Cell-cycle delay is induced in cells of a U937 promonocytic cell line by low-intensity light irradiation at 660 nm

Visible-light irradiation (VLI) at 660 nm and 11.5 J/cm(2) inhibits proliferation of cells of the U937 promonocytic cell line, as monitored by autoradiographical analysis. The S-phase cell population is reduced at 6 h post-radiation treatment. Flow cytometric analysis confirms this, and also shows that light irradiation of cells induces a statistically significant increase in G2/M cells at 6 h post-radiation treatment. It has been postulated that VLI at 660 nm can alter cell-cycle progression by affecting intracellular concentrations of ions, in particular pH and calcium. However, no significant effects of light irradiation on these intracellular ions have been observed. These effects of VLI are not a consequence of radiation-induced DNA strand breaks, therefore events other than direct DNA damage are involved. These findings demonstrate a direct photobiological effect of VLI at 660 nm on the cell cycle, and indicate a previously unsuspected mechanism for the induction of cell-cycle delay that is neither a result of changes in the concentration of intracellular ions nor initiated by DNA strand breaks. (C) 1999 Elsevier Science S.A. All rights reserved.