Combined lipase deficiency in the mouse. Evidence of impaired lipase processing and secretion

Lipase maturation factor 1 affects redox homeostasis in the endoplasmic reticulum

Lipoprotein lipase (LPL) is a secreted lipase that clears triglycerides from the blood. Proper LPL folding and exit from the endoplasmic reticulum (ER) require lipase maturation factor 1 (LMF1), an ER-resident transmembrane protein, but the mechanism involved is unknown. We used proteomics to identify LMF1-binding partners necessary for LPL secretion in HEK293 cells and found these to include oxidoreductases and lectin chaperones, suggesting that LMF1 facilitates the formation of LPL's five disulfide bonds. In accordance with this role, we found that LPL aggregates in LMF1-deficient cells due to the formation of incorrect intermolecular disulfide bonds. Cells lacking LMF1 were hypersensitive to depletion of glutathione, but not DTT treatment, suggesting that LMF1 helps reduce the ER Accordingly, we found that loss of LMF1 results in a more oxidized ER Our data show that LMF1 has a broader role than simply folding lipases, and we identified fibronectin and the low-density lipoprotein receptor (LDLR) as novel LMF1 clients that contain multiple, non-sequential disulfide bonds. We conclude that LMF1 is needed for secretion of some ER client proteins that require reduction of non-native disulfides during their folding.

Angiopoietin-like 4 promotes intracellular degradation of lipoprotein lipase in adipocytes

LPL hydrolyzes triglycerides in triglyceride-rich lipoproteins along the capillaries of heart, skeletal muscle, and adipose tissue. The activity of LPL is repressed by angiopoietin-like 4 (ANGPTL4) but the underlying mechanisms have not been fully elucidated. Our objective was to study the cellular location and mechanism for LPL inhibition by ANGPTL4. We performed studies in transfected cells, ex vivo studies, and in vivo studies with Angptl4(-/-) mice. Cotransfection of CHO pgsA-745 cells with ANGPTL4 and LPL reduced intracellular LPL protein levels, suggesting that ANGPTL4 promotes LPL degradation. This conclusion was supported by studies of primary adipocytes and adipose tissue explants from wild-type and Angptl4(-/-) mice. Absence of ANGPTL4 resulted in accumulation of the mature-glycosylated form of LPL and increased secretion of LPL. Blocking endoplasmic reticulum (ER)-Golgi transport abolished differences in LPL abundance between wild-type and Angptl4(-/-) adipocytes, suggesting that ANGPTL4 acts upon LPL after LPL processing in the ER. Finally, physiological changes in adipose tissue ANGPTL4 expression during fasting and cold resulted in inverse changes in the amount of mature-glycosylated LPL in wild-type mice, but not Angptl4(-/-) mice. We conclude that ANGPTL4 promotes loss of intracellular LPL by stimulating LPL degradation after LPL processing in the ER.

Lipase maturation factor 1 (lmf1) is induced by endoplasmic reticulum stress through activating transcription factor 6α (Atf6α) signaling

Lipase maturation factor 1 (Lmf1) is a critical determinant of plasma lipid metabolism, as demonstrated by severe hypertriglyceridemia associated with its mutations in mice and human subjects. Lmf1 is a chaperone localized to the endoplasmic reticulum (ER) and required for the post-translational maturation and activation of several vascular lipases. Despite its importance in plasma lipid homeostasis, the regulation of Lmf1 remains unexplored. We report here that Lmf1 expression is induced by ER stress in various cell lines and in tunicamycin (TM)-injected mice. Using genetic deficiencies in mouse embryonic fibroblasts and mouse liver, we identified the Atf6α arm of the unfolded protein response as being responsible for the up-regulation of Lmf1 in ER stress. Experiments with luciferase reporter constructs indicated that ER stress activates the Lmf1 promoter through a GC-rich DNA sequence 264 bp upstream of the transcriptional start site. We demonstrated that Atf6α is sufficient to induce the Lmf1 promoter in the absence of ER stress, and this effect is mediated by the TM-responsive cis-regulatory element. Conversely, Atf6α deficiency induced by genetic ablation or a dominant-negative form of Atf6α abolished TM stimulation of the Lmf1 promoter. In conclusion, our results indicate that Lmf1 is an unfolded protein response target gene, and Atf6α signaling is sufficient and necessary for activation of the Lmf1 promoter. Importantly, the induction of Lmf1 by ER stress appears to be a general phenomenon not restricted to lipase-expressing cells, which suggests a lipase-independent cellular role for this protein in ER homeostasis.

Lipase maturation factor 1: a lipase chaperone involved in lipid metabolism

Mutations in lipase maturation factor 1 (LMF1) are associated with severe hypertriglyceridemia in mice and human subjects. The underlying cause is impaired lipid clearance due to lipase deficiency. LMF1 is a chaperone of the endoplasmic reticulum (ER) and it is critically required for the post-translational activation of three vascular lipases: lipoprotein lipase (LPL), hepatic lipase (HL) and endothelial lipase (EL). As LMF1 is only required for the maturation of homodimeric, but not monomeric, lipases, it is likely involved in the assembly of inactive lipase subunits into active enzymes and/or the stabilization of active dimers. Herein, we provide an overview of current understanding of LMF1 function and propose that it may play a regulatory role in lipase activation and lipid metabolism. Further studies will be required to test this hypothesis and elucidate the full spectrum of phenotypes in combined lipase deficiency. This article is part of a Special Issue entitled Triglyceride Metabolism and Disease.

Lipase maturation factor 1 is required for endothelial lipase activity

Lipase maturation factor 1 (Lmf1) is an endoplasmic reticulum (ER) membrane protein involved in the posttranslational folding and/or assembly of lipoprotein lipase (LPL) and hepatic lipase (HL) into active enzymes. Mutations in Lmf1 are associated with diminished LPL and HL activities ("combined lipase deficiency") and result in severe hypertriglyceridemia in mice as well as in human subjects. Here, we investigate whether endothelial lipase (EL) also requires Lmf1 to attain enzymatic activity. We demonstrate that cells harboring a (cld) loss-of-function mutation in the Lmf1 gene are unable to generate active EL, but they regain this capacity after reconstitution with the Lmf1 wild type. Furthermore, we show that cellular EL copurifies with Lmf1, indicating their physical interaction in the ER. Finally, we determined that post-heparin phospholipase activity in a patient with the LMF1(W464X) mutation is reduced by more than 95% compared with that in controls. Thus, our study indicates that EL is critically dependent on Lmf1 for its maturation in the ER and demonstrates that Lmf1 is a required factor for all three vascular lipases, LPL, HL, and EL.

Lipase maturation factor LMF1, membrane topology and interaction with lipase proteins in the endoplasmic reticulum

Lipase maturation factor 1 (LMF1) is predicted to be a polytopic protein localized to the endoplasmic reticulum (ER) membrane. It functions in the post-translational attainment of enzyme activity for both lipoprotein lipase and hepatic lipase. By using transmembrane prediction methods in mouse and human orthologs, models of LMF1 topology were constructed and tested experimentally. Employing a tagging strategy that used insertion of ectopic glycan attachment sites and terminal fusions of green fluorescent protein, we established a five-transmembrane model, thus dividing LMF1 into six domains. Three domains were found to face the cytoplasm (the amino-terminal domain and loops B and D), and the other half was oriented to the ER lumen (loops A and C and the carboxyl-terminal domain). This representative model shows the arrangement of an evolutionarily conserved domain within LMF1 (DUF1222) that is essential to lipase maturation. DUF1222 comprises four of the six domains, with the two largest ones facing the ER lumen. We showed for the first time, using several naturally occurring variants featuring DUF1222 truncations, that Lmf1 interacts physically with lipoprotein lipase and hepatic lipase and localizes the lipase interaction site to loop C within DUF1222. We discuss the implication of our results with regard to lipase maturation and DUF1222 domain structure.

Lipoprotein lipase: from gene to obesity

Lipoprotein lipase (LPL) is a multifunctional enzyme produced by many tissues, including adipose tissue, cardiac and skeletal muscle, islets, and macrophages. LPL is the rate-limiting enzyme for the hydrolysis of the triglyceride (TG) core of circulating TG-rich lipoproteins, chylomicrons, and very low-density lipoproteins (VLDL). LPL-catalyzed reaction products, fatty acids, and monoacylglycerol are in part taken up by the tissues locally and processed differentially; e.g., they are stored as neutral lipids in adipose tissue, oxidized, or stored in skeletal and cardiac muscle or as cholesteryl ester and TG in macrophages. LPL is regulated at transcriptional, posttranscriptional, and posttranslational levels in a tissue-specific manner. Nutrient states and hormonal levels all have divergent effects on the regulation of LPL, and a variety of proteins that interact with LPL to regulate its tissue-specific activity have also been identified. To examine this divergent regulation further, transgenic and knockout murine models of tissue-specific LPL expression have been developed. Mice with overexpression of LPL in skeletal muscle accumulate TG in muscle, develop insulin resistance, are protected from excessive weight gain, and increase their metabolic rate in the cold. Mice with LPL deletion in skeletal muscle have reduced TG accumulation and increased insulin action on glucose transport in muscle. Ultimately, this leads to increased lipid partitioning to other tissues, insulin resistance, and obesity. Mice with LPL deletion in the heart develop hypertriglyceridemia and cardiac dysfunction. The fact that the heart depends increasingly on glucose implies that free fatty acids are not a sufficient fuel for optimal cardiac function. Overall, LPL is a fascinating enzyme that contributes in a pronounced way to normal lipoprotein metabolism, tissue-specific substrate delivery and utilization, and the many aspects of obesity and other metabolic disorders that relate to energy balance, insulin action, and body weight regulation.

Mutations in LMF1 cause combined lipase deficiency and severe hypertriglyceridemia

Hypertriglyceridemia is a hallmark of many disorders, including metabolic syndrome, diabetes, atherosclerosis and obesity. A well-known cause is the deficiency of lipoprotein lipase (LPL), a key enzyme in plasma triglyceride hydrolysis. Mice carrying the combined lipase deficiency (cld) mutation show severe hypertriglyceridemia owing to a decrease in the activity of LPL and a related enzyme, hepatic lipase (HL), caused by impaired maturation of nascent LPL and hepatic lipase polypeptides in the endoplasmic reticulum (ER). Here we identify the gene containing the cld mutation as Tmem112 and rename it Lmf1 (Lipase maturation factor 1). Lmf1 encodes a transmembrane protein with an evolutionarily conserved domain of unknown function that localizes to the ER. A human subject homozygous for a deleterious mutation in LMF1 also shows combined lipase deficiency with concomitant hypertriglyceridemia and associated disorders. Thus, through its profound effect on lipase activity, LMF1 emerges as an important candidate gene in hypertriglyceridemia.

KIT is required for hepatic function during mouse post-natal development

Background

The Kit gene encodes a receptor tyrosine kinase involved in various biological processes including melanogenesis, hematopoiesis and gametogenesis in mice and human. A large number of Kit mutants has been described so far showing the pleiotropic phenotypes associated with partial loss-of-function of the gene. Hypomorphic mutations can induce a light coat color phenotype while complete lack of KIT function interferes with embryogenesis. Interestingly several intermediate hypomorphic mutations induced in addition growth retardation and post-natal mortality.

Results

In this report we investigated the post-natal role of Kit by using a panel of chemically-induced hypomorphic mutations recently isolated in the mouse. We found that, in addition to the classical phenotypes, mutations of Kit induced juvenile steatosis, associated with the downregulation of the three genes, VldlR, Lpin1 and Lpl, controlling lipid metabolism in the post-natal liver. Hence, Kit loss-of-functions mimicked the inactivation of genes controlling the hepatic metabolism of triglycerides, the major source of energy from maternal milk, leading to growth and viability defects during neonatal development.

Conclusion

This is a first report involving KIT in the control of lipid metabolism in neonates and opening new perspectives for understanding juvenile steatosis. Moreover, it reinforces the role of Kit during development of the liver and underscores the caution that should be exerted in using KIT inhibitors during anti-cancer treatment.

The cld mutation: narrowing the critical chromosomal region and selecting candidate genes

Combined lipase deficiency (cld) is a recessive, lethal mutation specific to the tw73 haplotype on mouse Chromosome 17. While the cld mutation results in lipase proteins that are inactive, aggregated, and retained in the endoplasmic reticulum (ER), it maps separately from the lipase structural genes. We have narrowed the gene critical region by about 50% using the tw18 haplotype for deletion mapping and a recombinant chromosome used originally to map cld with respect to the phenotypic marker tf. The region now extends from 22 to 25.6 Mbp on the wild-type chromosome, currently containing 149 genes and 50 expressed sequence tags (ESTs). To identify the affected gene, we have selected candidates based on their known role in associated biological processes, cellular components, and molecular functions that best fit with the predicted function of the cld gene. A secondary approach was based on differences in mRNA levels between mutant (cld/cld) and unaffected (+/cld) cells. Using both approaches, we have identified seven functional candidates with an ER localization and/or an involvement in protein maturation and folding that could explain the lipase deficiency, and six expression candidates that exhibit large differences in mRNA levels between mutant and unaffected cells. Significantly, two genes were found to be candidates with regard to both function and expression, thus emerging as the strongest candidates for cld. We discuss the implications of our mapping results and our selection of candidates with respect to other genes, deletions, and mutations occurring in the cld critical region.

Combined lipase deficiency (cld/cld) in mice affects differently post-translational processing of lipoprotein lipase, hepatic lipase and pancreatic lipase

Lipoprotein lipase (LPL) and hepatic lipase (HL), which act on plasma lipoproteins, belong to the same gene family as pancreatic lipase. LPL is synthesized in heart, muscle and adipose tissue, while HL is synthesized primarily in liver. LPL is also synthesized in liver of newborn rodents. The active form of LPL is a dimer, whereas that of HL has not been established. Combined lipase deficiency (CLD) is an autosomal recessive mutation (cld) in mice which impairs post-translational processing of LPL and HL. Cld/cld mice have very low LPL and HL activities (< 5% of normal), yet normal pancreatic lipase activity. They develop massive hypertriglyceridemia and die within 3 days after birth. The CLD mutation allows synthesis, glycosylation and dimerization of LPL, but blocks activation and secretion of the lipase. Thus, dimerization per se does not result in production of active LPL. Immunofluorescence studies showed that LPL is retained in endoplasmic reticulum (ER) in cld/cld cells. Translocation of Golgi components to ER by treatment with brefeldin A (BFA) enabled synthesis of active LPL in cultured cld/cld brown adipocytes. Thus, production of inactive LPL in cld/cld cells results from inability of the cells to transport LPL from ER. The CLD mutation allows synthesis and glycosylation of HL, but blocks activation of the lipase. Immunofluorescence studies located HL mostly outside of cells in liver, liver cell cultures and incubated adrenal tissue of normal and cld/cld mice and mostly inside of cells in liver cell cultures and adrenal tissues treated with monensin (to block secretion of protein). These findings demonstrate synthesis and secretion of HL by both liver and adrenal cells of normal and cld/cld mice. Thus, the CLD mutation allows secretion of inactive HL by liver and adrenals. However, it does not block synthesis or secretion of active pancreatic lipase. Our findings indicate that LPL, HL and pancreatic lipase, although closely related, are processed differently.

Obesity QTLs on mouse chromosomes 2 and 17

The inheritance of obesity has been analyzed in an intercross between the mouse strains AKR/J and C57L/J. Two novel obesity quantitative trait loci (QTLs) have been identified using the strategy of selective DNA pooling. One QTL affecting adiposity, Obq3, was mapped to a 39-cM segment near the middle of Chromosome 2, with a peak lod score (5.1) just distal to the D2Mit15 locus. The AKR/J Obq3 allele confers increased adiposity in a nearly additive manner, and males are more affected than females. A second obesity QTL (Obq4) maps to the centromeric end of Chromosome 17, with a lod score peak of 4.6 at D17Mit143. The obesity-conferring allele is contributed by C57L/J and acts in a recessive or an additive manner. Obq4 also has more influence in males and affects the inguinal fat depot differentially. Obq3 and Obq4 account for 7.0 and 6.1% of the phenotypic variance in adiposity (gender-merged data), respectively. The possible relationships between these QTLs and previously described obesity QTLs and candidate genes are discussed. The large number of different obesity QTLs that have been described in mice and the relatively small effects contributed by individual loci suggest considerable genetic complexity.

Molecular pathobiology of the human lipoprotein lipase gene

Lipoprotein lipase (LPL; E.C. 3.1.1.34) is a key enzyme in the metabolism of lipids. Many diseases, including obesity, coronary heart disease, chylomicronemia (pancreatitis), and atherosclerosis, appear to be directly or indirectly related to abnormalities in LPL function. Human LPL is a member of a superfamily of lipases that includes hepatic lipase and pancreatic lipase. These lipases are characterized by extensive homology, both at the level of the gene and the mature protein, suggesting that they have a common evolutionary origin. A large number of natural mutations have been discovered in the human LPL gene, which are located at different sites in the gene and affect different functions of the mature protein. There is a high prevalence of two of these mutations (207 and 188) in the Province of Québec, and one of them (207) is almost exclusive to the French-Canadian population. A study of these and other naturally occurring mutant LPL molecules, as well as those created in vitro by site-directed mutagenesis, indicate that the sequence of LPL is organized into multiple structural and functional units that act in concert in the normal enzyme. In this review, we discuss the interrelationships of LPL structure and its function, the molecular etiology of abnormal LPL in humans, and the clinical and therapeutic aspects of LPL deficiency.

COOH-terminal disruption of lipoprotein lipase in mice is lethal in homozygotes, but heterozygotes have elevated triglycerides and impaired enzyme activity

The role of the enzyme lipoprotein lipase (LPL) in atherosclerosis is uncertain. To generate an animal model of LPL deficiency, we targeted the LPL gene in embryonic stem cells with a vector designed to disrupt the COOH terminus of the protein and used these cells to generate LPL-deficient mice. Germ line transmission of the disrupted LPL allele was achieved with two chimeric males, and offspring from each of these animals were phenotypically identical. Pups homozygous (-/-) for LPL deficiency died within 48 h of birth with extreme elevations of serum triglycerides (13,327 mg/dl) associated with essentially absent LPL enzyme activity in heart and carcass. Newborn heterozygous (+/-) LPL-deficient pups had lower LPL enzyme activity and higher triglycerides (370 versus 121 mg/dl) than wild type (+/+) littermates. Adult heterozygotes had higher triglycerides than wild type mice with ad libitum feeding (236 mg/dl for +/- versus 88 mg/dl for +/+) and after fasting for 4 h (98 mg/dl for +/- versus 51 for +/+) or 12 h (109 mg/dl for +/- versus 56 mg/dl for +/+). Triglycerides were present as very low density lipoprotein particles and chylomicrons, but high density lipoprotein cholesterol levels were not decreased in +/- animals. Plasma heparin-releasable LPL activity was 43% lower in +/- versus +/+ adult animals. LPL activity, mRNA, and protein were lower in the tissues of +/- versus +/+ mice. Homozygous LPL deficiency caused by disruption of the COOH terminus of the enzyme is lethal in mice. Heterozygous LPL deficiency caused by this mutation is associated with mild to moderate hypertriglyceridemia without affecting static HDL cholesterol levels. Heterozygous LPL-deficient mice could be useful for determining if hypertriglyceridemia, independently or in combination with other discrete defects, influences atherosclerosis.

Tissue-specific expression of human lipoprotein lipase. Effect of the 3'-untranslated region on translation

Lipoprotein lipase (LPL) is a central enzyme in lipoprotein metabolism and is expressed predominantly in adipose tissue and muscle. In these tissues, the regulation of LPL is complex and often opposite in response to the same physiologic stimulus. In addition, much regulation of LPL occurs post-transcriptionally. The human LPL cDNA is characterized by a long 3'-untranslated region, which has two polyadenylation signals. In this report, human adipose tissue expressed two LPL mRNA species (3.2 and 3.6 kb) due to an apparent random choice of sites for mRNA polyadenylation, whereas human skeletal and heart muscle expressed predominantly the longer 3.6-kb mRNA form. To determine whether there was any functional significance to this tissue-specific mRNA expression, poly(A)-enriched RNA from adipose tissue and muscle were translated in vitro, and the poly(A)-enriched RNA from muscle was more efficiently translated into LPL protein. The increased translatability of the 3.6-kb form was also demonstrated by cloning the full-length 3.2- and 3.6-kb LPL cDNA forms, followed by in vitro translation of in vitro prepared transcripts. To confirm that this increased efficiency of translation occurred in vivo, Chinese hamster ovary cells were transfected with the 3.2- and 3.6-kb LPL cDNAs. Cells transfected with the 3.6-kb construct demonstrated increased LPL activity and synthesis, despite no increase in levels of LPL mRNA. Thus, human muscle expresses the 3.6-kb form of LPL due to a non-random choice of polyadenylation signals, and this form is more efficiently translated than the 3.2-kb form.

Reduced dimerization of lipoprotein lipase in post-heparin plasma of a patient with hyperchylomicronemia

As in post-heparin plasma of control subjects, post-heparin plasma of a patient with hyperchylomicronemia contained lipoprotein lipase (LPL) subunits with M(r) = 57,000. But although the amount of LPL was the same as in post-heparin plasma of controls, no LPL activity was detectable. Nearly all the LPL in post-heparin plasma of controls bound to heparin-Sepharose and this LPL bound was mainly eluted with 1.5 M NaCl in parallel with the activity. In post-heparin plasma of the patient, 58% of the LPL subunits did not bind to heparin-Sepharose and 23% was eluted with 0.6 M NaCl. Studies by sucrose density gradient centrifugation showed that almost all the LPL in post-heparin plasma of controls was recovered in the peak with a sedimentation coefficient of 6.8 S, corresponding to the position of a dimeric form of LPL, in parallel with the activity; little LPL was recovered in the peak with a sedimentation coefficient of 4.0 S, corresponding to the position of a monomeric form of LPL. In post-heparin plasma of the patient, 35% of the LPL subunits was recovered in fractions with larger sedimentation coefficients at the bottom of the centrifuge tube, indicating the presence of an aggregated form(s) of LPL; the amount of the monomeric form of LPL was increased, while that of the dimeric form was decreased. Thus, defect of LPL activity in post-heparin plasma of the patient with hyperchylomicronemia could result from reduced dimerization of LPL subunits.

Characterization of cDNA encoding the mouse hepatic triglyceride lipase and expression by in vitro translation

A cDNA coding for the mouse hepatic triglyceride lipase has been isolated from a mouse liver cDNA library. The nucleotide sequence of the cDNA shows an open reading frame encoding a polypeptide of 510 amino acids that is 91.5% and 86% homologous to rat and human hepatic lipase, respectively. The most drastic protein sequence divergence is found at the carboxyterminal end which was speculated to harbour one heparin-binding site. By in vitro translation of cRNA in the presence of pancreatic membranes the hepatic lipase was shown to be glycosylated and to have an electrophoretic mobility of 53 kDa.