Proteases in Helicobacter pylori Infection

Introduction
Proteolytic enzymes are essential for the survival of all organisms, and their genes make up ≈2% of the human genome. Proteases constitute a diverse group of enzymes that share one single property, the capability of hydrolysing peptide bonds. Based on their catalytic mechanisms, they are classified in eight ‘catalytic types’ among which aspartic, serine, cysteine, threonine and metalloproteases are the most relevant types in mammalians. In order to regulate the activity of these enzymes and to prevent detrimental effects to cells and tissues, several control mechanisms have evolved including: expression as inactive proforms (zymogens); compartmentalisation in granules or lysosomes; inactivation by unfavourable conditions (redox status, pH, temperature); and inhibition by protease inhibitors. As well as having a role in protein catabolism, the role of proteases and their inhibitors in inflammation and cancer is now well established.

Helicobacter pylori infection is the most common cause of chronic gastritis, peptic ulcer disease and mucosa-associated lymphoid tissue (MALT) lymphoma. H. pylori is recognised to be the most important risk factor for the development of gastric cancer. The development of various clinical outcomes of the H. pylori infection depend on specific virulence factors of the bacterium, gene polymorphisms of the host and environmental conditions. The interplay of these factors define the topographic expression of chronic gastritis. The interplay of these factors defines the expression of chronic gastritis. Predominant antrum gastritis increases the risk for duodenal ulceration, whereas atrophic gastritis enhances the risk for gastric ulcer and adenocarcinoma. The latter condition is frequently associated with histomorphological modifications of the gastric architecture, the development of atrophic gastritis and intestinal metaplasia, which both represent premalignant conditions with a high potential to progress to gastric cancer. The key findings related to H. pylori-mediated gastritis are discussed below.

Proteins require hydrolysation for absorption by the intestine. Proteolytic enzymes responsible for degrading proteins are produced by three organs: stomach (pepsins), pancreas (chymotrypsin) and small intestine (e.g. aminopeptidases). Pepsins are aspartic gastric proteases, belonging to the peptidase family A1, and are secreted as zymogens. To date, pepsinogens A, B, C, F and prochymosin are known. In humans, pepsinogen A and pepsinogen C represent the two major gastric proteases. Pepsinogen A and C are secreted from corpus and antral-duodenal mucosa, respectively. The release of pepsinogens is regulated by centrally acting hormones like acetylcholine, adrenaline and somatostatin and gastric peptides such as gastrin-17/-34 or ghrelin. The biosynthesis of most hormones including gastrin-17 and ghrelin includes proteolytic processing by proteases such as subtilisinlike proconvertases (PC-1, PC-2) as well as lysosomal cathepsins and exopeptidases. In addition to the stomach-specific expression of pepsinogens, the gastric mucosa expresses a variety of additional proteases such as lysosomal proteases (e.g. cathepsins B, L, S and D) and matrix-metalloproteases that are involved in the antigen presentation and the remodelling of the extracellular matrix by epithelial cells.

Genomic analyses demonstrated that among the 1,500- 1,600 different genes usually encoded by a typical H. pylori genome, at least 20 are protease-encoding genes. So far, there are no available data concerning the relevance of these enzymes with respect to gastroduodenal diseases. Based on their homology to genes of other bacteria (Escherichia coli, Salmonella spec.), putative functions of some H. pylori-encoded proteases were proposed. ClpXP protease and the serine protease HtrA are considered to be stressregulated proteins that function both as chaperone and protease. The serine protease HtrA is secreted and belongs to the 150 most abundant proteins in H. pylori, and represents an antigen recognised by the majority of sera obtained from H. pylori-infected patients. Salama et al. identified the HtrA protease as an essential gene for the survival of the bacterium. Furthermore, a secreted collagenase (HP0169) was described as virulence factor for H. pylori colonisation. H. pylori is also capable of capturing host proteases on its membrane and activating them. Two plasminogenbinding proteins of H. pylori were identified and found in a variety of strains suggesting that the capture of plasminogen and its subsequent activation at the bacterial surface might be a general phenomenon that may enhance the virulence of H. pylori.