MPA induces c-Jun and c-Fos phosphorylation and AP-1 activation via p42/p44 MAPKs. (A) to (D) Cells were pretreated with RU486 or U0126, transfected with PR siRNAs or PR expression vectors and were then treated with MPA. Western blots (WB) were performed with phospho (p)-c-Jun and pp42/44MAPKs antibodies and filters were re-probed with the respective total antibody, or with a c-Fos antibody and re-probed with an actin antibody. Experiments in A to D were repeated five times with similar results. Signal intensities of phospho-proteins were analyzed by densitometry and normalized to total protein bands. Data analysis showed a significant increase in protein phosphorylation by MPA in comparison with untreated cells and a significant inhibition of MPA-induced phosphorylation by RU486 or UO126 (P <0.001). See also Additional file 1: Figure 1. MAPKs, Mitogen-activated protein kinases; MPA, Medroxyprogesterone acetate; PR, Progesterone receptor.