This finding resulted from a series of experiments designed to understand how solar ultraviolet (UV) radiation causes melanoma. The results of this study suggest that interferon-gamma, which has been thought to contribute to an innate defense system against cancer, under some circumstances may promote melanoma and incite the development of tumors. The work, led by researchers from the National Cancer Institute (NCI), part of the National Institutes of Health, appeared online in Nature, Jan. 19, 2011.
Cutaneous melanoma is a highly aggressive and frequently drug-resistant cancer with rising incidence rates. The major environmental risk factor for melanoma is UV radiation exposure, usually from the sun, with the highest risk associated with intermittent burning doses, especially during childhood.
Over the past 10 years, the researchers used genetically engineered mice first to prove, and then to try to understand, the connection between exposure to UV radiation and the initiation of melanoma. The current work was the latest attempt to define the molecular mechanisms of this cause and effect relationship. The results of this study offer the possibility that the inhibition of interferon-gamma immediately after sunburn might block the carcinogenic activation of the skin's pigment-producing cells, known as melanocytes, making it a potentially effective preventive strategy against UV radiation-induced melanoma, according to the scientists.
The key to the experiments, led by Glenn Merlino, Ph.D., Laboratory of Cancer Biology and Genetics, NCI, and research fellow and first author M. Raza Zaidi, Ph.D., was the development of a unique genetically engineered mouse in which the melanocytes were exclusively labeled with a green fluorescent protein. This fluorescent tag allowed visual tracking and specific purification of melanocytes from the mouse skin. For the first time this enabled researchers to evaluate the response of melanocytes to UV radiation exposure while residing in the natural skin environment of a living animal.
The researchers observed that UV radiation doses equivalent to what would cause sunburn in human skin resulted in increased numbers and movement of melanocytes within the mouse skin. A detailed analysis of gene expression changes associated with this melanocyte activation revealed abnormal expression of a number of genes known to be responsive to interferon-gamma.
When the function of interferon-gamma was inhibited at the time of UV radiation, the number of melanocytes and their movement remained normal, suggesting that interferon-gamma was responsible for the UV radiation-induced activation of the melanocytes.
The source of interferon-gamma within the skin was determined to be macrophages-cells that normally protect against infection-that had infiltrated the skin after UV exposure. The pro-melanoma potential of these macrophages was revealed when they were found to enhance the growth of melanomas when transplanted under the skin of mice.
This effect was abolished when interferon-gamma was blocked, corroborating its importance in promoting melanoma development. Moreover, when the scientists examined human melanoma tissue samples, they found interferon-gamma-producing macrophages in 70 percent of the tumors, supporting the idea that these macrophages can significantly contribute to the initiation and/or progression of human melanoma.
"We anticipate that this discovery may change how interferons are used in the clinic as anticancer agents," said Merlino. "Our findings raise the possibility that targeting the interferon-gamma pathway may represent a novel, less toxic therapeutic alternative for effective treatment of malignant melanoma patients, who currently have poor cure rates."
These studies were made possible through long-term collaborations with Edward De Fabo, Ph.D., and Frances Noonan, Ph.D., of George Washington University Medical Center, Washington, D.C.