Wnt/beta-catenin signaling is essential to early development. Activation of Frizzled-1 by Wnts induces nuclear accumulation of beta-catenin and activation of Lef/Tcf-dependent gene expression. Casein kinase 2 has been shown to affect Wnt/beta-catenin signaling. How casein kinase 2 exerts an influence in Wnt signaling is not clear; casein kinase 2 has been reported to be constitutively active (i.e. not regulated). Herein we show to the contrary that casein kinase 2 activity is rapidly and transiently increased in response to Wnt3a stimulation and is essential for Wnt/beta-catenin signaling. Chemical inhibition of casein kinase 2 or suppression of its expression blocks Frizzled-1 activation of Lef/Tcf-sensitive gene expression. Treatment with pertussis toxin or knock down of Galpha(q) or Galpha(o) blocks Wnt stimulation of casein kinase 2 activation, as does suppression of the phosphoprotein Dishevelled, demonstrating that casein kinase 2 is downstream of heterotrimeric G proteins and Dishevelled. Expression of a constitutively active mutant of either Galpha(q) or Galpha(o) stimulates casein kinase 2 activation and Lef/Tcf-sensitive gene expression. Thus, casein kinase 2 is shown to be regulated by Wnt3a and essential to stimulation of the Frizzled-1/beta-catenin/Lef-Tcf pathway.
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