Selective phospholipid adsorption and atherosclerosis

Adsorption of surfactant to bronchial epithelium: possible role of receptor 'unmasking' in asthma

BACKGROUND

In a recent study in animals it has been shown how surface-active phospholipid (SAPL) in the form of a commercially available micronized (5 micromphi) dry powder (ALECT/PumactantT) was able to reduce afferent neural feedback to the brainstem in response to a methacholine challenge by the same order of magnitude as drugs commonly prescribed for asthma. The underlying theory assumed that adsorption of SAPL to bronchial epithelium masked irritant receptors eliciting the bronchoconstrictor reflex, thus providing a barrier to noxious stimuli entering the lungs.

OBJECTIVE

To test the underlying assumption that SAPL was actually adsorbed (i.e., bound to bronchial epithelium), especially the major and most surface-active component of lung surfactant, namely dipalmitoyl phosphatidylcholine (DPPC). A secondary objective was to investigate any role of phosphatidylglycerol (PG) in promoting the adsorption of DPPC.

METHODS

Radiolabeled DPPC dispersed ultrasonically in saline was used to incubate excised sections of porcine bronchial epithelium. The adsorbed DPPC was then quantified by rigorously rinsing the tissue of adhering fluid and then digesting it for beta-scintillation counting. Each test (n=8 runs) was repeated for ratios of DPPC:PG of 9:1, 7:3 (as per ALECT/PumactantT) and 1:1 for both dipalmitoyl PG (DPPG) and EggPG (as incorporated in ALECT/PumactantT).

RESULTS

Despite rigorous rinsing postincubation, bronchial epithelium was found to adsorb DPPC at a level roughly equivalent to one close-packed monolayer; whereas both DPPG and EggPG promoted the adsorption of DPPC in a dose-dependent manner, reaching an approximate threefold increase for 7:3 DPPC:PG.

CONCLUSION

DPPC adsorbs to bronchial epithelium in amounts necessary for the masking of receptors, and this adsorption (probably chemisorption) is quite strongly promoted by PG either in its indigenous state (DPPG) or in the form (EggPG) used in ALECT to suppress the sensitivity of bronchial irritant receptors in our previous study and in clinical trials just completed.

Plasma sphingomyelin level as a risk factor for coronary artery disease

Only a fraction of the clinical complications of atherosclerosis are explained by known risk factors. Animal studies have shown that plasma sphingomyelin (SM) levels are closely related to the development of atherosclerosis. SM carried into the arterial wall on atherogenic lipoproteins may be locally hydrolyzed by sphingomyelinase, promoting lipoprotein aggregation and macrophage foam cell formation. A novel, high-throughput, enzymatic method to measure plasma SM levels has been developed. Plasma SM levels were related to the presence of coronary artery disease (CAD) in a biethnic angiographic case-control study (279 cases and 277 controls). Plasma SM levels were higher in CAD patients than in control subjects (60+/-29 versus 49+/-21 mg/dL, respectively; P:<0. 0001). Moreover, the ratio of SM to SM+phosphatidylcholine (PC) was also significantly higher in cases than in controls (0.33+/-0.13 versus 0.29+/-0.10, respectively; P:<0.0001). Similar relationships were observed in African Americans and whites. Plasma SM levels showed a significant correlation with remnant cholesterol levels (r=0.51, P:<0.0001). By use of multivariate logistic regression analysis, plasma SM levels and the SM/(SM+PC) ratio were found to have independent predictive value for CAD after adjusting for other risk factors, including remnants. The odds ratio (OR) for CAD was significantly higher for the third and fourth quartiles of plasma SM levels (OR 2.81 [95% CI 1.66 to 4.80] and OR 2.33 [95% CI 1.38 to 3. 92], respectively) as well as the SM/(SM+PC) ratio (OR 1.95 [95% CI 1.10 to 3.45] and OR 2.33 [95% CI 1.34 to 4.05], respectively). The findings indicate that human plasma SM levels are positively and independently related to CAD. Plasma SM levels could be a marker for atherogenic remnant lipoprotein accumulation and may predict lipoprotein susceptibility to arterial wall sphingomyelinase.

Boundary lubrication in vivo

Evidence is reviewed for the concept that the body employs essentially the same lubrication system in many sites in the body where tissues slide over each other with such ease. This system consists of fluid adjacent to surfaces coated with an oligolamellar lining of surface-active phospholipid (SAPL) acting as a back-up boundary lubricant wherever the fluid film fails to support the load--a likely event at physiological velocities. Particular attention is paid to the load-bearing joints, where the issue of identifying the vital active ingredient in synovial fluid is reviewed, coming down--perhaps predictably--in favour of SAPL. It is also explained how Lubricin and hyaluronic acid (HA) could have 'carrier' functions for the highly insoluble SAPL, while HA has good wetting properties needed to promote hydrodynamic lubrication of a very hydrophobic articular surface by an aqueous fluid wherever the load permits. In addition to friction and wear, release is included as another major role of boundary lubricants, especially relevant in environments where proteins are found, many having adhesive properties. The discussion is extended to a mention of the lubrication of prosthetic implants and to disease states where a deficiency of boundary lubricant is implicated, particular attention being paid to osteoarthritis.

An alternative view of the role(s) of surfactant and the alveolar model

Currently, the study of surfactant proteins is much in vogue, but, in the early days, the physics underlying surfactant function was treated somewhat superficially, leaving assumptions that have become culturally embedded, such as the "bubble" model of the alveolus. This review selectively reexamines these assumptions, comparing each combination of alveolar model and role of surfactant for compatibility with the major features of pulmonary mechanics and alveolar stability, morphology, and fluid balance.

Abnormal acyltransferase activities and accelerated cholesteryl ester transfer in patients with nephrotic syndrome

To determine the effects of the nephrotic syndrome (NS) on atherogenic risk, we studied the lipoprotein composition and the activities of lecithin-cholesterol acyltransferase (LCAT), lysolecithin acyltransferase (LAT), and cholesteryl ester transfer (CET) in the plasma of 11 NS patients and 10 control subjects. NS plasma had lower ratios of high-density lipoprotein (HDL) to low-density lipoprotein (LDL) and HDL2/HDL3 and an elevated free cholesterol (FC) to phosphatidyl choline (PC) ratio (1.09 +/- 0.27 in NS and 0.72 +/- 0.21 in controls, P < .02), all of which indicate an increased atherogenic potential. LCAT activity was normal in NS plasma when assayed with an exogenous substrate, but was 40% lower than in control plasma when assayed with the endogenous substrates. However, in vitro addition of serum albumin to NS plasma failed to normalize the LCAT activity. The LAT reaction, which is catalyzed by LCAT protein in the presence of LDL, was 60% to 80% higher in NS plasma, and consequently the ratio of LAT/LCAT activities was increased twofold. CET activity was significantly increased (+160% of control), and this abnormality was attributable to changes in both the acceptor (very-low-density lipoprotein [VLDL] + LDL) and donor (HDL) lipoproteins and possibly in CET protein. These results suggest that the NS may increase the risk of atherosclerosis not only by adversely affecting the concentrations of lipoproteins, but also by altering their composition and function.

Multiple roles for surface-active phospholipid in hypertension

The elusiveness of the agent responsible for primary hypertension and the diversity of its impact upon the body, as reflected by the widely differing and almost independent avenues of research in this field, indicate that the answer could lie with a particularly common substance in the body acting at various levels of fundamental physiological function. This hypothesis pursues some basic physics of phospholipid whereby a change in quantity or quality can affect the capability to generate extreme surface activity manifest as the numerous properties which this agent can impart to blood and to adjacent surfaces by adsorption. Nine possible roles are traced by which surface-active phospholipid could impinge upon neurogenic control of blood pressure, the effects of circulating relaxing factors, blood rheology, atherosclerosis, and the major renal aspects of control by diuresis and association with the antihypertensive neutral renomedullary lipid. This multi-faceted approach offers mechanisms by which diet can affect blood pressure in addition to the traditional emphasis upon the deposition of atheroma.

A high-performance liquid chromatographic procedure for the determination of disaturated phosphatidylcholine in human plasma

Plasma disaturated phosphatidylcholine (DSPC) concentration has been implicated as a risk factor for atherosclerosis. However, suitable methods for the estimation of these compounds in plasma are not available. In this paper, a method for the estimation of DSPC using argentation thin-layer chromatography and high-performance liquid chromatography is described. It is quantitative for the measurement of individual and total DSPC species and is not dependent on fatty acid chain length. The method employs hydrolysis of total plasma phosphatidyl choline by phospholipase C, followed by benzoylation of the diacylglycerols. The benzoates are then fractionated on silver nitrate-impregnated silica gel thin-layer chromatography plates, and the disaturated species separated and quantitated by high-performance liquid chromatography. The method is sensitive and reproducible and allows many samples to be done at once. With this method, the amounts of DSPC were found to be significantly higher in a group of normolipidemic diabetic subjects, compared to age-matched controls.

A physical approach to coeliac disease

The morphological changes used to confirm a diagnosis of coeliac disease (CD) are precisely those predicted if the surface energy of the interface between duodenal epithelium and the luminal contents were raised to levels comparable to those demonstrated by the hydrophobic nature of the stomach wall. This hypothesis pursues the concept that, in coeliac patients, the gastric mucosal barrier (GMB) is effectively extended from the stomach to coat the duodenum, thus closing the window of absorption in the Gl tract. If studies are correct which indicate that the GMB is a physical reality provided by an adsorbed monolayer of surface-active phospholipid (surfactant) which renders surfaces hydrophobic, then it could also act as a barrier to the absorption of nutrients in the duodenum. Gluten, or its more active polypeptide fragment gliadin, exacerbates the situation in CD by acting as very effective carriers for the insoluble phospholipid which is deposited at the duodenal surface where they are digested by brush-border and pancreatic enzymes.

Plasma lipids and acyltransferase activities in experimental nephrotic syndrome

Lecithin:cholesterol acyltransferase (LCAT) and lysolecithin acyltransferase (LAT) are two activities carried out by the same plasma enzyme, but require different apoprotein activators. The LCAT reaction takes place primarily on high density lipoproteins (HDL) and is activated by serum albumin, whereas LAT takes place on low density lipoproteins (LDL) and is inhibited by albumin. In nephrotic syndrome (NS), the levels of serum albumin are reduced, whereas the LDL levels are increased, and therefore, the ratio of LAT/LCAT activities should be increased. To test this hypothesis, we estimated the lipid levels and the two enzyme activities in experimental NS induced in rats by the injection of anti-Fx1A antibody (passive Heymann nephritis). As found in other nephrotic conditions, the plasma lipid levels rose progressively as the proteinuria increased and the serum albumin concentration declined. In addition, the ratio of LAT/LCAT activities increased by about fourfold after nine days of induction of nephritis. The LCAT activity correlated positively and the LAT activity negatively with serum albumin levels. The esterified cholesterol correlated positively with LCAT activity in normal rats but negatively in nephrotic animals, indicating that most of the cholesteryl esters in NS may be non-LCAT derived. The free cholesterol/lecithin ratio, a known risk factor for atherosclerosis, increased significantly in nephrotic rats. Furthermore, since the increase in the LAT activity produces more disaturated lecithins, another putative risk factor, the cumulative risk of coronary heart disease may be increased in long-term NS.

Possible role of adsorbed surfactant in controlling membrane permeability and function

It is well established in the physical sciences that the adsorption of a monolayer of certain surfactants onto the surface of a synthetic membrane used for ultrafiltration can greatly modify its permeability to water and its ability to transmit small solute molecules and ions of physiological interest. In this hypothesis, it is proposed that, when indigenous surfactant is adsorbed to certain membranes in the body, it can similarly modify their permeability. Since adsorption can be a rapidly reversible process, this would provide a simple physical means of controlling the overall level of physiological activity of the membrane and, possibly, an additional means of differentiating membranes according to function. The hypothesis raises many questions concerning its applicability to the general structure of biological membranes, the nature of the surfactant, its ability to adsorb to solid surfaces and the reasons why such a coating may have been missed. There are then the questions of which membranes might benefit most and what happens if the coating is too sparse or is removed unintentionally.

Adsorption of low density lipoproteins onto selected biomedical polymers

This study examines the interaction of human low density lipoprotein (LDL) with a select group of biomedical polymers. The adsorption characteristics of LDL on cured filler-free poly(dimethyl Siloxane) (C-PDMS), Biomer, Cardiomat 610, Kraton 1650, poly(hydroxyethyl methacrylate) (PHEMA) and glass are presented. Adsorption of LDL to charged hydrophilic glass control surfaces occurred rapidly, reaching plateau concentrations within one minute (0.19 +/- 0.01 ug/cm2). Adsorption of LDL to polymer surfaces appeared to be dependent upon both the polymer hydrophobicity (or apolar nature), and flexibility (or dynamic nature) at the interface. Increased surface concentrations were observed for Biomer (0.32 +/- 0.01 ug/cm2) as well as other polymers which exhibited both hydrophobic and elastomeric properties. Temperature changes between 25 degrees C and 37 degrees C were found to significantly influence the surface concentration of LDL on Biomer (0.16 +/- 0.01 ug/cm2 at 25 degrees C versus 0.32 +/- 0.01 ug/cm2 at 37 degrees C). A lipid core phase transition at 36 degrees C was believed to be responsible for the temperature influence. Preliminary competitive adsorption studies of LDL with albumin (HSA) and serum on silicone surfaces suggests that LDL adsorption occurred rapidly and preferentially (0.25 +/- 0.01 ug/cm2 for LDL alone; 0.33 +/- 0.01 ug/cm2 for LDL + HSA; 0.15 +/- 0.01 ug/cm2 LDL + serum). Preliminary studies on the role of LDL in calcification were not conclusive. It can be concluded that LDL adsorption is dependent upon polymer hydrophobicity, flexibility and temperature. Competitive adsorption experiments suggests that LDL may have a substantial influence on protein adsorption.

Premature rupture of membranes and surface energy: possible role of surfactant

Thirty-one human placental membranes of various gestational ages (24 1/2 to 42 weeks) have been studied for their surface properties by use of a goniometer to observe drops of saline and olive oil applied at regular intervals to the amniotic and chorionic surfaces as they dried in air. Results show that the epithelial surfaces of the chorion and amnion and their interface are all appreciably hydrophobic. Surface energy decreased on the amniotic surface with fetal maturity while the surface energy of the chorion was significantly (137%) higher in those displaying premature rupture of the membranes than in others of comparable gestational age. This is explained on the basis that low-energy surfaces (for instance, Teflon) do not stick and provide boundary lubrication which facilitates movement. These properties are needed to avoid the buildup of local mechanical stress which could initiate rupture. It is speculated that the release agent-lubricant is surfactant derived from amniotic fluid and directly adsorbed onto the epithelial walls. On other tissues, the same surfactants have been shown to reduce surface energy, rendering them hydrophobic and imparting many desirable properties such as release, lubrication, and resistance to erosion.

Surfactants identified in synovial fluid and their ability to act as boundary lubricants

Thin-layer chromatography has been used to identify phospholipids extracted from canine synovial fluid, the major component (45%) being phosphatidyl choline (PC). The extracts and their components have been shown to be surface active in reducing the surface tension of water and to be readily adsorbed to hydrophilic solids, whose surfaces then become hydrophobic. These adsorbed monolayers of synovial surfactant were then found to be excellent boundary lubricants in vitro, reducing the coefficient of kinetic friction (mu) in the dry state and under physiological loading by up to 97% for extracts and 99% for PC alone, reaching mu = 0.01. Surface-active phospholipid is put forward as the possible active ingredient in joint lubrication and shown to be consistent with previous biochemical studies to elucidate its identity. The model essentially follows the classical Hardy model for boundary lubrication imparted by surfactants. It is discussed in relation to a new approach in providing artificial lubrication and facilitating tissue release in patients with arthritis.

Surfactant as a release agent opposing the adhesion of tumor cells in determining malignancy

It is well established that adhesion of metastatic cells to the endothelium of the host organ is of major importance in initiating their arrest prior to invasion. However, wherever there is the potential for adhesion and subsequent disruption of normal physiological function in several other situations in vivo unrelated to cancer, the body provides a release agent in the form of surface-active phospholipids. It is proposed here that these surfactants also oppose the inherent adhesiveness of circulating tumour cells, arrest depending upon the relative potencies of glue and anti-glue. Thus, malignancy is determined not just by the "stickiness" of the metastatic emboli but by a general deficiency of surfactant or one of the host in failing to provide an adequately adsorbed layer of this release agent repelling adhesion and overall "rooting" of the seed cells. Moreover, a difference in surface activity of the release agent(s) at different sites can provide an explanation for any differential affinity observed when comparing the detachment of a cell from a primary tumour with its attachment to the host or any difference in metastatic potential of the same cells in different organs.

Characterization and quantification of human plasma lipids from crude lipid extracts by field desorption mass spectrometry

The main classes of human plasma lipids, free fatty acids, cholesterol, cholesterol esters, triglycerides and phosphatidylcholines were identified and quantified from samples of human plasma by field desorption mass spectrometry. A crude lipid extract was prepared from 50 microliters of human plasma and a 1-5% aliquot thereof was subjected directly to field desorption mass spectrometry without any chromatographic separation of the extract constituents. Within the classes of free fatty acids, triglycerides and phosphatidylcholines the fatty acid profile was obtained directly from the mass spectrum and accurate quantification could be achieved by the addition of stable isotope labelled internal standards.

Fatty-acid composition of serum lipids predicts myocardial infarction

During a follow-up of five to seven years 33 out of 1222 middle-aged men initially free of coronary heart disease sustained fatal or non-fatal myocardial infarction or died suddenly. The fatty-acid composition of serum triglycerides, phospholipids, and cholesterol esters had been measured at the start of the surveillance in these men and in a control group of 64 men matched for age, serum cholesterol and triglyceride concentrations, blood pressure, obesity, smoking, and one-hour glucose tolerance. Palmitic and stearic acids of phospholipids were significantly higher and linoleic and most polyunsaturated fatty acids, including arachidonic acid and eicosapentaenoic acid, of phospholipids were lower in the subjects who sustained coronary events compared with the controls. Linoleic acid tended to correlate negatively with blood pressure while other polyunsaturated fatty acids, especially eicosapentaenoic acid, exhibited a negative correlation with blood pressure and relative body weight in the controls but not in the subjects who sustained coronary events. These findings suggest that the fatty-acid pattern of serum phospholipids is an independent risk factor for coronary heart disease.

Quantitative analysis of disaturated lecithins in human plasma by ammonia chemical ionization mass spectrometry

Three physiologically important disaturated lecithins have been analyzed quantitatively as the acetate derivatives using a solids inlet probe and ammonia chemical ionization. Concentration independent response factors have been determined over a tenfold range that brackets human plasma levels. The results obtained serve as an independent corroboration of gas chromatographic analyses.