Silencing c-Myc Translation as a Therapeutic Strategy through Targeting PI 3 K Delta and CK 1 Epsilon in Hematological Malignancies Short title : Targeting PI 3 K and CK 1 in Myc Driven Lymphoma

Phosphoinositide 3-kinase (PI3K) and the proteasome pathway are both involved in activating the mechanistic target of rapamycin (mTOR). Because mTOR signaling is required for initiation of mRNA translation, we hypothesized that co-targeting the PI3K and proteasome pathways might synergistically inhibit translation of c-Myc. We found that a novel PI3K delta isoform inhibitor TGR-1202, but not the approved PI3Kδ inhibitor idelalisib, was highly synergistic with the proteasome inhibitor carfilzomib in lymphoma, leukemia, and myeloma cell lines and primary lymphoma and leukemia cells. TGR-1202 and carfilzomib (TC) synergistically inhibited phosphorylation of the eukaryotic translation initiation factor 4E (eIF4E)-binding protein 1 (4E-BP1), leading to suppression of c-Myc translation and silencing of c-Myc dependent transcription. The synergistic cytotoxicity of TC was rescued by overexpression of eIF4E or c-Myc. TGR1202, but not other PI3Kδ inhibitors, inhibited casein kinase-1 (CK1) epsilon. Targeting CK1ε using a selective chemical inhibitor or shRNA complements the effects of idelalisib, as a single agent or in combination with carfilzomib, in repressing phosphorylation of 4E-BP1 and the protein level of c-Myc. These results suggest that TGR-1202 is a dual PI3Kδ/CK1ε inhibitor, which may in part explain the clinical activity of TGR-1202 in aggressive lymphoma not found with idelalisib. Targeting CK1ε should become an integral part of therapeutic strategies targeting translation of oncogenes such as c-Myc. For personal use only. on December 11, 2016. by guest www.bloodjournal.org From