Macrophage catabolism of lipid A is regulated by endotoxin stimulation

Lipopolysaccharide (LPS) is a Gram-negative bacterial glycolipid that is believed to cause, by virtue of its stimulatory actions on macrophages and other eukaryotic cells, the life-threatening symptoms associated with Gram-negative infections. Macrophages both respond to and catabolically deactivate LPS. The lipid A moiety of LPS is responsible for the stimulatory actions of LPS on macrophages. We have previously developed methods employing a radiolabeled bioactive lipid A precursor, 4'-32P-lipid IVA, to study the interaction of this class of lipids with animal cells (Hampton, R. Y., Golenbock, D. T., and Raetz, C. R. H. (1988). J. Biol. Chem. 263, 14802-14807). In the current work, we have examined the uptake and catabolism of 4'-32P-lipid IVA by the RAW 264.7 cell line in serum-containing medium at physiological temperatures and have studied the effect of LPS stimulation on the ability of these cells to catabolize lipid IVA. RAW 264.7 macrophage-like cells avidly take up 4'-32P-lipid IVA under cell culture conditions at nanomolar concentrations. Uptake of lipid IVA was accompanied by lysosomal dephosphorylation of a fraction of the lipid to yield 4'-monophosphoryl lipid IVA. Chemically generated 4'-monophosphoryl lipid IVA was found to be substantially less bioactive than lipid IVA in the RAW cell, indicating that this catabolic dephosphorylation results in detoxification. In uptake experiments of 3-4 h duration, all metabolism of lipid IVA is blocked by ligands of the macrophage scavenger receptor. In longer experiments (24 h), both scavenger receptor-dependent and -independent uptake are responsible for the lysosomal catabolism of lipid IVA. Preincubation of RAW 264.7 cells with LPS caused dose-dependent inhibition of lipid IVA dephosphorylation. Sufficient LPS stimulation resulted in essentially complete inhibition of lipid IVA catabolism in both short- and long-term uptake experiments. This effect occurred at physiologically relevant concentrations of LPS (IC50 less than 1 ng/ml), and our data indicate that LPS-induced blockade of lipid IVA catabolism was due to the resultant physiological stimulation of the cells, and not inhibition of dephosphorylation by competition for uptake or enzymatic sites or by simple sequestration of labeled lipid IVA by LPS aggregates. We suggest that in the macrophage, LPS can modulate its own catabolism by virtue of its pharmacological properties. This effect of LPS could play a role in LPS pathophysiology as well as in macrophage biology.     

Dose dependent radiation-induced hypotension in the canine

Radiation-induced early transient incapacitation (ETI) is often accompanied by severe systemic hypotension. However, postradiation hypotension does not occur with equal frequency in all species and is not reported with consistency in the canine. In an attempt to clarify the differences in reported canine postradiation blood pressures, canine systemic blood pressures were determined both before and after exposure to gamma radiation of either 80 Gy or 100 Gy. Data obtained from six sham-radiated beagles and 12 radiated beagles indicated that 100 Gy, whole-body, gamma radiation produced a decrease in systemic mean blood pressure while 80 Gy, whole-body, gamma radiation did not. Analysis of this data could be consistent with a quantal response to a gamma radiation dose between 80 Gy and 100 Gy.     

Lipid A binding sites in membranes of macrophage tumor cells

Lipopolysaccharide affects a variety of eukaryotic cells and mammalian organisms. These actions are involved in the pathogenesis of Gram-negative septicemia. Many of the actions of lipopolysaccharide are believed to be caused by its active moiety, lipid A. Our laboratory has previously identified a bioactive lipid A precursor, termed lipid IVA (Raetz, C. R. H., Purcell, S., Meyer, M. V., Qureshi, N., and Takayama, K. (1985) J. Biol. Chem. 260, 16080-16888), which can be labeled with 32P of high specific activity and purified. In this work we have used the labeled probe, 4'-32P-lipid IVA, to develop a novel assay for the specific binding of lipid IVA to whole cells. We have also demonstrated its use in a ligand blotting assay of immobilized cellular proteins. Using the whole cell assay, we show that 4'-32P-lipid IVA specifically binds to RAW 264.7 macrophage-like cultured cells. The binding is saturable, is inhibited with excess unlabeled lipid IVA, and is proteinase K-sensitive. It displays cellular and pharmacological specificity. Using the ligand blotting assay, we show that several RAW 264.7 cell proteins can bind 4'-32P-lipid IVA. The two principal binding proteins have Mr values of 31 and 95 kDa, as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Fractionation studies indicate that the 31-kDa protein is enriched in the nuclear fraction and may be a histone, whereas the 95-kDa protein is enriched in the membrane fraction. The binding assays that we have developed should lead to a clearer understanding of lipid A/animal cell interactions.     

Isolation of a biologically active fragment from the carboxy terminus of the fetal rat binding protein for insulin-like growth factors

We have purified a 14 kDa fragment of the 30 kDa binding protein for insulin-like growth factors (IGFs) from BRL-3A cell conditioned medium. The fragment binds IGF-I and IGF-II with similar specificity to the 30 kDa binding protein, but with lower affinity. It corresponds to the carboxy terminus of the native binding protein (residues 148-270), and is thought to arise by proteolysis. We infer that this region of the native binding protein contains, at least in part, the IGF binding domain.     

Effects of changes in osmolality on the stability and function of cultured chromaffin cells and the possible role of osmotic forces in exocytosis

Recent evidence indicates that osmotic forces may play a role in exocytosis. To examine this possibility and to investigate the osmotic properties of storage granules within cells, we investigated the effects of changes of osmolality on stability and function of cultured bovine chromaffin cells. Cell volume measurements indicated that the cells behaved as osmometers and that the intracellular osmolality rapidly equilibrated with the osmolality of the extracellular medium. Hyperosmotic solutions strongly inhibited nicotinic agonist-stimulated secretion but did not alter nicotinic agonist-stimulated Ca(2+) uptake. Hyperosmotic solutions also strongly inhibited elevated potassium- stimulated secretion but only weakly inhibited elevated K(+)-stimulated Ca(2+) uptake. Thus, hyperosmotic solutions inhibited secretion at a step after calcium entry. Cells exposed to 165 mOs(1) solutions did not lyse and retained their capacity to store and secrete catecholamine upon stimulation. Significant intracellular lysis of chromaffin granules occurred within cells exposed to lower osmolalities. In contrast, 75 percent of the catecholamine was released from granules from cultured cells or from fresh adrenal medulla incubated in vitro at 210 mOs. The data provide evidence for a role for osmotic forces in exocytosis and suggest that if osmotic stress of the granule occurs during exocytosis, then water influx into chromaffin granules increases granule volume by at least 70 percent. The results also indicate that the osmotic properties of the granules are altered upon homogenization and subcellular fractionation of the cells.     

Explanation for the apparent inefficiency of reduced nicotinamide adenine dinucleotide in energizing amino acid transport in membrane vesicles

Lineweaver-Burk plots of reduced nicotinamide adenine dinucleotide (NADH) oxidation by membrane preparations from Bacillus subtilis are biphasic, with two K(m) values for NADH. The higher K(m) corresponds to the only K(m) observed for NADH oxidation by whole cells, whereas the lower K(m) corresponds to that observed with open cell envelopes. Membrane preparations apparently contain a small fraction of open or inverted vesicles which is responsible for the low K(m) reaction, whereas entry of NADH into the larger portion of closed, normally oriented vesicles is rate limiting and responsible for the high K(m) reaction. In contrast, the oxidation of l-alpha-glycerol-phosphate (glycerol-P) by membrane preparations shows only one K(m) that corresponds to that of glycerol-P oxidation by whole cells or lysates. Since glycerol-P dehydrogenase (NAD independent) has the same K(m), this enzyme reaction rather than entry of glycerol-P into vesicles represents the rate-limiting step for glycerol-phosphate oxidation. The K(m) for amino acid uptake by vesicles in the presence of NADH corresponds to the high K(m) for NADH oxidation, indicating that NADH energizes transport only if it enters closed, normally oriented vesicles. Studies with rotenone and proteolytic enzymes support this interpretation. The apparent efficiency of NADH in energizing uptake seems to be lower than that of glycerol-P because, under the experimental conditions usually employed, open or inverted vesicles that do not participate in amino acid uptake are responsible for the major portion of NADH oxidation. When the results are corrected for this effect, the efficiency of NADH is essentially the same as that of l-alpha-glycerol-P.