The Cancer Specific Ubiquitin Ligase MAGE-A3/6-TRIM28 Drives Tumorigenesis by Ubiquitination and Proteasomal Degradation of AMPK

The genes MAGE-A3 and MAGE-A6 (MAGE-A3/6) have a unique expression pattern in which they are normally expressed in the adult testis but are aberrantly expressed in cancer. It is known that when expressed in cancer, MAGE-A3/6 is a negative prognostic indicator and cancer cells are dependent on it for survival. Using the knowledge that MAGE-A3/6 binds and regulates the E3 ubiquitin ligase TRIM28, I investigated its biochemical role in cancer. I used unbiased methods to identify 19 novel substrates of MAGE-A3/6-TRIM28, including the known tumor suppressor AMPK. Ubiquitination of AMPK by MAGE-A3/6-TRIM28 induces its proteasomal degradation, thereby enhancing mTOR signaling and inhibiting autophagy within cells. Through this modulation of AMPK, MAGE-A3/6 is also able to act as an oncogene, inducing anchorage independent growth and the growth of tumors in vivo. Understanding the mechanism by which MAGE-A3/6 acts as an oncogene has revealed potential avenues of therapeutic intervention. Treatment of MAGE-A3/6 expressing cells with AMPK agonists reverses oncogenic properties in vitro. Ultimately, these studies have revealed how a germline protein functions in cancer and the potential points for therapeutic intervention.