Elevated expression of pp60c-src alters a selective morphogenetic property of epithelial cells in vitro without a mitogenic effect

Fundamental control of grade-specific colorectal cancer morphology by Src regulation of ezrin-centrosome engagement

The phenotypic spectrum of colorectal cancer (CRC) is remarkably diverse, with seemingly endless variations in cell shape, mitotic figures and multicellular configurations. Despite this morphological complexity, histological grading of collective phenotype patterns provides robust prognostic stratification in CRC. Although mechanistic understanding is incomplete, previous studies have shown that the cortical protein ezrin controls diversification of cell shape, mitotic figure geometry and multicellular architecture, in 3D organotypic CRC cultures. Because ezrin is a substrate of Src tyrosine kinase that is frequently overexpressed in CRC, we investigated Src regulation of ezrin and morphogenic growth in 3D CRC cultures. Here we show that Src perturbations disrupt CRC epithelial spatial organisation. Aberrant Src activity suppresses formation of the cortical ezrin cap that anchors interphase centrosomes. In CRC cells with a normal centrosome number, these events lead to mitotic spindle misorientation, perturbation of cell cleavage, abnormal epithelial stratification, apical membrane misalignment, multilumen formation and evolution of cribriform multicellular morphology, a feature of low-grade cancer. In isogenic CRC cells with centrosome amplification, aberrant Src signalling promotes multipolar mitotic spindle formation, pleomorphism and morphological features of high-grade cancer. Translational studies in archival human CRC revealed associations between Src intensity, multipolar mitotic spindle frequency and high-grade cancer morphology. Collectively, our study reveals Src regulation of CRC morphogenic growth via ezrin-centrosome engagement and uncovers combined perturbations underlying transition to high-grade CRC morphology. © 2020 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

Scribble modulates the MAPK/Fra1 pathway to disrupt luminal and ductal integrity and suppress tumour formation in the mammary gland

Polarity coordinates cell movement, differentiation, proliferation and apoptosis to build and maintain complex epithelial tissues such as the mammary gland. Loss of polarity and the deregulation of these processes are critical events in malignant progression but precisely how and at which stage polarity loss impacts on mammary development and tumourigenesis is unclear. Scrib is a core polarity regulator and tumour suppressor gene however to date our understanding of Scrib function in the mammary gland has been limited to cell culture and transplantation studies of cell lines. Utilizing a conditional mouse model of Scrib loss we report for the first time that Scrib is essential for mammary duct morphogenesis, mammary progenitor cell fate and maintenance, and we demonstrate a critical and specific role for Scribble in the control of the early steps of breast cancer progression. In particular, Scrib-deficiency significantly induced Fra1 expression and basal progenitor clonogenicity, which resulted in fully penetrant ductal hyperplasia characterized by high cell turnover, MAPK hyperactivity, frank polarity loss with mixing of apical and basolateral membrane constituents and expansion of atypical luminal cells. We also show for the first time a role for Scribble in mammalian spindle orientation with the onset of mammary hyperplasia being associated with aberrant luminal cell spindle orientation and a failure to apoptose during the final stage of duct tubulogenesis. Restoring MAPK/Fra1 to baseline levels prevented Scrib-hyperplasia, whereas persistent Scrib deficiency induced alveolar hyperplasia and increased the incidence, onset and grade of mammary tumours. These findings, based on a definitive genetic mouse model provide fundamental insights into mammary duct maturation and homeostasis and reveal that Scrib loss activates a MAPK/Fra1 pathway that alters mammary progenitor activity to drive premalignancy and accelerate tumour progression.

The role of MAPK in drug-induced kidney injury

This paper focuses on the role that mitogen-activated protein kinases (MAPKs) play in drug-induced kidney injury. The MAPKs, of which there are four major classes (ERK, p38, JNK, and ERK5/BMK), are signalling cascades which have been found to be broadly conserved across a wide variety of organisms. MAPKs allow effective transmission of information from the cell surface to the cytosolic or nuclear compartments. Cross talk between the MAPKs themselves and with other signalling pathways allows the cell to modulate responses to a wide variety of external stimuli. The MAPKs have been shown to play key roles in both mediating and ameliorating cellular responses to stress including xenobiotic-induced toxicity. Therefore, this paper will discuss the specific role of the MAPKs in the kidney in response to injury by a variety of xenobiotics and the potential for therapeutic intervention at the level of MAPK signalling across different types of kidney disease.

Tissue organization by cadherin adhesion molecules: dynamic molecular and cellular mechanisms of morphogenetic regulation

This review addresses the cellular and molecular mechanisms of cadherin-based tissue morphogenesis. Tissue physiology is profoundly influenced by the distinctive organizations of cells in organs and tissues. In metazoa, adhesion receptors of the classical cadherin family play important roles in establishing and maintaining such tissue organization. Indeed, it is apparent that cadherins participate in a range of morphogenetic events that range from support of tissue integrity to dynamic cellular rearrangements. A comprehensive understanding of cadherin-based morphogenesis must then define the molecular and cellular mechanisms that support these distinct cadherin biologies. Here we focus on four key mechanistic elements: the molecular basis for adhesion through cadherin ectodomains, the regulation of cadherin expression at the cell surface, cooperation between cadherins and the actin cytoskeleton, and regulation by cell signaling. We discuss current progress and outline issues for further research in these fields.

Heat shock protein 90 inhibitors attenuate LPS-induced endothelial hyperpermeability

Endothelial hyperperme ability leading to vascular leak is an important consequence of sepsis and sepsis-induced lung injury. We previously reported that heat shock protein (hsp) 90 inhibitor pretreatment improved pulmonary barrier dysfunction in a murine model of sepsis-induced lung injury. We now examine the effects of hsp90 inhibitors on LPS-mediated endothelial hyperpermeability, as reflected in changes in transendothelial electrical resistance (TER) of bovine pulmonary arterial endothelial cells (BPAEC). Vehicle-pretreated cells exposed to endotoxin exhibited a concentration-dependent decrease in TER, activation of pp60(Src), phosphorylation of the focal adhesion protein paxillin, and reduced expression of the adherens junction proteins, vascular endothelial (VE)-cadherin and beta-catenin. Pretreatment with the hsp90 inhibitor, radicicol, prevented the decrease in TER, maintained VE-cadherin and beta-catenin expression, and inhibited activation of pp60(Src) and phosphorylation of paxillin. Similarly, when BPAEC hyperpermeability was induced by endotoxin-activated neutrophils, pretreatment of neutrophils and/or endothelial cells with radicicol protected against the activated neutrophil-induced decrease in TER. Increased paxillin phosphorylation and decreased expression of beta-catenin and VE-cadherin were also observed in mouse lungs 12 h after intraperitoneal endotoxin and attenuated in mice pretreated with radicicol. These results suggest that hsp90 plays an important role in sepsis-associated endothelial barrier dysfunction.

Involvement of the SH3 domain in Ca2+-mediated regulation of Src family kinases

When cells are treated with Ca(2+) and Ca(2+)-ionophore, c-Src kinase activity increases, whereas c-Yes kinase activity decreases. This opposite modulation can be reproduced in an in vitro reconstitution assay and is dependent on Ca(2+) and on soluble factors present in cell lysates. Since c-Src and c-Yes share a high degree of homology, with the exception of their N-terminal "unique" domains, their activity was thought to be coordinately regulated. To assess the mechanism of regulation we generated stable cell lines expressing eight different constructs containing wild type c-Src and c-Yes, as well as swaps of the unique domain alone, unique and Src homology 3 (SH3) domains together and the SH3 domain alone. Swapping of the unique domains was not sufficient to reverse the regulation of the chimeric molecules. On the other hand, chimeras containing swaps of the unique plus the SH3 domains displayed reverse regulation, implicating both domains in the regulation of kinase activity by Ca(2+). To rule out the participation of the unique domain, we used chimeric molecules with swapped SH3 domains only and found that the SH3 domain is necessary and sufficient to confer Ca(2+)-mediated regulation of Src and Yes tyrosine kinases.

p120 catenin and phosphorylation: Mechanisms and traits of an unresolved issue

p120 catenin is a scaffold protein that interacts with cadherin cytoplasmic domain and acts as a crucial component of the signalling that regulates the cycle of adherens junction formation and disassembly. Here, we review the nature of stimuli that modulate p120ctn function and are translated as serine/threonine and tyrosine phosphorylation events at this multisite substrate for a variety of protein kinases. We also highlight recent findings that tentatively link phosphorylation of p120ctn to its role as a signal integrator capable to influence the state of the cadherin adhesive bond, the cytoskeleton and cell motility.

Modelling glandular epithelial cancers in three-dimensional cultures

Little is known about how the genotypic and molecular abnormalities associated with epithelial cancers actually contribute to the histological phenotypes observed in tumours in vivo. 3D epithelial culture systems are a valuable tool for modelling cancer genes and pathways in a structurally appropriate context. Here, we review the important features of epithelial structures grown in 3D basement membrane cultures, and how such models have been used to investigate the mechanisms associated with tumour initiation and progression.

Src kinase contributes to the metastatic spread of carcinoma cells

The involvement of Src kinase during carcinoma metastasis has been explored by using the NBT-II rat carcinoma cell line, which can be induced to scatter in vitro through Src activity. Here we show that Src activity was not required for growth of tumors derived from NBT-II cells injected into nude mice. In contrast, the presence of micrometastases was strictly dependent on Src, since the percentage of mice bearing metastases was dramatically reduced by the expression of a dominant-negative mutant of Src (SrcK-) or of Csk, the natural inhibitor of Src. Furthermore, metastatic cells originating from NBT-II cells displayed a Src activity higher than the parental cells, confirming that Src gives a selective advantage during the metastatic process. Finally, anatomopathological analysis of the primary tumors arising from NBT-II cells expressing Csk or SrcK- constructs revealed a highly differentiated epithelial phenotype contrasting with the poor differentiation of tumors derived from parental cells. The differentiated phenotype correlated with the presence of desmosomes at the cell periphery and the absence of vimentin intermediate filaments. Altogether, these data demonstrate that Src activity correlates with the loss of epithelial differentiation concomitantly with the increase of the metastatic potential of carcinoma cells.

Mesoderm-independent regulation of gastrulation movements by the src tyrosine kinase in Xenopus embryo

In vitro studies have demonstrated the involvement of Src kinases in several aspects of cell scattering, including cell dissociation and motility. We have therefore sought to explore their functions in the context of the whole organism. Loss-of-function microinjection studies indicate that the ubiquitous Src, Fyn, and Yes tyrosine kinases are specifically implicated in Xenopus gastrulation movements. Injection of mRNAs coding for dominant negative forms of the ubiquitous members of the Src family, namely Fyn, Src, and Yes, perturbs gastrulation movements, resulting in the inability to close the blastopore. Injection of mRNA coding for Csk, a natural inhibitor of Src kinase activity, produces the same phenotypic alterations. The ubiquitous Src kinases have redundant functions in gastrulation movements since overexpression of one member of the family can compensate for the inhibition of another. Interfering mutants of the Src family also inhibit activin-induced morphogenetic movements of animal cap explants isolated from injected embryos. In contrast, these mutants do not interfere with mesoderm induction, as inferred from the presence of mesoderm derivatives and from the expression of early mesodermal markers in injected embryos. In addition, Src kinase activity measured by an in vitro kinase assay is elevated in gastrulating embryos and in FGF- and activin-treated animal caps, confirming the implication of Src enzymatic activity during gastrulation. Altogether, our results demonstrate that Src kinases are essential components of the machinery that drives gastrulation movements independent of mesoderm induction and suggest that Src activity is primarily implicated in cellular movements that take place during the process of cell intercalation.

pp60(c-src) modulates microvascular endothelial phenotype and in vitro angiogenesis

The tyrosine kinase c-src associates with the platelet-derived growth factor (PDGF) receptor. Overexpression of wild-type c-src, a kinase-negative c-src mutant, and v-src in microvascular endothelial cells modulated the mitogenic effect of PDGF, suggesting that c-src kinase activity inhibits PDGF signals. Analyses of cell morphology in two-dimensional culture revealed changes in cell shape and size induced by the overexpression of c-src proteins. Investigations in three-dimensional culture unveiled a modulatory role of c-src during in vitro angiogenesis. Overexpression of c-src resulted in an increased diameter of tube-like structures, and the number of branching segments was decreased. Expression of the kinase-negative c-src mutant resulted in abortive tube formation consisting of disconnected multicellular fragments. These results indicate that the c-src tyrosine kinase exerts regulatory effects on endothelial proliferation, size, and cytoskeletal organization in two-dimensional culture and on the formation of a differentiated multicellular network in three-dimensional culture.

Development of transgenic Xenopus laevis with a high C-src gene expression

Xenopus laevis larvae with an elevated expression of c-src were generated by mating a transgenic X. laevis male frog carrying proviral Rous sarcoma virus (RSV) long terminal repeat (LTR) and most of the pol gene sequences in its sperm DNA and a normal X. laevis female frog. Offspring (15-20%) with a higher dosage of c-Src, detected in disorganized myotomal musculature and in cerebral and spinal neuronal cells by immunohistochemical analysis, developed abnormally, with edemas (in most cases), head deformities, and eye and axial system defects. In the remaining embryos, a small increase in c-src expression seemed to be compatible with normal embryogenesis. The dosage of c-Src correlated with the dosage of RSV LTR integrated in frog DNA as revealed by Southern and polymerase chain reaction (PCR) analyses. Authenticity of the integrated RSV LTR including enhancer sequence was proved by sequencing. Probing of total RNA from aberrant larvae demonstrated several times higher dosage of c-src mRNA in their tissues than in control tadpoles. We hypothesize that the integrated RSV regulatory sequences can stimulate the expression of c-src proto-oncogene of X. laevis above a threshold that interferes with the early developmental program of frog embryos.

Involvement of c-src in beta-casein expression by mammary epithelial cells

HC11 mouse mammary epithelial cells were stably transfected with c-src or a dominant negative mutant of c-src driven by a MMLV promoter. Prolactin increased c-src activity in control and c-src transfected cells, while dominant negative c-src reduced the prolactin-induced increase in c-src activity. Dominant negative c-src also reduced the ability of insulin, hydrocortisone, and prolactin to induce beta-casein accumulation to levels one-half to one-third that of control cells. This effect was uncorrelated with any change in cell proliferation or laminin accumulation and was observed both in cultures grown to confluency in the presence of EGF and in cultures grown on laminin.

Identification of four distinct pools of catenins in mammalian cells and transformation-dependent changes in catenin distributions among these pools

Catenins are cytoplasmic proteins that were initially identified in a complex with cadherins, a superfamily of transmembrane glycoproteins important for cell adhesion in normal and disease states. We have used gel filtration to identify four complexes of catenins in extracts from normal and transformed epithelial cells. In normal Madin-Darby canine kidney epithelial cells, a significant fraction of alpha- and beta-catenin and plakoglobin co-elute with cadherin in a high molecular weight complex (complex I). A portion of alpha-catenin and the remainder of beta-catenin and plakoglobin co-elute in a high molecular weight complex that does not contain cadherin (complex II). The remainder of alpha-catenin elutes in a low molecular weight fraction (complex III). In extracts from two colon carcinoma cell lines, HCT116 and SW480, beta-catenin elutes in an additional low molecular weight pool (complex IV) not present in Madin-Darby canine kidney cell extracts. In two subclones derived from SW480 cells, SW-E8 and SW-R2, beta-catenin is distributed evenly between high and low molecular weight pools in SW-E8 cells, whereas it elutes primarily in the low molecular weight pool (complex IV) in SW-R2 cells. These changes in beta-catenin elution profiles correlate with an increase in transformed phenotype and decreased cell-cell adhesion in the SW-R2 cells.

Activation of pp60c-src is necessary for human vascular smooth muscle cell migration

BACKGROUND

The most widely distributed nonreceptor tyrosine kinase is pp60c-src (src), yet the role of this intracellular signaling protein in cell migration has not been defined. Given that smooth muscle cell (SMC) migration is essential for the development of intimal hyperplasia, we investigated the importance of src in locomotion of human vascular SMC.

METHODS

SMC migration was evaluated using a microchemotaxis chamber assay and videomicroscopy. Src kinase activity was determined by measuring phosphorylation of a synthetic derivative of p34cdc2, a specific substrate for src. Blocking antibodies to src were introduced using a cytoplasmic microinjection technique.

RESULTS

Stimulation of SMC with platelet-derived growth factor (PDGF)-BB and AB resulted in an increase in src activation, whereas PDGF-AA did not consistently enhance src activity. These findings correlated with the ability of the PDGF isotypes to stimulate SMC chemotaxis; PDGF-BB and AB produced 7.4 +/- 0.3- and 5.3 +/- 0.5-fold increases in SMC chemotaxis, whereas PDGF-AA inhibited chemotaxis. SMC migration in response to PDGF-BB and serum was significantly inhibited by intracellular injection of a blocking antibody.

CONCLUSIONS

Our findings reveal an association between agonist-induced src activation and chemotaxis. Moreover, an antibody that inhibits src activation dramatically inhibits migration of individual SMC. We conclude that activation of src is necessary for SMC migration. Because of its importance in SMC migration, either molecular or pharmacologic inhibitors of src may be useful in the control of intimal hyperplasia.

High-level expression of human c-Src can cause a spherical morphology without loss of anchorage-dependent growth of NIH 3T3 cells

To investigate whether overexpression of human c-Src leads to cell rounding in anchorage-dependent NIH 3T3 fibroblasts, we have established c-Src-inducible cell lines using a lac repressor-operator system. RN1 cells, which expressed c-Src at a high level after induction, exhibited a spherical morphology and ceased to grow in monolayer culture. RN1 cells, however, exhibited neither focus-forming ability nor anchorage-independent growth potential with or without induction. Induced RN1 c-Src was phosphorylated at Ser75, a previously reported spherical cell-associated site, and at Tyr419. These data demonstrated for the first time that highly elevated human c-Src tyrosine kinase activity can cause NIH 3T3 cells to have a spherical morphology without loss of anchorage-dependent growth. The inducible cell line should be useful to study the mechanism for cell rounding by c-Src.

pp60c-src is required for the induction of a quiescent mesangial cell phenotype

The tyrosine kinase c-src associates with growth factor receptors, focal contacts and cytoskeletal proteins and is involved in signaling events. The aim of this study was to investigate the role of src in the regulation of mesangial cell (MC) proliferation and differentiation in three-dimensional (3D) culture in collagen gels. Using retroviral gene transfer we have overexpressed wild-type c-src, a kinase-negative c-src mutant (c-src295) and transforming v-src in MC. The MC differentiation in 3D culture was characterized by the formation of a nonproliferating multicellular network in control cells and in cells expressing wild-type c-src. Immunoblotting demonstrated a rapid down-regulation of the alpha-smooth muscle actin expression. The kinase-negative MC (c-src295) failed to differentiate, maintained a significant proliferative rate, and the alpha-smooth muscle actin expression remained stable during 3D culture. MC transformed with v-src showed a high level of tyrosine phosphorylation and proliferation in 3D culture. Analyses of proteins involved in cell cycle regulation demonstrated dephosphorylation of the retinoblastoma protein (Rb) during 3D culture in control and c-src transfected cells. Expression of v-src resulted in sustained Rb phosphorylation. Zymographic analysis of plasminogen activator (u-PA) revealed an inhibition of u-PA secretion in MC transfected with c-src295. These results indicate that c-src exerts regulatory effects on MC proliferation, cytoskeletal organization, matrix proteases and differentiation. Targeted manipulation of the c-src kinase may be useful in modulating MC behavior in vivo.

c-Src protein expression is increased in human breast cancer. An immunohistochemical and biochemical analysis

In human breast cancer, c-Src activity is elevated compared to normal breast tissue. It is not yet known whether this increase in c-Src activity is accompanied by an increase in c-Src protein expression. In this study, c-Src activity and protein expression were determined in a series of human breast cancers and in normal breast tissue, using immune complex kinase assays and immunoblotting. As the heterogeneity of breast cancer is not taken into account in these biochemical experiments, immunohistochemistry was also used to distinguish between normal and malignant cells. In human breast cancers, the c-Src activity is increased 4- to 30-fold, compared with normal breast tissue. This enhanced activity is accompanied by an increase in c-Src protein expression, as shown by both immunoblotting and immunohistochemistry. Immunohistochemistry indicates that the majority of c-Src appears to be concentrated around the nucleus in malignant cells, whereas in normal cells, it is distributed more evenly in the cytoplasm. These data confirm that c-Src activity is increased in human breast cancer. In addition, this study provides strong immunohistochemical evidence that the c-Src protein is also overexpressed, enabling a distinction to be made between normal and malignant cells.

Loss of MDCK cell alpha 2 beta 1 integrin expression results in reduced cyst formation, failure of hepatocyte growth factor/scatter factor-induced branching morphogenesis, and increased apoptosis

Cellular interactions with collagen in a model of kidney tubulogenesis were investigated using Madin-Darby canine kidney (MDCK) cells in an in vitro morphogenetic system. MDCK cells adhered to collagen types I and IV in a Mg(2+)-dependent manner, typical of the alpha 2 beta 1 integrin. Collagen-Sepharose affinity chromatography and immunoblotting demonstrated the presence and collagen binding activity of the alpha 2 beta 1 integrin on MDCK cells. To assess the function of alpha 2 beta 1 integrin, MDCK cells were transfected with a plasmid pRSV alpha 2′ which allowed the expression of alpha 2-integrin subunit antisense RNA. Three G418-resistant clones showing reduced adhesion to collagen, stable genomic integration of the antisense construct, decreased alpha 2-integrin subunit mRNA and decreased alpha 2-integrin subunit protein expression were selected for analysis in morphogenetic experiments. MDCK cells and plasmid-only control transfectants, cultured in three-dimensional collagen type I gels, showed normal cyst formation, whereas the antisense RNA transfectants showed increased apoptosis and formed small rudimentary cysts. Stimulation with hepatocyte growth factor/scatter factor-containing 3T3 fibroblast-conditioned medium or recombinant hepatocyte growth factor/scatter factor resulted in extensive branching of the preformed control cysts whereas the surviving small cysts formed by antisense expressing cells increased in size but failed to elongate and branch upon stimulation. We conclude that alpha 2 beta 1 integrin collagen interactions play a crucial role in the hepatocyte growth factor/scatter factor-induced tubulogenesis and branching morphogenesis of MDCK cells in collagen gels as well as an important role in cell survival.

Beta-catenin: a common target for the regulation of cell adhesion by Wnt-1 and Src signaling pathways

Beta-catenin is a cytosolic protein originally identified through its association with the cadherin class of cell-adhesion proteins. However, recent studies have demonstrated that there are cadherin-independent pools of beta-catenin and that beta-catenin binds at least one other protein, the product of the tumor-suppressor gene APC. Furthermore, beta-catenin is the target of two signal transduction pathways mediated by the proto-oncogenes src and wnt-1. This raises the possibility that beta-catenin plays a pivotal role in balancing cellular responses to both adhesive and proliferative signals.

Modulation of pp60c-src activity and cellular localization during differentiation of human trophoblast cells in culture

The morphological and functional differentiation of human trophoblast cells ends with the formation of terminally differentiated multinucleated syncytial trophoblasts. This in vivo differentiation is mimicked in vitro during the primary culture of extravillous cytotrophoblasts: isolated mononuclear cytotrophoblasts aggregate and fuse to form syncytia. This in vitro differentiation is associated with an increase in epidermal growth factor receptor (EGF-R) expression and a transitory increase in E-cadherin expression during cell aggregation. In the present study, we investigated the expression of pp60c-src during morphological differentiation of trophoblast cells. Cultures were terminated at various time intervals and pp60c-src was analysed by immunocytochemistry using a specific antibody. In addition, pp60c-src was investigated by western blot analysis and its tyrosine kinase activity was measured concomitantly. In mononuclear cytotrophoblasts, pp60c-src was localized at cell-matrix contacts and during the aggregation of cytotrophoblasts, pp60c-src was distributed on the cell surface at points of cell-cell contact being colocalized with EGF-R and E-cadherin. The kinase activity of the pp60c-src protein increased significantly at day 2 when cells were completely aggregated and started to fuse, and remained elevated while cells underwent further differentiation. Inhibition of pp60c-src by herbimycin A at 0.25 to 1 microgram/ml during the first day of culture was associated with a decreased expression of tyrosine kinase activity of EGF-R and an increase in E-cadherin expression. These data suggest that pp60c-src is involved in the modulation of trophoblast cell aggregation and fusion leading to syncytial formation.

Cytostellin: a novel, highly conserved protein that undergoes continuous redistribution during the cell cycle

Cytostellin, a 240 kDa protein, has been purified from mammalian cells by immunoaffinity chromatography using monoclonal antibody H5. Immunofluorescence microscopy shows diffuse and punctate cytostellin immunoreactivity in interphase nuclei. Nuclease digestion and salt extraction are not required to expose the epitope. The onset of prophase is marked by the appearance of multiple intensely immunofluorescent cytostellin-containing ‘bodies’ within the nucleus. Nuclear disassembly is heralded by the movement of cytostellin bodies from the nucleus to multiple positions throughout the cell. Cytostellin bodies in metaphase, anaphase and telophase cells are widely dispersed, including some in cell processes far removed from the mitotic spindle apparatus. However, a distinct subset of larger, more intensely staining bodies surrounds the mitotic spindle apparatus. Cytostellin bodies remain in the cytoplasm of the daughter cells and disappear after the appearance of nascent nuclei. Cytostellin is immunologically distinct from other nuclear and cytoplasmic proteins, and it has been detected by immunoblot analysis in all species tested from yeast to humans. Based upon these findings, we postulate that cytostellin has a cell cycle-dependent function which is conserved in higher and lower eukaryotic cells.

Quantitative dissociation between EGF effects on c-myc and c-fos gene expression, DNA synthesis, and epidermal growth factor receptor tyrosine kinase activity

The exact relationship between EGF-stimulated tyrosine phosphorylation, induction of the cellular proto-oncogenes c-myc and c-fos, and DNA synthesis remains uncertain. Madin-Darby Canine Kidney (MDCK) cells possess EGF receptor sites with high binding capacity, and in contrast to A431 cells, respond to EGF by increasing DNA synthesis. Following EGF stimulation of intact MDCK cells, there was a rapid and marked increase in the autophosphorylation of the EGF receptor. This was associated with an increase in the tyrosine phosphorylation of a 120 kDa phosphoprotein believed to be an endogenous substrate of this receptor kinase. The ED50 for stimulation of phosphorylation of pp120 was approximately 0.05 nM versus 1.0 nM for receptor autophosphorylation, consistent with amplification of signalling at this step in EGF action. Stimulation of DNA synthesis occurred after 12 to 24 hours and revealed even further amplification with an ED50 of about 0.1 nM. Intermediate between these events was a time-dependent activation of c-fos and c-myc gene expression. However, the ED50 for these processes was approximately 10 nM, indicating a relatively lower sensitivity of EGF for stimulation of proto-oncogene expression. Tyrphostin (RG 50864), a compound reported to inhibit specifically the EGF receptor kinase, completely blocked EGF stimulation of proto-oncogene induction. Interestingly, under the same experimental conditions, EGF receptor autophosphorylation was decreased only 60%. These data, along with the dose-response studies, indicate that proto-oncogene induction requires near maximal stimulation of EGF receptor autophosphorylation. They also suggest that, in MDCK cells, the EGF dependent induction of the c-fos and c-myc genes is not strictly correlated to the extent of EGF receptor autophosphorylation or EGF-stimulated DNA synthesis, and that EGF stimulation of DNA synthesis likely involves additional rate-limiting intermediate steps.

Induction of epithelial tubular morphogenesis in vitro by fibroblast-derived soluble factors

We have designed an in vitro system in which Madin-Darby canine kidney (MDCK) epithelial cells are cocultured in collagen gels with fibroblasts under conditions precluding heterocellular contact. Using this experimental approach, we have obtained evidence that fibroblast-derived soluble factors play a crucial role in the control of epithelial morphogenesis. First, MDCK cells suspended alone in collagen gels form spherical cysts, whereas in the presence of fibroblasts they form branching tubules. Second, MDCK cells grown as a monolayer on fibroblast-containing collagen gels invade the underlying matrix, within which they form a network of tubules. Third, fibroblast-conditioned medium mimics the effects of coculture by eliciting tubulogenesis by MDCK cells. These results demonstrate the involvement of diffusible paracrine factors in morphogenetic epithelial-mesenchymal interactions and provide a strategy for their molecular characterization.

On the principles of functional ordering in biological membranes

Integrating the available data on lipid-protein interactions and ordering in lipid mixtures allows to emanate a refined model for the dynamic organization of biomembranes. An important difference to the fluid mosaic model is that a high degree of spatiotemporal order should prevail also in liquid crystalline, "fluid" membranes and membrane domains. The interactions responsible for ordering the membrane lipids and proteins are hydrophobicity, coulombic forces, van der Waals dispersion, hydrogen bonding, hydration forces and steric elastic strain. Specific lipid-lipid and lipid-protein interactions result in a precisely controlled yet highly dynamic architecture of the membrane components, as well as in its selective modulation by the cell and its environment. Different modes of organization of the compositionally and functionally differentiated domains would correspond to different functional states of the membrane. Major regulators of membrane architecture are proposed to be membrane potential controlled by ion channels, intracellular Ca2+, pH, changes in lipid composition due to the action of phospholipase, cell-cell coupling, as well as coupling of the membrane with the cytoskeleton and the extracellular matrix. Membrane architecture is additionally modulated due to the membrane association of ions, lipo- and amphiphilic hormones, metabolites, drugs, lipid-binding peptide hormones and amphitropic proteins. Intermolecular associations in the membrane and in the membrane-cytoskeleton interface are further selectively controlled by specific phosphorylation and dephosphorylation cascades involving both proteins and lipids, and regulated by the extracellular matrix and the binding of growth factors and hormones to their specific receptor tyrosine kinases. A class of proteins coined architectins is proposed, as a notable example the pp60src kinase. The functional role of architectins would be in causing specific changes in the cytoskeleton-membrane interface, leading to specific configurational changes both in the membrane and cytoskeleton architecture and corresponding to (a) distinct metabolic/differentiation states of the cell, and (b) the formation and maintenance of proper three dimensional membrane structures such as neurites and pseudopods.

Proto-oncogenes in cell differentiation

Proto-oncogene products may be multi-functional proteins with various roles in cell differentiation as well as cell proliferation. The molecular biology of the gene products of three well characterized proto-oncogenes (c-fos, c-myc and c-src) are described, and the roles of three other proto-oncogene products, involved in hormone and growth factor reception, are reviewed.

Tyrosine phosphorylated proteins accumulate in junctional regions of the developing chick neural retina

Antibodies specific for protein phosphotyrosyl residues were used to localize sites of action of tyrosine-specific protein kinases in developing chick neural retina by immunoperoxidase staining. Phosphotyrosine-modified proteins became prominent in growth cone- and process-rich regions of embryonic retina during neuronal differentiation. Maximal levels accumulated in the synaptic layers and limiting membranes of the adult retina, where numerous junctional complexes reside. Two major phosphotyrosine-modified proteins in adult retina (80, 42 kDal) increased markedly during maturation. In contrast, the synaptic layers of optic tectum and other brain regions exhibited low protein phosphotyrosine levels. These results suggest a specific role for protein tyrosine phosphorylation in the retina at sites of synapses and other intercellular junctions.

Mitosis-specific phosphorylation of p60c-src by p34cdc2-associated protein kinase

As cells enter mitosis, the protein-tyrosine kinase, p60c-src, is known to be extensively phosphorylated on threonine in its amino-terminal region. In the present work, extracts of mitotic cells were searched for the protein kinase responsible for this phosphorylation. HeLa cells and Xenopus eggs were found to contain a mitosis-specific protein kinase activity capable of phosphorylating highly purified p60c-src in vitro on threonine residues. Tryptic phosphopeptide maps indicate that the mitotic HeLa kinase phosphorylates the same sites in vitro as those used during mitosis in vivo. In addition, this mitotic HeLa kinase comigrates on gel filtration with p34cdc2-associated histone H1 kinase, a well known regulator of mitotic events. Finally, antibodies to the C-terminal peptide of human p34cdc2 specifically deplete p60c-src-phosphorylating activity from mitotic extracts. These results suggest that p60c-src may act as an effector of p34cdc2 in certain mitotic processes.