Myc proteins as therapeutic targets

Abstract

Myc proteins (c-myc, Mycn and Mycl) target proliferative and apoptotic pathways vital for progression in cancer. Amplification of the MYCN gene has emerged as one of the clearest indicators of aggressive and chemotherapy-refractory disease in children with neuroblastoma, the most common extracranial solid tumor of childhood. Phosphorylation and ubiquitin-mediated modulation of Myc protein influence stability and represent potential targets for therapeutic intervention. Phosphorylation of Myc proteins is controlled in-part by the receptor tyrosine kinase/phosphatidylinositol 3-kinase/Akt/mTOR signaling, with additional contributions from Aurora A kinase. Myc proteins regulate apoptosis in part through interactions with the p53/Mdm2/Arf signaling pathway. Mutation in p53 is commonly observed in patients with relapsed neuroblastoma, contributing to both biology and therapeutic resistance. This review examines Myc function and regulation in neuroblastoma, and discusses emerging therapies that target Mycn.

Figure 1

Schematic model of regulatory pathways involved in Mycn stabilization. In proliferating cells, Mycn is initially phosphorylated at serine 62 by cyclin-dependent kinase 1/CyclinB. The S62-phosphorylated protein is stabilized and competent to enter the nucleus. Nuclear Mycn binds to its partner, the helix-loop-helix transcription factor Max and stimulates the transcription of a target genes important in cell cycle, proliferation, differentiation and apoptosis. This priming phosphorylation at S62 allows the binding of Gsk3β, as well as Pin1 and PP2A in a complex also containing Axin. Active Gsk3β phosphorylates Mycn at threonine 58, producing doubly phosphorylated, stabilized and transcriptionally active Mycn. In a Pin1-mediated process, PP2A dephosphorylates the S62 phosphate, enabling binding of ubiquitin ligase Fbw7. Fbw7 subsequently drives poly-ubiquitination and proteasomal degradation of Mycn. Aurora A kinase can bind to and stabilize polyubiquitinated/phosphorylated Mycn, potentially leaving it competent to bind to Max and activate transcription. In this model, upstream signaling starts at the membrane with receptor tyrosine kinases (RTKs), which are either activated by ligand or by constitutively activating mutations (in Alk, for example). RTKs activate phosphatidylinositol 3-kinase (PI3K), which catalyzes the conversion of phosphatidylinositol-3,4-bisphosphate (PIP2) to phosphatidylinositol-3,4,5-triphosphate (PIP3). PIP3 then binds to Akt, localizing it to the membrane and allowing the phosphorylation and activation of Akt by membrane bound Pdk1 at threonine 308. Active Akt then phosphorylates Glycogen synthase kinase 3β (Gsk3β) and inactivates it, blocking Gsk3β-mediated phosphorylation of Mycn T58 and thereby stabilizing Mycn. Akt also activates mammalian target of rapamycin (mTOR) through several indirect signaling mechanisms including through Tsc2/Tsc1 and Rheb. mTOR exists in two distinct complexes. mTORC1 complex, the rapamycin sensitive form, consists of mTOR bound to multiple effector proteins, including Raptor and Lst8, and is important for the promotion of global translation through S6K and 4EBP. mTORC1 also directly phosphorylates and inhibits PP2A, enabling the accumulation of doubly phosphorylated, active Mycn and contributing to the general proliferation-promoting downstream effects of the PI3K/Akt/Mycn pathway (Cheng et al., 2007). The mTORC2 complex, also known as Pdk2, contains mTOR, rictor, Lst8 and mSin1. mTORC2 phosphorylates Akt at serine 473 further activating this kinase. Inhibitors of this pathway could potentially destabilize Mycn. These inhibitors are currently in clinical trials or in clinical development and are denoted by red octagons. The Aurora A Kinase inhibitor is denoted by an orange octagon, which represents targeted kinase inhibition, but no anticipated activity against Mycn itself. Relevant references are contained within the corresponding text.