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 hepatocel lular carcinoma have not been clearly identified. Methods: In vivo collagen synthesis was studied by in situ hybridization an d immunohistochemistry for types I, IV, V and VI collagen, together with im munolabeling 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, immunocytochemis try and RNase protection. Results: All collagens studied were expressed in the stroma of the tumor, w ith a higher level of type VI and IV collagens than of type I and V. The ma jority of the cells expressing collagen transcripts in human hepatocellular carcinoma stroma were alpha-actin positive and CD 34 negative. In vitro ex periments 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 pres ent in tumor cell conditioned medium. It was not explained by an increase i n 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 hepatocyte s increase extracellular matrix deposition by cultured myofibroblasts, prob ably by post-transcriptional mechanisms. The generation of hepatocellular c arcinoma stroma by myofibroblasts could thus be under control of tumoral ce lls.