The Wnt–β-catenin canonical signaling pathway is crucial for normal embryonic development, and aberrant expression of components of this pathway results in oncogenesis. Upon scanning for the mitogen-activated protein kinase (MAPK) pathways that might intersect with the canonical Wnt–β-catenin signaling pathway in response to Wnt3a, we observed a strong activation of p38 MAPK in mouse F9 teratocarcinoma cells. Wnt3a-induced p38 MAPK activation was sensitive to siRNAs against Gαq or Gαs, but not against either Gαo or Gα11. Activation of p38 MAPK is critical for canonical Wnt–β-catenin signaling. Chemical inhibitors of p38 MAPK (SB203580 or SB239063) and expression of a dominant negative-version of p38 MAPK attenuate Wnt3a-induced accumulation of β-catenin, Lef/Tcf-sensitive gene activation, and primitive endoderm formation. Furthermore, epistasis experiments pinpoint p38 MAPK as operating downstream of Dishevelleds. We also demonstrate that chemical inhibition of p38 MAPK restores Wnt3a-attenuated GSK3β kinase activity. We demonstrate the involvement of G-proteins and Dishevelleds in Wnt3a-induced p38 MAPK activation, highlighting a critical role for p38 MAPK in canonical Wnt–β-catenin signaling.

Scribble (SCRIB) localizes to cell–cell junctions and regulates establishment of epithelial cell polarity. Loss of expression of SCRIB functions as a tumor suppressor in Drosophila and mammals; conversely, overexpression of SCRIB promotes epithelial differentiation in mammals. Here, we report that SCRIB is frequently amplified, mRNA overexpressed, and protein is mislocalized from cell–cell junctions in human breast cancers. High levels of SCRIB mRNA are associated with poor clinical prognosis, identifying an unexpected role for SCRIB in breast cancer. We find that transgenic mice expressing a SCRIB mutant [Pro 305 to Leu (P305L)] that fails to localize to cell–cell junctions, under the control of the mouse mammary tumor virus long terminal repeat promoter, develop multifocal hyperplasia that progresses to highly pleomorphic and poorly differentiated tumors with basal characteristics. SCRIB interacts with phosphatase and tensin homolog (PTEN) and the expression of P305L, but not wild-type SCRIB, promotes an increase in PTEN levels in the cytosol. Overexpression of P305L, but not wild-type SCRIB, activates the Akt/mTOR/S6K signaling pathway. Human breast tumors overexpressing SCRIB have high levels of S6K but do not harbor mutations in PTEN or PIK3CA, identifying SCRIB amplification as a mechanism of activating PI3K signaling in tumors without mutations in PIK3CA or PTEN. Thus, we demonstrate that high levels of mislocalized SCRIB functions as a neomorph to promote mammary tumorigenesis by affecting subcellular localization of PTEN and activating an Akt/mTOR/S6kinase signaling pathway. Cancer Res; 74(11); 3180–94. ©2014 AACR.

Significance: Recent advances in DNA sequencing and bioinformatics tools have allowed the large-scale characterization of genetic alterations in human tumors. We exploited genomics data to identify commonly occurring alterations in G-protein–coupled receptors (GPCRs) in triple-negative breast cancer (TNBC), a subset of breast cancer with poor prognosis and lacking effective targeted therapy. We have discovered that the orphan GPCR, G-protein–coupled receptor 161 (GPR161), is overexpressed specifically in TNBC and correlates with poor prognosis. We identify GPR161 as a regulator of mammary epithelial cell proliferation and invasion in a mammalian target of rapamycin signaling-pathway–dependent manner. Down-regulation of GPR161 in a TNBC-derived cell line impairs cell growth, suggesting that GPR161 is a drug target for breast cancer.

In Drosophila, activation of Jun N-terminal Kinase (JNK) mediated by Frizzled and Dishevelled leads to signaling linked to planar cell polarity. A biochemical delineation of WNT-JNK planar cell polarity was sought in mammalian cells, making use of totipotent mouse F9 teratocarcinoma cells that respond to WNT3a via Frizzled-1. The canonical WNT–β-catenin signaling pathway requires both Gα_o and Gα_q heterotrimeric G-proteins, whereas we show that WNT-JNK signaling requires only Gα_o protein. Gα_o propagates the signal downstream through all three Dishevelled isoforms, as determined by epistasis experiments using the Dishevelled antagonist Dapper1 (DACT1). Suppression of either Dishevelled-1 or Dishevelled-3, but not Dishevelled-2, abolishes WNT3a activation of JNK. Activation of the small GTPases RhoA, Rac1 and Cdc42 operates downstream of Dishevelled, linking to the MEKK 1/MEKK 4-dependent cascade, and on to JNK activation. Chemical inhibitors of JNK (SP600125), but not p38 (SB203580), block WNT3a activation of JNK, whereas both the inhibitors attenuate the WNT3a–β-catenin pathway. These data reveal both common and unique signaling elements in WNT3a-sensitive pathways, highlighting crosstalk from WNT3a-JNK to WNT3a–β-catenin signaling.