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.

A zyxin-nectin interaction facilitates zyxin localization to cell-cell adhesions.

Cell-cell junction remodeling is associated with dramatic actin reorganizations. Several actin regulatory systems have been implicated in actin remodeling events as cell-cell contacts are assembled and disassembled, including zyxin/LPP-VASP complexes. These complexes facilitate strong cell-cell adhesion by maintaining actin-membrane connections. It has been proposed that zyxin and LPP localize to cell-cell junctions via a well-defined interaction with alpha-actinin. This was recently confirmed for LPP, but zyxin localization at cell-cell contacts occurs independently of alpha-actinin binding. Here we seek to map the zyxin sequence responsible for localization to cell-cell contacts and identify the protein that docks zyxin at this cellular location. Previous results have shown that a zyxin fragment excluding the alpha-actin binding site and the LIM domains (amino acids 51-392) can independently localize to cell-cell contacts. Here, expression of smaller zyxin fragments show that zyxin localization requires amino acids 230-280. A yeast-two-hybrid screen, using the central region of zyxin as bait, resulted in the identification of the cell-cell adhesion receptor nectin-4 as a zyxin binding partner. Further demonstrating zyxin-nectin interactions, zyxin binds the intracellular domain of nectin-2 in vitro. Depletion of nectin-2 from L cells expressing E-cadherin results in a loss of zyxin localization to cell-cell contacts, demonstrating that the zyxin-nectin interaction plays a critical role in zyxin targeting to these sites.

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.

Phosphorylation-Induced Motor Shedding Is Required at Mitosis for Proper Distribution and Passive Inheritance of Mitochondria.

While interphase mitochondria associate with microtubules, mitotic mitochondria dissociate from spindle microtubules and localize in the cell periphery. Here, we show that this redistribution is not mediated by mitochondrial active transport or tethering to the cytoskeleton. Instead, kinesin and dynein, which link mitochondria to microtubules, are shed from the mitochondrial surface. Shedding is driven by phosphorylation of mitochondrial and cytoplasmic targets by CDK1 and Aurora A. Forced recruitment of motor proteins to mitotic mitochondria to override this shedding prevents their proper symmetrical distribution and disrupts the balanced inheritance of mitochondria to daughter cells. Moreover, when mitochondria with bound dynein bind to the mitotic spindle, they arrest cell-cycle progression and produce binucleate cells. Thus, our results show that the regulated release of motor proteins from the mitochondrial surface is a critical mitotic event.

Coordinate activation of Shh and PI3K signaling in PTEN-deficient glioblastoma: new therapeutic opportunities.

In glioblastoma, phosphatidylinositol 3-kinase (PI3K) signaling is frequently activated by loss of the tumor suppressor phosphatase and tensin homolog (PTEN). However, it is not known whether inhibiting PI3K represents a selective and effective approach for treatment. We interrogated large databases and found that sonic hedgehog (SHH) signaling is activated in PTEN-deficient glioblastoma. We demonstrate that the SHH and PI3K pathways synergize to promote tumor growth and viability in human PTEN-deficient glioblastomas. A combination of PI3K and SHH signaling inhibitors not only suppressed the activation of both pathways but also abrogated S6 kinase (S6K) signaling. Accordingly, targeting both pathways simultaneously resulted in mitotic catastrophe and tumor apoptosis and markedly reduced the growth of PTEN-deficient glioblastomas in vitro and in vivo. The drugs tested here appear to be safe in humans; therefore, this combination may provide a new targeted treatment for glioblastoma.