Alpha3beta1-integrin as a critical mediator of the hepatic differentiation response to the extracellular matrix

Targeting integrin pathways: mechanisms and advances in therapy

Integrins are considered the main cell-adhesion transmembrane receptors that play multifaceted roles as extracellular matrix (ECM)-cytoskeletal linkers and transducers in biochemical and mechanical signals between cells and their environment in a wide range of states in health and diseases. Integrin functions are dependable on a delicate balance between active and inactive status via multiple mechanisms, including protein-protein interactions, conformational changes, and trafficking. Due to their exposure on the cell surface and sensitivity to the molecular blockade, integrins have been investigated as pharmacological targets for nearly 40 years, but given the complexity of integrins and sometimes opposite characteristics, targeting integrin therapeutics has been a challenge. To date, only seven drugs targeting integrins have been successfully marketed, including abciximab, eptifibatide, tirofiban, natalizumab, vedolizumab, lifitegrast, and carotegrast. Currently, there are approximately 90 kinds of integrin-based therapeutic drugs or imaging agents in clinical studies, including small molecules, antibodies, synthetic mimic peptides, antibody-drug conjugates (ADCs), chimeric antigen receptor (CAR) T-cell therapy, imaging agents, etc. A serious lesson from past integrin drug discovery and research efforts is that successes rely on both a deep understanding of integrin-regulatory mechanisms and unmet clinical needs. Herein, we provide a systematic and complete review of all integrin family members and integrin-mediated downstream signal transduction to highlight ongoing efforts to develop new therapies/diagnoses from bench to clinic. In addition, we further discuss the trend of drug development, how to improve the success rate of clinical trials targeting integrin therapies, and the key points for clinical research, basic research, and translational research.

Fibrillar Collagen Type I Participates in the Survival and Aggregation of Primary Hepatocytes Cultured on Soft Hydrogels

Liver is an essential organ that carries out multiple functions such as glycogen storage, the synthesis of plasma proteins, and the detoxification of xenobiotics. Hepatocytes are the parenchyma that sustain almost all the functions supported by this organ. Hepatocytes and non-parenchymal cells respond to the mechanical alterations that occur in the extracellular matrix (ECM) caused by organogenesis and regenerating processes. Rearrangements of the ECM modify the composition and mechanical properties that result in specific dedifferentiation programs inside the hepatic cells. Quiescent hepatocytes are embedded in the soft ECM, which contains an important concentration of fibrillar collagens in combination with a basement membrane-associated matrix (BM). This work aims to evaluate the role of fibrillar collagens and BM on actin cytoskeleton organization and the function of rat primary hepatocytes cultured on soft elastic polyacrylamide hydrogels (PAA HGs). We used rat tail collagen type I and Matrigel® as references of fibrillar collagens and BM respectively and mixed different percentages of collagen type I in combination with BM. We also used peptides obtained from decellularized liver matrices (dECM). Remarkably, hepatocytes showed a poor adhesion in the absence of collagen on soft PAA HGs. We demonstrated that collagen type I inhibited apoptosis and activated extracellular signal-regulated kinases 1/2 (ERK1/2) in primary hepatocytes cultured on soft hydrogels. Epidermal growth factor (EGF) was not able to rescue cell viability in conjugated BM but affected cell aggregation in soft PAA HGs conjugated with combinations of different proportions of collagen and BM. Interestingly, actin cytoskeleton was localized and preserved close to plasma membrane (cortical actin) and proximal to intercellular ducts (canaliculi-like structures) in soft conditions; however, albumin protein expression was not preserved, even though primary hepatocytes did not remodel their actin cytoskeleton significantly in soft conditions. This investigation highlights the important role of fibrillar collagens on soft hydrogels for the maintenance of survival and aggregation of the hepatocytes. Data suggest evaluating the conditions that allow the establishment of optimal biomimetic environments for physiology and cell biology studies, where the phenotype of primary cells may be preserved for longer periods of time.

Extracellular Matrix and Integrins in Embryonic Stem Cell Differentiation

Embryonic stem cells (ESCs) are pluripotent cells with great therapeutic potentials. The in vitro differentiation of ESC was designed by recapitulating embryogenesis. Significant progress has been made to improve the in vitro differentiation protocols by toning soluble maintenance factors. However, more robust methods for lineage-specific differentiation and maturation are still under development. Considering the complexity of in vivo embryogenesis environment, extracellular matrix (ECM) cues should be considered besides growth factor cues. ECM proteins bind to cells and act as ligands of integrin receptors on cell surfaces. Here, we summarize the role of the ECM and integrins in the formation of three germ layer progenies. Various ECM–integrin interactions were found, facilitating differentiation toward definitive endoderm, hepatocyte-like cells, pancreatic beta cells, early mesodermal progenitors, cardiomyocytes, neuroectoderm lineages, and epidermal cells, such as keratinocytes and melanocytes. In the future, ECM combinations for the optimal ESC differentiation environment will require substantial study.

Maturation of induced pluripotent stem cell derived hepatocytes by 3D-culture

Induced pluripotent stem cell derived hepatocytes (IPSC-Heps) have the potential to reduce the demand for a dwindling number of primary cells used in applications ranging from therapeutic cell infusions to in vitro toxicology studies. However, current differentiation protocols and culture methods produce cells with reduced functionality and fetal-like properties compared to adult hepatocytes. We report a culture method for the maturation of IPSC-Heps using 3-Dimensional (3D) collagen matrices compatible with high throughput screening. This culture method significantly increases functional maturation of IPSC-Heps towards an adult phenotype when compared to conventional 2D systems. Additionally, this approach spontaneously results in the presence of polarized structures necessary for drug metabolism and improves functional longevity to over 75 days. Overall, this research reveals a method to shift the phenotype of existing IPSC-Heps towards primary adult hepatocytes allowing such cells to be a more relevant replacement for the current primary standard.

Keratin 8/18 modulation of protein kinase C-mediated integrin-dependent adhesion and migration of liver epithelial cells

Hepatocyte and hepatoma cell IFs are made solely of keratins 8/18 (K8/K18). Cell adhesion and migration involve integrin interactions with focal adhesion kinase (FAK) and protein kinase C (PKC). Here we report a new regulatory function for K8/K18 IFs in the PKC-mediated integrin/FAK-dependent adhesion and migration of simple epithelial cells.

Keratins are intermediate filament (IF) proteins of epithelial cells, expressed as pairs in a lineage/differentiation manner. Hepatocyte and hepatoma cell IFs are made solely of keratins 8/18 (K8/K18), the hallmark of all simple epithelia. Cell attachment/spreading (adhesion) and migration involve the formation of focal adhesions at sites of integrin interactions with extracellular matrix, actin adaptors such as talin and vinculin, and signaling molecules such as focal adhesion kinase (FAK) and member(s) of the protein kinase C (PKC) family. Here, we identify the novel PKCδ as mediator of the K8/K18 modulation of hepatoma cell adhesion and migration. We also demonstrate a K8/K18-dependent relationship between PKCδ and FAK activation through an integrin/FAK-positive feedback loop, in correlation with a reduced FAK time residency at focal adhesions. Notably, a K8/K18 loss results to a time course modulation of the receptor of activated C-kinase-1, β1-integrin, plectin, PKC, and c-Src complex formation. Although the K8/K18 modulation of hepatocyte adhesion also occurs through a PKC mediation, these differentiated epithelial cells exhibit minimal migrating ability, in link with marked differences in protein partner content and distribution. Together, these results uncover a key regulatory function for K8/K18 IFs in the PKC-mediated integrin/FAK-dependent adhesion and migration of simple epithelial cells.

Hepatocyte differentiation

Increasingly, research suggests that for certain systems, animal models are insufficient for human toxicology testing. The development of robust, in vitro models of human toxicity is required to decrease our dependence on potentially misleading in vivo animal studies. A critical development in human toxicology testing is the use of human primary hepatocytes to model processes that occur in the intact liver. However, in order to serve as an appropriate model, primary hepatocytes must be maintained in such a way that they persist in their differentiated state. While many hepatocyte culture methods exist, the two-dimensional collagen "sandwich" system combined with a serum-free medium, supplemented with physiological glucocorticoid concentrations, appears to robustly maintain hepatocyte character. Studies in rat and human hepatocytes have shown that when cultured under these conditions, hepatocytes maintain many markers of differentiation including morphology, expression of plasma proteins, hepatic nuclear factors, phase I and II metabolic enzymes. Functionally, these culture conditions also preserve hepatic stress response pathways, such as the SAPK and MAPK pathways, as well as prototypical xenobiotic induction responses. This chapter will briefly review culture methodologies but will primarily focus on hallmark hepatocyte structural, expression and functional markers that characterize the differentiation status of the hepatocyte.

Rapid prototyping as a tool for manufacturing bioartificial livers

Rapid prototyping (RP) technologies are a set of manufacturing processes that can produce very complex structures directly from computer-aided design models without structure-specific tools or knowledge. These technologies might eventually enable the manufacture of human livers to create functional substitutes for treating liver failure or dysfunctionality. However, the approaches used currently face many challenges, such as the complex branched vascular and bile ductular systems and the variety of cell types, matrices and regulatory factors involved in liver development. Here, we discuss the challenges and provide evidence for the usefulness of RP in overcoming them.

Hepatocytes maintain their function on basement membrane formed by epithelial cells

To establish liver tissue engineering, the effective substratum for hepatocytes culture should be developed. Up to now, it is believed that Matrigel, which contains several basement membrane proteins produced by sarcoma cells, is the most effective substratum. Matrigel does not contain extracellular matrix molecules derived from epithelial cells although the space of Disse contains the molecules such as laminin-511/521 (laminin-10/11). Therefore, the basement membrane formed by epithelial cells can be more effective substratum than Matrigel. In this study, we evaluated hepatocytes behavior on basement membrane (rBM) formed by alveolar epithelial cells. The viability of hepatocytes on rBM is higher than that of Matrigel within 5 days. Also, the expression of Cyp1a2 induced by beta-naphthoflavone can be observed in hepatocytes on rBM but not in Matrigel. These results indicate that rBM is a more effective substratum for hepatocyte culture than Matrigel.

Molecular Mechanism of Liver Development and Regeneration

The liver is the central organ for metabolism and has strong regenerative capability. Although the liver has been studied mostly biochemically and histopathologically, genetic studies using gene-targeting technology have identified a number of cytokines, intracellular signaling molecules, and transcription factors involved in liver development and regeneration. In addition, various in vitro systems such as fetal liver explant culture and primary culture of fetal liver cells have been established, and the combination of genetic and in vitro studies has accelerated investigation of liver development. Identification of the cell-surface molecules of liver progenitors has made it possible to identify and isolate liver progenitors, making the liver a unique model for stem cell biology. In this review, we summarize progresses in understanding liver development and regeneration.

The effect of natural extracellular matrix deposited on synthetic polymers on cultured primary hepatocytes

It is well known that natural extracellular matrix (ECM) molecules are deposited on the surface of biomaterials during culture of cells and affect cellular behaviors. However, it has not been fully understood what kinds of ECM molecules are deposited on the surface of biomaterials although the cellular behaviors were affected by deposited ECM. In this study, to investigate the effect of deposited natural ECM on behaviors of hepatocytes cultured on biomaterials such as poly (N-p-vinylbenzyl-4-O-beta-D-galactopyranosyl-D-gluconamide) (PVLA) as a hepatocyte-specific matrix and poly (L-lysine) (PLL) as a non-specific one during the culture of hepatocytes in vitro, we investigated expression pattern of ECM genes and adsorption of ECM molecules onto PVLA- and PLL-coated surfaces. It was found that the expression levels of type I collagen and fibronectin genes in the hepatocytes cultured on PVLA-coated surface were different from them in the hepatocytes cultured on PLL-coated one. Also, the results showed that laminin was dominantly deposited on PVLA-coated surface whereas fibronectin was dominantly deposited on PLL-coated one. Hepatocytes maintained liver-specific functions on PVLA- and laminin-coated surfaces. It is thought that deposited laminin during the culture of hepatocytes affects the liver-specific functions of hepatocytes cultured on PVLA-coated surface.

Immunolocalization of extracellular matrix components and integrins during mouse liver development

Intrahepatic biliary cell differentiation takes place in periportal hepatoblasts under the influence of the subjacent connective tissue, the mechanism of which is still unclear. This study was undertaken to analyze the immunolocalization of extracellular matrix components and their cellular receptors during mouse liver development, with special attention given to biliary differentiation and vascular development. In young fetal mouse liver, primitive structures of sinusoids were developed between hepatic cords associated with hematopoietic cells demonstrated by immunohistochemistry of basal laminar components, the alpha6 integrin subunit, and PECAM-1. Portal veins and hepatic veins showed different staining intensities of alpha2, alpha3, and alpha6 integrin subunits from early stages of development. Anti-beta4 integrin subunit antibodies reacted with portal veins, but not with hepatic veins after perinatal stages. Their different phenotypes may be related to the preferential differentiation of periportal bile ducts. In intrahepatic bile duct development, periportal hepatoblasts adjacent to the connective tissue were immunostained for each basal laminar component on the basal side at almost the same time; alpha3, alpha5, alpha6, and beta4 integrin subunits were immunohistochemically detectable later than the basal laminar components. These staining patterns of intrahepatic bile duct cells clearly differed from those of extrahepatic bile duct cells from the beginning of their development, suggesting that these ducts are of different origins. In conclusion, the vascular structures, including sinusoids, portal veins, and hepatic veins, develop from early stages of liver development, and the extracellular matrix components may play important roles in biliary differentiation and vascular development. Supplementary material for this article can be found on the HEPATOLOGY website (http://interscience.wiley.com/jpages/0270-9139/suppmat/index.html).

Physiological and Pathological Roles of α3β1 Integrin

Alpha3beta1 integrin has been considered to be a mysterious adhesion molecule due to the pleiotropy in its ligand-binding specificity. However, recent studies have identified laminin isoforms as high-affinity ligands for this integrin, and demonstrated that alpha3beta1 integrin plays a number of essential roles in development and differentiation, mainly by mediating the establishment and maintenance of epithelial tissues. Furthermore, alpha3beta1 integrin is also implicated in many other biological phenomena, including cell growth and apoptosis, angiogenesis and neural functions. This integrin receptor forms complexes with various other membrane proteins, such as the transmembrane-4 superfamily proteins (tetraspanins), cytoskeletal proteins and signaling molecules. Recently, lines of evidence have been reported showing that complex formation regulates integrin functions in cell adhesion and migration, signal transduction across cell membranes, and cytoskeletal organization. In addition to these roles in physiological processes, alpha3beta1 integrin performs crucial functions in various pathological processes, especially in wound healing, tumor invasion and metastasis, and infection by pathogenic microorganisms.

Progression of HCC in mice is associated with a downregulation in the expression of hepatocyte nuclear factors

Hepatocyte nuclear factors (HNF) play a critical role in development of the liver. Their roles during liver tumorigenesis and progression of hepatocellular carcinomas (HCC) are, however, poorly understood. To address the role of HNFs in tumor progression, we generated a new experimental model in which a highly differentiated slow-growing transplantable mouse HCC (sgHCC) rapidly gives rise in vivo to a highly invasive fast-growing dedifferentiated variant (fgHCC). This in vivo model has allowed us to investigate the fundamental mechanisms underlying HCC progression. A complete loss of cell polarity, a decrease in cell-cell and cell-extracellular matrix (ECM) adhesion, elevation of telomerase activity, and extinction of liver-specific gene expression accompanies tumor progression. Moreover, cells isolated from fgHCCs acquired the ability to proliferate rapidly in culture. These alterations were coupled with a reduced expression of several liver transcription factors including HNF4, a factor essential for hepatocyte differentiation. Forced re-expression of HNF4alpha1 in cultured fgHCC cells reversed the progressive phenotype and induced fgHCC cells to re-establish an epithelium and reform cell-ECM contacts. Moreover, fgHCC cells that expressed HNF4alpha1 also re-established expression of the profile of liver transcription factors and hepatic genes that are associated with a differentiated hepatocyte phenotype. Importantly, re-expression of HNF4alpha1 in fgHCC reduced the proliferation rate in vitro and diminished tumor formation in congenic recipient mice. In conclusion, loss of HNF4 expression is an important determinant of HCC progression. Forced expression of this factor can promote reversion of tumors toward a less invasive highly differentiated slow-growing phenotype.

Phenobarbital responsiveness as a uniquely sensitive indicator of hepatocyte differentiation status: requirement of dexamethasone and extracellular matrix in establishing the functional integrity of cultured primary rat hepatocytes

We used a serum-free, highly defined primary hepatocyte culture model to investigate the mechanisms whereby dexamethasone (Dex) and extracellular matrix (ECM) coordinate cell differentiation and transcriptional responsiveness to the inducer, phenobarbital (PB). Low nanomolar levels of Dex and dilute concentrations of ECM overlay were essential in the maintenance of normal hepatocyte physiology, as assessed by cell morphology, LDH release, expression of the hepatic nuclear factors C/EBPalpha, -beta, -gamma, HNF-1alpha, -1beta, -4alpha, and RXRalpha, expression of prototypical hepatic marker genes, including albumin and transferrin, and ultimately, cellular capacity to respond to PB. The loss of hepatocyte integrity produced by deficiency of these components correlated with the activation of several stress signaling pathways including the MAPK, SAPK/JNK, and c-Jun signaling pathways, with resulting nuclear recruitment of the activated protein-1 (AP-1) complex. In Dex-deficient cultures, normal cellular function, including the PB induction response, was largely restored in a dose-dependent manner by reintroduction of nanomolar additions of the hormone, in the presence of ECM. Our results demonstrate critical and cooperative roles for Dex and ECM in establishing hepatocyte integrity and in the coordination of an array of liver-specific functions. These studies further establish the PB gene induction response as an exceptionally sensitive indicator of hepatocyte differentiation status.

The role of collagen structure in mitogen stimulation of ERK, cyclin D1 expression, and G1-S progression in rat hepatocytes

Adhesion to type 1 collagen can elicit different cellular responses dependent upon whether the collagen is in a fibrillar form (gel) or monomeric form (film). Hepatocytes adherent to collagen film spread extensively, express cyclin D1, and increase DNA synthesis in response to epidermal growth factor, whereas hepatocytes adherent to collagen gel have increased differentiated function, but lower DNA synthesis. The signaling mechanisms by which different forms of type I collagen modulate cell cycle progression are unknown. When ERK MAP kinase activation was analyzed in hepatocytes attached to collagen film, two peaks of ERK activity were demonstrated. Only the second peak, which correlated with an increase of cyclin D1, was required for G1-S progression. Notably, this second peak of ERK activity was absent in cells adherent to collagen gel, but not required in the presence of exogenous cyclin D1. Expression of activated mutants of the Ras/Raf/MEK signaling pathway in cells adherent to collagen gel restored ERK phosphorylation and DNA synthesis, but differentially affected cell shape. Although Ras, Raf, and MEK all increased expression of cyclin D1 on collagen film, only Ras and Raf significantly up-regulated cyclin D1 levels on collagen gel. These results demonstrate that adhesion to polymerized collagen induces growth arrest by inhibiting the Ras/ERK-signaling pathway to cyclin D1 required in late G1.

Expression of a membrane-bound form of the ferroxidase ceruloplasmin by leptomeningeal cells

Ceruloplasmin is a key enzyme involved in detoxifying ferrous iron, which can generate free radicals. The secreted form of ceruloplasmin is produced by the liver and is abundant in serum. We have previously identified a membrane-bound glycosylphosphatidylinositol (GPI)-anchored form of ceruloplasmin (GPI-Cp) that is expressed by astrocytes in the central nervous system (CNS) (Patel and David. 1997. J Biol Chem 272:20185-20190). We now provide direct evidence that rat leptomeningeal cells, which cover the surface of the brain, also express GPI-Cp. The expression of GPI-Cp on the surface of these cells increases with postnatal development and is regulated in vitro by cell density, time in culture, and various extracellular matrix molecules. The expression of GPI-Cp also appears to be regulated differently in astrocytes and leptomeningeal cells in vitro. The abundant expression of GPI-Cp on the surface of leptomeningeal cells suggests that these cells play a role in antioxidant defense along the surface of the postnatal CNS possibly by detoxifying the cerebrospinal fluid.

Beta-catenin antisense studies in embryonic liver cultures: role in proliferation, apoptosis, and lineage specification

BACKGROUND & AIMS

Wnt/beta-catenin pathway activation occurs during liver growth in hepatoblastomas, hepatocellular cancers, and liver regeneration. The aim of this study was to investigate the role of beta-catenin, a key component of the Wnt pathway, in liver development as well as its normal distribution in developing liver.

METHODS

Embryonic liver cultures and beta-catenin antisense phosphorodiamidate morpholino oligomer (PMO) were used to elucidate the role of beta-catenin in liver development. Livers from embryos at 10 days of gestational development were cultured in the presence of antisense or control PMO for 72 hours and analyzed.

RESULTS

Beta-catenin shows stage-specific localization and distinct distribution compared with known markers in developing liver. A substantial decrease in beta-catenin protein was evident in the organs cultured in the presence of antisense. Beta-catenin inhibition decreased cell proliferation and increased apoptosis in these organ cultures. Presence of antisense resulted in loss of CK19 immunoreactivity of the bipotential stem cells. Beta-catenin inhibition also promoted c-kit immunoreactivity of the hepatocytes.

CONCLUSIONS

We conclude that the PMO antisense to beta-catenin effectively inhibits synthesis of its protein. Beta-catenin modulates cell proliferation and apoptosis in developing liver. It may play a significant role in early biliary lineage commitment of the bipotential stem cells and also seems to be important in hepatocyte maturation.

Mechanisms controlling early development of the liver

Over the last decade significant advances have been made in our understanding of the molecular mechanisms that control early aspects of mammalian liver development. Studies using tissue explant cultures and molecular biology techniques as well as the analysis of transgenic and knockout mice have identified signaling molecules and transcription factors that are necessary for the onset of hepatogenesis. This review presents an overview of these studies and discusses the role of individual factors during hepatic development.

Purification of fetal mouse hepatoblasts by magnetic beads coated with monoclonal anti-e-cadherin antibodies and their in vitro culture

A simple, rapid, and reproducible method of fetal hepatoblast purification was established to investigate mechanisms controlling interactions between hepatoblasts and nonparenchymal cells during liver development. Because E-cadherin is exclusively expressed on the cell membrane of hepatoblasts, magnetic beads coated with monoclonal antibodies to an extracellular epitope of its molecule were used to purify hepatoblasts from a cell suspension prepared from 12.5-day fetal mouse livers. The purity and yield in the hepatoblast fraction prepared in our protocol were more than 90% and approximately 30%, respectively. The nonparenchymal fraction rarely contained hepatoblasts; the rate of hepatoblast contamination in this fraction was less than 1%. Separate cultures of these two fractions were compared with cocultures of both fractions. In culture of the hepatoblast fraction, hepatoblasts formed aggregates similar to a bunch of grapes via their loose adhesion, floating in the medium after 24 h, and dissociated into single cells from the aggregates after 120 h of culture. By contrast, in the mixed culture, the majority of hepatoblasts formed multicellular spheroids after 24 h, and these spheroids changed into monolayer cell sheets after 120 h of culture. The cells comprising these monolayer sheets abundantly expressed albumin and carbamoylphosphate synthase I. In the mixed culture, fibroblastic cells also proliferated extensively with spreading on glass slides and surrounded the hepatoblast or hepatocyte colonies. On the other hand, fibroblastic cells spreading on glass slides decreased gradually in cultures of the nonparenchymal cell fraction alone. These findings indicated that the coexistence of hepatoblasts and nonparenchymal cells may be essential for their mutual survival, proliferation, differentiation, and morphogenesis. The conditioned medium of fetal liver cell cultures could partially replace the effects of the nonparenchymal cells on hepatoblasts in vitro. Our isolation protocol for fetal mouse hepatoblasts using immunobeads can greatly facilitate studies on mechanisms of cell-cell interactions during liver development.

Retinoid signaling controls mouse pancreatic exocrine lineage selection through epithelial-mesenchymal interactions

BACKGROUND & AIMS

The early embryonic pancreas gives rise to exocrine (ducts and acini) and endocrine lineages. Control of exocrine differentiation is poorly understood, but may be a critical avenue through which to manipulate pancreatic ductal carcinoma. Retinoids have been shown to change the character of pancreatic ductal cancer cells to a less malignant phenotype. We have shown that 9-cis retinoic acid (9cRA) inhibits acinar differentiation in the developing pancreas, in favor of ducts, and we wanted to determine the role of retinoids in duct versus acinar differentiation.

METHODS

We used multiple culture systems for the 11-day embryonic mouse pancreas.

RESULTS

Retinoic acid receptor (RAR)-selective agonists mimicked the acinar suppressive effect of 9cRA, suggesting that RAR-RXR heterodimers were critical to ductal differentiation. RARalpha was only expressed in mesenchyme, whereas RXRalpha was expressed in epithelium and mesenchyme. Retinaldehyde dehydrogenase 2, a critical enzyme in retinoid synthesis, was expressed only in pancreatic epithelium. 9cRA did not induce ductal differentiation in the absence of mesenchyme, implicating a requirement for mesenchyme in 9cRA effects. Mesenchymal laminin is necessary for duct differentiation, and retinoids are known to enhance laminin expression. In 9cRA-treated pancreas, immunohistochemistry for laminin showed a strong band of staining around ducts, and blockage of laminin signaling blocked all 9cRA effects. Western blot and RT-PCR of pancreatic mesenchyme showed laminin-beta1 protein and mRNA induction by 9cRA.

CONCLUSIONS

Retinoids regulate exocrine lineage selection through epithelial-mesenchymal interactions, mediated through up-regulation of mesenchymal laminin-1.

Transforming growth factor-beta1 triggers hepatocellular carcinoma invasiveness via alpha3beta1 integrin

Metastasis occurrence in the course of hepatocellular carcinoma (HCC) severely affects prognosis and survival. We have shown that HCC invasive cells express alpha3beta1-integrin whereas noninvasive cells do not. Here we show that transforming growth factor (TGF)-beta1 stimulates alpha3-integrin expression at a transcriptional level in noninvasive HCC cells, causing transformation into a motile and invasive phenotype. Such activities are inhibited by neutralizing anti-alpha3- but not anti-alpha6-integrin monoclonal antibodies. HCC invasive cells secrete abundant levels of active TGF-beta1 in comparison with noninvasive cells, but in the latter, addition of active matrix metalloproteinases-2 increases the concentration of active TGF-beta1. In this way, the cells express alpha3-integrin at a transcriptional level and acquire motility on Ln-5. By contrast, an anti-TGF-beta1-neutralizing antibody reduces alpha3-integrin expression and the invasive ability of HCC invading cells. In HCC patients, TGF-beta1 serum concentrations and alpha3-integrin expression are strongly correlated. The integrin, absent in normal and peritumoral liver parenchyma, is abundantly expressed in HCC primary and metastatic tissue. In particular, patients with metastasis show higher levels of TGF-beta1 serum concentrations and stronger expression of TGF-beta1 and alpha3-integrin in HCC tissues. In conclusion, TGF-beta1 may play an important role in HCC invasiveness by stimulating alpha3-integrin expression, and could therefore be an important target for new therapies.

Maturation of fetal hepatocytes in vitro by extracellular matrices and oncostatin M: induction of tryptophan oxygenase

Previously, we described that embryonic day 14.5 (E14.5) mouse fetal hepatocytes differentiate to express tyrosine amino transferase (TAT) and glucose-6-phosphatase, which are expressed in the perinatal liver, in response to oncostatin M (OSM) or in high-cell-density culture. However, under such conditions, fetal hepatic cells failed to express genes for adult liver-specific enzymes, such as tryptophan oxygenase (TO). Although phenobarbital (PB) and dimethylsulfoxide (DMSO) have been known to maintain the functions of adult hepatocytes in vitro, they failed to induce TO expression in fetal hepatic cells. Thus far, no system has been developed that reproduces terminal differentiation of fetal hepatocytes in vitro. Here, we describe that extracellular matrices derived from Engelbreth-Holm-Swarm sarcoma (EHS) in combination with OSM or high-cell-density culture induced expression of TO as well as cytochrome P450 genes that are involved in detoxification. However, EHS alone was insufficient to induce expression of TO, although it induced TAT expression in fetal hepatocytes. In addition, high-density culture further augmented differentiation. In conclusion, the combination of signals by cytokines, cell-cell contact, and cell-matrix interaction is required for induction of adult liver functions in fetal hepatocytes in vitro. This primary culture system will be useful for studying the mechanism of liver development.

Functional and genomic implications of global gene expression profiles in cell lines from human hepatocellular cancer

Global gene expression profiles in cancer have impacted both classification of tumors and definition of molecular pathways in neoplasia. To explore the possibility of employing human tumor cell lines to obtain information on the functional genomics of the early stages of tumorigenesis, we have characterized variation in gene-expression patterns in a cytogenetically well-defined series of cell lines derived from human hepatocellular carcinoma (HCC). Microarrays containing 6,720 sequence-verified human cDNAs were used in this study. Nineteen well-characterized HCC cell lines were analyzed, and a nontumorigenic liver-derived epithelial cell line (Chang) was used as a reference. Each sample was examined at least twice by switching fluorescent dyes, Cy-5 and Cy-3, and average values of 2 experiments on each sample were used for further analysis. Analysis of the clustered data revealed 2 distinctive subtypes of gene-expression patterns among the 19 cell lines, suggesting a degree of heterogeneity among the gene-expression profiles of cell lines. Remarkably, expression of alpha-fetoprotein (AFP) was highly correlated with the molecular subtypes of HCC. Although the 3 most distinctive gene-expression modules represented the signatures of 2 different subgroups of HCC, most of the cell lines shared many coexpressed genes. However, sets of coexpressed genes that are specific for the subtypes of HCC were identified. Furthermore, our results indicate that the comparison between gene-expression patterns and structural alterations in chromosomes is potentially useful in identifying genes critical in early stages of tumorigenesis. In conclusion, these results not only identified unrecognized subtypes of HCC, but also provided potential molecular markers for each subtype that can be useful for diagnostic and/or therapeutic purposes.

The role of fetal and adult hepatocyte extracellular matrix in the regulation of tissue-specific gene expression in fetal and adult hepatocytes

We explored the effect of extracellular matrix (ECM) produced by fetal and adult hepatocytes on tissue-specific gene expression and proliferation of fetal and adult hepatocytes. Adult hepatocytes ECM strongly induced expression of both albumin and HNF-4 in adult hepatocytes. In contrast, fibroblast ECM reduced the expression of mRNAs for albumin and alpha-fetoprotein in fetal hepatocytes. Adult hepatocytes ECM also increased the activity of liver-specific enzymes of adult hepatocytes (DPP IV and glucose-6-phosphatase) in both fetal and adult hepatocytes, while fetal hepatocyte-derived ECM increased activity of the fetal hepatocyte enzyme GGT in fetal hepatocytes. Fibroblast ECM was inhibitory for the activity of all enzymes assayed. Removal of heparin chains from the various matrices by pretreatment of the ECM with heparinase resulted in reduction of glucose-6-phosphatase and DPP IV in adult hepatocytes. Removal of chondroitin sulfate chains from fetal hepatocyte-derived ECM resulted in loss of induction of GGT in the fetal cells. Fetal hepatocytes proliferated best on adult hepatocyte-derived ECM. Adult hepatocytes showed only modest proliferation on both fetal and adult hepatocytes ECM and their growth was inhibited by fibroblast ECM. In conclusion, adult hepatocyte ECM better supports the expression of adult genes, whereas fetal hepatocyte ECM induced expression of fetal genes. Fibroblast derived-ECM was inhibitory for both proliferation and tissue-specific gene expression in fetal and adult hepatocytes. The data support a role for heparan sulfate being the active element in adult ECM, and chondroitin sulfate being the active element in fetal ECM.

Smad proteins and hepatocyte growth factor control parallel regulatory pathways that converge on beta1-integrin to promote normal liver development

Smads serve as intracellular mediators of transforming growth factor beta (TGF-beta) signaling. After phosphorylation by activated type I TGF-beta receptors, Smad proteins translocate to the nucleus, where they serve as transcription factors and increase or decrease expression of TGF-beta target genes. Mice lacking one copy each of Smad2 and Smad3 suffered midgestation lethality due to liver hypoplasia and anemia, suggesting essential dosage requirements of TGF-beta signal components. This is likely due to abnormal adhesive properties of the mutant hepatocytes, which may result from a decrease in the level of the beta1-integrin and abnormal processing and localization of E-cadherin. Culture of mutant livers in vitro revealed the existence of a parallel developmental pathway mediated by hepatocyte growth factor (HGF), which could rescue the mutant phenotype independent of Smad activation. These pathways merge at the beta1-integrin, the level of which was increased by HGF in the cultured mutant livers. HGF treatment reversed the defects in cell proliferation and hepatic architecture in the Smad2(+/-); Smad3(+/-) livers.

Epithelial development and differentiation in the mammary gland is not dependent on alpha 3 or alpha 6 integrin subunits

In the mammary gland, both laminin and integrins have been shown to be required for normal ductal morphogenesis during development in vivo, and for functional differentiation in culture models. Major integrin receptors for laminins in the mammary gland are alpha 3 beta 1, alpha 6 beta 1, and alpha 6 beta 4. However, the specific subunits that contribute to laminin-mediated mammary cell function and development have not been identified. In this study, we use a genetic approach to test the hypothesis that laminin-binding integrins are required for the function of the mammary gland in vivo. Rudiments of embryonic mammary gland were shown to develop in the absence of these integrin subunits. Postnatal development of the mammary gland was studied in integrin null tissue that had been transplanted into the mammary fat pads of syngeneic hosts. In mammary epithelium lacking alpha 6 integrin, the beta 4 subunit was not apparent and hemidesmosome formation was only rudimentary. However, despite this deficiency, normal ductal morphogenesis and branching of the mammary gland occurred and myoepithelial cells were distributed normally with respect to luminal cells. Mammary alveoli devoid of alpha 3 or alpha 6 integrin formed in pregnancy and were histologically and functionally identical to those in wild-type mammary gland. The tissue underwent full morphological differentiation, and the epithelial cells retained the ability to synthesize beta-casein. This work demonstrates that mammary tissue genetically lacking major laminin-binding integrin receptors is still able to develop and function.

Human hepatocellular carcinoma (HCC) cells require both alpha3beta1 integrin and matrix metalloproteinases activity for migration and invasion

Hepatocellular carcinoma (HCC) is the most frequent malignant tumor of the liver; prognosis depends on the tendency to metastasize. Cancer cell invasion is regulated by proteolytic remodeling of extracellular matrix components and by integrin expression. We have shown that matrix metalloproteinase-2 (MMP-2) and membrane-type-1 matrix metalloproteinase (MT1-MMP) cleave Laminin-5 (Ln-5), stimulating cell migration. Here we report that all HCC cells express MT1-MMP, migrate on Ln-1 and Collagen IV, whereas only HCC cells that express alpha3beta1 integrin secrete detectable levels of gelatinases, migrate on Ln-5, and invade through a reconstituted basement membrane (BM). Migration on Ln-5 is blocked by BB-94, an MMP inhibitor, and by MIG1, a monoclonal antibody that hinders migration on MMP-2-cleaved Ln-5. Invasion through a reconstituted BM is also inhibited by BB-94. HCC alpha3beta1-negative cells migrate on Ln-1 and Collagen IV, but not on Ln-5, and do not invade through a reconstituted BM, although they express MT1-MMP. Anti-alpha3beta1 blocking antibodies inhibit gelatinase activation, cell motility, and cell invasion through MATRIGEL: In vivo, alpha3beta1 integrin and Ln-5 are expressed in HCC tissue but not in normal liver. In conclusion, our data suggest that both alpha3beta1 integrin and gelatinase activity are required for HCC migration and invasion.

Functions of alpha3beta1 integrin

alpha3beta1 integrin is a laminin receptor with apparently diverse functions. In epithelial cells it acts as a receptor for the basement membrane, whereas in neuronal and possibly tumor cells it mediates migration. Interactions of alpha3beta1 integrin with tetraspanin proteins may provide clues to how it transduces signals that affect cell behavior.

Review article: liver support systems in acute hepatic failure

The treatment of acute hepatic failure has developed rapidly over the last 40 years, reducing morbidity and mortality from this syndrome. Whilst this has been partly attributed to significant improvements in the specialist medical management of these patients, advances in surgical techniques and pharmaceutical developments have led to the establishment of successful liver transplantation programmes, which have improved mortality significantly. This review will examine the clinical impact of alternative methods that have been used to provide extra-corporeal hepatic support. Non-biological, bio- logical and hybrid hepatic extra-corporeal support will be explored, offering a comprehensive historical overview and an appraisal of present and future advances.

Early liver differentiation: genetic potentiation and multilevel growth control

Hepatocytes undergo distinct phases of differentiation as they arise from the gut endoderm, coalesce to form the liver, and mature by birth. Gene inactivation and in vivo footprinting studies in mouse embryos have identified regulatory transcription factors and cell signaling molecules that control some but not all of these transitions. The latest studies reveal DNA-binding proteins that appear to potentiate gene activation during liver specification and the importance of signals between early hepatocytes and other cell types that promote early liver growth.

Conformational effects of volatile anesthetics on the membrane-bound acetylcholine receptor protein: facilitation of the agonist-induced affinity conversion

The rate of the carbamylcholine-induced affinity conversion of the membrane-bound acetylcholine receptor protein from Torpedo californica is enhanced by pretreatment of the membranes under an atmosphere of 3% halothane or 1% chloroform. The enhancement is much more pronounced in the presence of low rather than high concentrations of carbamylcholine since the volatile anesthetics alter the apparent dissociation constant for carbamylcholine from 17 to 3 microM without affecting the first-order rate constant for the ligand-induced conversion (0.07 s-1). These results indicate that the acetylcholine receptor is assuming a conformational form with intermediate affinity for carbamylcholine in addition to the previously described low- and high-affinity forms. The dissociation constants for carbamylcholine obtained from kinetic studies of the carbamylcholine-induced transition are 3-15-fold lower than those obtained as inhibition constants from the rate of 125I-labeled alpha-bungarotoxin binding to the low-affinity conformer of the acetylcholine receptor protein. This pattern, observed in both the presence and absence of anesthetic, provides further evidence that the acetylcholine receptor has nonequivalent ligand binding sites for carbamylcholine.