HEPATIC AND LIPOPROTEIN LIPASE INDUCED REMNANT CLEARANCE

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Two lipases, lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL), play central roles in catabolism of chylomicrons and very low-density lipoproteins (VLDL). The current proposal will build on in vitro findings from our laboratory which indicate that 1) LPL promotes clearance of VLDL by direct interation with LDL receptors and the LDL receptor-related protein (LRP) and 2) HTGL is internalized and degraded via LRP. These two receptors mediate chylomicron and VLDL remnant clearance in vivo. We will test the importance of the receptor-binding functions of HTGL and LPL for chylomicron and VLDL clearance in vivo and the role that these lipases play in activating apoE to bind LRP. Aim 1 will test the hypothesis that HTGL promotes clearance of chylomicrons and VLDL via LDL receptors and LRP by direct interaction with these receptors in vitro. Studies will be done in normal and mutant cell lines that express both LRP and LDL receptors or lack one or the other receptor and in solid-phase assays using paretically purified LRP or LDL receptors, enzymatic activity, and lipoprotein binding. Aim 2 will test the hypothesis that HTGL and LPL ~activate~ apoE to bind LRP in vitro. In the absence of lipases, LRP does not bind VLDL or beta-VLDL from cholesterol-fed animals despite the fact that both lipoproteins bind to LDL receptors via apoE. In Aim 3 the contribution of receptor binding by LPL and HTGL to the clearance of chylomicrons and VLDL via LRP and LDL receptors will be tested in vivo using adenoviral gene transfer. Effects of adenoviral expression of human LPL, HTGL, or variants of these lipases that lack receptor binding or enzymatic activity will be studied in apoE knockout (KO) mice. Apoe KO mice that lack either LDL receptors or normal LRP function will be studied to assess the relative contributions of these two receptor pathways to clearance of triglyceriderich lipoproteins. Remnants of triglyceride- rich lipoproteins are atherogenic as are LDL, which are derived from VLDL. The proposed studies will increase our understanding of the roles that LPL and HTGL play in preventing the accumulation in plasma of these atherogenic lipoproteins. Studies with adenoviral vectors may support the feasibility of future gene transfer therapy in the treatment of hyperlipidemia.