Photobiological implications of melanin photoprotection after UVB-induced tanning of human skin but not UVA-induced tanning

Abstract

Repetitive suberythemal UVA and/or UVB exposures were used to generate comparable UV-induced tans in human skin over the course of 2 weeks. To evaluate the potential photoprotective values of those UVA- and/or UVB- induced tans and to avoid the confounding issue of residual UV-induced DNA damage, we waited 1 week before challenging those areas with a 1.5 MED of UVA+UVB after which we measure DNA damage. The results show that the type of UV used to induce skin pigmentation affects the redistribution of melanin in the skin and/or de novo melanin synthesis. The UVA-induced tans failed to even provide a minimal SPF of 1.5, which suggests that producing a tan with UVA-rich sunlamps prior to a holiday or vacation is completely counterproductive.

Keywords: DNA damage; photoprotection; pigmentation; skin; ultraviolet.

Figure 1

Protocol for UVA- and/or UVB- tanning and challenge, and effects on erythema and pigmentation of the skin. (A) Timeline of suberythemal repetitive irradiations with UVA and/or UVB for 2 weeks. In week 1, a 0.4 MED dose of UV was applied daily (Monday through Friday) and 0.5 MED UV was applied daily in week 2. One week later, biopsies were taken from each of the 4 skin areas on each subject. The following week, a 1.5 MED UVA+UVB challenge was applied to each skin area and biopsies were taken immediately after the challenge and also 4 days later. Images of skin at the different biopsy time points are depicted; shown for subject #16531. (B) Diffuse reflectance spectroscopy indicates minimal to no erythema developed prior to and immediately after the UV challenge. There were no statistically significant differences between any of the UV treatment areas or time points with the exception of the UVB-induced tan area that showed a statistically significant difference in erythema (P<0.05) prior to the UV challenge compared to the 4 days post-UV challenge. (C) Pigmentation levels increased after UVA, UVB and UVA+UVB (P<0.01, P<0.002 and P<0.05, respectively) compared to control area prior to UV challenge, corroborating the visual tan development and remained elevated at 4 days post-UV challenge (P<0.05). In addition, pigmentation levels increased significantly with UVA only (P<0.05) and UVB only (P<0.005) 4 days after the UV challenge compared to prior to the repeated UV regimen. The UV challenge induced significant pigmentation increases in the unexposed control area (P<0.01). Data points correspond to mean values calculated from apparent concentration diffuse reflectance spectroscopy values acquired from 5 subjects ± SE; a.u. = arbitrary units.

Figure 2

UVA-induced pigmentation is not photoprotective. (A) Melanin content of human skin sections detected by Fontana-Masson silver staining show clear visual increases in melanin with the 3 UV treatments compared to the unexposed control; representative micrographs are shown for subject #18910; scale bar = 50 μm. The brightfield images were taken with an AxioCam MRc Rev 3 color camera (Carl Zeiss, Inc., Oberkochen, Germany) attached to a DMRB microscope (Leica Microsystems, Bannockburn, IL, USA). (B) Micrographs (subject #18910) depict no CPD-positive cells 7 days after 2 weeks of suberythemal repetitive UV treatments prior to UV challenge. Following the 1.5 MED UV challenge, the UVA-induced pigmentation provided no photoprotection immediately after the UV challenge. Immunofluorescence images were taken with a ORCA-ER Black and White CCD digital camera (Hamamatsu Photonics K.K., Hamamatsu, Japan) attached to the Axiovert S100 inverted microscope (Carl Zeiss, Inc., Oberkochen, Germany); scale bar = 50 μm. (C) The percentages of CPD-positive cells (means ± SEM, n=5) were significantly increased compared to unexposed areas for UVA-induced and unexposed areas receiving the 1.5 MED UV challenge (red bars; P<0.05).

Figure 3

Heatmaps of UV-induced genes show UV spectral expression changes for genes according to the types of UV treatment and times. Microarray analysis is shown prior to UV challenge, immediately after the UV Challenge, and 4 days after the UV Challenge for the UV-irradiated sites compared to unexposed control skin. Heatmap clustering was generated for UVA (A), UVB (B) and UVA+UVB (C) vs the unexposed control (red (max = +3) to blue (min = −3) color gradient). (D) Top 20 gene probes identified by treatment effect P value determined by the Repeated Measure Analysis of Variance model.