SUMMARY A substantial amount of mitochondrial energy is required for cell cycle progression. However, the mechanisms coordinating the mitochondrial respiration with G2/M transition, a critical step in cell division, remains to be elucidated. Here we show that a fraction of cell cycle CyclinB1/Cdk1 proteins localizes into the matrix of mitochondria and phosphorylates a cluster of mitochondrial proteins including the complex I (CI) subunits in the respiratory chain. The CyclinB1/Cdk1-mediated CI subunit phosphorylation enhances CI activity, whereas deficiency of such phosphorylation in each of the relevant CI subunits results in impairment of CI function. Mitochondria-targeted CyclinB1/Cdk1 increases mitochondrial respiration with enhanced oxygen consumption and ATP generation, which provides cells with efficient bioenergy for G2/M transition and shortens overall cycling time. Thus, CyclinB1/Cdk1-mediated phosphorylation of mitochondrial substrates allows cells to sense and respond to an increased energy demand for G2/M transition, and subsequently to up-regulate mitochondrial respiration for a successful cell cycle progression.

Cancer cells acquire resistance to DNA-damaging therapeutic agents, such as cisplatin, but the genetic mechanisms through which this occurs remain unclear. We show that the c-MYC oncoprotein increases cisplatin resistance by decreasing production of the c-MYC inhibitor BIN1 (bridging integrator 1). The sensitivity of cancer cells to cisplatin depended on BIN1 abundance, regardless of the p53 gene status. BIN1 bound to the automodification domain of and suppressed the catalytic activity of poly(ADP-ribose) polymerase 1 (PARP1, EC 2.4.2.30), an enzyme essential for DNA repair, thereby reducing the stability of the genome. The inhibition of PARP1 activity was sufficient for BIN1 to suppress c-MYC-mediated transactivation, the G(2)-M transition, and cisplatin resistance. Conversely, overexpressed c-MYC repressed BIN1 expression by blocking its activation by the MYC-interacting zinc finger transcription factor 1 (MIZ1) and thereby released PARP1 activity. Thus, a c-MYC-mediated positive feedback loop may contribute to cancer cell resistance to cisplatin.

The molecular mechanisms governing acquired tumor resistance during radiotherapy remain to be elucidated. In breast cancer patients, overexpression of HER2 (human epidermal growth factor receptor 2) is correlated with aggressive tumor growth and increased recurrence. In the present study, we demonstrate that HER2 expression can be induced by radiation in breast cancer cells with a low basal level of HER2. Furthermore, HER2-postive tumors occur at a much higher frequency in recurrent invasive breast cancer (59%) compared to the primary tumors (41%). Interestingly, NF-κB is required for radiation-induced HER2 transactivation. HER2 was found to be co-activated with basal and radiation-induced NF-κB activity in radioresistant but not radiosensitive breast cancer cell lines after long-term radiation exposure, indicating that NF-κB-mediated HER2 overexpression is involved in radiation-induced repopulation in heterogeneous tumors. Finally, we found that inhibition of HER2 resensitizes the resistant cell lines to radiation. Since HER2 is shown to activate NF-κB, our data suggest a loop-like HER2-NF-κB-HER2 pathway in radiation-induced adaptive resistance in breast cancer cells.

Mechanisms governing inducible resistance to ionizing radiation in untransformed epithelial cells pre-exposed to low-dose ionizing radiation (LDIR; V10 cGy) are not well understood. The present study provides evidence that preexposure to 10 cGy X-rays increases clonogenic survival of mouse skin JB6P+ epithelial cells subsequently exposed to 2 Gy doses of ;-rays. To elucidate the molecular pathways of LDIR-induced adaptive radioresistance, the transcription factor nuclear factor-KB (NF-KB) and a group of NF-KBrelated proteins [i.e., p65, manganese superoxide dismutase (MnSOD), phosphorylated extracellular signal-regulated kinase, cyclin B1, and 14-3-3Z] were identified to be activated as early as 15 min after LDIR. Further analysis revealed that a substantial amount of both 14-3-3Z and cyclin B1 accumulated in the cytoplasm at 4 to 8 h when cell survival was enhanced. The nuclear 14-3-3Z and cyclin B1 were reduced and increased at 4 and 24 h, respectively, after LDIR. Using YFP-fusion gene expression vectors, interaction between 14-3-3Z and cyclin B1 was visualized in living cells, and LDIR enhanced the nuclear translocation of the 14-3-3Z/cyclin B1 complex. Treatment of JB6P+ cells with the NF-KB inhibitor IMD-0354 suppressed LDIR-induced expression of MnSOD, 14-3-3Z, and cyclin B1 and diminished the adaptive radioresistance. In addition, treatment with small interfering RNA against mouse MnSOD was shown to inhibit the development of LDIR-induced radioresistance. Together, these results show that NF-KB, MnSOD, 14-3-3Z, and cyclin B1 contribute to LDIR-induced adaptive radioresistance in mouse skin epithelial cells. [Cancer Res 2007;67(7):3220-8]