Estimation of energy pathway fluxes in cancer cells - Beyond the Warburg effect

Citrate oscillations during cell cycle are a targetable vulnerability in cancer cells

Cell cycle progression is timely interconnected with oscillations in cellular metabolism. Here, we first describe how these metabolic oscillations allow cycling cells to meet the bioenergetic needs specifically for each phase of the cell cycle. In parallel, we highlight how the cytosolic level of citrate is dynamically regulated during these different phases, being low in G1 phase, increasing in S phase, peaking in G2/M, and decreasing in mitosis. Of note, in cancer cells, a dysregulation of such citrate oscillation can support cell cycle progression by promoting a deregulated Warburg effect (aerobic glycolysis), activating oncogenic signaling pathways (such as PI3K/AKT), and promoting acetyl-CoA production via alternative routes, such as overconsumption of acetate. Then, we review how administration of sodium citrate (at high doses) arrests the cell cycle in G0/G1 or G2/M, inhibits glycolysis and PI3K/AKT, induces apoptosis, and significantly reduces tumor growth in various in vivo models. Last, we reason on the possibility to implement citrate administration to reinforce the effectiveness of cell cycle inhibitors to better cure cancer.

Energy Metabolism Behavior and Response to Microenvironmental Factors of the Experimental Cancer Cell Models Differ from that of Actual Human Tumors

Analysis of the biochemical differences in the energy metabolism among bi-dimensional (2D) and tri-dimensional (3D) cultured cancer cell models and actual human tumors was undertaken. In 2D cancer cells, the oxidative phosphorylation (OxPhos) fluxes range is 2.5-19 nmol O2/min/mg cellular protein. Hypoxia drastically decreased OxPhos flux by 2-3 times in 2D models, similar to what occurs in mature multicellular tumor spheroids (MCTS), a representative 3D cancer cell model. However, mitochondrial protein contents and enzyme activities were significantly different between both models. Moreover, glycolytic fluxes were also significantly different between 2D and MCTS. The glycolytic flux range in 2D models is 1-34 nmol lactate/min/mg cellular protein, whereas in MCTS the range of glycolysis fluxes is 60-80 nmol lactate/min/mg cellular. In addition, sensitivity to anticancer canonical and metabolic drugs was greater in MCTS than in 2D. Actual solid human tumor samples show lower (1.6-4.5 times) OxPhos fluxes compared to normoxic 2D cancer cell cultures. These observations indicate that tridimensional organization provides a unique microenvironment affecting tumor physiology, which has not been so far faithfully reproduced by the 2D environment. Thus, the analysis of the resemblances and differences among cancer cell models undertaken in the present study raises caution on the interpretation of results derived from 2D cultured cancer cells when they are extended to clinical settings. It also raises awareness about detecting which biological and environmental factors are missing in 2D and 3D cancer cell models to be able to reproduce the actual human tumor behavior.

Aggregation assisted enhancement of singlet oxygen generation by 4-ethynylphenyl substituted porphyrin photosensitizer for photodynamic therapy

Modulating the photophysical properties of photosensitizers is an effective approach to enhance singlet oxygen generation for photodynamic therapy. Porphyrins are the most widely used photosensitizers due to their biocompatible nature. Aggregation-induced emission (AIE) characteristics of photosensitizers are one of the advantageous features that will enhance fluorescence, intersystem crossing, and efficient triplet state generation. Herein, we demonstrate two glycosylated porphyrin photosensitizers, ZnGEPOH (with two ethynyl groups) and ZnGPOH (without two ethynyl groups), which exhibit AIE. Detailed studies revealed that ZnGEPOH exhibited a two-fold increase in singlet oxygen production than ZnGPOH due to AIE. The photo-cytotoxicity of ZnGPOH and ZnGEPOH were evaluated using cancer cell lines A549 and AGS. ZnGEPOH shows superior photo-cytotoxicity with cell viability of 21% and 19% for A549 and AGS, respectively, at 250 μg/mL concentration in 48 h. Moreover, ZnGEPOH exhibits minimal photo-cytotoxicity towards the control cell line HEK 293.

Synergistic celecoxib and dimethyl-celecoxib combinations block cervix cancer growth through multiple mechanisms

OBJECTIVE

The synergistic inhibitory effect of celecoxib (CXB) and dimethyl-celecoxib (DMC) plus paclitaxel (PA) or cisplatin (CP) on human cervix HeLa and SiHa cells was assessed at multiple cellular levels in order to elucidate the biochemical mechanisms triggered by the synergistic drug combinations.

METHODS

The effect of CXB (5 μM)/CP (2 μM) or CXB (5 μM)/PA (15 μM) and DMC (15 μM)/CP (5 μM) or DMC (15 μM)/PA (20 μM) for 24 h was assayed on cancer cell proliferation, energy metabolism, mitophagy, ROS production, glycoprotein-P activity, DNA stability and apoptosis/necrosis.

RESULTS

Drug combinations synergistically decreased HeLa and SiHa cell proliferation (>75%) and arrested cellular cycle by decreasing S and G2/M phases as well as the Ki67 content (HeLa) by 7.5-30 times. Cell viability was preserved (>90%) and no apparent effects on non-cancer cell growth were observed. Mitochondrial and glycolytic protein contents (44-95%) and ΔΨm (45-50%) in HeLa cells and oxidative phosphorylation and glycolysis fluxes (70-90%) in HeLa and SiHa cells were severely decreased, which in turn promoted a drastic fall in the ATP supply (85-88%). High levels of mitophagy proteins in HeLa cells and active mitochondrial digestion in HeLa and SiHa cells was observed. Mitochondrial fission and microtubule proteins were also affected. Intracellular ROS content (2-2.3-fold) and ROS production was stimulated (2.3-4 times), whereas content and activity of glycoprotein-P (45-85%) were diminished. DNA fragmentation was not observed and apoptosis/necrosis was not detected suggesting that cell death could be mainly associated to mitophagy induction.

CONCLUSIONS

CXB or DMC combination with canonical chemotherapy may be a promising chemotherapy strategy against cervical cancer growth, because it can selectively block multiple cell processes including inhibition of energy pathways and in consequence ATP-dependent processes such as cell proliferation, glycoprotein-P activity, ROS production and mitophagy, with no apparent effects on non-cancer cells.

Repurposing auranofin and meclofenamic acid as energy-metabolism inhibitors and anti-cancer drugs

OBJECTIVE

Cytotoxicity of the antirheumatic drug auranofin (Aur) and the non-steroidal anti-inflammatory drug meclofenamic acid (MA) on several cancer cell lines and isolated mitochondria was examined to assess whether these drugs behave as oxidative phosphorylation inhibitors.

METHODS

The effect of Aur or MA for 24 h was assayed on metastatic cancer and non-cancer cell proliferation, energy metabolism, mitophagy and metastasis; as well as on oxygen consumption rates of cancer and non-cancer mitochondria.

RESULTS

Aur doses in the low micromolar range were required to decrease proliferation of metastatic HeLa and MDA-MB-231 cells, whereas one or two orders of magnitude higher levels were required to affect proliferation of non-cancer cells. MA doses required to affect cancer cell growth were one order of magnitude higher than those of Aur. At the same doses, Aur impaired oxidative phosphorylation in isolated mitochondria and intact cells through mitophagy induction, as well as glycolysis. Consequently, cell migration and invasiveness were severely affected. The combination of Aur with very low cisplatin concentrations promoted that the effects on cellular functions were potentiated.

CONCLUSION

Aur surges as a highly promising anticancer drug, suggesting that efforts to establish this drug in the clinical treatment protocols are warranted and worthy to undertake.

Mitochondrial Proteins as Metabolic Biomarkers and Sites for Therapeutic Intervention in Primary and Metastatic Cancers

Accelerated aerobic glycolysis is one of the main metabolic alterations in cancer, associated with malignancy and tumor growth. Although glycolysis is one of the most studied properties of tumor cells, recent studies demonstrate that oxidative phosphorylation (OxPhos) is the main ATP provider for the growth and development of cancer. In this last regard, the levels of mRNA and protein of OxPhos enzymes and transporters (including glutaminolysis, acetate and ketone bodies catabolism, free fatty acid β-oxidation, Krebs Cycle, respiratory chain, phosphorylating system- ATP synthase, ATP/ADP translocator, Pi carrier) are altered in tumors and cancer cells in comparison to healthy tissues and organs, and non-cancer cells. Both energy metabolism pathways are tightly regulated by transcriptional factors, oncogenes, and tumor-suppressor genes, all of which dictate their protein levels depending on the micro-environmental conditions and the type of cancer cell, favoring cancer cell adaptation and growth. In the present review paper, variation in the mRNA and protein levels as well as in the enzyme/ transporter activities of the OxPhos machinery is analyzed. An integral omics approach to mitochondrial energy metabolism pathways may allow for identifying their use as suitable, reliable biomarkers for early detection of cancer development and metastasis, and for envisioned novel, alternative therapies.

The pro-oncogenic protein IF1 does not contribute to the Warburg effect and is not regulated by PKA in cancer cells

The endogenous inhibitor of mitochondrial F1Fo-ATPase (ATP synthase), IF1, has been shown to exert pro-oncogenic actions, including reprogramming of cellular energy metabolism (Warburg effect). The latter action of IF1 has been reported to be hampered by its PKA-dependent phosphorylation, but both reprogramming of metabolism and PKA-dependent phosphorylation are intensely debated. To clarify these critical issues, we prepared stably IF1-silenced clones and compared their bioenergetics with that of the three parental IF1-expressing cancer cell lines. All functional parameters: respiration rate, ATP synthesis rate (OXPHOS), and mitochondrial membrane potential were similar in IF1-silenced and control cells, clearly indicating that IF1 cannot inhibit the ATP synthase in cancer cells when the enzyme works physiologically. Furthermore, all cell types exposed to PKA modulators and energized with NAD+-dependent substrates or succinate showed similar OXPHOS rate regardless of the presence or absence of IF1. Therefore, our results rule out that IF1 action is modulated by its PKA-dependent phosphorylated/dephosphorylated state. Notably, cells exposed to a negative PKA modulator and energized with NAD+-dependent substrates showed a significant decrease of the OXPHOS rate matching previously reported inactivation of complex I. Overall, this study definitively demonstrates that IF1 inhibits neither mitochondrial ATP synthase nor OXPHOS in normoxic cancer cells and does not contribute to the Warburg effect. Thus, currently the protection of cancer cells from severe hypoxia/anoxia and apoptosis remain the only unquestionable actions of IF1 as pro-oncogenic factor that may be exploited to develop therapeutic approaches.

Warburg-associated acidification represses lactic fermentation independently of lactate, contribution from real-time NMR on cell-free systems

Lactate accumulation and acidification in tumours are a cancer hallmark associated with the Warburg effect. Lactic acidosis correlates with cancer malignancy, and the benefit it offers to tumours has been the subject of numerous hypotheses. Strikingly, lactic acidosis enhances cancer cell survival to environmental glucose depletion by repressing high-rate glycolysis and lactic fermentation, and promoting an oxidative metabolism involving reactivated respiration. We used real-time NMR to evaluate how cytosolic lactate accumulation up to 40 mM and acidification up to pH 6.5 individually impact glucose consumption, lactate production and pyruvate evolution in isolated cytosols. We used a reductive cell-free system (CFS) to specifically study cytosolic metabolism independently of other Warburg-regulatory mechanisms found in the cell. We assessed the impact of lactate and acidification on the Warburg metabolism of cancer cytosols, and whether this effect extended to different cytosolic phenotypes of lactic fermentation and cancer. We observed that moderate acidification, independently of lactate concentration, drastically reduces the glucose consumption rate and halts lactate production in different lactic fermentation phenotypes. In parallel, for Warburg-type CFS lactate supplementation induces pyruvate accumulation at control pH, and can maintain a higher cytosolic pyruvate pool at low pH. Altogether, we demonstrate that intracellular acidification accounts for the direct repression of lactic fermentation by the Warburg-associated lactic acidosis.