Hidden Targets in RAF Signalling Pathways to Block Oncogenic RAS Signalling

Abstract

Oncogenic RAS (Rat sarcoma) mutations drive more than half of human cancers, and RAS inhibition is the holy grail of oncology. Thirty years of relentless efforts and harsh disappointments have taught us about the intricacies of oncogenic RAS signalling that allow us to now get a pharmacological grip on this elusive protein. The inhibition of effector pathways, such as the RAF-MEK-ERK pathway, has largely proven disappointing. Thus far, most of these efforts were aimed at blocking the activation of ERK. Here, we discuss RAF-dependent pathways that are regulated through RAF functions independent of catalytic activity and their potential role as targets to block oncogenic RAS signalling. We focus on the now well documented roles of RAF kinase-independent functions in apoptosis, cell cycle progression and cell migration.

Keywords: ASK; MST2; PLK; RAF kinase-independent; RAS; RHO-α; apoptosis; cancer therapy; cell cycle.

Figure 1

The RAF-MEK-ERK pathway is activated by H/K and NRAS upon extracellular stimuli. ERK1/2 phosphorylate over 50 substrates and control different cell fate.

Figure 2

RAF1 kinase-independent regulation of ASK1 proapoptotic signalling. FGF activation promotes RAF1–ASK1 complex localization in the mitochondria. Oxidative stress prevents the inhibitory binding of RAF1 to ASK1 and leads to activation of stress MAPK.

Figure 3

RAF1 protein structure. The phosphorylation sites indicate the residues that are known to be phosphorylated when RAF1 binds to its kinase-independent effectors.

Figure 4

RAF1 negatively regulates the proapoptotic Hippo pathway by binding to MST2 upon growth factor stimulation. RASSF1A rescues MST2 from the inhibitory binding of RAF1 and regulates the activation of the core proteins of the Hippo pathway upon death receptor activation. Oncogenic KRAS also promotes the activation of the proapoptotic Hippo pathway while wildtype RAS isoforms promote RAF1–MST2 interaction.

Figure 5

RAF1 and BRAF regulate the activation of apoptosis by modulating PKCθ phosphorylation of BAD.

Figure 6

RAF1 interacts and inhibits the kinase activity of ROK-α. RHO binding to ROK-α rescues this kinase from the inhibitory effect of RAF1 and promotes the activation of cell migration and cell differentiation. FAS activation increases the formation of RAF1–ROK-α complex increasing apoptotic threshold.

Figure 7

RAF1 kinase-independent regulation of cell cycle and mitosis check points. (A) RAF1 binds to PLK1 and Aurora A in the mitotic spindle and activates these kinases. (B) PAK1 promotes the interaction of RAF1 with CHK2 upon genotoxic effect, promotes DNA repair and prevents the activation of the DNA damage apoptotic pathway.