Paclitaxel (PX) binds to and stabilizes tubulin, preventing depolymerization, and resulting in cell death. Based on a previous report showing the activity of phosphatidylinositol kinase (PIK) on tubulin, we investigated the effect of the PI4K inhibitor orobol and the PI3K activator platelet derived growth factor (PDGF) on PX sensitivity. Drug sensitivity was examined by classical colony forming assay. Tubulin isotype expression was determined by semi-quantitative RT-PCR. Microtubule texture was observed by laser confocal microscope using anti-β-tubulin antibody. Apoptotic activity was estimated by frequency of condensed nuclear chromatin with Hoechst 33342 stain. Orobol enhanced PX sensitivity of human ovarian carcinoma 2008 cells by 18.9±1.2-fold (N=3; P<0.01). In contrast, pretreatment with PDGF rendered cells resistant to PX by 2.3±0.4-fold (N=3; P<0.01). Neither orobol nor PDGF showed any effect on cell growth. Orobol produced a 2.5-fold sensitization in cisplatin-resistant 2008/C13*5.25 (C13) cells, and PDGF rendered the cells 2.3-fold resistant to PX. Orobol suppressed the β4a-tubulin isotype expression by 85% and other isotypes by 20%. In contrast, PDGF induced β4a-tubulin isotype expression by 1.3-fold, while it supressed all the other isotypes by 20-40%. Orobol produced thick microtubules and PDGF generated ring condensed microtubules. Orobol promoted PX-induced apoptosis, while PDGF caused 50% reduction of apoptosis. These results indicate that orobol and PDGF regulate PX sensitivity by reciprocally altering the proportion of tubulin isotype expression and PX-induced apoptotic signaling.

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