Novel mechanism of Wnt signalling inhibition mediated by Dickkopf-1 interaction with LRP6/Arrow

Wnt signalling has an important role in cell fate determination, tissue pat terning and tumorigenesis(1-4). Secreted antagonists of Wnt include Frizzle d (Fz)-related proteins (FRPs)(5-7), Cerberus(8), Wnt inhibitory factor (WI F)(9) and Dickkopf (Dkk)(10,11). FRPs, Cerberus and WIF have all been shown to act by binding and sequestering Wnt, We report a novel mechanism of Wnt -signalling inhibition by human Dkk-1, Dkk-1 demonstrated no interaction wi th Wnt but bound a single cell surface site with high affinity (K-D = 0.39 nM). Its receptor was detectable in a complex with a relative molecular mas s of 240,000 (M-r 240K) with [I-125] Dkk-1 by covalent affinity cross-linki ng. Wnt signalling through beta -catenin is mediated by the Fz receptor(12) and a recently identified low-density-lipoprotein-receptor-related co-rece ptor, LRP6/Arrow(13-15). Overproduction of the 200K LRP6 protein, but not o f Fz, strikingly increased Dkk-1 binding as well as the amount of the 240K crosslinked complex, which was shown to be composed of Dkk-1 and LRP6. More over, Dkk-1 function was completely independent of Fz but LRP6 dramatically interfered with the Dkk-1 inhibition of Wnt signalling. Thus, unlike Wnt a ntagonists, which exert their effects by molecular mimicry of Fz(5-7) Or Wn t sequestration through other mechanisms(8,9), Dkk-1 specifically inhibits canonical Wnt signalling by binding to the LRP6 component of the receptor c omplex.