Differential properties of mitosis-associated events following CHK1 and WEE1 inhibitor treatments in human tongue carcinoma cells.

CHK1 and WEE1 play pivotal roles in G2/M checkpoint following exogenous DNA damage and regulation of DNA replication under normal cellular conditions. Here, we monitored and compared the cell cycle kinetics of mitosis-associated events after CHK1 and WEE1 inhibitor treatments in a human tongue cancer cell line (SAS). A fluorescent ubiquitination-based cell cycle indicator (Fucci) that reflects SCFSKP2 and APCCDH1 E3 ligase activities was used to monitor cell cycle progression. Numerous γH2AX-positive cells were observed within the S phase population of cells following CHK1 inhibitor treatment, and polyploid cells exhibiting DNA damage emerged via abortive mitosis (endomitosis) at 24 h post treatment. While WEE1 inhibitor-treated cells exhibited similar polyploidy via endomitosis at later time points, they possessed fewer γH2AX foci during S phase, and polyploid cells exhibiting DNA damage were scarce. Instead, mitosis duration greatly extended and was accompanied by an abnormal emission of Fucci red fluorescence. Kinetic analysis of Fucci fluorescence revealed that abnormal emission occurred at early M phase in a manner independent of green fluorescence degradation as a marker of APCCDH1 activation. When an inhibitor of the essential spindle checkpoint factor MPS1 was co-treated with a WEE1 inhibitor, the elongated mitosis duration and abnormal red fluorescence were abrogated, and WEE1-induced reduction of clonogenic survival was offset. We demonstrate novel differential effects on mitosis-associated events following CHK1 and WEE1 inhibitor treatments.

Induction of endomitosis-like event in HeLa cells following CHK1 inhibitor treatment.

The effects of CHK1 inhibitor on cell cycle kinetics have not been fully investigated yet. In this study, we closely analyzed this kinetics using a CHK1 inhibitor (PF00477736) in HeLa cells expressing fluorescent ubiquitination-based cell cycle indicator (Fucci). This system allowed us to visualize cell cycle progression following CHK1 inhibitor treatment in real-time. FACS analysis showed that high levels of DNA damage as determined by γH2AX immunostaining was induced in S phase and that polyploid cells harboring the same levels of DNA damage appeared thereafter. Surprisingly, time-lapse imaging of Fucci fluorescence revealed that many cells entered M phase at once and exhibited prolonged mitosis; eventually progressing to G1 phase not accompanied by cytokinesis; this is an endomitosis-like event. Most of these cells then underwent S/G2 phases at least once, which corroborated the appearance of polyploid cells. However, a small fraction of cells with 2 N DNA content still remained 24 h after the treatment. When co-treated with MAD2 inhibitor, a core factor constituting spindle checkpoint, the 2 N DNA cell fraction disappeared and almost all cells exhibited endomitosis, leading to enhanced sensitivity. Detailed cell cycle analysis revealed that induction of an endomitosis-like event might be associated with CHK1 inhibitor-induced cell death in HeLa cells.

Fluctuation in radioresponse of HeLa cells during the cell cycle evaluated based on micronucleus frequency.

In this study, we examined the fluctuation in radioresponse of HeLa cells during the cell cycle. For this purpose, we used HeLa cells expressing two types of fluorescent ubiquitination-based cell cycle indicators (Fucci), HeLa-Fucci (CA)2 and HeLa-Fucci (SA), and combined this approach with the micronucleus (MN) assay to assess radioresponse. The Fucci system distinguishes cell cycle phases based on the colour of fluorescence and cell morphology under live conditions. Time-lapse imaging allowed us to further identify sub-positions within the G1 and S phases at the time of irradiation by two independent means, and to quantitate the number of MNs by following each cell through M phase until the next G1 phase. Notably, we found that radioresponse was low in late G1 phase, but rapidly increased in early S phase. It then decreased until late S phase and increased in G2 phase. For the first time, we demonstrated the unique fluctuation of radioresponse by the MN assay during the cell cycle in HeLa cells. We discuss the difference between previous clonogenic experiments using M phase-synchronised cell populations and ours, as well as the clinical implications of the present findings.

WEE1 inhibition enhances sensitivity to hypoxia/reoxygenation in HeLa cells.

Hypoxia/reoxygenation (H/R) treatment reportedly induces DNA damage response (DDR), including DNA double-strand break (DSB) repair and G2 arrest, resulting in reduction of clonogenic survival. Because WEE1 plays a key role in the G2/M checkpoint along with CHK1/2, we investigated the effect of WEE1 inhibition on H/R-induced DDR using HeLa cells. The H/R treatment combined with WEE1 inhibitor abrogated G2 arrest, subsequently leading to the cells entering the M phase, and finally resulting in mitotic catastrophe after prolonged mitosis. Colony-forming assay showed an enhanced decrease in the surviving fraction and the focus formation of BRCA1 was significantly reduced. We demonstrate for the first time that WEE1 inhibition enhances H/R-induced cell death accompanied by mitotic catastrophe and that the process may be mediated by homologous recombination.