Effects of ethyl-CPIB (clofibrate) on tissue lipoprotein lipase and plasma post-heparin lipolytic activity in rats

Peroxisome proliferator activated receptors and lipoprotein metabolism

Plasma lipoproteins are responsible for carrying triglycerides and cholesterol in the blood and ensuring their delivery to target organs. Regulation of lipoprotein metabolism takes place at numerous levels including via changes in gene transcription. An important group of transcription factors that mediates the effect of dietary fatty acids and certain drugs on plasma lipoproteins are the peroxisome proliferator activated receptors (PPARs). Three PPAR isotypes can be distinguished, all of which have a major role in regulating lipoprotein metabolism. PPARalpha is the molecular target for the fibrate class of drugs. Activation of PPARalpha in mice and humans markedly reduces hepatic triglyceride production and promotes plasma triglyceride clearance, leading to a clinically significant reduction in plasma triglyceride levels. In addition, plasma high-density lipoprotein (HDL)-cholesterol levels are increased upon PPARalpha activation in humans. PPARgamma is the molecular target for the thiazolidinedione class of drugs. Activation of PPARgamma in mice and human is generally associated with a modest increase in plasma HDL-cholesterol and a decrease in plasma triglycerides. The latter effect is caused by an increase in lipoprotein lipase-dependent plasma triglyceride clearance. Analogous to PPARalpha, activation of PPARbeta/delta leads to increased plasma HDL-cholesterol and decreased plasma triglyceride levels. In this paper, a fresh perspective on the relation between PPARs and lipoprotein metabolism is presented. The emphasis is on the physiological role of PPARs and the mechanisms underlying the effect of synthetic PPAR agonists on plasma lipoprotein levels.

Modulation of plasma TG lipolysis by Angiopoietin-like proteins and GPIHBP1

There is evidence that elevated plasma triglycerides (TG) serve as an independent risk factor for coronary heart disease. Plasma TG levels are determined by the balance between the rate of production of chylomicrons and VLDL in intestine and liver, respectively, and their rate of clearance in peripheral tissues. Lipolytic processing of TG-rich lipoproteins is mediated by the enzyme lipoprotein lipase (LPL), which is tethered to the capillary endothelium via heparin sulphate proteoglycans. In recent years the Angiopoietin-like proteins ANGPTL3 and ANGPTL4 have emerged as novel modulators of LPL activity. Studies in transgenic animals supported by in vitro experiments have demonstrated that ANGPTL3 and ANGPTL4 impair plasma TG clearance by inhibiting LPL activity. In humans, genetic variation within the ANGPTL3 and ANGPTL4 genes contributes to variation in plasma TG and HDL levels, thereby validating the importance of ANGPTLs in the regulation of lipoprotein metabolism in humans. Combined with the discovery of GPIHBP1 as a likely LPL anchor, these findings have led to a readjustment of the mechanism of LPL function. This review provides an overview of our current understanding of the role and regulation of ANGPTL3, ANGPTL4 and GPIHBP1, and places the newly acquired knowledge in the context of the established function and mechanism of LPL-mediated lipolysis.

Induced mutant mice expressing lipoprotein lipase exclusively in muscle have subnormal triglycerides yet reduced high density lipoprotein cholesterol levels in plasma

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.

Effects of KRN4884, a novel pyridinecarboxamidine type KATP channel opener, on serum triglyceride levels in rats

1. The effects of KRN4884, a novel pyridinecarboxamidine type KATP channel opener, on serum triglyceride levels were investigated in Sprague-Dawley rats. 2. Oral administration of KRN4884 (3 mg kg-1) for 10 days caused a significant reduction in serum triglyceride levels, which was comparable to that of clofibrate (160 mg kg-1). Reduction in serum triglyceride levels by KRN4884 and clofibrate were accompanied by a reduction in triglyceride levels both in chylomicron and in very low density lipoprotein. KRN4884 treatment did not affect serum concentrations of total cholesterol and phospholipid, but did increase free fatty acid levels. Clofibrate reduced total cholesterol, phospholipid and free fatty acid levels. 3. Administration of clofibrate significantly decreased triglyceride secretion rate as measured by the Triton WR-1339 injection procedure, while KRN4884 did not. 4. Rats receiving KRN4884 exhibited an increase in lipoprotein lipase (LPL) activity both in adipose tissue and in skeletal muscle. There was an inverse correlation between serum triglyceride levels and tissue LPL activities. KRN4884 did not change hepatic triglyceride lipase (HTGL) activity. Clofibrate affected neither LPL nor HTGL activities. 5. It is concluded that administration of KRN4884 results in reduced serum triglyceride levels which may be due to the enhancement of LPL activity in peripheral tissues.

Regulation of lipoprotein lipase activity and mRNA content in rat epididymal adipose tissue in vitro by recombinant tumour necrosis factor

Tumour necrosis factor (TNF) has previously been shown to decrease lipoprotein lipase (LPL) activity and mRNA levels in 3T3-L1 cells and in adipose tissue from rats and guinea pigs when injected in vivo, but not to alter LPL activity in human adipocytes incubated in vitro. The effect of recombinant human TNF on LPL activity and mRNA levels in rat epididymal adipose tissue incubated in vitro was examined. LPL activity and mRNA levels fell in adipose tissue taken from fed rats and incubated in Krebs-Henseleit bicarbonate medium with glucose. The addition of insulin and dexamethasone prevented these falls. TNF (400 ng/ml) produced a fall of approx. 50% in LPL activity after 2 h of incubation and of approx. 30% in LPL mRNA levels after 3 h. TNF did not decrease LPL activity in isolated adipocytes. These results demonstrate that rat adipose tissue incubated in vitro is responsive to TNF whereas isolated adipocytes are not.

The acylation of proteins by xenobiotic amphipathic carboxylic acids in cultured rat hepatocytes

Three xenobiotic amphipathic carboxylates, namely MEDICA 16, nafenopin and bezafibrate, which differ remarkably in their hydrophobic backbones, were found to acylate membrane and cytosolic liver proteins in cultured rat hepatocytes. The acylation patterns observed were time- and dose-dependent, and the acylated residue consisted of the original xenobiotic. The acylation patterns generated by the three xenobiotic carboxylates included common proteins which were acylated by the three xenobiotics (e.g. proteins of 32, 52, 56 and 72 kDa) as well as unique proteins which were specifically acylated by the respective xenobiotics. The acylation of liver proteins by either MEDICA 16 or nafenopin remained unaffected under conditions where protein synthesis was completely inhibited by cycloheximide. Protein acylation thus offers a common mode of action of xenobiotic amphipathic carboxylates, which may, however, result in diverse xenobiotyl-protein adducts. The xenobiotyl-acylated proteins might be involved in triggering some of the biological effects exerted by xenobiotic amphipathic carboxylates employed as hypolipidaemic effectors, peroxisomal proliferators or preadipocyte convertors.