2.6. Cell Treatment and UVB Irradiation

HaCaT cells were pretreated with various concentrations of PE (0, 10, 50, 100, and 1000 µg/mL) and further cultured for 6 h before UVB irradiation. The non-UV control groups were treated with cell culture medium only. Prior to UVB exposure (40 mJ/cm²) with the BIO SUN Ultraviolet Irradiation System (Vilber Lourmat, Marne La Vallée, France), the cells were washed twice with 4 °C cold phosphate buffer saline (PBS). Then, an adequate amount of cold PBS was layered on the cells to cover the cells, in 100-mm culture dishes. After UVB irradiation, the cells were washed again with cold PBS, and then, the PBS was replaced with fresh DMEM and the cells were incubated for 18 h for further experiments.