Epithelial cytoskeletal framework and nuclear matrix-intermediate filament scaffold: three-dimensional organization and protein composition

Cell density and cell shape-related regulation of vimentin and cytokeratin synthesis. Inhibition of vimentin synthesis and appearance of a new 45 kD cytokeratin in dense epithelial cell cultures

The pattern of the intermediate type filament protein synthesis was examined in cultured bovine mammary gland epithelial (BMGE) cells under conditions of varied cell shape and cell-cell contact. In dense monolayer and suspension cultures BMGE cells expressed a new cytokeratin of 45 kD identified as a member of the acidic subfamily of cytokeratins. This polypeptide has a phosphorylated component and is dissociated from the cytokeratins complex in the presence of 6.5 M urea. The mRNA of the new cytokeratin accumulated in dense cell cultures, as revealed by in vitro translation in a cell-free system. In BMGE-H cells that express also vimentin, the synthesis of vimentin decreased dramatically in dense cell cultures, while the synthesis of the 45 kD cytokeratin was maximal under these conditions. The results suggest that the expression of certain cytokeratins and that of vimentin can be coordinately regulated by factors in the cellular environment that effect cell shape and cell surface contacts.

Matrix-cytoskeletal interactions in the developing eye

The embryonic avian corneal epithelium in vitro responds to extracellular matrix (ECM) molecules in either soluble or polymerized form by flattening its basal surface, organizing the basal cortical actin cytoskeleton, and stepping up its production of corneal stroma twofold. Embryonic corneal epithelia, like hepatocytes and mammary gland cells, seem to contain heparan sulfate proteoglycan (HSPG) in their plasmalemma, which may interact with actin on the one hand or underlying collagen on the other. Work on the corneal epithelium suggests that, in addition to HSPG, specific glycoprotein receptors for laminin and collagen exist in the basal plasmalemma and play the critical role in actually organizing the basal epithelial cytoskeleton. As yet, uncharacterized proteins may link such receptors to actin. We suggest that ECM-dependent organization of the cytoskeleton is responsible for ECM enhancement of corneal epithelial differentiation. Cell shape and exogenous ECM also affect mesenchymal cell differentiation. In the case of the corneal fibroblast migrating in collagen gels, an actin cortex present around the elongate cell seems to interact with myosin in the cytosol to bring about pseudopodial extension. Both microtubules and actin microfilaments are involved in fibroblast elongation in collagen gels. It follows from the rules presented in this review that the mesenchymal cell surface is quite different from the epithelial cell surface in its organization. Nevertheless, epithelial cell surface-ECM interaction can be modified in the embryo at particular times to permit predesignated epithelial-mesenchymal transformations, as for example at the primitive streak. Though basal surfaces of definitive, nonmalignant epithelia adhere rather strictly to the rules of epithelium-ECM interaction and do not invade underlying ECM, the environment can be manipulated in vitro to cause these epithelia to send out pseudopodia and give rise aberrantly to mesenchymal cells in collagen gels. Further study of this phenomenon should cast light on the manner in which epithelial and mesenchymal cells organize receptors for matrix molecules on their cell surfaces and develop appropriate cytoskeletal responses to the extracellular matrix.

Mechanisms of hormone action: parallels in receptor-mediated signal propagation for steroid and peptide effectors

The purpose of this contribution is to provide in brief form growing evidence in support of an integrated concept of hormone action that appears to shed fresh light on the information gap between the triggering and the effectuation of outcome of the action of given hormones. In accord with these new concepts there has now arisen a substantial body of data from a wide variety of effectors and target cells that demonstrates an astonishing unity in the actions of hormones of widely dissimilar chemical structure. In a nutshell, it now appears that primary recognition sites for both peptide and steroidal agonists occur at the outer cell surface. For steroid hormones, as exemplified by estradiol-17 beta, these sites possess several of the hallmarks of true receptors. Moreover, capture of this ligand is associated with unmistakable signs of membrane perturbation. And at a still very early stage in the signal propagation sequence, activation of a very limited fraction of the cellular lysosomal population may be identified following the application of steroid, as well as peptide, hormones. In turn, there is mounting evidence for cellular entry and even lysosomal uptake of peptidal effectors, the significance of which is still under debate. Likewise, there occur clear signs of limited reorganization of components of the cellular architecture at the surface, in the cytoplasm, and in the nucleus and its subcompartments, which are consistent with minimal recompartmentation of 'microquanta' of lysosomal constituents. These observations may be made within seconds to minutes following application of tropic hormone of either class to its selective targets, and thus, at times preceding those relatively more distal responses of augmented transcriptional and translational activities.

Epithelial structure revealed by chemical dissection and unembedded electron microscopy

Cytoskeletal structures obtained after extraction of Madin-Darby canine kidney epithelial cell monolayers with Triton X-100 were examined in transmission electron micrographs of cell whole mounts and unembedded thick sections. The cytoskeleton, an ordered structure consisting of a peripheral plasma lamina, a complex network of filaments, and chromatin-containing nuclei, was revealed after extraction of intact cells with a nearly physiological buffer containing Triton X-100. The cytoskeleton was further fractionated by extraction with (NH4)2SO4, which left a structure enriched in intermediate filaments and desmosomes around the nuclei. A further digestion with nuclease and elution with (NH4)2SO4 removed the chromatin. The stable structure that remained after this procedure retained much of the epithelial morphology and contained essentially all of the cytokeratin filaments and desmosomes and the chromatin-depleted nuclear matrices. This structural network may serve as a scaffold for epithelial organization. The cytoskeleton and the underlying nuclear matrix intermediate filament scaffold, when examined in both conventional embedded thin sections and in unembedded whole mounts and thick sections, showed the retention of many of the detailed morphological aspects of the intact cells, which suggests a structural continuum linking the nuclear matrix, the intermediate filament network, and the intercellular desmosomal junctions. Most importantly, the protein composition of each of the four fractions obtained by this sequential procedure was essentially unique. Thus, the proteins constituting the soluble fraction, the cytoskeleton, the chromatin fraction, and the underlying nuclear matrix-intermediate filament scaffold are biochemically distinct.

Tumor promoters induce a specific morphological signature in the nuclear matrix-intermediate filament scaffold of Madin-Darby canine kidney (MDCK) cell colonies

Tumor promoters such as phorbol 12-tetradecanoate 13-acetate (TPA), mezerein, teleocidin, aplysiatoxin, and benzoyl peroxide, although structurally unrelated, induce similar, profound changes in morphology in differentiated epithelial Madin-Darby canine kidney (MDCK) cell colonies. The alteration is evident in the organization of intermediate filaments in intact cells and in whole mounts of the nuclear matrix-intermediate filament (NM-IF) scaffold of the epithelial sheet. This substructure, obtained by salt extraction of the cytoskeletal framework, represents only 5% of the total cell protein but contains all of the intermediate filaments, nuclear matrix, and desmosomal core proteins arranged essentially as in the intact cell. The NM-IF is profoundly reorganized after exposure to TPA and retains the morphological changes observed in intact cells. These include bundling of the intermediate filaments, disruption of cell-cell borders, and marked deformation of the polygonal geometry of epithelia. Thus, TPA and all other complete or second-stage tumor promoters examined have a characteristic morphological signature that is not induced by mitogens, metabolic inhibitors, or agents known to disrupt microtubules or microfilaments. This signature, characteristic of tumor promoters, occurs in the absence of both protein and RNA synthesis. These results suggest that this response is prior to and independent of other biochemical markers for tumor promoters. Of the major filament systems, the cytokeratin network is implicated as an early or possibly primary site of tumor-promoter action because characteristics of the promoted cytoskeletal signature are observed in epithelial colonies after prior exposure to colchicine or cytochalasin D. Despite the massive reorganization of cytoskeletal morphology induced by TPA, the distribution of prelabeled proteins into structural fractions (i.e., cytoskeletal, chromatin, and the NM-IF) remains essentially unchanged. The sensitivity and specificity of the epithelial cell response suggest its possible use as a screen for promoting compounds.