In extrahepatic tissues lipoprotein lipase (LPL) hydrolyzes triglycerides thereby generating FFA for tissue uptake and metabolism. To study the effects of increased FFA uptake in muscle tissue, transgenic mouse lines were generated with a human LPL minigene driven by the promoter of the muscle creatine kinase gene. In these mice human LPL was expressed in skeletal muscle and cardiac muscle, but not in other tissues. In proportion to the level of LPL overexpression, decreased plasma triglyceride levels, elevated FFA uptake by muscle tissue, weight loss, and premature death were observed in three independent transgenic mouse lines. The animals developed a severe myopathy characterized by muscle fiber degeneration, fiber atrophy, glycogen storage, and extensive proliferation of mitochondria and peroxisomes. This degree of proliferation suggests that FFA play an important role in the biogenesis of these organelles. Our experiments indicate that LPL is rate limiting for the supply of muscle tissue with triglyceride-derived FFA. Improper regulation of muscle LPL can lead to major pathological changes and may be important in the pathogenesis of some human myopathies. Muscle-specific LPL transgenic mouse lines will serve as a useful animal model for the investigation of myopathies and the biogenesis of mitochondria and peroxisomes. (J. Clin. Invest. 1995. 96:976-986.)

Hormone-sensitive lipase (HSL) is believed to play an important role in the mobilization of fatty acids from triglycerides (TG), diglycerides, and cholesteryl esters in various tissues. Because HSL-mediated lipolysis of TG in adipose tissue (AT) directly feeds non-esterified fatty acids (NEFA) into the vascular system, the enzyme is expected to affect many metabolic processes including the metabolism of plasma lipids and lipoproteins. In the present study we examined these metabolic changes in induced mutant mouse lines that lack HSL expression (HSLko mice). During fasting, when HSL is normally strongly induced in AT, HSL-ko animals exhibited markedly decreased plasma concentrations of NEFA (؊40%) and TG (؊63%), whereas total cholesterol and HDL cholesterol levels were increased (؉34%). Except for the increased HDL cholesterol concentrations, these differences were not observed in fed animals, in which HSL activity is generally low. Decreased plasma TG levels in fasted HSL-ko mice were mainly caused by decreased hepatic very low density lipid lipoprotein (VLDL) synthesis as a result of decreased NEFA transport from the periphery to the liver. Reduced NEFA transport was also indicated by a depletion of hepatic TG stores (؊90%) and strongly decreased ketone body concentrations in plasma (؊80%). Decreased plasma NEFA and TG levels in fasted HSL-ko mice were associated with increased fractional catabolic rates of VLDL-TG and an induction of the tissue-specific lipoprotein lipase (LPL) activity in cardiac muscle, skeletal muscle, and white AT. In brown AT, LPL activity was decreased. Both increased VLDL fractional catabolic rates and increased LPL activity in muscle were unable to provide the heart with sufficient NEFA, which led to decreased tissue TG levels in cardiac muscle. Our results demonstrate that HSL deficiency markedly affects the metabolism of TG-rich lipoproteins by the coordinate down-regulation of VLDL synthesis and up-regulation of LPL in muscle and white adipose tissue. These changes result in an "anti-atherogenic" lipoprotein profile.In mammals, white adipose tissue (WAT) 1 is the most important storage organ of TG. The mobilization of TG during fasting or periods of increased energy demand, and the release of non-esterified fatty acids (NEFA) is an essential process that supplies non-adipose organs with substrates for energy conversion (1, 2). NEFA absorbed by skeletal and cardiac muscle are predominantly used for oxidation and energy production. In the liver, NEFA are also used for oxidation but, in addition, are utilized for several other metabolic processes. NEFA can be stored as hepatic TG droplets, used for the synthesis of ketone bodies, or incorporated into VLDL (3, 4). Once formed, VLDL particles are secreted from the liver into the vascular system where they are lipolyzed by endothelial cell associated lipoprotein lipase (LPL) (5, 6). This process supplies peripheral tissues such as AT with NEFA, thereby closing an inter-tissue cycle of fatty acid transport.An important enzyme for the mo...

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Endothelial lipase (EL) is a phospholipase with little triacylglycerol lipase activity. To assess structural and functional properties of EL-HDL (EL-modified high-density lipoprotein), HDL was incubated with conditioned medium from Cos-7 cells infected with adenovirus encoding human EL. After re-isolation of HDL by ultracentrifugation, TLC and HPLC analyses revealed that EL-HDL was markedly depleted in phosphatidylcholine and enriched in lyso-phosphatidylcholine compared with LacZ-HDL (control HDL) incubated with conditioned medium from Cos-7 cells infected with adenovirus encoding beta-galactosidase. The EL-HDL was enriched in non-esterified fatty acids and, as revealed by lipid electrophoresis, was more negatively charged than control HDL. The HDL particle size as well as the total cholesterol, free cholesterol and triacylglycerol content of HDL were not significantly altered after EL modification. The ability of EL-HDL to mediate 3H-cholesterol efflux from SR-BI (scavenger receptor B type I) overexpressing Chinese-hamster ovary cells was impaired and markedly lower compared with LacZ-HDL at HDL concentrations of 100 microg/ml and above. Studies with 125I-labelled HDL showed almost unaltered binding affinity (K(m) values) and a slightly but significantly decreased binding capacity (B(max) values) of EL-HDL to SR-BI, compared with LacZ-HDL. The ATP-binding-cassette transporter A1-dependent cholesterol and phospholipid effluxes were not affected by EL modification. From these results, we concluded that EL modification alters chemical composition and physical properties of HDL, resulting in its decreased binding capacity to SR-BI and a diminished ability to mediate SR-BI-dependent cholesterol efflux.

Lipoprotein lipase (LPL) is the rate-limiting enzyme for the hydrolysis of triglycerides and the subsequent uptake of free fatty acids in extrahepatic tissues. Deficiency of LPL in humans (Type I hyperlipoproteinemia) is associated with massive chylomicronemia, low high density lipoprotein (HDL) cholesterol levels, and recurrent attacks of pancreatitis when not controlled by a strict diet. In contrast to humans, homozygous LPL knock-out mice (L0) do not survive suckling and die between 18 and 24 h after birth. In this study, an adenovirus-based protocol was utilized for the transient expression of LPL during the suckling period in an effort to rescue L0 mice. After a single intraperitoneal injection of 5؋10 9 plaque-forming units of LPL-expressing virus immediately after birth, more than 90% of L0 mice survived the first days of life. 3% of L0 mice survived the entire suckling period and lived for up to 20 months, although LPL activity in mouse tissues and postheparin plasma was undetectable in all animals after 6 weeks of age. Adult LPL-deficient mice were smaller than their littermates until 2-3 months of age and exhibited very high triglyceride levels in the fed (4997 ؎ 1102 versus 113.4 ؎ 18.7 mg/dl) and fasted state (2007 ؎ 375 versus 65.5 ؎ 7.4 mg/dl). Plasma total cholesterol levels, free fatty acids, and ketone bodies were elevated in L0 mice, whereas plasma glucose was normal. Most strikingly, L0 mice lacked apoA-I-containing pre␤-HDL particles as well as mature HDL resulting in undetectable HDL cholesterol and HDL-apoA-I levels. HDL deficiency in plasma was evident despite normal apoA-I mRNA levels in the liver and normal apoA-I protein levels in plasma, which were predominantly found in the chylomicron fraction. The absence of pre␤-HDL and mature HDL particles supports the concept that the lipolysis of triglyceride-rich lipoproteins is an essential step for HDL maturation.The major function of LPL is the enzymatic cleavage of acyl-glycerol esters in triglycerides (TG) 1 of very low density lipoproteins (VLDL) and chylomicrons. Following its synthesis in parenchymal cells such as adipocytes and muscle cells, the enzyme is translocated and bound to the intimal side of the capillary endothelium by its interaction with sulfated glucosaminoglycans (for a review, see Refs. 1-3). Free fatty acids (FFA), the products of plasma TG hydrolysis, are absorbed by the underlying tissue for storage (adipose tissue) (4) or energy production (muscle) (5). Besides this important enzymatic function, LPL has also been shown to act as a ligand or bridging factor for the receptor-mediated cellular uptake of various lipoproteins (6 -8). Additionally, LPL facilitates the selective uptake of lipids and lipophilic vitamins (9 -11). Both enzymatic and nonenzymatic LPL-mediated processes greatly affect the metabolism of plasma lipoproteins and energy homeostasis in all vertebrates. LPL deficiency (type I hyperlipoproteinemia) (12) is a rare autosomal, recessively inherited disease characterized by elevated plasma TG levels, low p...