APOA-I KNOCKOUT MICE - CHARACTERIZATION OF HDL METABOLISM IN HOMOZYGOTES AND IDENTIFICATION OF A POST-RNA MECHANISM OF APOA-I UP-REGULATIONIN HETEROZYGOTES

The major high density lipoprotein (HDL) apolipoprotein, apoA-I, was k nocked out by gene targeting in ES cells to provide a model for the st udy of HDL metabolism and its relationship to plasma and tissue choles terol metabolism. HDL and non-HDL cholesterol (HDLC) were reduced in a poA-I-deficient mice. Feeding a high fat-high cholesterol diet raised HDL-C minimally in apoA-I knockout compared to the large increase seen in control mice, suggesting an interaction between diet and apoA-I ge notype. In apoA-I-deficient mice, HDL was normal in size but altered i n composition. Compared to control mice there was more triglyceride an d free cholesterol and less cholesteryl ester (CE), suggesting that ap oA-I-deficient HDL is a poor substrate for hepatic lipase and lecithin :cholesterol acyltransferase (LCAT). The metabolic basis of the low HD LC levels in the apoA-I knockout mice was decreased flux into the HDL CE pool. The absolute delivery of HDL CE to both peripheral tissues an d liver was also decreased. As tissue cholesterol levels and synthesis were unchanged, the decreased flux of cholesterol into the HDL CE poo l was most likely due to decreased efflux of cholesterol from the peri pheral tissues and decreased functional LCAT activity. The low HDLC st ate in the apoA-I-deficient mouse was associated with an absolute decr ease in unidirectional transport of cholesterol from peripheral tissue s to the liver but this did not lead to cholesterol accumulation in th e periphery or a cholesterol deficit in the liver; nor was there alter ed peripheral tissue HMG-CoA reductase activity. The only sign of decr eased cholesterol flux to the liver was a 2.3-fold decrease in liver c holesterol 7 alpha-hydroxylase mRNA, suggesting decreased bile acid sy nthesis. In the apoA-I knockout mouse model it appears that low HDL le vels create a new steady state in which decreased cholesterol is deliv ered to both peripheral tissues and the liver.