Fibrosis may be described as extensive scar formation, observed as increased deposition and abnormal distribution of extracellular matrix (ECM) components, such as collagen and proteoglycans. Liver fibrosis is a serious complication of chronic inflammatory liver diseases arising from diverse infectious, inflammatory or toxic causes . Progression of fibrosis eventually leads to liver cirrhosis, which is a major global health problem accounting for approximately 800,000 deaths per year worldwide [1–3].
Histopathological examination of liver biopsies is the gold standard for diagnosis and staging of chronic liver diseases and is of significance when evaluating the effect of therapeutic intervention. Liver biopsy, however, has two significant drawbacks. It is invasive, and prone to variation in the length and size of the tissue specimen which leads to low reproducibility and high intrapatient variation [4, 5]. Therefore, the development of non-invasive biomarkers of fibrogenesis may be important for the staging and monitoring of chronic liver disease, and may provide additional information on the pathogenesis.
Matrix remodelling is a normal integrated process of tissue homeostasis and maintenance but it changes under certain disease conditions [6, 7]. The central pathological feature of fibrosis is uncontrolled ECM remodelling [4–6, 8, 9]. The liver ECM is composed mainly of fibrous proteins such as collagens and proteoglycans [6, 7, 10]. During fibrogenesis the quality, quantity, and distribution of the ECM in the liver changes, which results in excessive accumulation of fibrous tissue (that is, scar tissue), and an overall increase in ECM density [6, 7, 10]. A cirrhotic liver may contain up to six times more collagen than that of a healthy liver [6–8, 11].
The formation and degradation of ECM components is accompanied by the release of protein breakdown products into the circulation [7, 12]. Thus, circulating levels of these byproducts may potentially be used as biochemical markers for assessing the extent of disease and prognosis, and for monitoring response to treatment . Although several biomarkers such as hyalronic acid, collagen markers, and laminin are available for diagnosis and follow-up of liver fibrosis, the accuracy of these biomarkers for detection of fibrosis is highly variable [4, 5].
In the healthy human liver the most abundant collagens are the fibril-forming types I and III. Fibril-forming collagens are synthesised as precursor molecules with large propeptide extensions at both the N-terminal and C-terminal of the molecule . The mature propeptides are cleaved from procollagen by N-terminal or C-terminal proteinases, and mature collagen is integrated into the ECM [8, 15, 16]. During fibrogenesis, type I collagen levels increase up to eightfold [4, 5, 9, 17]. Notably, type I collagen levels increase significantly more than type III, changing the ratio from 1:1 in the healthy liver to 1:2 in the cirrhotic liver [4, 5, 9, 17]. Measuring serum levels of the procollagen type I N-terminal propeptide (PINP) released during collagen formation may be useful as a marker of fibrogenesis, either alone, in ratios or in combination with other techniques.
Several animal models for liver fibrosis have been developed, most of them using small rodents , each with individual strengths and weaknesses. Bile duct ligation (BDL) has been used as an animal model of chronic liver injury due to its resemblance to hepatocyte damage, hepatic stellate cell activation, and liver fibrosis observed in human cholestatic liver disease . Even though a range of investigators have used BDL rats as models of liver fibrosis, the measurement of serological biochemical markers of liver ECM turnover has not been presented. This may in part be due to the lack of procollagen markers for rodent use. The current study is, to our knowledge, the first to use the type I collagen turnover marker, PINP, to monitor the development of liver fibrosis in BDL rats. In contrast to previous researchers, we used mature rats of 6 months of age, because in young rodents collagen turnover is highly elevated during skeletal growth and remodelling of the growth plate .
Bone turnover can be measured by serological biochemical markers . Bone formation can be assessed by both type I collagen propeptides and osteocalcin, which is synthesised and secreted by osteoblasts during bone formation . Collagen type I constitutes 90% of bone, but it is also present in many other tissues including liver, skin and tendons [19–21]. In contrast, osteocalcin is one of the major non-collagenous proteins of bone constituting approximately 50% of the total non-collagenous proteins , and is considered more bone specific.
The aim of the current study was to investigate whether PINP levels, indicating collagen type I formation, could be a potential marker for liver fibrosis in an experimental model. We compared levels of PINP with those of osteocalcin, a bone-specific marker, to investigate the specificity for liver and bone. We evaluated liver fibrosis by quantitative histology.