Essential Role for Oxidative Phosphorylation in Cancer Progression

Cancers are often affected by derangements in mitochondrial (mt) function, as well as mtDNA mutations. In this issue, Tan et al., 2015Tan A.S. Baty J.W. Dong L.-F. Bezawork-Geleta A. Endaya B. Goodwin J. Bajzikova M. Kovarova J. Peterka M. Yan B. et al.Cell Metab. 2015; 21 (this issue): 81-94Abstract Full Text Full Text PDF PubMed Scopus (452) Google Scholar demonstrate that only mtDNA-depleted cancer cells capable of recovering mtDNA from the host form metastasizing cancers in vivo, revealing an essential requirement for oxidative phosphorylation in tumor progression. Cancers are often affected by derangements in mitochondrial (mt) function, as well as mtDNA mutations. In this issue, Tan et al., 2015Tan A.S. Baty J.W. Dong L.-F. Bezawork-Geleta A. Endaya B. Goodwin J. Bajzikova M. Kovarova J. Peterka M. Yan B. et al.Cell Metab. 2015; 21 (this issue): 81-94Abstract Full Text Full Text PDF PubMed Scopus (452) Google Scholar demonstrate that only mtDNA-depleted cancer cells capable of recovering mtDNA from the host form metastasizing cancers in vivo, revealing an essential requirement for oxidative phosphorylation in tumor progression. For close to one century, it has been known that cancer cells tend to assimilate glucose and other building blocks for anabolic reactions at a higher pace than their normal counterparts and that this peculiarity is coupled to comparatively low levels of oxidative phosphorylation (Galluzzi et al., 2013Galluzzi L. Kepp O. Vander Heiden M.G. Kroemer G. Nat. Rev. Drug Discov. 2013; 12: 829-846Crossref PubMed Scopus (521) Google Scholar). One explanation that has advanced for this so-called "Warburg phenomenon" is a mitochondrial defect that would be explained by mutations in mitochondrial DNA (mtDNA), reductions in mtDNA copy numbers, and/or reduced transcription and translation of nuclear genes coding for mitochondrial proteins (Brandon et al., 2006Brandon M. Baldi P. Wallace D.C. Oncogene. 2006; 25: 4647-4662Crossref PubMed Scopus (632) Google Scholar, Wallace, 2012Wallace D.C. Nat. Rev. Cancer. 2012; 12: 685-698Crossref PubMed Scopus (1433) Google Scholar). While it is as yet impossible to engineer stable cell lines that lack mitochondria, it is feasible to culture cells in conditions that preclude the replication of mtDNA in specific auxotrophic media (that contain extra pyruvate, as well as uridine), thus resulting in the generation of mtDNA null (or so-called ρ°) cells, in which four out of the five respiratory chain complexes lack mtDNA-encoded subunits. ρ° cells exhibit a complete deficiency in oxidative phosphorylation and hence contain mitochondria that consume rather than generate ATP to maintain their essential metabolic functions (Wallace, 2012Wallace D.C. Nat. Rev. Cancer. 2012; 12: 685-698Crossref PubMed Scopus (1433) Google Scholar). Tan et al., 2015Tan A.S. Baty J.W. Dong L.-F. Bezawork-Geleta A. Endaya B. Goodwin J. Bajzikova M. Kovarova J. Peterka M. Yan B. et al.Cell Metab. 2015; 21 (this issue): 81-94Abstract Full Text Full Text PDF PubMed Scopus (452) Google Scholar now show that two murine cancer cell lines (B16 melanoma and 4T1 breast carcinoma), which had been rendered ρ° in vitro, formed tumors in vivo with a latency of several weeks compared to their mtDNA-sufficient (ρ+) parental controls. Once these delayed tumors emerged, they progressed as rapidly as ρ+ controls. The authors recovered the in vivo derivatives of the originally ρ° cells at different stages of malignant progression by isolating primary tumors from the subcutaneous inoculation site, circulating tumor cells, as well as lung metastases. In contrast to ρ° cells cultured in vitro, their in vivo products rapidly formed tumors upon re-injection into mice. Moreover, the in vivo derivatives of ρ° cells exhibited a tumor progression-related recovery in mitochondrial biochemistry, ultrastructure, and function. Hence, developing and progressing tumors were characterized by the reformation of respiratory chain supercomplexes, the reacquisition of normal mitochondrial morphology with cristae, as well as the restoration of oxidative phosphorylation. This gradual recovery was correlated with a relative increase in the abundance of mtDNA, which augmented with each step of tumor progression, reaching close-to-normal levels as the tumor cells attained the blood stream for metastatic dissemination (Figure 1A). This mtDNA was transcribed in a normal fashion, hence fully contributing to mitochondrial biogenesis in formerly ρ° cells. Taking advantage of polymorphisms in the mtDNA sequence between different mouse strains and cell lines, Tan et al., 2015Tan A.S. Baty J.W. Dong L.-F. Bezawork-Geleta A. Endaya B. Goodwin J. Bajzikova M. Kovarova J. Peterka M. Yan B. et al.Cell Metab. 2015; 21 (this issue): 81-94Abstract Full Text Full Text PDF PubMed Scopus (452) Google Scholar discovered that the mtDNA contained in these progressing cancers was not amplified from residual mtDNA that might have contaminated the original cancer cell line, but rather was acquired from the host. Hence, ρ° cells somehow recover mtDNA from the host, which restores their metabolic capacities to a level compatible with tumor progression. How can mtDNA be transferred from one cell to another? Although it is technically feasible to transfect mtDNA isolated from one cell line into another one, this procedure generally does not cause the mtDNA to be introduced into the mitochondrial matrix and hence fails to restore normal mitochondrial biogenesis. To functionally transmit mtDNA, it is necessary to transfer entire mtDNA-containing mitochondria from ρ+ cells to ρ° cells. Although this can be achieved in vitro by microinjecting isolated ρ+ organelles into ρ° cells, the standard procedure to transfer mtDNA from one cell to the other is more sophisticated. Usually, the mtDNA-containing cell is enucleated (meaning that its only DNA is mitochondrial) and then fused with the ρ° cell (whose only DNA is nuclear) resulting in a cytoplasmic hybrid (or "cybrid") that re-unites mtDNA from one cell and nuclear DNA from another cell (Schon et al., 2012Schon E.A. DiMauro S. Hirano M. Nat. Rev. Genet. 2012; 13: 878-890Crossref PubMed Scopus (512) Google Scholar). How, then, is mtDNA from the host transferred into the initially ρ° cancer cell? Given the aforementioned experience in laboratory technology, it appears improbable that ρ° cells subjected to Darwinian selection in vivo would have developed a strategy to selectively take up mtDNA from the host (Figure 1B). It appears far more plausible that they incorporate entire host mitochondria (Rogers and Bhattacharya, 2013Rogers R.S. Bhattacharya J. Physiology (Bethesda). 2013; 28: 414-422Crossref PubMed Scopus (61) Google Scholar). How could this be achieved? Tumor cells might acquire host cell organelles by phagocytosis, although the engulfed cargo would then usually be targeted for lysosomal destruction, unless the host mitochondrion manages to escape the phagocytic vesicle (Figure 1C). Such an event of "escape" might occur at a low frequency, explaining why ρ° cancers become palpable approximately 20 days later than their mtDNA-sufficient counterparts (Tan et al., 2015Tan A.S. Baty J.W. Dong L.-F. Bezawork-Geleta A. Endaya B. Goodwin J. Bajzikova M. Kovarova J. Peterka M. Yan B. et al.Cell Metab. 2015; 21 (this issue): 81-94Abstract Full Text Full Text PDF PubMed Scopus (452) Google Scholar). Alternatively, tumor cells might fuse with host cells (Figure 1D). Such somatic cell-cell fusion events have been documented in oncogenesis (Vitale et al., 2011Vitale I. Galluzzi L. Senovilla L. Criollo A. Jemaà M. Castedo M. Kroemer G. Cell Death Differ. 2011; 18: 1403-1413Crossref PubMed Scopus (106) Google Scholar). However, at difference with the cybrid technology described above, host cells, which usually are nucleated, would be expected to contribute both their mtDNA and nuclear DNA to the resulting somatic hybrid cells. Cell fusion among somatic cells is well documented to result in a non-physiological increase in ploidy (with consequent DNA damage responses and endoplasmic reticulum stress), usually resulting in apoptotic suicide, senescence, or immune system-mediated elimination of the hyperploid product (Senovilla et al., 2012Senovilla L. Vitale I. Martins I. Tailler M. Pailleret C. Michaud M. Galluzzi L. Adjemian S. Kepp O. Niso-Santano M. et al.Science. 2012; 337: 1678-1684Crossref PubMed Scopus (314) Google Scholar). However, at least occasionally such hyperploid cells may succeed in reducing their ploidy, hence generating highly tumorigenic cells containing a close-to-normal number of chromosomes (Vitale et al., 2011Vitale I. Galluzzi L. Senovilla L. Criollo A. Jemaà M. Castedo M. Kroemer G. Cell Death Differ. 2011; 18: 1403-1413Crossref PubMed Scopus (106) Google Scholar). In order to discriminate between these potential scenarios, it will be important to analyze the cancers generated from originally ρ° cells for changes in their ploidy, as well as for the relative contribution of nuclear DNA from tumor and host cells. The polyploidization-depolyploidization cycle that frequently accompanies malignant progression (Vitale et al., 2011Vitale I. Galluzzi L. Senovilla L. Criollo A. Jemaà M. Castedo M. Kroemer G. Cell Death Differ. 2011; 18: 1403-1413Crossref PubMed Scopus (106) Google Scholar) could have caused mixing of the nuclear genomes from tumor and host cells accompanied by recovery of mtDNA. Irrespective of this incognita, the results by Tan et al., 2015Tan A.S. Baty J.W. Dong L.-F. Bezawork-Geleta A. Endaya B. Goodwin J. Bajzikova M. Kovarova J. Peterka M. Yan B. et al.Cell Metab. 2015; 21 (this issue): 81-94Abstract Full Text Full Text PDF PubMed Scopus (452) Google Scholar underscore the probable importance of mtDNA-dependent mitochondria biogenesis for oncogenesis and tumor progression. If mitochondrial respiration were absolutely required for the fitness of cancer stem cells, it would be possible to gauge tumor progression by administering pharmacological inhibitors of respiratory chain complexes, as this has been exemplified for KRAS-induced cancers (Shackelford et al., 2013Shackelford D.B. Abt E. Gerken L. Vasquez D.S. Seki A. Leblanc M. Wei L. Fishbein M.C. Czernin J. Mischel P.S. Shaw R.J. Cancer Cell. 2013; 23: 143-158Abstract Full Text Full Text PDF PubMed Scopus (420) Google Scholar, Viale et al., 2014Viale A. Pettazzoni P. Lyssiotis C.A. Ying H. Sánchez N. Marchesini M. Carugo A. Green T. Seth S. Giuliani V. et al.Nature. 2014; 514: 628-632Crossref PubMed Scopus (787) Google Scholar). The report by Tan et al. now suggests that mtDNA-dependent metabolic processes, most likely oxidative phosphorylation, can be therapeutically targeted in a vast spectrum of different cancers. Irrespective of this speculation, the paper by Tan et al. highlights the malicious capacity of tumor cells to acquire mtDNA from benign bystander cells. Mitochondrial Genome Acquisition Restores Respiratory Function and Tumorigenic Potential of Cancer Cells without Mitochondrial DNATan et al.Cell MetabolismJanuary 06, 2015In BriefMitochondrial genome acquisition from cells in the tumor microenvironment restores tumorigenicity and respiration in cells lacking mtDNA. Cell lines derived from primary and circulating tumor cells and lung metastases that grew from these cells showed stepwise recovery of tumorigenicity and respiration that was associated with respirasome and complex II assembly. Full-Text PDF Open Archive