1. ¹Instituto de Investigaciones Biomédicas 'Alberto Sols', Consejo Superior de Investigaciones Científicas-Universidad Autónoma de Madrid, Arturo Duperier 4, E-28029 Madrid, Spain
2. ²Hospital Universitario Puerta de Hierro, E-28035 Madrid, Spain
3. ³Departamento de Bioquímica y Biologia Molecular, Instituto Universitario de Oncología, Universidad de Oviedo, E-33006 Oviedo, Spain
4. ⁴Unitat de Biologia Cel.lular i Molecular, Institut Municipal d'Investigació Mèdica-Universitat Pompeu-Fabra, E-08003 Barcelona, Spain

Correspondence: A Muñoz, E-mail: amunoz@iib.uam.es

⁵These authors contributed equally to this work

Received 14 June 2004; Revised 4 October 2004; Accepted 20 October 2004; Published online 13 December 2004.

Abstract

Wnt glycoproteins regulate homeostasis and development by binding to membrane Frizzled-LRP5/6 receptor complexes. Wnt signaling includes a canonical pathway involving cytosolic -catenin stabilization, nuclear translocation and gene regulation, acting as a co-activator of T-cell factor (TCF) proteins, and noncanonical pathways that activate Rho, Rac, JNK and PKC, or modulate Ca²⁺ levels. DICKKOPF-1 (DKK-1) encodes a secreted Wnt antagonist that binds to LRP5/6 and induces its endocytosis, leading to inhibition of the canonical pathway. We show that activation of canonical signaling by Wnt1 or ectopic expression of active -catenin, TCF4 or LRP6 mutants induces transcription of the human DKK-1 gene. Multiple -catenin/TCF4 sites in the DKK-1 gene promoter contribute to this activation. In contrast, Wnt5a, which signals through noncanonical pathways, does not activate DKK-1. Northern and Western blot studies show that activation of the Wnt/-catenin pathway by treatment with lithium or Wnt3a-conditioned medium, or by stable expression of either Wnt1 or -catenin, increases DKK-1 RNA and protein, thus initiating a negative feedback loop. However, we found that DKK-1 expression decreases in human colon tumors, which suggests that DKK-1 acts as a tumor suppressor gene in this neoplasia. Our data indicate that the Wnt/-catenin pathway is downregulated by the induction of DKK-1 expression, a mechanism that is lost in colon cancer.