Distribution of fibronectin and fibronectin-binding proteins, AGp110 and integrin alpha 5 beta 1, during chemically induced hepatocarcinogenesis in adult rats

Hepatocyte polarity

Hepatocytes, like other epithelia, are situated at the interface between the organism's exterior and the underlying internal milieu and organize the vectorial exchange of macromolecules between these two spaces. To mediate this function, epithelial cells, including hepatocytes, are polarized with distinct luminal domains that are separated by tight junctions from lateral domains engaged in cell-cell adhesion and from basal domains that interact with the underlying extracellular matrix. Despite these universal principles, hepatocytes distinguish themselves from other nonstriated epithelia by their multipolar organization. Each hepatocyte participates in multiple, narrow lumina, the bile canaliculi, and has multiple basal surfaces that face the endothelial lining. Hepatocytes also differ in the mechanism of luminal protein trafficking from other epithelia studied. They lack polarized protein secretion to the luminal domain and target single-spanning and glycosylphosphatidylinositol-anchored bile canalicular membrane proteins via transcytosis from the basolateral domain. We compare this unique hepatic polarity phenotype with that of the more common columnar epithelial organization and review our current knowledge of the signaling mechanisms and the organization of polarized protein trafficking that govern the establishment and maintenance of hepatic polarity. The serine/threonine kinase LKB1, which is activated by the bile acid taurocholate and, in turn, activates adenosine monophosphate kinase-related kinases including AMPK1/2 and Par1 paralogues has emerged as a key determinant of hepatic polarity. We propose that the absence of a hepatocyte basal lamina and differences in cell-cell adhesion signaling that determine the positioning of tight junctions are two crucial determinants for the distinct hepatic and columnar polarity phenotypes.

Autocrine-controlled formation and function of tissue-like aggregates by primary hepatocytes in micropatterned hydrogel arrays

The liver carries out a variety of essential functions regulated in part by autocrine signaling, including hepatocyte-produced growth factors and extracellular matrix (ECM). The local concentrations of autocrine factors are governed by a balance between receptor-mediated binding at the cell surface and diffusion into the local matrix and are thus expected to be influenced by the dimensionality of the cell culture environment. To investigate the role of growth factor and ECM-modulated autocrine signaling in maintaining appropriate primary hepatocyte survival, metabolic functions, and polarity, we created three-dimensional cultures of defined geometry using micropatterned semisynthetic polyethylene glycol-fibrinogen hydrogels to provide a mechanically compliant, nonadhesive material platform that could be modified by cell-secreted factors. We found that in the absence of exogenous peptide growth factors or ECM, hepatocytes retain the epidermal growth factor (EGF) receptor ligands (EGF and transforming growth factor-α) and the proto-oncogenic mesenchymal epithelial transition factor (c-MET) ligand hepatocyte growth factor (HGF), along with fibronectin. Further, hepatocytes cultured in this three-dimensional microenvironment maintained high levels of liver-specific functions over the 10-day culture period. Function-blocking inhibitors of α5β1 or EGF receptor dramatically reduced cell viability and function, suggesting that signaling by both these receptors is needed for in vitro survival and function of hepatocytes in the absence of other exogenous signals.

Liver repopulation and carcinogenesis: two sides of the same coin?

Liver repopulation by transplanted normal hepatocytes has been described in a number of experimental settings. Extensive repopulation can also occur from the selective proliferation of endogenous normal hepatocytes, both in experimental animals and in the human liver. This review highlights the intriguing association between clinical and experimental conditions related to liver repopulation and an increased risk for development of hepatocellular carcinoma. It is suggested that any microenvironment that is able to sustain the clonal growth of normal transplanted (or endogenous) hepatocytes is also geared to select for the emergence of rare resistant cells with an altered phenotype. Whereas the first pathway leads to liver repopulation with normal histology, the latter results in the growth of focal proliferative lesions and carries an increased risk of neoplastic disease. The implications of this association are discussed, both in terms of pathogenetic significance and possible therapeutic exploitation.

Phenobarbital-induced cytoplasmic accumulation of beta1-integrin in rat liver enzyme altered foci; an immunohistological study

In this immunohistological study we investigated integrin expression in EAF in female rats treated with diethylnitrosamine (DEN) as initiator and phenobarbital (PB) as promotor (DEN-PB treatment) for up to 32 weeks. Using a beta1-integrin antibody, there was an increased cytoplasmic staining and a decreased sinusoidal staining in EAF, as compared to non-EAF areas. The majority of small EAF and all larger EAF exhibited this altered distribution of beta1-integrin. The increased cytoplasmic staining was not found in EAF after a 10 week treatment-free period. In periportal areas in partial hepatectomized control rats a similar increase in cytoplasmic staining was seen. EAF in DEN-initiated and DEN-promoted rats (DEN-DEN treatment) were also studied. This protocol induced rapidly growing EAF. Most lesions did not show the increased cytoplasmic staining. However, after partial hepatectomy of DEN-DEN-treated rats, a cytoplasmic staining was seen in EAF. It is concluded that PB induced a reversible cytoplasmic beta1-integrin expression in many EAF and in all larger EAF. It is suggested that the alteration constitutes part of hepatocyte resistance to toxicological stress and apoptosis in EAF.

Synergistic induction by collagen and fibronectin of liver-specific genes in rat primary cultured hepatocytes

The extracellular matrix plays an important role for maintaining liver functions. We examined the effects of type I collagen and fibronectin on the expression of liver-specific genes in rat primary hepatocytes. When primary culture hepatocytes were overlaid with a type I collagen-gel, the expression of liver-specific genes (tyrosine aminotransferase, aldolase B, and albumin) increased by 4-5 times, compared with not overlaid hepatocytes. In contrast, the expression of non-liver-specific genes (GAPDH and beta-actin) was suppressed under the same conditions. The addition of fibronectin together with type I collagen-gel further enhanced the expression of liver-specific genes by 1.4-1.8 times. The addition of GRGDS peptide instead of fibronectin with the collagen-gel had a similar effect on hepatic gene expression to that of fibronectin. Addition of fibronectin alone exhibited had no effect on gene expression. These results suggest that type I collagen and fibronectin synergistically induce liver-specific genes.

Expression of the integrin alpha5 subunit and its mediated cell adhesion in hepatocellular carcinoma

Tumor invasion and metastasis are complex processes, requiring the ability of tumor cells to interact with proteins of the extracellular matrix through cell-adhesion molecules on the cell surface. Integrins are heterodimeric membrane glycoproteins, consisting of alpha and beta subunits, which enable cells to recognize adhesive substrates in the extracellular matrix. The roles of the integrin alpha5beta1 in tumor invasion are highlighted by finding that some tumor cells have lost or reduced alpha5beta1 expression. It therefore functions as a negative signaling regulator. Expression of alpha5beta1 and its mediation of cell adhesion in hepatocellular carcinoma (HCC) have not been elucidated. In surgical specimens of HCC we found, by immunohistochemistry and Northern blot analysis, that the alpha5-positive rates in cancerous tissues were lower than the corresponding rates in non-cancerous tissues. Reduced expression of the integrin alpha5 occurred more frequently in HCC with more malignant phenotypes, such as poor differentiation, large size (more than 10-cm in diameter), absence of capsule and high invasion. Reverse transcription/polymerase chain reaction, a more sensitive assay, was used to detect the alpha5 mRNA level in LCID20, a highly metastatic model of human HCC, and LCID35, a low-metastasis model. The results showed that integrin alpha5 was negative in the former and positive in the latter. Cell adhesion assays showed the maximal percentage inhibition of anti-alpha5 mAb on SMMC 7721 cell adhesion to fibronectin to be 68.9 +/- 4.9% at the saturation concentrations of each antibody (200 microg/ml). If anti-alpha5 mAb was combined with anti-beta1 mAb, the inhibition was 74.1 +/- 11.1%. It is concluded that reduced expression of the integrin alpha5 subunit is correlated with more malignant phenotypes of human HCC. Any change in the adhesion of hepatocellular carcinoma cells to fibronectin is mainly dependent upon the function of the integrin alpha5beta1.

Fibronectin in human bile fluid for diagnosis of malignant biliary diseases

In a preliminary study, we demonstrated a strong association between the concentration of the glycoprotein fibronectin (FN) in human bile fluid and the presence of malignant biliary diseases. We now present the results of measurements of total FN (tFN) and cellular FN (cFN) within a larger group of 71 patients. Bile fluid was collected during routine endoscopic retrograde cholangiography or by transhepatic puncture, respectively, from patients admitted for examination/treatment of biliary obstruction. Determination of tFN in bile was performed using a previously described time-resolved fluorescence immunoassay (TRFIA). For cFN, a newly developed TRFIA, using a specific monoclonal antibody for the EDA epitope of cFN, was applied. Within the noncarcinoma group of patients (n=50), consistently low concentrations of tFN (median = 5 ng/mL) were found. In most of these cases, the corresponding concentrations of cFN were below the detection limit (2.6 ng/mL) of this assay. Highly significantly elevated concentrations were found for both tFN (median = 1,220 ng/mL) and cFN (median = 243 ng/mL) in the carcinoma group (n = 21) in comparison with the noncarcinoma group (P < or = .01). By adopting cutoff values of 60 ng/mL for tFN and >0 ng/mL for cFN, diagnostic sensitivities for carcinoma of the biliary tract of 0.89 and 0.92, and specificities of 0.96 and 0.98, respectively, were computed. FN in bile fluid is suggested as a sensitive, specific, and easily determined marker for differential diagnosis of malignant and benign diseases of the biliary tract.