Effect of infrared radiation A on photoaged hairless mice harboring eumelanin and pheomelanin in the epidermis

Infrared radiation A (IRA) is absorbed by melanin and generates heat. Therefore, the effect of IRA could be well analyzed using skin, which contains melanin in the epidermis. Hairless mice harboring epidermal melanocytes that produce eumelanin, pheomelanin, or non‐melanin were generated by backcrossing K14‐stem cell factor mice, recessive yellow mice, and then albino hairless mice. High‐dose IRA was irradiated over 18 weeks after the establishment of photoaged mice by irradiation with ultraviolet B (UVB) three times a week for 14 weeks. Tumor formation was assessed every week. The formation of cyclobutane pyrimidine dimer and apoptotic cells by the irradiation of IRA and UVB was evaluated. Repetitive irradiation of IRA did not promote tumor formation in all types of mice. Pre‐irradiation of IRA to UVB, but not post‐irradiation, accelerated the elimination of cyclobutane pyrimidine dimers and enhanced apoptosis; these effects were most obvious in eumelanin‐producing mice. Real‐time polymerase chain reaction analysis showed downregulation of FLICE (cellular caspase 8)‐like inhibitory protein and B‐cell lymphoma‐extra large and upregulation of Bcl‐2‐associated X protein by UVB, but further enhancement of these molecules by pre‐irradiation of IRA was not observed. These results indicate that IRA does not confer the promotion of UVB‐induced carcinogenesis in photoaged mice harboring epidermal melanocytes and that photochemical reaction between IRA and melanin might be involved in the induction of apoptosis and the elimination of cyclobutane pyrimidine dimers by UVB. The enhancement of apoptosis by pre‐irradiation of IRA to UVB might be induced by mechanisms other than the modification of the mRNA expression of FLICE (cellular caspase 8)‐like inhibitory protein, B‐cell lymphoma‐extra large, and Bcl‐2‐associated X.