An animal study of the effects on p16 and PCNA expression of repeated treatment with high-energy laser and intense pulsed light exposure

Background and Objective: Non-ablative skin rejuvenation treatments that involve the use of laser/light sources together with cooling devices have gained much popularity in recent years due to the lack of down time that is associated with them. One important but neglected issue is long-term safety. Does the repeated use of non-ablative skin rejuvenation lead to photoaging? Are we creating another sun-bed phenomenon? Recently, we performed an in vitro study to examine the effect of sub-lethal QS 755 nm lasers on the expression of p16INK4a on melanoma cell lines, and found that sub-lethal laser damage could increase DNA damage, which led to an increase in p16 expression. Our objective was to assess the cutaneous effect of repeated exposure to high-energy lasers and intense pulsed light sources on male Institute of Cancer Research (ICR) mice. Study Design/Materials and Methods: Twenty-eight male ICR mice were divided into four groups. Other than the control group, all groups received either laser (585 nm pulsed dye laser or 1,320 nm Nd:YAG laser) or intense pulsed light (IPL) treatment. All four groups were anesthetized with a mixture of Hypnorm/Dormicum before treatment. The animals were irradiated twice a week for 6 months. Signs of toxicity such as mortality and weight loss were checked once a week. Skin tumor formation was evidenced by lesions of greater than 1 mm in diameter that persisted for 2 weeks. At the end of the 6 months, the expression of proliferating cell nuclear antigen (PCNA) and p16 in the mouse skin was determined by immunohistochemical staining and immunoblotting using specific monoclonal antibodies for mouse PCNA and p16. The results were expressed as mean standard error of the mean (SEM). Statistical difference was assessed by multiple ANOVA. A P-value of <0.05 was considered to be significant. Results: At the end of the 6 months, none of the animals had developed any signs of toxicity such as mortality or weight lost. There was no evidence of tumor formation. There were significant elevations of p16 and PCNA in all treated groups as compared to the control group (ANOVA P < 0.05). This particularly applied to the group that was treated with the 1,320 nm Nd:YAG laser. Conclusion: The repeated use of high-energy laser and intense pulsed light source did not cause any toxicity in mice. The changes in p16 and PCNA imply that further studies are necessary to consider the implications of repeated exposure to longer wavelength radiation in human skin.