High molecular protein of Helicobacter pylori responsible for inhibition of ornithine decarboxylase activity of human gastric cultured cells

Background: Ornithine decarboxylase (ODC), a rate-limiting enzyme of polyamine biosynthesis, mediates epithelial cell proliferation and plays a critical role in the optimal repair of gastric mucosal damage. Several studies have shown that Helicobacter pylori inhibits the growth and proliferation of gastric cells in vitro . Aim: To test whether H. pylori extract affects ODC mRNA expression and its enzyme activity in gastric cells and to examine the partial characterization of the molecule responsible for this effect. Methods: Human gastric cells (MKN-45) were used. Bacterial extracts from various E. coli or H. pylori strains, namely (1) cagA (+) , vacA (+) , CagA(+) , VacA(+) ; (2) cagA (+) , vacA (+) , CagA(+) VacA(-) ; or (3) cagA (-) , vacA (+) , CagA(-) , VacA(-) were added to the cells. Cell proliferation was assessed by [(3) H]-thymidine incorporation, viability by MTT assay and LDH release test, ODC enzyme activity by (14) CO2 counts from L-[1(14) C]ornithine, and ODC mRNA by Northern blotting. Results: H. pylori and E. coli extract did not affect viability of gastric cells. H. pylori extract, especially extracts containing a protein greater than 50 kDa, significantly inhibited proliferation and ODC activity of gastric cells while E. coli extract had no effect. Inhibition of ODC activity was found in extracts of all H. pylori strains, irrespective of CagA and VacA protein expression. Serum stimulation induces an increase in ODC mRNA while H. pylori extract did not affect ODC mRNA expression. Conclusion: High molecular weight (greater than 50 kDa) proteins of H. pylori extract without CagA or VacA protein inhibited proliferation and ODC activity of human gastric cells, but did not affect ODC mRNA expression, suggesting that inhibition of ODC activity is regulated at the post-transcriptional level.