High Density Lipoprotein Apoprotein Research Articles

Effects of intralipid-induced hypertriglyceridemia on plasma high-density lipoprotein metabolism in the cynomolgus monkey

Low plasma levels of high-density lipoprotein (HDL) and apolipoprotein (apo) A-I often accompany human hypertriglyceridemia. In an animal model of hypertriglyceridemia, the lipoprotein lipase (LPL)-inhibited cynomolgus monkey, we reported that plasma levels of apo A-I were decreased and the fractional catabolic rate (FCR) of HDL apo was increased. To explore whether hypertriglyceridemia alone would alter plasma apo A-I levels and catabolism, hypertriglyceridemia was produced by intravenous (IV) infusion of 20% Intralipid into female cynomolgus monkeys. Baseline plasma triglyceride (TG) levels averaged 106 mg/dL. With infusion of 200 mg/kg/h intralipid TG, plasma TG levels peaked at 967 mg/dL (range, 413 to 1,069; n = 6). More prolonged or more severe hypertriglyceridemia caused serious complications in several monkeys. Despite the severe hypertriglyceridemia, HDL TG content, HDL apoproteins, and plasma apo A-I levels did not markedly change, suggesting that very little HDL remodeling had occurred. Kinetic studies of HDL protein and apo A-I were performed in four pairs of monkeys. The two tracers were removed from the plasma at identical rates. In five pairs of animals, apo A-I turnover during control and intralipid-induced hypertriglyceridemia was not significantly different. We hypothesize that apo A-I FCR is a function of HDL composition. Because Intralipid infusion did not alter HDL composition to the same degree as did LPL inhibition, its effects on HDL apo catabolism were not apparent.

Effect of a moderate alcohol intake on the lipoproteins of normotriglyceridemic obese subjects compared with normoponderal controls

Moderate alcohol intake is frequently associated with an elevated concentration of high-density lipoprotein (HDL), which is one of the potential causes for the relative decrease in cardiovascular risk reported in moderate drinkers. Conversely, low HDL concentrations, particularly HDL 2, in obese subjects may be a risk factor. The effect of 30 g alcohol daily (wine) during 14 days following a period of abstinence was studied in seven normolipidemic obese subjects (body mass index [BMI], 30 ± 1.7 kg/m 2) compared with seven normoponderal controls (BMI, 22 ± 1.2 kg/m 2). Alcohol caused apolipoprotein (apo) AI and apo AII concentrations to increase in all controls by 12% and 16% ( P < .05), but not in obese subjects. Lipoprotein (Lp) AI HDL particles (without AII) were initially in the same proportions in the two groups. Their increase in controls only ( P < .03) was not matched by an increase in HDL 2 in all subjects. In obese subjects, neither Lp AI nor HDL 2 were increased by alcohol, but their HDL-triglyceride (TG) contents, initially elevated, were normalized. Cholesterol ester (CE) transfer activity was not different in controls and obese subjects during abstinence (105.7 ± 40.8 v 104.8 ± 34.5 mmol/mg protein/h). It was notably depressed by alcohol in controls (74.2 ± 27.4, P < .002), but not in obese subjects. It is concluded that (1) The reduced pool of HDL 2 in obese subjects is not due to insufficient Lp AI nor to excess CE transfer; (2) Alcohol acts on HDL both by an increase of the HDL apoprotein pool and by a decrease of cholesterol ester transfer protein (CETP); (3) Obesity inhibits both effects.

Lipid utilization by human lymphocytes is correlated with high-density-lipoprotein binding site activity.

The nature and physiological importance of high-density lipoprotein (HDL) binding sites on unstimulated (resting) and mitogen-activated (blast) human peripheral blood lymphocytes were investigated. Specific HDL binding on resting and blast T-lymphocytes was saturable at 50 micrograms of 125I-HDL/ml and of high affinity, with Kd values of 8.1 x 10(-8) M and 6.5 x 10(-8) M, respectively, and Bmax. values of 79 ng and 180 ng/mg of cell protein respectively at 4 degrees C. Binding of HDL double-labelled with fluorescent dioctadecylindocarbocyanine (Dil) and isotope (125I) as well as of single fluorescence- or isotope-labelled HDL was inhibited competitively by HDL apoproteins. Studies of the cholesterol flux between the cells and HDL showed that HDL, low-density lipoprotein (LDL) or BSA at a concentration of 100 micrograms/ml in the tissue culture medium did not result in a significant difference in exogenous [3H]cholesterol efflux from the cell membrane at 37 degrees C. Proliferating T-blasts incorporated more cholesterol from HDL or LDL than did resting lymphocytes. When the cells were pulsed with 125I-HDL and chased in fresh lipid-free medium, up to 80% of the radioactivity released was not precipitable with trichloroacetic acid. This percentage decreased in a competitive manner when unlabelled HDL was present in the chase incubation medium. Finally, cultivation of lymphocytes with conditioned medium from macrophages increased Dil-HDL binding/uptake, while it was decreased by mevinolin-induced inhibition of hydroxymethylglutaryl-coA reductase. In conclusion, human lymphocytes possess a HDL binding site (receptor) responsible for lipid binding/uptake and concomitant internalization and degradation of apoproteins from HDL, but not for reverse cell membrane cholesterol transport. The activity of the binding site is up-regulated during cell proliferation and down-regulated during cell growth suppression.

Localization of an apolipoprotein A-I epitope critical for activation of lecithin-cholesterol acyltransferase.

Apolipoprotein (apo) A-I, the major apoprotein of human high density lipoprotein, is a vital cofactor for lecithin-cholesterol acyltransferase (LCAT), the plasma enzyme responsible for esterification of free cholesterol associated with high density lipoprotein. This esterification is an important component of the reverse cholesterol transport process. An immunochemical approach was used to test the hypothesis that a discrete region of apoA-I was important for LCAT activation. Three human apoA-I-specific monoclonal antibodies were found to inhibit LCAT activation in vitro in a manner directly proportional to their ability to bind to apoA-I-proteoliposomes in fluid phase immunoassays. This relationship was not observed with another four apoA-I-specific antibodies that also were able to bind to the apoA-I proteoliposomes. The use of synthetic peptides representing short amino acid sequences of the apoA-I molecule facilitated the identification of discrete but overlapping apoA-I epitopes for those antibodies that interfered with LCAT-mediated cholesterol esterification. These epitopes spanned amino acid residues 95-121 of mature apoA-I. Therefore, this region is most likely involved in the activation of LCAT by apoA-I.

Effect of corn and coconut oil-containing diets with and without cholesterol on high density lipoprotein apoprotein A-I metabolism and hepatic apoprotein A-I mRNA levels in cebus monkeys.

The mechanism(s) by which diets containing corn or coconut oil (31% of energy as fat) totally free of cholesterol or with 0.1% added cholesterol by weight (0.3 mg/kcal) affect plasma high density lipoprotein cholesterol (HDL-C), apoprotein (apo) A-I levels, apo A-I kinetics, and hepatic apo A-I mRNA concentrations were investigated in 26 cebus monkeys. Coconut oil-fed monkeys had elevated levels of plasma total cholesterol (217%), very low density lipoprotein plus low density lipoprotein cholesterol (331%), HDL-C (159%), and apo A-I (117%) compared with corn oil-fed animals. Although the addition of cholesterol to the corn oil diet significantly increased these parameters, no such effects were seen when cholesterol was added to the coconut-oil diet. Both the type of fat and cholesterol in the diet significantly affected HDL apo A-I metabolism by decreasing apo A-I fractional catabolic rate and increasing apo A-I production rate in the coconut oil-fed groups. The decrease in apo A-I fractional catabolic rate in the coconut oil-fed animals was also associated with an increase in the HDL core lipid to surface ratio. Liver apo A-I mRNA abundance was elevated in the coconut oil-fed groups; however, dietary cholesterol had no affect on these levels. The lack of parallel effects of dietary fat and cholesterol on apo A-I production rate and liver apo A-I mRNA levels suggests that the increase in the apo A-I production rate observed in the coconut oil-fed groups resulted from the fat-induced rise in liver apo A-I mRNA abundance, whereas the cholesterol-induced rise in the apo A-I production rate resulted from a mechanism other than changes in liver apo A-I mRNA levels.

Bovine lipoprotein and apolipoprotein profiles as influenced by sex and growth

Three (intact) Angus males and females that were half-sibs and born within 21 d of each other were selected for this study. Each animal was bled periodically from birth to slaughter (18 mo) to determine the qualitative composition of plasma lipoproteins and apolipoproteins during its growth and development. Major components observed were: 1) very low-density (VLDL), 2) low-density (LDL), and 3) high-density lipoproteins (HDL). Individual amounts of triglycerides, cholesterol, and proteins for the VLDL were not different (P greater than .05) between sexes at any time during growth and development. At 1 yr of age and 15 mo of age, females had significantly larger (143.4 and 93.5 mg/dl) amounts of protein in the HDL than males (67.0 and 93.5 mg/dl), respectively. Within the male group, the LDL triglyceride concentration of calves was significantly (P less than .05) higher (7.4 mg/dl) than at all other bleeding times. Within the female group, cholesterol values for the VLDL were significantly (P less than .05) larger as calves and weanlings (16.5 and 21.7 mg/dl respectively) than for other bleeding periods. At all stages of growth and development, the HDL apoprotein profiles showed a distinct band with a weight of about 28,000 Da, which represented apolipoprotein-A-I. During the suckling stage, pooling of LDL fractions provided two components on the acrylamide gel (7.5 to 20%), apolipoprotein-B and a low molecular weight band. At 12 and 15 mo, no low molecular weight band was present in the pooled LDL fraction.(ABSTRACT TRUNCATED AT 250 WORDS)

High density lipoprotein apolipoproteins mediate removal of sterol from intracellular pools but not from plasma membranes of cholesterol-loaded fibroblasts.

Cultured cells possess high-affinity binding sites (receptors) for high density lipoprotein (HDL) that appear to mediate removal of excess intracellular cholesterol from cells. To examine the role of intact HDL apoproteins in receptor-mediated cholesterol removal, HDL3 apoproteins were digested with the proteolytic enzymes trypsin and pronase, and the residual particles were used in sterol efflux experiments. Protease treatment abolished the interaction of HDL3 with the 110-kd cell membrane protein postulated to represent the HDL receptor molecule, indicating that this interaction is mediated by HDL apoproteins rather than lipids. Compared with native HDL3 protease-modified HDL3 had a markedly reduced ability to selectively remove sterol from intracellular pools, even though modified particles promoted greater cholesterol efflux from the plasma membrane than did native particles. These results indicate that whereas sterol efflux from plasma membranes is mediated by HDL lipids, removal of excess intracellular sterol from cells is mediated by HDL apoproteins. These findings are consistent with the hypothesis that receptor binding of HDL apoproteins stimulates translocation of excess intracellular sterol to the cell surface where it becomes accessible for removal by HDL or other lipid-rich acceptor particles.

The Thyroxine-Binding Site of Human Apolipoprotein-A-I: Location in the N-Terminal Domain

We tested the ability of nine monoclonal antibodies (MAb) against human apolipoprotein-A-I (apoA-I), the 28.3-kDa major apoprotein of high density lipoproteins (HDL), to inhibit its photoaffinity labeling with [125I]T4. Two forms were evaluated: isolated lipid-free apoA-I (Sigma or Calbiochem) and lipid-complexed apoA-I [HDL2, (density, 1.063-1.125 g/ml) and HDL3 (density, 1.125-1.210 g/ml)]. After labeling with 0.5 nM [125I]T4 in the presence of MAb or normal mouse IgG, the products were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and subsequent densitometric quantitation of radioactivity associated with the 28.3-kDa band. Group I MAbs, namely those having epitopes in the N-terminal portion of apoA-I, include MAb 16 (epitopes at residues 1-16), 4 and 14 (residues 1-86), and 18 (residues 98-105); group II includes MAbs 7,10, 15, and 17 (epitopes at residues 87-148); group III includes MAb 9 (residues 149-243). All group I MAbs inhibited [125I]T4 binding to isolated apoA-I with this order of potency: MAb 16 (-50% to -61%) greater than MAb 14 (-37% to -41%) greater than MAb 4 (-27% to -33%) greater than MAb 18 (-19% to -27%). In the case of lipid-associated apoA-I, the pattern of hierarchy was variable, presumably related to the known markedly polydisperse nature of HDL, but a constant feature, in contrast to the case of isolated apoA-I, was that MAb 4 was more potent than MAb 14. Group II MAbs gave less than 3% inhibition in both isolated and lipid-complexed apoA-I. Group III MAb 9 either failed to inhibit or gave 18-27% inhibition (one preparation each of HDL2 and HDL3). We conclude that the T4 site of apoA-I is in the N-terminal domain of apoA-I, closer to the epitope for MAb 16 than to that for MAb 18, and that conformational changes occurring when apoA-I is associated with lipids in the HDL particle alter the spatial relationship between some epitopes and the T4 site. Our definition of the T4 site of apoA-I is consistent with another set of data showing that heparin failed to inhibit [125I]T4 binding to isolated apoA-I. Heparin is known to interact with clusters of basic residues, and these residues are concentrated in the midregion of apoA-I.

Serum amyloid A protein in very low density – and high density lipoproteins during the course of acute myocardial infarction

In a preceding paper we reported on the detection and characterization of human serum amyloid A protein (SAA) in very low density lipoproteins (VLDL) and high density lipoproteins (HDL) of patients after acute myocardial infarction. Here we describe the time course of the occurrence of SAA in VLDL and HDL in the postinfarction period. SAA reached its maximum in VLDL and HDL approximately 53 h after the acute event. At the peak of the acute-phase response, SAA comprised as much as 38% of the total apoproteins of VLDL and HDL. SAA appeared at the same points in time and with nearly the same concentrations in VLDL and HDL. We conclude that SAA is not exchanged in plasma between lipoproteins of different densities and that this protein is secreted on its own by hepatocytes and not as a part of an already constituted lipoprotein particle.

Cholesterol esters selectively taken up from high-density lipoproteins are hydrolyzed extralysosomally

High-density lipoprotein (HDL) cholesterol esters (CE) are taken up by many cells without parallel uptake of HDL apoproteins. This selective uptake is mediated by reversible incorporation of HDL CE into a plasma membrane pool, from which the CE are internalized. We now show that selectively taken up CE are directed to an extralysosomal destination where they are hydrolyzed and available to the steroidogenic pathway. Cultured human fibroblasts take up HDL CE predominantly by selective uptake. Wolman's disease fibroblasts, which are deficient in lysosomal cholesterol esterase, effectively hydrolyzed CE from HDL, but not CE taken up in low density lipoproteins (LDL); normal fibroblasts hydrolyzed both effectively. Analogously, the lysosomotropic agent chloroquine effectively blocked hydrolysis of LDL CE but not HDL CE. A similar effect of chloroquine was seen in primary cultures of rat adrenal cells, which are very active in selective uptake. More than 50% of HDL CE taken up by adrenal cells appeared in the medium as corticosterone. To examine the subcellular destination of selectively taken up CE, non-hydrolyzable tracers of HDL and LDL CE were simultaneously injected into rats. On fractionation of adrenal glands 24 h after injection, 83% of the HDL CE tracer and 48% of the LDL CE tracer were recovered in cytoplasmic lipid droplets; that LDL tracer in the lipid droplets was accounted for by selective uptake of CE from LDL. Thus, selectively taken up cholesterol esters are processed by a mechanism distinct from the classical endosomal lysosomal pathway, and are delivered to a cytoplasmic compartment.

Lipoproteins of human peripheral lymph

The concentration of cholesterol in peripheral lymph is roughly one tenth of that in the blood plasma of the same subject. In lymph, there is virtually no very low density lipoprotein (VLDL), probably due to low permeability of the vascular endothelium for particles of this size. More than 95% of apo B-containing lipoproteins of lymph have the density of plasma low density lipoproteins (LDL). The concentration of apo A-I and apo A-II in lymph is about 15% of that in plasma; yet about 50% of the total mass of both these main HDL apoproteins is present extravascularly. High density lipoproteins (HDL) of lymph appear square-packing, and the presence of such large HDL particles is the most conspicuous difference between lipoproteins in plasma and in the extravascular fluids. It remains to be seen whether raising plasma apo A-I concentration per se will increase the initial stages of reverse cholesterol transport and also be clinically beneficial.

Ecdysteroid biosynthesis in the crustacean Y‐organ and control by an eyestalk neuropeptide

AbstractThe endocrine axis is described that controls growth, molting, and regeneration in crustaceans: The ecdysial glands (Y‐organs), sources of ecdysteroid hormones, and eyestalk neurosecretory centers, sources of a putative peptide (molt‐inhibiting hormone, MIH) that exerts negative control of Y‐organ function. Current knowledge of the physiology of these components is briefly reviewed, and in this context Y‐organs are discussed as useful models for study of steroidogenesis generally in animals at the cellular and molecular levels. Data are emphasized that demonstrate the utilization of cholesterol by Y‐organs for biosynthesis of at least two ecdysteroid secretion products, and suppression of the process by MIH. Evidence from radioactive tracer experiments is presented to show that (a) cholesterol in circulating hemolymph and in vitro in the presence of hemolymph serum is quantitatively bound to high‐density lipoprotein (HDL), (b) Y‐organs take up cholesterol by an active process, and (c) uptake is greatly accelerated in the absence of MIH (eyestalks absent) or inhibited by eyestalk extract. Results of in vitro experiments with 125I‐labeled HDL suggest that the HDL‐cholesterol complex is taken into Y‐organ cells mediated by receptors for the HDL apoprotein. Measured as a function of HDL concentration in the medium, cellular acid‐precipitable radioactivity (HDL uptake), acid‐soluble radioactivity (HDL degradation), and ecdysone secretion each exhibited saturation kinetics. Also, at saturation levels of external HDL, uptake, degradation, and secretion were significantly higher in Y‐organs from de‐eyestalked crabs than in glands from intact crabs.

In vitro incorporation of radiolabeled cholesteryl esters into high and low density lipoproteins.

We have developed and validated a method for in vitro incorporation of radiolabeled cholesteryl esters into low density (LDL) and high density lipoproteins (HDL). Radiolabeled cholesteryl esters dissolved in absolute ethanol were mixed with LDL or HDL in the presence of lipoprotein-deficient serum (LPDS) as a source of core lipid transfer activity. The efficiency of incorporation was dependent on: a) the core lipid transfer activity and quantity of LPDS, b) the mass of added radiolabeled cholesteryl esters, c) the length of incubation, and d) the amount of acceptor lipoprotein cholesterol. The tracer incorporation was documented by repeat density gradient ultracentrifugation, agarose gel electrophoresis, and precipitation with heparin-MnCl2. The radiolabeling conditions did not affect the following properties of the lipoproteins: 1) chemical composition, 2) electrophoretic mobility on agarose gels, 3) hydrated density, 4) distribution of apoproteins on SDS gels, 5) plasma clearance rates, and 6) immunoprecipitability of HDL apoproteins A-I and A-II. Rat HDL containing radiolabeled cholesteryl esters incorporated in vitro had plasma disappearance rates identical to HDL radiolabeled in vivo.

In vivo apoprotein catabolism of high density lipoproteins in copper-deficient, hypercholesterolemic rats.

High density lipoprotein (HDL) apoprotein catabolism was examined in male Sprague-Dawley rats deficient in dietary copper. Twenty-four rats were randomly divided into two groups: copper-adequate (control, 5 mg of copper/kg diet) and copper-deficient (0.6 mg of copper/kg diet). After 5 weeks, animals were administered a tracer dose of iodinated HDL protein previously isolated from donor rats that were subjected to the same dietary treatments as the test animals. Copper-deficient rats exhibited a 54% increase in plasma volume and a 26% increase in HDL protein concentration above controls. Consequently, the intravascular pool of total HDL protein was increased 2-fold. The fractional catabolic rate of total HDL protein was similar between groups. However, because of the increased intravascular HDL pool in copper-deficient animals, the absolute catabolic rate was greater (640 +/- 49 micrograms/hr vs 316 +/- 12 micrograms/hr in controls). Tissue uptake of total HDL protein in copper-deficient rats tended to be greater in the kidneys, spleen, and testes compared with controls; the heart exhibited a significant 2.3-fold increase. In contrast, the catabolic rate of HDL protein in the liver and adrenal gland were not different between treatment groups. That an obligatory increase in HDL protein uptake was not observed in the liver and adrenal gland (organs which are sensitive to and can further metabolize cholesterol) suggests that these organs may be regulated, possibly contributing to the observed hypercholesterolemia in this model. These data imply that total HDL apoprotein catabolism is increased in response to the increased intravascular pool of HDL in copper-deficient rats.

Hepatic Synthesis of Apoproteins of Very Low Density and High Density Lipoproteins in Perfused Rat Liver: Influence of Chronic Heavy and Moderate Doses of Ethanol

The effects of 6 weeks of heavy and moderate ethanol feeding to rats upon lipids and lipoprotein metabolism were determined. As compared to the control group, the heavy ethanol feeding resulted in the following changes: liver weight/kilogram body weight increased by 48% (p less than 0.001) with a concomitant 52% increase (p less than 0.001) in liver protein/kilogram body weight and a 2.75-fold (p less than 0.001) increase in liver total lipids/kilogram body weight. In contrast, liver DNA/kilogram body weight or per liver was not affected significantly. Plasma cholesterol and triglycerides were higher by 53% (p less than 0.01) and 77% (p less than 0.01), respectively. Liver cholesterol and triglycerides were 4.4-fold and 3.8-fold higher (p less than 0.001), respectively. Plasma total A1 was 1.72-fold higher (p less than 0.001), whereas there was no significant difference in plasma apo E levels between the two groups. However, plasma high density lipoproteins (HDl) apo E was 48% lower (p less than 0.02) while the very low density lipoproteins (VLDL) E was 2.15-fold higher (p less than 0.02). Hepatic total protein synthetic rate in the ethanol group was not significantly different from the control group. In contrast, labeled leucine incorporation into the total secretory proteins was inhibited by 36% (p less than 0.01) in ethanol-fed group.(ABSTRACT TRUNCATED AT 250 WORDS)

Fat-storing (Ito) cells of rat liver synthesize and secrete apolipoproteins: comparison with hepatocytes

Fat-storing cells of the liver store most of the vitamin A of the body. Vitamin A is present in a few large fat droplets within the cell. The aim of our study was to investigate apolipoprotein biosynthesis in isolated fat-storing cells from rat liver during the time in culture. Isolated rat hepatocytes were studied for comparison. Proteins were biosynthetically labeled and further identified by immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis. Apoproteins in culture supernatants were identified by density gradient ultracentrifugation and by one- and two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of aliquots of the gradient. Rat plasma was processed in the same way and used for comparison. Fat-storing cells synthesized and secreted apoprotein E, apoprotein A-I, apoprotein A-IV, and low amounts of apoprotein C. The synthesis of these proteins increased during the culture time, reaching a maximum at the fifth day after isolation. The proteins were identified mostly as apoproteins of high-density lipoprotein. Hepatocytes synthesized and secreted apoproteins of all classes of lipoproteins. The distribution of high-density lipoprotein apoproteins was similar to that of fat-storing cells but hepatocytes produced larger amounts of the apoproteins.

The Primary Structure of Equine Serum Amyloid A (SAA) Protein

The complete amino acid sequence of equine serum amyloid A (SAA) was elucidated. The protein consists of 110 amino acid residues and contains an 8-amino acid residue insertion tentatively located between positions 69 and 70, as compared with human SAA. Microheterogeneities were detected at positions 16, 44, and 59, compatible with the existence of more than one SAA gene in the horse. This corresponds to the situation in man and mouse. Pronounced homology with SAA from man and several animal species was observed, thus confirming the conserved structure of this acute phase reactant and apoprotein of high-density lipoprotein (HDL).

Relationships of plasma lipoprotein concentrations to unbound, albumin‐bound and sex hormone‐binding globulin‐bound fractions of gonadal steroids in men

Most epidemiologic studies of plasma lipoproteins and gonadal steroids have measured total hormone concentrations only. Gonadal steroids are transported in plasma in unbound, albumin-bound and sex hormone-binding globulin (SHBG)-bound forms, only the first two of which are biologically active. In the present study the associations of plasma lipoproteins with the different fractions of testosterone, oestradiol and 5 alpha-dihydrotestosterone (5 alpha DHT) were explored by multiple regression in 70 men aged 52-67 years (mean, 59 years). The principal finding was that the distribution of cholesterol between the two major subclasses of high density lipoprotein (HDL) was correlated with the concentrations of unbound and albumin-bound oestradiol, but not with SHBG-bound oestradiol or any fraction of testosterone or 5 alpha DHT. These associations were independent of plasma triglyceride. Apoproteins A-I and A-II were not correlated with any hormone fraction. Thus, in middle-aged/elderly men HDL subclasses appear to be influenced by circulating estrogenic activity, but not by androgenic activity, through a mechanism that is unrelated to triglyceride transport or HDL apoprotein metabolism.

Promotion of Lymphocyte Growth by High Density Lipoproteins (HDL): Physiological Significance of the HDL Binding Site

The characteristics and physiological relevance of the high density lipoprotein (HDL) binding site on unstimulated and mitogen activated human peripheral blood lymphocytes have been investigated. At 37 °C, specific binding/uptake of fluorescent (dioctadecylin-docarbocyanine, DiI) HDL was observed by cells from healthy donors as well as by those from low density lipoprotein receptor-defective patients; mitogen activated T-blasts exhibited a markedly elevated DiI-HDL uptake compared to resting T-cells. Binding was saturable at 37 °C and of high affinity, with a Kd of 5 × 10−8 M. It was blocked by anti-apoAI polyclonal antibodies (F(ab)2 fraction), but not by anti-apolipoprotein (apo)E, anti-apoAII, or anti-apoB, and was inhibited competitively by HDL apoproteins and an apoAI-protein A fusion protein. T-cell associated DiI-HDL was increased by trypsin treatment (of the cells) and decreased by activation in the presence of HDL or low density lipoprotein. Comparison of the concentration dependencies of growth promotion and specific cell association of HDL indicated that two mechanisms of lipid exchange may be in operation: one a binding-dependent mechanism of cholesterol exchange, with maximal effect in the HDL concentration range (20-200 μg/ml) in which specific binding increases rapidly, and the other a binding-independent exchange of lipids effective at concentrations in which specific binding is saturated (300-5000 μg/ml).

Rat tissues express serum amyloid A protein-related mRNAs.

Serum amyloid A (SAA) is a small (12 kDa) acute-phase apoprotein of high density lipoprotein found in mammals. It is also the precursor to amyloid protein A, the main protein constituent of fibrils found in amyloidosis secondary to chronic or recurrent inflammation--e.g., rheumatoid arthritis. However, rats do not develop amyloidosis and SAA is not an apoprotein of rat high density lipoprotein; thus rats appear to be an exception in regard to expression of SAA genes. We report here that rats do have representatives of the SAA gene family and express two distinct SAA mRNAs. Moreover, the pattern of genes expressed among tissues, and their induction by inflammatory agents, is similar to that of related mouse genes. RNA from various tissues of normal and injured rats was examined by RNA blot hybridization with SAA cDNA and complementary RNA probes for the three murine SAA genes. A SAA mRNA of approximately 400 nucleotides related to mouse SAA1 and SAA2 mRNAs reached a high level in liver 24 hr after injection of bacterial lipopolysaccharide. No extra-hepatic tissues were found to express the SAA1/SAA2-related mRNA. Turpentine induced two hepatic SAA1/SAA2-related mRNAs of approximately 400 and approximately 500 nucleotides in length. Liver SAA1/SAA2-related mRNA hybrid selected and translated in a wheat germ protein-synthesizing system, from lipopolysaccharide- and turpentine-injected rats, produced a single protein with an estimated molecular mass of 8 kDa. This rat liver SAA-related mRNA appears to lack a highly conserved coding region for portions of two amphipathic helical domains and the joining sequence. An mRNA related to mouse SAA3 was found expressed at a high level in lung after lipopolysaccharide but not following turpentine injection. This mRNA was also expressed at high levels in ileum and large intestine of control rats and was not found in the liver of control or challenged rats. These observations show that the SAA gene family is present and expressed in rats and that its expression is found under situations similar to those found in mice. This lends support for the importance of the SAA gene family in the response to injury by vertebrates.