Distribution of cell surface LETS protein in co-cultures of normal and transformed cells

Evidence for an interaction between the cell surface and intermediate filaments in cultured fibroblasts

Intermediate filaments (IF) were found in close proximity to the plasma membrane in substrate attached baby hamster kidney cells (BHK-21) and chick embryo fibroblasts (CEF) as well as cells removed from their substrate in the absence of trypsin. However, in cells removed with trypsin, it appeared that IF had retracted away from the membrane. In cells with abundant extracellular matrix (ECM), colchicine induced massive cables of IF, which appeared to interact with specialized areas of the inner plasma membrane. In cells lysed to extract most microfilaments and cytoplasmic constituents, the intact IF network which remained was closely associated with the ECM. From these ultrastructural observations it was concluded that IF interact in some way with a "cell membrane complex" defined as comprising the plasma membrane and molecules attached to its inner and outer surfaces. In order to investigate the possibility that components of the membrane complex may co-isolate with IF, native intermediate filaments (NIF) were prepared. In addition to the structural subunits and other associated polypeptides, a approximately 220 kd species which reacted specifically with antibodies directed against the ECM protein fibronectin (FN) was observed; 220 kd was still present after NIF were isolated under pH conditions where FN is more soluble, suggesting that its presence was not simply due to the coprecipitation of two insoluble proteins. Immunofluorescence and immunogold localization confirmed that FN is a component of the cell membrane complex with which IF appeared to interact.

Concurrent modulation of cell surface fibronectin and adhesion to fibronectin in hepatoma cells

Rat hepatoma cells grown in vitro were poorly adhesive to plastic surfaces coated with fibronectin and lacked cell surface fibronectin matrix. They synthesized soluble fibronectin into the medium. The cell surface fibronectin matrix and the ability to attach to fibronectin-coated surface were restored in the 7777 cells upon passage as a tumor in rats and by coculturing these cells with normal liver-derived cells in vitro. Fibronectin matrix and the ability of cells to attach to fibronectin were thus modulated in a coordinated fashion, suggesting that the formation of a cell surface fibronectin matrix is dependent on the cell surface property that enables cells to interact with fibronectin.

Fibronectins--adhesive glycoproteins of cell surface and blood

A recently characterised class of adhesive, high molecular weight glycoproteins is present on the surfaces of cells, in connective tissue matrices, and in extracellular fluids. These proteins may have important roles in cellular adhesion, malignant transformation, reticuloendothelial system function, and embryonic differentiation.

High molecular weight, cell surface-associated glycoprotein (fibronectin) lost in malignant transformation

Fibronectin is a polymorphic glycoprotein found in blood and tissues of vertebrates and in cultures of adherent vertebrate cells. There are several forms of fibronectin is composed of two high molecular weight subunits held together by forms found in tissues and on and around the surfaces of cultured cells. Soluble fibronectin is composed of two high molecular weight subunits held together by disulfide bonds. Insoluble fibronectin may be covalently cross-linked in larger complexes. Fibronectin has affinities for collagen, fibrin, heparin, and cell surfaces. In culture, fibronectin in growth medium may mediate attachment of cells to substratum, and fibronectin synthesized by cells may mediate adhesion to substratum. The widespread occurrence of fibronectin in basal lamina indicates that may different cell types in vivo abut against a fibronectin-containing matrix. Cultured transformed cells usually lack cell-surface fibronectin, also called large, external transformation-sensitive (LETS) protein. The failure of transformed cells to synthesize or bind fibronectin is paralleled (at least in some systems) by failures to synthesize or bind collagen and proteoglycans. Abnormal synthesis of fibronectin and other matrix components and abnormal interactions with the tissue matrix may account for several phenotypic characteristics of transformed cultured cells and for some of the malignant behavior of neoplastic cells in vivo.

Effects of cocultivation with transformed cells on surface proteins of normal cells

Virally transformed fibroblasts do not have on their surface a major protein (large external transformation-sensitive, LETS) which is present in normal cells. Cocultivation of the transformed cells with normal cells whose surface proteins have been prelabelled induces an accelerated release of the LETS protein from the normal cells. We have investigated various conditions which affect this phenomenon. Our results show that alteration of cell surface proteins by cocultivation with the transformed cells is time and dose-dependent and requires cell contact. Serum was depleted at least 99% of plasminogen by affinity chromatography and used in the cocultivation experiments. It was found that activation of plasminogen was not required for the accelerated turnover of the LETS protein. Other diffusible proteases are also unlikely to be involved. The possibility that transformed cells have a membrane bound activity is discussed. The role of plasminogen activation was also tested for its relevance in transformation related proteolysis, growth and morphology of cells.

Studies on intercellular LETS glycoprotein matrices

Intercellular matrices secreted by chick embryo fibroblasts in culture were studied by scanning electron microscopy. Cell-cell contact is a prerequisite for the expression of such matrices. The smallest fiber detected by transmission electron microscopy is 5--10 nm in diameter. These matrix fibers tend to cluster to form bundles. Immunofluorescence and immunoferritin procedures reveal that LETS protein is one of the components of the matrices. The matrices are isolated from other cellular organelles by detergent treatment. More than 90% of the proteins in cell-free matrices are LETS protein, suggesting that the matrices are probably made of only one component--LETS protein. Since the solubilization of matrices requires beta-mercaptoethanol, LETS protein matrices may be the first known polymer system in nature to use disulfide linkage as an intermolecular polymerization vehicle. Collagen does not appear to be involved in such matrices. The LETS protein matrix supports the morphological conversion of rounded cells into spindle-shaped, and also promotes myoblast fusion. It does not, however, exert an effect upon cell growth, the rate of glucose uptake or protease production.