Apoptosis-Inducing Galactolipids from a Cultured Marine Diatom, Phaeodactylum tricornutum

Abstract

Two monogalactosyl diacylglycerols, 1 and 2, were isolated from the marine diatom Phaeodactylum tricornutum, using the patented ApopScreen cell-based screen for apoptosis-inducing, potential anticancer compounds. The molecular structures of the galactolipids were determined using a combination of NMR, mass spectrometry, and chemical degradation. The bioactivities were confirmed using a specific apoptosis induction assay based on genetically engineered mammalian cell lines with differential, defined capacities for apoptosis. The galactolipids induce apoptosis in micromolar concentrations. This is the first report of apoptosis induction by galactolipids.The oceans are the largest ecosystem of Earth and hold great, unexplored potential for drug discovery. One of the most important photosynthetic eukaryotes in marine ecosystems are the diatoms, 1 single-celled algae surrounded by a silica-derived wall. There are estimated to bẽ 10 6 species of diatoms, 2,3 which makes them ~4-fold more diverse than angiosperms. These organisms rose to ecological prominence over the past 40 million years, and in part, their success has been attributed not only to efficient resource acquisition strategies but also to allelochemical production that may favor their selection over competitors. Hence, we examined whether this group of organisms produces novel compounds capable of inducing apoptosis.The screening of organic extracts from marine algae and cyanobacteria for mechanism-based anticancer agents has become increasingly efficient and has led to the discovery of new chemotypes showing antiproliferative properties. 4,5 Cytotoxic chemo-therapeutics currently in use for the treatment of cancer rely on the ability to selectively target proliferating cells, which are enriched in tumors. Tumor cells progressively evolve genetic mutations that enable not only cell proliferation but also resistance to apoptosis, a cell suicide pathway that is the cellular response to oncogene activation or irreparable cellular damage. 6-9 Effective cancer therapeutic strategies often rely on preferential and efficient induction of apoptosis in tumor cells. Progressive exposure to such molecules commonly leads to selection of resistant cells that are therapeutically associated with both tumor progression and resistance to chemotherapy. [6][7][8][9] While many conventional cytotoxic chemotherapeutics trigger apoptosis indirectly by inflicting cellular damage, recent efforts have been directed to developing agents that specifically target or activate a caspase cascade that leads to apoptosis. 10In our ongoing effort to discover and develop new marine natural product biomedicinals, we have screened extracts from several marine organisms and have found that those isolated from a cultured marine diatom, Phaeodactylum tricornutum, possessed an ability to specifically induce apoptosis in immortal mammalian epithelial cells. Subsequently, extracts were subjected to ApopScreen bioas-say-guided purification, which resulted in the isolation of two monogalactos...

In vitro studies of anti-inflammatory and anticancer activities of organic solvent extracts from cultured marine microalgae

“…To our knowledge, few of reports have been published with the extracts of bioactive metabolites from the cultured marine microalgae. Recently, two monogalactosyl diacylglycerols were isolated from the cultured marine P. tricornutum and reported as inducing apoptosis in two genetically-matched immortal mouse epithelial cell lines (Andrianasolo et al 2008). Moreover, these results indicate the potential of cultured marine microalgae species for anticancer activities and should lead to study further on secondary metabolites.…”

Antibacterial constituents from Leucosceptrum canum

“…The E-configuration of C2 0 -C3 0 olefinic bond was assigned on the basis of NOESY spectrum, where cross-signals between H-1 0 (d 4.61) and H-19 0 (d 1.71) as well as between H-2 0 (d 5.36) and H-4 0 (d 2.04) were observed. The double bond configuration was supported by the d value (33.5) of the allylic carbon, as explained for compound 1 (Ishii et al, 2006;Andrianasolo et al, 2008). The absolute configuration of asymmetric centers in compound 2 could not assign due to same reason as mentioned for compound 1.…”

“…The Econfiguration of the C-2/C-3 olefinic bond was assigned on the basis of the NOESY spectrum, where the correlations between H-2 and H-4 as well as between H-20 and H-1 were observed. This was supported by the shift value d 33.8 of the allylic carbon, since allylic carbon signals of similar E-and Z-isomers were observed at d 31-33 and 26-28 respectively (Ishii et al, 2006;Andrianasolo et al, 2008). The absolute configuration of C-7 and C-11 asymmetric center could not be assigned due to the lack of appropriate chemical methods and literatures.…”

“…The Zconfiguration of C-2/C-3 olefinic bond was assigned on the basis of NOESY spectrum and analysis of 13 C and 1 H NMR spectrum. Moreover, the cis-geometry (Z) of the double bond was determined from the d value (26.9) of the allylic carbons as discussed for compound 1 (Ishii et al, 2006;Andrianasolo et al, 2008). Like that of compounds 1 and 2, the absolute configuration of compound 3 could not be assigned.…”

“…The Z-configurations of all olefins in compound 4 was also assigned with the similar justification given for compound 3 that all allylic carbon signals of Z-isomer observed at d ca. 27 (Ishii et al, 2006;Andrianasolo et al, 2008). Based upon this information, compound 4 was assigned (6Z,9Z,12Z,15Z,18Z,21Z,24Z)-4,27-dibutyl-6,9,12,15,15,18,21,24-triacontaheptaenedioc acid, trivially named as leucoenoic acid.…”