We have provided the first evidence that SIRT6 overexpression is selectively toxic to cancer cell lines but not non-transformed cell lines. This toxicity is a consequence of SIRT6-driven apoptosis. This cancer-specific cytotoxicity is dependent on the mono-ADP ribosyltransferase activity of SIRT6. Mechanistically, SIRT6 appears to promote apoptosis through either p53 or p73 signaling and requires ATM to initiate this response.
We observed that a broad spectrum of cancer cell lines were sensitive to SIRT6 overexpression. This may stem from the ability of SIRT6 to stimulate apoptosis redundantly, through both the p53 and the p73 programmed cell death pathways. While inactivating mutations in p53 are the single most common type of genetic lesion observed in cancer, loss of function mutations in p73 are far less common.20
p53 and p73 are typically executors of upstream pro-apoptotic signals. The precise mechanism by which SIRT6 overexpression selectively activates these programmed cell death pathways in cancer cells remains unclear, however. One hypothesis was that SIRT6 overexpression would shut down or attenuate glycolysis and glucose uptake in cancer cells, thereby denying the cancer cells their primary energy source. However, we were unable to detect significant changes in glucose intake prior to cancer cell death after SIRT6 overexpression, suggesting that an alternative mechanism drives apoptosis in these cells.
The dependency of killing on ATM signaling and SIRT6 mono-ADP ribosylation activity provides some hints as to how SIRT6 overexpression may selectively induce cancer cell death. One possibility is that SIRT6 potentiates or activates a dormant DNA damage response (DDR) in cancer cells. The oncogene expression and rapid proliferation that characterize many cancer cells typically induces large amounts of DNA damage. Normally, this type of damage is recognized by ATM and other members of the DDR pathway, and cells enter into cell cycle arrest or undergo either p53- or p73-dependent apoptosis. In cancer cells, however, the DDR signaling is often subverted to avoid cell death. It is possible that SIRT6 overexpression sensitizes cancer cells to DNA damage. Exactly how SIRT6 would exert this type of effect remains unclear, but it is likely that SIRT6 mono-ADP-ribosylates a target protein to achieve these ends. To date, the only known, in vivo, target of SIRT6 mono-ADP ribosylation is PARP1. Our data indicates that SIRT6 overexpression induces apoptosis independently of PARP1, however. This suggests that there are additional targets for SIRT6 mono-ADP ribosylation in vivo.
Protocols that induce cancer cell death but do not harm healthy tissues are naturally of intense scientific and medical interest. Here, we have provided evidence that SIRT6 overexpression or hyperactivation may constitute such a therapy. Several cellular and animal studies have indicated that dietary restriction and glucose deprivation are sufficient to increase the cellular titer of SIRT6.21
It will be interesting to see if cancer cells exhibit a similar response to starvation and, if so, whether the subsequent increase in SIRT6 levels is lethal. There is evidence that at least a subset of human cancers are sensitive to dietary restriction,22
and it is possible that SIRT6 plays a role in mediating this effect.21
Chemical intervention also represents an intriguing method for stimulating SIRT6 activity. SIRT6 is a member of the sirtuin gene family, and several lines of evidence indicate that the activity of the founding member of this family in mammals, SIRT1, can be stimulated by small molecules.23
Analogously, it may be possible to synthesize compounds that can stimulate SIRT6 activity. Any such compounds could potentially be useful in treating cancer and a variety of other age-related diseases. We propose, therefore, that genetic or pharmacological activation of SIRT6 may constitute a valuable new strategy for treating and attenuating neoplastic disease.