The liver has the unique capacity to regulate its mass after loss of functional liver cells due to liver disease, injury, and/or toxicity. Unfortunately, in the course of chronic liver disease this meticulously regulated regeneration process is imbalanced resulting in a decreased regenerative capacity. Like in humans, most liver diseases in dogs are chronic and occur through persistent inflammation due to different causes. On-going liver cell damage leads to a reduction of the functional liver cell mass and progressive deposition of fibrous tissue in the liver. We found that regenerative and fibrotic molecular pathways of canine cirrhosis resembled those in human liver samples with cirrhosis, reinforcing the current paradigm of a highly comparable pathophysiology of human and canine hepatic disease. Dogs with liver diseases are therefore relevant animal models for the evaluation of novel therapeutic approaches, such as the stimulation of different cell types (mature or immature) to participate in the regeneration process by growth factor therapy. In the present thesis, two options of approach are proposed; (1) the use of Hepatocyte Growth Factor (HGF) and (2) liver progenitor cell-based therapies. (1) HGF is the principal factor for stimulating the liver to grow and regenerate and suppress fibrosis and apoptosis. Therefore, recombinant HGF was produced and tested for its biological activity. The first clinical trial with recombinant HGF was initiated in dogs with a congenital portosystemic shunt (CPSS). (2) When the proliferation of mature liver cells is inhibited, the proliferation and differentiation (activation) of normally quiescent liver progenitor cells (LPC) are observed. LPC activation has recently been demonstrated in spontaneous canine and human liver diseases and has opened a new field of research in regenerative medicine. A tool to gain insight into the molecular mechanisms and regulatory events leading to LPC activation is provided by culturing these cells in vitro. However, purification of LPCs is complicated due to their scarce number in healthy livers and the lack of specific LPC markers. We described two techniques to isolate and culture LPCs from healthy mature dog livers. These cells were characterized at a molecular level and may be ideally suited for future cell therapies. In conclusion, the results in the present thesis can be mutually beneficial for canine and human biomedical research and provide possible novel means of therapy for liver disease. The research was conducted at the Department of Clinical Sciences of Companion Animals (Faculty Core Programme Tissue Repair and the University Focus Point Growth and Differentiation. Collaborations include Departments of Pathobiology (Prof. De Bruin) and Biochemistry & Cell Biology (Prof. Helms). Collaborations outside the faculty include the Departments of Molecular Pathology (Prof. Roskams) and Molecular Cell Biology (Dr. Vankelecom), University Hospitals of Leuven (Belgium), Zenoaq (Japan), and Intervet B.V. (Boxmeer)
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