Abstract
We have previously elucidated the signaling events that are required for TGFβ1 autoinduction (Yue, J., and Mulder, K. M. (2000) J. Biol. Chem. 275, 30765-30773). Further, we have reported that the TGFβ receptor (TβR)-interacting protein km23-1 plays an important role in TGFβ signal transduction (Jin, Q., Ding, W., and Mulder, K. M. (2007) J. Biol. Chem. 282, 19122-19132). Here we examined the role of km23-1 in TGFβ1 autoinduction in TGFβ-sensitive epithelial cells. siRNA blockade of km23-1 reduced TGFβ1 mRNA expression, as well as DNA binding and transcriptional activation of the relevant activator protein-1 site in the human TGFβ1 promoter. Further, knockdown of km23-1 inhibited TGFβ-mediated activation of ERK and JNK, phosphorylation of c-Jun, and transactivation of the c-Jun promoter. Sucrose gradient analyses indicate that km23-1 was present in lipid rafts together with Ras and TβRII after TGFβ treatment. Immunoprecipitation/blot analyses revealed the formation of a TGFβ-inducible complex between Ras and km23-1 in vivo within minutes of TGFβ addition. Moreover, we demonstrate for the first time that km23-1 is required for Ras activation by TGFβ. Our results indicate that km23-1 is required for TGFβ1 autoinduction through Smad2-independent Ras/ERK/JNK pathways. More importantly, our findings demonstrate that km23-1 functions as a critical adaptor coupling TβR activation to activation of Ras effector pathways downstream.