High de novo synthesis of glycerolipids compared to deacylation-reacylation in rat liver microsomes

Insertion of functional intact insulin receptors into hepatocytes

Isolated insulin receptors were inserted into hepatocytes by fusion with receptor-lysophospholipid micelles. The number of receptors was increased approximately two times as monitored by insulin binding experiments. Addition of exogenous receptors resulted in increased receptor autophosphorylation. The exogenous added receptors could be coupled to the intracellular effector system as shown by increased phosphorylation of the insulin receptor substrate 1, IRS-1, and by increased glycogen synthesis. The co-incorporated lysophospholipid was partly metabolized to phospholipid. This method for introducing insulin receptors into cells represents a novel system to study the pathway of insulin signalling and may provide information as to the role of variation of receptor number in insulin action in cellular metabolism of specific cells.

Long chain fatty acid utilization of T-cells from autoimmune MRL-lpr/lpr mice

To test the hypothesis that T-cells which exhibit abnormal immunological behavior manifest derangements in the de novo synthesis of phospholipids, the utilization of [3H]palmitic acid in B220+ T-cells from autoimmune MRL-lpr/lpr mice was investigated. The rate of incorporation of [3H]palmitic acid into membrane phospholipids was markedly increased in intact B220+ T-cells compared to that in T-cells from immunologically normal mice. The activities of two key enzymes involved in the de novo synthesis of palmitoyl-phospholipids, acyl-coenzyme (CoA) ligase and acyl-CoA; sn-glycerol-3-phosphate acyl transferase, were significantly higher in homogenates from B220+ T-cell membranes compared with those in controls. Despite these findings, the molar concentration of individual palmitoyl glycerolipids was equivalent in the membranes of B220+ T-cells and control lymph node T-cells. The results indicate that T-cells from lupus mice exhibit complex defects in the biosynthesis and turnover of membrane phospholipids and suggest the possibility that these aberrations contribute to T-cell dysfunction in autoimmune diseases.

Insertion of isolated insulin receptors into placental membrane vesicles

Purified human insulin receptors were inserted into placental plasma-membrane vesicles by fusion of membranes with receptor-lysophosphatidylcholine micelles. Scatchard analysis of insulin binding showed that about 10-15% of the added receptors became inserted into the membrane. The receptor number could be increased about 3-fold, corresponding to approx. 5 pmol of receptor/mg of membrane protein. The receptors became firmly bound to the membrane, as they could not be removed by extensive wash. The insertion of exogenous receptors could be demonstrated by immunoblotting. The inserted insulin receptor had the same insulin-binding affinity as the isolated receptor and the endogenous receptor of the membrane. Insulin binding in the presence or absence of Triton X-100 revealed that more than 80% of the exogenous receptors had a right-side-out orientation. Function of the inserted receptors, as observed by insulin-stimulated autophosphorylation, could be demonstrated. About 80% of the added lysophospholipid, corresponding to approx. 160 nmol of lysophospholipid/mg of membrane protein, became integrated into the membrane and was partly metabolized to phospholipid and to non-esterified fatty acid. The method of insertion of isolated insulin receptors using the natural detergent, lysophospholipid, may be a method for insertion of receptors into intact cells, where the lysophospholipid, as in the plasma-membrane vesicles, will be acylated to phospholipid.

Biosynthesis of glycerolipids by hepatoma and liver microsomes. I. Fatty acyl-CoA ligase and acyl-CoA:sn-glycerol-3-phosphate acyltransferase

The intracellular membranes of hepatomas exhibit an altered content and composition of lipid compared to the membranes of normal liver. In order to elucidate the role of lipid biosynthetic enzymes in these membrane differences, we first examined the fatty acyl-CoA ligase and acyl-CoA:sn-glycerol 3-phosphate (Gro-3P) acyltransferase activities and acyl specificities of microsomes from liver, Morris hepatoma 7288C, and hepatoma tissue culture (HTC) cells. Based upon incorporation of fatty acid and Gro-3P, it is concluded that acyl-CoA:sn-Gro-3P acyltransferase activities are markedly elevated (6-30-fold) in the microsomes of Morris Hepatoma 7288C and HTC cells compared to microsomes from liver, whereas the fatty acyl-CoA ligase activity is reduced (30-50-fold). Therefore, the low phospholipid content of these tumor cells does not appear to result from reduced acyltransferase activity. Though diminished ligase activity may play a role, it appears that activation of fatty acid may not be rate-limiting, even at the low levels of fatty acyl-CoA ligase present in the tumor and HTC cells. Preliminary evidence suggests that another factor that may be responsible for the low tumor phospholipid content is the limited availability of Gro-3P, a lipid precursor. The phospholipid in hepatoma 7288C is also characterized by an elevated ratio of monenoic to dienoic fatty acid. We have found that this change does not reflect an altered specificity of acyl-CoA:sn-Gro-3P acyltransferase.

Incorporation of cytochrome b5 into endoplasmic reticulum vesicles as protein-lysophospholipid micelles

Cytochrome b5 is incorporated into vesicles of the endoplasmic reticulum as protein-lysophosphatidylcholine micelles. Cytochrome b5 becomes firmly bound to the membrane and at the same time lysophosphatidylcholine is acylated by acyltransferases of the endoplasmic reticulum and converted into the membrane component phosphatidylcholine. The possibility of an insertion of cytochrome b5 into the endoplasmic reticulum in vivo by this mechanism is discussed.

The molecular species of phosphatidic acid, diacylglycerol and phosphatidylcholine synthesized from sn-glycerol 3-phosphate in rat lung microsomes

The species pattern of phosphatidic acid, diacylglycerol and phosphatidylcholine synthesized from [14C]glycerol 3-phosphate was measured using a newly developed HPLC technique yielding 13 molecular species. A direct comparison of these species patterns presupposes determination of the lipolytic activity of lung microsomes. The lipolytic activity was quantitatively determined by measuring the changes of the endogenous concentration of diacylglycerol, triacylglycerol and free fatty acids. The species pattern of endogenous diacylglycerol measured in the time-course of lipolysis did not show any changes up to an incubation period of 20 min, suggesting that the lipolytic activity showed only a very low selectivity for individual substrate species. Diisopropylfluorophosphate (5 mumol/mg microsomal protein) strongly decreased the lipolytic activities as well as the microsomal phosphatidate phosphohydrolase activity, as measured by means of exogenous phosphatidic acid, and also the generation of phosphatidic acid from [14C]glycerol 3-phosphate. In lung microsomes, labeled phosphatidic acid and diacylglycerols were synthesized from the endogenous free fatty acids and sn-[14C]glycerol 3-phosphate, which had previously been added. By addition of CDPcholine to the prelabeled microsomes the synthesis of phosphatidylcholine was measured. After hydrolysis of phosphatidic acid and phosphatidylcholine with cytoplasmatic phosphatidate phosphohydrolase or phospholipase C, respectively, the de novo synthesized species patterns of these two lipids and of the diacylglycerol were determined. Comparison of the species pattern of de novo synthesized phosphatidic acid with that of diacylglycerol largely showed the same distribution of radioactivity among the individual species, except that the relative proportion of label was higher in the 16:0/16:0 and 16:0/18:0 species of phosphatidic acid and lower in the 16:0/20:4 and 18:0/20:4 species than in the corresponding species of diacylglycerol. The species pattern of de novo-synthesized diacylglycerol showed no differences from that of the phosphatidylcholine synthesized from it. From this result we concluded that the cholinephosphotransferase of lung microsomes is nonselective for individual species of the diacylglycerol substrate. The 16:0/18:1 and 16:0/18:2 species of phosphatidic acid, diacylglycerol and phosphatidylcholine showed a higher synthesis rate than their 18:0 counterparts, whereas the 16:0 or 18:0 analogues of species containing 20:4 and 22:6 fatty acids showed nearly the same synthesis rates.(ABSTRACT TRUNCATED AT 400 WORDS)

The role of Mg2+-dependent phosphatidate phosphohydrolase in pulmonary glycerolipid biosynthesis

Rat lung microsomes washed with increasing concentrations of NaCl show a displacement of protein from microsomes to the wash supernatant. Among the proteins removed from the microsomal surface was the Mg2+-dependent phosphatidate phosphohydrolase, while the Mg2+-independent activity remained associated with the microsomes. The Mg2+-dependent activity could be quantitatively assayed in the wash supernatant. Microsomes washed with increasing concentrations of NaCl showed a progressive impairment in the synthesis of labelled neutral lipid and phosphatidylcholine from [14C]glycerol 3-phosphate with a concomitant increase in the labelling of phosphatidic acid. The impairment was sigmoidal and correlated highly with the decrease in Mg2+-dependent phosphatidate phosphohydrolase activity. When Mg2+-dependent phosphatidate phosphohydrolase from wash supernatant was incubated with microsomes previously washed with high salt concentrations, the labelling of neutral lipid and phosphatidylcholine was returned to control levels. Labelling of neutral lipids and phosphatidylcholine could be restored upon addition of a cytosolic Mg2+-dependent phosphatidate phosphohydrolase isolated by gel filtration. Mg2+-independent phosphatidate phosphohydrolase isolated from cytosol was incapable of restoring the labelling of neutral lipids and phosphatidylcholine. These findings confirm that the Mg2+-dependent phosphatidate phosphohydrolase of rat lung is involved in pulmonary glycerolipid biosynthesis. The role of the Mg2+-independent phosphatidate phosphohydrolase activity remains unknown.

Glycerophosphate acylation by microsomes and mitochondria of normal and dystrophic human muscle

The incorporation of [14C]glycerophosphate into phosphatidic acid, diacylglycerol, triacylglycerol and phosphatidylcholine by microsomes and mitochondria prepared from normal and dystrophic human muscle incubated in vitro in the presence of 0.3 mmol/l CDP-choline was measured. In mitochondria only phosphatidic acid and diacylglycerol are labelled; the rate of incorporation into these two compounds showed no difference between dystrophic and normal mitochondria. In dystrophic microsomes the incorporation into phosphatidic acid was delayed and decreased. No incorporation of glycerol into diacylglycerol, phosphatidylcholine and triacylglycerol could be measured. Thus in dystrophic muscle microsomes only PA was labelled during an incubation of up to 45 min. In both types of microsomes the concentration of endogenous free fatty acids and diacylglycerol was nearly identical. The level of phosphatidylcholine was 186 and 79 nmol/mg microsomal protein in normal and dystrophic muscle microsomes, respectively. Whether the results could be explained as secondary changes was discussed. Despite some unsolved problems the conclusion was drawn that a better explanation of the results is to assume a primary defect involving microsomal-bound phosphatidic acid phosphohydrolase and possibly glycerol-P-acyltransferases.