Postprandial lipoprotein metabolism: VLDL vs chylomicrons

Abstract

Since Zilversmit first proposed postprandial lipemia as the most common risk of cardiovascular disease, chylomicrons (CM) and CM remnants have been thought to be the major lipoproteins which are increased in the postprandial hyperlipidemia. However, it has been shown over the last two decades that the major increase in the postprandial lipoproteins after food intake occurs in the very low density lipoprotein (VLDL) remnants (apoB-100 particles), not CM or CM remnants (apoB-48 particles). This finding was obtained using the following three analytical methods; isolation of remnant-like lipoprotein particles (RLP) with specific antibodies, separation and detection of lipoprotein subclasses by gel permeation HPLC and determination of apoB-48 in fractionated lipoproteins by a specific ELISA. The amount of the apoB-48 particles in the postprandial RLP is significantly less than the apoB-100 particles, and the particle sizes of apoB-48 and apoB-100 in RLP are very similar when analyzed by HPLC. Moreover, CM or CM remnants having a large amount of TG were not found in the postprandial RLP. Therefore, the major portion of the TG which is increased in the postprandial state is composed of VLDL remnants, which have been recognized as a significant risk for cardiovascular disease.

Figure 1

After fat intake, the intestine secretes chylomicrons (CM), the triglycerides of which are lipolyzed by lipoprotein lipase (LPL). The LPL reaction constitutes the initial process in theformation of triglyceride-rich lipoprotein (TRL) remnants. The VLDL secretion process is partly regulated by the rate of FFA influx to the liver. VLDL triglycerides are lipolyzed by endothelial-bound lipoprotein lipase and VLDL remnant particles are formed. The final TRL remnant composition is modulated by the cholesterol ester transfer protein (CETP) reaction with HDL, hepatic lipase (HL), and the exchange of soluble apolipoproteins such as C-I, C-II, C-III and E. (A) The great majority of the remnants are removed from plasma by receptor-mediated processes and the principal receptors are the LDL receptor and the LDL-receptor-related protein (LRP). It is probable that the CM remnants use both of these routes, whereas the VLDL remnants are more likely to use only the LDL receptor.

Figure 2

The amino acid sequence of the epitope region of the anti-apoB-100 antibody (JI-H) is homologous to an amphipathic helical region of apoE, which suggests that apoE can compete for binding of the antibody with the B-51 amphipathic helical epitope (2270–2321) ( Ref 14). (Nakajima et al. J Clin Ligand Assay 1996;19:177–83.)

Figure 3

Twenty four-hour circulating plasma TG concentrations in subjects before and after 2, 8 and 10 weeks of consuming fructose-sweetened beverages (n = 17). The plasma TG levels were found to be significantly increased during the day associated with food intake in a fructose treatment study. Only in the early morning did the TG levels in all cases returned to the basal levels, and were highest in the middle of the night. Blood samples were collected every hour for 24hs. Between blood samplings, each subject consumed a standardized breakfast (9:00 h), lunch (13:00 h) and dinner (18:00 h) containing 55% of the meal energy as carbohydrate, 30% fat and 15% protein. (Ref. 43) (Stanhope et al. J Clin Invest. 2009; 119:1322–34)

Figure 4

Postprandial changes in the plasma TG, apoB-48 and apoB concentrations before and after an oral fat load. TG displayed a close correlation with apoB-48, but not with apoB (Ref.48) (Nakano et al., Ann Clin Biochem. 2011; 48: 57–64)

Figure 5

Typical profiles of RLP-TG and RLP-C in the postprandial plasma of a normolipidemic (TG<150 mg/dL) and a hyperlipidemic (TG>150 mg) subject after oral fat load. The RLP fraction was monitored by TC and TG with an HPLC system. After 5μL serum was incubated with 50μL of immnoaffinity gel (300 μL of gel suspension solution) for 2h as the RLP-C assay, 100μL of the supernatant of the unbound fraction were applied to the HPLC system. RLP was always detected with a large VLDL particle size (thin line; RLP-C, thick line; RLP-TG) and a small peak in the void fraction comprised of the CM size. The clearance of RLP-C and RLP-TG was delayed in the hyperlipidemic case, while clearance in the normolipidemic case returned to the basal level in 6h ( Ref. 97) (Nakano et al., Clin Chim Acta 2011; 412: 71–78)

Figure 6

The plasma TG, RLP-C, RLP-TG and ApoB-48 concentrations after a fat-load. The bars indicate the S.E.M and each time point is shown for the homo-, heterozygotes and controls. The open circles indicate controls; half-shaded circles indicate heterozygotes; closed circles indicate data from a CETP-deficient homozygote as a reference. CETP deficiency results in a significantly reduced TRL remnant formation in the postprandial state. (Ref. 85)(Inazu et al. Atherosclersois 2008; 196: 953-57).

Figure 7

A typical profile of postprandial RLP in a hyperlipidemic subject. RLP was isolated and fractionated by HPLC from the postprandial plasma of a hyperlipidemic subject (fasting level: TC; 238, TG; 196, HDL-C; 53, LDL-C; 111, RLP-C; 7.0, RLP-TG; 69, apoB; 92, apoB48; 0.86 mg/dL) (in) 4 h after an oral fat load. The RLP fractionated by HPLC was monitored for total cholesterol (TC) and triglycerides (TG) (top) as well as for apoB-48 and apoB-100 (bottom). The scale (perpendicular axis) of the apoB-100 (left) concentration is 4 fold higher than that of apoB-48 (right), but displays a similar area under the curve. The major particles in RLP were of VLDL size as monitored by apoB-100, and the comparatively smaller size of CM as monitored by apoB-48 ( Ref. 48) (Nakano et al., Ann Clin Biochem. 2011; 48: 57–64.)