THE LISTERIAL EXOTOXINS LISTERIOLYSIN AND PHOSPHATIDYLINOSITOL-SPECIFIC PHOSPHOLIPASE-C SYNERGIZE TO ELICIT ENDOTHELIAL-CELL PHOSPHOINOSITIDE METABOLISM

Exotoxins such as listeriolysin (LLO) and phosphatidylinositol-specifi c phospholipase C (PlcA) have been implicated in listerial infection a nd sepsis, Employing different Listeria strains, mutated in individual ly known virulence genes, we examined exotoxin-related induction of en dothelial cell signaling, Listeria monocytogenes was a potent inductor of phosphatidylinositol (Ptdlns) metabolism in HUVEC. This effect was completely absent in a LLO-negative strain, Using a recombinant Liste ria innocua strain, engineered to produce high levels of LLO, Ptdlns m etabolism was restored to similar to 30% of that produced by the paren tal L. monocytogenes strain, A recombinant L. innocua strain expressin g only PlcA did not induce any Ptdlns metabolism, Even higher than wil d-type levels of Ptdlns hydrolysis products were, however, evoked when engineered bacteria secreted both LLO and PlcA, These effects occurre d in the absence of bacterial uptake by the endothelial cells, Corresp onding results were observed with regard to endothelial diacylglycerol (DAC) generation, The amplification of endothelial cell signaling cou ld be reproduced by engaging purified LLO and PlcA in the absence of b acteria, In these experiments, the unrelated performing agent staphylo coccal a-toxin, a very weak stimulus for endothelial phosphoinositide metabolism by itself, substituted for LLO to allow marked Ptdlns hydro lysis when co-applied with PlcA, We conclude that the listerial exotox ins LLO and PlcA cooperate to provoke potent second messenger synthesi s in endothelial cells, in the absence of cell invasion by the bacteri a, This is an impressive example of synergism between a pore-forming a nd an enzymatic bacterial exotoxin in prove king cell signaling and in flammatory events.