SIRT6 is a mammalian homolog of the yeast Sir2 deacetylase that promotes longevity in yeast and invertebrates. Mice deficient for SIRT6 exhibit premature aging and genome instability. Here we show that in mammalian cells subjected to oxidative stress SIRT6 is recruited to the sites of DNA double-strand breaks (DSBs) and strongly stimulates both pathways of DSB repair, nonhomologous end joining and homologous recombination. We found that SIRT6 physically associates with PARP1 leading to stimulation of PARP1 poly-ADP-ribose polymerase activity. Mono-ADP-ribosylation activity of SIRT6 is sufficient for the activation of PARP1 in vitro, while both mono-ADP-ribosylation and deacetylation activities are required for the stimulation of DSB repair in vivo. Our results suggest that SIRT6 mono-ADP-ribosylates PARP1 on lysine 521 thereby stimulating PARP1 activity and enhancing DSB repair under oxidative stress. We propose that SIRT6 functions as a regulator integrating oxidative stress signaling and DNA damage response.

The naked mole-rat displays exceptional longevity, with a maximum lifespan exceeding 30 years1–3. This is the longest reported lifespan for a rodent species and is especially striking considering the small body mass of the naked mole-rat. In comparison, a similarly sized house mouse has a maximum lifespan of 4 years4,5. In addition to their longevity, naked mole-rats show an unusual resistance to cancer. Multi-year observations of large naked mole-rat colonies did not detect a single incidence of cancer2,6. Here we identify a mechanism responsible for the naked mole-rat’s cancer resistance. We found that naked mole-rat fibroblasts secrete extremely high molecular weight hyaluronan (HA), which is over five times larger than human or mouse HA. This high molecular weight HA accumulates abundantly in naked mole rat tissues due to the decreased activity of HA-degrading enzymes and a unique sequence of hyaluronan synthase 2 (HAS2). Furthermore, the naked mole-rat cells are more sensitive to HA signaling, as the naked mole rat cells have a higher affinity to HA than the mouse or human cells. Perturbation of the signaling pathways sufficient for malignant transformation of mouse fibroblasts fails to transform naked mole-rat cells. However, once high molecular weight HA is removed by either knocking down HAS2 or overexpressing the HA-degrading enzyme, Hyal2, naked mole-rat cells become susceptible to malignant transformation and readily form tumors in mice. We speculate that naked mole-rats have evolved a higher concentration of HA in the skin to provide skin elasticity needed for life in underground tunnels. This trait may have then been co-opted to provide cancer resistance and longevity to this species.

Summary Dietary restriction (DR) without malnutrition encompasses numerous regimens with overlapping benefits including longevity and stress resistance, but unifying nutritional and molecular mechanisms remain elusive. In a mouse model of DR-mediated stress resistance, we found that sulfur amino acid (SAA) restriction increased expression of the transsulfuration pathway (TSP) enzyme cystathionine γ-lyase (CGL), resulting in increased hydrogen sulfide (H2S) production and protection from hepatic ischemia reperfusion injury. SAA supplementation, mTORC1 activation, or chemical/genetic CGL inhibition reduced H2S production and blocked DR-mediated stress resistance. In vitro, the mitochondrial protein SQR was required for H2S-mediated protection during nutrient/oxygen deprivation. Finally, TSP-dependent H2S production was observed in yeast, worm, fruit fly and rodent models of DR-mediated longevity. Together, these data are consistent with evolutionary conservation of TSP-mediated H2S as a novel mediator of DR benefits with broad implications for clinical translation.

The naked mole-rat is the longest living rodent with a maximum lifespan exceeding 28 years. In addition to its longevity, naked mole-rats have an extraordinary resistance to cancer as tumors have never been observed in these rodents. Furthermore, we show that a combination of activated Ras and SV40 LT fails to induce robust anchorage-independent growth in naked mole-rat cells, while it readily transforms mouse fibroblasts. The mechanisms responsible for the cancer resistance of naked mole-rats were unknown. Here we show that naked mole-rat fibroblasts display hypersensitivity to contact inhibition, a phenomenon we termed ''early contact inhibition.'' Contact inhibition is a key anticancer mechanism that arrests cell division when cells reach a high density. In cell culture, naked mole-rat fibroblasts arrest at a much lower density than those from a mouse. We demonstrate that early contact inhibition requires the activity of p53 and pRb tumor suppressor pathways. Inactivation of both p53 and pRb attenuates early contact inhibition. Contact inhibition in human and mouse is triggered by the induction of p27 Kip1 . In contrast, early contact inhibition in naked mole-rat is associated with the induction of p16 Ink4a . Furthermore, we show that the roles of p16 Ink4a and p27 Kip1 in the control of contact inhibition became temporally separated in this species: the early contact inhibition is controlled by p16 Ink4a , and regular contact inhibition is controlled by p27 Kip1 . We propose that the additional layer of protection conferred by two-tiered contact inhibition contributes to the remarkable tumor resistance of the naked mole-rat.longevity ͉ p16Ink4a ͉ p53 ͉ pRb ͉ tumor suppressor

SummaryIn multicellular organisms, telomerase is required to maintain telomere length in the germline but is dispensable in the soma. Mice, for example, express telomerase in somatic and germline tissues, while humans express telomerase almost exclusively in the germline. As a result, when telomeres of human somatic cells reach a critical length the cells enter irreversible growth arrest called replicative senescence. Replicative senescence is believed to be an anticancer mechanism that limits cell proliferation. The difference between mice and humans led to the hypothesis that repression of telomerase in somatic cells has evolved as a tumor-suppressor adaptation in large, long-lived organisms. We tested whether regulation of telomerase activity coevolves with lifespan and body mass using comparative analysis of 15 rodent species with highly diverse lifespans and body masses. Here we show that telomerase activity does not coevolve with lifespan but instead coevolves with body mass: larger rodents repress telomerase activity in somatic cells. These results suggest that large body mass presents a greater risk of cancer than long lifespan, and large animals evolve repression of telomerase activity to mitigate that risk.

Summary Angiogenesis, the formation of new blood vessels by endothelial cells (EC), is an adaptive response to oxygen/nutrient deprivation orchestrated by vascular endothelial growth factor (VEGF) upon ischemia or exercise. Hypoxia is the best-understood trigger of VEGF expression via the transcription factor HIF1α. Nutrient deprivation is inseparable from hypoxia during ischemia, yet its role in angiogenesis is poorly characterized. Here, we identified sulfur amino acid restriction as a proangiogenic trigger, promoting increased VEGF expression, migration and sprouting in EC in vitro, and increased capillary density in mouse skeletal muscle in vivo, via the GCN2/ATF4 amino acid starvation response pathway independent of hypoxia or HIF1α. We also identified a requirement for cystathionine-γ-lyase in VEGF-dependent angiogenesis via increased hydrogen sulfide (H2S) production. H2S mediated its proangiogenic effects in part by inhibiting mitochondrial electron transport and oxidative phosphorylation, resulting in increased glucose uptake and glycolytic ATP production.

DNA is a precious molecule. It encodes vital information about cellular content and function. There are only two copies of each chromosome in the cell, and once the sequence is lost no replacement is possible. The irreplaceable nature of the DNA sets it apart from other cellular molecules, and makes it a critical target for age-related deterioration. To prevent DNA damage cells have evolved elaborate DNA repair machinery. Paradoxically, DNA repair can itself be subject to age-related changes and deterioration. In this review we will discuss the changes in efficiency of mismatch repair (MMR), base excision repair (BER), nucleotide excision repair (NER) and double-strand break (DSB) repair systems during aging, and potential changes in DSB repair pathway usage that occur with age. Mutations in DNA repair genes and premature aging phenotypes they cause have been reviewed extensively elsewhere, therefore the focus of this review is on the comparison of DNA repair mechanisms in young versus old.

Blind mole rats Spalax (BMR) are small subterranean rodents common in the Middle East. BMR is distinguished by its adaptations to life underground, remarkable longevity (with a maximum documented lifespan of 21 y), and resistance to cancer. Spontaneous tumors have never been observed in spalacids. To understand the mechanisms responsible for this resistance, we examined the growth of BMR fibroblasts in vitro of the species Spalax judaei and Spalax golani. BMR cells proliferated actively for 7-20 population doublings, after which the cells began secreting IFN-β, and the cultures underwent massive necrotic cell death within 3 d. The necrotic cell death phenomenon was independent of culture conditions or telomere shortening. Interestingly, this cell behavior was distinct from that observed in another long-lived and cancer-resistant African mole rat, Heterocephalus glaber, the naked mole rat in which cells display hypersensitivity to contact inhibition. Sequestration of p53 and Rb proteins using SV40 large T antigen completely rescued necrotic cell death. Our results suggest that cancer resistance of BMR is conferred by massive necrotic response to overproliferation mediated by p53 and Rb pathways, and triggered by the release of IFN-β. Thus, we have identified a unique mechanism that contributes to cancer resistance of this subterranean mammal extremely adapted to life underground. BMRs are very long-lived for their size. The maximum lifespan documented for the animals kept in our animal facility is 21 y (3). In comparison, mice and rats belonging to the same superfamily have a maximum lifespan of 4 y (4, 5). Furthermore, BMRs show a striking resistance to cancer. Our observation of thousands of captive animals did not show a single case of spontaneous tumor development over a 40-y period. Cancer accounts for ∼23% of human mortality (6). In mice and rats, cancer mortality is very high, reaching 90% in some strains (7,8).Animals have evolved multiple mechanisms to protect themselves from cancer. These mechanisms include cell-cycle checkpoints, DNA repair, programmed cell death, and replicative senescence controlled by a network of tumor-suppressor genes, such as p53 and Rb. Anticancer adaptations differ between species, which may explain the differences in cancer susceptibility (9-11). Mice have been used extensively as models for cancer research. However, mice are more prone to cancer and are likely to possess fewer anticancer defenses, thus limiting a potential for discovery of novel anticancer pathways. Hence, there is great value in studying anticancer mechanisms in cancer-resistant species. Subterranean mammals are good candidates for these studies.We showed previously that the BMR p53 gene contains an arginine to lysine substitution at a site corresponding to human p53 position 174, where mutations are often found in human tumors (12). The R174K substitution affects the DNA-binding domain of p53; the resulting protein is capable of inducing cellcycle arrest but is defective in initiating apoptosis. We hypothes...