Adhesion-dependent mechanisms regulating mitosis

Abstract: Integrin-mediated cell adhesion is required for normal cell cycle progression during G1-S transition and for the completion of cytokinesis. Cancer cells have ability to grow anchorage-independently, but the underlying mechanisms and the functional significance for cancer development are unclear. The current thesis describes new data on the adhesion-linked molecular mechanisms regulating cytokinesis and centrosomes.       Non-adherent fibroblast failed in the last step of the cytokinesis process, the abscission. This was due to lack of CEP55-binding of ESCRT-III and its associated proteins to the midbody (MB) in the intercellular bridge (ICB), which in turn correlated with too early disappearance of PLK1 and the consequent premature CEP55 accumulation. Integrin-induced FAK activity was found to be an important upstream step in the regulation of PLK1 and cytokinetic abscission. Under prolonged suspension culture, the MB disappeared but septin filaments kept the ICB in the ingressed state. Upon re-plating on fibronectin, such cells were found to divide through traction-based abscission. Non-adherent cytokinetic cells maintained septin filaments around the ICB for >24 hours, but septin was gradually depolymerized later on and furrow-regressed binucleated cells were generated (<15%). Binucleated non-transformed cells were halted in G1 and became senescent, possibly via PIDDosome formation by two centrosomes merging. In contrast to normal fibroblasts, ras-transformed fibroblasts were able to recruit the ESCRT-III-associated protein ALIX to MB under non-adherent condition. Live-cell imaging and septin-7 immuno-staining showed that cytokinetic abscission occurred in non-adherent ras-transformed fibroblast.        Non-adherent fibroblasts, as well as adherent cells lacking FAK expression, were also delayed in early stages of mitosis progression and exhibited defect centrosome separation and abnormal spindle formation. Our data show that integrin-dependent FAK activity promotes centrosome separation via a FAK-PLK1-Eg5 route during early mitosis.