ARHGAP21 protein, a new partner of α-tubulin involved in cell-cell adhesion formation and essential for epithelial-mesenchymal transition.

Cell-cell adhesions and the cytoskeletons play important and coordinated roles in cell biology, including cell differentiation, development, and migration. Adhesion and cytoskeletal dynamics are regulated by Rho-GTPases. ARHGAP21 is a negative regulator of Rho-GTPases, particularly Cdc42. Here we assess the function of ARHGAP21 in cell-cell adhesion, cell migration, and scattering. We find that ARHGAP21 is localized in the nucleus, cytoplasm, or perinuclear region but is transiently redistributed to cell-cell junctions 4 h after initiation of cell-cell adhesion. ARHGAP21 interacts with Cdc42, and decreased Cdc42 activity coincides with the appearance of ARHGAP21 at the cell-cell junctions. Cells lacking ARHGAP21 expression show weaker cell-cell adhesions, increased cell migration, and a diminished ability to undergo hepatocyte growth factor-induced epithelial-mesenchymal transition (EMT). In addition, ARHGAP21 interacts with α-tubulin, and it is essential for α-tubulin acetylation in EMT. Our findings indicate that ARHGAP21 is a Rho-GAP involved in cell-cell junction remodeling and that ARHGAP21 affects migration and EMT through α-tubulin interaction and acetylation.

Competitive enhancement of HGF-induced epithelial scattering by accessory growth factors.

HGF signaling induces epithelial cells to disassemble cadherin-based adhesion and increase cell motility and invasion, a process termed epithelial-mesenchymal transition (EMT). EMT plays a major role in cancer metastasis, allowing individual cells to detach from the primary tumor, invade local tissue, and colonize distant tissues with new tumors. While invasion of vascular and lymphatic networks is the predominant route of metastasis, nerves also can act as networks for dissemination of cancer cell to distant sites in a process termed perineual invasion (PNI). Signaling between nerves and invasive cancer cells remains poorly understood, as does cellular decision making that selects the specific route of invasion. Here we examine how HGF signaling contributes to PNI using reductionist culture model systems. We find that TGFß, produced by PC12 cells, enhances scattering in response to HGF stimulation, increasing both cell-cell junction disassembly and cell migration. Further, gradients of TGFß induce migratory mesenchymal cells to undergo chemotaxis towards the source of TGFß. Interestingly, VEGF suppresses TGFß-induced enhancement of scattering. These results have broad implications for how combinatorial growth factor signaling contributes to cancer metastasis, suggesting that VEGF and TGFß might modulate HGF signaling to influence route selection during cancer progression.

Zyxin phosphorylation at serine 142 modulates the zyxin head-tail interaction to alter cell-cell adhesion.

Zyxin is an actin regulatory protein that is concentrated at sites of actin-membrane association, particularly cell junctions. Zyxin participates in actin dynamics by binding VASP, an interaction that occurs via proline-rich N-terminal ActA repeats. An intramolecular association of the N-terminal LIM domains at or near the ActA repeats can prevent VASP and other binding partners from binding full-length zyxin. Such a head-tail interaction likely accounts for how zyxin function in actin dynamics, cell adhesion, and cell migration can be regulated by the cell. Since zyxin binding to several partners, via the LIM domains, requires phosphorylation, it seems likely that zyxin phosphorylation might alter the head-tail interaction and, thus, zyxin activity. Here we show that zyxin point mutants at a known phosphorylation site, serine 142, alter the ability of a zyxin fragment to directly bind a separate zyxin LIM domains fragment protein. Further, expression of the zyxin phosphomimetic mutant results in increased localization to cell-cell contacts of MDCK cells and generates a cellular phenotype, namely inability to disassemble cell-cell contacts, precisely like that produced by expression of zyxin mutants that lack the entire regulatory LIM domain region. These data suggest that zyxin phosphorylation at serine 142 results in release of the head-tail interaction, changing zyxin activity at cell-cell contacts.