Microtubule inhibitors differentially affect translational movement, cell surface expression, and endocytosis of transferrin receptors in K562 cells

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Abstract
We used quantitative fluorescence microscopy and fluorescence photobleaching recovery techniques to investigate the translational movement, cell surface expression, and endocytosis of transferrin receptors in K562 human erythroleukemia cells. Receptors were labeled with fluorescein‐conjugated transferrin (FITC‐Tf). Coordinated decreases in surface fluorescence counts, the photobleachig parameter K, and transferrin receptor fractional mobility were observed as FITC‐Tf was cleared from the cell surface by receptor‐mediated endocytosis. Based on the kinetics of decrease in these parameters, first order rate constants for FITC‐Tf uptake at 37°C and 21°C were calculated to be 0.10‐0.15 min−1 and 0.02–0.03 min, respectively. K562 cells were treated with colchicine or vinblastine to investigate the role of microtubules in transferrin receptor movement and endocytosis. Treatment of cells for 1 hr with a microtubule inhibitor prevented transferrin receptor endocytosis but had no effect on the translational mobility of cell surface receptors. In contrast, drug treatment for 3 hr caused translational immobilization of cell surface receptors as well as inhibition of endocytosis. These effects were not produced by β‐lumicolchicine, an inactive colchicine analog, or by cytochalasin, a microfilament inhibitor. The effect of microtuble inhibitors on transferrin receptor mobility was reversed by pretreating cells with taxol, a microtubule‐stabilizing agent. Microtubule inhibitors had no effect on the translational mobility of cell surface glycophorins or phospholipids, indicating that intact microtubules were not required for translational movement of these molecules. We conclude that the translational movement of cell surface transferrin receptors is directed by a subpopulation of relatively drug‐resistant microtubules. In contrast, transferrin receptor endocytosis depends on a subpopulation of microtubules that is relatively sensitive to the action of inhibitors. These results appear to demonstrate at least two functional roles for microtubules in receptor‐mediated transferrin uptake in K562 cells.

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