Before the health hazards of ultraviolet (UV) light exposure were fully appreciated, sun worshippers applied lotions hoping to tan rather than burn. Skin tanning results from the production of the pigment melanin, which absorbs UV radiation and can partially protect cells from the UV-induced DNA damage that can ultimately cause skin cancer. Without melanin, cells are highly susceptible to sunlight; sunburn is the body's response to this damage.
Cui et al. show that the tumor suppressor p53, which functions as a transcription factor and is one of the most intensely studied proteins in biology, plays a crucial role in UV-induced melanin production. Studying p53-deficient mice as well as normal human skin samples, they find that UV light activates p53 in skin keratinocytes (the outermost cells) and that p53 activates the gene encoding pro-opiomelanocortin (POMC). The POMC protein is then cut in several places to generate peptides, including α-melanocyte-stimulating hormone, which stimulates melanocytes (cells located at the base of the epidermis) to produce melanin. Interestingly, POMC proteolysis also generates the opioid peptide β-endorphin, which the authors speculate might contribute to sun-seeking behavior in humans. — PAK
Cell 128, 853 (2007).