Negative Feedback Loop of Wnt Signaling through Upregulation of Conductin/Axin2 in Colorectal and Liver Tumors

Abstract

Activation of Wnt signaling through β-catenin/TCF complexes is a key event in the development of various tumors, in particular colorectal and liver tumors. Wnt signaling is controlled by the negative regulator conductin/axin2/axil, which induces degradation of β-catenin by functional interaction with the tumor suppressor APC and the serine/threonine kinase GSK3β. Here we show that conductin is upregulated in human tumors that are induced by β-catenin/Wnt signaling, i.e., high levels of conductin protein and mRNA were found in colorectal and liver tumors but not in the corresponding normal tissues. In various other tumor types, conductin levels did not differ between tumor and normal tissue. Upregulation of conductin was also observed in the APC-deficient intestinal tumors of Min mice. Inhibition of Wnt signaling by a dominant-negative mutant of TCF downregulated conductin but not the related protein, axin, in DLD1 colorectal tumor cells. Conversely, activation of Wnt signaling by Wnt-1 or dishevelled increased conductin levels in MDA MB 231 and Neuro2A cells, respectively. In time course experiments, stabilization of β-catenin preceded the upregulation of conductin by Wnt-1. These results demonstrate that conductin is a target of the Wnt signaling pathway. Upregulation of conductin may constitute a negative feedback loop that controls Wnt signaling activity.

We thank T. Kirchner for performing β-catenin staining on intestinal tumors of Min mice and for helpful discussions, L. Sorokin for critical reading of the manuscript, A. Brown for the Rat2/Wnt-1 and Rat2/CV7 cells, K. Willert for the antiaxin antibodies, and J. Sussman for the dishevelled-2 expression plasmid. We also thank S. Goletz for help in the preparation of the C/G7 antibody; N. Sochnikova and J. Hülsken for help with the in situ hybridizations; K. Feller, A. Koberling, and I. Wendler for excellent technical assistance; and A. Döbler for secretarial support.

This work was supported by a grant from the Deutsche Forschungsgemeinschaft to J.B. (SFB 366).