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Mitochondria, “cellular power plants”, are double membrane organelles where the inner membrane holds a series of redox catalysts (i.e., pyridine nucleotides, flavoproteins, iron sulfur proteins, ubiquinones, and cytochromes) that are assembled into four major complexes. The electron transport chain (ETC) proteins are arranged according to their redox potentials; the flow of electrons down the chain is accompanied by the pumping of protons out of the inner mitochondrial membrane creating a proton gradient that drives the production of ATP from ADP + Pi. Oxygen is the terminal electron acceptor for the ETC. Compared to glycolysis, mitochondrial respiration and OXPHOS are more efficient in generating ATP from glucose. Mitochondria being the major source of reactive oxygen species (ROS), it is widely believed that in addition to energy production mitochondria may have a regulatory role in numerous cellular processes, including proliferation.
Cancer is a disease of aberrant proliferation. The possible role of mitochondrial function and carcinogenesis was first proposed by the German scientist Otto Warburg as a shift in cancer cell energy production from respiration to aerobic glycolysis, even in the presence of abundant oxygen (Warburg effect)1. The cellular and molecular mechanisms of the Warburg effect and its role in carcinogenesis are not completely understood.
Numerous studies report the association of mutations in the mitochondrial DNA (mtDNA) with the carcinogenesis process: somatic mutations, reduced amount of mtDNA, and decreased expression of the mitochondrial genes2. However, the causality of mtDNA mutations in carcinogenesis is not completely understood. The mitochondrial genome is a 16.6 kb closed circular double-helix that encodes for 13 polypeptides of the oxidative phosphorylation (OXPHOS) subunits, 22 tRNAs, 2 rRNAs, and the displacement loop (D-loop). The 13 polypeptides include 7 subunits of complex I, 1 subunit of complex III, 3 subunits of complex IV, and 2 subunits of complex V. All other mitochondrial proteins are encoded by nuclear genes, and the proteins are transported into the mitochondrion. In general, mammalian cells contain approximately 10,000 copies of mtDNA. This is an enormous technical challenge for designing experiments to determine whether mtDNA mutations regulate the carcinogenesis process.
In this issue of Cancer Biology & Therapy Kulawiec et al.3 applied a state of the art cytoplasmic hybrid, cybrid, technology to determine if mutations in mtDNA confer cancer cell metastasis. A cybrid is a hybrid cell that contains the nuclear genome from one source and the mitochondrial genome from another source. This state of the art technology allows researchers to distinguish the genetic contributions of the mitochondrial genome from that of the nuclear genome. The authors performed an entire mtDNA genome sequence analysis of MDA-MB-435, MDA-MB-231, and MCF-7 human breast cancer cells. These results revealed mutations in the D-loop region, complex I (ND4 and ND5), and complex IV (COI) that were common in all three cell types as well as mutations that were specific to each cell type. The authors identified an A to G transition mutation at position 12308 in the tRNA LeuCUN gene that was specific to the MDA-MB-435 cell line. Due to the pathogenicity associated with A12308G mutation and a higher incidence of mtDNA mutations, Kulawiec et al.3 selected the MDA-MB-435 cell line to generate cybrids. Cybrids carrying mtDNA mutations exhibited a higher frequency of lung metastasis compared to cybrids carrying wild type mtDNA. These results unequivocally demonstrated a regulatory role of mtDNA mutations, possibly due to the mutation in the tRNA LeuCUN gene, in cancer cell metastasis.
A similar observation was recently reported by Ishikawa et al.4. These authors also used the cybrid technology to swap mitochondria in mouse tumor cells with low (P29) and high (A11) metastatic potentials. Cybrids carrying mitochondria from A11 cells exhibited higher incidence of lung metastasis compared to cybrids carrying mitochondria from P29 cells. mtDNA sequence analysis of A11 cells identified mutations (missense, G13997A, and frame-shift, 13885insC) in the gene encoding NADH (reduced form of nicotinamide adenine dinucleotide) dehydrogenase subunit 6 (ND6), which was associated with a decrease in complex I activity. Interestingly, the transfer of mtDNA from A11 metastatic mouse tumor cells to non-transformed NIH3T3 mouse fibroblasts did not induce tumorigenicity or metastasis, suggesting that a defect in complex I activity alone might not be sufficient for transformation and metastasis.
Results from the Kulawiec et al.3 and Ishikawa et al.4 suggest that mutations in mtDNA could confer cancer cell metastasis. Both of these two studies attempted to address the downstream signaling pathways regulating mtDNA mutations-related cancer cell metastasis. Kulawiec et al.3 suggested that an AKT-dependent but reactive species (ROS) independent pathway could regulate mtDNA mutations-related cancer cell metastasis. In contrast, Ishikawa et al.4 suggested that the ROS-signaling and expression of ROS-sensitive genes could link mtDNA mutations to cancer cell metastasis.
In recent years, cellular oxidation and reduction (redox) environment has gained significant attention as a critical regulator of human health and disease. Cellular redox environment is a balance between the production of ROS and their removal by antioxidants. ROS are oxygen containing molecules that are highly reactive in redox reactions. ROS are primarily produced intracellularly by two metabolic sources: the mitochondrial electron transport chain (ETC) and oxygen metabolizing enzymatic reactions (Figure 1). Approximately 98–99% of all mitochondrial oxygen consumption is efficiently reduced by Complex IV5. Despite this high efficiency, the 1-electron reduction of oxygen at Complexes I and III is known to generate superoxide. Once formed, superoxide is rapidly converted to hydrogen peroxide via the spontaneous (105 mol−1 sec−1) or enzymatic (109 mol−1 sec−1) driven dismutation reaction. Hydrogen peroxide is neutralized both by catalase and glutathione peroxide. Hydrogen peroxide reacts with transition metal ions (e.g. cuprous and ferrous ions) through Fenton and Haber-Weiss chemistry generating the highly reactive hydroxyl radical that is well known to cause damage to cellular macromolecules (Figure 1).
Superoxide could alter the redox-state of metal cofactors (e.g. Fe) present in many kinases and phosphatases (one-electron reactions), while hydrogen peroxide could influence the redox state of protein thiols (two-electron reactions). Phosphatases are susceptible to oxidation by hydrogen peroxide because of the lowered pKa of the active site cysteine. The PTEN tumor suppressor (Phosphatase and Tensin homolog deleted from chromosome 10) is a protein tyrosine phosphatase that is known to regulate entry into the cell cycle, metastasis, motility, apoptosis, cell growth and size6. Thiol-disulfide redox reactions between cys71 and cys124 reversibly regulate PTEN phosphatase activity; the oxidized form is inactive and the reduced form has phosphatase activity7. Redox regulation of PTEN increases PIP3 levels, which facilitates recruitment of protein kinase-B (AKT) to the cell membrane. AKT is phosphorylated at threonine-308 by phosphoinositide dependent kinase 1 (PDK-1) and at serine 473 by PDK-2 (also known as mTORC2, mammalian target of rapamycin)8. Phosphorylated AKT is a serine/threonine kinase that phosphorylates numerous proteins including proteins that are involved in cellular metabolism, proliferation, metastasis, and inhibition of cell death (Figure 1).
Kulaweic et al.3 showed an increase in the basal level of AKT-phosphorylation (Ser-473) in the cybrids carrying mtDNA mutations correlating with an increase in resistance to etoposide-induced apoptosis. However, results from the flow cytometry measurements of DCF and DHE fluorescence did not show any measurable difference between the wild type and mutant mtDNA carrying cybrids, suggesting that mtDNA mutations, primarily tRNA LeuCUN, might not perturb cellular ROS levels. The earlier study by Ishikawa et al.4 demonstrated an increase in ROS levels and ROS-sensitive gene expressions correlating with the mtDNA mutations-related, primarily mutation in ND6, cancer cell metastasis. The authors conclude that superoxide and hydrogen peroxide generating mtDNA mutations could regulate cancer cells metastasis. However, the specificity of these assays for the measurements of superoxide and hydrogen peroxide were not addressed. The complexity of the DHE and DCF chemistry9 and the failure to evaluate the specificity of these assays for measurements of superoxide and hydrogen peroxide in both of these studies led to a conclusion of ROS independent3 and dependent4 regulation of mtDNA mutations-related cancer cell metastasis. In summary, while both studies3, 4 did provide compelling evidence for mtDNA mutations and cancer cell metastasis, the quest for a mechanism remains elusive.
I thank Ms. Amanda L. Kalen for critical reading of the manuscript and Mr. Gareth Smith for help with the illustration. Funding from NIH CA 111365 supported this work.
There is no conflict of interest