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.

Purpose To understand the role of HER2-associated signaling network in breast cancer stem cells (BCSCs); using radiation-resistant breast cancer cells and clinical recurrent breast cancers to evaluate HER2-targeted therapy as a tumor eliminating strategy for recurrent HER2−/low breast cancers. Experimental Design HER2-expressing BCSCs (HER2+/CD44+/CD24−/low) were isolated from radiation-treated breast cancer MCF7 cells and in vivo irradiated MCF7 xenograft tumors. Tumor aggressiveness and radiation resistance were analyzed by gap filling, Matrigel invasion, tumor-sphere formation, and clonogenic survival assays. The HER2/CD44 feature was analyzed in 40 primary and recurrent breast cancer specimens. Protein expression profiling in HER2+/CD44+/CD24−/low versus HER2−/CD44+/CD24−/low BCSCs was conducted with 2-D DIGE and HPLC-MS/MS analysis and HER2-mediated signaling network was generated by MetaCore™ program. Results Compared to HER2-negative BCSCs, HER2+/CD44+/CD24−/low cells showed elevated aldehyde dehydrogenase (ALDH) activity and aggressiveness tested by matrigel invasion, tumor sphere formation and in vivo tumorigenesis. The enhanced aggressive phenotype and radioresistance of the HER2+/CD44+/CD24−/low cells were markedly reduced by inhibition of HER2 via siRNA or Herceptin treatments. Clinical breast cancer specimens revealed that cells co-expressing HER2 and CD44 were more frequently detected in recurrent (84.6%) than primary tumors (57.1%). In addition, 2-D DIGE and HPLC-MS/MS of HER2+/CD44+/CD24−/low versus HER2−/CD44+/CD24−/low BCSCs reported a unique HER2-associated protein profile including effectors involved in tumor metastasis, apoptosis, mitochondrial function and DNA repair. A specific feature of HER2-STAT3 network was identified. Conclusion This study provides the evidence that HER2-mediated pro-survival signaling network is responsible for the aggressive phenotype of breast cancer stem cells that could be targeted to control the therapy-resistant HER2−/low breast cancer.

Significance: The mitochondrial antioxidant manganese superoxide dismutase (MnSOD) is encoded by genomic DNA and its dismutase function is fully activated in the mitochondria to detoxify free radical O 2 -generated by mitochondrial respiration. Accumulating evidence shows an extensive communication between the mitochondria and cytoplasm under oxidative stress. Not only is the MnSOD gene upregulated by oxidative stress, but MnSOD activity can be enhanced via the mitochondrial protein influx (MPI). Recent Advances: A cluster of MPI containing cytoplasmic/nuclear proteins, such as cyclins, cyclin-dependent kinases, and p53 interact with and alter MnSOD activity. These proteins modulate MnSOD superoxide scavenging activity via post-translational modifications in the mitochondria. In addition to well-established pathways in gene expression, recent findings suggest that MnSOD enzymatic activity can also be enhanced by phosphorylation of specific motifs in mitochondria. This review attempts to discuss the pre-and post-translational regulation of MnSOD, and how these modifications alter MnSOD activity, which induces a cell adaptive response to oxidative stress. Critical Issues: MnSOD is biologically significant to aerobic cells. Its role in protecting the cells against the deleterious effects of reactive oxygen species is evident. However, the exact network of MnSOD-associated cellular adaptive reaction to oxidative stress and its post-translational modifications, especially its enzymatic enhancement via phosphorylation, is not yet fully understood. Future Directions: The broad discussion of the multiple aspects of MnSOD regulation, including gene expression, protein modifications, and enzymatic activity, will shed light onto the unknown mechanisms that govern the prosurvival networks involved in cellular and mitochondrial adaptive response to genotoxic environment. Antioxid. Redox Signal. 20, 1599Signal. 20, -1617

A unique feature of cancer cells is to convert glucose into lactate to produce cellular energy, even under the presence of oxygen. Called aerobic glycolysis [The Warburg Effect] it has been extensively studied and the concept of aerobic glycolysis in tumor cells is generally accepted. However, it is not clear if aerobic glycolysis in tumor cells is fixed, or can be reversed, especially under therapeutic stress conditions. Here, we report that mTOR, a critical regulator in cell proliferation, can be relocated to mitochondria, and as a result, enhances oxidative phosphorylation and reduces glycolysis. Three tumor cell lines (breast cancer MCF-7, colon cancer HCT116 and glioblastoma U87) showed a quick relocation of mTOR to mitochondria after irradiation with a single dose 5 Gy, which was companied with decreased lactate production, increased mitochondrial ATP generation and oxygen consumption. Inhibition of mTOR by rapamycin blocked radiation-induced mTOR mitochondrial relocation and the shift of glycolysis to mitochondrial respiration, and reduced the clonogenic survival. In irradiated cells, mTOR formed a complex with Hexokinase II [HK II], a key mitochondrial protein in regulation of glycolysis, causing reduced HK II enzymatic activity. These results support a novel mechanism by which tumor cells can quickly adapt to genotoxic conditions via mTOR-mediated reprogramming of bioenergetics from predominantly aerobic glycolysis to mitochondrial oxidative phosphorylation. Such a “waking-up” pathway for mitochondrial bioenergetics demonstrates a flexible feature in the energy metabolism of cancer cells, and may be required for additional cellular energy consumption for damage repair and survival. Thus, the reversible cellular energy metabolisms should be considered in blocking tumor metabolism and may be targeted to sensitize them in anti-cancer therapy.

The tumor adaptive resistance to therapeutic radiation remains to be a barrier for further improvement of local cancer control. SIRT3, a member of the sirtuin family of NAD+-dependent protein deacetylases in mitochondria, promotes metabolic homeostasis through regulation of mitochondrial protein deacetylation and plays a key role in prevention of cell aging. Here, we demonstrate that SIRT3 expression is induced in an array of radiation-treated human tumor cells and their corresponding xenograft tumors including colon cancer HCT-116, glioblastoma U87 and breast cancer MDA-MB231 cells. The SIRT3 transcriptional activation is due to SIRT3 promoter activation controlled by the stress transcription factor NF-κB. Post-transcriptionally, the SIRT3 enzymatic activity is further enhanced via Thr150/Ser159 phosphorylation by Cyclin B1/CDK1, which is also induced by radiation and relocated to mitochondria together with SIRT3. Cells expressing the Thr150Ala/Ser159Ala mutant SIRT3 show a reduction in the mitochondrial protein lysine deacetylation, ΔΨm, MnSOD activity and mitochondrial ATP generation. The clonogenicity of Thr150Ala/Ser159Ala mutant transfectants is lower and significantly decreased under radiation. Tumors harboring the Thr150Ala/Ser159Ala mutant SIRT3 show inhibited growth and sensitivity to in vivo local irradiation. These results demonstrate that enhanced SIRT3 transcription and post-translational modifications in mitochondria contribute to the adaptive radioresistance in tumor cells. The CDK1-mediated SIRT3 phosphorylation is a potential effective target to sensitize tumor cells to radiotherapy.