Lipoprotein lipase activity in cultured mesenchymal rat heart cells Part 5. effect on enzyme activity of the glycosylation inhibitors, 2-deoxyglucose and tunicamycin

The effect of individual N-glycans on enzyme activity

In a series of investigations, N-glycosylation has proven to be a key determinant of enzyme secretion, activity, binding affinity and substrate specificity, enabling a protein to fine-tune its activity. In the majority of cases elimination of all putative N-glycosylation sites of an enzyme results in significantly reduced protein secretion levels, while removal of individual N-glycosylation sites often leads to the expression of active enzymes showing markedly reduced catalytic activity, with the decreased activity often commensurate with the number of glycosylation sites available, and the fully deglycosylated enzymes showing only minimal activity relative to their glycosylated counterparts. On the other hand, several cases have also recently emerged where deglycosylation of an enzyme results in significantly increased catalytic activity, binding affinity and altered substrate specificity, highlighting the very unique and diverse roles that individual N-glycans play in regulating enzyme function.

Regulation of lipoprotein lipase by dibutyryl cAMP, cholera toxin, Hepes and heparin in F1 heart-cell cultures

Regulation of lipoprotein lipase was studied in mesenchymal rat heart-cell cultures. Treatment of the cultures with dibutyryl cyclic AMP or with cholera toxin resulted in an increase in LPL activity and a comparable increase in LPL mRNA. When the cells were exposed to 100 mM Hepes for 24 h, total enzyme activity rose 2-fold and LPL mRNA increased 2.4-fold. After 72 h, there was a 3-fold increase in LPL mRNA and a 4-fold rise in cellular LPL activity, while medium activity increased 20-fold. Exposure of the cultures to heparin for 24 h resulted in a 3.2-fold increase in total activity and a 36-fold increase in medium activity. This increase was not accompanied by any rise in LPL mRNA. Addition of actinomycin D to control dishes for 24 h resulted in a 33% reduction in LPL mRNA and a 43% reduction in enzyme activity. These values were 71% and 56%, respectively, in Hepes-treated cells, indicating that no stabilization of LPL mRNA occurred under these conditions. It can be concluded that in mesenchymal rat heart-cells in culture cAMP and cholera toxin upregulate lipoprotein lipase at the level of transcription. The increase in LPL activity after 24 h exposure to Hepes could be compatible with transcriptional regulation, while exposure to heparin is not accompanied by a change in LPL mRNA.

Intracellular regulation of lipoprotein lipase in human monocyte-derived macrophages

The intracellular pathway of lipoprotein lipase (LPL) has been examined in human monocyte-derived macrophages in culture. These cells were previously shown to synthesize and constitutively secrete LPL. The secretion is dependent on new enzyme synthesis. 6-d-old human monocytes have stores of mRNA for linear release of LPL up to 24 h. Enzyme activity in cells and in culture medium was almost completely inhibited by 24 h treatment with tunicamycin, an inhibitor of glycosylation. In monensin-treated cells a pronounced increase in enzyme activity was found, whereas the secreted activity was markedly reduced. This indicates that LPL in human monocytes is processed through a pH sensitive part of the Golgi complex and that the terminal glycosylation is not needed for the expression of its catalytic activity. Our results suggest that lysosomal function is not important in secretion of the enzyme, whereas vesicular transport seem to be involved in regulating LPL in human monocyte-derived macrophages in culture.

Increasing effect of vanadate on lipoprotein lipase activity in isolated rat fat pads

Vanadate increased lipoprotein lipase (LPL) activity in the isolated fat pads in a time- and dose-dependent manner. The increasing effect of vanadate was inhibited by amiloride, similar to that of insulin, and it also was not additive to that of insulin. Although the increasing effects of vanadate and insulin were preserved in K(+)-free medium, appreciable decreases in both effects were observed by replacement of Na+ with choline ion or omission of Ca2+ in the medium. Vanadate showed the full effect in the presence of cycloheximide at concentrations that inhibited protein synthesis of the fat pads, suggesting that the action of vanadate is not due to the increase in protein synthesis. Tetrakis (acetoxymethyl) ester of quin 2 at 50 microM concentration never inhibited the action of vanadate though it showed a little inhibition at a concentration of 300 microM. No inhibition of the action of vanadate was observed with ruthenium red. These results suggest that vanadate increases the LPL activity via a process less sensitive to the intracellular Ca2+ concentration. Adrenaline, dibutyryl cyclic AMP, and 3-isobutyl-1-methylxanthine all inhibited the action of vanadate, suggesting that the action is inhibited with increase in the intracellular concentration of cyclic AMP. Monensin and carbonyl cyanide m-chlorophenylhydrazone inhibited the action of vanadate. In contrast, the action of insulin was never inhibited by monensin. Tunicamycin and 2-deoxyglucose, at rather high concentrations, inhibited both actions. These findings suggest that vanadate increases the LPL activity through mechanisms of action involving amiloride- and monensin-sensitive pathways dependent on energy.

Rapid modulation of rat adipocyte lipoprotein lipase: effect of calcium, A23187 ionophore, and thrombin

The effect of calcium ions on the lipoprotein lipase (LPL) activity in rat adipocytes has been investigated. Incubation of the cells in the absence of extracellular calcium produced a rapid decline of LPL activity in the cells. The enzyme, however, could be immediately reactivated in less than 3 min by the addition of calcium. The degree of reactivation was proportional to the concentration of extracellular calcium. alpha 1 agonists phenylephrine and methoxamine affected LPL activity only slightly, as did vasopressin and angiotensin II. In contrast, calcium ionophore A23187 elicited a quick and transient enzyme activation which reached its peak 4 min after the addition of the drug. Thrombin (0.1 U/ml) produced the most rapid and intense response. The effect of thrombin was already evident 10 s after its addition, and the enzyme activity almost doubled above the basal level. Extracellular calcium was necessary to achieve thrombin activation. Contrary to previous thought, these data support the conclusion that LPL may undergo rapid activation, and that calcium ions are critically involved in this activation process. Thrombin rapidly raises LPL activity and may be one of its physiological activators in vivo.

Tunicamycin-treated rat heart cell cultures synthesize an inactive nonreleasable lipoprotein lipase

Cells isolated from newborn rat hearts were cultured in the presence of 100 mM Hepes (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid). Lipoprotein lipase activity was present in an intracellular and heparin-releasable pool and was also secreted into the culture medium. Treatment of the cultures with 5 micrograms/ml tunicamycin caused almost complete loss of lipoprotein lipase activity in all three compartments. In control cultures, immunoblotting of lipoprotein lipase derived from all three pools revealed a single band of lipoprotein lipase with an apparent Mr of 56,000. In the tunicamycin-treated cultures, the enzyme appeared only intracellularly and had an apparent Mr of 49,000. No immunoreactive enzyme was found in the medium. Thus, glycosylation of lipoprotein lipase in heart cell cultures is mandatory for enzyme activity and translocation from an intracellular to the heparin-releasable pool and for secretion into the medium.

Enhanced release and synthesis of lipoprotein lipase in rat heart cell cultures exposed to high concentrations of Hepes

While attempting to optimize conditions for synthesis of lipoprotein lipase by cultured heart cells, we encountered an unexpected rise in enzyme activity when media were supplemented inadvertently with 100 mM Hepes buffer (4-(2-hydroxyethyl)-1-piperazineethanesulphonic acid). This finding was further investigated and optimal results were obtained at pH 7.0-7.2. The increase in lipoprotein lipase activity was time dependent; after 3-6 h there was a rise in medium activity but cellular activity increased only after 24 h. The increased enzyme activity was defined as lipoprotein lipase by inhibition with antiserum to rat adipose tissue lipoprotein lipase. A 72-h exposure to Hepes resulted in a 30% increase in the incorporation of [35S]methionine into cellular proteins and a 2-fold increase into heparin-releasable proteins. Using heparin Sepharose chromatography and stepwise elution, a lipoprotein lipase enriched fraction was recovered with 2 M NaCl. The amount of [35S]methionine and [3H]galactose incorporated into protein of this fraction derived from Hepes-treated cells was 2-6-fold that of controls. A 4-fold increase in cellular lipoprotein lipase mass in Hepes-treated cells was shown by immunoblotting. Results obtained with Hepes-conditioned medium suggest the presence of cell-derived compounds that enhance release and subsequent synthesis of lipoprotein lipase. The effect of Hepes-conditioned medium on lipoprotein lipase resembled to some extent that of the addition of heparin. Therefore, it appears that when Hepes is first added to the culture medium, it might promote a release of heparan sulfate or related compounds, possibly by virtue of its negatively charged sulfonic acid residue. The accumulated heparan sulfate could then promote a sustained release of lipoprotein lipase into the culture medium which in turn leads to increased enzyme synthesis.

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.

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.

Importance of the different steps of glycosylation for the activity and secretion of lipoprotein lipase in rat preadipocytes studied with monensin and tunicamycin

Lipoprotein lipase synthesized by cultured rat preadipocytes is present in three compartments: an intracellular, a surface-related 3-min heparin-releasable, and that secreted into the culture medium. 30 min after addition of 6 microM monensin, the lipoprotein lipase activity in the heparin-releasable compartment starts to decrease; by 4 h of monensin treatment the lipoprotein lipase activity in the heparin-releasable pool and in the culture medium is about 10% of that found in control dishes. The intracellular activity, which had been identified as lipoprotein lipase by an antiserum to lipoprotein lipase, increases slowly and doubles by 24 h. However, since the cellular compartment accounts for 10-25% of total activity, this increase does not account for the missing enzyme activity. To determine whether this enzyme molecule is synthesized but is not active, incorporation of labeled leucine, mannose and galactose into immunoadsorbable lipoprotein lipase was studied in control, monensin- or tunicamycin-treated cells. Addition of tunicamycin (5 micrograms/ml) for 24 h caused a 30-50% reduction in immunoadsorbable lipoprotein lipase, but the enzyme activity was reduced by 90%. On the other hand, 4 h monensin treatment reduced both incorporation of [3H]leucine into immunoadsorbable lipoprotein lipase and heparin-releasable and medium lipoprotein lipase activity by 57 to 77%. The immunoadsorbable lipoprotein lipase in the intracellular compartment has a [14C]mannose to [3H]galactose ratio of 0.15 and this ratio increased 6-fold in monensin-treated cells. The intracellular lipoprotein lipase in monensin-treated cells had the same affinity for both the native and synthetic substrate as the lipoprotein lipase in control cells, yet its spontaneous secretion into the culture medium and its release by 3 min heparin treatment was markedly decreased. The present results indicate that: the presence of asparagine-linked oligosaccharide (formation of which is inhibited by tunicamycin) is mandatory for the expression of lipoprotein lipase activity; lipoprotein lipase is active also in a high mannose form; and terminal glycosylation and oligosaccharide processing, which is inhibited by monensin, may be important for the appearance of heparin-releasable lipoprotein lipase and secretion of lipoprotein lipase into the medium.

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.

Effects of hormones, amino acids and specific inhibitors on rat heart heparin-releasable lipoprotein lipase and tissue neutral lipase activities during long-term perfusion

Rat hearts were perfused for long periods in the presence of 14C-labeled amino acids. From these hearts, postheparin-effluent and a tissue homogenate containing lipoprotein lipase and neutral lipase, respectively, were derived. Lipolytic activity and 14C-labeled protein in both preparations were characterized by affinity chromatography, immunoprecipitation and SDS-polyacrylamide gel electrophoresis. Lipase activity and 14C-labeled protein co-eluted from heparin-Sepharose 4B at 1.2 M NaCl and were inhibited and precipitated by preincubation with anti-lipoprotein lipase gamma-globulins. Gel electrophoresis of both preparations showed the presence of 14C-labeled protein with a molecular weight of 35 000. These data strongly suggest similarity between lipoprotein lipase and neutral lipase and their possible precursor-product relationship and indicate that during perfusion continuous synthesis, secretion and vascular binding of lipase molecules occur. Cycloheximide perfusion induced a dramatic decrease of lipoprotein lipase and neutral lipase activity, indicating a half-life of less than 90 min for both enzymes. Tunicamycin present during perfusion also induced a drop in lipoprotein lipase and tissue neutral lipase activity, indicating that glycosylation is necessary for secretion of lipoprotein lipase. Long-term perfusion of rat hearts in the presence of norepinephrine, glucagon or tyrosine leads to reciprocal alterations in lipoprotein lipase and neutral lipase activities, i.e., lipoprotein lipase activity increased and neutral lipase activity decreased, whereas total lipase activity (lipoprotein lipase + neutral lipase) remained unaltered. During perfusion in the presence of insulin, no net change in lipase activities was observed. Also, insulin did not affect the glucagon-induced inverse effects on either lipase activity. The reciprocal changes in lipase activities occurring during norepinephrine perfusion were hampered by colchicine and propranolol, pointing towards beta-receptor and microtubular mediation of tissue lipase processing and endothelial binding. Our data suggest that the tissue flux and vascular binding of lipase protein may be important sites of hormonal regulation of lipoprotein lipase homeostasis.

Modulation of lipoprotein lipase activity in cultured rat mesenchymal heart cells and preadipocytes by dibutyryl cyclic AMP, cholera toxin and 3-isobutyl-1-methylxanthine

We have compared the effects of cellular cyclic AMP modulation on the regulation of lipoprotein lipase in cultures of rat epididymal pad preadipocytes and mesenchymal heart cells. Addition of dibutyryl cyclic AMP (dibutyryl cAMP) or 3-isobutyl-1-methylxanthine (IBMX) to preadipocytes grown in serum-containing culture medium resulted in a progressive decrease in lipoprotein lipase activity released into the culture medium so that at 6-8 h enzyme activity ranged between 20 and 30% of that recovered in the control dishes. Similar short-term (6-8 h) studies of the heart cell cultures showed a variable and much less pronounced depression of lipoprotein lipase activity. Thus, following dibutyryl cAMP and IBMX treatment, lipoprotein lipase activity ranged between 70 and 95% of control values. Incubation for 6 h with cholera toxin was followed by a 4-fold rise in the concentration of cellular cyclic AMP in both types of culture, but while in heart cell cultures enzyme activity was unchanged, lipoprotein lipase activity in preadipocytes decreased to 30% of control value. After 24 h incubation with all three effectors, an increase in lipoprotein lipase activity was seen. In the preadipocytes the increase ranged between 50 and 150% above control value, in the heart cell cultures it was 100-250%. 24-h incubation of heart cell cultures with dibutyryl cAMP resulted in a 6-fold increase of heparin-releasable lipoprotein lipase activity while residual activity was doubled. The rise in surface-bound lipoprotein lipase was evidenced also by an increase in the lipolysis of chylomicron triacylglycerol. In the presence of cycloheximide, the dibutyryl cAMP-induced heparin-releasable and residual lipoprotein lipase activity declined at the same rate as the basal activity. The reason for the difference in response of cultured preadipocytes and heart cells to the effectors during the first 8 h of incubation has not been elucidated, but could be related to a possible absence of hormone-sensitive lipase in the heart cells, and hence in a difference in intracellular metabolism of triacylglycerol. On the other hand, a common mechanism can be postulated for the long-term effect of cyclic AMP on the induction of lipoprotein lipase activity in both types of cultures. It probably involves mRNA and protein synthesis, which culminates in an increase in enzyme activity.