Dependence of energy and aerobic metabolism of initiation of DNA synthesis and mitosis by G1 and G2 cells

Modulation of oxidative phosphorylation augments antineoplastic activity of mitotic aurora kinase inhibition

Uncontrolled mitosis is one of the most important features of cancer, and mitotic kinases are thought to be ideal targets for anticancer therapeutics. However, despite numerous clinical attempts spanning decades, clinical trials for mitotic kinase-targeting agents have generally stalled in the late stages due to limited therapeutic effectiveness. Alisertib (MLN8237) is a promising oral mitotic aurora kinase A (AURKA, Aurora-A) selective inhibitor, which is currently under several clinical evaluations but has failed in its first Phase III trial due to inadequate efficacy. In this study, we performed genome-wide CRISPR/Cas9-based screening to identify vulnerable biological processes associated with alisertib in breast cancer MDA-MB-231 cells. The result indicated that alisertib treated cancer cells are more sensitive to the genetic perturbation of oxidative phosphorylation (OXPHOS). Mechanistic investigation indicated that alisertib treatment, as well as other mitotic kinase inhibitors, rapidly reduces the intracellular ATP level to generate a status that is highly addictive to OXPHOS. Furthermore, the combinational inhibition of mitotic kinase and OXPHOS by alisertib, and metformin respectively, generates severe energy exhaustion in mitotic cells that consequently triggers cell death. The combination regimen also enhanced tumor regression significantly in vivo. This suggests that targeting OXPHOS by metformin is a potential strategy for promoting the therapeutic effects of mitotic kinase inhibitors through the joint targeting of mitosis and cellular energy homeostasis.

Mitochondrial Ca2+ and cell cycle regulation

It has been demonstrated for more than 40 years that intracellular calcium (Ca²⁺) controls a variety of cellular functions, including mitochondrial metabolism and cell proliferation. Cytosolic Ca²⁺ fluctuation during key stages of the cell cycle can lead to mitochondrial Ca²⁺ uptake and subsequent activation of mitochondrial oxidative phosphorylation and a range of signaling. However, the relationship between mitochondrial Ca²⁺ and cell cycle progression has long been neglected because the molecule responsible for Ca²⁺ uptake has been unknown. Recently, the identification of the mitochondrial Ca²⁺ uniporter (MCU) has led to key advances. With improved Ca²⁺ imaging and detection, effects of MCU-mediated mitochondrial Ca²⁺ have been observed at different stages of the cell cycle. Elevated Ca²⁺ signaling boosts ATP and ROS production, remodels cytosolic Ca²⁺ pathways and reprograms cell fate-determining networks. These findings suggest that manipulating mitochondrial Ca²⁺ signaling may serve as a potential strategy in the control of many crucial biological events, such as tumor development and cell division in hematopoietic stem cells (HSCs). In this review, we summarize the current understanding of the role of mitochondrial Ca²⁺ signaling during different stages of the cell cycle and highlight the potential physiological and pathological significance of mitochondrial Ca²⁺ signaling.

Fueling the Cell Division Cycle

Cell division is a complex process with high energy demands. However, how cells regulate the generation of energy required for DNA synthesis and chromosome segregation is not well understood. Recent data suggest that changes in mitochondrial dynamics and metabolic pathways such as oxidative phosphorylation (OXPHOS) and glycolysis crosstalk with, and are tightly regulated by, the cell division machinery. Alterations in energy availability trigger cell-cycle checkpoints, suggesting a bidirectional connection between cell division and general metabolism. Some of these connections are altered in human disease, and their manipulation may help in designing therapeutic strategies for specific diseases including cancer. We review here recent studies describing the control of metabolism by the cell-cycle machinery.

The plant cell cycle--15 years on

The basic components of the plant cell cycle are G1 (postmitotic interphase), S-phase (DNA synthesis phase), G2 (premitotic interphase) and mitosis/cytokinesis. Proliferating cells are phosphoregulated by cyclin-dependent protein kinases (CDKs). Plant D-type cyclins are sensors of the G0 to G1 transition, and are also important for G2/M. At G1/S, the S-phase transcription factor, E2F, is released from inhibitory retinoblastoma protein. Negative regulation of G1 events is through KRPs (Kip-related proteins). Plant S-phase genes are similar to animal ones, but timing of expression can be different (e.g. CDC6 at the start of S-phase) and functional evidence is limited. At G2/M, A-type and the unique B-type CDKs when bound to A, B and D cyclins, drive cells into division; they are negatively regulated by ICK1/2 and perhaps also by WEE1 kinase. In Arabidopsis, a putative CDC25 lacks a regulatory domain. Mitosis depends on correct temporal activity of CDKs, Aurora kinases and anaphase promotion complex; CDK-cyclin B activity beyond metaphase is catastrophic. Endoreduplication (re-replication of DNA in the absence of mitosis) is characterized by E2F expression and down-regulation of mitotic cyclins. Some cell size data support, whilst others negate, the idea of cell size having an impact on development.

Sugar control of the plant cell cycle: differential regulation of Arabidopsis D-type cyclin gene expression

In most plants, sucrose is the major transported carbon source. Carbon source availability in the form of sucrose is likely to be a major determinant of cell division, and mechanisms must exist for sensing sugar levels and mediating appropriate control of the cell cycle. We show that sugar availability plays a major role during the G(1) phase by controlling the expression of CycD cyclins in Arabidopsis. CycD2 mRNA levels increase within 30 min of the addition of sucrose; CycD3 is induced after 4 h. This corresponds to induction of CycD2 expression early in G(1) and CycD3 expression in late G(1) near the S-phase boundary. CycD2 and CycD3 induction is independent both of progression to a specific point in the cell cycle and of protein synthesis. Protein kinase activity of CycD2- and CycD3-containing cyclin-dependent kinases is consistent with the observed regulation of their mRNA levels. CycD2 and CycD3 therefore act as direct mediators of the presence of sugar in cell cycle commitment. CycD3, but not CycD2, expression responds to hormones, for which we show that the presence of sugars is required. Finally, protein phosphatases are shown to be involved in regulating CycD2 and CycD3 induction. We propose that control of CycD2 and CycD3 by sucrose forms part of cell cycle control in response to cellular carbohydrate status.

Metabolic aspects of cell cycle regulation in normal and cancer cells

Several studies are reviewed dealing with the mechanisms which regulate the cell cycle progression in normal and cancer cells. Using Yoshida AH 130 ascites tumor cells, it has been found that the G1-S transition of these cells is impaired by specific inhibitors of the electron flow through the respiratory chain (antimycin A), although respiratory ATP can be replaced by glycolytic ATP. The above transition can be also inhibited by the addition of physiologic substrates, mainly pyruvate, by a mechanism which appears linked to a modification of the cellular redox state and can be totally reversed by adding adenine to the culture medium. Adenine equally removes the block produced by antimycin A, pointing out a respiration-linked step of purine metabolism restricting the cell recruitment into S. A substantial protection of this step against the inhibitory effects of pyruvate and antimycin A has been obtained by the addition of folate and tetrahydrofolate, suggesting that the respiration-linked limiting step of tumor cell cycling involves folate metabolism and its connection to purine synthesis. The biologic relevance of these findings is stressed by the fact that pyruvate addition also inhibits the proliferation of concanavalin A-stimulated lymphocytes as well as of bone marrow hemopoietic cells in the presence of colony-stimulating factors. On the other hand, pyruvate only slightly affects the growth kinetics of malignant lymphoblasts and of Friend erythroleukemia cells either in the absence or in the presence of the differentiation inducer dimethylsulfoxide.

The role of respiration in tumor cell transition from the noncycling to the cycling state

Resting Yoshida AH130 hepatoma cells, harvested at the plateau of tumor development in vivo, were recruited into the cycling state following transfer to an in vitro system whereby these cells were incubated in the autologous ascites plasma diluted with buffered saline and enriched with glucose. In this system, cell recruitment into the phase of DNA synthesis (S phase) strictly depends on the activity of the respiratory chain and is abolished by anaerobiosis as well as by antimycin A, although the intracellular levels of ATP and the rate of protein synthesis are practically unaffected by these treatments. Furthermore, 2,4-dinitrophenol, at concentrations which uncouple the respiratory phosphorylation and hence enhance both glycolysis and oxygen consumption, does not hinder cell promotion into S phase. Thus, the absolute respiration dependence of cycling resumption by resting ascites cells does not seem to rely on respiratory ATP supply, but rather is linked to the electron flow through the respiratory chain.

Anaerobiosis and oxygen recovery: changes in cell cycle distribution of Ehrlich ascites tumor cells grown in vitro

The cell cycle distribution of in vitro cultured Ehrlich ascites tumor (EAT) cells was analysed by pulse-cytophotometry to characterize the growth cessation observed under anaerobic conditions. DNA histograms provided evidence that in the absence of oxygen EAT cells accumulate in the G1 and early S phase of the cell cycle while in the presence of oxygen an increase in G2 was observed during 24h culture period. Cellular recovery from anaerobiosis was observed soon after transfer of the cells into fresh aerobic culture medium but occurred slowly if the cells were only resupplied with air. Cell cycle analyses as well as (14C)-thymidine incorporation suggest considerable synchronization results from the introduction of anaerobiosis.

Recovery of Pisum root meristems after mitotic-inhibitory treatments with 3H-thymidine. Inhibition of cell-cycle progression by unincorporated 3H-thymidine

Primary root meristems of Pisum sativum recover form a 3H-thymidine-induced reduction in mitotic activity once the roots are no longer exposed to exogenous 3H-thymidine. Cells arrested in G2 during 3H-thymidine treatment apparently do not divide for at least 16 hours after treatment, whereas cells remaining in G1 and S do divide and thereby account for recovery. Recovery occurs only when meristems are no longer exposed to exogenous (i.e. unincorporated) 3H-thymidine, suggesting that cytoplasmic irradiation from unincorporated 3H-thymidine prevents cellular recovery from 3H-thymidine-induced inhibition of cell progression through the mitotic cycle. Concentrations of 14C-thymidine which result in cytoplasmic irradiation nearly equivalent to that achieved with 3H-thymidine, but much lower levels of nuclear irradiation, also prevent recovery from 3H-thymidine-induced inhibition of mitotic activity, but do not alone produced such inhibition. These results support the contention that cytoplasmic irradiation prevents recovery from the effects of nuclear irradiation. Unincorporated 3H-thymidine also prevents recovery from sucrose deprivation in stationary phase G2 cells which have not incorporated 3H-thymidine into nuclear DNA.

Experimental control of the activity of the quiescent centre in excised root tips of Zea mays

Quiescent centres have been demonstrated in cultured excised root tips of both Pisum sativum and Zea mays. Upon addition of sucrose to Zea roots which have been deprived of carbohydrate, the cells of the quiescent zone as well as those of the rest of the meristem undergo DNA synthesis. Following the onset of proliferative activity in the meristem, DNA synthesis in the quiescent-centre cells is again arrested. It is suggested that the dividing cells of the meristem are responsible for the maintenance of the quiescent centre. It has also been shown that DNA-synthesising cells do occur within the quiescent centre and that they appear to be localised in specific regions.