Mechanisms of photochemotherapy-induced apoptotic cell death in lymphoid cells

Abstract

Previous studies we performed showed that 8-methoxypsoralen in combination with ultraviolet A light (photochemotherapy) caused DNA damage and that this caused nucleotide depletion in peripheral blood leukocytes, secondary to an active form of programmed cell death, poly(ADP-ribosyl)ation. Further studies revealed that 24 h after exposure to 10 J/cm2 ultraviolet A light and 8-methoxypsoralen (300 ng/mL), apoptotic cells increased from 3 (control) to 31% (p < 0.001). Ultraviolet A light alone also significantly increased the number of apoptotic cells. These morphological changes were confirmed by parallel findings on DNA electrophoresis. Treatment with 2 to 5 J/cm2 of ultraviolet A light and 8-methoxypsoralen caused an approximately 30% increase in cytosolic free calcium levels in peripheral blood leukocytes 1 h after exposure. Associated with this was a 51% increase in 45Ca2+ uptake over the first 60 min. Similar findings in a different lymphoid cell (CCRF–CEM) confirmed the results obtained with peripheral blood leukocytes. The use of calcium-free medium prevented a rise in cytosolic free calcium and decreased the number of cells undergoing apoptotic cell death. Cycloheximide inhibited ultraviolet A light – 8-methoxypsoralen induced apoptosis in CCRF–CEM cells; it also decreased calcium levels in control CCRF–CEM cells. This study shows that ultraviolet A light – 8-methoxypsoralen caused apoptotic cell death in lymphoid cells; this appeared to be associated with calcium influx, presumably because of the requirement of endogenous endonucleases for calcium.Key words: 8-methoxypsoralen, poly(ADP-ribosyl)ation, DNA damage.