Paclitaxel and nocodazole differentially alter endocytosis in cultured cells.

Abstract

Microtubule-based transport facilitates the endocytosis of exogenous macromolecules. We have determined how microtubule accumulation and disassembly alter endocytosis. The effects of paclitaxel, which promotes microtubule assembly, and nocodazole, which promotes microtubule disassembly, on fluid-phase and receptor-mediated endocytosis were measured using uptake of horseradish peroxidase and 125I-transferrin, respectively. Changes in membrane and microtubule organization were examined by fluorescence microscopy. Neither paclitaxel (4 microM, 60 min pretreatment) nor nocodazole (1 microgram/ml, 60 min pretreatment) significantly inhibited fluid-phase endocytosis. However, paclitaxel caused a redistribution of fluorescent fluid-phase marker to the periphery. Both paclitaxel and nocodazole treatment significantly (p < or = 0.05) reduced the initial uptake of 125I-transferrin at 5 min to approximately 50% of control. Despite the similarity of the effects on initial endocytic uptake, the effects on steady state accumulation of 125I-transferrin were quite distinct. Exposure of CV-1 cells to paclitaxel for an additional 30, 60 or 90 min also showed reduced accumulation of 125I-transferrin up to a maximum significant (p < or = 0.05) inhibition of 48% +/- 10% of control at 90 min. In contrast, nocodazole caused an initial significant (p < or = 0.05) increase in 125I-transferrin accumulation after 30 min (159% +/- 13% of control), while by 90 min 125I-transferrin accumulation had returned to control levels. Microtubule content, particularly of stable microtubules, was increased in CV-1 cells by paclitaxel, but abolished by nocodazole treatment. Our data show that changes in the microtubule array can alter the dynamics of receptor movement through the endosomal pathway. However, microtubule assembly versus disassembly have different effects.