Oncogenic BRAF regulates β-Trcp expression and NF-κB activity in human melanoma cells

Abstract

Mutational activation of BRAF is a frequent event in human malignant melanomas suggesting that BRAF-dependent signaling is conducive to melanoma cell growth and survival. Previously published work reported that melanoma cells exhibit constitutive anti-apoptotic nuclear factor κB (NF-κB) transcription factor activation triggered by proteolysis of its inhibitor IκB. IκB degradation is dependent upon its phosphorylation by the IκB kinase (IKK) complex and subsequent ubiquitination facilitated by β-Trcp E3 ubiquitin ligase. Here, we report that melanocytes expressing a conditionally oncogenic form of BRAF^V600E exhibit enhanced β-Trcp expression, increased IKK activity and a concomitant increase in the rate of IκBα degradation. Conversely, inhibition of BRAF signaling using either a broad-spectrum Raf inhibitor (BAY 43-9006) or by selective knock-down of BRAF^V600E expression by RNA interference in human melanoma cells leads to decreased IKK activity and β-Trcp expression, stabilization of IκB, inhibition of NF-κB transcriptional activity and sensitization of these cells to apoptosis. Taken together, these data support a model in which mutational activation of BRAF in human melanomas contributes to constitutive induction of NF-κB activity and to increased survival of melanoma cells.