Mechanism of Action of Coumarin Natural Products that Inhibit Cell Proliferation and Potentiate the Anticancer Drug Taxol
Investigations of ancient botanical medicinal formulations have proven to provide a rich source of drugs with therapeutic merit. An analysis of medicinal plant formulations used by traditional healers of ancient Persia to induce abortions is reported here. By investigating the medical and pharmacological literature on the medicinal properties of the plant species used, I found that many of the plant species demonstrate antineoplastic or cytotoxic activities and are rich in coumarin compounds. In studies on the anti-proliferative actions of coumarin compounds, dicoumarol (a coumarin anticoagulant) inhibited the first cleavage of sea urchin embryos in a concentration dependent manner with 50% inhibition occurring at a concentration of 10 µM. Because first cleavage in sea urchin embryos is highly selective for microtubule-targeted agents, I thought the active compounds might inhibit cell division by interacting with tubulin or microtubules. I found that dicoumarol binds to bovine brain tubulin with a Kd of 22 µM, and that 0.1 µM dicoumarol strongly stabilizes the growing and shortening dynamics at the plus ends of the microtubules in vitro. Dicoumarol reduced the rate and extent of shortening, it increased the percentage of time the microtubules spent in an attenuated (paused) state, and it reduced the overall dynamicity of the microtubules. The anti-mitotic effects of the widely used cancer chemotherapeutic agent taxol (paclitaxel) are also mediated by suppressing microtubule dynamics. Indirect immunofluorescence microscopic imaging of microtubules and 4,6-diamidino-2-phenylindole imaging of in cell chromatin characterized the effects of the drugs on the organization of microtubules and chromosomes in metaphase spindles. Dicoumarol produced abnormal organization of mitotic spindle microtubules with chromosomes that had not congressed to the metaphase plate. In addition, exposure to combinations of taxol and dicoumarol potentiated the inhibition of cell division of sea urchin embryos. The results suggest that the anti-proliferative mechanism of action of dicoumarol and possibly related pharmacophores are mediated by tubulin binding and the stabilization of spindle microtubule dynamics. Because of its low toxicity and simple chemical structure, there is potential interest to explore combinations of anti-mitotic coumarins with other chemotherapeutic agents to improve efficacy and lower toxicity.