Targeting Translation Bypasses Pim Kinase Activity, a Common and Adverse Prognostic Marker In Lymphoma

Abstract 119

The PI3K/AKT/mTOR pathway is frequently activated in lymphoma, but rapamycin-analog (rapalog) mTOR inhibitors have shown only modest benefits in clinical trials on lymphoma patients. To better understand the resistance of lymphomas to rapalogs, we have undertaken a study of the Pim family kinases, which signal in parallel to PI3K/AKT/mTOR and whose expression has been detected in multiple subtypes of non-Hodgkin lymphoma (NHL). The two pathways converge on activation of cap-dependent translation, suggesting new treatment possibilities by targeting this common downstream output. To assess the clinical relevance of Pim activity, we have quantified Pim1 and Pim2 expression in multiple NHL subtypes using tissue microarray (TMA) technology. We find common expression of Pim1, Pim2, or both proteins in diffuse large B-cell lymphoma (DLBCL, 65.5%), follicular lymphoma (58%), small lymphocytic lymphoma (76.5%) and mantel cell lymphoma (89.7%). Importantly, Kaplan-Meier survival analysis of clinical data linked to our DLBCL TMAs show a strong trend toward a worse overall survival when Pim expression is present in diagnostic tumor samples compared to Pim-negative tumors (p=0.0965). Studies in vivo demonstrate Pim's ability to accelerate oncogenesis in a manner similar to AKT in model systems specific to Burkitt's lymphoma (Eμ -Myc) and follicular lymphoma (VavP-Bcl2). Treatment studies in secondary recipient animals show that Pim promotes resistance to anthracycline chemotherapy like AKT. However, Pim tumors completely resist rapamycin in stark contrast to AKT tumors. To elaborate on these findings, we have employed a genetically defined system of rapamycin sensitivity lacking mTOR's upstream repressor TSC2. These studies show that Pim's ability to mediate rapamycin resistance is dependent on its ability to maintain inhibitory phosphorylation of the translation repressor 4EBP1. Specifically, a phosphorylation-site deficient mutant of 4EBP1 completely abrogates Pim's ability to maintain the viability of the TSC2 −/− cells. We have expanded on this finding using the drug silvestrol, which inhibits cap-dependent translation by targeting eIF4A. Silvestrol shows high potency against Pim-expressing TSC2 −/− cells (IC₅₀ < 1 nM) and also against a panel of Pim-expressing lymphoma cell lines (IC₅₀ 1–10 nM). Indeed, targeting cap-dependent translation appeared more effective than the Pim kinase inhibitor SGI-1776 (IC₅₀ 1–10 μ M against lymphoma cell lines), which has significantly higher potency against Pim1 than Pim2. In conclusion, we have more clearly defined Pim kinase activity as a major mediator of oncogenesis in multiple NHL subtypes and as a likely negative prognostic marker in DLBCL. Our mechanistic and treatment studies provide a strong rational basis for targeting cap-dependent translation as a treatment strategy to bypass Pim activity and improve lymphoma patients' responses to both cytotoxic and rapalog therapies.