Taxol and discodermolide: Functional similarities and differences
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Taxol is used successfully for the treatment of human breast, ovarian, and lung carcinomas. Taxol binds to the beta-tubulin subunit in the microtubule polymer and acts by stabilizing microtubules. Despite the success of Taxol, the development of Taxol-resistant tumors has led to a search for novel natural products.;The epothilones, eleutherobin, and discodermolide are structurally unrelated to Taxol and have been identified as promising candidates with mechanisms of action similar to that of Taxol. In the A549-T12 Taxol-resistant human lung carcinoma cell line, the epothilones and eleutherobin, but not discodermolide, demonstrated cross-resistance. A549-T12 cells also were found to be Taxol-dependent and this property was used to examine the substitution of Taxol by the natural compounds. Both immunofluorescence and flow cytometry revealed that the epothilones and eleutherobin, but not discodermolide, could support the normal growth of A549-T12 cells. Using median effect analysis, a synergistic interaction in four human carcinoma cell lines was observed. These results suggest that Taxol and discodermolide may represent a promising chemotherapeutic combination.;To further explore the relationship between Taxol and discodermolide, discodermolide analogs were employed to perform structure-activity relationship (SAR) studies. Small changes to the discodermolide structure resulted in a dramatic decrease in the nucleating activity of the discodermolide analogs. The X-ray/solution structure of discodermolide was docked into the Taxol-binding site of beta-tubulin and revealed two possible binding modes, one of which concurred more closely with the SAR analysis. This study provides important new information on the structural requirements responsible for the nucleation activity and potency of discodermolide.;The identification of drug-resistance mechanisms also is important to the development of better chemotherapeutic strategies. The A549-T12 cell line is Taxol-resistant and to address the possibility that a mutation in beta-tubulin could be responsible for the resistance, the predominant beta-tubulin isotype was sequenced. The results did not reveal a mutation in betatubulin, so the predominant alpha-tubulin isotype was sequenced. Primers were designed for full-length Kalpha1, the ubiquitous alpha-tubulin isotype, and a heterozygote point mutation in alpha-tubulin was discovered in the A549 Taxol-resistant cell lines. This is the first report of an alpha-tubulin mutation in a Taxol-resistant cell line.