|LE Magazine January 2001 |
and Cellular Senescence
What the life cycles of cells and proteins tell us about mortality
Page 1 of 3
What makes cells mortal? Research findings on cellular senescence may explain not only the life span potential of cells, but also cancer and human mortality.
Research findings on cellular senescence may explain not only the life span potential of cells, but also cancer and human mortality.
Most cells regenerate themselves by dividing to form a pair of new cells. In 1961, scientist L. Hayflick discovered that cells eventually reach a limit beyond which they cannot continue to divide (Hayflick L et al., 1961; Hayflick L, 1965). In a now-famous series of experiments, Hayflick demonstrated that cultured human fibroblasts (connective tissue cells) can divide only about 60 to 80 times. When a cell reaches this “Hayflick Limit” it enters into a twilight state called cellular senescence. Senescent cells are very much alive—yet they are distorted in both form and function.
Cultures of senescent cells cannot be mistaken for younger cells, which are uniform in appearance and line up in parallel arrays. By contrast, senescent cells exhibit a grainy appearance and take on odd shapes and sizes. They lose the ability to organize themselves in a regular pattern. These striking changes are called the senescent phenotype. A dipeptide (chemical union of two amino acids) called carnosine has been shown to rejuvenate cells displaying the senescent phenotype, quickly restoring the juvenile phenotype (McFarland GA et al., 1999; McFarland GA et al., 1994).
Senescent cells also behave in deviant ways. For example, senescent dermal (skin) cells generate more metalloproteinase enzymes that break down proteins in the surrounding extracellular matrix (the fabric that holds together cells, lymph nodes and blood vessels). They also generate more of the proinflammatory cytokine (hormone-like proteins involved in cellular signaling) interleukin 1-a. Senescent endothelial cells, which line blood vessel walls, generate higher levels of an adhesion molecule that contributes to atherosclerosis. By secreting damaging molecules and cytokines, senescent cells can disrupt the surrounding tissue microenvironment. Relatively few cells could in this way exert far-reaching deleterious effects upon tissue integrity and organ function (Campisi J, 1997).
Is cellular senescence then tantamount to aging? There are several lines of evidence supporting this conclusion. Cells from older people senesce after only a fraction of the cell divisions that fetal cells can undergo. Cells from short-lived animal species senesce faster than cells from long-lived species. Cells from people with genetic premature aging syndromes senesce prematurely, suggesting that the same genes regulate life span in the cell and the organism. Finally, senescent cells accumulate with age in organs and tissues, where they resist programmed cell death (apoptosis) and contribute to age-related degeneration (Campisi J, 1997).
The cancer connection
There is another direct connection between cellular senescence, aging and mortality. Surprisingly, cellular senescence appears to be controlled by tumor suppressor genes, including p53 and Rb (Bringold F et al., 2000; Campisi J, 2000, 1997). Most tumors contain cells that continue to divide beyond normal limits or indefinitely. Tumor suppressor genes are thought to act in part by inducing cellular senescence, which puts a halt to cell division. This has led scientists to the intriguing hypothesis that cellular senescence evolved as a defense against cancer. In support of this theory, recent research shows that cells can respond to carcinogenic stimuli such as DNA damage and the activation of cancer-promoting genes by entering a senescent state.
If oxidized proteins are not broken down, they tend to cross-link and aggregate (as, for example, in cataracts or senile plaques). Rapid effective proteolysis is therefore an anti-aging mechanism.
The double-edged sword of cellular senescence thereby consigns cells to mortality in order to protect them against cancer. Ironically, cellular senescence alters the microenvironment around the cell in two ways that are thought to contribute to both aging and carcinogenesis. First, senescent cells may impair the structural integrity of the microenvironment, allowing a cell harboring an oncogenic mutation to proliferate. For example, the enzymes secreted by senescent dermal fibroblasts may be able to destroy the basement membrane and underlying stroma (the tissue framework for an organ) that keep potentially cancerous cells in check. Second, senescent cells overproduce growth factors and cytokines that could stimulate the growth of precancerous cells. These derangements of the structure and function of the cellular microenvironment could synergize with accumulating mutations to favor the early stages of tumorigenesis (Campisi J, 2000; Campisi J, 1997).
In addition, disturbances in cell cycle control due to inefficient protein removal can set the stage for cancer, as we shall see below.
The protein life cycle
The body is made up largely of proteins. The health of the body's stock of proteins depends upon its freedom from damage (as through oxidation or cross-linking), and upon its timely removal as part of normal protein turnover.
The body's antioxidant system and other lines of defense cannot completely protect proteins. Nature's second line of defense is the body's system for repairing or removing damaged proteins. While some protein repair mechanisms exist, there are no known ways to repair most protein damage, including even simple oxidative damage to the amino acids which are the building blocks of proteins. Thus it is essential for the body to efficiently remove aberrant and unneeded proteins, a process called proteolysis.
Timely proteolysis removes damaged proteins before they do significant harm, and removes undamaged proteins before they become damaged or disruptive. For example, if oxidized proteins are not broken down, they tend to cross-link and aggregate (as, for example, in cataracts or senile plaques). Rapid effective proteolysis is therefore an anti-aging mechanism (Grune T et al., 1997).
The main proteolytic enzyme complex is called the proteasome. It removes proteins that have been tagged for degradation by a peptide called ubiquitin. Through its role in protein disposal, the proteasome-ubiquitin pathway helps regulate many basic cellular processes including the cell cycle and cell division, cell differentiation, cellular signaling, cellular metabolism and DNA repair (Ciechanover A, 1998). Thus a malfunctioning proteasomal system has far-reaching consequences.
As cells age, after many cell divisions, proteasome activity declines (Sitte N et al., 2000; Merker K et al., 2000). At the same time, more and more proteins undergo damage through a process called carbonylation. Thus the proteolytic system becomes increasingly inadequate to deal with the increasing numbers of abnormal or unneeded proteins, which can irreversibly form cross-links and turn cellular processes awry.
New research shows that when the population of carbonylated proteins permanently increases—as in aging—proteasome activity is depressed (Petropoulos I et al., 2000; Keller JN et al., 2000; Burcham PC et al., 1997). A vicious circle develops of age-related decline in proteasomal activity, age-related increase in protein carbonylation and further inhibition of the proteasome. The life cycles of proteins become blocked, and the normal turnover of protein declines.
Is there a way to block this vicious circle? The body contains a dipeptide called carnosine that both protects proteins from carbonylation and helps reverse proteasomal decline. As in the aging body, proteolysis declines in cultured cells as they approach senescence. Australian scientists showed that carnosine enhances intracellular proteolytic activity in human connective tissue cells (Hipkiss AR et al., 1995). Carnosine enhanced proteolysis the most in old cells, and to a lesser extent in “middle aged” cells, compensating for age-related proteolytic decline (for details see “Carnosine—Nature's pluripotent life extension agent” from this issue).
Continued on Page 2
References on Page 3