Transforming growth factor (TGF)-beta 1 can cause fibrosis diseases by enhancing production of collagen. However, the intracellular signaling mechanism for TGF-beta 1 stimulation of this process has not been fully elucidated. The present study focused on this mechanism and the cross-talk between the MAPK and Smad pathways. Extracellular signal-regulated kinase (ERK)2 ablation by a small interfering RNA led to marked inhibition of TGF-beta 1-induced collagen synthesis and enhanced phosphorylation of the Smad2 linker site in NIH/3T3 fibroblast cells. However, ERK1 ablation had minimal effects. Ablation of either ERK2 or ERK1 had no effect on the phosphorylation of the Smad2 C-terminal site. Furthermore, a Smad2 mutant with reduced phosphorylation of the Smad2 linker site inhibited TGF-beta 1-induced collagen synthesis. These results indicate that ERK2, rather than ERK1, plays a predominantly positive role in TGF-beta 1-induced collagen synthesis, and that ERK2 enhances collagen synthesis, at least partially, through activation of the Smad2 linker site. (C) 2009 Elsevier Inc. All rights reserved.