Membrane-microfilament interactions in ascites tumor cell microvilli. Identification and isolation of a large microfilament-associated membrane glycoprotein complex

Dasatinib inhibits primary melanoma cell proliferation through morphology-dependent disruption of Src-ERK signaling

New strategies for the treatment of advanced melanoma are urgently required. The RAS/RAF/MAPK pathway and c-Src are deregulated in the majority of malignant melanomas, suggesting that they may interact functionally and are involved in the development and progression of the malignancy. Preclinical studies have demonstrated variable inhibition of melanoma cell growth by dasatinib in vitro. Src may act through different downstream signaling pathways. In the present study, we demonstrate that dasatinib induces changes in cell morphology, characterized by an arborized and contracted appearance, and accompanied by a reduction in cell proliferation in primary melanoma cells. This morphological change is demonstrated to be associated with the inhibition of nuclear translocation of activated ERK1/2. Together, these results indicate that Src may promote cell proliferation through the activation of the ERK signaling pathway in melanoma oncogenesis.

Role of microvillar cell surfaces in the regulation of glucose uptake and organization of energy metabolism

Experimental evidence suggesting a type of glucose uptake regulation prevailing in resting and differentiated cells was surveyed. This type of regulation is characterized by transport-limited glucose metabolism and depends on segregation of glucose transporters on microvilli of differentiated or resting cells. Earlier studies on glucose transport regulation and a recently presented general concept of influx regulation for ions and metabolic substrates via microvillar structures provide the basic framework for this theory. According to this concept, glucose uptake via transporters on microvilli is regulated by changes in the structural organization of the microfilament bundle, which is acting as a diffusion barrier between the microvillar tip compartment and the cytoplasm. Both microvilli formation and the switch of glucose metabolism from "metabolic regulation" to "transport limitation" occur during differentiation. The formation of microvillar cell surfaces creates the essential preconditions to establish the characteristic functions of specialized tissue cells including the coordination between glycolysis and oxidative phosphorylation, regulation of cellular functions by external signals, and Ca(2+) signaling. The proposed concept integrates various aspects of glucose uptake regulation into a ubiquitous cellular mechanism involved in regulation of transmembrane ion and substrate fluxes.

Association of the Ras to mitogen-activated protein kinase signal transduction pathway with microfilaments. Evidence for a p185(neu)-containing cell surface signal transduction particle linking the mitogenic pathway to a membrane-microfilament association site

Microvilli of the aggressive 13762 ascites mammary adenocarcinoma contain a large, microfilament-associated signal transduction particle whose scaffolding is a stable glycoprotein complex (Li, Y., Hua, F., Carraway, K. L., and Carraway, C. A. C. (1999) J. Biol. Chem. 274, 25651-25658) associated with the growth factor receptor p185(neu). The receptor is constitutively tyrosine-phosphorylated in the cells and microvilli, predicting that it should recruit mitogenic pathway components to this membrane-microfilament interaction site. Immunoprecipitation of cell lysates with anti-phosphotyrosine and immunoblotting showed phosphorylated forms of the mitogenic pathway proteins Shc and MAPK in addition to p185(neu), suggesting that the Ras to MAPK mitogenic pathway is activated. Immunoblotting of p185(neu)-containing microvillar fractions revealed the presence in each of stably associated Shc, Grb-2, Sos, Ras, Raf, mitogen-activated protein kinase kinase, and mitogen-activated protein kinase/extracellular signal-regulated kinase, as well as the transcription factor-phosphorylating kinase Rsk. All of these pathway components co-immunoprecipitated with p185(neu) from cleared lysates of microvilli solubilized under microfilament-depolymerizing conditions. The recruitment of constitutively phosphorylated p185(neu) and the activated mitogenic pathway proteins to this membrane-microfilament interaction site provides a physical model for integrating the assembly of the mitogenic pathway with the transmission of growth factor signal to the cytoskeleton. This linkage is probably a requisite step in the global cytoskeleton remodeling accompanying mitogenesis.

The p185(neu)-containing glycoprotein complex of a microfilament-associated signal transduction particle. Purification, reconstitution, and molecular associations with p58(gag) and actin

Microfilaments associate with the microvillar membrane of 13762 ascites mammary adenocarcinoma cells via a large transmembrane complex (TMC) comprising the major glycoproteins TMC-gp120, -110, -80, -65, and -55, the receptor kinase p185(neu), and the cytoplasmic proteins actin and p58(gag), linking the receptor with microfilaments in a signal transduction particle. Immunoblot screening with polyclonal antisera to TMC glycoproteins showed selective epithelial expression in normal rat tissues and epithelially derived tumor cells. The TMC glycoproteins were isolated by solubilization of microfilament core preparations in SDS, dilution, and separation on a concanavalin A-agarose affinity column. The large p185(neu)-containing complex was reconstituted from the column eluate after displacement of SDS with nonionic detergent, demonstrated by gel filtration and co-immunoprecipitation of the glycoproteins with anti-gp55 or anti-p185(neu). Exhaustive biotinylation of the glycoproteins gave a stoichiometry of gp120:gp110:gp80:gp65:gp55 of approximately 1:1:1:0.5:1. Overlay blots with biotinylated actin and in vitro translated, [(35)S]methionine-labeled p58(gag), respectively, showed specific interactions of actin with gp55 and gp120 and of p58(gag) with gp65 and gp55. These results provide evidence for a specific complex of microfilament-associated glycoproteins containing p185(neu) and p58(gag) and suggest a role for the complex in signal transduction scaffolding.

c-Src association with and phosphorylation of p58gag, a membrane- and microfilament-associated retroviral Gag-like protein in a xenotransplantable rat mammary tumor

The retroviral Gag-like protein p58gag expressed in a highly metastatic ascites rat mammary adenocarcinoma has been implicated in cell surface changes contributing to xenotransplantability. p58gag is present in the cells in a plasma membrane- and microfilament-associated signal transduction particle containing Src and is phosphorylated on tyrosine. Overlay analyses and affinity chromatography with glutathione S-transferase (GST) fusion proteins of Src homology-3 (SH3) domains showed direct binding of the Src but not the Crk SH3 domain to p58gag. This association was confirmed by co-immunoprecipitation of partially purified p58gag from ascites cell lysates with platelet Src. Further, a GST-p58gag fusion protein bound full length c-Src from either platelets or c-Src-expressing insect cells. The GST-p58gag fusion protein, but not GST, was phosphorylated by platelet or insect cell-expressed c-Src, but not by a kinase negative c-Src variant. The binding of GST-p58gag to c-Src was almost completely abolished by a 50-fold excess of the GST-SH3 domain of Src, and a parallel decrease in tyrosine phosphorylation of p58gag was observed. These results demonstrate that p58gag is tyrosine-phosphorylated as a consequence of its specific association with c-Src via its SH3 domain. These observations suggest a mechanism by which Gag proteins may contribute to retroviral maturation or pathogenesis through binding and relocalization of SH3 domain-containing proteins such as Src-like tyrosine kinases to sites of association of microfilaments with the plasma membrane.

Interaction of IgG immunoglobulins with the guinea pig peritoneal macrophage Fc gamma receptors. Effect on the association of the receptors with the membrane skeleton and the cytoskeleton

Binding of ligands to cell surface receptors may induce an interaction of the receptors with the cytoskeleton and/or membrane skeleton and decrease the solubility of the receptors in nonionic detergents. Cytochalasins, reagents affecting the structure of microfilaments, inhibit some cell functions induced by cross-linking of the receptors with ligands. Information concerning the function of the cytoskeleton in insolubilization of Fc gamma receptors (Fc gamma R) and in Fc gamma R-mediated signal transmission is rather limited. The aim of this work was to investigate the effect of binding of homologous (guinea pig IgG1 and IgG2) and heterologous (rabbit IgG) immunoglobulins to guinea pig peritoneal macrophages on association of the macrophage Fc gamma receptors with the membrane skeleton and cytoskeleton. Cross-linking the macrophage Fc gamma receptors with immunoglobulin ligands induced insolubilization of the receptors in nonionic detergents suggesting association of the receptors with the membrane skeleton and the cytoskeleton. The ligands showed differential effects depending on a subclass and origin of the IgG used. The process of association of the Fc gamma receptors with the skeletons was fast and did not depend on temperature. Treatment of insoluble complexes with cytochalasin D, DNAse I or colchicine showed that actin microfilaments and microtubules play a role, at least partially, in insolubilization of the cross-linked macrophage Fc gamma receptors. Inhibition of insolubilization of the macrophage Fc gamma receptors by genistein indicated that tyrosine kinases are involved in the process of insolubilization. The association with the skeletons might be a part of the process of transduction of a signal which depended on the subclass and origin of IgG used and on the type of the Fc gamma receptor.

Membrane interactions with the actin cytoskeleton

Recent advances have been made in our understanding of the direct binding of actin to integral membrane proteins. New information has been obtained about indirect actin-membrane associations through spectrin superfamily members and through proteins at the cytoplasmic surfaces of focal contacts and adherens junctions.

Complex intermolecular interactions maintain a stable linkage between the photoreceptor connecting cilium axoneme and plasma membrane

Microtubule-membrane cross-linkers in motile and nonmotile cilia are supramolecular structures, held together by strong interactions between the constituent molecules. We have characterized these interactions in the photoreceptor connecting cilium, where cross-linkers co-fractionate and maintain their in situ location after Triton X-100 extraction of axonemes. In bovine photoreceptor cells, the transmembrane assemblage that is cross-linked to the connecting cilium axoneme contains three high molecular mass glycoconjugates of 425, 600, and 700 kDa (Horst et al., 1987). The relative amounts of the three glycoconjugates, as judged from band intensity in electrophoretograms, depend strongly on sample treatment prior to electrophoresis. The electrophoretic pattern was reproducible after several weeks of storage of the axoneme fraction in extraction buffer containing 50% sucrose. Removal of sucrose from the buffer by dialysis eliminated the 600 kDa and 700 kDa, and decreased the detected amount of the 425 kDa glycoconjugate. When samples were incubated in Laemmli sample buffer at increasing temperatures (23 degrees, 60 degrees, 95 degrees C), a gradual reduction in the intensity of the three bands was observed. The quantitative reduction of high molecular mass glycoconjugates was accompanied by the appearance of novel protein species of lower molecular mass, as detected by lectin and antibody overlays of axonemal transblots. These results suggest that the previously characterized cross-linker glycoconjugates are complex, SDS-resistant multi-molecular conglomerates. We have further used fluorescent lectins to monitor the presence of glycoconjugates on whole-mounted axonemes, in conditions aimed to selectively solubilize the cross-linkers. The cross-linker complexes could not be dissociated from the axoneme by incubation with buffers containing 1 M of either Na2SO4 or NaI. The results indicate that the connecting cilium-specific cross-linker complexes are bound via high-affinity interactions to both axoneme and overlying plasma membrane.

A monoclonal antibody study of protein distribution in the membrane skeleton of the ciliate Pseudomicrothorax

The membrane skeleton, or epiplasm, of the ciliated protozoon Pseudomicrothorax dubius is a chemically and structurally complex layer. It is responsible for the cell shape and the positioning of some cortical organelles. One may expect that its possible morphogenetic role can be achieved only via a regional differentiation of the protein distribution in the epiplasm. We have tried to demonstrate such differentiation by preparing an epiplasm extract, which consists predominantly of concanavalin A (ConA)-positive glycoproteins. This fraction, either untreated or deglycosylated, was used to raise monoclonal antibodies (mAbs), whose specificity was tested on western blots of either untreated or deglycosylated epiplasm. The recognized polypeptides were then localized in situ by fluorescence and electron microscopic immunocytochemistry. Six mAbs are presented here. Four of them are directed against ConA-positive glycoproteins and show a localization of the latter on the outer surface of the epiplasm. The two others are directed against other epiplasmic polypeptides: one is specific for a common epitope shared by most of the epiplasmic proteins, but not by the glycoproteins, and labels the entire membrane skeleton, whereas the other recognizes three minor polypeptides, which seem localized to the inner part of the epiplasm.