Urinary excretion of the collagen cross-link pyridinoline increases during liver fibrogenesis

Collagen cross-linking mediated by lysyl hydroxylase 2: an enzymatic battlefield to combat fibrosis

The hallmark of fibrosis is an excessive accumulation of collagen, ultimately leading to organ failure. It has become evident that the deposited collagen also exhibits qualitative modifications. A marked modification is the increased cross-linking, leading to a stabilization of the collagen network and limiting fibrosis reversibility. Not only the level of cross-linking is increased, but also the composition of cross-linking is altered: an increase is seen in hydroxyallysine-derived cross-links at the expense of allysine cross-links. This results in irreversible fibrosis, as collagen cross-linked by hydroxyallysine is more difficult to degrade. Hydroxyallysine is derived from a hydroxylysine in the telopeptides of collagen. The expression of lysyl hydroxylase (LH) 2 (LH2), the enzyme responsible for the formation of telopeptidyl hydroxylysine, is universally up-regulated in fibrosis. It is expected that inhibition of this enzyme will lead to reversible fibrosis without interfering with the normal repair process. In this review, we discuss the molecular basis of collagen modifications and cross-linking, with an emphasis on LH2-mediated hydroxyallysine cross-links, and their implications for the pathogenesis and treatment of fibrosis.

Essential modification of the Sircol Collagen Assay for the accurate quantification of collagen content in complex protein solutions

Collagen contains the unique imino acid hydroxyproline (HyPro), which is involved in the stabilization of this triple helical molecule. The concentration of HyPro is customarily used to calculate the total collagen content in a cell culture environment and in acid hydrolysates of normal and pathophysiological tissues. Radiolabelling, chromatographic and calorimetric assays have been developed over the years for the accurate determination of collagen content through HyPro estimation. Recently, the Sircol Collagen Assay (SCA) has been almost exclusively adopted as the fastest and simplest colorimetric method for the determination of collagen concentration in complex protein solutions. We show here that the colorimetric SCA, which is based on the binding of Sirius red (SR) to collagen, is flawed by interference of non-collagenous proteins (e.g. serum). In fact, we demonstrate that SCA in cell culture systems and tissue hydrolysates results in a dramatic overestimation of collagen content ranging from 3- to 24-fold. In order to rescue this otherwise very practical assay, we introduce a simple purification procedure that allows the removal of interfering non-collagenous proteins from culture media and tissue samples so that accurate measurements with SCA are now possible.

Impact of treatment with praziquantel, silymarin and/or β-glucan on pathophysiological markers of liver damage and fibrosis in mice infected with Mesocestoides vogae (Cestoda) tetrathyridia

Mesocestoides vogae tetrathyridia infection in mice causes hepatocyte injury, hepatic granulomatous inflammmation, liver fibrosis and chronic peritonitis manifested with portal hypertension. To reduce the detrimental effect of parasites on the host liver, the effect of the anthelmintic drug praziquantel (PZQ) in combination with natural products silymarin (an antioxidant) and β-glucan (an immunomodulator) was investigated. The therapeutic effect of drugs was assessed by means of aminotransferase (alanine aminotransferase (ALT) and aspartate aminotransferase (AST)) activities, content of albumin, total proteins and hyaluronic acid (HA) in sera of ICR mice infected with M. vogae larvae. Animals were treated with PZQ suspended in oil emulsion (Group 1), PZQ combined with silymarin incorporated into lipid microspheres (LMS) (Group 2), PZQ combined with β-glucan incorporated into liposomes (LG) (Group 3), PZQ co-administered with LMS and LG (Group 4). Untreated animals (Group 5) served as the control. Treatment of animals started at the early chronic phase of infection (day 14 p.i.) and lasted 10 days; serum samples were collected on days 0, 7, 14, 25, 28, 31, 35 and 45 p.i. ALT and AST activities were significantly (P < 0.05) decreased in Groups 2, 3 and 4. HA content was significantly (P < 0.05 and 0.01) lower in Groups 2 and 4. Albumin levels were decreased in Groups 2 and 4, total protein concentration decreased in Groups 1 and 3 (P < 0.05 and 0.01). These results showed that combined treatment of PZQ with silymarin and/or β-glucan was able to ameliorate or suppress fibrogenesis in the liver, protect liver cells from oxidative damage and, possibly, stimulate regeneration of the parenchyma.

Antifibrogenic role of the cannabinoid receptor CB2 in the liver

BACKGROUND & AIMS

Hepatic myofibroblasts are central for the development of liver fibrosis associated with chronic liver diseases, and blocking their accumulation may prevent fibrogenesis. Cannabinoids are the active components of marijuana and act via 2 G-protein-coupled receptors, CB1 and CB2. Here, we investigated whether liver fibrogenic cells are a target of cannabinoids.

METHODS

CB2 receptors were characterized in biopsy specimens of normal human liver and active cirrhosis by immunohistochemistry, and in cultures of hepatic stellate cells and hepatic myofibroblasts by reverse-transcription polymerase chain reaction (RT-PCR), immunocytochemistry, and GTPgammaS assays. Functional studies were performed in cultured hepatic myofibroblasts and activated hepatic stellate cells. Carbon tetrachloride-induced liver fibrosis was studied in mice invalidated for CB2 receptors.

RESULTS

In liver biopsy specimens from patients with active cirrhosis of various etiologies, CB2 receptors were expressed in nonparenchymal cells located within and at the edge of fibrous septa in smooth muscle alpha-actin-positive cells. In contrast, CB2 receptors were not detected in normal human liver. CB2 receptors were also detected in cultured hepatic myofibroblasts and in activated hepatic stellate cells. Their activation triggered potent antifibrogenic effects, namely, growth inhibition and apoptosis. Growth inhibition involved cyclooxygenase-2, and apoptosis resulted from oxidative stress. Finally, mice invalidated for CB2 receptors developed enhanced liver fibrosis following chronic carbon tetrachloride treatment as compared with wild-type mice.

CONCLUSIONS

These data constitute the first demonstration that CB2 receptors are highly up-regulated in the cirrhotic liver, predominantly in hepatic fibrogenic cells. Moreover, this study also highlights the antifibrogenic role of CB2 receptors during chronic liver injury.

Potential use of collagen and elastin degradation markers for monitoring liver fibrosis in schistosomiasis

Liver fibrosis is a serious complication of schistosomiasis infection, is associated with increased amounts of collagen and the collagen cross-link, pyridinoline. Non-invasive markers of liver fibrosis have been developed. Serum and urinary markers of collagen synthesis and degradation have been studied to assess the balance between collagen synthesis, measured with markers of collagen synthesis such as amino-terminal propeptide of type III procollagen (PIIINP), and markers of degradation such as pyridinoline or pyridinoline cross-linked carboxyterminal telopeptide of type I collagen (ICTP). It has been shown that mice infected with Schistosomiasis mansoni excrete excess pyridinoline cross links in urine and this was correlated with the collagen content of granulomas from the liver. Treatment of infected mice with an anti-parasitic drug, praziquantel, decreased the collagen content of parenchyma and excretion of pyridinoline in the urine. Although the connective tissue protein, elastin, is present in the liver, the role of elastin in liver fibrosis has not been investigated. However, it has been shown that the urinary concentration of elastin specific crosslinks, desmosine and isodesmosine, as well as the urinary concentration of the collagen crosslink, pyridinoline, correlated well with liver fibrosis score in biopsy specimens from patients with liver disease secondary to hepatitis C virus and alcohol. Each biopsy specimen was reviewed by two pathologists who were blinded as to the clinical data. The pathological evaluation generated scores for both inflammation and fibrosis. No correlation was seen between the urinary markers and inflammation scores. The measurement of non-invasive markers of collagen synthesis and degradation may be useful in monitoring the reversal of fibrosis following therapeutic intervention in schistosome infections.

Diagnostic value of connective tissue metabolites in Schistosoma mansoni related liver disease

Reliable non-invasive markers of hepatosplenic involvement in schistosomiasis are needed for determination of morbidity levels in endemic populations and for diagnosis and follow-up of affected individuals. Serum levels of connective tissue metabolites have been investigated as fibrosis markers in various hepatic disorders, but their accuracy in the detection of hepatosplenic schistosomiasis under endemic conditions has not been fully elucidated. 206 adult inhabitants of a Tanzanian village highly endemic for schistosomiasis mansoni (prevalence 88%) underwent clinical, parasitological and sonographic work-up; sera were tested for aminoterminal procollagen III-peptide (PIIIP), carboxyterminal procollagen IV peptide (NC1) and laminin. Connective tissue marker levels did not correlate with the presence or intensity of infection. NC1 levels were significantly correlated with periportal liver fibrosis (P < 0.001), splenomegaly (P < 0.002), portal vein dilatation (P < 0.004) and the presence of portosystemic collaterals (P < 0.001); for PIIIP and laminin, none of the respective relationships was significant. Due to wide overlap of NC1 levels between individuals with normal sonography findings and those with advanced periportal fibrosis and portal hypertension, the sensitivity and positive predictive value of this markers to detect these individuals were low (< 40%), although specificity and overall accuracy in the given setting were good (80-90%). It is concluded that PIIIP and laminin are not useful as diagnostic serum markers of hepatosplenic schistosomiasis at the community level; NC1 was significantly related to various indices of hepatosplenic involvement, but its low sensitivity precludes its use as a screening tool under endemic conditions.

Monitoring of extracellular matrix metabolism and cross-linking in tissue, serum and urine of patients with chromoblastomycosis, a chronic skin fibrosis

BACKGROUND

Chromoblastomycosis is a fungal disease leading to a granulomatous reaction associated with dermal fibrosis.

METHODS

In an attempt to elucidate the mechanisms leading to improvement in the cutaneous lesions after treatment with terbinafine, a new antifungal drug, we analysed collagen content and cross-linking before and at the end of the treatment. The turnover of extracellular matrix was monitored for 1 year by following up serum and urinary metabolites.

RESULTS

The serum levels of type III collagen and its N-terminal propeptide were correlated with the lesion size (P < 0.035) after 4 and 12 months of treatment respectively. After 4 months of treatment, urinary pyridinoline was higher (P = 0.04) in patients whose lesion size was reduced by more than 50% and serum hyaluronan was lower in patients who had lesions active for less than 5 years (P < 0.05). The treatment increased pyridinoline and pentosidine cross-links in the lesions but significantly reduced the collagen content (P = 0.05).

CONCLUSION

This is the first demonstration that, in addition to its fungicidal activity, terbinafine acts in vivo as an antifibrotic drug.