Original Article

Oncogene (2008) 27, 3301–3312; doi:10.1038/sj.onc.1211003; published online 4 February 2008

Mutant B-RAF mediates resistance to anoikis via Bad and Bim

K Boisvert-Adamo1 and A E Aplin1

1Center for Cell Biology and Cancer Research, Albany Medical College, Albany, NY, USA

Correspondence: Dr AE Aplin, Center for Cell Biology and Cancer Research, Albany Medical College, 47 New Scotland Avenue, Albany, MC-165, NY 12208, USA. E-mail: aplina@mail.amc.edu

Received 3 April 2007; Revised 12 November 2007; Accepted 26 November 2007; Published online 4 February 2008.

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Abstract

Normal cells undergo anoikis when they lose adhesion to or encounter an inappropriate extracellular matrix. By contrast, oncogenic signaling in tumor cells enables resistance to anoikis, a trait that contributes to tumor progression. The B-RAF serine-threonine kinase is mutated in multiple cancers and functions as an oncogene in melanoma. Previously, we demonstrated that B-RAF and downstream mitogen-activated protein/extracellular signal-regulated kinase kinase (MEK) signaling are necessary for protection from anoikis in mutant B-RAF-expressing melanoma cells. Regulation of Bcl-2 family members in melanoma and their role in B-RAF-mediated survival is poorly defined. Here, we provide evidence that B-RAF-MEK signaling protects against anoikis through alterations in two proapoptotic Bcl-2 family proteins: Bcl-xL/Bcl-2-associated death promoter (Bad) and Bcl-2-interacting mediator of cell death (Bim). B-RAF-MEK signaling regulates phosphorylation of the inhibitory serine-75 residue of Bad, and decreases Bad mRNA expression. RNA interference and overexpression experiments demonstrate that Bad contributes to the susceptibility of B-RAF-depleted cells to anoikis. Additionally, B-RAF-MEK signaling regulates the expression of BimEL, mainly through control of protein turnover. Increased BimEL levels induce apoptosis in suspended cells and are required for anoikis in B-RAF-depleted cells. Depletion of Bim together with Bad has an additive effect on protecting B-RAF knockdown cells from anoikis. Together, our data show that Bad and Bim are major B-RAF responsive proteins regulating apoptosis in melanoma cells.

Keywords:

apoptosis, Bad, Bim, B-RAF, melanoma

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