Symbols of the passing of time, gray hair could actually have another meaning, from a biological point of view. They could play a role in protecting against cancer, according to the findings of a new study. Here’s how.
To be or not to be… The dilemma of damaged cells
Over time, or under the influence of factors such as stress, sun exposure, certain chemical agents or even cellular aging, the DNA of cells can be altered. Faced with this damage, stem cells are faced with a decisive choice: cease their activity or continue their division with the risk of making defective “copies” capable of causing cancer.
“Somatic tissues undergo functional decline with age, exhibiting phenotypes characteristic of aging, including graying and cancer. However, genotoxins – substances or agents capable of damaging a cell’s genetic material – signals and cellular mechanisms specific to each phenotype remain largely unknown.” specify researchers from the University of Tokyo (Japan).
These scientists focused on the cells responsible for the color of our hair. The latter are called melanocytes, themselves derived from pigment stem cells housed in hair follicles. Their role is to produce melanin, the pigment responsible for coloring hair and skin. They hypothesized that these cells were faced with the same dilemma in the event of DNA damage (continue their division or cease their activity, a visible death with graying of the hair) and that this change in color could thus reflect a natural defense reaction against the risk of cancer.
What if gray hair was the result of a beneficial cellular sacrifice
To support this hypothesis, Professor Emi Nishimura’s team used long-term in vivo lineage tracing and gene expression profiling in mice to analyze the response of melanocyte stem cells to different types of DNA damage.
Result: they observed that melanocytic stem cells react to certain DNA damage by entering into a very specific process. Specifically, in the event of DNA double-strand breaks, stem cells activate the p53–p21 pathway, which triggers transformation into mature melanocytes before programmed death, also called apoptosis. Consequence: fewer cells capable of producing pigment and therefore hair which turns gray.
“This process, driven by the activation of the p53–p21 pathway, during which cells differentiate irreversibly before being lost, causes hair to gray and effectively protects against melanoma, a skin cancer.” note the researchers.
This phenomenon, called senodifferentiation, corresponds to differentiation coupled with cellular senescence. In short, the cells specialize irreversibly before being eliminated, causing hair to turn gray.
Published in the journal Nature Cell Biologyso this research shows how some cells choose to shut down rather than become dangerous.
A protection that can sometimes turn against us
But it all depends on the type of stress experienced. Exposed to certain carcinogens, such as 7,12-dimethylbenz(a)anthracene or UVB rays, these cells no longer follow this sacrifice program. In this case, “McSCs (melanocytic stem cells) bypass this protective differentiation program, even in the presence of DNA damage. They then retain their capacity for self-renewal and multiply clonally, a process supported by the KIT ligand secreted both by the local niche and in the epidermis. This niche-derived signal inhibits senodifferentiation, rendering McSCs vulnerable to tumors” specify the scientists.
In other words, when this protective mechanism is interrupted, the body loses one of its natural defenses against the proliferation of abnormal cells. Senodifferentiation therefore represents a protective pathway induced by stress, which eliminates potentially dangerous cells. “Our results show that the same population of stem cells can follow two opposite fates depending on the signals it receives: exhaustion or expansion.” explains Emi Nishimura. “This allows us to rethink hair graying and melanoma not as independent events, but as divergent outcomes of stem cell responses to stress.” he believes.
To conclude, we cannot say that graying protects against cancer but it is the consequence of a cellular cleaning process: potentially dangerous cells prefer to be harakiri before becoming dangerous and being able to induce a cancerous process.