Cholesterol and phospholipid composition of erythroblasts isolated from mouse spleen after Rauscher leukemia virus infection

Translocator protein 2 is involved in cholesterol redistribution during erythropoiesis

Translocator protein (TSPO) is an 18-kDa cholesterol- and drug-binding protein conserved from bacteria to humans. While surveying for Tspo-like genes, we identified its paralogous gene, Tspo2, encoding an evolutionarily conserved family of proteins that arose by gene duplications before the divergence of avians and mammals. Comparative analysis of Tspo1 and Tspo2 functions suggested that Tspo2 has become subfunctionalized, typical of duplicated genes, characterized by the loss of diagnostic drug ligand-binding but retention of cholesterol-binding properties, hematopoietic tissue- and erythroid cell-specific distribution, and subcellular endoplasmic reticulum and nuclear membrane localization. Expression of Tspo2 in erythroblasts is strongly correlated with the down-regulation of the enzymes involved in cholesterol biosynthesis. Overexpression of TSPO2 in erythroid cells resulted in the redistribution of intracellular free cholesterol, an essential step in nucleus expulsion during erythrocyte maturation. Taken together, these data identify the TSPO2 family of proteins as mediators of cholesterol redistribution-dependent erythroblast maturation during mammalian erythropoiesis.

Lipid characteristics of Friend erythroleukaemia cells differentiated by dimethyl sulphoxide or hexamethylenebisacetamide, and of non-inducible clones treated with the inducers

We studied the lipid changes in Friend erythroleukaemia (FEL) cells differentiated by dimethyl sulphoxide (DMSO) or hexamethylenebisacetamide (HMBA). We also examined clones of FEL cells non-inducible to differentiation by DMSO or HMBA for lipid changes not related to the differentiation process. A decrease in triacylglycerols was found in both DMSO- or HMBA-differentiated FEL cells, whereas a decrease in phosphatidylethanolamine was observed only in DMSO-differentiated FEL cells. The characteristics of the phospholipid fatty acid profiles of FEL cells, which proved to be similar to those of other transformed cells, were not significantly affected by erythroid differentiation.

Incorporation into the tissues and turnover of arachidonic acid after administration to normal and essential fatty acid deficient rats

A comparison was made of the incorporation into the tissues and metabolism of [1-14C] arachidonic acid (AA) after i.v. administration to normal and EFA-deficient rats. At different times, ultra-thin whole body sections were prepared and the distribution of the radioactivity determined by autoradiograms. After 5 min, a considerable incorporation occurs in the following organs: subcutaneous and perispinal fat, liver, heart muscle, kidney and adrenal. The EFA deficient rats show a similar distribution but the radioactivity is longer retained. The total amount of radioactivity in the heart, liver, kidney and adrenal was measured at different times. A decline occurs in the heart, and an increase in the adrenal. In the urine, the highest amount of radioactivity is excreted on the first day. The excretion is lower in the EFA-deficient rats. Small amounts of radioactive metabolites with the chromatographic characteristics of PGE2 and 13,14 dihydro-15ketoPGE2 were isolated from urine. The amounts of 14CO2 produced were determined after the administration of [1-14C] AA. Half times were: 39 +/- 2.9 min in the EFA-deficient and 28 +/- 2.8 min in the normal rats. In the heart, AA is incorporated into phospholipids and neutral lipids. The following percentages were determined: phosphatidylinositol: 6.9 +/- 0.6%, phosphatidylcholine: 44 +/- 4.1%, phosphatidylethanolamine: 10.0 +/- 1.0%, neutral lipids: 9.3 +/- 1.6%. Several explanations can be given for the higher requirements of some tissues for AA. It could be, that this substance is used in the formation of particular membranes with a high AA content. Differences in the amounts of metabolites produced may also play a role.

Reorientation rates and asymmetry of distribution of lysophospholipids between the inner and outer leaflet of the erythrocyte membrane

Labelled lysophospholipids were inserted into the outer layer of the erythrocyte membrane and their reorientation (flip) to the inner layer quantified by following the increase of the fraction of lysophospholipids not extractable by albumin. Flip rate constants were calculated from the kinetics of equilibration of the lysophospholipids between two compartments, the outer and the inner leaf of the bilayer, in the early phase of the flip kinetics where correction for non-enzymatic hydrolysis and acylation could be omitted. The distribution of a lysophospholipid finally attained reflects its affinity for the two layers. Whereas lysophosphatidylcholine has a slight preference for the outer layer of the membrane, lysophosphatidylserine spontaneously concentrates in the inner layer up to a ratio of 4:1. This asymmetry mimics the distribution of phosphatidylserine in the native membrane. Flip rates depend on membrane lipid compositions. They are enhanced by cholesterol depletion. Comparison of various mammalian species demonstrates that erythrocytes with a higher phosphatidylcholine/sphingomyelin ratio and high content of polyunsaturated fatty acids (mouse and rat) have a high transbilayer mobility, in contrast to erythrocytes with a low phosphatidylcholine/sphingomyelin ratio and a low content of polyunsaturated fatty acids (ox). Molecular properties of lysophospholipids influence their transbilayer mobility. Flip rates of lysophospholipids are enhanced not only by unsaturation of their fatty acid, but also by a negative net charge on the headgroup. This indicates that the strongly asymmetric distribution of phosphatidylserine in the native erythrocyte membrane, which is maintained for the lifespan of the cell, does not result from a lack of transbilayer mobility.

Isolation and characterization of plasma membranes from Friend erythroleukaemic cells. A study with sphingomyelinase C

Plasma membranes have been prepared from Friend erythroleukaemic cells using a Dounce homogenization technique followed by differential and sucrose gradient centrifugations. (I) A plasma membrane fraction was obtained which showed a 20- to 30-fold enrichment in 5'-nucleotidase, alkaline phosphodiesterase I, alkaline phosphatase and in 32P-labeled (poly)phosphoinositides. About 1% of the total protein, 6-7% of phospholipid, 8-9% of cholesterol and 12-15% of each of the above markers were recovered in the plasma membrane fraction with an average yield of 15-20%. The plasma membrane was characterized by a high cholesterol to phospholipid molar ratio (0.626), a 2-fold enrichment in sphingomyelin and in phosphatidylserine as compared to the whole cell and by the complete absence of diphosphatidylglycerol. (2) When compared to the phospholipid composition of the mature mouse erythrocyte membrane, the plasma membrane of the Friend cell only differs by a higher phosphatidylcholine and a lower phosphatidylethanolamine content, whereas the levels of sphingomyelin and phosphatidylinositol plus phosphatidylserine are similar. (3) Friend cells were treated with sphingomyelinase C (S. aureus) under non-lytic conditions and subsequently submitted to subcellular fractionation. The results showed that the plasma membrane accounted for 38.5% of the total phospholipid, 64.1% of the total cholesterol and about 4.4% of the total protein content of Friend cells. (4) Sphingomyelin appeared to be asymmetrically distributed in the plasma membrane of Friend cells, with about 85% of this phospholipid being present in the outer monolayer.