UV and You

 

Conor MacEvilly, Ph.D.
LPI Skin Research Group


The only known beneficial biochemical effect of sun exposure is the formation of vitamin D, which prevents rickets. Numerous adverse affects are associated with both acute and chronic UV radiation, including erythema (sunburn), systemic immunosuppression, photoallergic dermatitis, photoaging, and, most importantly, photocarcinogenesis. Skin cancer is now about as common as all other cancers combined, and approximately one million Americans will develop skin cancer this year. Skin cancers are generally divided into two main classes: non-melanoma types (basal cell and squamous cell carcinoma) and malignant melanomas. The latter, although less common, are more lethal because of their ability to metastasize and lack of curative treatment for advanced cases. Melanomas are responsible for 75 percent of the nearly 10,000 skin cancer deaths each year.

It is well established that non-melanoma skin cancers are associated with chronic sun exposure, whereas melanomas appear to be associated with acute, intense exposure or blistering sunburn. However, the relationship between melanoma development and sun exposure is complex. For example, only about 20 percent of melanomas occur on the most exposed area of the body, the face and neck. In addition, whereas non-melanoma skin cancers are very rare in heavily pigmented individuals compared to Caucasians, melanomas are about one-fifth as common in heavily pigmented individuals as in Caucasians.

Dr. Linus Pauling and a diagram of skin.
(Photographer and date unknown.)
Skin, the bodys largest organ, consists of two main layers: the outermost epidermis and the underlying dermis. Skin cells (keratinocytes), which are continuously dying and being replaced, make up about 95 percent of the cell mass of the epidermis. Keratinocytes are responsible for the production of specialized proteins (e.g., keratins) and lipids (e.g., ceramides) that contribute to the structural integrity and barrier function of the skin. The epidermis, for example, acts as a barrier against viral and bacterial infiltration and prevents water loss.

Two components of natural sunlight, ultraviolet A (UVA) and ultraviolet B (UVB), are responsible for most of the adverse effects associated with sun exposure. Ninety percent of UVB radiation is absorbed by the epidermis, with DNA in skin cells a primary target of UVB radiation.

Unlike UVB radiation, a significant amount of UVA radiation can penetrate to the dermal layer beneath the epidermis and is thought to be responsible for aging effects. The majority of commercially available topical lotions to protect against sun exposure depend on chemical ingredients to absorb or reflect UV radiation. Some of these chemicals, however, such as para-aminobenzoic acid (PABA), can result in adverse allergic reactions in certain individuals following sun exposure.

At LPI, we are interested in understanding how antioxidants and other natural compounds protect the skin from the adverse effects of the sun. Though not as drastic as skin cancer, the aging effects of UV radiation on the skin, such as pigmentation changes, loss of elasticity and wrinkling, are of considerable concern. Human and animal research has shown that the skin contains antioxidants, such as vitamins C and E, as well as antioxidant enzymes like superoxide dismutase and catalase. The highest concentration of these molecules is in the epidermal layer of the skin, which is exposed to the highest levels of radiation, implying a protective effect for these antioxidants. In addition, it has been shown that the levels of these antioxidants drop following an acute dose of UV radiation and that some require a number of days to reestablish their pre-exposure levels. Published studies suggest both dietary and topical antioxidants may have beneficial effects in protecting the skin, and our cell culture experiments have provided additional evidence for the beneficial effects of antioxidants and other natural compounds.

Until fairly recently, dermatologists had considered the skin as primarily a large protective coat around the body, i.e., a giant Saran-wrap. Recently, it has become increasingly clear that keratinocytes have properties similar to some of the cells that comprise the bodys immune system, such as leukocytes and lymphocytes. Keratinocytes are capable of releasing numerous molecules that can initiate both immunological and inflammatory effects in the skin following exposure to environmental stimuli such as contact allergens (e.g., poison ivy) and UV radiation. It has also been shown that exposing human volunteers to relatively large doses of UV radiation may produce a systemic immunosuppressive effect. A number of recent studies have suggested that while sunscreens prevent the skin from becoming sunburned, they may not be capable of preventing the immunosuppressive effects of UV radiation, which may be partly responsible for the formation of skin cancers. However, this remains a controversial area.

With increases in the levels of UV radiation reaching the earths surface well documented, it is of substantial importance to analyze the mechanisms by which epidermal keratinocytes, as the primary target cells, are affected by UV radiation, in addition to investigating strategies for preventing or reversing damage.

Our program in skin research is supported by Unilever Research U.S. and Elizabeth Arden Company.

Last updated November, 1996


Honoring a Scientific Giant with Research Toward Longer, Better Lives

Please send any comments, suggestions, or questions about The Linus Pauling Institute to lpi@oregonstate.edu