Stabilization of liver lipase in vitro by heparin or by binding to non-parenchymal liver cells

Inactive hepatic lipase in rat plasma

Hepatic lipase activity is detectable in liver but also in adrenal glands, ovaries, and plasma. The subunit size of hepatic lipase in liver, adrenal glands, and nonheparin plasma was compared. Hepatic lipase in liver and adrenal glands appeared as a 55 kDa band. In liver, a faint band of lower size was also detected. In nonheparin plasma, hepatic lipase appeared as a doublet of 57 kDa and 59 kDa. When activity/mass ratio was calculated, similar values were obtained for liver and adrenal glands. In plasma this value was much lower. After heparin administration in vivo, hepatic lipase activity in plasma increased nearly 100-fold with appearance of an additional 55 kDa band in postheparin plasma. This band coeluted with activity after preparative polyacrylamide gel electrophoresis. Differences in size persisted after digestion with peptide-N-glycosidase F. A progressive increase in 57 kDa and 59 kDa in postheparin plasma followed disappearance of the 55 kDa band, suggesting that these larger bands originate from the smaller form. In plasma, both smaller and larger forms were associated with HDL, but not with LDL or VLDL. We conclude that rat plasma contains a larger form of hepatic lipase that is inactive in in vitro assay.

Lower plasma levels of lipoprotein lipase after infusion of low molecular weight heparin than after administration of conventional heparin indicate more rapid catabolism of the enzyme

The functional pool of lipoprotein lipase (LPL) is anchored to heparan sulfate at the vascular endothelium. Injection of heparin releases the enzyme into the circulating blood. Animal experiments have shown that the enzyme is then extracted and degraded by the liver. Low molecular weight (LMW) heparin preparations are widely used in the clinic and are supposed to release less LPL. In this study, we infused a LMW heparin into healthy volunteers for 8 hours. The peak of LPL activity was only about 30% and the subsequent plateau of LPL activity only about 40% compared with those seen with conventional heparin. When a bolus of heparin was given after 4 hours' infusion of LMW or conventional heparin, only relatively small, and similar, amounts of LPL entered plasma. This suggests that the difference between LMW and conventional heparin lay in the ability to retain LPL in the circulating blood, not in the ability to release the lipase. Triglycerides (TGs) decreased when the heparin infusion was started, as expected from the high circulating LPL activities. After 1 to 2 hours, TG levels increased again, and after 8 hours they were about twice as high as before the heparin infusion. This indicates that the amount of LPL available for lipoprotein metabolism had become critically low in relation to TG transport rates. This study indicates that LMW heparin compared with conventional heparin causes as much or more depletion of LPL and subsequent impairment of TG clearing.

Secretion of hepatic lipase by perfused liver and isolated hepatocytes

Hepatic lipase is found in liver and in adrenal glands and ovaries. Because in adult rats, neither adrenals nor ovaries synthesize this enzyme, it is assumed that the liver is the origin of their hepatic lipase. Our aim was to study the secretion of hepatic lipase by the liver. We observed that plasma of both fed and fasted rats contained hepatic lipase activity. This activity was significantly correlated with that in the liver. Isolated livers, perfused with heparin-free medium, secreted fully active hepatic lipase to the perfusate. The addition of heparin resulted in a rapid and larger release of hepatic lipase to the perfusate. In isolated hepatocytes, heparin did not affect the secretion of hepatic lipase mass, although it increased the stability of the enzyme activity. To study the degradation of hepatic lipase by hepatocytes, protein synthesis was blocked with cycloheximide, and both secreted and intracellular hepatic lipases were analyzed by Western blotting. We observed that the amount of hepatic lipase secreted equaled the decrease of intracellular mass. The total mass of the enzyme (inside and outside the cells) remained constant, at least for 90 min. In the next experiment, 0.7 nM 125I-hepatic lipase was added to hepatocyte suspensions, and the appearance of trichloracetic acid-soluble products was analyzed. Only 12% of the radioactivity added was associated with the cells after 90 min of incubation, and less than 2% of the hepatic lipase added was degraded. Although the association was decreased in the presence of heparin, the amount of 125I-hepatic lipase degraded was not affected. Taking all these results into account, we propose a model for the continuous secretion of hepatic lipase by the liver.

Hepatic lipase affects both HDL and ApoB-containing lipoprotein levels in the mouse

Transgenic mice were created overproducing a range of human HL (hHL) activities (4-23-fold increase) to further examine the role of hepatic lipase (HL) in lipoprotein metabolism. A 5-fold increase in heparin releasable HL activity was accompanied by moderate (approx. 20%) decreases in plasma total and high density lipoprotein (HDL) cholesterol and phospholipid (PL) but no significant change in triglyceride (TG). A 23-fold increase in HL activity caused a more significant decrease in plasma total and HDL cholesterol, PL and TG (77%, 64%, 60%, and 24% respectively), and a substantial decrease in lipoprotein lipids amongst IDL, LDL and HDL fractions. High levels of HL activity diminished the plasma concentration of apoA-I, A-II and apoE (76%, 48% and 75%, respectively). In contrast, the levels of apoA-IV-containing lipoproteins appear relatively resistant to increased titers of hHL activity. Increased hHL activity was associated with a progressive decrease in the levels and an increase in the density of LpAI and LpB48 particles. The increased rate of disappearance of 125I-labeled human HDL from the plasma of hHL transgenic mice suggests increased clearance of HDL apoproteins in the transgenic mice. The effect of increased HL activity on apoB100-containing lipoproteins was more complex. HL-deficient mice have substantially decreased apoB100-containing low density lipoproteins (LDL) compared to controls. Increased HL activity is associated with a transformation of the lipoprotein density profile from predominantly buoyant (VLDL/IDL) lipoproteins to more dense (LDL) fractions. Increased HL activity from moderate (4-fold) to higher (5-fold) levels decreased the levels of apoB100-containing particles. Thus, at normal to moderately high levels in the mouse, HL promotes the metabolism of both HDL and apoB-containing lipoproteins and thereby acts as a key determinant of plasma levels of both HDL and LDL.

Heparin-decasaccharides impair the catabolism of chylomicrons

On intravenous injection to rats, decasaccharides gave rise to a short-lived peak of lipoprotein lipase (LPL) activity, whereas octa- and hexasaccharides caused only marginal increases. In isolated hearts perfused by a single pass, decasaccharides released LPL more efficiently than conventional heparin on a mass basis. Octa- and hexasaccharides were much less efficient. Similar results were obtained for hepatic lipase, which was studied both in vivo and by liver perfusion. In the intact rat, the heparin fragments themselves disappeared rapidly from the circulating blood. The decay of hepatic lipase activity after the early peak roughly paralleled the decay of decasaccharide concentration, but for LPL the decay was faster, presumably because the liver extracted this lipase from plasma. To assess the lipase activities remaining in contact with blood a large dose of conventional heparin was injected at a series of times after the decasaccharides. LPL was decreased by 40% after 1 h. At that time, the LPL activity that could be released from isolated hearts by single-pass perfusion with heparin for 2 min ("functional LPL') was decreased by 75%. Chylomicrons labelled in vivo with [14C]oleic acid (primarily in triacylglycerols, providing a tracer for lipolysis) and [3H]retinol (primarily in ester form, providing a tracer for the particles) were injected intravenously to explore the effects of the LPL depletion on lipoprotein metabolism. Triacylglycerol lipolysis and particle clearance was markedly delayed from 30 min to 2 h after injection of decasaccharides. After 1 h the fractional catabolic rate was only one-third of the control value and the catabolism of chylomicron triacylglycerols by perfused hearts was delayed to a similar extent. Thus injection of decasaccharides leads to accelerated turnover of LPL with loss of functional LPL from extrahepatic tissues. This in turn leads to a period of delayed lipolysis and removal of chylomicron particles.

Remodeling of HDL containing apoA-I but not apoA-II (LpA-I) by lipoprotein-deficient plasma and hepatic lipase: its effect on the structure and cellular cholesterol-reducing capacity of LpA-I

We investigated the effects of lipoprotein-deficient plasma (LDP) and hepatic lipase (HL) on the structure and cellular cholesterol-reducing capacity of subclasses of LpA-I (HDL containing apoA-I but not apoA-II). LpA-I is composed of large (11.1 nm; L-LpA-I), medium (8.8 nm: M-LpA-I) and small (7.7 nm: S-LpA-I) particles. L-LpA-I and M- and S-LpA-I combined (MS-LpA-I) were incubated with lipoprotein-deficient plasma and HL in the presence of very low density lipoprotein (VLDL). After incubation of L-LpA-I, the proportions of cholesteryl esters and phospholipids decreased and as a result, the proportion of protein increased. The remodeled L-LpA-I particles were generally smaller (spherical: 7.8-8.8 nm) in diameter. A small number of disc-shaped particles were also found in electron photomicrographs. These changes coincided with a slower electrophoretic mobility of remodeled L-LpA-I. In the case of MS-LpA-I, only the proportion of free cholesterol increased after incubation, and MS-LpA-I particles did not change in size. The cholesterol-reducing capacities of remodeled L-LpA-I and MS-LpA-I from macrophage foam cell were slightly higher and lower than their respective original counterparts, although neither of these differences was statistically significant. These results suggest that LDP and HL mainly contribute to the remodeling of L-LpA-I particles, and may not affect the cellular cholesterol-reducing capacity of these particles.

Depletion of lipoprotein lipase after heparin administration

Some or most of the turnover of lipoprotein lipase (LPL) occurs by dissociation from vascular endothelial sites in extrahepatic tissues and further degradation in the liver. Heparin greatly enhances this dissociation and delays but does not abolish uptake in the liver, raising the possibility that heparin could lead to accelerated catabolism of functional LPL. To investigate this, we determined time curves for heparin (anti-factor Xa activity) and for LPL and hepatic lipase after injection in rats of two doses of conventional unfractionated heparin (UFH) or low-molecular-weight heparin (LMWH). The high dose (250 U/kg) of both heparins resulted in similar initial levels of LPL activity in plasma, but at 30 minutes the activity with LMWH had declined by more than 80%, whereas with UFH it remained essentially unchanged during this time. In contrast, time curves for heparin activity in blood were similar for the two heparins. The low dose (50 U/kg) led to lower initial levels of LPL activity with LMWH in spite of slower elimination of heparin activity from the blood. These results agree with previous studies that indicate that LMWH has a similar ability as UFH to release LPL, but a lesser ability to delay its removal by the liver. Only slight differences were noted in the time curves for hepatic lipase with the two heparins. To assess the possible depletion of the lipases, we administered a second large dose of conventional heparin. One hour after the first injection, the second injection resulted in lower plasma LPL activities in all four groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of size-fractionated heparins with lipoprotein lipase and hepatic lipase in the rat

Heparin and heparin partially depolymerized by enzymic digestion were separated into six size fractions. Hep 1 (tetrasaccharides), with a mean M(r) of 1200, did not release significant amounts of either lipoprotein lipase (LPL) or hepatic lipase (HL) on intravenous injection into rats. Hep 2 (mainly octa- and deca-saccharides), with a mean M(r) of 2400-3000, released both lipases. To evoke the same plasma activity of LPL and HL required about 10 times more by weight, or about 40 times more molecules, of this heparin than of hep 5 (mean M(r) 12,000, similar to conventional heparin). Hep 5 impeded binding and degradation of 125I-labelled bovine LPL by perfused rat livers. In contrast, hep 2 had no detectable effect on these processes. This demonstrates a difference between the sites in the liver that mediate binding, uptake and degradation of LPL, and the extrahepatic sites that bind functional LPL, and the hepatic sites that bind functional HL. After injection of 3.25 mg of hep 5/kg body weight, plasma LPL activity rapidly rose and then remained high for at least 1 h. With hep 2, plasma LPL also rose rapidly, but then decreased to almost basal by 1 h. When a labelled triacylglycerol emulsion was injected 1 h after the heparins, the fractional catabolic rate was enhanced in the rats that had received conventional heparin, as expected from the high plasma LPL activity, but decreased compared with controls in rats that had received hep 2, indicating that available LPL had been depleted through enhanced transport to and uptake in the liver.

Cholesteryl ester transfer protein and hepatic lipase activity promote shedding of apo A-I from HDL and subsequent formation of discoidal HDL

The effects of lipid transfers and hepatic lipase (HL) on the concentration, composition, particle size distribution and morphology of high density lipoproteins (HDL) have been investigated. Human plasma supplemented with additional very low density lipoproteins (VLDL), cholesteryl ester transfer protein (CETP) and HL has been incubated at 37 degrees C for up to 8 h. The HDL became depleted of cholesteryl esters and reduced in particle size. Within 2 h of such incubation they had also lost about 30% of their apo A-I. However, with extension of the incubations beyond 2 h, the apo A-I returned progressively to the HDL fraction until, after 8 h, the concentration of apo A-I in HDL was identical to that in non-incubated samples. This return of apo A-I to the HDL density range was accompanied by a progressive appearance in electron micrographs of discoidal HDL particles. Thus, the depletion of the core lipid content and the reduction in particle size of HDL promoted by lipid transfers and HL activity in vitro is accompanied by a shedding of apo A-I which forms the nucleus of new discoidal HDL particles. The potential physiological importance of such a process is considerable.

Heterogeneity among ovarian blood vessels: endogenous hepatic lipase is concentrated in blood vessels of rat corpora lutea

We used indirect immunofluorescence and immunogold light microscopy to examine the distribution of hepatic lipase, an enzyme involved in lipoprotein metabolism, in ovaries of gonadotropin-treated immature rats. Antibodies utilized were rabbit anti-rat hepatic lipase IgG, anti-rat von Willebrand factor (VWF, an endothelial cell marker), and goat anti-rabbit IgG conjugated to gold particles or rhodamine. Immunoreagents were applied to fresh frozen sections of unfixed ovary or liver (positive control) or were delivered to ovaries by vascular perfusion before fixation in situ and silver-enhancement of sections. Appropriate controls verified that the immunolocalizations were specific. Immunofluorescence implied that luteal but not stromal blood vessels of ovaries were positive for hepatic lipase, whereas luteal and stromal blood vessels bore VWF. The improved morphology gained by perfusing ovaries with antibodies allowed precise localization of the enzyme. Hepatic lipase was concentrated within thin-walled vessels of corpora lutea but not those of stroma in ovaries at the time of peak steroidogenic activity. Quantification of hepatic lipase-labeled vessels in stromal and luteal compartments confirmed our visual impression. Many images suggested that stromal vessels lacking hepatic lipase gained this enzyme upon contact with luteal tissue. Perfusion of ovaries with cationized ferritin labeled all ovarian vessels equally well, ruling out the possibility that the observed distribution of hepatic lipase was artifactual. These findings demonstrate that ovarian blood vessels are heterogeneous for hepatic lipase. Moreover, they imply that luteal tissue, perhaps luteal cells, may influence expression of hepatic lipase binding sites by endothelial cells.

Estradiol increases the secretion of hepatic lipase by rat hepatocyte cultures

Hepatic lipase (EC 3.1.1.3) is synthesized and secreted by parenchymal hepatocytes and binds to endothelial cells of liver sinusoids. The present study shows that the activity of hepatic lipase secreted by hepatocyte cultures from male rats in increased approx. 6-fold after 10 h culture with 10 microM 17 beta-estradiol. The stimulatory effect of 17 beta-estradiol is biphasic and declines at higher concentrations. In hepatocytes from male rats: progesterone, unlike 17 beta-estradiol, had only a small stimulatory effect when present as the sole hormone and a small inhibitory effect in the presence of 17 beta-estradiol, while testosterone and dexamethasone had no effect. Hepatocyte cultures from female rats had a higher basal rate of hepatic lipase secretion than cells from male rats and showed a smaller stimulation by 17 beta-estradiol. These results suggest that 17 beta-estradiol might regulate the secretion of hepatic lipase by hepatocytes, and presumably the activity of the enzyme at either the endothelial surface of the liver sinusoids or at extrahepatic sites.

Hepatic lipase promotes a loss of apolipoprotein A-I from triglyceride-enriched human high density lipoproteins during incubation in vitro

Studies have been performed to investigate a possible mechanism to account for the low concentrations of apolipoproteins A-I (apo A-I) in subjects with hypertriglyceridemia. Incubation of human plasma in vitro with canine hepatic lipase resulted in the hydrolysis of approximately half the triglyceride in the high density lipoproteins (HDLs), but little change in the concentrations of other HDL constituents. However, when the plasma was supplemented with cholesteryl ester transfer protein and very low density lipoproteins to enrich the HDL with triglyceride, hepatic lipase promoted not only a significant reduction in HDL triglyceride acquired by the lipid transfer process but also an enhanced transfer of cholesteryl esters out of the particles. These changes were accompanied by a marked loss of apo A-I from HDL, which was demonstrated independently by ultracentrifugation, size-exclusion chromatography, and gradient gel-immunoblot analysis. The apo A-I lost from HDL was recovered in the "lipoprotein-free" fraction of plasma. The results of these studies indicate that primary reductions in the concentration of HDL core lipids in vitro are accompanied by a secondary loss of apo A-I from HDL. While recognizing the need for caution in any extrapolation from observations made in vitro to what may occur in vivo, these studies are nevertheless consistent with a proposition that the low concentration of apo A-I in subjects with hypertriglyceridemia is secondary to the reduced concentration of HDL core lipids in such subjects.

Inhibition of phospholipase A2 by heparin

Phospholipase A2 (PLA2) is an important enzyme in the regulation of cell behavior. The hydrolysis of phosphatidylcholine in vitro catalyzed by porcine pancreatic PLA2 was inhibited by heparin. Other glycosaminoglycans inhibited PLA2 activity to a significantly lesser extent, with a pattern of inhibition: heparin much greater than chondroitin sulfate (CS)-C greater than CS-A greater than CS-B greater than keratan sulfate. Hyaluronic acid and heparan sulfate caused no inhibition. Heparin's ability to inhibit PLA2 activity did not depend on substrate concentration, but did depend on ionic strength, with inhibition decreasing with increasing ionic strength. Heparin inhibition also varied with pH, being more effective at pH 5-8 than at pH 10. As a consequence, heparin induced a shift of the pH optimum of PLA2 from 7 to 8. Histone IIA and protamine sulfate, heparin-binding proteins, reversed heparin-induced PLA2 inhibition. The concentration of heparin which inhibited PLA2 activity by 50% increased with increasing enzyme concentration. Furthermore, PLA2 bound to heparin-Affigel. The data indicate that the catalytic potential of PLA2 can be regulated by heparin or heparin-like molecules and that inhibition is contingent on the formation of a heparin-PLA2 complex.

Lipoprotein lipase prevents the hepatic lipase-induced reduction in particle size of high density lipoproteins during incubation of human plasma

Human plasma lipoproteins or human whole plasma have been incubated in vitro with canine hepatic lipase (HL) and bovine milk lipoprotein lipase (LPL) to determine the effects of lipases on the particle size distribution of HDL. Confirming previous reports, HL preferentially hydrolysed high density lipoprotein (HDL) triacylglycerol while LPL hydrolysed predominantly very low density lipoprotein (VLDL) triacylglycerol; however, neither lipase altered HDL particle size unless both VLDL and cholesteryl ester transfer protein (CETP) were present. Under these conditions HL promoted marked reduction in HDL particle size in a process dependent on the concentration of VLDL triacylglycerol while LPL was virtually without effect. When both LPL and HL were included in the same incubation, however, LPL prevented the effects of HL. These results are consistent with a proposition that HL has a direct effect on HDL particle size in a process which is dependent on concurrent lipid transfers between HDL and VLDL and that LPL reduces the effect of HL by reducing the concentration of VLDL triacylglycerol.

Synergistic effects of lipid transfers and hepatic lipase in the formation of very small high-density lipoproteins during incubation of human plasma

Studies have been performed to determine the involvement of very-low-density lipoproteins (VLDL), cholesteryl ester transfer protein (CETP) and hepatic lipase (HL) in the formation of very small HDL particles. Human whole plasma has been incubated for 6 h at 37 degrees C in the absence and in the presence of various additions. There was minimal formation of very small HDL in incubations of non-supplemented plasma or in plasma supplemented with either VLDL, CETP or HL alone; nor were small HDL prominent after incubating plasma supplemented with mixtures of VLDL plus CETP, VLDL plus HL or CETP plus HL. By contrast, when plasma was supplemented with a mixture containing all three of VLDL, CETP and HL, incubation resulted in an almost total conversion of the HDL fraction into very small particles of radius 3.7 nm. The appearance of these very small HDL was independent of activity of lecithin: cholesterol acyltransferase. It was, however, dependent on both duration of incubation and on the concentrations of the added VLDL, CETP and HL. The effects of these incubations was also assessed in terms of changes to the concentration and distribution of lipid constituents across the lipoprotein spectrum. It was found that not only did lipid transfers and HL exhibit a marked synergism in promoting a reduction in HDL particle size but also that HL, although deficient in intrinsic transfer activity, enhanced the CETP-mediated transfers of cholesteryl esters from HDL to other lipoprotein fractions.

Heparin decreases the degradation rate of hepatic lipase in Fu5AH rat hepatoma cells. A model for hepatic lipase efflux from hepatocytes

The mechanism for the stimulation of hepatic lipase secretion by heparin was studied in cultured Fu5AH rat hepatoma cells. Quantitative immunoprecipitation followed by electrophoresis and fluorography were used to isolate and quantitate the radioactive enzyme; hepatic lipase protein mass was quantitated by ELISA. Addition of heparin to the medium resulted in a 2-fold increase in lipase secretion rate, whereas cell-surface-associated and intracellular lipase decreased by 76 and 20%, respectively. Rates of synthesis of hepatic lipase measured by incorporation of Trans 35S-label into enzyme protein were not different in control or heparin-treated dishes. In pulse-chase studies, it was estimated that the degradation rate constants for control and heparin-treated cultures were 0.51 +/- 0.09 and 0.14 +/- 0.13 h-1 for control and heparin-treated cultures, respectively. 52% of the synthesized enzyme was degraded in control cultures; addition of heparin to the culture medium reduced this figure to 11% of the synthetic rate. Equilibrium binding data of highly purified 125I-hepatic lipase to Fu5AH cells at 4 degrees C demonstrate the presence of a class of high-affinity binding sites. At 37 degrees C, cell-surface-bound 125I-hepatic lipase is internalized and either degraded or recycled to the medium. The half-intracellular residence times of hepatic lipase were 55 and 31 min in control and heparin-treated cultures, respectively. Radioactivity incorporated in the 55.4 kDa high-mannose-containing lipase and the mature 57.6 kDa species was measured as a means of locating the enzyme in the secretory pathway before or beyond the medial Golgi. The disappearance of the 55.4 kDa species from the cell is similar in control and heparin-treated cultures with half-intracellular residence times of 29 and 25 min, respectively. In contrast, the amount of radiolabeled 57.6 kDa species in control cells remained constant from 15 min to 2 h, whereas it decreased by 79% in heparin-treated cells. The above data demonstrate that the increase in hepatic lipase secretion is due to a decreased degradation rate with no change in synthetic rate and that heparin primarily affected the residence time of hepatic lipase in the medial Golgi-plasma membrane region.

The rabbit as an animal model of hepatic lipase deficiency

A natural deficiency of hepatic lipase in rabbits has been exploited to gain insights into the physiological role of this enzyme in the metabolism of plasma lipoproteins. A comparison of human and rabbit lipoproteins revealed obvious species differences in both low-density lipoproteins (LDL) and high-density lipoproteins (HDL), with the rabbit lipoproteins being relatively enlarged, enriched in triacylglycerol and depleted of cholesteryl ester. To test whether these differences related to the low level of hepatic lipase in rabbits, whole plasma or the total lipoprotein fraction from rabbits was either kept at 4 degrees C or incubated at 37 degrees C for 7 h in (i) the absence of lipase, (ii) the presence of hepatic lipase and (iii) the presence of lipoprotein lipase. Following incubation, the lipoproteins were recovered and subjected to gel permeation chromatography to determine the distribution of lipoprotein components across the entire lipoprotein spectrum. An aliquot of the lipoproteins was subjected also to gradient gel electrophoresis to determine the particle size distribution of the LDL and HDL. Both hepatic lipase and lipoprotein lipase hydrolysed lipoprotein triacylglycerol and to a much lesser extent, also phospholipid. There were, however, obvious differences between the enzymes in terms of substrate specificity. In incubations containing hepatic lipase, there was a preferential hydrolysis of HDL triacylglycerol and a lesser hydrolysis of VLDL triacylglycerol. By contrast, lipoprotein lipase acted primarily on VLDL triacylglycerol. When more enzyme was added, both lipases also acted on LDL triacylglycerol, but in no experiment did lipoprotein lipase hydrolyse the triacylglycerol in HDL. Coincident with the hepatic lipase-induced hydrolysis of LDL and HDL triacylglycerol, there were marked reductions in the particle size of both lipoprotein fractions, which were now comparable to those of human LDL and HDL3, respectively.

Chylomicron-remnant uptake by freshly isolated hepatocytes. Effect of heparin and of hepatic triacylglycerol lipase

Chylomicron remnants labelled biologically with [3H]cholesterol were efficiently taken up by freshly isolated hepatocytes during a 3 h incubation in Krebs bicarbonate medium. Their [3H]cholesteryl ester was hydrolysed (74% net hydrolysis), and 0.1 mM-chloroquine could partially inhibit this hydrolysis, provided that hepatocytes were first preincubated for 2 h 30 min at 37 degrees C. This hydrolysis was also measured in preincubated cells with remnants double-labelled (3H and 14C) on their free cholesterol moiety; [3H]cholesterol arising from [3H]cholesteryl ester hydrolysis was recovered in the free [3H]cholesterol pool. A dose-response study showed saturation of remnant uptake at 180 micrograms of remnant protein/10(7) cells. Heparin (10 units/ml) increased remnant uptake by 63% (P less than 0.01), [3H]cholesteryl ester accumulation in the cell pellet by 110% (P less than 0.025) and hepatic lipase activity secreted in the medium by 2.4-fold (P less than 0.01) and by 3.3-fold (P less than 0.01) at the end of the preincubation and incubation periods respectively. Addition of 100 munits of semi-purified hepatic lipase preparation/flask stimulated remnant uptake by 44-69%, and [3H]cholesteryl ester accumulation in the presence of chloroquine by 2.1-fold (P less than 0.025). When hepatic lipase was incubated solely with the remnants, it decreased their triacylglycerol and phospholipid contents by 24% and 26% respectively. Thus freshly isolated hepatocytes may be used to study chylomicron-remnant uptake. Hepatic lipase, which seems to underly the stimulating effect of heparin, facilitates remnant uptake in vitro, and this could be mediated by at least one (or both) of its hydrolytic properties.

Secretion of liver lipase activity by periportal and perivenous hepatocytes

Periportal and perivenous hepatocytes were separated by isopycnic centrifugation through Percoll, or selectively isolated by combined digitonin/collagenase perfusion. With either method, secretion of liver lipase activity was 2-2.5-fold higher in periportal than perivenous cells. This acinar heterogeneity parallels that of cholesterol de novo synthesis and bile formation reported previously.

Effect of lipoprotein lipase and hepatic triglyceride lipase activity on the distribution of apolipoprotein E among the plasma lipoproteins

The independent roles of human lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL) in determining the distribution of apolipoprotein E (apo E) among the plasma lipoproteins has been studied in vitro. In one series of three studies, postheparin plasma (10%) was incubated for 2 h with autologous plasma and the changes in the lipoprotein association of apo E after lipase exposure were determined after lipoprotein fractionation on 4% agarose columns. Specificity for LPL or HTGL was achieved by inhibition with goat anti-human HTGL or with 1 M NaCl, respectively. In another study, LPL and HTGL were partially purified from human postheparin plasma. The independent effects of these enzymes on the lipoprotein association of apo E were then examined after incubation of plasma in the absence or presence of one or both lipases. Data from both types of in vitro study showed that LPL-mediated triglyceride hydrolysis in the absence of HTGL activity was accompanied by a loss of apo E from triglyceride-rich lipoproteins, a gain or no change in the apo E-containing lipoproteins the size of intermediate density lipoproteins (IDL) and inconsistent changes in the apo E mass associated with high density lipoproteins (HDL). HTGL activity, on the other hand, in the absence of LPL, resulted in a redistribution of apo E from lipoproteins the size of IDL and a gain by those of HDL size. These studies thus support previous in vivo studies which pointed toward a specific role for HTGL in the processing of apo E containing IDL.

Isolation of a high-density-lipoprotein conversion factor from human plasma. A possible role of apolipoprotein A-IV as its activator

1. A high-density-lipoprotein (HDL) conversion factor was partially purified from human plasma by precipitation with (NH4)2SO4, ultracentrifugation, cation-exchange chromatography, anion-exchange chromatography and chromatography on a column of hydroxyapatite. 2. This factor modulates the particle size of HDL by converting a homogeneous population into new populations of particles, some of which are smaller and others larger than those in the original population. 3. The isolated HDL conversion factor appeared as one major band and at least three minor bands on SDS/polyacrylamide-gel electrophoresis; attempts to purify this factor further resulted in loss of conversion activity. 4. Preparations of the HDL conversion factor were stable after heating to 58 degrees C for 1 h, and were shown not to possess proteolytic activity. 5. The conversion factor was distinct from the known apolipoproteins, none of which had HDL conversion activity. 6. Addition of apolipoprotein A-IV had a dose-dependent potentiating effect on the process promoted by the HDL conversion factor.

Monoclonal antibodies against salt-resistant rat liver lipase. Cross-reactivity with lipases from rat adrenals and ovaries

To obtain monoclonal antibodies against rat salt-resistant liver lipase, mice were immunized with enzyme purified from heparin-containing rat liver perfusates. Hybridomas were screened for antibody production by means of an enzyme-linked immunosorbent assay (ELISA) and an immunoprecipitation assay. Five hybridoma cell lines secreting antibodies against rat liver lipase indicated as A, B, C, D and E, have been obtained. All antibodies possess gamma one (gamma 1) heavy chains and kappa (kappa) light chains. The antibodies precipitate salt-resistant lipase from rat post-heparin plasma, are positive in ELISA, inhibit liver lipase activity and bind monospecifically with the enzyme as shown by immunoblotting. The monoclonal antibodies showed no significant reactivity with human liver lipase. The salt-resistant lipases of rat adrenals and ovaries are also precipitated by the monoclonal antibodies directed against the liver enzyme. Therefore, the heparin-releasable lipases of the liver, adrenals and ovaries possess identical epitopes.

Synthesis and secretion of triacylglycerol lipase by cultured rat hepatocytes

Rat hepatocytes isolated by collagenase perfusion were cultured for 48-72 h and examined for synthesis and secretion of hepatic triacylglycerol lipase activity. Low levels of enzyme activity found in the culture medium increased with time of incubation, and a 3-10-fold rise was encountered in the presence of optimal concentrations of heparin (5 U/ml). After interruption of enzyme synthesis by cycloheximide, plateauing of enzyme activity in the medium occurred, indicating that addition of heparin may not only stabilize but also enhance hepatic triacylglycerol lipase secretion. Synthesis and secretion of hepatic triacylglycerol lipase was not related to cell density, and enzyme secretion was encountered in subconfluent cultures. Release of enzyme activity into the medium was not sensitive to chlorpromazine, a lysosomal enzyme inhibitor, but was completely inhibited by treatment with tunicamycin, an inhibitor of glycosylation. As release of enzyme activity could be maintained for 12 h in the absence of serum, possible hormonal regulation was sought. Under the present experimental conditions, no modulation of hepatic triacylglycerol lipase was encountered by either gonadal or thyroid hormones. Addition of cyclic AMP to the culture medium resulted in a 30% decrease in enzyme activity. The dependence of hepatic triacylglycerol lipase secretion on the intactness of the Golgi apparatus and on vesicular transport was demonstrated by the treatment with monensin. The present results show that cultured rat hepatocytes provide a good model system by which the regulation of synthesis and secretion of hepatic triacylglycerol lipase can be studied.

Regulation of liver lipase. I. Evidence for several regulatory sites, studied in corticotrophin-treated rats

The activity of liver lipase, an enzyme that can be released from the liver by heparin, varies under several hormonal conditions. The site(s) at which regulation of the enzyme activity may occur was investigated in vitro. As a model, rats were used which had been treated with a corticotrophin analogue, to induce hypercortisolism, a condition in which liver lipase activity is lowered. Lipases isolated from heparin-containing perfusates of livers from ACTH or control rats were identical with respect to heat stability and specific activity as determined by immunotitration and binding to isolated non-parenchymal liver cells, indicating that the enzyme structure was not affected by the treatment. The secretion of liver lipase by isolated parenchymal liver cells was studied. During incubation of parenchymal cells derived from ACTH rats, less enzyme activity was found to be secreted when compared with hepatocytes isolated from control rats (ACTH rats, 2.30 +/- 0.2 mU/10(6) cells; control rats, 3.3 +/- 0.3 mU/10(6) cells). Liver lipase partially purified from control rats could be bound specifically to saturation by non-parenchymal cells, isolated from ACTH or control rats. Non-parenchymal cells from ACTH rats bound less lipase activity (29 mU/mg cell protein) than cells from control rats (50 mU/mg cell protein). This reduction in binding capacity seems to be due to a diminished number of binding sites, since the affinity based on Scatchard analysis and half-maximal binding was not different. These results suggest that the lowered liver lipase activity found during hypercortisolism may be due to an impaired synthesis and/or secretion of the enzyme by the parenchymal cells and to a reduced binding capacity of the non-parenchymal cells for liver lipase.

Liver lipase and high-density lipoprotein. Lipoprotein changes after incubation of human serum with rat liver lipase

Human sera were incubated with rat liver lipase after inactivation of lecithin:cholesterol acyltransferase, and the changes in serum lipoprotein composition were measured. In the presence of liver lipase serum triacylglycerol and phosphatidylcholine were hydrolyzed. The main changes in the concentrations of these lipids were found in the high-density lipoprotein fraction. Subfractionation of high-density lipoprotein by rate-zonal ultracentrifugation showed a prominent decrease in all constituents of high-density lipoprotein2, a smaller decrease in the 'light' high-density lipoprotein3 and an increase in the 'heavy' high-density lipoprotein3. These data support a concept in which liver lipase is involved in high-density lipoprotein2 phospholipid and triacylglycerol catabolism and suggest that as a result of this action high-density lipoprotein2 is converted into high-density lipoprotein3.