability at 50% (IC(50)) and the inhibitory concentration of activity at 50% (EC(50)) values in APO- and APO+ cells with the selective NF-kappa B inhibitor, BAY 11-7082 (BAY). The apoptotic reporter assay was used to determine the effects of the transfection of the inhibitory kappa B-dominant negative (I kappa B-DN) construct in conjunction with TNF, paclitaxel, or doxorubicin treatments in these cells.

RESULTS

The concentrations at which 50% of cell viability is inhibited (IC(50)) and at which 50% of NF-kappa B activity is inhibited (EC(50)) for BAY in APO- and APO+ cells were 95.24 micromol/L and 1.53 micromol/L, respectively, and 7.62 micromol/L and 2.64 micromol/L, respectively. The IC(50) and the EC(50) values were equivalent for the APO+ cells (P =.665), but not for the APO- cells (P =.025). I kappa B-DN--transfection alone, or with TNF, doxorubicin, or paclitaxel treatments resulted in cell death of both APO- and APO+ cells as compared with vector-control; however, greater cytotoxicity was seen in the APO+ cells. Direct comparison of the APO+ cells versus the APO- cells revealed that these differences were significant (P =.05).

CONCLUSIONS

Pharmacologic or molecular inhibition of the NF-kappa B pathway blocked cell survival in MCF-7 APO+ cells, while only molecular inhibition induced cytotoxicity in the APO- cells. Selective manipulation of the NF-kappa B pathway in combination with standard chemotherapeutic agents may lead to an increased potency and efficacy of these agents.

Separable and redundant regulatory determinants in Cactus mediate its dorsal group dependent degradation

Dorsal-ventral polarity within the Drosophila syncytial blastoderm embryo is determined by the maternally encoded dorsal group signal transduction pathway that regulates nuclear localization of the transcription factor Dorsal. Nuclear uptake of Dorsal, a Rel/NFκB homolog, is controlled by the interaction with its cognate IκB inhibitor protein Cactus, which is degraded on the ventral side of the embryo in response to dorsal group signaling. Previous studies have suggested that an N-terminally located kinase target motif similar to that found in IκB proteins is involved in the spatially controlled degradation of Cactus. We report studies of the in vivo function and distribution of fusion proteins comprising segments of Cactus attached to Escherichia coli β-galactosidase (lacZ). Full-length Cactus-lacZ expressed in vivo normalizes the ventralized phenotype of embryos that lack Cactus and faithfully reconstitutes dorsal group-regulated degradation, while fusion protein constructs that lack the first 125 amino acids of Cactus escape dorsal group-dependent degradation. Furthermore, Cactus-lacZ constructs that lack only the putative IκB-dependent kinase target-like motif can nevertheless undergo spatially regulated dorsal group-dependent degradation and we have identified the regulatory determinant responsible for dorsal group-dependent degradation of Cactus in the absence of this motif. Taken together, our studies indicate the presence of two distinct redundantly acting determinants in the N terminus of Cactus that direct dorsal group-dependent degradation. Strikingly, the regulatory domain of human IκBα can also direct polarized degradation of Cactus-lacZ fusion protein.

Rac1 mediates STAT3 activation by autocrine IL-6

The activity of the small GTPase, Rac1, plays a role in various cellular processes including cytoskeletal rearrangement, gene transcription, and malignant transformation. In this report constitutively active Rac1 (Rac V12) is shown to stimulate the activation of STAT3, a member of the family of signal transducers and activators of transcription (STATs). The activity of Rac1 leads to STAT3 translocation to the nucleus coincident with STAT3-dependent gene expression. The expression of Vav (Delta1-187), a constitutively active guanine nucleotide exchange factor for the Rho GTPases, or activated forms of Ras or Rho family members, leads to STAT3-specific activation. The activation of STAT3 requires tyrosine phosphorylation at residue 705, but is not dependent on phosphorylation of Ser-727. Our studies indicate that Rac1 induces STAT3 activation through an indirect mechanism that involves the autocrine production and action of IL-6, a known mediator of STAT3 response. Rac V12 expression results in the induction of the IL-6 and IL-6 receptor genes and neutralizing antibodies directed against the IL-6 receptor block Rac1-induced STAT3 activation. Furthermore, inhibition of the nuclear factor-kappaB activation or disruption of IL-6-mediated signaling through the expression of IkappaBalpha S32AS36A and suppressor of cytokine signaling 3, respectively, blocks Rac1-induced STAT3 activation. These findings elucidate a mechanism dependent on the induction of an autocrine IL-6 activation loop through which Rac1 mediates STAT3 activation establishing a link between oncogenic GTPase activity and Janus kinase/STAT signaling.

Degradation of IkappaBalpha is limited by a postphosphorylation/ubiquitination event

Regulation of IkappaBalpha during activation was examined using EGFP. Single cell analysis showed that both localisation- and cytokine-induced degradation of IkappaBalpha are dependent on expression levels. Cells expressing higher levels of the inhibitor demonstrated an increase in nuclear IkappaBalphaEGFP with a pronounced enhancement in the nuclear/cytoplasmic ratio. Enhancing the levels of the endogenous IkappaBalpha by relA transfection caused significant reduction in IL-1-mediated degradation of the fusion protein. Similarly, IkappaBalphaEGFP-transfected cells showed an inverse correlation between the level of the fusion protein and IL-1-mediateddegradation. Comparing absolute levels demonstrated a biphasic response, with reduction in cells expressing over 15-fold that of endogenous levels. Further experiments using Western analysis showed a positive correlation between both phosphorylation and ubiquitination of IkappaBalphaEGFP, and the level the inhibitor. In contrast, and in agreement with the singlecell analysis, while IL-1 stimulation caused the expected degradation at lower levels of the fusion protein,breakdown of IkappaBalphaEGFP was totally inhibited at the higher transfection levels. The data show that turnover of IkappaBalpha is saturable and suggest that limitation of the pathway by enhanced inhibitor expression is regulated through a post phosphorylation/ubiquitination event, at the level of degradation.

Blockade of NF-kappaB activity in human prostate cancer cells is associated with suppression of angiogenesis, invasion, and metastasis

Since the NF-kappaB/relA transcription factor is constitutively activated in human prostate cancer cells, we determined whether blocking NF-kappaB/relA activity in human prostate cancer cells affected their angiogenesis, growth, and metastasis in an orthotopic nude mouse model. Highly metastatic PC-3M human prostate cancer cells were transfected with a mutated IkappaBalpha (IkappaBalphaM), which blocks NF-kappaB activity. Parental (PC-3M), control vector-transfected (PC-3M-Neo), and IkappaBalphaM-transfected (PC-3M-IkappaBalphaM) cells were injected into the prostate gland of nude mice. PC-3M and PC-3M-Neo cells produced rapidly growing tumors and regional lymph node metastasis, whereas PC-3M-IkappaBalphaM cells produced slow growing tumors with low metastatic potential. NF-kappaB signaling blockade significantly inhibited in vitro and in vivo expression of three major proangiogenic molecules, VEGF, IL-8, and MMP-9, and hence decreased neoplastic angiogenesis. Inhibition of NF-kappaB activity in PC-3M cells also resulted in the downregulation of MMP-9 mRNA and collagenase activity, resulting in decreased invasion through Matrigel. Collectively, these data suggest that blockade of NF-kappaB activity in PC-3M cells inhibits angiogenesis, invasion, and metastasis.

Caspase-3-mediated cleavage of the NF-kappa B subunit p65 at the NH2 terminus potentiates naphthoquinone analog-induced apoptosis

The transcription factor nuclear factor kappaB (NF-kappaB) plays a crucial role in immune and inflammatory response, and protects cells from apoptosis. In this report, we investigate whether the NF-kappaB signaling pathway is blocked during apoptosis induced by 2,3-dichloro-5,8-dihydroxy-1,4-naphthoquinone (NA), an analog of naphthoquinone. It is observed that NA triggers apoptotic cell death in HeLa cells and destroys resistance to apoptosis caused by tumor necrosis factor-alpha. Data presented in this study establish that p65/RelA, a subunit of NF-kappaB, is cleaved at Asp(97) by caspase-3 during apoptosis. Caspase-3-cleaved p65 loses transcriptional activity and potentiates NA-induced apoptosis, in contrast to an uncleavable mutant of p65, which protects the cell from apoptosis. Caspase-3, which is responsible for the cleavage of p65, is activated via the cytochrome c/caspase-9 signaling pathway rather than Fas/caspase-8 pathway during NA-induced apoptosis. Our results suggest that NA induces apoptosis by the negative regulation of cell survival through caspase-3-mediated cleavage of p65.

Direct phosphorylation of NF-kappaB1 p105 by the IkappaB kinase complex on serine 927 is essential for signal-induced p105 proteolysis

The p105 precursor protein of NF-kappaB1 acts as an NF-kappaB inhibitory protein, retaining associated Rel subunits in the cytoplasm of unstimulated cells. Tumor necrosis factor alpha (TNFalpha) and interleukin-1alpha (IL-1alpha) stimulate p105 degradation, releasing associated Rel subunits to translocate into the nucleus. By using knockout embryonic fibroblasts, it was first established that the IkappaB kinase (IKK) complex is essential for these pro-inflammatory cytokines to trigger efficiently p105 degradation. The p105 PEST domain contains a motif (Asp-Ser(927)-Gly-Val-Glu-Thr), related to the IKK target sequence in IkappaBalpha, which is conserved between human, mouse, rat, and chicken p105. Analysis of a panel of human p105 mutants in which serine/threonine residues within and adjacent to this motif were individually changed to alanine established that only serine 927 is essential for p105 proteolysis triggered by IKK2 overexpression. This residue is also required for TNFalpha and IL-1alpha to stimulate p105 degradation. By using a specific anti-phosphopeptide antibody, it was confirmed that IKK2 overexpression induces serine 927 phosphorylation of co-transfected p105 and that endogenous p105 is also rapidly phosphorylated on this residue after TNFalpha or IL-1alpha stimulation. In vitro kinase assays with purified proteins demonstrated that both IKK1 and IKK2 can directly phosphorylate p105 on serine 927. Together these experiments indicate that the IKK complex regulates the signal-induced proteolysis of NF-kappaB1 p105 by direct phosphorylation of serine 927 in its PEST domain.

NF-kappa B signaling promotes both cell survival and neurite process formation in nerve growth factor-stimulated PC12 cells

Nerve growth factor binds to the TrkA and p75(NTR) (p75) and generates signals leading to neuronal cell survival, differentiation, and programmed cell death. Here we describe a series of experiments involving selective activation of either TrkA or p75 in which distinct cell-signaling intermediates promote different cellular consequences. We analyzed pheochromocytoma 12 (PC12) cells stably expressing chimeras consisting of the extracellular domain of PDGF receptor (PDGFR) fused to the transmembrane and cytoplasmic segments of p75 or TrkA. Because PC12 cells lack endogenous PDGFR, addition of PDGF to these cell lines permits selective activation of the p75 or TrkA responses without stimulating endogenous receptors. Although both p75 and TrkA activated nuclear factor-kappaB (NF-kappaB), we show that distinct proximal-signaling intermediates are used by each receptor. A dominant-negative mutant of TRAF6 blocked p75- but not TrkA-mediated induction of NF-kappaB. Conversely a dominant-negative mutant of Shc inhibited TrkA but not p75 activation of NF-kappaB. Both of these distinct signaling pathways subsequently converge, leading to activation of the IkappaB kinase complex. Moreover, the activation of NF-kappaB by these distinct pathways after stimulation of either TrkA or p75 leads to different physiological consequences. Blocking p75-mediated activation of NF-kappaB by ecdysone-inducible expression of a nondegradable mutant of IkappaBalpha significantly enhanced apoptosis. In contrast, blocking NF-kappaB induction via TrkA significantly inhibited neurite process formation in PC12 cells. Together these findings indicate that, although both of these receptors lead to the activation of NF-kappaB, they proceed via distinct proximal-signaling intermediates and contribute to different cellular outcomes.

Activation of NF-kappa B is required for hypertrophic growth of primary rat neonatal ventricular cardiomyocytes

The transcription factor NF-kappaB regulates expression of genes that are involved in inflammation, immune response, viral infection, cell survival, and division. However, the role of NF-kappaB in hypertrophic growth of terminally differentiated cardiomyocytes is unknown. Here we report that NF-kappaB activation is required for hypertrophic growth of cardiomyocytes. In cultured rat primary neonatal ventricular cardiomyocytes, the nuclear translocation of NF-kappaB and its transcriptional activity were stimulated by several hypertrophic agonists, including phenylephrine, endothelin-1, and angiotensin II. The activation of NF-kappaB was inhibited by expression of a "supersuppressor" IkappaBalpha mutant that is resistant to stimulation-induced degradation and a dominant negative IkappaB kinase (IKKbeta) mutant that can no longer be activated by phosphorylation. Furthermore, treatment with phenylephrine induced IkappaBalpha degradation in an IKK-dependent manner, suggesting that NF-kappaB is a downstream target of the hypertrophic agonists. Importantly, expression of the supersuppressor IkappaBalpha mutant or the dominant negative IKKbeta mutant blocked the hypertrophic agonist-induced expression of the embryonic gene atrial natriuretic factor and enlargement of cardiomyocytes. Conversely, overexpression of NF-kappaB itself induced atrial natriuretic factor expression and cardiomyocyte enlargement. These findings suggest that NF-kappaB plays a critical role in the hypertrophic growth of cardiomyocytes and may serve as a potential target for the intervention of heart disease.

IκB Kinase Is Critical for TNF-α-Induced VCAM1 Gene Expression in Renal Tubular Epithelial Cells

The expression of VCAM1 is up-regulated in renal proximal tubular epithelial cells (TEC) in a variety of inflammatory renal diseases, a prominent example of which is acute renal allograft rejection. VCAM1 may play an important role in these diseases because it binds to the integrins very late Ag-4 and alpha(4)beta(7) on lymphocytes and monocytes, thereby providing a potential mechanism to recruit these leukocytes to sites of inflammation. The molecular mechanisms underlying VCAM1 regulation in renal TEC are essentially unknown. We now report that VCAM1 mRNA is dramatically up-regulated in C1, a cell line derived from renal TEC, on exposure to TNF-alpha. Two NF-kappaB binding sites in the VCAM1 promoter are critical for the TNF-alpha-induced VCAM1 transcriptional up-regulation, and both sites bind to p65-p50 NF-kappaB complexes. TNF-alpha induces activation of inhibitor of NF-kappaB (IkappaB) kinase-beta (IKK-beta), a protein kinase that phosphorylates the NF-kappaB inhibitor IkappaB, and thereby targets the latter for degradation via the ubiquitin-proteasome pathway. Moreover, dominant negative versions of IKK inhibit TNF-alpha activation of a VCAM1 promoter reporter. We conclude that the IKK/NF-kappaB pathway is critical in the TNF-alpha-induced up-regulation of VCAM1 mRNA in renal TEC.

Novel NEMO/IkappaB kinase and NF-kappa B target genes at the pre-B to immature B cell transition

The IkappaB kinase (IKK) signaling complex is responsible for activating NF-kappaB-dependent gene expression programs. Even though NF-kappaB-responsive genes are known to orchestrate stress-like responses, critical gaps in our knowledge remain about the global effects of NF-kappaB activation on cellular physiology. DNA microarrays were used to compare gene expression programs in a model system of 70Z/3 murine pre-B cells versus their IKK signaling-defective 1.3E2 variant with lipopolysaccharide (LPS), interleukin-1 (IL-1), or a combination of LPS + phorbol 12-myristate 13-acetate under brief (2 h) or long term (12 h) stimulation. 70Z/3-1.3E2 cells lack expression of NEMO/IKKgamma/IKKAP-1/FIP-3, an essential positive effector of the IKK complex. Some stimulated hits were known NF-kappaB target genes, but remarkably, the vast majority of the up-modulated genes and an unexpected class of repressed genes were all novel targets of this signaling pathway, encoding transcription factors, receptors, extracellular ligands, and intracellular signaling factors. Thirteen stimulated (B-ATF, Pim-2, MyD118, Pea-15/MAT1, CD82, CD40L, Wnt10a, Notch 1, R-ras, Rgs-16, PAC-1, ISG15, and CD36) and five repressed (CCR2, VpreB, lambda5, SLPI, and CMAP/Cystatin7) genes, respectively, were bona fide NF-kappaB targets by virtue of their response to a transdominant IkappaBalphaSR (super repressor). MyD118 and ISG15, although directly induced by LPS stimulation, were unaffected by IL-1, revealing the existence of direct NF-kappaB target genes, which are not co-induced by the LPS and IL-1 Toll-like receptors.

Rac1 protects epithelial cells against anoikis

Rho family members play a critical role in malignant transformation. Anchorage-independent growth and the ability to avoid apoptosis caused by loss of anchorage (anoikis) are important features of transformed cells. Here we show that constitutive activation of Rac1 inhibits anoikis in Madin-Darby canine kidney (MDCK) epithelial cells. Constitutively active Rac1-V12 decreases DNA fragmentation and caspase activity by 50% in MDCK cells kept in suspension. In addition, expression of Rac1-V12 in MDCK cells in suspension conditions causes an increase in the number of surviving cells. We also investigated the signaling pathways that are activated by Rac1 to stimulate cell survival. We show that expression of Rac1-V12 in MDCK cells in suspension stimulates a number of signaling cascades that have been implicated in the control of cell survival, including the p42/44 ERK, p38, protein kinase B, and nuclear factor kappaB pathways. Using specific chemical or protein inhibitors of these respective pathways, we show that Rac1-mediated cell survival strongly depends on phosphatidylinositol 3-kinase activity and that activation of ERK, p38, and NF-kappaB are largely dispensable for Rac1 survival signaling. In conclusion, these studies demonstrate that Rac1 can suppress apoptosis in epithelial cells in anchorage-independent conditions and suggest a potential role for Rac1-mediated survival signaling in cell transformation.

Interaction between hnRNPA1 and IkappaBalpha is required for maximal activation of NF-kappaB-dependent transcription

Transcriptional activation of NF-kappaB is mediated by signal-induced phosphorylation and degradation of its inhibitor, IkappaBalpha. NF-kappaB activation induces a rapid resynthesis of IkappaBalpha which is responsible for postinduction repression of transcription. Following resynthesis, IkappaBalpha translocates to the nucleus, removes template bound NF-kappaB, and exports NF-kappaB to the cytoplasm in a transcriptionally inactive form. Here we demonstrate that IkappaBalpha interacts directly with another nucleocytoplasmic shuttling protein, hnRNPA1, both in vivo and in vitro. This interaction requires one of the N-terminal RNA binding domains of hnRNPA1 and the C-terminal region of IkappaBalpha. Cells lacking hnRNPA1 are defective in NF-kappaB-dependent transcriptional activation, but the defect in these cells is complemented by ectopic expression of hnRNPA1. hnRNPA1 expression in these cells increased the amount of IkappaBalpha degradation, compared to that of the control cells, in response to activation by Epstein-Barr virus latent membrane protein 1. Thus in addition to regulating mRNA processing and transport, hnRNPA1 also contributes to the control of NF-kappaB-dependent transcription.

Micrococci and peptidoglycan activate TLR2-->MyD88-->IRAK-->TRAF-->NIK-->IKK-->NF-kappaB signal transduction pathway that induces transcription of interleukin-8

This study was done to elucidate the signal transduction pathway of interleukin-8 (IL-8) induction by gram-positive bacteria. Bacteria (micrococci) and peptidoglycan (PGN) induced transcription of IL-8 in HEK293 cells expressing Toll-like receptor 2 (TLR2) and CD14 but not in those expressing TLR1 or TLR4. A mutation within the NF-kappaB site in the IL-8 promoter abrogated transcriptional induction of IL-8 by the two stimulants. Dominant negative myeloid differentiation protein (MyD88), IL-1 receptor-associated kinase (IRAK), NFkappaB-inducing kinase (NIK), and IkappaB kinase (IKK) mutant forms completely inhibited micrococcus- and PGN-induced activation of NF-kappaB and expression of the gene for IL-8. Induction of NF-kappaB was partially inhibited by dominant negative tumor necrosis factor receptor-associated kinase 6 (TRAF6) but not TRAF2, whereas induction of IL-8 gene was partially inhibited by both TRAF6 and TRAF2. These data indicate that micrococci and PGN induce TLR2-dependent activation of the gene for IL-8 and that this activation requires MyD88, IRAK, NIK, IKK, and NF-kappaB and may also utilize TRAF6 and, to a lesser extent, TRAF2.

Activation of NF-kappaB by hepatitis B virus X protein through an IkappaB kinase-independent mechanism

pX, the hepatitis B virus-encoded transcription coactivator, is involved in viral infection in vivo. pX stimulates the activity of several transcription factors including nuclear factor-kappaB (NF-kappaB), but the mechanism of activation is poorly understood. The IkappaB kinase complex (IKK) mediates activation of NF-kappaB in response to various extracellular stimuli, including inflammatory cytokines like tumor necrosis factor and interleukin 1, human T cell lymphoma virus 1 Tax protein, and tumor promoters like phorbol esters. It is not known whether IKK also mediates activation of NF-kappaB by pX. Here we report that IKK was not essential for activation of NF-kappaB by pX. Expression of pX resulted in the degradation of IkappaBalpha in the absence of its phosphorylation at Ser(32) and Ser(36) residues. Although pX stimulated the activity of cotransfected IKK-beta when it was overexpressed, it failed to activate endogenous IKK. Furthermore, expression of pX stimulated NF-kappaB nuclear translocation and transcriptional activity in IKK-gamma-null fibroblast 5R cells. Our data indicate that pX stimulates NF-kappaB activity through a mechanism that is dependent on IkappaBalpha degradation but not on IKK activation.

Differential effects of Ras signaling through NFkappaB on skeletal myogenesis

Oncogenic Ras (H-Ras G12V) inhibits skeletal myogenesis through multiple signaling pathways. Previously, we demonstrated that the major downstream effectors of Ras (i.e., MEK/MAPK, RalGDS and Rac/Rho) play a minor, if any, role in the differentiation-defective phenotype of Ras myoblasts. Recently, NFkappaB, another Ras signaling target, has been shown to inhibit myogenesis presumably by stimulating cyclin D1 accumulation and cell cycle progression. In this study, we address the involvement of NFkappaB activation in the Ras-induced inhibition of myogenesis. Using H-Ras G12V and three G12V effector-loop variants, we detect high levels of NFkappaB transcriptional activity in C3H10T1/2-MyoD cells treated with differentiation medium. Myogenesis is blocked by all Ras proteins tested, yet only in the case of H-Ras G12V are cyclin D1 levels increased and cell cycle progression maintained. Expression of IkappaBalpha SR, an inhibitor of NFkappaB, does not reverse the differentiation-defective phenotype of Ras expressing cultures, but does induce differentiation in cultures treated with tumor necrosis factor (TNFalpha) or in cultures expressing the RelA/p65 subunit of NFkappaB. These data confirm that NFkappaB is a target of Ras and suggest that the cellular actions of NFkappaB require additional signals that are discriminated by the Ras effector-loop variants. Results with IkappaBalpha SR convincingly demonstrate that H-Ras G12V does not rely on NFkappaB activity to block myogenesis, an observation that continues to implicate another unidentified signaling pathway(s) in the inhibition of skeletal myogenesis by Ras.