Hepatic CD36 downregulation parallels steatosis improvement in morbidly obese undergoing bariatric surgery

BACKGROUND

The notion that hepatic expression of genes involved in lipid metabolism is altered in obese patients is relatively new and its relationship with hepatic steatosis and cardiometabolic alterations remains unclear.

OBJECTIVE

We assessed the impact of Roux-en-Y gastric bypass surgery (RYGB) on the expression profile of genes related to metabolic syndrome in liver biopsies from morbidly obese individuals using a custom-made, focused cDNA microarray, and assessed the relationship between the expression profile and hepatic steatosis regression.

MATERIALS AND METHODS

Plasma and liver samples were obtained from patients at baseline and 12 months after surgery. Samples were assayed for chemical and gene expression analyses, as appropriate. Gene expression profiles were assessed using custom-made, focused TaqMan low-density array cards.

RESULTS

RYGB-induced weight loss produced a favorable reduction in fat deposits, insulin resistance (estimated by homeostasis model assessment of insulin resistance (HOMA-IR)), and plasma and hepatic lipid levels. Compared with the baseline values, the gene expression levels of key targets of lipid metabolism were significantly altered: CD36 was significantly downregulated (-40%; P=0.001), whereas APOB (+27%; P=0.032) and SCARB1 (+37%; P=0.040) were upregulated in response to surgery-induced weight reduction. We also observed a favorable reduction in the expression of the PAI1 gene (-80%; P=0.007) and a significant increase in the expression of the PPARA (+60%; P=0.014) and PPARGC1 genes (+36%; P=0.015). Notably, the relative fold decrease in the expression of the CD36 gene was directly associated with a concomitant reduction in the cholesterol (Spearman's r=0.92; P=0.001) and phospholipid (Spearman's r=0.76; P=0.04) contents in this tissue.

CONCLUSIONS

For the first time, RYGB-induced weight loss was shown to promote a favorable downregulation of CD36 expression, which was proportional to a favorable reduction in the hepatic cholesterol and phospholipid contents in our morbidly obese subjects following surgery.

Effect of oral oleoyl-estrone treatment on plasma lipoproteins and tissue lipase activities of Zucker lean and obese female rats

OBJECTIVE

To study the effect of oral oleoyl-estrone on the plasma lipoprotein profile and tissue lipase activities in order to determine the handling of circulating lipids by adipose tissue, liver and muscle of obese female rats.

DESIGN

Lean (Fa/?) and obese (fa/fa) female Zucker rats treated for 10 days with a daily gavage of 0.2 ml sunflower oil containing 0 (controls) or 10 micromol/kg of oleoyl-estrone. After sacrifice, samples of tissues and plasma were taken.

MEASUREMENTS

Plasma lipoprotein classes and composition; lipoprotein lipase and hepatic lipase activities in plasma, liver, skeletal muscle and periovaric and mesenteric white adipose tissue (WAT).

RESULTS

Oleoyl-estrone decreased plasma cholesterol (mainly in HDLs: 76%) of lean rats, but dramatically decreased all lipid classes in obese rats, in which chylomicra and VLDL lost most of their triacylglycerols (95 and 81%, respectively). Hepatic lipase activity decreased markedly with oleoyl-estrone in all groups, both in plasma (79% lean, 100% obese) and liver (62% in both groups). Lipoprotein lipase activity was largely unchanged by oleoyl-estrone in lean rats, but in the obese it decreased in WAT (82% in periovaric, and 49% in mesenteric), and increased in plasma (x4) and in skeletal muscle (x5); liver levels showed no change.

CONCLUSIONS

The shift observed in obese rats from a decrease in liver and WAT lipoprotein lipase and hepatic lipase activities to an increase in muscle lipoprotein lipase is coincident with the hypolipemic effect of oleoyl-estrone, especially in obese rats, and indicates that muscle is a key site for the disposal of endogenous fat mobilized due to oleoyl-estrone treatment.

TNFalpha expression of subcutaneous adipose tissue in obese and morbid obese females: relationship to adipocyte LPL activity and leptin synthesis

INTRODUCTION

Tumor necrosis factor (TNFalpha) has been invoked as an adipostat. Accordingly, the adipose tissue expression of TNFalpha has been shown to be proportional to the degree of adiposity. The regulatory role of TNFalpha in obesity may be controlled by several mechanisms. These include the inhibitory effect on LPL activity, the mediation on glucose homeostasis or the effect on leptin. To assess the role of TNFalpha in obesity we measured adipocyte TNFalpha expression in 96 females with a wide range of adiposity and with or without type 2 diabetes. We analysed the relationship between TNFalpha expression, adipocyte LPL activity, insulin resistance and leptin in this population.

RESULTS

The TNFalpha and leptin expression of the adipose tissue in obese and morbid obese patients were significantly higher than in controls. Obese and morbid obese patients had slightly higher levels of LPL activity, but these differences were not significant. We observed a significant relationship between adipose TNFalpha expression and body mass index (r=0.35, P<0.001). TNFalpha expression was negatively related to LPL activity (r=-0.28, P<0.05) and positively related to leptin expression (r=0.35, P<0.001).

CONCLUSION

Our results indicate that obese women, even those with morbid obesity, over-express TNFalpha in subcutaneous adipose tissue in proportion to the magnitude of the fat depot and independently of the presence of type 2 diabetes. The TNFalpha system may be a homeostatic mechanism that prevents further fat deposition by regulating LPL activity and leptin production.

Anomalous lipoproteins in obese Zucker rats

AIMS

Obesity is characterized by dislipoproteinaemia with increased cholesterol and triacylglycerol levels and lower chylomicra disposal rates. We studied here whether these alterations were related to lipoprotein number and/or size and composition.

METHODS

Plasma from lean and obese Zucker rats was fractionated into lipoprotein classes (chylomicra, very low density lipoprotein (VLDL), low density lipoprotein (LDL) and high density lipoprotein (HDL)) by differential centrifugation. The apoprotein and lipid composition of each fraction were measured. Lipoprotein particle size was estimated by dynamic light scattering and used to tabulate the mean diameter and volume of lipoprotein micelles. Particle mass was calculated from the density and volume. The mass of lipids and protein in each fraction/ml of plasma allowed the estimation of mean particle concentration and then the number of molecules of lipid and protein/unit of lipoprotein micelle.

RESULTS

A large part of hyperlipidaemia of obese rats is due to the accumulation of chylomicra: 1.3 +/- 0.2 mg/ml in lean rats [LR] (34% of all lipoproteins) and 8.2 +/- 0.9 mg/ml in the obese rats [OR] (66% of all lipoproteins). Lipid percentage composition of lipoproteins was similar in both groups. The particle size of LDL and HDL was much higher in OR than in LR: LDLs weighed 31.1 +/- 7.5 ag (LR) vs. 273 +/- 81 ag (OR), and HDLs weighed 31.7 +/- 12.6 ag (LR) and 375 +/- 103 ag (OR). In chylomicra and VLDL there was a relative scarcity of apoproteins in OR compared with LR. The whole architecture of LDLs is altered in OR, with a predominance of surface lipids: phospholipid and free cholesterol, and lower amounts of core lipids: triacylglycerols and cholesterol esters, with surface/core lipids ratios of 0.74 (LR) and 1.89 (OR). The consequences of anomalous LDL and HDL composition, size and overall structure may result in magnified lipoprotein metabolism alterations that hamper their ability to transfer apolipoproteins to larger chylomicra and VLDL, and to alter cholesterol transfer and binding of their apoproteins to cell surface receptors. The smaller number of LDL and HDL particles may further compound these difficulties and thus change the free to esterified cholesterol ratios observed in OR.

CONCLUSIONS

The main conclusions of this study are the key importance of chylomicron analysis for a better understanding of the transfer of lipids, and the altered lipoprotein size and apoprotein distribution in obese rats, which seriously hamper cholesterol interchange, resulting in hypercholesterolaemia, and thus triggering even more far-reaching consequences for the well-being of the obese.

Lipoprotein lipase and cholesterol transfer activities of lean and obese Zucker rats

Adult female lean and obese Zucker rats maintained under standard conditions were used for the estimation of plasma, liver and white adipose tissue (WAT) activity of lipoprotein lipase, plasma and liver hepatic lipase and plasma lecithin-cholesterol acyltransferase. No differences in plasma or tissue levels of lipoprotein lipase between lean and obese rats were detected, but the larger WAT size of the obese rats resulted in higher lipase activity per unit of rat weight. Hepatic lipase levels in plasma were higher in the obese, but in liver, the higher activity was found in lean rats. No significant differences were found for lecithin-cholesterol acyltransferase activity, except when the levels in the HDL fraction were expressed per unit of protein weight, showing lower activity in the obese rats. In conclusion, the essentially maintained enzyme activities in obese rat tissues suggest that they cannot explain the deficient lipoproteins processing of obese rats, and, consequently their dislipidaemia.

Effect of 24-h food deprivation on lipoprotein composition and oleoyl-estrone content of lean and obese Zucker rats

BACKGROUND

Food deprivation induces the mobilization of fat reserves and, consequently, the transport of lipids in plasma. Zucker obese rats are grossly hyperlipidemic and do not use lipids as an efficient energy substrate. They also have lower circulating levels of acyl-estrone than expected because of their large fat stores.

AIM OF THE STUDY

To measure the effect of 24 h food deprivation on hyperlipidemia and acyl-estrone distribution in plasma in Zucker obese rats.

METHODS

The plasma lipoprotein distribution and composition of Zucker lean (Fa/?) and obese (fa/fa) rats was determined after 24 hours of food deprivation. Lipid classes: phospholipid, free and esterified cholesterol and triacylglycerols, and protein and total (mainly acyl-) estrone were also measured in total plasma and lipoprotein fractions.

RESULTS

Food-deprived rats showed lower triacylglycerol levels than fed rats, but obese rats maintained high lipid levels, mainly in the VLDL fraction. The ratio of total plasma free-to-esterified cholesterol was lower in fed lean rats (0.29) than in the obese (0.61); the situation improved slightly after 24-h starvation, since the corresponding ratios were 0.30 and 0.41. Acyl estrone levels changed little with 24-h food deprivation. The chylomicra + VLDL total estrone compartment was essentially unchanged in lean and obese fed and starved groups, but the HDL pool decreased with food deprivation in the obese.

CONCLUSION

Short-term starvation helped to enhance the differences between lean and obese Zucker rats in the handling of lipoprotein lipids, the latter showing a marked impairment in their ability to dispose of circulating lipids. The different pace of plasma lipid utilization may compound the problems of cholesterol transfer, partly explaining the dyslipemia that characterizes this animal model of obesity. The differences in acyl-estrone distribution also indicate that fat mass is preserved more effectively in obese rats even after food deprivation.

Metabolic effects of rexinoids: tissue-specific regulation of lipoprotein lipase activity

Hypertriglyceridemia is a frequent complication accompanying the treatment of patients with either retinoids or rexinoids, [retinoid X receptor (RXR)-selective retinoids]. To investigate the cellular and molecular basis for this observation, we have studied the effects of rexinoids on triglyceride metabolism in both normal and diabetic rodents. Administration of a rexinoid such as LG100268 (LG268) to normal or diabetic rats results in a rapid increase in serum triglyceride levels. LG268 has no effect on hepatic triglyceride production but suppresses post-heparin plasma lipoprotein lipase (LPL) activity suggesting that the hypertriglyceridemia results from diminished peripheral processing of plasma very low density lipoproteins particles. Treatment of diabetic rats with rexinoids suppresses skeletal and cardiac muscle but not adipose tissue LPL activity. This effect is independent of changes in LPL mRNA. In C2C12 myocytes, LG268 suppresses the level of cell surface (i.e., heparin-releasable) LPL activity without altering LPL mRNA. This effect is very rapid (t(1/2) = 2 h) and is blocked by the transcriptional inhibitor actinomycin D. These studies demonstrate that RXR ligands can have dramatic effects on the post-translational processing of LPL and suggest that skeletal muscle may be an important target of rexinoid action. In addition, these data underscore that the metabolic consequences of RXR activation are distinct from either retinoic acid receptor or peroxisome proliferator-activated receptor activation.

Activation of PPARdelta alters lipid metabolism in db/db mice

Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors, which heterodimerize with the retinoid X receptor and bind to peroxisome proliferator response elements in the promoters of regulated genes. Despite the wealth of information available on the function of PPARalpha and PPARgamma, relatively little is known about the most widely expressed PPAR subtype, PPARdelta. Here we show that treatment of insulin resistant db/db mice with the PPARdelta agonist L-165041, at doses that had no effect on either glucose or triglycerides, raised total plasma cholesterol concentrations. The increased cholesterol was primarily associated with high density lipoprotein (HDL) particles, as shown by fast protein liquid chromatography analysis. These data were corroborated by the chemical analysis of the lipoproteins isolated by ultracentrifugation, demonstrating that treatment with L-165041 produced an increase in circulating HDL without major changes in very low or low density lipoproteins. White adipose tissue lipoprotein lipase activity was reduced following treatment with the PPARdelta ligand, but was increased by a PPARgamma agonist. These data suggest both that PPARdelta is involved in the regulation of cholesterol metabolism in db/db mice and that PPARdelta ligands could potentially have therapeutic value.

Effect of fasting on hepatic lipase activity in the liver of developing rats

The effect of fasting on hepatic lipase was studied during postnatal development in the rat. It was found that fasting produced a significant decrease in hepatic lipase only in neonatal (1-day-old) and adult (60-day-old) rats. We studied the effect of fasting on the distribution of hepatic lipase between extracellular (heparin-releasable) and intracellular (liver-retained or residual) compartments in perfused livers, and on the secretion of hepatic lipase by isolated hepatocytes. Fasting had similar effects in neonates and adults: it decreased both the heparin-releasing and the residual activities in perfused livers, and also decreased the rate of hepatic lipase secretion by isolated hepatocytes. Finally, the effect of fasting on hepatic lipase mRNA relative abundance in developing rat livers was determined. No difference was observed among the groups studied. It is concluded that the mechanisms involved in the effect of fasting on hepatic lipase appear to be similar in neonates and adult animals and may involve the post-translational processing of the enzyme.

Modulation of rat liver apolipoprotein gene expression and serum lipid levels by tetradecylthioacetic acid (TTA) via PPARalpha activation

3-Thia fatty acids are modified fatty acids that promote hepatic peroxisome proliferation and decrease serum triacylglycerol, cholesterol and free fatty acid levels in rats. In vivo administration of tetradecylthioacetic acid (TTA) to rats led to a significant decrease in liver apolipoproteins apoA-I, A-II, A-IV, and C-III mRNA levels, and to an increase of liver acyl-CoA oxidase (ACO), carnitine palmitoyltransferase-II, and 3-hydroxy-3-methylglutaryl coenzyme A synthase (HMG-CoA synthase) mRNA levels and activities. By contrast, no significant changes of lipoprotein lipase (LPL) mRNA levels were detected in rat epididymal adipose tissue. Liver carnitine palmitoyltransferase-I, apoB, apoE, and LDL receptor mRNA levels were not significantly affected. When tested in vitro, TTA increased rat ACO and carnitine palmitoyltransferase-I mRNA levels in primary rat hepatocytes and also LPL mRNA levels in 3T3-L1 preadipocytes. TTA also enhanced the transcriptional activity of chimeras containing the DNA binding domain of the yeast transcription factor Gal4 fused to the ligand binding domain of either human PPARalpha or human PPARgamma. The effect depended on the concentration tested and the cell type. In conclusion, our data suggest that in vitro, TTA activates both PPARalpha and PPARgamma, but the latter with much lower affinity. TTA affects serum lipid levels in vivo in rats by acting mainly on the liver via PPARalpha where it decreases the liver expression of genes involved in vascular lipid transport and increases the expression of genes involved in intracellular fatty acid metabolism. -Raspé, E., L. Madsen, A-M. Lefebvre, I. Leitersdorf, L. Gelman, J. Peinado-Onsurbe, J. Dallongeville, J-C. Fruchart, R. Berge, and B. Staels. Modulation of rat liver apolipoprotein gene expression and serum lipid levels by tetradecylthioacetic acid (TTA) via PPARalpha activation.

Distribution of oleoyl-estrone in rat plasma lipoproteins

Pooled adult normal rat plasma was used for the separation of lipoprotein fractions: VLDL, LDL and HDL, from which a total lipids extract was obtained. The presence of fragments with the MW of estrone and oleoyl-estrone in the lipoprotein fractions was analyzed by HPLC-MS. The results show that oleoyl-estrone is the major estrone component in lipoproteins; this molecular species was present in all three lipoprotein lipid extracts. The lipoprotein fractions were used for the analysis of protein and lipid classes: triacylglycerols, total and esterified cholesterol and phospholipids as well as acyl-estrone. About half of the total acyl-estrone was in the HDL fraction and only about 10% in the VLDL fraction. HDLs contained about one molecule in 50 particles, LDLs one molecule per particle and VLDLs 15 molecules per particle, i.e. given their size, the larger lipoproteins contained more oleoyl-estrone than the HDLs. The distribution of this hormone suggests that oleoyl-estrone is lost with other lipids as the lipoproteins shrink. The results presented show that oleoyl-estrone is a molecule found naturally in rat lipoproteins in low concentrations - the lowest in HDLs - that are consistent with its postulated role in the control of body weight.

Acquired lipoprotein lipase deficiency associated with chronic urticaria. A new etiology for type I hyperlipoproteinemia

Type I hyperlipoproteinemia (type I HLP) is a rare disorder of lipid metabolism characterized by fasting chylomicronemia and reduced postheparin plasma lipoprotein lipase (LPL) activity. Most cases of type I HLP are due to genetic defects in the LPL gene or in its activator, the apolipoprotein CII gene. Several cases of acquired type I HLP have also been described in the course of autoimmune diseases due to the presence of circulating inhibitors of LPL. Here we report a case of type I HLP due to a transient defect of LPL activity during puberty associated with chronic idiopathic urticaria (CIU). The absence of any circulating LPL inhibitor in plasma during the disease was demonstrated. The LPL genotype showed that the patient was heterozygous for the D9N variant. This mutation, previously described, can explain only minor defects in the LPL activity. The presence of HLP just after the onset of CIU, and the elevation of the LPL activity with remission of the HLP when the patient recovered from CIU, indicate that type I HLP was caused by CIU. In summary, we report a new etiology for type I HLP - a transient decrease in LPL activity associated with CIU and with absence of circulating inhibitors. This is the first description of this association, which suggests a new mechanism for type I HLP.

Partial hepatectomy and/or surgical stress provoke changes in the expression of lipoprotein lipase and actin in liver and extrahepatic tissues

The expression of lipoprotein lipase (LPL) and actin genes was examined in heart, muscle and white adipose tissue (WAT) and the expression of albumin and actin genes was examined in regenerating liver after 2/3 hepatectomy. Both surgical stress and partial hepatectomy (PH) affected LPL and actin mRNA levels in muscle and WAT, but not in heart. The changes in LPL mRNA suggest transcriptional regulation of the enzyme during hepatic regeneration. Our results show for the first time that the LPL gene expression in the different tissues studied is altered not only by the surgical stress, but also by PH per se. Actin expression is also affected in some tissues. In liver, PH and surgical stress altered the expression of albumin and total mRNA. The total mRNA of the other tissues studied did not change. The changes observed in LPL in different tissues, especially in WAT and muscle, may be responsible for some of the changes in lipidic metabolism, thus allowing for some plasma lipoproteins to be used as substrates by the LPL activity that arises in the liver during hepatic regeneration. The fatty acids derived from these lipoproteins would constitute not only an energy source but also the building material needed in the process of restoration of the lost hepatic mass. It is suggested that hormonal changes taking place after surgery are responsible for the variation in the levels of the different mRNAs studied.

3-Hydroxy-3-methylglutaryl CoA reductase inhibitors reduce serum triglyceride levels through modulation of apolipoprotein C-III and lipoprotein lipase

Statins are hypolipidemic drugs which not only improve cholesterol but also triglyceride levels. Whereas their cholesterol-reducing effect involves inhibition of de novo biosynthesis of cellular cholesterol through competitive inhibition of its rate-limiting enzyme 3-hydroxy-3-methylglutaryl CoA reductase, the mechanism by which they lower triglycerides remains unknown and forms the subject of the current study. Treatment of normal rats for 4 days with simvastatin decreased serum triglycerides significantly, whereas it increased high density lipoprotein cholesterol moderately. The decrease in triglyceride concentrations after simvastatin was caused by a reduction in the amount of very low density lipoprotein particles which were of an unchanged lipid composition. Simvastatin administration increased the lipoprotein lipase mRNA and activity in adipose tissue and heart. This effect on lipoprotein lipase was accompanied by decreased mRNA as well as plasma levels of the lipoprotein lipase inhibitor apolipoprotein C-III. These results suggest that the triglyceride-lowering effect of statins involves a stimulation of lipoprotein lipase-mediated clearance of triglyceride-rich lipoproteins.

Regulation of lipoprotein metabolism by thiazolidinediones occurs through a distinct but complementary mechanism relative to fibrates

Thiazolidinediones are antidiabetic agents, which not only improve glucose metabolism but also reduce blood triglyceride concentrations. These compounds are synthetic ligands for PPAR gamma, a transcription factor belonging to the nuclear receptor subfamily of PPARs, which are important transcriptional regulators of lipid and lipoprotein metabolism. The goal of this study was to evaluate the influence of a potent thiazolidinedione, BRL49653, on serum lipoproteins and to determine whether its lipid-lowering effects are mediated by changes in the expression of key genes implicated in lipoprotein metabolism. Treatment of normal rats for 7 days with BRL49653 decreased serum triglycerides in a dose-dependent fashion without affecting serum total and HDL cholesterol and apolipoprotein (apo) A-I and apo A-II concentrations. The decrease in triglyceride concentrations after BRL49653 was mainly due to a reduction of the amount of VLDL particles of unchanged lipid and apo composition. BRL49653 treatment did not change triglyceride production in vivo as analyzed by injection of Triton WR-1339, indicating a primary action on triglyceride catabolism. Analysis of the influence of BRL49653 on the expression of LPL and apo C-III, two key players in triglyceride catabolism, showed a dose-dependent increase in mRNA levels and activity of LPL in epididymal adipose tissue, whereas liver apo C-III mRNA levels remained constant. Furthermore, addition of BRL49653 to primary cultures of differentiated adipocytes increased LPL mRNA levels, indicating a direct action of the drug on the adipocyte. Simultaneous administration of BRL49653 and fenofibrate, a hypolipidemic drug that acts primarily on liver through activation of PPAR alpha both decreased liver apo C-III and increased adipose tissue LPL mRNA levels, resulting in a more pronounced lowering of serum triglycerides than each drug alone. In conclusion, both fibrates and thiazolidinediones exert a hypotriglyceridemic effect. While fibrates act primarily on the liver by decreasing apo C-III production, BRL49653 acts primarily on adipose tissue by increasing lipolysis through the induction of LPL expression. Drugs combining both PPAR alpha and gamma activation potential should therefore display a more efficient hypotriglyceridemic activity than either compound alone and may provide a rationale for improved therapy for elevated triglycerides.

PPARalpha and PPARgamma activators direct a distinct tissue-specific transcriptional response via a PPRE in the lipoprotein lipase gene

Increased activity of lipoprotein lipase (LPL) may explain the hypotriglyceridemic effects of fibrates, thiazolidinediones and fatty acids, which are known activators (and/or ligands) of the various peroxisome proliferator-activated receptors (PPARs). Treatment with compounds which activate preferentially PPARalpha, such as fenofibrate, induced LPL expression exclusively in rat liver. In contrast, the antidiabetic thiazolidinedione BRL 49653, a high affinity ligand for PPARgamma, had no effect on liver, but induced LPL expression in rat adipose tissue. In the hepatocyte cell line AML-12, fenofibric acid, but not BRL 49653, induced LPL mRNA, whereas in 3T3-L1 preadipocytes, the PPARgamma ligand induced LPL mRNA levels much quicker and to a higher extent than fenofibric acid. In both the in vivo and in vitro studies, inducibility by either PPARalpha or gamma activators, correlated with the tissue distribution of the respective PPARs: an adipocyte-restricted expression of PPARgamma, whereas PPARalpha was expressed predominantly in liver. A sequence element was identified in the human LPL promoter that mediates the functional responsiveness to fibrates and thiazolidinediones. Methylation interference and gel retardation assays demonstrated that a PPARalpha or gamma and the 9-cis retinoic acid receptor (RXR) heterodimers bind to this sequence -169 TGCCCTTTCCCCC -157. These data provide evidence that transcriptional activation of the LPL gene by fibrates and thiazolidinediones is mediated by PPAR-RXR heterodimers and contributes significantly to their hypotriglyceridemic effects in vivo. Whereas thiazolidinediones predominantly affect adipocyte LPL production through activation of PPARgamma, fibrates exert their effects mainly in the liver via activation of PPARalpha.

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.

Decrease in the expression of hepatic lipase activity following partial hepatectomy

Variations in hepatic lipase (HL) activity were studied for the first time in liver, plasma and adrenal glands of partially hepatectomized (70%), sham-operated and intact rats. Activity profiles performed during 7 days in liver, plasma and adrenal glands of sham-operated rats were similar to those obtained in intact animals. However, HL activity in intact animals appeared to be slightly higher during the first 24 hours. Following surgery, hepatectomized animals showed a reduction of about 300 U in liver HL activity which persisted for 7 days. Plasma HL activity of hepatectomized rats was undetectable at 6 hours post-surgery but in increased afterwards. A high correlation between liver and adrenal gland HL activity was found in hepatectomized but not in sham-operated animals. HL mRNA levels in hepatectomized rats showed a 40% decrease during the first 24 hours after surgery, but they returned to the normal range later. On the other hand, HL mRNA values increased in sham-operated rats but no increase in HL activity was detected in these animals. To conclude, our results show that HL activity decreases dramatically during hepatic regeneration due to a concomitant decrement in the expression of the gene that encodes the enzyme and to other undetermined factors.

Expression and regulation of the lipoprotein lipase gene in human adrenal cortex

Lipoprotein lipase (LPL), an enzyme which hydrolyzes triglycerides and participates in the catabolism of remnant lipoproteins, plays a crucial role in energy and lipid metabolism. The goal of this study was to analyze the expression and regulation of the LPL gene in human adrenals. Reverse transcriptase-polymerase chain reaction amplification and sequence analysis demonstrated the presence of LPL mRNA in fetal and adult human adrenal cortex. Furthermore, the human adrenocortical carcinoma cell line, NCI-H295, expresses LPL mRNA and protein, which is localized to the outer cellular membrane as demonstrated by immunofluorescence confocal microscopy and can be released in the medium by heparin addition. To asses whether the LPL gene is regulated by agents regulating adrenal steroidogenesis, NCI-H295 cells were treated with activators of second messenger systems. Whereas the calcium-ionophore A23187 did not affect LPL gene expression, treatment with phorbol 12-myristate 13-acetate decreased LPL mRNA levels in a time- and dose-dependent manner. This decrease after phorbol 12-myristate 13-acetate was associated with diminished heparin-releasable LPL mass and activity in the culture medium. Addition of the cAMP analog 8-Br-cAMP to NCI-H295 cells resulted in a rapid, but transient dose-dependent induction of LPL mRNA. Treatment with the protein synthesis inhibitor cycloheximide gradually induced, whereas simultaneous addition of cAMP and cycloheximide superinduced LPL mRNA levels. Nuclear run-on analysis indicated that the effects of cAMP and cycloheximide occurred at the transcriptional and post-transcriptional level, respectively. Transient co-transfection assays demonstrated that the first 230 base pairs of the proximal LPL promoter contain a cAMP-responsive element activated by protein kinase A and transcription factors belonging to the CREB/CREM family. These data indicate that LPL is expressed in human adrenal cortex and regulated in NCI-H295 adrenocortical carcinoma cells by activators of the protein kinase A and protein kinase C second messenger pathways in a manner comparable to P450scc, which catalyzes the first step in adrenal steroidogenesis. These observations suggest a role for LPL in adrenal energy and/or lipid metabolism and possibly in steroidogenesis.

Lipoprotein lipase in highly vascularized structures of the eye

Ocular tissues are highly dependent on lipid turnover and metabolism, which requires an uptake mechanism for fatty acids from lipoproteins. We studied the activity and expression of lipoprotein lipase (LPL), which catalyzes the hydrolysis of plasma triglycerides, in different ocular regions. Human and bovine eyes were dissected and various specialized anatomical areas were assayed for LPL activity, mRNA, and immunoreactivity. Variable levels of LPL activity were detected in all structures in human and bovine eyes. LPL activity was much higher in vascularized structures, such as ciliary body, iris, and retina than in avascular eye structures, such as vitreous body, lens, and cornea. In both human and bovine eyes, ciliary body contained the highest LPL lipolytic activity. LPL mRNA was detected by reverse transcription followed by polymerase chain reaction (RT-PCR) in all regions of human eyes. By RT-PCR analysis it was shown that bovine eyes contained high levels of LPL mRNA in ciliary body and iris, lower levels were found in retina, optic nerve, and lens, whereas no LPL mRNA could be found in bovine cornea. RT-PCR data, obtained in bovine eyes, agree with the results obtained by Northern blot experiments, confirming the high levels of LPL mRNA in iris and ciliary body. Immunofluorescence experiments performed on human eye samples indicated that the LPL protein is mostly distributed on the choroides, the choriocapillaris, and on the vessels of ciliary body, iris, optic nerve, and retina. The present study demonstrates that active LPL protein is synthesized, secreted, and located among microvessels in several specialized regions of the eye, and suggests that LPL could be involved in the uptake of fatty acids by the ocular tissues.

Hormonal regulation of lipoprotein lipase activity from 5-day-old rat hepatocytes

Lipoprotein lipase (LPL) activity is known to be synthesized, active and functional in the 1-day-old rat liver: it peaks just at birth triggered by parturition. During suckling LPL mRNA, LPL synthesis and LPL activity are still high at 5 days and then fade reaching adult values at weaning. How LPL expression is gradually extinguished is not known. Therefore we studied the effect of different doses of several hormones on LPL activity released by incubated hepatocytes from 5-day-old rats. In the presence of heparin the release of LPL activity in the medium was linear until 3 h and was always significantly increased vs. without heparin. At 3 h in the presence of heparin the main hormonal effects were: dose-dependent increase (30-60%) with dexamethasone; dose-dependent increase (20-60%) with glucagon; dose-independent decrease (50-60%) with ethinylestradiol, testosterone, progesterone and prolactin; no effect with insulin; 20-40% increase with adrenaline < 1 mM but 40-50% decrease with noradrenaline < 10 microM. Increase of LPL release by glucagon and adrenaline agrees with the increased LPL expression we previously found in an undifferentiated hepatoma cell line when the adenylate cyclase/protein kinase A pathway was activated. The effect of glucagon is concordant with our previous observations that fasting increases liver LPL activity in neonatal rats. The high estradiol levels known to be present in male and female 9-19-day-old rats might contribute to liver LPL extinction during suckling.

Na+,K(+)-ATPase expression during the early phase of liver growth after partial hepatectomy

Na+,K(+)-ATPase expression has been studied in the early phase of liver growth after partial hepatectomy to ascertain whether its increased activity is due to stable effects, involving de novo synthesis and insertion of pumps into the plasma membrane. Na+,K(+)-ATPase activity progressively increases after partial hepatectomy, reaching a three-fold induction above basal values 12 h after surgery. mRNA amounts of both alpha 1 and beta 1 subunits are rapidly increased up to two-fold for alpha 1 and nearly three-fold for beta 1, at 9 and 12 h post-hepatectomy, respectively. This correlates with increased abundance of both subunit proteins. The results prove that the increase of Na+,K(+)-ATPase activity correlates with higher expression of both subunit proteins and mRNAs, although the characteristics of the induction suggest that some translational and post-translational events may be equally involved in the increased activity of the pump.

Lipoprotein lipase activity in developing rat brain areas

Lipoprotein lipase (LPL) is a key extracellular enzyme that enables tissue to import fatty acids from triacylglyceride-rich lipoproteins. LPL is present in most tissues of the body, but in the brain its functional significance remains unclear. Lipids constitute the main components of myelin and undergo significant changes during maturation. However, nothing is known of the postnatal evolution of LPL activity in the brain areas during postnatal development. Here we found that LPL activity is relatively high in the newborn brain and peaks between the 5th and the 10th days after birth, reaching activities 5 times higher than in the adult brain. In all the areas studied (olfactory bulbs, cortex, thalamus, cerebellum, hippocampus, striatum, brain-stem and spinal cord) LPL also increases sharply during postnatal development. Hippocampus shows the highest LPL activity levels, which are between 5 and 11 times higher than in the other regions. The significance of these high LPL activity levels is discussed.

Epidermal growth factor interferes with the effect of adrenaline on glucose production and on hepatic lipase secretion in rat hepatocytes

We studied the interaction of epidermal growth factor (EGF) and adrenaline in the control of several metabolic functions in isolated hepatocytes from fed rats. EGF did not modulate glucose release, urea production or hepatic lipase secretion, but interfered with the stimulatory effect of adrenaline on both glucose and urea production and also with the inhibitory effect of this hormone on hepatic lipase secretion. EGF also interfered with the effect of both angiotensin II and vasopressin on glucose release and on hepatic lipase secretion. While the effect of EGF interfering with the action of adrenaline on glucose release was potentiated in the absence of extracellular calcium, the effect on the inhibition of hepatic lipase secretion was abolished. These results suggest that EGF interfered with catecholamine actions in the liver at a site distal from the generation of the calcium signal.

Effects of sex steroids on hepatic and lipoprotein lipase activity and mRNA in the rat

In humans, sex steroids have been implicated in the regulation of hepatic and lipoprotein lipase activity. Therefore, the effects of orchidectomy and subsequent androgen or estrogen administration on hepatic lipase (HL) and adipose tissue and heart lipoprotein lipase (LPL) were examined. Relative to intact controls, orchidectomy of male rats resulted in no significant change in HL activity and mRNA, or in heart and adipose tissue LPL activity and mRNA levels. Subsequently, a subcutaneous silastic tubing, delivering either testosterone, dihydrotestosterone, nandrolone, or 17 beta-estradiol, was implanted for 5 weeks. All substitution treatments had a tendency to reduce HL activity and to induce HL mRNA levels. This effect was, however, only significant for testosterone which resulted in a decrease in HL activity (238 +/- 15 vs. 328 +/- 31 mU/g tissue; p vs. control < 0.05) and an increase in HL mRNA (166 +/- 11 vs. 100 RAU; p vs. control < 0.01). No significant effects of androgens on LPL expression either in heart or adipose tissue were observed. Adipose tissue LPL activity (20 +/- vs. 35 +/- 4 mU/g; p vs. control < 0.05) and mRNA (28 +/- 4 vs. 100 RAU; p vs. control < 0.001) levels, but not heart LPL, however, were diminished substantially after 17 alpha-estradiol treatment. In conclusion, rat HL is influenced by testosterone, while adipose tissue, but not heart LPL, is reduced after estrogen administration.

Lipoprotein lipase expression in undifferentiated hepatoma cells is regulated by progesterone and protein kinase A

Recently, it was shown that lipoprotein lipase (LPL) was produced in neonatal but not in adult rat liver. In an attempt to further define the mechanism involved in liver LPL expression, we identified a neonatal mouse hepatoma cell line, BWTG3, capable of producing LPL. The regulation of LPL expression by various extracellular stimuli was investigated in this cell line. Progesterone caused a rise in LPL production by BWTG3 cells. Other hormones tested, such as insulin, glucagon, adrenalin, testosterone, and thyroid hormone, had no effect on LPL production. The effects of progesterone on LPL production showed slow kinetics reaching a maximum 24 h after addition. Cotransfection of a progesterone receptor expression vector with a 5'-LPL-CAT reporter construct resulted in an induction of CAT activity, suggesting that the increase in LPL accumulation after progesterone was linked to transcriptional induction of the LPL gene. Stimuli causing an elevation of protein kinase A activity in the cells also increased LPL production. Three agents capable of elevating intracellular cAMP levels, i.e., forskolin, dBcAMP, and choleratoxin, caused an elevation of LPL production. The increase in LPL activity caused by forskolin and choleratoxin was paralleled by an elevation of LPL mRNA levels, while dBcAMP only induced a small elevation of LPL mRNA levels. The increase in LPL production was shown to be linked to the stimulation of the PKA signal transduction pathway and was apparently transmitted via the transcription factor CREB. No effect of the stimulation of protein kinase C or calcium/calmodulin-dependent kinase on LPL production was detected.(ABSTRACT TRUNCATED AT 250 WORDS)

Neonatal extinction of liver lipoprotein lipase expression

In contrast to the complete absence of lipoprotein lipase (LPL) mRNA in adult rat liver, fetal and neonatal rat liver contain substantial amounts of LPL mRNA, which is translated in active LPL protein as can be deduced from the presence of LPL activity in this organ. At this neonatal stage, both the relative abundance of LPL mRNA and LPL activity increased with starvation. During the suckling period, LPL mRNA and LPL activity gradually decreased until both parameters were undetectable. While the administration of L-thyroxine or hydrocortisone enhanced the disappearance of LPL mRNA, induced hypothyroidism delayed its disappearance. In adult animals induced hypothyroidism could not reactivate LPL mRNA production in the liver. The data presented suggest that liver LPL production responds to changes in the nutritional state and becomes extinguished during development, in a fashion reminiscent to the extinction of alpha-fetoprotein. This extinction of LPL gene expression is influenced by hormonal factors.

Lipoprotein lipase and hepatic lipase activities are differentially regulated in isolated hepatocytes from neonatal rats

Lipoprotein lipase and hepatic lipase are members of the lipase gene family sharing a high degree of homology in their amino acid sequences and genomic organization. We have recently shown that isolated hepatocytes from neonatal rats express both enzyme activities. We show here that both enzymes are, however, differentially regulated. Our main findings are: (i) fasting induced an increase of the lipoprotein lipase activity but a decrease of the hepatic lipase activity in whole liver, being in both cases the vascular (heparin-releasable) compartment responsible for these variations. (ii) In isolated hepatocytes, secretion of lipoprotein lipase activity was increased by adrenaline, dexamethasone and glucagon but was not affected by epidermal growth factor, insulin or triiodothyronine. On the contrary, secretion of hepatic lipase activity was decreased by adrenaline but was not affected by other hormones. (iii) The effect of adrenaline on lipoprotein lipase activity appeared to involve beta-adrenergic receptors, but stimulation of both beta- and alpha 1-receptors seemed to be required for the effect of this hormone on hepatic lipase activity. And (iv), increased secretion of lipoprotein lipase activity was only observed after 3 h of incubation with adrenaline and was blocked by cycloheximide. On the contrary, decreased secretion of hepatic lipase activity was already significant after 90 min of incubation and was not blocked by cycloheximide. We suggest that not only synthesis of both enzymes, but also the posttranslational processing, are under separate control in the neonatal rat liver.

Down-regulation of hepatic lipase gene expression and activity by fenofibrate

The influence of the hypolipidemic drug, fenofibrate, on hepatic lipase (HL) gene expression and activity was investigated in the rat. Fenofibrate treatment provoked a dose-dependent decrease in HL mRNA levels. At a dose of 0.5% (w/w), HL mRNA levels were reduced to nearly 50% the levels in untreated controls. This decrease was parallelled by a comparable reduction in liver HL activity. The decrease in HL mRNA levels was already observed after 1 day of fenofibrate treatment. Whole liver perfusion experiments showed that the heparin-releasable HL activity in fenofibrate-treated livers dropped to 10% the activity in control livers. In conclusion, treatment with fenofibrate decreases HL gene expression, leading to a lowered activity of endothelium bound HL in fenofibrate-treated livers.

Involvement of catecholamines in the effect of fasting on hepatic endothelial lipase activity in the rat

The effect of fasting on hepatic endothelial lipase activity in the liver of adult rats was investigated. We found that, both in male and female rats, fasting produced a progressive decrease of the hepatic endothelial lipase activity. Upon refeeding, the activity returned to control values in 48 h. In isolated livers from fed male rats, a sharp peak of hepatic endothelial lipase activity appeared in the perfusate upon heparin addition. It accounted for 75% of the total activity (heparin-released + residual) of the tissue. Fasting (24 h) decreased the heparin-releasable activity, and this effect was responsible for most of the decrease found in whole tissue. We suggest that the effect might be due to a decreased synthesis and/or secretion of the enzyme by hepatocytes, since isolated hepatocytes from fasted rats, incubated at 37 C, released 65% less activity to the incubation medium than hepatocytes from fed rats. Adrenaline, but not insulin, glucagon, dexamethasone, epidermal growth factor, or T3, decreased the amount of hepatic endothelial lipase activity released by hepatocytes isolated from fed rats. The effect of adrenaline appears to be mediated by alpha 1-receptors since phenylephrine but not isoprenaline reproduced, and prazosin but not propranolol blocked, the effect of the catecholamine. In the presence of cycloheximide, adrenaline also decreased the amount of activity released. We suggest that, in our incubation conditions (up to 3 h), the hormone affects the posttranslational processing of the enzyme. In vivo administration of prazosin blocked the effect of both noradrenaline and fasting on hepatic endothelial lipase activity in whole liver. Those results suggest that catecholamines are involved in the decreased hepatic endothelial lipase activity found in the liver of fasted rats, and points out the role of these hormones in the acute modulation of an enzyme involved in reverse cholesterol transport.