Here we identify a novel antimicrobial activity associated with chemerin, and show that host-derived cat L and cat K can cleave and activate the leukocyte attractant activity of chemerin, as well as enhance its antibacterial effects.
Various serine proteases have been reported to effectively convert chemerin to a potent chemoattractant in vitro
. There is also a single example of a cysteine protease, S. aureus
-derived staphopain B (SspB) that can efficiently activate human chemerin. In addition, host-originating cathepsin S and calpains have been reported to process mouse chemerin, although in this case the proteolysis of the C-terminus generates chemerin variants equipped primarily with anti-inflammatory properties (13
). Cysteine cathepsins of the papain-like family are normally confined to the endosomal/lysosomal network. However, there is evidence that certain cathepsins are also active extracellularly, either in association with the cell surface or in soluble form (14
). Some cells such as macrophages and fibroblasts constitutively secrete cysteine cathepsins as zymogens (14
). Moreover, macrophages have been reported to deploy enzymatically active cathepsin B, L and S and exhibit an elastin-degrading phenotype, indicating that macrophages can mobilize cysteine cathepsins to participate in the pathophysiologic remodeling of the extracellular matrix (17
). Massive amounts of extracellular cathepsins, probably released from macrophages, are found in the bronchial tree of patients suffering from acute pulmonary inflammation (24
). In addition, cat K is strongly implicated in maintaining the homeostasis of the extracellular matrix in the lung (25
). Since chemerin mRNA is abundantly produced in lung (1
), collectively these data suggest that either cat L or cat K may be involved in chemerin processing in this organ. Alternatively, significant expression and/or activity of cat K and cat L in the joints of patients with rheumatoid arthritis and skin dermatoses, respectively (26
), together with reported chemerin immunoreactivity and/or bioactivity in psoriasis skin and inflamed synovial fluid (1
) suggests that these cathepsins may play a role in chemerin cleavage in joints and skin. Since cathepsin-mediated processing releases chemerin attractant activity, these enzymes may have an important regulatory role in immune cell migration. Notably, the presence of pDC in lung as well as the inflamed joints and psoriatic skin (1
) supports the notion that cat K and cat L, through the generation of active chemerin, may contribute to pDC recruitment to these sites.
Our data also uncover a novel role for chemerin as a host-expressed antibacterial agent in host defense. Despite low primary sequence homology between chemerin and antibacterial cathelicidins, the conserved positioning of key cysteine residues leads to a predicted shared similar tertiary structure, although recent NMR assignment of human chemerin does not exclude a different fold (30
). LL-37, the 37-aa C-terminal derivative of human cathelicidin hCAP18 is well known for its potent and broad-spectrum bacterial killing activity. However, chemerin is structurally similar to the cathelin-like N-terminal region. Interestingly, the cathelin-like domain of hCAP18 has been reported to possess antimicrobial activity, although the mechanism by which it inhibits bacterial growth is not known (20
Chemerin may exert antimicrobial activity on the surface of skin and/or lung where it is locally expressed (1
). For example, the respiratory surface is continually exposed to pathogenic organisms, such as K. pneumoniae,
which, as shown in this report, might be a direct chemerin target. Although either pro-chemerin or the C-term truncated chemerin forms displayed antibacterial activity, C-terminal processing augmented the inhibitory effect of chemerin on the growth of Enterobacteriaceae
. However, our data suggest that pro-chemerin is also processed by bacterial proteases during incubation, although the protease(s) responsible remain to be identified. It will be interesting to map the specific chemerin domains/regions responsible for its anti-microbial activity. Our preliminary data suggest that most of the anti-bacterial activity is associated with the chemerin region(s) located within 65-115aa (data not shown). This is consistent with our data showing that Fc-chemS157 and Fc-chemR125 have similar antibacterial activity, although the inhibitory C-term peptide must be removed for full antibacterial effects.
Although the antimicrobial effects of chemerin on E. coli
and K. pneumoniae
were less potent compared with the classical antibacterial peptide LL-37, chemerin showed bactericidal properties at much lower concentrations. In general, pore-forming antimicrobial peptides, such as LL-37, require micromolar concentrations for activity. However, some antibacterial peptides, such as Lactocococcus
-derived nisin operate in the nanomolar range (31
). This ability is attributed to docking to a specific component on the bacteria cell wall for subsequent pore formation, or to the dual killing mechanisms of the peptide, which in addition inhibits bacterial cell wall biosynthesis (32
). Chemerin might employ a similar strategy to exert its antimicrobial activity in the nanomolar range. However, since antimicrobial properties can be sensitive to pH and ionic composition of the peptide environment (31
), it will be important to determine whether chemerin operates in conditions similar to those found in the skin and/or bronchial tree.
Thus, our present work uncovers a novel antibacterial property of chemerin and characterizes the activation of chemerin by host-derived cysteine proteases of the cathepsin family, and adds a new dimension to the ways chemerin may modulate and augment immunity.