Triglyceride Uptake Research Articles

Up-Regulation of Fetal Rat Lung Parathyroid Hormone-Related Protein Gene Regulatory Network Down-Regulates the Sonic Hedgehog/Wnt/βcatenin Gene Regulatory Network

Lung development depends on endodermal Sonic Hedgehog (Shh) signaling to mesodermal Wingless/int/beta catenin (Wnt/betacatenin), followed by parathyroid hormone-related protein (PTHrP) signaling from endoderm to mesoderm. Fluid distension of fetal rat lung explants up-regulates PTHrP signaling and down-regulates Shh/Wnt/betacatenin signaling, marked by decreases in Patched, Gli, Frizzled, and Dishevelled, inducing fibroblast triglyceride uptake, type II cell saturated phosphatidylcholine, and surfactant protein-B expression. Bumetanide, which inhibits fluid distension, blocked down-regulation of the Shh/Wnt/betacatenin pathway and up-regulation of the PTHrP pathway, whereas PTHrP (1-34, 5 x 10(-7) M) treatment overcame bumetanide inhibition, and the PTHrP receptor antagonist PTHrP (7-34) amide (5 x 10(-6) M) mimicked bumetanide, indicating that PTHrP signaling mediates fluid distension-induced alveolar differentiation. Fetal rat lung explant automaturation was characterized by decreased Wnt/betacatenin signaling and increased PTHrP/PTHrP receptor signaling, up-regulating fibroblast-specific adipocyte differentiation related protein (ADRP) and peroxisome proliferator-activated receptor gamma. Wnt/betacatenin agonists (LiCl or SB415268) maintained Shh/Wnt/betacatenin signaling, blocking spontaneous up-regulation of the PTHrP pathway, whereas PTHrP or cAMP down-regulated Shh/Wnt/betacatenin signaling and stimulated PTHrP signaling for fibroblast and type II cell differentiation. This is the first evidence that alveolar fluid distension is an organizing principle for PTHrP signaling down-regulation of the Shh/Wnt/betacatenin pathway.

EVIDENCE FOR THE PRESENCE OF LIPOFIBROBLASTS IN HUMAN LUNG

The lipid-containing alveolar interstitial fibroblast (lipofibroblast) is known to be critically involved in rodent lung development, homeostasis, and injury/repair. However, there is lack of information on their presence and function in the human lung. Based on a number of morphological (lipid staining), molecular (presence of characteristic lipogenic and absence of myogenic markers), and functional (triglyceride uptake) characteristics that are the hallmarks of the rodent lung lipofibroblast, using human lung fibroblasts of embryonic (WI-38) and adult origin and lung tissue from human autopsy specimens, the authors for the first time clearly demonstrate the presence of lipofibroblasts in the human lung.

CD36 deficiency in mice impairs lipoprotein lipase-mediated triglyceride clearance

CD36 is involved in high-affinity peripheral FFA uptake. CD36-deficient (cd36(-)(/)(-)) mice exhibit increased plasma FFA and triglyceride (TG) levels. The aim of the present study was to elucidate the cause of the increased plasma TG levels in cd36(-)(/)(-) mice. cd36(-)(/)(-) mice showed no differences in hepatic VLDL-TG production or intestinal [(3)H]TG uptake compared with wild-type littermates. cd36(-)(/)(-) mice showed a 2-fold enhanced postprandial TG response upon an intragastric fat load (P < 0.05), with a concomitant 2.5-fold increased FFA response (P < 0.05), suggesting that the increased FFA in cd36(-/-) mice may impair LPL-mediated TG hydrolysis. Postheparin LPL levels were not affected. However, the in vitro LPL-mediated TG hydrolysis rate as induced by postheparin plasma of cd36(-)(/)(-) mice in the absence of excess FFA-free BSA was reduced 2-fold compared with wild-type plasma (P < 0.05). This inhibition was relieved upon the addition of excess FFA-free BSA. Likewise, increasing plasma FFA in wild-type mice to the levels observed in cd36(-)(/)(-) mice by infusion prolonged the plasma half-life of glycerol tri[(3)H]oleate-labeled VLDL-like emulsion particles by 2.5-fold (P < 0.05). We conclude that the increased plasma TG levels observed in cd36(-)(/)(-) mice are caused by decreased LPL-mediated hydrolysis of TG-rich lipoproteins resulting from FFA-induced product inhibition of LPL.

Mechanism of nicotine-induced pulmonary fibroblast transdifferentiation

We tested the hypothesis that in vitro nicotine exposure disrupts specific epithelial-mesenchymal paracrine signaling pathways and results in pulmonary interstitial lipofibroblast (LIF)-to-myofibroblast (MYF) transdifferentiation, resulting in altered pulmonary development and function. Studies were done to determine whether nicotine induces LIF-to-MYF transdifferentiation and to elucidate underlying molecular mechanism(s) involved and to determine whether nicotine-induced LIF-to-MYF transdifferentiation could be prevented by stimulating specific alveolar interstitial fibroblast lipogenic pathway. WI38 cells, a human embryonic pulmonary fibroblast cell line, were treated with nicotine with or without specific agonists of alveolar fibroblast lipogenic pathway, PTHrP, DBcAMP, or the potent PPARgamma stimulant rosiglitazone (RGZ) for 7 days. Expression of key lipogenic and myogenic markers was examined by RT-PCR, Western hybridization, and immunohistochemistry. The effect of nicotine on triglyceride uptake by WI38 cells and PTHrP binding to its receptor was also determined. Finally, the effect of transfecting WI38 cells with a PPARgamma expression vector on nicotine-induced LIF-to-MYF transdifferentiation was determined. Nicotine treatment resulted in significantly decreased expression of lipogenic and increased expression of myogenic markers in a dose-dependent manner, indicating nicotine-induced LIF-to-MYF transdifferentiation. This was accompanied by decreased PTHrP receptor binding to its receptor. The nicotine-induced LIF-to-MYF transdifferentiation was completely prevented by concomitant treatment with PTHrP, DBcAMP, RGZ, and by transiently overexpressing PPARgamma. Our data suggest nicotine induces alveolar LIF-to-MYF transdifferentiation through a mechanism involving downregulation of lipogenic PTHrP-mediated, cAMP-dependent PKA signaling pathway, which can be prevented using specific molecular targets. Potential therapeutic implications of these observations against in utero nicotine-induced lung injury remain to be tested.

Effects of hypothyroidism on mammary and liver lipid metabolism in virgin and late-pregnant rats

Untreated maternal hypothyroidism (hypoT) has serious consequences in offspring development that may result from the effect on lactation of maternal metabolism dysfunction. We studied the effects of prolonged propylthiouracil (PTU)-induced hypoT (0.1% PTU in drinking water starting 8 days before mating until day 21 of pregnancy or for 30 days in virgin rats) on liver and mammary lipid metabolism and serum lipid concentrations. In virgins, hypoT reduced hepatic mRNAs associated with triglyceride (TG) and cholesterol synthesis (including fatty acid synthase and 3-hydroxy-3-methylglutaryl coenzyme A reductase), and induced lobuloalveolar mammary development. Pregnancy increased hepatic mRNAs associated with TG and cholesterol synthesis and uptake (including LDL receptor) and with lipid oxidation, such as acyl CoA oxidase. HypoT decreased mRNAs and the activity of proteins associated with TG synthesis, and mRNAs associated with cholesterol uptake and lipid oxidation. Pregnancy increased mammary mRNAs related to lipid oxidation and decreased cholesterol synthesis, whereas hypoT decreased mRNAs and activities of proteins associated with TG synthesis and decreased epithelial mammary tissue. Virgin and pregnant hypoT rats had increased circulating VLDL + LDL cholesterol. HypoT decreased circulating TGs in pregnant rats. The observed effects of hypoT may result in decreased mammary lipid availability. This, along with the decreased epithelial mammary tissue during lactogenesis, may contribute to the future lactational deficit of hypoT mothers.

Differential uptake of subfractions of triglyceride-rich lipoproteins by THP-1 macrophages

It is well known that raised plasma triglycerides (TG) are positively linked to the development of coronary heart disease. However, triglycerides circulate in a range of distinct lipoprotein subfractions and the relative atherogenicity of these subfractions is not clear. In this study, three fractions of triglyceride rich lipoprotein (TRL) were isolated from normolipidaemic males according to their differing Svedberg flotation ( S f) rates: chylomicron (CM, S f > 400), very low-density lipoprotein (VLDL)-1 ( S f 60–400) and VLDL-2 ( S f 20–60). These fractions were incubated with THP-1 monocyte-derived macrophages for determination of cholesterol and TG accumulation, in the presence and absence of the lipoprotein lipase (LPL) inhibitor orlistat. Expression of LDL receptor related protein (LRP) and apolipoprotein B48 receptor (apoB48R) was also examined in both differentiating monocytes, and monocyte-derived macrophages, incubated with TRL. VLDL-1 caused a significantly greater accumulation of TG within macrophages compared to VLDL-2. Binding studies also tended to show a greater preference for VLDL-1. No change in expression of LRP or apoB48R was observed in fully differentiated macrophages incubated with VLDL-1, VLDL-2 or CM, although a greater expression of LRP mRNA was observed in differentiating monocytes exposed to VLDL-1, compared to those incubated with CM or VLDL-2. TG loading in response to all three TRL fractions was blocked by orlistat, suggesting that it is likely that the major pathway for uptake of TG was hydrolysis by LPL. Calculations suggested that direct uptake of particles accounts for between 12 and 25% of total TAG uptake. In conclusion, THP monocyte-derived macrophages demonstrate a preference for VLDL-1, both through the LPL pathway and by direct uptake of whole particles.

Peroxisome Proliferator-Activated Receptor Agonists Modulate Heart Function in Transgenic Mice with Lipotoxic Cardiomyopathy

hLpL(GPI) transgenic mice that overexpress human lipoprotein lipase (hLpL) with a glycosylphosphatidylinositol anchor on cardiomyocytes develop lipotoxic cardiomyopathy associated with increased cardiac uptake of plasma lipids. We hypothesized that peroxisome proliferator-activated receptor (PPAR)alpha, PPARgamma, or a PPARalpha/gamma agonist would alter cardiac function by modulating lipid uptake by the heart. hLpL(GPI) mice were administered rosiglitazone (10 mg/kg/day), fenofibrate (100 mg/kg/day), or DRF2655, an alkoxy propanoic acid analog (10 mg/kg/day), for 16 days. Rosiglitazone reduced plasma triglyceride (TG) from 107.63 +/- 6.98 to 77.61 +/- 3.98 mg/dl, whereas fenofibrate had no effect. DRF2655 reduced TG to 33.17 +/- 4.12 mg/dl. Rosiglitazone and DRF2655 decreased heart TG and total cholesterol; fenofibrate had no effect. Molecular markers for cardiac dysfunction, atrial natriuretic factor, brain natriuretic peptide, and tumor necrosis factor-alpha were decreased with rosiglitazone and increased with fenofibrate. Echocardiographic measurements showed reduced fractional shortening and increased left ventricular systolic dimension with fenofibrate. No changes in these parameters were observed with rosiglitazone or DRF2655 treatment. Muscle-specific carnitine palmitoyltransferase-1 and fatty acid transporter protein-1 gene expression were increased with fenofibrate and DRF2655 treatment; no change in expression of these genes was noted with rosiglitazone treatment. Rosiglitazone and DRF2655 reduced TG uptake by the heart, and fenofibrate treatment increased fatty acid uptake. Thus, in a lipotoxic cardiomyopathy mouse model, a PPARgamma agonist reduced cardiac lipid and markers of cardiomyopathy, whereas an agonist of PPARalpha did not improve cardiac lipids and worsened heart function. These changes were paralleled by alterations in heart lipid uptake. Overall, PPAR activators exhibit differential effects in this model of lipotoxic dilated cardiomyopathy.

Evidence for an Exaggerated Postprandial Lipemia in Chronic Paraplegia

Background/Objective: Excessive delay in triglyceride (TG) metabolism after ingestion of dietary fatrepresents a significant cardiovascular disease (CVD) risk. The objective of this study was to compare thepostprandial lipemic responses of individuals with paraplegia with those of healthy nondisabled individuals.Methods: The ability of 3 recreationally active individuals with paraplegia having normal fasting TG(mean= 103 mg/dl) to metabolize TG after ingestion of a high-fat test meal was compared with apreviously published cohort of 21 recreationally active individuals without paraplegia (TG mean= 86 mg/dl)who underwent identical testing. The subjects with paraplegia had venous blood taken under fastingconditions, and then ingested a milkshake containing premium ice cream blended with heavy whippingcream(~ 92% of calories from fat). Additional blood samples were obtained at 2, 4, and 6 hours afteringestion. The area under the curve (AUC) for TG clearance for both subject groups was measured with anarea planimeter.Results: TG uptake for both groups was almost identical for the first 2 hours after ingestion. At 4 and 6 hoursafter ingestion, the TG levels were 50 and 35 mg/dl higher, respectively, in subjects with paraplegia than innondisabled subjects. When corrected for small baseline differences in TG concentrations (16 mg/dl), theAUC was 46.5% greater for the group with paraplegia than in the nondisabled group. A near mirrorassociation across time was observed between postprandial serum high-density lipoprotein cholesterol(HDL-C) and TG levels in subjects with paraplegia.Conclusion: This case series finds an exaggerated postprandial lipemia (PPL) in persons with paraplegiawith normal fasting TGs. This finding is the first evidence, in a small population, of an unreported potentialCVD risk in persons with paraplegia.

Population prevalence of APOE, APOC3 and PPAR-α mutations associated to hypertriglyceridemia in French Canadians

Hypertriglyceridemia (HTG) is known as a common metabolic disorder associated with increased production, decrease catabolism and/or decreased hepatic uptake of triglyceride (TG)-rich particles. We assessed, in the Quebec City population, the allele frequency and haplotype distributions of mutations in genes related to HTG, such as the apolipoprotein E (APOE) (C112R and C158R), the apolipoprotein CIII (APOC3) (C-482T and C3238G) and the peroxisome proliferator-activated receptor alpha (PPARalpha) (L162V) genes. A total of 938 anonymous unlinked newborns from the metropolitan Quebec City area have been genotyped. Allele frequencies observed in the Quebec City population differed from known frequencies determined in other Caucasian populations. The co-transmitted allele distribution between the two-marker genotypes APOE/APOC3(C3238G) and APOC3(C-482T)/PPARalpha(L162V) presented a weak deviation from the assumption of genetic independence. Also, we observed a non-independent distribution of the T-482/G3238 allele combinations within the APOC3 gene, suggesting strong linkage disequilibrium between the C-482T and C3238G polymorphisms. Moreover, comparisons of allele frequencies observed in the population of Québec City to those obtained in other Caucasian populations suggested that the population of Québec City may be at a lower risk of developing HTG due to APOE, APOC3 and PPARalpha genetic variants. However, the strong linkage disequilibrium and the two-marker genotype distributions observed in the APOC3 gene suggest that these two variants may functionally interact in the Québec City population.

Circulating soluble CD14 monocyte receptor is associated with increased alanine aminotransferase.

Recent studies have suggested that abnormal hepatocellular function is associated with obesity, insulin resistance, and type 2 diabetes (1)(2)(3). Nonancoholic fatty liver disease (NAFLD) is the preferred term to describe a spectrum of liver damage, ranging from steatosis to steatohepatitis, liver fibrosis, and cirrhosis, that can be found in insulin-resistant (obese and type 2 diabetic) and hyperlipidemic patients (4). Insulin sensitivity and liver function exhibit a bidirectional relationship: a normally functioning liver determines, in part, the normal body response to insulin, whereas abnormal insulin sensitivity causes liver damage (5)(6). The insulin-resistant state is characterized by a failure to suppress hepatic glucose production and glycogenolysis (5), usually accompanied by fat accumulation in hepatocytes as a result of increased hepatic uptake and synthesis of triglycerides. The latter is known to impair liver function and, consequently, to worsen insulin resistance (5)(6). Aminotransferases are considered indicators of hepatocellular health. Alanine aminotransferase (ALT) is found primarily in the liver, whereas aspartate aminotransferase (AST) and γ-glutamyltranspeptidase (γGT) are also found in other tissues. NAFLD is characterized by mild-to-moderate increases in ALT and AST (7). Recent prospective studies have shown that increased ALT and γGT are predictors of the development of type 2 diabetes (1)(2). CD14, a 55-kDa glycoprotein, is a multifunctional receptor constitutively expressed in considerable amounts on the surface of mature monocytes, macrophages, and neutrophils (8). CD14 has specificity for lipopolysaccharide (LPS) and other bacteria-wall-derived components (9). LPS signaling triggers a cascade that leads to cytokine production and shedding of the extracellular domain of CD14 (sCD14) (10)(11)(12)(13). The buffering of LPS is crucial not only during acute inflammatory and infectious processes; in healthy humans, triglyceride-rich lipoproteins contain detectable amounts of endogenous LPS that are presumably scavenged in …

Post-transcriptional Regulation of Sterol Regulatory Element-binding Protein-1 by Ethanol Induces Class I Alcohol Dehydrogenase in Rat Liver

Members of the sterol regulatory element-binding protein (SREBP) family of transcription factors control the synthesis and uptake of cholesterol, fatty acids, triglycerides, and phospholipids. Continuous intragastric infusion of ethanol-containing diets as part of total enteral nutrition generates well defined 6-day cycles (pulses) of urine ethanol concentrations (UECs) in rats. Pulsatile UECs are the result of cyclical expression and activity of the principal alcohol-metabolizing enzyme, hepatic Class I alcohol dehydrogenase (ADH), and this mechanism involves regulated CCAAT/enhancer-binding protein-beta expression and binding to the ADH promoter. In this study, we further explore the molecular mechanism for ethanol-induced ADH expression during the UEC pulse in adult male rats fed ethanol by total enteral nutrition for 21-30 days. In hypophysectomized rats, in which the ADH protein increased by approximately 6-fold, the nuclear form of SREBP-1 decreased by approximately 7-fold. Because the ADH promoter contains two canonical sterol response element (SRE) sites (-63 to -53 and -52 to -40 relative to the transcription start site), electrophoretic mobility shift assays were conducted using an ADH-specific SRE site. Hepatic nuclear protein binding decreased by 2.4-fold on the ascending limbs and by 3.6-fold on the descending limbs of UEC pulses (p < 0.05). The specificity of nuclear protein binding to the ADH-SRE site was confirmed using antibody and UV cross-link assays. The in vivo binding status of SREBP-1 to ADH-SRE sites, as measured by the chromatin immunoprecipitation assay, had a pattern very similar to the electrophoretic mobility shift assay results. Functional analysis of the ADH-SREs demonstrated these sites to be essential for ADH transcription. In vitro transcription assays demonstrated that depletion of the SREBP-1 protein from nuclear extracts increased transcription activity by approximately 5-fold and that the liver X receptor agonist T0901317 (a known activator of SREBP-1c transcription) reduced in vitro expression of ADH mRNA by 2-fold. We conclude that SREBP-1 is a negative regulator of the ADH gene and may work in concert with the CCAAT/enhancer-binding proteins to mediate ethanol induction of ADH in vivo.

Systemic and forearm triglyceride metabolism: fate of lipoprotein lipase-generated glycerol and free fatty acids.

Little is known about the fate of the lipolytic products produced by the action of lipoprotein lipase (LPL) on circulating triglyceride-rich lipoproteins in humans. We studied eight lean, healthy male subjects after an overnight fast. Subjects received infusions of lipid emulsions containing triolein labeled with (3)H on both the glycerol backbone and the fatty acid portion of the molecule; (14)C glycerol and (14)C oleate were coinfused to quantify the systemic and forearm release of (3)H glycerol and (3)H oleate resulting from LPL action. There was significant forearm uptake of both whole plasma triglyceride (presumed to represent primarily VLDL; extraction fraction 2.6 +/- 0.6%, P < 0.005 vs. zero) and radiolabeled triglyceride derived from the lipid emulsion (a surrogate for chylomicrons; extraction fraction 31 +/- 4%, P < 0.005 vs zero). Systemic clearance and forearm fractional extraction of glycerol was greater than that of oleate (P < 0.001 and P < 0.02, respectively). The systemic and forearm fractional release of LPL-generated glycerol were similar at 51 +/- 4 and 59 +/- 1%, respectively (NS). In contrast, the forearm fractional release of LPL-generated oleate was less than systemic fractional release (14 +/- 2 vs. 36 +/- 4%, P < 0.0001). These results indicate that there is escape, or spillover, of the lipolytic products of LPL action on triglyceride-rich lipoproteins in humans. They further suggest that LPL-mediated fatty acid uptake is an inefficient process, but may be more efficient in muscle than in adipose tissue.

Suppression of skeletal muscle lipoprotein lipase activity during physical inactivity: a molecular reason to maintain daily low-intensity activity.

We have examined the regulation of lipoprotein lipase (LPL) activity in skeletal muscle during physical inactivity in comparison to low-intensity contractile activity of ambulatory controls. From studies acutely preventing ambulatory activity of one or both the hindlimbs in rats, it was shown that approximately 90-95 % of the heparin-releasable (HR) LPL activity normally present in rat muscle with ambulatory activity is lost, and thus dependent on local contractile activity. Similarly, approximately 95 % of the differences in LPL activity between muscles of different fibre types was dependent on ambulatory activity. The robustness of the finding that physical inactivity significantly decreases muscle LPL activity was evident from confirmatory studies with different models of inactivity, in many rats and mice, both sexes, three muscle types and during both acute and chronic (11 days) treatment. Inactivity caused a local reduction of plasma [3H]triglyceride uptake into muscle and a decrease in high density lipoprotein cholesterol concentration. LPL mRNA was not differentially expressed between ambulatory controls and either the acutely or chronically inactive groups. Instead, the process involved a rapid loss of the HR-LPL protein mass (the portion of LPL largely associated with the vascular endothelium) by an actinomycin D-sensitive signalling mechanism (i.e. transcriptionally dependent process). Significant decreases of intracellular LPL protein content lagged behind the loss of HR-LPL protein. Treadmill walking raised LPL activity approximately 8-fold (P < 0.01) within 4 h after inactivity. The striking sensitivity of muscle LPL to inactivity and low-intensity contractile activity may provide one piece of the puzzle for why inactivity is a risk factor for metabolic diseases and why even non-vigorous activity provides marked protection against disorders involving poor lipid metabolism.

Hyperoxia augments pulmonary lipofibroblast-to-myofibroblast transdifferentiation.

Bronchopulmonary dysplasia (BPD) remains a major cause of morbidity and mortality in premature infants, and despite many advances, its pathophysiology remains incompletely understood. Exposure of the premature lung to hyperoxia is commonly implicated in its pathogenesis. However, the exact link between hyperoxia and BPD, particularly its role in the generation of myofibroblasts, the signature cell-type for lung fibrosis, is undetermined. There is increasing evidence that lipid interstitial fibroblasts play an important role in injury-repair mechanisms in various organ systems. This study demonstrates that exposure to hyperoxia augments the transdifferentiation of pulmonary lipofibroblasts to myofibroblasts. Fetal rat lung fibroblasts (FRLF) from embryonic (e) (term = e22) 18 and e21 gestation were studied. After initial culture in minimum essential medium (MEM) and 10% fetal bovine serum (FBS) in 21% O2 / 5% CO2 at 37 degrees C, FRLF were maintained in MEM and 10%FBS at 37 degrees C under control (21% O2 / 5% CO2) and under experimental conditions (24-hour exposure to 95% O2 /5% CO2) at passage (P) 1 and 5. At each passage, cells were allowed to attach to 100 cm2 culture dishes and grow in 21% O2 before being subjected to the experimental conditions. Passage 1 and 5 cells were analyzed for the expression of well-characterized lipogenic and myogenic markers based on semiquantitative competitive RT-PCR (for parathyroid hormone-related protein receptor [PTHrPR]), adipose differentiation related protein (ADRP), and alpha smooth muscle actin (alphaSMA), triglyceride uptake, and leptin assay. Serial passage and maintenance of cells in 21% O2 resulted in a significant decrease in the expression of the lipogenic markers from P1 to P5, spontaneously. This decrease was greater for e18 than for e21 FRLF. However, exposing cells to 95% O2 augmented the loss of the lipogenic markers and gain of the myogenic marker from P1 to P5 in comparison to cells maintained in 21% O2. These changes were also greater for e18 vs e21 lipofibroblasts. These changes in mRNA expression were accompanied by decreased triglyceride uptake and leptin secretion on exposure to hyperoxia. These results suggest that exposure to hyperoxia (95% O2) augments the transdifferentiation of pulmonary lipofibroblasts to myofibroblasts.

Routes of FA delivery to cardiac muscle: modulation of lipoprotein lipolysis alters uptake of TG-derived FA.

Long-chain fatty acids (FA) supply 70-80% of the energy needs for normal cardiac muscle. To determine the sources of FA that supply the heart, [(14)C]palmitate complexed to bovine serum albumin and [(3)H]triolein [triglyceride (TG)] incorporated into Intralipid were simultaneously injected into fasted male C57BL/6 mice. The ratio of TG to FA uptake was much greater for hearts than livers. Using double-labeled Intralipid with [(3)H]cholesteryl oleoyl ether (CE) and [(14)C]TG, we observed that hearts also internalize intact core lipid. Inhibition of lipoprotein lipase (LPL) with tetrahydrolipstatin or dissociation of LPL from the heart with heparin reduced cardiac uptake of TG by 82 and 64%, respectively (P < 0.01). Palmitate uptake by the heart was not changed by either treatment. Uptake of TG was 88% less in hearts from LPL knockout mice that were rescued via LPL expression in the liver. Our data suggest that the heart is especially effective in removal of circulating TG and core lipids and that this is due to LPL hydrolysis and not its bridging function.

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

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. 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. 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.

Stretch-stimulated surfactant synthesis is coordinated by the paracrine actions of PTHrP and leptin.

Intrauterine lung development, culminating in physiological pulmonary surfactant production by epithelial type II (TII) cells, is driven by fluid distension through unknown mechanisms. Differentiation of alveolar epithelial and mesenchymal cells is mediated by soluble factors like parathyroid hormone-related protein (PTHrP), a stretch-sensitive TII cell product. PTHrP stimulates pulmonary surfactant production by a paracrine feedback loop mediated by leptin, a soluble product of the mature lipofibroblast (LF). When LFs and TIIs are stretched in coculture, there is a fivefold increase in surfactant phospholipid synthesis that can be "neutralized" by inhibitors of PTHrP or leptin, implicating a paracrine feedback loop in this mechanism. Stretching LFs stimulates PTHrP binding (2.5-fold) and downstream stimulation of triglyceride uptake quantitatively (15-25%) due to upregulation of adipose differentiation-related protein expression. Stretching TII cells increases leptin stimulation of their surfactant phospholipid synthesis threefold, suggesting that retrograde signaling by leptin to TII cells is also stretch sensitive. We conclude that the effect of stretch on alveolar LF and TII differentiation is coordinated by PTHrP, leptin, and their receptors.

SREBPs: activators of the complete program of cholesterol and fatty acid synthesis in the liver.

Lipid homeostasis in vertebrate cells is regulated by a family of membrane-bound transcription factors designated sterol regulatory element–binding proteins (SREBPs). SREBPs directly activate the expression of more than 30 genes dedicated to the synthesis and uptake of cholesterol, fatty acids, triglycerides, and phospholipids, as well as the NADPH cofactor required to synthesize these molecules (1–4). In the liver, three SREBPs regulate the production of lipids for export into the plasma as lipoproteins and into the bile as micelles. The complex, interdigitated roles of these three SREBPs have been dissected through the study of ten different lines of gene-manipulated mice. These studies form the subject of this review.

Integration of biochemical and physiologic effects of insulin on glucose metabolism.

The major effects of insulin on tissues are: (1) Carbohydrate metabolism: (a) It increases the rate of transport of glucose across the cell membrane in adipose tissue and muscle, (b) it increases the rate of glycolysis in muscle and adipose tissue, (c) it stimulates the rate of glycogen synthesis in a number of tissues, including adipose tissue, muscle, and liver. It also decreases the rate of glycogen breakdown in muscle and liver, (d) it inhibits the rate of glycogenolysis and gluconeogenesis in the liver. (2) Lipid metabolism: (a) It decreases the rate of lipolysis in adipose tissue and hence lowers the plasma fatty acid level, (b) it stimulates fatty acid and triacylglycerol synthesis in tissues, although only to a minor extent in humans, (c) it increases the rate of very-low-density lipoprotein formation in the liver, (d) it increases the uptake of triglyceride from the blood into adipose tissue and muscle, (e) it decreases the rate of fatty acid oxidation in muscle and liver, (f) it increases the rate of cholesterol synthesis in liver. (3) Protein metabolism: (a) It increases the rate of transport of some amino acids into tissues, (b) it increases the rate of protein synthesis in muscle, adipose tissue, liver, and other tissues, (c) it decreases the rate of protein degradation in muscle (and perhaps other tissues), (d) it decreases the rate of urea formation.--These insulin effects serve to encourage the synthesis of carbohydrate, fat and protein.

Hydrogenated fat consumption affects acylation-stimulating protein levels and cholesterol esterification rates in moderately hypercholesterolemic women

To determine whether hydrogenated fat consumption alters triglyceride metabolism and cholesterol esterification rates, 14 women (65–71 years of age) were provided with each of four diets for 5-week periods according to a randomized cross-over design. The experimental diets contained either soybean oil (SO), low trans squeeze (SQM), medium trans tub (TM), or high trans stick (SM) margarines. Triglyceride uptake by adipose tissue was determined by measuring plasma acylation-stimulating protein (ASP), FFA, glucose, and insulin levels, while rates of transfer and esterification rate of newly synthesized cholesterol (ER) were derived by using plasma CETP levels and the deuterium incorporation methodology. Plasma ASP levels were lowest (P < 0.05) in subjects on the SM diet (33.4 ± 12.7 nM) compared with the SO (48.7 ± 17.0 nM) and SQM (50.7 ± 15.7 nM) diets. Conversely, FFA were highest (P < 0.05) on the SM diet (0.86 ± 0.45 mM) relative to all the other diets. No differences were observed in plasma glucose and insulin levels among diets. A trend toward higher CETP levels after consumption of the SM diet was observed. However, the ER was lowest (P < 0.05) after the SM (0.111 ± 0.062 g·day−1) diet and highest after consumption of the SQM (0.216 ± 0.123 g·day−1) diet. In addition, ASP levels were negatively correlated with FFA (r = −0.63, P < 0.05), LDL cholesterol (r = −0.56, P < 0.05), and TG (r = −0.41, P < 0.05), whereas FFA was positively correlated with apolipoprotein B-containing lipoproteins (r = 0.58 and 0.47, for VLDL and LDL cholesterol, P < 0.05), and negatively correlated with HDL cholesterol (r = −0.51, P < 0.05). The ER was found to positively correlate with HDL cholesterol and HDL2 subfraction (r = 0.53 and 0.45, respectively, P < 0.05). Taken together, these data demonstrate that the alterations in circulating lipid levels, commonly observed with consumption of hydrogenated fat-rich diets, can be explained in part by changes in ASP activity as well as newly synthesized cholesterol ER.—Matthan, N. R., K. Cianflone, A. H. Lichtenstein, L. M. Ausman, M. Jauhiainen, and P. J. H. Jones. Hydrogenated fat consumption affects acylation-stimulating protein levels and cholesterol esterification rates in moderately hypercholesterolemic women.