The Brn-2 Transcription Factor Links Activated BRAF to Melanoma Proliferation

Abstract

Malignant melanoma, an aggressive and increasingly common cancer, is characterized by a strikingly high rate (70%) of mutations in BRAF, a key component of the mitogen-activated protein (MAP) kinase signaling pathway. How signaling events downstream from BRAF affect the underlying program of gene expression is poorly understood. We show that the Brn-2 POU domain transcription factor is highly expressed in melanoma cell lines but not in melanocytes or melanoblasts and that overexpression of Brn-2 in melanocytes results in increased proliferation. Expression of Brn-2 is strongly upregulated by Ras and MAP kinase signaling. Importantly, the Brn-2 promoter is stimulated by kinase-activating BRAF mutants and endogenous Brn-2 expression is inhibited by RNA interference-mediated downregulation of BRAF. Moreover, silent interfering RNA-mediated depletion of Brn-2 in melanoma cells expressing activated BRAF leads to decreased proliferation. The results suggest that the high levels of Brn-2 expression observed in melanomas link BRAF signaling to increased proliferation.

We thank Nicola Bentley and Giulia Gambino for undertaking early work on the investigation of the Brn-2 promoter and Y. Hara and M. Nirenberg for providing the original Brn-2 genomic clone. The Ras and β-catenin expression vectors used in this study were a gift from Alan Hall and Rudi Grosschedl, respectively. Thanks also to Ian Hart, who provided the Ras-transformed melan-a cells and several melanoma cell lines, to Dot Bennett for the primary mouse melanocytes, to Hartmut Land for the pBabe vectors, and to Chris Marshall for the MEK.EE retroviral clone.

This work was supported by the Marie Curie Cancer Care, the Association for International Cancer Research, the Medical Research Council, and Cancer Research UK.