Liver fibrogenesis is a complex process which comprises of a sequence of events including inflammatory cell infiltration and proliferation of matrix-producing mesenchymal cells, with an increased accumulation of the extracellular matrix (ECM) components, including type I collagen. At the center of the fibrogenic process are the hepatic stellate cells (HSC). Recently we have shown that HSC phagocytose apoptotic bodies (AB) from dying hepatocytes and this induces NADPH oxidase (NOX) activation and resulting superoxide production and release both into the intracellular and intracellular milieu. Intracellular superoxide then induces procollagen a 1(I) upregulation, thereby contributing to the fibrogenic process. Phagocytosis by HSC is likely to be a common, universal pathway leading to fibrosis independent of the etiology of the liver disease, linking chronic liver injury with apoptosis of hepatocytes to clearance of AB with resulting scar formation. As NOX activation appears to be a critical element in inducing HSC activation, and it is unknown whether the phagocytic NOX (NOX2) plays role in engulfment, our HYPOTHESIS is that AB engulfment leads specifically to NOX2 activation, and that this via the Rac-GTPase activates signaling pathways, leading to increased motility and activation of HSC.
The SPECIFIC AIMS of this proposal will be answering two key questions generated by this hypothesis: 1. Is NOX2 playing a role in the phagocytosis of AB in HSC? And 2. Does PS binding to its receptor (PS-R) on HSC, and the subsequent engulfment of AB, induce NOX2 and downstream signaling pathways? In this proposal we develop new areas of investigation that are the natural extension of the specific aims described in the original K08 application, and based on data derived from the studies described in the K08 grant. The importance of studying the proposed signaling events is enhanced by the fact that these likely represent common pathways in liver fibrosis, independent of etiology.
