Taming the Hippo: Raf-1 controls apoptosis by suppressing MST2/Hippo

The Raf-1 kinase has a well established role in activating the MEK-ERK/MAPK pathway. However, accumulating evidence including the phenotype of Raf-1(-/-) mice suggested that Raf-1 may have other functions independent of its role as MEK activator, in particular pertaining to protection against apoptosis. We have recently demonstrated a new role of Raf-1 by showing that Raf-1 controls the proapoptotic kinase MST2/Hippo. In mammalian cells MST2 is activated by stress signals and causes apoptosis when overexpressed. Its Drosophila homologue Hippo regulates apoptosis and cell cycle arrest during differentiation. Raf-1 inhibits MST2 by preventing its dimerisation and recruiting a phosphatase that removes activating phosphorylations on MST2. Both functions require Raf-1 binding to MST2, but are independent of Raf-1's kinase activity and the ERK pathway. Downregulation of MST2 by siRNA reverts the apoptosis hypersensitivity of Raf-1(-/-) mouse fibroblasts. In contrast, the downregulation of Raf-1 in Raf-1(+/+) cells and human cancer cell lines enhances susceptibility to Fas induced apoptosis, which is rescued by concomitant downregulation of both Raf-1 and MST2. The MST2:Raf-1 complex is dissociated by stress signals as well as mitogens. Stress signals robustly activate MST2 and trigger apoptosis. Mitogens only make MST2 permissive for activation by releasing it from Raf-1, and in addition activate survival pathways allowing proliferation. Thus, by linking mitogenic and apoptotic signalling the MST:Raf-1 complex may serve as a safeguard against unlicensed proliferation.