Briaviolides K-N, New Briarane-Type Diterpenoids from Cultured Octocoral Briareum violaceum

Abstract

Four new briarane diterpenoids, briaviolides K-N (1-4), have been obtained from the cultured-type octocoral Briareum violaceum. Using a spectroscopic approach, the structures of briaranes 1-4 were identified. This study employed an in vitro model of lipopolysaccharide (LPS)-induced inflammation in the murine macrophage RAW 264.7 cell line, and found that among the four briaranes, briarane 2 possessed anti-inflammatory activity against inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) protein expressions in cells. In addition, principal component analysis using the chemical global positioning system (ChemGPS) for natural products (ChemGPS-NP) was employed in order to analyze the structure-activity relationship (SAR), and the results indicated that the ring conformation of the compound has a leading role in suppressing the expressions of pro-inflammatory iNOS and COX-2 proteins in macrophages.

Keywords: Briareum violaceum; COX-2; ChemGPS-NP; briarane; briaviolide; iNOS.

Figure 1

Cultured octocoral B. violaceum and structures of briaviolides K–N (1–4), excavatoid F (5), and (1S*,2S*,5Z,7S*,8S*,9S*,10S*,11R*,12R*,13Z,17R*)-2,12-diacetoxy-8,17-epoxy-9-hydroxybriara-5, 13-dien-18-one (6). The 1′2′3′4′ means the serial numbers of the carbon atom of the n-butyrate moiety.

Figure 2

Model of 1 generated by computer-aided analyses based on MM2 force field calculations and selected protons with key NOESY correlations.

Figure 3

Model of 2 generated by computer-aided analyses based on MM2 force field calculations and selected protons with key NOESY correlations.

Figure 4

Model of 3 generated by computer-aided analyses based on MM2 force field calculations and selected protons with key NOESY correlations.

Figure 5

Effects of compounds 1–4 on the expression of pro-inflammatory iNOS and COX-2 proteins in murine RAW264.7 macrophage cell. Using immunoblot analysis, briarane 2 was demonstrated to reduce lipopolysaccharide (LPS)-induced expressions of these two pro- inflammatory proteins. Data were normalized to those of cells treated with LPS alone, and cells treated with dexamethasone (10 μM) were used as a positive control (which has been shown to reduce the levels of iNOS and COX-2 to 36.52% and 6.18%, respectively). Data are expressed as the mean ± SEM (n = 3). * Significantly different from cells treated with LPS (p < 0.05).

Figure 6

ChemGPS-NP-based analysis of the chemical space plotted for a series of briarane-type diterpenoids reported by our lab (red and yellow, which were active in inhibition assays of iNOS and COX-2), as well as previously studied 342 iNOS (light blue) and 2,592 COX-2 (light green) inhibitors sorted from the ChEMBL database. In the score plot of the three dimensions (principle component), PC1 (red) describes the size, shape, and polarizability; PC2 (orange) represents the aromatic- and conjugation-related properties; and PC3 (green) depicts lipophilicity, polarity, and H-bond capacity.