PURPOSE:During our search for new microtubule effectors as anticancer agents, we have found that a small synthetic molecule designated GS-164 interferes with the assembly of porcine microtubule proteins and has cytotoxic activity against a wide range of human tumor cell lines. In this study, we investigated mode of action of the compound in comparison with Taxol and colcemid.
METHODS:To gain an insight into the mode of action of GS-164, we used an in vitro microtubule polymerization assay and a flow-cytometric measurement technique. Microtubule organization and the level of tubulin polymerization in HeLa cells were also examined by immunofluorescence microscopy and cytoskeletal protein analyses, respectively.
RESULTS:GS-164 stimulated assembly of microtubule proteins in vitro in a concentration-dependent and a GTP-independent manner. Furthermore, as with Taxol, the microtubule polymerization induced by GS-164 was antagonized by podophyllotoxin, a tubulin polymerization inhibitor, and microtubules formed by GS-164 were resistant to disassembly by calcium or low temperatures. GS-164 in the micromolar range arrested the cell cycle of HeLa cells in the mitotic phase leading to cell death. GS-164 also increased the amounts of cellular microtubules in HeLa cells, resulting in the formation of microtubule bundles.
CONCLUSION:These results indicate that GS-164 stimulates microtubule assembly by a similar mechanism to that of Taxol. A comparative conformational analysis of GS-164 and Taxol suggested that the structure of the former mimics the minimum essential sites of Taxol required to exert the Taxol-like activities described above. Although the cytotoxicity of GS-164 against human tumor cells was 1000-fold lower than that of Taxol and GS-164 was one-tenth as active as Taxol in vitro, these findings pave the way for synthesizing clinically useful anticancer agents using GS-164 as a lead compound.
