Radiosensitivity and transcription factor NF-kappaB inhibition-progress and pitfalls

Severe Acute Enteritis in a Multiple Myeloma Patient Receiving Bortezomib and Spinal Radiotherapy: Case Report

Proteasome inhibitors have been reported to enhance radiosensitivity in vitro. A case of potential clinical interaction between bortezomib, a proteasome inhibitor, and spine radiation is reported. A woman undergoing palliative radiotherapy to the T12 -S2 spine with concurrent bortezomib developed unexpectedly severe, acute radiation enteritis requiring hospital admission. Clinicians are advised to consider the potential for interactions of bortezomib with radiotherapy when the two agents are used simultaneously in the clinic.

Blockade of NFκB activity by Sunitinib increases cell death in Bortezomib-treated endometrial carcinoma cells

Endometrial carcinoma is one of the most common malignancies in the female genital tract, usually treated by surgery and radiotherapy. Chemotherapy is used when endometrial carcinoma is associated with widespread metastasis or when the tumor recurs after radiation therapy. In the present study, we demonstrate that the tyrosine kinase receptor inhibitor Sunitinib reduces cell viability, proliferation, clonogenicity and induces apoptotic cell death in endometrial carcinoma cell lines, which is not due to its action through the most known targets like VEGFR, nor through EGFR as demonstrated in this work. Interestingly, Sunitinib reduces NFκB transcriptional activity either at basal level or activation by EGF or TNF-α. We observed that Sunitinib was able to inhibit the Bortezomib-induced NFκB transcriptional activity which correlates with a decrease of the phosphorylated levels of IKKα and β, p65 and IκBα. We evaluated the nature of the interaction between Sunitinib and Bortezomib by the dose effect method and identified a synergistic effect (combination index < 1). Analogously, silencing of p65 expression by lentiviral-mediated short-hairpin RNA delivery in Bortezomib treated cells leads to a strongly increased sensitivity to Bortezomib apoptotic cell death. Altogether our results suggest that the combination of Sunitinib and Bortezomib could be considered a promising treatment for endometrial carcinoma after failure of surgery and radiation.

Induction of autophagic cell death and radiosensitization by the pharmacological inhibition of nuclear factor-kappa B activation in human glioma cell lines

OBJECT

The intrinsic radioresistance of certain cancer cells may be closely associated with the constitutive activation of nuclear factor-kappa B (NF-kappaB) activity, which may lead to protection from apoptosis. Recently, nonapoptotic cell death, or autophagy, has been revealed as a novel response of cancer cells to ionizing radiation. In the present study, the authors analyzed the effect of pitavastatin as a potential inhibitor of NF-kappaB activation on the radiosensitivity of A172, U87, and U251 human glioma cell lines.

METHODS

The pharmacological inhibition of NF-kappaB activation was achieved using pitavastatin, an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase. Growth and radiosensitivity assays were performed using a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Hoechst 33258 staining, supravital acridine orange staining, and electron microscopy were performed utilizing 3 glioma cell lines with or without pitavastatin pretreatment to identify apoptosis or autophagy after irradiation.

RESULTS

The growth of these 3 glioma cell lines was not significantly inhibited by pitavastatin at a concentration of up to 1 microM. Treatment with 0.1 microM of pitavastatin enhanced radiation-induced cell death in all glioma cell lines, with different sensitivity. Apoptosis did not occur in any pretreated or untreated (no pitavastatin) cell line following irradiation. Instead, autophagic cell changes were observed regardless of the radiosensitivity of the cell line. An inhibitor of autophagy, 3-methyladenine suppressed the cytotoxic effect of irradiation with pitavastatin, indicating that autophagy is a result of an antitumor mechanism. Using the most radiosensitive A172 cell line, the intracellular localization of p50, a representative subunit of NF-kappaB, was evaluated through immunoblotting and immunofluorescence studies. The NF-kappaB of A172 cells was immediately activated and translocated from the cytosol to the nucleus in response to irradiation. Pitavastatin inhibited this activation and translocation of NF-kappaB.

CONCLUSIONS

Autophagic cell death rather than apoptosis is a possible mechanism of radiation-induced and pitavastatin-enhanced cell damage, and radiosensitization by the pharmacological inhibition of NF-kappaB activation may be a novel therapeutic strategy for malignant gliomas.

NF-kappaB modulation and ionizing radiation: mechanisms and future directions for cancer treatment

NF-kappaB transcription factor regulates important cellular processes ranging from establishment of the immune and inflammatory responses to regulation of cell proliferation or apoptosis, through the induction of a large array of target genes. NF-kappaB is now considered as an important actor in the tumorigenic process mainly because it exerts strong anti-apoptotic functions in cancer cells. NF-kappaB is triggered by chimio- and radio-therapeutic strategies that are intended to eliminate cancerous cells through induction of apoptosis. Numerous studies have demonstrated that inhibition of NF-kappaB by different means increased sensitivity of cancer cells to the apoptotic action of diverses effectors such as TNFalpha or chemo- or radio-therapies. From these studies as emerged the concept that NF-kappaB blockade could be associated to conventional therapies in order to increase their efficiency. This review focuses on the current knowledge on NF-kappaB regulation and discusses the therapeutic potential of targeting NF-kappaB in cancer in particular during radiotherapy.

Inhibition of constitutively activated nuclear factor-κB radiosensitizes human melanoma cells

Melanoma tumors and cultured cell lines are relatively resistant to the cytotoxic effects of ionizing radiation, thereby limiting the use of radiotherapy for the clinical treatment of melanoma. New strategies for sensitizing melanoma cells therefore deserve examination. In an attempt to identify and target signaling pathways that contribute to radioresistance, we investigated the role of nuclear factor-κB (NF-κB), a transcription factor known to inhibit apoptosis induced by a variety of stimuli and promote radioresistance. Two human metastatic melanoma cell lines, A375 and MeWo, were used to examine the radiosensitizing effects of inhibitors of the NF-κB pathway. Nuclear extracts from these cell lines were tested for active NF-κB using the electrophoretic mobility shift assay. Both melanoma cell lines had constitutively activated NF-κB as observed by electrophoretic mobility shift assay. In an attempt to reverse NF-κB activity, cells were treated either with vehicle alone (DMSO) or with a proteasome inhibitor Z-Leu-Leu-Leu-H (MG132; 10 μmol/L for 2 hours prior to irradiation) that inhibited both constitutive and radiation-induced NF-κB activity. The clonogenic cell survival assay showed that pretreatment with MG132 enhanced tumor cell radiosensitivity with the survival factor at 2 Gy being reduced from 48 ± 0.8% and 48 ± 1.6% in vehicle-treated cells to 27.7 ± 0.32% and 34.3 ± 0.7% in MG132-treated MeWo and A375 cells, respectively. To test the role of NF-κB in radioresistance more directly, MeWo cells were stably transfected with a dominant-negative mutant IκBα construct, which led to the inhibition of both constitutive and radiation-induced NF-κB activity. A modest restoration of radiosensitivity was also observed in the stably transfected MeWo cells with survival factor at 2 Gy values being reduced from 47 ± 0.8% in parental MeWo cells to 32.9 ± 0.7% in stable transfectants. Because constitutively activated mitogen-activated protein kinase kinase (MEK) pathway has been shown to lead to activated NF-κB, we wanted to determine the relative contribution of activated MEK in the human melanoma cells. To test this, MeWo and A375 melanoma cells were exposed to the MEK inhibitor PD184352. Treatment with PD184352 partially reversed NF-κB activity but did not impart radiation sensitivity to these cells. Our results indicate that activated NF-κB may be one of the pathways responsible for the radioresistance of melanoma cells and that strategies for inhibiting its influence may be useful in restoring the radioresponse of melanomas.

The p65 subunit of nuclear factor-κB is a molecular target for radiation sensitization of human squamous carcinoma cells

The transcription factor nuclear factor-κB (NF-κB) is activated in response to various stimuli including ionizing radiation. Disruption of NF-κB activation by mutant forms of the NF-κB inhibitor IκB-α or by proteasome inhibitors enhances both sensitivity to radiation and radiation-induced apoptosis. Human squamous carcinoma SCC-35 cells stably expressing a fragment (residues 1 to 84) of human p65 have been shown to exhibit down-regulation of both endogenous p65 mRNA and its protein. The mutant protein also inhibited radiation-induced NF-κB activation by preventing the proteolysis of IκB-α. This resulted in enhancement of cellular radiosensitivity and radiation-induced apoptosis. The NH2-terminal region of p65 is thus a potential molecular target for disruption of NF-κB activation and sensitization of tumors to radiotherapy.

Adenovirus-mediated gene transduction of truncated I kappa B alpha enhances radiosensitivity in human colon cancer cells

Nuclear factor kappa B (NF-kappa B) is a transcription factor that is known to regulate apoptosis when cells are exposed to DNA-damaging agents such as ionizing radiation and cytotoxic drugs. We sought to determine if inhibition of NF-kappa B could enhance radiosensitivity in human colon cancer cells in vitro and in vivo. To inhibit NF-kappa B activation specifically, we constructed a recombinant adenovirus vector expressing a truncated form of the inhibitor protein I kappa B alpha (I kappa B alpha Delta N) that lacks the phosphorylation sites essential for activation of NF-kappa B, and transfected two human colon cancer cell lines (HT29 and HCT15) with this vector. In vitro colony-forming assays revealed that the overexpression of the stable I kappa B alpha by AxI kappa B alpha Delta N infection significantly suppressed cell growth after irradiation in both cell lines as compared to infection with a control vector, AxLacZ. Treatment with AxI kappa B alpha Delta N and irradiation successfully inhibited the growth of HT29 xenografted subcutaneous tumors in nude mice with an 83.8% volume reduction on day 38 as compared to the untreated tumors. Furthermore, it was demonstrated that apoptosis was increased by adenovirus-mediated gene transduction of I kappa B alpha Delta N in vitro and in vivo. These results indicated that inhibition of NF-kappa B could enhance radiosensitivity through an increase in radiation-induced apoptosis. We believe that radio-gene therapy using adenovirus-mediated gene transduction of I kappa B alpha Delta N could be an attractive candidate as a treatment strategy for colorectal cancer.

NF-kappa B signaling pathway as a target for human tumor radiosensitization

NF-kappa B is a critical nuclear transcriptional factor that is activated in response to cellular stresses and regulates the expression of genes involved in cell proliferation and cell death. When regulated NF-kappa B activation is disrupted, cells undergo apoptosis. That is, constitutively elevated or dysregulated NF-kappa B activation leads to cell death in response to stress. These mechanisms have been shown experimentally by expressing dominant negative inhibitors of NF-kappa B (I kappa B-alpha) in cancer cells exposed to chemotherapeutic agents or to ionizing radiation. NF-kappa B also plays an important role in a novel, radiation-inducible signaling pathway that involves the ataxia-telangiectasia mutated (ATM) protein kinase. Cells from patients with ataxia-telangiectasia (AT) are exquisitely sensitive to ionizing radiation and exhibit impaired NF-kappa B activation in response to this stress. Restoration of NF-kappa B regulation in AT fibroblasts by introducing a dominant negative form of I kappa B-alpha has resulted in correction of radiation sensitivity and a reduction of ionizing radiation-induced apoptosis. Expression of introduced ATM in AT cells results in correction of NF-kappa B regulation and an increase in postradiation survival without reduction in radiation-induced apoptosis. Taken together, these observations support a central role for NF-kappa B regulation in cellular intrinsic radiation sensitivity and apoptosis after exposure to ionizing radiation. Therefore, we hypothesize that the signaling pathway involving ATM/NF-kappa B/I kappa B offers attractive potential molecular targets for radiation sensitization in strategies to enhance the therapeutic ratio in cancer treatment.

Enhancement of radiosensitivity by proteasome inhibition: implications for a role of NF-kappaB

PURPOSE

NF-kappaB is activated by tumor necrosis factor, certain chemotherapeutic agents, and ionizing radiation, leading to inhibition of apoptosis. NF-kappaB activation is regulated by phosphorylation of IkappaB inhibitor molecules that are subsequently targeted for degradation by the ubiquitin-proteasome pathway. PS-341 is a specific and selective inhibitor of the proteasome that inhibits NF-kappaB activation and enhances cytotoxic effects of chemotherapy in vitro and in vivo. The objective of this study was to determine if proteasome inhibition leads to enhanced radiation sensitivity.

METHODS AND MATERIALS

Inhibition of NF-kappaB activation in colorectal cancer cells was performed by treatment of LOVO cells with PS-341 or infection with an adenovirus encoding IkappaB super-repressor, a selective NF-kappaB inhibitor. Cells were irradiated at 0, 2, 4, 6, 8, and 10 Gy with or without inhibition of NF-kappaB. NF-kappaB activation was determined by electrophoretic mobility gel shift assay, and apoptosis was evaluated using the TUNEL assay. Growth and clonogenic survival data were obtained to assess effects of treatment on radiosensitization. In vitro results were tested in vivo using a LOVO xenograft model.

RESULTS

NF-kappaB activation was induced by radiation and inhibited by pretreatment with either PS-341 or IkappaBalpha super-repressor in all cell lines. Inhibition of radiation-induced NF-kappaB activation resulted in increased apoptosis and decreased cell growth and clonogenic survival. A 7-41% increase in radiosensitivity was observed for cells treated with PS-341 or IkappaBalpha. An 84% reduction in initial tumor volume was obtained in LOVO xenografts receiving radiation and PS-341.

CONCLUSIONS

Inhibition of NF-kappaB activation increases radiation-induced apoptosis and enhances radiosensitivity in colorectal cancer cells in vitro and in vivo. Results are encouraging for the use of PS-341 as a radiosensitizing agent in the treatment of colorectal cancer.