Identification of Acyloxyacyl Hydrolase, a Lipopolysaccharide Detoxifying Enzyme, in the Murine Urinary Tract
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Acyloxyacyl hydrolase (AOAH) is a lipase that removes the secondary fatty acyl chains that are substituted to the hydroxyl groups of glucosamine-linked 3-hydroxyacyl residues in lipid A, the bioactive center of Gram-negative bacterial lipopolysaccharides (LPS). Such limited deacylation has been shown to attenuate cytokine and chemokine responses to LPS, suggesting a role for AOAH in modulating (downregulating) inflammatory responses to invading Gramnegative bacteria. Prior to the experiments described in this report, AOAH had only been found in myeloid lineage cells (monocyte-macrophages, neutrophils and dendritic cells). In the work presented here, AOAH was found in murine renal proximal tubule cells and in human renal v cortex. Proximal tubule cells are known targets for invading Gram-negative uropathogens and we hypothesize that possessing AOAH may help them degrade the LPS contained within these bacteria. I further found that AOAH is secreted from proximal tubules in vitro and that it can be detected in murine urine, where it is able to deacylate purified LPS. AOAH may also associate with downstream bladder epithelial cells (which do not express AOAH) and be processed by them to its more enzymatically active, mature form. Bladder cells that have taken up AOAH in vitro are able to deacylate LPS. To determine the in vivo role of AOAH, I induced ascending urinary tract infections (UTIs) in wild type and AOAH null mice. To my surprise, AOAH null mice were able to clear bacteria from their urine faster than did wild type mice. An analysis of the immune response by histological analysis of bladder tissue and enumeration of neutrophils in the urine did not show a significant difference between wild type and AOAH null mice at any of the time points examined. Although I do not yet understand the mechanism for such increased clearance in AOAH null animals, we hypothesize that, due to their inability to deacylate LPS, they might have a more effective immune response to invading Gram-negative bacteria. A more detailed analysis of such responses to invading Gram-negative uropathogens will be important for understanding the in vivo role of AOAH in the urinary tract.