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BACKGROUND AND PURPOSEFrankincense, the gum resin derived from Boswellia species, showed anti-inflammatory efficacy in animal models and in pilot clinical studies. Boswellic acids (BAs) are assumed to be responsible for these effects but their anti-inflammatory efficacy in vivo and their molecular modes of action are incompletely understood. EXPERIMENTAL APPROACHA protein fishing approach using immobilized BA and surface plasmon resonance (SPR) spectroscopy were used to reveal microsomal prostaglandin E2 synthase-1 (mPGES1) as a BA-interacting protein. Cell-free and cell-based assays were applied to confirm the functional interference of BAs with mPGES1. Carrageenan-induced mouse paw oedema and rat pleurisy models were utilized to demonstrate the efficacy of defined BAs in vivo. KEY RESULTSHuman mPGES1 from A549 cells or in vitro-translated human enzyme selectively bound to BA affinity matrices and SPR spectroscopy confirmed these interactions. BAs reversibly suppressed the transformation of prostaglandin (PG)H2 to PGE2 mediated by mPGES1 (IC50 = 3-10 mM). Also, in intact A549 cells, BAs selectively inhibited PGE2 generation and, in human whole blood, b-BA reduced lipopolysaccharide-induced PGE2 biosynthesis without affecting formation of the COX-derived metabolites 6-keto PGF1a and thromboxane B2. Intraperitoneal or oral administration of b-BA (1 mg·kg -1 ) suppressed rat pleurisy, accompanied by impaired levels of PGE2 and b-BA (1 mg·kg -1 , given i.p.) also reduced mouse paw oedema, both induced by carrageenan. CONCLUSIONS AND IMPLICATIONSSuppression of PGE2 formation by BAs via interference with mPGES1 contribute to the anti-inflammatory effectiveness of BAs and of frankincense, and may constitute a biochemical basis for their anti-inflammatory properties.

The microsomal prostaglandin E2 synthase (mPGES)-1 is the terminal enzyme in the biosynthesis of prostaglandin (PG)E2 from cyclooxygenase (COX)-derived PGH2. We previously found that mPGES-1 is inhibited by boswellic acids (IC50 = 3–30 μM), which are bioactive triterpene acids present in the anti-inflammatory remedy frankincense. Here we show that besides boswellic acids, additional known triterpene acids (i.e., tircuallic, lupeolic, and roburic acids) isolated from frankincense suppress mPGES-1 with increased potencies. In particular, 3α-acetoxy-8,24-dienetirucallic acid (6) and 3α-acetoxy-7,24-dienetirucallic acid (10) inhibited mPGES-1 activity in a cell-free assay with IC50 = 0.4 μM, each. Structure–activity relationship studies and docking simulations revealed concrete structure-related interactions with mPGES-1 and its cosubstrate glutathione. COX-1 and -2 were hardly affected by the triterpene acids (IC50 > 10 μM). Given the crucial role of mPGES-1 in inflammation and the abundance of highly active triterpene acids in frankincence extracts, our findings provide further evidence of the anti-inflammatory potential of frankincense preparations and reveal novel, potent bioactivities of tirucallic acids, roburic acids, and lupeolic acids.

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Age-related diseases, such as osteoarthritis, Alzheimer’s disease, diabetes, and cardiovascular disease, are often associated with chronic unresolved inflammation. Neutrophils play central roles in this process by releasing tissue-degenerative proteases, such as cathepsin G, as well as pro-inflammatory leukotrienes produced by the 5-lipoxygenase (5-LO) pathway. Boswellic acids (BAs) are pentacyclic triterpene acids contained in the gum resin of the anti-inflammatory remedy frankincense that target cathepsin G and 5-LO in neutrophils, and might thus represent suitable leads for intervention with age-associated diseases that have a chronic inflammatory component. Here, we investigated whether, in addition to BAs, other triterpene acids from frankincense interfere with 5-LO and cathepsin G. We provide a comprehensive analysis of 17 natural tetra- or pentacyclic triterpene acids for suppression of 5-LO product synthesis in human neutrophils. These triterpene acids were also investigated for their direct interference with 5-LO and cathepsin G in cell-free assays. Furthermore, our studies were expanded to 10 semi-synthetic BA derivatives. Our data reveal that besides BAs, several tetra- and pentacyclic triterpene acids are effective or even superior inhibitors of 5-LO product formation in human neutrophils, and in parallel, inhibit cathepsin G. Their beneficial target profile may qualify triterpene acids as anti-inflammatory natural products and pharmacological leads for intervention with diseases related to aging.

Boswellic acids (BAs) possess anti-inflammatory properties in various biological models with similar features to those of glucocorticoids (GCs), such as suppression of the release of pro-inflammatory cytokines. Hence, the molecular mechanism of BAs responsible for their anti-inflammatory features might be attributable to interference with the human glucocorticoid receptor (GR). Due to obvious structural similarities with GCs, we conducted pharmacophore studies as well as molecular docking simulations of BAs as putative ligands at the ligand binding site (LBS) of the GR in distinct functional states. In order to verify receptor binding and functional activation of the GR by BAs, radiometric binding assays as well as GR response element-dependent luciferase reporter assay were performed with dexamethasone (DEX) as a functional positive control. With respect to the observed position of GCs in GR crystal complexes in the active antagonist state, BAs docked in a flipped orientation with estimated binding constants reflecting nanomolar affinities. For validation, DEX and other steroids were successfully redocked into their crystal poses in similar ranges as reported in the literature. In line with the pharmacophore and docking models, the BAs were strong GR binders (radiometric binding assay), albeit none of the BAs activated the GR in the reporter gene assay, when compared to the GC agonist DEX. The flipped scaffolds of all BAs dislodge the known C-11 function from its receiving amino acid (Asn564), which may explain the silencing effects of receptor-bound BAs in the reporter gene assay. Together, our results constitute a compelling example of rigid keys acting in an adaptable lock qualifying as a reversed induced fit mechanism, thereby extending the hitherto published knowledge about molecular target interactions of BAs.

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