Editors' ChoiceWnt signaling

Wnt to YAP pathway

Sci. Signal.  25 Aug 2015:
Vol. 8, Issue 391, pp. ec236
DOI: 10.1126/scisignal.aad2809

The transcriptional regulator β-catenin is activated by some ligands of the Wnt family, referred to as the Wnt/β-catenin pathway or the “canonical” pathway. Wnt signaling also occurs through poorly characterized “alternative” pathways, which can promote or inhibit the Wnt/β-catenin pathway depending on the specific ligand and receptors involved. YAP and TAZ (collectively YAP/TAZ) constitute another family of transcriptional regulators and are controlled through inhibitory phosphorylation by the kinases in the Hippo pathway. Park et al. found that Wnt signaling activated YAP/TAZ through a pathway independent of β-catenin, thereby identifying a Wnt-YAP/TAZ pathway. Compared with control mice, mice genetically engineered to overexpress Wnt1 in the mammary tissue exhibited increased abundance of YAP/TAZ and CTGF, the product of a YAP/TAZ target gene, whereas mice engineered to express a constitutively active form of β-catenin in the mammary tissue did not exhibit these effects. Stimulation of a mammary epithelial cell line with Wnt3a, which can stimulate β-catenin signaling, increased both TAZ and β-catenin. Application of Wnt5a or Wnt3a to various cultured cells or stable expression of Wnt5a or Wnt3a in L cells resulted in the accumulation of dephosphorylated YAP/TAZ, but only Wnt3a application or expression stimulated an increase in β-catenin, and only β-catenin accumulation was blocked by antagonists of Wnt/β-catenin signaling. Wnt5b promoted the nuclear accumulation of YAP/TAZ in mouse embryo fibroblasts (MEFs). Cultured cells produce Wnt, and signaling through β-catenin or alternative pathways can be induced or enhanced by introduction of specific receptors. Expression of the Wnt receptor Frizzled 1 alone, but not Frizzled 2 or Frizzled 5, in HEK293A cells resulted in YAP dephosphorylation and TAZ accumulation without an increase in β-catenin. Frizzled 2 and Frizzled 5 required the coexpression of the co-receptor ROR1 or the adhesion receptor CELSR, another Frizzled co-receptor, to stimulate YAP/TAZ accumulation. Knockdown of Gα12/13 (two related α-subunits of the heterotrimeric G protein family) blocked YAP/TAZ accumulation in HEK293A cells expressing Frizzled 1 or in other cells exposed to Wnt3a or Wnt5b, whereas impairing signaling downstream of or by Gαq/11 or Gαi/o did not. Experiments with double-knockout MEFs for Gαq and Gα11 or Gα12 and Gα13 confirmed that Gα12 or Gα13 was necessary for Wnt5b to stimulate YAP/TAZ accumulation. Gα12/13 signal through the guanosine triphosphatases of the Rho family, which activate YAP/TAZ downstream of G protein–coupled receptors by inhibiting the kinases Lats1 and Lats2 (collectively Lats1/2) and prevent their inhibitory phosphorylation of YAP/TAZ. Impairing Rho activity in various ways or analysis of Wnt-mediated YAP/TAZ activation in cells lacking various components of the core Hippo pathway (Lats1/2, the upstream kinases MST1 and MST2, or the adaptor SAV1) confirmed that RhoA or Rac1 transmitted the signal from Wnt3a to YAP/TAZ by inhibiting Lats1/2 activity. Knockdown of YAP and TAZ blocked Wnt4-stimulated osteogenic differentiation of bone marrow–derived mesenchymal stem cells (MSCs) and the increased migratory behavior of cells expressing Frizzled 1. Expression of constitutively active forms of YAP or TAZ in MCF10A cells blocked the accumulation of β-catenin in response to Wnt3a, consistent with the known inhibitory role of the “alternative” Wnt pathway on the canonical Wnt/β-catenin pathway. Conditioned medium from the cells expressing the constitutively active TAZ mutant blocked Wnt3a-induced β-catenin accumulation, and mRNA analysis revealed that the YAP-overexpressing MCF10A cells or cells expressing the constitutively active YAP or TAZ mutants exhibited enhanced expression of genes encoding inhibitors of Wnt/β-catenin. Analysis of mice with inducible YAP in the liver showed that whereas expression of YAP/TAZ target genes was increased, expression of β-catenin target genes was suppressed. Alternative Wnt signaling promotes adipocyte differentiation, whereas Wnt/β-catenin signaling is inhibitory. Addition of Wnt5b to 3T3-L1 preadipocytes induced the accumulation of YAP/TAZ. Disrupting the interaction of YAP/TAZ with its transcription factor partner TEAD or knocking down TEAD reduced the expression of YAP/TAZ target genes, including those encoding inhibitors of Wnt/β-catenin signaling, and increased the expression of a β-catenin target gene. Not only do these results define a G protein–mediated pathway for Wnt signaling to YAP/TAZ, they also indicate that the Wnt-YAP/TAZ pathway provides a mechanism for the Wnt-mediated inhibition of Wnt/β-catenin signaling.

H. W. Park, Y. C. Kim, B. Yu, T. Moroishi, J.-S. Mo, S. W. Plouffe, Z. Meng, K. C. Lin, F.-X. Yu, C. M. Alexander, C.-Y. Wang, K.-L. Guan, Alternative Wnt signaling activates YAP/TAZ. Cell 162, 780–794 (2015). [PubMed]