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How does a cell initiate DNA repair?

Understanding how molecules and genes repair aging DNA has implications for both a longer life and stabilizing our current quality of life. More research needs to be done, but recent work may help scientists' assist repair of DNA.


The latest work by Vera Gorbunova PhD, and Andrei Seluanov PhD, both in the Department of Biology at the University of Rochester in New York, is beginning to shed light on molecular mechanisms driving the aging process. Their previous work involved understanding an inferior DNA repair system, which occurs later in life, in addition to how DNA fragments — called 'jumping genes' — are usually kept inactive.

Seluanov believes their current work may help pharmaceutical research to one day design drugs that activate SIRT6 to reduce molecular damage. "Such drugs may be used to protect our genomes from damage, and could ultimately prevent cancer and extend a healthy lifespan," he believes. The findings are published in the journal Cell Reports.


One of the biggest hazards posed by oxidative stress is DNA damage, in particular — DNA double-strand breaks (DSBs). DSBs are especially toxic as both strands of the DNA backbone are severed.


The longevity gene called SIRT6, has been identified as key in stimulating DSB repair, particularly in response to oxidative stress. DSBs are repaired primarily by two pathways in humans, homologous recombination (HR), in which sequences are exchanged between two similar or identical molecules of DNA. Or, by non-homologous end joining (NHEJ) of DNA. SIRT6 stimulates both of these responses to oxidative stress.

To find out what activates SIRT6, a team of researchers alternately applied chemical inhibitors to human skin cells held in petri dishes, to determine which proteins were essential for repairing broken DNA strands. They discovered one protein — c-Jun N-terminal kinase, or simply JNK.


When JNK was inhibited, SIRT6 did not activate and the broken strands of DNA did not repair effectively.


Responding to stress signals within cells, JNK adds phosphates to proteins. The Rochester study found amino acid residue on SIRT6 had been modified by JNK. Now, SIRT6 can attract the enzyme PARP1 (Poly [ADP-ribose] polymerase 1) to the damage site, where PARP1 begins DNA repair.


In effect, the JNK activated gene serves as a first responder, recruiting DNA repair enzymes to the accident site and setting them to work.


Despite the significant progress in understanding the SIRT6 role in maintaining genome stability, less had been known about its regulation. How SIRT6 was activated in response to oxidative stress needed to be explained.

This recent study now reveals how JNK adds the phosphate group (a process known as phosphorylation) to SIRT6, attracting PARP1 to the site to begin transfer of the high-energy phosphate molecule (Serine 10 or S10) to this target — all in response to oxidative stress. By defining this pathway, their results help definine how oxidative stress can signal DNA repair, and perhaps help pharmaceutical chemists advance therapeutic treatments.

Discussion
Our results indicate that SIRT6 is phosphorylated by JNK specifically in response to oxidative stress at residue S10 and that this modification is necessary for SIRT6 to stimulate DNA DSB repair under stress conditions. Once phosphorylated, SIRT6 is rapidly mobilized to DSB break sites, and it potentiates the activity and recruitment of the apical DNA repair factor, PARP1, which in turn stimulates DNA DSB repair (Figure 6). Our work provides evidence of a post-translational modification stimulating the activity of SIRT6. In summary, our results delineate the pathway connecting oxidative stress response and DSB repair, indicating that S10 phosphorylation plays a central role in stimulating DSB repair in response to oxidative stress.

Interestingly, SIRT6-mediated stimulation of DSB repair is reminiscent of a hormetic response, wherein mild doses of stress have beneficial effects on an organism by promoting stress and survival pathways. Hormesis has been linked to the lifespan-extending effects of caloric restriction (Masoro, 2006, Merksamer et al., 2013). We hypothesize that the JNK-SIRT6 axis represents a hormetic response pathway, promoting longevity by stimulating DNA DSB repair under moderately stressful conditions.
We demonstrated that SIRT6 recruitment to DNA damage sites is stimulated by oxidative stress. Oxidative damage is one of the more frequent types of damage occurring to DNA during the life course of an organism. It is also hypothesized to be the major type of damage to macromolecules contributing to the aging process. It is possible that the JNK-SIRT6 axis has evolved specifically to modulate the repair of oxidatively damaged DNA. It would be interesting to study whether other physiological types of stress, such as starvation or exercise, may similarly stimulate SIRT6 phosphorylation and/or recruitment to DNA upon DNA damage.

Studies in invertebrates have revealed that mild activation of JNK signaling is longevity promoting, whereas constitutive activation of JNK drives apoptosis, inflammation, and cell death (Wang et al., 2014). It is also interesting to note that JNK has been reported to phosphorylate SIRT1, a gene also linked to longevity maintenance in mammals, thereby stimulating the activity of this sirtuin protein (Ford et al., 2008, Nasrin et al., 2009).

In mammals, several studies have indicated that activation of SIRT6 represents a novel strategy for delaying the onset, and potentially reversing the pathology, of multiple age-related diseases. For example, SIRT6 overexpression protects against diet-induced obesity, has anti-tumorigenic effects, and extends lifespan in mice (Kanfi et al., 2010, Kanfi et al., 2012, Min et al., 2012, Van Meter et al., 2011b). As such, identifying mechanisms for modulating the activity of SIRT6 is of intense therapeutic and pharmacological interest. Several studies have indicated that caloric restriction and reduction of glucose intake can upregulate SIRT6 expression levels (Kanfi et al., 2008). Our study suggests that the activity of SIRT6 may be modulated post-translationally as well, since phosphorylation of SIRT6 by JNK stimulates the ability of the protein to mediate DNA DSB repair. This finding provides new avenues for developing SIRT6 activators potentially having beneficial effects on health.

Authors
Michael Van Meter, Matthew Simon, Gregory Tombline, Alfred May, Timothy D. Morello, Basil P. Hubbard, Katie Bredbenner, Rosa Park, David A. Sinclair, Vilhelm A. Bohr, Vera Gorbunova (4
4 Lead Contact), Andrei Seluanov.
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Sep 12, 2016   Fetal Timeline   Maternal Timeline   News   News Archive   



When JNK is inhibited, SIRT6 does not activate and broken strands of DNA do not repair.
Image Credit:Vera Gorbunova


 


 

Phospholid by Wikipedia