Morphologic features of the liver in abetalipoproteinemia

Microsomal transfer protein inhibition in humans

PURPOSE OF REVIEW

Microsomal triglyceride transfer protein (MTP) is a key protein in the secretion of apolipoprotein B-containing lipoproteins. Its pharmacological inhibition is associated with a decrease in LDL cholesterol (LDL-C) and triglycerides. However, the clinical use of MTP inhibitors has been uncertain because of the gastrointestinal adverse events and the increase in liver fat content observed during their administration.

RECENT FINDINGS

Lomitapide, a systemic MTP inhibitor, significantly reduces LDL-C in homozygous familial hypercholesterolemia (hoFH) when administered concurrently with other lipid-lowering therapies, including apheresis. Its lipid-lowering effect is additive to that of existing drugs. In the presence of an up-titration regiment and low-fat diet, lomitapide is generally well tolerated and liver fat accumulation stabilizes after the initial increase. Elevation of alanine aminotranferase levels greater than 3 times the upper limit of normal can be managed successfully with temporary dose reduction. Drug-drug interaction studies show that concomitant treatment of lomitapide with other lipid-lowering drugs is generally safe. Based on these findings, lomitapide was recently approved for the treatment of hoFH as add-on therapy.

SUMMARY

MTP inhibition is a valuable therapeutic approach for hoFH. Long-term safety consequences of liver fat accumulation will need to be assessed.

Genetic determinants of hepatic steatosis in man

Hepatic steatosis is one of the most common liver disorders in the general population. The main cause of hepatic steatosis is nonalcoholic fatty liver disease (NAFLD), representing the hepatic component of the metabolic syndrome, which is characterized by type 2 diabetes, obesity, and dyslipidemia. Insulin resistance and excess adiposity are considered to play key roles in the pathogenesis of NAFLD. Although the risk factors for NAFLD are well established, the genetic basis of hepatic steatosis is largely unknown. Here we review recent progress on genomic variants and their association with hepatic steatosis and discuss the potential impact of these genetic studies on clinical practice. Identifying the genetic determinants of hepatic steatosis will lead to a better understanding of the pathogenesis and progression of NAFLD.

Dirlotapide: a review of its properties and role in the management of obesity in dogs

Dirlotapide is a microsomal triglyceride transfer protein (MTP) inhibitor developed specifically for canine weight reduction. MTP catalyzes the assembly of triglyceride-rich apolipoprotein-B containing lipoproteins to form chylomicrons in the intestinal mucosa and very low-density lipoproteins in the liver. Following oral administration, dirlotapide has in vivo selectivity for intestinal MTP compared with hepatic MTP. In addition to reducing intestinal fat absorption, dirlotapide also reduces food intake in a dose-dependent manner, probably via increased release of peptide YY into the circulation. The decrease in food intake is responsible for the majority of the weight reduction effect. In clinical use, it is recommended to adjust the dose according to the observed weight loss of each individual. The initial dose of 0.05 mg/kg is doubled after 14 days and then adjusted monthly, the maximum permitted daily dose is 1.0 mg/kg, although doses as high as 10 mg/kg have been administered to dogs without severe adverse experience in safety studies. Dirlotapide can be used without necessitating changes to the current feeding or exercise regimens, but it is desirable to monitor the food intake during weight-stabilization to establish revised feeding and exercise routines that will minimize the risk of weight regain post-treatment. The drug offers a novel approach that is applicable in cases where dietary management alone has proved to be unsuccessful.

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.

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.

Diffuse fatty liver in familial heterozygous hypobetalipoproteinemia

A 34-year-old man had asymptomatic hepatomegaly, slightly increased serum alanine aminotransferase and gamma-glutamyl transpeptidase levels, and a sonographic pattern suggesting diffuse hepatic steatosis. Liver biopsy revealed fatty change in 25% to 50% of hepatocytes. The patient also had low serum levels of cholesterol and triglycerides and met clinical, biochemical, and familial diagnostic criteria of heterozygous hypobetalipoproteinemia. We could not relate his hepatic steatosis to any already known cause of fatty liver and could only attribute it to heterozygous hypobetalipoproteinemia. Familial heterozygous hypobetalipoproteinemia should be ruled out in patients with unexplained hepatic steatosis.

Inhibition of the microsomal triglyceride transfer protein blocks the first step of apolipoprotein B lipoprotein assembly but not the addition of bulk core lipids in the second step

The microsomal triglyceride transfer protein (MTP) is required for assembly and secretion of the lipoproteins containing apolipoprotein B (apoB): very low density lipoproteins and chylomicrons. Evidence indicates that the subclasses of these lipoproteins that contain apoB-48 are assembled in a distinct two-step process; first a relatively lipid-poor primordial lipoprotein precursor is produced, and then bulk neutral lipids are added to form the core of these spherical particles. To determine if either step is mediated by MTP, a series of clonal cell lines stably expressing apoB-53 and MTP was established in non-lipoprotein-producing HeLa cells. MTP activity in these cells was approximately 30%, and apoB secretion was 7-33% of that in HepG2 cells on a molar basis. Despite having robust levels of triglyceride and phospholipid synthesis, these cell lines, as exemplified by HLMB53-59, secreted >90% of the apoB-53 on relatively lipid-poor particles in the density range of 1.063-1.21 g/ml. These results suggested that coexpression of MTP and apoB only reconstituted the first but not the second step in lipoprotein assembly. To extend this observation, additional studies were carried out in McArdle RH-7777 rat hepatoma cells, in which the second step of apoB-48 lipoprotein assembly is well defined. Treatment of these cells with the MTP photoaffinity inhibitor BMS-192951 before pulse labeling with [35S]methionine/cysteine led to an 85% block of both apoB-48 and apoB-100 but not apoAI secretion, demonstrating inhibition of the first step of lipoprotein assembly. After a 30-min [35S]methioneine/cysteine pulse labeling and 120 min of chase, all of the nascent apoB-48 was observed to have a density of high density lipoproteins (1.063-1.21 g/ml), indicating that only the first step of lipoprotein assembly had occurred. The addition of oleic acid to the cell culture media activated the second step as evidenced by the conversion of the apoB-48 high density lipoproteins to very low density lipoproteins (d < 1.006 g/ml) during an extended chase period. Inactivation of MTP after completion of the first step, but before stimulation of the second step by the addition of oleic acid, did not block this conversion. Thus, inhibition of MTP did not hinder the addition of bulk core lipid to the primordial lipoprotein precursor particles, indicating that MTP is not required for the second step of apoB-48 lipoprotein assembly.

Angioid streaks associated with abetalipoproteinemia

Angioid streaks were observed in two patients with abetalipoproteinemia. The progression of the angioid streaks was minimal over the years that these patients received vitamin A and E supplementation, though in one patient the development of subretinal neovascular membranes within the angioid streaks was the cause of rapid central visual loss. The simultaneous appearance of two rare entities in unrelated individuals strengthens the relationship between these two disorders that has been suggested by previous case studies. The authors propose a common metabolic pathway involving trace element deficiencies that may account for this relationship as well as the association of angioid streaks with other rare disorders such as Paget's disease, hypoparathyroidism, lead poisoning, hyperphosphatemia, and a number of hemoglobinopathies. Their study of these two patients underscores the need for further investigations as to the role of copper, zinc and omega-3 fatty acids in the pathogenesis of retinopathy in abetalipoproteinemia.

Intestinal and hepatic apolipoprotein B gene expression in abetalipoproteinemia

A 20-year-old woman with abetalipoproteinemia underwent orthotopic liver transplantation for cirrhosis, affording access to her liver and small intestine for study. Before transplantation, her plasma apolipoprotein B concentration was less than 1 mg/dL according to enzyme-linked immunosorbent assay, whereas after transplantation her plasma apolipoprotein B concentration was 76 mg/dL (all apolipoprotein B-100). Apolipoprotein B content was reduced in her intestine and liver compared with normal and cirrhotic controls. Cultured hepatocytes from the patient's explanted liver secreted a 1.006 g/mL less than or equal to d less than or equal to 1.063 g/mL lipoprotein rich in apolipoprotein E and a 1.063 g/mL less than or equal to d less than or equal to 1.21 g/mL lipoprotein containing apolipoproteins E and A-I with no immunodetectable apolipoprotein B in the culture medium. Normal hepatocytes secreted very low-density lipoprotein and low-density lipoprotein containing apolipoprotein B-100. Abetalipoproteinemic intestinal apolipoprotein B messenger RNA concentration was 4-5-fold higher than control values. However, the patient's liver apolipoprotein B messenger RNA level was one fifth that of control normal and cirrhotic liver. Analysis of the patient's intestinal and hepatic apolipoprotein B messenger RNA for posttranscriptional stop-codon insertion revealed normally edited transcripts. These results suggest that apolipoprotein B is synthesized as the product of a normally edited messenger RNA transcript, but not secreted, in abetalipoproteinemia.

Hepatic peroxisomal abnormalities in abetalipoproteinemia

Spinocerebellar degeneration in a 17-yr-old boy with abetalipoproteinemia was associated with vitamin E deficiency and hepatic steatosis. In liver biopsy samples before and after 15 mo of vitamin E treatment, hepatocellular peroxisomes were morphologically abnormal; pleomorphic, with a broadened range of sizes; often larger than normal; and with marginal bars in some. In the first sample, peroxisomes exhibited matrical heterogeneity and dense nucleoids. Peroxisomes in the second biopsy sample lacked nucleoids and contained more homogeneous matrices. The mean peroxisomal diameter increased from 0.77 +/- 0.33 to 0.86 +/- 0.32 microM (normal, 0.62 +/- 0.14). These observations raise the possibility that peroxisomes may be involved in the metabolism of apolipoprotein B or may be affected by the disturbances of hepatocellular lipid metabolism caused by this disease.