AOAH explained

Acyloxyacyl hydrolase, also known as AOAH, is a eukaryotic protein encoded by the AOAH gene. AOAH is produced by macrophages (including Kupffer cells and microglia), dendritic cells (especially in the colon), NK cells, ILC1 cells, neutrophils and renal proximal tubule cells.

Species distribution

The AOAH gene has been found in many invertebrates and in all vertebrates studied to date except fish. Although mice have other well-established mechanisms for preventing LPS signaling, none of these has prevented long-term persistence of stimulatory LPS in animals that lack AOAH.

Structure

The enzyme's 2 disulfide-linked subunits are encoded by a single mRNA. The smaller subunit is a member of the saposin-like (SAPLIP) protein family and the larger subunit, which contains the active site serine, is a GDSL lipase. The enzyme's 3D structure and catalytic mechanism were reported by Gorelik et al.

Function

Acyloxyacyl hydrolase (AOAH) is a lipase that selectively releases the secondary (acyloxyacyl-linked) fatty acyl chains from the hexaacyl lipid A moiety found in many bacterial lipopolysaccharides (LPSs, also called endotoxins). The resulting tetraacyl LPS is non-stimulatory and can be a potent inhibitor of LPS sensing via the MD-2--Toll-like Receptor 4 (TLR4). The enzyme's other known substrates include bacterial lipopeptides and several host glycerolipids, including lyso-and oxidized phospholipids.

Animal studies

Absence of the enzyme in genetically engineered mice has been associated with distinctive phenotypes. AOAH-deficient animals are unable to inactivate even small amounts of LPS in most tissues; the LPS remains bioactive and may pass from cell to cell in vivo for many weeks. The LPS-injected mice develop strikingly high titers of polyclonal antibodies, prolonged hepatomegaly, and innate immune "tolerance" that results in slow and inadequate responses to a bacterial challenge. Absence of the enzyme renders mice more likely to develop severe lung injury and die if they are challenged with intratracheal LPS, Gram-negative bacteria, or acid (AOAH may also inactivate oxidized phospholipids). Other studies have found that AOAH reduces the stimulatory potency of LPS that translocates from the gastrointestinal tract to the liver and other organs. AOAH may also prevent LPS-induced arterial foam cell formation in vivo.