Myc Commands an Aurora Kinase – Sumoylation Circuit Required for B Cell Lymphoma Growth and Survival

Abstract

▪Myc oncoproteins (c-Myc, N-Myc and L-Myc) are transcription factors that regulate cell growth, cell division and metabolism under physiologic conditions. Myc overexpression is a hallmark of Burkitt lymphoma (BL) harboring MYC/IG translocations, and is frequently present in many advanced cancers. Myc overexpression is associated with aggressive disease, which is in part due to the destruction of select targets by the ubiquitin-proteasome system, for example Skp2SCF-directed destruction of the Cdk inhibitior p27Kip1 (Keller et al., EMBO 2007, Old et al., Mol. Canc. Res. 2010). We have identified a related means of post-translational protein modification, SUMOylation, as a pathway activated by Myc. Consequently, Myc-driven human BL and manifest Eµ-Myc mouse lymphomas are characterized by a hyperSUMOylation phenotype. Targeting SUMOylation in such lymphomas by a genetic approach results in growth inhibition and blocks lymphoma maintenance in vitro and in vivo. We further found that the SUMO pathway is in particular required for intact transitioning of the G2-M cell cycle checkpoint and through mitosis. The use of pharmacologic SUMOylation inhibitors (SUMOi) accordingly resulted in cell cycle arrest, polyploidy and cell death (Hoellein et al., Blood 2014).In this current report we aimed to identify crucial players of the Myc-SUMOylation axis by applying a mass spectrometry approach that used purified SUMOylated proteins in a conditional Myc-inducible B cell model. Stable isotope labeling permitted quantitative analysis of protein SUMOylation in the Myc-on versus Myc-off state. This screen identified Aurora kinase A as a Myc-regulated target of SUMOylation. Aurora kinases are essential regulators of mitosis and cytokinesis that are indispensable for Myc-driven transformed cells, and inhibition of Aurora kinase function results in similar effects as blocking SUMOylation, namely G2-M arrest, impaired cytokinesis resulting in polyploidy, and apoptosis (den Hollander et al., Blood 2010). We confirmed Myc-specific SUMO modification of Aurora kinase A and B and found reduced SUMOylation of both kinases upon treatment with pharmacologic SUMOylation inhibitors or shRNA targeting the key SUMOylation enzymes. Overexpression of Aurora kinase A and B mutants harboring a mutated SUMOylation motif resulted in a phenotype reminiscent of pharmacological SUMOi. This phenotype is independent of the Aurora kinase function since the SUMO motif mutation does not impair kinase activity. Moreover, in a comprehensive screen for specific E3-ligases that regulate the SUMO modification of the Aurora kinases we identified several SUMO E3 ligases that are also Myc induced.In summary, we propose a Myc-Aurora-kinase-SUMOylation circuit where Myc activation allows the induction of Aurora kinase transcription as well as the transcription of critical SUMOylation pathway genes. This mechanism contributes to sufficient Aurora kinase SUMOylation allowing faithful cell cycle passage and cytokinesis. In lymphoma and possibly other Myc-dependent cancers this pathway represents a target for synthetic lethal drug applications. DisclosuresNo relevant conflicts of interest to declare.