Cell Fate Control by Integrated E2F, FoxO and PI3K Signaling in Retinoblastoma

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DESCRIPTION (provided by applicant): The E2F transcription factors induce proliferation but also can promote cell death as an innate anti-cancer mechanism to kill cells with a spontaneous oncogenic mutation that might otherwise go on to form a cancer. Little is known about the molecular circuitry that tips E2F1 balance towards proliferation in some settings (normal growth), apoptosis in others (oncogenic stress/incipient cancers), and which pathways mediate this decision. Lack of such knowledge is an important unmet medical problem, because without it, acquiring the ability to restore latent E2F1 mediated apoptotic activity therapeutically in tumors to accelerate tumor cell death is highly unlikely. We discovered that the PI3K/Akt pathway, which is frequently activated in human cancer, is a key negative regulator of E2F1 induced apoptosis and apoptotic target gene expression. These findings suggest that function of E2F1, a traditionally "undruggable" target molecule (transcription factor) can be modulated towards apoptosis by interfering with PI3K activity. We also show that the FoxO1 transcription factor physically interacts with E2F1 and can bind to the same promoters to regulate gene expression and apoptosis. The major hypothesis tested in this proposal is that E2F1 and FoxO1 transcription factors coordinate a tumor suppressive apoptotic transcriptional program that can be inhibited by Akt activation. Aim 1 will focus on a) identifying domains in E2F1 and FoxO1 required for interaction and function, b) will determine if other E2Fs and FoxOs interact, and c) employ an IP-Mass Spec strategy to determine other E2F1 binding partners that may regulate the function of the E2F1/FoxO1 complex and coordinate its pro-apoptotic function. Aim 2 delves into identification of co-regulated E2F1 and FoxO1 target genes and the mechanisms of gene expression using qPCR, RNA-Seq, ChIP, and ChIP-Seq. Aim 3 tests FoxO and Pten/PI3K/Akt pathway regulation of Rb/E2F apoptosis and tumorigenesis in vivo in the mouse retina. We also propose to use our mouse model, which develops aggressive bilateral retinoblastoma, for pre-clinical testing anti-PI3K therapy with or without current standard of care treatment. Our finding that FoxO and PI3K activity facilitate control of the E2F1 apoptotic and proliferative balance suggest that these studies will provide significant insight into the mechanistic relationship between these proteins in normal and cancerous cells. The experiments outlined in this proposal will provide important information about the function of the E2F1/FoxO complex, its regulation by PI3K signaling, and the extent that inhibiting PI3K in vivo may be a therapeutic option aiming at restoring E2F1/FoxO apoptosis stimulation in tumors. Considering that dysfunction in Rb/E2F and PI3K/Pten pathways feature prominently in numerous human tumors, we expect that our findings may be applicable to other tumor types.