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 micro-fluorimetry 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 × 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