Mitochondrial dysfunction in cancer

Abstract

A mechanistic understanding of how mitochondrial dysfunction contributes to cell growth and tumorigenesis is emerging beyond Warburg as an area of research that is under-explored in terms of its significance for clinical management of cancer. Work discussed in this review focuses less on the Warburg effect and more on mitochondria and how dysfunctional mitochondria modulate cell cycle, gene expression, metabolism, cell viability, and other established aspects of cell growth and stress responses. There is increasing evidence that key oncogenes and tumor suppressors modulate mitochondrial dynamics through important signaling pathways and that mitochondrial mass and function vary between tumors and individuals but the significance of these events for cancer are not fully appreciated. We explore the interplay between key molecules involved in mitochondrial fission and fusion and in apoptosis, as well as in mitophagy, biogenesis, and spatial dynamics of mitochondria and consider how these distinct mechanisms are coordinated in response to physiological stresses such as hypoxia and nutrient deprivation. Importantly, we examine how deregulation of these processes in cancer has knock on effects for cell proliferation and growth. We define major forms of mitochondrial dysfunction and address the extent to which the functional consequences of such dysfunction can be determined and exploited for cancer diagnosis and treatment.

Keywords: cancer metabolism; cell cycle control; mitochondria; mitochondrial biogenesis; mitochondrial fusion and fission; mitophagy; oxidative phosphorylation.

Figure 1

Defining mitochondrial dynamics. (A) Mitochondrial fusion requires the action of fusion proteins, OPA-1 at the IMM and Mitofusin 1 and Mitofusin 2 at the OMM promoting the fusion of membranes of juxtaposed mitochondria. Mitochondrial fusion is selective for polarized mitochondria and promoted by growth on oxidative carbon sources, such as galactose, that also induces respiratory chain protein expression, increased cristae density and formation of respiratory chain supercomplexes and increased OXPHOS. Fusion likely also contributes to increased respiration and mitochondrial metabolism by promoting increased diffusion of intermediate metabolites and reducing agents. Fusion also limits mitophagy and apoptosis. (B) Mitochondrial fission is promoted by the GTPase activity of the dynamin-related protein (DRP1) that is recruited to mitochondria in response to stresses, such as hypoxia, where DRP1 interacts with its mitochondrial receptors (Mff1, Fis1 and others) to pinch off mitochondria into smaller units. Mitochondrial fission depolarizes mitochondria but mitochondria generally recover. Failure to restore membrane potential is thought to target mitochondria for degradation by autophagy or depending on other stresses, result in apoptosis. Cleavage of OPA-1 promotes apoptosis. (C) Biogenesis is induced by nutrient deprivation and in response to oxidative stress and requires the coordinated expression of nuclear and mitochondrial encoded genes that are co-regulated by transcription factors NRF1/2, PPARγ, ERRα, β, γ, and the key transcriptional co-factor, PGC-1α. Mitochondrial biogenesis is required for cell growth to produce increased metabolites and energy and defects in biogenesis are frequently lethal to cells and organisms. (D) Mitophagy is a specialized form of autophagy in which mitochondria are targeted to nascent phagophores and engulfed by autophagosomes that fuse with lysosomes to degrade the encapsulated mitochondria. Mitochondrial fragmentation is required for mitophagy and induction of fusion protects mitochondria from degradation under starvation conditions. Mitophagy is promoted by a number of different mechanisms including Pink1/Parkin-mediated pathways and also the BNIP3/NIX pathway. (E) Apoptosis is a terminal event that is promoted by the activity of BH3-only members of the Bcl-2 superfamily of cell death regulators. When apoptosis is inhibited, novel functions for BAK/BAX and other Bcl-2 family members has been revealed. (F) Mitochondrial spatial dynamics is relatively under-studied but mitochondria respond to key stresses, including hypoxia and calcium flux in the cell, by changing their sub-cellular localization, including coalescing around the nucleus and changing their proximity to the ER. Mitochondrial migration in cells is modulated by Miro, a Ca²⁺ dependent small G protein as well as by poorly understood effects of Parkin and HDAC6.

Figure 2

Dual and apparently opposing roles of Bcl-2 family members and fission/fusion proteins in apoptosis and mitochondrial dynamics. Mitochondrial fission and fusion proteins appear to modulate apoptosis through activities that are distinct from their roles in mitochondrial dynamics but which involve members of the Bcl-2 family. Conversely, Bcl-2 family members modulate mitochondrial fission and fusion in a manner that appears to be independent of their functions in apoptosis.

Figure 3

Variation in mitochondrial staining in human breast cancers. Immunohistochemical staining for mitochondrial 60 kDa antigen (Biogenex clone 113-1) reveals marked variations in mitochondrial staining between different primary human breast cancers with some tumors showing very low staining (left) and others very high staining (right). Differences in mitochondrial mass between different primary tumors examined in this study was greater than intra-tumor heterogeneity in mitochondrial mass. The significance of these differences in mitochondrial mass for tumor growth, progression, and therapy response is unknown.

Figure 4

Signaling pathways regulating biogenesis in response to stress. Mitochondrial mass is increased in response to nutrient stress through increased mitochondrial biogenesis. The peroxisome-proliferator activator receptor-gamma co-activator 1-alpha (PGC-1α) is key to coordinating responses to nutrient availability and induction of biogenesis through its co-activation of NRF1/2, ERRα, β, γ, and PPARγ. These transcription factors induce expression of key nuclear-encoded genes, such as Tfam and mitochondrial polymerases, but also induce expression of metabolic enzymes active at the mitochondria and other proteins that are imported into the mitochondria. Mitochondrial encoded proteins required for respiratory chain function are also induced secondary to the actions of PGC-1α and its related proteins, PGC-1β, and PRC. Both p53 (through inhibition of PGC-1α) and c-MYC (through activation of PGC-1β) modulate biogenesis. Recent data highlights a role for MITF-induced PGC-1α activity and mitochondrial metabolism in a subset of human melanomas, that is sensitive to B-Raf inhibitors (since B-Raf blocks the action of MITF on PGC-1α).

Figure 5

Mitophagy pathways. Turnover of mitochondria at the autophagosome (mitophagy) is required to maintain a healthy pool of mitochondria. Defects in mitophagy result in accumulation of dysfunctional mitochondria. Two major pathways have been identified that regulate targeting of mitochondria to the autophagosome: (A) the Pink1/Parkin pathway in which activation of Pink1 kinase following mitochondrial depolarization leads to phosphorylation of Mitofusin by Pink1, that then acts as a receptor for Parkin. Recruitment of Parkin, a E3 ubiquitin ligase, results in ubiquitination of multiple mitochondrial substrates but how this leads to mitophagy is still a matter for debate, and possible explanations for how Parkin functions are highlighted in the inset box; (B) An alternative pathway involves the activity of BNIP3 and NIX, both of which are hypoxia inducible but also regulated by FoxOs, E2Fs, and p53. BNIP3 and NIX have both been shown to interact directly with processed LC3 through a conserved LIR motif in their amino terminal ends. This interaction has been proposed to explain how BNIP3 and NIX target mitochondria to the autophagosome. Both BNIP3 and NIX are also known to interact with Rheb, and with Bcl-2/Bcl-X_L but the significance of these interactions for mitophagy are not clear.

Figure 6

Types of mitochondrial dysfunction. We have attempted to define “mitochondrial dysfunction” in this review and the figure summarizes the major types of mitochondrial dysfunction that are known. (A) Mitochondrial inner membrane de-polarization (ΔΨ) during which there is loss of membrane potential; (B) mitochondrial membrane permeability transition (MPT) during which opening of the permeability transition pore (consisting of VDAC, ANT and usually association of Cyclophilin D) can lead to non-apoptotic cell death; (C) defective respiration/oxygen consumption due to altered expression of respiratory chain components, poisoning with respiratory complex inhibitors or many other stresses; (D) the Unfolded Mitochondrial Protein Response (UPR^mt) can arise when there is an imbalance in expression of mitochondrial encoded mitochondrial proteins relative to nuclear encoded mitochondrial proteins, resulting in dysfunctional mitochondria; (E) damage to the mitochondrial genome most commonly reported as a result of oxidative damage to bases arising from respiratory chain defects; (F) defects in the production of iron-sulfur complexes in the mitochondrial matrix leading to defects not just in respiratory chain components but also other cellular enzymes; (G) release of cytochrome c anchored at the inner mitochondrial membrane via cardiolipin can result in formation of the apoptosome and activation of caspases leading to apoptosis. Some of these aberrant mitochondrial behaviors are inter-dependent, for example, membrane depolarization is a factor in mitochondrial permeability transition, defective respiration, and apoptosis amongst other consequences, but frequently can stand alone as a signal, for example to promote mitophagy. The consequences for the cell of these different forms of mitochondrial dysfunction are described in the text and below in Figure 7.

Figure 7

Retrograde signaling from mitochondria to nucleus. The role of the nucleus in regulating mitochondrial function has been examined extensively but the importance of mitochondrial events in signaling to the nucleus and to other cell growth mechanisms has been relatively under-studied. The figure summarizes some key signaling consequences of dysfunctional mitochondria. (A) Mitochondrial control of nuclear gene expression through effects of altered production of certain metabolites, such as α-ketoglutarate and succinate, on epigenetic modification of histones, stabilization of key transcription factors, such as HIF, in addition to effects on other enzymes and proteins. (B) Altered production of NAD⁺, ATP, and other changes in mitochondrial metabolism can modulate key signaling molecules in the cell, such as AMPK and the Sirtuins. (C) Mitochondria play a key role in buffering against Ca²⁺ flux into the cytosol from the extra-cellular environment or following release from the ER and failure of the mitochondria to execute this key function can result in altered Ca²⁺ signaling in the cell. (D) Mitochondrial ROS production has been one of the most extensively studied mediators of mitochondrial dysfunction and activity that elicits its effects on transcription factor activity as well as activity of key enzymes in the cell, such as caspases and phosphatases. (E) Important cellular kinases are known to localize to the mitochondria and altered mitochondrial dynamics and function may modulate the activity of these kinases not just at the mitochondria but at other sub-cellular locations if released from the mitochondria.

Figure 8

Oncogene and tumor suppressor gene regulation of mitochondria. The key activities of major oncogenes and tumor suppressors are increasingly being linked to effects on mitochondrial function. In particular, both p53 and Myc, inarguably two of the most significant tumor related genes in the human genome, have been shown to modulate several different aspects of mitochondrial function and in some instances, this has been shown to be key to their role in cancer, as discussed in greater length in the text.