Tissue-specific regulation of guinea pig lipoprotein lipase; effects of nutritional state and of tumor necrosis factor on mRNA levels in adipose tissue, heart and liver

β-Estradiol Enhanced Secretion of Lipoprotein Lipase from Mouse Mammary Tumor FM3A Cells

The role of β-estradiol (E₂) in lipoprotein metabolism in mammary tumors is unclear, therefore, we investigated the effect of E₂ on the secretion of lipoprotein lipase (LPL) from mouse mammary tumor FM3A cells. E₂-treated cells increased the secretion of active LPL from FM3A cells in a time- and dose-dependent manner. Activity of mitogen-activated protein kinase (MAPK) was increased in the tumor cells treated with E₂, and enhanced secretion of LPL was suppressed by MAPK kinase 1/2 inhibitor, PD98059, extracellular signal-regulated kinase (ERK) 1/2 inhibitor, FR180204, p38 MAPK inhibitor, SB202190, and phosphatidyl inositol 3-kinase (PI3K) inhibitor, LY294002. In addition, the effect of E₂ on LPL secretion was markedly suppressed by an inhibitor of mammalian target of rapamycin complex (mTORC) 1 and 2, KU0063794, but were not by a mTORC1 inhibitor, rapamycin. Furthermore, a small interfering RNA (siRNA)-mediated decrease in the expression of rapamycin-insensitive companion of mTOR (Rictor), a pivotal component of mTORC2, suppressed secretion of LPL by E₂. These results suggest that the stimulatory secretion of LPL by E₂ from the tumor cells is closely associated with an activation of mTORC2 rather than mTORC1 possibly via the MAPK cascade.

Physiological regulation of lipoprotein lipase

The enzyme lipoprotein lipase (LPL), originally identified as the clearing factor lipase, hydrolyzes triglycerides present in the triglyceride-rich lipoproteins VLDL and chylomicrons. LPL is primarily expressed in tissues that oxidize or store fatty acids in large quantities such as the heart, skeletal muscle, brown adipose tissue and white adipose tissue. Upon production by the underlying parenchymal cells, LPL is transported and attached to the capillary endothelium by the protein GPIHBP1. Because LPL is rate limiting for plasma triglyceride clearance and tissue uptake of fatty acids, the activity of LPL is carefully controlled to adjust fatty acid uptake to the requirements of the underlying tissue via multiple mechanisms at the transcriptional and post-translational level. Although various stimuli influence LPL gene transcription, it is now evident that most of the physiological variation in LPL activity, such as during fasting and exercise, appears to be driven via post-translational mechanisms by extracellular proteins. These proteins can be divided into two main groups: the liver-derived apolipoproteins APOC1, APOC2, APOC3, APOA5, and APOE, and the angiopoietin-like proteins ANGPTL3, ANGPTL4 and ANGPTL8, which have a broader expression profile. This review will summarize the available literature on the regulation of LPL activity in various tissues, with an emphasis on the response to diverse physiological stimuli.

Regulation of lipoprotein lipase by Angptl4

Triglyceride (TG)-rich chylomicrons and very low density lipoproteins (VLDL) distribute fatty acids (FA) to various tissues by interacting with the enzyme lipoprotein lipase (LPL). The protein angiopoietin-like 4 (Angptl4) is under sensitive transcriptional control by FA and the FA-activated peroxisome proliferator activated receptors (PPARs), and its tissue expression largely overlaps with that of LPL. Growing evidence indicates that Angptl4 mediates the physiological fluctuations in LPL activity, including the decrease in adipose tissue LPL activity during fasting. This review focuses on the major ambiguities concerning the mechanism of LPL inhibition by Angptl4, as well as on the physiological role of Angptl4 in lipid metabolism, highlighting its function in a variety of tissues, and uses this information to make suggestions for further research.

Effect of starvation on activities and mRNA expression of lipoprotein lipase and hormone-sensitive lipase in tilapia (Oreochromis niloticus x O. areus)

A 4-week study was conducted to determine the effect of starvation on activities and mRNA expression of lipoprotein lipase (LPL) and hormone-sensitive lipase (HSL) in hybrid tilapia (Oreochromis niloticus x O. areus). The tissue samples were sampled once a week. Results showed that body weight (BW) and hepatosomatic index (HSI) were decreased significantly (P < 0.05) during starvation. The percentages of crude fat and crude protein in the whole body and the fat content in muscle decreased significantly (P < 0.05), while the rate of moisture and crude ash increased significantly (P < 0.05). The response of LPL, HSL activities and mRNA expression in tissues was tissue dependent. The activities of LPL and HSL in muscle at day 7 were elevated by 2.5 times (P < 0.05) and 11.8 times (P < 0.05) of the value at day 0, respectively, and both then decreased to pre-starvation levels at day 14 and finally stabilized at a certain level afterward. LPL and HSL mRNA abundance in muscle remained relatively stable between 0 and 14 day; then, a significant increase was seen after 14 days. In the liver, LPL activity maintained a significantly increasing trend during starvation, while HSL activity rose dramatically at day 7 of starvation by 2.35 times (P < 0.05) and finally stabilized at a certain level. The mRNA abundance of liver LPL increased significantly during the whole process of starvation (P < 0.05), whereas the mRNA abundance of liver HSL decreased significantly at day 7 of starvation, elevating significantly afterward (P < 0.05).

Effect of globin digest on the liver injury and hepatic gene expression profile in galactosamine-induced liver injury in SD rats

AIMS

We investigated the effect of globin digest (GD) on the liver injury and hepatic gene expression profile in galactosamine (GalN)-induced liver injury.

MAIN METHODS

The effect of GD on the liver injury was examined by measuring the activities of serum transferases and hepatic antioxidant enzymes, histopathological analysis, gene expression profile, and proteins of the peroxisome proliferator-activated receptor alpha (PPARα) and met proto-oncogene (c-Met) in SD rats at 24 h after GalN administration. The effect of GD on the expression of PPARα and its target gene in AML-12 mouse hepatocytes was also examined.

KEY FINDINGS

GD suppressed the elevated activities of serum transferases in GalN-induced liver injury in SD rats. The thiobarbituric acid reactive substance content in GalN-injured liver was a decreasing tendency by GD. GD suppressed the increased oxidized glutathione content, and increased the decreased protein, reduced glutathione contents, and catalase activity in GalN-injured liver. GD may improve the antioxidant defense system and protein synthesis in GalN-injured liver. GD suppressed the elevated expression of the genes related to the inflammation, and decreased the histopathological grade value of inflammatory cell infiltration in GalN-injured liver. GD increased the expression of PPARα protein in GalN-injured liver, and also increased the expression of PPARα and its target gene in AML-12 hepatocytes. The total and phosphorylated c-Met proteins in GalN-injured liver were the increasing tendencies by GD.

SIGNIFICANCE

These findings indicate that GD has the hepatoprotective effect on GalN-induced liver injury in SD rats.

Lipoprotein lipase: from gene to obesity

Lipoprotein lipase (LPL) is a multifunctional enzyme produced by many tissues, including adipose tissue, cardiac and skeletal muscle, islets, and macrophages. LPL is the rate-limiting enzyme for the hydrolysis of the triglyceride (TG) core of circulating TG-rich lipoproteins, chylomicrons, and very low-density lipoproteins (VLDL). LPL-catalyzed reaction products, fatty acids, and monoacylglycerol are in part taken up by the tissues locally and processed differentially; e.g., they are stored as neutral lipids in adipose tissue, oxidized, or stored in skeletal and cardiac muscle or as cholesteryl ester and TG in macrophages. LPL is regulated at transcriptional, posttranscriptional, and posttranslational levels in a tissue-specific manner. Nutrient states and hormonal levels all have divergent effects on the regulation of LPL, and a variety of proteins that interact with LPL to regulate its tissue-specific activity have also been identified. To examine this divergent regulation further, transgenic and knockout murine models of tissue-specific LPL expression have been developed. Mice with overexpression of LPL in skeletal muscle accumulate TG in muscle, develop insulin resistance, are protected from excessive weight gain, and increase their metabolic rate in the cold. Mice with LPL deletion in skeletal muscle have reduced TG accumulation and increased insulin action on glucose transport in muscle. Ultimately, this leads to increased lipid partitioning to other tissues, insulin resistance, and obesity. Mice with LPL deletion in the heart develop hypertriglyceridemia and cardiac dysfunction. The fact that the heart depends increasingly on glucose implies that free fatty acids are not a sufficient fuel for optimal cardiac function. Overall, LPL is a fascinating enzyme that contributes in a pronounced way to normal lipoprotein metabolism, tissue-specific substrate delivery and utilization, and the many aspects of obesity and other metabolic disorders that relate to energy balance, insulin action, and body weight regulation.

A highly conserved motif within the NH2-terminal coiled-coil domain of angiopoietin-like protein 4 confers its inhibitory effects on lipoprotein lipase by disrupting the enzyme dimerization

Lipoprotein lipase (LPL) is a principal enzyme responsible for the clearance of chylomicrons and very low density lipoproteins from the bloodstream. Two members of the Angptl (angiopoietin-like protein) family, namely Angptl3 and Angptl4, have been shown to inhibit LPL activity in vitro and in vivo. Here, we further investigated the structural basis underlying the LPL inhibition by Angptl3 and Angptl4. By multiple sequence alignment analysis, we have identified a highly conserved 12-amino acid consensus motif that is present within the coiled-coil domain (CCD) of both Angptl3 and Angptl4, but not other members of the Angptl family. Substitution of the three polar amino acid residues (His(46), Gln(50), and Gln(53)) within this motif with alanine abolishes the inhibitory effect of Angptl4 on LPL in vitro and also abrogates the ability of Angptl4 to elevate plasma triglyceride levels in mice. The CCD of Angptl4 interacts with LPL and converts the catalytically active dimers of LPL to its inactive monomers, whereas the mutant protein with the three polar amino acids being replaced by alanine loses such a property. Furthermore, a synthetic peptide consisting of the 12-amino acid consensus motif is sufficient to inhibit LPL activity, although the potency is much lower than the recombinant CCD of Angptl4. In summary, our data suggest that the 12-amino acid consensus motif within the CCD of Angptl4, especially the three polar residues within this motif, is responsible for its interaction with and inhibition of LPL by blocking the enzyme dimerization.

Genes involved in fatty acid partitioning and binding, lipolysis, monocyte/macrophage recruitment, and inflammation are overexpressed in the human fatty liver of insulin-resistant subjects

OBJECTIVE

The objective of this study is to quantitate expression of genes possibly contributing to insulin resistance and fat deposition in the human liver.

RESEARCH DESIGN AND METHODS

A total of 24 subjects who had varying amounts of histologically determined fat in the liver ranging from normal (n = 8) to steatosis due to a nonalcoholic fatty liver (NAFL) (n = 16) were studied. The mRNA concentrations of 21 candidate genes associated with fatty acid metabolism, inflammation, and insulin sensitivity were quantitated in liver biopsies using real-time PCR. In addition, the subjects were characterized with respect to body composition and circulating markers of insulin sensitivity.

RESULTS

The following genes were significantly upregulated in NAFL: peroxisome proliferator-activated receptor (PPAR) gamma 2 (2.8-fold), the monocyte-attracting chemokine CCL2 (monocyte chemoattractant protein [MCP]-1, 1.8-fold), and four genes associated with fatty acid metabolism (acyl-CoA synthetase long-chain family member 4 [ACSL4] [2.8-fold], fatty acid binding protein [FABP]4 [3.9-fold], FABP5 [2.5-fold], and lipoprotein lipase [LPL] [3.6-fold]). PPARgamma coactivator 1 (PGC1) was significantly lower in subjects with NAFL than in those without. Genes significantly associated with obesity included nine genes: plasminogen activator inhibitor 1, PPARgamma, PPARdelta, MCP-1, CCL3 (macrophage inflammatory protein [MIP]-1 alpha), PPAR gamma 2, carnitine palmitoyltransferase (CPT1A), FABP4, and FABP5. The following parameters were associated with liver fat independent of obesity: serum adiponectin, insulin, C-peptide, and HDL cholesterol concentrations and the mRNA concentrations of MCP-1, MIP-1 alpha, ACSL4, FABP4, FABP5, and LPL.

CONCLUSIONS

Genes involved in fatty acid partitioning and binding, lipolysis, and monocyte/macrophage recruitment and inflammation are overexpressed in the human fatty liver.

A transcription-dependent mechanism, akin to that in adipose tissue, modulates lipoprotein lipase activity in rat heart

The enzyme lipoprotein lipase (LPL) releases fatty acids from lipoprotein triglycerides for use in cell metabolism. LPL activity is rapidly modulated in a tissue-specific manner. Recent studies have shown that in rat adipose tissue this occurs by a shift of extracellular LPL toward an inactive form catalyzed by an LPL-controlling protein whose expression changes in response to the nutritional state. To explore whether a similar mechanism operates in other tissues we injected actinomycin D to block transcription of the putative LPL controlling protein(s). When actinomycin was given to fed rats, heparin-releasable LPL activity increased by 160% in heart and by 150% in a skeletal muscle (soleus) in 6 h. Postheparin LPL activity in blood increased by about 200%. To assess the state of extracellular LPL we subjected the spontaneously released LPL in heart perfusates to chromatography on heparin-agarose, which separates the active and inactive forms of the lipase. The amount of lipase protein released remained relatively constant on changes in the nutritional state and/or blockade of transcription, but the distribution between the active and inactive forms changed. Less of the LPL protein was in the active form in perfusates from hearts from fed compared with fasted rats. When glucose was given to fasted rats the proportion of LPL protein in the active form decreased. Actinomycin D increased the proportion that was active, in accord with the hypothesis that the message for a rapidly turning over LPL-controlling protein was being removed.

Angiopoietin-like protein 4 converts lipoprotein lipase to inactive monomers and modulates lipase activity in adipose tissue

Lipoprotein lipase (LPL) has a central role in lipoprotein metabolism to maintain normal lipoprotein levels in blood and, through tissue specific regulation of its activity, to determine when and in what tissues triglycerides are unloaded. Recent data indicate that angiopoietin-like protein (Angptl)-4 inhibits LPL and retards lipoprotein catabolism. We demonstrate here that the N-terminal coiled-coil domain of Angptl-4 binds transiently to LPL and that the interaction results in conversion of the enzyme from catalytically active dimers to inactive, but still folded, monomers with decreased affinity for heparin. Inactivation occurred with less than equimolar ratios of Angptl-4 to LPL, was strongly temperature-dependent, and did not consume the Angptl-4. Furthermore, we show that Angptl-4 mRNA in rat adipose tissue turns over rapidly and that changes in the Angptl-4 mRNA abundance are inversely correlated to LPL activity, both during the fed-to-fasted and fasted-to-fed transitions. We conclude that Angptl-4 is a fasting-induced controller of LPL in adipose tissue, acting extracellularly on the native conformation in an unusual fashion, like an unfolding molecular chaperone.

Influence of trauma on plasma elimination of exogenous fat and on lipoprotein lipase activity and mass

BACKGROUND

Trauma is followed by an increased plasma clearance and oxidation of exogenous fat but the underlying mechanism is not fully understood.

AIM

To examine the influence of a surgical trauma on the plasma elimination of exogenous triglycerides (TG) and its relationship with lipoprotein lipase (LPL) activity and LPL mass.

METHODS

Nine patients underwent a hypertriglyceridaemic clamp and a lipolytic capacity test before and after open abdominal surgery. The infusion rate was adjusted to maintain a stable TG concentration of 4 mmol x l(-1) during 180 min. The lipolytic capacity was determined as the change in LPL activity and mass following a bolus dose of 100 IU x kg BW(-1) heparin sodium.

RESULTS

Postoperatively, the plasma elimination rate of fat was 2.6 times higher (P<0.001). Infusion of lipids in the postoperative state was followed by a smaller rise in free fatty acids (P<0.05) in comparison with the preoperative situation. The postoperative basal fasting LPL activity was half of that in the preoperative state and the LPL activity rose almost two-fold during the clamp. The heparin-induced rises in LPL activity and LPL mass were similar (n.s.) before and after surgery.

CONCLUSIONS

A moderate surgical trauma is accompanied by a greater than two-fold rise in plasma elimination rate of exogenous fat despite a lower basal LPL activity and a virtually unchanged LPL pattern during infusion of lipids. Our study demonstrates that although trauma may substantially enhance the fat elimination capacity a significant proportion of the infused fat is not utilized for metabolic purposes.

Effects of infection and inflammation on lipid and lipoprotein metabolism: mechanisms and consequences to the host

Infection and inflammation induce the acute-phase response (APR), leading to multiple alterations in lipid and lipoprotein metabolism. Plasma triglyceride levels increase from increased VLDL secretion as a result of adipose tissue lipolysis, increased de novo hepatic fatty acid synthesis, and suppression of fatty acid oxidation. With more severe infection, VLDL clearance decreases secondary to decreased lipoprotein lipase and apolipoprotein E in VLDL. In rodents, hypercholesterolemia occurs attributable to increased hepatic cholesterol synthesis and decreased LDL clearance, conversion of cholesterol to bile acids, and secretion of cholesterol into the bile. Marked alterations in proteins important in HDL metabolism lead to decreased reverse cholesterol transport and increased cholesterol delivery to immune cells. Oxidation of LDL and VLDL increases, whereas HDL becomes a proinflammatory molecule. Lipoproteins become enriched in ceramide, glucosylceramide, and sphingomyelin, enhancing uptake by macrophages. Thus, many of the changes in lipoproteins are proatherogenic. The molecular mechanisms underlying the decrease in many of the proteins during the APR involve coordinated decreases in several nuclear hormone receptors, including peroxisome proliferator-activated receptor, liver X receptor, farnesoid X receptor, and retinoid X receptor. APR-induced alterations initially protect the host from the harmful effects of bacteria, viruses, and parasites. However, if prolonged, these changes in the structure and function of lipoproteins will contribute to atherogenesis.

The distribution of lipoprotein lipase in rat adipose tissue. Changes with nutritional state engage the extracellular enzyme

Lipoprotein lipase (LPL) acts at the vascular endothelium. Earlier studies have shown that down-regulation of adipose tissue LPL during fasting is post-translational and involves a shift from active to inactive forms of the lipase. Studies in cell systems had indicated that during fasting LPL might be retained in the endoplasmic reticulum. We have now explored the relation between active/inactive and intra/extracellular forms of the lipase. Within adipocytes, neither LPL mass nor the distribution of LPL between active and inactive forms changed on fasting. Extracellular LPL mass also did not change significantly, but shifted from predominantly active to predominantly inactive. To explore if changes in secretion were compensated by changes in turnover, synthesis of new protein was blocked by cycloheximide. The rates at which intra- and extracellular LPL mass and activity decreased did not change on fasting. To further explore how LPL is distributed in the tissue, heparin (which detaches the enzyme from the endothelial surface) was injected. Tissue LPL activity decreased by about 10% in 2 min and by 50% in 1 h. Heparin released mainly the active form of the lipase. There was no change of LPL activity or mass within adipocytes. The fraction of extracellular LPL that heparin released and the time course were the same in fed and fasted rats, indicating that active, extracellular LPL was distributed in a similar way in the two nutritional states. This study suggests that the nutritional regulation of LPL in adipose tissue determines the activity state of extracellular LPL.

Postprandial regulation of blood lipids and adipose tissue lipoprotein lipase in type 2 diabetes patients and healthy control subjects

BACKGROUND/AIM

In type 2 diabetes and other insulin-resistant conditions, postprandial hypertriglyceridaemia is an important metabolic perturbation. To further elucidate alterations in the clearance of triglyceride-rich lipoproteins in type 2 diabetes we focused on the nutritional regulation of adipose tissue lipoprotein lipase (LPL).

SUBJECTS AND METHODS

Eight subjects with type 2 diabetes and eight age-, sex- and body mass index (BMI)-matched control subjects underwent subcutaneous abdominal adipose tissue biopsies in the fasting state and 3.5 h following a standardized lipid-enriched meal. LPL activity and mass were measured in adipose tissue and also in plasma after an intravenous injection of heparin.

RESULTS

Postprandial, but not fasting, triglycerides were significantly higher in the diabetic subjects than in the control subjects (3.0+/-0.4 vs 2.0+/-0.2 mmol/l, P=0.028). Adipose tissue LPL activity was increased following the meal test by approximately 35-55% (P=0.021 and 0.004, respectively). There was no significant difference between the groups in this respect. The specific enzyme activity of LPL was not altered in the postprandial state. Fasting and postprandial adipose tissue LPL activity as well as post-heparin plasma LPL activity tended to be lower among the diabetes patients (NS). There was a significant and independent inverse association between insulin resistance (homeostasis model assessment insulin resistance (HOMA-IR) index) vs post-heparin plasma LPL activity and postprandial triglyceride levels, respectively. Adipose tissue LPL activity was related to insulin action in vitro on adipocyte glucose transport, but not to HOMA-IR.

CONCLUSION

Following food intake adipose tissue LPL activity is enhanced to a similar degree in patients with type 2 diabetes and in healthy control subjects matched for BMI, age and gender. If LPL dysregulation is involved in the postprandial hypertriglyceridaemia found in type 2 diabetes, it should occur in tissues other than subcutaneous fat.

Lipoprotein lipase: genetics, lipid uptake, and regulation

Lipoprotein lipase (LPL) regulates the plasma levels of triglyceride and HDL. Three aspects are reviewed. 1) Clinical implications of human LPL gene variations: common mutations and their effects on plasma lipids and coronary heart disease are discussed. 2) LPL actions in the nervous system, liver, and heart: the discussion focuses on LPL and tissue lipid uptake. 3) LPL gene regulation: the LPL promoter and its regulatory elements are described.

Lipoprotein lipase: the regulation of tissue specific expression and its role in lipid and energy metabolism

PURPOSE OF REVIEW

The aim of this review is to summarize and discuss recent advances in the understanding of the physiological role of lipoprotein lipase in lipid and energy metabolism.

RECENT FINDINGS

Studies on the transcriptional and the posttranscriptional level of lipoprotein lipase expression have provided new insights into the complex mechanisms that are involved in the regulation of the enzyme. Additionally a large body of evidence from both human studies and animal models suggests that the level of lipoprotein lipase expression in a given tissue is the rate limiting process for the uptake of triglyceride derived fatty acids. Imbalances in the partitioning of fatty acids among peripheral tissues have major metabolic consequences. For example, in mice both decreased lipoprotein lipase activities in adipose tissue and increased activity in muscle are associated with resistance to obesity; lack of lipoprotein lipase activity in macrophages is correlated with a decreased susceptibility to develop atherosclerotic lesions and overexpression of the enzyme in muscle is associated with increased blood glucose levels and insulin resistance.

SUMMARY

Considering the central role of lipoprotein lipase in energy metabolism it is a reasonable goal to discover and develop new drugs that affect the tissue specific expression pattern of the enzyme.

Organization of the lipoprotein lipase gene of red sea bream Pagrus major

Lipoprotein lipase (LPL) is a key enzyme of lipid deposition and metabolism. To investigate the mechanism of lipid deposition in fish, as a first step, we have characterized the LPL gene of a marine teleost red sea bream Pagrus major by cDNA and genomic structure analysis. The red sea bream LPL gene encodes 511 amino acids and spans approximately 6.3 kb of the genome. The coding region is organized into ten exons and nine introns. In comparison with the LPL of other animals, the deduced amino acid sequence shows a high degree of similarity with a conservation of functional domains, e.g. catalytic triad, N-glycosylation sites, lipid and heparin binding regions. The 1.1 kb of 5' flanking region contains two CCAAT, sequences homologous to Oct-I site and response elements for hormones including glucocorticoid, insulin and thyroid hormone. The results of the present study will facilitate further study of the function and regulation of the LPL in non-mammalian vertebrates.

The effects of feeding condition and dietary lipid level on lipoprotein lipase gene expression in liver and visceral adipose tissue of red sea bream Pagrus major

The effects of feeding condition and dietary lipid level on lipoprotein lipase (LPL) gene expression in the liver and visceral adipose tissue of red sea bream Pagrus major were investigated by competitive polymerase chain reaction. Not only visceral adipose tissue but also liver of red sea bream showed substantial LPL gene expression. In the liver, starvation (at 48 h post-feeding) drastically stimulated LPL gene expression in the fish-fed low lipid diet, but had no effect in the fish fed high lipid diet. Dietary lipid level did not significantly affect the liver LPL mRNA level under fed condition (at 5 h post-feeding). In the visceral adipose tissue, LPL mRNA number per tissue weight was significantly higher in the fed condition than in the starved condition, irrespective of the dietary lipid levels. Dietary lipid levels did not affect the visceral adipose tissue LPL mRNA levels under fed or starved conditions. Our results demonstrate that both feeding conditions and dietary lipid levels alter the liver LPL mRNA levels, while only the feeding conditions but not dietary lipid levels cause changes in the visceral adipose LPL mRNA level. It was concluded that the liver and visceral adipose LPL gene expression of red sea bream seems to be regulated in a tissue-specific fashion by the nutritional state.

Food deprivation increases post-heparin lipoprotein lipase activity in humans

OBJECTIVE

To study the effect of fasting on lipoprotein lipase (LPL) activity in human post-heparin plasma, representing the functional pool of LPL.

DESIGN

Fourteen healthy volunteers were recruited for the study. The subjects were fasted for 30 h. Activities of LPL and hepatic lipase (HL), and LPL mass, were measured in pre- and post-heparin plasma in the fed and in the fasted states, respectively. For comparison, LPL and HL activities were measured in pre- and post-heparin plasma from fed and 24-h-fasted guinea pigs.

RESULTS

Fasting caused a significant drop in the levels of serum insulin, triglycerides and glucose in the human subjects. Post-heparin LPL activity increased from 79 +/- 6.4 mU mL-1 in the fed state to 112 +/- 10 mU mL-1 in the fasted state (P < 0.01), while LPL mass was 361 +/- 29 in the fed state and 383 +/- 28 in the fasted state, respectively (P = 0.6). In contrast, fasting of guinea pigs caused an 80% drop in post-heparin LPL activity. The effect of fasting on human and guinea pig post-heparin HL activity were moderate and statistically not significant.

CONCLUSIONS

In animal models such as rats and guinea pigs, post-heparin LPL activity decreases on fasting, presumably due to down-regulation of adipose tissue LPL. In humans, fasting caused increased post-heparin LPL activity.

Regulation of lipoprotein lipase by the oxysterol receptors, LXRalpha and LXRbeta

Lipoprotein lipase (LPL) is a key enzyme for lipoprotein metabolism and is responsible for hydrolysis of triglycerides in circulating lipoproteins, releasing free fatty acids to peripheral tissues. In liver, LPL is also believed to promote uptake of high density lipoprotein (HDL)-cholesterol and thereby facilitate reverse cholesterol transport. In this study we show that the Lpl gene is a direct target of the oxysterol liver X receptor, LXRalpha. Mice fed diets containing high cholesterol or an LXR-selective agonist exhibited a significant increase in LPL expression in the liver and macrophages, but not in other tissues (e.g. adipose and muscle). Studies in Lxr-deficient mice confirmed that this response was dependent more on the presence of LXRalpha than LXRbeta. Analysis of the Lpl gene revealed the presence of a functional DR4 LXR response element in the intronic region between exons 1 and 2. This response element directly binds rexinoid receptor (RXR)/LXR heterodimers and is sufficient for rexinoid- and LXR agonist-induced transcription of the Lpl gene. Together, these studies further distinguish the roles of LXRalpha and beta and support a growing body of evidence that LXRs function as key regulators of lipid metabolism and are anti-atherogenic.

The influence of fasting/refeeding on the lipoprotein lipase activity of adipose tissue and muscle

Lipoprotein lipase activity in adipose tissue and muscle is modulated by changes in the pattern of food intake. We have measured total lipoprotein lipase activity in adipose tissue and muscle of male Wistar rats (N = 6-10), weighing 200-250 g (~12 weeks), during the refeeding/fasting state following 24 h of fasting. Lipoprotein lipase activity in tissue homogenates was evaluated using a [3H]-triolein-containing substrate, and released [3H]-free fatty acids were extracted and quantified by liquid scintillation. Adipose tissue lipoprotein lipase activity did not completely recover within 2 h of refeeding (60% of refed ad libitum values). Cardiac lipoprotein lipase activity remained increased even 2 h after refeeding (100% of refed ad libitum values), whereas no significant changes were observed in the soleus and diaphragm muscles. Adipose tissue lipoprotein lipase activities were consistently higher than the highest skeletal muscle or heart values. It is therefore likely that adipose tissue, rather than muscle makes the major contribution to triacylglycerol clearance. There was concomitant relatively high lipoprotein lipase activity in both adipose tissue and cardiac muscle during the first few hours of refeeding, therefore cardiac muscle may contribute significantly to triacylglycerol clearance during this period. The results suggest that during fasting, increased lipoprotein lipase activity provides a complementary source of free fatty acids from circulating triacylglycerol, allowing the heart to maintain its continuous, high-energy expenditure.

A fluorescent reverse transcription-polymerase chain reaction assay to quantify the lipoprotein lipase messenger RNA

Relative quantitative reverse transcription-polymerase chain reaction (rqRT-PCR), which allows an accurate quantification of the amount of mRNA in samples potentially differing in the quality of their RNA preparation, was used to quantify lipoprotein lipase (LPL) mRNA in ovine adipose tissue. A comparative evaluation of four rqRT-PCR procedures was carried out. The amount of LPL mRNA was assayed relative to either that of gamma-actin (ACT) or cyclophilin (CYC) mRNA, used as endogenous standard. Independent (INACT and INCYC procedures) or simultaneous (COACT and COCYC procedures) amplifications have been compared. Fluorescently labelled primers yielded PCR products which were quantitatively analysed using an automated DNA sequencer. After optimizing the PCR cycle number and verifying that the amounts of ACT and CYC mRNA varied only weakly according to the nutritional conditions studied, we have tested the ability of the four procedures to quantify specific variations in LPL mRNA. The repeatability of each step and the overall assay reproducibility were also examined. The COACT and INCYC procedures were finally retained to accurately quantify LPL mRNA in AT from nine underfed or refed ewes, and gave highly correlated results (r=0.98, p<0.01). In addition, significant correlations (r=0.83, p<0.01 and r=0.92, p<0.01 for COACT and INCYC, respectively) were observed with the LPL activity in AT.

The translational regulation of lipoprotein lipase in diabetic rats involves the 3'-untranslated region of the lipoprotein lipase mRNA

Adipose tissue lipoprotein lipase (LPL) activity is decreased in patients with poorly controlled diabetes, and this contributes to the dyslipidemia of diabetes. To study the mechanism of this decrease in LPL, we studied adipose tissue LPL expression in male rats with streptozotocin-induced diabetes. Heparin releasable and extractable LPL activity in the epididymal fat decreased by 75-80% in the diabetic group and treatment of the rats with insulin prior to sacrifice reversed this effect. Northern blot analysis indicated no corresponding change in LPL mRNA levels. However, LPL synthetic rate, measured using [(35)S]methionine pulse labeling, was decreased by 75% in the diabetic adipocytes, and insulin treatment reversed this effect. These results suggested regulation of LPL at the level of translation. Diabetic adipocytes demonstrated no change in the distribution of LPL mRNA associated with polysomes, suggesting no inhibition of translation initiation. Addition of cytoplasmic extracts from control and diabetic adipocytes to a reticulocyte lysate system demonstrated the inhibition of LPL translation in vitro. Using different LPL mRNA transcripts in this in vitro translation assay, we found that the 3'-untranslated region (UTR) of the LPL mRNA was important in controlling translation inhibition by the cytoplasmic extracts. To identify the specific region involved, gel shift analysis was performed. A specific shift in mobility was observed when diabetic cytoplasmic extract was added to a transcript containing nucleotides 1818-2000 of the LPL 3'-UTR. Thus, inhibition of translation is the predominant mechanism for the decreased adipose tissue LPL in this insulin-deficient model of diabetes. Translation inhibition involves the interaction of a cytoplasmic factor, probably an RNA-binding protein, with specific sequences of the LPL 3'-UTR.

Lipoprotein lipase activity and mRNA are up-regulated by refeeding in adipose tissue and cardiac muscle of sheep

Previous studies in rodents have shown that the lipoprotein lipase (LPL) regulation is complex and often opposite in adipose tissue (AT) and muscle in response to the same nutritional treatment. However, neither LPL responses nor the molecular mechanisms involved in the nutritional regulation have been studied in both AT and muscle of ruminant species. To explore this, we measured the LPL activity and mRNA levels in perirenal AT and cardiac muscle (CM) of control, 7-d-underfed or 14-d-refed ewes. Underfeeding decreased (P < 0.01) LPL activity both in AT (-59%) and CM (-31%), and these activities were restored (P < 0.01) by refeeding (AT, +248%; CM, +34%). Variations of LPL mRNA level measured by real-time reverse transcription-polymerase chain reaction or by Northern blot followed variations of LPL activity: underfeeding decreased AT- and CM-LPL mRNA levels (-58 and -53%, respectively), and refeeding restored (P < 0.01) them in CM (+117%) and increased them over the baseline in AT (+640%). Quantification of either 3.4- or 3.8-kb LPL mRNA levels revealed a predominant (P < 0.001) expression of the 3.4-kb mRNA in AT (60%) and of the 3.8-kb mRNA in CM (56%), without any preferential regulation of one of these mRNA species by the nutritional status. This work reveals a tissue-specific expression pattern of the ovine LPL gene and a pretranslational nutritional regulation of its expression, which is achieved in the same direction in perirenal AT and CM. The different regulation of CM-LPL between ewes and rats probably arises from peculiarities of ruminant species for nutrient digestion and absorption and liver lipogenesis.

Nutritional regulation of binding sites for lipoprotein lipase in rat heart

Several laboratories have shown that when rats are fasted, the amount of lipoprotein lipase (LPL) at the vascular endothelium in heart (monitored as the amount released by heparin) increases severalfold without corresponding changes in the production of LPL. This suggests that there is a change in endothelial binding of LPL. To study this, (125)I-labeled bovine LPL was injected. The fraction that bound in the heart was more than twice as high in fasted than in fed rats, 4.3% compared with 1.9% of the injected dose. Refeeding reversed this in 5 h. When unlabeled LPL was injected before the tracer, the fraction of (125)I-LPL that bound in heart decreased, indicating that the binding was saturable. When isolated hearts were perfused at 4 degrees C with a single pass of labeled LPL, twice as much bound in hearts of fasted rats. We conclude that fasting causes a change in the vascular endothelium in heart such that its ability to bind LPL increases.

Local and systemic effects of macrophage cytokines in intestinal inflammation

The activation of macrophages and newly recruited monocytes appears to be common to both Crohn's disease and ulcerative colitis, despite different inductive stimuli. Similar activation occurs acutely during the course of invasive intestinal infections such as shigellosis, but is then usually downregulated. The macrophage cytokines tumor necrosis factor-alpha and interleukin-1 (IL-1) are centrally involved in the local inflammatory response, and blockade of either cytokine greatly attenuates the inflammatory lesion. Induction of focal vascular thrombosis and matrix degradation are thought to be an important component of this focal damage. Both cytokines and IL-6 are now recognized to contribute to the systemic effects of intestinal disease, including growth suppression, anorexia, and chronic anemia. Disturbance of sleep patterns, mood, and affect may also occur, and recent evidence points towards bidirectional interplay between macrophage cytokines and central nervous system function.

Lipoprotein lipase expression exclusively in liver. A mouse model for metabolism in the neonatal period and during cachexia

Lipoprotein lipase (LPL), the rate-limiting enzyme in triglyceride hydrolysis, is normally not expressed in the liver of adult humans and animals. However, liver LPL is found in the perinatal period, and in adults it can be induced by cytokines. To study the metabolic consequences of liver LPL expression, transgenic mice producing human LPL specifically in the liver were generated and crossed onto the LPL knockout (LPL0) background. LPL expression exclusively in liver rescued LPL0 mice from neonatal death. The mice developed a severe cachexia during high fat suckling, but caught up in weight after switching to a chow diet. At 18 h of age, compared with LPL0 mice, liver-only LPL-expressing mice had equally elevated triglycerides (10,700 vs. 14,800 mg/dl, P = NS), increased plasma ketones (4.3 vs. 1.7 mg/dl, P < 0.05) and glucose (28 vs. 15 mg/dl, P < 0.05), and excessive amounts of intracellular liver lipid droplets. Adult mice expressing LPL exclusively in liver had slower VLDL turnover than wild-type mice, but greater VLDL mass clearance, increased VLDL triglyceride production, and three- to fourfold more plasma ketones. In summary, it appears that liver LPL shunts circulating triglycerides to the liver, which results in a futile cycle of enhanced VLDL production and increased ketone production, and subsequently spares glucose. This may be important to sustain brain and muscle function at times of metabolic stress with limited glucose availability.

Diurnal rhythms and effects of fasting and refeeding on rat adipose tissue lipoprotein lipase

The activity of lipoprotein lipase (LPL) in adipose tissue is modulated by changes in the nutritional status. We have measured LPL activity, mass, and mRNA levels in rat adipose tissue during normal feeding cycles, during short- and long-term fasting, and during refeeding after fasting. LPL activity displayed a diurnal rhythm. The activity was highest during the night and early morning, decreased to a minimum during the early afternoon, and then increased again. These changes corresponded to the feeding pattern. The increases and/or decreases resulted from changes in LPL synthetic rate compounded by posttranslational mechanisms. During short-term fasting, LPL specific activity decreased to < 30% of control. The specific activity was restored within 4 h by refeeding. On longer fasting, LPL mRNA decreased. This became significant from 36 h. On refeeding, it took 12 h to restore the mRNA levels, whereas tissue LPL activity and mass could not be fully restored by 36 h of refeeding. These data show that LPL activity during short-term fasting is regulated posttranscriptionally, which allows for quick upregulation after refeeding. On longer fasting, other mechanisms affecting LPL transcription and synthesis come into play, and upregulation after refeeding is slowed down.

Hepatic regeneration induces changes in lipoprotein lipase activity in several tissues and its re-expression in the liver

We examined the expression of lipoprotein lipase (LPL) gene and LPL activity following a two-thirds hepatectomy and during liver regeneration. In most of the tissues studied, LPL activity increased a few hours after partial hepatectomy, but soon returned to normal levels. The greatest increase was found in the adrenal glands, plasma and liver. This increase in LPL activity in the liver could be partially due to an increase in the influx of the enzyme from extrahepatic tissues. There is, however, also a re-expression of LPL mRNA in the liver after partial hepatectomy (during the first hours). It is well known that LPL is expressed in the liver of neonatal animals, but progressively decreases during post-natal development, to reach adult levels around the time of weaning. Our results show by the first time that the remaining liver re-expresses LPL gene during the regeneration process and that the hepatocytes de-differentiate and acquire some of the neonatal characteristics. The increase in LPL mRNA will contribute to the rise in LPL activity after hepatectomy. This presence of LPL could enable the liver to take up fatty acids from the circulating triacylglycerols, which are needed as energetic and plastic substrates during the process of hepatic regeneration.

Forms of lipoprotein lipase in rat tissues: in adipose tissue the proportion of inactive lipase increases on fasting

Previous studies have shown that the ratio of lipoprotein lipase (LPL) catalytic activity to LPL mass in tissues differs in different conditions, but it is not clear whether this occurs by a change in the catalytic efficiency of the LPL molecules, or because of a shift in the relation between active and inactive forms of the enzyme. To explore this, we have measured LPL activity and mass in detergent extracts of rat tissues. LPL specific activity was high and similar in heart, skeletal muscle, lung and brain. The liver had significantly lower specific activity, which is in accord with previous findings that the liver takes up and catabolizes LPL. The specific activity was also low in adipose tissue from fasted rats. When tissue extracts were applied to columns of heparin-agarose and eluted by a gradient of NaCl, a peak of active LPL was eluted at 1.0 M NaCl, but there was also a peak of inactive LPL protein, which was eluted at 0.6 M NaCl. In adipose tissue, LPL activity decreased by 70-80% during an overnight fast, whereas LPL mass decreased by only 20-40%. The mass ratio between inactive and active LPL, as separated by heparin-agarose chromatography, increased from 0.5 to over 2 during the fast. In hearts there was no significant difference between fed and fasted rats in total LPL activity, LPL mass or in the distribution between inactive and active forms. The results indicate that the relation between inactive (probably monomeric) and active (dimeric) forms of LPL is a target for post-translational regulation in adipose tissue.

Hypertriglyceridemia associated with tumor necrosis factor-alpha in hamster cheek-pouch carcinogenesis

We demonstrated for the first time that 9,10-dimethyl-1,2-benzanthracene (DMBA)-treated hamsters showed hypertriglyceridemia followed by cachexia. Hypertriglyceridemia is believed to be caused in part by the decreased lipoprotein lipase (LPL) activity, and by cytokines such as tumor necrosis factor (TNF)-alpha. In addition, TNF-alpha action is associated with the LPL activity. Therefore, we determined the content of triglyceride (TG), LPL, and TNF-alpha in the serum from DMBA-treated hamsters. Elevated TG concentration in the serum of tumor-bearing hamsters was more remarkable and preceded the increase in other lipids, whereas the activity of LPL, the key enzyme of TG metabolism in vivo, was drastically reduced. TNF-alpha, known as an endogenous inhibitor of LPL activity, was detected in both the sera and the extract of tumors from DMBA-treated hamsters, whereas it was not detectable in any control samples. Pre-incubation of control sera with exogenous recombinant human TNF-alpha resulted in a potent inhibition of endogenous LPL activity in a dose-dependent manner in vitro. Therefore, the presence of TNF-alpha might lead to the increase in plasma TG mediated by LPL in tumor-bearing hamsters.

Tissue-specific expression of human lipoprotein lipase. Effect of the 3'-untranslated region on translation

Lipoprotein lipase (LPL) is a central enzyme in lipoprotein metabolism and is expressed predominantly in adipose tissue and muscle. In these tissues, the regulation of LPL is complex and often opposite in response to the same physiologic stimulus. In addition, much regulation of LPL occurs post-transcriptionally. The human LPL cDNA is characterized by a long 3'-untranslated region, which has two polyadenylation signals. In this report, human adipose tissue expressed two LPL mRNA species (3.2 and 3.6 kb) due to an apparent random choice of sites for mRNA polyadenylation, whereas human skeletal and heart muscle expressed predominantly the longer 3.6-kb mRNA form. To determine whether there was any functional significance to this tissue-specific mRNA expression, poly(A)-enriched RNA from adipose tissue and muscle were translated in vitro, and the poly(A)-enriched RNA from muscle was more efficiently translated into LPL protein. The increased translatability of the 3.6-kb form was also demonstrated by cloning the full-length 3.2- and 3.6-kb LPL cDNA forms, followed by in vitro translation of in vitro prepared transcripts. To confirm that this increased efficiency of translation occurred in vivo, Chinese hamster ovary cells were transfected with the 3.2- and 3.6-kb LPL cDNAs. Cells transfected with the 3.6-kb construct demonstrated increased LPL activity and synthesis, despite no increase in levels of LPL mRNA. Thus, human muscle expresses the 3.6-kb form of LPL due to a non-random choice of polyadenylation signals, and this form is more efficiently translated than the 3.2-kb form.

A novel regulatory pathway of brown fat thermogenesis. Retinoic acid is a transcriptional activator of the mitochondrial uncoupling protein gene

The mitochondrial uncoupling protein (UCP) is responsible for the thermogenic function of brown fat, and it is a molecular marker of the brown adipocyte cell type. Retinoic acid (RA) increased UCP mRNA levels severalfold in brown adipocytes differentiated in culture. This induction was independent of adrenergic pathways or protein synthesis. RA stimulated ucp gene expression regardless of the stage of brown adipocyte differentiation. In transient transfection experiments RA induced the expression of chloramphenicol acetyltransferase vectors driven by 4.5 kilobases of the 5'-noncoding region of the rat ucp gene, and co-transfection of expression vectors for RA receptors enhanced the action of RA. Retinoic acid receptor alpha was more effective than retinoid X receptor in promoting RA action, whereas a mixture of the two was the most effective. The RA-responsive region in the ucp gene was located at -2469/-2318 and contains three motifs (between -2357 and -2330) of the consensus half-sites characteristic of retinoic acid response elements. This 27-base pair sequence specifically binds purified retinoic acid receptor alpha as well as related proteins from brown fat nuclei. In conclusion, a novel potential regulatory pathway of brown fat development and thermogenic function has been recognized by identifying RA as a transcriptional activator of the ucp gene.

Distinct immunoreactivities suggest the existence of potential tissue variants in rat lipoprotein lipase

Lipoprotein lipases (LPL) isolated from rat cardiac muscle and bovine milk were each used as immunogens to produce polyclonal anti-LPL sera and two anti-LPL monoclonal antibodies. The immunological reactivities of these antibody sources with LPL purified from rat cardiac muscle, lung, adipose tissue, mammary gland and skeletal muscle were compared by an e.l.i.s.a. and by Western blotting. Differences between the immunoreactivities of LPL from the distinct tissue sources were revealed in both systems. A synthetic peptide with a sequence corresponding to the heparin-binding site of LPL (Ser-Arg-Thr-Asn-Thr-Lys-Val-Ser-Arg-Ile-Thr-Gly-Leu) was produced and used as an immunogen. The antiserum produced against the synthetic peptide was found to bind specifically to the region of the heparin-binding site, as determined by use of a competition e.l.i.s.a. In use against the five tissue LPL preparations, this antiserum revealed only minor variations between the tissue sources, compared with the hierarchy of reactivity observed when antibodies raised against the whole molecule were used. In combination with the outcome of previous studies on some of the physical properties of these preparations [Soteriou and Cryer (1993) Int. J. Biochem. 25, 1483-1490], the observations reported here on the distinct immunoreactivities exhibited by LPL prepared from the different tissue sources of a single species indicate the necessity to characterize fully the nature of these differences.

Identification of the 5' regulatory elements of avian lipoprotein lipase gene: synergistic effect of multiple factors

The organization of cis-acting regulatory elements of the chicken lipoprotein lipase gene was investigated in 5.4 kb of 5' flanking sequences. Various lengths of 5' flanking sequence were linked to the bacterial chloramphenicol acyltransferase (CAT) gene and transfected into primary cultures of chicken adipocytes by DEAE-dextran transfection method. Negative elements are present between -1947 and -139 of the 5' flanking sequence. Removal of these sequences revealed the presence of positive elements located within 138 bp upstream of the major transcription start site. Sequence analysis showed that the region from the major transcription start site to -138 contains an inverted GC box (ACCACGCCCC), a CCAAT element and two direct repeats of the octamer motif, ATTTGCAT. DNase I footprinting assays using a probe extending from -175 to +191, identified three sites protected by nuclear factors. Site I (-126 to -123), a C-rich sequence, GCCC, was identified only on the coding strand. Site II covered the sequence from -95 to -68 and includes the GC box. Site III, from -53 to -26, contained two octamer repeats. Site I is the 5' portion of a 10 bp sequence (CCCTCCCCCC; -126/-116) which is perfectly conserved in the avian and the human promoter. Single or multiple copies of a 37 bp DNA fragment (-138/-102) containing the 10 bp conserved sequence were cloned into LPLCAT-51, upstream or downstream of the major transcription start site and in both orientations; transfection and CAT activity assays with these constructs indicate that the -138/-102 fragment has an enhancer like activity. Additional 5' and internal deletions of LPLCAT-138 suggest that the factors binding to the C-rich element, the GC box and the two octamer repeats have a synergistic effect on promoter activity.

Increased expression of the mRNA for hormone-sensitive lipase in adipose tissue of cancer patients

The expression of genes coding for regulatory enzymes involved in the uptake, synthesis and mobilisation of lipid was measured in adipose tissue of cancer patients. Total RNA was isolated from subcutaneous adipose tissue of control and cancer patients and the various mRNAs measured by Northern blot analysis. The total lipoprotein lipase enzymic activity and the relative levels of the mRNAs for lipoprotein lipase and for fatty acid synthase were not significantly different between cancer patients and control patients. However, there was a significant two-fold increase in the relative level of mRNA for hormone-sensitive lipase (HSL) in adipose tissue of cancer patients compared with control patients. The cancer patients also exhibited a two-fold elevation in serum triacylglycerol levels and serum free fatty acid levels. There was a significant correlation between the serum free fatty acid level and expression of HSL mRNA in the adipose tissue. The serum levels of insulin and tumour necrosis factor-alpha were not different between cancer and control patients. The results suggest that at least one of the mechanisms for depletion of lipid from adipose tissue in cancer patients operates at the level of increased expression of mRNA of the lipolytic regulatory enzyme, hormone-sensitive lipase.

The effects of exercise and feeding on the activity of lipoprotein lipase in nine different adipose depots of guinea pigs

1. The activity of lipoprotein lipase (LPL) was measured in whole adipose tissue from 9 identified adipose depots of sedentary, fasting adult guinea pigs and following 30 min of exercise or voluntary ingestion of chow, and in adipocyte and stromal-vascular fractions from exercised specimens. 2. In sedentary, fasting specimens, LPL activity was up to 4 times higher in the small intermuscular depots than in the perirenal and epididymal depot (Table 1). 3. LPL activity increased significantly after feeding only in the large superficial depot, groin, and in the perirenal depot. LPL activity decreased after exercise only in the 2 intermuscular depots and in small anterior superficial depots. These effects of exercise were consistently greater in males than in females (Table 3). 4. Following exercise, there was up to twice as much LPL in the adipocytes as in the stromal-vascular fraction of the intermuscular depots, about 50% more in adipocytes from the minor superficial depots and about equal quantities in the 2 fractions of the intra-abdominal and groin depots (Table 2). 5. The data demonstrate the physiological inhomogeneity of both superficial and internal adipose depots, and are consistent with the hypothesis that LPL originating from adipose tissue may enter the circulation.

Characterization of the human lipoprotein lipase (LPL) promoter: evidence of two cis-regulatory regions, LP-alpha and LP-beta, of importance for the differentiation-linked induction of the LPL gene during adipogenesis

When preadipocytes differentiate into adipocytes, several differentiation-linked genes are activated. Lipoprotein lipase (LPL) is one of the first genes induced during this process. To investigate early events in adipocyte development, we have focused on the transcriptional activation of the LPL gene. For this purpose, we have cloned and fused different parts of intragenic and flanking sequences with a chloramphenicol acetyltransferase reporter gene. Transient transfection experiments and DNase I hypersensitivity assays indicate that several positive as well as negative elements contribute to transcriptional regulation of the LPL gene. When reporter gene constructs were stably introduced into preadipocytes, we were able to monitor and compare the activation patterns of different promoter deletion mutants at selected time points representing the process of adipocyte development. We could delimit two cis-regulatory elements important for gradual activation of the LPL gene during adipocyte development in vitro. These elements, LP-alpha (-702 to -666) and LP-beta (-468 to -430), contain a striking similarity to a consensus sequence known to bind the transcription factors HNF-3 and fork head. Results of gel mobility shift assays and DNase I and exonuclease III in vitro protection assays indicate that factors with DNA-binding properties similar to those of the HNF-3/fork head family of transcription factors are present in adipocytes and interact with LP-alpha and LP-beta. We also demonstrate that LP-alpha and LP-beta were both capable of conferring a differentiation-linked expression pattern to a heterolog promoter, thus mimicking the expression of the endogenous LPL gene during adipocyte differentiation. These findings indicate that interactions with LP-alpha and LP-beta could be a part of a differentiation switch governing induction of the LPL gene during adipocyte differentiation.

Regulation of the synthesis, processing and translocation of lipoprotein lipase

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Characterization of the human lipoprotein lipase (LPL) promoter: evidence of two cis-regulatory regions, LP-alpha and LP-beta, of importance for the differentiation-linked induction of the LPL gene during adipogenesis

When preadipocytes differentiate into adipocytes, several differentiation-linked genes are activated. Lipoprotein lipase (LPL) is one of the first genes induced during this process. To investigate early events in adipocyte development, we have focused on the transcriptional activation of the LPL gene. For this purpose, we have cloned and fused different parts of intragenic and flanking sequences with a chloramphenicol acetyltransferase reporter gene. Transient transfection experiments and DNase I hypersensitivity assays indicate that several positive as well as negative elements contribute to transcriptional regulation of the LPL gene. When reporter gene constructs were stably introduced into preadipocytes, we were able to monitor and compare the activation patterns of different promoter deletion mutants at selected time points representing the process of adipocyte development. We could delimit two cis-regulatory elements important for gradual activation of the LPL gene during adipocyte development in vitro. These elements, LP-alpha (-702 to -666) and LP-beta (-468 to -430), contain a striking similarity to a consensus sequence known to bind the transcription factors HNF-3 and fork head. Results of gel mobility shift assays and DNase I and exonuclease III in vitro protection assays indicate that factors with DNA-binding properties similar to those of the HNF-3/fork head family of transcription factors are present in adipocytes and interact with LP-alpha and LP-beta. We also demonstrate that LP-alpha and LP-beta were both capable of conferring a differentiation-linked expression pattern to a heterolog promoter, thus mimicking the expression of the endogenous LPL gene during adipocyte differentiation. These findings indicate that interactions with LP-alpha and LP-beta could be a part of a differentiation switch governing induction of the LPL gene during adipocyte differentiation.