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Background: MMP-1 (Matrix metalloproteinase-1) promotes carcinogenesis and distant metastasis in different cancers. Regulation of MMP-1 could occur at multiple levels: epigenetically, post-transcriptionally, or post-translationally. An increasing body of evidence supports that the cytoplasmic transcription factor STAT3 (signal transducer and activator of transcription 3) is activated constitutively in a variety of cancers wherein it significantly affects the growth of tumors and also facilitates metastasis. In addition, STAT3 has been found to regulate nuclear activity pro-inflammatory transcriptional factor, NF-κB signaling, especially, the alternative one (RelB/p100) by directly interacting with them METHOD AND RESULTS: In this proof of concept study, we tested the hypothesis that STAT3 interacts with RelB to promote tumor invasion by positively regulating MMP-1 in colon cancer. We found that RelB and STAT3 were constitutively localized in the nucleus of colon cancer in surgically-resected specimens with use of Western blot analysis, which was further confirmed by immunofluorescence (IF) staining in colon carcinoma cell line HT29. We further observed that STAT3/RelB knockdown resulted in reduced MMP-1. Our results from chromatin immunoprecipitation studies further established that association between RelB and MMP-1 promoter decreased when STAT3 was depleted, and conversely, STAT3 association with MMP-1 decreased with the knockdown of RelB., Conclusion: These results suggest that STAT3 and ReB constitute a minimal activator complex for positive regulation of MMP-1 in colon cancer., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Background: Human immunodeficiency virus type 1 (HIV-1) Tat protein plays an essential role in HIV-1 gene transcription. Tat transactivates HIV-1 long terminal repeat (LTR)-directed gene expression through direct interactions with the transactivation-responsive region (TAR) element and other cis elements in the LTR. The TAR-independent Tat-mediated LTR transactivation is modulated by several host factors, but the mechanism is not fully understood., Results: Here, we report that Tat interacts with the Rel homology domain of RelB through its core region. Furthermore, RelB significantly increases Tat-mediated transcription of the HIV-1 LTR and viral gene expression, which is independent of the TAR. Both Tat and RelB are recruited to the HIV-1 promoter, of which RelB facilitates the recruitment of Tat to the viral LTR. The NF-κB elements are key to the accumulation of Tat and RelB on the LTR. Knockout of RelB reduces the accumulation of RNA polymerase II on the LTR, and decreases HIV-1 gene transcription. Together, our data suggest that RelB contributes to HIV-1 transactivation., Conclusions: Our results demonstrate that RelB interacts with Tat and enhances TAR-independent activation of HIV-1 LTR promoter, which adds new insights into the multi-layered mechanisms of Tat in regulating the gene expression of HIV-1.

Background: The development of radioresistance is one of main causes for therapeutic failure of prostate cancer (PCa). The present study aims to investigate the function and the related mechanism by which HZ08 sensitizes radiotherapeutic efficiency to treat aggressive PCa cells., Methods: PCa cells were pretreated with HZ08 (6,7-dimethoxy-1-(3,4-dimethoxy) benzyl-2-(N-n-octyl-N'-cyano) guanyl-1,2,3,4-tetrahydroisoquinoline) and followed by ionizing radiation (IR) treatment. Cytotoxicity in the treated cells was analyzed to assess the radiosensitization capacity of HZ08 by flow cytometry, MTT and colony survival assays. The cellular levels of reactive oxygen species (ROS) and oxygen consumption rates (OCR) were measured using specific ROS detection probes and a Seahorse XF96 Analyzer, respectively. RelB binding to the NF-κB intronic enhancer region of the human SOD2 gene was determined using a ChIP assay. The levels of phosphorylation of PI3K, Akt and IKKα were quantified and further confirmed using a PI3K inhibitor. Finally, the synergistic effect of HZ08 on radiosensitization of PCa cells was validated using a mouse xenograft tumor model., Results: HZ08 enhanced radiosensitivity of PCa cells through increasing ROS and declining mitochondrial respiration due to suppression of mitochondrial antioxidant enzyme MnSOD. Mechanistically, HZ08 appeared to inhibit PI3K/Akt/IKKα signaling axis, resulting in transcriptional repression of MnSOD expression by preventing RelB nuclear translocation., Conclusions: HZ08 can serve as a useful radiosensitizing agent to improve radiotherapy for treating aggressive PCa cells with high level of constitutive RelB. The present study suggests a promising approach for enhancing radiotherapeutic efficiency to treat advanced PCa by inhibiting antioxidant defense function.

Epstein-Barr virus (EBV) infection in tumor cells is usually restricted to the latent form, indicating that the induction of viral lytic infection may present a novel approach for the treatment of EBV‑associated tumors. By contrast, EBV lytic replication is inhibited by high‑levels of nuclear factor (NF)‑κB, which suggests that NF‑κB inhibitors may activate lytic replication from the latent form. In the current study, the addition of NF‑κB inhibitors (Bay11‑7082, Z‑LLF‑CHO and aspirin) was observed to induce the EBV lytic genes BZLF1, BRLF1 and BMRF1 in EBV‑positive gastric cancer (GC) cells. Both EBV‑positive and ‑negative GC cells were treated with different concentrations of anti‑herpes agents and the cytotoxic effects were measured at different time points following induction of EBV lytic replication. A marginal dose‑ and time‑dependent reduction in cell viability was observed for EBV‑positive and‑negative GC cells. The cytotoxic effects of NF‑κB inhibitors on EBV‑positive GC cells were enhanced by the addition of the anti‑herpes agents, ganciclovir, acyclovir, foscarnet and brivudine (P<0.05). However, there was no significant synergistic effect on EBV‑negative GC cells. The combination of 5 mM aspirin and ganciclovir exhibited the highest cytotoxic effect in EBV‑positive GC cells (CC50=7.2 µg/ml).

Vascular calcification (VC) is associated with increased cardiovascular mortality in aging, chronic kidney disease (CKD), type 2 diabetes mellitus (T2DM) and atherosclerosis. TNF-like weak inducer of apoptosis (TWEAK) recently emerged as a new biomarker for the diagnosis and prognosis of cardiovascular diseases. TWEAK binding to its functional receptor Fn14 was reported to promote several steps of atherosclerotic plaque progression. However, no information is currently available on the role of TWEAK/Fn14 on the development of medial calcification, which is highly prevalent in aging, CKD and T2DM. This study explored the involvement of TWEAK in human vascular smooth muscle cells (h-VSMCs) calcification in vitro. We report that TWEAK binding to Fn14 promotes inorganic phosphate-induced h-VSMCs calcification, favors h-VSMCs osteogenic transition, decreasing acta2 and myh11 and increasing bmp2 mRNA and tissue non-specific alkaline phosphatase (TNAP), and increases MMP9 activity. Blockade of the canonical NFκB pathway reduced by 80% TWEAK pro-calcific properties and decreased osteogenic transition, TNAP and MMP9 activity. Blockade of non-canonical NFκB signaling by a siRNA targeting RelB reduced by 20% TWEAK pro-calcific effects and decreased TWEAK-induced loss of h-VSMCs contractile phenotype and MMP9 activity, without modulating bmp2 mRNA or TNAP activity. Inhibition of ERK1/2 activation by a MAPK kinase inhibitor did not influence TWEAK pro-calcific properties. Our results suggest that TWEAK/Fn14 directly favors inorganic phosphate-induced h-VSMCs calcification by activation of both canonical and non-canonical NFκB pathways. Given the availability of neutralizing anti-TWEAK strategies, our study sheds light on the TWEAK/Fn14 axis as a novel therapeutic target in the prevention of VC.

Overexpression of the antiviral DNA cytosine deaminase APOBEC3B has been linked to somatic mutagenesis in many cancers. Human papillomavirus infection accounts for APOBEC3B upregulation in cervical and head/neck cancers, but the mechanisms underlying nonviral malignancies are unclear. In this study, we investigated the signal transduction pathways responsible for APOBEC3B upregulation. Activation of protein kinase C (PKC) by the diacylglycerol mimic phorbol-myristic acid resulted in specific and dose-responsive increases in APOBEC3B expression and activity, which could then be strongly suppressed by PKC or NF-κB inhibition. PKC activation caused the recruitment of RELB, but not RELA, to the APOBEC3B promoter, implicating noncanonical NF-κB signaling. Notably, PKC was required for APOBEC3B upregulation in cancer cell lines derived from multiple tumor types. By revealing how APOBEC3B is upregulated in many cancers, our findings suggest that PKC and NF-κB inhibitors may be repositioned to suppress cancer mutagenesis, dampen tumor evolution, and decrease the probability of adverse outcomes, such as drug resistance and metastasis., (©2015 American Association for Cancer Research.)

Levels of the nuclear factor‑kappa B (NF‑κB) alternative pathway member RelB have been shown to correlate with the effect of radiation therapy in prostate cancer. RelB expression was evaluated by immunohistochemistry in normal prostate, benign prostate hyperplasia and prostate cancer specimens. RM‑1 cells were pretreated with RelB siRNA prior to radiation therapy, and RelB expression in cytoplasmic and nuclear extracts was detected by real‑time polymerase chain reaction and western blot analysis. The apoptotic rates of experimental RM‑1 cell groups were assessed by flow cytometry. A clonogenic growth array was used to evaluate the radiosensitivity of RM‑1 cell groups. The NF‑κB family member RelB was expressed at a high level in prostate cancer specimens. Compared with irradiated control cells, RM‑1 cells transfected with RelB siRNA and treated with radiation therapy demonstrated a significant downregulation of RelB expression in the cytoplasm and nucleus. Notably, flow cytometry revealed that pretreatment of RM‑1 cells with RelB siRNA enhanced the apoptotic rate in response to radiation therapy compared with controls. Clonogenic growth assay results revealed enhanced radiosensitivity of RelB siRNA cells at various dosage points compared with control groups. Blockage of the alternative NF‑κB pathway via RelB silencing is a promising approach to enhance the radiosensitivity of prostate cancer.

AMR-Me, a C-28 methylester derivative of triterpenoid compound Amooranin isolated from Amoora rohituka stem bark and the plant has been reported to possess multitude of medicinal properties. Our previous studies have shown that AMR-Me can induce apoptosis through mitochondrial apoptotic and MAPK signaling pathways by regulating the expression of apoptosis related genes in human breast cancer MCF-7 cells. However, the molecular mechanism of AMR-Me induced apoptotic cell death remains unclear. Our results showed that AMR-Me dose-dependently inhibited the proliferation of MCF-7 and MDA-MB-231 cells under serum-free conditions supplemented with 1 nM estrogen (E2) with an IC50 value of 0.15 µM, 0.45 µM, respectively. AMR-Me had minimal effects on human normal breast epithelial MCF-10A + ras and MCF-10A cells with IC50 value of 6 and 6.5 µM, respectively. AMR-Me downregulated PI3K p85, Akt1, and p-Akt in an ERα-independent manner in MCF-7 cells and no change in expression levels of PI3K p85 and Akt were observed in MDA-MB-231 cells treated under similar conditions. The PI3K inhibitor LY294002 suppressed Akt activation similar to AMR-Me and potentiated AMR-Me induced apoptosis in MCF-7 cells. EMSA revealed that AMR-Me inhibited nuclear factor-kappaB (NF-κB) DNA binding activity in MDA-MB-231 cells in a time-dependent manner and abrogated EGF induced NF-κB activation. From these studies we conclude that AMR-Me decreased ERα expression and effectively inhibited Akt phosphorylation in MCF-7 cells and inactivate constitutive nuclear NF-κB and its regulated proteins in MDA-MB-231 cells. Due to this multifactorial effect in hormone-dependent and independent breast cancer cells AMR-Me deserves attention for use in breast cancer prevention and therapy., (© 2013 Wiley Periodicals, Inc.)

Fucoxanthin is a carotenoid present in the chloroplasts of brown seaweeds. When ingested, it is metabolized mainly to fucoxanthinol in the gastrointestinal tract by digestive enzymes. These compounds have been shown to have many beneficial health effects. The present study was designed to evaluate the molecular mechanisms of action of fucoxanthin and/or of its metabolite fucoxanthinol against viability of estrogen-sensitive MCF-7 and estrogen-resistant MDA-MB-231 breast cancer cell lines. Fucoxanthin and fucoxanthinol reduced the viability of MCF-7 and MDA-MB-231 cells in dose- and time-dependent manners as a result of increased apoptosis. Furthermore, fucoxanthinol-induced apoptosis was more potent than that of fucoxanthin and correlated, for MDA-MB-231 cells, with inhibitory actions on members of the NF-κB pathway p65, p50, RelB, and p52. Being overexpressed and regulated by NF-κB in different types of cancers, the transcription factor SOX9 was also decreased at the nuclear level by fucoxanthin and fucoxanthinol in MDA-MB-231. Taken together, the current results suggest that fucoxanthinol and fucoxanthin could be potentially effective for the treatment and/or prevention of different types of cancers, including breast cancer.

In this study, we determined the respective roles of RelA and RelB NF-κB subunits in Epstein-Barr virus (EBV)-transformed B cells. Using different EBV-immortalized B-cell models, we showed that only RelA activation increased both survival and cell growth. RelB activity was induced secondarily to RelA activation and repressed RelA DNA binding by trapping the p50 subunit. Reciprocally, RelA activation repressed RelB activity by increasing expression of its inhibitor p100. To search for such reciprocal inhibition at the transcriptional level, we studied gene expression profiles of our RelA and RelB regulatable cellular models. Ten RelA-induced genes and one RelB-regulated gene, ARNTL2, were repressed by RelB and RelA, respectively. Apart from this gene, RelB signature was included in that of RelA Functional groups of RelA-regulated genes were for control of energy metabolism, genetic instability, protection against apoptosis, cell cycle and immune response. Additional functions coregulated by RelA and/or RelB were autophagy and plasma cell differentiation. Altogether, these results demonstrate a cross-inhibition between RelA and RelB and suggest that, in fine, RelB was subordinated to RelA. In the view of future drug development, RelA appeared to be pivotal in both classical and alternative activation pathways, at least in EBV-transformed B cells.

Background: The induction of specific immune tolerance for alloantigen is the best method for solving transplant rejection. We have previously reported T-cell tolerance induced by RNA interference (RNAi) RelB dendritic cells (DCs), supporting the possibility of immunologic tolerance in liver transplantation., Methods: A stable model of acute rejection was established in Lewis (RT11) rats that had received a liver graft from dark agouti-RT1a rats. To evaluate the immune tolerance of DCs of different maturity, the rats were randomly assigned to four groups (12 donor/recipient pairs): (1) control-DC group, recipient rats without preinjection; (2) RelB short hairpin (sh)RNAi-DC group, recipient rats with preinjection of tolerogenic DCs by way of RelB silencing; (3) imDC group, recipient rats with preinjection of immature DCs; and (4) lipopolysaccharide-DC group, recipient rats with preinjection of mature DCs. The immune tolerance of the grafts was evaluated by liver function tests (aspartate transaminase, total bilirubin), cytokines (interleukin [IL]-2, IL-4, IL-10 and interferon-γ), and histopathologic examination during the 2 wk after transplantation. The survival time of the rats was also observed., Results: Compared with the other three groups, the graft survival time was significantly prolonged in the RelB shRNAi-DC group. In addition, RelB shRNAi-DCs resulted in the reduced secretion of IL-2 and interferon-γ and increased levels of IL-10 and IL-4. The symptoms of rejection were obviously alleviated in the RelB shRNAi-DC group, and the rejection activity index was still reduced after 2 wk., Conclusions: Injection of RelB-silenced DCs contributed to the reduced incidence of graft rejection and prolonged the graft survival time. The potential mechanisms involved the regulation and induction of immune-incompetent T cell by DCs., (Copyright © 2013 Elsevier Inc. All rights reserved.)

Background: RNA interference targeting RelB can significantly attenuate ischemia/reperfusion (I/R)-induced renal dysfunction, but its roles in liver I/R injury remain to be defined. We have investigated whether siRNA targeting of RelB in liver could also elicit protection against I/R injury., Materials and Methods: A RelB miRNA RNAi expression plasmid, a scrambled plasmid, or phosphate-buffered saline (50 μg of the plasmid diluted in phosphate-buffered saline, 8% wt/vol) were rapidly injected, within 6-8 s, into mouse tail veins 24 h before liver I/R. Mice were subjected to 30 min of 70% hepatic ischemia or to a sham operation. Six h after reperfusion, blood and liver tissue samples were collected for subsequent assays., Results: The expression level of RelB was reduced in the RelB RNAi group compared with the control group, while it was increased in the I/R group. In the sham group, malondialdehyde, myeloperoxidase (MPO), and superoxide dismutase serum levels were almost the same, but the alanine aminotransferase level in the untreated group was 20- to 25-fold lower than in the other groups. In I/R-treated mice, although alanine aminotransferase, malondialdehyde, and MPO serum levels in the RelB RNAi group were lower than in other groups, all were higher than in the sham group. Silencing RelB could inhibit the decrease of superoxide dismutase activity and the upregulation of MPO and tumor necrosis factor α induced by I/R injury., Conclusions: Silencing RelB can protect the liver against I/R-induced damage. Therefore, it is a promising therapeutic target for protection against I/R injury in the liver., (Copyright © 2012 Elsevier Inc. All rights reserved.)

Following the TLR-dependent initiation phase of acute systemic proinflammatory responses such as sepsis, an adaptive phase represses or activates a specific pattern of gene expression until the inflammation resolves. Here, we used the THP-1 sepsis cell model of bacterial LPS/endotoxin tolerance to show that TLR4-induced miR-146a supports the feed-forward adaptive processes that silence transcription and disrupt translation of acute proinflammatory genes. First, we found that miR-146a regulates a pathway that promotes the binding of transcription repressor RelB to the TNF-α promoter, a step known to precede histone and DNA modifications, which generate facultative heterochromatin to silence acute proinflammatory genes. However, once RelB binding occurred, miR-146a inhibition could not reverse compacted chromatin, and endotoxin tolerance persisted. Second, we observed that miR-146a regulates a pathway that supports assembly of the translation repressor complex of TNF-α by preventing the interaction of the RNA-binding protein effector Ago2 and RBM4. We also determined that once endotoxin tolerance is established, and specific genes have been reprogrammed, transcription and translation disruption can be reversed only by simultaneously depleting RelB and inhibiting miR-146a. Thus, miR-146a induction supports the TLR4-dependent shift from initiation to gene-specific repression at two levels. Our results also imply that therapies designed to reverse endotoxin tolerance as potential therapies for sepsis should be directed at the transcription and translation pathways of reprogramming.

Virtual screening against NF-kappaB p50 using docking simulations was applied by starting from a three-dimensional (3D) database containing more than 4.6 million commercially available structures. This database was filtered by specifying a subset of commercially available compounds sharing a (2E,Z)-3-(2-hydroxyphenyl)-2-propenoate substructure and relevant druglike properties. Docking to p50 NF-kappaB was performed with a test set of six known inhibitors of NF-kappaB-DNA interactions. In agreement with docking results, the highest-scored compound displayed a high level of inhibitory activity in electrophoretic mobility shift assay (EMSA) experiments (inhibition of NF-kappaB-DNA interactions) and on biological functions dependent on NF-kappaB activity (inhibition of IL-8 gene expression in cystic fibrosis IB3-1 cells). We found that this in silico screening approach is suitable for the identification of low-molecular-weight compounds that inhibit NF-kappaB-DNA interactions and NF-kappaB-dependent functions. Information deduced from the discovery of the new lead compound and its binding mode could result in further lead optimization resulting in more potent NF-kappaB inhibitors.

Aberrant constitutive expression of the NF-kappaB c-Rel and RelA subunits in breast cancer cells was shown to promote their survival. Recently, we demonstrated that aggressive breast cancers constitutively express high levels of the RelB subunit, which promotes their more invasive phenotype via induction of the BCL2 gene. As these cancers are frequently resistant to therapy, here we tested the hypothesis that RelB promotes their survival. High RelB expressing Hs578T and MDA-MB-231 breast cancer cells were more resistant to gamma-radiation than MCF7 and ZR-75 cells, which express lower RelB levels. Knockdown of RelB in Hs578T led to decreased survival in response to gamma-irradiation, while conversely ectopic expression of RelB in MCF7 cells protected these cells from radiation. Similar data were obtained upon treatment of Hs578T or MCF7 cells with the chemotherapeutic agent doxorubicin. High serum levels of 25-hydroxyvitamin D are associated with decreased breast cancer risk and mortality, although, the mechanisms of its protective actions have not been fully elucidated. Treatment of Hs578T and Her-2/neu-driven NF639 cells with 1,25-dihydroxyvitamin D3 decreased RelB/RELB gene expression and levels of pro-survival targets Survivin, MnSOD and Bcl-2, while increasing their sensitivity to gamma-irradiation. Thus, RelB, which promotes survival and a more highly invasive phenotype of breast cancer cells, is a target of 1,25-dihydroxyvitamin D3, providing one mechanism for the observed protective role of 25-hydroxyvitamin D in patients with breast cancer.

NF-kappaB plays a major role in regulating the immune system. Therefore, alterations in NF-kappaB activity have profound effects on many immunopathologies, including inflammation, autoimmunity, and lymphoid neoplasia. We investigated the effects of estrogen (17beta-estradiol) on NF-kappaB in C57BL/6 mice since estrogen is a natural immunomodulator and we have recently reported that estrogen up-regulates several NF-kappaB-regulated proteins (inducible NO synthase, IFN-gamma, and MCP-1). We found that in vivo estrogen treatment had differential effects on NF-kappaB family members. Estrogen profoundly blocked the nuclear translocation of p65, c-Rel, and Rel-B, partially blocked p52, but permitted translocation of p50. Despite blockade of both the classical (p65/p50) and alternative (RelB/p52) NF-kappaB activation pathways, estrogen induced constitutive NF-kappaB activity and increased the levels of cytokines regulated by NF-kappaB (IL-1 alpha, IL-1 beta, IL-10, and IFN-gamma). Studies involving a NF-kappaB inhibitor confirmed a positive regulatory role of NF-kappaB on these cytokines. Remarkably, estrogen selectively induced B cell lymphoma 3 (Bcl-3), which is known to associate with p50 to confer transactivation capabilities, thereby providing a potential link between observed p50 DNA-binding activity and estrogen up-regulation of NF-kappaB transcriptional activity. Chromatin immunoprecipitation assays confirmed that Bcl-3 bound to the promoter of the NF-kappaB-regulated inducible NO synthase gene in cells from estrogen-treated mice. Estrogen appeared to act at the posttranscriptional level to up-regulate Bcl-3 because mRNA levels in splenocytes from placebo- and estrogen-treated mice were comparable. The novel findings of differential regulation of NF-kappaB proteins by estrogen provide fresh insight into potential mechanisms by which estrogen can regulate NF-kappaB-dependent immunological events.

Dendritic cells (DC), the most potent APCs, can initiate the immune response or help induce immune tolerance, depending upon their level of maturation. DC maturation is associated with activation of the NF-kappaB pathway, and the primary NF-kappaB protein involved in DC maturation is RelB, which coordinates RelA/p50-mediated DC differentiation. In this study, we show that silencing RelB using small interfering RNA results in arrest of DC maturation with reduced expression of the MHC class II, CD80, and CD86. Functionally, RelB-silenced DC inhibited MLR, and inhibitory effects on alloreactive immune responses were in an Ag-specific fashion. RelB-silenced DC also displayed strong in vivo immune regulation. An inhibited Ag-specific response was seen after immunization with keyhole limpet hemocyanin-pulsed and RelB-silenced DC, due to the expansion of T regulatory cells. Administration of donor-derived RelB-silenced DC significantly prevented allograft rejection in murine heart transplantation. This study demonstrates for the first time that transplant tolerance can be induced by means of RNA interference using in vitro-generated tolerogenic DC.

Daxx is a nuclear protein that localizes to PML oncogenic domains, sensitizes cells to apoptosis, and functions as a transcriptional repressor. We found that Daxx represses the expression of several antiapoptotic genes regulated by nuclear factor-kappaB, including cIAP2, in human tumor cell lines. Daxx interacts with RelB and inhibits RelB-mediated transcriptional activation of the human cIAP2 gene promoter. Daxx also forms complexes with RelB while bound to its target sites in the cIAP2 promoter, as shown by electrophoretic mobility shift assays and chromatin immunoprecipitation experiments. Using cells from daxx-/- mouse embryos, we observed that levels of the corresponding murine c-IAP mRNA and protein are increased in cells lacking Daxx. Conversely, c-IAP mRNA and protein levels were reduced in relB-/- cells. Taken together, these observations provide a mechanism that links two previously ascribed functions of Daxx: transcriptional repression and sensitization to apoptosis.

The relationship between NF-kappaB and resistance to radiation treatment in many tumor cell types has been generally well recognized. However, which members of the NF-kappaB family contribute to radiation resistance is unclear. In the present study, we demonstrate that RelB plays an important radioprotective role in aggressive prostate cancer cells, in part by the induction of antioxidant and antiapoptotic manganese superoxide dismutase (MnSOD) gene. RelB is both constitutively present and is inducible by radiation in aggressive prostate cancer cells. Using ectopically expressed dominant negative inhibitor, p100 mutant, and the siRNA approach, we demonstrate that selective inhibition of RelB significantly decreases the levels of MnSOD resulting in a significant increase in the sensitivity of prostate cancer cells to radiation treatment. These results demonstrate that RelB plays an important role in redox regulation of the cell and protects aggressive prostate cancer cells against radiation-induced cell death. Thus, inhibition of RelB could be a novel mechanism to radiosensitize prostate cancer.