ULTRAVIOLET-B-INDUCED APOPTOSIS OF KERATINOCYTES IN MURINE SKIN IS REDUCED BY MILD LOCAL HYPERTHERMIA

Two components of sunlight, ultraviolet (UV)B(290-320 nm) and infrared (greater than 700 nm), each cause damage to the skin, However, we recently identified a protective response in which heat reduces UVB-induced killing of cultured keratinocytes. Here, this investigation is extended to the living epidermis. The effects of hyperthermic preconditioning upon UVB-induced apoptosis were studied morphologically with hematoxylin and eosin staining, and biochemically with TUNEL (terminal deoxynucleotide transferase nick-end labeling) to measure endonucleolytic cleavage of DNA in situ. Anesthetized SKH-1 hairless mice were exposed to UVB light (0 to 120 mJ/cm(2)), after which their skin was biopsied at 24 h and paraffin sections were stained with hematoxylin and eosin or with TUNEL, Apoptotic keratinocytes were found to increase after UVB in a dose-related manner, In contrast, if one flank of the mouse was pretreated at 40 degrees C for 1 h and both flanks subsequently were UVB-irradiated at 6 h, the resulting formation of apoptotic cells was reduced twofold or more in the heated flank. Protection appeared by 3 h, reached a maximum at 6 h, and disappeared by 12 h, In summary, heat induces a transient protective effect that reduces UVB-mediated death of keratinocytes in skin at physiologically attainable doses of heat and UVB.