Permeation of human plasma lipoproteins in human carotid endarterectomy tissues: measurement by optical coherence tomography

Atherosclerosis is an inflammatory process occurring in arterial tissue, involving the subintimal accumulation of LDL. Measurement of the rate at which LDL and other lipoproteins, such as HDL and VLDL, enter and exit the tissue can provide insight into the mechanisms involved in the development of atherosclerotic lesions. Permeation of VLDL, LDL, HDL, and glucose was measured for both normal and atherosclerotic human carotid endarterectomy tissues (CEA) at 20°C and 37°C using optical coherence tomography (OCT). The rates for LDL permeation through normal CEA tissue were (3.16 ± 0.37) × 10(-5) cm/s at 20°C and (4.77 ± 0.48) × 10(-5) cm/s at 37°C, significantly greater (P < 0.05) than the rates for atherosclerotic CEA tissue at these temperatures [(1.97 ± 0.34) × 10(-5) cm/s at 20°C and (2.01 ± 0.23) × 10(-5) cm/s at 37°C]. This study effectively used OCT to measure the rates at which naturally occurring lipoproteins enter both normal and diseased carotid intimal tissue.

Sirolimus changes lipid concentrations and lipoprotein metabolism in kidney transplant recipients

BACKGROUND

Sirolimus (Rapammune, rapamycin, RAPA) is a strong immunosuppressive agent that reduces kidney transplant rejection. Hyperlipidemia is a significant side effect of sirolimus treatment and often leads to vascular disease. We have studied the repeatability, reversibility, and dose dependence of the plasma lipid and apoprotein changing effects of sirolimus and attempted to determine the mechanism by which sirolimus induces hypertriglyceridemia in some kidney transplant recipients.

METHODS

Six patients with renal allografts maintained on cyclosporine A and prednisone were selected on the basis of their previous hyperlipidemic response to short-term (14 days) sirolimus administration. For longer-term treatment, each patient was started on 10 mg/d sirolimus and continued as tolerated for 42 days to reinduce hyperlipidemia. Timed blood samples were analyzed for lipid, apoprotein, and sirolimus levels.

RESULTS

During sirolimus administration, mean total plasma cholesterol increased from 214 to 322 mg/dL (+50%); low density lipoprotein-cholesterol levels changed in a similar pattern. Mean triglyceride level rose from 227 to 432 mg/dL (+95%). ApoB-100 concentration rose from 124 to 160 mg/dL (+28%). ApoC-III level increased from 28.9 to 55.5 mg/dL (+92%). These lipid and apoprotein changes were found to be repeatable, reversible, and dose dependent. [(13)C(4)]-palmitate metabolic studies in four patients with hypertriglyceridemia indicated that the free fatty acid pool was expanded by sirolimus treatment (mean = 42.3%). Incorporation of [(13)C(4)]-palmitate into triglycerides of very low density lipoprotien, intermediate density lipoprotein, low density lipoproteins was decreased 38.3%, 42.1%, and 38.4%, respectively, by sirolimus treatment of these patients.

CONCLUSIONS

These results suggest that sirolimus alters the insulin signaling pathway so as to increase adipose tissue lipase activity, decrease lipoprotein lipase activity, or both, resulting in increased hepatic synthesis of triglyceride, increased secretion of VLDL, and increased hypertriglyceridemia.

Isolation, quantitation, and characterization of a stable complex formed by Lp[a] binding to triglyceride-rich lipoproteins

Lipoprotein [a] (Lp[a]) is a cholesterol-rich lipoprotein resembling LDL to which a large polymorphic glycoprotein, apolipoprotein [a] (apo[a]), is covalently coupled. Lp[a] usually exists as a free-standing particle in normolipidemic subjects; however, it can associate noncovalently with triglyceride-rich lipoproteins in hypertriglyceridemic (HTG) subjects. In this study, 10-78% of the Lp[a] present in five HTG subjects was found in the triglyceride-rich lipoprotein (TRL) fraction. The Lp[a]-TRL complex was resistant to dissociation by ultracentrifugation (UCF) alone, but was quantitatively dissociated by UCF in the presence of 100 mM proline. Of this dissociated Lp[a], 70-88% was in the form of a lipoprotein resembling conventional Lp[a]. Incubation of Lp[a]-depleted TRL with native Lp[a] resulted in a reconstituted Lp[a]-TRL complex that closely resembled the native isolates in all examined properties. Complex formation was inhibited by several compounds in the order proline > tranexamate > epsilon-aminocaproate >> arginine > lysine. Neither plasminogen nor LDL inhibited binding of Lp[a] to TRL. We observed the preferential binding of Lp[a] containing higher apparent molecular weight apo[a] polymorphs to TRL both in native and reconstituted Lp[a]-TRL complexes. A disproportionate amount of Lp[a] was bound to the larger TRL particles. Although most apo[a] bound to TRL was in the form of conventional Lp[a] particles, lipid-free recombinant apo[a] was observed to bind TRL. These results provide unequivocal evidence of the existence of an Lp[a]-TRL complex under pathophysiologic conditions. The metabolic fate of the Lp[a]-TRL complex, which is more abundant in hypertriglyceridemia, may be different from that of conventional Lp[a], and may contribute uniquely to the progression or severity of cardiovascular disease.

Selective oxidation in vitro by myeloperoxidase of the N-terminal amine in apolipoprotein B-100

In contrast to the multiple low abundance 2,4-dinitrophenylhydrazine-reactive tryptic peptides formed by oxidation of LDL with reagent HOCl in vitro, myeloperoxidase-catalyzed oxidation produces a dominant product in considerably greater yield and selectivity. This modified peptide had a single amino-terminal sequence corresponding to amino acids 53-66 of apolipoprotein B-100 (apoB-100), but its mass spectra indicated a significantly higher mass than could be reconciled with simple modifications of this peptide. Subsequent studies indicate that this product appears to result from N-chlorination of the N-terminal amino group of apoB-100 and dehydrohalogenation to the corresponding imine, which may form the hydrazone derivative directly, or after hydrolysis to the ketone. The methionine residue is oxidized to the corresponding sulfoxide, and the primary sequence peptide (residues 1-14 of apoB-100) is linked by the intramolecular disulfide bond between C-12 and C-61 to the peptide composed of residues 53-66, as we have observed previously (Yang, C-Y., T. W. Kim, S. A. Weng, B. Lee, M. Yang, and A. M. Gotto, Jr. 1990. Proc. Natl. Acad. Sci. USA. 87: 5523-5527) in unmodified LDL. The selective oxidation by myeloperoxidase of the N-terminal amine suggests strong steric effects in the approach of substrate to the enzyme catalytic site, an effect that may apply to other macromolecules and to cell surface molecules.

Effect of sirolimus on the metabolism of apoB100- containing lipoproteins in renal transplant patients

BACKGROUND

Sirolimus (Rapamune, rapamycin, RAPA) is a potent immunosuppressive drug that has reduced the rate of acute rejection episodes by more than 40% in phase III trials when added to an immunosuppression regimen of cyclosporine (CsA) and prednisone. However, RAPA treatment tends to increase lipid levels, particularly among patients with pre-existing hyperlipidemia.

METHODS

To identify the metabolic pathway(s) leading to RAPA-mediated hyperlipidemia, five patients with renal transplants maintained on CsA+/-prednisone+/- azathioprine (AZA) were studied before and after 6 weeks of treatment with RAPA (off RAPA and on RAPA, respectively). Each study patient was infused with a single bolus of [2H4]-lysine to derive metabolic parameters for apoB100-containing lipoproteins by using kinetic analysis based upon quantitation of isotopic enrichment by gas chromatography-mass spectrometry.

RESULTS

Serial lipid measurements revealed that four patients displayed increased plasma triglyceride levels after RAPA treatment, which coincided with significantly higher plasma VLDL-apoB100 concentrations (21.7+/-12.1 mg/dl off RAPA vs. 38.7+/-14.8 mg/dl on RAPA, mean+/-SD, P<0.05). Kinetic analysis showed that the RAPA-induced increase in VLDL-apoB100 concentrations was due to a significant reduction in the fractional catabolic rate (FCR) of very low-density lipoprotein (VLDL) apoB100 (0.83+/-0.65 off RAPA vs. 0.24+/-0.10 on RAPA, mean+/-SD, P<0.05), rather than an enhanced VLDL-apoB100 synthesis. In one patient, RAPA treatment induced hypercholesterolemia but not hypertriglyceridemia. This hypercholesterolemia was due to elevated low-density lipoprotein (LDL) cholesterol levels, which coincided with a decreased FCR of LDL-apoB100. Heparin-induced lipoprotein lipase activity was significantly lower in the immunosuppressed hyperlipidemic patients than in normolipidemic controls. However, RAPA treatment did not significantly alter basal lipoprotein lipase activity in renal transplant patients in this study.

CONCLUSIONS

This study indicates that for renal transplant patients in whom RAPA treatment induces hyperlipidemia, this effect is the result of reduced catabolism of apoB100-containing lipoproteins.

Secreted caveolin-1 stimulates cell survival/clonal growth and contributes to metastasis in androgen-insensitive prostate cancer

Caveolin-1 is an integral protein of caveolae, known to play important roles in signal transduction and lipid transport. We demonstrate that caveolin-1 expression is significantly increased in primary and metastatic human prostate cancer after androgen ablation therapy. We also show that caveolin-1 is secreted by androgen-insensitive prostate cancer cells, and that this secretion is regulated by steroid hormones. Significantly, caveolin-1 was detected in the MDL(3) fraction of serum specimens from patients with advanced prostate cancer and to a lesser extent in normal subjects. Conditioned media from high passage caveolin-1 secreting, androgen-insensitive, LNCaP cells stimulated increased viability and clonal growth of low passage, caveolin-1-negative, androgen-sensitive, LNCaP cells in vitro, and this effect was blocked by treating the media with caveolin-1 antibody. i.p. injections of caveolin-1 antibody suppressed the orthotopic growth and spontaneous metastasis of highly metastatic, androgen-insensitive caveolin-1-secreting mouse prostate cancer. Overall, our results establish caveolin-1 as an autocrine/paracrine factor that is associated with androgen-insensitive prostate cancer. We demonstrate the potential for caveolin-1 as a therapeutic target for this important malignancy.

Evaluation of Lp[a] and other independent risk factors for CHD in Asian Indians and their USA counterparts

Conventional risk factors for coronary heart disease (CHD) do not completely account for the observed increase in premature CHD in people from the Indian subcontinent or for Asian Indians who have immigrated to the USA. The objective of this study was to determine the effect of immigration to the USA on plasma levels of lipoprotein [a] (Lp[a]) and other independent risk factors for CHD in Asian Indians. Three subject groups were studied: group 1, 57 subjects living in India and diagnosed with CHD (CHD patients); group 2, 46 subjects living in India and showing no symptoms of CHD (control subjects); group 3, 206 Asian Indians living in the USA. Fasting blood samples were drawn to determine plasma levels of triglyceride (TG), total cholesterol (TC), low density lipoprotein [LDL cholesterol (LDL-Chol)], high density lipoprotein [HDL cholesterol (HDL-Chol)], apolipoprotein B-100 (apoB-100), and Lp[a]. Apolipoprotein [a] (apo[a]) size polymorphism was determined by immunoblotting. Plasma TG, apoB-100, and Lp[a] concentrations were higher in CHD patients than in control and USA groups. CHD patients had higher levels of TC and LDL-Chol and lower HDL-Chol than control subjects. However, the USA population had higher levels of TC, LDL-Chol, and apoB-100 and lower HDL-Chol than control subjects. Plasma Lp[a] levels were inversely correlated with the relative molecular weight of the more abundant of each subject's two apo[a] isoforms (MAI), and CHD patients showed higher frequencies of lower relative molecular weights among MAI. Our observed changes in lipid profiles suggest that immigrating to the USA may place Asian Indians at increased risk for CHD. This study suggests that elevated plasma Lp[a] confers genetic predisposition to CHD in Asian Indians, and nutritional and environmental factors further increase the risk of CHD. This is the first report implicating MAI size as a predictor for development of premature CHD in Asian Indians. Including plasma Lp[a] concentration and apo[a] phenotype in screening procedures may permit early detection and preventive treatment of CHD in this population.

Quantification of cholesteryl esters in human and rabbit atherosclerotic plaques by magic-angle spinning (13)C-NMR

Accumulation of cholesteryl esters (CEs) is a key event in the formation of atherosclerotic plaques. More recent work suggests a role for CEs in plaque rupture leading to thrombosis, which can result in an acute event such as myocardial infarction or stroke. In this study, we present nuclear magnetic resonance (NMR) protocols for quantification of CEs in plaques in situ. Total CEs quantified by (13)C magic-angle spinning (MAS) NMR in excised plaques from human carotid arteries and rabbit aortic arteries were in good agreement with the amounts determined by subsequent standard chemical assays. The latter analysis is disadvantageous because it requires that plaque lipids be extracted from the tissue, resulting in the loss of all phase information of CEs as well as other major plaque components. With our MAS-NMR protocol, the plaque components are preserved in their native phases. Combining MAS and off-MAS NMR, we were able to quantitatively distinguish isotropic (liquid) CEs from anisotropic (liquid-crystalline) CEs in plaque tissues. In a recent study, we applied a different (13)C MAS-NMR protocol to quantify crystalline cholesterol monohydrate in plaques. Together, these 2 studies describe a new, noninvasive MAS-NMR strategy for the identification and quantification of the major lipid components in plaques in situ. This approach will be useful for investigation of the relationship between plaque rupture and specific lipids in their biologically relevant phases.

Quantification in situ of crystalline cholesterol and calcium phosphate hydroxyapatite in human atherosclerotic plaques by solid-state magic angle spinning NMR

Because of renewed interest in the progression, stabilization, and regression of atherosclerotic plaques, it has become important to develop methods for characterizing structural features of plaques in situ and noninvasively. We present a nondestructive method for ex vivo quantification of 2 solid-phase components of plaques: crystalline cholesterol and calcium phosphate salts. Magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra of human carotid endarterectomy plaques revealed (13)C resonances of crystalline cholesterol monohydrate and a (31)P resonance of calcium phosphate hydroxyapatite (CPH). The spectra were obtained under conditions in which there was little or no interference from other chemical components and were suitable for quantification in situ of the crystalline cholesterol and CPH. Carotid atherosclerotic plaques showed a wide variation in their crystalline cholesterol content. The calculated molar ratio of liquid-crystalline cholesterol to phospholipid ranged from 1.1 to 1.7, demonstrating different capabilities of the phospholipids to reduce crystallization of cholesterol. The spectral properties of the phosphate groups in CPH in carotid plaques were identical to those of CPH in bone. (31)P MAS NMR is a simple, rapid method for quantification of calcium phosphate salts in tissue without extraction and time-consuming chemical analysis. Crystalline phases in intact atherosclerotic plaques (ex vivo) can be quantified accurately by solid-state (13)C and (31)P MAS NMR spectroscopy.

The role of lipoprotein[a] in atherosclerosis

Recent studies confirm and extend previous evidence that lipoprotein (Lp) plays a significant role in atherosclerosis and is one of the top five or six risk factors for cardiovascular disease. In Japanese patients, Lp levels and apo phenotypes are significant predictors for myocardial infarction. Lp levels are significantly higher in ischemic stroke patients than in controls. However, plasma concentrations of Lp are not predictive of ischemic cerebral infarction in either men or women. Serum Lp levels are significantly higher in patients with carotid plaques or measurable intima-media thickness than in controls without. Despite these associations, there is no significant relationship between Lp level and arterial endothelial function, smooth muscle response, or carotid wall thickness, even though other lipid risk factors like low-density lipoprotein cholesterol (LDL-C) and LDL-C/high-density lipoprotein cholesterol (HDL-C) ratio are correlated with abnormal arterial function and structure. There is new evidence that the association of Lp with extracellular matrix (ECM) secreted by arterial smooth muscle cells increases two- to threefold the subsequent specific binding of LDL. Alpha-defensins released from activated or senescent neutrophils stimulate the binding of Lp to ECM of endothelial cells. Several factors that affect the accumulation of Lp and oxidized LDL in the arterial intima have been identified. Several recent studies have provided new insights into the physiologic role that Lp might play in compromising fibrinolysis. The interaction of Lp with cells is clearly distinct from that with ECM and with fibrinogen; the regulation sites within Lp and plasminogen for these regulatory molecules are not identical. These recent advances bring us significantly closer to understanding how Lp exerts its atherogenic and thrombogenic properties.

Identification of different lipid phases and calcium phosphate deposits in human carotid artery plaques by MAS NMR spectroscopy

Accumulation of lipids in atherosclerotic plaques causes progressive narrowing of the arterial lumen, often followed by thrombosis and ischemia. Currently several different methods, most requiring disruption of the plaque, are used to study the physical properties of lipids accumulated in plaques, and lipid composition is typically determined by chemical analysis of completely disrupted plaques. In this study, 13C magic angle spinning NMR spectroscopy (MAS NMR) was used to determine in situ the lipid composition and molecular organization of all lipid phases in human carotid artery plaques (ex vivo). Protocols were developed to observe signals from one lipid phase without interference from other phases. In addition, 31P MAS NMR detected calcification in plaques by the signals from inorganic phosphate complexed to calcium. Together, 13C and 31P MAS NMR comprise a powerful nondisruptive approach for determining the quantity and phase state of components in arterial plaques.

Lipoprotein(a) in plasma, arterial wall, and thrombus from patients with aortic aneurysm

The plasma concentration of lipoprotein(a) [Lp(a)] is highly correlated with the incidence of cardiovascular and peripheral vascular disease. A positive physiological role for Lp(a) has not yet been clearly identified, although elevated plasma levels in pregnant women, long-distance runners, subjects given growth hormone, patients after cardiovascular surgery, and patients with cancer, diabetes, or renal disease suggest its involvement in tissue synthesis and repair. The hypothesis that Lp(a) is involved in repair/reinforcement of the aorta was tested in 38 patients undergoing surgery for aortic aneurysm. In 29 patients 1 day before surgery, the mean plasma Lp(a) protein level was 10.7 mg/dl. At about 1, 2, and 8 weeks after surgery, the level was 14.1, 15.1, and 15.2 mg/dl, respectively. These levels are significantly higher than those of a comparable group of normal subjects (6.4 mg/dl; n = 274). Specimens of resected aortic aneurysm showed extensive medial degeneration, discontinuous elastic fibers, and deposition of mucopolysaccharides; these specimens were treated with a detergent-containing buffer to extract entrapped lipoproteins. The mean Lp(a) protein level in aortic wall extracts was 14.6 ng/mg tissue; these individual values were significantly associated with plasma Lp(a) levels before surgery (r2 = 0.31, p = 0.0003). The mean Lp(a) protein level in aortic thrombus extracts was substantially higher at 69.6 ng/mg tissue; these individual levels also were significantly associated with plasma Lp(a) concentrations before surgery (r2 = 0.68, p < 0.0001). The observations that: (i) plasma Lp(a) protein is about 1.7-fold higher in patients with aortic aneurysms than in normal subjects; and (ii) that Lp(a) protein in the aneurysmic thrombus is about 4.8-fold higher than in the aortic wall suggest that this lipoprotein plays a significant and direct role in thrombus formation and in reinforcement of the aneurysmic aortic wall.

Template-based docking of a prolactin receptor proline-rich motif octapeptide to FKBP12: implications for cytokine receptor signaling

A conserved proline-rich motif (PRM) in the cytoplasmic domain of cytokine receptors has been suggested to be a signaling switch regulated by the action of the FK506 binding protein (FKBP) family of peptidylprolyl isomerases (O'Neal KD, Yu-Lee LY, Shearer WT, 1995, Ann NY Acad Sci 766:282-284). We have docked the prolactin receptor PRM (Ile1-Phe2-Pro3-Pro4-Val5-Pro6-Gly7-Pro8) to the ligand binding site of FKBP12. The procedure involved conformational search restricted by NMR restraints (O'Neal KD et al., 1996, Biochem J 315:833-844), energy minimization of the octapeptide conformers so obtained, template-based docking of a selected conformer to FKBP12, and energy refinement of the resulting complex. The template used was the crystal structure of a cyclic FK506-peptide hybrid bound to FKBP12. Val5-Pro6 of the PRM was taken to be the biologically relevant Xaa-Pro bond. The docked conformer is stabilized by two intramolecular hydrogen bonds, N7H7-->O4 and N2H2-->O8, and two intermolecular ones, Ile56; N-H-->O = C:Pro6 and Tyr82:O-H-->O = C:Gly7. This conformer features a Type I beta-turn and has extensive hydrophobic contacts with the FKBP12 binding surface. The observed interactions support the hypothesis that FKBP12 catalyzes cis-trans isomerization in the PRM when it is part of the longer cytoplasmic domain of a cytokine receptor, and suggest a significant role for the PRM in signal transduction.

Pathophysiology and treatment of lipid perturbation after cardiac transplantation

In this review we examine the complex interactions between lipoprotein metabolism, immunosuppressive drug therapy, and inflammation and the potential benefits of lipid-lowering drug therapy after heart transplantation. The newer formulations of cyclosporine, Neoral (Novartis Pharmaceuticals; Basle, Switzerland), and other newer agents such as tacrolimus may have advantages in regard to lipid metabolism as compared with traditional triple-drug immunosuppression. Lipoprotein levels may influence both the toxicity and efficacy of cyclosporine. Dyslipidemia may adversely influence inflammation and rejection in the allograft. Two recent clinical trials have shown that lipid-lowering therapy with a 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor alone or in combination with low-density lipoprotein apheresis may confer significant benefits toward preventing transplant coronary artery disease.

Effect of gemfibrozil on levels of lipoprotein[a] in type II hyperlipoproteinemic subjects

Plasma lipoprotein[a] (Lp[a]) levels are highly correlated with angiographically demonstrable coronary heart disease, and elevated Lp[a] is an independent risk factor for atherosclerosis. Previous studies have provided evidence that the levels of Lp[a] and triglyceride are related, suggesting that Lp[a] might be altered by gemfibrozil, a drug well known for its efficacy in reducing plasma triglycerides. Accordingly, 18 type IIa and 16 type IIb hyperlipoproteinemic males aged 35-58 were treated for 3 months with 600 mg of gemfibrozil twice daily. The efficacy of the drug in altering lipid and lipoprotein levels was different in the two type groups. In type IIa and IIb subjects the respective changes in median levels were: total cholesterol, -7.5 and -8.5% triglycerides, -35.6 and -54.4%; HDL-cholesterol, +9.0 and +11.0%; and Lp[a], -17.2 and +6.1%. Before and after gemfibrozil treatment, 7 type IIa and 10 type IIB subjects were given a 100 g/2 m2 oral-fat load; triglycerides and Lp[a] were measured post-prandially at 0, 2, 4, 6, 8, and 10 h. The differences between before- and after-gemfibrozil post-prandial curve integrated areas (PPCIA) were compared for triglycerides and Lp[a]. The changes in median PPCIA for triglycerides in types IIa and IIB were -54% and -53%, and for Lp[a] were -8% and +8%, respectively. These results indicate i) that the levels of Lp[a] are about 2 times higher in type IIa than IIb subjects, and ii) that although gemfibrozil elicits a rather uniform decrease in fasting and post-prandial triglyceride levels in type IIa and IIb patients, the drug causes heterogeneous changes in Lp[a], suggesting that different metabolic mechanisms may be dominant in subjects showing opposing effects.

Multiple cis-trans conformers of the prolactin receptor proline-rich motif (PRM) peptide detected by reverse-phase HPLC, CD and NMR spectroscopy

An eight-amino-acid synthetic peptide (IIe1-Phe2-Pro3-Pro4-Val5-Pro6-Gly7-Pro8) corresponding to the conserved proline-rich motif (PRM) of the intracellular domain of the prolactin receptor (PRL-R) was studied by one- and two-dimensional (1D and 2D) proton NMR spectroscopy in water and DMSO in order to characterize its conformational dynamics. The purified PRL-R PRM peptide eluted as two partially resolved peaks in equilibrium on reverse-phase HPLC (RP-HPLC) at 20 degrees C with a ratio of 60:40. At 30 degrees C, the two peaks coalesced into a single peak The two RP-HPLC peaks correspond to two peptide conformers resulting from the slow cis-trans isomerization of one of the four proline amide bonds. Although the peptide has only three amide (NH) protons, its ID NMR spectrum in water contains approximately 15 discernible NH region peaks, providing evidence for multiple conformers. The amide resonances were assigned on the basis of 2D-COSY spectra, chemical shift values resonance splitting patterns and temperature coefficients. The cis:trans ratio for each proline in water, calculated from integrated intensities and/or peak heights of the appropriate resonances, were Phe2-Pro3 (35:65), Pro3-Pro4 (40:60), Val5-Pro6 (70:30), and Gly7-Pro8 (30:70). Temperature studies (25-70 degrees C) were used to semi-quantitatively estimate the rates of isomerization for the different prolines. In water, Pro8 undergoes rapid isomerization; Pro3 isomerizes at an intermediate rate; while Pro4 and Pro6 both appear to isomerize very slowly since no coalescence of amide resonances was observed. In DMSO, only Pro4 displayed slow isomerization. Slow kinetics combined with a similar 60:40 ratio of conformers determined by RP-HPLC and NMR suggests that isomerization of the Pro3-Pro4 bond generates the two RP-HPLC peaks. Both proximal and distal proline isomerization effects were observed in NMR experiments. All of the 16 theoretical (24 = 16) proline configurations appear to exist in equilibrium in water The predominant (19%) conformation, trans3-trans4-cis6-trans8, may reflect the configuration of the PRM prolines in the native PRL-R. Isomerization of Pro6 from cis to trans generates an interaction between the peptide N-and C-termini, suggesting an overall pseudo-cyclic conformation. This all-trans proline configuration may play an important biochemical role in the function of cytokine/haematopoietin receptors. A model is proposed which suggests that isomerization of the PRM by an immunophilin such as the FK 506-binding protein (FKBP) serves as an on-off switch for cytokine receptor activation.

Race and gender differences in the association of Lp(a) with carotid artery wall thickness. The Atherosclerosis Risk in Communities (ARIC) Study

The association of lipoprotein(a) [Lp(a)] with preclinical atherosclerotic disease is not well established in any race group, particularly African Americans. This report examined the association of Lp(a) with preclinical extracranial carotid atherosclerosis in middle-aged black and white participants in the Atherosclerosis Risk in Communities (ARIC) Study. Study participants (15 124: 2417 black women, 1522 black men, 5907 white women, and 5278 white men) who were 45 to 64 years old at baseline were examined during the period 1987 to 1989. Carotid intimal-medial far-wall thickness was determined by B-mode ultrasonography and expressed as the overall wall thickness mean at six sites to approximate atherosclerosis in the carotid system. Lp(a) was measured as its total protein component, Lp(a) protein, by a double-antibody ELISA for apolipoprotein(a) detection. Mean Lp(a) protein levels were higher in blacks than whites (169.1 and 147.0 microgram/mL in black women and black men, respectively, compared with 86.6 and 75.1 micrograms/mL in white women and white men). Mean carotid wall thickness (in millimeters) varied by race and gender: 0.798 in white men, 0.779 in black men, 0.718 in black women and 0.695 in white women. Multivariable-adjusted Lp(a) protein was independently associated with wall thickness (in millimeters) in white men and black men; among women, however, this association appeared to be stronger when smoking and diabetes were present. A 100-microgram/mL difference in Lp(a) protein was associated with 0.049- and 0.043-mm higher wall thickness values in black men and white men, respectively. Among white women who smoked, the difference in wall thickness was 0.051 mm compared with 0.032 mm for former/never smokers and 0.21 mm in black female diabetics compared with 0.031 mm in black female nondiabetics. These results suggest that Lp(a) is associated with preclinical carotid atherosclerosis in both blacks and whites, but that this association may be affected by the presence of other cardiovascular risk factors, particularly in women.

Effect of L-thyroxine therapy on lipoprotein fractions in overt and subclinical hypothyroidism, with special reference to lipoprotein(a)

The effect of L-thyroxine therapy on lipoprotein fractions was assessed in 15 patients with overt hypothyroidism (14 women and one man aged 45 +/- 3.9 years; thyrotropin [TSH]: mean +/- SEM, 42 +/- 6.5 mIU/L; range, 20.5 to 106.5) and 14 patients with subclinical hypothyroidism (13 women and one man aged 41 +/- 4 years; TSH: mean +/- SEM, 9.1 +/- 1 mIU/L ; range 5.1 to 17.3). Fasting serum lipid levels were measured initially and 4 months after achievement of a euthyroid state with incremental L-thyroxine therapy (TSH: mean +/- SEM, 1.8 +/- 0.4 mIU/L; range, 0.3 to 4.9 for both groups). In the overtly hypothyroid group, restoration of a euthyroid state was associated with a significant reduction in total cholesterol, and apo B. In the subclinically hypothyroid group, there was a significant reduction of only total cholesterol (199.6 +/- 13.2 v 183.4 +/- 11.6 mg/dL) and LDL-C (13.6 +/- 8.4 v 114 +/- 9.25 mg/dL). In contrast, lipoprotein(a) [Lp(a)] was unaffected by the incremental adjustment of L-thyroxine therapy in both groups (overt, 34.3 +/- 8.8 v 35.6 +/- 6.7 mg/dL; subclinical, 23.0 +/- 8.6 v 29.4 +/- 9.5 mg/dL). We conclude that restoration of a euthyroid state in patients with overt hypothyroidism has no significant effect on Lp(a) levels, and confirm that subclinical hypothyroidism is associated with a significant increase in LDL-C, known to have an atherogenic effect.

Human Lp(a): regions in sequences of apoproteins similar to domains in signal transduction proteins

The major apoproteins of Lp(a)--apo(a) and apo B-100--are linked by only one intermolecular disulfide bond. This linkage has been suggested to be located between apo(a) Cys4057 and apo B-100 Cys3734. Several studies, however, have suggested other noncovalent interactions between different regions of apo(a) and apo B-100. One possible mechanism for these interactions may involve the apo(a) proline-rich interkringle regions that share sequence similarities with the proline-rich regions of Src homology 3 (SH3) domain-binding proteins such as 3BP-1. SH3 and SH2 domains, and their respective ligands, proline-rich regions, and phosphotyrosine motifs, are noncatalytic segments common to signal transduction proteins. Therefore, we used sequence comparison algorithms and molecular modeling programs to identify corresponding SH3 and SH2 candidate regions as well as potential phosphotyrosine sites in the apo B-100 sequence. Six SH2 and 16 SH3 candidate regions, along with 21 potential phosphotyrosine sites, are contained in the apo B-100 sequence. In Lp(a), these regions of apo B-100 may be involved in the noncovalent, protein-protein interactions between apo(a) and apo B-100. The presence of candidate SH3 and SH2 regions in apo B-100, and potential phosphotyrosine sites in apo B-100, apo(a), and apo A-I, suggests an alternative signaling pathway unrelated to the known B/E receptor.

Structure and metabolism of lipoprotein (a)

An understanding of the complex structural and metabolic features of lipoprotein (a) is essential because this lipoprotein particle, with poorly recognized physiological functions, has been implicated in both thrombogenesis and atherogenesis. Studies aimed at investigating the structure-function relationship of apolipoprotein (a) have generated new useful information. In-vivo and in-vitro studies have provided insights into the synthesis, secretion, and association of apolipoprotein (a) with apolipoprotein B100 and the mechanisms regulating the lipoprotein (a) concentration in the bloodstream.

Rapid identification of common human pathogens by high-resolution proton magnetic resonance spectroscopy

Routine procedures for recovery of bacteria from clinical specimens involve culturing the latter on various nonselective and selective agar media. The bacteria are then identified by means of biochemical and immunological test procedures. Reduction of the time required to identify the bacteria is highly desirable for rapid clinical diagnosis. Towards this end the potential of proton nuclear magnetic resonance (NMR) spectroscopy for providing a "fingerprint" within the proton spectrum of five bacterial genera, reflecting their characteristic cell wall constituents, has been investigated. Establishing a database of high-resolution proton NMR spectra of a large number of bacterial species is a prerequisite for attaining this objective. A database has been established for five common human pathogens: Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Staphylococcus aureus, and Enterococcus faecalis. On the basis of the presence of characteristic resonances in their spectra, a simple algorithm has been developed to differentiate and identify these microorganisms. The NMR spectra of E. coli and S. aureus showed no dependency on the type of growth medium, growth density, or incubation time.

Quantitation of lipid in biological tissue by chemical shift magnetic resonance imaging

A method combining several previously used approaches is described for the rapid, accurate quantitation of the fat content of biological tissue based on chemical shift images (CSI) corrected for magnetic field inhomogeneity, and compensated for T1 and T2 effects. The gravimetrically determined lipid content of fatty tissues (pork fat, rabbit and human liver) that had been differentially depleted of lipid by chloroform extraction correlated well (r = 0.99) with the lipid image intensities of the respective tissues. This multi-point CSI method was used to quantitate lipid in fresh fatty human liver tissue (wet and dry) containing varying amounts of lipid. Plots of integrated lipid intensity versus tissue lipid content gave straight parallel lines for hydrated (r = 0.94) and dehydrated (r = 0.98) tissues, permitting determination of a proportionality constant for measuring absolute amounts of lipid present in a specific biological tissue. These results suggest the feasibility of using the method in vivo for absolute quantitation of lipid in tissues of agricultural (e.g. pork, beef) and medical (e.g. human liver) interest.

Attenuation of the neurotoxic effect of A beta amyloid peptide by apolipoprotein E

Alzheimer's disease patients have increased frequency of apolipoprotein E allele c4, suggesting apoE4 is a risk factor determining disease. ApoE binds A beta amyloid peptide with great avidity in vitro and in the neuritic plaque. Potentially, binding of A beta to apolipoprotein E could increase A beta neurotoxicity. However, in hippocampal cultures, 0.1 microM apolipoprotein E eliminated the neurotoxicity of 10 microM A beta. Neuronal rescue was dose-dependent and occurred even after 48 hours exposure to A beta, but was overwhelmed by excess A beta. Thus, interaction between these proteins does not directly increase A beta neurotoxicity, and the role of ApoE in Alzheimer's disease remains to be elucidated.

Electron cryomicroscopy and digital image processing of lipoprotein(a)

Electron cryomicroscopy was used to study the structure of human lipoprotein(a) (Lp(a)), a plasma lipoprotein implicated in cardiovascular disease. An individual Lp(a) particle consists of a neutral lipid core within a shell of phospholipid, cholesterol and glycoprotein. In principle, electron cryomicroscopy images of single particles should contain structural detail attributable to the density differences among these components and the surrounding buffer. We observed such structural detail in images of frozen, hydrated Lp(a) particles. Lp(a) particles appeared to be roughly spherical in shape with an average diameter of 210 A. As is generally true for unstained samples in vitreous ice, imaged with a low electron dose, these images have low contrast with low signal-to-noise ratios. To increase the signal-to-noise ratio, we averaged classes of similar particles. We began with a set of 5813 randomly oriented Lp(a) particles and generated classes using a linear multivariate statistical method, followed by hierarchical ascendant classification. Our initial classification, based on only the first eight eigenvectors, separated particles on the basis of gross size and shape. After a rough reference-free alignment step, a second classification used the finer details in the images. This approach yielded class averages with structural detail only faintly visible in the raw, single images.

Long distance runners and body-builders exhibit elevated plasma levels of lipoprotein(a)

A one-point cross-sectional study of 20 sedentary individuals, 20 low-aerobic athletes (body-builders), and 20 high-aerobic athletes (long distance, endurance runners) was conducted in Mexico City, Mexico to determine the influence of these diverse life-styles on the plasma levels of Lp(a). Only non-obese male subjects, aged 23-33, who were nonsmokers, non-alcoholics, and had never used anabolic steroids were included in this study. Blood samples were drawn 24 h following the last period of physical activity, and after a 12-14-h fast-period and a 15-min sitting-rest. Plasma levels of Lp(a) and other parameters, including postheparin lipoprotein lipase (LPL) and hepatic lipase (HL) activities, triglycerides (TG), total cholesterol (TC), LDL cholesterol (LDL-C), and HDL cholesterol (HDL-C), as well as % body fat and muscle mass, and maximum aerobic capacity (VO2max) were measured to determine possible correlations with Lp(a) and to serve as convenient internal standards. Mean Lp(a) concentrations were significantly higher in the runners (52 +/- 19 mg/dl) than in the body-builders (40 +/- 6.4 mg/dl, P < 0.05) and the sedentary subjects (24 +/- 5 mg/dl, P < 0.001). Positive correlations between Lp(a) and Vo2max (P < 0.001), HDL-C (P < 0.005) and HDL2-C subfraction (P < 0.005), and a negative correlation with TG were determined. Agglomerative cluster methods suggested three close-distance clusters and a fourth cluster which is composed of four runners who exhibited low LDL-C/HDL-C and high LPL/HL ratios, high mean Lp(a), HDL2-C, and Vo2max levels, but low TG levels. These data show that some individuals who maintain a life-style of very high level physical exertion may have remarkably elevated plasma Lp(a) concentrations. The highly increased concentrations of Lp(a) in high exercise athletes may represent a normal metabolic response to repeated small tissue injuries resulting from frequent and prolonged large muscle movement.

Recent advances in lipoprotein and atherosclerosis research at Baylor College of Medicine. Apolipoprotein B, lipoprotein[a], and transplantation arteriopathy

A multidisciplinary team approach with focused objectives characterizes research at Baylor College of Medicine into the causes, prevention, and treatment of atherosclerotic disease. Current clinical activities range from programs to modify lifestyle for the primary prevention of coronary artery disease to a large, angiographically monitored lipid-lowering trial. In basic research, much attention has been focused on the plasma lipoproteins and their roles in atherogenesis. The current review highlights recent advances in ongoing basic research involving 1) apolipoprotein (apo) B, whose form apo B-100 serves as a ligand for the low-density lipoprotein receptor; 2) lipoprotein[a], elevated plasma levels of which are predictive of atherosclerotic disease; and 3) transplantation arteriopathy, which impedes long-term survival of cardiac transplant recipients.

Hormonal agents used in lowering lipoprotein(a)

Lipoprotein(a) (Lp(a)) plasma concentrations in Caucasian populations are classified as a quantitative genetic trait. Although the prevailing view has been that Lp(a) levels are affected by age and gender, recent data are beginning to indicate otherwise. Lp(a) levels change throughout life especially in females after menopause. Lp(a) levels decrease in women treated with anabolic steroids such as stanozolol and danazol. The Lp(a) plasma concentration is also profoundly affected by sex hormone variations during pregnancy. In men with prostatic cancer Lp(a) levels are reduced about 50% by estrogen therapy, and increased 20% by orchidectomy. We have evaluated the changes in Lp(a) and lipid levels in postmenopausal women following estrogen/progestogen replacement therapy. The mean level of Lp(a) in treated women was about 50% lower after 6 and 12 months of replacement therapy. A significant correlation between basal Lp(a) levels and the changes at either 6 or 12 months was observed, suggesting that therapy was particularly efficacious in those women with high basal Lp(a) levels. One year after therapy cessation, Lp(a) concentrations tended to return to pre-therapy values. In addition estrogen-progestogen treatment significantly lowered total-cholesterol (12%) and LDL-cholesterol (28%), and increased HDL-cholesterol (18%). From these studies it appears that sex hormones are actively involved in the modulation of plasma Lp(a) levels and that both female and male sex hormones possess a lowering effect. The results confirm a direct effect of sex hormones on Lp(a) metabolism and suggest that estrogen-progestogen treatment of postmenopausal women can improve the lipid profile not only by lowering total- and LDL-cholesterol and raising HDL cholesterol, but also by lowering plasma Lp(a).

Interaction of LDL and Lp[a] with human skin fibroblasts

We have studied the interaction of LDL and Lp[a] with fibroblasts. Our studies suggest that Lp[a] does not effectively compete with LDL for binding to the LDL receptor, and does not efficiently suppress the activity of the intracellular enzyme HMG-CoA reductase. However, Lp[a-], formed by reduction of the disulfide bond between apo[a] and apoB, behaves much like homologous LDL, whether or not apo[a] is removed from the mixture, and in spite of the fact that one or more apoB disulfides may also have been cleaved. In our studies we also noted that Lp[a] often enhanced binding of 125I-LDL by fibroblasts. Further investigation has suggested that this interaction is time-dependent. Experiments in receptor-negative fibroblasts indicate that the enhancement is not related to the presence of the LDL receptor; however, it is inhibited by the removal of calcium from the medium. The presence of sialic acid at millimolar concentrations in the medium inhibits much of the Lp[a]-enhanced binding of 125I-LDL to the cells. These studies suggest that Lp[] may in some way enhance LDL binding to cells, perhaps via interaction with cell surface glycosaminoglycans or proteoglycans or with collagen.

Identification and quantification of apolipoproteins in addition to apo[a] and apo B-100 in human lipoprotein[a]

The protein moiety of Lp[a] is widely believed to consist of one molecule of apo B-100 and one molecule of apo[a] per particle, linked by at least one disulfide bond. In this study we have re-examined the composition of Lp[a] to determine if other less abundant apolipoproteins might be present. Analysis of Lp[a] by sodium dodecyl sulfate-polyacrylamide electrophoresis under reducing conditions showed bands corresponding to < 200 kD but > 50 kD, 40 kD, 26 kD, 23 kD and 9 kD when stained with silver. Western immunoblot analysis of three preparations of Lp[a] revealed the presence of apoE and apoD. Enzyme-linked immunoassays were used to quantify apoA-I, apoA-II, apoC-I, apoC-II, apoC-III, apoE and apo B-100 in Lp[a] and autologous LDL isolated from three healthy males. There is a significant amount of apoA-I in the Lp[a], although the levels varied widely among the different samples. ApoE concentrations were consistent in the three Lp[a] samples and were between 22 and 26% of relative apo B-100 concentrations. Relatively minor amounts of apoA-II and no apoCs were detectable in the three Lp[a] preparations. In contrast, the autologous LDL preparations contained relatively higher amounts of apoA-I, apoA-II, apoE, apoC-I, apoC-II and apoC-III. The identity of the multiple bands corresponding to < 200 kD and > 54 kD and 9 kD is not established.

The relation of lipoprotein[a] concentrations and apolipoprotein[a] phenotypes with asymptomatic atherosclerosis in subjects of the Atherosclerosis Risk in Communities (ARIC) Study

Plasma levels of lipoprotein[a] (Lp[a]) are associated with increased risk of coronary artery disease and show an inverse correlation with apolipoprotein[a] (apo[a]) molecular weight. We determined Lp[a] levels and apo[a] phenotypes in 171 cases with preclinical extracranial carotid atherosclerosis as ascertained by B-mode ultrasound and in 274 control subjects free of carotid atherosclerosis. Lp[a] protein levels measured by enzyme-linked immunosorbent assay ranged from 4 to 361 micrograms/mL in cases and from 2 to 392 micrograms/mL in controls, but median levels of Lp[a] were higher in cases than in controls (51 micrograms/mL versus 33 micrograms/mL, P < .003). In both groups, all 11 apo[a] polymorphs that are resolved by the procedure used were present, resulting in 43 and 39 different apo[a] phenotypes in cases and controls, respectively. An inverse relation between apo[a] polymorph size and Lp[a] level was observed in both cases (r = -0.49, P < .001) and controls (r = -0.34, P < .001). Apo[a] phenotype distributions were similar in cases and controls. However, in 17 phenotypes with three or more subjects per group, the difference of mean Lp[a] concentrations between cases and controls was 32 +/- 36 micrograms/mL (mean +/- SD). Thus, the higher Lp[a] levels in cases were not associated with a greater prevalence of small apo[a] polymorphs. Stepwise logistic regression analyses of known risk factors for coronary heart disease showed that plasma Lp[a] concentration was an independent predictor of case-control status, while Lp[a] phenotype was not, irrespective of the presence or absence of Lp[a] concentration in the model.(ABSTRACT TRUNCATED AT 250 WORDS)

Acrylodan can label amino as well as sulfhydryl groups: results with low-density lipoprotein, lipoprotein[a], and lipid-free proteins

Human plasma lipoprotein[a] and autologous low-density lipoprotein were reacted with the fluorescent probe 6-acryloyl-2-(dimethylamino)naphthalene (acrylodan) previously reported to be specific for sulfhydryl groups. Reaction kinetics were biphasic in both cases. The reaction of bovine serum albumin with acrylodan was also biphasic. Monophasic kinetics were observed when protein free sulfhydryl groups were blocked by carboxamidomethylation prior to acrylodan reaction. A significant increase in total fluorescence was observed in the reaction of acrylodan with proteins containing no free sulfhydryl groups and with polylysine. The rates of these reactions were highly sensitive to pH. Fluorescence changes due to dissolution of probe into hydrophobic protein or lipid domains were minimal as was reaction of probe with phospholipid head groups. When isolated from acrylodan-labeled Lp[a], apo[a], which contains no free sulfhydryl groups, contained covalently bound acrylodan. These results suggest that acrylodan can modify the lysine residues of lipid-free proteins and may modify not only the free sulfhydryl groups of low-density lipoprotein and lipoprotein[a] but also reactive amino groups. We conclude that under these conditions, the use of this probe to quantify free sulfhydryl groups in these lipoproteins is infeasible.

The lowering of lipoprotein[a] induced by estrogen plus progesterone replacement therapy in postmenopausal women

OBJECTIVE

To evaluate whether estrogen plus progesterone replacement therapy influences the plasma lipoprotein[a] (Lp[a]) levels in postmenopausal women.

DESIGN

Fifty-five women who had been menopausal for at least 1 year were followed up for 12 months. Twenty-four subjects served as the control group and 31 subjects served as the therapy group. The therapy consisted of conjugated estrogen (1.25 mg/d) and medroxyprogesterone acetate (10 mg/d for 10 days a month). Blood samples were obtained before the start of therapy and at 6 months and 12 months after therapy. Nine subjects in the therapy group were followed up for an additional year after the treatment was suspended (washout group).

SETTINGS

All subjects were healthy women (mean age, 52 years) who had natural menopause at least 1 year before the beginning of recruitment. None of the women had received exogenous sex steroids or drugs known to influence lipid and lipoprotein metabolism in the previous 12 months.

MAIN RESULTS

In the control group, no change was noted in the plasma Lp[a] concentrations during the study. In the treatment group, the mean plasma Lp[a] concentrations decreased 50% after 6 months (P < .01) and remained at this level 12 months after treatment was started. In the washout group, mean plasma Lp[a] levels tended to return to pretherapy values. In addition, estrogen plus progesterone treatment significantly lowered total cholesterol levels by 15% and low-density lipoprotein cholesterol levels by 30%; it increased high-density lipoprotein cholesterol levels by 19%.

CONCLUSION

The results suggest that in estrogen plus progesterone-treated postmenopausal women, the lipid profile is improved not only by lowering low-density lipoprotein cholesterol levels and raising high-density lipoprotein levels, but also by lowering plasma Lp[a] concentrations.

Lipoprotein[a] as a risk factor for preclinical atherosclerosis

Elevated mean levels of lipoprotein[a] (Lp[a]) have been associated with symptomatic cardiovascular diseases such as clinically manifest myocardial infarction (MI), coronary artery disease, restenosis of coronary artery vein grafts after bypass, and a family history of MI. Associations of Lp[a] with arterial wall thickening in asymptomatic individuals previously have not been addressed and are evaluated in this report among participants of the Atherosclerosis Risk in Communities (ARIC) Study. Intima-media wall thickening in the extracranial carotid arteries was assessed noninvasively with B-mode ultrasonography; Lp[a] was measured as its total protein component. Individuals with wall thickening > or = 90th percentile of the population maximum far-wall thickness were pair matched to participants < 75th percentile of wall thickness by race, gender, center, 10-year age group, and time of examination. These selection criteria yielded 492 matched pairs, with 395 white pairs and 97 black pairs. The mean Lp[a] protein level for all black participants was 174.6 micrograms/mL compared with 77.8 micrograms/mL for whites. Conditional logistic regression analysis for the association of Lp[a] with case-control status yielded a statistically significant prevalence odds ratio (OR) estimate of 1.49, based on a 1-SD difference in Lp[a] protein, after adjusting for age, low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, fibrinogen, hypertension, and cigarette smoking. None of these risk factors significantly altered the OR, in agreement with reports that Lp[a] is unaffected by environmental influences. In addition, no differential effect of Lp[a] protein on case-control status (effect modification) was observed by race, gender, low-density lipoprotein cholesterol, or fibrinogen in this population.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of ligand-binding sites of modeled apo[a] kringle-like sequences in human lipoprotein[a]

Human lipoprotein[a] contains at least two high-molecular-weight, disulfide-linked apolipoproteins, apo[a] and apo B-100. Apo[a] is a highly glycosylated, hydrophilic apoprotein that somewhat resembles plasminogen by containing an extended kringle domain and a carboxyl-terminal serine protease domain. The apo[a] kringle domain is composed of 11 distinct kringle types. Ten of these display high sequence homology to plasminogen kringle 4 (PGK4). The crystallographic coordinates for PGK4 were used to generate three-dimensional molecular models of the apo[a] kringle types, and the lysine-binding region of PGK4 was used to compare the different potential receptor-ligand and ligand-binding sites contained in each different PGK4-like kringle of apo[a]. A receptor-ligand site can be proposed for each kringle type. Potential serine protease cleavage sites, containing arginine-threonine and threonine-arginine, are located on the surface of the kringles. The ligand-binding site of one apo[a] kringle model is almost identical to that of PGK4 and may be a lysine-binding site of apo[a]. Four other apo[a] kringle models appear to have structurally similar lysine-binding sites, but with differences that may influence ligand-polypeptide specificity. Five apo[a] kringle models have ligand-binding sites that probably do not bind lysine; one of these is the highly repeated kringle in the known apo[a] polymorph.

Proposed mechanisms for binding of apo[a] kringle type 9 to apo B-100 in human lipoprotein[a]

The protein component of human lipoprotein[a] consists primarily of two apolipoproteins, apo[a] and apo B-100, linked through a cystine disulfide(s). In the amino acid sequence of apo bd, Cys4057 located within a plasminogen kringle 4-like repeat sequence (3991-4068) is believed to form a disulfide bond with a specific cysteine residue in apo B-100. Our fluorescence-labeling experiments and molecular modeling studies have provided evidence for possible interactions between this apo[a] kringle type and apo B-100. The fluorescent probe, fluorescein-5-maleimide, was used in parallel experiments to label free sulfhydryl moieties in lipoprotein[a] and low-density lipoprotein (LDL). In apo B-100 of LDL, Cys3734 was labeled with the probe, but this site was not labeled in autologous lipoprotein[a]. The result strongly implicates Cys3734 of apo B-100 as the residue forming the disulfide linkage with Cys4057 of apo[a]. To explore possible noncovalent interactions between apo B-100 and apo[a], the crystallographic coordinates for plasminogen kringle 4 were used to generate molecular models of the apo[a] kringle-repeat sequence (3991-4068, LPaK9), the only plasminogen kringle 4 type repeat in apo[a] having an extra cysteine residue not involved in an intramolecular disulfide bond. The Cys4057 residue (henceforth designated as Cys67 in the LPaK9 sequence) is believed to form an intermolecular disulfide bond with a cysteine of apo B-100. In computer graphics molecular models of LPaK9, Cys67 is located on the surface of the kringle near the lysine ligand binding site. Selected segments of the LDL apo B-100 sequence that contain free sulfhydryl cysteines were subjected to energy minimization and docking with the ligand binding site and adjacent regions of the LPaK9 model. In the docking experiments, apo B-100 segment 3732-3745 (PSCKLDFREIQIYK) displayed the best fit and the largest number of van der Waals contacts with models of LPaK9. Other apo B-100 peptides with sulfhydryl cysteine were found to be less compatible when minimized with this kringle. These results support and extend previously suggested mechanisms for a complex interaction between apo[a] and apo B-100 that involve more than a simple covalent disulfide bond.

Carbohydrate composition of protein and lipid components in sialic acid-rich and -poor low density lipoproteins from subjects with and without coronary artery disease

Low density lipoprotein (LDL) from patients with coronary heart disease (CHD) caused 78-286% increase in accumulation of cholesterol in human aortic subendothelial cells compared to 2-17% caused by LDL from normal subjects. Ricin-Sepharose affinity chromatography was used to separate LDL into two subfractions, one sialic acid-rich (SAR) and the other sialic acid-poor (SAP). SAP-LDL from CHD patients caused 156-307% increase in accumulation of cellular cholesterol, whereas SAR-LDL from these patients caused only 14-21% increase. SAP-LDL from normal healthy subjects caused 50-86% increased accumulation, whereas their SAR-LDL induced only 2-12% increase. Carbohydrate analysis of SAP-LDL protein isolated from four CHD patients revealed mean values of 59, 25, 61, and 11 nmoles of N-acetyl glucosamine, galactose, mannose, and sialic acid per mg protein, respectively. Mean values for SAR-LDL protein from these patients were 59, 31, 77, and 24 nmol/mg protein, respectively. Analysis of SAP-LDL protein from four normal healthy subjects indicated respective mean values of 58, 29, 72, and 22 nmol/mg, whereas SAR-LDL protein from normals contained 59, 29, 72, and 29 nmol/mg. The carbohydrate content of LDL lipids represents about 25% of the total carbohydrate present in the lipoprotein. The mean values for SAP-LDL lipids from four CHD patients were about 2, 2, 18, 18, and 2 nmol/mg protein for N-acetyl galactosamine, N-acetyl glucosamine, galactose, glucose, and sialic acid, respectively. The mean values for SAR-LDL lipids from these patients were 3, 4, 34, 41, and 5 nmol/mg, respectively. Analysis of SAP-LDL lipids from four normal healthy subjects indicated respective mean values of 4, 6, 30, 31, and 3 nmol/mg, whereas SAR-LDL lipids from these subjects contained 6, 9, 41, 46, and 7 nmol/mg. These results suggest that the different biological properties of SAR-LDL and SAP-LDL are related to their different carbohydrate compositions.

Proceedings of a National Cancer Institute workshop: MR spectroscopy and tumor cell biology

In December 1991, the National Cancer Institute held a workshop to evaluate the role of magnetic resonance (MR) spectroscopy in human cancer biology. The clinical and basic cancer research issues requiring use of MR spectroscopy, the advantages and limitations of MR spectroscopy, and future directions in MR spectroscopy of cancer were discussed. Consensus-building panels were formed on the following four topics: cell membrane biochemistry, tumor therapeutic response or drug resistance, appropriate model systems, and potential clinical applications of MR spectroscopy. The workshop members concluded that large prospective clinical studies as well as in vivo animal and human studies to define prognostic variables should be performed, with correlation between MR spectroscopic results and biochemical and physiologic features. Studies of phospholipid metabolism, the pharmacokinetics of anticancer agents, and effects of new cancer treatments on the tumor vasculature and normal tissues are needed.

Method for calculating 3-D coordinates from molecular stereograms

The three-dimensional coordinates for the alpha-carbon atoms of crambin and basic pancreatic trypsin inhibitor (BPTI) were determined from the respective alpha-carbon trace stereograms using an improved Simplex algorithm. This algorithm was used in a two-step process to estimate the z-coordinate values. In one approach, an average interatomic distance value, an approximate viewing angle, and a table of digitized values for xleft, yleft and xright, yright are provided in the first step. In the second step, the z-coordinate values are derived by varying z to minimize the bond distance error (Rossmann and Argos, 1980). In another approach, only a reference bond distance table is provided along with the table of xleft, yleft and xright, yright digitized values. In the first step, the viewing angle (phi), a combined scale and viewing distance parameter (q), a rotational angular distortion from digitizing and/or photocopying (z), and translational distortion factors (xerr and yerr) are calculated. In the second step, the z-coordinate values are varied to minimize the bond distance error. RMS difference values of less than 1.5 A were obtained for both crambin and BPTI alpha-carbon atoms.

Triglyceride metabolism in 3T3-L1 cells. An in vivo 13C NMR study

13C nuclear magnetic resonance spectroscopy has been used to study triglyceride metabolism in 3T3-L1 cells incubated with [1-13/14C] acetate, myristate, palmitate, stearate, or oleate. Labeled cells embedded in agarose filaments were perfused in a specially fitted NMR tube within the spectrometer magnet. Incubation of 3T3-L1 cells with a specific fatty acid enriched the cellular triglycerides with that fatty acid; the NMR signal observed in the carbonyl region of the cell spectrum was due in large part to that fatty acid. NMR data demonstrated that cellular enzymes preferentially esterified saturated fatty acids at the glyceride sn-1,3 position and unsaturated fatty acids at the sn-2 position. cellular triglyceride hydrolysis by hormone-sensitive lipase was monitored by measuring the decrease in the integrated intensities of resonances arising from fatty acyl carbonyls esterified at glycerol carbons sn-1,3 and sn-2. Under basal conditions, the time courses were first-order, and the average rates were 0.14% of signal/min at both carbonyl positions. Under isoproterenol stimulated conditions, these rates were still first-order and increased 6.4-fold at the sn-1,3 position and 2.4-fold at the sn-2 position. The observation that the hydrolysis time courses were first-order suggested that only a small amount of cellular triglyceride was available to hormone-sensitive lipase, supporting the view that lipolytic enzymes operate at lipid surfaces where only small amounts of neutral lipid may be soluble. Attempts to correlate the measured rates with the rates of hydrolysis at the sn-1,3 and sn-2 positions were hindered by the fact that the chemical shifts of the carbonyl carbons of the diglyceride hydrolysis product did not overlie those of the triglyceride. Analysis of hydrolysis kinetics revealed that hormone-sensitive lipase exhibited little preference for a particular esterified fatty acid under basal conditions; however, under stimulated conditions, the enzyme exhibited a preference for certain triglyceride species.

Sialic acid content of human low density lipoproteins affects their interaction with cell receptors and intracellular lipid accumulation

Low density lipoproteins (LDL) isolated from the plasma of patients with angiographically demonstrable coronary heart disease (CHD) induced accumulation of triglycerides, free cholesterol, and cholesteryl esters in cultured macrophages, smooth muscle cells, and endothelial cells derived from uninvolved intima of human aorta, but not in skin fibroblasts or hepatoma cells. The sialic acid content of LDL from CHD patients was 40-75% lower than that from healthy donors. There was a negative correlation between LDL sialic acid content and the LDL-induced accumulation of total intracellular cholesterol. Neuraminidase treatment of LDL from normal healthy donors produced sialic acid-depleted LDL (Ds-LDL) which was able to stimulate intracellular lipid accumulation. Neuraminidase treatment of LDL from CHD patients further increased its capacity to induce intracellular lipid accumulation. Sialic acid-poor LDL isolated by affinity chromatography of LDL from CHD patients induced a 2- to 4-fold increase of free and esterified cholesterol in human intimal smooth muscle cells. Binding, uptake, and degradation of 125I-labeled Ds-LDL by macrophages and endothelial cells were 1.5- to 2-fold higher than for native LDL. Binding and uptake of Ds-LDL was inhibited 64-93% by the addition of 20-fold excess acetylated LDL (Ac-LDL); in the inverse experiment, the level of inhibition was 35-54%. These data indicate that a sialic acid-poor form of LDL isolated from CHD patients can interact with both native and scavenger LDL receptors. A sialic acid-poor form of LDL may be a naturally occurring ligand that interacts with the scavenger receptor(s) on macrophages and endothelial cells.

A structural assessment of the apo[a] protein of human lipoprotein[a]

Apolipoprotein[a], the highly glycosylated, hydrophilic apoprotein of lipoprotein[a] (Lp[a]), is generally considered to be a multimeric homologue of plasminogen, and to exhibit atherogenic/thrombogenic properties. The cDNA-inferred amino acid sequence of apo[a] indicates that apo[a], like plasminogen and some zymogens, is composed of a kringle domain and a serine protease domain. To gain insight into possible positive functions of Lp[a], we have examined the apo[a] primary structure by comparing its sequence with those of other proteins involved in coagulation and fibrinolysis, and its secondary structure by using a combination of structure prediction algorithms. The kringle domain encompasses 11 distinct types of repeating units, 9 of which contain 114 residues. These units, called kringles, are similar but not identical to each other or to PGK4. Each apo[a] kringle type was compared with kringles which have been shown to bind lysine and fibrin, and with bovine prothrombin kringle 1. Apo[a] kringles are linked by serine/threonine- and proline-rich stretches similar to regions in immunoglobulins, adhesion molecules, glycoprotein Ib-alpha subunit, and kininogen. In comparing the protease domains of apo[a] and plasmin, apo[a] contains a region between positions 4470 and 4492 where 8 substitutions, 9 deletions, and 1 insertion are apparent. Our analysis suggests that apo[a] kringle-type 10 has a high probability of binding to lysine in the same way as PGK4. In the only human apo[a] polymorph sequenced to date, position 4308 is occupied by serine, whereas the homologous position in plasmin is occupied by arginine and is an important site for proteolytic cleavage and activation. An alternative site for the proteolytic activation of human apo[a] is proposed.

Lipoprotein(a) and apolipoprotein changes after cardiac transplantation

Although lipoprotein changes after cardiac transplantation have been documented, the effects of transplantation and subsequent immunosuppressive therapy (particularly the combination of prednisone, azathioprine and cyclosporine) on apolipoprotein levels and lipoprotein(a) have not been reported. Fasting cholesterol, triglycerides, high density lipoprotein (HDL) cholesterol, low density lipoprotein (LDL) cholesterol, apolipoprotein A-1 and B-100 and lipoprotein(a) were evaluated in 69 consecutive patients during the waiting period before cardiac transplantation. There were 28 deaths before donor organ identification and 41 patients received a cardiac allograft. The lipoprotein levels of transplant recipients were again assayed 3 months postoperatively. Mean (+/- SEM) values increased for total plasma cholesterol (from 180 +/- 8 to 228 +/- 8 mg/dl, p less than or equal to 0.001), triglycerides (from 126 +/- 11 to 207 +/- 14 mg/dl; p less than or equal to 0.001), HDL cholesterol (from 39 +/- 2 to 49 +/- 3 mg/dl; p less than or equal to 0.002) and LDL cholesterol (from 119 +/- 7 to 138 +/- 7 mg/dl; p less than 0.02). Apolipoprotein A-1 and B-100 also increased, but lipoprotein(a) decreased from 11.7 +/- 1.7 to 6.8 +/- 1.1 mg/dl; p less than or equal to 0.0001) after transplantation. Although total cholesterol, triglycerides, LDL cholesterol, apolipoprotein A-1 and B-100 increased dramatically after cardiac transplantation, so did HDL cholesterol, thereby keeping the LDL/HDL cholesterol ratio constant. The surprising decrease in lipoprotein(a) after cardiac transplantation suggests that metabolism of lipoprotein(a) is independent of LDL cholesterol and that immunosuppressive drugs either decrease the synthesis or increase catabolism of lipoprotein(a).

Cholesterol metabolism in hypercholesterolemia-resistant rabbits

Normal rabbits typically respond to a diet high in cholesterol with a large increase in the concentration of plasma cholesterol. We have previously described the breeding and partial characterization of a variant rabbit which does not respond to a high cholesterol diet with changes in plasma cholesterol concentration. In the present report we have characterized three components involved in cholesterol homeostasis: the B/E (LDL) receptor, 3-hydroxy-3-methylglutaryl coenzyme A reductase activity (HMG-CoA reductase, EC 1.1.1.34) and acyl-coenzyme A: cholesterol acyltransferase activity (ACAT, EC 2.3.1.26) in the livers of the hypercholesterolemia-resistant rabbits. Using normal cholesterol-fed rabbit [125I] beta-VLDL as a ligand, liver membranes prepared from resistant rabbits fed a low-cholesterol diet had 70% higher binding capacity than membranes from normal rabbits fed the same diet. Similar experiments demonstrated that the resistant rabbits had a 240% higher B/E receptor binding capacity compared to normal animals when liver membranes were prepared from animals fed a 0.25% cholesterol-enriched diet. No difference in the binding affinity of [125I]beta-VLDL was detected in membranes prepared from normal or resistant animals. When fed a low-cholesterol diet, the resistant rabbits had approximately 2-fold higher hepatic HMG-CoA reductase activity (97.4 +/- 3.5 pmol product/mg/min in resistant animals compared to 45 +/- 1.1 pmol product/min/mg in normal animals). The difference was exaggerated in animals fed the 0.25% cholesterol-enriched diet, 73.3 +/- 5.5 vs 2.4 +/- 0.56 pmol product/min/mg for resistant and normal membranes respectively. The basal activity of ACAT in hepatic membranes was significantly lower in the resistant rabbits compared to normal rabbits (138 +/- 11 vs 268 +/- 19 pmol cholesteryl ester/min/mg in resistant and normal rabbits respectively); when fed a 0.25% cholesterol-enriched diet, the enzyme was induced 6-fold in normal animals but was increased only 2-fold in the resistant animal. These biochemical data suggested that the resistant rabbit maintained low intracellular cholesterol even when fed a cholesterol-enriched diet. Direct measurement of cellular cholesterol and cholesteryl esters demonstrated that the concentration of these lipids was significantly lower in the resistant animal than in normal animals with the largest differences found in the cytoplasmic rather than the membrane compartment. These studies demonstrate that the resistant rabbit manifests several quantitative differences in cholesterol metabolism and in the regulation of cholesterol metabolism; but these studies do not directly explain the underlying cause of the resistance to hypercholesterolemia in the resistant rabbit.

Amino acid sequence of rabbit apolipoprotein E

The complete amino acid sequence of rabbit apolipoprotein E (apoE) was determined by generating three sets of peptides using cyanogen bromide, endoproteinase AspN, and Staphylococcus aureus V8 protease to cleave the protein. Through twenty cycles of sequence analysis on the whole protein, glutamic acid was identified as the N-terminal residue of rabbit apoE; the C-terminus of the protein was identified as glutamine. Based on the sequence of 294 amino acid residues determined by protein structure analysis, the molecular weight of rabbit apoE was determined to be 33,684. The protein sequence differed from the cDNA inferred sequence in 19 positions, only one of which could be attributed to microheterogeneity. The corrected amino acid sequence of rabbit apoE shares 80% homology with the human apoE sequence, 4% greater homology than that inferred from the cDNA sequence. The great similarity in the amino acid sequences of human and rabbit apoE suggests that their physical and physiological properties may also be similar. This homology and the relative ease with which apoE is isolated from rabbit plasma make it possible to conduct some in vitro experiments with the rabbit apoprotein that would have direct relevance to human apoE, but would be difficult or impossible with the human counterpart because of the quantity of protein required.