Propionate induces polymorphonuclear leukocyte activation and inhibits formylmethionyl-leucyl-phenylalanine-stimulated activation

Abstract

Short-chain carboxylic acids (SCCA) are metabolic by-products of bacterial pathogens which can alter cytoplasmic pH and inhibit a variety of polymorphonuclear leukocyte (PMN) motile functions. Since cytoskeletal F-actin alterations are central to PMN mobility, in this study we examined the effects of SCCA on cytoskeletal F-actin. Initially, we tested nine SCCA (formate, acetate, propionate, butyrate, valerate, caproate, lactate, succinate, and isobutyrate). We document here that while eight altered cytoplasmic pH, only six altered cytoskeletal F-actin. We then selected one SCCA that altered both F-actin and cytoplasmic pH (propionate) and one SCCA that altered only cytoplasmic pH (lactate) for further study. Propionate, but not lactate, caused an irregular cell shape and F-actin distribution. Furthermore, propionate, but not lactate, inhibited formylmethionyl-leucyl-phenylalanine (fMLP)-stimulated PMN polarization, F-actin localization, and cytoplasmic pH oscillation. Propionate-induced changes in cytoskeletal F-actin and cytoplasmic acidification were not affected by the fMLP receptor antagonist N-t-BOC-1-methionyl-l-leucyl-l-phenylalanine; however, alkalinization was affected. Pertussis toxin treatment completely inhibited propionate-induced changes in F-actin but had no effect on propionate-induced cytoplasmic pH oscillation. These results indicate that propionate (i) bypasses the fMLP receptor and G protein(s) to induce cytoplasmic pH oscillation, (ii) operates through G protein(s) to induce actin oscillation, cell shape changes (to irregular), and F-actin localization, and (iii) inhibits fMLP-stimulated cytoplasmic pH and actin oscillation, PMN polarization, and F-actin localization.Neutrophils provide the first line of host defense against bacterial pathogens. How and why the delicate balance between the polymorphonuclear leukocytes (PMNs) and bacteria is swayed toward the bacteria in disease is often not clear. It is clear that chemical mediators which activate or inactivate the PMNs initially interact with receptors, secondarily activate biochemical responses, and finally activate cellular responses (5, 37, 50). Bacterial mediators found at the sites of infection may use similar pathways to activate or inactivate PMN function (1,10,11,25,43,59).Short-chain carboxylic acids (SCCA) are metabolic byproducts from pathogenic anaerobic bacteria and appear in millimolar concentrations at the sites of infection (1,6,17,28,51,58). Studies from several laboratories indicate that SCCA inhibit stimulated PMN functions, such as chemotaxis, phagocytosis, degranulation, oxidative burst, and phagocytic killing of ingested bacteria (9, 4244,55,59).Despite considerable evidence regarding the effects of SCCA on PMNs, the mechanism(s) of SCCA-induced inhibition is unknown. Paradoxically, some SCCA can stimulate cytoskeletal F-actin polymerization, right-angle light scatter, and changes in cytoplasmic pH (25, 54, 59) yet inhibit motile functions (1). The purpose of this study was to examine the mechanism of SCCA-mediated PMN...

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