Remodeling of HDL by CETP in vivo and by CETP and hepatic lipase in vitro results in enhanced uptake of HDL CE by cells expressing scavenger receptor B-I

The transport of HDL cholesteryl esters (CE) from plasma to the liver involves a direct uptake pathway, mediated by hepatic scavenger receptor B-I (SR-BI), and an indirect pathway, involving the exchange of HDL CE for triglycerides (TG) of TG-rich lipoproteins by cholesteryl ester transfer protein (CETP). We carried out HDL CE turnover studies in mice expressing human CETP and/or human lecithin:cholesterol acyltransferase (LCAT) transgenes on a background of human apoA-I expression. The fractional clearance of HDL CE by the liver was delayed by LCAT transgene, while the CETP transgene increased it. However, there was no incremental transfer of HDL CE radioactivity to the TG-rich lipoprotein fraction in mice expressing CETP, suggesting increased direct removal of HDL CE in the liver. To evaluate the possibility that this might be mediated by SR-BI, HDL isolated from plasma of the different groups of transgenic mice was incubated with SR-BI transfected or control CHO cells. HDL isolated from mice expressing CETP showed a 2- to 4-fold increase in SR-BI-mediated HDL CE uptake, compared to HDL from mice lacking CETP. The addition of pure CETP to HDL in cell culture did not lead to increased selective uptake of HDL CE by cells. However, when human HDL was enriched with TG by incubation with TG-rich lipoproteins in the presence of CETP, then treated with hepatic lipase, there was a significant enhancement of HDL CE uptake. Thus, the remodeling of human HDL by CETP, involving CE;-TG interchange, followed by the action of hepatic lipase (HL), leads to the enhanced uptake of HDL CE by cellular SR-BI. These observations suggest that in animals such as humans in which both the selective uptake and CETP pathways are active, the two pathways could operate in a synergistic fashion to enhance reverse cholesterol transport.

Biochemical and physical properties of remnant-HDL2 and of pre beta 1-HDL produced by hepatic lipase

The hepatic lipase acting on triglyceride-rich high-density lipoprotein2 (HDL2) induces the formation of pre beta 1-HDL, leaving a residual alpha-migrating HDL particle that was named "remnant-HDL2" (Barrans, A., Collet, X., Barbaras, R., Jaspard, B., Manent, J., Vieu, C., Chap, H., and Perret, B. (1994) J. Biol. Chem. 269, 11572-11577.]. In this study, these two product particles generated by hepatic lipase were isolated by density gradient ultracentrifugation. Particles were first characterized in terms of chemical composition, density, and mass. The pre beta 1-HDL obtained in vitro contain one to two molecules of apoA-I, associated with phospholipids, and free and esterified cholesterol. When compared to triglyceride-rich HDL2, remnant-HDL2 have lost on average one molecule of apoA-I, 60% of triacylglycerols, and 15% of phospholipids. The estimated composition is concordant with the hypothesis of the splitting of a substrate particle into one pre beta 1-HDL and one remnant-HDL2. Spectroscopic studies were carried out to monitor changes in lipid fluidity upon lipolysis. The fluorescence anisotropy was measured using (1,6)-diphenyl-hexa-(1,3, 5)-triene as a probe, and the degree of order was calculated from electron spin resonance spectra using the 5-nitroxy-derivative of stearic acid. Both approaches showed a decreased lipid fluidity in remnant-HDL2, as compared to triglyceride-rich HDL2. The immunoreactivity of apoA-I toward several monoclonal antibodies was assayed as a reflection of changes of apoA-I conformation. In remnant-HDL2, as compared to triglyceride-rich HDL2, a lower reactivity was noted with the 2G11 antibody, which interacts in the NH2 terminal part of apoA-I. Finally, remnant-HDL2 was clearly different from HDL3 with respect to all of the parameters studied, demonstrating that hepatic lipase does not promote the direct conversion of HDL2 to HDL3. Thus, hepatic lipase produces remnant-HDL2 particles, which display modifications of apoA-I conformation and of fluidity of the lipid environment. This newly described HDL2 subfraction may play a major role in the reverse cholesterol transport.

Remnant high density lipoprotein2 particles produced by hepatic lipase display high-affinity binding and increased endocytosis into a human hepatoma cell line (HEPG2)

We had previously shown that hepatic lipase plays a prominent role in promoting the generation of pre-beta HDL particles from triglyceride rich HDL2, leaving an alpha-HDL particle of decreased size that was named "remnant HDL2" [Barrans, A., et al. (1994) J. Biol. Chem. 269, 11572-11577]. Interestingly, this remnant HDL2 was rapidly cleared by the liver, suggesting a particularly high affinity of those remnant HDL2 for liver cells. In the present study, we attempted to characterize the interaction of remnant HDL2 with HepG2 cells, as compared to those of native triglyceride rich HDL2. Two main observations were made. First, while triglyceride rich HDL2 particles were able to bind only the low-affinity binding sites, the remaining particle generated after hepatic lipase lipolysis the remnant HDL2 was further able to bind to the high-affinity binding sites. Competition experiments indicate that these two remnant HDL2 binding sites were the same as the two HDL3 binding sites previously described [Barbaras, R., et al. (1994) Biochemistry 33, 2335-2340]. This is the first observation on the remodeling dependence of HDL binding onto hepatocytes. Second, following binding on those two binding sites, the remnant HDL2 were faster internalized and in higher amounts than the native triglyceride rich HDL2. All together, these observations suggest that the continuous remodeling of HDL induces different binding and internalization characteristics of the HDL particles and that the high-affinity HDL binding sites might trigger the internalization of apo HDL through the low-affinity binding sites.