Corepressor MMTR/DMAP1 is an intrinsic negative regulator of CAK kinase to regulate cell cycle progression.

We have previously reported that MMTR (MAT1-mediated transcriptional repressor) is a co-repressor that inhibits TFIIH-mediated transcriptional activity via interaction with MAT1 (Kang et al., 2007). Since MAT1 is a member of the CAK kinase complex that is crucial for cell cycle progression and that regulates CDK phosphorylation as well as the general transcription factor TFIIH, we investigated MMTR function in cell cycle progression. We found that MMTR over-expression delayed G1/S and G2/M transitions, whereas co-expression of MAT1 and MMTR rescued the cell growth and proliferation rate. Moreover, MMTR was required for inhibition of CAK kinase-mediated CDK1 phosphorylation. We also showed that the expression level of MMTR was modulated during cell cycle progression. Our data support the notion that MMTR is an intrinsic negative cell cycle regulator that modulates the CAK kinase activity via interaction with MAT1.

ATP depletion during mitotic arrest induces mitotic slippage and APC/CCdh1-dependent cyclin B1 degradation.

ATP depletion inhibits cell cycle progression, especially during the G1 phase and the G2 to M transition. However, the effect of ATP depletion on mitotic progression remains unclear. We observed that the reduction of ATP after prometaphase by simultaneous treatment with 2-deoxyglucose and NaN3 did not arrest mitotic progression. Interestingly, ATP depletion during nocodazole-induced prometaphase arrest resulted in mitotic slippage, as indicated by a reduction in mitotic cells, APC/C-dependent degradation of cyclin B1, increased cell attachment, and increased nuclear membrane reassembly. Additionally, cells successfully progressed through the cell cycle after mitotic slippage, as indicated by EdU incorporation and time-lapse imaging. Although degradation of cyclin B during normal mitotic progression is primarily regulated by APC/CCdc20, we observed an unexpected decrease in Cdc20 prior to degradation of cyclin B during mitotic slippage. This decrease in Cdc20 was followed by a change in the binding partner preference of APC/C from Cdc20 to Cdh1; consequently, APC/CCdh1, but not APC/CCdc20, facilitated cyclin B degradation following ATP depletion. Pulse-chase analysis revealed that ATP depletion significantly abrogated global translation, including the translation of Cdc20 and Cdh1. Additionally, the half-life of Cdh1 was much longer than that of Cdc20. These data suggest that ATP depletion during mitotic arrest induces mitotic slippage facilitated by APC/CCdh1-dependent cyclin B degradation, which follows a decrease in Cdc20 resulting from reduced global translation and the differences in the half-lives of the Cdc20 and Cdh1 proteins.

The p90 ribosomal S6 kinase 2 specifically affects mitotic progression by regulating the basal level, distribution and stability of mitotic spindles.

RSK2, also known as RPS6KA3 (ribosomal protein S6 kinase, 90 kDa, polypeptide 3), is a downstream kinase of the mitogen-activated protein kinase (MAPK) pathway, which is important in regulating survival, transcription, growth and proliferation. However, its biological role in mitotic progression is not well understood. In this study, we examined the potential involvement of RSK2 in the regulation of mitotic progression. Interestingly, depletion of RSK2, but not RSK1, caused the accumulation of mitotic cells. Time-lapse analysis revealed that mitotic duration, particularly the duration for metaphase-to-anaphase transition was prolonged in RSK2-depleted cells, suggesting activation of spindle assembly checkpoint (SAC). Indeed, more BubR1 (Bub1-related kinase) was present on metaphase plate kinetochores in RSK2-depleted cells, and depletion of BubR1 abolished the mitotic accumulation caused by RSK2 depletion, confirming BubR1-dependent SAC activation. Along with the shortening of inter-kinetochore distance, these data suggested that weakening of the tension across sister kinetochores by RSK2 depletion led to the activation of SAC. To test this, we analyzed the RSK2 effects on the stability of kinetochore-microtubule interactions, and found that RSK2-depleted cells formed less kinetochore-microtubule fibers. Moreover, RSK2 depletion resulted in the decrease of basal level of microtubule as well as an irregular distribution of mitotic spindles, which might lead to observed several mitotic progression defects such as increase in unaligned chromosomes, defects in chromosome congression and a decrease in pole-to-pole distance in these cells. Taken together, our data reveal that RSK2 affects mitotic progression by regulating the distribution, basal level and the stability of mitotic spindles.

Co-treatment with hepatocyte growth factor and TGF-beta1 enhances migration of HaCaT cells through NADPH oxidase-dependent ROS generation.

Wound healing requires re-epithelialization from the wound margin through keratinocyte proliferation and migration, and some growth factors are known to influence this process. In the present study, we found that the co-treatment with hepatocyte growth factor (HGF) and TGF-beta1 resulted in enhanced migration of HaCaT cells compared with either growth factor alone, and that N-acetylcysteine, an antioxidant agent, was the most effective among several inhibitors tested, suggesting the involvement of reactive oxygen species (ROS). Fluorescence-activated cell sorter analysis using 2,7-dichlorofluorescein diacetate (DCF-DA) dye showed an early (30 min) as well as a late (24 h) increase of ROS after scratch, and the increase was more prominent with the growth factor treatment. Diphenyliodonium (DPI), a potent inhibitor of NADPH oxidase, abolished the increase of ROS at 30 min, followed by the inhibition of migration, but not the late time event. More precisely, gene knockdown by shRNA for either Nox-1 or Nox-4 isozyme of gp91phox subunit of NADPH oxidase abolished both the early time ROS production and migration. However, HaCaT cell migration was not enhanced by treatment with H((2))O((2)). Collectively, co-treatment with HGF and TGF-beta1 enhances keratinocyte migration, accompanied with ROS generation through NADPH oxidase, involving Nox-1 and Nox-4 isozymes.

Hepatocyte growth factor at S phase induces G2 delay through sustained ERK activation.

The effect of growth factors on the cell cycle progression, except G1/S transition, is poorly understood. Herein, we examined the effect of hepatocyte growth factor (HGF) treated at S phase on the cell cycle progression of HeLa cells. Interestingly, the treatment resulted in G2 delay, evidenced by flow cytometric and mitotic index analyses. The delay corresponded with the delay of degradation of cyclin A and cyclin B, and the delay of decrease of Cdk1/cyclin B and Cdk2/cyclin A kinase activities. As for the signaling responsible, sustained activation of ERK, but neither of p38MAPK nor of JNK, was observed after HGF treatment at S phase. Furthermore, U0126, an inhibitor of MEK1, and DN-MEK partially abrogated the G2 delay, indicating that activation of MEK-ERK pathway is involved. Taken together, HGF treatment of HeLa cells at S phase induces G2 delay partially through sustained activation of ERK signaling.