Haptoglobin (Hp) induces extracellular signal-regulated kinase (ERK) activation in monocytes. U937 cells were serum starved with 1% serum overnight. Cells were then pretreated or not treated with U0126 (10 min, 1 μM) or RS102895 (30 min, 5 μM) prior to incubation with 10% serum, monocyte chemoattractant protein 1 (MCP1) (200 ng/ml) or Hp (0.5 mg/ml) for 2 min. Harvested cells were lysed and extracted proteins were separated on 12% SDS polyacrylamide gel (50 μg per lane). Activation of ERK1/2 was detected with anti-phospho-ERK antibody. The membrane was stripped and reprobed with anti-ERK1/2 antibody for internal control. In the bottom panel, the bar graph shows the quantification of pERK1/2. Data are expressed as means ± standard error of the mean (SEM) for three experiments. Student t test on the effect of serum, MCP1 and Hp versus serum starved cells on ERK1/2 activation. ***P < 0.001. Two-way analysis of variance (ANOVA) on the effect of pretreatment with U0126 or RS102895 on ERK1/2 activation induced by serum, MCP1 and Hp, P < 0.0001. Bonferroni post-test. ^^^P < 0.001 versus serum stimulated cells, §§P < 0.01 versus MCP1 stimulated cells, °°P < 0.01 versus Hp stimulated cells.