P53 misregulation and cancer have become almost synonymous in the scientist's imagination. Mutations in this prominent tumor suppressor gene have been found in the majority of human cancers, and downregulating the protein increases tumor incidence in animal models. P53 can halt cell division and induce DNA repair in response to stress and trigger cell death if the cell is beyond repair. One would think, then, that upregulating this protective protein could treat cancers, and therapies to that effect are being developed. The only trouble, according to a report in last Thursday's Nature10, is that too much p53 may accelerate aging.

Larry Donehauer of the Baylor College of Medicine in Houston, Texas, and colleagues at several other institutions report that they have, by mistake, created a mutant mouse that overproduces p53. The reason for the overproduction appears to be that one of the two alleles is considerably shortened and induces its normal counterpart to overproduce the protein. As expected, these mice are more resistant to tumors, but rather than living longer, their life spans were only about 80 percent that of their wildtype littermates. This surprising finding is accompanied by numerous signs of early aging, including osteoporosis, generalized organ atrophy, and diminished stress tolerance.

"The influence of p53 on life span may result from a delicate balance between its antitumour and pro-ageing effects, such that too little p53 increases mortality from cancer whereas too much p53 increases mortality," write Gerardo Ferbeyre of Universite de Montreal in Canada and Scott Lowe of Coldspring Harbor Laboratory in New York in an accompanying News and Views article. They add, "The results raise the disturbing possibility that the DNA-damaging drugs used to treat cancer in young people might prompt p53 into action and accelerate age-related disorders later on."

. . . And of Coenzyme Q

A molecule that is better known in conjunction with aging is coenzyme Q (Q). The enzyme, from both endogenous and dietary sources, is an important player in maintaining the proton gradient across the mitochondrial membrane that drives ATP synthesis. One of the byproducts of this movement of charged particles is the production of reactive oxygen species (ROSs), free radicals that have been linked to aging in general and the destruction of cells in particular. In the current issue of Science, Paula Larsen and Catherine Clarke of the University of California, Los Angeles, show that removing Q from the diet of normal nematodes (Caenorhabditis elegans) dramatically extends their life spans.

The study extends work showing that dietary Q could rescue worms genetically altered to block the production of endogenous Q, allowing them to develop normally and live longer. The researchers wanted to avoid interfering with normal larval development, which depends on Q, in order to isolate the effects of reduced Q on adult aging. They found that wildtype nematodes fed a Q-less diet beginning in adulthood lived approximately 60 percent longer. The data support a model whereby a mitochondrial membrane protein gradient deprived of dietary Q, relying solely on endogenous sources, will produce fewer ROSs. This in turn could help slow aging. Larsen and Clarke also show that they can further extend the life of several nematode mutants (clk-1, daf-2, daf-12, daf-16), which already live longer than wildtype nematodes. They authors suggest that this additive effect on longevity may result from reduced ROS production combined with increased breakdown of ROS.

"I think the article is very interesting," said nematode aging expert Cynthia Kenyon of the University of California, San Francisco. "It will also be interesting to learn how, at the molecular level, this extension of lifespan occurs." An accompanying News and Views article discusses how the paper points to interactions in the mitochondrion between Q and the insulin signaling pathway.—Hakon Heimer

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  1. I find the study interesting because our work suggests that SIR2 will be a universal regulator of aging (definitely in yeast and worms) and we recently found that it functions as a negative regulator of p53 in mammalian cells. My hunch is that this is only one of several relevant SIR2 activities that may impact aging. This paper shows that up-regulation of p53 in a whole mouse can elicit symptoms that are aging-like. One hopes there is a window in which SIR2 can be up-regulated (pharmacologically?) to slow aging without increasing cancer. One cautionary note—it is always difficult to say that a mutant phenotype is premature aging rather than pathology. It is much more convincing to make the organism live longer by genetic intervention.

  2. Homeostatic Control: Relevance to Alzheimer Disease
    Larsen and Clarke highlight that endogenous homeostatic control over energetics and endogenous antioxidant defenses make cells and organisms better adapted to survival than exogenous intervention. These authors show that Caenorhabditis elegans deprived of exogenous Coenzyme Q (CoQ) have an extended life span over those replete with endogenous CoQ provided by biosynthesis plus external food-derived CoQ. Genetic manifestations of food sources and worms shows that the life span extension is specific to exogenous CoQ restriction, even though exogenous CoQ is required to complete the life cycle/development of worms unable to synthesize it. The authors and attached commentary (Tatar and Rand, 2002) interpret the effect through signaling or oxidative stress, but an additional consideration is the tight metabolic control that operates on factors proscribed by metabolic need over those that are only limited by ingestion. Massive quantities of a metabolic cofactor or antioxidant will disrupt this delicate balance; this is why vitamins generally are not promising therapeutics, and it underscores the importance of a varied diet. Therefore, indiscriminate administration of metabolic cofactors and antioxidant therapy may, in fact, be detrimental without an in-depth understanding of basic homeostatic mechanisms.

    The role of endogenous responses and compensations is apparent in Alzheimer's disease, where strict regulatory controls exist for the response to oxidative stress by redox-balance (Russell et al. 1999), amyloid (Nunomura et al., 2001; Takeda et al. 2000) and neurofilament (Wataya et al., High molecular weight neurofilament proteins are physiological substrates of adduction by the lipid peroxidation product hydroxynonenal. J Biol Chem, in press.) It is critical to consider these basic processes when developing novel therapeutic modalities in Alzheimer's and other degenerative diseases since, despite evidence of redox stress in these disorders, this data may in fact explain the modest efficacy of antioxidant treatments.

    References:

    . Extension of life-span in Caenorhabditis elegans by a diet lacking coenzyme Q. Science. 2002 Jan 4;295(5552):120-3. PubMed.

    . Aging. Dietary advice on Q. Science. 2002 Jan 4;295(5552):54-5. PubMed.

    . Increased neuronal glucose-6-phosphate dehydrogenase and sulfhydryl levels indicate reductive compensation to oxidative stress in Alzheimer disease. Arch Biochem Biophys. 1999 Oct 15;370(2):236-9. PubMed.

    . Oxidative damage is the earliest event in Alzheimer disease. J Neuropathol Exp Neurol. 2001 Aug;60(8):759-67. PubMed.

    . In Alzheimer's disease, heme oxygenase is coincident with Alz50, an epitope of tau induced by 4-hydroxy-2-nonenal modification. J Neurochem. 2000 Sep;75(3):1234-41. PubMed.

    . High molecular weight neurofilament proteins are physiological substrates of adduction by the lipid peroxidation product hydroxynonenal. J Biol Chem. 2002 Feb 15;277(7):4644-8. PubMed.

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Primary Papers

  1. . p53 mutant mice that display early ageing-associated phenotypes. Nature. 2002 Jan 3;415(6867):45-53. PubMed.
  2. . The price of tumour suppression?. Nature. 2002 Jan 3;415(6867):26-7. PubMed.
  3. . Extension of life-span in Caenorhabditis elegans by a diet lacking coenzyme Q. Science. 2002 Jan 4;295(5552):120-3. PubMed.
  4. . Aging. Dietary advice on Q. Science. 2002 Jan 4;295(5552):54-5. PubMed.