Federica Madia

Growth hormone receptor deficiency is associated with a major reduction in pro-aging signaling, cancer, and diabetes in humans.

Ecuadorians who have a genetic mutation in the growth hormone receptor almost never die of cancer or diabetes complications, possibly because of high resistance to oxidative damage and low circulating insulin. Clues to a Cancer- and Diabetes-Free Life In the 1958 film Live Fast and Die Young, two reckless sisters threaten to burn out early. Similarly, one theory of aging predicts that a faster metabolism leads to a shorter life. Does this trade-off also apply to age-related disease? A new study by Guevara-Aguirre et al. offers clues that address this seminal question. The authors’ findings stem from studies of a unique group of Ecuadorian people who have a mutation in the growth hormone receptor (GHR) gene and a resulting insulin-like growth factor–1 (IGF-1) deficiency, which stunts their growth. These descendants of Spanish conversos, Jews who converted to Christianity to avoid the Inquisition, almost never get diabetes or cancer as a result, the authors postulate, of the privileged metabolic status that arises from their altered hormonal state. Relative to controls, these subjects show lower insulin concentrations and higher insulin sensitivity, and when stressed, their cells tend to self-destruct rather than accumulate mutations and DNA damage—all features that are known to promote cell protection in model organisms. For 22 years, this group of 99 related Ecuadorians—most of whom are homozygous for an A-to-G splice site mutation at position 180 in exon 6 of the GHR gene—has been monitored extensively, so that their health details are well documented. From this reservoir of data, plus information about the diseases of family members as well as causes of death of those relatives who have died, the authors deciphered that the Ecuadorian subjects who carried the GHR mutation had an abnormally low incidence of cancer and diabetes. The group showed only one case of nonlethal cancer and no cases of diabetes, whereas the controls—unaffected relatives—developed cancer (17%) and diabetes (5%) at rates similar to those of the Ecuadorian population as a whole. To illuminate the underlying reason for the subjects’ freedom from these diseases, the authors focused on the components carried in their blood. In experiments on cultured human epithelial cells, Guevara-Aguirre et al. found that low concentrations of one of these, IGF-1, was responsible for preventing oxidative DNA damage when the cells were exposed to the oxidizing agent H2O2 and for promoting cell death when stress-related DNA damage did occur, a checkpoint that averts cancer-promoting behavior by abnormal cells. Analysis of the participating cell signaling pathways identified activation of the transcription factor FoxO under conditions of low IGF-1 as a likely mediator of these effects. Further, the lower blood insulin concentrations and higher insulin sensitivity in these subjects likely account for the absence of diabetes in this population. Although it is difficult to prove that alterations in IGF-1 amounts are responsible for the cancer- and diabetes-free lives of these Ecuadorian people, genetic work from several model organisms suggests that this is so. In yeast, mutations in genes that encode components of a growth-promoting pathway protect against age-dependent genomic instability, and mutations in the insulin/IGF-1–like signaling pathway increase life span and reduce abnormal cellular proliferation in worms. Mice with defects in GH and IGF-1 live exceptionally long lives, with delayed appearance of age-dependent mutations and cancer. The Ecuadorians do not live longer-than-normal lives compared with their compatriots, but rather die in due course from causes of death other than cancer and diabetes complications. Thus, the metabolic inverse of “live fast and die young”—a slowed metabolism yields a longer life—is not supported by the current findings. But a life free from two dreaded diseases may be considered a desirable trade-off. Mutations in growth signaling pathways extend life span, as well as protect against age-dependent DNA damage in yeast and decrease insulin resistance and cancer in mice. To test their effect in humans, we monitored for 22 years Ecuadorian individuals who carry mutations in the growth hormone receptor (GHR) gene that lead to severe GHR and IGF-1 (insulin-like growth factor–1) deficiencies. We combined this information with surveys to identify the cause and age of death for individuals in this community who died before this period. The individuals with GHR deficiency exhibited only one nonlethal malignancy and no cases of diabetes, in contrast to a prevalence of 17% for cancer and 5% for diabetes in control subjects. A possible explanation for the very low incidence of cancer was suggested by in vitro studies: Serum from subjects with GHR deficiency reduced DNA breaks but increased apoptosis in human mammary epithelial cells treated with hydrogen peroxide. Serum from GHR-deficient subjects also caused reduced expression of RAS, PKA (protein kinase A), and TOR (target of rapamycin) and up-regulation of SOD2 (superoxide dismutase 2) in treated cells, changes that promote cellular protection and life-span extension in model organisms. We also observed reduced insulin concentrations (1.4 μU/ml versus 4.4 μU/ml in unaffected relatives) and a very low HOMA-IR (homeostatic model assessment–insulin resistance) index (0.34 versus 0.96 in unaffected relatives) in individuals with GHR deficiency, indicating higher insulin sensitivity, which could explain the absence of diabetes in these subjects. These results provide evidence for a role of evolutionarily conserved pathways in the control of aging and disease burden in humans.

Low Protein Intake Is Associated with a Major Reduction in IGF-1, Cancer, and Overall Mortality in the 65 and Younger but Not Older Population

Mice and humans with growth hormone receptor/IGF-1 deficiencies display major reductions in age-related diseases. Because protein restriction reduces GHR-IGF-1 activity, we examined links between protein intake and mortality. Respondents aged 50-65 reporting high protein intake had a 75% increase in overall mortality and a 4-fold increase in cancer death risk during the following 18 years. These associations were either abolished or attenuated if the proteins were plant derived. Conversely, high protein intake was associated with reduced cancer and overall mortality in respondents over 65, but a 5-fold increase in diabetes mortality across all ages. Mouse studies confirmed the effect of high protein intake and GHR-IGF-1 signaling on the incidence and progression of breast and melanoma tumors, but also the detrimental effects of a low protein diet in the very old. These results suggest that low protein intake during middle age followed by moderate to high protein consumption in old adults may optimize healthspan and longevity.

Starvation-dependent differential stress resistance protects normal but not cancer cells against high-dose chemotherapy

Strategies to treat cancer have focused primarily on the killing of tumor cells. Here, we describe a differential stress resistance (DSR) method that focuses instead on protecting the organism but not cancer cells against chemotherapy. Short-term starved S. cerevisiae or cells lacking proto-oncogene homologs were up to 1,000 times better protected against oxidative stress or chemotherapy drugs than cells expressing the oncogene homolog Ras2val19. Low-glucose or low-serum media also protected primary glial cells but not six different rat and human glioma and neuroblastoma cancer cell lines against hydrogen peroxide or the chemotherapy drug/pro-oxidant cyclophosphamide. Finally, short-term starvation provided complete protection to mice but not to injected neuroblastoma cells against a high dose of the chemotherapy drug/pro-oxidant etoposide. These studies describe a starvation-based DSR strategy to enhance the efficacy of chemotherapy and suggest that specific agents among those that promote oxidative stress and DNA damage have the potential to maximize the differential toxicity to normal and cancer cells.

Tor1/Sch9-Regulated Carbon Source Substitution Is as Effective as Calorie Restriction in Life Span Extension

The effect of calorie restriction (CR) on life span extension, demonstrated in organisms ranging from yeast to mice, may involve the down-regulation of pathways, including Tor, Akt, and Ras. Here, we present data suggesting that yeast Tor1 and Sch9 (a homolog of the mammalian kinases Akt and S6K) is a central component of a network that controls a common set of genes implicated in a metabolic switch from the TCA cycle and respiration to glycolysis and glycerol biosynthesis. During chronological survival, mutants lacking SCH9 depleted extracellular ethanol and reduced stored lipids, but synthesized and released glycerol. Deletion of the glycerol biosynthesis genes GPD1, GPD2, or RHR2, among the most up-regulated in long-lived sch9Δ, tor1Δ, and ras2Δ mutants, was sufficient to reverse chronological life span extension in sch9Δ mutants, suggesting that glycerol production, in addition to the regulation of stress resistance systems, optimizes life span extension. Glycerol, unlike glucose or ethanol, did not adversely affect the life span extension induced by calorie restriction or starvation, suggesting that carbon source substitution may represent an alternative to calorie restriction as a strategy to delay aging.

Reduced levels of IGF-I mediate differential protection of normal and cancer cells in response to fasting and improve chemotherapeutic index.

Inhibitors of the insulin-like growth factor-I (IGF-I) receptor have been widely studied for their ability to enhance the killing of a variety of malignant cells, but whether IGF-I signaling differentially protects the host and cancer cells against chemotherapy is unknown. Starvation can protect mice, but not cancer cells, against high-dose chemotherapy [differential stress resistance (DSR)]. Here, we offer evidence that IGF-I reduction mediates part of the starvation-dependent DSR. A 72-hour fast in mice reduced circulating IGF-I by 70% and increased the level of the IGF-I inhibitor IGFBP-1 by 11-fold. LID mice, with a 70% to 80% reduction in circulating IGF-I levels, were protected against three of four chemotherapy drugs tested. Restoration of IGF-I was sufficient to reverse the protective effect of fasting. Sixty percent of melanoma-bearing LID mice treated with doxorubicin achieved long-term survival whereas all control mice died of either metastases or chemotherapy toxicity. Reducing IGF-I/IGF-I signaling protected primary glia, but not glioma cells, against cyclophosphamide and protected mouse embryonic fibroblasts against doxorubicin. Further, S. cerevisiae lacking homologs of IGF-I signaling proteins were protected against chemotherapy-dependent DNA damage in a manner that could be reversed by expressing a constitutively active form of Ras. We conclude that normal cells and mice can be protected against chemotherapy-dependent damage by reducing circulating IGF-I levels and by a mechanism that involves downregulation of proto-oncogene signals.

Longevity mutation in SCH9 prevents recombination errors and premature genomic instability in a Werner/Bloom model system

Werner and Bloom syndromes are human diseases characterized by premature age-related defects including elevated cancer incidence. Using a novel Saccharomyces cerevisiae model system for aging and cancer, we show that cells lacking the RecQ helicase SGS1 (WRN and BLM homologue) undergo premature age-related changes, including reduced life span under stress and calorie restriction (CR), G1 arrest defects, dedifferentiation, elevated recombination errors, and age-dependent increase in DNA mutations. Lack of SGS1 results in a 110-fold increase in gross chromosomal rearrangement frequency during aging of nondividing cells compared with that generated during the initial population expansion. This underscores the central role of aging in genomic instability. The deletion of SCH9 (homologous to AKT and S6K), but not CR, protects against the age-dependent defects in sgs1Δ by inhibiting error-prone recombination and preventing DNA damage and dedifferentiation. The conserved function of Akt/S6k homologues in lifespan regulation raises the possibility that modulation of the IGF-I–Akt–56K pathway can protect against premature aging syndromes in mammals.

Oncogene homologue Sch9 promotes age-dependent mutations by a superoxide and Rev1/Polζ-dependent mechanism

Oncogenes contribute to tumorigenesis by promoting growth and inhibiting apoptosis. Here we examine the function of Sch9, the Saccharomyces cerevisiae homologue of the mammalian Akt and S6 kinase, in DNA damage and genomic instability during aging in nondividing cells. Attenuation of age-dependent increases in base substitutions, small DNA insertions/deletions, and gross chromosomal rearrangements (GCRs) in sch9Δ mutants is associated with increased mitochondrial superoxide dismutase (MnSOD) expression, decreased DNA oxidation, reduced REV1 expression and translesion synthesis, and elevated resistance to oxidative stress-induced mutagenesis. Deletion of REV1, the lack of components of the error-prone Polζ, or the overexpression of SOD1 or SOD2 is sufficient to reduce age-dependent point mutations in SCH9 overexpressors, but REV1 deficiency causes a major increase in GCRs. These results suggest that the proto-oncogene homologue Sch9 promotes the accumulation of superoxide-dependent DNA damage in nondividing cells, which induces error-prone DNA repair that generates point mutations to avoid GCRs and cell death during the first round of replication.

Tor-Sch9 deficiency activates catabolism of the ketone body-like acetic acid to promote trehalose accumulation and longevity

In mammals, extended periods of fasting leads to the accumulation of blood ketone bodies including acetoacetate. Here we show that similar to the conversion of leucine to acetoacetate in fasting mammals, starvation conditions induced ketone body‐like acetic acid generation from leucine in S. cerevisiae. Whereas wild‐type and ras2Δ cells accumulated acetic acid, long‐lived tor1Δ and sch9Δ mutants rapidly depleted it through a mitochondrial acetate CoA transferase‐dependent mechanism, which was essential for lifespan extension. The sch9Δ‐dependent utilization of acetic acid also required coenzyme Q biosynthetic genes and promoted the accumulation of intracellular trehalose. These results indicate that Tor‐Sch9 deficiency extends longevity by switching cells to an alternative metabolic mode, in which acetic acid can be utilized for the storage of stress resistance carbon sources. These effects are reminiscent of those described for ketone bodies in fasting mammals and raise the possibility that the lifespan extension caused by Tor‐S6K inhibition may also involve analogous metabolic changes in higher eukaryotes.

Studying age-dependent genomic instability using the S. cerevisiae chronological lifespan model.

Studies using the Saccharomyces cerevisiae aging model have uncovered life span regulatory pathways that are partially conserved in higher eukaryotes1-2. The simplicity and power of the yeast aging model can also be explored to study DNA damage and genome maintenance as well as their contributions to diseases during aging. Here, we describe a system to study age-dependent DNA mutations, including base substitutions, frame-shift mutations, gross chromosomal rearrangements, and homologous/homeologous recombination, as well as nuclear DNA repair activity by combining the yeast chronological life span with simple DNA damage and mutation assays. The methods described here should facilitate the identification of genes/pathways that regulate genomic instability and the mechanisms that underlie age-dependent DNA mutations and cancer in mammals.

Aging and Dietary Restriction: The Yeast Paradigm

The only intervention to have been consistently shown to extend the life span in organisms ranging from yeast to monkeys is calorie restriction (CR). Saccharomyces cerevisiae is perhaps the simplest among the major model organisms established for studies of calorie restriction and aging since it is unicellular, easily manipulated genetically, amenable to a number of powerful genome/proteome-wide screening techniques, and the genes that regulates its life span play similar roles in higher eukaryotes. Here we briefly review some of the methods and discoveries related to dietary restriction (DR) and aging in yeast and discuss the potential genes and mechanisms mediated its effect on life span.