In silico modeling of the dynamics of low density lipoprotein composition via a single plasma sample

Abstract

Lipoproteins play a key role in the development of CVD, but the dynamics of lipoprotein metabolism are difficult to address experimentally. This article describes a novel two-step combined in vitro and in silico approach that enables the estimation of key reactions in lipoprotein metabolism using just one blood sample. Lipoproteins were isolated by ultracentrifugation from fasting plasma stored at 4°C. Plasma incubated at 37°C is no longer in a steady state, and changes in composition may be determined. From these changes, we estimated rates for reactions like LCAT (56.3 µM/h), β-LCAT (15.62 µM/h), and cholesteryl ester (CE) transfer protein-mediated flux of CE from HDL to IDL/VLDL (21.5 µM/h) based on data from 15 healthy individuals. In a second step, we estimated LDL's HL activity (3.19 pools/day) and, for the very first time, selective CE efflux from LDL (8.39 µM/h) by relying on the previously derived reaction rates. The estimated metabolic rates were then confirmed in an independent group (n = 10). Although measurement uncertainties do not permit us to estimate parameters in individuals, the novel approach we describe here offers the unique possibility to investigate lipoprotein dynamics in various diseases like atherosclerosis or diabetes.

Keywords: lecithin:cholesterol acyltransferase; lipase/hepatic; lipid transfer proteins; lipoproteins/kinetics; low density lipoprotein/metabolism.

Fig. 1.

Concept of reaction rate inference using two steps. Top: We consider six enzymatic reactions, as well as production and clearance of LDL. Particles shrink during their residence in plasma from LDL-I to LDL-III. Cellular components consist of erythrocytes, endothelial cells, and so forth. First step: There are no cellular components in isolated plasma, and only three reactions take place. Production and clearance of lipoproteins, as well as interaction with cellular components, do not take place. Particles are no longer in steady state, and dynamic changes in composition take place, enabling us to infer the three respective reaction rates. Second step: Estimated reaction rates are taken over from step 1 (dotted, gray lines). The clearance rate λ is adapted from the literature (dashed lines). Using the steady-state compositional data at baseline, the remaining three reaction rates and composition of LDL particles entering the LDL range are finally estimated by parameter inference.

Fig. 2.

Flux of CE due to CETP and LCAT between lipoprotein fractions. CE flux mediated by LCAT and CETP estimated by the model in µM/h. LCAT produces CE in LDL and HDL and is estimated by the difference in estimated CE change and CE change observed after 1 h of plasma storage at 37°C. LCAT’s contribution to IDL and VLDL is disregarded. For the CETP reaction, CE hetero- (single arrows) and homoexchange (double arrows) are displayed. As the CETP reaction is assumed to be equimolar, the TG heteroexchanges (not shown) correspond to the opposite CE heteroexchanges.

Fig. 3.

Predicted flux of TGs, FC, and esterified cholesterol in the LDL fraction. Flux is represented in µM/h. For the CETP reaction and the FC influx and efflux reaction, only the net flux is displayed. Besides erythrocytes, cellular components also include FC of lipoproteins like nascent VLDL and chylomicrons, which are transferred to LDL via aqueous diffusion.