Human plasma lipid exchange protein(s): a method for separation of donor and acceptor lipoproteins by heparin-Sepharose chromatography

Progesterone stimulation of HMG-CoA reductase activity in cultured cells

In a previous study we showed that progesterone (PG) stimulated HMG-CoA reductase (HMGR) activity in rat intestinal epithelial cells (IEC-6) incubated in the presence or absence of low-density lipoprotein (LDL) [1,2]. In the present study we examined further the mechanism of this stimulation. We observed that the stimulation of HMGR activity by PG was completely prevented by cycloheximide. Turnover studies utilizing immunoprecipitation of HMGR-labeled with [35S]methionine revealed that PG increased reductase activity by inhibiting HMGR degradation without affecting the synthesis of HMGR. The stimulation of HMGR activity by progesterone could be accounted for by a continuous synthesis of HMGR while its degradation was retarded. In the presence of LDL, the activity of HMGR in IEC-6 cells was effectively inhibited, however PG was able to stimulate HMGR in the presence of LDL. This effect was not due to an interference of normal cellular metabolism of LDL, since PG had no effect on the cellular uptake and lysosomal degradation of 125I-LDL. PG did not affect of the rate of lysosomal hydrolysis of [3H]cholesteryl linoleate-LDL. The free [3H]cholesterol derived from [3H]cholesteryl linoleate-LDL moved to the cell membrane and effluxed to HDL3 in the medium at the same rate in the presence or absence of PG. Although PG did not affect LDL metabolism, pre-treatment of cells with LDL delayed the onset of HMGR stimulation by PG. In IEC-6 cells deprived of LDL for 24 h, the HMGR activity was stimulated immediately following PG addition. In cells pre-treated with LDL for 24 h, the stimulation was delayed by 4 h. Treatment of cells with 25-hydroxycholesterol completely prevented PG stimulation of HMGR activity. We propose that the stimulation of HMGR activity in the presence or absence of LDL is related to the ability of PG to attenuate the formation and/or action of intracellular HMGR repressor molecules which accelerate the degradation of HMGR.

Human plasma lipid transfer protein catalyzes the speciation of high density lipoproteins

The role of purified plasma lipid transfer protein complexes in determining the particle size distribution of human plasma high density lipoproteins (HDL) was examined in vitro. Incubation of HDL2 or HDL3, isolated from normolipemic subjects with very low density lipoproteins (VLDL) or VLDL-remnants and lipid transfer protein complex had little or no effect on HDL particle size. In contrast, HDL isolated from patients with hypertriglyceridemia, designated HDL3D, showed speciation of particle size distribution when incubated with VLDL-remnants and the transfer protein. Incubation of HDL3D with VLDL-remnants and lipid transfer complex resulted in the production of two particles of radius 4.3 and 3.7 nm; incubation with VLDL or in the absence of the transfer protein did not result in a redistribution of particle size. We suggest that the action of lipid transfer protein complex on triacylglycerol-rich lipoprotein remnants and HDL accounts for the low levels of HDL-cholesterol observed in subjects with severe hypertriglyceridemia.

Spontaneous and plasma factor-mediated transfer of pyrenyl cerebrosides between model and native lipoproteins

A series of pyrenyl glucocerebrosides was synthesized by reacylation of psychosine with pyrene-labeled fatty acids having 3-11 methylene units. When incorporated into model high-density lipoproteins consisting of dimyristoylphosphatidylcholine-apolipoprotein A-II complexes and incubated with unlabeled complexes, these lipids exhibited spontaneous transfer. Half times of transfer varied from 1.5 min to 365 min at 37 degrees C. The logarithm of the rate of transfer was linearly related to the number of fatty acyl methylene units and HPLC retention time. Transfer occurred by passage of lipid monomers through the aqueous phase. Spontaneous transfer of the glycolipids also occurred when they were incorporated into native high-density lipoproteins. Rates of transfer between native high-density lipoprotein particles were higher than those observed between model high-density lipoprotein particles. A partially purified lipid exchange protein from plasma, as well as unfractionated lipoprotein-deficient serum, stimulated the transfer of fluorescent glycolipid between model high-density lipoprotein or native high-density lipoprotein and low-density lipoprotein 2-24 fold. The protein also stimulated the transfer of tritiated ganglioside GM3 between native low-density lipoprotein and high-density lipoprotein. This protein may play a role in glycolipid exchange in vivo.

Fluorescence assay of the specificity of human plasma and bovine liver phospholipid transfer proteins

The specificities of a human plasma and bovine liver phospholipid transfer protein were studied using a fluorescence assay based on the transfer of pyrenyl phospholipids. This method was used previously to determine the mechanism of spontaneous transfer of phospholipids between model lipoproteins (Massey, J.B., Gotto, A.M., Jr. and Pownall, H.J. (1982) Biochemistry 21, 3630-3636). The pyrenyl phospholipids varied in the headgroup moiety; pyrenyl phosphatidylcholines contained different fatty acyl chains in the sn-1 position. Model high-density lipoproteins (R-HDL) consisting of apolipoprotein A-I and 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) were used as donor and acceptor particles. As previously shown, the bovine liver protein mediated the transfer of only phosphatidylcholine. In contrast, the human plasma protein transferred all species studied which included a phosphatidylserine, phosphatidylcholine, phosphatidylglycerol, phosphatidylethanolamine, phosphatidic acid, sphingomyelin, galactosylcerebroside, and a diacylglycerol. The activity of these transfer proteins was only slightly affected by changes in the acyl chain composition of the transferring lipid. Pyrenyl and radioactive ([3H]POPC) phospholipids were transferred with equal rates by the human transfer protein, suggesting that this protein has similar binding characteristics for pyrenyl and natural phospholipids. Spontaneous phospholipid transfer occurs by the aqueous diffusion of monomeric lipid where the rate is highly dependent on fatty acyl chain composition. In this study, no correlation between the rate of spontaneous transfer and protein-mediated transfer was found. The apparent Km values for R-HDL and low-density lipoprotein (LDL), when used as acceptors, were similar when based on the number of acceptor particles. The apparent Vmax for the bovine liver protein was identical for R-HDL and LDL but for the plasma protein Vmax was slightly higher for R-HDL. These results suggest that, like the bovine liver protein, the plasma protein functions as a phospholipid-binding carrier that exchanges phospholipids between membrane surfaces. The assay of lipid transfer proteins by pyrenyl-labeled lipids is faster and easier to perform than other current methods, which require separation of donor and acceptor particles, and is suitable for studies on the function and mechanism of action of lipid transfer proteins.

Measurement and prediction of the rates of spontaneous transfer of phospholipids between plasma lipoproteins

The purpose of this report is to develop a correlation between the hydrophobicity of a phospholipid as measured by reversed-phase high-performance liquid chromatography and its rate of spontaneous transfer and to use this correlation to predict the rate of transfer of any homologous lipid from any lipoprotein. We have studied the mechanism of transfer of a series of fluorescent or radiolabeled phospholipids among natural and reassembled serum lipoproteins. Fluorescent phosphatidylcholines included those with 9-(1-pyrenyl)nonanoic acid in the sn-2 position and lauric, myristic, palmitic, stearic, oleic or linoleic acid at sn-1. The radioactive phosphatidylcholines contained [3H]oleic acid in the sn-2 position and lauric, myristic, or palmitic acid at sn-1. The kinetics of transfer of the pyrene-labeled lipid were followed by changes in the excimer fluorescence, and that of the radioactive lipids by separation of the donor (lipid-apolipoprotein recombinant) from the acceptor (single bilayer vesicles) on a column of Sephacryl S-200. The retention time of each lipid was measured by high-performance hydrophobic chromatography through a Waters radially compressed C18 column eluted with 75% isopropanol and 25% triethylammonium phosphate (0.15 M). A linear relationship was observed between the rate-constant of transfer and the retention time which suggest that the rate of desorption of phosphatidylcholines from lipoproteins and vesicles is controlled predominately by the hydrophobic effect. For a homologous series of lipids, the rate of transfer can be predicted from retention times obtained from hydrophobic chromatography. The kinetics of transfer of 1-lauroyl-2-[9-(1-pyrenyl)nonanoyl] phosphatidylcholine between isolated human serum lipoproteins exhibits a linear correlation between the transfer half-time and the size of the donor lipoproteins. As a consequence, transfer from very-low-density lipoprotein is 10-times slower than that observed from high-density lipoproteins. The observed correlations between phospholipid transfer rates and both the Stokes radius of the donor and the retention time of the phospholipid on a hydrophobic column permit one to calculate the rate of transfer of homologous molecules between lipid-protein complexes. The results predict that the spontaneous transfer of phospholipids between plasma lipoproteins would be too slow to be a physiologically important phenomena.