Intracellular activation and cytotoxic action of RS-1541 against cultured human tumor cells |
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Authors: | Toshiro Takatori Takeshi Koizumi Taro Tokui Yoshihiro Mitsuhashi Akio Shiraishi Takashi Tsuruo |
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Institution: | (1) Biological Research Laboratories, Sankyo Co., Ltd., Shinagawa-ku, 140 Tokyo, Japan;(2) Analytical and Metabolic Research Laboratories, Sankyo Co., Ltd., Shinagawa-ku, 140 Tokyo, Japan;(3) Institute of Molecular and Cellular Bioscience, University of Tokyo, Yayoi, Bunkyo-ku, 113 Tokyo, Japan |
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Abstract: | RS-1541, an acyl-derivative of rhizoxin (Fig. 1), is a potent antitumor compound. This agent showed cytotoxicity in vitro on some cultured human tumor cells, although it was less potent than rhizoxin. Rhizoxin exhibited antitumor effects by inhibiting the polymerization of tubulin, whereas RS-1541 did not inhibit tubulin polymerization in vitro. However, cell cycle analysis in vivo showed that the two agents had the same mode of action. The cytotoxicity of RS-1541 was enhanced when the initial cell density of the cells was increased. The cytotoxicity was also enhanced when the membrane fraction of St-4 cells, which were the most sensitive to RS-1541 among the cell lines tested, was added to the target cells. When St-4 cells were incubated with 14C]-RS-1541, significant amounts of 14C]-rhizoxin were produced within the cells. Further fractionation of the crude membrane showed that the activity that enhanced the cytotoxicity of RS-1541 (RS-1541-enhancing activity) belonged to the mitochondrial-lysosomal fraction, not to the microsomal fraction. Both the enhancing activity and the activity that converting 14C]-RS-1541 to 14C]-rhizoxin (RS-1541-converting activity) were inhibited by treatment with chloroquine, an inhibitor of lysosomal function. Cholesterol esterase derived fromCandida cylindracea had RS-1541-enhancingand-converting activities. These data suggest that RS-1541 exerts its cytotoxic action after being converted to rhizoxin within the cells by a lysosomal enzyme such as cholesterol esterase.Abbreviations
DMSO
Dimethylsulfoxide
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PBS(-) Ca2+
Mg2+-free phosphate-buffered saline
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HCO60
hydrogenated castor oil polyethylene glycor ether
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DMA
dimethylacetamide
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RSB
reticulocyte standard buffer, consisting of 10mM NaCl, 1.5 mM MgCl2, and 10 mM TRIS-HCl, (pH 7.4)
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TLC
thin-layer chromatography
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ara-C
1- -D-arabinofuranosylcytosine
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LDL
low-density lipoprotein |
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Keywords: | RS-1541 Rhizoxin Cytotoxicity |
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