UC Irvine

One of the most frequently transformed signaling networks in cancer is the PI3K/mTOR /eIF4F signaling pathway. Through harmonized regulation of mTOR and eIF4F, as well as other overlapping signaling pathways, cancer cells achieve elevated proliferation, cell growth (increase in size), metabolism and survival. The key aim of this dissertation work is to identify and characterize critical signaling components of the PI3K/mTOR/eIF4F signaling pathway using both aggressive non-Hodgkin’s lymphoma (NHL) and Philadelphia chromosome-positive (Ph+) acute leukemia models. Our goal is to identify molecular mechanisms that could be exploited in order to improve responses to specific targeted agents: venetoclax (BCL2 inhibitor) in the case of aggressive NHL (chapter 2), and the tyrosine kinase inhibitor dasatinib for Ph+ acute leukemia (chapter 3).

The BCL2 inhibitor venetoclax has shown efficacy in several hematologic malignancies, with the greatest response rates in indolent blood cancers such as chronic lymphocytic leukemia (CLL). In aggressive NHL, including diffuse large B-cell lymphoma (DLBCL) and mantle cell lymphoma (MCL), there is a lower response rate to venetoclax monotherapy. At the same time, indications about the importance of mTOR signaling pathway and regulation of mRNA translation via eIF4F complex assembly have been accumulating.

In chapter 2 of this work, we demonstrate that small molecule inhibitors of cap-dependent mRNA translation sensitize NHL cells to apoptosis induced by venetoclax. We compared the mTOR kinase inhibitor (TOR-KI) MLN0128 with SBI-756, a compound targeting eukaryotic translation initiation factor 4G1 (eIF4G1), a scaffolding protein in the eIF4F complex. Treatment of NHL cells with SBI-756 synergized with venetoclax to induce apoptosis in vitro, and enhanced venetoclax efficacy in vivo at a tolerable dose. SBI-756 prevented eIF4E-eIF4G association and cap-dependent translation in a dose-dependent manner, while retaining mTOR substrate phosphorylation – indication of a great advantage over TOR-KIs. In addition, SBI-756 retained its ability to sensitize DLBCL cells to venetoclax even when those cells were lacking eIF4E binding protein-1 (4E-BP1), while these cells were insensitive to TOR-KI treatment. Moreover, SBI-756 treatment selectively reduced translation of mRNAs encoding ribosomal proteins and translation factors, hence leading to a delayed suppression in protein synthesis rates in sensitive cells. Furthermore, when human peripheral blood mononuclear cells were treated with SBI-756, only B lymphocytes had displayed reduced viability.

Moreover, we show the importance of eIF4E to support B-ALL survival. In chapter 3 of this work we demonstrate that treatment of Ph+ B-ALL cells with SBI-756 was able to reduce eIF4F complex formation (prevention of eIF4E-eIF4G interaction) in vitro and in vivo. At the same time, we developed a novel inducible system which enabled control of eIF4E availability. In both chemical and genetic approaches, we have shown that reduction of eIF4F complex formation sensitizes Ph+ B-ALL cells to dasatinib treatment. Similarly to the findings in NHL, both chemical and genetic suppression of eIF4F complex formation have resulted in reduced protein synthesis, reduced survival and sensitization to dasatinib. Lastly, treatment with SBI-756 was found cytotoxic with selectivity for B cells, while the compound was well tolerated in vivo.

Together, our findings using both NHL and B-ALL systems indicate that the eIF4F complex formation is an “Achilles heel” of aggressive blood malignancies and could be exploited for targeted therapy. Specifically, our abilities (chemically and genetically) to reduce eIF4F complex formation and mRNA translation provide a druggable and specific target with potential to improve treatment of NHL, B-ALL patients, and maybe others as well.