H. pylori colonizes the stomach for the life of the host because the immune response is ineffective. Persistence of the infection is the underlying cause of peptic ulcer disease and gastric cancer. We have reported that H. pylori induces apoptosis of macrophages. Because of their role in innate immunity, death of macrophages is likely to be an important cause of H. pylori persistence. We demonstrated that macrophage apoptosis is dependent on H. pylori-induced production of polyamines, which occurs by activation of arginase that converts L-arginine to Lornithine, and of ornithine decarboxylase (ODC) that converts L-ornithine to the polyamines putrescine, spermidine, and spermine. Backconversion of spermine to spermidine and spermidine to putrescine is an oxidative process that releases H2O2. An inducible form of polyamine oxidase (PAO1), also known as spermine oxidase, directly converts spermine to spermidine. We show that H. pylori induces PAO1 in macrophages, which results in release of H202, mitochondrial membrane depolarization, cytochrome c release, caspase-3 activation, and apoptosis. Inhibition of PAO1 attenuates these events, while transfection of PAO1 induces apoptosis. We have demonstrated a causal role for spermine oxidation by PAO1 in H. pylori-induced apoptosis and DNA damage in gastric epithelial cells. H. pylori induces expression of ODC in macrophages, and the spermine generated inhibits iNOS translation and NO production, resulting in enhanced survival of H. pylori. Additionally, we show that macrophage ODC and gastric epithelial cell PAO1 promoter activity is induced by H. pylori. We hypothesize that H. pylori disrupts polyamine homeostasis by inducing ODC and PAO1, leading to oxidative stress, apoptosis, DMA damage, and failure of innate immune response.
Our specific aims are: 1) To establish the molecular mechanisms by which H. pylori induces ODC and PAO1, assessing A) mechanisms in monocytes and macrophages, B) relevance of the identified promoter elements in (A), and C) mechanisms in gastric epithelial cells; 2) To determine the role of alterations in polyamine synthesis and oxidation in H. py/or/-induced immune dysregulation and DNA damage, assessing A) apoptotic mechanisms, B) DMA damage, and C) effects on iNOS; and 3) To demonstrate that the ODC-PAO pathway has an important role in H. pylori pathogenesis in vivo, assessing A) expression of ODC and PAO1 in H. pylori gastritis tissues, and B) the effect of inhibition of PAO in mouse H. pylori infection. We anticipate that our studies of this previously unrecognized pathway of mucosal immune dysregulation and injury will provide new insights into H. pylori pathogenesis and associated inflammation and carcinogenesis, and may provide insight into other forms of mucosal inflammation.
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