Viral FLIP Enhances Wnt Signaling Downstream of Stabilized β-Catenin, Leading to Control of Cell Growth

Abstract

Death receptor-mediated apoptosis is potently inhibited by viral FLIP (FLICE/caspase 8 inhibitory protein), which is composed of two tandemly repeated death effector domains (DEDs), through reduced activation of procaspase 8. Here, we show that equine herpesvirus 2-encoded viral FLIP E8 enhances Wnt/β-catenin signaling in a variety of cell lines. E8 was shown to strikingly augment Wnt3a signaling, as shown both in a luciferase assay for T-cell factor/β-catenin and through induction of endogenous cyclin D1. The effect of E8 was independent of its direct binding activity with DED-containing signaling molecules, including caspase 8 and FADD, in death receptor-mediated apoptosis. E8 enhanced Wnt signaling downstream of stabilized β-catenin, while a long form of cellular FLIP (c-FLIP_L) enhanced stabilization of β-catenin in 293T cells. Consequently, coexpression of E8 and c-FLIP_L synergistically increased Wnt signaling in 293T cells. Moreover, E8-mediated stimulation of Wnt signaling induced dramatic growth retardation in untransformed cell lines but not in transformed cell lines. Thus, viral FLIP E8 not only inhibits death receptor-mediated apoptosis but also enhances Wnt signaling pathways that are closely related to those of both ontogenesis and oncogenesis.

SUPPLEMENTAL MATERIAL

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

ACKNOWLEDGMENTS

We are grateful to M. Hijikata, K. Shimotohno, K. Ueda, and J. Fujisawa for kindly providing research materials, and we thank H. Sakamaki, K. Lee, T. Shimaoka, and K. Okamoto for useful discussions.

This work was supported in part by grants-in-aid from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of the Japanese Government. S. Nakagiri was supported by the 21st Century COE Program of the MEXT, awarded to the Graduate School of Biostudies and Institute for Virus Research, Kyoto University.