NF-κB Mediates Mitogen-Activated Protein Kinase Pathway-Dependent iNOS Expression in Human Melanoma

Tumor expression of inducible nitric oxide synthase (iNOS) predicts poor outcomes for melanoma patients. We have reported the regulation of melanoma iNOS by the mitogen-activated protein kinase (MAPK) pathway. In this study, we test the hypothesis that NF-κB mediates this regulation. Western blotting of melanoma cell lysates confirmed the constitutive expression of iNOS. Western blot detected baseline levels of activated nuclear extracellular signal-regulated kinase and NF-κB. Indirect immunofluorescence confirmed the presence of NF-κB p50 and p65 in melanoma cell nuclei, with p50 being more prevalent. Electrophoretic mobility shift assay demonstrated baseline NF-κB activity, the findings confirmed by supershift analysis. Treatment of melanoma cells with the MEK inhibitor U0126 decreased NF-κB binding to its DNA recognition sequence, implicating the MAPK pathway in NF-κB activation. Two specific NF-κB inhibitors suppressed iNOS expression, demonstrating regulation of iNOS by NF-κB. Several experiments indicated the presence of p50 homodimers, which lack a transactivation domain and rely on the transcriptional coactivator Bcl-3 to carry out this function. Bcl-3 was detected in melanoma cells and co-immunoprecipitated with p50. These data suggest that the constitutively activated melanoma MAPK pathway stimulates activation of NF-κB hetero- and homodimers, which, in turn, drive iNOS expression and support melanoma tumorigenesis.