Figure 5.

Pharmacologic inhibition of MEK-ERK in combination with ATRA promotes apoptosis. A549 cells were serum-starved and treated or nontreated (NT) with 5 μM of ATRA alone or in combination with 25 μM of PD98059 for 48 h. Cells irradiated with 20 J/m² of UV light for 2 min were used as a positive control for apoptosis (+). DNA fragmentation was detected by TUNEL. The apoptotic cells are stained brown. Percentages of TUNEL-positive nuclei were quantified by counting 50 cells from five random microscopic fields (means ± SEM, ^** P < 0.001; ^*** P < 0.0001 compared with NT cells, analysis of variance, and Newman-Keuls test).