Melanoma Metastasis Depends on ARF6 Regulation of the Balance Between Beta-Catenin and N-Cadherin

Blocking the activity of the ARF6 (adenosine diphosphate ribosylation factor 6) protein pathway stabilized melanoma cells in culture and reduced melanoma metastasis into the lungs in a mouse model.

ARF6 is a member of the ADP ribosylation factor family of GTP-binding proteins. It has a variety of cellular functions that are frequently involved in trafficking of biological membranes and transmembrane protein cargo. ARF6 has specifically been implicated in endocytosis of plasma membrane proteins and also, to a lesser extent, plasma membrane protein recycling.

Investigators at the University of Utah (Salt Lake City, USA) examined the role of the ARF6 pathway in melanoma especially as to its relation to the balance between beta-catenin and N-cadherin. Beta-catenin has a dual function in cells: fortifying cadherin-based adhesion at the plasma membrane and activating transcription in the nucleus. It is part of a complex of proteins that constitute adherens junctions (AJs). AJs are necessary for the creation and maintenance of epithelial cell layers by regulating cell growth and adhesion between cells. Beta-catenin also anchors the actin cytoskeleton and may be responsible for transmitting the contact inhibition signal that causes cells to stop dividing once the epithelial sheet is complete. The gene that codes for beta-catenin can function as an oncogene. An increase in beta-catenin production has been noted in those people with basal cell carcinoma and leads to the increase in proliferation of related tumors.

Cadherins are members of a family of calcium-dependent cell adhesion proteins that preferentially interact in a homophilic manner in cell-cell interactions. They are type I membrane proteins that contribute to the sorting of heterogeneous cell types. The names of different classes indicate the tissues in which they were first found: E-cadherin is present on many types of epithelial cells; N-cadherin is present on nerve, muscle, and lens cells; and P-cadherin is present on placental and epidermal cells. Typically, cadherins have five similar extracellular domains, the outermost three of which have Ca2+-binding sites, and an intracellular C-terminal domain that interacts with the actin cytoskeleton.

The investigators reported in the March 5, 2013, online edition of the journal Science Signaling that in melanoma cells, the protein WNT5A (wingless-type MMTV integration site family, member 5A) stimulated the disruption of N-cadherin and beta-catenin complexes by activating ARF6. Binding of WNT5A to the Frizzled 4–LRP6 (low-density lipoprotein receptor–related protein 6) receptor complex activated ARF6, which liberated beta-catenin from N-cadherin, thus increasing the pool of free beta-catenin, enhancing beta-catenin–mediated transcription, and stimulating cellular invasion.

Inhibition of ARF6 with the small molecule SecinH3 stabilized adherens junctions, blocked beta-catenin signaling and invasiveness of melanoma cells in culture, and reduced spontaneous pulmonary metastasis in mice.

“Metastasis is what makes melanoma lethal, but our knowledge of how melanoma spreads is limited,” said first author Dr. Allie Grossmann, research fellow in molecular genetic pathology at the University of Utah. “By improving our understanding of the cellular machinery responsible for melanoma metastasis, we can better identify targeted therapies that may stop the spread of cancer cells.”

“The surprising discovery that SecinH3 seems to inhibit the entire metastatic signaling cascade represents a potential breakthrough that may lead to improvements in the efficacy of current drug therapy. Our findings are exciting because the clinical implications extend beyond melanoma to other cancers, such as breast cancer and a type of brain cancer known as glioblastoma, in which invasion is also mediated by ARF6.”

While SecinH3 is not a suitable compound for treatment of melanoma in human patients, it serves as a useful model of potential small-molecule inhibitors of ARF6. Investigators at the University of Utah will be collaborating with researchers at the biotech company Navigen, Inc. (Salt Lake City, UT, USA) in an effort to better understand the ARF6 signaling pathway and develop new compounds to prevent metastasis in preclinical models of melanoma and other cancers.

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University of Utah
Navigen, Inc.