Biomarkers of hepatic fibrosis, fibrogenesis and genetic pre-disposition pending between fiction and reality

Abstract

Fibrosis is a frequent, life-threatening complication of most chronic liver diseases. Despite major achievements in the understanding of its pathogenesis, the translation of this knowledge into clinical practice is still limited. In particular, non-invasive and reliable (serum-) biomarkers indicating the activity of fibrogenesis are scarce. Class I biomarkers are defined as serum components having a direct relation to the mechanism of fibrogenesis, either as secreted matrix-related components of activated hepatic stellate cells and fibroblasts or as mediators of extracellular matrix (ECM) synthesis or turnover. They reflect primarily the activity of the fibrogenic process. Many of them, however, proved to be disappointing with regard to sensitivity and specificity. Up to now hyaluronan turned out to be the relative best type I serum marker. Class II biomarkers comprise in general rather simple standard laboratory tests, which are grouped into panels. They fulfil most criteria for detection and staging of fibrosis and to a lesser extent grading of fibrogenic activity. More than 20 scores are currently available, among which Fibrotest is the most popular one. However, the diagnostic use of many of these scores is still limited and standardization of the assays is only partially realized. Combining of panel markers in sequential algorithms might increase their diagnostic validity. The translation of genetic pre-disposition biomarkers into clinical practice has not yet started, but some polymorphisms indicate a link to progression and outcome of fibrogenesis. Parallel to serum markers non-invasive physical techniques, for example, transient elastography, are developed, which can be combined with serum tests and profiling of serum proteins and glycans.

1

Schematic presentation of the pathogenetic sequence of liver fibrosis and cirrhosis based on the activation of hepatic stellate cells (HSC) and transdifferentiation to matrix-synthesizing myofibroblasts (MFB). The inset of the electron micrograph shows retinoid-filled lipid droplets of HSC indenting the nucleus. Surrogate pathogenetic mechanisms contributing to the expansion of the myofibroblast pool in fibrotic liver are indicated: epithelial-mesenchymal-transition (EMT) of biliary epithelial cells or even hepatocytes, transformation of circulating monocytes at the site of injury to fibroblasts and the influx of bone marrow-derived fibrocytes into damaged tissue. Examples of serum biomarkers reflecting the pathogenetic sequence are given, but a considerable overlap is noticeable. Abbreviations: see Table 2, CRP, C-reactive protein; CSF, colony-stimulating factor; CTGF, connective tissue growth factor; GLDH, glutamate-dehydrogenase; PIVKA, prothrombin induced by vitamin K absence

2

Network of resident liver cells (red) and inflammatory non-liver resident cells (black) with hepatic stellate cells in the process of activation and transdifferentiation to myofibroblasts. Major molecular mediators are indicated. The influx of inflammatory and immune competent cells from the circulation into the damaged liver tissue is illustrated. Secreted products of resident liver cells leading to biochemical changes in blood of liver fibrotic patients are exemplified. Abbreviations: AcAld, acetalde-hyde; α₂M, α₂-macroglobulin; CTGF, connective tissue growth factor; EGF, epidermal growth factor; ET-1, endothelin-1; HNE, 4-hydroxynonenal; HSC, hepatic stellate cells; ICAM-1, intercellular adhesion molecule-1; IGFBP, IGF-binding proteins; ROS, reactive oxygen species; VEGF, vascular endothelial growth factor.

3

Pathobiochemical mechanisms of elevation of class I biomarkers of fibrosis exemplified by collagens hyaluronan and laminin, respectively. Increased production by activated hepatic stellate cells due to paracrine stimulation via TGF-β by interacting cells, mechanical stress and hypoxia leads to stimulated secretion and consecutive deposition as incomplete basement membranes in the space of Disse and perisinusoidal fibrosis. As a consequence of newly developed sub-endothelial basement membrane and cellular insufficiency, the clearance function of the sinusoidal compartment for circulating matrix components is decreased and intrahepatic hemodynamic resistance is elevated. The latter leads to perihepatic shunting of blood reducing further the elimination of matrix components and their fragments from the blood.

4

Receiver-operating-characteristic (ROC) curves of the diagnostic power of serum CTGF and of the CTGF/platelet ratio for fibrosis and cirrhosis, respectively. AUC, area under the curve. Data compiled from ref. [55].