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Rhizochalin [(2R,3R,26R,27R)-2,27-diamino-3-hydroxy-26-[(2R,3R,4S,6R)-3,4,5-trihydroxy-6-(hydroxymethyl)tetrahydro-2H-pyran-2-yloxy]octacosan-11-one], an antimicrobial and cytotoxic marine two-headed sphingolipid-like natural product, isolated from the sponge Rhizochalina incrustata, and some related compounds were studied as anticarcinogenic and proapoptotic agents. The corresponding effects were tested on the mouse skin epidermal JB6 P(+) Cl 41 cell line, its stable transfectants, THP-1, HeLa, and SNU-C4 human tumor cells using a variety of assessments, including cell viability (MTS), flow cytometry, anchorage-independent soft agar, and luciferase assays. At 5-10 muM concentrations, rhizochalin was effective as an inhibitor of the malignant transformation of JB6 P(+) Cl 41 cells or colony formation of human tumor cells, which exerted its action, at least in part, through the induction of p53-dependent apoptosis. Structure-activity relationship study showed aglycon of rhizochalin to be the most active while peracetylated aglycon was the least active among the compounds studied.

A new fatty acid, (5Z,9Z)-22-methyl-5,9-tetracosadienoic acid (1a), and a rare fatty acid, (5Z,9Z)-23-methyl-5,9-tetracosadienoic acid (2a), the predominant constituents of the free fatty acid fraction from the lipids of the sponge Geodinella robusta, were isolated and partly separated by reversed phase high-performance liquid chromatography, followed by multifold crystallization from MeOH to give 1a and 2a in 70% and 60% purity, respectively. These fatty acids were identified as (5Z,9Z)-22- and (5Z,9Z)-23-methyl-5,9-tetracosadienoic acids by nuclear magnetic resonance techniques, including distortionless enhancement by polarization transfer, heteronuclear multiple quantum connectivity, and correlation spectroscopy experiments, as well as from mass-spectrometric data for their methyl esters, the methyl esters of their perhydro derivatives, and their pyrrolidides. Mixtures of 1a and 2a showed cytotoxic activity against mouse Ehrlich carcinoma cells and a hemolytic effect on mouse erythrocytes. The sterol fraction from the same sponge was analyzed by gas-liquid chromatography-mass spectrometry, and 24-methylenecholesterol was identified as a main constituent of this fraction. The implications of the co-occurrence of membranolytic long-chain fatty acids and 24-methylenecholesterol as a main membrane sterol are discussed in terms of the phenomenon of biochemical coordination.

Rhizochalinin (Rhiz) is a novel marine natural sphingolipid-like compound, which shows promising in vitro and in vivo activity in human castration-resistant prostate cancer. In the present study, a global proteome screening approach was applied to investigate molecular targets and biological processes affected by Rhiz in castration-resistant prostate cancer. Bioinformatical analysis of the data predicted an antimigratory effect of Rhiz on cancer cells. Validation of proteins involved in the cancer-associated processes, including cell migration and invasion, revealed downregulation of specific isoforms of stathmin and LASP1, as well as upregulation of Grp75, keratin 81, and precursor IL-1β by Rhiz. Functional analyses confirmed an antimigratory effect of Rhiz in PC-3 cells. Additionally, predicted ERK1/2 activation was confirmed by Western blotting analysis, and revealed prosurvival effects in Rhiz-treated prostate cancer cells indicating a potential mechanism of resistance. A combination of Rhiz with MEK/ERK inhibitors PD98059 (non-ATP competitive MEK1 inhibitor) and FR180204 (ATP-competitive ERK1/2 inhibitor) resulted in synergistic effects. This work provides further insights into the molecular mechanisms underlying Rhiz bioactivity. Furthermore, our research is exemplary for the ability of proteomics to predict drug targets and mode of action of natural anticancer agents.

Pibocin 1 (1), the first representative of marine ergoline alkaloids, was isolated from the Far-Eastern ascidian Eudistoma sp. Its structure and absolute stereochemistry have been established as (8β)-2-bromo-6,8-dimethylergoline on the basis of spectroscopic and X-Ray analysis data. Pibocin showed moderate cytotoxic activity against mouse Ehrlich carcinoma cells.