How do we age? That was the weighty question to kick off a presentation by Barbara A. Gilchrest, M.D., Boston University School of Medicine, who presented at MauiDerm earlier this year.
Wailea, Hawaii - How do we age? That was the weighty question to kick off a presentation by Barbara A. Gilchrest, M.D., Boston University School of Medicine, who presented at MauiDerm earlier this year.
In the skin there are two distinct components of aging: intrinsic and extrinsic aging processes, Dr. Gilchrest notes. Intrinsic aging entails the clinical, histologic and physiologic changes realized in sun-protected skin of older adults. Extrinsic aging is what is commonly called photoaging, or the clinical, histologic and physiologic changes seen in the habitually sun-exposed skin of older adults.
“There is a striking contrast clinically between these two processes,” she says.
Intrinsic skin aging has relatively minor impact on skin appearance, according to Dr. Gilchrest, but there are well-documented functional deficits that range from poor healing, poor thermal regulation and compromised immune function.
“Photoaged skin, in contrast, has major impact on appearance. There is further loss of immune function and other physiologic deficits. Very importantly, it is associated with photocarcinogensis - a huge problem in our practices,” Dr. Gilchrest says.
Another concept widely held in the gerontology community, she says, is that aging and photoaging are consequences of safeguarding the genome. From this perspective, cancer is the failure of this safeguard mechanism.
Major mechanisms of skin aging, according to documented literature in the late 20th and early 21st centuries, include changes in signaling, oxidative stress and cumulative DNA damage. Lastly, and of great importance to Dr. Gilchrest, is telomere shortening and dysfunction. These mechanisms are inter-related, and among them, telomere signaling offers the most comprehensive and unifying framework.
Telomeres are the ends of chromosomes. At the very tips of all eukaryotic chromosomes are telomeres. In human cells, telomeres are 7,000 to 10,000 base pairs in length. Their job is to cap chromosome ends.
“If you remove telomeres experimentally from chromosomes, the chromosomes fuse and it’s a catastrophic event for cells,” Dr. Gilchrest says.
Researchers have shown that telomeres function to limit cell division and are essentially the biologic clock (Harley CB, Futcher AB, Greider CW. Nature. 1990;345:458-460).
“Because of the end replication problem, every time the cell divides it’s not possible to replicate the final 50 to 100 base pairs at the end of the chromosome,” she says. “The telomeres shorten each time the cell divides. And ultimately telomeres reach a critically short length after which the cell will never divide again no matter what mitotic signals are provided.”
More recently, laboratories have been examining the important function of telomeres in triggering DNA damage responses. For example, Rockefeller University’s de Lange Laboratory has shown disruption of the telomere loop causes DNA damage signaling.
“A thought that is critical to consider in skin aging is that telomeres are excellent targets for DNA damage. The TTAGGG sequence is completely conserved in all mammalian organisms. TT (adjacent thymidines) are the target for a great majority of UV-induced DNA damage. Guanine is half of the telomere sequence, and is the target for almost all oxidative damage.
“It’s been shown experimentally by groups examining DNA damage for other reasons that if you use any agent to damage the DNA in a cell, you’ll damage DNA throughout the genome, of course, but you get much more damage in the telomeres,” she says. “Nature has given us a wonderful mechanism for sensing when damage is occurring.”
Dr. Gilchrest also notes that telomeres have been strongly implicated in human aging. Telomeres are known to shorten with age both in vitro and in vivo. Telomeres shorten progressively over the lifespan and shortening is associated with age-associated diseases, such as cardiovascular disease and diabetes.
Telomere length also correlates with longevity. In recent studies, it has been shown that if you take a group of people at age 60 and measure their telomeres and wait and see how long these individuals live, those with longer telomeres at age 60 average a longer life span (Cawthon RM, Smith KR, O’Brien E, et al. Lancet. 2003;361(9355):393-395; Valdes AM, Andrew T, Gardner JP, et al. Lancet. 2005;366(9486):662-664).
Additionally, virtually all diseases of premature aging are all characterized by very short telomeres. Those include Werner syndrome, progeria and other progeroid syndromes.
One other molecule critical in the telomere story, according to Dr. Gilchrest, is telomerase, the enzyme complex responsible for lengthening telomeres. This enzyme complex is expressed in germline cells, stem cells, and greater than 90 percent of malignant cells, where it maintains telomere length. Importantly, it also slows but does not prevent telomere shortening in normal somatic cells.
What is the consequence of activating or upregulating this telomerase enzyme activity?
“If you activate telomerase, you immortalize cells. They will then divide indefinitely in a culture dish, and they do not age,” Dr. Gilchrest says.
There is a downside, however. Stewart et al found that if you activate telomerase in an organism, you also promote carcinogenesis, “because you have removed the safeguard against infinite proliferation of abnormal cells. That was very unfortunate,” Dr. Gilchrest says (Stewart SA, Hahn WC, O’Conner BF, et al. Proc Natl Acad Sci U S A. 2002;99(20):12606-12611).
In another study, it was shown that when examining skin explant models, activating telomerase results in tissue rejuvenation by a number of criteria (Funk WD, Wang CK, Shelton DN, et al. Exp Cell Res. 2000;258(2):270-278).
More recently, increased telomerase activity in combination with cancer resistance appears to delay aging in a mouse model (Tomas-Loba et al. Cell, 2008). According to Dr. Gilchrest, by activating telomerase to a moderate level in a mouse experiment, study authors found no increase in incidence of cancer, median and maximum lifespan was substantially increased, there was decrease in clinical and molecular aging markers, and telomere length in cells in animals was lengthened.
“At least in a mouse model, it gets around the yin/yang issue of aging and cancer,” she says.
The telomerase activation in mice also resulted in much less inflammation in skin, epidermis and subcutaneous fat layers were thicker, and there was increased resistance to ulcer formation. Further, senescent markers for DNA damage signaling were much decreased in skin.
Dr. Gilchrest cites another mouse model of aging in which researchers genetically deleted the telomerase-active enzyme in these animals, such that there was no telomerase activity (Rudolf et al. Cell. 1999).
“In the first generation of the mice, the animals seemed completely normal. But as they bred them, by the fourth generation, animals aged very prematurely. They were disease prone, infertile, unhealthy and died at a much younger age,” Dr. Gilchrest says.
In this model of mice with short telomeres and no ability to lengthen because of no telomerase activity, what was interesting was what happened if telomerase was reintroduced into these mice. In as little as four weeks, “there is a striking change in the animals. Telomeres are lengthened, fibroblasts are proliferative, fertility increases and litter sizes increased,” she says.
Additionally, when the animals were assessed for rejuvenation psychologically, the animals with longer telomeres were much more youthful in responses to olfactory stimuli, and they lived longer. According to Dr. Gilchrest, the animals did not develop cancers in this four-week period. Also, there was less DNA damage signaling (indicative of cell senescence) in tissues.
“We now know that telomere shortening leads to apoptosis or senescence of cells in skin and other tissues, but what causes aging of remaining cells?” Dr. Gilchrest says.
She cites more work showing that as telomeres shorten, their gene expression patterns change (Lou Z, Wei J, Riethman H, et al. AGING. 2009;1(7):608-621). Telomere dysfunction (or shortening) can also compromise cellular metabolism by activating p53, according to study findings, which, in turn, compromises mitochondrial energy production (Sahin E, Colla S, Liesa M, et al. Nature. 2011;475:254).
Lastly, how does stress spur aging?
“The mechanism by which stress interacts with aging process remains largely unknown,” Dr. Gilchrest says. Epel et al. looked at the effect of stress on aging from the perspective of the telomere (Epel ES, Blackburn EH, Lin J, et al. Proc Natl Acad Sci U S A. 2004;101(49):17312-17315). A total of 58 healthy women were recruited for the study, 19 of who had a healthy child, while 39 had a chronically ill child. Study authors measured peripheral blood lymphocytes for these women for their telomere function and length.
Researchers found a “statistically significant difference” in telomere length between two groups. “Stressed mothers have shorter telomeres and lower telomerase activity,” Dr. Gilchrest says. “These women have compromised ability to maintain telomere length.”
Future studies may examine how to find a balance between slowing aging and preventing cancer, she says.
“Telomere-based signaling acts first to reduce DNA damage, slow senescence and protect the genome. Telomerase is documented to be critical for genome protection, cancer prevention and regulating aging. So we are left with a choice of aging or cancer until we learn how to manipulate the system better,” Dr. Gilchrest says. “Fortunately, means of achieving a better balance seem possible, at least in mice.”
Disclosures: Dr. Gilchrest reports no relevant financial interests.