The Pseudomonas aeruginosa Autoinducer N-3-Oxododecanoyl Homoserine Lactone Accelerates Apoptosis in Macrophages and Neutrophils

Abstract

Quorum-sensing systems are critical regulators of the expression of virulence factors of various organisms, including Pseudomonas aeruginosa. Las and Rhl are two major quorum-sensing components, and they are regulated by their corresponding autoinducers,N-3-oxododecanoyl homoserine lactone (3-oxo-C 12-HSL) and N-butyryl-l-homoserine lactone (C 4-HSL). Recent progress has demonstrated the potential of quorum-sensing molecules, especially 3-oxo-C 12-HSL, for modulation of the host immune system. Here we show the specific ability of 3-oxo-C 12-HSL to induce apoptosis in certain types of cells. When bone marrow-derived macrophages were incubated with synthetic 3-oxo-C 12-HSL, but when they were incubated not C 4-HSL, significant loss of viability was observed in a concentration (12 to 50 μM)- and incubation time (1 to 24 h)-dependent manner. The cytotoxic activity of 3-oxo-C 12-HSL was also observed in neutrophils and monocytic cell lines U-937 and P388D1 but not in epithelial cell lines CCL-185 and HEp-2. Cells treated with 3-oxo-C 12-HSL revealed morphological alterations indicative of apoptosis. Acceleration of apoptosis in 3-oxo-C 12-HSL-treated cells was confirmed by multiple criteria (caspases 3 and 8, histone-associated DNA fragments, phosphatidylserine expression). Structure-activity correlation experiments demonstrated that the fine structure of 3-oxo-C 12-HSL, the HSL backbone, and side chain length are required for maximal activity. These data suggest that Pseudomonas 3-oxo-C 12-HSL specifically promotes induction of apoptosis, which may be associated with 3-oxo-C 12-HSL-induced cytotoxicity in macrophages and neutrophils. Our data suggest that the quorum-sensing molecule 3-oxo-C 12-HSL has critical roles in the pathogenesis of P. aeruginosa infection, not only in the induction of bacterial virulence factors but also in the modulation of host responses.