Hyperactivated Wnt Signaling Induces Synthetic Lethal Interaction with Rb Inactivation by Elevating TORC1 Activities

Abstract

Inactivation of the Rb tumor suppressor can lead to increased cell proliferation or cell death depending on specific cellular context. Therefore, identification of the interacting pathways that modulate the effect of Rb loss will provide novel insights into the roles of Rb in cancer development and promote new therapeutic strategies. Here, we identify a novel synthetic lethal interaction between Rb inactivation and deregulated Wg/Wnt signaling through unbiased genetic screens. We show that a weak allele of axin, which deregulates Wg signaling and increases cell proliferation without obvious effects on cell fate specification, significantly alters metabolic gene expression, causes hypersensitivity to metabolic stress induced by fasting, and induces synergistic apoptosis with mutation of fly Rb ortholog, rbf. Furthermore, hyperactivation of Wg signaling by other components of the Wg pathway also induces synergistic apoptosis with rbf. We show that hyperactivated Wg signaling significantly increases TORC1 activity and induces excessive energy stress with rbf mutation. Inhibition of TORC1 activity significantly suppressed synergistic cell death induced by hyperactivated Wg signaling and rbf inactivation, which is correlated with decreased energy stress and decreased induction of apoptotic regulator expression. Finally the synthetic lethality between Rb and deregulated Wnt signaling is conserved in mammalian cells and that inactivation of Rb and APC induces synergistic cell death through a similar mechanism. These results suggest that elevated TORC1 activity and metabolic stress underpin the evolutionarily conserved synthetic lethal interaction between hyperactivated Wnt signaling and inactivated Rb tumor suppressor.

ESCRT-0 complex modulates Rbf mutant cell survival by regulating Rhomboid endosomal trafficking and EGFR signaling

“…Furthermore, eye-specific knockdown of Rbf and Stam, or Rbf and Hrs using RNA interference (RNAi) constructs dramatically reduced the size of adult eyes, whereas RNAi against each of the single genes only exhibited moderate effects ( Fig. S1G-L In Drosophila eye discs, cell death in rbf-mutant tissue is observed near the morphogenetic furrow, which requires dE2F1 activity and is mediated by increased Hid expression (Du, 2000;Li et al, 2010;Moon et al, 2005;Tanaka-Matakatsu et al, 2009;Zhang et al, 2014). We examined the effects of rbf and stam on the levels of Hid protein.…”

“…However, because stam and hrs mutants have been shown to affect the localization of a large number of receptors in different signaling pathways, the possibility that other pathways also contribute to the synergistic cell death of rbf stam double mutants cannot be excluded. Previously, mutations of axin, gig and Drosophila Tsc1 (dTsc1) have also been reported to induce synergistic cell death with rbf (Gordon et al, 2013;Hsieh et al, 2010;Li et al, 2010;Zhang et al, 2014). Interestingly, although synergistic cell death of rbf stam double-mutant cells is restricted in posterior eye discs, synergistic cell death in rbf axin, rbf gig or rbf dTsc1 double-mutant clones is more prominent in the anterior proliferating region of the eye disc.…”

“…The distinct patterns of synergistic cell death observed in different double-mutant clones are mediated by different mechanisms. The synergistic cell death in rbf axin, rbf gig and rbf dTsc1 doublemutant cells is mediated by excessive cellular stress and increased dE2F1 levels (Gordon et al, 2013;Hsieh et al, 2010;Li et al, 2010;Zhang et al, 2014), whereas synergistic cell death of rbf stam or rbf hrs mutants in posterior eye discs is mediated by reduced EGFR signaling. In addition, mutation of groucho, which causes derepression of Rhomboid expression and precocious EGFR signaling activation in eye discs (Zhang and Du, 2015), can suppress cell death in rbf mutants near the morphogenetic furrow (our unpublished results).…”

“…Fly stocks used in this study were: rbf 15aΔ (Zhang et al, 2014) (Freeman et al, 1992), UAS-Rho (Zhang and Du, 2015), UAS-sSpi (Miura et al, 2006), UASmSpi-GFP (Yogev et al, 2010)…”

“…The Drosophila developing eye provides a model system to identify genes that modulate the proliferation, differentiation or apoptosis of rbf-inactivated cells (Gordon et al, 2013;Li et al, 2010;Steele et al, 2009;Tanaka-Matakatsu et al, 2009;Zhang et al, 2014). Photoreceptor differentiation in the developing eye initiates in the morphogenetic furrow (Treisman, 2013).…”