Induced Mutant Mice Expressing Lipoprotein Lipase Exclusively in Muscle Have Subnormal Triglycerides yet Reduced High Density Lipoprotein Cholesterol Levels in Plasma

Abstract

To determine the contribution of muscle lipoprotein lipase (LPL) to lipoprotein metabolism, induced mutant mice were generated that express human LPL exclusively in muscle. By cross-breeding heterozygous LPL knockout mice with transgenic mice expressing human LPL only in muscle, animals were obtained that express human LPL primarily in skeletal muscle on either the null (L0-MCK) or normal (L2-MCK) LPL backgrounds, and these were compared with control littermates (L2). Fed and fasted post-heparin plasma (PHP) LPL activities were increased 1.4-and 2.3-fold, respectively, in L2-MCK mice and were normal in L0-MCK mice compared with controls. The specific enzyme activities of human LPL in mouse plasma was comparable to human LPL in human PHP. Skeletal muscle LPL activity was increased in both L2-MCK and L0-MCK mice in the fed (6.6-fold) and fasted (4.2-fold in L2-MCK; and 3.4-fold in L0-MCK) states. Adipose tissue LPL mRNA and activity were not detectable in L0-MCK mice. Growth and body mass composition were similar among all groups. In the fasted and fed state, L2-MCK mice had 31% and 53% reductions, respectively, in plasma triglycerides (TG), compatible with increased PHP LPL activity. Unexpectedly, both in the fasted and fed state the L0-MCK mice also had reduced TG (22%), despite normal PHP LPL activities. Very low density lipoprotein (VLDL) turnover studies revealed that the decreased TG were due to increased particle fractional catabolic rate in both L2-MCK and L0-MCK mice. Despite reduced TG, both L2-MCK and L0-MCK mice showed reduced high density lipoprotein (HDL) cholesterol levels (16% and 19%, respectively). HDL turnover studies indicated increased HDL cholesteryl ester fractional catabolic rate in the L2-MCK and L0-MCK compared with control mice. In summary, these studies suggest that muscle LPL is particularly potent with regard to VLDL metabolism and is sufficient to compensate for the lack of LPL in other tissues with regard to lipolyzing VLDL particles. With regard to HDL, muscle LPL expression does not result in normal levels due to enhanced breakdown either by mediating accelerated HDL clearance or by failing to establish normal HDL particles that are then cleared more quickly than normal. These studies provide new insights on the tissue-specific effects of LPL on lipoprotein metabolism.

Transgenic Mice Expressing Lipoprotein Lipase in Adipose Tissue

“…To detect the hLPL3Ј transgene, the following primers, amplifying a 5Ј 410 bp sequence of the hLPL cDNA, were used: 5Ј-GTT ACC GTC CAG CCA TGG ATC ACC A-3Ј and 5Ј-CCT CAA GGG AAA GCT GCC CAC-3Ј. PCR conditions were as described previously (25).…”

“…Ethidium bromide signal densities were integrated using Eagle Eye software. Values from varying cycle numbers (15,17,20,25,30,35,40, and 45 cycles) were obtained. Products were normalized for size with markers of known DNA quantities (GeneChoice DNA Ladder I, PGC Scientifics), and normalized densities were plotted with PCR cycle number to establish linearity.…”

“…PCR was performed at different cycle numbers to ensure linearity of the product. This gel is representative and shows 15,17,20,25,30, and 35 cycles of PCR, lanes 1-6 respectively, for each group. B, integrated densities of quantitative PCR products were normalized to markers of known quantities of DNA and values were plotted.…”

“…Initial reports of muscle-specific LPL transgenics described very high muscle LPL overexpression, such that the mice developed weight loss, increased NEFA uptake by muscle, and a fatal myopathy (23). In subsequent studies, however, more moderate muscle specific overexpressors had less carcass lipid, lower triglycerides, and a resistance to high fat diet-induced obesity (24,25). By breeding these mice onto the knockout background, mice were developed with LPL expression only in skeletal or cardiac muscle (22,25,26).…”

“…In subsequent studies, however, more moderate muscle specific overexpressors had less carcass lipid, lower triglycerides, and a resistance to high fat diet-induced obesity (24,25). By breeding these mice onto the knockout background, mice were developed with LPL expression only in skeletal or cardiac muscle (22,25,26). The muscle-driven LPL transgene resulted in a normalization of the lipid profile, thus demonstrating the importance of muscle LPL in triglyceride clearance.…”