Collagens in the liver extracellular matrix bind hepatocyte growth factor

Myofibroblasts are responsible for collagen synthesis in the stroma of human hepatocellular carcinoma: an in vivo and in vitro study

BACKGROUND/AIMS

Marked changes in extracellular matrix occur in the stroma of hepatocellular carcinoma, as compared to normal or cirrhotic liver. The cell types responsible for extracellular matrix synthesis within hepatocellular carcinoma have not been clearly identified.

METHODS

In vivo collagen synthesis was studied by in situ hybridization and immunohistochemistry for types I, IV, V and VI collagen, together with immunolabeling of alpha-smooth muscle actin, a myofibroblast marker, and CD34, an endothelial cell marker. In vitro, extracellular matrix deposition by cultured myofibroblasts was studied by reticulin staining, immunocytochemistry and RNase protection.

RESULTS

All collagens studied were expressed in the stroma of the tumor, with a higher level of type VI and IV collagens than of type I and V. The majority of the cells expressing collagen transcripts in human hepatocellular carcinoma stroma were alpha-actin positive and CD 34 negative. In vitro experiments demonstrated that the hepatocellular carcinoma cell lines HepG2, HuH7 and Hep3B markedly increased extracellular matrix deposition by human liver myofibroblasts. This increase was mediated by a soluble mediator present in tumor cell conditioned medium. It was not explained by an increase in mRNA levels of extracellular matrix components, nor by a decrease in the secretion of matrix-degrading proteinases by myofibroblasts.

CONCLUSIONS

Myofibroblasts are the main source of collagens in the stroma of hepatocellular carcinoma. Our data also indicate that tumoral hepatocytes increase extracellular matrix deposition by cultured myofibroblasts, probably by post-transcriptional mechanisms. The generation of hepatocellular carcinoma stroma by myofibroblasts could thus be under control of tumoral cells.

Interactions of fibroblasts with the extracellular matrix: implications for the understanding of fibrosis

The cellular organization and the compartmentalization in multicellular organisms is mediated by the extracellular matrix (ECM). This structure is composed by a wide variety of different macromolecules which carry distinct domains with defined structural and/or biological activities. Cells are known to interact with these molecules via specific receptors. Following activation, these receptors transduce signals either directly to the intracellular cytoskeleton or via different signalling cascades. Cell-matrix interactions, therefore, not only control the shape and orientation of cells but can also directly regulate cellular functions, including migration, differentiation, proliferation, and the expression of different genes. These cell-matrix interactions have been elucidated in detail for several biological processes, especially morphogenesis and differentiation, but also play an important role during pathological situations, e.g. wound healing and tumor progression. Although much less investigated, similar mechanisms are thought to regulate the biological behavior of fibroblastic cells, the final target cells in fibrosis. The activity of these cells depends in various ways on the presence of ECM molecules. First, some of the molecules are known to bind to and modulate the activity of those growth factors and cytokines, which lead to the activation of fibroblasts during the early phases of fibrosis. Second, deposition of large amounts of ECM molecules alters the environment and the mechanical load on the cells which are embedded in this matrix. Third, ECM molecules directly modulate fibroblast metabolism via certain integrin receptors. This review summarizes recent developments in all three domains. It mainly focuses on the direct role of ECM molecules in the biosynthetic activity of fibroblasts.