Blockage of MKP-1 expression results in decreased macrophage proliferation. (A-C) Macrophages were transfected with either an siRNA directed against MKP-1 or a control siRNA against luciferase. Twenty-four hours after transfection, the cells were stimulated with M-CSF for the indicated periods of time. (A) MKP-1 expression was analyzed by Western blotting, using β-actin expression as a control. (B) Total ERK activity was analyzed by Western blotting. Diphospho-ERK detection was carried out with specific antibodies. β-actin expression was measured as a control. (C) Thymidine incorporation was determined in triplicate as a measurement of macrophage proliferation. Error bars represent SD from triplicates in 1 representative experiment. (D) Macrophages from wild-type or MKP-1–deficient mice were stimulated with M-CSF for the indicated periods of time. Expression of MKP-4 was determined by rt-PCR using the expression values of L14 for normalization. (E) Macrophages from wild-type or MKP-1–deficient mice were either left untreated or stimulated with M-CSF for 45 minutes. Expression of CPG21, MKP-5, and -7 was determined by rt-PCR using the expression values of L14 for normalization. In all the sections, similar results were obtained in at least 3 independent experiments. Error bars in panels D and E indicate the variation between experiments.