New findings in a study of the process of skin tanning
reveal a promising way of protecting fair-skinned people from skin cancer caused
by exposure to sunlight.
In a new research done on mice scientists have developed a cream that
switched on the tanning machinery in the skin cells without exposing them to
sun. This new understanding of the process of skin tanning enabled them to
develop a promising way of protecting fair-skinned people from skin cancer
caused by exposure to sunlight.
People who tan easily, or have naturally dark skin, are far less likely to
develop skin cancer than fair-skinned individuals -- who tend to get sunburns
rather than tans -- the findings suggests that medicinally-induced tans can
protect at-risk individuals from the disease.
The study used a small molecule that essentially mimics the process that
occurs when skin cells are struck by ultraviolet light from the sun. While the
compound used in the study has not yet been tested in humans, the results
"demonstrate the principle that actual tanning can be 'rescued' by recognizing
the normal pathway and the precise step where it is blocked in people who do not
tan well. Melanoma, the fastest-increasing form of cancer in the world, occurs
when pigment-making skin cells called melanocytes begin dividing rampantly as a
result of damage to their DNA. If melanoma tumors are detected and surgically
removed before their cells spread to other parts of the body, patients have an
almost 100 percent chance of surviving. The odds drop sharply, however, if
treatment doesn't begin until the disease has spread, or metastasized.
One trigger for melanoma development appears to be ultraviolet (UV) light
from the sun, which can damage the skin's DNA. For most of human history,
fair-skinned people, who tan poorly, occupied regions with low sun exposure,
such as Nordic areas with winter months of darkness. As human populations have
scattered throughout the globe, increasing numbers of fair-skinned people have
come to live in sunny climes, and melanoma and other skin cancer rates have shot
up.
Red hair and pale skin humans do not tan easily because of a peculiarity in a
tiny pouch-like receptor, called MC1R, on the surface of melanocytes. When the
hormone MSH -- for Melanocyte Stimulating Hormone -- drops into the pouch, it
causes a surge in the melanocyte¡¯s production of the chemical cAMP. cAMP then
stimulates melanocytes to turn on a large number of genes, causing a pigment
called melanin to be produced. If cAMP levels are low, the melanocytes make
red/blond melanin. If cAMP levels are high, they make brown/black melanin. The
melanin is eventually discharged from melanocytes and taken up by keratinocytes.
MC1R is shaped differently in red-haired people, so that MSH cannot stimulate it
strongly. The result is that cAMP production stays at low levels. Less cAMP
means less red/blond pigment production, which results in fair skin.
Many scientists have theorized that tanning occurs when ultraviolet radiation
strikes the nuclei of melanocytes, causing DNA damage that prompts the
melanocytes to produce pigment. This supposition, however, conflicted with the
results of lead researcher Dr David Fisher's experiments. "Our work suggested
that a peculiarity in the MC1R receptor on melanocytes is responsible for a
failure to tan," Fisher relates. "But that sort of change on the cell surface
shouldn¡¯t impede UV radiation from reaching the melanocyte¡¯s DNA."
The new experiments demonstrated that, rather than acting directly on the
nuclei of melanocytes, UV radiation acts on keratinocytes (the most abundant as
well as superficial cells in the skin), causing them to produce and secrete MSH,
which attaches to adjacent melanocytes and starts the pigment-making process.
While Fisher's model adequately explains why redheads do not tan, it isn't
the only possibility. "Suppose that during the embryonic or fetal period MC1R
never activated cAMP production in developing melanocytes," Fisher proposes.
"Would mature melanocytes then be permanently 'crippled,' unable to respond to
UV, regardless of how its signals were transmitted?¡±
One way to disprove that "permanently crippled" scenario would be to see if
melanocytes with abnormal MC1R receptors can be coaxed into producing pigment in
adult mice. To attempt this, Fisher and his associates treated the skin of
red-haired, fair-skinned mice with a compound known to increase cAMP levels. The
compound, called forskolin, is derived from the root of the forskohlii plant
found in India. The mice involved in the experiment turned dark, proving that
melanocytes in redheads aren¡¯t inherently unable to make pigment if
appropriately stimulated.
Further experiments showed that not only can red-haired mice be given tans
without exposing them to UV light, but this sunless tanning process is virtually
indistinguishable from that in dark-haired mice that tan naturally.
The Dana-Farber researchers also showed that tans acquired through forskolin
conferred significant protection against skin cancer caused by exposure to UV
light. Fisher notes that while it is unknown whether forskolin will penetrate
deeply enough in human skin to activate melanocytes, these results suggest that
the search for other substances that do reach deep into the skin may well have
the same pigmentation effects in people.
"These studies suggest that a drug-induced 'rescue' of the tanning mechanism
may correspondingly rescue at least some aspect of skin cancer protection,"
Fisher observes. "Such sunless tanning may also dissuade sun-seeking behaviors,
which undoubtedly contribute significantly to high skin cancer incidence."