Degradation of AMPK by a cancer-specific ubiquitin ligase.

AMP-activated protein kinase (AMPK) is a master sensor and regulator of cellular energy status. Upon metabolic stress, AMPK suppresses anabolic and promotes catabolic processes to regain energy homeostasis. Cancer cells can occasionally suppress the growth-restrictive AMPK pathway by mutation of an upstream regulatory kinase. Here, we describe a widespread mechanism to suppress AMPK through its ubiquitination and degradation by the cancer-specific MAGE-A3/6-TRIM28 ubiquitin ligase. MAGE-A3 and MAGE-A6 are highly similar proteins normally expressed only in the male germline but frequently re-activated in human cancers. MAGE-A3/6 are necessary for cancer cell viability and are sufficient to drive tumorigenic properties of non-cancerous cells. Screening for targets of MAGE-A3/6-TRIM28 revealed that it ubiquitinates and degrades AMPKα1. This leads to inhibition of autophagy, activation of mTOR signaling, and hypersensitization to AMPK agonists, such as metformin. These findings elucidate a germline mechanism commonly hijacked in cancer to suppress AMPK.

Detection of in vivo enzyme activity with CatalyCEST MRI.

PURPOSE: CatalyCEST MRI compares the detection of an enzyme-responsive chemical exchange saturation transfer (CEST) agent with the detection of an unresponsive "control" CEST agent that accounts for other conditions that influence CEST. The purpose of this study was to investigate the feasibility of in vivo catalyCEST MRI. METHODS: CEST agents that were responsive and unresponsive to the activity of urokinase plasminogen activator were shown to have negligible interaction with each other. A CEST-fast imaging with steady state precession (FISP) MRI protocol was used to acquire MR CEST spectroscopic images with a Capan-2 pancreatic tumor model after intravenous injection of the CEST agents. A function of (super)-Lorentzian line shapes was fit to CEST spectra of a region-of-interest that represented the tumor. RESULTS: The CEST effects from each agent showed the same initial uptake into tumor tissues, indicating that both agents had the same pharmacokinetic transport rates. Starting 5 min after injection, CEST from the enzyme-responsive agent disappeared more quickly than CEST from the unresponsive agent, indicating that the enzyme responsive agent was being catalyzed by urokinase plasminogen activator, while both agents also experienced net pharmacokinetic washout from the tumor. CONCLUSION: CatalyCEST MRI demonstrates that dynamic tracking of enzyme-responsive and unresponsive CEST agents during the same in vivo MRI study is feasible.

Oncogenic MAGEA-TRIM28 ubiquitin ligase downregulates autophagy by ubiquitinating and degrading AMPK in cancer.

Autophagy is commonly altered in cancer and has a complicated, but important role in regulation of tumor growth. Autophagy is often tumor suppressive in the early stages of cancer development, but contributes to the late stages of tumor growth. Because of this, putative oncogenes that modulate autophagy signaling are especially interesting. Here we discuss our recent work detailing the function of the MAGEA-TRIM28 ubiquitin ligase as an oncogene product that targets PRKAA1/AMPKα1 for ubiquitination and proteasome-mediated degradation. Degradation of AMPK, a master cellular energy sensor and regulator, by MAGEA-TRIM28 results in significantly reduced autophagy and changes in cellular metabolism, including upregulation of MTOR signaling. Overall, expression of MAGEA3 (or MAGEA6) and degradation of AMPK is sufficient to induce transformation of normal cells and promote multiple hallmarks of cancer.