INTRODUCTION Vinblastine is a powerful antitumor drug in widespread use in cancer chemotherapy. The mechanism by which vin- blastine effects its unusually potent antiproliferative action

Vinblastine is a powerful antitumor drug in widespread use in cancer chemotherapy. The mechanism by which vinblastine effects its unusually potent antiproliferative action is by inhibition of mitosis (Jordan et al., 1991), and often has been ascribed to its capacity to depolymerize mitotic spindle microtubules (e.g. Malawista et al., 1968; Wilson and Bryan, 1974). However, in recent studies we found that low concentrations of vinblastine (0.1 6 nM) blocked progression of mitosis from metaphase to anaphase in HeLa cells without causing net depolymerization of microtubules (Jordan et al., 1991, 1992). Some blocked spindles appeared normal in organization and had apparently normal arrangements of microtubules, but in many cells the block induced by low vinblastine concentrations was associated with subtle alterations in the organization of the spindle microtubules, chromosomes and centrosomes. For example, in such cells astral microtubules were more numerous and prominent than those in control cells while the microtubules of the central spindle became shorter. Centrosomes were fragmented, and some chromosomes were stranded or locked at or near the spindle poles and appeared unable to congress to the otherwise compact metaphase plate. Spindle organization became increasingly distorted with increasing drug concentrations. Spindle microtubules are highly dynamic and the dynamics of some spindle microtubules change as the cell progresses through mitosis (Saxton et al., 1984; Mitchison et al., 1986; Mitchison, 1989; Hamaguchi et al., 1987; Gorbsky et al., 1987; Wadsworth et al., 1989; Hayden et al., 1990; Rieder and Alexander, 1990). For example, the rapid dynamic instability at the plus ends of centrosome-anchored microtubules that occurs during prometaphase appears to be involved in formation of the bipolar metaphase spindle and the attachment of microtubules to the kinetochores (Hayden et al., 1990; Rieder and Alexander, 1990). During metaphase, kinetochore microtubules incorporate tubulin at the kinetochores and undergo a continuous poleward treadmilling or flux of subunits (Hamaguchi et al., 1987; Mitchison, 1989) while during anaphase, the kinetochore microtubules selectively shorten by losing tubulin subunits at the kinetochores. At the same time, interpolar microtubules selectively lengthen by incorporation of tubulin subunits at their plus ends (Gorbsky et al., 1987, Saxton and McIntosh, 1987; Masuda and Cande, 1987). In our previous studies we hypothesized that the defects in spindle organization induced in HeLa cells by low vinblastine concentrations were caused by suppression of microtubule polymerization dynamics rather than by depolymerization of the spindle microtubules (Jordan et al., 1991, 1992). In further support of this idea, we also had 261 Journal of Cell Science 104, 261-274 (1993) Printed in Great Britain © The Company of Biologists Limited 1993