Obesity is a risk factor for """"""""nonalcoholic steatohepatitis"""""""" (NASH), a type of liver disease. NASH progresses gradually from an early stage of steatosis, to a more advanced form of liver injury (steatohepatitis), and eventually, to cirrhosis. However, the mechanisms that initiate steatosis and those which drive disease progression are unknown. Our long-term goal is to clarify the pathogenesis of NASH. To begin this work animal models that develop NASH either spontaneously or after treatment with ordinarily innocuous doses of lipopolysaccharide (LPS) will be studied. Ob/ob and db/db mice, winch are obese because they lack leptin (an adipocyte hormone that suppresses appetite and increases energy utilization) or its functional receptor, have steatosis and rapidly develop steatohepatitis after LPS treatment. Transgenic mice with a blockade at the terminal stages of adipocyte differentiation are also leptin-deficient as adults and develop steatohepatitis spontaneously. Our preliminary work in the obese leptin-""""""""defective"""""""" rodents suggests that their macrophages have depressed phagocytosis and produce cytokines abnormally. These studies also indicate that leptin may effect macrophage function by interacting with the transmembrane form of its receptor. Because macrophage cytokines particularly tumor necrosis factor alpha (TNF) and TNF-regulatory cytokines appear to be involved in the pathogenesis of alcohol-induced steatohepatitis, we proposed the following HYPOTHESIS: NASH is caused by an interruption of the leptin-dependent signals that modulate macrophage production of cytokines which regulate hepatic intermediary metabolism and sensitivity to LPS.
3 Specific Aims will be addressed: Alm number 1 will determine the importance of leptin itself in the pathogenesis of NASH by assessing if treatment with recombinant leptin improves NASH in ob/ob, db/db or the transgenic mice.
Aim number 2 will determine if and how leptin regulates proinflammatory cytokine production by evaluating the effects of leptin on cytokine expression in leptin-defective mice and in cultures of normal macrophages.
Aim number 3 will determine if altered expression of leptin-regulated cytokines causes hepatic ATP depletion, which may """"""""sensitize"""""""" the liver to injury. Hepatic ATP stores will be measured and correlated with expression of UCP-2, a mitochondrial protein that may uncouple ATP synthesis from oxidative phosphorylation; recombinant cytokines will also be added to hepatocyte cultures to determine if they influence UCP-2 expression and the effect of adenovirus-mediated transfer of the UCP-2 gene on hepatocyte ATP levels and viability will be assessed.
