Understanding Chylomicron Retention Disease Through Sar1b Gtpase Gene Disruption: Insight From Cell Culture.

BACKGROUND: Understanding the specific mechanisms of rare autosomal disorders has greatly expanded insights into the complex processes regulating intestinal fat transport. Sar1B GTPase is one of the critical proteins governing chylomicron secretion by the small intestine, and its mutations lead to chylomicron retention disease, despite the presence of Sar1A paralog. OBJECTIVE: The central aim of this work is to examine the cause-effect relationship between Sar1B expression and chylomicron output and to determine whether Sar1B is obligatory for normal high-density lipoprotein biogenesis. APPROACH AND RESULTS: The SAR1B gene was totally silenced in Caco-2/15 cells using the zinc finger nuclease technique. SAR1B deletion resulted in significantly decreased secretion of triglycerides (≈40%), apolipoprotein B-48 (≈57%), and chylomicron (≈34.5%). The absence of expected chylomicron production collapse may be because of the compensatory SAR1A elevation observed in our experiments. Therefore, a double knockout of SAR1A and SAR1B was engineered in Caco-2/15 cells, which led to almost complete inhibition of triglycerides, apolipoprotein B-48, and chylomicron output. Further experiments with labeled cholesterol revealed the downregulation of high-density lipoprotein biogenesis in cells deficient in SAR1B or with the double knockout of the 2 SAR1 paralogs. Similarly, there was a fall in the movement of labeled cholesterol from cells to basolateral medium containing apolipoprotein A-I, thereby limiting newly synthesized high-density lipoprotein in genetically modified cells. The decreased cholesterol efflux was associated with impaired expression of ABCA1 (ATP-binding cassette subfamily A member 1). CONCLUSIONS: These findings demonstrate that the deletion of the 2 SAR1 isoforms is required to fully eliminate the secretion of chylomicron in vitro. They also underscore the limited high-density lipoprotein production by the intestinal cells in response to SAR1 knockout.

Angptl3 deficiency is associated with increased insulin sensitivity, lipoprotein lipase activity, and decreased serum free fatty acids.

OBJECTIVE: Angiopoietin-like 3 (Angptl3) is a regulator of lipoprotein metabolism at least by inhibiting lipoprotein lipase activity. Loss-of-function mutations in ANGPTL3 cause familial combined hypolipidemia through an unknown mechanism. APPROACH AND RESULTS: We compared lipolytic activities, lipoprotein composition, and other lipid-related enzyme/lipid transfer proteins in carriers of the S17X loss-of-function mutation in ANGPTL3 and in age- and sex-matched noncarrier controls. Gel filtration analysis revealed a severely disturbed lipoprotein profile and a reduction in size and triglyceride content of very low density lipoprotein in homozygotes as compared with heterozygotes and noncarriers. S17X homozygotes had significantly higher lipoprotein lipase activity and mass in postheparin plasma, whereas heterozygotes showed no difference in these parameters when compared with noncarriers. No changes in hepatic lipase, endothelial lipase, paraoxonase 1, phospholipid transfer protein, and cholesterol ester transfer protein activities were associated with the S17X mutation. Plasma free fatty acid, insulin, glucose, and homeostatic model assessment of insulin resistance were significantly lower in homozygous subjects compared with heterozygotes and noncarriers subjects. CONCLUSIONS: These results indicate that, although partial Angptl3 deficiency did not affect the activities of lipolytic enzymes, the complete absence of Angptl3 results in an increased lipoprotein lipase activity and mass and low circulating free fatty acid levels. This latter effect is probably because of decreased mobilization of free fatty acid from fat stores in human adipose tissue and may result in reduced hepatic very low density lipoprotein synthesis and secretion via attenuated hepatic free fatty acid supply. Altogether, Angptl3 may affect insulin sensitivity and play a role in modulating both lipid and glucose metabolism.

PCSK9 dominant negative mutant results in increased LDL catabolic rate and familial hypobetalipoproteinemia.

OBJECTIVE: Proprotein convertase subtilisin/kexin type 9 (PCSK9) is a central player in the regulation of cholesterol homeostasis, increasing the low-density lipoprotein (LDL) receptor degradation. Our study aimed at exploring the pathogenic consequences in vivo and in vitro of a PCSK9 prodomain mutation found in a family with hypobetalipoproteinemia (FHBL). METHODS AND RESULTS: A white 49-year-old diabetic man had profound FBHL (LDLC: 16 mg/dL) whereas his daughter and sister displayed a milder phenotype (LDLC 44 mg/dL and 57 mg/dL, respectively), all otherwise healthy with a normal liver function. A monoallelic PCSK9 double-mutant R104C/V114A cosegregated with FBHL, with no mutation found at other FHBL-causing loci. A dose-effect was also found in FBHL relatives for plasma APOB and PCSK9 (very-low to undetectable in proband, approximately 50% decreased in sister and daughter) and LDL catabolic rate (256% and 88% increased in proband and daughter). Transient transfection in hepatocytes showed severely impaired processing and secretion of the double mutant which acted as a dominant negative over secretion of wild-type PCSK9. CONCLUSIONS: These results show that heterozygous PCSK9 missense mutations may associate with profound hypobetalipoproteinemia and constitute the first direct evidence in human that decrease of plasma LDLC concentrations associated to PCSK9 LOF mutations are attributable to an increased clearance rate of LDL.

Lipoprotein lipase and hepatic lipase activity after heparin administration in abetalipoproteinemia and hypobetalipoproteinemia.

The purpose of this study was to examine whether an absence of triglyceride-rich lipoproteins (chylomicrons and very-low-density lipoproteins) in plasma is associated with any changes in the enzyme activity of lipoprotein lipase or hepatic lipase after heparin administration. To study this, the activities of hepatic lipase and lipoprotein lipase were determined in control subjects, in two patients with heterozygous hypobetalipoproteinemia, and in three patients with phenotypic abetalipoproteinemia after administration of heparin. Both enzymes showed normal activity in the patients with hypobetalipoproteinemia, but showed consistently reduced activity in the patients with abetalipoproteinemia. Hepatic lipase activity in plasma samples from these three patients obtained 15 minutes after intravenous injection of heparin was 55%, 87%, and 46% of that of the controls, whereas corresponding values in plasma samples obtained 30 minutes after heparin were 47%, 70%, and 57%, respectively. Lipoprotein lipase activity in the three patients with abetalipoproteinemia was 46%, 29%, and 34% of that of the controls in the samples obtained 15 minutes after heparin injection, whereas the values obtained after 30 minutes were 53%, 64%, and 47% of that of the controls. We conclude that an inherent absence of triglyceride-rich lipoproteins, as occurs in abetalipoproteinemia, is associated with reduced enzyme activity of both hepatic lipase and lipoprotein lipase in plasma after heparin administration.

Persistent transaminase elevation due to heterozygous (familial) apolipoprotein B deficiency.

Homozygous apolipoprotein B deficiency can present with fatty liver and raised levels of transaminases. Subjects with heterozygous deficiency are almost always asymptomatic. We report an asymptomatic 26-year-old man with persistently raised transaminases, in whom the diagnosis of heterozygous (familial) apolipoprotein B deficiency was made on the basis of characteristic lipid profile.