Apoptosis of cells lacking mitochondrial DNA

Melanoma metabolism contributes to the cellular responses to MAPK/ERK pathway inhibitors

BACKGROUND

Besides its influence on survival, growth, proliferation, invasion and metastasis, cancer cell metabolism also greatly influences the cellular responses to molecular-targeted therapies.

SCOPE OF THE REVIEW

To review the recent advances in elucidating the metabolic effects of BRAF and MEK inhibitors (clinical inhibitors of the MAPK/ERK pathway) in melanoma and discuss the underlying mechanisms involved in the way metabolism can influence melanoma cell death and resistance to BRAF and MEK inhibitors. We also underlined the therapeutic perspectives in terms of innovative drug combinations.

MAJOR CONCLUSION

BRAF and MEK inhibitors inhibit aerobic glycolysis and induce high levels of metabolic stress leading to effective cell death by apoptosis in BRAF-mutated cancer cells. An increase in mitochondrial metabolism is required to survive to MAPK/ERK pathway inhibitors and the sub-population of cells that survives to these inhibitors are characterized by mitochondrial OXPHOS phenotype. Consequently, mitochondrial inhibition could be combined with oncogenic "drivers" inhibitors of the MAPK/ERK pathway for improving the efficacy of molecular-targeted therapy.

GENERAL SIGNIFICANCE

Metabolism is a key component of the melanoma response to BRAF and/or MEK inhibitors. Mitochondrial targeting may offer novel therapeutic approaches to overwhelm the mitochondrial addiction that limits the efficacy of BRAF and/or MEK inhibitors. These therapeutic approaches might be quickly applicable to the clinical situation.

Inhibition of mitochondrial respiration mediates apoptosis induced by the anti-tumoral alkaloid lamellarin D

Lamellarin D (Lam D), a marine alkaloid, exhibits a potent cytotoxicity against many different tumors. The pro-apoptotic function of Lam D has been attributed to its direct induction of mitochondrial permeability transition (MPT). This study was undertaken to explore the mechanisms through which Lam D promotes changes in mitochondrial function and as a result apoptosis. The use of eight Lam derivatives provides useful structure-apoptosis relationships. We demonstrate that Lam D and structural analogues induce apoptosis of cancer cells by acting directly on mitochondria inducing reduction of mitochondrial membrane potential, swelling and cytochrome c release. Cyclosporin A, a well-known inhibitor of MPT, completely prevents mitochondrial signs of apoptosis. The drug decreases calcium uptake by mitochondria but not by microsomes indicating that Lam D-dependent permeability is specific to mitochondrial membranes. In addition, upon Lam D exposure, a rapid decline of mitochondrial respiration and ATP synthesis occurs in isolated mitochondria as well as in intact cells. Evaluation of the site of action of Lam D on the electron-transport chain revealed that the activity of respiratory chain complex III is reduced by a half. To determine whether Lam D could induce MPT-dependent apoptosis by inhibiting mitochondrial respiration, we generated respiration-deficient cells (rho0) derived from human melanoma cells. In comparison to parental cells, rho0 cells are totally resistant to the induction of MPT-dependent apoptosis by Lam D. Our results indicate that functional mitochondria are required for Lam D-induced apoptosis. Inhibition of mitochondrial respiration is responsible for MPT-dependent apoptosis of cancer cells induced by Lam-D.

Overcoming chemoresistance of non-small cell lung carcinoma through restoration of an AIF-dependent apoptotic pathway

Non-small cell lung carcinomas (NSCLCs) are typically resistant against apoptosis induced by standard chemotherapy. We evaluated the effects of the two potential antitumor agents of the lamellarin class on a highly apoptosis-resistant NSCLC cell line. Both the marine alkaloid lamellarin-D and its synthetic amino derivative PM031379 induced the activation of Bax, the mitochondrial release of cytochrome c and apoptosis-inducing factor (AIF), as well as the activation of caspase-3. However, only PM031379 triggered cell death and sign of nuclear apoptosis coupled to the nuclear translocation of AIF. Depletion of AIF with small interfering RNA or microinjection of a neutralizing anti-AIF antibody largely prevented PM031379-induced cytotoxicity, underscoring the essential contribution of AIF to NSCLC killing. Using NSCLC cells lacking mitochondrial DNA, we showed that the generation of mitochondrial reactive oxygen species (ROS) was crucial for the PM031379-induced translocation of AIF to the nucleus and subsequently cell death. Pretreatment of NSCLC cells with menadione, a mitochondrial ROS generator, was able to restore the deficient chemotherapy-induced apoptosis of NSCLC cells. Altogether, these data suggest that mitochondrial ROS generation is crucial for overriding the chemoresistance of NSCLC cells. Moreover, this study delineates the unique mechanism of action of lamellarins as potential anticancer agents.

Apoptosis-inducing factor (AIF): a novel caspase-independent death effector released from mitochondria

Apoptosis-inducing factor (AIF) is a phylogenetically ancient mitochondrial intermembrane flavoprotein endowed with the unique capacity to induce caspase-independent peripheral chromatin condensation and large-scale DNA fragmentation when added to purified nuclei. In addition to its apoptogenic activity on nuclei, AIF can also participate in the regulation of apoptotic mitochondrial membrane permeabilization and exhibits an NADH oxidase activity. Under normal circumstances, AIF is secluded behind the outer mitochondrial membrane. However, upon apoptosis induction AIF translocates to the cytosol and the nucleus. Injection of anti-AIF antibodies or knockout of the AIF gene have demonstrated that AIF may be required for cell death occurring in response to some stimuli. In particular, inactivation of AIF renders embryonic stem cells resistant to cell death following growth factor withdrawal. Moreover, AIF is essential for programmed cell death during cavitation of embryoid bodies, the very first wave of (caspase-independent) cell death indispensable for mouse morphogenesis. We have recently found that AIF is neutralized by heat-shock protein (HSP) 70, in a reaction that appears to be independent of ATP or the ATP-binding domain (ABD) of HSP70 and thus differs from the previously described Apaf-1/HSP70 interaction (which requires ATP and the HSP70 ABD). Intriguingly, HSP70 lacking ABD (HSP70 Delta ABD) inhibits apoptosis induced by serum withdrawal, staurosporin, and menadione, three models of apoptosis which are also affected by micro-injection of anti-AIF antibody or genetic ablation of AIF. Altogether, these data suggest that AIF plays a role in the regulation of caspase-independent cell death.

The central role of the mitochondrial megachannel in apoptosis: evidence obtained with intact cells, isolated mitochondria, and purified protein complexes

The mitochondrial megachannel (also called permeability transition pore) is a polyprotein complex formed in the contact site between the inner and the outer mitochondrial membranes and participates in the regulation of mitochondrial membrane permeability. We have obtained three independent lines of evidence suggesting the implication of the mitochondrial megachannel in apoptosis. First, in intact cells, apoptosis is accompanied by an early dissipation of the mitochondrial transmembrane potential (delta psi m). In several models of apoptosis, specific agents inhibiting the mitochondrial megachannels prevent this delta psi m dissipation and simultaneously abolish the manifestations of caspase- and endonuclease activation, indicating that megachannel opening is a critical event of the apoptotic process. Second, mitochondria are rate-limiting for caspase and nuclease activation in several cell-free systems of apoptosis. Isolated mitochondria release apoptogenic factors capable of activating pro-caspases or endonucleases upon opening of the mitochondrial megachannel in vitro. Third, opening of the purified megachannel reconstituted into liposomes is inhibited by recombinant Bcl-2 or Bcl-XL, two apoptosis-inhibitory proteins which also prevent megachannel opening in cells and isolated mitochondria. This indicates that the megachannel is under the direct regulatory control of anti-apoptotic members of the Bcl-2 family. Altogether, our results suggest that megachannel opening is sufficient and (mostly) necessary for triggering apoptosis.

A cytofluorometric assay of nuclear apoptosis induced in a cell-free system: application to ceramide-induced apoptosis

Purified nuclei exposed to apoptogenic factors in vitro undergo morphological and biochemical changes in chromatin organization. Most cell-free models of nuclear apoptosis are based on the quantitation of endonuclease-mediated DNA fragmentation on agarose gels or on the changes of nuclear morphology revealed by the DNA-intercalating fluorochrome 4'-6-diamidino-2-phenylindole dihydrochloride. In this work we develop a cytofluorometric system for the accurate quantitation of nuclear DNA loss. This system has been used to determine the conditions of nuclear apoptosis induced by apoptosis-inducing factor (AIF) contained in the supernatant of mitochondria induced to undergo permeability transition. AIF can provoke significant nuclear DNA loss in < or = 5 min, acts over a wide pH range (pH 6 to 9), and resists cysteine protease inhibitors such as iodoacetamide and N-ethylmaleimide. Moreover, we applied this system to the question of how the proapoptotic second messenger ceramide would induce apoptosis in vitro: via a direct effect on nuclei, a direct effect on mitochondria, or via indirect mechanisms? Our data indicate that ceramide has to activate yet unknown cytosolic effectors that, in the presence of mitochondria, can induce nuclear apoptosis in vitro.

Origin of eukaryotic programmed cell death: a consequence of aerobic metabolism?

A marked feature of eukaryotic programmed cell death is an early drop in mitochondrial transmembrane potential. This results from the opening of permeability transition pores, which are composed of adenine nucleotide translocators and mitochondrial porins. The latter share striking similarities with bacterial porins, including down-regulation of their pore size by purine nucleotides), suggesting a common origin. The porins of some invasive bacteria play a crucial role during their accommodation inside the host cell and this coexistence resembles the endosymbiotic origin of mitochondria. The above observations suggest that early in eukaryotic evolution, former invaders may have used porin-type channels to enter their host and to induce its death when the levels of its cytoplasmic purine nucleotides were dropped. The appearance of adenosine nucleotide translocators in the primitive eukaryotes, which permitted usage of the oxidative metabolism of the invaders, provided the basis for the permeability transition phenomena, now linked to the apoptotic process. Bcl-2-type molecules, being able to modulate the permeability transition pores by interaction with adenosine nucleotide translocators, may have played an essential role in conferring a means of controlling apoptosis.

Apoptosis and necrosis in toxicology: a continuum or distinct modes of cell death?

Mounting evidence indicates that apoptosis rather than necrosis predominates in many cytolethal toxic injuries. Associated cell death models of apoptosis and necrosis are either: (1) totally separate death modes, (2) a continuum whereby they are extremes of biochemically overlapping death pathways, or (3) essentially distinct processes with only limited molecular and cell biology overlap. We conclude that the current balance of evidence favours the third of these options. The established axiom that, even when considering the same toxicant, injury amplitude (dose) is a primary determinant of whether cells die via active cell death (apoptosis) or failure of homeostasis (necrosis) remains valid. Tissue selectivity of toxicants can stem from the apoptotic or necrotic thresholds at which different cells die, as well as targeting factors such as toxicokinetics, receptor recognition, bioactivation, and cell-specific lesions.