Streptococcal serum opacity factor promotes cholesterol ester metabolism and bile acid secretion in vitro and in vivo

Abstract

Plasma high density lipoprotein-cholesterol (HDL-C) concentrations negatively correlate with atherosclerotic cardiovascular disease. HDL is thought to have several atheroprotective functions, which are likely distinct from the epidemiological inverse relationship between HDL-C levels and risk. Specifically, strategies that reduce HDL-C while promoting reverse cholesterol transport (RCT) may have therapeutic value. The major product of the serum opacity factor (SOF) reaction versus HDL is a cholesteryl ester (CE)-rich microemulsion (CERM), which contains apo E and the CE of ∼400,000 HDL particles. Huh7 hepatocytes take up CE faster when delivered as CERM than as HDL, in part via the LDL-receptor (LDLR). Here we compared the final RCT step, hepatic uptake and subsequent intracellular processing to cholesterol and bile salts for radiolabeled HDL-, CERM- and LDL-CE by Huh7 cells and in vivo in C57BL/6J mice. In Huh7 cells, uptake from LDL was greater than from CERM (2-4×) and HDL (5-10×). Halftimes for [14C]CE hydrolysis were 3.0 ± 0.2, 4.4 ± 0.6 and 5.4 ± 0.7 h respectively for HDL, CERM and LDL-CE. The fraction of sterols secreted as bile acids was ∼50% by 8 h for all three particles. HDL, CERM and LDL-CE metabolism in mice showed efficient plasma clearance of CERM-CE, liver uptake and metabolism, and secretion as bile acids into the gall bladder. This work supports the therapeutic potential of the SOF reaction, which diverts HDL-CE to the LDLR, thereby increasing hepatic CE uptake, and sterol disposal as bile acids.

Acylation of lysine residues in human plasma high density lipoprotein increases stability and plasma clearance in vivo

“…Of note, while CE uptake from HDL by SR-B1 is known to be selective for the lipid, this may not be the case for HexHDL-CE, where the increased HexHDL stability may result in significant CE uptake by holoparticle uptake. Given that Huh7 cells express SR-B1 [39, 51], these data suggest but do not prove that the reduced CE uptake of HexHDL-CE by Huh7 cells is at least, in part, via reduced SR-B1 uptake.…”

“…At various post injection times, mice were euthanized and their blood collected by heart puncture into EDTA vacutainer tubes; tissues were harvested for lipid analyses as described. [38, 39] Hepatic lipid extracts were separated by high performance thin layer chromatography (HPTLC) on HPTLC Silica Gel 60 plates (EMD Chemicals, Darmsted, Germany) as described. [28, 40] CE, free cholesterol (FC) and oxysterols were separated by developing the plate to 70% of plate length with hexane:ethyl acetate 8:2 v:v. The plate was dried and lipids visualized with iodine vapor.…”

“…Radioactivity in the HPTLC bands was quantified by β-counting. Partitioning [39, 41] was used to determine the fraction of total sterol converted to bile salts. Cold carrier lipids—cholate, taurocholate (30 μg each), cholesterol and CE (50 μg each)—were added to mouse gallbladder bile to minimize non-specific loss of the radioactive sterols.…”

Structural Stability of Streptococcal Serum Opacity Factor

“…During the SOF mediated host-guest association, the cholesteryl ester (CE) flows from the guest to the host and the now delipidated guest is released as a neo HDL. The major protein on the CERM is apo E, a ligand for receptor mediated uptake via multiple hepatic receptors, including LDLR [7, 8]. These findings provoked studies showing that SOF injection into mice reduces their plasma cholesterol ~40% in three hours [9], and according to cell studies, SOF increases hepatic CE uptake by diverting HDL-CE to the LDL receptor [7] and promotes CE metabolism, including bile acid secretion in vitro and in vivo.…”

“…These findings provoked studies showing that SOF injection into mice reduces their plasma cholesterol ~40% in three hours [9], and according to cell studies, SOF increases hepatic CE uptake by diverting HDL-CE to the LDL receptor [7] and promotes CE metabolism, including bile acid secretion in vitro and in vivo. [8] SOF is potent, showing measurable activity at 10 −14 M [9]. Given the novelty and potency of the SOF reaction and its cholesterol-lowering effects in mice we studied the stability of SOF and describe our findings here.…”