Transforming Growth Factor β-Dependent Sequential Activation of Smad, Bim, and Caspase-9 Mediates Physiological Apoptosis in Gastric Epithelial Cells

Abstract

Transforming growth factor β (TGF-β) has been implicated in the maintenance of homeostasis in various organs, including the gastric epithelium. In particular, TGF-β-induced signaling was shown to be required for the differentiation-associated physiological apoptosis of gastric epithelial cells, but its mechanism has not been well understood. In this study, the molecular mechanism of TGF-β-induced apoptosis was analyzed in a human gastric epithelial cell line, SNU16, as an in vitro model. Expression of Smad7 and Bcl-X_L, but not viral FLIP, was shown to prevent TGF-β-induced apoptosis, indicating an exclusive requirement of the activation of Smad signaling pathway and mitochondrial dysfunction followed by activation of caspase-9. In addition, treatment with TGF-β induced binding of Bim, a proapoptotic Bcl-2 homology domain 3 (BH3)-only protein, to Bcl-X_L, which is dependent on the activation of Smad, and reduction in the expression of Bim by RNA interference decreased the sensitivity to TGF-β-induced apoptosis. Moreover, we found abnormalities in the gastric epithelium of both Bim and caspase-9 knockout mice; these abnormalities were associated with a defect of physiological apoptosis in gastric epithelial cells. These results indicate for the first time that TGF-β is involved in the physiological loss of gastric epithelial cells by activating apoptosis mediated by Smad, Bim, and caspase-9.

SUPPLEMENTAL MATERIAL

Supplemental material for this article may be found at http://mcb.asm.org/.

ACKNOWLEDGMENTS

We thank K. Miyazono, E. Nishida, P. Bouillet, Y. Ito, and K. Ito for providing materials. We also thank K. Sakamaki, A. Murakami, K. K. Lee, and K. Okamoto for useful comments.

This work was supported in part by a Grant-in-Aid from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese government. M.O. was supported by the 21st century program of MEXT of the Japanese government for the Graduate School of Biostudies and the Institute for Virus Research, Kyoto University.