Protein kinase C is differentially stimulated by Wnt and Frizzled homologsin a G-protein-dependent manner

In studies of developmental signaling pathways stimulated by the Wnt protei ns and their receptors, Xenopus Wnt-5A (Xwnt-5A) and a prospective Wnt rece ptor, rat Frizzled 2 (Rfz2), have been shown to stimulate inositol signalin g and Ca2+ fluxes in zebrafish [1-3]. As protein kinase C (PKC) isoforms ca n respond to Ca2+ signals [4], we asked whether expression of different Wnt and Frizzled homologs modulates PKC. Expression of Rfz2 and Xwnt-5A result ed in translocation of PKC to the plasma membrane, whereas expression of ra t Frizzled 1 (Rfz1), which activates a Wnt pathway using beta-catenin but n ot Ca2+ fluxes [5], did not. Rfz2 and Xwnt-5A were also able to stimulate P KC activity in an in vitro kinase assay. Agents that inhibit Rfz2-induced s ignaling through G-protein subunits blocked Rfz2-induced translocation of P KC. To determine if other Frizzled homologs differentially stimulate PKC, w e tested mouse Frizzled (Mfz) homologs for their ability to induce PKC tran slocation relative to their ability to induce the expression of two target genes of beta-catenin, siamois and Xnr3. Mfz7 and Mfz8 stimulated siamois a nd Xnr3 expression but not PKC activation, whereas Mfz3, Mfz4 and Mfz6 reci procally stimulated PKC activation but not expression of siamois or Xnr3. T hese results demonstrate that some but not all Wnt and Frizzled signals mod ulate PKC localization and stimulate PKC activity via a G-protein-dependent mechanism. In agreement with other studies [1-3,6,7], these data support t he existence of multiple Wnt and Frizzled signaling pathways in vertebrates .