PACLITAXEL AND NOCODAZOLE DIFFERENTIALLY ALTER ENDOCYTOSIS IN CULTURED-CELLS

Purpose. Microtubule-based transport facilitates the endocytosis of ex ogenous macromolecules. We have determined how microtubule accumulatio n and disassembly alter endocytosis. Methods. The effects of paclitaxe l, which promotes microtubule assembly, and nocodazole, which promotes microtubule disassembly, on fluid-phase and receptor-mediated endocyt osis were measured using uptake of horseradish peroxidase and I-125-tr ansferrin, respectively. Changes in membrane and microtubule organizat ion were examined by fluorescence microscopy. Results. Neither paclita xel (4 mu M, 60 min pretreatment) nor nocodazole (1 mu g/ml, 60 min pr etreatment) significantly inhibited fluid-phase endocytosis. However, paclitaxel caused a redistribution of fluorescent fluid-phase marker t o the periphery. Both paclitaxel and nocodazole treatment significantl y (p less than or equal to 0.05) reduced the initial uptake of I-125-t ransferrin at 5 min to similar to 50% of control. Despite the similari ty of the effects on initial endocytic uptake, the effects on steady s tate accumulation of I-125-transferrin were quite distinct. Exposure o f CV-1 cells to paclitaxel for an additional 30, 60 or 90 min also sho wed reduced accumulation of I-125-transferrin up to a maximum signific ant (p less than or equal to 0.05) inhibition of 48% +/- 10% of contro l at 90 min. In contrast, nocodazole caused an initial significant (p less than or equal to 0.05) increase in I-125- transferrin accumulatio n after 30 min (159% +/- 13% of control), while by 90 min I-12S-transf errin accumulation had returned to control levels. Microtubule content , particularly of stable microtubules, was increased in CV-I cells by paclitaxel, but abolished by nocodazole treatment. Conclusions. Our da ta show that changes in the microtubule array can alter the dynamics o f receptor movement through the endosomal pathway. However, microtubul e assembly versus disassembly have different effects.