A novel Glu421Lys substitution in the lipoprotein lipase gene in pregnancy-induced hypertriglyceridemic pancreatitis

A protein of capillary endothelial cells, GPIHBP1, is crucial for plasma triglyceride metabolism

GPIHBP1, a protein of capillary endothelial cells (ECs), is a crucial partner for lipoprotein lipase (LPL) in the lipolytic processing of triglyceride-rich lipoproteins. GPIHBP1, which contains a three-fingered cysteine-rich LU (Ly6/uPAR) domain and an intrinsically disordered acidic domain (AD), captures LPL from within the interstitial spaces (where it is secreted by parenchymal cells) and shuttles it across ECs to the capillary lumen. Without GPIHBP1, LPL remains stranded within the interstitial spaces, causing severe hypertriglyceridemia (chylomicronemia). Biophysical studies revealed that GPIHBP1 stabilizes LPL structure and preserves LPL activity. That discovery was the key to crystallizing the GPIHBP1-LPL complex. The crystal structure revealed that GPIHBP1's LU domain binds, largely by hydrophobic contacts, to LPL's C-terminal lipid-binding domain and that the AD is positioned to project across and interact, by electrostatic forces, with a large basic patch spanning LPL's lipid-binding and catalytic domains. We uncovered three functions for GPIHBP1's AD. First, it accelerates the kinetics of LPL binding. Second, it preserves LPL activity by inhibiting unfolding of LPL's catalytic domain. Third, by sheathing LPL's basic patch, the AD makes it possible for LPL to move across ECs to the capillary lumen. Without the AD, GPIHBP1-bound LPL is trapped by persistent interactions between LPL and negatively charged heparan sulfate proteoglycans (HSPGs) on the abluminal surface of ECs. The AD interrupts the HSPG interactions, freeing LPL-GPIHBP1 complexes to move across ECs to the capillary lumen. GPIHBP1 is medically important; GPIHBP1 mutations cause lifelong chylomicronemia, and GPIHBP1 autoantibodies cause some acquired cases of chylomicronemia.

GPIHBP1 and ANGPTL4 Utilize Protein Disorder to Orchestrate Order in Plasma Triglyceride Metabolism and Regulate Compartmentalization of LPL Activity

Intravascular processing of triglyceride-rich lipoproteins (TRLs) is crucial for delivery of dietary lipids fueling energy metabolism in heart and skeletal muscle and for storage in white adipose tissue. During the last decade, mechanisms underlying focal lipolytic processing of TRLs along the luminal surface of capillaries have been clarified by fresh insights into the functions of lipoprotein lipase (LPL); LPL's dedicated transporter protein, glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1 (GPIHBP1); and its endogenous inhibitors, angiopoietin-like (ANGPTL) proteins 3, 4, and 8. Key discoveries in LPL biology include solving the crystal structure of LPL, showing LPL is catalytically active as a monomer rather than as a homodimer, and that the borderline stability of LPL's hydrolase domain is crucial for the regulation of LPL activity. Another key discovery was understanding how ANGPTL4 regulates LPL activity. The binding of ANGPTL4 to LPL sequences adjacent to the catalytic cavity triggers cooperative and sequential unfolding of LPL's hydrolase domain resulting in irreversible collapse of the catalytic cavity and loss of LPL activity. Recent studies have highlighted the importance of the ANGPTL3-ANGPTL8 complex for endocrine regulation of LPL activity in oxidative organs (e.g., heart, skeletal muscle, brown adipose tissue), but the molecular mechanisms have not been fully defined. New insights have also been gained into LPL-GPIHBP1 interactions and how GPIHBP1 moves LPL to its site of action in the capillary lumen. GPIHBP1 is an atypical member of the LU (Ly6/uPAR) domain protein superfamily, containing an intrinsically disordered and highly acidic N-terminal extension and a disulfide bond-rich three-fingered LU domain. Both the disordered acidic domain and the folded LU domain are crucial for the stability and transport of LPL, and for modulating its susceptibility to ANGPTL4-mediated unfolding. This review focuses on recent advances in the biology and biochemistry of crucial proteins for intravascular lipolysis.

Gene-environment interaction between APOA5 c.553G>T and pregnancy in hypertriglyceridemia-induced acute pancreatitis

BACKGROUND

The etiology of hypertriglyceridemia (HTG) and, consequently, HTG-induced acute pancreatitis (HTG-AP), is complex.

OBJECTIVE

Herein, we explore a possible gene-environment interaction between APOA5 c.553G>T (p.185Gly>Cys, rs2075291), a common variant associated with altered triglyceride levels, and pregnancy in HTG-AP.

METHODS

We enrolled 318 Chinese HTG-AP patients and divided them into 3 distinct groups: Group 1, male patients (n = 183); Group 2, female patients whose disease was unrelated to pregnancy (n = 105); and Group 3, female patients whose disease was related to pregnancy (n = 30). APOA5 rs2075291 genotype status was determined by Sanger sequencing. A total of 362 healthy Han Chinese subjects were used as controls. Data on body mass index, peak triglyceride level, age of disease onset, episode number, and clinical severity of HTG-AP were collected from each patient. Multiple comparisons, between patient groups, between patient groups and controls, or within each patient group, were performed.

RESULTS

A robust association of APOA5 rs2075291 with HTG-AP in general, and HTG-AP during pregnancy in particular, was demonstrated. The minor T allele showed a stronger association with Group 3 patients than with either Group 1 or Group 2 patients. This stronger association was due mainly to the much higher frequency of TT genotype in Group 3 patients (20%) than that (<6%) in Group 1 and Group 2 patients. Moreover, the TT genotype was associated with a significantly higher peak triglyceride level in Group 3 patients compared with the GG genotype.

CONCLUSION

Our findings provide evidence for an interaction between APOA5 rs2075291 and pregnancy in HTG-AP.

GPIHBP1 and Lipoprotein Lipase, Partners in Plasma Triglyceride Metabolism

Lipoprotein lipase (LPL), identified in the 1950s, has been studied intensively by biochemists, physiologists, and clinical investigators. These efforts uncovered a central role for LPL in plasma triglyceride metabolism and identified LPL mutations as a cause of hypertriglyceridemia. By the 1990s, with an outline for plasma triglyceride metabolism established, interest in triglyceride metabolism waned. In recent years, however, interest in plasma triglyceride metabolism has awakened, in part because of the discovery of new molecules governing triglyceride metabolism. One such protein-and the focus of this review-is GPIHBP1, a protein of capillary endothelial cells. GPIHBP1 is LPL's essential partner: it binds LPL and transports it to the capillary lumen; it is essential for lipoprotein margination along capillaries, allowing lipolysis to proceed; and it preserves LPL's structure and activity. Recently, GPIHBP1 was the key to solving the structure of LPL. These developments have transformed the models for intravascular triglyceride metabolism.

Lipoprotein lipase transporter GPIHBP1 and triglyceride-rich lipoprotein metabolism

Increased plasma triglyceride serves as an independent risk factor for cardiovascular disease (CVD). Lipoprotein lipase (LPL), which hydrolyzes circulating triglyceride, plays a crucial role in normal lipid metabolism and energy balance. Hypertriglyceridemia is possibly caused by gene mutations resulting in LPL dysfunction. There are many factors that both positively and negatively interact with LPL thereby impacting TG lipolysis. Glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1), a newly identified factor, appears essential for transporting LPL to the luminal side of the blood vessel and offering a platform for TG hydrolysis. Numerous lines of evidence indicate that GPIHBP1 exerts distinct functions and plays diverse roles in human triglyceride-rich lipoprotein (TRL) metabolism. In this review, we discuss the GPIHBP1 gene, protein, its expression and function and subsequently focus on its regulation and provide critical evidence supporting its role in TRL metabolism. Underlying mechanisms of action are highlighted, additional studies discussed and potential therapeutic targets reviewed.

The impact of lipoprotein lipase deficiency on health-related quality of life: a detailed, structured, qualitative study

Background

Lipoprotein lipase deficiency (LPLD) is an autosomal recessive inherited disorder caused by loss-of-function mutations in genes involved in the lipoprotein lipase pathway. It is characterised by chylomicronaemia, severe hypertriglyceridaemia and an increased risk of recurrent pancreatitis that often requires hospitalisation. This research aimed to improve our understanding of the debilitating impact that LPLD has on the daily lives of patients and their families.

Methods

The research comprised a 2-h interview with the patient and, where possible, a 1-h interview with a family member; a 1-week pre- and post-interview task (written and/or video diary); and a 30–45-min follow-up telephone interview. Feelings and thoughts at each stage of the disease journey were captured on a 0–10 rating scale, while the impact of disease on overall health status was measured via the EuroQoL 5 domains, 3 levels (EQ-5D-3L) questionnaire (descriptive and visual analogue scale).

Results

Of four patients identified, three (two female, one male) were recruited to participate in the study; the male patient did not complete the pre-interview task or consent to a family member interview. Demographics and medical history differed among patients in terms of age at symptom onset, their journey to LPLD diagnosis, treatments, the number of attacks of pancreatitis and lengths of hospitalisations. Health-related quality of life, assessed by the EQ-5D-3L, was poor during acute attacks of pancreatitis but was minimally impacted by their condition at interview. Patients described feeling apprehensive, frightened, anxious, depressed or frustrated during and after hospitalisations; spouses of the two female patients also reported being worried or afraid. LPLD affected many aspects of daily living, including diet; socialising and building relationships; state of mind (fear of another attack of pancreatitis or lack of disease control); college and working life (through absenteeism and consequent financial implications); and being reliant on family and friends for support.

Conclusions

The interviews of the three patients with LPLD highlighted several concerns and emphasised the need for improved education, support, dietary advice and appropriate disease management. Additional support services would ease the fear and uncertainty surrounding attacks of pancreatitis, and would allow for improved treatment during hospitalisations.

Electronic supplementary material

The online version of this article (10.1186/s13023-017-0706-1) contains supplementary material, which is available to authorized users.

An LPL-specific monoclonal antibody, 88B8, that abolishes the binding of LPL to GPIHBP1

LPL contains two principal domains: an amino-terminal catalytic domain (residues 1-297) and a carboxyl-terminal domain (residues 298-448) that is important for binding lipids and binding glycosylphosphatidylinositol-anchored high density lipoprotein binding protein 1 (GPIHBP1) (an endothelial cell protein that shuttles LPL to the capillary lumen). The LPL sequences required for GPIHBP1 binding have not been examined in detail, but one study suggested that sequences near LPL's carboxyl terminus (residues ∼403-438) were crucial. Here, we tested the ability of LPL-specific monoclonal antibodies (mAbs) to block the binding of LPL to GPIHBP1. One antibody, 88B8, abolished LPL binding to GPIHBP1. Consistent with those results, antibody 88B8 could not bind to GPIHBP1-bound LPL on cultured cells. Antibody 88B8 bound poorly to LPL proteins with amino acid substitutions that interfered with GPIHBP1 binding (e.g., C418Y, E421K). However, the sequences near LPL's carboxyl terminus (residues ∼403-438) were not sufficient for 88B8 binding; upstream sequences (residues 298-400) were also required. Additional studies showed that these same sequences are required for LPL binding to GPIHBP1. In conclusion, we identified an LPL mAb that binds to LPL's GPIHBP1-binding domain. The binding of both antibody 88B8 and GPIHBP1 to LPL depends on large segments of LPL's carboxyl-terminal domain.

GPIHBP1 and Plasma Triglyceride Metabolism

GPIHBP1, a GPI-anchored protein in capillary endothelial cells, is crucial for the lipolytic processing of triglyceride-rich lipoproteins (TRLs). GPIHBP1 shuttles lipoprotein lipase (LPL) to its site of action in the capillary lumen and is essential for the margination of TRLs along capillaries - such that lipolytic processing can proceed. GPIHBP1 also reduces the unfolding of the LPL catalytic domain, thereby stabilizing LPL catalytic activity. Many different GPIHBP1 mutations have been identified in patients with severe hypertriglyceridemia (chylomicronemia), the majority of which interfere with folding of the protein and abolish its capacity to bind and transport LPL. The discovery of GPIHBP1 has substantially revised our understanding of intravascular triglyceride metabolism but has also raised many new questions for future research.

The acidic domain of the endothelial membrane protein GPIHBP1 stabilizes lipoprotein lipase activity by preventing unfolding of its catalytic domain

GPIHBP1 is a glycolipid-anchored membrane protein of capillary endothelial cells that binds lipoprotein lipase (LPL) within the interstitial space and shuttles it to the capillary lumen. The LPL•GPIHBP1 complex is responsible for margination of triglyceride-rich lipoproteins along capillaries and their lipolytic processing. The current work conceptualizes a model for the GPIHBP1•LPL interaction based on biophysical measurements with hydrogen-deuterium exchange/mass spectrometry, surface plasmon resonance, and zero-length cross-linking. According to this model, GPIHBP1 comprises two functionally distinct domains: (1) an intrinsically disordered acidic N-terminal domain; and (2) a folded C-terminal domain that tethers GPIHBP1 to the cell membrane by glycosylphosphatidylinositol. We demonstrate that these domains serve different roles in regulating the kinetics of LPL binding. Importantly, the acidic domain stabilizes LPL catalytic activity by mitigating the global unfolding of LPL's catalytic domain. This study provides a conceptual framework for understanding intravascular lipolysis and GPIHBP1 and LPL mutations causing familial chylomicronemia.

DOI:http://dx.doi.org/10.7554/eLife.12095.001

Fat is an important part of our diet. The intestines absorb fats and package them into particles called lipoproteins. After reaching the bloodstream, the fat molecules (lipids) in the lipoproteins are broken down by an enzyme called lipoprotein lipase (LPL), which is located along the surface of small blood vessels. This releases nutrients that can be used by vital tissues – mainly the heart, skeletal muscle, and adipose tissues. LPL is produced by muscle and adipose tissue, but it is quickly swept up by a protein called GPIHBP1 and shuttled to its site of action inside the blood vessels.

Mutations that alter the structure of LPL or GPIHBP1 can prevent the breakdown of lipids, resulting in high levels of lipids in the blood. This can lead to inflammation in the pancreas and also increases the risk of heart attacks and strokes. Many earlier studies have examined the properties of LPL, but our understanding of GPIHBP1 has been limited, mainly because it has been difficult to purify GPIHBP1 for analysis.

Using genetically altered insect cells, Mysling et al. were able to purify two different forms of GPIHBP1 – a full-length version and a shorter version that lacked a small section at the end of the molecule known as the acidic domain. This revealed that the opposite end of the molecule – called the carboxyl-terminal domain – is primarily responsible for binding LPL and anchoring it inside blood vessels. Once LPL is bound to GPIHBP1, the acidic domain of GPIHBP1 helps to stabilize LPL. If GPIHBP1’s acidic domain is missing then LPL is more susceptible to losing its structure, rendering it incapable of breaking down the lipids in the blood.

Mysling et al. describe a new model for how LPL and GPIHBP1 interact that explains how specific mutations in the genes that encode these proteins interfere with the delivery of LPL to small blood vessels. In the future, this could help researchers to develop new strategies to treat people with high levels of lipids in their blood.

DOI:http://dx.doi.org/10.7554/eLife.12095.002

Pathogenic classification of LPL gene variants reported to be associated with LPL deficiency

BACKGROUND

Lipoprotein lipase (LPL) deficiency is a serious lipid disorder of severe hypertriglyceridemia (SHTG) with chylomicronemia. A large number of variants in the LPL gene have been reported but their influence on LPL activity and SHTG has not been completely analyzed. Gaining insight into the deleterious effect of the mutations is clinically essential.

METHODS

We used gene sequencing followed by in-vivo/in-vitro and in-silico tools for classification. We classified 125 rare LPL mutations in 33 subjects thought to have LPL deficiency and in 314 subjects selected for very SHTG.

RESULTS

Of the 33 patients thought to have LPL deficiency, only 13 were homozygous or compound heterozygous for deleterious mutations in the LPL gene. Among the 314 very SHTG patients, 3 were compound heterozygous for pathogenic mutants. In a third group of 51,467 subjects, from a general population, carriers of common variants, Asp9Asn and Asn291Ser, were associated with mild increase in triglyceride levels (11%-35%).

CONCLUSION

In total, 39% of patients clinically diagnosed as LPL deficient had 2 deleterious variants. Three patients selected for very SHTG had LPL deficiency. The deleterious mutations associated with LPL deficiency will assist in the diagnosis and selection of patients as candidates for the presently approved LPL gene therapy.

Severe gestational hypertriglyceridemia: A practical approach for clinicians

Severe gestational hypertriglyceridemia is a potentially life threatening and complex condition to manage, requiring attention to a delicate balance between maternal and fetal needs. During pregnancy, significant alterations to lipid homeostasis occur to ensure transfer of nutrients to the fetus. In women with an underlying genetic predisposition or a secondary exacerbating factor, severe gestational hypertriglyceridemia can arise, leading to devastating complications, including acute pancreatitis. Multidisciplinary care, implementation of a low-fat diet with nutritional support, and institution of a hierarchical therapeutic approach are all crucial to reduce maternal and fetal morbidity. To avoid maternal pancreatitis, close surveillance of triglycerides throughout pregnancy with elective hospitalization for refractory cases is recommended. Careful dietary planning is required to prevent neural and retinal complications from fetal essential fatty acid deficiency. Questions remain about the safety of fibrates and plasmapheresis in pregnancy as well as the optimal timing for induction and delivery of these women.

Acute pancreatitis associated with severe hypertriglyceridaemia; A retrospective cohort study

AIM

Acute Pancreatitis (AP) secondary to hypertriglyceridaemia (HTG) is a rare association of which little is known in the literature. This study investigates patient characteristics and outcomes (reoccurrence and mortality) in those presenting with AP secondary to HTG in one of the largest reported British cohorts.

METHODS

A retrospective observational case note review of all patients treated at our institution between 2004 and 2012. Data are expressed as mean and standard deviation if parametric and as median and range if non-parametric. Full fasting lipid profiles and patient demographics were recorded to elucidate further the cause of the severe hypertriglyceridaemia (>10 mmol/L fasting).

RESULTS

There were 784 patients admitted with AP admitted to our institution within the study period. APHTG was present in 18 patients (2.3%). Peak serum triglyceride concentration was 43.9 mmol/L, SD 18.9 mmol/L. Serum amylase activity was 'falsely' low (with raised urine amylase) in about 10% of the patients with acute pancreatitis and hypertriglyceridaemia. 67% of our patients had type 2 diabetes mellitus or impaired glucose tolerance, 28% had a fatty liver and 50% displayed alcohol excess all these conditions are known to be associated with HTG There was a 94.5% reduction in serum triglyceride between presentation and last follow-up visit. There were also no deaths or recurrent episodes of AP during the study period.

CONCLUSIONS

APHTG was present in 2.3% of patients presenting with AP. The reoccurrence and mortality rates were zero in this cohort. This may in part be due to aggressive serum triglyceride lowering by a multi-disciplinary team. Early clinical recognition is vital to provide targeted treatment and to try and reduce further episodes of AP.

A hepatic amino acid/mTOR/S6K-dependent signalling pathway modulates systemic lipid metabolism via neuronal signals

Metabolism is coordinated among tissues and organs via neuronal signals. Levels of circulating amino acids (AAs), which are elevated in obesity, activate the intracellular target of rapamycin complex-1 (mTORC1)/S6kinase (S6K) pathway in the liver. Here we demonstrate that hepatic AA/mTORC1/S6K signalling modulates systemic lipid metabolism via a mechanism involving neuronal inter-tissue communication. Hepatic expression of an AA transporter, SNAT2, activates the mTORC1/S6K pathway, and markedly elevates serum triglycerides (TGs), while downregulating adipose lipoprotein lipase (LPL). Hepatic Rheb or active-S6K expression have similar metabolic effects, whereas hepatic expression of dominant-negative-S6K inhibits TG elevation in SNAT2 mice. Denervation, pharmacological deafferentation and β-blocker administration suppress obesity-related hypertriglyceridemia with adipose LPL upregulation, suggesting that signals are transduced between liver and adipose tissue via a neuronal pathway consisting of afferent vagal and efferent sympathetic nerves. Thus, the neuronal mechanism uncovered here serves to coordinate amino acid and lipid levels and contributes to the development of obesity-related hypertriglyceridemia.

Genetic Variants Associated with Gestational Hypertriglyceridemia and Pancreatitis

Severe hypertriglyceridemia is a well-known cause of pancreatitis. Usually, there is a moderate increase in plasma triglyceride level during pregnancy. Additionally, certain pre-existing genetic traits may render a pregnant woman susceptible to development of severe hypertriglyceridemia and pancreatitis, especially in the third trimester. To elucidate the underlying mechanism of gestational hypertriglyceridemic pancreatitis, we undertook DNA mutation analysis of the lipoprotein lipase (LPL), apolipoprotein C2 (APOC2), apolipoprotein A5 (APOA5), lipase maturation factor 1 (LMF1), and glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1) genes in five unrelated pregnant Chinese women with severe hypertriglyceridemia and pancreatitis. DNA sequencing showed that three out of five patients had the same homozygous variation, p.G185C, in APOA5 gene. One patient had a compound heterozygous mutation, p.A98T and p.L279V, in LPL gene. Another patient had a compound heterozygous mutation, p.A98T & p.C14F in LPL and GPIHBP1 gene, respectively. No mutations were seen in APOC2 or LMF1 genes. All patients were diagnosed with partial LPL deficiency in non-pregnant state. As revealed in our study, genetic variants appear to play an important role in the development of severe gestational hypertriglyceridemia, and, p.G185C mutation in APOA5 gene appears to be the most common variant implicated in the Chinese population. Antenatal screening for mutations in susceptible women, combined with subsequent interventions may be invaluable in the prevention of potentially life threatening gestational hypertriglyceridemia-induced pancreatitis.

Biochemistry and pathophysiology of intravascular and intracellular lipolysis

All organisms use fatty acids (FAs) for energy substrates and as precursors for membrane and signaling lipids. The most efficient way to transport and store FAs is in the form of triglycerides (TGs); however, TGs are not capable of traversing biological membranes and therefore need to be cleaved by TG hydrolases ("lipases") before moving in or out of cells. This biochemical process is generally called "lipolysis." Intravascular lipolysis degrades lipoprotein-associated TGs to FAs for their subsequent uptake by parenchymal cells, whereas intracellular lipolysis generates FAs and glycerol for their release (in the case of white adipose tissue) or use by cells (in the case of other tissues). Although the importance of lipolysis has been recognized for decades, many of the key proteins involved in lipolysis have been uncovered only recently. Important new developments include the discovery of glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1), the molecule that moves lipoprotein lipase from the interstitial spaces to the capillary lumen, and the discovery of adipose triglyceride lipase (ATGL) and comparative gene identification-58 (CGI-58) as crucial molecules in the hydrolysis of TGs within cells. This review summarizes current views of lipolysis and highlights the relevance of this process to human disease.

Glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 and the intravascular processing of triglyceride-rich lipoproteins

Lipoprotein lipase (LPL) is produced by parenchymal cells, mainly adipocytes and myocytes, but is involved in hydrolysing triglycerides in plasma lipoproteins at the capillary lumen. For decades, the mechanism by which LPL reaches its site of action in capillaries was unclear, but this mystery was recently solved. Glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1), a glycosylphosphatidylinositol-anchored protein of capillary endothelial cells, 'picks up' LPL from the interstitial spaces and shuttles it across endothelial cells to the capillary lumen. When GPIHBP1 is absent, LPL is mislocalized to the interstitial spaces, leading to severe hypertriglyceridaemia. Some cases of hypertriglyceridaemia in humans are caused by GPIHBP1 mutations that interfere with the ability of GPIHBP1 to bind to LPL, and some are caused by LPL mutations that impair the ability of LPL to bind to GPIHBP1. Here, we review recent progress in understanding the role of GPIHBP1 in health and disease and discuss some of the remaining unresolved issues regarding the processing of triglyceride-rich lipoproteins.

Hypertriglyceridemia-induced pancreatitis created by oral estrogen and in vitro fertilization ovulation induction

Hypertriglyceridemia is one of the known causes of pancreatitis. Estrogen treatment can aggravate hypertriglyceridemia by increasing very low density lipoprotein secretion and reducing hepatic triglyceride lipase. In this paper, we present 3 patients who developed severe hypertriglyceridemia with conditions that increased estrogen. Two patients were found to have genetic lipoprotein lipase deficiency and were treated with birth control pills. The third was a patient with polycystic ovary disease who was receiving ovulation induction therapy for in vitro fertilization.

New wrinkles in lipoprotein lipase biology

PURPOSE OF REVIEW

We summarize recent progress on GPIHBP1, a molecule that transports lipoprotein lipase (LPL) to the capillary lumen, and discuss several newly studied molecules that appear important for the regulation of LPL activity.

RECENT FINDINGS

LPL, the enzyme responsible for the lipolytic processing of triglyceride-rich lipoproteins, interacts with multiple proteins and is regulated at multiple levels. Several regulators of LPL activity have been known for years and have been investigated thoroughly, but several have been identified only recently, including an endothelial cell protein that transports LPL to the capillary lumen, a microRNA that reduces LPL transcript levels, a sorting protein that targets LPL for uptake and degradation, and a transcription factor that increases the expression of apolipoproteins that regulate LPL activity.

SUMMARY

Proper regulation of LPL is important for controlling the delivery of lipid nutrients to tissues. Recent studies have identified GPIHBP1 as the molecule that transports LPL to the capillary lumen, and have also identified other molecules that are potentially important for regulating LPL activity. These new discoveries open new doors for understanding basic mechanisms of lipolysis and hyperlipidemia.

GPIHBP1, an endothelial cell transporter for lipoprotein lipase

Interest in lipolysis and the metabolism of triglyceride-rich lipoproteins was recently reignited by the discovery of severe hypertriglyceridemia (chylomicronemia) in glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1 (GPIHBP1)-deficient mice. GPIHBP1 is expressed exclusively in capillary endothelial cells and binds lipoprotein lipase (LPL) avidly. These findings prompted speculation that GPIHBP1 serves as a binding site for LPL in the capillary lumen, creating "a platform for lipolysis." More recent studies have identified a second and more intriguing role for GPIHBP1-picking up LPL in the subendothelial spaces and transporting it across endothelial cells to the capillary lumen. Here, we review the studies that revealed that GPIHBP1 is the LPL transporter and discuss which amino acid sequences are required for GPIHBP1-LPL interactions. We also discuss the human genetics of LPL transport, focusing on cases of chylomicronemia caused by GPIHBP1 mutations that abolish GPIHBP1's ability to bind LPL, and LPL mutations that prevent LPL binding to GPIHBP1.

Mutations in lipoprotein lipase that block binding to the endothelial cell transporter GPIHBP1

GPIHBP1, a glycosylphosphatidylinositol-anchored protein of capillary endothelial cells, shuttles lipoprotein lipase (LPL) from subendothelial spaces to the capillary lumen. An absence of GPIHBP1 prevents the entry of LPL into capillaries, blocking LPL-mediated triglyceride hydrolysis and leading to markedly elevated triglyceride levels in the plasma (i.e., chylomicronemia). Earlier studies have established that chylomicronemia can be caused by LPL mutations that interfere with catalytic activity. We hypothesized that some cases of chylomicronemia might be caused by LPL mutations that interfere with LPL's ability to bind to GPIHBP1. Any such mutation would provide insights into LPL sequences required for GPIHBP1 binding. Here, we report that two LPL missense mutations initially identified in patients with chylomicronemia, C418Y and E421K, abolish LPL's ability to bind to GPIHBP1 without interfering with LPL catalytic activity or binding to heparin. Both mutations abolish LPL transport across endothelial cells by GPIHBP1. These findings suggest that sequences downstream from LPL's principal heparin-binding domain (amino acids 403-407) are important for GPIHBP1 binding. In support of this idea, a chicken LPL (cLPL)-specific monoclonal antibody, xCAL 1-11 (epitope, cLPL amino acids 416-435), blocks cLPL binding to GPIHBP1 but not to heparin. Also, changing cLPL residues 421 to 425, 426 to 430, and 431 to 435 to alanine blocks cLPL binding to GPIHBP1 without inhibiting catalytic activity. Together, these data define a mechanism by which LPL mutations could elicit disease and provide insights into LPL sequences required for binding to GPIHBP1.

Hypertriglyceridemic acute pancreatitis during pregnancy: prevention with diet therapy and omega-3 fatty acids in the following pregnancy

Acute pancreatitis complicating pregnancy is rare and has previously been associated with high mortality rates. We report a case of repeated hypertriglyceridemia during pregnancy. During the patient's first pregnancy, acute pancreatitis was elicited in the third trimester by pregnancy-induced hypertriglyceridemia. The patient was treated successfully with a conservative treatment course. The hypertriglyceridemia recurred during her second pregnancy. She carried the pregnancy to term without incident while maintaining a diet low in fat diet and high in omega-3 fatty acids. Early diagnosis and intensive treatment can help to preserve the lives of the patient and the fetus. Prophylactic diet therapy and omega-3 fatty acids may prevent recurrent hypertriglyceridemia during pregnancy.

Hypertriglyceridemia-induced pancreatitis: A case-based review

Hypertriglyceridemia is an established cause of pancreatitis. In a case-based approach, we present a review of hypertriglyceridemia and how it can cause pancreatitis. We outline how to investigate and manage such patients. A 35 year old man presented to the emergency department with abdominal pain and biochemical evidence of acute pancreatitis. There was no history of alcohol consumption and biliary imaging was normal. The only relevant past medical history was that of mild hyperlipidemia, treated with diet alone. Physical exam revealed epigastric tenderness, right lateral rectus palsy, lipemia retinalis, bitemporal hemianopsia and a delay in the relaxation phase of his ankle reflexes. Subsequent laboratory investigation revealed marked hypertriglyceridemia and panhypopituarism. An enhanced CT scan of the head revealed a large suprasellar mass impinging on the optic chiasm and hypothalamus. The patient was treated supportively; thyroid replacement and lipid lowering agents were started. He underwent a successful resection of a craniopharyngioma. Post-operatively, the patient did well on hormone replacement therapy. He has had no further attacks of pancreatitis. This case highlights many of the factors involved in the regulation of triglyceride metabolism. We review the common causes of hypertriglyceridemia and the proposed mechanisms resulting in pancreatitis. The incidence and management of hypertriglyceridemia-induced pancreatitis are also discussed.

Combination of apolipoprotein E2 and lipoprotein lipase heterozygosity causes severe hypertriglyceridemia during pregnancy

Pregnancy is a physiological condition where plasma triglyceride levels are moderately increased. This results from raised synthesis of very-low-density lipoproteins (VLDL) in response to elevated estrogen levels. The occurrence of marked hypertriglyceridemia (HTG) is rare and may result from combination of heterozygote mutation in the lipoprotein lipase (LPL) gene and apolipoprotein E2 isoform, as reported in this case. This observation illustrates the interaction between genetic and environmental factors, since pregnancy may disclose a silent LPL deficiency. The risk of acute pancreatitis threatens both the mother and fetus lives. Early recognition of severe HTG and appropriate management are essential for a successful pregnancy outcome.

Genetic disorders of the pancreas

The venues opened to all by the remarkable studies of the genome are just starting to become manifest; they can now distinguish different variants of a disease; they are given the tools to better understand the pathophysiology of illness; they hope to be able to provide better treatment alternatives to our patients. The examples described in this review demonstrate the applicability of these concepts to pancreatic disorders. Researchers may be just scratching the surface at this time, but the potential is enormous. Many philosophic and ethical questions need to be answered as physicians move along: Should all family members of an index case be screened? Who should pay for testing? Who should get results? But, without the participation of so many patients, their family members, and numerous volunteers, researchers would not have witnessed the bridging of so many gaps as they have so far. All of us may now look forward to the application of this incredible knowledge to the therapeutic solutions so eagerly awaited.

Immunoadsorption and plasma exchange in pregnancy

BACKGROUND

During pregnancy, familial hyperlipidemia or systemic lupus erythematosus (SLE) can exacerbate having devastating consequences for both mother and fetus. Immunoadsorption is established for removal of pathogenic proteins lipoproteins or autoantibodies, but this procedure has only rarely been used in pregnancy.

METHODS

We evaluated retrospectively 126 extracorporeal treatments during six pregnancies. Forty low-density lipoprotein immunoadsorptions, 6 sole plasma exchanges and 36 combined procedures (plasma exchange followed by immunoadsorption) were performed for severe hypertriglyceridemia, complicated by acute pancreatitis. Forty-four IgG immunoadsorptions were executed in 2 pregnant women suffering from SLE with a disastrous course during prior pregnancies.

RESULTS

In hyperlipidemic pregnant women, mean triglyceride levels prior to treatment were 3,841 +/- 2,076 mg/dl (mean +/- SD) and total cholesterol was 617 +/- 354 mg/dl. Until delivery, a 27% reduction of triglycerides could be achieved. Clinical and serological signs of pancreatitis disappeared after initiation of extracorporeal therapy. Four healthy babies were delivered (birthweights between 2,250 and 3,360 g). In 1 woman suffering from SLE, intrauterine fetal death occurred in the 22nd week of gestation despite a reduction of cardiolipin antibodies by 69%. The second case (a twin pregnancy) was complicated by steroid-resistant antibody-mediated anemia. Due to frequent immunoadsorptions, red blood cell count improved (reduction of antierythrocyte antibodies by 66.6%) and 2 healthy babies (birthweights 2,120 and 2,350 g) were delivered by cesarean section.

CONCLUSION

Long-term antibody-based immunoadsorption has been demonstrated to be safe and well tolerated in pregnant women and enables normal intrauterine/fetal development. Although rarely indicated during pregnancy, this treatment modality might be a promising new technique for removal of autoantibodies and lipoproteins in patients with serious gestational complications without sufficient response to conventional therapy.

Lipoprotein lipase: genetics, lipid uptake, and regulation

Lipoprotein lipase (LPL) regulates the plasma levels of triglyceride and HDL. Three aspects are reviewed. 1) Clinical implications of human LPL gene variations: common mutations and their effects on plasma lipids and coronary heart disease are discussed. 2) LPL actions in the nervous system, liver, and heart: the discussion focuses on LPL and tissue lipid uptake. 3) LPL gene regulation: the LPL promoter and its regulatory elements are described.

Successful pregnancy outcome in a patient with severe chylomicronemia due to compound heterozygosity for mutant lipoprotein lipase

OBJECTIVES

Familial chylomicronemia syndrome is characterized by massive accumulation of plasma chylomicrons, which typically results from an absolute deficiency of lipoprotein lipase (LPL). Chylomicronemia in pregnancy is a rare, but serious clinical problem and can be found in patients with underlying molecular defects in the LPL gene. We report the course and treatment of an 18 yr-old primigravida who had LPL deficiency and hypertriglyceridemia since birth. We also analyzed the molecular basis of her LPL deficiency.

DESIGN AND METHODS

The patient's antenatal course was complicated by extreme elevations of plasma triglycerides. Her management included a very low fat diet, pharmacotherapy with gemfibrozil in the third trimester, and intermittent hospitalization with periods of fasting supplemented by IV glucose feeding. We used DNA sequencing to determine whether mutations in LPL were present.

RESULTS

At 38 weeks of gestation, labor was induced, and the patient delivered a healthy 2.77 kilogram male. Postnatal triglycerides fell to prenatal levels. DNA sequencing showed that she was a compound heterozygote for mutant LPL: I > T194 and R > H243.

CONCLUSIONS

This experience indicates that vigilance is required during pregnancy in patients with familial chylomicronemia due to mutant LPL. Gemfibrozil was used in this patient without apparent adverse effects. Compound heterozygosity for LPL mutations is an important underlying mechanism for LPL deficiency.

Necrotizing pancreatitis during pregnancy: a rare cause and review of the literature

Acute pancreatitis is an uncommon cause of abdominal pain during pregnancy, and rarely progresses to the necrotizing from of the disease in this clinical setting. Hyperlipidemia is an infrequent cause of acute pancreatitis. Whereas only 100 cases of hyperlipidemia-induced necrotizing pancreatitis have been reported in the literature to date, all of the cases were mild in severity and responsive to conservative medical management. Herein we present a case of life-threatening necrotizing pancreatitis, which developed in a hyperlipidemic pregnant woman and required multiple peripartum pancreatic necrosectomies. Additionally, we review the evaluation of pregnant patients with abdominal pain, the pathophysiology of hyperlipidemia-induced necrotizing pancreatitis, and the operative care of this challenging group of patients, revisiting an innovative technique for management of the retroperitoneum.

Lipoprotein lipase (LPL) deficiency: a new patient homozygote for the preponderant mutation Gly188Glu in the human LPL gene and review of reported mutations: 75 % are clustered in exons 5 and 6

We have investigated the lipoprotein lipase (LPL) gene of a 2-year-old patient presenting classical features of the familial LPL deficiency including undetectable LPL activity. DNA sequence analysis of exon 5 identified the patient as a homozygote for the Gly188Glu mutation, frequently involved in this disease. A review of cases of LPL deficiency with molecular study of the LPL gene showed a total number of 221 reported mutations involved in this disease. Gly188Glu was involved in 23.5 % of cases and 74.6 % of mutations were clustered in exons 5 and 6. Based on these observations, we propose a method of screening for mutations in this gene.

Estrogen effects on triglyceride metabolism in analbuminemic rats

BACKGROUND

Triglyceride (TG) levels are normally lower in female rats, while the opposite is the case in the Nagase analbuminemic rats (NAR). Increased TG levels in normal males are caused by a testosterone-mediated decrease in postheparin (PH) lipoprotein lipase (LpL). Castration of males reduces TG, while castration of females is without effect. TG levels are reduced by castration of the female NAR, suggesting that estrogen rather than testosterone causes hypertriglyceridemia in this strain. The mechanism for this increase is unknown.

METHODS

We measured secretion of very-low density lipoprotein (VLDL) TG using Triton WR 1339 clearance as the disappearance from blood of 3H-trioleate and 14C-cholesterol-labeled chylomicrons (CM), and the activity of the PH lipases: LpL and hepatic lipase (HL). All were determined in Sprague-Dawley (SD) and NAR female, male, and ovariectomized (OVX) rats.

RESULTS

TG levels were significantly greater in female NAR in comparison to all other groups. Ovariectomy of NAR significantly ameliorated hypertriglyceridemia. VLDL TG secretion was significantly greater in intact female NAR compared with all other groups. There were no other differences in VLDL TG secretion among the other groups. The clearance of CM was greatest in female SD rats, and OVX had no effect. NAR cleared CM less well than did SD rats (P < 0.001), but among NAR, clearance was greatest in OVX NAR and male NAR (P < 0. 002). Both PH LpL activity and HL activity were lowest in female NAR (P < 0.05). Ovariectomy partially corrected the defect in HL (P < 0. 05).

CONCLUSION

TG levels in female NAR are in part a result of increased VLDL-TG secretion, an effect mediated by estrogen. The presence of an estrogen-mediated catabolic defect that was alleviated by OVX was also observed. This catabolic defect is likely a result of an estrogen-mediated decrease both in LpL and HL expressed only in the presence of analbuminemia.

Ile225Thr loop mutation in the lipoprotein lipase (LPL) gene is a de novo event

Mutations in the lipoprotein lipase (LPL) gene are the most important cause of familial chylomicronemia with over 70 mutations being recorded to date. Thus far de novo mutations have not been described. Here we report on the molecular analysis of the family of a patient previously reported to be LPL deficient on the basis of compound heterozygosity for the Arg243His and Ile225Thr mutations, the latter being the first and only mutation identified in the loop region of LPL. Both parents of the propositus were screened for the presence of these two mutations to confirm their status as obligate heterozygotes and to determine the mutation allocation. Although paternal inheritance of the Arg243His allele could be established, maternal DNA did not show carrier status for the Ile225Thr substitution. An examination of maternity, using LPL restriction fragment length polymorphisms four polymorphic CA repeats and ApoE genotypes, was consistent with correct biological parentage for the propositus. Therefore, we conclude that the Ile225Thr mutation constitutes a de novo event, the first to be reported in the LPL gene.