Effect of combining GW0742 with retinoic acid on cell proliferation of HaCaT keratinocytes. HaCaT cells were treated with either GW0742 (0.2 μM), atRA (1.0 μM), or 9-cis RA (1.0 μM) (A and C) or combinations of GW0742, atRA, and 9-cis RA (arrow) (B and D) in the presence (A and B) or absence (C and D) of culture medium serum, and cell number was quantified as described under Materials and Methods. Values represent the mean ± S.E.M. *, significantly different values (P < 0.05) from vehicle (DMSO) at the particular time point, as determined by ANOVA and Bonferroni’s multiple comparison test.