Distribution of apolipoproteins A-I and A-IV among lipoprotein classes in rat mesenteric lymph, fractionated by molecular sieve chromatography

Multiple apolipoprotein kinetics measured in human HDL by high-resolution/accurate mass parallel reaction monitoring

Endogenous labeling with stable isotopes is used to study the metabolism of proteins in vivo. However, traditional detection methods such as GC/MS cannot measure tracer enrichment in multiple proteins simultaneously, and multiple reaction monitoring MS cannot measure precisely the low tracer enrichment in slowly turning-over proteins as in HDL. We exploited the versatility of the high-resolution/accurate mass (HR/AM) quadrupole Orbitrap for proteomic analysis of five HDL sizes. We identified 58 proteins in HDL that were shared among three humans and that were organized into five subproteomes according to HDL size. For seven of these proteins, apoA-I, apoA-II, apoA-IV, apoC-III, apoD, apoE, and apoM, we performed parallel reaction monitoring (PRM) to measure trideuterated leucine tracer enrichment between 0.03 to 1.0% in vivo, as required to study their metabolism. The results were suitable for multicompartmental modeling in all except apoD. These apolipoproteins in each HDL size mainly originated directly from the source compartment, presumably the liver and intestine. Flux of apolipoproteins from smaller to larger HDL or the reverse contributed only slightly to apolipoprotein metabolism. These novel findings on HDL apolipoprotein metabolism demonstrate the analytical breadth and scope of the HR/AM-PRM technology to perform metabolic research.

Apolipoprotein A-IV is a novel substrate for matrix metalloproteinases

Screening of matrix metalloproteinase (MMP)-14 substrates in human plasma using a proteomics approach previously identified apolipoprotein A-IV (apoA-IV) as a novel substrate for MMP-14. Here, we show that among the tested MMPs, purified apoA-IV is most susceptible to cleavage by MMP-7, and that apoA-IV in plasma can be cleaved more efficiently by MMP-7 than MMP-14. Purified recombinant apoA-IV (44-kDa) was cleaved by MMP-7 into several fragments of 41, 32, 29, 27, 24, 22 and 19 kDa. N-terminal sequencing of the fragments identified two internal cleavage sites for MMP-7 in the apoA-IV sequence, between Glu(185) and Leu(186), and between Glu(262) and Leu(263). The cleavage of lipid-bound apoA-IV by MMP-7 was less efficient than that of lipid-free apoA-IV. Further, MMP-7-mediated cleavage of apoA-IV resulted in a rapid loss of its intrinsic anti-oxidant activity. Based on the fact that apoA-IV plays important roles in lipid metabolism and possesses anti-oxidant activity, we suggest that cleavage of lipid-free apoA-IV by MMP-7 has pathological implications in the development of hyperlipidemia and atherosclerosis.

Effect of human Apo AIV against lipid peroxidation of very low density lipoproteins

Recent studies have demonstrated that Apo AIV exerts a protective effect against atherosclerosis. Moreover, Qin et al. (Am. J. Physiol. 274 (1998) H1836) have demonstrated that Apo AIV, isolated from rat plasma, exerts an inhibitory effect against Cu(2+)-induced lipid peroxidation of intestinal lymph and LDL. The aim of the study was to investigate whether human Apo AIV exerts a protective effect against Cu(2+)-induced lipid peroxidation. Our results demonstrated that human Apo AIV exerted an inhibitory effect against Cu(2+) and AAPH induced lipid peroxidation of VLDL, as shown by the lower increase in the levels of TBARS and conjugated dienes in lipoproteins preincubated with Apo AIV. In addition, the tryptophan (Trp) and probe 2-(dimethylamino)-6-lauroylnaphthalene (Laurdan) fluorescence studies demonstrated that the modifications of spectral properties in both lipoproteins preincubated with Apo AIV were lower with respect to ox-lipoproteins, suggesting that Apo AIV prevents the modification of physico-chemical properties due to peroxidation.

Intestinal synthesis and lymphatic secretion of apolipoprotein A-IV after cessation of duodenal fat infusion: mediation by bile

We tested whether secretion of apolipoprotein (apo) A-IV depends upon intestinal triglyceride (TG) transport by comparing output kinetics of TG and apo A-IV during and after duodenal lipid infusion in lymph-fistula rats. Lipid infusion (triolein, 40 mumol/h, 8 h) produced increases in lymphatic TG and apo A-IV output. After 8 h, triolein infusate was replaced with glucose-saline; TG output returned to basal levels 4-5 h later. However, apo A-IV output continued at significantly elevated levels until 20 h after the start of the experiment. Bile diversion blocked this continued output of A-IV during the post-lipid period, and resulted in basal TG output that was 75% lower than in bile-intact rats. Return of bile or low-dose triolein infusion (5 mumol/h) into the intestine reversed these effects. There were no differences in hepatic synthesis or filtration of plasma A-IV into lymph between bile-intact and bile-diverted groups. Intestinal A-IV synthesis was elevated in both groups even during the post-lipid period. The results support the hypothesis that intestinal triglyceride transport drives apo A-IV secretion, and suggest the existence of a bile-dependent, post-translational mechanism for the control of lymphatic apo A-IV output.

PYY stimulates synthesis and secretion of intestinal apolipoprotein AIV without affecting mRNA expression

We tested whether exogenous peptide YY (PYY) can stimulate synthesis and lymphatic secretion of intestinal apolipoprotein AIV (apo AIV). Rats with mesenteric lymph fistulas and right atrial cannulas were given continuous intravenous infusions of control vehicle or PYY at 25, 50, 75, 100, or 200 pmol . kg-1 . h-1. PYY (75-200 pmol . kg-1 . h-1) stimulated lymphatic apo AIV output from 1.5- to 3.5-fold higher than basal output. In separate experiments, PYY (100 pmol . kg-1 . h-1) produced a 60% increase in jejunal mucosal apo AIV synthesis but had no effect on mucosal apo AIV mRNA levels at doses up to 200 pmol . kg-1 . h-1. Finally, exogenous PYY infusion (100 pmol . kg-1 . h-1) produced a plasma PYY increment of 30 pM compared with an increment of 18.7 pM in response to ileal infusion of lipid. These results support the hypothesis that PYY may be an endocrine mediator of the effects of distal gut lipid on production and release of intestinal apo AIV, likely via a posttranscriptional mechanism of action.

Apolipoprotein AIV: a potent endogenous inhibitor of lipid oxidation

Overexpression of apolipoprotein (apo) AIV in transgenic mice confers significant protection against atherosclerosis in apoE knockout animals even in the presence of a more severe atherogenic lipid profile. Because lipoprotein oxidation has been recognized to be pivotal in development of atherosclerosis, the antioxidative activity of apoAIV was investigated. Fasting intestinal lymph was used to mimic conditions in the interstitial fluid, the potential site for lipoprotein oxidation in vivo. ApoAIV (10 micrograms/ml) significantly inhibited copper-mediated oxidation of lymph. This inhibitory effect was further evaluated using purified low-density lipoprotein. Addition of apoAIV (2.5 micrograms/ml) increased the time of 50% conjugated diene formation by 2.4-fold, whereas apoE or BSA did not show such a protection even at 20 micrograms/ml. Addition of apoAIV during the propagation phase also resulted in a dose-dependent inhibition. ApoAIV also protected macrophage-induced oxidation of fasting lymph. These results provide the first evidence that apoAIV is a potent endogenous antioxidant.

Sensitivity of serum apolipoprotein A-IV levels to changes in dietary fat content

The distinctive biological properties of apolipoprotein A-IV suggest that its concentration in serum should be specifically regulated by dietary triglyceride. To test this hypothesis, serum lipids, lipoprotein cholesterol, and apolipoprotein levels were measured in 10 normolipidemic male subjects whose dietary fat intake was isocalorically modified over a range of 10%-50% of total calories. Serum apolipoprotein A-IV levels changed significantly from baseline in response to each dietary modification. Apolipoprotein A-IV levels decreased by 21% during the first week of the low-fat (10%) diet, increased to 12% over baseline during the first week of the moderate-fat (25%) diet, and increased further to 35% over baseline during the first week of the high-fat (50%) diet. Unexpectedly, during the second week of each dietary period, apolipoprotein A-IV levels demonstrated statistically significant trends back toward baseline, which were opposite in direction and proportional in magnitude to the changes during the first week. Nonetheless, serum apolipoprotein A-IV levels, but not apolipoprotein A-I or apolipoprotein B levels, were significantly and positively correlated with the percent of total daily caloric intake ingested as dietary fat. It was concluded that serum apolipoprotein A-IV levels are extremely sensitive to acute changes in dietary fat content and over the range of fat intake examined in this study are significantly correlated with the percent of total calories consumed as dietary triglyceride. Furthermore, the results also suggest that apolipoprotein A-IV may be subject to a rapidly acting autoregulatory mechanism.

The effect of insulin deficiency on the metabolism of lipid emulsion models of triacylglycerol-rich lipoproteins in rats

Groups of control rats and rats made insulin deficient by treatment with streptozotocin were injected intravenously (IV) with triacylglycerol-cholesteryl oleate-cholesterol-phospholipid emulsions designed to model the composition of triacylglycerol-rich lipoproteins. Insulin deficiency decreased the removal rates of emulsion triacylglycerol and cholesteryl ester whether fed a regular diet or a high-fat diet. Injection of heparin to stimulate the action of lipoprotein lipase increased the removal rates in both control and insulin-deficient rats, but control values were not restored by heparin given to insulin-deficient rats and compared with controls the difference due to insulin deficiency persisted. When emulsions were injected into functionally hepatectomized insulin-deficient rats the removal of the emulsion triacylglycerols was faster than in controls. Preformed remnants made in functionally hepatectomized donor rats were removed less rapidly after plasma injection into insulin-deficient rats than in control rats. If the remnants were isolated from the plasma by ultracentrifugation this effect disappeared. An emulsion with a supra-physiological content of unesterified cholesterol had increased efficiency of removal of emulsion core lipids from the plasma of insulin-deficient rats, but had negligible influence in control rats. These effects correlated with changes in the apolipoproteins associated with the lipid particles. Compared with control rat plasma, more of apolipoproteins AI and AIV and less of apolipoprotein E isoforms associated with emulsions exposed to insulin-deficient rat plasma.