Transforming growth factor beta (TGF-beta) plays a significant role in the regulation of the tumor microenvironment. To explore the role of TGF-beta in oral cancer-induced bone destruction, we investigated the immunohistochemical localization of TGF-beta and phosphorylated Smad2 (p-Smad2) in 12 surgical specimens of oral squamous cell carcinoma (OSCC). These studies revealed TGF-beta and p-Smad2 expression in cancer cells in all tested cases. Several fibroblasts located between cancer nests and resorbing bone expressed TGF-beta in 10 out of 12 cases and p-Smad2 in 11 out of 12 cases. Some osteoclasts also exhibited p similar to Smad2 expression. The OSCC cell line, HSC3, and the bone marrow-derived fibroblastic cell line, ST2, synthesized substantial levels of TGF-beta. Culture media derived from HSC3 cells could stimulate Tgf-beta 1 mRNA expression in ST2 cells. Recombinant TGF-beta 1 could stimulate osteoclast formation induced by receptor activator of nuclear factor kappa-B ligand (RANKL) in RAW264 cells. TGF-beta 1 could upregulate the expression of p-Smad2 in RAW264 cells, and this action was suppressed by the addition of a neutralizing antibody against TGF-beta or by SB431542. Transplantation of HSC3 cells onto the calvarial region of athymic mice caused bone destruction, associated with the expression of TGF-beta and p-Smad2 in both cancer cells and stromal cells. The bone destruction was substantially inhibited by the administration of SB431542. The present study demonstrated that TGF-beta synthesized by both cancer cells and stromal cells participates in the OSCC-induced bone destruction. (C) 2015 Elsevier Inc. All rights reserved.