Boswellic acids target the human immune system-modulating antimicrobial peptide LL-37

Abstract

The antimicrobial peptide LL-37 is the sole member of the human cathelicidin family with immune system-modulating properties and roles in autoimmune disease development. Small molecules able to interact with LL-37 and to modulate its functions have not been described yet. Boswellic acids (BAs) are pentacyclic triterpene acids that are bioactive principles of frankincense extracts used as anti-inflammatory remedies. Although various anti-inflammatory modes of action have been proposed for BAs, the pharmacological profile of these compounds is still incompletely understood. Here, we describe the identification of human LL-37 as functional target of BAs. In unbiased target fishing experiments using immobilized BAs as bait and human neutrophils as target source, LL-37 was identified as binding partner assisted by MALDI-TOF mass spectrometry. Thermal stability experiments using circular dichroism spectroscopy confirm direct interaction between BAs and LL-37. Of interest, this binding of BAs resulted in an inhibition of the functionality of LL-37. Thus, the LPS-neutralizing properties of isolated LL-37 were inhibited by 3-O-acetyl-β-BA (Aβ-BA) and 3-O-acetyl-11-keto-β-BA (AKβ-BA) in a cell-free limulus amoebocyte lysate assay with EC50=0.2 and 0.8 μM, respectively. Also, LL-37 activity was inhibited by these BAs in LL-37-enriched supernatants of stimulated neutrophils or human plasma derived from stimulated human whole blood. Together, we reveal BAs as inhibitors of LL-37, which might be a relevant mechanism underlying the anti-inflammatory properties of BAs and suggests BAs as suitable chemical tools or potential agents for intervention with LL-37 and related disorders.

Keywords: 11-Keto-β-boswellic acid (PubChem CID:9847548); 3-O-Acetyl-11-keto-β-boswellic acid (PubChem CID: 71,463,896); 3-O-Acetyl-β-boswellic acid (PubChem CID: 11,386,458); Boswellic acid; Cathelicidin; Cytochalasin B (PubChem CID:5311281); Frankincense; Inflammation; LL-37; LL-37 (PubChem CID: 16,198,951); Lipopolysaccharide; Lipopolysaccharide (PubChem CID: 11,970,143); N-formyl-methionyl-leucyl-phenylalanin (PubChem CID: 4364); β-Boswellic acid (PubChem CID: 168,928).