Repeated exposures of human skin equivalent to low doses of ultraviolet-B radiation lead to changes in cellular functions and accumulation of cyclobutane pyrimidine dimers

Chronic exposure to sunlight may induce skin damage such as photoaging and photocarcinogenesis. These harmful effects are mostly caused by ultraviolet -B (UVB) rays. Yet, less is known about the contribution of low UVB doses t o skin damage. The aim of this study was to determine the tissue changes in duced by repeated exposure to a suberythemal dose of UVB radiation. Human k eratinocytes in monolayer cultures and in skin equivalent were irradiated d aily with 8 mJ/cm(2) of UVB. Then structural, ultrastructural, and biochemi cal alterations were evaluated. The results show that exposure to UVB led t o a generalized destabilization of the epidermis structure. In irradiated s kin equivalents, keratinocytes displayed differentiated morphology and a re duced capacity to proliferate. Ultrastructural analysis revealed, not only unusual aggregation of intermediate filaments, but also disorganized desmos omes and larger mitochondria in basal cells. UVB irradiation also induced t he secretion of metalloproteinase-9, which may be responsible for degradati on of type IV collagen at the basement membrane. DNA damage analysis showed that both single and repeated exposure to UVB led to formation of (6-4) ph otoproducts and cyclobutane pyrimidine dimers. Although the (6-4) photoprod ucts were repaired within 24 h after irradiation, cyclobutane pyrimidine di mers accumulated over the course of the experiment. These studies demonstra te that, even at a suberythemal dose, repeated exposure to UVB causes signi ficant functional and molecular damage to keratinocytes, which might eventu ally predispose to skin cancer.