Proc. Indian Acad. Sci. (Chem. Sci.), Vol. It 1, No. 2, April 1999, pp. 331-342 © Indian Academy of Sciences
Hepatocyte-matrix interaction
P R S U D H A K A R A N *Department o f Biochemistry, University o f Kerala, Kariavattom, Trivandrum 695 581, India
e-mail:dlcampus @ md2.vsnl.net.in
Abstract. Various components of extracellular matrices have unique patterns of distribution in the liver, which change under hepatic regeneration and in pathological conditions. In vitro studies using isolated hepatocytes maintained in culture on different matrix proteins or tissue biomatrices revealed that the matrix influences the biochemical activity of hepatocytes in culture on a selective basis. Hepatocyte-matrix interactions are mediated by specific receptors for matrix proteins belonging to both integrin and non-integrin group of cell surface molecules. The level of air1 integrin, which is a comtnon receptor for Col IV and Ln in liver appears to change under hepatic regeneration; cat ions appear to modulate its interaction with Col IV and Ln in a differential manner. A number of other integrin receptors including that for fibronectin (a.fl0 and vitronectin ( a ~ ) are also present in liver. Apart from integrin receptors, non-integrin receptors such as 67 kDa laminin binding protein, 68 kDa collagen IV binding protein, 110 kDa Fn receptor are also present in liver cells. HSPG present on the hepatocyte surface also appears to have an augmenting effect in mediating cell adhesion. Although matrix components appear to exert their effect through interaction with matrix receptors on cell surface, details about the intracellular signalling process in liver cells are not clear.
Keywords. Extracellular matrix; hepatocyte-matrix interaction; hepatocytes; integrins; matrix receptors; molecular recognition.
1. I n t r o d u c t i o n
The extracellular matrix produced by all metazoan cells is a c o m p l e x network o f macromolecules that interact with one another as well as with cells in tissues. Cell adhesion to the extracellular matrix plays a critical role in complex biological processes including cell migration, growth, differentiation and survival. Though the extracellular matrix (ECM) has been known to contain signals affecting cellular activity, progress in studying its influence was hampered until recently, mainly due to the chemical complexity and the inherently insoluble nature o f its components. Collagens, elastins, proteoglycans and glycoproteins such as fibronectin, laminin, tenasin, entactin etc. constitute the major components o f the E C M in various tissues. The nature o f the E C M interacting with the cells has important regulatory and structural consequences for the cell. An extensive literature documenting the biological effects o f extracellular matrix is now available. More recently, the focus o f research in matrix biology has shifted to cell surface receptors *For correspondence
This paper was part of the collagen symposium organized on the occasion of the Golden Jubilee of the Central Leather Research Institute, Chennai in January 1999.
332 P R Sudhakaran
for matrix macromolecules and generation and transduction of signals intracellularly. Although the proportion of ECM and the connective tissue in relation to the parenchymal tissue is very small in the liver, the ECM has an important biological role. Cell-cell and cell-matrix interaction in liver are mediated by specific cell surface molecules. A brief review on the molecular mechanisms involved in the interaction of liver cells with the components of ECM is presented here.
2. Extracellular matrix in liver
ECM in the liver has to provide maximum mechanical stability to the parenchymal tissue without causing any hindrance to free transport of metabolites between parenchymal cells and the blood stream. Hepatocytes, which constitute more than 90% of the liver mass in most mammals are structurally and functionally polarised and ~ have three distinct membrane domains, viz. sinusoidal (basal), lateral and canalicular (apical). Hepatocytes have a belt of apical surface dividing the two basolateral surfaces that are in contact with ECM. Components of the ECM in liver have been characterised by biochemical and immunological techniques 1~6. The major types of collagen are Col I, III and IV. The matrix glycoprotems, fibronectin, laminin and heparan sulphate proteoglycans are also present. Ultrastructural localisation studies revealed the presence of typical collagen fibrils and small bundles in portal tracts and Disse's space. The presence of Col I in the liver capsule, portal stroma and perisinusoidal space suggests a structural role to support the hepatocyte layer at the intralobular region. Type III collagen is essentially a stromal component and is not in direct contact with the hepatocyte surface. Basal surface of hepatocytes is in contact with basement membrane components such as fibronectin, laminin, Col IV and heparan sulphate proteoglycan. Fibronectin appears on all cell surface domains of hepatocytes 7. It is present in perisinusoidal space in direct contact with hepatocyte microvilli. 1.7,s. It is also present in liver capsules, portal stroma and bile ducts. Col IV and laminin are present in ductai, neural and vascular membranes and also as small discrete deposits between hepatocyte and endothelial sinusoids3. The cellular source of these matrix components in liver appears to be of mixed origin. In vitro studies with isolated liver cells in culture suggest that hepatocytes and hepatic stellate cells are two major sources of these matrix components in liver 9'1°.
Using isolated hepatocytes maintained in serum-free medium on specific matrix protein substrate or tissue biomatrix or collagen gels, it has been demonstrated that the expression of various hepatocyte specific functions varied with the nature of the substratum N-14. Cells cultured on Col IV retained differentiated phenotype more efficiently than those maintained on laminin or other substrata, as measured by the synthesis of albumin and a-fetoprotein 14. These and a number of other similar observations from our laboratory and from elsewhere demonstrated the influence of the matrix substrata on the metabolic activity of hepatocyte in culture. Adult rat hepatocyte maintained on a reconstituted ECM gel from EHS tumour showed a differential regulation of the expression of the major cytoskeletal genes and of the liver specific genes for albumin and al antitrypsin 15. The modulation of the biochemical activity of hepatocytes in culture by matrix substratum was further demonstrated by maintaining hepatocytes on. different tissue biomatrix substrate16. Comparison of the kinetics of synthesis and secretion of proteins by cells maintained on different tissue biomatrices showed that those maintained on hepatic biomatrix synthesised more albumin and apo B, the major apoprotein of VLDL, than those maintained on heterologous tissue biomatrices. Induction
Hepatocyte-matrix interaction 333 of tyrosine aminotransferase by dexamethasone was maximum in cells maintained on hepatic biomatrix when compared to that on cells maintained on aortic biomatrix. Cyt P4so is a major constituent of microsomal systems involved in the oxidative metabolism of a wide variety of xenobiotics and endogenous substrates. A few isoforms of Cyt P450 such as P450 IAI and P450 III A are induced in the primary cultures of rat hepatocytes with suitable inducers. But when the cells were maintained on tissue biomatrix or on matrigel substrata, and treated with phenobarbital, new forms of P450 such as P450 II B, were expressed ~0.17. Results of a series of such experiments showed that depending on the nature of the matrix with which the cells are in contact, the hepatocytes show difference with respect to these functions indicating that the matrix can influence, on a selective basis, the metabolic activity of hepatocytes.
D-penicillamine which inhibits collagen fiber formation inhibited DNA synthesis while ascorbic acid which stimulates production of mature collagen enhanced DNA synthesis in primary cultures of rat hepatocytes 18, suggesting that the elaboration of an endogenous extracellular matrix is a critical factor influencing the survival and metabolic activity of hepatocytes in culture. The regulation of cellular function by ECM was also demonstrated in a number of other systems such as synthesis of milk proteins in response to hormones by mammary epithelial cells in culture 19-2z, production of matrix metalloproteinases by substratum adhered cells 23, induction of colony stimulating factor in monocytes etc. 24.
3. Receptors for matrix proteins
As indicated above, the macromolecules of the ECM, apart from contributing significantly to anchorage, polarity and migration of cells, exert a number of biological effects 25. The diverse cellular effects of ECM are mediated through interaction of matrix macromolecules with specific components generally referred to as matrix receptors. In recent years several membrane molecules with affinity for individual ECM components have been isolated from a variety of cells and tissues either by using specific antibodies capable of perturbing cellular adhesive events or by direct approach involving affinity chromatography using matrix proteins or their cell-binding peptide fragments as affinity matrix. The matrix-binding molecules identified so far on the cell surface include a group of integral membrane proteins called integrins and a series of non-integrin proteins and glycolipids (table 1).
3.1 Integrins
A group of matrix protein receptors on the cell surface having similar structural and functional characteristics have been grouped into a supergene family of receptors called integrins 25-32. Each integrin is a heterodimeric transmembrane glycoprotein composed of two subunits viz. a and ft. Sequence analysis using cDNAs indicated that each integrin molecule has a large extracellular domain, a transmembrane segment and a cytoplasmic tail. There is about 40% amino acid homology for the two subunits and these are glycosylated by asparagine-linked oligosaccharides. The specifi~ pattern of noncovalent association between the a and fl subunits creates dimers with specificity for different extracellular matrix proteins (figure 1). At present 15a and 8,6 subunits have been identified, which combine in various combinations to form over 20 different types of integrins. The different isoforms of integrin subunits are formed due to alternative
334 P R Sudhakaran
Table 1. Matrix receptors.
Family Receptor Ligand
lntegrin
Non-integrin
alfll Col I, Col IV, Ln.
crzflt Col I, Col IV, Ln. Fn.
asfll Col I, Col IV, Ln. Fn. En.
a4fl) Fn asfll Fn. a6fll Ln. aTfll Ln. avfll Fn. a~fl6 Fn. a~fl~ Vn. avfl3 Vn, Fb, vWF, Tsp. Fn a~fl2 Fb. OlMfl 2 C3BI, Fx, Fb
atfl2 ICAM1, ICAM2, ICAM3
a~fl4 Ln.
astir Vn, Fn.
agfll Tn.
a4fl 7 VCAM-1, MAd CAM-I
67 K Ln Bp Ln. 68 K Col I, Col IV 120 K Ln. Fibroblast receptor Fn. AGp 110 K Fn. Syndecan Fn. Hyaladherins Hyaluronectin HA 85 kDa HA receptor HA CD 44 HA 34 K HA 38 K HSPG 67 K elastin Abbreviations as in figure 1
splicing of their precursor mRNAs. Alternative splicing appears to play an important role in integrin localisation at focal adhesion sites and in intracellular signal transduction.
Using a variety of electron microscopic methods 33 integrins have been shown to have a globular head region formed from substantial portions of both a and fl subunits and mediates ligand binding. The globular head region is at a height of about 20 nm above the cell surface and is held by semirigid legs that penetrate through the plasma membrane. The carboxyterminal cytoplasmic domains protruding into the cytoplasm interact with various cytoplasmic molecules that regulate extracellular ligand binding affinity. The transmembrane organisation permits integrins to bind to extracellular ligands and transmit signals intracellularly 30,32
The diversity of integrins provides cells with varied capabilities to recognise adhesive substrates and ECM. There are at least three modes of integrin mediated adhesion, (a) adhesion of cells to ECM by binding to ECM macromolecules, (b) ceil-cell adhesion by forming bridge-like structure between adjacent cells and (c) cell aggregation through insoluble proteins. The ligand-binding specificity of the integrins contributes significantly
/
Ct~3.~
Col
I
(xt MAO CAM-I 0"4 \'~ o \'~ Ln / ~°11I~ F'b, vWF, Vn TSP~ Fn~ RGD 3 Ctllb (~L\ ..~.L*cg
C3bl, Fx, Fb \ ' Vn, RGD~v
13s
a.,t
13
2
Figure 1. Integrin family: The known subunit combinations and ligands for these integrins are shown. Abbreviations: Fn- fibronectin, Ln- laminin, Vn- vitronectin, Fn (alt)- fibronectin alternatively spliced, Tn- tenascin, Fb - fibrinogen, En - entactin, vWF -von Willebrand factor, Col - collagen, TSP - thrombospondin, C3b! - complement component, ICAM - intercellular adhesion molecule, VCAM - vascular cell adhesion molecule, Fx - factor X. I3 36 P R Sudhakaran
to cell behaviour. As can be seen from figure 1, integrins binding to Fn, Ln, Col IV, Col I etc. have been identified and these integrins mediate cell adhesion to specific ECM molecules. Matrix-binding integrins of adherent cells appear to be constitutively activated but those on non-adherent ceils such as platelets, monocytes, lymphocytes etc. need to be activated. Ligation of T-cell receptor results in activation of LFA1 on lymphocytes 34. Cells may also regulate integrin specificity, a~31 integrin on platelets is a collagen receptor but in certain other cells such as endothelial cells it binds to Ln and Fn in addition to collagen. Apart from the state of activation and spatial distribution, a major determinant of the specificity of binding to matrix proteins is the type and concentration of integrin receptors. For e.g., during epithelial wound repair, there is increase in the level of O~5~ 1 35,36 integrin which binds to Fn. This is particularly important as adult human keratinocytes, which are initially unable to adhere to Fn, adhere to fibroblasts after in vivo wounding. This process is accompanied by changes in the glycosylation state of fll subunits and the levels o f a ~ l integrin 35,36
Cell adhesion is based on protein-protein recognition and a series of earlier studies have identified molecular recognition sequences on adhesive matrix proteins. A common element of molecular recognition sequence originally identified on Fn is a small adhesive peptide sequence RGD 37. Synthetic peptides containing RGD sequences produce many of the cellular effects of matrix proteins including cell adhesion, migration etc. The RGD sequence is the cell recognition sequence in a number of ECM proteins including Fn, Vn, collagen, fibrinogen, vWF, osteopontin, thrombospondin, tenascin and entactin. Integrins bind to extracellular matrix protein at specialised cell attachment sites that often have the tripeptide RGD sequence 38. In fact this property of the RGD peptide has been exploited to develop methods for the isolation of integrins. It appears that the specificity of the RGD sequence for binding to integrin resides in the conformation of RGD tripeptide and the role of the surrounding sequences may be to force the RGD into an appropriate conformation. A peptide sequence different from RGD has been identified as the target sequence of the a4fll integrin on Fn 39. This sequence GPEILDVPSI is present in one of the alternatively spliced segments of Fn.
A series of studies aimed at identifying ligand recognition sites on integrin, has defined the sites on several a and fl subunits that bind to matrix protein 40-44. The major platelet receptor a llb~3 has been studied in great detail. Amino acid residues 114-128 and 212-219 in the f13 subunit combined with contributions of residues 294-314 and 657-665 in the allb subunit appear to constitute the ligand-binding pocket 43'44. It is possible to have more than one binding site on the integrin as is seen in the case OfalJb subunit where a 11-residue sequence (TDVNGDGRHDL 296-306) by itself mimics the capacity of the entire integrin to bind to fibrinogen. 45. An important element of ligand recognition process for RGD peptide may be the formation of a putative divalent cation binding site similar to the cation binding motif characteristic of molecules like calmodulin. Two distinct domains involved in integrin binding to non-RGD ligand also appear to exist on lymphocyte function associated antigen (LFA-1) molecule (aLflZ) 46.47
3.2 Signal transduction by integrins
Integrins transduce signals that initiate cellular activities such as division, secretion and gene expression. One of the concepts of transmission of signals by integrins is by organising the cytoskeleton, thus regulating cell shape and integral cellular archite- cture 48"49. Cytoplasmic domains of the integrins interact with the cytoskeleton
Hepatocyte-matrix interaction 337 components 5o and ECM-integrin mediated adhesion serve as nucleation foci for cyto- skeletal assembly 51. Cell shape and cytoskeletal organisation regulate the biosynthetic capabilities of the cell and thus contribute to cell growth or differentiation 52.53
An alternate pathway of integrin mediated signalling is to give rise to biochemical signals within the cells. Ligation of integrins alters cellular patterns of tyrosine phosphorylation 54. For example, integrin mediated cell adhesion to Fn, Ln, collagen, vitronectin lead to enhanced tyrosine phosphorylation of pp 125 FAK (focal adhesion kinase), a novel cytoplasmic tyrosine kinase. Activation of pp 125 FAK is related to the anchorage dependence of cell growth 55
Another phosphotyrosine kinase syk, is activated by integrin 56. Activation of the 72 K
syk in platelets is regulated by integrin-dependent and -independent mechanisms both of which are necessary for maximal activation, f12 integrins mediate activation offgr, a src family of phosphotyrosine kinase 57. Binding of fibroblast integrins to extracellular matrix ligand also can trigger intracellular tyrosine phosphorylation pathways involved in the activation of transcription factor. The kinases termed extracellular regulated kinase I and 2 (ERK1 and ERK2), which belong to the group of mitogen activated kinases, (MAP), are transiently activated after adhesion of cells to Fn 58,59. This activation is accompanied by translocation into nucleus where certain genes may get transcriptionally activated. Another pathway, termed Jun kinase or stress-activated protein kinase (SAP) pathway can also be activated by integrin ligands 6°, but with different kinetics than ERKs. It is significant that interaction of ceils with extracellular matrix ligands like Fn through integrin leads to the activation of two major signalling pathways, viz. the Jun kinase pathway which is generally activated by stress and inflammatory cytokines such as TNF a and the ERK pathway which is specified for cytokines like PDGF and EGF. In addition to tyrosine phosphorylation, a number of other integrin related signalling events have been reported. In platelets integrin ligation can affect calcium activated proteinase 6~, Na÷/H ÷ antiportor 62, and subcellular distribution of phosphoinositide 3 kinases 63. Integrin mediated events induce calcium transients in osteoclasts 64 and monocytes 65, while cAMP 66 and Ca 2+67 are affected by f12 integrin-mediated events in neutrophils. In endothelial cells, this rise in intracellular Ca z÷ on adhesion to Fn is due to an influx of extracellular Ca 2÷ mediated by a~ integrin 68, probably via association with a putative calcium transporter termed integrin-associated-protein 69'7°. The complex grouping of signalling pathways and the organisation of various cytoskeletal components after cellular adhesion through integrins, may provide multiple points of regulation of integrin response by both extracellular and intracellular stimuli.
4. Matrix receptors in liver
4.1 Hepatoeyte-matrix interaction
The interaction of liver cells with various components of ECM have been studied using isolated liver cells. In vitro studies have shown that hepatocytes attach, spread and adapt significantly different morphologies on non-collagenous components such as Fn, Ln, or collagen and on isolated tissue biomatrix which is a mixture of glycoproteins such as Fn, Ln, tenascin, proteoglycans like HSPG and collagen 71-77 The ability of hepatocytes to bind directly to collagen, Fn, and Ln coated substrata, by a distinctly different and saturable kinetic pattern, suggests the occurrence of specific cell surface receptors for each matrix protein or shared receptor having different affinities for each iigand. This was
338 P R Sudhakaran
further suggested by binding of matrix proteins to cells in in vitro binding assays
(Sudhakaran, unpublished data) and their competitive inhibition by cell recognition peptides. Binding of cell-binding fragment of Fn to hepatocytes was inhibited by peptides containing the cell recognition tripeptide sequence RGD. Binding of laminin and collagen IV to hepatocytes as well as to hepatic plasma membrane in solid phase assays also indicated the presence of specific cell surface molecules that are involved in the interaction, with high affinities and specificities, to matrix proteins. These early observations suggested the occurrence of receptors for matrix proteins on hepatocytes.
4.2 Integrin receptors in liver
A series of studies have led to the identification of integrin receptors in hepatocytes and liver tissue. Using adhesion blocking antibodies a 115 kDa protein associated with 170 kDa protein was identified 78. This was subsequently shown to belong to the integrin group of receptors. Heterodimeric alfll integrin isolated from rat liver cells has been shown to bind to Ln 79 collagen 180 and collagen IV st. Proteolytic fragments of Ln, have been shown to promote attachment of adult rat hepatocytes and cqfll integrin appears to
bind to two structurally distinct domains of laminin, viz. pepsin fragment PI from the central region and the elastase fragment E8 from the distal arm of the cross shaped Ln molecule 79
Another matrix receptor which has been studied in detail is the fibronectin receptor. Hepatocyte adhesion to Fn is mediated by a ~ l integrin in a Ca ~ dependent manner through the RGD sequence in the cell binding domains of Fn 82,83. Immunofluorescence and interference reflection microscopy studies using hepatocytes attached to fibronectin showed colocalisation of the termini of actin filaments, and vinculin and fibronectin and its receptor a ~ l integrin at focal contact sites 84-86. Immunocytochemical studies have demonstrated the presence of integrin receptors in intact liver. Apart from ctl and as subunits described above, at least three other integrin a subunits have been detected in adult rat liver in association with/31 subunits. Histochemical studies have demonstrated the presence of a2, a3 and a6 subunits,/34 subunit and the receptor for vitronectin ad31 in normal bile duct epithelium and their ectopic expression in neoplastic human parenchymal biopsies ST.
Cell matrix interactions play a crucial role in hepatogenesis as well as in hepatic regeneration after liver injury. Comparison of the attachment of adult and fetal hepatocytes and those from regenerating rat liver showed that hepatocytes from fetal liver and regenerating liver adhere more to Ln and suggested the occurrence of more laminin binding cell surface molecules in fetal liver 10.90. cqfll integrin has been isolated from fetal hepatic plasma membrane by iigand affinity chromatography. It has been shown to bind to both Ln and Col IV in a cation dependent manner. These studies also suggested that cq/3t integrin in liver is a common receptor for Ln and Col IV 10. However, binding studies using radioiodinated ligands showed that different cations like Ca ~, Mg ++, Mn ~ affected the binding of cq/31 integrin to Ln and Col IV differently suggesting a possible role for
cations in modulating the interaction of the common receptor with different ligands in liver 88. The relative amounts of al/31 integrin appeared to be at least two-fold higher in regenerating liver than in adult rat liver (Sudhakaran, unpublished data).
The synthesis and distribution of integrin was studied using isolated rat liver cells or HepG2 cells in culture. Sites of hepatocyte-matrix interaction have been identified ultrastructurally as regions of cell surface thickening adjacent to ECM components. These
Hepatocyte-matrix interaction 339 focal contacts contain ECM components, cytoskeletal proteins such as actin, talin, vinculin, a-actinin and focal adhesion kinase. Immunocytochemical analysis of hepatocyte adhered to fibronectin or laminin showed distribution of integrins at focal contact sites 84-86. Immunostaining of HepG2 cells with antibodies against al and fll integrin subunits followed by laser scanning microscopic analysis showed that the signal intensity was more towards the basal area than the top surface indicating clustering of integrins to these adhesion sites lo. In cells treated with glycosylation inhibitor such as tunicamycin, there was a significant reduction in signal intensity in the basal area indicating the importance of post-translational glycosylation of integrin subunits in their distribution into adhesion sites (Sudhakaran, unpublished data).
4.3 Non-integrin receptors
Apart from integrin receptors, nonintegrin receptors for matrix proteins have also been shown to be present in liver. Clement et al. s9 showed the presence of a group of laminin- binding proteins on the hepatocyte surface which include a 68 kDa protein that recognises the Y1GSR cell recognition sequence on Ln. A 67-kDa laminin binding protein has also been isolated from hepatic plasma membrane 9o. Although this protein has been shown to bind to laminin with very high affinity, its role as a true receptor for Ln in vivo is controversial. We have also isolated from rat liver another 68-kDa protein by affinity chromatography, that bind specifically to Col IV. This protein appeared to be different from the Ln BP 67-kDa protein and is similar to the collagen-binding 68-kDa non- integrin receptor present in platelets and other types of cells.
By successive affinity chromatography of the membrane extracts of rat hepatocytes on WGA sepharose and Fn-sepharose, a ll0-kDa protein (AGp 1 10) that binds to Fn in a Ca ++ dependent and RGD independent manner has also been isolated. AGp 110 is an integral membrane glycoprotein. This protein has been shown to be localised at focal contact sites along with Fn, integrins, and the termini of actin and vinculin 85.s6
Numerous studies have shown that proteoglycans, particularly heparan sulphate proteoglycans are present at the surface of different types of cells, where they are directly bound to the plasma membrane either as integral or peripheral membrane components. Membrane intercalated HSPG have been demonstrated on rat hepatocytes, mouse mammary epithelial cells, human fibroblasts, glial cells and endothelial cells 91. Syndecans constitute the major form of cell surface HSPG 92. Matrix glycoproteins such as Fn and Ln bind to HS. Heparin binding domains have been identified in these adhesion proteins 93. Codistribution of proteoglycans, Fn and actin in cells undergoing spreading on substratum also suggest a role for HSPG in mediating cell adhesion to matrix substrata91 Syndecan has been suggested to serve as a receptor for Fn on mammary epithelial cells. Although most of the adhesive matrix molecules contain sites that can interact with glycosaminoglycans, particularly HS, the cell binding fragments of these matrix molecules appear to lack affinity for HS. The binding of cell surface HSPG to HS binding sites on these matrix proteins may play an augmenting role in cell adhesion. Apparent affinity of Fn for its cell surface receptor on hepatocytes or peritoneal macrophages is increased if heparan sulphate is added 91. It therefore appears that the HSPG on the hepatocyte surface may act as a coreceptor which may facilitate interaction of cells with matrix proteins.
Apart from adhesion receptors mediating cell-matrix interaction, molecules that promote cell-cell interaction have also been identified in liver. Cadherins represent a
340 P R Sudhakaran
major group of cell adhesion receptors mediating Ca ++ dependent cell-cell interaction. They bind cells by means of homophilic interaction and are important in establishing and maintaining intercellular connections. These are transmembrane glycoproteins consisting of about 700-750 amino acid residues 94. The extracellular region is folded into five domains each containing about 100 amino acid residues. The short cytoplasmic tail interacts with the cytoskeleton by means of proteins called catenins, which play a crucial role in mediating cadherin function. E-cadherin which is a major component of intermediate junction has been detected in hepatocytes 86. L-CAM is a Ca ++ dependent cell adhesion molecule, belonging to the cadherin group, that is present on the liver cell surface 95. This is an acidic glycoprotein with three to four regions of internal homology of 110 amino acids each. cDNA analysis of L-CAM and cadherin showed that these are specified by different genes having close structural relationship and homology. Low level expression of intercellular adhesion molecule (ICAM-1) and leucocyte function associated antigen (LFA-3) have also been found on sinusoidal cells
96.
As can be seen there is a great deal of overlap and redundancy in the binding of integrins to various ligands. Thus in liver, cqfll integrin binds to more than one ligand viz. Col and Ln. On the other hand one ligand binds to more than one integrin as in the case of Fn binding to a ~ l and cod31 integrin. Further, some of the adhesion proteins are recognised both by integrins and by other types of receptors. Selective distribution and expression pattern of the matrix receptors and the adhesion proteins in embryonic and adult and regenerating conditions and their alteration in inflammatory conditions highlight the importance of matrix receptors and their interactions in the development and maintenance of hepatic parenchymal architecture and functions.
Acknowledgements
The work from the author's laboratory reported here, was supported by Department of Science and Technology, Government of India. Part of the work was done when the author was a Guest Professor at the laboratory of Prof von Figura in the University of G6ttingen, Germany.
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