Fibronectin assembly in the crypts of cytokinesis-blocked multilobular cells promotes anchorage-independent growth

Initiation of fibronectin fibrillogenesis is an enzyme-dependent process

Fibronectin fibrillogenesis and mechanosensing both depend on integrin-mediated force transmission to the extracellular matrix. However, force transmission is in itself dependent on fibrillogenesis, and fibronectin fibrils are found in soft embryos where high forces cannot be applied, suggesting that force cannot be the sole initiator of fibrillogenesis. Here, we identify a nucleation step prior to force transmission, driven by fibronectin oxidation mediated by lysyl oxidase enzyme family members. This oxidation induces fibronectin clustering, which promotes early adhesion, alters cellular response to soft matrices, and enhances force transmission to the matrix. In contrast, absence of fibronectin oxidation abrogates fibrillogenesis, perturbs cell-matrix adhesion, and compromises mechanosensation. Moreover, fibronectin oxidation promotes cancer cell colony formation in soft agar as well as collective and single-cell migration. These results reveal a force-independent enzyme-dependent mechanism that initiates fibronectin fibrillogenesis, establishing a critical step in cell adhesion and mechanosensing.

Contribution of integrin adhesion to cytokinetic abscission and genomic integrity

Recent research shows that integrin-mediated adhesion contributes to the regulation of cell division at two key steps: the formation of the mitotic spindle at the mitotic entry and the final cytokinetic abscission at the mitotic exit. Failure in either of these processes will have a direct impact on the other in each round of the cell cycle and on the genomic integrity. This review aims to present how integrin signals are involved at these cell cycle stages under normal conditions and some safety mechanisms that may counteract the generation of aneuploid cells in cases of defective integrin signals.

Septin and Ras regulate cytokinetic abscission in detached cells

Background

Integrin-mediated adhesion is normally required for cytokinetic abscission, and failure in the process can generate potentially oncogenic tetraploid cells. Here, detachment-induced formation of oncogenic tetraploid cells was analyzed in non-transformed human BJ fibroblasts and BJ expressing SV40LT (BJ-LT) ± overactive HRas.

Results

In contrast to BJ and BJ-LT cells, non-adherent BJ-LT-Ras cells recruited ALIX and CHMP4B to the midbody and divided. In detached BJ and BJ-LT cells regression of the cytokinetic furrow was suppressed by intercellular bridge-associated septin; after re-adhesion these cells divided by cytofission, however, some cells became bi-nucleated because of septin reorganization and furrow regression. Adherent bi-nucleated BJ cells became senescent in G1 with p21 accumulation in the nucleus, apparently due to p53 activation since adherent bi-nucleated BJ-LT cells passed through next cell cycle and divided into mono-nucleated tetraploids; the two centrosomes present in bi-nucleated BJ cells fused after furrow regression, pointing to the PIDDosome pathway as a possible mechanism for the p53 activation.

Conclusions

Several mechanisms prevent detached normal cells from generating tumor-causing tetraploid cells unless they have a suppressed p53 response by viruses, mutation or inflammation. Importantly, activating Ras mutations promote colony growth of detached transformed cells by inducing anchorage-independent cytokinetic abscission in single cells.

Tension-induced cytokinetic abscission in human fibroblasts

Previous studies have shown that cytokinetic abscission at the end of mitosis is executed by the ESCRT machinery in mammalian cells, and that the process is dependent on adhesion-induced integrin signalling via a FAK-PLK1-CEP55-TSG101/Alix-CHMP4B pathway. The present study identified an alternative abscission mechanism driven by mechanical force. In the absence of integrin signals (non-adherent conditions), cytokinesis in non-transformed human fibroblasts proceeds to CEP55 accumulation at the midbody, but after prolonged time (>3 hours) the major midbody components Aurora B, MKLP1 and CEP55 were no longer detected in the area. Upon adhesion to fibronectin, such cells were able to complete abscission without re-appearance of midbody proteins. Live-cell imaging revealed that re-plating on stiff fibronectin matrix (64 KPa) allowed >95% of the cells to complete abscission within 9 hours while the corresponding number was 40% on soft fibronectin matrix (0.5 KPa). The cells re-plated on poly-L-lysine were not able to generate tension and did not divide. Thus, mechanical tension can cause cytokinetic abscission by stretching of the intercellular bridge between the two daughter cells until it eventually ruptures without the involvement of ESCRT complexes. Importantly, regression of the cleavage furrow and formation of bi-nucleated cells did not occur in most of the suspension-treated mitotic cells after re-plating on fibronectin. Septin, which stabilizes the membrane associated with the midbody, was found to remain along the ingressed membrane, suggesting that this filament system maintains the membrane bridge although the midbody had dissolved, thereby preventing regression and allowing tension to act on the narrow intercellular bridge.

Integrin signaling via FAK-Src controls cytokinetic abscission by decelerating PLK1 degradation and subsequent recruitment of CEP55 at the midbody

Adhesion to extracellular matrix is required for cell cycle progression through the G1 phase and for the completion of cytokinesis in normal adherent cells. Cancer cells acquire the ability to proliferate anchorage-independently, a characteristic feature of malignantly transformed cells. However, the molecular mechanisms underlying this escape of the normal control mechanisms remain unclear. The current study aimed to identify adhesion-induced reactions regulating the cytokinesis of non-transformed human fibroblasts.The adhesion-dependent control of cytokinesis was found to occur at a late stage close to the abscission, during which the endosomal sorting complex required for transport (ESCRT) severs the thin intercellular bridge connecting two nascent daughter cells. CEP55, a key protein involved in the abscission process, was localized at the midbody in both adherent and non-adherent fibroblasts, but it was unable to efficiently recruit ALIX, TSG101, and consequently the ESCRT-III subunit CHMP4B was missing in the non-adherent cells. PLK1, a kinase that prevents premature recruitment of CEP55 to the midbody, disappeared from this site more rapidly in the non-adherent cells. A FAK-Src signaling pathway downstream of integrin-mediated cell adhesion was found to decelerate both PLK1 degradation and CEP55 accumulation at the midbody. These data identify the regulation of PLK1 and CEP55 as steps where integrins exert control over the cytokinetic abscission.