Aging and Disease: 2.1 – Cell senescence, Why Cells Divide

• Aging
• Alzheimer's
• APOE4
• Dementia
• microglia
• Telomeres

Why do some people age faster than others? We’ve all seen people – high school reunions come to mind – who have the same chronological age, but different biological ages: with the same “age”, one person looks ten years older (or younger) than another. If aging is related to cell senescence and cell senescence depends on cell division, then why do some people’s cells divide more than other people’s cells? Why don’t people age at the same rate?

Why does he look old, but she doesn’t, even at the same “age”?

And why do our own organs and tissues age at different rates? We’ve all seen people whose skin looks old, but they have no evidence of osteoarthritis or dementia; equally, we’ve seen other people with terrible osteoarthritis, but no heart disease or dementia. Not only do we age at different rates when we compare different people, but our tissues sometimes age at different rates even within the same person. If aging is related to cell senescence and cell senescence depends on cell division, then why do people vary internally, having some cells (in one tissue) divide more frequently than other cells (in another tissue)? Why don’t all of our tissues age in parallel?

Why does he have bad knees, but she has a bad heart, even at the same “age”?

The easy – and naïve – answer is to say the magic word “genes” and nod knowingly.

The real – and more complex – answer demands a lot more thought. It requires that we reexamine both the data and our assumptions. It requires, in a word, that we think about what’s really going on. Part of this complex answer begins easily. We notice that people who were exposed to too much sun (and too many sun burns), for example, have skin that ages faster than people who avoided sun damage to their skin, and this is true even with identical genes, as in identical twins. We have discussed the fact that aging is not simple a matter of genes, but it’s a balance between damage and maintenance. “It’s not the years, it’s the miles.” Indeed, the degree to which we pile damage onto our tissues shows a good correlation to how fast those tissues show aging and age-related disease. Most of us know this without really thinking about it. For example, we automatically assume that smoking causes COPD, “bad” diets increase your risk of heart attacks, and so forth. These assumptions are now part of our cultural baggage and (true or not) have attained the status of medical wisdom. In fact, to a large extent these are supported by a fair amount of good evidence, although it’s always a bit more complex than the current culturally accepted facts would have you believe. For example, it may or may not (depending on the decade we’re talking about) be accepted that dietary cholesterol has a direct impact on the cholesterol deposits in your coronary arteries, but the evidence that dietary intake (unspecified for the moment, but not just cholesterol) has a long-term impact on coronary artery disease is fairly good.

In short, your behavior (diet, exercise, stress, etc.) can accelerate or decelerate not only your overall rate of aging, but the rate of aging (and age-related disease) in a number of specific tissues. To give a few more examples, people engaged in high-impact activities (think basketball) have a higher incidence of osteoarthritis of the knees than do people engaged in low-impact activities (think yoga). People who get repeated head injuries (think pugilists and American football players) have a higher incidence of Alzheimer’s and other dementias. In both of these cases – osteoarthritis and dementia – those at high risk not only have a higher incidence of the age-related disease in old age, but they get the specific age-related disease at a younger age than do those at lower risk. They are both more likely to get the disease and more likely to get it earlier. What this tells us is not surprising: aging is related to what you do behaviorally, not just who you are genetically. In short, it’s not just your genes.

Genes do, of course, play a fundamental role but they do it in complex relationship with the damage that accrues over a lifetime. If you really want to avoid osteoarthritis, you not only want to have parents who never had osteoarthritis, but you want to avoid repetitive high-impacts to your joints. If you really want to avoid dementia, you not only want a double allele of APOE-2 (instead of two APOE-4 alleles), but you want to avoid boxing or playing football. But then if these sorts of behavior cause age-related disease, and cell senescence underlies age-related disease, what is the relationship?

The key relationship is the rate of cell division. If your cells are forced to divide more frequently, you force them to senesce faster. If, for example, you damage your knees (forcing your chondrocytes to divide and replace the damaged cells) then you will accelerate aging in your knees (as those cells divide, lose telomeres, and change gene expression). The more you damage your knee joints, the more rapidly your chondrocytes divide, and the more rapidly you develop osteoarthritis. If you damage your head (forcing glial cells to divide and replace the damaged cells), then you will accelerate aging in your brain (as those cells divide, lose telomeres, and change gene expression). The more you damage your brain, the more rapidly your glial cells divide, and the more rapidly you develop dementia.

The details, the pathology, the reality of these age-related diseases are wildly more complex than this cursory review suggests, but the basic theme is valid. Given equivalent genes, people who engage in a lifestyle that increases cell turnover will increase their rate of aging. Likewise, your particular lifestyle may increase cell turnover preferentially in one organ or tissue and that will accelerate the rate at which that organ or tissue develops age-related disease.

Any cell in your body (in any tissue) has a baseline “rate of cell division” (i.e., rate of tissue aging). Skin cells, gastrointestinal lining cells, and hematopoietic stem cells divide frequently, while neurons, muscle cells, etc. divide very infrequently in the adult (an in some cases, not at all). Anything that accelerates cell division, accelerates aging. Anytime you increase the rate of damage to a tissue, you increase the rate of cell division (i.e., the rate of tissue aging) and the result is increased aging and increased age-related disease. The same is true between individuals. We each (based on our own genetics) have what you might think of as a “baseline rate of aging” for our body. If you take care of yourself, you still age inexorably, but relatively slowly. If you engage in a high-risk lifestyle, you will age not only inexorably, but relatively quickly.

Aging is caused by cell senescence and cell senescence is cause by cell division, but while you need your cells to divide in order to survive, the relative rate of cell division is, to an extent, controlled by your lifestyle. Cells divide because you’re alive, but the way you live has an impact on how fact those cells divide and how fast you age.

So, let’s answer our initial question. We have been making the case that aging occurs because cells divide, shortening telomeres, which changes gene expression, which results in dysfunctional cells, dysfunctional tissues, and tissue aging (and disease). This is true, but it begs the question of “if cell division causes aging, then what causes cell division?”

The answer is that cell division is both a natural result of being you (your genes, your personality, your culture, and the simple fact that you are alive and some of your cells MUST divide to keep you alive) and the result of what you do to yourself. You have a baseline rate of cell division (and hence aging). If you have a high-risk lifestyle, you age faster; if you have a low-risk lifestyle, you age a bit more slowly. You can increase or decrease your rate of aging – to a degree – depending on what you do. There is (so far) nothing you can do to STOP aging, but can certainly make it a bit slower, or a lot faster.

Next time: 2.2 Cell senescence, Telomeres