Bile salts and cholesterol in the pathogenesis of target cells in obstructive jaundice

Selective loss of calcium permeability on maturation of reticulocytes

Calcium and sodium permeability of human reticulocytes have been studied and compared to mature erythrocytes. Mature erythrocytes had extremely low Ca2+ permeability which was less than 0.1% of values published for squid axon or HeLa cells. Calcium entry was markedly increased in reticulocyte-rich suspensions and the uptake was linearly related to the percentage of reticulocytes present. The data suggest that reticulocytes are 43-fold more permeable to Ca2+ than mature cells although their Ca2+ concentration is not increased. Sodium influx into reticulocyte-rich suspensions was also increased in direct proportion to the percent of reticulocytes present. Reticulocytes are sixfold more permeable to Na+ than mature cells so the ratio of Ca2+:Na+ permeability falls by sevenfold as the reticulocyte changes to an erythrocyte. [3H]Ouabain binding was increased in reticulocyte-rich cell suspensions and the correlation suggested a value of about 4,000 sites per reticulocyte compared with 362+/-69 per mature cell. Maturation of the human reticulocyte produces disproportionate changes in cation permeability and in particular a selective loss of Ca2+ permeability.

Hemolytic anemias. Failure of the red cell membrane

The normal erythrocyte membrane is composed of nearly equivalent amounts of lipid and protein. The lipid portion of the membrane has been well studied. Even though de novo synthesis of lipid does not occur in human red cells, many biochemical pathways exist which facilitate detoxification of lipid breakdown products and lipid renewal. Rare defects in these processes are associated with hemolytic disorders. Recent studies have revealed that the membrane proteins are diverse and suggest that protein dysfunction may also account for clinical disease. Protein and lipid are entwined in a physicochemical relationship which is probably best depicted by the classic lipid bilayer with interspersed proteins in both the inner and outer surfaces and also spanning the bilayer. Membrane failure results in hemolytic anemia. This failure can be intrinsic, caused by abnormal lipid or protein constituents; or extrinsic, with a normal membrane being unable to counteract physical, chemical or immunologic stress. Clinical examples of membrane failure and hemolytic anemia can be separated into three groups according to the predominant mechanism of the hemolysis: fragmentation, whole-cell lysis, and filtration and entrapment. Although these mechanisms can act separately or in concert, the final hemolytic destruction of the cell can usually be traced to a failure of membrane function.

The fusion of abnormal plasma lipoprotein (LP-X) and the erythrocyte membrane in patients with cholestasis studied by electronmicroscopy

Adhesion followed by fusion of LP-X vesicles with the erythrocyte membrane is an important contribution to the erythrocyte enlargement in patients with intra or extra hepatic cholestasis. Adhesion of LP-X vesicles is demonstrated by thin section and freeze-etch electronmicroscopy. Fusion of LP-X with the erythrocyte membrane is deduced from biochemical data and freeze-etch electronmicroscopy in that the uptake of cholesterol and lecithin coincides with the increase in smooth areas on the fracture faces of the erythrocyte membrane.

Changes of nonelectrolyte permeability in cholesterol-loaded erythrocytes

Membrane cholesterol in porcine and bovine erythrocytes was elevated up to 165% of its normal value by incubation of the cells in cholesterol/phosphatidylcholine dispersions with or without serum. This alteration of membrane lipid composition brought about only a minor (10-40%) decrease of the permeability to glycerol, erythritol and to organic acids penetrating by non-ionic diffusion, although additional cholesterol had actually been incorporated into the lipid bilayer, as indicated by determinations of cell surface area from the critical hemolytic volume, in combination with quantitative evaluation of freeze-etch electron micrographs. On the basis of this finding and of the previously demonstrated (Grunze, M. and Deuticke, B. (1974) Biochim. Biophys. Acta 356, 125-130) considerable increase of permeability in cholesterol-depleted cells, it is proposed that in the erythrocyte membrane a pronounced "specific" reduction of permeability by cholesterol occurs only up to a molar ratio cholesterol/polar lipid of 0.6. At higher ratios cholesterol affects permeability only slightly, owing to an "unspecific" rigidifying effect on the membrane lipid phase.

The relationship of red cell membrane lipid content to red cell morphology and survival in patients with liver disease

The relationship of red blood cell (RBC) membrane lipid content to RBC morphology and survival was studied in patients with liver diseases. An increase in RBC cholesterol and phospholipid was detected in most patients with hepatocellular disease or cholestatic jaundice but the alteration in RBC lipid content did not correlate with RBC survival. The main abnormality of RBC morphology observed was the presence of macrocytes and target cells. In a small proportion of patients (approximately 3%) with severe hepatocellular disease, significant numbers of severely deformed ("spur") cells were seen. In these patients haemolysis was moderately severe and the RBC lipid profile showed increased membrane cholesterol without a concomitant increase in phospholipids. It is concluded that only in patients with "spur" cell anaemia do the morphological alterations lead to premature removal of cells from the circulation. The cause of the shortened RBC survival in jaundiced patients without "spur" cells remains to be determined.

Endothelial injury. Association with elevations of serum bile acid and cholesterol concentration in biliary-obstructed rats

An association between atherosclerosis, biliary obstruction and hyperlipidemia has been reported in the literature. In previous study from this laboratory, ultrastructural evidence of coronary artery endothelial damage was obtained in rats following ligation-induced biliary obstruction. In the present investigation, serum bile acids, total cholesterol and alkaline phosphatase levels were studied in association with similarly induced biliary obstruction and related to electron-microscopic observations of coronary artery endothelium. The results disclosed marked elevation of all serum parameters in as short a time as 24 hr following ligation compared with shamoperated controls. Animals exhibiting increases of serum bile acids and cholesterol also revealed severe configurational changes of endothelial cells which manifesed as buckling, detachment from the underlying internal elastic lamina, and vacuole formation. The role of elevated circulating bile acids and hypercholesterolemia as possible factors in producing arterial injury through membrane interaction is discussed. These observations suggest that biliary obstruction, even of short duration, may act as a potentially atherogenic mechanism in the experimental animal.

Modification of red cell membrane structure by cholesterol-rich lipid dispersions. A model for the primary spur cell defect

Cholesterol-rich membranes are the hallmark of "spur" red cells. Spur cells accumulate cholesterol from cholesterol-rich serum lipoproteins. Previous studies suggested that this added cholesterol is responsible for both the altered morphology and the destruction of spur cells. To examine this process in the absence of other serum factors, cholesterol-lecithin dispersions with varying amounts of unesterified cholesterol (C) relative to phospholipid (P) were prepared, and their influence on normal human red cells was studied. Cholesterol-rich lipid dispersions (C/P mole ration greater 1.0) transferred cholesterol to both red cell membranes and serum lipoproteins, and cholesterol-poor dispersions (C/P mole ration less 1.0) depleted red cells of cholesterol. Changes in membrane cholesterol paralleled changes in membrane surface area, as calculated from osmotic fragility, with a 0.22 percent variation in surface area per 1.0 percent variation in cholesterol content. Cold-induced compression of membrane surface area was increased in cholesterol-poor red cells (C/P equals 0.4), whereas the surface area of cholesterol-rich membranes (C/P equals 1.80) underwent no compression. Although the Na and K permeability of red cells severely depleted of cholesterol was increased, lesser degrees of depletion had no effect, and the permeability of cholesterol-rich cells was normal. However, increasing membrane cholesterol caused a progressive decrease in red cell deformability, as measured by filtration. Cholesterol-poor red cells were spherocytic in appearance and cholesterol-rich cells were broad and flat, indicative of their surface areas. In addition, cholesterol-rich cells had an irregular contour due to folding of the periphery of the cell. This shape abnormality was identical to that of both spur cells after splenectomy and normal red cells incubated in spur serum. Normalization of the C/P of spur serum by added phospholipid prevented the increase in membrane cholesterol and surface area and the transformation of cell shape. These studies establish that the cholesterol content of red cells is dependent on the C/P of their milieu, either lipoproteins or cholesterol-lecithin dispersions. Moreover, the surface area, deformability, and contour of cholesterol-rich red cells are a direct function of their increased membrane C/P. Although cholesterol-rich spur cells are further modified in the circulation of patients with spleens, this abnormality of the membrane lipid bilayer, induced by cholesterol-rich cholesterol-lecithin dispersions, represents the primary spur cell defect.

Haemolysis in experimental cholestasis: possible role of erythrocyte sialic acid

Because previous studies have suggested that red cell membrane sialic acid might be a determinant of survival in vivo, we examined the relationship between haemolysis, erythrocyte membrane sialic acid, and hepatic neuraminidase (sialidase) levels in experimental cholestatic jaundice in rats. Treatment of normal rat erythrocytes with neuraminidase in vitro resulted in a significant reduction in their survival in vivo and removal by the spleen. The degree of haemolysis was comparable to that previously found 12 days after bile duct ligation in rats. Bile duct ligation resulted in a significant increase in hepatic neuraminidase and a decrease in erythrocyte sialic acid. The findings are consistent with the hypothesis that one or more neuraminidases present in inreased amounts in the bile duct-ligated livers are responsible for the removal of sialic acid from circulating erythrocytes and subsequent removal of these cells in the spleen.

An analysis of lipoproteins, bile acids, and red cell membranes associated with target cells and spur cells in patients with liver disease

Most patients with stable cirrhosis of the alcoholic have "target" red cells; however, a minority have "spur" cells and severe hemolytic anemia. These two syndromes were studied in 27 patients with target cells and 17 patients with spur cells, all of whom had advanced cirrhosis. The cholesterol and phospholipid content of red cell membranes effectively distinguished target cells from spur cells. Target cells alone were rich in lecithin, and both the cholesterol/phospholipid and cholesterol/lecithin mole ratios were greater in spur cells. The cholesterol/phospholipid mole ratio of both types of red cells correlated closely with the free cholesterol saturation of serum lipoproteins, as defined by the amount of free cholesterol relative to phospholipid and protein in these lipoproteins. Lecithin: cholesterol acyltransferase (LCAT) activity was decreased in most patients with target cells and spur cells; however, the relationship between this activity and the lipid abnormalities observed was weak. Serum bile acid levels also correlated poorly with serum and cell lipids. However, in patients with target cells the amount of cholic and deoxycholic acids in serum was approximately equal to the amount of chenodeoxycholic acid, whereas in patients with spur cells chenodeoxycholic acid (the precursor of lithocholic acid) predominated.

Loss of membrane components in the pathogenesis of antibody-induced spherocytosis

The infusion of hyperimmune agglutinating antibodies into man or animals causes spherocytosis and hemolysis. The mechanism of spherocytosis was studied in rats given rabbit anti-rat red cell antiserum intravenously. During the 18 hr after antibody infusion, a time before the onset of reticulocytosis, hematocrits fell from 40.6 to 27.6%. However, no change occurred in mean cell volume, mean cell hemoglobin content, or the red cell concentrations of potassium or adenosine triphosphate (ATP). There was a progressive loss of membrane constituents and membrane surface area which followed first order kinetics. At 18 hr membrane cholesterol had decreased 23.5%, phospholipid 26.3%, protein 4.7%, and surface area (calculated from a measure of osmotic fragility) 14.2%. There was no change in the per cent composition of the various phospholipids. Similar changes occurred in animals splenectomized before receiving antibody.These studies demonstrate that spherocytosis induced by heterologous agglutinating antibodies in vivo results from a loss of surface area with no accompanying change in cell volume or in the concentration of the major intracellular constituents. It is caused by a process acting at the cell surface leading to the loss of lipid-rich, protein-poor components of the red cell membrane.

Anemia with spur cells: a red cell defect acquired in serum and modified in the circulation

The sera and red cells of three patients with severe liver disease and "spur cells" were studied. In each case the per cent of serum cholesterol which was free (unesterified) was elevated, and the serum lecithin: cholesterol acyltransferase activity was depressed. Lipoproteins with beta mobility were increased, but exhibited immune reactivity with antisera to both alpha- and beta-lipoproteins. Serum bile salt concentrations were markedly elevated and consisted primarily of chenodeoxycholic acid, with small amounts of lithocholic acid present as well.SPUR CELLS MANIFESTED A STRIKING INCREASE IN CHOLESTEROL CONTENT AND IN THE CHOLESTEROL: phospholipid ratio, but a normal osmotic fragility. When incubated in heated normal serum, spur cells lost their excess cholesterol and became spherocytic and osmotically fragile. Conversely, sera from patients with spur cells readily transferred up to one-third of their free cholesterol to normal red cells, causing normal cells to become resistant to osmotic lysis. In addition, these sera caused normal red cells to acquire thorny membrane projections. Cholesterol transfer to normal cells also occurred from normal serum which had previously been incubated with spur cells. Changes in cell cholesterol were induced by all of the lipoprotein fractions of spur serum. When transfused into a patient with spur cells, normal red cells became more resistant to osmotic lysis over the course of 24 hr. However, over the subsequent 7 days they underwent a progressive increase in osmotic fragility. These normal cells, as well as the patient's own cells, had a shortened survival. Correlating with the moderate decrease in the filterability of red cells spurred in vitro, red cell destruction occurred predominently in the spleen. Red cells in this disorder appear to serve as repositories for free cholesterol loosely bound to serum lipoproteins. Cholesterol acquisition by the red cell membrane increases its surface area and causes the red cell to be resistant to osmotic lysis. The associated alteration in red cell shape leads to further changes in the cell membrane during circulation in vivo resulting in the loss of membrane surface area and culminating in the cell's premature destruction in the spleen.

The selective and conjoint loss of red cell lipids

The pattern of lipid loss from the membrane of red cells incubated in serum is influenced by the availability of glucose. Under homeostatic conditions with respect to glucose, cholesterol alone is lost. This results from esterification of free cholesterol in serum by the serum enzyme, lecithin:cholesterol acyltransferase, and is associated with a proportional decrease in membrane surface area, reflected by an increased osmotic fragility. This selective loss of membrane cholesterol also occurs in hereditary spherocytosis (HS) red cells, even after incubation for 65 hr in the presence of glucose. The loss of free cholesterol from red cells relative to its loss from serum, under these conditions, is greatest at higher hematocrits, similar to those found in the spleen. Although the selective loss of membrane cholesterol increases the spherodicity of normal red cells, it does not lead to a change in their rate of glucose consumption, and both the loss of cholesterol and the increase in osmotic fragility are reversible in vitro. Moreover, normal red cells made osmotically fragile by cholesterol depletion in vitro rapidly become osmotically normal and survive normally after their reinfusion in vivo.In contrast to this selective loss of membrane cholesterol, red cells incubated in the absence of glucose lose both cholesterol and phospholipid. This occurs more rapidly in HS than normal red cells and is followed by a disruption of cation gradients and then by hemolysis. Cholesterol and phospholipid lost under these conditions is not restored during subsequent incubations in vitro. Selective loss of membrane cholesterol is a physiologic event secondary to an altered state of serum lipids. It is reversible both in vitro and in vivo and neither influences cellular metabolism nor impairs viability. Conjoint loss of phospholipid and cholesterol, however, results from intrinsic injury to the red cell membrane which results from prolonged metabolic depletion.

The role of membrane lipids in the survival of red cells in hereditary spherocytosis

Red cells in hereditary spherocytosis (HS) have a decreased ratio of membrane surface area to cell volume and therefore a spheroidal shape. This abnormality in shape predisposes them to pooling and destruction in the spleen. Although splenectomy prevents hemolysis in HS, the red cell defect, as manifested by spheroidicity, increased autohemolysis, excesive permeability to sodium, and hypermetabolism, persists. The role of membrane lipids in these manifestations in vitro and in cell survival in vivo was examined. Before splencetomy, and in spite of the presence of a young cell population, the cholesterol and phospholipid content of HS red cells is decreased. After splenectomy lipid values are similar to those obtained in normal subjects with spleens. However, after splenectomy for conditions other than HS the lipid content of red cells is greater than normal. Thus, when compared with the red cells of patients without HS who have also undergone splenectomy, HS cells after splenectomy are deficient in both cholesterol and phospholipid. Obstructive jaundice causes an increase in membrane lipid, primarily cholesterol, and a decrease in the osmotic fragility of normal red cells. When HS red cells are transfused into patients with obstructive jaundice they also become less osmotically fragile. Moreover, when incubated in obstructive jaundice serum, they gain cholesterol. This acquistion of membrane lipid in vitro does not result in a change in their rate of glucose utilization or sodium efflux. However, the transformation to a less spheroidal shape in vivo permits them to traverse better the splenic circulation and survive longer.