Cytokinesis defects, polyploidy and premature senescence in SSeCKS/Gravin/AKAP12-null mouse embryo fibroblasts

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SSeCKS/Gravin/AKAP12 (“SSeCKS”) regulates a number of proliferative and cytoskeletal signaling events by scaffolding a number of key mediators of G1→S progression, such as protein kinase (PK) A and C, cyclins and calmodulin. SSeCKS expression is suppressed by Src and Ras oncogenes and in many cancer cell lines and human cancer tissues. Moreover, the re-expression of SSeCKS to physiologic levels can suppress oncogenic growth by disengaging active Src from MEK-ERK and PI3K proliferation pathways and by re-establishing a normalized actin cytoskeletal architecture through the inhibition of Rho-family GTPases (1-3). Although the overexpression of SSeCKS in untransformed cells is known to induce G1 arrest through the suppression of cyclin D1 expression (4), the precise mechanism of how SSeCKS controls normal cell cycle progression is not known. In this study, we have examined a role of SSeCKS on cellular functions by using SSeCKS-null mouse embryonic fibroblasts (KO-MEF). KO-MEF exhibit premature-senescence (between passages 6 and 7), coincident with the expression of senescence-associated β-galactosidase and with a very high level of multi-nucleation and/or tetraploidy. Preliminary evidence suggests that the senescence in KO-MEF is both Rb- and p53-dependent. Using a combination of live cell imaging and immunofluorescence staining, we show that SSeCKS is enriched around nascent segregated daughter chromosomes in anaphase cells and that KO cells are defective in both the polarity of anaphase spindles and the ability of late-stage cytokinetic plates to septate. Evidence will be shown suggesting that SSeCKS controls normal cell cycle progression by attenuating RhoA-mediated late-stage events in cytokinesis.