BRAF-induced tumorigenesis is IKKα-dependent but NF-κB-independent.

Abstract

KRAS mutations contribute to cell proliferation and survival in numerous cancers, including colorectal cancers (CRC). One pathway through which mutant KRAS acts is an inflammatory pathway that involves the kinase IKK and activates the transcription factor NF-κB. BRAF, a kinase that is downstream of KRAS, is mutated in a subset of CRC and is predictive of poor prognosis and therapeutic resistance. We found that, in contrast to mutant KRAS, mutant BRAF (BRAF(V600E)) did not trigger NF-κB activation but instead triggered the phosphorylation of a proteolytic fragment of IKKα (p45-IKKα) in CRC cells. BRAF(V600E) CRC cells had a high abundance of phosphorylated p45-IKKα, which was decreased by a RAF inhibitor. However, the abundance and DNA binding of NF-κB in these cells were unaffected by the RAF inhibitor, and expression of BRAF(V600E) in human embryonic kidney-293T cells did not activate an NF-κB reporter. Moreover, BRAF-induced transformation of NIH-3T3 cells and BRAF-dependent transcription required phosphorylation of p45-IKKα. The kinase TAK1, which was associated with the endosomal compartment, phosphorylated p45-IKKα. Inhibition of endosomal vacuolar adenosine triphosphatase (V-ATPase) with chloroquine or bafilomycin A1 blocked p45-IKKα phosphorylation and induced apoptosis in BRAF-mutant CRC cells independent of autophagy. Treating mice with V-ATPase inhibitors reduced the growth and metastasis of BRAF(V600E) xenograft tumors in the cecum of mice.