We have discovered a novel pathway for cellular catabolism of apoB-rich atherogenic lipoproteins: a non-enzymatic action of lipoprotein lipase (LpL) enhances their binding to cell-surface heparin sulfate proteoglycans (HSPGs), and thereby mediates substantial retention, internalization, and lysosomal degradation of the lipoproteins. The pathway is independent of the LDL and scavenger receptors and is not regulated by cellular sterol content.
Aim I is to identify and characterize the relevant HSPGs from hepatocytes, the major cell to clear LpL from plasma. We will focus on the HSPG core proteins, which form distinct genetic and metabolic families. Based on our initial success with 21-mer anti-sense thioated oligonucleotides against the syndecan family of HSPG core proteins, we propose a series of molecular manipulations of intact cells. We shall prepare full- or long-length sense and anti-sense cDNA constructs driven by a strong promoter, for use in transient transfections to determine the roles of specific core proteins in the uptake of LpL-apoB complexes. Some HSPG core patients are phosphatidylinositol (PI) anchored, and so we shall also determine the effects of digestion with PI-specific phospholipase C. Because these manipulations are limited to known species or classes of core proteins, we also propose a standard protein purification. We will rely on unusual features of the HSPGs, namely, rapid metabolic incorporation of 35SO4, high negative charge, and binding to LpL-lipoprotein complexes. Previously cloned hepatic HSPGs will be identified immunologically. Core proteins from previously uncharacterized HSPGs will be microsequenced by Edman degradation, and degenerate oligonucleotides based on the microsequences will be used to screen cDNA libraries, to obtain probes for molecular studies.
Aim II is to test directly the atherogenicity of the LpL-HSPG pathway in vivo. We have hypothesized that this pathway is anti-atherogenic in liver because it promotes hepatic re-uptake of nascent apoB-rich atherogenic lipoproteins, thereby reducing their net secretory output, yet it would be atherogenic in the arterial wall because it promotes cellular and interstitial accumulation of cholesteryl esters in peripheral cells. As part of a separately funded pilot program, we are making transgenic mice that express normal and mutated human Lpl in their hepatocytes. In this application, we propose to examine these liver- specific transgenics for their resistance to atherosclerosis from diet or from a genetic absence of apoE. We also propose to make transgenics that express LpL in vascular smooth muscle cells. Smooth muscle-specific transgenics will be examined for their susceptibility to atherosclerosis. Overall, these studies will establish the molecular basis and the physiologic important of this novel, non-enzymatic action of LpL in atherogenesis.
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| Williams, Kevin Jon (2008) Molecular processes that handle -- and mishandle -- dietary lipids. J Clin Invest 118:3247-59 |
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