Project 1. Macrophage-specific targeting of LXRs in CVD and NASH Fatty liver diseases account for rapidly growing morbidity in the United States, where it is estimated that 80 to 100 million individuals have non-alcoholic fatty liver disease (NAFLD) and 6 to 16 million have the more severe liver disease, nonalcoholic steatohepatitis (NASH). NAFLD is a spectrum of liver conditions strongly coupled with obesity, insulin resistance, CVD, and type-2 diabetes mellitus. Long term prospective studies indicate that the presence and severity of NAFLD independently predicts fatal and nonfatal CVD events. The development of NAFLD and CVD is influenced by combinations of genetic and environmental factors, some of which are disease-specific and others that affect both disease processes. In this Project, we will investigate the central hypothesis that impaired function of liver X receptors in Kupffer cells in the liver and macrophages within the artery wall represent a common underlying mechanism that contributes to both NAFLD and atherosclerosis, and that this mechanism can be reversed by treatment with desmosterol mimetics. A major limitation in targeting LXRs for treatment of atherosclerosis is that most synthetic agonists cause marked hypertriglyceridemia by inducing the expression of SREBP1c in hepatocytes. Our studies of macrophage foam cells led to the finding that desmosterol, an intermediate in the cholesterol biosynthetic pathway, is the most abundant endogenous LXR agonist. Unlike conventional agonists that selectively bind to LXRs, desmosterol also binds to SCAP, thereby inhibiting processing of SREBP1 and SREBP2. Unexpectedly, we recently discovered that desmosterol and synthetic desmosterol mimetics do not activate LXR or suppress SREBP target genes in hepatocytes. In vivo studies with a synthetic desmosterol mimetic further demonstrated activation of LXR target genes in Kupffer cells but not in the liver as a whole. Our findings reveal cell-specific differences in LXR responses to natural and synthetic ligands in macrophages and hepatocytes that provide a conceptually new basis for prevention of NASH and atherosclerosis.
Three Specific Aims are proposed.
Specific Aim 1 will test the hypothesis that LXR activity in Kupffer cells is required for normal liver homeostasis and that Kupffer cell-specific deletion of LXRs results in exaggerated NASH and atherosclerosis. These studies will exploit new mouse models allowing Kupffer cell-specific deletion of LXRs.
Specific Aim 2 will use a combination of pharmacologic and genetic approaches to test the hypothesis that selective activation of LXRs in Kupffer cells with synthetic desmosterol mimetics protects mice from NASH and atherosclerosis independent of effects of these ligands within the artery wall.
Specific Aim 3, performed in collaboration with Project 4, will test the hypothesis that monocyte gene expression signatures and epigenetic landscapes in peripheral blood monocytes correlate with CVD phenotypes and the extent of fibrosis in human subjects. The proposed studies may result in qualitative advances in understanding roles of LXRs in regulation of NASH and atherosclerosis and establish the potential of desmosterol mimetics to be advanced for clinical investigation.
