Activation of p38 MAPK by IL-1β. p38 phosphorylation by IL-1β in chondrocyte/agarose constructs cultured under free-swelling conditions (a) in the presence or absence of 10 ng/ml IL-1β for up to 45 minutes or (b) with IL-1β and 10 μmol/l SB203580 for 20 minutes. Phospho-p38 MAPK was analyzed for each test condition using a phosphorylation state specific anti-p38 (Thr180/Tyr182) antibody (upper panel), against total p38 MAPK (middle panel) and α-tubulin as a loading control (lower panel). All cell extracts were subjected to Western blot analysis. Each band corresponds to three constructs pooled from two separate experiments. MAPK, mitogen-activated protein kinase; UT, untreated.