In vivo metabolism of ApoB, ApoA-I, and VLDL triglycerides in a form of hypobetalipoproteinemia not linked to the ApoB gene

ANGPTL3 deficiency impairs lipoprotein production and produces adaptive changes in hepatic lipid metabolism

Angiopoietin-like protein 3 (ANGPTL3) is a hepatically secreted protein and therapeutic target for reducing plasma triglyceride-rich lipoproteins and low-density lipoprotein (LDL) cholesterol. Although ANGPTL3 modulates the metabolism of circulating lipoproteins, its role in triglyceride-rich lipoprotein assembly and secretion remains unknown. CRISPR-associated protein 9 (CRISPR/Cas9) was used to target ANGPTL3 in HepG2 cells (ANGPTL3-/-) whereupon we observed ∼50% reduction of apolipoprotein B100 (ApoB100) secretion, accompanied by an increase in ApoB100 early presecretory degradation via a predominantly lysosomal mechanism. Despite defective particle secretion in ANGPTL3-/- cells, targeted lipidomic analysis did not reveal neutral lipid accumulation in ANGPTL3-/- cells; rather ANGPTL3-/- cells demonstrated decreased secretion of newly synthesized triglycerides and increased fatty acid oxidation. Furthermore, RNA sequencing demonstrated significantly altered expression of key lipid metabolism genes, including targets of peroxisome proliferator-activated receptor α, consistent with decreased lipid anabolism and increased lipid catabolism. In contrast, CRISPR/Cas9 LDL receptor (LDLR) deletion in ANGPTL3-/- cells did not result in a secretion defect at baseline, but proteasomal inhibition strongly induced compensatory late presecretory degradation of ApoB100 and impaired its secretion. Additionally, these ANGPTL3-/-;LDLR-/- cells rescued the deficient LDL clearance of LDLR-/- cells. In summary, ANGPTL3 deficiency in the presence of functional LDLR leads to the production of fewer lipoprotein particles due to early presecretory defects in particle assembly that are associated with adaptive changes in intrahepatic lipid metabolism. In contrast, when LDLR is absent, ANGPTL3 deficiency is associated with late presecretory regulation of ApoB100 degradation without impaired secretion. Our findings therefore suggest an unanticipated intrahepatic role for ANGPTL3, whose function varies with LDLR status.

Sar1b mutant mice recapitulate gastrointestinal abnormalities associated with chylomicron retention disease

Chylomicron retention disease (CRD) is an autosomal recessive disorder associated with biallelic Sar1b mutations leading to defects in intracellular chylomicron (CM) trafficking and secretion. To date, a direct cause-effect relationship between CRD and Sar1b mutation has not been established, but genetically modified animal models provide an opportunity to elucidate unrecognized aspects of these mutations. To examine the physiological role and molecular mechanisms of Sar1b function, we generated mice expressing either a targeted deletion or mutation of human Sar1b using the CRISPR-Cas9 system. We found that deletion or mutation of Sar1b in mice resulted in late-gestation lethality of homozygous embryos. Moreover, compared with WT mice, heterozygotes carrying a single disrupted Sar1b allele displayed lower plasma levels of triglycerides, total cholesterol, and HDL-cholesterol, along with reduced CM secretion following gastric lipid gavage. Similarly, decreased expression of apolipoprotein B and microsomal triglyceride transfer protein was observed in correlation with the accumulation of mucosal lipids. Inefficient fat absorption in heterozygotes was confirmed via an increase in fecal lipid excretion. Furthermore, genetically modified Sar1b affected intestinal lipid homeostasis as demonstrated by enhanced fatty acid β-oxidation and diminished lipogenesis through the modulation of transcription factors. This is the first reported mammalian animal model with human Sar1b genetic defects, which reproduces some of the characteristic CRD features and provides a direct cause-effect demonstration.

Identifying the Lipidomic Effects of a Rare Loss-of-Function Deletion in ANGPTL3

BACKGROUND

The identification and understanding of therapeutic targets for atherosclerotic cardiovascular disease is of fundamental importance given its global health and economic burden. Inhibition of ANGPTL3 (angiopoietin-like 3) has demonstrated a cardioprotective effect, showing promise for atherosclerotic cardiovascular disease treatment, and is currently the focus of ongoing clinical trials. Here, we assessed the genetic basis of variation in ANGPTL3 levels in the San Antonio Family Heart Study.

METHODS

We assayed ANGPTL3 protein levels in ≈1000 Mexican Americans from extended pedigrees. By drawing upon existing plasma lipidome profiles and genomic data we conducted analyses to understand the genetic basis to variation in ANGPTL3 protein levels, and accordingly the correlation with the plasma lipidome.

RESULTS

In a variance components framework, we identified that variation in ANGPTL3 was significantly heritable (h²=0.33, P=1.31×10^-16). To explore the genetic basis of this heritability, we conducted a genome-wide linkage scan and identified significant linkage (logarithm of odds =6.18) to a locus on chromosome 1 at 90 centimorgans, corresponding to the ANGPTL3 gene location. In the genomes of 23 individuals from a single pedigree, we identified a loss-of-function variant, rs398122988 (N121Kfs*2), in ANGPTL3, that was significantly associated with lower ANGPTL3 levels (β=-1.69 SD units, P=3.367×10^-13), and accounted for the linkage signal at this locus. Given the known role of ANGPTL3 as an inhibitor of endothelial and lipoprotein lipase, we explored the association of ANGPTL3 protein levels and rs398122988 with the plasma lipidome and related phenotypes, identifying novel associations with phosphatidylinositols.

CONCLUSIONS

Variation in ANGPTL3 protein levels is heritable and under significant genetic control. Both ANGPTL3 levels and loss-of-function variants in ANGPTL3 have significant associations with the plasma lipidome. These findings further our understanding of ANGPTL3 as a therapeutic target for atherosclerotic cardiovascular disease.

ANGPTL3 Inhibition With Evinacumab Results in Faster Clearance of IDL and LDL apoB in Patients With Homozygous Familial Hypercholesterolemia-Brief Report

Supplemental Digital Content is available in the text.

Objective:

The mechanism by which evinacumab, a fully human monoclonal antibody directed against ANGPTL3 (angiopoietin-like 3 protein) lowers plasma LDL (low-density lipoprotein) cholesterol levels in patients with homozygous familial hypercholesterolemia is unknown. We investigated apoB (apolipoprotein B) containing lipoprotein kinetic parameters in patients with homozygous familial hypercholesterolemia, before and after treatment with evinacumab.

Approach and Results:

Four patients with homozygous familial hypercholesterolemia underwent apoB kinetic analyses in 2 centers as part of a substudy of a trial evaluating the efficacy and safety of evinacumab in patients with homozygous familial hypercholesterolemia. The enrichment of apoB with the stable isotope (5,5,5-²H₃)-Leucine was measured in VLDL (very LDL), IDL (intermediate-density lipoprotein), and LDL at different time points before and after intravenous administration of 15 mg/kg evinacumab. Evinacumab lowered LDL-cholesterol by 59±2% and increased IDL apoB and LDL apoB fractional catabolic rate in all 4 homozygous familial hypercholesterolemia subjects, by 616±504% and 113±14%, respectively. VLDL-apoB production rate decreased in 2 of the 4 subjects.

Conclusions:

In this small study, ANGPTL3 inhibition with evinacumab is associated with an increase in the fractional catabolic rate of IDL apoB and LDL apoB, suggesting that evinacumab lowers LDL-cholesterol predominantly by increasing apoB-containing lipoprotein clearance from the circulation. Additional studies are needed to unravel which factors are determinants in this biological pathway.

Registration:

URL: https://www.clinicaltrials.gov; Unique identifier: NCT04722068.

Genetics of familial hypercholesterolemia: a tool for development of novel lipid lowering pharmaceuticals?

PURPOSE OF REVIEW

Familial hypercholesterolemia is characterized by high LDL cholesterol and an elevated risk to develop coronary heart disease. Mutations in LDL receptor-mediated cholesterol uptake are the main cause of familial hypercholesterolemia. However, multiple mutations in various other genes are also associated with high LDL cholesterol and even familial hypercholesterolemia. Thus, pharmaceuticals that target these genes and proteins might be attractive treatment options to reduce LDL cholesterol. This review provides an overview of the recent developments and clinical testing of such pharmaceuticals.

RECENT FINDINGS

About 80 genes are associated with hypercholesterolemia but only pharmaceuticals that inhibit cholesteryl ester transfer protein (CETP), angiopoietin-related protein 3 (ANGPTL3), and apolipoprotein C-III (apoC-III) have recently been tested in clinical trials. Inhibition of CETP and ANGPTL3 lowered LDL cholesterol. ANGPTL3 inhibition had the largest effect and was even effective in familial hypercholesterolemia patients. The effect of apoC-III inhibition on LDL cholesterol is not conclusive.

SUMMARY

Of the many potential pharmaceutical targets involved in LDL cholesterol, only a few have been studied so far. Of these, pharmaceuticals that inhibit CETP or ANGPTL3 are promising novel treatment options to reduce LDL cholesterol but the effect of apoC-III inhibition requires more research.

Threshold Effects of Circulating Angiopoietin-Like 3 Levels on Plasma Lipoproteins

CONTEXT

Angiopoietin-like 3 (ANGPTL3) deficiency in plasma due to loss-of-function gene mutations results in familial combined hypobetalipoproteinemia type 2 (FHBL2) in homozygotes. However, the lipid phenotype in heterozygotes is much milder and does not appear to relate directly to ANGPTL3 levels. Furthermore, the low-density lipoprotein (LDL) phenotype in carriers of ANGPTL3 mutations is unexplained.

OBJECTIVE

To determine whether reduction below a critical threshold in plasma ANGPTL3 levels is a determinant of lipoprotein metabolism in FHBL2, and to determine whether proprotein convertase subtilisin kexin type 9 (PCSK9) is involved in determining low LDL levels in this condition.

DESIGN

We studied subjects from 19 families with ANGPTL3 mutations and subjects with familial combined hypobetalipoproteinemia type 1 (FHBL1) due to truncated apolipoprotein B (apoB) species.

RESULTS

First, total cholesterol, high-density lipoprotein (HDL) cholesterol, triglycerides, and HDL and LDL particle concentration correlated with plasma ANGPTL3 levels but only when the latter was <25% of normal (<60 ng/dL). Second, the very low-density lipoprotein particle concentration correlated strongly with plasma ANGPTL3 when the latter was <58% of normal. Third, both FHBL1 and FHBL2 subjects showed low levels of mature and LDL-bound PCSK9 and higher levels of its furin-cleaved form. Finally, LDL-bound PCSK9 is protected from cleavage by furin and binds to the LDL receptor more strongly than apoB-free PCSK9.

CONCLUSIONS

Our results suggest that the hypolipidemic effects of ANGPTL3 mutations in FHBL2 are dependent on a threshold of plasma ANGPTL3 levels, with differential effects on various lipoprotein particles. The increased inactivation of PCSK9 by furin in FHBL1 and FHBL2 is likely to cause increased LDL clearance and suggests novel therapeutic avenues.

Utility of free and total target measurements as target engagement and efficacy biomarkers in biotherapeutic development--opportunities and challenges

For biotherapeutics directed against soluble targets, most often monoclonal antibodies (mAbs), their therapeutic efficacy theoretically is driven by the magnitude and duration of free target suppression. However, for soluble targets of rapid turnover and low abundance, it can be technically challenging to directly measure the lowering of free target following treatment with biologics. The opportunities, challenges, and practical approaches to assess free and bound soluble targets and the utility of free and bound target measurements as biomarkers for target engagement and efficacy are covered in this review. In particular, case examples are presented to illustrate the interplay between drug and free/bound target, and how an integrated bioanalytical and pharmacokinetic/target engagement/pharmacodynamic (PK/TE/PD) modeling approach can be used to assess the target engagement for biologics directed against soluble targets with rapid turnover. Important caveats of the modeling approach in the absence of free target measurements are also discussed.

Clinical and genetic analysis of a family diagnosed with familial hypobetalipoproteinemia in which the proband was diagnosed with diabetes mellitus

OBJECTIVE

To perform clinical and genetic analysis of a family with familial hypobetalipoproteinemia in which the proband had been diagnosed with diabetes mellitus.

METHODS

Direct sequencing was performed on candidate genes such as APOB, PCSK9, and ANGPTL3. The effect of the mutant gene on lipid profile was investigated using biochemical methods.

RESULTS

A novel mutation Y344S in ANGPTL3 was identified but no variants were found in PCSK9 or APOB. Lipid profiles showed the levels of TG, TC, and LDL-C to be significantly lower in Y344S carriers than in non-carriers in this family. The levels of HDL-C and plasma concentrations of ANGPTL3 showed no significant differences. Western blot analysis revealed that the mutant ANGPTL3 proteins could not be secreted into the medium.

CONCLUSION

A novel mutation Y344S was found in ANGPTL3 gene in two diabetic patients with familial hypobetalipoproteinemia. The family study and genetic analysis suggest that this set of gene mutation may be a genetic basis for the lipid phenotypes, and may become a vascular protective factor in the probands with high risk of atherosclerosis.

Clinical characteristics and plasma lipids in subjects with familial combined hypolipidemia: a pooled analysis

Angiopoietin-like 3 (ANGPTL3) regulates lipoprotein metabolism by modulating extracellular lipases. Loss-of function mutations in ANGPTL3 gene cause familial combined hypolipidemia (FHBL2). The mode of inheritance and hepatic and vascular consequences of FHBL2 have not been fully elucidated. To get further insights on these aspects, we reevaluated the clinical and the biochemical characteristics of all reported cases of FHBL2. One hundred fifteen FHBL2 individuals carrying 13 different mutations in the ANGPTL3 gene (14 homozygotes, 8 compound heterozygotes, and 93 heterozygotes) and 402 controls were considered. Carriers of two mutant alleles had undetectable plasma levels of ANGPTL3 protein, whereas heterozygotes showed a reduction ranging from 34% to 88%, according to genotype. Compared with controls, homozygotes as well as heterozygotes showed a significant reduction of all plasma lipoproteins, while no difference in lipoprotein(a) [Lp(a)] levels was detected between groups. The prevalence of fatty liver was not different in FHBL2 subjects compared with controls. Notably, diabetes mellitus and cardiovascular disease were absent among homozygotes. FHBL2 trait is inherited in a codominant manner, and the lipid-lowering effect of two ANGPTL3 mutant alleles was more than four times larger than that of one mutant allele. No changes in Lp(a) were detected in FHBL2. Furthermore, our analysis confirmed that FHBL2 is not associated with adverse clinical sequelae. The possibility that FHBL2 confers lower risk of diabetes and cardiovascular disease warrants more detailed investigation.

Familial hypobetalipoproteinemia-induced nonalcoholic steatohepatitis

Familial hypobetalipoproteinemia (FHBL) is a rare genetic disorder of lipid metabolism that is associated with abnormally low serum levels of low-density lipoprotein (LDL) cholesterol and apolipoprotein B. It is an autosomal co-dominant disorder, and depending on zygosity, the clinical manifestations may vary from none to neurological, endocrine, hematological or liver dysfunction. Nonalcoholic fatty liver disease is common in persons with FHBL, however progression to nonalcoholic steatohepatitis is unusual. We describe here a patient with a novel APOB mutation, V703I, which appears to contribute to the severity of the FHBL phenotype. He had liver enzyme abnormalities, increased echogenicity of the liver consistent with steatosis, very low LDL cholesterol at 0.24 mmol/l (normal 1.8–3.5 mmol/l) and an extremely low apolipoprotein B level of 0.16 g/l (normal 0.6–1.2 g/l). APOB gene sequencing revealed him to be a compound heterozygote with two mutations (R463W and V703I). APOB R463W has previously been reported to cause FHBL. Genetic sequencing of his first-degree relatives identified the APOB V703I mutation in his normolipidemic brother and father and the APOB R463W mutation in his mother and sister, both of whom have very low LDL cholesterol levels. These results suggest that the APOB V703I mutation alone does not cause the FHBL phenotype. However, it is possible that it has a contributory role to a more aggressive phenotype in the presence of APOB R463W.

Mutations in the ANGPTL3 gene and familial combined hypolipidemia: a clinical and biochemical characterization

CONTEXT

Familial combined hypolipidemia causes a global reduction of plasma lipoproteins. Its clinical correlates and metabolic implications have not been well defined.

OBJECTIVE

The objective of the study was to investigate the genetic, clinical, and metabolic characteristics of a cohort of subjects with familial combined hypolipidemia.

DESIGN

The design of the study included candidate gene screening and the comparison of the clinical and metabolic characteristics between carrier and noncarrier individuals.

SETTING

The study was conducted in a general community.

SUBJECTS

Participants in the study included individuals belonging to nine families with familial combined hypolipidemia identified in a small town (Campodimele) as well as from other 352 subjects living in the same community.

MAIN OUTCOMES MEASURES

Serum concentrations of lipoproteins, Angiopoietin-like 3 (Angptl3) proteins, and noncholesterol sterols were measured.

RESULTS

The ANGPTL3 S17X mutation was found in all probands, 20 affected family members, and 32 individuals of the community. Two additional frame shift mutations, FsE96del and FsS122, were also identified in two hypocholesterolemic individuals. Homozygotes for the ANGPTL3 S17X mutation had no circulating Angptl3 and a marked reduction of all plasma lipids (P < 0.001). Heterozygotes had 42% reduction in Angptl3 level compared with noncarriers (P < 0.0001) but a significant reduction of only total cholesterol and high-density lipoprotein cholesterol. No differences were observed in the plasma noncholesterol sterols between carriers and noncarriers. No association between familial combined hypolipidemia and the risk of hepatic or cardiovascular diseases were detected.

CONCLUSIONS

Familial combined hypolipidemia segregates as a recessive trait so that apolipoprotein B- and apolipoprotein A-I-containing lipoproteins are comprehensively affected only by the total deficiency of Angptl3. Familial combined hypolipidemia does not perturb whole-body cholesterol homeostasis and is not associated with adverse clinical sequelae.

In vivo human apolipoprotein E isoform fractional turnover rates in the CNS

Apolipoprotein E (ApoE) is the strongest genetic risk factor for Alzheimer's disease and has been implicated in the risk for other neurological disorders. The three common ApoE isoforms (ApoE2, E3, and E4) each differ by a single amino acid, with ApoE4 increasing and ApoE2 decreasing the risk of Alzheimer's disease (AD). Both the isoform and amount of ApoE in the brain modulate AD pathology by altering the extent of amyloid beta (Aβ) peptide deposition. Therefore, quantifying ApoE isoform production and clearance rates may advance our understanding of the role of ApoE in health and disease. To measure the kinetics of ApoE in the central nervous system (CNS), we applied in vivo stable isotope labeling to quantify the fractional turnover rates of ApoE isoforms in 18 cognitively-normal adults and in ApoE3 and ApoE4 targeted-replacement mice. No isoform-specific differences in CNS ApoE3 and ApoE4 turnover rates were observed when measured in human CSF or mouse brain. However, CNS and peripheral ApoE isoform turnover rates differed substantially, which is consistent with previous reports and suggests that the pathways responsible for ApoE metabolism are different in the CNS and the periphery. We also demonstrate a slower turnover rate for CSF ApoE than that for amyloid beta, another molecule critically important in AD pathogenesis.

Exome sequencing, ANGPTL3 mutations, and familial combined hypolipidemia

We sequenced all protein-coding regions of the genome (the "exome") in two family members with combined hypolipidemia, marked by extremely low plasma levels of low-density lipoprotein (LDL) cholesterol, high-density lipoprotein (HDL) cholesterol, and triglycerides. These two participants were compound heterozygotes for two distinct nonsense mutations in ANGPTL3 (encoding the angiopoietin-like 3 protein). ANGPTL3 has been reported to inhibit lipoprotein lipase and endothelial lipase, thereby increasing plasma triglyceride and HDL cholesterol levels in rodents. Our finding of ANGPTL3 mutations highlights a role for the gene in LDL cholesterol metabolism in humans and shows the usefulness of exome sequencing for identification of novel genetic causes of inherited disorders. (Funded by the National Human Genome Research Institute and others.).

Effect of apolipoprotein-B synthesis inhibition on liver triglyceride content in patients with familial hypercholesterolemia

To investigate the impact of mipomersen, an apolipoprotein B-100 (apoB) synthesis inhibitor, on intra-hepatic triglyceride content (IHTG content), we conducted a randomized, double-blind, placebo-controlled study in 21 patients with familial hypercholesterolemia (FH). Subjects received a weekly subcutaneous dose of 200 mg mipomersen or placebo for 13 weeks while continuing conventional lipid lowering therapy. The primary endpoint was change in IHTG content from week 0 to week 15 as measured by localized proton magnetic resonance spectroscopy (1H-MRS). Thirteen weeks of mipomersen administration reduced LDL-cholesterol by 22.0 (17.8) % and apoB by 19.9 (17.4) % (both P < 0.01). One of 10 patients (10%) in the mipomersen-treated group developed mild hepatic steatosis at week 15, which was reversible following mipomersen discontinuation. For the group, there was a trend toward an increase in IHTG content [placebo; baseline: 1.2% and week 15: 1.1%; change -0.1 (0.9). Mipomersen; baseline: 1.2% and week 15: 2.1%; change 0.8 (1.7) (P = 0.0513)]. Mipomersen administration for 13 weeks to subjects with FH is associated with a trend toward an increase in IHTG content. Future studies evaluating the effects of long-term use of mipomersen reaching more profound reductions in apoB are required prior to broader use of this compound.

Stable isotope labeling tandem mass spectrometry (SILT) to quantify protein production and clearance rates

In all biological systems, protein amount is a function of the rate of production and clearance. The speed of a response to a disturbance in protein homeostasis is determined by turnover rate. Quantifying alterations in protein synthesis and clearance rates is vital to understanding disease pathogenesis (e.g., aging, inflammation). No methods currently exist for quantifying production and clearance rates of low-abundance (femtomole) proteins in vivo. We describe a novel, mass spectrometry-based method for quantitating low-abundance protein synthesis and clearance rates in vitro and in vivo in animals and humans. The utility of this method is demonstrated with amyloid-beta (Abeta), an important low-abundance protein involved in Alzheimer's disease pathogenesis. We used in vivo stable isotope labeling, immunoprecipitation of Abeta from cerebrospinal fluid, and quantitative liquid chromatography electrospray-ionization tandem mass spectrometry (LC-ESI-tandem MS) to quantify human Abeta protein production and clearance rates. The method is sensitive and specific for stable isotope-labeled amino acid incorporation into CNS Abeta (+/-1% accuracy). This in vivo method can be used to identify pathophysiologic changes in protein metabolism and may serve as a biomarker for monitoring disease risk, progression, or response to novel therapeutic agents. The technique is adaptable to other macromolecules, such as carbohydrates or lipids.

Studying apolipoprotein turnover with stable isotope tracers: correct analysis is by modeling enrichments

Lipoprotein kinetic parameters are determined from mass spectrometry data after administering mass isotopes of amino acids, which label proteins endogenously. The standard procedure is to model the isotopic content of the labeled precursor amino acid and of proteins of interest as tracer-to-tracee ratio (TTR). It is shown here that even though the administered tracer alters amino acid mass and turnover, apolipoprotein synthesis is unaltered and hence the apolipoprotein system is in a steady state, with the total (labeled plus unlabeled) masses and fluxes remaining constant. The correct model formulation for apolipoprotein kinetics is shown to be in terms of tracer enrichment, not of TTR. The needed mathematical equations are derived. A theoretical error analysis is carried out to calculate the magnitude of error in published results using TTR modeling. It is shown that TTR modeling leads to a consistent underestimation of the fractional synthetic rate. In constant-infusion studies, the bias error percent is shown to equal approximately the plateau enrichment, generally <10%. It is shown that, in bolus studies, the underestimation error can be larger. Thus, for mass isotope studies with endogenous tracers, apolipoproteins are in a steady state and the data should be fitted by modeling enrichments.

Human amyloid-beta synthesis and clearance rates as measured in cerebrospinal fluid in vivo

Certain disease states are characterized by disturbances in production, accumulation or clearance of protein. In Alzheimer disease, accumulation of amyloid-beta (Abeta) in the brain and disease-causing mutations in amyloid precursor protein or in enzymes that produce Abeta indicate dysregulation of production or clearance of Abeta. Whether dysregulation of Abeta synthesis or clearance causes the most common form of Alzheimer disease (sporadic, >99% of cases), however, is not known. Here, we describe a method to determine the production and clearance rates of proteins within the human central nervous system (CNS). We report the first measurements of the fractional production and clearance rates of Abeta in vivo in the human CNS to be 7.6% per hour and 8.3% per hour, respectively. This method may be used to search for novel biomarkers of disease, to assess underlying differences in protein metabolism that contribute to disease and to evaluate treatments in terms of their pharmacodynamic effects on proposed disease-causing pathways.

Gastric bypass surgery improves metabolic and hepatic abnormalities associated with nonalcoholic fatty liver disease

BACKGROUND & AIMS

Most patients with extreme obesity have nonalcoholic fatty liver disease (NAFLD). Although gastric bypass (GBP) surgery is the most common bariatric operation performed in obese patients in the United States, the effect of GBP surgery-induced weight loss on the metabolic and hepatic abnormalities associated with NAFLD are not clear.

METHODS

Whole-body glucose, fatty acid and lipoprotein kinetics, liver histology, and hepatic cellular factors involved in inflammation and fibrogenesis were evaluated in 7 extremely obese subjects (body mass index, 58 +/- 4 kg/m(2)) before and 1 year after GBP surgery.

RESULTS

At 1 year after surgery, subjects lost 29% +/- 5% of initial body weight (P < .01); palmitate rate of appearance in plasma, an index of adipose tissue lipolysis, decreased by 47% +/- 4% (P < .01); endogenous glucose production rate decreased by 27% +/- 7% (P < .01); and very-low-density lipoprotein-triglyceride secretion rate decreased by 44% +/- 9% (P < .05). In addition, GBP surgery-induced weight loss decreased hepatic steatosis but did not change standard histologic assessments of inflammation and fibrosis. However, there was a marked decrease in hepatic factors involved in regulating fibrogenesis (collagen-alpha1(I), transforming growth factor-beta1, alpha-smooth muscle actin, and tissue inhibitor of metalloproteinase 1 expression and alpha-smooth muscle actin content) and inflammation (macrophage chemoattractant protein 1 and interleukin 8 expression) (P < .05, compared with values before weight loss).

CONCLUSIONS

These data demonstrate that weight loss induced by GBP surgery normalizes the metabolic abnormalities involved in the pathogenesis and pathophysiology of NAFLD and decreases the hepatic expression of factors involved in the progression of liver inflammation and fibrosis.

Absence of fatty liver in familial hypobetalipoproteinemia linked to chromosome 3p21

Our aim was to ascertain whether fatty liver may be present in the genetic form of familial hypobetalipoproteinemia (FHBL) linked to a susceptibility locus on chromosome 3p21. Three genetic forms of FHBL exist: (a) FHBL caused by truncation-specifying mutations of apolipoprotein B (apoB), (b) FHBL linked to chr3p21, and (c) FHBL not linked either to APOB or to chr3p21. Fatty liver is common in apoB-defective FHBL. Hepatic fat contents were quantified by magnetic resonance spectroscopy in 16 subjects with 3p21-linked FHBL, 32 subjects with apoB-defective FHBL, and 39 sex- and age-matched controls. Mean liver fat of 3p21 subjects was similar to controls and approximately 60% lower than apoB-defective FHBL subjects ( P = .0012). Indices of adiposity (body mass index, waist/hip ratio) and masses of abdominal subcutaneous, retroperitoneal, and intraperitoneal adipose tissue (IPAT) were quantified by MR imaging. Mean measures of adiposity were similar in the 3 groups, suggesting that adiposity per se was not responsible for differences in liver fat. Liver fat content was positively correlated with IPAT. The intercepts of regression lines of IPAT on liver fat content were similar in controls and 3p21, but higher in apoB-defective FHBL subjects. The slopes of the lines were steepest in apoB-defective, intermediate in 3p21, and flattest in controls. Lipoprotein profiles and very low density lipoprotein-apoB100 kinetics of 3p21 and apoB-defective groups also differed. Thus, 2 genetic subtypes of FHBL also differ in several phenotypic features.

[Hepatic steatosis associated with heterozygotic familial hypobetalipoproteinemia]

Fatty liver disease is now recognized as a major health burden, due to the greater number of cases that are being diagnosed. This trend could partly be explained by the increased use of liver ultrasonography in asymptomatic patients for various reasons, mainly persistent transaminase elevation. The most commonly reported risk factors associated with fatty liver disease are chronic alcohol intake, obesity, type 2 diabetes mellitus, hyperlipidemia, and some drugs. When these factors have been ruled out in a patient with a fatty liver, less frequent causes such as certain inherited metabolic disorders should be considered. Familial hypobetalipoproteinemia is characterized by an alteration of apolipoprotein B (apo B) synthesis, leading to the secretion of truncated forms of the protein, which in turn leads to a marked reduction in excretion of very low-density lipoproteins from the liver and consequently to lipid deposits, especially triglycerides, in the hepatocytes. We report the case of a 23-year-old man who met the diagnostic criteria for heterozygous familial hypobetalipoproteinemia. He presented with mild transaminase elevation and fatty liver. Total cholesterol, low-density lipoprotein cholesterol, very low-density lipoprotein cholesterol and apo B were below normal limits, while levels of high-density lipoprotein cholesterol were normal. Lipid profile determination and liver ultrasonography of first and second-degree relatives were also performed. Molecular studies of the index case revealed an unaffected apo B gene.

Fatty liver in familial hypobetalipoproteinemia: roles of the APOB defects, intra-abdominal adipose tissue, and insulin sensitivity

Fatty liver is frequent in the apolipoprotein B (apoB)-defective genetic form of familial hypobetalipoproteinemia (FHBL), but interindividual variability in liver fat is large. To explain this, we assessed the roles of metabolic factors in 32 affected family members with apoB-defective FHBL and 33 related and unrelated normolipidemic controls matched for age, sex, and indices of adiposity. Two hour, 75 g oral glucose tests, with measurements of plasma glucose and insulin levels, body mass index, and waist-hip ratios were obtained. Abdominal subcutaneous, intraperitoneal (IPAT), and retroperitoneal adipose tissue masses were quantified by MR imaging, and hepatic fat was quantified by MR spectroscopy. Mean +/- SD liver fat percentage values of FHBL and controls were 14.8 +/- 12.0 and 5.2 +/- 5.9, respectively (P = 0.001). Means for these measures of obesity and insulin action were similar in the two groups. Important determinants of liver fat percentage were FHBL-affected status, IPAT, and area under the curve (AUC) insulin in both groups, but the strongest predictors were IPAT in FHBL (partial R(2) = 0.55, P < 0.0002) and AUC insulin in controls (partial R(2) = 0.59, P = 0.0001). Regression of liver fat percentage on IPAT fat was significantly greater for FHBL than for controls (P < 0.001). In summary, because apoB-defective FHBL imparts heightened susceptibility to liver triglyceride accumulation, increasing IPAT and insulin resistance exert greater liver fat-increasing effects in FHBL.

Familial hypobetalipoproteinemia: a review

We review the genetics and pathophysiology of familial hypobetalipoproteinemia (FHBL), a mildly symptomatic genetically heterogeneous autosomal trait. The minority of human FHBL is caused by truncation-specifying mutations of the APOB gene on chromosome 2. In seven families, linkage to chromosome 2 is absent, linkage is instead to chromosome 3 (3p21). In others, linkage is absent to both APOB and to 3p21. Apolipoprotein B-100 (apoB-100) levels are approximately 25% of normal, instead of the 50% expected based on the presence of one normal allele due to reduced rates of production. The presence of the truncating mutation seems to have a "dominant recessive" effect on apoB-100 secretion. Concentrations of apoB truncations in plasma differ by truncation but average at approximately 10% of normal levels. Lipoproteins bearing truncated forms of apoB are cleared more rapidly than apoB-100 particles. In contrast with apoB-100 particles cleared primarily in liver via the LDL receptor, most apoB truncation particles are cleared in renal proximal tubular cells via megalin. Since apoB defects cause a dysfunctional VLDL-triglyceride transport system, livers accumulate fat. Hepatic synthesis of fatty acids is reduced in compensation. Informational lacunae remain about genes affecting fat accumulation in liver, and the modulation of liver fat in the presence apoB truncation defects.

Fatty liver in familial hypobetalipoproteinemia: triglyceride assembly into VLDL particles is affected by the extent of hepatic steatosis

Familial hypobetalipoproteinemia (FHBL) subjects may develop fatty liver. Liver fat was assessed in 21 FHBL with six different apolipoprotein B (apoB) truncations (apoB-4 to apoB-89) and 14 controls by magnetic resonance spectroscopy (MRS). Liver fat percentages were 16.7 +/- 11.5 and 3.3 +/- 2.9 (mean +/- SD) (P = 0.001). Liver fat percentage was positively correlated with body mass index, waist circumference, and areas under the insulin curves of 2 h glucose tolerance tests, suggesting that obesity may affect the severity of liver fat accumulation in both groups. Despite 5-fold differences in liver fat percentage, mean values for obesity and insulin indexes were similar. Thus, for similar degrees of obesity, FHBL subjects have more hepatic fat. VLDL-triglyceride (TG)-fatty acids arise from plasma and nonplasma sources (liver and splanchnic tissues). To assess the relative contributions of each, [2H2]palmitate was infused over 12 h in 13 FHBL subjects and 11 controls. Isotopic enrichment of plasma free palmitate and VLDL-TG-palmitate was determined by mass spectrometry. Non-plasma sources contributed 51 +/- 15% in FHBL and 37 +/- 13% in controls (P = 0.02). Correlations of liver fat percentage and percent VLDL-TG-palmitate from liver were r = 0.89 (P = 0.0001) for FHBL subjects and r = 0.69 (P = 0.01) for controls. Thus, apoB truncation-producing mutations result in fatty liver and in altered assembly of VLDL-TG.

Effect of weight loss on VLDL-triglyceride and apoB-100 kinetics in women with abdominal obesity

The effects of obesity and weight loss on lipoprotein kinetics were evaluated in six lean women [body mass index (BMI): 21 +/- 1 kg/m(2)] and seven women with abdominal obesity (BMI: 36 +/- 1 kg/m(2)). Stable isotope tracer techniques, in conjunction with compartmental modeling, were used to determine VLDL-triglyceride (TG) and apolipoprotein B-100 (apoB-100) secretion rates in lean women and in obese women before and after 10% weight loss. VLDL-TG and VLDL-apoB-100 secretion rates were similar in lean and obese women. Weight loss decreased the rate of VLDL-TG secretion by approximately 40% (from 0.41 +/- 0.05 to 0.23 +/- 0.03 micromol x kg fat-free mass(-1) x min(-1); P < 0.05). The relative decline in VLDL-TG produced from nonsystemic fatty acids, derived from intraperitoneal and intrahepatic TG, was greater (61 +/- 7%) than the decline in VLDL-TG produced from systemic fatty acids, predominantly derived from subcutaneous TG (25 +/- 8%; P < 0.05). Weight loss did not affect VLDL-apoB-100 secretion rate. We conclude that weight loss decreases the rate of VLDL-TG secretion in women with abdominal obesity, primarily by decreasing the availability of nonsystemic fatty acids. There is a dissociation in the effect of weight loss on VLDL-TG and apoB-100 metabolic pathways that may affect VLDL particle size.

Apolipoprotein B metabolism: tracer kinetics, models, and metabolic studies

The study of apolipoprotein (apo) B metabolism is central to our understanding of lipoprotein metabolism. However, the assembly and secretion of apoB-containing lipoproteins is a complex process. Specialized techniques, developed and applied to in vitro and in vivo studies of apoB metabolism, have provided insights into the mechanisms involved in the regulation of this process. Moreover, these studies have important implications for understanding both the pathophysiology as well as the therapeutic options for the dyslipidemias. The purpose of this review is to examine the role of apoB in lipoprotein metabolism and to explore the applications of kinetic analysis and multicompartmental modeling to the study of apoB metabolism. New developments and significant advances over the last decade are discussed.