Researchers Discover Second Protective Role for Tumor-Suppressor

ATM, a protein that reacts to DNA damage by ordering repairs or the suicide of the defective cell, plays a similar, previously unknown role in response to oxidative damage outside of the nucleus, researchers report this week in the online version of the Proceedings of the National Academy of Sciences.

"This tumor-suppressor that works in the nucleus to prevent replication of defective cells also has a second life out in the cytoplasm, which was totally unexpected," said senior author Cheryl Walker, Ph.D., professor in The University of Texas M. D. Anderson Cancer Center Department of Carcinogenesis.

"ATM recognizes damage caused by reactive oxygen species (ROS) and tells the cell to stop growing by suppressing the protein-synthesizing pathway mTORC1 or orders the cell to consume itself, a process called autophagy," Walker said. This pathway parallels the protein's role of damage recognition and response in the nucleus.

Reactive oxygen species are a byproduct of cellular metabolism and in small amounts play a role in cell signaling. Their ability to react with other molecules makes them toxic, and they are kept in check by antioxidant enzymes. When that natural balance is disrupted, elevated levels of these volatile molecules damage proteins, lipids and DNA, Walker said.

The authors note that elevated ROS has been linked to more than 150 diseases, including diabetes, cancer, neurodegenerative diseases and atherosclerosis.

In its previously known role, ATM (short for Ataxia-Telangiectasia Mutated) senses DNA damage, orders the cell to repair the damage and halts cell division pending repair via the tumor suppressor p53. If repair is not possible, ATM sets off apoptosis - programmed cell death. ATM is commonly mutated in cancer.

The added protective role discovered by the researchers also points to a potential way to activate the tumor-suppressor without damaging DNA.

Walker's lab was studying another tumor-suppressing protein called TSC2 that is active in the cellular cytoplasm and found that ATM appeared to be associated with TSC2 activation.

In a series of experiments, the research team uncovered the molecular pathway that begins with ROS activation of ATM which then:

* Activates the tumor suppressor LKB1, which in turn phosphorylates and activates the AMP kinase (AMPK), a key player in energy sensing and growth factor signaling.

* AMPK switches on the tumor-suppressor TSC2 (tuberous sclerosis complex 2).

* TSC2 then suppresses the kinase mTOR (mammalian Target of Rapamycin), which shuts down the mTORC1 signaling pathway, an important regulator of protein creation and cell growth.

* Because TORC1 suppresses autophagy, when TORC1 is suppressed by TSC2, autophagy is free to occur.

During autophagy, membranes form around organelles in the cytoplasm, which are subsequently digested. Autophagy plays a normal role in cell growth and stability, and is a natural cellular defense mechanism, providing nutrients for a starving cell, for example.