β-Catenin has a dual function in cells: fortifying cadherin-based adhesion at the plasma membrane and activating transcription in the nucleus. We found that in melanoma cells, WNT5A stimulated the disruption of N-cadherin and β-catenin complexes by activating the guanosine triphosphatase adenosine diphosphate ribosylation factor 6 (ARF6). Binding of WNT5A to the Frizzled 4–LRP6 (low-density lipoprotein receptor–related protein 6) receptor complex activated ARF6, which liberated β-catenin from N-cadherin, thus increasing the pool of free β-catenin, enhancing β-catenin–mediated transcription, and stimulating invasion. In contrast to WNT5A, the guidance cue SLIT2 and its receptor ROBO1 inhibited ARF6 activation and, accordingly, stabilized the interaction of N-cadherin with β-catenin and reduced transcription and invasion. Thus, ARF6 integrated competing signals in melanoma cells, thereby enabling plasticity in the response to external cues. Moreover, small-molecule inhibition of ARF6 stabilized adherens junctions, blocked β-catenin signaling and invasiveness of melanoma cells in culture, and reduced spontaneous pulmonary metastasis in mice, suggesting that targeting ARF6 may provide a means of inhibiting WNT/β-catenin signaling in cancer.

SUMMARY Activating mutations in Gαq proteins, which form the a subunit of certain heterotrimeric G proteins, drive uveal melanoma oncogenesis by triggering multiple downstream signaling pathways, including PLC/PKC, Rho/Rac, and YAP. Here we show that the small GTPase ARF6 acts as a proximal node of oncogenic Gαq signaling to induce all of these downstream pathways as well as β-catenin signaling. ARF6 activates these diverse pathways through a common mechanism—the trafficking of GNAQ and β-catenin from the plasma membrane to cytoplasmic vesicles and the nucleus, respectively. Blocking ARF6 with a small molecule reduces uveal melanoma cell proliferation and tumorigenesis in a mouse model, confirming the functional relevance of this pathway and suggesting a therapeutic strategy for Gα-mediated diseases.

Mps1 is a dual specificity protein kinase that is essential for the bipolar attachment of chromosomes to the mitotic spindle and for maintaining the spindle assembly checkpoint until all chromosomes are properly attached. Mps1 is expressed at high levels during mitosis and is abundantly expressed in cancer cells. Disruption of Mps1 function induces aneuploidy and cell death. We report the identification of MPI-0479605, a potent and selective ATP competitive inhibitor of Mps1. Cells treated with MPI-0479605 undergo aberrant mitosis, resulting in aneuploidy and formation of micronuclei. In cells with wild-type p53, this promotes the induction of a postmitotic checkpoint characterized by the ATM-and RAD3-related-dependent activation of the p53-p21 pathway. In both wild-type and p53 mutant cells lines, there is a growth arrest and inhibition of DNA synthesis. Subsequently, cells undergo mitotic catastrophe and/or an apoptotic response. In xenograft models, MPI-0479605 inhibits tumor growth, suggesting that drugs targeting Mps1 may have utility as novel cancer therapeutics. Mol Cancer Ther; 10(12); 2267-75. Ó2011 AACR.

The low molecular weight protein-tyrosine phosphatase (low M(r) PTPase) is an 18-kDa cytoplasmic enzyme of unknown function that has been previously found in several vertebrates. Using an oligonucleotide probe derived from the active site sequence of the mammalian low M(r) PTPases, a Saccharomyces cerevisiae gene that encodes a homolog of this enzyme was cloned by low stringency hybridization. This gene, LTP1, together with a neighboring gene, TKL1, is shown to be located on the right arm of chromosome XVI. The deduced amino acid sequence of its 161-amino acid residue product shows a 39% average identity with that of the mammalian enzymes. The yeast Ltp1 protein was expressed in Escherichia coli, purified to homogeneity, and shown to possess PTPase activity. The recombinant Ltp1 efficiently hydrolyzes phosphotyrosine and a phosphotyrosine-containing peptide, Tyr531-fyn, but it shows low activity toward phosphoserine and phosphothreonine. The catalytic activity of Ltp1 toward a number of substrates was approximately 30-fold lower than the corresponding values measured for the bovine low M(r) PTPase. However, the yeast enzyme was markedly activated by adenine and some purine nucleosides and nucleotides, including cAMP and cGMP. In the case of adenine, the activity of Ltp1 was increased by approximately 30-fold. The high degree of evolutionary conservation of the low M(r) PTPases implies a significant role for this enzyme. However, neither the disruption of the LTP1 gene nor an approximately 10-fold overexpression of its product in S. cerevisiae caused any apparent phenotypic changes under the conditions tested. No proteins related to Ltp1 could be detected in extracts of the ltp1 null mutant, either by immunoblotting or by gel-filtration analysis accompanied by extended kinetic assays, consistent with the conclusion that LTP1 is the only low M(r) PTPase-encoding gene in S. cerevisiae.

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