Inhibition of the mitogen-activated protein kinase pathway prevents tumor necrosis factor alpha (TNF-α) and epidermal growth factor (EGF)-mediated loss of aggrecan and type II collagen mRNA. Confluent chondrocytes were pretreated with U0124 (10 μm, inactive analog of U0126) or U0126 (10 μm, a MEK1/2 inhibitor), for 15 min, followed by treatment with TNF-α (30 ng/ml), EGF (10 ng/ml) or TNF-α + EGF for 24 hours. Levels of (a) aggrecan and (b) type II collagen mRNA were assessed by northern blot analysis of total RNA (10 μg). Levels were normalized to levels of 18S rRNA and data are expressed as the percentage of respective control ± standard error of the mean (n = 5). a Significant difference from respective control (P < 0.001), b significant difference from cultures treated individually with TNF-α or EGF (P < 0.01), c significant difference from cultures treated with U0124 followed by addition of TNF-α + EGF (P < 0.05).