Lynn King

Phagocytosis-induced release of arachidonic acid from human neutrophils

Abstract The phospholipids of human neutrophils were labeled with [3H] arachidonic acid and [14C] palmitic acid. Phagocytosis of opsonized zymosan resulted in rapid release of free arachidonic acid but not of palmitic acid. Arachidonic acid was not released when the cells were exposed to unopsonized zymosan, zymosan-activated serum, or phorbol myristate acetate. These observations suggest that phagocytosis of opsonized zymosan results in the activation of a phospholipase A2.

Regulation of arachidonic acid metabolism in Madin-Darby canine kidney cells. Comparison of A23187 and 12-O-tetradecanoyl-phorbol-13-acetate

Challenge of Madin-Darby canine kidney (MDCK) cells with the divalent cation ionophore A23187 caused a marked increase in the deacylation of [3H]arachidonic acid but not of [14C]palmitic acid. When the cells were treated with 12-O-tetradecanoyl-phorbol-13-acetate (TPA) and A23187, there was an additional increase in the deacylation of [3H]arachidonic acid compared to that observed with either agent alone. In contrast to deacylation, the stimulation of prostaglandin production by A23187 was small compared to the stimulation by TPA. Cycloheximide inhibited synthesis of prostaglandins in TPA-treated cells, but did not block the stimulated deacylation caused by either TPA or A23187. These data indicate that, while both TPA and A23187 stimulated the deacylation of [3H]arachidonic acid, TPA had an additional, cycloheximide-sensitive effect that was required for efficient conversion of the release fatty acids to prostaglandins. Thus, although required, deacylation appeared to be independent of and insufficient to stimulate maximum prostaglandin synthesis in these cells.

Stimulation of deacylation in Madin-Darby canine kidney cells. 12-O-tetradecanoyl-phorbol-13-acetate stimulates rapid phospholipid deacylation.

The tumor-promoting phorbol diester, 12-O-tetradecanoyl-phorbol-13-acetate (TPA), stimulates Madin-Darby canine (MDCK) cells to deacylate cellular phospholipid and to produce prostaglandins. We have used this system to characterize the kinetics of deacylation of [3H]arachidonate and the further metabolism of arachidonate by the cyclooxygenase system. Stimulation of the appearance of [3H]arachidonic acid in extracellular fluids was found to be maximal 2 h after treatment with TPA and its subsequent removal. The production of prostaglandins then followed for up to 24 h. Phospholipase activity was not inhibited by indomethacin over the range of 0.01-100 micrograms/ml. In contrast, prostaglandin synthesis was inhibited at 1 microgram/ml indomethacin. Further, there was a significant stimulation of deacylation within 15 min in the presence of TPA that increased to nearly 30% of the total radioactivity within 1 h. Likewise, stimulation of prostaglandin production was detected within 15 min, but, unlike the deacylation process, did not increase significantly during TPA treatment. The source of arachidonic acid in the early stimulation period was found to be primarily phosphatidylethanolamine, but phosphatidylcholine and phosphatidylinositol were also deacylated. The results presented here argue that the phospholipase and cyclooxygenase are not tightly coupled in this system. Furthermore, we conclude that the earliest effect of TPA with regard to increased prostaglandin production in the MDCK cell is the direct stimulation of phospholipase activity.

[42] Phospholipase activity of bacterial toxins

Publisher Summary This chapter describes the phospholipase activity of bacterial toxins. The chapter describes methods for the preparation of radiolabeled phospholipid substrates and methods for determining phospholipase activity and specificity. The assay of a recently recognized phospholipase A 2 (PLA2) produced by Vibrio vulnificus and the phospholipase D (PLD) activity of a recently isolated toxin produced by Vibrio damsela can be used as representative examples of the phospholipase determinations. Bacterial toxins may possess phospholipase activity that contributes to their membrane-damaging properties. These bacterial phospholipases have a variety of substrate specificities and include phospholipases A, C, and D. The methods described in the chapter provide a simple and sensitive method for determining the phospholipase activity and phospholipid class specificity of bacterial phospholipases. These procedures are easily modified to determine the ion requirements and pH optima of the enzymes. However, these procedures are not recommended for kinetic analysis of the enzymes because of problems of substrate insolubility in aqueous solutions.

Lipid synthesis in cultured human embryonic fibroblasts.

We describe here the pathways by which human embryonic fibroblasts synthesize lipids. In these studies, we quantitated the phospholipids by their phosphorus content and by their acyl components. These determinations defined both the chemical composition of the cellular membranes as well as their metabolic turnover. Using radiolabeled precursors, we have shown (a) synthesis of the glycerol moiety via glycolysis and the action of glycerokinase, (b) utilization of both exogenously added and endogenously synthesized fatty acids, (c) synthesis de novo of phosphatidyl choline and phosphatidyl ethanolamine from their base precursors, and (d) the methylation of phosphatidyl ethanolamine yielding phosphatidyl choline. Dividing cells synthesized phosphoglyceride more rapidly than cells in the stationary phase. However, considerable turnover of cellular lipid did occur in the stationary phase.

Phospholipid synthesis in human embryo fibroblasts infected with herpes simplex virus type 2

The effect of herpes simplex virus type 2 infection on the synthesis of phospholipids in human embryo fibroblasts was determined at temperatures permissive (35 C) or nonpermissive (42 C) for virus replication. Incorporation of [32P]i was decreased by herpes simplex virus type 2 in fection after 6 hr, which corresponds to the time of initiation of progeny virus production. No differences were observed in the relative incorporation of [32P]i into phospholipid classes. In another series of experiments, cells were labeled with [3H] ethanolamine before infection and with [14C] ethanolamine after infection. The incorporation of [14C] ethanolamine was also decreased after 6 hr of infection. When choline was substituted for ethanolamine, a similar, although less pronounced, decrease in incorporation was seen in infected cells compared to mock-infected cells. During abortive infection at 42 C, incorporation of [3H] thymidine into cellular DNA was stimulated, but the incorporation of phospholipid precursors was decreased. Total phospholipid composition and phospholipid acyl group composition were not changed appreciably during abortive or productive infection, regardless of whether the cells were labeled before or after infection. In conclusion, these data indicated that, during herpes simplex virus type 2 infection, the incorporation of lipid prescursors into phospholipid was decreased. The stimulation of cellular DNA synthesis previously observed during abortive infection at 42 C was not paralleled by a detectable stimulation of total phospholipid synthesis. Neither productive nor abortive infection resulted in significant phospholipid compositional changes in the host cell; however, both resulted in a marked inhibition of phospholipid synthesis.