A SUBSTRATE-GUIDED APPROACH FOR IDENTIFICATION OF PATHWAYS REGULATED BY PIM KINASES; NEW BIOMARKERS AND PUTATIVE CO-THERAPEUTIC STRATEGIES FOR CANCER TREATMENT

Abstract

Kinase inhibition is a major strategy for therapeutic intervention as oncogenic kinase activity is a common feature shared by most cancers. Pim kinases are deregulated in certain solid tumors and hematopoietic cancers, notably in acute myeloid leukemia (AML). While cancer cells can become dependent on Pim kinase activity to sustain proliferation, these kinases appear to be dispensable in most normal adult tissues. These features make Pim kinases an attractive target for cancer therapy, however, their physiological roles have not been fully characterized. Pim kinases are considered a potential therapeutic target against AML as Pim kinase inhibitors can disrupt proliferation and survival of AML cell lines and primary patient cells in vitro. However, single-agent Pim inhibition therapy may not be effective as resistance to Pim inhibitors is frequently observed in AML patients. Combination of Pim inhibition along with other inhibitors might effectively overcome primary resistance and preemptively limit relapse by acquired resistance. Here, we describe a new approach to identify novel and effective synergistic combinations with kinase inhibitors in a substrate-guided fashion. Research in our lab has identified ~570 Pim kinase substrates. In this work two Pim substrates- hnRNPA1 and rpS6 were validated as in vivo targets for Pim inhibitor-induced de-phosphorylation. Interestingly, a majority of novel Pim substrates were involved in mRNA splicing and rRNA processing pathways. Pharmacological inhibition of Pim kinases using AZD1208 led to large-scale changes in splicing patterns of cellular mRNAs, and induced several specific defects in rRNA processing. Further, we selected drug combinations based on the knowledge of these pathways regulated by Pim kinases, and demonstrate their synergistic potential against AML cell lines, irrespective of the response to single-agent Pim inhibition. Moreover, novel Pim substrates, Pim inhibitor-induced splicing changes, and rRNA processing changes can act as putative biomarkers for Pim activity and inhibitor responsiveness. Using Pim kinase as an example, we demonstrate a substrate-guided approach to biomarker identification and combination-therapy selection yielding drug synergy.