Apolipoprotein synthesis in normal and abetalipoproteinemic intestinal mucosa

Efficacy and safety of lomitapide in familial chylomicronaemia syndrome

BACKGROUND AND AIMS

Familial chylomicronaemia syndrome (FCS) is a rare autosomal recessive disorder, resulting in elevated triglycerides (TGs), abdominal pain and pancreatitis. Treatment options are limited. Lomitapide, a microsomal triglyceride transfer protein inhibitor, is approved for the treatment of homozygous familial hypercholesterolaemia. Whether its therapeutic use may be extended to FCS remains unknown. The aim of this study was to evaluate the efficacy and safety of lomitapide in adult patients with FCS.

METHODS

The open-label, single-arm 'LOCHNES' study of lomitapide in FCS enrolled patients >18 years with genetically confirmed FCS, elevated fasting TG ≥ 750 mg/dL and history of pancreatitis. Patients were administered lomitapide to the maximum tolerated dose for 26 weeks. The primary endpoint was the percent change in TGs from baseline to Week 26.

RESULTS

Eighteen patients were enrolled with median baseline TG levels 1803.5 mg/dL (97.5% CI, 1452-2391 mg/dL). At Week 26, median fasting TGs were reduced to 305 mg/dL (97.5% CI 219-801 mg/dL; 70.5% reduction); median lomitapide dose was 35 mg/day; 13 patients achieved TGs ≤750 mg/dL. Adverse events were mild to moderate and mainly related to gastrointestinal tolerability. Liver imaging at baseline and Week 26 revealed hepatic fat increases from median 12.0%-32.5%, while median hepatic stiffness remained normal. No patient experienced acute pancreatitis or severe abdominal pain during lomitapide treatment.

CONCLUSIONS

Lomitapide is effective and well tolerated in reducing TGs in FCS patients with a history of pancreatitis. Larger studies are warranted to determine lomitapide effectiveness in FCS.

Intestinal lymphatic transport for drug delivery

Intestinal lymphatic transport has been shown to be an absorptive pathway following oral administration of lipids and an increasing number of lipophilic drugs, which once absorbed, diffuse across the intestinal enterocyte and while in transit associate with secretable enterocyte lipoproteins. The chylomicron-associated drug is then secreted from the enterocyte into the lymphatic circulation, rather than the portal circulation, thus avoiding the metabolically-active liver, but still ultimately returning to the systemic circulation. Because of this parallel and potentially alternative absorptive pathway, first-pass metabolism can be reduced while increasing lymphatic drug exposure, which opens the potential for novel therapeutic modalities and allows the implementation of lipid-based drug delivery systems. This review discusses the physiological features of the lymphatics, enterocyte uptake and metabolism, links between drug transport and lipid digestion/re-acylation, experimental model (in vivo, in vitro, and in silico) of lymphatic transport, and the design of lipid- or prodrug-based drug delivery systems for enhancing lymphatic drug transport.

The mechanism of the formation and secretion of chylomicrons

The purpose of this review is to update the reader on our current understanding of the uptake and secretion of dietary lipid by the enterocyte to the periphery. This is a multi-stage process that first involves luminal digestion, followed by cellular uptake and processing, and subsequent extracellular transport of chylomicrons. We discuss the importance of acid and pancreatic lipase in lipid digestion. Micellar solubilization of fatty acids and 2-mono-acyl glycerol is critical to uptake by enhancing enterocyte exposure. There is controversy regarding the mechanism of fatty acid uptake by the enterocyte and whether this is mediated by a carrier-dependent process. The mechanism of fatty acid transport to the endoplasmic reticulum is discussed including the role of fatty acid binding proteins. Intracellularly, 2-monoacylglycerol and fatty acid are reconstituted to form triacylglycerol by the action of MGAT and DGAT. We focus on the mechanisms of intracellular chylomicron formation and secretion into lymph. Chylomicron and VLDL particles differ not only by an operational definition but likely represent two distinct pathways of intestinal lipoprotein formation. The physiologic role of apo B-48 in the intestine is presented as well as clinical disease of chylomicron metabolism, specifically abetalipoproteinemia and Anderson's disease.

Carboxyl ester lipase: structure-function relationship and physiological role in lipoprotein metabolism and atherosclerosis

Carboxyl ester lipase (CEL), previously named cholesterol esterase or bile salt-stimulated (or dependent) lipase, is a lipolytic enzyme capable of hydrolyzing cholesteryl esters, tri-, di-, and mono-acylglycerols, phospholipids, lysophospholipids, and ceramide. The active site catalytic triad of serine-histidine-aspartate is centrally located within the enzyme structure and is partially covered by a surface loop. The carboxyl terminus of the protein regulates enzymatic activity by forming hydrogen bonds with the surface loop to partially shield the active site. Bile salt binding to the loop domain frees the active site for accessibility by water-insoluble substrates. CEL is synthesized primarily in the pancreas and lactating mammary gland, but the enzyme is also expressed in liver, macrophages, and in the vessel wall. In the gastrointestinal tract, CEL serves as a compensatory protein to other lipolytic enzymes for complete digestion and absorption of lipid nutrients. Importantly, CEL also participates in chylomicron assembly and secretion, in a mechanism mediated through its ceramide hydrolytic activity. Cell culture studies suggest a role for CEL in lipoprotein metabolism and oxidized LDL-induced atherosclerosis. Thus, this enzyme, which has a wide substrate reactivity and diffuse anatomic distribution, may have multiple functions in lipid and lipoprotein metabolism, and atherosclerosis.

The role of the microsomal triglygeride transfer protein in abetalipoproteinemia

The microsomal triglyceride transfer protein (MTP) is a dimeric lipid transfer protein consisting of protein disulfide isomerase and a unique 97-kDa subunit. In vitro, MTP accelerates the transport of triglyceride, cholesteryl ester, and phospholipid between membranes. It was recently demonstrated that abetalipoproteinemia, a hereditary disease characterized as an inability to produce chylomicrons and very low-density lipoproteins in the intestine and liver, respectively, results from mutations in the gene encoding the 97-kDa subunit of the microsomal triglyceride transfer protein. Downstream effects resulting from this defect include malnutrition, very low plasma cholesterol and triglyceride levels, altered lipid and protein compositions of membranes and lipoprotein particles, and vitamin deficiencies. Unless treated, abetalipoproteinemic subjects develop gastrointestinal, neurological, ophthalmological, and hematological abnormalities.

Analysis of the role of microsomal triglyceride transfer protein in the liver of tissue-specific knockout mice

A deficiency in microsomal triglyceride transfer protein (MTP) causes the human lipoprotein deficiency syndrome abetalipoproteinemia. However, the role of MTP in the assembly and secretion of VLDL in the liver is not precisely understood. It is not clear, for instance, whether MTP is required to move the bulk of triglycerides into the lumen of the endoplasmic reticulum (ER) during the assembly of VLDL particles. To define MTP's role in hepatic lipoprotein assembly, we recently knocked out the mouse MTP gene (Mttp). Unfortunately, achieving our objective was thwarted by a lethal embryonic phenotype. In this study, we produced mice harboring a "floxed" Mttp allele and then used Cre-mediated recombination to generate liver-specific Mttp knockout mice. Inactivating the Mttp gene in the liver caused a striking reduction in VLDL triglycerides and large reductions in both VLDL/LDL and HDL cholesterol levels. The Mttp inactivation lowered apo B-100 levels in the plasma by >95% but reduced plasma apo B-48 levels by only approximately 20%. Histologic studies in liver-specific knockout mice revealed moderate hepatic steatosis. Ultrastructural studies of wild-type mouse livers revealed numerous VLDL-sized lipid-staining particles within membrane-bound compartments of the secretory pathway (ER and Golgi apparatus) and few cytosolic lipid droplets. In contrast, VLDL-sized lipid-staining particles were not observed in MTP-deficient hepatocytes, either in the ER or in the Golgi apparatus, and there were numerous cytosolic fat droplets. We conclude that MTP is essential for transferring the bulk of triglycerides into the lumen of the ER for VLDL assembly and is required for the secretion of apo B-100 from the liver.

Knockout of the abetalipoproteinemia gene in mice: reduced lipoprotein secretion in heterozygotes and embryonic lethality in homozygotes

Abetalipoproteinemia, an inherited human disease characterized by a near-complete absence of the apolipoprotein (apo) B-containing lipoproteins in the plasma, is caused by mutations in the gene for microsomal triglyceride transfer protein (MTP). We used gene targeting to knock out the mouse MTP gene (Mttp). In heterozygous knockout mice (Mttp+/- ), the MTP mRNA, protein, and activity levels were reduced by 50%, in both liver and intestine. Compared with control mice (Mttp+/+), chow-fed Mttp+/- mice had reduced plasma levels of low-density lipoprotein cholesterol and had a 28% reduction in plasma apoB100 levels. On a high-fat diet, the Mttp+/- mice exhibited a marked reduction in total plasma cholesterol levels, compared with those in Mttp+/+ mice. Both the livers of adult Mttp+/- mice and the visceral endoderm of the yolk sacs from Mttp+/- embryos manifested an accumulation of cytosolic fat. All homozygous embryos (Mttp-/-) died during embryonic development. In the visceral endoderm of Mttp-/- yolk sacs, lipoprotein synthesis was virtually absent, and there was a marked accumulation of cytosolic fat droplets. In summary, half-normal MTP levels do not support normal levels of lipoprotein synthesis and secretion, and a complete deficiency of MTP causes lethal developmental abnormalities, perhaps because of an impaired capacity of the yolk sac to export lipids to the developing embryo.

A-beta-lipoproteinemia: clinical and laboratory features, therapeutic manipulations, and follow-up study of three members of a Greek family

We describe the clinicoepidemiologic features, natural history, and therapeutic manipulations in three Greek patients with A-beta-lipoproteinemia (two brothers aged 15 and 29 years, respectively, and one sister aged 30 years). Diarrhea started in infancy in the two brothers and from the age of 13 in the sister. During the second decade of life, central nervous system symptoms became prominent, with fatigue and disturbance in gait and balance. Night blindness developed at a later phase of the disease in the brothers, whereas cavus developed in both legs in the sister. Apolipoprotein B was absent in all patients, and each had more than 50% of acanthocytes present on peripheral smear. The diagnosis of A-beta-lipoproteinemia was established on the basis of small bowel histology and serum lipid estimations. Family studies revealed normal lipid profiles in all healthy members. The human leukocyte antigen (HLA) pattern in the two most severely affected patients was identical. The only detectable difference between the severely ill patients and other members of the family, however, was homozygosity for the HLA B18 antigen, whereas the third patient had no alleles for the HLA B18 antigen. Treatment consisted of a low-fat diet and high doses of vitamins A and E. A modified diet substituting medium-chain triglycerides for dietary fat was also given, with significant improvement in the nutritional status of patients but not in symptoms related to advanced disease, such as retinal and cardiac manifestations. We conclude that the course of the disease in untreated patients is characterized by continuous symptoms. Some of the symptoms, however, especially those related to malabsorption, as well as some anthropometric parameters can be improved by the application of a modified diet including medium-chain triglycerides. We suggest the routine measurement of plasma lipids and apoproteins not only in children with failure to thrive, with unexplained malabsorption, or with neurologic symptoms, but also in adults with chronic diarrhea accompanied by neurologic symptoms or clinical and laboratory signs of malabsorption.

Normal intestinal dietary fat and cholesterol absorption, intestinal apolipoprotein B (ApoB) mRNA levels, and ApoB-48 synthesis in a hypobetalipoproteinemic kindred without any ApoB truncation

The purpose of this study was to characterize intestinal apolipoprotein B (apoB) metabolism in subjects with familial hypobetalipoproteinemia (FHBL), where segregation analysis supports linkage to the apoB gene but no apoB truncations are present. We investigated cholesterol and fat absorption, intestinal apoB mRNA synthesis and editing, as well as apoB-48 synthesis. Plasma triglycerides (TG) and retinyl palmitate in the chylomicron fractions were analyzed after 12 hours of fasting and then repeatedly for 14 hours after ingestion of a vitamin A-containing high-fat meal. Cholesterol absorption was assessed using a dual stable-isotope method. Mean peak times and concentrations and areas under the curve (AUCs) for fat absorption and mean percentages of cholesterol absorption were comparable in affected and nonaffected family members. Intestinal biopsies were extracted for total RNA and also incubated with 35S-methionine for measurements of apoB synthesis. Similar quantities of apoB mRNA were found to be expressed in the intestine in affected and control subjects by RNase protection assay. ApoB mRNA editing assay showed that the majority of apoB-100 mRNA was edited to the apoB-48 form to a similar extent in both groups. Virtually no apoB-100 protein was synthesized by the intestine in any subject, and apoB-48 protein synthesis was not significantly different in the affected individuals. These data are consistent with in vivo metabolism data that show normal production rates for liver-derived apoB-100 but increased apoB-100 fractional catabolic rates in affected members of this family. Thus, the molecular defect probably does not affect transcription, translation, or secretion of apoB-containing lipoproteins, but may instead affect their clearance.

Chylomicron assembly and catabolism: role of apolipoproteins and receptors

Chylomicrons are lipoproteins synthesized exclusively by the intestine to transport dietary fat and fat-soluble vitamins. Synthesis of apoB48, a translational product of the apob gene, is required for the assembly of chylomicrons. The apob gene transcription in the intestine results in 14 and 7 kb mRNAs. These mRNAs are post-transcriptionally edited creating a stop codon. The edited mRNAs chylomicrons from the shorter apoB48 peptide remains to be elucidated. In addition, the roles of proteins involved in the assembly pathway, e.g. apobec-1, MTP and apoA-IV, needs to be studied. Cloning of enzymes involved in the intestinal biosynthesis of triglycerides will be crucial to fully appreciate the assembly of chylomicrons. There is a need for cell culture and transgenic animal models that can be used for intestinal lipoprotein assembly. The catabolism of chylomicrons is far more complex and efficient than the catabolism of VLDL. Even though the major steps involved in the catabolism of chylomicrons are now known, the determinants for apolipoprotein exchange, processing of remnants in the space of Disse, as well as the mechanism of uptake of these particles by extra-hepatic tissue needs further exploration.