A MEK-independent role for CRAF in mitosis and tumor progression

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Abstract
Raf kinase activity is deregulated in cancers and is thought to promote tumor growth by inducing proliferation signaling through MEK/Erk. This report identifies a new role for c-Raf independent of MEK/Erk and relying on phosphorylation of Ser338. Phospho–C-Raf interacts with the mitotic kinases Aurora-A and Plk1, activating the latter to promote mitotic progression and increase cell division, and this pathway is a crucial mediator for the protumorigenic effect of c-Raf in vivo. RAF kinases regulate cell proliferation and survival and can be dysregulated in tumors1,2. The role of RAF in cell proliferation has been linked to its ability to activate mitogen-activated protein kinase kinase 1 (MEK) and mitogen-activated protein kinase 1 (ERK). Here we identify a MEK-independent role for RAF in tumor growth. Specifically, in mitotic cells, CRAF becomes phosphorylated on Ser338 and localizes to the mitotic spindle of proliferating tumor cells in vitro as well as in murine tumor models and in biopsies from individuals with cancer. Treatment of tumors with allosteric inhibitors, but not ATP-competitive RAF inhibitors, prevents CRAF phosphorylation on Ser338 and localization to the mitotic spindle and causes cell-cycle arrest at prometaphase. Furthermore, we identify phospho-Ser338 CRAF as a potential biomarker for tumor progression and a surrogate marker for allosteric RAF blockade. Mechanistically, CRAF, but not BRAF, associates with Aurora kinase A (Aurora-A) and Polo-like kinase 1 (Plk1) at the centrosomes and spindle poles during G2/M. Indeed, allosteric or genetic inhibition of phospho-Ser338 CRAF impairs Plk1 activation and accumulation at the kinetochores, causing prometaphase arrest, whereas a phospho-mimetic Ser338D CRAF mutant potentiates Plk1 activation, mitosis and tumor progression in mice. These findings show a previously undefined role for RAF in tumor progression beyond the RAF-MEK-ERK paradigm, opening new avenues for targeting RAF in cancer.