Thalicthuberine: A Novel Mitotic Inhibitor in Prostate Cancer Cells

Bioassay-guided fractionation of a cytotoxic extract derived from the Australian endemic plant Hernandia albiflora (twigs) resulted in the isolation of the two known compounds: deoxypodophyllotoxin and thalicthuberine. Both compounds were tested for their inhibition of LNCaP cell viability. Thalicthuberine (TH, IC50=2.0 µM) was selected for mechanism of action studies. Ongoing investigations have shown that TH induced a strong accumulation of LNCaP cells in mitosis, severe mitotic spindle defects and asymmetric cell divisions, ultimately leading to mitotic catastrophe accompanied by cell death through apoptosis. However, unlike microtubule-targeting drugs (vinblastine, paclitaxel and nocodazole), TH did not affect tubulin polymerization in a cell-free system, while it reduced cellular microtubule polymer mass, suggesting that TH indirectly targets microtubule dynamics through inhibition of critical regulators or tubulin-associated proteins. Based on phenotypic comparisons, we excluded the mitotic regulators Aurora kinase A and Polo-like kinase 1 as potential targets of TH. TH did not directly interact with DNA (cell-free assay) or induce DNA damage in cells. Further experiments will attempt to address the molecular target(s) and inhibition mechanism of TH. This is the first report of a mechanism of action study for this natural product. Based on these results, TH has great promise for further development into a potent non-tubulin-targeting mitotic inhibitor. The dataset was derived from three biological repeats of each of the treatment groups DMSO, vinblastine and TH.