Nephronectin (Npnt) is an extracellular matrix protein known to be a ligand for the integrin alpha 8 beta 1. We previously demonstrated that Npnt expression was suppressed by TGF-beta through ERK1/2 and JNK in osteoblasts. In this study, we found that inhibition of a TGF-beta type I receptor (TGF-beta R1, Alk5) by a specific inhibitor {2-[3-(6-Methylpyridin-2-yl)-1H-pyrazol-4-yl]-1,5-naphthyridine} strongly induced Npnt expression in osteoblast-like MC3T3-E1 cells. The Alk5 inhibitor-induced increase of Npnt expression occurred in both time- and dose-dependent manners, while that expression was also induced by introduction of an siRNA for Smad2, a central intracellular mediator of TGF-beta, signaling. These results suggest that the expression of Npnt is regulated by the Alk5-SMAD signaling pathway in osteoblasts. (c) 2012 Elsevier Inc. All rights reserved.