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Cellular Function and Aging
Tumor Suppression Protein 53 and Effects on Cellular Function and Aging
The concept of aging has many intrinsic and extrinsic factors that act as markers on an individual organism. Ignoring mortality associated with external environmental factors, very few organisms can be said to have cellular immortality with no decrease in cellular function or repeat division in normal diploid cells. Cellular senescence is a normal process that halts cellular division after a set of cycles of replication. Senescent cells can remain completely functional but lose the programmed process of replication. The normal pathway for senescent cells is either aging with metabolic pathways continuing for the cell or programmed cell death which is known as apoptosis that occurs when cellular function changes, a specific lifetime is reached for the cell or the cell is damaged. The multicellular cnidarians known as a Hydra has been shown to have a complete lack of senescence in cellular function with cells dividing frequently and continuously and being sloughed off at the tips of appendages and new stem cells continuously repopulating (Watanabe 2009). The hydra organism effectively shows no aging (Martinez 1998) and studies of the Hydra genome show that the organism has a mutation in the expression of the p53 gene that manifests as a lack of p53 protein in hydra cells (Rutkowski 2010). The link between a lack of p53 expression and aging has been studied exhaustively with the inverse relationship between tumor suppression and cell immortality at balance with the expression of the protein. What has not been studied under such significant scrutiny has been the relationship between p53 expression and cellular senescence which is the halting of cellular processes to form a dormant cell. The tradeoff for having no pathway to halt cellular activity is continuous cell division and replenishment which the hydra has exploited to live an immortal life. For a more complex animal with differentiated organ systems the nature of p53 tumor suppression and "immortality" is a legitimate tradeoff between insuring that cancer cells become dormant and undergo apoptosis (programmed cell death) and renewing the organ systems of the body.
A recent publication titled Senescence-Associated Secretory Phenotypes Reveal Cell-Nonautonomous Functions of Oncogenic RAS and the p53 Tumor Suppressor by Jean Phillippe Coppe etal observes the relationship between the byproducts of cells that have been made senescent (dormant) and the healthy cells surrounding them. The results of the research in the paper show that human cells induced into senescence by the suppression of the p53 tumor suppression protein and the upregulation of oncogenic RAS secrete proteins that are both toxic and can lead to expression of cancer in normal cells surrounding the senescent ones. Aging is dependent on the rate of cellular regeneration throughout all of a person's organs. As the rate of regeneration slows and cells either die off or are induced into senescence, organs age. At the surface, it appears that halting apoptosis and cellular senescence would be a ticket to immortality similar to the hydra; this immortality is short lived as dampening pathways to cellular senescence and apoptosis leads to the development of cancers brought on by DNA damage or other mutations (Lowe 2003). Senescence appears to be an important mechanism for decreasing the capability of cancer cells to multiply and divide; if cells with DNA damage or malignant tendencies can be turned off and placed in a "holding pattern" then the effect should be beneficial to the organism. A paradox is apparent when both the capability to regenerate new cells and the potential for secreting damaging molecules occurs through the very mechanism designed to protect an organism from developing cancer. This tug-of-war is termed antagonistic pleiotropy and is the heart of the relationship balancing cancer prevention and aging where each is both beneficial and inversely proportional to the other (Kirkwood 2000).
Senescence as a cellular state is a fairly broad definition; however the specific pathology is an inactivation of the cell's ability to divide while still maintaining metabolic function. The cell may or may not continue to function with the original processes within tissue but specific behaviors, pathways and secretions continue to occur with senescent cells and these cells can have effects on surrounding cells. Research has shown that the number of senescent cells in mammalian tissue increases with increasing age (Campisi 2005) Researchers studying aging and possible solutions to slow the effects of the process have focused attention on p53 function and senescent cells. The "Catch 22" associated with inhibiting p53 and increases in cancer or retaining senescent cells has not been solved, however, one area of research that could benefit the development of new therapies is an understanding of the class of secretions produced by senescent cells and the effects of those secretions on surrounding cells and tissue. Since senescent cells increase with normal lifespan and age-related diseases are found associated with senescent cells (Faragher 2000), an understanding of exactly what genes are expressed and what proteins are secreted from these cells could help further treatments for those diseases.
The article by Coppe looks at how senescent cells secrete proteins and which genes are turned on within these cells. Comparing senescent cells secretions and gene upregulation with normal cells can explain how these senescent cells behave in tissue and what effects they have on neighboring cells. The authors looked at five cells lines of fibroblast cells derived from embryonic lung tissue, foreskin, and breast tissue. The cells were studied as "normal" cells in culture and after either stressing the cells by repeatedly dividing them until nearly all division stopped or by exposing them to high dose radiation. When all five cell lines had cultures of both "normal" cells and senescent cells present the cell lines were studied.
The authors made use of several techniques to determine what genetic pathways were being expressed and which proteins were secreted. Cells from each culture were incubated in clean cell culture media solution and the solution was then analyzed. Proteins are detected quickest with antibodies which are part of the detection array in the immune system. Since a given antibody is specific for a given part of a protein, known cellular proteins can be detected using an available antibody array. The array is a chip coated with known antibodies and when the antibodies contact a specific protein in solution, enzymes linked to fluorescent molecules can be used to show levels of antibody detection simply by analyzing how the fluorescence intensity. A cell line was incubated for 24 hours then the secreted solutions could be analyzed. An initial review of the data shows that there are much higher levels of specific proteins for the senescent cells. Without assigning a specific level of expression, the authors show in Figure 1A of the article that the senescent cells have clearly visible higher levels of multiple protein types in solution after 24 hours of incubation. For the 120 proteins assayed in the study, 41 were significantly higher in the senescent cell lines. Not every protein in the array was expressed at higher levels; seventy nine proteins in the cell lines were the same for normal cells as the senescent counterparts indicating that some specific proteins were upregulated and expressed for the senescent cells. The authors called these specifically upregulated expressed proteins "senescence-associated secretory phenotypes" (SASP)s which means a specific group of proteins that are only expressed by senescent cells.
Further review of the SASPs beyond the simple analysis of the numbers of expressed proteins showed that the group of secreted molecules was complex. The group included inflammatory cytokines, cell surface molecules, immune responding molecules, and survival factors that help insure a cell remains viable. It was found that each of the five different cell types had a range of expressed SASP's and the cells behaved differently. However, it was also found that the manner of inducing senescence did not affect the SASP types. Cells of one specific type, lung tissue for example, expressed the same SASPs after inducing senescence whether that senescent behavior was caused by radiation or forcing the cells to divide many times until division would no longer occur. The authors also learned that simple DNA damage to cells did not induce SASP unless the cells underwent full conversion to senescence; simply irradiating the cells with damaging but non-senescence inducing levels of radiation did not produce SASP's in the cultures.
The authors compared fibroblast cells from the early study with epithelial cells from human prostate to observe the effect of senescence on organ-specific cells. The comparison of normal cells of fibroblasts and normal epithelial cells derived from the prostate found a direct match of all secreted proteins in the array showing that the cells have very similar amounts and types of proteins secreted if cell division and behavior is normal despite being different cell types. The senescent cells for both radiation and cell division-induced senescence for prostate derived epithelial cells expressed high secretion of different proteins. These SASP's were different from those found in senescent fibroblasts…[continue]
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