The inhibitory effect of decorin is independent of the EGF or IFN-γ receptors.

(A) Decorin and IFN-γ, but not EGF, inhibit macrophage proliferation. Macrophages were either treated with 20 nM EGF, 300 U/mL IFN-γ, or 10 μg/mL decorin or remained untreated for 24 hours in the presence of 1000 U/mL M-CSF, and proliferation was determined as indicated in “Materials and methods.” Each determination was made in triplicate, and the values represented correspond to the mean ± SD of one representative of 3 independent experiments. (B) Decorin and IFN-γ, but not EGF, induce the expression of p21^Waf1 mRNA in BMDMs. Macrophages were treated for 3 hours with either 20 nM EGF, 300 U/mL IFN-γ, or 10 μg/mL decorin or remained untreated. Expression of p21^Waf1 was determined by Northern blotting. (C) Decorin, but not EGF, elongates the M-CSF–induced activation of ERK. Quiescent macrophages were stimulated with one or a combination of the following: 1000 U/mL M-CSF, 20 nM EGF, or 10 μg/mL decorin for the indicated times. ERK activity was determined by in-gel kinase assay. (D) Decorin inhibits MKP-1 expression induced by M-CSF. Quiescent macrophages cultured on plates precoated with BSA or 10 μg/mL decorin were stimulated with M-CSF for 30 minutes. The expression of MKP-1 was analyzed by Northern blotting. The levels of the 18S rRNA transcript were used as a loading and transfer control. (E) Decorin inhibits the proliferation of macrophages from IFN-γ receptor knock-out mice. The 10⁵ macrophages from control (░) and IFN-γ receptor knock-out (▪) mice were cultured in the presence of 1000 U/mL M-CSF and treated with 300 U/mL IFN-γ or 100 μg/mL decorin. Proliferation was determined after 24 hours by [³H]-thymidine uptake. Each determination was made in triplicate, and the values represented correspond to the mean ± SD of 2 independent experiments.