Cutaneous vitamin D synthesis versus skin cancer development: The Janus faces of solar UV-radiation

Abstract

In scientific and public communities, there is an ongoing discussion how to balance between positive and negative effects of solar UV-exposure. On the one hand, solar UV-radiation represents the most important environmental risk factor for the development of non-melanoma skin cancer. Consequently, UV protection is an important measure to prevent these malignancies, especially in risk groups. Otherwise, approximately 90% of all vitamin D needed by the human body has to be formed in the skin through the action of UV-radiation. This dilemma represents a serious problem, for an association of vitamin D-deficiency and multiple independent diseases including various types of cancer, bone diseases, autoimmune diseases, infectious diseases, cardiovascular diseases and hypertension has now been reported in a large number of investigative and epidemiologic studies. As a consequence, it has been assumed that for the general population in the US, Europe and other countries, the net effects of solar UV B-radiation on human health are beneficial at or near current levels. We and others have shown that strict sun protection causes vitamin D-deficiency/insufficiency and that detection and treatment of vitamin D-deficiency in sun deprived risk groups is of high importance. Although further work is necessary to define an adequate vitamin D-status and adequate guidelines for solar and artificial UV-exposure, it is at present mandatory that public health campaigns and sun protection recommendations to prevent skin cancer consider these facts. In this review, we analyze the present literature to help developing well-balanced recommendations on sun protection that ensure an adequate vitamin D-status. These recommendations will hopefully protect us against adverse effects of UV protection without significantly increasing the risk to develop UV-induced skin cancer.

Keywords: cancer; solar UV radiation; vitamin D.

Figure 1

Schematic illustration of the cutaneous vitamin D endocrine system in human skin. Please note that the skin represents an unique tissue in the human body’s vitamin D endocrine system, producing various vitamin D metabolites for endocrine, paracrine and autocrine signalling pathways. Importantly, vitamin D is photosynthesized in the skin (epidermis and dermis) by solar or artificial UV-B-radiation (before it is transferred to the blood for endocrine signalling to cover the body’s needs in vitamin D), and biologically active 1,25-dihydroxyvitamin D₃ is synthesized in many skin cells, where it acts locally and regulates a broad variety of independent cellular functions via autocrine/paracrine pathways.