A Survivin-Ran Complex Regulates Spindle Formation in Tumor Cells

Abstract

Aberrant cell division is a hallmark of cancer, but the molecular circuitries of this process in tumor cells are not well understood. Here, we used a high-throughput proteomics screening to identify novel molecular partners of survivin, an essential regulator of mitosis overexpressed in cancer. We found that survivin associates with the small GTPase Ran in an evolutionarily conserved recognition in mammalian cells and Xenopus laevis extracts. This interaction is regulated during the cell cycle, involves Ran-GTP, requires a discrete binding interface centered on Glu65 in survivin, and is independent of the Ran effector Crm1. Disruption of a survivin-Ran complex does not affect the assembly of survivin within the chromosomal passenger complex or its cytosolic accumulation, but it inhibits the delivery of the Ran effector molecule TPX2 to microtubules. In turn, this results in aberrant mitotic spindle formation and chromosome missegregation in tumor, but not normal, cells. Therefore, survivin is a novel effector of Ran signaling, and this pathway may be preferentially exploited for spindle assembly in tumor cells.Deregulated cell division is a hallmark of cancer (31), which results in unrestrained cell proliferation, abrogation of cell cycle checkpoints, and propensity to aneuploidy (27). These processes hinge on spatial-temporal assembly of a bipolar mitotic spindle (23), where microtubules nucleating from duplicated centrosomes or assembled in proximity of mitotic chromosomes "search and capture" chromatids and ultimately segregate them between daughter cells (46). Reconstitution experiments in model organisms, particularly cycled Xenopus laevis extracts, uncovered a pivotal role of the small GTPase Ran in the mechanism of spindle formation (36). We now know that a gradient of Ran-GTP assembles on mitotic chromosomes (20), largely through the chromatin-associated activity of the nucleotide exchange factor RCC1. In turn, this releases Ran effector molecules implicated in spindle formation (9, 15), including microtubule-stabilizing TPX2 (14), from an inhibitory interaction with importin ␣/␤ receptors (16, 45). Depletion of Ran in Xenopus extracts (9) or mammalian cells (15) or targeting its effector molecules (26, 41, 43) profoundly impairs spindle formation, causing the appearance of flattened mitotic spindles, severely depleted of microtubules, and abnormal chromosomal segregation.Among the regulators of cell division aberrantly overexpressed in cancer is survivin (3), a member of the inhibitor of apoptosis (IAP) gene family with dual roles in suppression of cell death and control of mitosis (1, 29). The latter has been linked to a multiplicity of functions, including kinetochore targeting of the chromosomal passenger complex (22), enhancement of Aurora B kinase activity (7), control of kinetochore-microtubule interactions for proper chromosomal alignment (39), participation in the spindle assembly checkpoint (11), and regulation of microtubule dynamics for spindle formation (37). Although these functions are ess...

Nuclear transport and the mitotic apparatus: an evolving relationship

“…However, in contrast to the situation observed for the RRSU complex (see above), short (20-40 min) treatments of late G2 or of metaphase cells with LMB does not affect the localization of already assembled CPCs at the centromeres [77]. A more recent study reveals that survivin also interacts with RanGTP in a Crm1-and NES-independent fashion [78]. This association seems to have little if any role in either CPC assembly or localization.…”

RHAMM Promotes Interphase Microtubule Instability and Mitotic Spindle Integrity through MEK1/ERK1/2 Activity

“…Intriguingly, Ran, like RHAMM, is overexpressed in human cancers in vivo, and a number of human cancer cell lines exhibit dependence on Ran-GTP for successful mitosis. Silencing Ran expression in tumor cells results in aberrant mitotic spindle formation and apoptosis, whereas mitosis and survival of normal cell lines are largely unaffected (57,58). Therefore, Randirected mitosis may predominate in diseased and/or stressed tissues, and RHAMM may also participate in spindle formation under these conditions.…”

Tumor suppressor function of miR-483-3p on squamous cell carcinomas due to its pro-apoptotic properties

“…[46][47][48] Moreover, RAN and survivin form complexes that were shown to be crucial for mitotic spindle formation and chromosome segregation in tumor cells. 49 Interestingly, RAN is also targeted by miR-203, whose expression is decreased in healing epidermis, thus favoring keratinocytes migration and proliferation. 50 Finally, despite that API5 was previously shown (1) to protect cells from growth factor deprivation-or drug-induced apoptosis 51,52 and (2) to be targeted by other miRNAs, 53,54 it seemed non-essential for the pro-apoptotic effects of miR-483-3p.…”