The Hippo signalling pathway has emerged as a key regulator of organ size, tissue homeostasis, and patterning. Recent studies have shown that two effectors in this pathway, YAP/TAZ, modulate Wnt/β‐catenin signalling through their interaction with β‐catenin or Dishevelled, depending on biological contexts. Here, we identify a novel mechanism through which Hippo signalling inhibits Wnt/β‐catenin signalling. We show that YAP and TAZ, the transcriptional co‐activators in the Hippo pathway, suppress Wnt signalling without suppressing the stability of β‐catenin but through preventing its nuclear translocation. Our results show that YAP/TAZ binds to β‐catenin, thereby suppressing Wnt‐target gene expression, and that the Hippo pathway‐stimulated phosphorylation of YAP, which induces cytoplasmic translocation of YAP, is required for the YAP‐mediated inhibition of Wnt/β‐catenin signalling. We also find that downregulation of Hippo signalling correlates with upregulation of β‐catenin signalling in colorectal cancers. Remarkably, our analysis demonstrates that phosphorylated YAP suppresses nuclear translocation of β‐catenin by directly binding to it in the cytoplasm. These results provide a novel mechanism, in which Hippo signalling antagonizes Wnt signalling by regulating nuclear translocation of β‐catenin.

Mitogen‐activated protein kinase (MAPK) pathways play central roles in controlling diverse cellular functions. They are finely regulated by several mechanisms, including scaffolding of their components, and phosphorylation/dephosphorylation and compartmentalization of MAPKs. A number of molecules have been identified as regulators involved in these mechanisms. They modulate the magnitude and the specificity of MAPK signaling, and thereby regulate the wide variety of signaling outputs. Recent studies have identified novel functions of the MAPK signaling pathways. It is becoming clear that strict regulation of the MAPK pathways underlies their manifold functions in numerous biological processes. iubmb Life, 58: 312‐317, 2006

Many chemical mediators regulate neutrophil recruitment to inflammatory sites. Although the actions of each chemical mediator have been demonstrated with neutrophils in vitro, how such chemical mediators act cooperatively or counteractively in vivo remains largely unknown. Here, by in vivo two-photon excitation microscopy with transgenic mice expressing biosensors based on Förster resonance energy transfer, we time-lapse–imaged the activities of extracellular signal–regulated kinase (ERK) and protein kinase A (PKA) in neutrophils in inflamed intestinal tissue. ERK activity in neutrophils rapidly increased during spreading on the endothelial cells and showed positive correlation with the migration velocity on endothelial cells or in interstitial tissue. Meanwhile, in the neutrophils migrating in the interstitial tissue, high PKA activity correlated negatively with migration velocity. In contradiction to previous in vitro studies that showed ERK activation by prostaglandin E2 (PGE2) engagement with prostaglandin receptor EP4, intravenous administration of EP4 agonist activated PKA, inhibited ERK, and suppressed migration of neutrophils. The opposite results were obtained using nonsteroidal antiinflammatory drugs (NSAIDs). Therefore, NSAID-induced enteritis may be caused at least partially by the inhibition of EP4 receptor signaling of neutrophils. Our results demonstrate that ERK positively regulates the neutrophil recruitment cascade by promoting adhesion and migration steps.