Protein kinase C is differentially stimulated by Wnt and Frizzled homologs in aG-protein-dependent manner

Abstract

In studies of developmental signaling pathways stimulated by the Wnt proteins and their receptors, Xenopus Wnt-5A (Xwnt-5A) and a prospective Wnt receptor, rat Frizzled 2 (Rfz2), have been shown to stimulate inositol signaling and Ca²⁺ fluxes in zebrafish [1], [2], [3]. As protein kinase C (PKC) isoforms can respond to Ca²⁺ signals [4], we asked whether expression of different Wnt and Frizzled homologs modulates PKC. Expression of Rfz2 and Xwnt-5A resulted in translocation of PKC to the plasma membrane, whereas expression of rat Frizzled 1 (Rfz1), which activates a Wnt pathway using β-catenin but not Ca²⁺ fluxes [5], did not. Rfz2 and Xwnt-5A were also able to stimulate PKC activity in an in vitro kinase assay. Agents that inhibit Rfz2-induced signaling through G-protein subunits blocked Rfz2-induced translocation of PKC. To determine if other Frizzled homologs differentially stimulate PKC, we tested mouse Frizzled (Mfz) homologs for their ability to induce PKC translocation relative to their ability to induce the expression of two target genes of β-catenin, siamois and Xnr3. Mfz7 and Mfz8 stimulated siamois and Xnr3 expression but not PKC activation, whereas Mfz3, Mfz4 and Mfz6 reciprocally stimulated PKC activation but not expression of siamois or Xnr3. These results demonstrate that some but not all Wnt and Frizzled signals modulate PKC localization and stimulate PKC activity via a G-protein-dependent mechanism. In agreement with other studies [1], [2], [3], [6], [7] these data support the existence of multiple Wnt and Frizzled signaling pathways in vertebrates.