Activating mutations in genes that drive cell proliferation are required but not sufficient on their own to give rise to cancer as they typically require other co-transforming mutations. The reason is that mammalian cells are equipped with mechanisms that sense apparent growth signals that could lead to transforming cells into a tumorigenic threat to the organism. Those mechanisms sense such uncontrolled growth signals and coordinate the activation of responses such as cell cycle arrest, senescence or cell death (suicide) that aim to eliminate the threat. Only when such responses are disabled by other co-transforming mutations, can transformation occur.
Ras are important oncoproteins found mutated in 25-30% of all human tumors (Fernández-Medarde et al., 2011). Ras shuffle between active and inactive status and regulate several signalling pathways that control growth and proliferation. In an attempt to understand how cells react to increased activity of H-Ras, my previous work has shown that cells respond to mutational activation of H-Ras by triggering massive autophagy which seems to act as a fail-safe mechanism to restrain tumorigenesis (Elgendy et al., 2011). H-Ras-induced autophagy was associated with the induction of Beclin 1 and Noxa, which all act in the same pathway to mediate autophagy induction. To get further insight into how Ras mechanistically leads to the induction of Beclin 1 and Noxa and to dissect this pathway further, I used pharmacological inhibitors of several pathways downstream of Ras. Inhibition of the MEK/ERK pathway completely abolished Ras-induced Beclin 1 and Noxa expression. Taking that further, I used siRNA to knockdown several known factors that act downstream of ERK. My unpublished observations show that ablation of ATF4 (and to much less extent c-jun) completely abolishes the induction of Beclin 1 and Noxa. Since ATF4 is known to be an essential mediator of ER stress (Szegezdi et al., 2006) and given that ER stress has been shown to mediate the induction of autophagy in several contexts, I assessed the effect of ATF4 knockdown on Ras-induced autophagy. Ablation of ATF4 abrogated Ras-induced autophagy as assessed by percentage of cells showing LC3 puncta, a widely used marker of autophagy (Klionsky et al., 2014). These results indicate that ATF4 acts as the master regulator of the responses triggered by mutational activation of H-Ras. This proposal aims to dissect the interplay between ATF4 and H-Ras, to further explore whether ATF4 controls other mediators besides Beclin 1 and Noxa, and how it co-ordinates the whole ER stress-autophagy response to ultimately understand the implications for H-Ras-driven tumorigenesis.