Negative growth control of osteosarcoma cell by Bowman-Birk protease inhibitor from soybean; involvement of connexin 43. Bowman-Birk protease inhibitor (BBI) from soybean acts as a potential chemopreventive agent in several types of tumors. However, the mechanism is still unclear. The present study was undertaken to estimate a mechanism of BBI-dependent negative growth control of human osteosarcoma cell (U2OS cell). BBI had negative growth control of the cells via induction of connexin (Cx) 43, a tumor suppressor gene in U2OS cells. This negative growth control by BBI was abrogated under down-regulation of Cx43 induced by a Cx43 antisense nucleotide treatment. It was also found that the BBI-dependent induction of Cx43 was due to elevation of Cx43 mRNA and stabilization of Cx43 protein. Especially, BBI-dependent inhibition of chymotrypsin-like activity in proteasome contributed to stabilization of Cx43 protein. These results suggest that a major negative growth effect of BBI is based on the restoration of Cx43 expression in U2OS cells.