The c-Myc oncoprotein is essential for normal cell growth and proliferation. However, overexpression of c-Myc occurs in most human cancers. Thus, its level and activity must be tightly regulated during normal cell homeostasis. The ubiquitination-proteasome system plays a key role in controlling c-Myc levels and activity. c-Myc normally undergoes rapid ubiquitin-dependent proteolysis, but it is transiently stabilized by key phosphorylation events in response to growth signals. Phosphorylation of Serine 62 (S62) stabilizes c-Myc, whereas phosphorylation of Threonine 58 (T58) promotes c-Myc ubiquitination by the SCFFbw7 ubiquitin ligase and proteasomal degradation, mainly in the nucleolus. Like other post-translational modifications, ubiquitination can be reversed by the action of deubiquitinating enzymes (DUBs). While several ubiquitin ligases have been identified for c-Myc, only one DUB, USP28, has been reported to target c-Myc. We have recently discovered that the nucleolar deubiquitinating enzyme USP36 is a novel c-Myc regulator. USP36 binds to c-Myc and deubiquitinates c-Myc in cells and in vitro. Overexpression of wild-type USP36, but not its catalytic-inactive C131A mutant, stabilizes c-Myc and enhances c-Myc-driven transcription. Knockdown of USP36 reduces c-Myc levels and drastically suppresses cell proliferation. Importantly, USP36 interacts with the nucleolar Fbw7? and abolishes Fbw7?-mediated c-Myc degradation. In contrast, USP28 antagonizes Fbw7?-mediated c-Myc degradation. Since the bulk of c-Myc is degraded in the nucleolus, our discovery leads to the novel hypothesis that USP36 functions as a crucial regulator of c-Myc by deubiquitinating c-Myc in the nucleolus. Interestingly, we found that USP36 itself is a c-Myc target gene, suggesting that USP36 and c-Myc form a positive feed-forward regulatory loop. To gain further insight into the role of USP36 in the regulation of c-Myc protein stability, activity and oncogenicity, we will investigate the molecular and biochemical mechanisms underlying the regulation of c-Myc by USP36 in Aim 1, including how USP36 interplays with Fbw7? to regulate c-Myc in the nucleolus, whether it interplays with USP28 in the dynamic control of c-Myc ubiquitination, and the importance of c-Myc-USP36 feed-forward regulation. We will elucidate the functional consequences of USP36 regulation of c-Myc in cells in Aim 2 by analyzing whether USP36 regulates c-Myc binding and turnover at target gene promoters, whether it promotes c-Myc-dependent ribosome biogenesis, and whether it promotes c-Myc's oncogenic potential in cells and in vivo. Finally, we will elucidate whether USP36 is a therapeutic target using cell based and mouse models as proposed in Aim 3, including the investigation of USP36 deregulation in human breast cancers, whether deletion of USP36 inhibits c-Myc-driven mammary tumorigenesis in mice, and high-throughput screening of small molecule inhibitors for USP36. Achieving these goals will provide critical insight into how c-Myc is properly regulated by dynamic ubiquitination and deubiquitination, how deregulation of this dynamic contributes to tumorigenesis, and how USP36 can be targeted in human cancers.
Deregulated high-level expression of the c-Myc protein contributes significantly to cancer formation and progression. This study aims at elucidating the molecular mechanisms controlling c-Myc protein levels and activity by the deubiquitinating enzyme USP36, which could yield important insight into how to target USP36 for cancer therapy.
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