Cooperative repression of cell death by the tsc2 and RB tumor suppressors during Drosophila development.

摘要

The retinoblastoma tumor suppressor RB is an important regulator of cell proliferation, and is one of the most frequently affected genes in human cancer. A principle function of pRb and its related pocket proteins is repression of E2F-dependent transcription, and many studies have focused on the effects of E2F deregulation by either RB loss or by its functional inactivation due to cooperative mutations in other tumor suppressors. E2F1 in particular has garnered much attention due to its ability to activate transcription of a diverse range of target genes, including many involved in proliferation as well as apoptosis. Therefore RB is also an inhibitor of cell death, and E2F1 has the paradoxical function of being an oncogene and a tumor suppressor depending on the context of its activity. Understanding the specific roles of E2F1 in the absence of RB and how signaling pathways affect them is an important aim in the field of Cancer Biology. The Drosophila model organism provides a useful experimental system in which to study RB-E2F biology because this pathway is functionally conserved yet genetically simple in that flies have only two pocket protein-encoding genes, RBF and RBF2 and two E2Fs, dE2F1 and dE2F2. Further, studies have shown that loss of RBF during fly development results in context-dependent apoptosis due to deregulation of dE2F1. We performed a forward genetic screen to identify mutations in genes that modify the phenotype of RBF loss and recovered a mutation in dTSC2/GIGAS. dTSC2 is orthologous to the tuberous sclerosis complex tumor suppressor TSC2, which functions to regulate cell growth by inhibiting mTOR in the TORC1 complex. My characterization of the dtsc2 mutation reveals that, similar to other dtsc2 alleles, it causes cell growth and enlargement of tissues by promoting TORC1 activity. However this phenotype requires RBF, as rbf mutation suppresses the growth effect of dtsc2 in proliferating cells by inducing them to undergo dE2F1-dependent apoptosis. The mechanism by which rbf suppresses dtsc2 requires cell death and involves the synergistic induction of S-phase progression and coordinated activation of the pro-apoptotic gene HID. My experiments show that loss of dTSC2 results in dS6K-dependent dJnk/Bsk activation and that dJnk promotes HID gene expression, but Rbf prevents apoptosis in this context by repressing dE2f1 activity. These studies demonstrate that RBF and dTSC2 cooperate to regulate dE2f1-induced cell death by preventing the stress-related activation of dJnk in response to excessive TORC1 signaling.