ErbB-2 signals through Plexin-B1 to promote breast cancer metastasis

Diagnosis of metastatic breast cancer is associated with a very poor prognosis.New therapeutic targets are urgently needed, but their development is hampered by a lack of understanding of the mechanisms leading to tumor metastasis.Exemplifying this is the fact that the approximately 30% of all breast cancers overexpressing the receptor tyrosine kinase ErbB-2 are characterized by high metastatic potential and poor prognosis, but the signaling events downstream of ErbB-2 that drive cancer cell invasion and metastasis remain incompletely understood.Here we show that overexpression of ErbB-2 in human breast cancer cell lines leads to phosphorylation and activation of the semaphorin receptor Plexin-B1.This was required for ErbB-2-dependent activation of the pro-metastatic small GTPases RhoA and RhoC and promoted invasive behavior of human breast cancer cells.In a mouse model of ErbB-2-overexpressing breast cancer, ablation of the gene encoding Plexin-B1 strongly reduced the occurrence of metastases.Moreover, in human patients with ErbB-2-overexpressing breast cancer, low levels of Plexin-B1 expression correlated with good prognosis.Our data suggest that Plexin-B1 represents a new candidate therapeutic target for treating patients with ErbB-2-positive breast cancer. Results Overexpression of ErbB-2 results in activation of Plexin-B1 and Rho GTPases.To test whether overexpression of ErbB-2 is sufficient to phosphorylate and activate Plexin-B1, we overexpressed wild-type or constitutively active ErbB-2 in HEK293 cells.This resulted in tyrosine phosphorylation of Plexin-B1 (Figure 1A) as well as in activation of RhoA (Figure 1B) and RhoC (Figure 1D), but not of RhoB (Figure 1C), independent of a Plexin-B1 ligand.Expression of a Plexin-B1 mutant that lacks the intracellular domain blocked RhoA and RhoC activation, indicating that Plexin-B1 signaling is indeed required for RhoA and RhoC activation downstream of ErbB-2 (Figure 1B and Supplemental Figure 1; supplemental material available online with this article; doi:10.1172/JCI60568DS1).In addition to its ability to mediate Rho signaling, Plexin-B1 has been shown to be a GTPase-activating protein (GAP) for R-Ras (29).Consistent with earlier studies showing that R-RasGAP activity is independent of Plexin-B1 phosphorylation by ErbB-2 (18), overexpression of wild-type or Conflict of interest:The authors have declared that no conflict of interest exists.