HU210 (1 μM) induces a time-dependent increase in (a) p44 (ERK1), (b) p42 (ERK2), and (c) p38 mitogen-activated protein kinase (MAPK) phosphorylation (filled bars) in fibroblast-like cells derived from the synovia of patients with rheumatoid arthritis or osteoarthritis compared with basal levels. Pre-incubation (20 hours) of cells with pertussis toxin (PTX) attenuated HU210-induced phosphorylation. Data are presented as mean percentage of unstimulated basal MAPK phosphorylation levels ± standard error of the mean (n = 3 synovia). Comparisons between groups were made using one-way analysis of variance. *P < 0.05, **P < 0.01 versus basal levels, φ P < 0.05 versus corresponding PTX-stimulated time point. ERK, extracellular signal-regulated kinase.