The mode of mitochondrial degeneration in gliomas

Cancer Etiology: A Metabolic Disease Originating from Life's Major Evolutionary Transition?

A clear understanding of the origins of cancer is the basis of successful strategies for effective cancer prevention and management. The origin of cancer at the molecular and cellular levels is not well understood. Is the primary cause of the origin of cancer the genomic instability or impaired energy metabolism? An attempt was made to present cancer etiology originating from life's major evolutionary transition. The first evolutionary transition went from simple to complex cells when eukaryotic cells with glycolytic energy production merged with the oxidative mitochondrion (The Endosymbiosis Theory first proposed by Lynn Margulis in the 1960s). The second transition went from single-celled to multicellular organisms once the cells obtained mitochondria, which enabled them to obtain a higher amount of energy. Evidence will be presented that these two transitions, as well as the decline of NAD+ and ATP levels, are the root of cancer diseases. Restoring redox homeostasis and reactivation of mitochondrial oxidative metabolism are important factors in cancer prevention.

Targeting mitochondria as therapeutic strategy for metabolic disorders

Mitochondria are critical regulator of cell metabolism; thus, mitochondrial dysfunction is associated with many metabolic disorders. Defects in oxidative phosphorylation, ROS production, or mtDNA mutations are the main causes of mitochondrial dysfunction in many pathological conditions such as IR/diabetes, metabolic syndrome, cardiovascular diseases, and cancer. Thus, targeting mitochondria has been proposed as therapeutic approach for these conditions, leading to the development of small molecules to be tested in the clinical scenario. Here we discuss therapeutic interventions to treat mitochondrial dysfunction associated with two major metabolic disorders, metabolic syndrome, and cancer. Finally, novel mechanisms of regulation of mitochondrial function are discussed, which open new scenarios for mitochondria targeting.

Cancer as a metabolic disease

Emerging evidence indicates that impaired cellular energy metabolism is the defining characteristic of nearly all cancers regardless of cellular or tissue origin. In contrast to normal cells, which derive most of their usable energy from oxidative phosphorylation, most cancer cells become heavily dependent on substrate level phosphorylation to meet energy demands. Evidence is reviewed supporting a general hypothesis that genomic instability and essentially all hallmarks of cancer, including aerobic glycolysis (Warburg effect), can be linked to impaired mitochondrial function and energy metabolism. A view of cancer as primarily a metabolic disease will impact approaches to cancer management and prevention.

Cardiolipin and electron transport chain abnormalities in mouse brain tumor mitochondria: lipidomic evidence supporting the Warburg theory of cancer

Otto Warburg first proposed that cancer originated from irreversible injury to mitochondrial respiration, but the structural basis for this injury has remained elusive. Cardiolipin (CL) is a complex phospholipid found almost exclusively in the inner mitochondrial membrane and is intimately involved in maintaining mitochondrial functionality and membrane integrity. Abnormalities in CL can impair mitochondrial function and bioenergetics. We used shotgun lipidomics to analyze CL content and composition in highly purified brain mitochondria from the C57BL/6J (B6) and VM/Dk (VM) inbred strains and from subcutaneously grown brain tumors derived from these strains to include an astrocytoma and ependymoblastoma (B6 tumors), a stem cell tumor, and two microgliomas (VM tumors). Major abnormalities in CL content or composition were found in all tumors. The compositional abnormalities involved an abundance of immature molecular species and deficiencies of mature molecular species, suggesting major defects in CL synthesis and remodeling. The tumor CL abnormalities were also associated with significant reductions in both individual and linked electron transport chain activities. A mathematical model was developed to facilitate data interpretation. The implications of our findings to the Warburg cancer theory are discussed.

Pharmacologic manipulations of mitochondrial membrane potential (DeltaPsim) selectively in glioma cells

Metabolic control theory applies principles of bioenergetics for the control or management of complex diseases. Since metabolism is a general process underlying all biologic phenotypes, changes in metabolism can potentially modify phenotype. Therefore, it is reasonable to assume that experimental modulation of the availability of cellular energy can potentially alter cell phenotypes and cell functions critical to tumor progression including cell division. The purpose of this study was to determine if OMX-2, a methylquinone system designed to shuttle electrons from mitochondrial complexes, was able to target mitochondria in cancer cells and trigger cell death. Using flow cytometry, cell viability assays, and ATP measurements, we found that OMX-2 differentially decreased DeltaPsim without triggering cell death. In contrast, known blockers of the Electron Transport Chain (ETC) decreased DeltaPsim and triggered cell death. When normal cells were treated with OMX-2, neither DeltaPsim or cell death was triggered. Furthermore, OMX-2 modulated intracellular ATP and decreased cell numbers of glioma cells. Cell cycle analysis indicated that OMX-2 induced a reversible cell cycle arrest in G1/S. Finally, impairment of glycolysis by 2-Deoxyglucose (2-DOG) acted synergistically with OMX-2 to trigger cell death. Overall, these results indicate that it is possible to selectively target cancer cells by decreasing DeltaPsim and induced cell cycle arrest without triggering cell death. Moreover, pharmacological approaches designed to act on both glycolysis and oxidative phosphorylation can be considered as a new approach to selectively kill cancer cells.

Chlorimipramine: A novel anticancer agent with a mitochondrial target

Mitochondria have been suggested to be a potential intracellular target for cancer chemotherapy. In this report, we demonstrate the ability of the tricyclic antidepressant chlorimipramine to kill human glioma cells in vitro by a molecular mechanism resulting in an increase in caspase 3 activity following inhibition of glioma oxygen consumption. Studies with isolated rat mitochondria showed that chlorimipramine specifically inhibited mitochondrial complex III activity, which causes decreased mitochondrial membrane potential as well as mitochondrial swelling and vacuolation. The use of chlorimipramine in human as an effective, non-toxic cancer therapeutic having a strong selectivity between cancer cells and normal cells on the basis of their mitochondrial function is discussed.

Quantitative evaluation of leukemic mitochondria with a computer-controlled image analyzer

Mitochondria from 25 patients with acute lymphoblastic leukemia (ALL) and 25 patients with acute myelogenous leukemia (AML) were compared in terms of their number, area, and shape index using a computer-controlled image analyzer. The number of mitochondria was greater in the AML than in the ALL patients. However, their size, as measured in electron micrographic profiles was similar in the two groups, in disagreement with conventional reports that mitochondria are small in granulocytes but large in lymphocytes. Two ALL patients had giant mitochondria. The mitochondria of the ALL cells were more irregular than those of the AML cells, and furthermore, within the ALL group, the degree of the irregularity was greater in those with a poor prognosis than in those in longstanding remission. The number of mitochondria was significantly greater in B-cell ALL than in null cell and T-cell ALL.

Reacting ultrastructure of the human oligodendrocyte (a study of cerebral cortex distant to brain tumours)

Electron microscopical examination of adjacent human cerebral cortex from patients with brain tumour revealed the following morphological distinctive features of the oligodendrocyte: the presence of a diffuse electron dense material, large quantities of free ribosomes or ribosomal rosettes, eccentric nucleus with irregular clumps of chromatin, swollen perinuclear cisterna, extensive cytoplasmic microtubules, well-developed Golgi apparatus opposite the eccentric nuclei, dilated endoplasmic reticulum and mitochondria. The majority of oligodendrocytes contain large indefinable heterogeneous electron-dense structures within their perikaryon or processes. Oligodendrocytes with similar inclusion bodies are seen in the vicinity of capillaries. For these reasons a possible phagocytic activity of the human oligodendrocyte seems likely.

The brown bowel syndrome: a possible smooth muscle mitochondrial myopathy?

Two cases of lipofuscinosis of the gastrointestinal tract are described, and a mitochondrial origin of the pigment is proposed. The mechanism of formation of lipofuscin is discussed, with particular reference to the maintenance of structurally and functionally intact mitochondrial membranes. The central role of vitamin E is considered, and its biochemical significance to mitochondrial metabolism is emphasized. Comparisons are drawn between the abnormalities demonstrated within the smooth muscle cells of the two cases described, and the muscle cells observed in cases of skeletal muscle myopathies of mitochondrial origin. It is proposed that lipofuscinosis of the gastrointestinal tract, otherwise known as the 'brown bowel syndrome', may be regarded as a smooth muscle myopathy of mitochondrial origin.