Summary Mixed lineage kinase 3 (MLK3) is a MAP3K that activates the JNK-dependent MAPK pathways. Here we show that MLK3 is required for cell migration in a manner independent of its role as a MAP3K or MLK3 kinase activity. Rather, MLK3 functions in a regulated way to limit levels of the activated GTPase, Rho, by binding to the Rho activator, p63RhoGEF/GEFT, which, in turn, prevents its activation by Gαq. These findings demonstrate a scaffolding role for MLK3 in controlling the extent of Rho activation that modulates cell migration. Moreover, they suggest that MLK3 functions as a network hub that links a number of signaling pathways.

Wnt proteins constitute a family of secreted signaling molecules that regulate highly conserved pathways essential for development and, when aberrantly activated, drive oncogenesis in a number of human cancers. A key feature of the most widely studied Wnt signaling cascade is the stabilization of cytosolic β-catenin, resulting in β-catenin nuclear translocation and transcriptional activation of multiple target genes. In addition to this canonical, β-catenin-dependent pathway, Wnt3A has also been shown to stimulate RhoA GTPase. While the importance of activated Rho to non-canonical Wnt signaling is well appreciated, the potential contribution of Wnt3A–stimulated RhoA to canonical β-catenin-dependent transcription has not been examined and is the focus of this study. We find that activated Rho is required for Wnt3A–stimulated osteoblastic differentiation in C3H10T1/2 mesenchymal stem cells, a biological phenomenon mediated by stabilized β–catenin. Using expression microarrays and real-time RT-PCR analysis, we show that Wnt3A–stimulated transcription of a subset of target genes is Rho-dependent, indicating that full induction of these Wnt targets requires both β-catenin and Rho activation. Significantly, neither β–catenin stabilization nor nuclear translocation stimulated by Wnt3A is affected by inhibition or activation of RhoA. These findings identify Rho activation as a critical element of the canonical Wnt3A–stimulated, β–catenin-dependent transcriptional program.