Maturation and secretion of lipoprotein lipase in cultured adipose cells. I. Intracellular activation of the enzyme

Enzymes involved in triglyceride hydrolysis

The lipolytic enzymes LPL and HL play important roles in the metabolism of lipoproteins and participate in lipoprotein interconversions. LPL was originally recognized to be the key enzyme in the hydrolysis of chylomicrons and triglyceride, but it also turned out to be one determinant of HDL concentration in plasma. When LPL activity is high, chylomicrons and VLDL are rapidly removed from circulation and a concomitant rise of the HDL2 occurs. In contrast, low LPL activity impedes the removal of triglyceride-rich particles, resulting in the elevation of serum triglycerides and a decrease of HDL (HDL2). Concordant changes of this kind in LPL and HDL2 are induced by many physiological and pathological perturbations. Finally, the operation of LPL is also essential for the conversion of VLDL to LDL. This apparently clear-cut role of LPL in lipoprotein interconversions is contrasted with the enigmatic actions of HL. The enzyme was originally thought to participate in the catalyses of chylomicron and VLDL remnants generated in the LPL reaction. However, substantial in vitro and in vivo data indicate that HL is a key enzyme in the degradation of plasma HDL (HDL2) in a manner which opposes LPL. A scheme is presented for the complementary actions of the two enzymes in plasma HDL metabolism. In addition, recent studies have attributed a role to HL in the catabolism of triglyceride-rich lipoproteins, particularly those containing apo E. However, this function becomes clinically important only under conditions where the capacity of the LPL-mediated removal system is exceeded. Such a situation may arise when the input of triglyceride-rich particles (chylomicrons and/or VLDL) is excessive or LPL activity is decreased or absent.

Antibody against human lipoprotein lipase

A polyclonal antibody against human lipoprotein lipase (LPL) was prepared. LPL from post-heparin plasma was first purified by heparin Sepharose 4B affinity chromatography. Protein impurities co-eluted with LPL were then eliminated by electrophoresis in the presence of ampholytes. Antithrombin III was identified in this fraction of protein impurities by immunodiffusion against a human antithrombin antiserum, while no antithrombin III could be detected in the purified LPL fraction. Immunodiffusion revealed a single line of precipitation between this antibody and human post-heparin plasma LPL. When pre-incubated with a constant activity of highly purified post-heparin plasma LPL (2.7 mU/75 microliters), an equal volume of the anti-LPL antiserum, either pure or diluted to 1/32 caused complete inhibition of the enzyme activity. Half maximal inhibition was observed at a dilution of approximately 1/200. By using a secondary antibody, it was shown that antiserum inhibited LPL activity by means of its immunoglobulins. This antibody was able to inhibit LPL from human adipose tissue, indicating that human LPL released from endothelial cell membranes has common antigenic determinants with adipose tissue LPL.

Expression of the mitochondrial uncoupling protein in brown adipocytes. Absence in brown preadipocytes and BFC-1 cells. Modulation by isoproterenol in adipocytes

The expression of the uncoupling protein has been compared in cells of BFC-1 clonal line established from mouse brown adipose tissue (BAT) and in preadipocytes, as well as in adipocytes from mouse BAT, both in primary culture. The results of immunoblots show that, after one week in culture, adipocytes have a reduced level of the 32 kD protein. This level can be raised 2-3.5-fold by a 24-h exposure to isoproterenol. Thus a direct modulation by a beta-agonist drug in the expression of the uncoupling protein is observed. Under the same conditions as well as under various other conditions, preadipocytes in primary culture and BFC-1 cells do not express the uncoupling protein. At the same time these cells are able both to differentiate into adipose cells, as demonstrated by the emergence of enzyme markers and triglyceride accumulation, and to respond to isoproterenol. Thus isoproterenol is not sufficient to trigger the expression of the uncoupling protein and behaves as a mere modulator once the cells have acquired the capacity to express it. Injection of undifferentiated BFC-1 cells into athymic mice bearing catecholamine-containing mini-osmotic pumps, or co-cultures of BFC-1 cells and pheochromocytoma PC-12 cells do not allow BFC-1 cells to express the uncoupling protein. Taken together, the results suggest that the formation of brown preadipocytes is critically linked during development to the release by sympathetic nerves of specific trophic factors acting locally.

Development of a chemically defined serum-free medium for differentiation of rat adipose precursor cells

Stromal-vascular cells from the epididymal fat pad of 4-week-old rats, when cultured in a medium containing insulin or insulin-like growth factor, IGF-I, triiodothyronine and transferrin, were able to undergo adipose conversion. Over ninety percent of the cells accumulated lipid droplets and this proportion was reduced in serum-supplemented medium. The adipose conversion was assessed by the development of lipoprotein lipase (LPL) and glycerol-3-phosphate dehydrogenase (GPDH) activities, [14C]glucose incorporation into polar and neutral lipids, triacylglycerol accumulation and lipolysis in response to isoproterenol. Similar results were obtained with stromal-vascular cells from rat subcutaneous and retroperitoneal adipose tissues. Stromal-vascular cells required no adipogenic factors in addition to the components of the serum-free medium. Insulin was required within a physiological range of concentrations for the emergence of LPL and at higher concentrations for that of GPDH. When present at concentrations ranging from 2 to 50 nM, IGF-I was able to replace insulin for the expression of both LPL and GPDH. The development of a serum-free, chemically defined medium for the differentiation of diploid adipose precursor cells opens up the possibility of characterizing inhibitors or activators of the adipose conversion process.

A preadipocyte clonal line from mouse brown adipose tissue. Short- and long-term responses to insulin and beta-adrenergics

A clonal cell line has been established from the interscapular brown adipose tissue (BAT) of the C57 BL/6J +/+ mouse. The line, designated BFC-1, is aneuploid and exhibits both morphological and biochemical properties characteristics of mature adipocytes. Adipose conversion begins after confluence and is accompanied by an early emergence of lipoprotein lipase; a later emergence of glycerol-3-phosphate dehydrogenase and acid: CoA ligase; an increase in the average triglyceride content. Adipose conversion, estimated by activities of enzyme markers, is enhanced at any given time by the continuous presence in the culture medium of insulin and triiodothyronine, both within their physiological range of concentrations. In addition to both hormones, chronic exposure of confluent cells to beta-adrenergics brings similar long-term effects on adipose conversion. The uptake of labelled 2-deoxyglucose by differentiated BFC-1 cells is stimulated by insulin; the half-maximum effect is observed at 1 nM insulin. Differentiated BFC-1 cells, in which endogenous triglycerides have been prelabelled on the fatty acid moiety, do respond to beta-adrenergics by releasing radioactive fatty acids. The agonist potency order and the EC50 value for each agonist are BRL 37344 (0.5 nM) greater than isoproterenol (1.5 nM) greater than norepinephrine (3 nM) greater than epinephrine (7 nM) greater than salbutamol (15 nM). The half-maximally and maximally effective concentrations of corticotropin to stimulate lipolysis are found to be 4 and 100 nM, respectively. The lipolytic response to isoproterenol is counteracted by prior addition of insulin or simultaneous addition of propranolol. Parallel studies performed on Ob17 cells, a clonal line established from mouse white adipose tissue (Négrel et al., Proc natl acad sci US 75 (1978) 6054), show that the agonist potency order and the EC50 value for each agonist are BRL 37344 (3 nM) greater than isoproterenol (10 nM) greater than norepinephrine (20 nM) greater than epinephrine (40 nM). Thus both BFC-1 cells and Ob17 cells show an atypical beta-adrenoreceptor similar to that described in rat adipocytes (Arch et al., Nature 309 (1984) 163), but the sensitivity of BFC-1 cells toward beta-agonists is found to be 6-fold higher than that of Ob17 cells. Thus the BFC-1 line represents a useful model for the study of short- and long-term responses to beta-adrenergics.

Regulation of the secretion of lipoprotein lipase by mouse macrophages

The regulation of the secretion of lipoprotein lipase was studied in primary cultures of mouse peritoneal macrophages and in the murine macrophage cell line J774. As previously reported, both cell types secrete a lipase with the characteristics of lipoprotein lipase. Incubation of macrophages with insulin, insulin-like growth factor, and L-thyroxine had no effect on lipoprotein lipase secretion. Incubation with dexamethasone and with several agents which increase intracellular cyclic AMP led to a decrease in lipoprotein lipase secretion by mouse peritoneal macrophages. These results suggest that the hormonal regulation of lipoprotein lipase in macrophages is different from that in adipose tissue and heart muscle. Incubation of the macrophages with heparin caused a marked increase in the secretion of lipoprotein lipase. Short incubations with heparin (5 min) caused a release of the enzyme into the media, while longer incubations caused a 2-8-fold increase in net lipoprotein lipase secretion which was maximal after 2-16 h depending on cell type, and persisted for 24 h. The effect of heparin was dose-dependent and specific (it was not duplicated by other glycosaminoglycans). The mechanism of heparin-induced increase in lipoprotein lipase secretion was explored. The increase was not caused by the release of a presynthesized intracellular pool of lipoprotein lipase or by the stabilization of lipoprotein lipase by heparin after secretion. The heparin-induced increase in lipoprotein lipase secretion was dependent on protein synthesis. The secretion of lipoprotein lipase by macrophages in response to low levels of heparin may be a significant factor in the formation of atherosclerotic lesions.

Regulation of lipoprotein lipase synthesis and 3T3-L1 adipocyte metabolism by recombinant interleukin 1

When fully differentiated 3T3-L1 adipocytes were exposed to purified, recombinant murine interleukin 1 (rIL-1), a dose-dependent suppression of lipoprotein lipase activity was observed. The loss of activity reached a maximum of 60-70% of control and appeared to be due to an effect on the synthesis of the enzyme as judged by a suppression of the ability to incorporate [35S]methionine into immunoprecipitable lipoprotein lipase. There was no general effect on protein synthesis as determined by radiolabel incorporation into acid precipitable protein; however, after a 17 h exposure of the 3T3-L1 cells to recombinant interleukin 1, the synthesis of two proteins (molecular weights, 19,400 and 165,000 daltons) was enhanced several-fold. When the effect of Il-1 on the major metabolic pathways of the adipocyte was investigated, lipolysis as measured by glycerol release from the cells was markedly enhanced after a 17 h incubation with the hormone, while no effect was observed on de novo fatty acid synthesis. These effects on the metabolism of the adipocytes occur at concentration on a basis of molecules per cell, similar (only a 3-fold difference) to those required for stimulation of [3H]thymidine incorporation into mouse thymocyte DNA, suggesting that IL-1 may be a physiologically significant effector of adipocyte metabolism.

Hyperlipidemia in mast cell-deficient W/WV mice

Approximately 70% of the W/WV mice lacking mast cells due to a genetic defect showed hypertriglyceridemia combined with hypercholesterolemia. Increases of various magnitudes in chylomicrons, very-low-density lipoprotein, and intermediate-density lipoprotein were observed in the plasma of W/WV mice compared to those in the plasma of congenic normal mice. The increase in these lipoproteins was seen even in normolipidemic W/WV mice. Activities of both lipoprotein lipase and hepatic triacylglycerol lipase in the plasma after heparin injection were markedly lower in the W/WV mice than in the congenic normal mice, although activities of both lipoprotein lipase in the heart and adipose tissue and hepatic triacylglycerol lipase in the liver were not decreased. These results suggest that the W/WV mice have genetic defects in one or more of the following: secretion of both lipases from their synthesising cells, transport to the endothelium, and anchoring to the endothelial surface. Heparin deficiency in these mice may be responsible for the impairment and, thereby, may partially contribute to the hyperlipidemia.

Effects of adrenaline on the turnover of lipoprotein lipase in rat adipose tissue

The mechanisms by which adrenaline brings about a reduction in the lipoprotein lipase activity of adipose tissue in vitro were investigated. The incorporation of [3H]leucine into lipoprotein lipase was measured during 1-h pulse incubations of rat epididymal fat bodies that had been preincubated for 4 h in the presence of glucose, insulin and dexamethasone. When adrenaline was added to the incubation medium at the start of the pulse, the incorporation of [3H]leucine was markedly reduced, suggesting that the rate of the enzyme's synthesis had decreased. On the other hand, the degradation of lipoprotein lipase, as measured by the loss of 3H-labelled enzyme protein during pulse-chase incubations of the epididymal fat bodies, was found to be significantly increased by the addition of adrenaline to the incubation medium at the start of the chase period. It is concluded that adrenaline is able both to inhibit the synthesis of lipoprotein lipase and to stimulate its degradation.

Beta-adrenergic stimulation enhances translocation, processing and synthesis of lipoprotein lipase in rat heart cells

Cells isolated from newborn rat hearts were cultured for 10-14 days, and lipoprotein lipase activity was present in an intracellular and heparin-releasable pool. Treatment of the cultures with 10(-7) M isoproterenol for 3 min resulted in a 3-fold increase in heparin-releasable lipoprotein lipase and a concomitant decrease in residual cellular enzyme activity. Similar results were obtained by treatment with dibutyryl cAMP. Treatment with isoproterenol or dibutyryl cAMP for 2 h affected glycosylation of immunoadsorbable lipoprotein lipase, so that the ratio of [3H]galactose to [14C]mannose in the heparin-releasable enzyme increased from 3.8 (control) to 13.0 (isoproterenol-treated). The change in the ratio of the sugars in the cellular fraction of the enzyme was from 3.1 to 9.9. 2 h treatment with isoproterenol did not enhance new enzyme synthesis, as determined by incorporation of [3H]leucine into immunoadsorbable lipoprotein lipase. 24 h after addition of either isoproterenol or dibutyryl cAMP to the culture medium, stimulation of enzyme synthesis was demonstrated. The present results permit three effects of isoproterenol on lipoprotein lipase to be distinguished: stimulation of translocation from a cellular to heparin-releasable pool; enhanced processing of mannose residues and terminal glycosylation; stimulation of synthesis of enzyme protein.

Transformation of Ob17 cells promotes proliferation and differentiation of Ob17 preadipocytes via distinct extracellular intermediates

Conditioned serum-free medium of Ob17 cells transformed by the middle-T-only gene of polyoma virus (Ob17MT cells) is able to support growth and adipose conversion of the parental Ob17 cells. Conditioned media from 3T3-F442A cells (untransformed preadipocyte clonal line) and MTT4 cells (middle-T-transformed non-preadipocyte clonal line) are inactive. The serum-free conditioned medium of Ob17MT cells is also active on growth and adipose conversion of 3T3-F442A cells. The morphological differentiation of Ob17 cells is accompanied by the expression of early (lipoprotein lipase, LPL) and late (glycerol-3-phosphate dehydrogenase, GPDH) biochemical markers of adipose conversion. Bio-Gel P-60 chromatography and SDS-PAGE have allowed characterization of a mitogenic fraction of apparent MW approximately equal to 28 Kd distinct from an adipogenic fraction of apparent MW less than 10 Kd. This adipogenic fraction is only required for the acquisition of the GPDH activity and is therefore active on terminal differentiation.

Characterization of rat adipose tissue lipoprotein lipase using a monospecific antibody

An antibody to a highly pure enzyme preparation was developed to facilitate detailed studies of rat adipose tissue lipoprotein lipase regulation. Lipoprotein lipase was purified by heparin-Sepharose affinity chromatography followed by preparative isoelectric focusing. The enzyme migrated as a single broad band on SDS disc gel and two-dimensional gel electrophoresis with an apparent molecular mass of 67 000 and 62 000 Da, respectively. The amino acid composition of the purified rat enzyme was virtually identical to that of bovine milk. A major protein component with no lipase activity co-eluted with the enzyme from the affinity column, but was separated by the isoelectric focusing step. The molecular mass was slightly lower (58 000 Da) but the amino acid composition of this protein was similar to that of the enzyme. An antibody raised against the purified rat enzyme was highly potent and was effective in inhibiting rat heart lipoprotein lipase, but not the salt-resistant hepatic lipase. Analysis of crude acetone-ether adipose tissue preparation on SDS slab polyacrylamide gel coupled to Western blotting revealed five protein bands = (62 000, 56 000, 41 700, 22 500, 20 000 Da). Similarly, following affinity purification by immunoadsorption, the purified antibody reacted with five equivalent protein bands. Fluorescent concanavalin A binding data indicated that the 56 kDa band is a glycosylated form of lipoprotein lipase. Pretreatment of adipose tissue with proteinase inhibitors revealed that the lower molecular mass proteins (41 700 and 20 000 Da) were degradation products of lipoprotein lipase, and the 22 500 Da band could be accounted for by non-specific binding.

Nutritional regulation of lipoprotein lipase in guinea pig tissues

Glucose transport in guinea pig adipocytes has been shown to be markedly resistant to stimulation by insulin. Lipoprotein lipase is another transport catalyst in adipose tissue which is believed to be regulated by insulin. We have therefore studied how feeding-fasting affects lipoprotein lipase activity in guinea pig tissues. There was an even more marked decrease in adipose tissue lipoprotein lipase activity on fasting in guinea pigs (10-20 fold) than in rats or mice (4-5 fold). In adipocytes, the activity decreased only 2.5-4.5 fold; most of the change was in extracellular lipoprotein lipase. On glucose refeeding, the activity was rapidly restored. In the first 4 hours after glucose administration extracellular lipoprotein lipase activity increased to more than 10 times the amount present in adipocytes. After cycloheximide, lipoprotein lipase activity decreased with a half-life of 22 min. It is concluded that lipoprotein lipase is rapidly produced and turned over in guinea pig adipose tissue, and that the system is quite sensitive to feeding-fasting. In contrast to adipose tissue, there was no significant change in lipoprotein lipase activity in any other tissue on fasting. There was a strong correlation between the activities in heart and diaphragm muscle, but this correlation was independent of feeding-fasting.

Maturation and secretion of lipoprotein lipase in cultured adipose cells. II. Effects of tunicamycin on activation and secretion of the enzyme

The effects of N-linked glycosylation on the activation and secretion of lipoprotein lipase were studied in Ob17 cells. The cells were first depleted of any activity and enzyme content by cycloheximide treatment and of precursors of oligosaccharide chains by tunicamycin. The repletion of lipoprotein lipase content was studied in these cells maintained in the presence of tunicamycin after cycloheximide removal. During the repletion phase, the EC50 values of inhibition by tunicamycin (approx. 0.2 microgram/ml) of the incorporation of labeled glucose, mannose or galactose into trichloroacetic acid-insoluble material were found to be identical. Under these conditions, the rate of protein synthesis was maximally decreased by 30%. The results showed clearly that the recovery in lipoprotein lipase activity was parallel to the recovery in hexose incorporation, no activity being recovered in the absence of glycosylation. An inactive form of lipoprotein lipase from tunicamycin-treated cells was detected by competition experiments with mature active lipoprotein lipase for the binding to immobilized antilipoprotein lipase antibodies, as well as by immunofluorescence staining. SDS-polyacrylamide gel electrophoresis and Western blots of cellular extracts and of extracellular media, obtained after tunicamycin-treated cells were exposed to heparin, revealed a single immunodetectable Mr 52 000 protein, whereas a single Mr 57 000 protein was detected in control cells. Therefore, the results indicate that the acquisition by lipoprotein lipase of a catalytically active conformation is linked directly or indirectly to glycosylation. Despite this lack of activation, the lipoprotein lipase molecule was able to migrate intracellularily and to undergo secretion after heparin stimulation of the tunicamycin-treated cells.

A continuous flow method for the study of lipoprotein lipase secretion in adipose cells

The secretion of lipoprotein lipase has been examined in Ob17 adipose cells. No spontaneous secretion is detected. The activity of the heparin-releasable enzyme shows a first-order process of inactivation. This constant rate of inactivation, coupled with a decreased rate of secretion, prevents any significant determination of enzyme secretion in heparin-containing media. Thus, a perifusion system, with which the rate of enzyme inactivation is minimal and systematic, has been devised and used. The data show that the secretion of a pool of pre-existing lipoprotein lipase molecules is followed by the secretion of newly synthesized enzyme molecules. The results are discussed with respect to the significance of the determinations of the heparin-releasable enzyme in most studies as well as with respect to the intracellular localization of lipoprotein lipase in Ob17 cells.

Intracellular localization of lipoprotein lipase in adipose cells

Subcellular localization of lipoprotein lipase has been examined in differentiated Ob17 adipose cells. No patent activity is detectable in carefully homogenized cells. All latent activity can be unmasked by disrupting membrane structures with neutral detergents. The sequestration of lipoprotein lipase in closed membrane structures is supported by experiments of immunotitration with anti-lipoprotein lipase antibodies and by experiments showing a full protection of the masked activity against proteolytic attack by trypsin. The intracellular distribution of lipoprotein lipase investigated by immunofluorescence staining and by isopycnic centrifugation indicates that a large proportion of the enzyme is located in the Golgi apparatus, in which the activation of the enzyme is likely to take place (C. Vannier et al. (1985) J. Biol. Chem. 260, 4424-4431). Altogether, the results are in favor of a localization of lipoprotein lipase in adipose cells as being typical of that of a secretory protein and underline the absence of lipoprotein lipase in the cell cytoplasm.

Fetuin modulates growth and differentiation of Ob17 preadipose cells in serum-free hormone-supplemented medium

A serum-free hormone-supplemented medium able to support the growth of rodent adipose precursor cells has been used to characterize additional components from serum required for the differentiation of preadipose Ob17 cells into adipose-like cells. Fetuin is shown to behave as a growth-promoting agent for these cells. In addition to growth hormone, triiodothyronine and a low-molecular weight component(s) also purified from serum, fetuin is required for the full expression of the differentiation program. Other serum proteins as well as other mitogenic factors are unable to substitute for fetuin. A possible role of fetuin in the development of adipose tissue is discussed.