Molecular determinants of melanoma malignancy: Selecting targets for improved efficacy of chemotherapy

The BRAF^V600E mutation is common in human melanoma. This mutation enhances IκB kinase (IKK)/nuclear factor-κB (NF-κB) and extracellular signal-regulated kinase/activator protein signaling cascades. In this study, we evaluated the efficacy of targeting either B-Raf or IKKβ in combination with the DNA alkylating agent temozolomide for treatment of advanced metastatic melanoma. Xenografts of Hs294T human metastatic melanoma cells exhibiting the BRAF^V600E mutation were treated with inhibitors of IKKβ (BMS-345541), B-Raf (BAY 54-9085), and/or temozolomide. Drug response was mechanistically analyzed in vitro and in vivo. In this study, we determined that the antitumor activity of all three drugs depends on inhibition of NF-κB. BMS-345541 inhibits IKKβ-mediated phosphorylation of IκBα and thus blocks the nuclear localization of NF-κB, whereas BAY 54-9085 inhibits activation of NF-κB through a mechanism that does not involve stabilization of IκBα. Moreover, BMS-345541, but not BAY 54-9085, activates the death pathways of p53 and c-Jun-NH ₂-kinase, contributing to the killing of melanoma cells. Temozolomide inhibits both NF-κB and extracellular signal-regulated kinase activity, conferring effective in vivo antitumor activity. Thus, temozolomide, but not BAY 54-9085, has a synergistic in vivo antitumor effect with BMS-345541. We conclude that the efficacy of antimelanoma therapy depends on inhibition of expression of antiapoptotic genes transcriptionally regulated by NF-κB. In contrast, drug targeting of the extracellular signal-regulated kinase/mitogen-activated protein kinase pathway alone in melanoma cells is ineffective for melanoma therapy in cases where NF-κB is not also targeted.