In vivo influx of free and esterified plasma cholesterol into human aortic tissue without atherosclerotic lesions

Atherogenesis, Transcytosis, and the Transmural Cholesterol Flux: A Critical Review

The recently described phenomenon of cholesterol-loaded low-density lipoproteins (LDL) entering the arterial wall from the lumen by transcytosis has been accepted as an alternative for the long-held concept that atherogenesis involves only passive LDL movement across an injured or dysfunctional endothelial barrier. This active transport of LDL can now adequately explain why plaques (atheromas) appear under an intact, uninjured endothelium. However, the LDL transcytosis hypothesis is still questionable, mainly because the process serves no clear physiological purpose. Moreover, central components of the putative LDL transcytosis apparatus are shared by the counter process of cholesterol efflux and reverse cholesterol transport (RCT) and therefore can essentially create an energy-wasting futile cycle and paradoxically be pro- and antiatherogenic simultaneously. Hence, by critically reviewing the literature, we wish to put forward an alternative interpretation that, in our opinion, better fits the experimental evidence. We assert that most of the accumulating cholesterol (mainly as LDL) reaches the intima not from the lumen by transcytosis, but from the artery's inner layers: the adventitia and media. We have named this directional cholesterol transport transmural cholesterol flux (TCF). We suggest that excess cholesterol, diffusing from the avascular (i.e., devoid of blood and lymph vessels) media's smooth muscle cells, is cleared by the endothelium through its apical membrane. A plaque is formed when this cholesterol clearance rate lags behind its rate of arrival by TCF.

Cytokine Circuits in Cardiovascular Disease

Arterial inflammation is a hallmark of atherosclerosis, and appropriate management of this inflammation represents a major unmet therapeutic need for cardiovascular disease patients. Here, we review the diverse contributions of immune cells to atherosclerosis, the mechanisms of immune cell activation in this context, and the cytokine circuits that underlie disease progression. We discuss the recent application of these insights in the form of immunotherapy to treat cardiovascular disease and highlight how studies on the cardiovascular co-morbidity that arises in autoimmunity might reveal additional roles for cytokines in atherosclerosis. Currently, data point to interleukin-1β (IL-1β), tumor necrosis factor (TNF), and IL-17 as cytokines that, at least in some settings, are effective targets to reduce cardiovascular disease progression.

Apolipoprotein M promotes mobilization of cellular cholesterol in vivo

OBJECTIVE

The HDL associated apolipoprotein M (apoM) protects against experimental atherosclerosis but the mechanism is unknown. ApoM increases prebeta-HDL formation. We explored whether plasma apoM affects mobilization of cholesterol from peripheral cells in mice.

METHODS AND RESULTS

ApoM-enriched HDL from apoM-transgenic mice increased the in vitro efflux of 3H-cholesterol from macrophages by 24 +/- 3% (p < 0.05) as compared with HDL from wild type (WT) mice, thus confirming previous findings. However, apoM-free HDL was not poorer than that of WT HDL to mobilize 3H-cholesterol. 3H-cholesterol-labeled foam cells were implanted in the peritoneal cavity of apoM-/-, WT and apoM-transgenic mice to assess the mobilization of cholesterol from foam cells in vivo and subsequent excretion into feces. The results showed a statistically non-significant trend towards increased mobilization of cellular cholesterol to plasma with increasing plasma apoM. However, the apoM-genotype did not affect the excretion of 3H-cholesterol in feces. Nevertheless, when apoM-/-, apoM-transgenic and WT mice received a constant intravenous infusion of 13C2-cholesterol/intralipid for 5 h, the rate of enrichment of blood free cholesterol with free 13C2-cholesterol was significantly lower (consistent with an increase in flux of unlabeled free cholesterol into the plasma) in the apoM-transgenic (3.0 +/- 0.9 per thousand/h) as compared to WT (5.7 +/- 0.9 per thousand/h, p < 0.05) and apoM-/- (6.5 +/- 0.6 per thousand/h, p < 0.01) mice.

CONCLUSION

The present data indicate that the plasma apoM levels modulate the ability of plasma to mobilize cellular cholesterol, whereas apoM has no major effect on the excretion of cholesterol into feces.

LDL-cholesterol lowering or HDL-cholesterol raising for cardiovascular prevention

A number of reports have indicated that both lowering low density lipoprotein (LDL)-cholesterol and raising high density lipoprotein (HDL)-cholesterol can result in significant cardiovascular benefit, both in terms of reduction of events and also, to a variable extent, of atheromatous lesions. LDL and HDL have opposite roles in body cholesterol regulation and, in theory, both reduced deposition (LDL reduction) and increased removal (raised HDL) can improve vascular disease. A number of reports over the last 30 years have attempted to quantitate with cholesterol balance/turnover studies, the correlations between LDL and HDL levels and body cholesterol pool sizes. More recently, these studies have evaluated the effects of LDL or HDL changes on cholesterol elimination. Data have, at times, been fully consistent with theoretical expectations, whereas at others they have not. Evaluation of these, at times, historical data provides, however, an important clue to the understanding of current results with different medications for the management of lipoprotein disorders.

Retention of aggregated LDL by cultured human coronary artery endothelial cells

Aggregated LDL (AgLDL) accumulates within the subendothelial space of developing atherosclerotic lesions. We were interested to learn whether endothelial cells can interact with AgLDL. Incubation of endothelial cells with AgLDL resulted in apparent cholesterol retention. Microscopic examination revealed that cholesterol retention resulted mainly from endothelial cell surface attachment of AgLDL. Little AgLDL entered endothelial cells consistent with the small amount of endothelial cell degradation of AgLDL. Although endothelial cell retention of AgLDL was inhibited by LDL, AgLDL retention was not blocked by lactoferrin, C7 anti-LDL receptor monoclonal antibody, or receptor-associated protein, suggesting that LDL receptor family members did not mediate this retention. Surface retention of AgLDL depended on microtubule function and could be regulated by the protein kinase C activator, PMA. Treatment of endothelial cells with PMA either before or during, but not after incubation with AgLDL, inhibited retention of AgLDL. Our findings show that endothelial cells can retain AgLDL but internalize and metabolize little of this AgLDL. Thus, it is unlikely that endothelial cells can transport AgLDL out of atherosclerotic lesions, but it is likely that retention of AgLDL affects endothelial function.

Alterations in high-density lipoprotein metabolism and reverse cholesterol transport in insulin resistance and type 2 diabetes mellitus: role of lipolytic enzymes, lecithin:cholesterol acyltransferase and lipid transfer proteins

Insulin resistance and type 2 diabetes mellitus are generally accompanied by low HDL cholesterol and high plasma triglycerides, which are major cardiovascular risk factors. This review describes abnormalities in HDL metabolism and reverse cholesterol transport, i.e. the transport of cholesterol from peripheral cells back to the liver for metabolism and biliary excretion, in insulin resistance and type 2 diabetes mellitus. Several enzymes including lipoprotein lipase (LPL), hepatic lipase (HL) and lecithin: cholesterol acyltransferase (LCAT), as well as cholesteryl ester transfer protein (CETP) and phospholipid transfer protein (PLTP), participate in HDL metabolism and remodelling. Lipoprotein lipase hydrolyses lipoprotein triglycerides, thus providing lipids for HDL formation. Hepatic lipase reduces HDL particle size by hydrolysing its triglycerides and phospholipids. A decreased postheparin plasma LPL/HL ratio is a determinant of low HDL2 cholesterol in insulin resistance. The esterification of free cholesterol by LCAT increases HDL particle size. Plasma cholesterol esterification is unaltered or increased in type 2 diabetes mellitus, probably depending on the extent of triglyceride elevation. Subsequent CETP action results in transfer of cholesteryl esters from HDL towards triglyceride-rich lipoproteins, and is involved in decreasing HDL size. An increased plasma cholesteryl ester transfer is frequently observed in insulin-resistant conditions, and is considered to be a determinant of low HDL cholesterol. Phospholipid transfer protein generates small pre beta-HDL particles that are initial acceptors of cell-derived cholesterol. Its activity in plasma is elevated in insulin resistance and type 2 diabetes mellitus in association with high plasma triglycerides and obesity. In insulin resistance, the ability of plasma to promote cellular cholesterol efflux may be maintained consequent to increases in PLTP activity and pre beta-HDL. However, cellular cholesterol efflux to diabetic plasma is probably impaired. Besides, cellular abnormalities that are in part related to impaired actions of ATP binding cassette transporter 1 and scavenger receptor class B type I are likely to result in diminished cellular cholesterol efflux in the diabetic state. Whether hepatic metabolism of HDL-derived cholesterol and subsequent hepatobiliary transport is altered in insulin resistance and type 2 diabetes mellitus is unknown. Specific CETP inhibitors have been developed that exert major HDL cholesterol-raising effects in humans and retard atherosclerosis in animals. As an increased CETP-mediated cholesteryl ester transfer represents a plausible metabolic intermediate between high triglycerides and low HDL cholesterol, studies are warranted to evaluate the effects of these agents in insulin resistance- and diabetes-associated dyslipidaemia.

Accumulation of lipoprotein fractions and subfractions in the arterial wall, determined in an in vitro perfusion system

A large proportion of a dense subfraction of LDL in plasma is coupled with an increased risk of coronary artery disease, CAD. This may reflect an increased inflow of such LDL subfractions into the intima, since the inflow of lipoproteins is supposed to be inversely related to the size of the particles. In order to evaluate this possibility we used an in vitro perfusion system for aortic intima-media from rabbits with experimental atherosclerosis. The uptake of human VLDL, LDL, HDL and subfractions of LDL (LDL1, 1.019-1.035 and LDL2, 1.035-1.063 g/ml) in lesions and non-involved areas was studied. Our results indicate that particle size is an important factor for the clearance of lipoproteins into the arterial tissue, both for plaques (VLDL 7.6, LDL 25, HDL 58 nl/mg wet wt./h) and in other areas (VLDL 3.8, LDL 4.1, HDL 12 nl/mg wet wt./h). Interestingly, the uptake of LDL2 was as much as 1.5-1.9 times higher than LDL1. This supports the view that an increased lipid load in the arterial wall may be one mechanism behind the association between denser LDL and CAD. Our data also suggest that the difference between LDL uptake in plaque (576 nl/mg wet wt.) and other areas (48 nl/mg wet wt.) not only reflects a rapid clearance but a large distribution volume of the intima (plaque > 60%, non-involved areas 5.7%).

Accelerated cholesterol accumulation in homologous arterial transplants in cholesterol-fed rabbits. A surgical model to study transplantation atherosclerosis

Accelerated coronary artery disease has become a major complication to heart transplantation in humans. Therefore, we have developed a surgical model in the rabbit, with transplantation of the thoracic aorta as a bypass graft onto the abdominal aorta of another rabbit. The model permits the study of cholesterol metabolism in transplanted arteries. The graft did not accumulate cholesterol for as long as 298 days, provided that the rabbits were normocholesterolemic, i.e., with plasma cholesterol levels of 0.3-0.7 mmol/l. However, after a few weeks of cholesterol feeding resulting in plasma cholesterol levels of 2-5 mmol/l, the homologous graft accumulated cholesterol compared with intact aortic tissue in the rabbits and also compared with autologous aortic grafts. The intimal clearance of plasma cholesteryl ester, mainly high density lipoprotein cholesteryl ester, in the luminal layer of the aortic graft was 60-150 nl x cm-2 x hr-1 1-2 hours after transplantation. The intimal clearance in the corresponding intact thoracic aorta of the recipient animal was 5-20 nl x cm-2 x hr-1. The values were 1,500-3,000 nl x cm-2 x hr-1 51-298 days after transplantation, while the intimal clearance of the rabbit's own aorta remained unchanged. A pronounced increase in plasma lipoprotein permeability is thus an early event in transplanted arteries. It results in a higher cholesteryl ester influx that leads to cholesterol accumulation in the artery, but only if the rabbits are fed a cholesterol-enriched diet. This rabbit model may be useful in the search for interventional measures to prevent or diminish the accelerated coronary artery disease in transplanted hearts in humans.

Quantitative studies of transfer in vivo of low density, Sf 12-60, and Sf 60-400 lipoproteins between plasma and arterial intima in humans

To assess the potential of various plasma lipoprotein classes to contribute to the lipid content of the arterial intima, influx and efflux of these plasma lipoprotein fractions into and from the intima of human carotid arteries were measured in vivo. While low density lipoprotein (LDL) is known to transfer from plasma into the arterial wall, there is less information on the atherogenic potential of lipoproteins of intermediate density (Sf 12-60) or of very low density (Sf 60-400). Aliquots of the same lipoprotein (LDL, Sf 12-60 lipoprotein particles, or Sf 60-400 lipoprotein particles) iodinated with iodine-125 and iodine-131 were injected intravenously 18-29 hours and 3-6 hours, respectively, before elective surgical removal of atheromatous arterial tissue, and the intimal clearance of lipoproteins, lipoprotein influx, and fractional loss of newly entered lipoproteins were calculated. Intimal clearance of Sf 60-400 particles was not detectable (less than 0.3 microliter x hr-1 x cm-2), whereas the average value for both LDL and Sf 12-60 lipoprotein particles was 0.9 microliter x hr-1 x cm-2. Since the fractional loss of newly entered LDL and Sf 12-60 lipoprotein particles was also similar, the results suggest similar modes of entry and exit for these two particles. However, due to lower plasma concentrations of Sf 12-60 lipoproteins as compared with LDL, the mass influx of cholesterol in the Sf 12-60 particles was on the order of one 10th of that in LDL, and that of apolipoprotein B was about one 20th.(ABSTRACT TRUNCATED AT 250 WORDS)

Different efflux pathways for high and low density lipoproteins from porcine aortic intima

To study the efflux of high (HDL) and low (LDL) density lipoproteins from the arterial wall in vivo, a surgical model in pigs was used. An isolated segment of the lesion-free thoracic aorta was pulse labeled from the lumen of the artery with 3H-cholesteryl ester labeled HDL and 14C-cholesteryl ester labeled LDL. Subsequently, the labeled aortic segment was exposed to cold chase in vivo. The transfer of HDL cholesteryl ester from plasma into intima expressed as intimal clearance was three to seven times greater than that of LDL cholesteryl ester. At least 50%, but possibly as much as 95%, of the HDL cholesteryl ester that entered the arterial intima during a period of 4 hours penetrated the arterial wall beyond the internal elastic lamina. In contrast, less than 15% of the LDL cholesteryl ester that entered the arterial intima in the same period penetrated beyond the luminal layer. After 24 hours of cold chase in vivo, more than 80% of both labeled HDL esterified cholesterol and labeled LDL esterified cholesterol had disappeared from the arterial wall. Transmural profiles after 9 hours of cold chase showed that labeled HDL was present throughout the entire arterial wall, whereas labeled LDL in quantitative amounts was present only in the luminal layer. The results suggest that the most important efflux route for HDL esterified cholesterol is through the vasa vasorum and lymphatics in the outer media and adventitia, whereas LDL esterified cholesterol predominantly leaves intima via the lumen of the artery.

Fate of fibrinogen in human arterial intima

Fibrinogen and fibrinogen/fibrin-related antigen (total FRA) was measured in human normal intima and different types of atherosclerotic lesions and mural thrombi. The amount showed marked variation between groups of tissue samples, but within each group there was a significant correlation between levels of total FRA and low density lipoprotein (LDL), suggesting that some common factor must influence their influx or retention. The total FRA were analyzed by gradient sodium dodecyl sulfate polyacrylamide gel electrophoresis and immunoblotting with antisera to whole fibrinogen and fragments D and E, and fibrinopeptide A (FPA). All intimal samples (but not thrombi) contained fragment X, the first product of plasmin digestion of fibrinogen, but fragment Y was present in only half the samples, and no core-fragment E containing FPA was detected in any sample, suggesting that fibrinogenolysis is limited. By contrast, all samples contained fragment E, which was negative for FPA, so presumably derived from fibrin; they also contained fragments D-dimer and DY, which are characteristic degradation products of cross-linked fibrin. There were no differences between samples obtained during reconstructive vascular surgery and samples obtained at autopsy, so the patterns appear to represent the steady state. This implies that within the intima there is continuous formation of cross-linked fibrin and continuous fibrinolysis, both processes generating fragments that may have atherogenic properties.

A surgical model to study in vivo efflux of cholesterol from porcine aorta. Evidence for cholesteryl ester transfer through the aortic wall

We describe a surgical procedure in pigs which makes it possible to follow the influx into, the penetration through and the efflux from the arterial wall of labeled lipoproteins. After 4 h exposure of the luminal side of the arterial wall to labeled lipoproteins, labeled esterified cholesterol was found in all layers of the aortic wall, whereas labeled free cholesterol gained access only to the most luminal layer. The data suggest that at least 40%, if not 80-90%, of the cholesteryl ester that enters the aortic wall from the luminal side, passes through the entire wall and leaves the aortic wall through vasa vasorum and lymphatics. They also suggest that free cholesterol in the lipoproteins exchanges extensively with cellular free cholesterol while the lipoproteins penetrate through the most luminal layer. More than 90% of the labeled cholesteryl ester disappeared from the arterial wall during a cold chase period of 4 days. By the simultaneous use of [3H]cholesterol and [14C]cholesterol it was shown that only 10-20% of this disappearance can be explained by cholesteryl ester hydrolysis in the arterial wall.

In vivo transfer of cholesteryl ester from high and low density plasma lipoproteins into human aortic tissue

For the study of cholesteryl ester transfer from different plasma lipoproteins into human aortic tissue, patients scheduled for reconstructive aortic surgery were intravenously injected with autologous in vitro labeled lipoproteins 20 to 24 hours before aortic intima-media samples were obtained during the operation. The injectate contained high density lipoproteins (d greater than 1.063) labeled with 3H-cholesteryl ester and lipoproteins of lower density (d less than 1.063) labeled with 14C-cholesteryl ester or lipoproteins with the opposite labeling. In 16 aortic tissue samples (some with visible atherosclerosis) from 11 normocholesterolemic patients, the aortic influx of total cholesteryl ester was 1 to 50 nmol x cm-2 x day-1. Some 39% +/- 3% (mean +/- SEM) of the influx was derived from high density lipoproteins, which in plasma accounted for only 22% +/- 2% (mean +/- SEM) of the esterified cholesterol. The findings suggest that: 1) esterified cholesterol from the two lipoprotein fractions in plasma enter the aortic intima by the same mechanism, and 2) influx of cholesteryl ester from the smaller, high density lipoproteins is greater than influx from the larger, lower density lipoproteins considering their concentrations in plasma. In some patients, the cholesterol content in the intima-media tissue with no visible atherosclerosis corresponded to only a few months of continuous cholesteryl ester influx. This time is short considering the age of the patients and, therefore, indicates that removal of esterified cholesterol from the intima-media is of major importance in preventing cholesterol deposition in the arterial wall.

Glycosaminoglycans and apolipoproteins B and A-I in human aortas. Chemical and immunological analysis of lesion-free aortas from children and adults

To study changes in the contents of plasma lipoproteins in human arteries with age and the relationship of lipoproteins with other arterial constituents, we analyzed the contents of apolipoproteins B (apo B) and A-I (apo A-I), free and esterified cholesterol, and glycosaminoglycans (GAG) in lesion-free aortic intimas of 30 children and adults. The content of apo B increased significantly with age, whereas that of apo A-I remained relatively constant. Apo B and apo A-I had significant positive correlations with the content of chondroitin sulphates A + C (CS A + C), which comprised 35% to 47% of the aortic GAG. The correlations remained significant after correction for the effect of age. Aortic apo B, but not apo A-I, also showed significant positive correlations with the contents of intimal free and esterified cholesterol. The results indicate that: considerable amounts of apo B and apo A-I can be found in lesion-free aortic intimas; there is an age-related rise in the content of apo B and a fall in the ratio of apo A-I to apo B, which are unfavorable developments in the light of current views on atherogenesis; the contents of the apolipoproteins are proportional to that of CS A + C, which might have a role in the retention of lipoproteins in the arteries.

Mechanisms and consequences of cellular cholesterol exchange and transfer

It is apparent from consideration of the reactions involved in cellular cholesterol homeostasis that passive transfer of unesterified cholesterol molecules plays a role in cholesterol transport in vivo. Studies in model systems have established that free cholesterol molecules can transfer between membranes by diffusion through the intervening aqueous layer. Desorption of free cholesterol molecules from the donor lipid-water interface is rate-limiting for the overall transfer process and the rate of this step is influenced by interactions of free cholesterol molecules with neighboring phospholipid molecules. The influence of phospholipid unsaturation and sphingomyelin content on the rate of free cholesterol exchange are known in pure phospholipid bilayers and similar effects probably occur in cell membranes. The rate of free cholesterol clearance from cells is determined by the structure of the plasma membrane. It follows that the physical state of free cholesterol in the plasma membrane is important for the kinetics of cholesterol clearance and cell cholesterol homeostasis, as well as the structure of the plasma membrane. Bidirectional flux of free cholesterol between cells and lipoproteins occurs and rate constants characteristic of influx and efflux can be measured. The direction of any net transfer of free cholesterol is determined by the relative free cholesterol/phospholipid molar ratios of the donor and acceptor particles. Cholesterol diffuses down its gradient of chemical potential generally partitioning to the phospholipid-rich particle. Such a surface transfer process can lead to delivery of cholesterol to cells. This mechanism operates independently of any lipoprotein internalization by receptor-mediated endocytosis. The influence of enzymes such as lecithin-cholesterol acyltransferase and hepatic lipase on the direction of net transfer of free cholesterol between lipoproteins and cells can be understood in terms of their effects on the pool sizes and the rate constants for influx and efflux. Excess accumulation of free cholesterol in cells stimulates the rate of cholesteryl ester formation and induces deposition of cholesteryl ester inclusions in the cytoplasm similar to the situation in the 'foam' cells of atherosclerotic plaque. Clearance of cellular cholesteryl ester requires initial hydrolysis to free cholesterol followed by efflux of this free cholesterol. The rate of clearance of cholesteryl ester from cytoplasmic droplets is influenced by the physical state of the cholesteryl ester; liquid-crystalline cholesteryl ester is removed more slowly than cholesteryl ester in a liquid state.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of cholestanol feeding and cholestyramine treatment on the tissue sterols in the rabbit

Rabbits were fed diets enriched with cholestanol or cholesterol (3.5 g/wk) for 4-12 weeks. During cholestanol feeding, the concentration of cholestanol in blood serum, liver, heart and aorta increased 15-30 times. In serum and liver, the concentration of cholesterol also increased. Cholestanol-fed rabbits developed inflammatory changes in the liver, with proliferation of small bile ducts. Liver tests were only slightly abnormal. Morphological atherosclerosis of the aorta was only occasionally seen in rabbits receiving cholestanol for eight weeks or less. During cholesterol feeding, the amounts of cholesterol in different tissues increased dramatically, most in the aorta. Morphological atherosclerosis in the aorta was found in all rabbits fed cholesterol-enriched diets for more than four weeks. Brain cholestanol was doubled in rabbits fed cholestanol for eight weeks, whereas brain sterols did not change significantly during cholesterol feeding. After an additional regression period with cholestyramine for eight weeks, the increased content of cholestanol in the brain was unchanged in cholestanol-fed rabbits. These observations are discussed in relation to the cholestanolosis of the brain that develops in the rare inherited human disease cerebrotendinous xanthomatosis.

Cholesteryl ester efflux from extracellular and cellular elements of the arterial wall. Model systems in culture with cholesteryl linoleyl ether

Cholesteryl ester (CE) accretion in human atheroma is a slow process during which lipoproteins infiltrate the arterial extracellular space and then gain entry into the cellular components. The present aim was to simulate this process in model systems in culture to learn whether cholesteryl ester transfer protein (CETP) may promote CE efflux at different stages of atheroma formation. To simulate CE efflux from arterial interstitium, cationized LDL labeled with 3H-cholesteryl linoleyl ether (3H-CLE) was added to fixed aortic smooth muscle cells (SMC) or to extracellular matrix. To study efflux of 3H-CLE taken up by cells via receptor-mediated endocytosis of LDL, the SMC cultures were fixed and permeabilized prior to the determination of CE efflux. The cellular model included macrophages, which had ingested acetylated LDL labeled with 3H-CLE. Efflux of 3H-CLE and 14C-CE was studied during postincubation of the labeled cultures with human lipoprotein deficient serum (LPDS) or partially purified CETP. As controls, we used SMC cultures incubated with albumin. In all systems, a 3- to 12-fold increase of 3H-CLE or 14C-CE efflux was found in the postincubation medium containing human LPDS or partially purified CETP when compared to controls. Permeabilization of the cells with saponin enhanced cellular 3H-CLE and 14C-CE efflux in the presence of human LPDS. The findings indicate that CETP may promote CE efflux from aortic interstitium or disintegrating cells. We propose that CETP may play an important role in aortic CE homeostasis under physiological and pathological conditions.