Daniel Poeckel

Boswellic acids: biological actions and molecular targets.

Gum resin extracts of Boswellia species have been traditionally applied in folk medicine for centuries to treat various chronic inflammatory diseases, and experimental data from animal models and studies with human subjects confirmed the potential of B. spec extracts for the treatment of not only inflammation but also of cancer. Analysis of the ingredients of these extracts revealed that the pentacyclic triterpenes boswellic acids (BAs) possess biological activities and appear to be responsible for the respective pharmacological actions. Approaches in order to elucidate the molecular mechanisms underlying the biological effects of BAs identified 5-lipoxygenase, human leukocyte elastase, toposiomerase I and II, as well as IkappaB kinases as molecular targets of BAs. Moreover, it was shown that depending on the cell type and the structure of the BAs, the compounds differentially interfere with signal transduction pathways including Ca(2+/-) and MAPK signaling in various blood cells, related to functional cellular processes important for inflammatory reactions and tumor growth. This review summarizes the biological actions of BAs on the cellular and molecular level and attempts to put the data into perspective of the beneficial effects manifested in animal studies and trials with human subjects related to inflammation and cancer.

Carnosic acid and carnosol potently inhibit human 5-lipoxygenase and suppress pro-inflammatory responses of stimulated human polymorphonuclear leukocytes.

Carnosic acid (CA) and carnosol (CS) are phenolic diterpenes present in several labiate herbs like Rosmarinus officinalis (Rosemary) and Salvia officinalis (Sage). Extracts of these plants exhibit anti-inflammatory properties, but the underlying mechanisms are largely undefined. Recently, we found that CA and CS activate the peroxisome proliferator-activated receptor gamma, implying an anti-inflammatory potential on the level of gene regulation. Here we address short-term effects of CA and CS on typical functions of human polymorphonuclear leukocytes (PMNL). We found that (I), CA and CS inhibit the formation of pro-inflammatory leukotrienes in intact PMNL (IC(50)=15-20 microM [CA] and 7 microM [CS], respectively) as well as purified recombinant 5-lipoxygenase (EC number 1.13.11.34, IC(50)=1 microM [CA] and 0.1 microM [CS], respectively), (II) both CA and CS potently antagonise intracellular Ca(2+) mobilisation induced by a chemotactic stimulus, and (III) CA and CS attenuate formation of reactive oxygen species and the secretion of human leukocyte elastase (EC number 3.4.21.37). Together, our findings provide a pharmacological basis for the anti-inflammatory properties reported for CS- and CA-containing extracts.

Identification of human cathepsin G as a functional target of boswellic acids from the anti-inflammatory remedy frankincense.

Frankincense preparations, used in folk medicine to cure inflammatory diseases, showed anti-inflammatory effectiveness in animal models and clinical trials. Boswellic acids (BAs) constitute major pharmacological principles of frankincense, but their targets and the underlying molecular modes of action are still unclear. Using a BA-affinity Sepharose matrix, a 26-kDa protein was selectively precipitated from human neutrophils and identified as the lysosomal protease cathepsin G (catG) by mass spectrometry (MALDI-TOF) and by immunological analysis. In rigid automated molecular docking experiments BAs tightly bound to the active center of catG, occupying the same part of the binding site as the synthetic catG inhibitor JNJ-10311795 (2-[3-{methyl[1-(2-naphthoyl)piperidin-4-yl]amino}carbonyl)-2-naphthyl]-1-(1-naphthyl)-2-oxoethylphosphonic acid). BAs potently suppressed the proteolytic activity of catG (IC50 of ∼600 nM) in a competitive and reversible manner. Related serine proteases were significantly less sensitive against BAs (leukocyte elastase, chymotrypsin, proteinase-3) or not affected (tryptase, chymase). BAs inhibited chemoinvasion but not chemotaxis of challenged neutrophils, and they suppressed Ca2+ mobilization in human platelets induced by isolated catG or by catG released from activated neutrophils. Finally, oral administration of defined frankincense extracts significantly reduced catG activities in human blood ex vivo vs placebo. In conclusion, we show that catG is a functional and pharmacologically relevant target of BAs, and interference with catG could explain some of the anti-inflammatory properties of frankincense.

Boswellic Acids Stimulate Arachidonic Acid Release and 12-Lipoxygenase Activity in Human Platelets Independent of Ca2+ and Differentially Interact with Platelet-Type 12-Lipoxygenase

Boswellic acids inhibit the transformation of arachidonic acid to leukotrienes via 5-lipoxygenase but can also enhance the liberation of arachidonic acid in human leukocytes and platelets. Using human platelets, we explored the molecular mechanisms underlying the boswellic acid-induced release of arachidonic acid and the subsequent metabolism by platelet-type 12-li-poxygenase (p12-LO). Both β-boswellic acid and 3-O-acetyl-11-keto-boswellic acid (AKBA) markedly enhanced the release of arachidonic acid via cytosolic phospholipase A2 (cPLA2), whereas for generation of 12-hydro(pero)xyeicosatetraenoic acid [12-H(P)ETE], AKBA was less potent than β-boswellic acid and was without effect at higher concentrations (≥30 μM). In contrast to thrombin, β-boswellic acid-induced release of ara-chidonic acid and formation of 12-H(P)ETE was more rapid and occurred in the absence of Ca2+. The Ca2+-independent release of arachidonic acid and 12-H(P)ETE production elicited by β-boswellic acid was not affected by pharmacological inhibitors of signaling molecules relevant for agonist-induced arachidonic acid liberation and metabolism. It is noteworthy that in cell-free assays, β-boswellic acid increased p12-LO catalysis approximately 2-fold in the absence but not in the presence of Ca2+, whereas AKBA inhibited p12-LO activity. No direct modulatory effects of boswellic acids on cPLA2 activity in cell-free assays were evident. Therefore, immobilized KBA (linked to Sepharose beads) selectively precipitated p12-LO from platelet lysates but failed to bind cPLA2. Taken together, we show that boswellic acids induce the release of arachidonic acid and the synthesis of 12-H(P)ETE in human platelets by unique Ca2+-independent routes, and we identified p12-LO as a selective molecular target of boswellic acids.

Discovery of a Highly Selective Tankyrase Inhibitor Displaying Growth Inhibition Effects against a Diverse Range of Tumor Derived Cell Lines

The availability of high quality probes for specific protein targets is fundamental to the investigation of their function and their validation as therapeutic targets. We report the utilization of a dedicated chemoproteomic assay platform combining affinity enrichment technology with high-resolution protein mass spectrometry to the discovery of a novel nicotinamide isoster, the tetrazoloquinoxaline 41, a highly potent and selective tankyrase inhibitor. We also describe the use of 41 to investigate the biology of tankyrase, revealing the compound induced growth inhibition of a number of tumor derived cell lines, demonstrating the potential of tankyrase inhibitors in oncology.

Defined Structure-Activity Relationships of Boswellic Acids Determine Modulation of Ca2+ Mobilization and Aggregation of Human Platelets by Boswellia serrata Extracts

Boswellic acids constitute a group of unique pentacyclic triterpene acids from Boswellia serrata with multiple pharmacological activities that confer them anti-inflammatory and anti-tumoral properties. A subgroup of boswellic acids, characterized by an 11-keto group, elevates intracellular Ca2+ concentrations [Ca2+]i and causes moderate aggregation of human platelets. How different BAs and their mixtures in pharmacological preparations affect these parameters in activated platelets has not been addressed, so far. Here, we show that boswellic acids either antagonize or induce Ca2+ mobilization and platelet aggregation depending on defined structural determinants with inductive effects predominating for a B. serrata gum resin extract. 3-O-Acetyl-11-keto-β-boswellic acid potently suppressed Ca2+ mobilization (IC50 = 6 µM) and aggregation (IC50 = 1 µM) when platelets were activated by collagen or the thromboxane A2 receptor agonist U-46619, but not upon thrombin. In contrast, β-boswellic acid and 3-O-acetyl-β-boswellic acid, which lack the 11-keto moiety, were weak inhibitors of agonist-induced platelet responses, but instead they elicited elevation of [Ca2+]i and aggregation of platelets (≥ 3 µM). 11-Keto-β-boswellic acid, the structural intermediate between 3-O-acetyl-11-keto-β-boswellic acid and β-boswellic acid, was essentially inactive independent of the experimental conditions. Together, our study unravels the complex agonizing and antagonizing properties of boswellic acids on human platelets in pharmacologically relevant preparations of B. serrata gum extracts and prompts for careful evaluation of the safety of such extracts as herbal medicine in cardiovascular risk patients.