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

Abstract

Abstract 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.