Tissue and age-specificity of post-translational modifications of N-cadherin during chick embryo development

TGF-β-dependent reprogramming of amino acid metabolism induces epithelial-mesenchymal transition in non-small cell lung cancers

Epithelial–mesenchymal transition (EMT)—a fundamental process in embryogenesis and wound healing—promotes tumor metastasis and resistance to chemotherapy. While studies have identified signaling components and transcriptional factors responsible in the TGF-β-dependent EMT, whether and how intracellular metabolism is integrated with EMT remains to be fully elucidated. Here, we showed that TGF-β induces reprogramming of intracellular amino acid metabolism, which is necessary to promote EMT in non-small cell lung cancer cells. Combined metabolome and transcriptome analysis identified prolyl 4-hydroxylase α3 (P4HA3), an enzyme implicated in cancer metabolism, to be upregulated during TGF-β stimulation. Further, knockdown of P4HA3 diminished TGF-β-dependent changes in amino acids, EMT, and tumor metastasis. Conversely, manipulation of extracellular amino acids induced EMT-like responses without TGF-β stimulation. These results suggest a previously unappreciated requirement for the reprogramming of amino acid metabolism via P4HA3 for TGF-β-dependent EMT and implicate a P4HA3 inhibitor as a potential therapeutic agent for cancer.

Through metabolome and transcriptome analyses, Nakasuka et al find that TGF-β-induced epithelial–mesenchymal transition (EMT) in non-small cell lung cancer cells is associated with reprogramming of amino acid metabolism. They also identify P4HA3 as a key enzyme involved in these changes altogether providing insights into potential mechanisms of metastasis.

Epithelial phenotype and the RPE: is the answer blowing in the Wnt?

Cells of the human retinal pigment epithelium (RPE) have a regular epithelial cell shape within the tissue in situ, but for reasons that remain elusive the RPE shows an incomplete and variable ability to re-develop an epithelial phenotype after propagation in vitro. In other epithelial cell cultures, formation of an adherens junction (AJ) composed of E-cadherin plays an important early inductive role in epithelial morphogenesis, but E-cadherin is largely absent from the RPE. In this review, the contribution of cadherins, both minor (E-cadherin) and major (N-cadherin), to RPE phenotype development is discussed. Emphasis is placed on the importance for future studies of actin cytoskeletal remodeling during assembly of the AJ, which in epithelial cells results in an actin organization that is characteristically zonular. Other markers of RPE phenotype that are used to gauge the maturation state of RPE cultures including tissue-specific protein expression, protein polarity, and pigmentation are described. An argument is made that RPE epithelial phenotype, cadherin-based cell-cell adhesion and melanization are linked by a common signaling pathway: the Wnt/beta-catenin pathway. Analyzing this pathway and its intersecting signaling networks is suggested as a useful framework for dissecting the steps in RPE morphogenesis. Also discussed is the effect of aging on RPE phenotype. Preliminary evidence is provided to suggest that light-induced sub-lethal oxidative stress to cultured ARPE-19 cells impairs organelle motility. Organelle translocation, which is mediated by stress-susceptible cytoskeletal scaffolds, is an essential process in cell phenotype development and retention. The observation of impaired organelle motility therefore raises the possibility that low levels of stress, which are believed to accompany RPE aging, may produce subtle disruptions of cell phenotype. Over time these would be expected to diminish the support functions performed by the RPE on behalf of photoreceptors, theoretically contributing to aging retinal disease such as age-related macular degeneration (AMD). Analyzing sub-lethal stress that produces declines in RPE functional efficiency rather than overt cell death is suggested as a useful future direction for understanding the effects of age on RPE organization and physiology. As for phenotype and pigmentation, a role for the Wnt/beta-catenin pathway is also suggested in regulating the RPE response to oxidative stress. Exploration of this pathway in the RPE therefore may provide a unifying strategy for advancing our understanding of both RPE phenotype and the consequences of mild oxidative stress on RPE structure and function.

Partial reversal of transformed fusiform phenotype by overexpression of calreticulin

Calreticulin, a Ca²⁺-storage and chaperone protein of the ER, has also been shown to affect cell adhesiveness. To examine the effects of differential expression of calreticulin on cellular adhesiveness, we used L fibroblast cell lines stably expressing either elevated or reduced amounts of full length, ER-targeted calreticulin. Overexpression of calreticulin correlates with an increase in adhesiveness of L fibroblasts such that these transformed cells acquire epithelioid morphology and form an epithelial-cell sheet when crowded. Functionally, the “reversal” of transformed phenotype in L fibroblasts differentially overexpressing calreticulin can be accounted for by changes in levels of expression of N-cadherin and vinculin. Structurally, however, although the form and extent of cell-cell contacts in L fibroblasts overexpressing calreticulin mimicked those in normal epithelia, electron microscopical examination revealed that cell-cell junctions formed by these transformed cells bore only superficial resemblance to those of normal epithelia in culture. Our data imply that overexpression of calreticulin, while partially reverses fusiform transformed phenotype is in itself insufficient to re-establish bona fide zonulae adherens in transformed fibroblasts.

Cell phenotype in normal epithelial cell lines with high endogenous N-cadherin: comparison of RPE to an MDCK subclone

PURPOSE

Unlike most monolayer epithelial cells, cultured RPE are competent to form a zonular adhesion of N- rather than E-cadherin. To determine whether other normal epithelial cells do likewise, cells with high endogenous N-cadherin were cloned from the typically E-cadherin dominant epithelial line Madin-Darby canine kidney cells (MDCK) to analyze cell and junction phenotype in the presence of N-cadherin.

METHODS

A MDCK subclonal line, clone-YH, was selected for high endogenous N-cadherin and was compared with the RPE line hTERT-RPE1 with regard to cell phenotype, cadherin gene expression and cadherin protein distribution, glycosylation state, and catenin complex composition.

RESULTS

In early cultures, hTERT-RPE1 cells are moderately epithelioid with junctional N-cadherin, but clone-YH cells are initially highly fusiform with N-cadherin in multiple sites. With time, N-cadherin in clone-YH becomes deglycosylated, resistant to detergent extraction, and zonular, and cells become epithelioid. Treatment with the N-glycosylation inhibitor tunicamycin induces an epithelioid phenotype in clone-YH, like time in culture but disrupts the hTERT-RPE1 phenotype. N-cadherin traffics to surface membranes and complexes with catenins regardless of cell type or glycosylation state, although catenin complex composition varied, showing enriched alpha-catenin under the cell-type-specific conditions in which N-cadherin was junctional. Clone-YH continued to express E-cadherin as a very minor cadherin, which trafficked to membranes but did not accumulate at junctions.

CONCLUSIONS

RPE cells are not unique in localizing N-cadherin to a zonular adhesion typical of a monolayer epithelium, because even epithelial cells derived from a typically E-cadherin dominant line (clone-YH) form a zonular N-cadherin junction if the protein is abundant. However, there are cell and cadherin differences in mechanisms of cadherin accumulation in a zonular pattern, and a previously unrecognized cell-type-specific role for protein glycosylation in epithelial phenotype development.

Effects of Thimerosal on NGF Signal Transduction and Cell Death in Neuroblastoma Cells

Signaling through neurotrophic receptors is necessary for differentiation and survival of the developing nervous system. The present study examined the effects of the organic mercury compound thimerosal on nerve growth factor signal transduction and cell death in a human neuroblastoma cell line (SH-SY5Y cells). Following exposure to 100 ng/ml NGF and increasing concentrations of thimerosal (1 nM-10 microM), we measured the activation of TrkA, MAPK, and PKC-delta. In controls, the activation of TrkA MAPK and PKC-delta peaked after 5 min of exposure to NGF and then decreased but was still detectable at 60 min. Concurrent exposure to increasing concentrations of thimerosal and NGF for 5 min resulted in a concentration-dependent decrease in TrkA and MAPK phosphorylation, which was evident at 50 nM for TrkA and 100 nM for MAPK. Cell viability was assessed by the LDH assay. Following 24-h exposure to increasing concentrations of thimerosal, the EC50 for cell death in the presence or absence of NGF was 596 nM and 38.7 nM, respectively. Following 48-h exposure to increasing concentrations of thimerosal, the EC50 for cell death in the presence and absence of NGF was 105 nM and 4.35 nM, respectively. This suggests that NGF provides protection against thimerosal cytotoxicity. To determine if apoptotic versus necrotic cell death was occurring, oligonucleosomal fragmented DNA was quantified by ELISA. Control levels of fragmented DNA were similar in both the presence and absence of NGF. With and without NGF, thimerosal caused elevated levels of fragmented DNA appearing at 0.01 microM (apoptosis) to decrease at concentrations >1 microM (necrosis). These data demonstrate that thimerosal could alter NGF-induced signaling in neurotrophin-treated cells at concentrations lower than those responsible for cell death.

Evidence that tyrosine phosphorylation regulates N-cadherin turnover during retinal development

N-cadherin, a member of the cadherin family of calcium-dependent cell adhesion molecules, mediates adhesive and signaling interactions between cells during development. N-Cadherin undergoes dynamic spatiotemporal changes in expression which correlate with morphogenetic movements of cells during organogenesis and histogenesis. We have previously shown that N-cadherin expression during development is regulated by several mechanisms, including mRNA expression, cytokine modulation, and proteolytically mediated turnover, yielding the NCAD90 protein. The present study was directed at determining the extent to which N-cadherin in primary embryonic cells is the target of endogenous kinases and phosphatases, as well as the effects of modulation of these enzymes on NCAD90 expression. The results of phosphoamino acid analyses, peptide mapping, and measurements of N-cadherin and NCAD90 expression in embryonic tissues indicate that N-cadherin is indeed the target of endogenous kinase and phosphatase action, and that modulation of different classes of these enzymes can result in either stimulation or inhibition of NCAD90 production. These results provide a mechanistic explanation for observations that cadherin function is downregulated following expression of exogenously introduced viral tyrosine kinases and provide a function for the tyrosine phosphatases recently found in association with cadherins. The results indicate that N-cadherin expression during retinal development is possibly regulated in part by modulation of its phosphorylation state, the balance of which may determine whether N-cadherin remains stably expressed or is targeted for proteolytically mediated turnover to produce NCAD90.

Development of the visual system of the chick. I. Cell differentiation and histogenesis

This review summarizes present knowledge on the embryonic development of the avian visual projections, based on the domestic chick as a model system. The reductionist goal to understand formation and function of complex neuroanatomical systems on a causal level requires a synthesis of classic developmental biology with recent advances on the molecular mechanisms of cell differentiation and histogenesis. It is the purpose of this article. We are discussing the processes underlying patterning of the anterior neural tube, when the retina and optic tectum are specified and their axial polarity is determined. Then the development of these structures is described from the molecular to the anatomical level. Following sections deal with the establishment of secondary visual connections, and the developmental interactions between compartments of the retinotectal system. Using this latter pathway, from the retina to the optic tectum, many investigations aimed at mechanisms of axonal pathfinding and connectivity have accumulated a vast body of research, which will be covered by a following review.

Cell-cell adhesion molecules and the development of an epithelial phenotype in cultured human retinal pigment epithelial cells

For most epithelial cells, the adherens junction protein E-cadherin is an epithelial morphogen, inducing the development of an epithelial phenotype in vitro after cell contact at confluency. Here retinal pigment epithelial cells (RPE), which lack E-cadherin but express a cadherin that is also found in many non-epithelial cells (N-cadherin), were examined for the ability to produce an epithelial phenotype in vitro. Subpopulations of grossly epithelioid or fusiform cells were selected for analysis from RPE cultures derived from adult human donors. After confluency, epithelioid RPE cells were observed to undergo time-dependent changes that were similar to those previously found in epithelial cells expressing E-cadherin: the cadherin gradually developed a zonular distribution of detergent-resistant protein that co-localized with forming circumferential actin bundles; Na/K ATPase accumulated at cell contact sites, then polarized to its tissue-specific domain (the apical membrane for RPE); the cells formed elevated domes on the impermeant culture substrate. In contrast to cells expressing E-cadherin, these events in RPE required weeks rater than days at confluency. Additional proteins were examined in epithelioid RPE cells revealing that cytokeratins reorganized after confluency producing a zonular array, and several other adhesion proteins (alpha5beta1 integrin, ICAM-1, PECAM-1, NCAM) became enriched at cell-cell contact sites, each developing a distinct pattern at a distinct postconfluency interval. In contrast to epithelioid RPE, in fusiform RPE the adhesion molecules did not develop discrete distribution patterns after confluency, although the same complement of adhesion proteins was expressed. In cells expressing E-cadherin, the absence of epithelial properties is often due to underexpression of the cadherin or of the catenins, adherens junction proteins that link the cadherin to actin. Fusiform RPE, however, were not deficient in these proteins, expressing amounts of N-cadherin, alpha-catenin, beta-catenin, plakoglobin, p120, alpha-actinin and vinculin that were equivalent to epithelioid cells. It appears, therefore, that a subset of epithelial cells that express N-cadherin can produce a highly-developed epithelial phenotype in vitro through a slow morphogenetic process. However, the expression alone of adhesion molecules, including those with a morphoregulatory function in other cells, is insufficient to produce an epithelial phenotype in all cells derived from the pigment epithelium.

Protein insolubility and late-stage morphogenesis in long-term postconfluent cultures of MDCK epithelial cells

Epithelial morphogenesis in vitro has been studied in cultures soon after cell-cell contact at confluency when several actin-associated proteins (fodrin, adherens junction molecules E-cadherin and catenins) localize to specific subcellular domains and become resistant to extraction with non-ionic detergents. Here we demonstrate that early confluency is followed by a long postconfluent period of several weeks during which these proteins and actin itself become progressively enriched in the detergent-resistant fraction of MDCK epithelial cells. Cultures from another tissue (human retinal pigment epithelium) which produces weakly epithelialized monolayers in culture do not exhibit similar late-stage increases in protein insolubility. After confluency some cells in MDCK cultures undergo additional morphogenetic changes giving rise to cord-like structures, and the MDCK adherens junction becomes more stable to disrupting agents. These results indicate that in vitro morphogenesis is not restricted to early confluency in MDCK cells but rather molecular stabilization and dynamic changes in cell shape occur over a protracted postconfluent interval.

N-cadherin expression and function in cultured oligodendrocytes

N-Cadherin is a major cell adhesion molecule that is expressed in the developing nervous system where it has been implicated in neural migration and axon growth. Recently, a role for N-cadherin in oligodendrocyte differentiation has been identified [23]. Oligodendrocyte precursors adhere to N-cadherin and mature rapidly to produce myelin sheets. Since this implies that oligodendrocytes express N-cadherin, we examined the expression of N-cadherin by oligodendrocytes in culture. N-Cadherin was expressed by O-2A progenitors, immature oligodendrocytes and mature oligodendrocytes, but at a lower level than in type 1 astrocytes in the same cultures. On mature oligodendrocytes, the N-cadherin was concentrated on the major processes emerging from the soma. The ability of N-cadherin and merosin to promote oligodendrocyte precursor migration was also studied. Average migration rates were significantly higher on merosin (11.2 microns/h) than on N-cadherin (5.6 microns/h). These results suggest that N-cadherin is not likely to function predominantly as a substrate that stimulates migration of O-2A progenitors, but may be more important in initiating early oligodendrocyte-axon interactions that promote the process of myelination.

Cloning and expression studies of cDNA for a novel Xenopus cadherin (XmN-cadherin), expressed maternally and later neural-specifically in embryogenesis

From a Xenopus tailbud cDNA library, we obtained the cDNA for a novel cadherin which was named as XmN-cadherin (Xenopus maternally expressed neural cadherin). The cDNA consisted of 3690 bp and encoded 922 amino acid residues. XmN-cadherin preserved five extracellular cadherin motifs, a single transmembrane domain, and a cytoplasmic domain, and was closely related by its sequence to R- and N-cadherin. In the adult frog, XmN-cadherin mRNA was detected strongly in ovary, testis, brain, eye, and kidney, and weakly in stomach, and intestine. In the egg, the mRNA occurred as a maternal mRNA at a relatively high level, and its level became very low by the neurula stage, then increased steadily thereafter. Dissection experiments with 8-cell stage and neurula stage embryos revealed that the maternally inherited mRNA was relatively uniformly distributed within the embryo. By a sharp contrast, whole mount in situ hybridization revealed that the zygotically expressed mRNA occurred almost exclusively in neural tissues such as brain, the anterior part of spinal cord, and the optic and otic vesicles. Thus, XmN-cadherin appears to have at least triple functions; it probably contributes in early embryos to cell-type non-specific cell adhesion, but in post-neurula embryos may be responsible for the development and/or maintenance of anterior neural tissues, and may be used in adult frog for the development and/or maintenance of neural, endodermal and reproductive organs.

Adhesion molecules. I: Keratinocyte-keratinocyte interactions; cadherins and pemphigus

During the last few years, considerable progress has been made in our understanding of the structure and function of cadherins and of the pathophysiology of pemphigus. Cadherins are a multiple gene family of Ca(++)-dependent cell adhesion molecules with a typical single-spanning transmembrane structure. Cadherins have two major subfamilies, classic cadherin and desmosomal cadherin. Classic cadherins, including E-, P-, and N-cadherins, are characterized by a homophilic binding specificity. They localize at adherens junctions and mediate physiologic interaction with the involvement of cytoplasmic anchoring molecules, catenins, and the actin-based cytoskeleton network. Desmosomal cadherins, the desmocollins and desmogleins, localize at desmosomes and are linked to the intermediate keratin filaments network via plakoglobin and desmoplakin. Molecular cloning has demonstrated that the autoantigens of both pemphigus vulgaris and pemphigus foliaceus are members of the desmoglein subfamily of the cadherin supergene family. Thus, pemphigus is characterized as an anti-cadherin autoimmune disease. Furthermore, a baculovirus recombinant protein of pemphigus vulgaris antigen was capable of absorbing out the pathogenic autoantibodies from patients' sera, providing a possibility of antigen-specific therapeutic strategies for pemphigus.

Purification and characterization of NCAD90, a soluble endogenous form of N-cadherin, which is generated by proteolysis during retinal development and retains adhesive and neurite-promoting function

The cadherins are calcium-dependent cell adhesion molecules which regulate cell-cell interactions during morphogenesis. During development, cadherin expression is subject to dynamic patterns of regulation. We have previously demonstrated that expression of N-cadherin, the predominant cadherin of neural tissues, is sharply down-regulated during development of the retina and brain during later stages of histogenesis (Lagunowich and Grunwald, Dev Biol 135:158-171, 1989; Lagunowich et al., J Neurosci Res 32:202-208, 1992), and that this down-regulation is due to multiple factors, including decreased mRNA levels and turnover apparently mediated by endogenous metalloproteolytic activity (Roark et al., Development 114:973-984, 1992). In the present study, we describe metabolic studies which provide direct biochemical evidence for turnover of 130-kDa N-cadherin in embryonic retina tissues, yielding a soluble 90-kDa N-terminal fragment. We demonstrate that this form of N-cadherin, which we refer to as NCAD90, accumulates in vivo during development. We further demonstrate that purified NCAD90, obtained from embryonic vitreous humor, retains biological function and promotes cell adhesion and neurite growth in a dose-dependent fashion among chick embryo neural retina cells when present in a substrate-bound form. The morphology of retinal cells and neurites grown on a substrate of NCAD90 differs strikingly from that seen on a laminin substrate, in a manner similar to that described for intact 130-kDa N-cadherin. We conclude that proteolysis of N-cadherin at the cell surface during embryonic retinal histogenesis is an endogenous mechanism for regulating N-cadherin expression which generates a novel and functional form of the protein. The results further indicate that an intact cytoplasmic domain is not essential for all cadherin functions.

Molecular biology of cadherins in the nervous system

Cadherins are cell-cell adhesion molecules belonging to the Ca(2+)-dependent cadherin superfamily. In the last few years the number of cadherins identified in the nervous system has increased considerably. Cadherins are integral membrane glycoproteins. They are structurally closely related and interspecies homologies are high. The function is mediated through a homophilic binding mechanism, and intracellular proteins, directly or indirectly connected to the cadherins and the cytoskeleton, are necessary for cadherin activity. Cadherins have been implicated in segregation and aggregation of tissues at early developmental stages and in growth and guidance of axons during nervous system development. These functions are modified by changes in type(s) and amount of cadherins expressed at different developmental stages. The regulatory elements guiding cadherin expression are currently being elucidated.

Regulation of E-cadherin-mediated adhesion by muscarinic acetylcholine receptors in small cell lung carcinoma

We present the first evidence that adhesion mediated by a member of the cadherin gene family can be regulated by a G protein-coupled receptor. We show that activating the M3 muscarinic acetylcholine receptor (mAChR) rapidly induces E-cadherin-mediated adhesion in a small cell lung carcinoma (SCLC) cell line. This response is inhibited by E-cadherin antibodies, and does not occur in another SCLC cell line which expresses functional mAChR but reduced levels of E-cadherin. Protein kinase C may be involved, since phorbol 12-myristate 13-acetate also induces E-cadherin-mediated aggregation. Immunofluorescence analyses indicate that mAChR activation does not grossly alter E-cadherin surface expression or localization at areas of cell-cell contact, suggesting mAChR activation may increase E-cadherin binding activity. Our findings suggest that G protein-coupled receptors may regulate processes involving cadherin-mediated adhesion, such as embryonic development, neurogenesis, and cancer metastasis.

Intracellular associations of adhesion molecules

Significant advances have recently been made in our understanding of the cytoplasmic anchorage of adhesion molecules. The identification of catenins, a new class of proteins involved in the cytoplasmic anchorage of cadherins that are structurally homologous to other peripheral cytoplasmic proteins, emphasizes the existence of protein families that modulate the function of cell-substrate and cell-cell adhesion molecules.

Cadherin-mediated cell-cell adhesion is perturbed by v-src tyrosine phosphorylation in metastatic fibroblasts

Rat 3Y1 cells acquire metastatic potential when transformed with v-src, and this potential is enhanced by double transformation with v-src and v-fos (Taniguchi, S., T. Kawano, T. Mitsudomi, G. Kimura, and T. Baba. 1986. Jpn. J. Cancer Res. 77:1193-1197). We compared the activity of cadherin cell adhesion molecules of normal 3Y1 cells with that of v-src transformed (SR3Y1) and v-src and v-fos double transformed (fosSR3Y1) 3Y1 cells. These cells expressed similar amounts of P-cadherin, and showed similar rates of cadherin-mediated aggregation under suspended conditions. However, the aggregates or colonies of these cells were morphologically distinct. Normal 3Y1 cells formed compacted aggregates in which cells are firmly connected with each other, whereas the transformed cells were more loosely associated, and could freely migrate out of the colonies. Overexpression of exogenous E-cadherin in these transformed cells had no significant effect on their adhesive properties. We then found that herbimycin A, a tyrosine kinase inhibitor, induced tighter cell-cell associations in the aggregates of the transformed cells. In contrast, vanadate, a tyrosine phosphatase inhibitor, inhibited the cadherin-mediated aggregation of SR3Y1 and fosSR3Y1 cells but had little effect on that of normal 3Y1 cells. These results suggest that v-src-mediated tyrosine phosphorylation perturbs cadherin function directly or indirectly, and the inhibition of tyrosine phosphorylation restores cadherin action to the normal state. We next studied tyrosine phosphorylation on cadherins and the cadherin-associated proteins, catenins. While similar amounts of catenins were expressed in all of these cells, the 98-kD catenin was strongly tyrosine phosphorylated only in SR3Y1 and fosSR3Y1 cells. Cadherins were also weakly tyrosine phosphorylated only in the transformed cells. The tyrosine phosphorylation of these proteins was enhanced by vanadate, and inhibited by herbimycin A. Thus, the tyrosine phosphorylation of the cadherin-catenin system itself might affect its function, causing instable cell-cell adhesion.

Immunohistochemical and biochemical analysis of N-cadherin expression during CNS development

The expression of the calcium-dependent adhesion molecule N-cadherin during chick embryo central nervous system (CNS) development was examined by immunohistochemistry and electrophoresis and immunoblotting. During histogenesis, N-cadherin is expressed at high levels in a uniform fashion in many regions of the CNS. However, during later stages of development, expression becomes restricted to the ependymal cells lining the ventricular system and in the choroid plexus. This down-regulation was confirmed by both immunohistochemical and biochemical techniques. The program of expression lags behind in the cerebellum in concert with the delayed development of this region of the brain. A high level of N-cadherin was found to be expressed in the brainstem and spinal cord floorplate, while a low level was detected at the optic nerve head. The results indicate that while, in general, the program of N-cadherin expression is similar in the retina and the brain, certain structures unique to the eye and brain express locally high or low levels of this adhesion protein.