Cell cycle analysis of cultured mammalian cells after exposure to 4,5',8-trimethylpsoralen and long-wave ultraviolet light

Cell cycle analysis was used to study the effect of 4,5',8-trimethylpsoralen (TMP) and long-wave ultraviolet light (UV-A) on cultured mammalian cells. DNA distribution patterns were measured for murine melanoma cells (a cloned line of Cloudman S91) and a strain of diploid human skin fibroblasts (CRL 1295) using both a microfluorimetry procedure and flow cytometry. The untreated cells and those receiving TMP alone and UV-A alone had identical DNA content as assessed at several posttreatment intervals (0-72 hr). The majority of cells in control groups contained a G₁ DNA content, whereas exposure to TMP (2 x 10^-7 M) plus UV-A (1 Joule/cm²) led to the accumulation of cells in the G₂ phase. These observations were similar for each cell type and both analytical techniques were in excellent agreement. The finding that psoralen plus UV-A induces a phase-specific G₂ blockade in cultured cells has important implications for understanding the mechanisms which account for enhanced pigmentation and suppression of cellular proliferation following exposure to these agents in vivo.