The Role of Mitochondria in Cancer and Other Chronic Diseases. Bioenergetics of Human Cancer Cells and Normal Cells During Proliferation and Differentiation

The Role of Mitochondria in Cancer and Other Chronic Diseases Bioenergetics of Human Cancer Cells and Normal Cells During Proliferation and Differentiation

Research into the energy metabolism of cancer cells began in the early 20th century with Otto Warburg, who observed that tumor tissues appear defective in respiration and have abnormally high rates of aerobic glycolysis. This led Warburg to propose that cancer arouse as a result of mitochondrial injury, as even under conditions of plentiful oxygen, cancer cells still choose to switch to glucose fermentation with subsequent lactic acid formation. It has been explained by various possible mechanisms such as the metabolic adaptation to the hypoxic environment; by a direct effect of hypoxia-inducible factor on mitochondrial bioenergetics, mutations in oncogenes and proteins related to signal transduction pathways that interfere with mitochondrial bioenergetics and by mutations in mitochondrial DNA or in nuclear genes involved in the metabolic and bioenergetic functions. Since then several controversies have ensued related to tumor cell metabolism and mitochondrial function.  Two of the most well-known and accepted features of tumor cell metabolism are the “Crabtree effect” and the “Pasteur effect”.  The former refers to inhibition of cancer cell respiration by elevated glucose concentrations, while the latter refers to inhibition of glycolysis by elevated oxygen concentration.   It has also been observed in some cancer cells that the consumption rate of one nutrient (oxygen or glucose) increases when concentration of the other nutrient is reduced, suggesting an ability of cancer cells to adjust their metabolism based on micro-environment. These observations support the concept of cancer as a metabolic disease.

A variety of abnormalities in cancer cell mitochondrial structure and function have been reported. These include increases in, and alteration of, mitochondrial DNA, elevation of hexokinase production, lysis of cristae structures  and altered mitochondrial protein and lipid content.  Interestingly, genetic alterations in cancer cells, such as deregulation of the PI3K/Akt pathway, or imbalances in activity of c-MYC, HIF, or p53, can alter glucose and amino acid metabolism.  In addition, cancer cells have abnormal content and composition of cardiolipin, a key mitochondrial lipid that is necessary for proper cell respiration.  Cardiolipin normally resides in the inner membrane of mitochondria, where it plays a role in chemiosmosis.  A generalized increase in anabolism characterizes nearly all cancer types, perhaps indicating consumption of metabolic intermediates toward anabolic reactions, and concomitantly less conversion of pyruvate to oxaloacetate, leaning toward an augmented formation of lactic acid thus indicating an increase dependency on glycolysis as energy fuel.

We examine differences between cancer cells and normal cells in three parameters related to mitochondria:  ATP production, cardiolipin concentration, and mitochondrial potentials.  Moreover, we examine how, in cancer cells and normal cells, these parameters are affected by ATP synthesis inhibitors and, in case of one cancer cell type, chemically induced differentiation.

Cancer cells are known to have different metabolic properties than normal cells, particularly their tendency to undergo glycolysis even under aerobic favoring conditions. This has created interest in how mitochondrial function in tumor cells may differ from that in normal cells. Using human malignant cells (SW-620, PC-3, HT-1080, SK-MEL, HL-60, K-562 and MOLT-3), human fibroblast (CCL-153) and human T Cells, we investigated three key parameters that have been typically to describe mitochondrial function: cellular ATP production, mitochondrial potential and cellular cardiolipin levels. On average, tumor cancer cells had more ATP production and greater mitochondrial potentials. For example, ATP levels in malignant cells ranged from 20 to 69 µmole/10⁶ cells, with a cancer cell average of 40±18 µmole/10⁶ cells. For normal cells, the ATP level range went from 9 to 24 µmole/106 cells, for an average of 15±11 µmole/106 cells. Total mitochondrial potentials tended to be three times higher in cancer cells, perhaps because overall mitochondrial mass (as measured by relative cardiolipin levels) were twice as high in cancer cells.

Higher total mitochondrial masses are consistent with proliferation. Proliferating cells in general showed higher mitochondrial function compared to quiescent cells (confluent monolayers), and HL-60 cells showed reductions in all three mitochondrial parameters measured here when the cells were exposed to the differentiating agent TPA.

The effects of ATP production inhibitors CCCP and oligomycin on mitochondrial function in normal and cancer cells were also compared. In general, in these experiments, cancer cell mitochondrial inhibition with these agents produced a decrease ATP levels by 30-40% while in normal cells ATP production was reduced by 60%. These results provide evidence of a mitochondrial dysfunction in cancer cells. Cancer cells appear to better withstand interference with ATP synthesis in mitochondria since they rely mainly on glycolysis as an energy producing mechanism.

Our results can be summarized as following:

• Our data support the idea that cancer cells rely more in glycolysis as their main energy production mechanism. We found an increased glycolysis in transformed cells in comparison with normal cells.
• Inhibition of ATP production by OxPhos by addition of CCCP, a pure uncoupler that acts as ionophore, completely dissipating the chemiosmotic gradient, but leaving the  electrotransport system uninhibited resulted in a decrease mitochondrial potential. The addition of CCCP with the antibiotic oligomycin or oligomycin alone, that acts by binding ATP synthase in such a way that blocks the proton channel, in which the ETC runs but no ATP synthesis occurs, the inhibition of ATP production was 60%-70% in normal cells in comparison 30%-40% in cancer cells showing that energy production in cancer cells is mainly from glycolysis.
• The same results were found for transformed and differentiated leukemia cells HL-60. Cells were differentiated by incubation with different concentrations of TPA. As the result of differentiation the mitochondrial potential and ATP production were decreased 2-3 times. The evaluation of the contribution of glycolysis and OxPhos to the maintenance of ATP content showed the decreased level of ATP (approximately 2 times for the highest concentrations of inhibitors) in differentiated cells in comparison with the transformed cells.
• To find if the shift to a glycolytic pathway is increased in cancer cells because glycolysis is required for cellular proliferation, we compared the levels of ATP and mitochondrial potential for normal fibroblast cells (CCD153) at the stages of confluence and proliferation. According to our data there was increase in the mitochondrial potential, mass and energy production in proliferative cells in comparison to confluent cells.
• We also showed that the growth of tumorigenic and non-tumorigenic cells in typical cell culture media increase cardiolipin, the signature phospholipid of the inner mitochondrial membrane. According to our data the amount of these proteins correlated with mitochondrial potential.
• In the experiments with depolarization of mitochondria in normal and transformed blood cells (T cells, K-562, HL-60, and Molt-3, normal and transformed fibroblasts: SK-MES (skin melanoma), CCCD -18lu (normal lung fibroblast) and CCD-18co (normal colon fibroblast)), cells were exposed to different concentrations of proton ionophore CCCP during 1-2 hours and level of potential was analyzed in intact cells without lysis. For all cell lines, level of intensity at 595nm due to accumulation JC-1 in mitochondria was decreased and depended on CCCP concentration, but there were not greater reduction of mitochondrial potential in cancer cells in comparison to normal cells.
• Bioenergetics of Human Cancer Cells and Normal Cells. Nina A. Mikirova. British Journal of Medicine and Medical Research, ISSN: 2231-0614,Vol.: 7, Issue.: 12

Nutrition is the foundation and basis of good health; therefore, it stands to reason that a proper diet would assist in the prevention of common 21st century chronic diseases such as heart disease, diabetes, neurodegenerative diseases, and cancer. In this article we explain the roles of mitochondria in health, and the biochemistry of mitochondria in degenerative disease. We examine the role of oxygen in both (aerobic) oxidative phosphorylation (OxPhos) and (anaerobic) glycolysis, and how the latter may contribute to chronic disease states. We discuss the biochemical mechanisms behind adenosine triphosphate production and the simultaneous production of Reactive Oxygen Species (ROS) (free radicals), and the chronic effects of cellular ROS damage. Lastly, we discuss the cellular health-enhancing effects of reductive molecules (antioxidants) and an alkaline environment, and how this contrasts with an acidic environment/ diet, which contributes to chronic disease and the pathological state.

The review of the role of mitochondria in cancer and other diseases was published in journal:

• The Role of Mitochondria in Cancer and Other Chronic Diseases. Dorothy D Zeviar, Michael J Gonzalez, Jorge R Miranda Massari, Jorge Duconge, Nina Mikirova. Journal of Orthomolecular Medicine, 2014, Vol 29, No 4: 157-166