Since its discovery in 1979, p53 has never stopped surprising researchers. For a decade, it was thought to be an oncogene (a gene that causes cancer). In 1989, it was found to have the opposite role of suppressing tumors. (ref) It was originally thought to be a transcriptional activator, until it was discovered to also be a repressor. (ref) It was then discovered that not only did p53 function within the cell nucleus, but also in the cytoplasm, regulating mitochondrial activity. (ref) p53 is known to play a very surprising role in the cancer-protective process of UV skin tanning. (ref) A similar response to UV light is known to have protected our invertebrate ancestors from the potentially harmful effects of radiation one billion years ago, indicating that the function of p53 has been preserved for, at least, one million millenia. (ref) A further surprise came when p53 was found to have an important role in aging and longevity, which will be the focus of this discussion.
A more detailed discussion of p53 in cancer prevention can be found in an earlier blog entry by Dr. Guiliano, Turning p53 On in Cancer Cells.
What exactly is p53?
p53 actually refers to a family of closely related transcription factors consisting of p53, p63, and p73, each of which has various isoforms with distinct effects. (ref) (An isoform is different form, or variant of the same protein which may result from alternative splicing of the same gene.) Although they are referred to as “transcription factors”, due to their ability to interact with the process of genetic transcription, their effects are not limited to nuclear transcription, but also include post translational modifications, and many protein-protein interactions involved in a variety of signaling pathways. All three subfamilies target the same genes; and all three play important roles in cell-cycle regulation, including the hallmark ability to suppress tumorigenesis by inducing cell-cycle arrest. Such arrest may be temporary, allowing for DNA repair (quiesence), or permanent (senescence), or result in cellular death (apoptosis). Knockout mice studies have shown that p63 is especially important for regulation of epithelial tissues, including normal development and repair, while p73 is crucial for normal differentiation and development of neurons. (ref)
Interplay between p53 subfamilies and isoforms.
Various members of the p53 family play distinct, but similar, and complementary roles in suppressing tumorigenesis, and in the regulation of other cellular functions. A great deal of interaction between different p53 subfamilies and their various isoforms is known to take place. The importance of p53 subfamily interaction is exemplified by the fact that, without the presence of p63 and p73, p53 alone is unable to induce apoptosis in DNA-damaged cells. (ref) However, this interaction is not always, complementary. Truncated isoforms, lacking the N-terminal or transactivation domain, are constitutively active, meaning they are always active; for this reason, they are also said to be “unregulated”. Regulated, transactivating isoforms are activated in response to cellular stress signals. (Cellular stress can be the result of nutrient scarcity, aberrant metabolic activity, reactive oxygen species, inflammation, etc.) Truncated isoforms of all three subfamilies are known to antagonize the regulated activity of the complete, transactivaing isoforms. (ref, ref) To distinguish them from their truncated variants, transactivating isoforms are often written with a TA prefix, i.e. TAp53, TAp63, etc. Mutant p53 variants are also known to form mutant complexes with normal isoforms, thereby altering their function or deactivating them. (ref)
Guardians of the genome.
All three subfamilies play complementary roles in maintaining the genomic integrity and viability of germ line cells. For this reason, the p53 family has been collectively referred to as the “guardian of the genome”, while the p63, which is constitutively expressed in female germ cells during meiotic arrest, and is necessary for apoptosis of DNA-damaged oocytes, has been referred to as “the guardian of the female germ line”. See: p63 protects the female germ line , p73 knockout shows genomic instability with infertility –, and A male germ cell tumor-susceptibility-determining locus — .
p53 is also known as the “guardian of the somatic genome”:
“Within the higher vertebrates, p63 and p73 have taken on new functions in development of tissues and organs, whereas p53 has become the guardian of the somatic genome and a tumor suppressor. With the advent of employing large numbers of stem cells and tissue regeneration as a strategy for an organism’s growth, development, and maintenance, there is a greater need for stem cell surveillance to prevent cancers from arising. p53 evolved to fill this role. It is of some interest that p53 has recently been shown to regulate the efficiency of induced pluripotent stem cell production from differentiated cells (13–17) indicating a new possible role for p53 in enforcing the direction of developmental processes in a cell. There is an intimate relationship among p53, stem cell development, and epigenetic regulation of these processes, and it began to evolve in the fishes.” (ref)
The aging connection: Antagonistic Pleiotropy.
Originally proposed in 1957, the basic idea behind the theory of antagonistic pleiotropy is that the forces of natural selection will give preference to genes that promote survival in the earlier stages of life, even if they are detrimental to survival in later life. Clearly, if an organism doesn’t survive the early stages, then genes that promote survival in later life are meaningless. Furthermore, once an organism passes its reproductive stage, the genomic value of continued survival becomes less clear. Some have considered p53 an ideal candidate for such a gene. They reason that there is a tradeoff between youthful health, and longevity. By inducing cell-cycle arrest in suspect cells, p53 protects against cancer and other degenerative diseases, but at the cost of reducing future supplies of viable cells, leading to senescence and premature aging. See: The common biology of cancer and ageing. See also: Stem Cell Supply Chain Breakdown.
What is the role of the p53 family in aging? Examining the evidence.
In 2002 and 2004, two landmark studies demonstrated that mice with increased p53 activity, while extraordinarily resistant to cancer, showed multiple signs of accelerated aging, and died prematurely. See: p53 mutant mice that display early ageing-associated phenotypes, and Modulation of mammalian life span by the short isoform of p53. Overexpression of p53 was thought to result in chronic apoptosis resulting in cancer protection, at the cost of tissue atrophy or dysfunction, leading to premature death. (ref) A more recent 2009 study shows that overexpression of p53 results in depletion of neural stem supplies, directly leading to decline in normal brain function.
Regenerative capacity of neural precursors in the adult mammalian brain is under the control of p53: “We determined that the impaired ability of NSCs to proliferate does indeed limit the supply of newly generated neurons in the adult brain in an age-dependent way. We also determined that brain function (olfaction) and stem and progenitor cell proliferation declined in parallel. We propose that p53 is a central regulator of neurogenesis in the adult mammalian brain. . . Our results suggest that during aging this regulatory mechanism deteriorates, resulting in disruptions in the ability of stem cells to proliferate. As a consequence, neurogenic regions in the adult brain lose the capacity to replace neurons lost through attrition and normal brain function declines.”
Not all studies have shown a reduction in longevity associated with increased p53 activity. Mice with an extra copy of the wild-type p53 gene, showed increased p53 activity, and increased cancer resistance, but with no signs of accelerated aging, and a normal lifespan. See: “Super p53” mice exhibit enhanced DNA damage response, are tumor resistant and age normally. In another model, p53 overexpression was achieved by reducing activity of a known p53 inhibitor, murine double-minute gene 2 (Mdm2). See: Tumor suppression and normal aging in mice with constitutively high p53 activity. Once again, extraordinary cancer resistance was accomplished, with no adverse effects on aging or lifespan. In a third study, “genetically manipulated mice with increased, but otherwise normally regulated, levels of Arf and p53 present strong cancer resistance and have decreased levels of age-associated damage. These observations extend the protective role of Arf/p53 to aging, revealing a previously unknown anti-aging mechanism and providing a rationale for the co-evolution of cancer resistance and longevity.” See: Delayed ageing through damage protection by the Arf/p53 pathway. (Arf is an upstream regulator of p53.) These mice were not only cancer-resistant, but they had a significantly increased lifespan. In these studies increased p53 activity was associated with a reduction in age-associated DNA damage, and the accumulation of damaged cells.
Explaining the Contradiction.
These examples clearly demonstrate that p53 activity can be increased, providing improved cancer resistance, without adversely affecting lifespan, and in some cases, actually extending lifespan. Therefore, the lifespan reduction seen in the previous examples, must be the result of some other factor, and is not a necessary consequence of the cancer suppressive effects of increased p53 activity per se. There must be something else involved in the methods used to increase p53 activity that explains the reduced lifespan, and the reduction in neural stem cell supply. What do the methodologies of those studies all have in common? Answer: They all used a truncated isoform of p53. Apparently, the reduction in lifespan is the result of the use of this unregulated, truncated variant. As previously mentioned, this truncated isoform is known to antagonize the effects of the regulated, transactivating isoforms, TAp53, TAp63 and TAp73. One way the truncated variant may inactivate the TA isoforms is by directly binding to their transactivation domain, preventing activation and binding to target genes. Mice lacking the p63 gene have shortened lifespans. It is very plausible that the truncated p53 isoform could reduce lifespan by interfering with normal, regulated activity of TAp63. From TAp63: The fountain of youth:
“The mice exhibiting signs of premature aging contain truncated p53 mutants [4,5] while those that display a normal lifespan upregulate p53 by other mechanisms, such as the expression of a p53 transgene in addition to the endogenous p53 alleles or a hypomorphic allele of mdm2 [6,7]. One potential explanation of the discrepancy in the phenotypes of these mice is that TAp63 interacts with point mutant p53 rendering TAp63 functionally inactive. Consequently, mice expressing mutant p53 would exhibit phenotypes similar to those observed in the TAp63-/- mice. Previous studies have shown this to occur in the context of tumorigenesis and metastasis [8,9]. Mice engineered to express point mutants of p53 in Li-Fraumeni Syndrome inactivate p63 and p73 in tumors by binding to them and preventing the transactivation of their target genes [8,9,10]. These mouse models exhibit a metastatic phenotype similar to that observed in p53+/-;p63+/- and p53+/-;p73+/- mice illustrating an intricate relationship between the p53 family members [11,12]. Yet, another unexplored and possible explanation is that expression levels of the p53 family members change in mice that lack one or more of the family members, i.e. gene compensation. Such family member compensation has been observed in other families of genes including the Rb family [13,14,15]. In mouse models expressing abnormally high levels of p53, TAp63 levels may be dampened commensurate with an increase in p53 protein expression. p53 protein levels are known to be high in mice expressing mutated versions of p53 [8,9,10]. Thus, loss of TAp63 in these mouse models may again result in an acceleration of organismal aging.”
p53 is NOT the “central regulator of neurogenesis in the adult mammalian brain”.
As previously mentioned, p73 plays an important role in the maintenance of neurological tissues. Multiple, recent research has demonstrated that p73 maintains neural stem cell pools, not p53 (and most certainly not the truncated p53 variant used in the previously mentioned study). Apparently, the loss of neural stem cells was, once again, due to interference of this truncated p53 variant in the normal, regulated function of TAp73 in maintaining adequate neural stem cell supplies. p73 deficiency results in impaired self renewal and premature neuronal differentiation of mouse neural progenitors independently of p53: “p73 deficiency increases the population of neuronal progenitors ready to differentiate into neurons at the expense of depleting the pool of undifferentiated neurosphere-forming cells. Analysis of the neurogenic niches demonstrated that p73-loss depletes the number of neural-progenitor cells, rendering deficient niches in the adult mice. Altogether, our study identifies TAp73 as a positive regulator of self-renewal with a role in the maintenance of the neurogenic capacity. Thus, proposing p73 as an important player in the development of neurodegenerative diseases and a potential therapeutic target.” See also:
Just as TAp73 maintains neural stem cell supplies, TAp63 maintains stem cell supplies in epithelial and other tissues.
“TAp63 maintains adult stem cells. The mysterious mechanisms that regulate aging are an area of active research. The induction of senescence or apoptosis in stem and progenitor cells is thought to trigger premature organismal aging . Consistent with this idea, we found that the TAp63-/- mice had a significantly shortened life span compared to its wild-type littermates . These mice exhibited phenotypes associated with premature aging including kyphosis, impaired wound healing, alopecia, epithelial and muscular atrophy, and chronic nephritis. These phenotypes suggest a critical role for TAp63 in the maintenance of adult stem cells in multiple epithelial and non-epithelial tissues. Indeed, we found that TAp63 maintains dermal stem cells by transcriptionally activating the cyclin dependent kinase inhibitor, p57, thereby preventing hyperproliferation of these cells (Figure1A) [1,3]. Similar to the phenotype identified in dermal and epidermal progenitor and stem cells, other adult stem cells in the TAp63-/- mice may be hyperproliferative early in life and through similar senescence mechanisms that we delineated may result in a depletion of these stem cells and premature organismal aging â€‹(Figure1B) .”
“The cellular mechanisms that regulate the maintenance of adult tissue stem cells are still largely unknown. We show here that the p53 family member, TAp63, is essential for maintenance of epidermal and dermal precursors and that, in its absence, these precursors senesce and skin ages prematurely. Specifically, we have developed a TAp63 conditional knockout mouse and used it to ablate TAp63 in the germline (TAp63(-/-)) or in K14-expressing cells in the basal layer of the epidermis (TAp63(fl/fl);K14cre+). TAp63(-/-) mice age prematurely and develop blisters, skin ulcerations, senescence of hair follicle-associated dermal and epidermal cells, and decreased hair morphogenesis. These phenotypes are likely due to loss of TAp63 in dermal and epidermal precursors since both cell types show defective proliferation, early senescence, and genomic instability. These data indicate that TAp63 serves to maintain adult skin stem cells by regulating cellular senescence and genomic stability, thereby preventing premature tissue aging.”
In what other ways does the p53 family promote longevity?
As we have seen, increased, regulated p53 activity can result in increased longevity, as well as increased cancer resistance. Clearly, one way in which regulated activity of the p53 family may contribute to increased longevity is by maintaining increased stem cell supplies in various bodily tissues. Are there other ways in which the p53 family could contribute to longevity? p53 promotes longevity thru multiple mechanisms and signaling pathways. I may discuss some of these pathways in greater detail in a future post. For now, I will just briefly mention a couple of the many longevity-promoting mechanisms of p53.
Aging is characterized by increased local and systemic inflammation. Senescent cells are known to secrete large amounts cytokines (signaling molecules) with multiple harmful effects, including increased inflammation. (ref) The accumulation of senescent cells with aging may, in part, explain age-associated chronic inflammation, which, in turn, contributes to many degenerative conditions, including heart disease, AD, metabolic disorders and cancer. (ref) One of several ways, by which p53 activity reduces inflammation is by directly reducing the harmful secretions of senescent cells. (ref) Suprisingly, p53 also functions to actually suppress cellular senescence, thereby reducing the number of senescent cells excreting harmful cytokines. (ref , ref, ref).
Reactive oxygen species are free radicals which damage cellular structures and interfere with healthy physiologic processes. See: Oxidative Damage. p53 can both increase and decrease ROS. (ref) ROS play an important role in the process of cellular arrest. Mitochondrial energy production depends upon ROS. p53 contributes to controlling the harmful effects of ROS in several ways. p53 regulates mitochondrial function reducing the production of ROS in times of stress, and increases mitochondrial biogenesis. (ref) P53 also maintains mitochondrial DNA, which further reduces ROS. (ref) It is interesting that p53 actually increases ROS within the mitochondria, but generates antioxidants, like manganese superoxide dismutase, in order to prevent the ROS from causing damage outside of the mitochondria. (ref) It is interesting to note that much of the anticancer protection afforded by p53, results directly from its antioxidant function. This is demonstrated by the fact that the tumors in mice lacking p53 were prevented by supplementation of the antioxidant, NAC (a precursor to glutathione). (ref)
Other anti-aging target pathways of p53 activity:
FoxO family of longevity genes.
The p53 family does not cause depletion of stem cell pools. On the contrary, it actively maintains those pools. Moreover, the p53 family appears to promote longevity thru multiple mechanisms/pathways. As with anything known to have multiple, complex functions, in addition to anti-aging effects, p53 undoubtedly also has some pro-aging effects. As I have often said, there are no exclusively “good” or “bad” compounds, or processes. Optimal health is about maintaining balance in the function of regulatory processes. The p53 family is a very promising target for both the prevention of cancer and age-related decline. These two objectives appear to be complementary, not mutually exclusive.