Plasma cell tumors in humans most commonly occur as multiple myeloma, an incurable form of cancer. Myeloma cells appear to be responsive to a number of growth factors including IL-6 and Insulin-like growth factor I (IGF-I) which likely contribute to both survival and proliferation. One of the hall marks of myeloma is dissemination throughout the bone marrow yet little is known about the mechanisms affecting this process. Wnt proteins have been shown to be critical elements regulating development and inappropriate expression of Wnts has been observed in human cancers. We have recently described activation of the 'canonical' Wnt/beta catenin and the Wnt/RhoA pathways in myeloma plasma cells. Myeloma cells exposed to Wnt-3a undergo striking morphological changes and extensive rearrangement of the actin cytoskeleton. These morphological changes are associated with the Wnt/RhoA pathway and suggest possible alterations in cell motility. Using a transmigration assay, it was demonstrated that Wnt-3a can act as a chemotactic factor promoting the migration/invasion of myeloma cells through vascular endothelial cells or bone marrow stromal cell lines. Migration is associated with activation of both RhoA and PKCs alpha, beta and mu. Rho associated kinase inhibitors block both PKC mu activation and migration. Thus, in Wnt induced migration, activation of PKC mu is regulated by RhoA. Furthermore, co-immunoprecipitation studies revealed association between RhoA and PKC mu and PKCs and upstream elements known as Dishevelleds suggesting a macromolecular signaling complex regulating Wnt signaling. These results indicate that Wnts may also function as migration/invasion promoting factors and thus be important in the movement of myeloma cells during disease progression.
