Abstract PR12: Targeting eIF2α phosphorylation for cancer therapy

Solid tumor tumors contain hypoxic regions. Tumor hypoxia is co-present with reduced nutrients and usually intracellular acidification. Lack of O2 and nutrients, and intracellular acidification causes endoplasmic reticulum stress (ER). In addition to canonical hypoxia response, tumor cells activate integrated ER-stress response (IERSR) in a spatially and temporally regulated manner to enter into dormancy under hypoxia and de-activate IERSR to return to a proliferative state once blood supply is restored. Phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2 (eIF2α) branch of IERSR is a critical determinant of tumor survival under hypoxia. Temporally and spatially regulated eIF2α phosphorylation attenuates overall translation while favoring translation of a subset of mRNAs coding for transcription factors and accessory proteins. This results in re-programming of gene expression that favors tumor-cell survival. Once the hypoxic conditions are ameliorated eIF2α phosphorylation is reversed and tumor cells exit dormancy. Sustained eIF2α phosphorylation on the other hand not only reduces overall protein synthesis to inhibit proliferation but also re-programs gene expression to induce expression of pro-apoptotic proteins such as CHOP to induce cell death. We therefore hypothesized that pharmacological activation of eIF2α kinases to induce sustained eIF2a phosphorylation will cause selective demise of tumors. To test this hypothesis and develop mechanism specific anti-cancer agents we generated high throughput reporter assays and screened large chemical libraries for inducers of eIF2α phosphorylation. Our Screening effort combined with functional genetic studies, and synthesis and evaluation of focused libraries resulted in identification of highly potent and specific activators of three eIF2α kinases, heme-regulated inhibitor (HRI), Protein kinase R, and PKR-like ER-kinase (PERK). These agents downregulate expression of pro-proliferative proteins such as cyclin D1 and cyclin E while upregulating the expression of pro-apoptotic proteins such as CHOP and tumor suppressor proteins such as BRCA1; all dependent on eIF2α phosphorylation. We will present unpublished data on the structure activity relationship studies of the second generation activators of eIF2α kinases. We will also present in vivo data demonstrating that inducers of eIF2α phosphorylation fully abrogate proliferation of human breast and melanoma xenografts growth with no apparent toxicity. At the molecular level, they induce eIF2α phosphorylation, expression of pro-apoptotic protein CHOP and cell cycle inhibitor p27Kip1 while inhibiting Cyclin D1 expression in xenograft tumors. Tumor cells acquire resistance to targeted therapy by bypassing the signaling node inhibited by treatment; activating alternate oncogenic pathway or downstream signaling node. Because translation initiation is in the apex of many oncogenic and proliferative pathways we hypothesized that translation initiation inhibitors will abrogate proliferation of targeted-therapy resistant cancer cell lines independent of the primary therapy or the mechanism of resistance. Our studies in melanoma cancer cells resistant to B-Raf or B-Raf and MEK inhibitors demonstrate that activators of eIF2α kinases or inhibitors of eIF4F complex inhibit the proliferation of parental and drug resistant cancer-cells with the same efficiency. We will also present these data and on our progress in resolving the crystal structure of full-length HRI which should allow us to accelerate drug development by improving the specificity and potency of our current lead compounds. We conclude that our second generation activators of eIF2α kinases are potent and mechanism specific agents that can be developed for treatment of naive and drug resistant tumors. This abstract is also being presented as Poster B20. Citation Format: Revital Yefidoff-Freedman, Ronghui Du, Fan Jing, Quingwen Zhang, Bertal H. Aktas. Targeting eIF2α phosphorylation for cancer therapy. [abstract]. In: Proceedings of the AACR Special Conference on Translational Control of Cancer: A New Frontier in Cancer Biology and Therapy; 2016 Oct 27-30; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2017;77(6 Suppl):Abstract nr PR12.