Glycosylation, activity and secretion of lipoprotein lipase in cultured brown adipocytes of newborn mice. Effect of tunicamycin, monensin, 1-deoxymannojirimycin and swainsonine

Physiological response of mussel to rayon microfibers and PCB's exposure: Overlooked semi-synthetic micropollutant?

Rayon microfibers, micro-sized semi-synthetic polymers derived from cellulose, have been frequently detected and reported as "micropollutants" in marine environments. However, there has been limited research on their ecotoxicity and combined effects with persistent organic pollutants (POPs). To address these knowledge gaps, thick-shell mussels (Mytilus coruscus) were exposed to rayon microfibers at 1000 pieces/L, along with polychlorinated biphenyls (PCBs) at 100 and 1000 ng/L for 14 days, followed by a 7-day recovery period. We found that rayon microfibers at the environmentally relevant concentration exacerbated the irreversible effects of PCBs on the immune and digestive systems of mussels, indicating chronic and sublethal impacts. Furthermore, the results of 16 s rRNA sequencing demonstrated significant effects on the community structure, species richness, and diversity of the mussels' intestinal microbiota. The branching map analysis identified the responsive bacteria to rayon microfibers and PCBs belonging to the Proteobacteria, Actinobacteriota, and Bacteroidota phyla. Despite not being considered a conventional plastic, the extensive and increasing use of rayon fibers, their direct toxicological effects, and their interaction with POPs highlight the need for urgent attention, investigation, and regulation to address their contribution to "micropollution".

Sex-specific digestive performance of mussels exposed to warming and starvation

As global climate change has dramatically impacted the ocean, severe temperature elevation and a decline in primary productivity has frequently occurred, which has affected the structure of coastal biomes. In this study, the sex-specific responses to temperature change and food availability in mussels were determined in terms of digestive performance. The thick-shelled mussels Mytilus coruscus (male and female) were exposed to different temperature and nutritional conditions for 30 days. The results showed that the digestive enzymes of mussels were significantly affected by temperature, food, sex, and their interactions. High temperature (30°C) and starvation significantly decreased amylase, lysozyme, and pepsase activities of female mussels, while trypsin and trehalase did not change significantly at the experimental end. The activity of amylase, trypsin, and trehalase was significantly reduced in males at high temperature (30°C) under starvation treatment, but high temperature (30°C) elevated pepsase. Unsurprisingly, starvation caused the reduction of lysozyme and pepsase under 25°C in males. Amylase, lipase, and trehalase were higher in female mussels compared with males, while the enzymatic activities of lysozyme, pepsase, and trypsin were higher in male mussels than females. Principal component analysis showed that different enzyme activity indexes were separated in male and female mussels, indicating that male and female mussels exhibited significantly different digestive abilities under temperature and food condition change. The study clarified sex-specific response difference in mussel digestive enzymes under warming and starvation and provided guidance for the development of mussel aquaculture (high temperature management and feeding strategy) under changing marine environments.

Abrogation of Insulin-like Growth Factor-I (IGF-I) and Insulin Action by Mevalonic Acid Depletion

The vasculoprotective effects of hydroxy-3-methylglutaryl (HMG)-CoA reductase inhibitors (statins) correlate with cholesterol lowering. HMG-CoA reductase inhibitors also disrupt cellular processes by the depletion of isoprenoids and dolichol. Insulin and insulin-like growth factor (IGF) signaling appear particularly prone to such disruption as intracellular receptor processing requires dolichol for correct N-glycosylation, whereas downstream signaling through Ras requires the appropriate prenylation (farnesol). We determined how HMG-CoA reductase inhibition affected the mitogenic effects of IGF-I and metabolic actions of insulin in 3T3-L1 cells and examined the respective roles of receptor glycosylation and Ras prenylation. IGF-I- and insulin-induced proliferation was significantly reduced by all statins tested, although cerivastatin (10 nm) had the greatest effect (p < 0.005). Although inhibitors of Ras prenylation induced similar results (10 microm FTI-277 89% +/- 7.4%, p < 0.01), the effect of HMG-CoA reductase inhibition could only be partially reversed by farnesyl pyrophosphate refeeding. Treatment with statins resulted in decreased membrane expression of receptors and accumulation of proreceptors, suggesting disruption of glycosylation-dependent cleavage. Glycosylation inhibitors inhibited IGF-I-induced proliferation (tunicamycin p < 0.005, castanospermine p < 0.01, deoxymannojirimycin p < 0.01). High concentrations of statin were necessary to impair insulin-mediated glucose uptake (300 nm = 33% +/- 12% p < 0.05), and this process was not effected by farnesyl transferase inhibition. Gycosylation inhibitors mimicked the effect of statin treatment (tunicamycin p < 0.001, castanospermine p < 0.05, deoxymannojirimycin p < 0.05), and there was insulin proreceptor accumulation. These data imply that HMG-CoA reductase inhibitors disrupt IGF-I signaling by combined effects on Ras prenylation and IGF receptor glycosylation, whereas insulin signaling is only affected by disrupted receptor glycosylation.

Calreticulin promotes folding/dimerization of human lipoprotein lipase expressed in insect cells (sf21)

Lipoprotein lipase (LPL) is a non-covalent, homodimeric, N-glycosylated enzyme important for metabolism of blood lipids. LPL is regulated by yet unknown post-translational events affecting the levels of active dimers. On co-expression of LPL with human molecular chaperones, we found that calreticulin had the most pronounced effects on LPL activity, but calnexin was also effective. Calreticulin caused a 9-fold increase in active LPL, amounting to about 50% of the expressed LPL protein. The total expression of LPL protein was increased less than 20%, and the secretion rates for active and inactive LPL were not significantly changed by the chaperone. Thus, the main effect was an increased specific activity of LPL both in cells and media. Chromatography on heparin-Sepharose and sucrose density gradient centrifugation demonstrated that most of the inactive LPL was monomeric and that calreticulin promoted formation of active dimers. Higher oligomers of inactive LPL were present in cell extracts, but only monomers and dimers were secreted to the medium. Interaction between LPL and calreticulin was demonstrated, and the effect of the chaperone was prevented by castanospermine, an inhibitor of N-glycan glucose trimming. Our data indicate an important role of endoplasmic reticulum-based chaperones for the folding/dimerization of LPL.

VLDL-induced triglyceride accumulation in human macrophages is mediated by modulation of LPL lipolytic activity in the absence of change in LPL mass

Mixed dyslipidemia of phenotype IIB is characterized by elevated levels of very low density lipoprotein (VLDL)-1 and VLDL-2 subfractions and of low density lipoprotein (LDL), which are associated with premature formation of atherosclerotic plaques, characterized by the presence of lipid-rich macrophage foam cells. Lipoprotein lipase (LPL) is a key factor in mediating macrophage lipid accumulation and foam-cell formation from native VLDL particles. The action of macrophage-derived LPL in the induction of intracellular lipid accumulation from triglyceride-rich lipoprotein (TRL) subfractions (VLDL-1, VLDL-2) is, however, indeterminate, as is the potential role of VLDL-1 and VLDL-2 in modulating macrophage LPL expression. We evaluated the role of LPL in the interaction of type IIB VLDL-1 and VLDL-2 with human macrophages. Both VLDL-1 and VLDL-2 subfractions induced significant accumulation of triglyceride (9.8-fold, P<0.0001, and 4.8-fold, P<0.0001, respectively) and of free cholesterol content (1.4-fold, P<0.001, and 1.2-fold, P=0.02, respectively). Specific inhibition (90%) of the lipolytic activity of endogenous LPL by tetrahydrolipstatin (THL) in the presence of VLDL-1 or VLDL-2 resulted in marked reduction in cellular loading of both triglycerides (-89%, P=0.008, and -89%, P=0.015, respectively) and free cholesterol (-76%, P=0.02, and -55%, P=0.06 respectively). Furthermore, VLDL-1 and VLDL-2 induced marked increase in macrophage-derived LPL enzyme activity (+81%, P=0.002, and +45%, P=0.02), but did not modulate macrophage-derived LPL mRNA and protein expression; consequently, LPL specific activity was significantly increased from 1.6 mU/microg at baseline to 4.1 mU/microg (P=0.01) and 3.1 mU/microg (P=0.05), in the presence of VLDL-1 and VLDL-2, respectively. We conclude that type IIB VLDL-1 and VLDL-2 induce triglyceride accumulation in human monocyte-macrophages primarily via the lipolytic action of LPL, which may involve stabilization and activation of the macrophage-secreted enzyme, rather than via modulation of enzyme production.

Active-dimeric form of lipoprotein lipase increases in the adipose tissue of patients with rheumatoid arthritis treated with prednisolone

The synthesis, activity and mass of LPL in adipose tissue were studied in patients with rheumatoid arthritis (RA) treated with prednisolone (PSL) (PSL-treated group) and untreated patients with osteoarthritis (untreated group). LPL activity and mass in the extracts of acetone/ether powder of adipose tissue were 2.4 and 1.6 times, respectively, higher in the PSL-treated group than in the untreated group. There were no differences in the amount of 35S incorporated into LPL during the 2-h incubation of adipose tissue with [35S]methionine between PSL-treated and untreated groups. These results indicate that degradation of LPL was inhibited in the adipose tissue of the PSL-treated group. In the adipose tissue of the untreated group, 72% of the LPL was the inactive-monomeric form, which was eluted with 0.4-0.75 M NaCl from the heparin-Sepharose column, and 28% was the active-dimeric form, which was eluted with 0.8-1.2 M NaCl. In the adipose tissue of the PSL-treated group, 40% was inactive-monomeric, and 60% was active-dimeric. Thus, the relative amount of the active-dimeric form of LPL was increased in the adipose tissue of the PSL-treated group. Taken together, our present results indicate that the higher level of LPL activity in the PSL-treated group was a result of the inhibition of the degradation of the active-dimeric form.

Synthesis of active high mannose-type lipoprotein lipase in human adipose tissues

Lipoprotein lipase (LPL) activity in the retroperitoneal adipose tissue of a patient with Cushing's syndrome and in the subcutaneous adipose tissue of a patient with aseptic necrosis of the femoral head was higher than that in the corresponding tissues of the control subjects. The amount of [35S]methionine incorporated into LPL was also higher in these patients than in control subjects. However, the ratio of activity and amount of radioactivity in the LPL of patients was identical to that of control subjects, indicating that LPL synthesized in the adipose tissues of patients had a normal specific activity. LPL with Mr = 57000 was composed of two types of subunits: one type was partially endo H-sensitive, yielding a product with Mr = 55000, and the other was totally endo H-sensitive, yielding a product with Mr = 52000. Both retroperitoneal and subcutaneous adipose tissues of control subjects contained nearly equal amounts of partially and totally endo H-sensitive subunits. In the retroperitoneal adipose tissue of a patient with Cushing's syndrome, 8% of subunits were partially endo H-sensitive and 92% were totally endo H-sensitive. In the subcutaneous adipose tissue of a patient with aseptic necrosis of the femoral head, 21% of subunits were partially endo H-sensitive and 79% were totally endo H-sensitive. The 24-h treatment of subcutaneous adipose tissue of a control subject with 1 mM 1-deoxymannojirimycin (dMM) caused the synthesis of active, but totally endo H-sensitive, LPL. Thus, in human adipose tissue, the processing of one oligosaccharide chain of an LPL subunit to a complex type chain in the trans Golgi was not necessary for the expression of activity.

Intracellular activation of rat hepatic lipase requires transport to the Golgi compartment and is associated with a decrease in sedimentation velocity

Hepatic lipase (HL) is an N-glycoprotein that acquires triglyceridase activity somewhere during maturation and secretion. To determine where and how HL becomes activated, the effect of drugs that interfere with maturation and intracellular transport of HL protein was studied using freshly isolated rat hepatocytes. Carbonyl cyanide m-chlorophenyl hydrazone (CCCP), castanospermine, monensin, and colchicin all inhibited secretion of HL without affecting its specific enzyme activity. The specific enzyme activity of intracellular HL was decreased by 25-50% upon incubation with CCCP or castanospermine, and increased 2-fold with monensin and colchicin. Glucose trimming of HL protein was not affected by CCCP, as indicated by digestion of immunoprecipitates with jack bean alpha-mannosidase. Pulse labeling experiments with [(35)S]methionine indicated that conversion of the 53-kDa precursor to the 58-kDa form, nor the development of endoglycosidase H-resistance, were essential for acquisition of enzyme activity. In sucrose gradients, HL protein from secretion media sedimented as a homogeneous band of about 5.8 S, whereas HL protein from the cell lysates migrated as a broad band extending from 5.8 S to more than 8 S. With both sources, HL activity was exclusively associated with the 5.8 S HL protein form. We conclude that glucose trimming of HL protein in the endoplasmic reticulum is not sufficient for activation; full activation occurs during or after transport from the endoplasmic reticulum to the Golgi and is associated with a decrease in sedimentation velocity.

A recessive deletion in the GlcNAc-1-phosphotransferase gene results in peri-implantation embryonic lethality

Formation of the dolichol oligosaccharide precursor is essential for the production of asparagine- (N-) linked oligosaccharides (N-glycans) in eukaryotic cells. The first step in precursor biosynthesis requires the enzyme UDP-GlcNAc: dolichol phosphate N-acetylglucosamine-1-phosphate transferase (GPT). Without GPT activity, subsequent steps necessary in constructing the oligosaccharide precursor cannot occur. Inhibition of this biosynthetic step using tunicamycin, a GlcNAc analog, produces a deficiency in N-glycosylation in cell lines and embryonic lethality during preimplantation development in vitro, suggesting that N-glycan formation is essential in early embryogenesis. In exploring structure-function relationships among N-glycans, and since tunicamycin has various reported biochemical activities; we have generated a germline deletion in the mouse GPT gene. GPT mutant embryos were analyzed and the phenotypes obtained were compared with previous studies using tunicamycin. We find that embryos homozygous for a deletion in the GPT gene complete preimplantation development and also implant in the uterine epithelium, but die shortly thereafter between days 4-5 postfertilization with cell degeneration apparent among both embryonic and extraembryonic cell types. Of cells derived from these early embryos, neither trophoblast nor embryonic endodermal lineages are able to survive in culture in vitro. These results indicate that GPT function is essential in early embryogenesis and suggest that N-glycosylation is needed for the viability of cells comprising the peri-implantation stage embryo.

Effect of long-term treatment of 3T3-L1 adipocytes with chlorate on the synthesis, glycosylation, intracellular transport and secretion of lipoprotein lipase

Lipoprotein lipase (LPL) is synthesized and glycosylated in the endoplasmic reticulum (ER), transported through the Golgi to the cell surface, and finally secreted. To examine the role of heparan sulphate proteoglycans (HSPG) in the synthesis, activity, intracellular transport and secretion of LPL, 3T3-L1 adipocytes were cultured for 7 days in the presence of 20 mM chlorate, an inhibitor of sulphation of HSPG. Treatment of cells with 20 mM chlorate for 7 days caused a 55% decrease in LPL activity in the intracellular compartment and a 79% decrease in the cell-surface compartment. The synthetic rate of LPL in chlorate-treated cells was identical with that in control cells as determined by biosynthetic labelling. The study with endoglycosidase H (endo H) showed that the treatment with chlorate increased the proportion of LPL subunits which were totally endo H-sensitive. The study with a heparin-Sepharose column showed that 3T3-L1 adipocytes contained three forms of LPL. The first form, accounting for 35% of the LPL, did not bind to the heparin-Sepharose column and had little or no activity; the second form, accounting for 32%, bound to the column and was eluted with 0.4-0.75 M NaCl but had no activity; the third form, accounting for 33%, bound to the column and was eluted with 0.8-1.2 M NaCl and had activity. In chlorate-treated cells, the first form accounted for 66% of the LPL, the second form 15% and the third form 19%. When cells were incubated for 1 h with brefeldin A, which translocates Golgi proteins to the ER [J. Lippincott-Schwartz, L.C. Yuan, J.S. Banifacino and R.D. Klausner (1989) Cell 56, 801-813; J. Lippincott-Schwartz, J. Glickman, J.E. Donaldson, J. Robbins, T.E. Kreis, K.B. Seamon, M.P. Sheetz and R.D. Klausner (1991) J. Cell Biol. 112, 567-577], the chlorate-induced decrease in cellular LPL activity was restored. These findings indicate that LPL synthesized in chlorate-treated cells can be processed to be fully active, but chlorate-treated cells are unable to transport LPL to the Golgi and accumulate inactive LPL with a lower affinity for heparin in the ER. The treatment with chlorate decreased the proportion of LPL subunits that were endo H-resistant, indicating that the processing of oligosaccharide chains of LPL in the trans-Golgi was impaired in chlorate-treated cells. The amount of 35S-labelled LPL secreted by chlorate-treated cells was identical with that secreted by control cells, whereas the level of LPL activity in the medium of chlorate-treated cells was 25% of that in the medium of control cells, indicating that most of the LPL secreted by chlorate-treated cells was inactive.

Effect of carbonyl cyanide m-chlorophenylhydrazone (CCCP) on the dimerization of lipoprotein lipase

Lipoprotein lipase (LPL), an enzyme playing the central role in triglyceride metabolism, is a glycoprotein and a homodimer of identical subunits. Dimerization and proper processing of oligosaccharide chains are important maturation steps in post-translational regulation of enzyme activity. Indirect evidences suggest that dimerization of LPL occurs in endoplasmic reticulum (ER) or Golgi. In this study, we investigated the dimerization status of LPL in 3T3-L1 adipocytes, using sucrose density gradient ultracentrifugation and carbonyl cyanide m-chlorophenylhydrazone (CCCP), an inhibitor of ER-Golgi protein transport. In the presence of CCCP, no increase of cellular LPL activity was detected during 2 b of recovery period after the depletion of LPL, with heparin and cycloheximide. Only endoglycosidase H (endo H)-sensitive subunits were found in CCCP-treated cells after endo H digestion, suggesting that inactive LPL was retained in ER. In the presence of castanospermine, an inhibitor of ER glucosidase I, LPL subunits of both control and CCCP-treated cells had same molecular weight, indicating that complete oligosaccharides were transferred to LPL subunits in the presence of CCCP. In sucrose density gradient ultracentrifugation, all the LPL protein synthesized in the presence of CCCP was found at the dimeric fractions as in control cells. Most of LPL protein in control cells showed high affinity for heparin, and there was no difference between the control and CCCP-treated cells. These results suggest that dimerization and acquisition of high affinity for heparin of LPL can occur in ER of CCCP-treated cells without acquisition of catalytic activity.

Brefeldin A enables synthesis of active lipoprotein lipase in cld/cld and castanospermine-treated mouse brown adipocytes via translocation of Golgi components to endoplasmic reticulum

Brown adipocytes cultured from newborn combined-lipase-deficient (cld/cld) mice and castanospermine (CST)-treated 3T3-L1 adipocytes synthesize lipoprotein lipase (LPL) which is inactive and retained in the endoplasmic reticulum (ER) [Masuno, Blanchette-Mackie, Chernick and Scow (1990) J.Biol. Chem. 265, 1628-1638; Masuno, Blanchette-Mackie, Schultz, Spaeth, Scow and Okuda (1992) J. Lipid Res.33, 1343-1349]. Brefeldin A (BFA), which is known to block protein transport from ER and translocate Golgi components to ER, was used here to study the effect of translocated Golgi enzymes on LPL retained in ER of cld/cld and CST-treated mouse brown adipocytes. Brown adipocytes cultured from newborn normal mice contained 3000-5000 m-units of LPL activity/mg of DNA and secreted 35 m-units of LPL activity/mg of DNA per h. BFA at 10 micrograms/ml doubled LPL activity in normal cells within 2 h as it stopped completely secretion of active LPL. LPL in mouse cells has two N-oligosaccharide chains per subunit. Analyses with SDS/PAGE and immunoblotting showed that about one-third of LPL subunits in untreated normal cells were totally endo-beta-N-acetylglucosaminidase (endo H)-resistant, one-third were partially endo H-resistant, and one-third were totally endo H-sensitive. BFA decreased to zero the proportion of subunits which were totally endo H-resistant, while it increased the proportion which were partially endo H-resistant. Thus, BFA blocked processing of one oligosaccharide chain per subunit to endo H-resistance. Sucrose-gradient centrifugation studies showed that BFA increased the proportion of LPL subunits in normal cells which were present as active dimers. LPL activity in cld/cld adipocytes was 120 m-units/mg of DNA and that in normal adipocytes treated with CST was 430 m-units/mg of DNA. Most LPL subunits in such cells were totally endo H-sensitive and some were partially endo H-resistant, but none were totally endo H-resistant. Some of the subunits, in both cld/cld and CST-treated cells, were present as inactive LPL dimers. BFA increased LPL activity in cld/cld cells to 2100 m-units/mg of DNA and that in CST-treated cells to 2600 m-units/mg of DNA within 2 h. BFA increased in both groups the proportion of LPL subunits which were partially endo H-resistant. BFA also increased the proportion which were present as active dimers. Immunofluorescence studies in normal and cld/cld adipocytes showed that BFA caused retention of LPL in large tubular and spherical structures and in ER, but not in Golgi. When BFA was withdrawn and protein synthesis was blocked with cycloheximide, LPL in normal cells was transferred to Golgi within 30 min and disappeared within 60 min, whereas LPL in cld/cld cells was retained in large vesicles and ER. The findings indicate that BFA enabled synthesis of active LPL in cld/cld and CST-treated cells via translocation of Golgi components to ER. Also, cld/cld cells synthesized LPL which could be processed to active lipase and the enzymes needed for activation of the lipase were present in Golgi of such cells. Production of inactive LPL in cld/cld adipocytes probably results from their inability to transport LPL from ER to Golgi.

Recombinant human tumour necrosis factor-alpha suppresses synthesis, activity and secretion of lipoprotein lipase in cultures of a human osteosarcoma cell line

The effect of recombinant human tumour necrosis factor-alpha (TNF-alpha) on synthesis, activity and secretion of lipoprotein lipase (LPL) was examined using a human osteosarcoma cell line, osteosarcoma Takase (OST). Treatment of OST cells with TNF-alpha decreased LPL synthesis, resulting in a decrease in expression of activity and secretion of LPL. When OST cells were incubated with glycerol tri[1-14C]palmitate, TNF-alpha decreased dose- and time-dependently the production of 14CO2 and the amounts of radioactivity incorporated into cellular triacylglycerol and phospholipid. The similar reduction of synthesis and activity of LPL as suppression of CO2 production and cellular lipid synthesis indicated that the suppression of 14CO2 production and 14C-labelled lipid synthesis was secondary. TNF-alpha also suppressed expression of proliferating cell nuclear antigen, indicating that it had an anti-proliferative activity on OST cells. The findings suggest that one cause of the anti-proliferative activity of TNF-alpha is the suppression of the LPL-mediated supply of non-esterified fatty acids as an energy source for growth.

The mutation Gly142-->Glu in human lipoprotein lipase produces a missorted protein that is diverted to lysosomes

While the molecular characterization of lipoprotein lipase (LPL) activation is progressing, the intracellular processing, transport, and secretion signals of LPL are still poorly known. The aim of this paper is to study are involvement of glycine 142 in LPL secretion and to elucidate the intracellular destination of the altered protein that remains inside the cell. We mutated the human LPL cDNA by site-directed mutagenesis in order to produce the G142e hLPL in which the glycine 142 was replaced by a glutamic acid. The wild type human LPL (WT hLPL) and the mutant G142E hLPL were expressed by transient transfection in COS1 cells. Using Western blot assays we identified a single band that had the same molecular weight for both proteins. However, Western blots of culture media did not reveal any specific band for the mutant protein, and ELISA experiments showed that the extracellular mass of the mutant LPL was only 25% of the WT protein, indicating defective secretion of the altered enzyme. Heparin increased LPL secretion in the case of the WT hLPL but did not have any stimulatory effect when acting on G142E hLPL-transfected cells. However, heparin-Sepharose chromatography revealed that both proteins presented the same heparin affinity. Metabolic labeling and radioimmunoprecipitation studies showed that both the WT and the mutant hLPL intracellular levels decreased upon chase time. Furthermore, leupeptin had a greater effect on the intracellular level of the mutant enzyme, thus indicating its higher intracellular degradation. Immunofluorescent studies using confocal microscopy indicated high colocalization of the LPL labeling and the Lamp1 lysosomal labeling in G142E hLPL-expressing cells. This result was confirmed using immunoelectron microscopy, which in addition showed gold labeling in Golgi stacks. This finding together with experiments performed with endoglycosidase H digestion of immunoprecipitated radiolabeled LPL, indicated that the mutant enzyme entered the Golgi compartment. The results reported in this paper show that the G142E hLPL is not efficiently secreted to the extracellular medium, but it is missorted to lysosomes for intracellular degradation. This finding suggests that lysosomal missorting might be a mechanism of cell quality control of secreted LPL.

Glycosylation, dimerization, and heparin affinity of lipoprotein lipase in 3T3-L1 adipocytes

The relationship between glycosylation, dimerization, and heparin affinity of lipoprotein lipase (LPL) was studied in 3T3-L1 adipocytes. Three forms of LPL subunits were found in normal cells; totally endo H-resistant (57 kDa), partially sensitive (54 kDa), and totally sensitive (51 kDa) forms. LPL in normal cells was active, dimeric, and showed high affinity for heparin. LPL in cells treated with tunicamycin, preventing the transfer of N-linked oligosaccharide chain, was unglycosylated (51 kDa) and inactive. LPL proteins were found as an aggregate, and had low affinity for heparin. After treatment with castanospermine, an inhibitor of ER glucosidase I, 80% of LPL activity was inhibited. Most of LPL proteins were totally endo H-sensitive, present as an aggregate, and had low affinity for heparin. LPL in cells treated with deoxymannojirimycin, an inhibitor of Golgi mannosidase I, was active, dimeric, and had high affinity for heparin as in normal cells. But LPL subunits were all endo H-sensitive. These results suggest that core glycosylation and subsequent removal of glucose residue is required, but processing after Golgi mannosidase I is not necessary for dimerization and acquisition of high heparin affinity of LPL.

Glycosylation and secretion of lipoprotein lipase by 3T3-L1 adipocytes: effects of brefeldin A

Time courses of synthesis and secretion of lipoprotein lipase (LPL) were examined in 3T3-L1 adipocytes. LPL was glycosylated in the endoplasmic reticulum (ER) within 10 min after synthesis, and was transported after 20-30 min to the trans Golgi where it was converted to the mature form with M(r) = 55,000-58,000, which was resistant to endoglycosidase H (endo H). LPL subunits with M(r) = 55,000-58,000 appeared in the medium within 30 min after synthesis. The effects of brefeldin A (BFA), which inhibits transport of glycoproteins in various types of cells, on secretion and glycosylation of LPL were also examined. BFA completely blocked release of LPL activity into the medium, causing accumulation of the activity in cells. The suppressive effect of BFA on release of LPL activity was reversible. BFA-treated cells synthesized LPL with M(r) = 53,000-55,000 consisting of 2 types of subunits, the main type being totally endo H-sensitive and the other partially endo H-sensitive. No LPL were secreted into the medium by BFA-treated cells.

Characterization of differentiation factor/leukaemia inhibitory factor effect on lipoprotein lipase activity and mRNA in 3T3-L1 adipocytes

Alterations in lipid metabolism characterized in major part by a decrease in lipoprotein lipase (LPL) activity in adipose tissue are a central feature of cachexia from chronic infection or malignancy. These metabolic derangements may be mediated in large part through endogenous host proteins produced in response to various pathological stimuli. Differentiation factor/leukaemia inhibitory factor (D-factor) is a cytokine whose functions overlap those of tumour necrosis factor-alpha (TNF), IL-6 and IL-1. Recombinant murine D-factor produced a dose- and time-dependent inhibition of heparin-releasable LPL activity in differentiated 3T3-L1 adipocytes. Although 2-10 fold less potent than recombinant murine TNF, D-factor inhibited LPL activity at concentrations of 1-10 ng/ml. When added together, D-factor and TNF produced a synergistic inhibition of LPL activity. Interleukin 6 (IL-6) was 100-fold less potent than D-factor; 0.1 ng/ml of D-factor or 10 ng/ml of IL-6 caused a 50% inhibition of LPL activity. D-factor and TNF increased IL-6 production in 3T3-L1 cells. Ten ng/ml of D-factor or 1.0 ng/ml of TNF stimulated the release of < 1 ng/ml of IL-6 and inhibited LPL activity to 11 +/- 3% and 3 +/- 2% of control, respectively, whereas 50 ng/ml of recombinant IL-6 was required to decrease LPL activity to 24 +/- 19% of control. TNF produced a marked decrease in LPL mRNA, whereas D-factor had minimal or no effect at doses which inhibited LPL activity almost completely. Western blot analysis of cell extracts showed that TNF caused a greater decrease in LPL protein production than D-factor.2+ with TNF, may contribute to the manifestations of cachexia.

Role of processing of the oligosaccharide chains in the affinity of lipoprotein lipase for heparin

The role of processing of the oligosaccharide chains in the affinity of lipoprotein lipase (LPL) for heparin was examined in 3T3-L1 adipocytes. 43% of 35S-labeled LPL subunits in tunicamycin (TUN)-treated cells did not bind to a heparin-Sepharose column and 46% was eluted with 0.6 M NaCl. 11% of LPL subunits in castanospermine (CSTP)-treated cells did not bind to the column and 38% was eluted with 0.6 M NaCl. In contrast, as in untreated cells, LPL subunits in 1-deoxymannojirimycin (dMM)-treated and swainsonine (SW)-treated cells almost all bound to the column and over 93% of the subunits bound were eluted with 1.5 M NaCl. Thus, core glycosylation and subsequent removal of the distal glucose residue from oligosaccharide chains of LPL in the endoplasmic reticulum (ER) is required for acquisition of a higher affinity for heparin.

Assembly of lipoprotein lipase in perfused guinea-pig hearts

It has been suggested that lipoprotein lipase (LPL) can be assembled into its catalytically active dimeric form only after its oligosaccharide chains have been processed in the Golgi. To study this in a complete organ, LPL was metabolically labelled with [35S]methionine in perfused guinea-pigs hearts. After 10 min pulse-labelling, LPL protein was eluted as two peaks from heparin-agarose: peak 1 at about 0.65 M NaCl, peak 2 at about 0.95 M NaCl. Catalytic activity was associated only with peak 2. Model studies with bovine LPL showed that active dimeric LPL is eluted in peak 2, but after treatments that dissociate the enzyme into inactive monomers it is eluted in peak 1. Pulse-labelled LPL in both peak 1 and peak 2 was fully sensitive to treatment with endoglycosidase (Endo) H. With chase, peak 1 disappeared and peak 2 acquired resistance to Endo H. These findings suggest that core glycosylated LPL is assembled into dimers already in the endoplasmic reticulum and that processing of the oligosaccharide chains occurs after dimerization.

Regulation of the synthesis, processing and translocation of lipoprotein lipase

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Activation of lipoprotein lipase in cardiac myocytes by glycosylation requires trimming of glucose residues in the endoplasmic reticulum

Incubation of cycloheximide-treated cardiac myocytes results in a time-dependent increase in cellular and heparin-releasable lipoprotein lipase (LPL) activities. N-Methyldeoxynojirimycin (1 mM) and castanospermine (100 micrograms/ml), inhibitors of glucosidases in the endoplasmic reticulum (ER), prevented the increase in cellular LPL activity. The glucosidase inhibitors did not influence the synthesis or turnover of LPL protein. Therefore activation of LPL by glycosylation in cardiac myocytes requires the trimming of glucose residues in oligosaccharide chains by glucosidases of the ER.

Glycosylation of lipoprotein lipase in human subcutaneous and omental adipose tissues

Human adipose tissues from the abdomen (subcutaneous), thigh (subcutaneous) and omentum were incubated for 2 h with [35S]methionine. Then glycosylation of lipoprotein lipase (LPL) was analyzed by sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of endoglycosidase H (endo H)-digested subunits of the 35S-labeled lipase. Adipose tissues from the abdomen, thigh, and omentum all synthesized LPL subunits with Mr = 57,000 composed of two types of subunits. One type was partially endo H-sensitive yielding a product with Mr = 55,000, indicating that it had one endo H-resistant and one endo H-sensitive oligosaccharide chain. The other type of subunit was totally endo H-sensitive yielding a product with Mr = 52,000. Subcutaneous adipose tissues contained nearly equal amounts of partially and totally endo H-sensitive subunits of LPL, whereas omental adipose tissues contained mainly partially endo H-sensitive subunits of LPL.

Lipoprotein lipase: recent contributions from molecular biology

Lipoprotein lipase (LPL) is a glycoprotein enzyme that is produced in several cells and tissues. LPL belongs to a large lipase gene family that includes, among others, hepatic lipase and pancreatic lipase. After secretion, LPL becomes anchored on the luminal surface of the capillary endothelial cells. There it hydrolyzes triglycerides in triglyceride-rich lipoproteins, generating free fatty acids that can serve either as a direct energy source or can be stored. Through this action LPL plays a pivotal role both in energy and in lipoprotein metabolism. LPL production is regulated in a tissue-specific fashion by developmental, hormonal, and nutritional factors. The recent availability of the regulatory sequences of the LPL gene will greatly facilitate these regulatory studies in the future. In man, several mutations resulting in familial LPL deficiency have been delineated at a molecular level. The study of these mutations is not only very beneficial from a clinical point of view but also contributes in a major way to our understanding of the structure-function relationship of LPL and other lipases. In this review major attention is given to molecular studies relating to the regulation of LPL production, to the defects underlying LPL deficiency, and to structure-function relationship of the lipases.