Purpose: Microtubule-based transport facilitates the endocytosis of exogenous macromolecules. We have determined how microtubule accumulation and disassembly alter endocytosis.
Methods: 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.
Results: 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.
Conclusions: 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.