Several large-scale population studies have identified the association between polymorphisms of the LDL receptor-related protein-1 (LRP1) gene with a wide spectrum of metabolic diseases, including atherosclerosis, obesity/diabetes, nonalcoholic fatty liver diseases (NAFLD), and poor prognosis of hepatocellular carcinoma (HCC). The latter observations imply that LRP1 dysfunction in the liver may accelerate the progression of NAFLD to steatohepatitis and cirrhosis with increased HCC risk. Studies from the past funding period of this project have shown that hepatocyte-specific Lrp1 gene inactivation or mutations that alter LRP1 endocytosis and cell surface recycling in mice potentiates fat-induced steatosis and cell death. Since total LRP1 gene deletion and mutations that affect LRP1 cell surface recycling have not been reported previously in humans, we initiated studies to examine if and how LRP1 expression at the cell surface of hepatocytes may be regulated physiologically. Our preliminary results showed that fatty acid and high fat diet suppress LRP1 translocation to the cell surface of hepatocytes, whereas insulin has the opposite effect and stimulates LRP1 cell surface translocation. The objective of this renewal application is to decipher the mechanisms underlying the regulation of LRP1 recycling and translocation to the cell surface. We will test the hypothesis that reduction of cell surface expression of LRP1 in hepatocytes directly contributes to fat-induced liver inflammation and hepatic cirrhosis, and is directly responsible for the accelerated HCC progression associated with high fat/cholesterol diet and fatty liver disease. Moreover, we propose that increasing LRP1 recycling to the cell surface is a viable strategy to alleviate these metabolic abnormalities.
Specific Aim 1 will identify the mechanism underlying lipid suppression of LRP1 translocation to plasma membrane, testing the hypothesis that lipid accumulation alters phosphorylation of the cytoplasmic domain of LRP1, thereby suppressing its interaction with adaptor proteins required for its translocation to the cel surface. We will also evaluate two strategies to increase LRP1 recycling and test their effectiveness to improve hepatocyte functions and prevent lipid-induced liver dysfunction and cirrhosis.
Specific Aim 2 will delineate the mechanism underlying insulin activation of LRP1 translocation to the cell surface, testing the hypothesis that insulin induces lipid kinase activit to generate phosphatidylinositol (poly)phosphates to increase the recruitment of LRP1-adaptor complex to the recycling endosomes for translocation to the cell surface. Follow up experiments will explore the potential of permeable inositide polyphosphates as a strategy to overcome insulin resistance to improve metabolic health of the liver.
Specific Aim 3 will test the hypothesi that LRP1 dysfunction in hepatocytes promotes lipid-induced liver cirrhosis, thereby providing an ideal local environment to accelerate liver tumor progression. The completion of these studies will identify mechanisms regulating LRP1 translocation to the cell surface, and offers novel strategies to suppress NAFLD progression to NASH, liver cirrhosis, and HCC.
Genetic association studies have identified polymorphisms of the LDL receptor related protein-1 (LRP1) as a risk factor for cardiometabolic diseases including NASH and liver cancer. This proposal will delineate the mechanism(s) underlying low-density lipoprotein receptor related protein-1 (LRP1) regulation under physiological conditions and to identify mechanisms responsible for the progression of nonalcoholic fatty liver diseases to end stage liver cirrhosis with increased risk of hepatocellular carcinoma. The clinical implications of this mechanism-based study are that results will establish new paradigms of liver disease progression and offer novel approaches for treatment of metabolic and liver diseases.
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