Peptidoglycan recognition protein 2 (N-acetylmuramoyl-L-Ala amidase) is induced in keratinocytes by bacteria through the p38 kinase pathway

Skin protects host tissues from invasion by microorganisms. Skin was once regarded as a merely mechanical barrier due to its thick layer of keratinized epithelium (49). While this function is important, in recent years it has become clear that skin is rich in antimicrobial peptides and proteins, such as β-defensins (20, 21, 45), dermcidin (44), cathelicidin (12, 48), RNases (22), psoriasin (18), bactericidal permeability-increasing protein (51), and others (32, 63, 64). Healthy intact skin contains low levels of antimicrobial peptides and proteins. Breakage of the keratinized layer, however, may expose keratinocytes to high numbers of microorganisms, which induce production of antimicrobial peptides and proteins. Antimicrobial peptides and proteins are also produced in the skin in response to proinflammatory cytokines released from macrophages and dendritic cells stimulated by microorganisms or in some inflammatory disease processes, such as psoriasis (6, 12, 13, 18, 22, 29, 38, 45, 48, 63, 64). In addition to antimicrobial peptides and proteins, body secretions, such as sweat and tears, contain a bacteriolytic enzyme, lysozyme (EC 3.2.1.17). Lysozyme hydrolyzes the glycosidic bond between β(1-4)-linked N-acetylmuramic acid (MurNAc) and N-acetylglucosamine (GlcNAc) of peptidoglycan, a polymer uniquely present in the cell walls of virtually all bacteria (3, 50). Lysozyme by itself is bacteriolytic for only some gram-positive bacteria, but it acts synergistically with antimicrobial peptides enhancing their antibacterial effect. Digestion of peptidoglycan by lysozyme is also important in reducing peptidoglycan's proinflammatory activity (14). Mammals also have another enzyme that digests peptidoglycan, N-acetylmuramoyl-l-alanine amidase (EC 3.5.1.28), which is primarily present in the serum and which hydrolyzes the amide bond between MurNAc and l-Ala and thus removes stem peptides from the peptidoglycan molecule (5, 24, 36, 56-58). Digestion of peptidoglycan with amidase reduces or eliminates biologic activities of polymeric peptidoglycan (23, 34). We have recently demonstrated that human peptidoglycan recognition protein L (PGRP L) has N-acetylmuramoyl-l-alanine amidase activity (60). PGRPs are a family of pattern recognition molecules that were discovered first in insects (27, 62, 66) and then in mammals (7, 9, 27, 31). Insects have up to 17 different PGRP proteins that recognize peptidoglycan and bacteria and have several functions. They initiate activation of the prophenoloxidase cascade (which generates antimicrobial melanin and reactive oxygen species), activate Toll and Imd pathways (which induce production of antimicrobial peptides), participate in phagocytosis of bacteria, and are peptidoglycan-lytic enzymes (2, 7, 9, 27, 52, 62). Mammals have a family of four PGRPs, which were initially named PGRP-S, PGRP-L, and PGRP-Iα and PGRP-Iβ, by analogy to insect PGRPs (31); they were recently renamed peptidoglycan recognition protein 1 (PGLYRP1) PGLYRP2, PGLYRP3, and PGLYRP4, respectively, by the Human Genome Organization Gene Nomenclature Committee. Although mammalian PGLYRPs were initially thought of as pattern recognition receptors similar to insect PGRPs (7, 31), it is now becoming clear that they do not function as cell-surface receptors but more likely as effector molecules. Mammalian PGLYRP1 is present in granulocyte granules and has antibacterial properties (10, 30, 55), and mammalian PGLYRP2 is an N-acetylmuramoyl-l-alanine amidase (15, 60). PGLYRP2 is constitutively produced in the liver (31, 60) and is secreted into the bloodstream (68) but is not constitutively produced in other tissues. The aim of this study was to determine whether production of PGLYRP2 could be induced in other tissues that come in contact with bacteria and especially in the skin and the eyes.