Oral imatinib treatment reduces early fibrogenesis but does not prevent progression in the long term
Introduction
Hepatic stellate cells (HSCs) have been identified as the key source of excess extracellular matrix (ECM) in the fibrotic liver. Following liver injury of any etiology, they become activated from their quiescent state and transform into proliferative, fibrogenic, and contractile myofibroblasts. This process is initiated by paracrine secretion of growth factors and cytokines through neighboring cells whereupon HSCs respond with enhanced growth factor expression and responsiveness [1].
Platelet-derived growth factor (PDGF) is the best characterized and most potent mitogen for cultured HSCs isolated from human or rodent liver. It binds to PDGF receptors (PDGFRs) that belong to the family of receptor tyrosine kinases (RTKs). Transdifferentiation of HSCs coincides with an increased expression of PDGF [2] and a switch in receptor subtype expression from PDGFRα to -β [3]. Expression of PDGF and PDGFRβ is closely correlated with the extent of inflammation in liver tissue of patients with chronic liver diseases [2]. In addition, PDGF is profibrogenic in conditions where inflammation is less evident, such as cholestatic liver injury, where PDGF can also be synthesized by cholangiocytes [4], [5]. PDGFRs, like other RTKs, contain an extracellular ligand-binding domain connected to the cytoplasmic domain by a transmembrane helix. Upon ligand binding, PDGFRs dimerize and become autophosphorylated. Binding of adaptor molecules to phosphotyrosines, in turn, triggers various signaling pathways, including the Ras-ERK and the PI3-Akt pathways resulting eventually in cell proliferation and survival (reviewed in [6]).
Imatinib, formerly known as STI571, is a potent, competitive inhibitor of three tyrosine kinases (PDGFR, Bcr-Abl and c-Kit) [7]. It is in clinical use for the treatment of chronic myeloid leucemia and gastrointestinal stroma tumors. Imatinib produces a dose-dependent inhibition of the proliferation induced by PDGF in cultured rat HSCs. When BDL-rats were treated with imatinib the number of proliferating HSCs 48 h after BDL was reduced by 60% [8]. Similarly, when PDGF was absorbed by a soluble PDGFRβ, fibrogenesis was effectively attenuated for up to 14 days after BDL [9], [10]. Recently, the prophylactic potential of imatinib has been underlined by a significant fibrosis reduction during 8 weeks of concomitant stimulation with pig serum [11].
The evaluation of the antifibrotic potential of a given agent requires in vivo models that most closely reproduce human fibrotic liver diseases [12]. Furthermore, prophylaxis (i.e. initiation of treatment before fibrogenesis has started) does not reflect the clinical application of a potential antifibrotic drug. We therefore used the rat model of secondary biliary fibrosis after BDL which is characterized by progressive fibrosis in the absence of inflammation or necrosis to test whether imatinib shows antifibrotic effects when treatment is continued beyond 48 h after BDL and whether it is effective once liver fibrosis is already established.
Section snippets
Animals, bile duct-ligation
Male Wistar rats were kept under a 12 h light-dark cycle with free access to rat chow (Provimi Kliba AG, Kaiseraugst, Switzerland) and water. Induction of biliary fibrosis by bile duct-ligation (BDL) and sham operations were performed under ether anesthesia as previously described [13]. All animals received humane care. Animal experiments were approved by a state-appointed board on animal ethics and were performed according to international guidelines concerning the conduct of animal
Animal characterization and haemodynamics
Four rats died during the first five days after BDL. Two belonged to the treatment group and two were untreated. No further animals died during the last 2 weeks of the experiment. One imatinib treated rat was found to have a restored bile flow on laparotomy and was excluded from analysis. Three weeks after BDL ABT showed comparably impaired liver function in both treated and untreated rats (Table 2). Liver weight was increased in all BDL-rats compared with sham-operated rats (P<0.0001), but
Discussion
Abundant evidence has confirmed the involvement of PDGF and its respective receptors in the activation process of HSCs. Therefore PDGF signaling has repeatedly been postulated as an attractive therapeutic target [8], [9], [18], [19]. To our knowledge, the present study is the first to investigate the therapeutic potential of imatinib, a potent inhibitor of the PDGF-R tyrosine kinase, for a period longer than 48 h after BDL or in established fibrosis.
We found that prophylactic imatinib markedly
Acknowledgements
This study was supported by a grant from the Swiss National Foundation for Scientific Research (3200B0-105601/1) to JR.
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