Specific Killing of Rb Mutant Cancer Cells by Inactivating TSC2

Abstract

SUMMARY The retinoblastoma (Rb) tumor suppressor is often inactivated in cancers. To identify genes that can be used to specifically target such cancers, we carried out a genetic screen in Drosophila. We identified gig (fly TSC2) and found that inactivation of rbf (fly Rb) and gig synergistically induced cell death. Interestingly, inactivation of TSC2 specifically kills Rb mutant cancer cells under stress conditions, which is correlated with an inhibition of tumor growth. We show that cancer cell killing induced by concomitant inactivation of Rb and TSC2 is mediated by increased cellular stress, including oxidative stress. Inactivation of TSC2 and Rb synergistically induce oxidative stress via increased protein synthesis, inhibited de novo lipid synthesis, and decreased ROS scavenger enzyme SOD2 induction.

Deregulated G1/S control and energy stress contribute to the synthetic lethal interactions between inactivation ofRBandTSC1/TSC2

“…1D-E9). As caspase activation can induce compensatory proliferation in flies (Ryoo et al, 2004), we tested whether the proliferative phenotype is a consequence of apoptosis by removing the apoptotic gene hid (Tanaka-Matakatsu et al, 2009) from rbf,gig-mutant clones, which significantly reduces the level of apoptosis (Li et al, 2010). As shown in Fig.…”

“…We previously showed that loss of gig causes synergistic apoptosis and ablation of rbf-mutant tissue in mosaic eyes and wings (Li et al, 2010). As mutation of either rbf or gig causes G1-S deregulation, we investigated the effect of inactivating both rbf and gig (hereafter indicated 'rbf,gig') on G1-S regulation and its impact on synergistic apoptosis.…”

“…We previously showed that removing the transcription activation function of de2f1 or inactivation of s6k suppressed the synergistic cell death effect of rbf,gig (Li et al, 2010). To investigate whether the effects of de2f1 or s6k mutation on the synergistic cell death of rbf,gig clones are correlated with their effects on increased S phase, we introduced either de2f1 or s6k mutation into the rbf,gig-mutant background and found that each suppresses ectopic S phase within the MF (Fig.…”

“…We conducted a forward screen to identify genes that are important for cell survival and differentiation in the absence of the RB ortholog Retinoblastoma family protein rbf (Steele et al, 2009;Sukhanova et al, 2011;Tanaka-Matakatsu et al, 2009) and identified the fly TSC2 ortholog gig (encoding the protein Gigas), which induces synergistic cell death upon rbf loss (Li et al, 2010). This synthetic-lethal interaction between RB and TSC2 is conserved between flies and human cancer cells, as we showed that inactivation of RB and TSC2 leads to the induction of excessive cellular stress, including ROS, which contributes to synergistic cell death in cancer cells in vitro and using a xenograft model (Danos et al, 2012;Li et al, 2010). However, the in vivo mechanisms that mediate the synthetic lethality of RB and TSC2 mutations are unclear.…”

RB1: a prototype tumor suppressor and an enigma

“…This genetic interaction is effective at suppressing tumorigenesis in mouse models but has yet to be applied to human tumors. Based on genetic interactions that were discovered in Drosophila, others have suggested targeting TSC2 to elevate reactive oxygen species (ROS) in RB1 mutant tumors (Li et al 2010;Gordon et al 2013). Potentially, other metabolic features of RB1 mutant cells, such as the changes in mitochondrial activity, depletion of nucleotide pools, or changes in autophagy, might provide alternative therapeutic strategies (Angus et al 2002;Tracy et al 2007;Macleod 2008).…”