Lipoproteins in lecithin-cholesterol-acyltransferase(LCAT)-deficiency. II. Further studies on the abnormal high-density-lipoproteins

Diabetes induces gender gap on LCAT levels and activity

AIM

High density lipoprotein (HDL) decreases in diabetic women more rapidly than what is observed in diabetic men. Here we aimed to study serum LCAT levels and LCAT activity in men and women with type 2 diabetes (T2DM) as well as healthy controls.

MAIN METHODS

We measured serum LCAT activity and levels in 40 patients with T2DM plus 40 age-sex and body mass index (BMI)-matched controls. The correlation between LCAT levels and activity was measured in the studied groups, stratified according to gender.

KEY FINDINGS

Patients had a lower LCAT activity, diastolic blood pressure, fasting blood sugar, triglyceride, low density lipoprotein cholesterol and a higher LCAT levels than controls. The lower LCAT activity in patients with T2DM was significant after multiple adjustments for age, LCAT levels and BMI, using general linear model (67.9 ± 1.8 vs. 86.5 ± 1.8; P<0.001). Women with T2DM had a lower LCAT activity and a higher LCAT and HDL levels compared to men. The lower LCAT activity in women remained significant after multiple adjustments for age, BMI and LCAT levels; (61.9 ± 3.1 vs. 74.17 ± 3.61; P<0.05). Statin treatment did not have any significant value on the results. HDL was not correlated with LCAT levels or activity in any of the studied populations.

SIGNIFICANCE

We showed that while LCAT activity is decreased in patients with T2DM, LCAT levels is increased. Patients with T2DM exhibit opposing effects on LCAT activity and LCAT production which is more severe in women. Future prospective studies may elucidate the underlying pathways for these observations.

Markedly accelerated catabolism of apolipoprotein A-II (ApoA-II) and high density lipoproteins containing ApoA-II in classic lecithin: cholesterol acyltransferase deficiency and fish-eye disease

Classic (complete) lecithin:cholesterol acyltransferase (LCAT) deficiency and Fish-eye disease (partial LCAT deficiency) are genetic syndromes associated with markedly decreased plasma levels of high density lipoprotein (HDL) cholesterol but not with an increased risk of atherosclerotic cardiovascular disease. We investigated the metabolism of the HDL apolipoproteins (apo) apoA-I and apoA-II in a total of five patients with LCAT deficiency, one with classic LCAT deficiency and four with Fish-eye disease. Plasma levels of apoA-II were decreased to a proportionately greater extent (23% of normal) than apoA-I (30% of normal). In addition, plasma concentrations of HDL particles containing both apoA-I and apoA-II (LpA-I:A-II) were much lower (18% of normal) than those of particles containing only apoA-I (LpA-I) (51% of normal). The metabolic basis for the low levels of apoA-II and LpA-I:A-II was investigated in all five patients using both exogenous radiotracer and endogenous stable isotope labeling techniques. The mean plasma residence time of apoA-I was decreased at 2.08 +/- 0.27 d (controls 4.74 +/- 0.65 days); however, the residence time of apoA-II was even shorter at 1.66 +/- 0.24 d (controls 5.25 +/- 0.61 d). In addition, the catabolism of apoA-I in LpA-I:A-II was substantially faster than that of apoA-I in LpA-I. In summary, genetic syndromes of either complete or partial LCAT deficiency result in low levels of HDL through preferential hypercatabolism of apoA-II and HDL particles containing apoA-II. Because LpA-I has been proposed to be more protective than LpA-I:A-II against atherosclerosis, this selective effect on the metabolism of LpA-I:A-II may provide a potential explanation why patients with classic LCAT deficiency and Fish-eye disease are not at increased risk for premature atherosclerosis despite markedly decreased levels of HDL cholesterol and apoA-I.

Inhibition of hepatic triglyceride secretion and exogenous triglyceride clearance in the cholestatic rat

Extrahepatic biliary obstruction in humans and rats leads to hypertriglyceridemia. The observed hypertriglyceridemia could result from either a defect of plasma triglyceride (TG) catabolism or hepatic over-production of TG. To examine these questions we have used the rat model to determine hepatic TG secretion by the Triton WR-1339 methodology (inhibition of peripheral lipolysis) and exogenous TG clearance (after i.v. injection of Intralipid). Four groups of rats were studied: group OB--48 h post-operative--bile-duct obstructed; group DV--bile diverted; group SC--sham-operated controls; and group FC--48 h fasted, unoperated controls. The hepatic TG secretion rate for group OB rats was a factor of 7 lower than that of either group SC or FC, and 5 times lower than that for group DV. There were no differences between the hepatic TG secretion rates of groups DV and FC or SC. After i.v. injection of Intralipid, plasma TG decreased with first-order kinetics. The rate constant was taken as the exogenous TG clearance rate (ETGCR). Mean ETGCR for group OB was a factor of 3 lower than that for either control group; while the ETGCR for group DV was equivalent to the control groups. Thus biliary diversion does not affect hepatic TG secretion or the ETGCR. The apparent cause of the hypertriglyceridemia of cholestasis in the bile-obstructed rat is impaired plasma TG catabolism.

Promotion of sterol efflux and net transport by apolipoprotein E in lecithin:cholesterol acyltransferase deficiency

In the plasma of 4 subjects homozygous for deficiency of lecithin:cholesterol acyltransferase, the level of many apolipoproteins (apo A-I, apo A-II, apo B, apo D) was greatly relative to normal, while that of apo E is increased 5-fold. The lipoprotein complex containing lecithin:cholesterol acyltransferase with apo A-I and apo D in normal plasma is completely absent. The major part of apo E is unassociated with other apolipoproteins. The apoprotein-dependence of sterol efflux and net transport from human skin fibroblasts into plasma was determined by immunoaffinity chromatography. In normal plasma the major component of efflux of sterol radioactivity from labeled fibroblasts was dependent upon unassociated apo A-I. In LCAT-deficient plasma, apoprotein-dependent efflux was largely a function of unassociated apo E. When fibroblasts were incubated with fibrinogen-free unfractionated LCAT-deficient plasma, there was no spontaneous net transport of sterol either into or from the cells, indicating that efflux and influx rates were in balance. When apo E was removed by affinity chromatography, there was net transport from plasma to cells. These findings suggest a novel metabolic role for apo E in the promotion of sterol transport uncoupled to LCAT-activity.

The separation of human serum high density lipoproteins by hydroxy apatite column chromatography. Evidence for the presence of discrete subfractions

Human serum high density lipoprotein subfractions 2 and 3, isolated by preparative ultracentrifugation after blocking the enzyme phosphatidylcholine: cholesterol acyl transferase, have been subfractionated further by hydroxyapatite column chromatography. From subfraction 2 we reproducibly obtained 5 and from subfraction 3, 6 fractions differing in chemical composition and apolipoprotein content. The fractions eluting at low salt concentrations were composed primarily of apolipoprotein-A polypeptides while those eluting at high salt concentrations consisted primarily of apolipoprotein-C. From all the 11 subfractions only one contained the "arginine-rich" polypeptide. The apolipoprotein-C-containing fractions were richer in triacylglycerol, phospholipids and free cholesterol as compared to the apolipoprotein-A-containing ones. Although small differences of their partial specific volumes existed, the obtained values indicate that all subfractions belonged to the parent density class. The implications of these results to the current view of lipoprotein metabolism are discussed.

Studies on the metabolic defect in Broad-beta disease (hyperlipoproteinaemia type III)

The apoprotein composition of the main lipoprotein fractions (VLDL, LDL-1, LDL-2 and HDL) was studied initially in 15 patients with Broad-beta disease. Analytical isoelectric focusing of urea-soluble apo-VLDL and apo LSL-1 demonstrated a variant pattern of the polymorphic Apoprotein E with a deficient Apo E-III band in all patients. The Apo E-III deficiency pattern was seen in only six out of 304 hyperlipidaemic controls. These six Apo E-III deficient controls had characteristic signs of Broad-beta disease, and thus represented patients not previously recognized as having the disorder. The Apo E focusing patterns were constant on repeated examinations and were stable under different metabolic conditions. The data show that Apo E-III deficiency in VLDL is a specific qualitative marker for Broad-beta disease, allowing an unequivocal diagnosis that had not been possible previously. Indirect evidence suggests that Apo E-III deficiency is the basic lipoprotein abnormality underlying the familial dyslipoproteinaemia.