Separation of molecular species of lipoprotein lipase from adipose tissue

When NH(4)OH-NH(4)Cl extracts of adipose acetone powder were applied to agarose gel chromatography columns, two peaks of lipoprotein lipase were eluted. The first activity peak (LPL(a)) was eluted with an elution volume of a protein of molecular weight approximately five times that of the second (LPL(b)). Addition of heparin to the eluted fractions markedly stimulated activity of LPL(a), but suppressed that of LPL(b). Both lipases had the characteristics that distinguish lipoprotein lipase from other tissue lipases: a requirement for serum for substrate activation, inhibition by 1 m NaCl, and an alkaline pH optimum (pH 8.0). It is concluded that these fractions represent two species of lipoprotein lipase.     

Hormonal mediation of rat heart lipoprotein lipase activity after fat feeding

The effect of acute fat feeding on the response of two fractions of lipoprotein lipase in heart was explored. In rats, previously fasted, lipoprotein lipase activity released into the perfusate by heparin increased approximately 50% 4 h after fat feeding. The lipase activity remaining in the heart tissue after heparin perfusion showed no significant difference. When rats maintained ad libitum were intubated with glucose 2 h before the fat dose, a relatively larger increase (5-10-fold) in the heparin-releasable lipase activity was observed. The capacity of these hearts to hydrolyze 14C-labeled chylomicrons was also increased 4-5-fold over the controls. Fat ingestion has been reported to elevated plasma corticosteroid levels in rats. When adrenalectomized rats were fed fat, no significant changes in the heparin-releasable lipase activity were observed Hydrocortisone and corticotropin treatment increased the heparin-releasable lipase activity to the same degree as observed with fat feeding. These data suggest that the increase in heart lipoprotein lipase activity following fat feeding is mediated via corticosteroids.     

Comparison of the metabolic effects of chylomicrons and their remnants on isolated hepatocytes

A comparison was made between the effects of chylomicrons and chylomicron remnants on metabolic processes of isolated hepatocytes. Since isolated triacylglycerol-rich lipoproteins are contaminated with nonesterified fatty acids, control incubations were conducted with an amount of fatty acid equivalent to the contaminating fatty acids present in the chylomicrons and the remnant preparations, respectively. Chylomicron remnants, produced in vitro by incubation of chylomicrons in postheparin rat plasma, caused marked inhibition of glycolysis, fatty acid synthesis, and cholesterol synthesis, along with marked stimulation of ketogenesis. These effects were traced to the release of nonesterified fatty acids from these remnant particles as a consequence of contamination with lipoprotein lipase, picked up by the particles during the incubation with rat plasma. Fatty acids inhibit glycolysis, cholesterol, and fatty acid synthesis, but enhance ketone body formation by isolated hepatocytes. Chylomicrons and remnants prepared in vivo by the injection of chylomicrons into functionally hepatectomized rats were not contaminated with lipoprotein lipase and did not inhibit glycolysis and cholesterol synthesis nor increase ketone body formation. These lipoprotein particles did, however, cause significant inhibition of fatty acid synthesis, with the chylomicrons being more effective on a protein basis than the remnants produced in vivo. The mechanism responsible for the inhibition of fatty acid synthesis by chylomicrons and remnants prepared in vivo remains to be resolved.     

The functional status of lipoprotein lipase in rat liver

1. Acetone-dried powders of liver and heart tissues from rats given a high-carbohydrate diet or a fat meal were assayed for lipoprotein lipase activity. Heart tissue showed typical lipoprotein lipase activity, whereas none was detected in liver by the usual assay procedures. 2. When mixed acetone-dried powders were prepared from heart plus liver, there was a marked suppression of the expected activity, indicating that an inhibitor was present in the liver. This inhibition was partially overcome in the presence of relatively large amounts of heparin. 3. Lipoprotein lipase was also detected in liver alone when large quantities of heparin were added to the assay system. 4. No increase in lipoprotein lipase activity in either liver or heart was detected when rats were given a fat meal. 5. It is concluded that the liver of the rat contains lipoprotein lipase that is normally present in an inactive state. The results imply that a heparinase is the agent responsible for the inactivation. 6. The significance of the non-functional status of lipoprotein lipase in the liver is discussed. The results support the view that direct hydrolysis of plasma triglycerides by the liver is not a significant physiological process.     

Lipoprotein lipase activity of rat cardiac muscle. Changes in the enzyme activity during incubations of isolated cardiac-muscle cells in vitro.

1. Isolated cardiac-muscle cells from the hearts of adult rats were shown to retain a high amount of viability during 4 h of incubation when viability was assessed by Trypan Bue stain exclusion and intracellular enzyme leakage. 2. The cells also retained their ability to take up O2 and utilize added substrates over the period of incubation at both 25 and 30 degrees C. 3. When cells from the hearts of fed rats were incubated in a buffered-salts solution at pH 7.4 in the presence of amino acids and heparin, lipoprotein lipase activity in the medium increased progressively. 4. During these incubations the intracellular activity of the enzyme remained constant and the total activity of lipoprotein lipase in the system (cells plus medium) increased by 80% over the 4 h of incubation at 25 degrees C. 5. In the absence of heparin only low amounts of enzyme activity were detectable in the medium and the total lipoprotein lipase activity in the system remained constant. 6. The measurement of lipoprotein lipase activity in either fresh homogenates of the cells or in homogenates of acetone/diethyl ether-dried powders of the cells had no effect on the overall pattern of activity change during the incubations, although as reported previously the total activity detected with acetone/diethyl either-dried preparations was approx. 3-fold higher than with fresh cell homogenates. 7. The observations were compared with published data on lipoprotein lipase activity changes in neonatal heart cell cultures maintained in vitro.     

Synthesis and secretion of lipoprotein lipase in 3T3-L1 adipocytes. Demonstration of inactive forms of lipase in cells

3T3-L1 adipocytes in culture incorporated [35S]methionine into a protein which could be immunoprecipitated with chicken antiserum to bovine lipoprotein lipase. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed this protein had an Mr of 55,000, similar to that of bovine lipoprotein lipase, and accounted for 0.1-0.5% of total protein synthesis in the adipocytes. Lipoprotein lipase protein was present in small amounts in confluent 3T3-L1 fibroblasts, and the amount increased many-fold as the cells differentiated into adipocytes. This increase was accompanied by parallel increases in cellular lipase activity and secretion. When cells were grown with [35S]methionine, the amount of label incorporated into lipoprotein lipase increased for 2 h and then leveled off. Pulse-chase experiments showed that half-life of newly synthesized lipase was about 1 h. Turnover of lipoprotein lipase in control cells involved both release to the medium and intracellular degradation. When N-linked glycosylation was blocked by tunicamycin, the cells synthesized a form of lipase that had a smaller Mr (48,000), was catalytically inactive, and was not released to the medium. Radioimmunoassay demonstrated that 3T3-L1 adipocytes contained an unexpectedly large amount of lipoprotein lipase protein. 55% of the enzyme protein in acetone/ether powder of the cells was insoluble in 50 mM NH3/NH4Cl at pH 8.1, a solution commonly used to extract lipoprotein lipase; 27% of the lipase protein was soluble but did not bind to heparin-Sepharose and had very low lipase activity; and the remaining 13% was soluble, bound to heparin-Sepharose, and had high lipolytic activity. About one-half of the lipase released spontaneously to the medium was inactive, and lipase inactivation proceeded in the medium with little loss of enzyme protein. Lipoprotein lipase released heparin, in contrast, was fully active and more stable. When protein synthesis was blocked by cycloheximide, the level of lipoprotein lipase activity in adipocytes decreased more rapidly than the amount of lipase protein in the cells. Most of the inactive lipoprotein lipase in adipocytes probably results from dissociation of active dimeric lipase, but some could be a precursor of active enzyme.     

Lipoprotein lipase activity in neonatal-rat liver cell types.

The lipoprotein lipase activity in the liver of neonatal (1 day old) rats was about 3 times that in the liver of adult rats. Perfusion of the neonatal liver with collagenase decreased the tissue-associated activity by 77%. When neonatal-rat liver cells were dispersed, hepatocyte-enriched (fraction I) and haemopoietic-cell-enriched (fraction II) populations were obtained. The lipoprotein lipase activity in fraction I was 7 times that in fraction II. On the basis of those activities and the proportion of both cell types in either fraction, it was estimated that hepatocytes contained most, if not all, the lipoprotein lipase activity detected in collagenase-perfused neonatal-rat livers. From those calculations it was also concluded that haemopoietic cells did not contain lipoprotein lipase activity. When the hepatocyte-enriched cell population was incubated at 25 degrees C for up to 3 h, a slow but progressive release of enzyme activity to the incubation medium was found. However, the total activity (cells + medium) did not significantly change through the incubation period. Cycloheximide produced a time-dependent decrease in the cell-associated activity. Heparin increased the amount of lipoprotein lipase activity released to the medium. Because the cell-associated activity was unchanged, heparin also produced a time-dependent increase in the total activity. In those cells incubated with heparin, cycloheximide did not affect the initial release of lipoprotein lipase activity to the medium, but blocked further release. The cell-associated activity was also decreased by the presence of cycloheximide in those cells. It is concluded that neonatal-rat hepatocytes synthesize active lipoprotein lipase.     

Effects of exogenous apo E-3 and of cholesterol-enriched meals on the cellular metabolism of human chylomicrons and their remnants

The effects of exogenous apo E-3 and of cholesterol-enriched meals on the binding, cell association and proteolytic degradation of human chylomicrons and their remnants were determined in cultured human skin fibroblasts. Chylomicrons were prepared from plasma of normolipemic humans 4 h after a fat meal with normal or high cholesterol content. Remnants were obtained after incubation of chylomicrons with lipoprotein lipase in vitro. Cellular metabolism of chylomicrons was minimal, less than 10% that of LDL. Exogenous apo E-2 enhanced chylomicron metabolism by 3-4-fold. The cellular metabolism of remnants was 2.5-3.5-fold higher as compared to intact chylomicrons but their response to exogenous apo E-3 was considerably lower. The cellular metabolism of chylomicrons and chylomicron remnants obtained from subjects eating cholesterol-enriched fat meal was the highest either without or with added exogenous apo E-3. Yet, even in the preparation that exhibits the highest metabolic activity (apo E-3 enriched remnants from cholesterol-enriched meals) the absolute proteolytic degradation was about two-thirds that of LDL. We conclude that although LDL-receptors take up and degrade chylomicron remnants, the rate of catabolism of remnants by this route can not explain the rapid and complete remnant removal process as observed in vivo.     

Regulation of lipoprotein lipase. Induction by insulin.

Lipoprotein lipase activity in intact epididymal adipose tissue of fasted rats increased rapidly after treatment with insulin in vivo. In contrast, lipoprotein lipase activity in adipocytes isolated from the contralateral fat pads remained essentially unchanged. When adipocytes were incubated for 30 min at ambient temperature in vitro, about 2 times more lipoprotein lipase activity was found in the medium of cells from insulin-treated rats than in medium from cells of control animals. Following insulin treatment, extracts of tissue acetone powders separated by gel chromatography showed increases in both enzyme activity fractions obtained (designated lipoprotein lipase a and b). However, no consistent differences were observed between fractions derived from adipocyte acetone powders of insulin-treated and control animals. All the observed effects of insulin on lipoprotein lipase activity were abolished by cycloheximide treatment in vivo. These data indicate that following insulin treatment, increased lipoprotein lipase activity in adipose tissue results from enhanced enzyme secretion by the fat cell and subsequent accumulation in the tissue, thus implicating the adipocyte secretory mechanism as a major site of regulation of lipoprotein lipase activity in adipose tissue.     

Lipoprotn lipase content and triglyceride fatty acid uptake in adipose tissue of rats of differing body weights

Increasing body weight appears to alter lipid metabolism in adipose tissue. We have measured the content of lipoprotein lipase and the uptake of chylomicron triglyceride fatty acids in epididymal fat pads of rats of different weights. In order that the results might be expressed in terms of cell numbers, the relationship between the weights of fat pads and the numbers and volumes of fat cells isolated from them was determined. Highly significant correlations were found between fat pad weight and both the number and the volume of the individual adipocytes. In rats weighing from 140 to 350 g, the increase in the size of fat pads was attributable almost equally to increases in cell size and in cell number. Lipoprotein lipase activity was measured in acetone powders of whole fat pads and of isolated fat cell preparations. With both, lipoprotein lipase activity per cell diminished significantly as the weight of fat tissue increased, i.e., larger fat cells contained less enzyme per cell than smaller cells. The uptake of triglyceride fatty acid radioactivity was measured after incubation of fat pads with radiolabeled rat lymph chylomicrons in flasks containing either buffer alone or with added glucose or glucose plus insulin. The addition of glucose and insulin led to a mean increase of 70% in the uptake of radioactivity, but larger adipocytes were stimulated less than smaller cells. This resulted in a significant negative correlation between the weights of fat pads and the uptake of radioactivity. Enlargement of fat cells also led to a diminution in their capacity to esterify fatty acids.     

Insulin regulation of lipoprotein lipase in cultured 3T3-L1 cells

Lipoprotein lipase activity is produced by the 3T3-L1 cell an established mouse fibroblast line which resembles an adipocyte after reaching a confluent stage of growth. Since insulin has been shown to be an important regulator of lipoprotein lipase in other mammalian systems, a two hour incubation period was utilized to determine if insulin could enhance an acute response of enzyme activity. Over the range of concentrations tested (0.4, 4.0 and 40 ng/ml), insulin increased lipoprotein lipase activity in acetone ether powders of cells (intracellular enzyme) and the activity secreted into the culture medium. A simultaneous decrease in lipoprotein lipase activity releasable with heparin in a subsequent incubation (membrane bound activity) indicates two distinct effects of insulin on the enzyme in this system.     

The metabolism of very low density lipoprotein and chylomicrons by purified lipoprotein lipase from rat heart and adipose tissue

Crude lipoprotein lipase, extracted from rat adipose tissue or heart acetone-ether powders, was purified about 300 and 350 fold respectively by affinity chromatography. Artifactual increments in the density of very low density lipoprotein, noted after incubation with the crude lipoprotein lipase extract from adipose tissue, were abolished when the purified enzyme was used. Purified enzymes from both tissues showed similar modifications of activity in the presence of activators and inhibitors. The triglyceride moieties of various natural substrates were preferentially hydrolysed in the order Very low density lipoprotein > Serum chylomicrons > Thoracic duct chylomicrons by both enzymes.     

Effects of colchicine and cycloheximide on the functional and non-functional lipoprotein lipase fractions of rat heart.

In response to food deprivation, total myocardial lipoprotein lipase activity increased gradually over a period of 9 h. Although lipoprotein lipase exists in a functional and non-functional form in the myocardium, most of the increas in activity occurred in the functional (heparin-releasable) lipoprotein lipase fraction. The administration of colchicine, while having no effect on the increase seen in total lipoprotein lipase activity, did inhibit the increase in the functional fraction, while at the same time, caused a marked rise in the activity of the non-functional (non-releasable) fraction. In rats injected with colchicine after a 24-h fast, total lipoprotein lipase activity was not affected, but activity levels in the functional fraction declined while that in the non-functional fraction increased. These results suggest that the functional lipoprotein lipase is constantly being formed in sites not readily accessible to heparin (presumably the myocardial cells) and transported to its site of action, the surface of the endothelial cells of the capillaries. Cycloheximide administration to rats starved for 24 h caused a decline in activity in both the functional (half-life of about 2 h) and the non-functional (half-life of about 4 h) lipoprotein lipase fractions. These results suggest that the functional and non-functional lipoprotein lipase fractions may correspond to two distinct enzyme species.     

Changes with starvation in the rat of the lipoprotein lipase activity and hydrolysis of triacylglycerols from triacylglycerol-rich lipoproteins in adipose tissue preparations.

Lipoprotein lipase activity was higher in fat-pad pieces than in isolated adipocytes from the same fed rats, whereas hydrolysis of triacylglycerols from triacylglycerol-rich lipoproteins was similar in the two preparations when incubated either in basal conditions or in the presence of heparin. In both preparations there was a similar release of lipoprotein lipase activity into the medium during basal incubation, enhanced by the presence of heparin. In fat-pad pieces, but not in isolated adipocytes, incubation with heparin produced a decrease in the lipoprotein lipase activity measured in the tissue preparation. In fat-pad pieces from 24 h-starved rats, lipoprotein lipase activity was the same as in isolated adipocytes from the same animals and incubation with heparin did not affect the appearance of lipoprotein lipase in the medium or the utilization of triacylglycerols from triacylglycerol-rich lipoproteins. These results support the following conclusions. (1) The effectiveness of lipoprotein lipase in adipose tissue preparations in vitro depends more on its availability to the substrate than on its total activity. (2) Heparin acts on adipose tissue preparations from fed animals both by enhancing the release of pre-existing extracellular enzyme (which is absent in isolated adipocytes) and by enhancing the transfer outside the cells of the intracellular (and mainly undetectable) enzyme that is activated in the secretion process. (3) In adipose tissue from starved animals there is not only a decrease in the active extracellular form of lipoprotein lipase activity but also a reduction in the intracellular (and mainly undetectable) pool of the enzyme.     

A comparison of lipoprotein lipase activity and adipocyte differentiation in growing male rats.

During adipose tissue development changes in lipoprotein lipase activity per adipocyte precede significant changes in fat cell size. Lipoprotein lipase activity per adipocyte increases fourfold from the second to seventh postnatal week. Furthermore, when isolated adipocytes and stromal--vascular cells are prepared by collagenase digestion of adipose tissue, there is a progressive shift in enzyme activity during development from the stromal-vascular compartment to the adipocyte fraction. The data support the concept that during normal development a "bed" of preadipocytes is synthesized during the suckling period. The data further suggest a regulatory role for lipoprotein lipase in the control of "lipid-filling" during early postnatal development.     

Established cell lines from rat adipose tissue that secrete lipoprotein lipase

A number of cell lines derived from the stromal-vascular fraction of rat adipose tissue have been established that represent a variety of morphologic types. Despite their differing morphology, all of these cell lines secrete lipoprotein lipase in response to heparin. Because lipoprotein lipase secretion has been attributed to the presence of preadipocytes in the stromal-vascular fraction, we examined these cell lines for adipocyte conversion. None of the cell lines converted to adipocyte morphology when held at confluency or when exposed to media supplemented with high concentrations of fatty acid or very low density lipoproteins. These cell lines therefore do not seem to be preadipocytes, despite the presence of lipoprotein lipase. Among these cell lines are several that display the "cobblestone" morphology of endothelial cells, although they lack angiotensin-converting enzyme activity, reactivity with Factor VIII antibodies, and Weibel-Palade bodies. A number of authentic endothelial cells were found to be negative for lipoprotein lipase secretion. These data suggest that the "endothelial-like" cell lines established from adipose tissue are not endothelial cells.     

Synthesis and secretion of active lipoprotein lipase in Chinese-hamster ovary (CHO) cells.

Cultured Chinese-hamster ovary cells (CHO cells) were found to produce and secrete a lipase, which was identified as a lipoprotein lipase by the following criteria. Its activity was stimulated by serum and apolipoprotein CII, and was inhibited by high salt concentration. The lipase bound to heparin-agarose and co-eluted with 125I-labelled bovine lipoprotein lipase in a salt gradient. A chicken antiserum to bovine lipoprotein lipase inhibited the activity and precipitated a labelled protein of the same apparent size as bovine lipoprotein lipase from media of CHO cells labelled with [35S]methionine. The lipase activity and secretion were similar in growing cells and in cells that had reached confluency. Hence, lipoprotein lipase appears to be expressed constitutively in CHO cells and is not linked to certain growth conditions, as in pre-adipocyte and macrophage cell lines. At 37 degrees C, but not at 4 degrees C, heparin increased the release of lipase to the medium 2-4-fold. This increased release occurred without depletion of cell-associated lipase activity, suggesting that heparin enhanced release of newly synthesized lipase.     

Distribution of lipoprotein lipase and hepatic lipase between plasma and tissues: effect of hypertriglyceridemia

Lipoprotein lipase and hepatic lipase were measured in rat plasma using specific antisera. Mean values for lipoprotein lipase in adult rats were 1.8-3.6 mU/ml, depending on sex and nutritional state. Values for hepatic lipase were about three times higher. Lipoprotein lipase activity in plasma of newborn rats was 2-4-times higher than in adults. In contrast, hepatic lipase activity was lower in newborn than in adult rats. Following functional hepatectomy there was a progressive increase in lipoprotein lipase activity in plasma, indicating that transport of the enzyme from peripheral tissues to the liver normally takes place. Lipoprotein lipase, but not hepatic lipase, increased in plasma after a fat meal. An even more marked increase, up to 30 mU/ml, was seen after intravenous injection of Intralipid. Plasma lipase activity decreased in parallel with clearing of the injected triacylglycerol. 125I-labeled lipoprotein lipase injected intravenously during the hyperlipemia disappeared somewhat slower from the circulation than in fasted rats, but the uptake was still primarily in the liver. Hyperlipemia, or injection of heparin, led to increased lipoprotein lipase activity in the liver. This was seen even when the animals had been pretreated with cycloheximide to inhibit synthesis of new enzyme protein. These results suggest that during hypertriglyceridemia lipoprotein lipase binds to circulating lipoproteins/lipid droplets which results in increased plasma levels of the enzyme and increased transport to the liver.     

Effects of dietary saturated and polyunsaturated fat on lipoprotein lipase and hepatic triglyceride lipase activity

The effects of saturated and polyunsaturated dietary fat on the lipolytic activity of post-heparin plasma, lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL) were studied in the rat. The lipolytic activity was studied from 0 to 60 min using labelled chylomicrons as the substrate. Triacylglycerol hydrolysis rate was higher for the plasma of rats fed high fat diets (14% fat by weight). Chylomicrons of rats fed saturated or unsaturated fats were hydrolyzed at the same rate within the first 15 min but afterwards hydrolysis of chylomicrons of rats fed saturated fat was slower. The activities of LPL and HTGL were increased by high fat diets. Unsaturated fat increased more LPL activity than saturated fat conversely, HTGL activity was enhanced more by saturated fat than by unsaturated fat.     

Lipoprotein lipase and acid lipase activity in rabbit brain microvessels.

A preparation of cerebral microvessels was used to demonstrate the presence of lipoprotein lipase and acid lipase activity in the microvasculature of rabbit brain. Microvessels, consisting predominantly of capillaries, small arterioles, and venules, were islated from rabbit brain. Homogenates were assayed for lipolytic activity using a glycerol-stabilized trioleoylglycerol-phospholipid emulsion as substrate. Lipoprotein lipase activity was characterized with this substrate by previously established criteria including an alkaline pH optimum, increased activity in the presence of heparin and heat-inactivated plasma, and reduced activity in the presence of NaCl and protamine sulfate. A different substrate, containing trioleoylglycerol incorporated into phospholipid vesicles, was used to reveal acid lipase activity that was not affected by heparin, plasma, NaCl, or protamine sulfate. Lipoprotein lipase did not show activity with the vesicle preparation as substrate. Intact microvessels, when incubated in the presence of heparin, release lipoprotein lipase into the incubation solution. In contrast, release of acid lipase activity from intact microvessels was not dependent on heparin. The data show the presence of both lipoprotein lipase and acid lipase in brain microvessels and suggest that lipoproteins are metabolized within the cerebral vasculature.