Mitochondria as Regulators of Nonapoptotic Cell Death in Cancer

Abstract

Mitochondria are involved in cell survival and metabolic processes including adenosine triphosphate production, heme biosynthesis, reactive oxygen species, and iron and calcium homeostasis. Although mitochondria are well known to contribute to apoptosis, a growing body of evidence indicates that mitochondria modulate nonapoptotic cell death (NACD) mechanisms, including autophagy, necroptosis, ferroptosis, paraptosis, pyroptosis, parthanatosis, and cuproptosis. These NACD pathways differ in molecular triggers, morphological characteristics, and immunological consequences, but they all involve mitochondria. For example, mitochondrial ROS and lipid peroxidation play a role in ferroptosis, whereas mitochondrial depolarization and the release of apoptosis inducing factor are paramount to parthanatosis. Mitochondrial swelling is a hallmark of paraptosis, whereas mitochondrial disruption is associated with pyroptosis. Autophagy, though primarily a survival mechanism, is also regulated by mitochondrial dynamics in cancer cells. In cuproptosis, mitochondrial protein aggregates when iron-sulfur cluster proteins are disrupted, resulting in copper-dependent cell death. There are many factors that influence NACD, including mitochondrial membrane potential, bioenergetics, calcium flux, metabolites, and interactions with the endoplasmic reticulum. The review comprehensively summarizes our understanding of mitochondrial and NACD interactions, particularly in cells resistant to classical apoptosis agents. Therapeutic vulnerabilities associated with mitochondria-mediated NACD could lead to next-generation therapies.

Keywords: autophagy; cancer; ferroptosis; mitochondria; necroptosis; nonapoptotic cell death.

FIGURE 1

Mitochondrial dynamics. Mitochondria undergoes a dynamic equilibrium of fission and fusion. In mitochondrial fusion, two or more mitochondria merge to form a single, larger mitochondrion. The outer mitochondrial membrane proteins mitofusin 1 (MFN1) and mitofusin 2 (MFN2), as well as the inner mitochondrial membrane protein optic atrophy 1 (OPA1), are essential for mitochondrial fusion. On the other hand, in mitochondrial fission, a single mitochondrion divides into two or more smaller mitochondria. The cytosolic dynamin‐related protein 1 (DRP1) initiates mitochondrial fission by translocating to the outer mitochondrial membrane. Mitochondrial fission 1 protein (FIS1), located on the outer mitochondrial membrane, then recruits DRP1 to the mitochondria, facilitating the fission process. The figure created with BioRender.com.

FIGURE 2

The role of mitochondria in necroptosis. RIPK3 and MLKL interact with mitochondrial serine/threonine protein phosphatase (PGAM5), which activates of cyclophilin‐D (Cyp‐D). Cyp‐D further increases ROS accumulation that leads to the opening of mitochondrial permeability transition pores (MPTP) and a decrease in ATP production, leading to necroptosis. RIPK3 regulates certain energy metabolism enzymes, such as glycogen phosphorylase (PYGL), glutamate‐ammonia ligase (GLUL), and glutamate dehydrogenase 1 (GLUD1), resulting in ETC complex I and II‐derived mtROS production. The proapoptotic proteins, BAX, BAK, and BH3‐only proteins (BMF, BNIP3) positively regulate necroptosis. Antiapoptotic proteins, such asBCL‐2, MCL‐1, BH3‐only proteins (e.g., BECLIN‐1), negatively regulate necroptosis. The figure created with BioRender.com.

FIGURE 3

The role of mitochondrial oxidative stress and energetics in autophagy. By inhibiting PI3K–Akt–mTOR‐mediated autophagy, mtROS impedes autophagy through phosphatase and tensin homolog deleted on chromosome ten (PTEN). An increase in mtROS also increases the likelihood of autophagy. Ubiquinol–cytochrome c reductase core protein 1 (UQCRB) increases mtROS levels, which results in transient receptor potential cation channel (TRPML) activation and the nuclear translocation of transcription factor EB (TFEB), which facilitates autophagy. The enzymes, glutamate dehydrogenase (GLUD) and α‐ketoglutarate (α‐KG) negatively regulate autophagy. GLUD1 limits mtROS production and activates mammalian target of rapamycin (mTOR). α‐KG inhibits mTOR by activating the enzyme, hypoxia‐inducible factor (HIF) prolyl hydroxylases (PHDs), where glutamine synthetase (GLS), UQCRC1, and protein kinase C beta (PRCKB) positively regulate autophagy. GLS inhibits mTOR and activates autophagy. UQCRC1 upregulates AMPK signaling and induces autophagy by increasing the activity of TRPML. A protein kinase family member, PRKCB, negatively regulates autophagy by causing the loss of the mitochondrial membrane potential (MMP). The figure created with BioRender.com.

FIGURE 4

The molecular mechanisms that produce mitophagy and mitochondria‐associated ER membranes (MAMs)‐mediated autophagy. The autophagic recognition of mitochondria occurs through the interaction of the LC3‐interacting region (LIR) of LC3, with several proteins, such as BH3‐only proteins (BNIP3 in its phosphorylated form and BNIP3/Nix), dephosphorylated FUNDC under hypoxic conditions, and the release of activating molecule in beclin‐1‐regulated autophagy (AMBRA1), PTEN‐induced kinase 1–parkin (PINK1–PARK2), and cardiolipin, during mitochondrial hyperpolarization. In PINK1–PARK2‐mediated autophagy, PINK1 is present on the OMM, and it recruits PARK2 by phosphorylating the protein, mitofusin 2 (MFN2). The activation of PARK2 ubiquitinates the protein, p62, which interacts with LC3, producing phagophore formation and ultimately, biodegradation by autophagy. Autophagy receptors, such as nuclear dot protein 52 kDa (NDP52) and tank‐binding kinase 1 (TBK1), can induce autophagy by activating and recruiting the ULK1 (ULK1) complex in the absence of LC3. A decrease in the interaction of the ER and mitochondria due to the loss of St8 alpha‐N‐acetyl‐neuraminide alpha‐2,8‐sialyltransferase 1 (VAPB) and protein tyrosine phosphatase interacting protein 51 (PTPIP51) induces autophagy. MAMs‐associated autophagy is positively regulated by the ER chaperone protein, the sigma‐1 receptor (Sig‐1R) and a ganglioside (GD3), after interaction with AMBRA1 and the WD‐repeat protein, interacting with phosphoInositides (WIPI), the soluble N‐ethylmaleimide‐sensitive factor (NSF) attachment protein receptors (SNARE) protein, and AMPK, upon interaction with phosphorylated MFN2. In contrast, IP3 receptor‐mediated Ca²⁺ negatively regulates MAMs‐associated autophagy. The figure created with BioRender.com.

FIGURE 5

The role of mitochondria in ferroptosis. Opening of voltage‐dependent anion channels (VDAC) increases the mitochondrial transmembrane potential and leads to mitochondrial hyperpolarization, which increases the levels of mtROS. Increased cellular Fe²⁺ levels by the Fenton reaction, ferritinophagy, mitochondrial ferritin, iron–sulfur cluster or upregulation of iron regulatory protein 2 (IRP‐2), also contribute to mtROS production. MtROS increases the likelihood of ferroptosis by increasing lipid peroxidation. The enzymes involved in the pentose phosphate pathway, such as glucose‐6‐phosphate dehydrogenase (G6PD) and phosphoglycerate dehydrogenase (PGD) and enzymes involved in glutaminolysis, such as glutaminase (GLS) and glutamic‐oxaloacetic transaminase 1 (GOT1), are positive regulators of ferroptosis. Ferroptosis suppressor protein 1 (FSP1) suppresses lipid peroxidation and inhibits ferroptosis. The figure created with BioRender.com.

FIGURE 6

The role of mitochondria in pyroptosis. mtDNA that leaks from mitochondria is a damage‐associated molecular pattern (DAMPs) molecule that can induce the activation of the Nod‐like receptor family pyrin domain containing 3 (NLRP3) inflammasome, which further activates caspase‐1 and gasdermin‐D (GSDMD), producing pyroptosis. mtROS is also generated by disrupting glycolytic flux, which produces increased NAD+, decreased ATP production and decreased lactate secretion, which induce inflammasome signaling and activate pyroptosis. The figure created with BioRender.com.

FIGURE 7

The role of mitochondria in paraptosis. The influx of Ca²⁺ ions occurs from the ER to the mitochondria due to the activation of inositol 1,4,5‐triphosphate receptors (IP₃Rs) and ryanodine receptors (RYRs) in the mitochondria. Ca²⁺ overload results in decreased ATP production and a loss of mitochondrial membrane potential, which ultimately causes paraptosis. Paraptosis involves mitochondrial dysfunction that is mediated by inhibition of ETC complex I and decreased ATP synthesis, which is followed by ROS generation, translocation of apoptosis‐inducing factor (AIF) from mitochondria to the nucleus, DNA fragmentation and cell death due to paraptosis. The figure created with BioRender.com.

FIGURE 8

The role of mitochondria in parthanatos. Excessive DNA damage by free radicals, ionizing radiation, hydroxy radicals, and activation of NMDA receptors, causes the overactivation of poly (ADP‐ribose) polymerase‐1 (PARP‐1). Free PAR polymers induce the opening of mitochondrial permeability transition pores (MPTP), which results in the loss of the mitochondrial membrane potential. This induces the translocation of AIF from mitochondria to the nucleus, where it binds to the protein, macrophage migration inhibitory factor (MIF), which further results in DNA fragmentation and activates the mitochondria. PARP over activation also produce a depletion of NAD+, resulting in decreased ATP production by inhibiting complex I of the ETC chain, inducing parthanatos. The figure created with BioRender.com.

FIGURE 9

The role of mitochondria in cuproptosis. It is initiated by elevated copper (Cu) levels and involves a series of molecular mechanisms. Elesclomol facilitates the binding of Cu ions (Cu²⁺) in the extracellular region and subsequently transports them intracellularly. Once inside, reductase enzymes convert Cu²⁺ to Cu⁺, allowing cellular entry. Cu importers, SLC31A1, modulates intracellular Cu⁺ concentrations. The buildup of Cu within cells leads to an increase in reactive oxygen species (ROS). Furthermore, the enzyme FDX1 reduces Cu²⁺ to Cu⁺, enhancing the lipidation and aggregation of DLAT enzymes, which are important in regulating the mitochondrial TCA cycle. FDX1 also induces destabilization of Fe–S cluster proteins, influencing cellular sensitivity to Cu‐induced apoptosis. The thiol‐containing copper chelator, glutathione (GSH), acts to inhibit cuproptosis. Copper‐induced damage to the mitochondrial respiratory chain results in the hyperactivation of the energy sensor AMPK, which increases cuproptosis and causes the release of the proinflammatory mediator HMGB1, a key player in the inflammatory response. The figure created with BioRender.com.