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Cellular Mechanisms Controlling Myc Protein Stability

Rosalie C Sears

2 Collaborator(s)

Funding source

National Cancer Institute (NIH)
The c-Myc oncoprotein is a critical regulator of cellular function and its overexpression has been tightly linked to human cancer. Expression of c-Myc is regulated at many levels including protein stability. A complex signaling pathway affects c-Myc protein stability through the sequential phosphorylation of two highly conserved sites, Serine 62 (S62) and Threonine 58 (T58). These phosphorylation sites have opposing effects on c-Myc stability, where phosphorylation at S62 can stabilize c-Myc; subsequent phosphorylation at T58 promotes c-Myc ubiquitin-dependent proteolysis. Mitogen stimulation induces S62 phosphorylation through a number of kinases including MAPKs and CDKs to allow transient stabilization of c-Myc following a cell growth response. Protein levels are then downregulated through T58 phosphorylation, mediated by GSK3 . The dually phosphorylated form of c-Myc is then recognized by a phosphorylation-directed prolyl isomerase, Pin1, which catalyzes a cis to trans isomerization at Proline 63. This allows the trans-specific protein phosphatase PP2A-B56 to remove the stabilizing S62 phosphate. T58 phosphorylated c-Myc is then a substrate for polyubiquitination and degradation by the E3 ubiquitin ligase SCFFBW7. Recent research demonstrates that the Axin1 scaffold protein coordinates this c-Myc degradation pathway. Importantly, this process can be impaired in human cancer as multiple tested samples show enhanced S62 phosphorylation, reduced T58 phosphorylation, and increased c-Myc stability; and lesions in Axin1 have been identified in human cancers with stabilized c-Myc. Moreover, Axin1 is present at Myc target gene promoters suggesting that c-Myc activity and degradation may be coupled. New data demonstrates that Pin1 plays a dual role in regulating c-Myc, both enhancing its transcriptional activity and stimulating its turnover. The Central hypothesis of this proposal is that Pin1 increases the transcriptional activity of S62 phosphorylated c-Myc by enhancing its recruitment to promoters, where it is subsequently shut off at the promoter by an Axin1-nucleated destruction complex containing GSK3 , PP2A-B56 and Pin1, and this process can be deregulated in cancer cells potentiating Myc's oncogenic activity. This hypothesis will be tested with the following three specific aims: 1) examine a role for Pin1 in coordinating c-Myc transcriptional activity with Axin1-mediated destruction; 2) analyze regulation of the Axin1-Myc destruction complex in non-transformed and cancer cells; and 3) investigate the biological relevance of Pin1-mediated activation and Axin1-mediated degradation of c- Myc in human cancer and model this in vitro and in vivo. Completion of these aims will reveal novel molecular mechanisms that control c-Myc activity and expression involving the tumor suppressor protein, Axin1, and the multifunctional Pin1 prolyl-isomerase. Together, these aims will provide critical new information about cellular mechanisms that regulate the tumorigenic potential of the c-Myc oncoprotein, which could greatly aid in the search for c-Myc targeted therapy to treat cancer patients.

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