Differential expression of laminin chains in hepatic lipocytes

Extracellular matrix proteins in metastases to the liver - Composition, function and potential applications

The rising evidence of the tumor microenvironment (TME) and its role in cancer have made this an area of increased research efforts. The focus is both on the primary tumor but also on the metastatic setting. The TME though, does not only consist of the non-malignant cells of a tumor, but also of the acellular compartment: The Extracellular Matrix (ECM). The liver is a common organ for metastasis of many cancers and for some of these cancers' liver surgery is a standard treatment with long-term cure, whereas for other cancers not considered meaningful. Blood supply and anatomical reasons plays one part for the establishment of liver metastasis. It is however a well-known fact that the "soil" of a metastatic organ is of utter importance in the process of metastasis. The "soil" consists of the TME where the ECM is a critical and active part. This review focuses what is known about the normal ECM of the human liver, what is known about ECM proteins in human liver metastasis, challenges of studying the ECM in liver metastases and lastly, potential applications of this field of knowledge.

Transient expression of laminin ?1 chain in regenerating murine liver: Restricted localization of laminin chains and nidogen-1

Most interstitia between epithelial and endothelial cells contain basal laminae (BLs), as defined by electron microscopy. However, in liver, the sinusoidal interstitium (called space of Disse) between hepatocytes and sinusoidal endothelial cells (SECs) lacks BLs. Because laminins are major components of BLs throughout the body, whether laminins exist in sinusoids has been a controversial issue. Despite recent advances, the distribution and expression of laminin chains have not been well defined in mammalian liver. Here, using a panel of antibodies, we examined laminins in normal and regenerating mouse livers. Of alpha chains, alpha5 was widely observed in all BLs except for sinusoids, while the other alpha chains were variously expressed in Glisson's sheath and central veins. Laminin gamma1 was also distributed to all BLs except for sinusoids. Although the beta2 chain was observed in all BLs and sinusoids, the expression of beta1 chain was restricted to Glisson's sheath. Detailed analysis of regenerating liver revealed that alpha1 and gamma1 chains appeared in sinusoids and were produced by stellate cells. The staining of alpha1 and gamma1 chains reached its maximum intensity at 6 days after two-thirds partial hepatectomy (PHx). Moreover, in vitro studies showed that alpha1-containing laminin promoted spreading of sinusoidal endothelial cells (SECs) isolated from normal liver, but not other hepatic cells. In addition, SECs isolated from regenerating liver elongated pseudopodia on alpha1-containing laminin more so than did cells from normal liver. The transient expression of laminin alpha1 may promote formation of sinusoids after PHx.

[Mechanisms of hepatic fibrogenesis]

Hepatic fibrosis is a scaring process leading to cirrhosis, a major complication of numerous chronic liver diseases. Hepatic stellate cells play a central role in the fibrotic process. After parenchymal or biliary injury, cytokines and growth factors allow the recruitment, proliferation, and activation, of stellate cells toward myofibroblasts, which secrete the extracellular matrix. Fibrosis, resulting from the failure of the balance between synthesis and degradation of extracellular matrix, is an evolutive and potentially reversible process. Histological examination is the main investigation to quantify fibrosis. Serological tests are warranted to allow a non invasive follow up of patients. Development of antifibrotic therapies should soon permit to slow down the evolution toward cirrhosis, limiting the needs for hepatic transplantation.

Laminin isoforms in non-tumoral and tumoral human livers. Expression of alpha1, alpha2, beta1, beta2 and gamma1 chain mRNA and an alpha chain homologous to the alpha2 chain

BACKGROUND/AIMS

Laminins, the major non-collagenous basement membrane components, are involved in various biological processes. Laminin isoforms have never been characterized in human livers. The expression of five laminin mRNA was investigated in livers with or without cancer and in hepatoma cells and, by comparison, in both rat hepatoma and hepatic stellate cells.

METHODS

Laminin alpha1, alpha2, beta1, beta2 and gamma1 mRNA was detected by northern blot and/or RT-PCR in livers without chronic disease (n=5), in both tumoral and non-tumoral areas of livers with hepatocellular carcinomas (n=13) or metastases (n=18), in human HBGC2 and rat Faza-567 hepatoma cell lines, and in 6-day-old rat hepatic stellate cell cultures.

RESULTS

Laminin alpha1, alpha2 and beta1 mRNA were found in 25-33% and gamma1 mRNA in 58% of the livers, the signal for laminin beta2 mRNA being faint in all the samples. Laminin alpha2, beta1, beta2 and gamma1 mRNA were expressed in hepatoma and stellate cells. The laminin alpha2 cDNA probe recognized a 3.5 kb mRNA different from the expected 9 kb mRNA. Using degenerated oligonucleotides, RT-PCR products from both rat hepatoma and stellate cells revealed 90% identity with the alpha2 chain sequence. Antibodies against peptide deduced from the conserved C-terminal domain of both alpha1 and alpha2 chains recognized polypeptides corresponding to the degradation products of alpha2 chain in liver extracts and both media and cell layers from hepatoma and stellate cells. In addition, a Mr=130000 polypeptide was revealed by these antibodies in liver extracts and cell layers, which was consistent with the expected size deduced from the 3.5 kb mRNA.

CONCLUSIONS

This first report on laminin isoforms in human livers indicates that laminin 1 (alpha1-beta1-gamma1), 2 (alpha2-beta1-gamma1), 3 (alpha1-beta2-gamma1) and 4 (alpha2-beta2-gamma1) mRNA and a polypeptide homologous to the alpha2 isoform, which could correspond to a truncated form of this chain, are usually expressed in non-tumoral and/or tumoral livers.

Development of hepatic sinusoidal structure with special reference to the Ito cells

To elucidate sinusoidal cell structure and function under normal conditions and their behavior in diseased settings, an understanding of their developmental aspects is needed. At day 10 of gestation in mice and rats or at 5 weeks of gestation in humans, the hepatic cords grow into the mesenchymal tissue of the septum transversum, and the primitive sinusoidlike structure is simultaneously observed between the liver cell cords. In the margin of the growing liver primordium, mesenchymal cells in the septum transversum are trapped in the subendothelial space. These subendothelial cells are at the early stages of organogenesis and become progenitors of the Ito cells. By days 12-14 of gestation in mice and rats or 8 weeks of gestation in humans, the basic structure of the sinusoids has developed. Embryonic hepatic sinusoids are usually lined by a continuous endothelium without basement membranes, and an incompletely fenestrated sinusoid appears at the middle gestational stage. In the late gestational stages, the Ito cells exhibit myofibroblastlike features in humans, mice, and rats. In association with this event, perisinusoidal reticular networks are gradually intensified. After birth until days 4-5 in mice and rats, the sinusoidal and perisinusoidal structures are almost completely formed, although slight morphological differences from those in adult livers still exist. What happens to sinusoidal endothelial cells and Ito cells in hepatic fibrosis-cirrhosis of the adult may be a deviated or uncontrolled occurrence of what goes on during the fetal period, i.e., a continuous nonfenestrated sinusoidal lining in the early embryonic stage and a myofibroblastlike transformation of Ito cells in late fetal life.

Expression of laminin gamma 1 cultured hepatocytes involves repeated CTC and GC elements in the LAMC1 promoter

Laminin gamma 1 chain is present in all basement membranes and is expressed at high levels in various diseases, such as hepatic fibrosis. We have identified cis- and trans-acting elements involved in the regulation of this gene in normal rat liver, as well as in hepatocyte primary cultures and hepatoma cell lines. Northern-blot analyses showed that laminin gamma 1 mRNA was barely detectable in freshly isolated hepatocytes and expressed at high levels in hepatocyte primary cultures, as early as 4 h after liver dissociation. Actinomycin D and cycloheximide treatment in vivo and in vitro indicated that laminin gamma 1 overexpression in cultured hepatocytes was under the control of transcriptional mechanisms. Transfection of deletion mutants of the 5' flanking region of murine LAMC1 gene in hepatoma cells that constitutively express laminin gamma 1 indicated that regulatory elements were located between -594 bp and -94 bp. This segment included GC- and CTC-containing motifs. Gel-shift analyses showed that two complexes were resolved with different affinity for the CTC sequence depending on the location of the GC box. The pattern of complex formation with nuclear factors from freshly isolated and cultured hepatocytes was different from that obtained with total liver and similar to that with hepatoma cells. Southwestern analysis indicated that several polypeptides bound the CTC-rich sequence. Affinity chromatography demonstrated that A M(r) 60,000 polypeptide was a major protein binding to the CTC motif. This polypeptide is probably involved in the transcriptional activation of various proto-oncogenes and extracellular matrix genes that are expressed at high levels in both hepatoma cells and early hepatocyte cultures.

Insulin induces a change in extracellular matrix glycoproteins synthesized by rat mesangial cells in culture

Extracellular matrix (ECM) accumulation within the glomerular mesangium is a hallmark of progressive forms of renal disease. We recently succeeded in propagating mesangial cells (MC) from the time of explant without supplemental insulin which exhibit a matrix profile analogous to normal mesangium in vivo. We used these cells to characterize insulin-induced changes in biosynthesis and accumulation of three important matrix glycoproteins, laminin, fibronectin, and thrombospondin. Two clones of MC derived from glomeruli from a single rat were compared. MC grown in the absence of supplemental insulin (SI-MC) assemble a matrix rich in fibronectin with much smaller accumulations of laminin and thrombospondin. In comparison, MC (SI+MC) grown chronically in the presence of 1 microM insulin have a greatly expanded ECM that immunostains less intensely with antibodies to fibronectin, but, it contains significant accumulations of laminin and thrombospondin. Following metabolic labeling of secreted proteins with 35S-methionine, total protein synthesis was measured, and specific ECM components were identified and quantitated by immunoprecipitation, SDS-PAGE and autoradiography. The rate of total protein synthesis was increased by 50% in SI+MC as compared to SI-MC, yet, individual proteins were increased or decreased. The rate of synthesis of fibronectin was decreased and the rate of synthesis of laminin and thrombospondin was increased by insulin. These changes were directionally correlated with the net accumulation of these proteins as shown by immunostaining. In addition to an increase in laminin synthesis, insulin treatment induced a change in the isoform of laminin expressed.(ABSTRACT TRUNCATED AT 250 WORDS)

Partial cloning of the M subunit of laminin from adult rat lipocytes: expression of the M subunit by cells isolated from normal and injured liver

Laminin is a heterotrimeric glycoprotein found in the perisinusoidal space of adult rat liver. The principal cellular source of laminin in liver is the lipocyte, with its three subunits measuring 324, 200 and 200 kD. The large subunit of lipocyte-derived laminin is distinct from the A subunit of murine laminin (440 kD); its size suggests that it represents a peptide, called M, recently cloned from human placenta. Using oligonucleotide primers derived from the human M-subunit cDNA, we amplified a 445-bp sequence encoding a fragment of M-laminin from adult rat lipocytes. The rat cDNA is 90% homologous to the human M-subunit cDNA and recognizes an mRNA in lipocytes measuring about 10 kb. M-subunit transcripts were identified only in lipocytes from normal adult liver; they could not be identified in hepatocytes, endothelial cells or Kupffer cells. Lipocytes were screened for M-subunit protein with a polyclonal M antiserum. Cells stained specifically for the M-subunit after 36 hr in primary culture; the protein was also identified in freshly isolated cells by means of immunoblotting. To determine whether lipocytes alter their expression of the laminin M subunit during liver injury, we monitored M-subunit mRNA in these cells at various intervals after carbon tetrachloride administration. M-subunit transcripts increased twofold within 12 hr of toxin exposure, returning to below baseline by 48 hr. The results indicate that lipocytes produce the M subunit of laminin in place of A. Production of this subunit by lipocytes may facilitate cell growth and reorganization during liver regeneration.

Enterocytic differentiation of the human Caco-2 cell line is correlated with down-regulation of fibronectin and laminin

Human intestinal Caco-2 cells were used to examine the expression of fibronectin (FN) and laminin (LN) during enterocytic differentiation. Combination of immunoprecipitation, Western and Northern blotting revealed that Caco-2 cells expressed a classical FN and a variant form of LN: besides B1 and B2 chains, LN contained a 350-kDa heavy chain instead of the 400-kDa A chain. Throughout Caco-2 cell differentiation, FN and LN synthesis decreased at both mRNA and protein levels. These data indicate that enterocytic differentiation involves both transcriptional and/or post-transcriptional down-regulation of FN and LN gene expression.

Identification of connective tissue gene transcripts in freshly isolated parenchymal, endothelial, Kupffer and fat-storing cells by northern hybridization analysis

The aim of the present study was to identify the cell types that express collagen alpha 1(I), alpha 1(III) and alpha 1(IV), fibronectin and laminin B1 genes in normal rat liver. Parenchymal, sinusoidal endothelial, Kupffer and fat-storing (Ito) cells were isolated and purified. Total RNA of the freshly isolated cells was subjected to Northern hybridization analysis. We also compared the steady state levels of specific mRNAs in freshly isolated fat-storing cells to the levels in myofibroblast-like cells obtained from purified fat-storing cells cultured for two passages. The average purity of each cell preparation, and the percentage of contaminating cells, were determined by transmission electron microscopy and by examining the presence of vitamin A-autofluorescent cells. Fibronectin and collagen alpha 1(III) mRNAs were detected in total RNA of purified parenchymal cells. In poly(A)+ enriched RNA, small amounts of collagen alpha 1(I) mRNA were also present. In total RNA of freshly isolated fat-storing cells, collagen alpha 1(III), alpha 1(IV), and laminin B1 transcripts were found, whereas collagen alpha 1(I) and fibronectin mRNAs were not detected. Cultured fat-storing cells, however, did contain high levels of collagen alpha 1(I) and fibronectin mRNAs. The molecular size of the latter transcript was larger than the fibronectin transcript found in parenchymal cells and the whole liver. Endothelial cells contained small amounts of alpha 1(IV) mRNA. Kupffer cells did not contain the investigated transcripts. We conclude that normal parenchymal, fat-storing and endothelial cells each express a typical pattern of connective tissue molecules. When fat-storing cells are allowed to differentiate into myofibroblast-like cells, they express high levels of collagen alpha 1(I) and fibronectin mRNAs, in addition to collagen alpha 1(III) and alpha 1(IV), and laminin B1 chain mRNAs.

Distribution and cellular origin of collagen VI during development and in cirrhosis

Collagen VI is a ubiquitous microfibrillar collagen that forms a network in most interstitial connective tissues, including soft organs and cartilage. The extracellular and intracellular distribution of collagen VI in human liver was studied by light and electron microscopy using the indirect immunoperoxidase method. In normal adult liver, collagen VI was seen mainly in portal spaces and formed a continuous layer in the sinusoids. Fetal liver contained more of collagen VI in the sinusoid than newborn and adult livers. In alcoholic fibrotic and cirrhotic livers, collagen VI antibodies intensely stained fibrous septa that invaded the lobule. Immunoelectron microscopy on normal liver showed that collagen VI antibodies labeled microfibrillar material and occasionally the surface of cells including hepatocytes. In both perinatal and fibrotic livers, electron-dense deposits were abundant in the space of Disse, intensely staining fibrils located around bundles of banded collagen. In both normal and fibrotic adult livers, collagen VI was abundant in the rough endoplasmic reticulum of Ito cells, while hepatocytes were constantly negative. In fetal livers, hepatocytes also contained collagen VI. These results suggest that collagen VI is a major constituent of the hepatic extracellular matrix. Furthermore, the cellular sources of collagen VI appear to be different in adult and developing livers.