Notch signalling has an important role in skin homeostasis, promoting keratinocyte differentiation and suppressing tumorigenesis. Here we show that this pathway also has an essential anti‐apoptotic function in the keratinocyte UVB response. Notch1 expression and activity are significantly induced, in a p53‐dependent manner, by UVB exposure of primary keratinocytes as well as intact epidermis of both mouse and human origin. The apoptotic response to UVB is increased by deletion of the Notch1 gene or down‐modulation of Notch signalling by pharmacological inhibition or genetic suppression of ‘canonical’ Notch/CSL/MAML1‐dependent transcription. Conversely, Notch activation protects keratinocytes against apoptosis through a mechanism that is not linked to Notch‐induced cell cycle withdrawal or NF‐κB activation. Rather, transcription of FoxO3a, a key pro‐apoptotic gene, is under direct negative control of Notch/HERP transcription in keratinocytes, and upregulation of this gene accounts for the increased susceptibility to UVB of cells with suppressed Notch signalling. Thus, the canonical Notch/HERP pathway functions as a protective anti‐apoptotic mechanism in keratinocytes through negative control of FoxO3a expression.

Restricted expression of caspase-14 in differentiating keratinocytes suggests the involvement of caspase-14 in terminal differentiation. We purified active caspase-14 from human cornified cells with sequential chromatographic procedures. Specific activity increased 764-fold with a yield of 9.1%. Purified caspase-14 revealed the highest activity on WEHD-methylcoumaryl-amide (MCA), although YVAD-MCA, another caspase-1 substrate, was poorly hydrolyzed. The purified protein was a heterodimer with 17 and 11 kDa subunits. N-terminal and C-terminal analyses demonstrated that the large subunit consisted of Ser(6)-Asp(146) and N-terminal of small subunit was identified as Lys(153). We successfully developed an antiserum (anti-h14D146) directed against the Asp(146) cleavage site, which reacted only with active caspase-14 but not with procaspase-14. Furthermore we confirmed that anti-h14D146 did not show any reactivity to the active forms of other caspases. Immunohistochemical analysis demonstrated that anti-h14D146 staining was mostly restricted to the cornified layer and co-localized with some of the TUNEL positive-granular cells in the normal human epidermis. UV radiation study demonstrated that caspase-3 was activated and co-localized with TUNEL-positive cells in the middle layer of human epidermis. In contrast, we could not detect caspase-14 activation in response to UV. Our study revealed tightly regulated action of caspase-14, in which only the terminal differentiation of keratinocytes controls its activation process.

The monitoring of the amount of t-UCA in the SC by confocal Raman spectroscopy is a good method to assess the efficacy of sun protective substances.

Urocanic acid (UCA) is a major component in the epidermis which absorbs ultraviolet light (UV) and is isomerized from the trans to the cis form upon UV exposure. This study aimed to evaluate the amounts of cis- and trans-UCA in the stratum corneum (SC) in vivo using Raman spectroscopy. UV damage has previously been investigated by calculating the amount of trans-UCA by fitting Raman spectra. cis-UCA at pH 5.0 was also included in the fitting algorithm in this study. For validation, the SC was stripped from the subjects and the levels of cis- and trans-UCA were measured using high performance liquid chromatography (HPLC). Comparison between these two methods was performed at UV exposure and non-UV exposure sites on the volar forearm of 10 healthy Japanese subjects. Finally, the protective capacity of sunscreen was evaluated by Raman spectroscopy on the volar forearm of 18 healthy subjects. The amount of cis-UCA increased and the amount of trans-UCA decreased after UV exposure when measured by Raman spectroscopy. This was confirmed by HPLC analysis of the SC stripped from the same area of skin used in the Raman measurement. The protection against the increase in the cis-UCA and decrease in the trans-UCA amount after UV exposure by the application of sunscreen products was observed using Raman spectroscopy. The results show this method can be used to measure cis- and trans-UCA levels in vivo.

Very little information exists on the amount of natural and artificial UV light required to cause sunburn and tanning in individuals with very pale skin who are at the greatest risk of developing skin cancer. We have investigated minimal erythema dose (MED) and minimal melanogenic dose (MMD) in a group of 31 volunteers with Fitzpatrick skin types I and II using an Oriel 1000 W xenon arc solar simulator and natural sunlight in Sydney, Australia. We measured the erythemal and melanogenic responses using conventional visual scoring, a chromameter and an erythema meter. We found that the average MED measured visually using the artificial UV source was 68.7 ± 3.3 mJ/cm2 (3.4 ± 0.2 standard erythema doses [SED]), which was significantly different from the MED of sunlight, which was 93.6 ± 5.6 mJ/cm2 (P < 0.001) (11.7 ± 0.7 SED). We also found significant correlations between the solar‐simulated MED values, the melanin index (erythema meter) and the L* function (chromameter). The average MMD (obtained in 16 volunteers only) using solar‐simulated light was 85.6 ± 4.9 mJ/cm2, which was significantly less than that measured with natural sunlight (118.3 ± 8.6 mJ/cm2; P < 0.05). We mathematically modeled the data for both the chromameter and the erythema meter to see if we were able to obtain a more objective measure of MED and differentiation between skin types. Using this model, we were able to detect erythemal responses using the erythema index function of the erythema meter and the a* function of the chromameter at lower UV doses than either the standard visual or COLIPA methods.