Justicidin B – a potent cytotoxic arylnaphtalene lignan from in vitro cultures of Linum leonii

The plants are one of the attractive sources of novel antitumor compounds. Isolation of pharmaceuticals from plants is difficult due to their extremely low concentrations. To extend the research to human clinical studies, we needed to find a reliable supply of plant material, produced target compounds. As part of our ongoing program on the investigation of Linum species, cell cultures of L. leonii F.W.Schultz, were examined. We have established several callus and suspension cultures and checked for the occurrence of lignans.

The main component in cell cultures of L. leonii was isolated and analyzed by means of GC-MS and NMR. The EI-MS of the isolated compound showed an ion at m/z 364 and mass fragmentation, which is consistent with the data for an arylnaphthalene lignan. the 1H NMR spectrum showed that the isolated compound is justicidin B.

Justicidin B produced by in vitro cultures of Linum leonii was tested for cytotoxic activity and induction of apoptosis in MDA-MB-231 and MCF-7 breast cancer derived cell lines. The tested lignan evoked strong, concentration-dependent cytotoxicity in both cell lines, whereby MCF-7 proved to be more sensitive. The 24h treatment of both cell lines increased the level of apoptotic DNA fragmentation; however the proapoptotic activity is completely inhibited if the cells are co-incubated with the non-selective pan-caspase inhibitor Boc-Asp(OMe)-fluoromethyl ketone (PCI), which implies that justicidin B, activates PCD via caspase-dependent mechanisms. Exposure of MDA-MB-231 cells with justicidin B leads to concentration-dependent decrease in the NFkB expression; strong NFkB expression is observed in MCF-7 cells.

Acknowledgement: Financial support from Ministry of Education and Science, Sofia, Bulgaria (grant D002–128/08 I. Ionkova) is acknowledged.