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

Three-dimensional co-culture of hepatocytes and stellate cells

Hepatocytes self-assemble in culture to form compacted spherical aggregates, or spheroids, that mimic the structure of the liver by forming tight junctions and bile canalicular channels. Hepatocyte spheroids thus resemble the liver to a great extent. However, liver tissue contains other cell types and has bile ducts and sinusoids formed by endothelial cells. Reproducing 3-D co-culture in vitro could provide a means to develop a more complex tissue-like structure. Stellate cells participate in revascularization after liver injury by excreting between hepatocytes a laminin trail that endothelial cells follow to form sinusoids. In this study we investigated co-culture of rat hepatocytes and a rat hepatic stellate cell line, HSC-T6. HSC-T6, which does not grow in serum-free spheroid medium, was able to grow under co-culture conditions. Using a three-dimensional cell tracking technique, the interactions of HSC-T6 and hepatocyte spheroids were visualized. The two cell types formed heterospheroids in culture, and HSC-T6 cell invasion into hepatocyte spheroids and subsequent retraction was observed. RT-PCR revealed that albumin and cytochrome P450 2B1/2 expression were better maintained in co-culture conditions. These three-dimensional heterospheroids provide an attractive system for in vitro studies of hepatocyte-stellate cell interactions.

Increased Sp1-dependent transactivation of the LAMgamma 1 promoter in hepatic stellate cells co-cultured with HepG2 cells overexpressing cytochrome P450 2E1

Laminin is a basement-membrane protein that increases in liver fibrosis. To study the role of oxidative stress on laminin expression, hepatic stellate cells (HSC) were co-cultured with HepG2 cells that do or do not express (E47 or C34 cells, respectively) CYP2E1, a potent generator of oxygen radicals. Co-incubation of HSC with E47 cells increased laminin beta1 and gamma1 proteins compared with co-incubation with C34 cells; this increase was prevented by antioxidants and CYP2E1 inhibitors. Similar results were observed in co-culture with primary hepatocytes from saline- or pyrazole-treated (with high levels of CYP2E1) rats. Laminin alpha1 chain was not detectable in the HSC in any of the systems; however, laminin alpha2 chain increased in HSC co-cultured with E47 cells. Synthesis but not turnover of laminin beta1 and gamma1 proteins was increased in HSC in the E47 co-culture. Laminin beta1 and gamma1 mRNAs were up-regulated in HSC in the E47 co-culture because of transcriptional activation of both genes. Transfection experiments in HSC with reporter constructs driven by the laminin gamma1 promoter showed maximal responsiveness with the -230/+106 and the -1400/+106 constructs in the E47 system. Gel-shift assays demonstrated an increase in Sp1 binding to the laminin gamma1 promoter in HSC when co-incubated with E47 cells, which was blocked by an anti-Sp1 antibody. Co-transfection of a Sp1 expression vector further increased the responsiveness of the -330LAMgamma1-CAT reporter vector in HSC in the HSC/E47 system. These results show that diffusable CYP2E1-derived oxidative-stress mediators induce synthesis of laminins by a transcriptional mechanism in HSC. Such interactions between hepatocytes and HSC may be important during liver fibrosis.

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.

Expression and potential role of the extracellular matrix in hepatic ontogenesis: a review

Studies from a number of laboratories have provided information on the temporal and spatial expression of a variety of extracellular matrix (ECM) components in the developing liver and insight into their potential roles in hepatogenesis. Collagen type IV and laminin are present in the basement membranes of the capsular mesothelium, vascular structures of the portal and hepatic vein branches, and the ductular elements of the developing liver. The mesothelial, vascular, and ductular epithelial cells synthesize laminin and type IV collagen. In contrast, fibronectin and type I collagen are restricted to the adjacent or surrounding interstitium of those ductal and vascular elements, but are not within the basement membrane proper. The hepatic perisinusoidal space (Space of Disse) of the fetal rat develops a delicate extracellular matrix by 12.5 days of gestation, which is characterized by banded collagen fibrils and bundles associated with filamentous and flocculent material. Fibronectin, laminin, and collagen types I, III, and IV are present in the developing perisinusoidal space by this early gestational date, with laminin being the most prevalent component detected. The laminin chains localized to that region in the fetal/neonatal period are alpha 2, beta 1, beta 2, and gamma 1, whereas the alpha 1 chain of laminin is absent from the developing Space of Disse. Similar data have been reported on the laminin phenotype in the perisinusoidal space during hepatic regeneration. Electron microscopy immunohistochemistry studies have demonstrated that the sinusoidal lining cells and hepatocytes synthesize these ECM proteins during hepatogenesis. By 6 to 8 weeks of postnatal life, laminin is not detectable in the perisinusoidal space. Both the transient expression of laminin and the similarity of the laminin chain phenotype expressed in the perisinusoidal space in the developing and regenerating liver suggests a role for this protein in the organization of the hepatic lobule in those forms of hepatic morphogenesis.

Pharmacological aspects of pentoxifylline with emphasis on its inhibitory actions on hepatic fibrogenesis

1. Pentoxifylline (PTX), a derivative of the methylxanthine theobromine, has been used for many years in the treatment of peripheral vascular diseases. Increased red blood cell flexibility, reduction of blood viscosity, and decreased potential of platelet aggregation are the basic actions of PTX, resulting in therapeutic benefits due to improved microcirculation and tissue oxygenation. 2. PTX's generally accepted mechanism of action is the inhibition of phosphodiesterases, leading to increased intracellular levels of cyclic adenosine monophosphate (cAMP). 3. A number of studies have shown PTX's effects on the cytokine network. The most relevant clinical results are the therapeutic benefits of PTX in attenuating the effects of tumor necrosis factor-alpha (TNF-alpha) in conditions such as septic shock. 4. PTX also has been found to exert antifibrogenic actions, using cultured fibroblasts or animal models of fibrosis, including liver fibrosis. 5. In hepatic stellate cell culture PTX has been shown to inhibit the basic reactions of liver fibrogenesis, being effective on cytokines and growth factors relevant in fibrogenesis of the liver, too. 6. Therefore, PTX might be an effective drug with few side effects in the treatment of liver fibrosis. Further clinical studies have to be done to establish the real therapeutic benefits of PTX in liver fibrosis and cirrhosis.

Characterization of monoclonal antibodies recognizing human merosin and their use in affinity purification of native merosin

Two monoclonal antibodies (MAbs) to human placenta laminin (pl-LAM), 1D8 (IgG1) and 6G5 (IgG2b) were generated and shown by ELISA and immunoblot analysis to recognize only native pl-LAM, but not denatured, reduced pl-LAM or mouse EHS laminin. Intact pl-LAM was easily isolated and purified in large scale from human placenta by 1D8-conjugated affinity chromatography. Electrophoretic analysis of the purified pl-LAM revealed the presence of a major 750-kDa component composed of 320-, 220-, and 200-kDa polypeptides and a minor 800-kDa component composed of 320-, 240-, and 220-kDa polypeptides. Neither molecule had a 400-kDa component corresponding to the A chain. It has already been shown that the 320-kDa polypeptide is identical to the M chain of human merosin (Hori et al. J. Biochem. 1994;116:1212-1219). Electron microscopy revealed that isolated merosin was composed of three short arms and one long arm. By immunohistochemistry, MAbs showed positive staining in human adult kidney and liver. These results indicate that these MAbs recognize only native merosin and can be used to study merosin structure and function by rapid purification of native merosin and by immunohistochemical analysis.