Effects of ultraviolet irradiation on human skin-derived epidermal cells in vitro

The effects of UVA, mixed UVA + B, and solar-simulated irradiation were examined in human keratinocytes and melanocytes cultured in vitro. Irradiation with UVA, UVA + B, or the solar simulator caused a dose-dependent decrease in keratinocyte cell numbers and thymidine incorporation at 24 hours, with recovery after 48 and 72 hours. Divided dose regimens reduced the inhibitory effect of ultraviolet (UV) irradiation on cell numbers measured 24 hours after the last irradiation. Exposure to both UVA and UVA + B increased formation of cornified envelopes. Similar irradiance doses of UVA 80 minutes (1.12 J/cm2) and UVA + B 40 minutes (1.04 J/cm2) caused 2.4- and 3.3-fold increases in cornified envelope formation, respectively. With solar-simulated irradiation, the cornified envelope formation was increased by 3.5-fold after exposure of 8 minutes (2.6 J/cm2). Irradiation of melanocytes with UVA, UVA + B, or solar-simulated irradiation resulted in a dose-dependent decrease in melanocyte numbers after 24 hours compared with sham-irradiated controls. As a result of UV irradiation, tyrosinase activity of melanocytes measured at 24 hours was stimulated. UVA + B irradiation (1.04 J/cm2) increased tyrosinase activity approximately twofold, while UVA alone (1.1 J/cm2) increased tyrosinase four to sixfold and solar-simulated irradiation (1.3 J/cm2) increased tyrosinase approximately twofold compared to the control cells. Melanin content increased in cells after both UVA and mixed UVA + B irradiation. These results indicate that both UVA and mixed UVA + B irradiation had qualitatively similar effects on the proliferative and functional activity of skin-derived cells but that the type of irradiation and the dosage regimen affect the dose-response relationship.