The carboxy-terminal region of human lipoprotein lipase is necessary for its exit from the endoplasmic reticulum

Severe hypertriglyceridemia due to two novel loss-of-function lipoprotein lipase gene mutations (C310R/E396V) in a Chinese family associated with recurrent acute pancreatitis

Lipoprotein lipase (LPL) is widely expressed in skeletal muscles, cardiac muscles as well as adipose tissue and involved in the catabolism of triglyceride. Herein we have systematically characterized two novel loss-of-function mutations in LPL from a Chinese family in which afflicted members were manifested by severe hypertriglyceridemia and recurrent pancreatitis. DNA sequencing revealed that the proband was a heterozygote carrying a novel c.T928C (p.C310R) mutation in exon 6 of the LPL gene. Another member of the family was detected to be a compound heterozygote who along with the c.T928C mutation also carried a novel missense mutation c.A1187T (p.E396V) in exon 8 of the LPL gene. Furthermore, COS-1 cells were transfected with lentiviruses containing the mutant LPL genes. While C310R markedly reduced the overall LPL protein level, COS-1 cells carrying E396V or double mutations contained similar overall LPL protein levels to the wild-type. The specific activity of the LPL mutants remained at comparable magnitude to the wild-type. However, few LPL were detected in the culture medium for the mutants, suggesting that both mutations caused aberrant triglyceride catabolism. More specifically, E396V and double mutations dampened the transport of LPL to the cell surface, while for the C310R mutation, reducing LPL protein level might be involved. By characterizing these two novel LPL mutations, this study has expanded our understanding on the pathogenesis of familial hypertriglyceridemia (FHTG).

Severe hypertriglyceridemia in a newborn with monogenic lipoprotein lipase deficiency: an unconventional therapeutic approach with exchange transfusion

Severe hypertriglyceridemia (sHTG) (plasma triglyceride level > 10 mmol/L) due to lipoprotein lipase (LPL) deficiency is a known risk factor for acute pancreatitis. A 23-day-old male with sHTG was admitted to the Neonatal Intensive Care Unit for plasmapheresis being at high risk for acute pancreatitis. Given the potential hazard of an extracorporeal technique in a very young infant, we decided to perform an exchange transfusion (ET), a procedure widely used by neonatologists and less invasive than plasmapheresis. ET led to a dramatic reduction in plasma triglyceride level, from 93.2 to 3.8 mmol/L at the end of the procedure, without adverse events. The subsequent administration of a special formula low in fat and high in medium-chain triglycerides was effective in keeping fasting plasma triglyceride level below 5.6 mmol/L during the first 5 months of life. The sequence of LPL gene revealed that the patient was apparently homozygous for a novel nucleotide deletion (c.840delG) in exon 6 leading to a premature termination codon (p.N281Mfs*23). However, family studies revealed that while the patient's mother was heterozygous for this mutation, the father was heterozygous for a novel deletion eliminating the whole LPL gene. The patient therefore turned out to be a compound heterozygous for two LPL gene mutations predicted to abolish LPL activity. This is the first case of sHTG treated with ET in a neonate reported in the literature. ET appears to be a safe procedure, alternative to plasmapheresis, to prevent acute pancreatitis in young infants with sHTG due to LPL deficiency.

Effect of titanium on lipoprotein lipase activity in vivo and in vitro

SUMMARY

Lipoprotein lipase (LPL) is a major lipolytic enzyme in the intravascular metabolism of postprandial triglyceride-rich lipoproteins. This enzyme is synthesized and secreted by tissues and transported to the capillary endothelial surface. Decreased activity of this enzyme is suggested to be involved in arterial sequestration of lipoproteins and thus in the progression of atherosclerosis. Titanium salts are widely used in industry, medicine, and pharmacy for tablet coating, pharmaceuticals and cosmetic products. In this study the effect of titanium on post-heparin LPL activity is reported in vivo and in vitro.

METHODS

Groups of Male Wistar rats were administered (i.p) with an acute dose of 2.5 mg/kg titanium chloride for 10 days and a chronic dose of 0.75 mg/kg for 30 and/or 60 days. Blood samples were then collected for LPL assay. For in vitro study, plasma aliquots were incubated in the presence of up to 50 mM titanium and the enzyme activity was measured.

RESULTS

Animals exposed to acute dose of titanium showed about 20% reduction in LPL activity, whereas 31% and 36% reductions were observed in animals chronically exposed for 30 and/or 60 days, respectively. Titanium in vitro also led to enzyme inhibition, so that a decrease of 28-53% was seen in the presence of 0.1-50 mM titanium. This inhibition by titanium was potentiated when citrate and/or bicarbonate was present.

CONCLUSION

Although the mechanism of titanium effect on LPL activity in vivo and in vitro demands more investigations, the inhibitory effect of titanium ion in vivo should be considered seriously in subjects exposed to this metal ion. Changes in LPL activity may affect whole body lipid metabolism, a condition favorable for development and progression of atherosclerosis.

In and out of the ER: protein folding, quality control, degradation, and related human diseases

A substantial fraction of eukaryotic gene products are synthesized by ribosomes attached at the cytosolic face of the endoplasmic reticulum (ER) membrane. These polypeptides enter cotranslationally in the ER lumen, which contains resident molecular chaperones and folding factors that assist their maturation. Native proteins are released from the ER lumen and are transported through the secretory pathway to their final intra- or extracellular destination. Folding-defective polypeptides are exported across the ER membrane into the cytosol and destroyed. Cellular and organismal homeostasis relies on a balanced activity of the ER folding, quality control, and degradation machineries as shown by the dozens of human diseases related to defective maturation or disposal of individual polypeptides generated in the ER.

Lipase localization in Rhizopus oryzae cells immobilized within biomass support particles for use as whole-cell biocatalysts in biodiesel-fuel production

To identify the lipase responsible for the methanolysis activity of fungus whole-cell biocatalysts, the lipase localization of Rhizopus oryzae cells was determined. Western blot analysis showed that R. oryzae cells produce two types of lipase with different molecular masses of 34 and 31 kDa; the former (ROL34) was bound to the cell wall, whereas the latter (ROL31) was mainly bound to the cell membrane. It was found that cell immobilization within reticulated polyurethane foam biomass support particles strongly inhibits the secretion of membrane-bound lipase into the culture medium. An investigation of the relationship between ROL34 and ROL31 suggested that ROL31 originates from the cleavage of a 28-amino-acid residue at the N-terminus of ROL34. The addition of olive oil to the culture medium led to the retention of increased amounts of lipase within the cell. This phenomenon was further confirmed by an immunofluorescence labeling of hyphal cells. When cells were cultivated with various substrate-related compounds, such as olive oil and oleic acid, the intracellular methanolysis activity strongly correlated with the relative amounts of the membrane-bound lipase, which suggests that ROL31 localized in the membrane plays a crucial role in the methanolysis activity of R. oryzae cells.

Traffic jam: a compendium of human diseases that affect intracellular transport processes

As sequencing of the human genome nears completion, the genes that cause many human diseases are being identified and functionally described. This has revealed that many human diseases are due to defects of intracellular trafficking. This 'Toolbox' catalogs and briefly describes these diseases.

Pseudodominance of lipoprotein lipase (LPL) deficiency due to a nonsense mutation (Tyr302>Term) in exon 6 of LPL gene in an Italian family from Sardinia (LPL(Olbia))

We analyzed the molecular defect in the lipoprotein lipase (LPL) gene of a young boy from Sardinia who had primary hyperchylomicronemia, pancreatitis, and a complete LPL deficiency in post-heparin plasma. Analysis of LPL gene was performed by using single strand conformation polymorphism (SSCP) and direct sequencing of SSCP-positive region. The proband was homozygous for a C > A transversion in exon 6, which converts the codon for tyrosine at position 302 into a termination codon and eliminates an RsaI restriction site; this allowed the rapid screening of the proband's family members, among whom nine heterozygotes and one additional homozygote were identified. The homozygote was the proband's paternal grandmother who had shown the first clinical manifestation (recurrent pancreatitis) of LPL deficiency at the age of 54 years. LPL mutation carriers showed a mild dyslipidemic phenotype characterized by a reduction of high density lipoprotein-cholesterol (HDL-C) levels, HDL-C/total cholesterol ratio, and low density lipoprotein (LDL) size, associated with a variable increase of triglyceride levels. Five of these carriers were also heterozygotes for beta-thalassemia (Q39X mutation). In these double mutation carriers, plasma HDL-C levels were higher and plasma triglycerides tended to be lower than in carriers of LPL mutation alone. The Tyr302 > Term mutation encodes a truncated protein of 301 amino acids that is probably not secreted by the LPL producing cells. This is the first mutation of LPL gene found in Sardinians.