Stimulation of Melanogenesis
Tanning is the principal physiologic photoprotective
response of the skin. Tanned skin is thought to provide an
SPF of between 2 and 4 and to reduce light-induced DNA
damage. In fact, an inverse relationship exists between the
melanin content of our skin and said DNA photoproducts. It
would appear that UV-induced DNA damage and its repair
are some of the initial triggers of melanogenesis. From this
we can deduce that stimulating the production of melanin
without exposure to the sun would reduce cutaneous DNA
damage.94
Forskolin
A good way of increasing skin pigmentation is to intervene in
any of the cell signals that induce melanogenesis. Forskolin
is a diterpene that penetrates the cell and activates
adenylate cyclase. This enzyme mediates the effect of a
melanocyte-stimulating hormone in melanocytes, bypassing
the melanocortin 1 receptor and thereby obviating the
need for prior UV exposure. A recent study demonstrated
that topical application over 3 months of forskolin in mice
induced eumelanin production and resulted in a resistant
photoprotective effect evidenced by an increase in minimal
erythema dose.95 Moreover, this effect was accompanied by
an increase in epidermal melanocytes and thickening of the
epidermis due to an accumulation of nucleated keratinocytes.
The additional advantage of this photoprotective substance
is its ability to stimulate photoprotection in individuals
with a functional disruption of the melanocortin 1 receptor,
who are characterized by blonde or red hair and fair skin
that does not tan. This effect has been demonstrated in
transgenic mouse models.96