A, GSK-3β−/− and GSK-3β+/+ cells were starved and incubated without or with LPA (10 μM). After 24 h, cells (including floating and adherent cells) were harvested, fixed, and stained via terminal deoxynucleotidyltransferase-mediated dUTP nick end-labeling (TUNEL) using ApopTag (Chemicon International). The percentages of TUNEL-stained positive apoptotic cells were determined by flow cytometry. The results are presented as means ± S.D. of duplicate data from three independent experiments. B, the mitogenic activity of LPA in GSK-3β−/− and GSK-3β+/+ cells was measured by [3H]thymidine incorporation as described under “Experimental Procedures.” The data were presented as fold increases (means ± S.D. of triplicate assays) with the activity in unstimulated control cells defined as one.