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. We have previously demonstrated a role for the IGF-I signaling pathway in myeloma both in vitro and in vivo. Activation of this pathway leads to enhanced proliferation and down regulation of apoptosis. Many of the downstream elements responsible for these two effects have been characterized and both are largely regulated through PI-3K/Akt activity. Recent studies have extended the analysis of IGF-I to the phenomenon of migration. Normal plasma cells and possibly early myeloma cells must first traverse blood vessels to enter the bone marrow where they reside and migrate to secondary bone marrow sites. During the terminal stages of disease, these cells again traverse blood vessels and may seed multiple other organs. Present experiments using a transmigration assay demonstrate that IGF-I promotes the migration and invasion of myeloma cells through either vascular endothelium or bone marrow stromal cell lines. Biochemical analysis revealed that IGF-I treatment lead to activation of both RhoA and PKC mu and inhibition of PI-3K prevented activation, placing both targets down stream of PI-3K. Interestingly, inhibition of Akt did not affect activation of either indicating that the observed activation was not associated with the branch of the PI-3K pathway regulating proliferation and apoptosis. Migration of plasma cells could be abrogated by inhibition of either PKCs or RhoA indicating a requirement for both in this process. These findings suggest a role for IGF-I as a chemotactic factor in myeloma development in addition to previously described proliferative and anti-apoptotic effects. 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 cells as described above for IGF-I. Migration is associated with activation of both RhoA and PKC mu. In contrast to IGF-I mediated migration, PI-3K inhibitors do not block this effect, whereas Rho associated kinase inhibitors block both PKC mu activation and migration. Thus, in Wnt induced migration, activation of PKC mu is regulated by RhoA. 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.
