Figure 4.

ERK activation is associated with proliferation on ATRA-resistant lung cancer cells. 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. The proliferative effect was assessed by BrdU labeling according to the manufacturer's instructions. The graph shows the results of three independent experiments (means ± SEM, ^* P < 0.05; ^*** P < 0.0001 compared with NT cells, analysis of variance, and Newman-Keuls test).