Reverse Mitochondrial Dysfunction: Mito~Direct™

http://www.ncbi.nlm.nih.gov/pubmed/23378588

Cold Spring Harb Perspect Biol. 2013 Feb 1

Relevance of mitochondrial genetics and metabolism in cancer development.

Abstract

Cancer cells are characterized in general by a decrease of mitochondrial respiration and oxidative phosphorylation, together with a strong enhancement of glycolysis, the so-called Warburg effect. The decrease of mitochondrial activity in cancer cells may have multiple reasons, related either to the input of reducing equivalents to the electron transfer chain or to direct alterations of the mitochondrial respiratory complexes. In some cases, the depression of respiratory activity is clearly the consequence of disruptive mitochondrial DNA (mtDNA) mutations and leads as a consequence to enhanced generation of reactive oxygen species (ROS). By acting both as mutagens and cellular mitogens, ROS may contribute directly to cancer progression.

On the basis of our experimental evidence, we suggest a deep implication of the supercomplex organization of the respiratory chain as a missing link between oxidative stress, energy failure, and tumorigenesis. We speculate that under conditions of oxidative stress, a dissociation of mitochondrial supercomplexes occurs, with destabilization of complex I and secondary enhanced generation of ROS, thus leading to a vicious circle amplifying mitochondrial dysfunction. An excellent model to dissect the role of pathogenic, disassembling mtDNA mutations in tumor progression and their contribution to the metabolic reprogramming of cancer cells (glycolysis vs. respiration) is provided by an often underdiagnosed subset of tumors, namely, the oncocytomas, characterized by disruptive mutations of mtDNA, especially of complex I subunits. Such mutations almost completely abolish complex I activity, which slows down the Krebs cycle, favoring a high ratio of α-ketoglutarate/succinate and consequent destabilization of hypoxia inducible factor 1α (HIF1α).

On the other hand, if complex I is partially defective, the levels of NAD(+) may be sufficient to implement the Krebs cycle with higher levels of intermediates that stabilize HIF1α, thus favoring tumor malignancy. The threshold model we propose, based on the population-like dynamics of mitochondrial genetics (heteroplasmy vs. homoplasmy), implies that below threshold complex I is present and functioning correctly, thus favoring tumor growth, whereas above threshold, when complex I is not assembled, tumor growth is arrested. We have therefore termed "oncojanus" the mtDNA genes whose disruptive mutations have such a double-edged effect.

Mitochondria have an essential role in powering cells by generating ATP following the metabolism of pyruvate derived from glycolysis. They are also the major source of generating reactive oxygen species (ROS), which have regulatory roles in cell death and proliferation. Mutations in mitochondrial DNA (mtDNA) and dysregulation of mitochondrial metabolism have been frequently described in human tumors. Although the role of oxidative stress as the consequence of mtDNA mutations and/or altered mitochondrial functions has been demonstrated in carciongenesis, a causative role of mitochondria in tumor progression has only been demonstrated recently.

Specifically, the subject of this mini-review focuses on the role of mitochondria in promoting cancer metastasis. Cancer relapse and the subsequent spreading of cancer cells to distal sites are leading causes of morbidity and mortality in cancer patients. Despite its clinical importance, the underlying mechanisms of metastasis remain to be elucidated. Recently, it was demonstrated that mitochondrial oxidative stress could actively promote tumor progression and increase the metastatic potential of cancer cells. The purpose of this mini-review is to summarize current investigations of the roles of mitochondria in cancer metastasis. Future development of diagnostic and therapeutic strategies for patients with advanced cancer will benefit from the new knowledge of mitochondrial metabolism in epithelial cancer cells and the tumor stroma.

Warburg effect is a hallmark of cancer manifested by continuous prevalence of glycolysis and dysregulation of oxidative metabolism. Glycolysis provides survival advantage to cancer cells. To investigate molecular mechanisms underlying the Warburg effect, we first compared oxygen consumption among hFOB osteoblasts, benign osteosarcoma cells, Saos2, and aggressive osteosarcoma cells, 143B. We demonstrate that, as both proliferation and invasiveness increase in osteosarcoma, cells utilize significantly less oxygen. We proceeded to evaluate mitochondrial morphology and function. Electron microscopy showed that in 143B cells, mitochondria are enlarged and increase in number.

Quantitative PCR revealed an increase in mtDNA in 143B cells when compared with hFOB and Saos2 cells. Gene expression studies showed that mitochondrial single-strand DNA-binding protein (mtSSB), a key catalyst of mitochondrial replication, was significantly up-regulated in 143B cells. In addition, increased levels of the mitochondrial respiratory complexes were accompanied by significant reduction of their activities. These changes indicate hyperactive mitochondrial replication in 143B cells. Forced overexpression of mtSSB in Saos2 cells caused an increase in mtDNA and a decrease in oxygen consumption. In contrast, knockdown of mtSSB in 143B cells was accompanied by a decrease in mtDNA, increase in oxygen consumption, and retardation of cell growth in vitro and in vivo.

In summary, we have found that mitochondrial dysfunction in cancer cells correlates with abnormally increased mitochondrial replication, which according to our gain- and loss-of-function experiments, may be due to overexpression of mtSSB. Our study provides insight into mechanisms of mitochondrial dysfunction in cancer and may offer potential therapeutic targets.

Mitochondria play essential roles in cellular metabolism, redox homeostasis, and regulation of cell death. Emerging evidences suggest that cancer cells exhibit various degrees of mitochondrial dysfunctions and metabolic alterations, which may serve as a basis to develop therapeutic strategies to preferentially kill the malignant cells. Mitochondria as a therapeutic target for cancer treatment is gaining much attention in the recent years, and agents that impact mitochondria with anticancer activity have been identified and tested in vitro and in vivo using various experimental systems. Anticancer agents that directly target mitochondria or indirectly affect mitochondrial functions are collectively classified as mitocans.

This review article focuses on several natural compounds that preferentially kill cancer cells with mitochondrial dysfunction, and discusses the possible underlying mechanisms and their therapeutic implications in cancer treatment. Mitocans that have been comprehensively reviewed recently are not included in this article. Important issues such as therapeutic selectivity and the relevant biochemical basis are discussed in the context of future perspectives.

The role of oncoproteins and tumor suppressor proteins in promoting the malignant transformation of mammalian cells by affecting properties such as proliferative signalling, cell cycle regulation and altered adhesion is well established. Chemicals, viruses and radiation are also generally accepted as agents that commonly induce mutations in the genes encoding these cancer-causing proteins, thereby giving rise to cancer. However, more recent evidence indicates the importance of two additional key factors imposed on proliferating cells that are involved in transformation to malignancy and these are hypoxia and/or stressful conditions of nutrient deprivation (e.g. lack of glucose). These two additional triggers can initiate and promote the process of malignant transformation when a low percentage of cells overcome and escape cellular senescence.

It is becoming apparent that hypoxia causes the progressive elevation in mitochondrial ROS production (chronic ROS) which over time leads to stabilization of cells via increased HIF-2alpha expression, enabling cells to survive with sustained levels of elevated ROS. In cells under hypoxia and/or low glucose, DNA mismatch repair processes are repressed by HIF-2alpha and they continually accumulate mitochondrial ROS-induced oxidative DNA damage and increasing numbers of mutations driving the malignant transformation process. Recent evidence also indicates that the resulting mutated cancer-causing proteins feedback to amplify the process by directly affecting mitochondrial function in combinatorial ways that intersect to play a major role in promoting a vicious spiral of malignant cell transformation. Consequently, many malignant processes involve periods of increased mitochondrial ROS production when a few cells survive the more common process of oxidative damage induced cell senescence and death. The few cells escaping elimination emerge with oncogenic mutations and survive to become immortalized tumors.

This review focuses on evidence highlighting the role of mitochondria as drivers of elevated ROS production during malignant transformation and hence, their potential as targets for cancer therapy. The review is organized into five main sections concerning different aspects of "mitochondrial malignancy". The first concerns the functions of mitochondrial ROS and its importance as a pacesetter for cell growth versus senescence and death. The second considers the available evidence that cellular stress in the form of hypoxic and/or hypoglycaemic conditions represent two of the major triggering events for cancer and how oncoproteins reinforce this process by altering gene expression to bring about a common set of changes in mitochondrial function and activity in cancer cells. The third section presents evidence that oncoproteins and tumor suppressor proteins physically localize to the mitochondria in cancer cells where they directly regulate malignant mitochondrial programs, including apoptosis. The fourth section covers common mutational changes in the mitochondrial genome as they relate to malignancy and the relationship to the other three areas. The last section concerns the relevance of these findings, their importance and significance for novel targeted approaches to anti-cancer therapy and selective triggering in cancer cells of the mitochondrial apoptotic pathway.