Hepatic fibrosis, the liver response to a variety of chronic insults, results from the activation, proliferation, and matrix-generating actions of hepatic stellate cells (HSC). These normally quiescent, vitamin A-storing cells of the liver undergo """"""""activation"""""""" under conditions of chronic liver injury, adopting a proliferative, myofibroblast-like phenotype. Through increased production of normal and abnormal extracellular matrix (ECM) components and changes in the regulation of matrix proteases and their inhibitors, activated HSC change the structure of the liver, resulting in hepatic fibrosis and ultimately cirrhosis. The cytokine transforming growth factor (TGF)-beta is the major fibrogenic stimulus for activated HSC. TGF-beta has multiple potential effects on a cell, including regulation of growth and modulation of the immune response. In HSC, TGF-beta is primarily an inducer of the ECM. Although our knowledge of TGF-beta signaling has increased significantly in recent years, the mechanism of its matrix-inducing effects and the basis for cell-type specific responses is still poorly understood. The goal of this proposal is to understand the mechanism of TGF-beta mediated fibrosis. Using as a model system primary rat HSC undergoing activation in culture, the investigator proposes to address the following specific questions: 1) what are the differences in TGF-beta signaling between quiescent (non-fibrogenic) and activated (fibrogenic) HSC? Specifically, what role do Smads and MAP kinases play; 2) what is the role of the high affinity TGF-beta receptors in mediating fibrosis; and 3) how does the potent mitogen PDGF interact with TGF-beta and its signaling pathways, and what are the implications for TGF-beta mediated fibrosis? Experiments to answer these questions will make use of rat HSC in primary culture as well as a line of SV4O-immortalized HSC. The matrix-inducing effects of TGF-beta will be assessed by measuring levels of matrix components and protease inhibitors. Specific methods include the use of signaling inhibitors and overexpression of wild type and dominant negative signaling molecules. This study will increase our understanding of TGF-beta mediated fibrosis in HSC and will enable the design of rational anti-fibrotic therapies.
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