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 poly- ubiquitination 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.
Elevated expression of the c-Myc protein is widely observed in many different human cancers. The purpose of this research is to reveal new cellular mechanisms that control both the activity and expression level of this potent protein. Understanding mechanisms that increase or decrease c-Myc's activity and expression is critical to the development of future therapies targeting this tumor promoting protein.
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