Epidermal cell kinetics of pig skin in vivo following UVB irradiation: apoptosis induced by UVB is enhanced in hyperproliferative skin condition.

We investigated the effects of ultraviolet B (UVB) irradiation on pig epidermal sunburn cell (apoptotic cell) formation. Expression of p53 tumor suppressor gene product, p21 (WAF1/CIP1), and proliferating cell nuclear antigen (PCNA) was also determined immunohistochemically. Apoptotic cells appeared at 12 h and reached a peak at 48 h following 2 MED-UVB irradiation. The formation of sunburn cells was confirmed by terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick-end labeling (TUNEL) method. p53-positive cells, and p21-positive cells appeared at 6 h, and 12 h, respectively, following the UVB-irradiation. The peak of p53-positive cells was observed at 24 h, and that of p21-positive cells was at 48 h. No expression of TUNEL-, p53-, or p21-positive cells was detected in non-irradiated epidermis. The increase in PCNA-positive cells was observed at 24 h and reached its peak at 96 h following the UVB-irradiation. Flow cytometric analyses indicated a decrease in S-phase cells at 24 h, that was followed by their increase at 96 h. Cells in G2/M phase were also considerably decreased at 6 h and 48 h following the UVB-irradiation, and was followed by their increase thereafter. The [3H]thymidine uptake and mitotic counts remained low up until 48 h, and then both parameters increased reaching their peaks at 72 96 h. Effects of UVB irradiation were also determined in tape stripping-induced hyperproliferative epidermis. The numbers of UVB-induced apoptotic cells and PCNA positive cells were markedly enhanced in the tape stripping-treated epidermis, while the numbers of p53- and p21-positive cells were not significantly altered. No induction of apoptosis, p53, or p21 was observed by tape stripping alone. Our results indicate that UVB irradiation induces G1 arrest, prolonged S, and G2/M block of epidermal keratinocytes as well as apoptosis. These processes provide a G1 check point and the elimination of possibly hazardous cells carrying DNA damage, respectively. Our results also indicate that the UVB-induced apoptotic process is enhanced in hyperproliferative skin condition suggesting that apoptosis is closely associated with cell cycle progression.