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The extracellular matrix of fibrotic liver is predominantly produced by mesenchymal cells, the in vivo phenotype of which is poorly defined. We report on the application of combined immunohistology and in situ hybridization with [35S]-labeled alpha 1(I) and alpha 1(IV) procollagen RNA probes. In CCl4-treated rats, all alpha 1(I) procollagen-producing cells were vimentin positive but cytokeratin negative; over 90% expressed desmin, a marker of rat liver stellate cells. alpha 1(I) Procollagen-expressing, desmin-negative cells were confined to portal tract and perivascular stroma. Similarly, alpha 1(I) procollagen gene transcripts were, in all instances, colocalized with vimentin in human liver. In fibrotic specimens, over 70% of these cells expressed alpha-actin. Antibodies against epithelial, endothelial, and Kupffer cells, granulocytes, and lymphocytes did not react with alpha 1(I) procollagen RNA-expressing cells. Localization, morphology, and immunophenotype of alpha 1(I) procollagen-expressing cells indicated stellate cells and portal/vascular fibroblasts, but not epithelial cells, to be sources of hepatic interstitial collagen. However, most alpha 1(IV)-expressing human liver cells were identified as endothelial cells, the remaining cells were (myo-)fibroblastic and bile duct epithelial cells, but not hepatocytes. This indicates synthesis of procollagen type IV from both sides of the basement membrane and suggests an active participation of endothelial cells in the process of sinusoidal capillarization.

The cellular localization of expression of various genes activated during the course of liver fibrosis and regeneration was studied by immunohistology and in situ hybridization in rat and human liver tissues. Mesenchymal cells proved to be the principal sources of extracellular matrix proteins and of fibrogenic growth factors, whereas the collagenase-activating protease transin/stromelysin gene was transcribed in parenchymal cells as well. Fibrogenesis by the mesenchymal compartment appears to be balanced by fibrolysis controlled by parenchymal cell functions. Continuous parenchymal damage may thus disrupt this balance between fibrogenesis and fibrolysis, resulting in fibrosis.

We have examined the expression of the extracellular matrix-degrading metalloprotease transin/stromelysin during the early phases of rat liver regeneration following toxic injury by a single dose of carbon tetrachloride (CCl4). In situ hybridization displayed cell type-specific spatial and temporal RNA expression patterns with high transcript levels in small proportions of hepatocytes and non-parenchymal cells, peaking at 24 and 48 h after intoxication, respectively. In agreement with the presence of c-fos and c-jun recognition sites on the transin gene, expression of these oncogenes preceded transin expression. Transin-expressing hepatocytes were largely localized in areas subsequently eliminated by necrosis due to CCl4 intoxication. As a consequence of these expression patterns and the key function of transin as an activator of interstitial collagenase, it seems that the hepatic fibrosis observed after CCl4 administration may be related to fibrogenesis unbalanced by fibrolysis due to altered transin expression.