DNA damage and mitochondria in cancer and aging

Abstract

Age and DNA repair deficiencies are strong risk factors for developing cancer. This is reflected in the comorbidity of cancer with premature aging diseases associated with DNA damage repair deficiencies. Recent research has suggested that DNA damage accumulation, telomere dysfunction and the accompanying mitochondrial dysfunction exacerbate the aging process and may increase the risk of cancer development. Thus, an area of interest in both cancer and aging research is the elucidation of the dynamic crosstalk between the nucleus and the mitochondria. In this review, we discuss current research on aging and cancer with specific focus on the role of mitochondrial dysfunction in cancer and aging as well as how nuclear to mitochondrial DNA damage signaling may be a driving factor in the increased cancer incidence with aging. We suggest that therapeutic interventions aimed at the induction of autophagy and mediation of nuclear to mitochondrial signaling may provide a mechanism for healthier aging and reduced tumorigenesis.

Accumulation of DNA damage with age alters nuclear to mitochondrial crosstalk, resulting in mitochondrial dysfunction which may explain the strong association between age and carcinogenesis. Induction of autophagy may abate this pathway and result in healthier aging and reduced cancer.

Introduction

Aging is associated with rising risks for various diseases, including cancer. According to the National Cancer Institute, 54% of all cancer diagnoses in the USA between 2012 and 2016 were in patients above the age of 65. The median age of all cancer patients was 66 and the average age of death from cancer was 72 (1). These statistics illustrate a large body of work (reviewed in (2)) showing a strong correlation between aging and tumorigenesis. Several factors may be responsible for this correlation, such as the accumulation of DNA damage, telomere shortening, and decrease in mitochondrial quality control, all of which have been observed to occur during aging.

On average, a single cell experiences 70 000 DNA lesions in a day (3). There are various endogenous and exogenous sources of DNA damage, resulting in a variety of lesions. Most of these lesions are repaired accurately through DNA damage recognition and repair mechanisms. In spite of the extensive repertoire of the DNA damage response (DDR), however, current evidence suggests that DNA damage accumulates and DDR efficiency declines during the aging process (4–6). Many premature aging syndromes result from mutations in DDR proteins, suggesting that an accumulation of DNA damage can exacerbate the aging process (7–9). DNA damage also plays an integral role in tumorigenesis, as many hereditary cancer predisposition syndromes (HCPSs) result from inherited mutations in DDR genes (10). Several premature aging diseases exhibit an increased risk of cancer development. For instance, ataxia-telangiectasia (A-T), Werner syndrome (WS), xeroderma pigmentosum (XP), Rothmund–Thomson syndrome and Bloom syndrome have all been associated with an increased risk of developing malignant neoplasms (10–13). Thus, DNA damage accumulation is widely accepted as a contributing factor toward the increase in cancer prevalence with age.

The degradation of telomeres has been suggested to contribute toward biological aging and cancer (14,15). Damage to telomeres, such as their shortening following cell division, causes cells to enter into a state of senescence, a stress response that is thought to prevent cell growth, division and ultimately tumorigenesis (16–18). Senescent cells, however, have the potential to induce tumorigenesis through the secretion of inflammatory cytokines, a phenomenon known as the senescence-associated secretory phenotype (SASP) (19,20). Telomeres are also associated with malignancies through telomere crisis, a state of extreme telomere dysfunction in cancer cells that can result in genome instability (15,21). Substantial evidence from current studies on telomeres suggests that these genomic components may also contribute to the increase in cancer with age.

Mitochondrial dysfunction is another key characteristic of cancer and aging. Aerobic glycolysis, the Warburg effect, is a hallmark of cancer and is thought to occur because of mitochondrial alterations (22,23). Changes in mitophagy have been observed in a number of cancers as well (22). Mitophagy is the autophagic process by which dysfunctional mitochondria are targeted for lysosomal destruction and are recycled for cellular utilization. The accumulation of damaged mitochondria and a decrease in mitophagy are hallmarks of the aging process (13,24–27). A number of pathways mediate mitophagy, including the PTEN-induced kinase 1 (PINK1)/Parkin and the BCL2/adenovirus E1B 19 kDa protein-interacting protein 3 (BNIP3)/NIX pathways (28,29). A deficiency in mitophagy has been suggested to result in metabolic inefficiency and increased reactive oxygen species (ROS), which can induce cellular stress responses such as apoptosis and senescence (30–33). It has also been suggested that diminished mitophagy occurs with deficiencies in DDR responses and DNA damage, demonstrated by studies that show decreased mitophagy in premature aging syndromes (13,24,34,35). The observed decline in mitophagy with age is thought to be a critical mechanism by which age-related diseases can be initiated and exacerbated, and the cellular outcomes such as increased oxidative stress associated with this phenotype suggest that it can specifically contribute toward the process of tumorigenesis (22,23,31,36,37).

In this review, we discuss the current understanding of the mechanisms linking aging and cancer, such as DNA damage, telomere dysfunction, mitochondrial dysfunction and mitophagy. We emphasize the importance of nucleus to mitochondria signaling in aging and cancer, suggesting that mitophagy induction through the modulation of signaling between the nucleus and mitochondria may be an effective therapeutic strategy in promoting healthier aging and preventing tumorigenesis.

DNA damage and repair in cancer and aging

Cells experience diverse types of genotoxic insults from both endogenous and exogenous sources. These insults can generate various DNA lesions. For instance, oxidative stress and alkylating agents can result in nitrogenous base modifications that alter base pairing and, if they persist in the genome, generate errors in DNA replication. Ultraviolet radiation and interstrand crosslinks can result in helical distortions in the DNA that stall replication and transcription. The persistence of lesions can also initiate translesion synthesis, an error-prone method to proceed with replication in the presence of lesions inhibiting replicative polymerases. These damage events can, in turn, severely interfere with cellular function.

In response to these deleterious lesions, cells employ a vast repertoire of molecular machinery that catalyzes their resolution. These responses have been extensively studied and are thoroughly described in the literature (38).

DNA damage and repair in aging

The somatic mutation theory of aging posits that the accumulation of somatic mutations and other DNA damage lesions drives the aging process (39). In support of this theory, it has been previously observed that DNA repair efficiency declines with age and that DNA damage accumulates within the genome with age (4–6). The hypothesis that these correlations are causal is supported by the existence of premature aging syndromes resulting from deficiencies in DDR genes (40).

Segmental premature aging syndromes are rare diseases in which patients exhibit many features of physiological aging at an accelerated rate, and experience mortality at an age earlier than the average lifespan. The manifestations associated with these syndromes vary as a result of the tissue-specific dependence on different DDR responses. However, some of the conserved presentations of these syndromes include skin atrophy, hair graying, and increased risk of developing age-related diseases. Examples of these syndromes are A-T, XP and WS. XP results from a deficiency in nucleotide excision repair (NER) function due to mutations in the genes of the XP complementation group. XP presents with increased sensitivity to ultraviolet radiation due to the role of NER in repairing genomic lesions resulting from this form of environmental stress. A-T is caused by a deficiency in the ataxia-telangiectasia mutated (ATM) protein, a critical signal transducer and mediator of the double strand break repair (DSBR) pathway. WS results from a deficiency in the WRN helicase, an integral member of the base excision repair and homologous recombination (HR) DDR responses (7–9). These and other progeroid syndromes are further detailed in Table 1.

Table 1.

Progeroid syndromes: associated cancer and mitochondrial phenotypes

Syndrome	Gene altered	Role in DNA repair	Associated cancer types	Associated mitochondrial dysfunction	References
Werner syndrome	WRN	DSBR	Sarcomas	Suggested	(41,42)
Ataxia-telangiectasia	ATM	DSBR	Lymphomas	Yes	(11–13)
Xeroderma pigmentosum	XP genes (XPA, XPC)	GG-NER	Melanoma	Yes	(34,43,44)
Cockayne syndrome	CSA, CSB	TC-NER	None	Yes	(35,40,45)
Trichothiodystrophy	XPB, XPD	NER	None	Suggested	(40,46)
Bloom’s syndrome	BLM	DSBR	Diversity in site and type	Suggested	(47,48)
Rothmund–Thomson syndrome	REQL4	DSBR	Osteosarcoma	Yes	(49–51)
Fanconi’s anemia	FANC	ICLR	Acute myeloid leukemia and solid tumors	Yes	(52–55)

ICLR, interstrand crosslink repair.

The connection between DNA damage and aging is bolstered by the existence of these premature aging syndromes, and the molecular mechanisms by which DNA damage leads to aging is an area of active research. Persistent DNA damage has been shown to cause apoptosis (56). The age-related accumulation of DNA damage within tissues may indeed induce apoptosis and contribute to the decline in tissue function with age (57). DNA damage has also been implicated in the initiation of senescence (58,59). Senescence has been strongly associated with a decline in tissue function with age, so it is possible that DNA damage may lead to aging phenotypes in this manner (60). DNA damage can lead to increased levels of extranuclear DNA, which in turn can lead to the activation of stimulator of interferon genes (STING)-mediated inflammation (61,62). Aged cells, in particular, display an accumulation of extranuclear DNA, which has been implicated in the occurrence of senescence and inflammation (63). This inflammatory phenotype is mediated by SASP, which is initiated by the detection of cytosolic chromatin fragments (CCFs) by cGAS-STING (64). It has been demonstrated that oxidative DNA damage can specifically lead to the formation CCFs (65).

Another area of active research is how DNA damage regulates nuclear to mitochondrial signaling. The accumulation of damaged mitochondria, as well as diminished mitophagy, have been observed in aging, and are thought to play critical roles in the pathogenesis of age-related diseases (24,37). The relationship between DNA damage and mitophagy will be discussed in further detail below.

DNA damage and repair in cancer

DNA damage plays a causal role in tumorigenesis. This is supported by the fact that carcinogens are typically genotoxic (66). DNA damage-induced mutations may occur in genes that regulate proliferation and survival, leading to tumorigenesis. Chromosomal aberrations caused by DSBs have also been detected in cancer cells and have been shown cause tumorigenesis (67). Genome instability, in general, has been characterized as a driving factor in the development of intratumor heterogeneity and cancer progression (68,69). Another prominent observation supporting the role of DNA damage in tumorigenesis is that DDR genes are found to be mutated in HCPSs (Table 2).

Table 2.

HCPSs: associated cancer and mitochondrial phenotypes

Hereditary cancer syndrome	Gene altered	Role in DNA repair	Associated cancer type	Associated mitochondrial dysfunction	References
Li–Fraumeni syndrome	P53, CHEK2	Cell cycle arrest	Diverse in site and type	Yes	(70–72)
Hereditary breast and ovarian cancer syndrome	BRCA1/2	HR	Breast and ovarian	Suggested	(52,73,74)
Turcot syndrome 1	MLH1, MSH2	MMR	Colorectal and glioblastoma multiforme	Suggested	(75,76)
Nijmegen breakage syndrome	NBN	DSBR	Lymphoma	Suggested	(77–79)
HNPCC (Lynch syndrome)	MLH1, MSH2	MMR	Colorectal	Suggested	(76,80)

HNPCC, hereditary non-polyposis colorectal cancer.

HCPSs are caused by inherited germline mutations that increase the risk of developing a specific tumor type relative to the general population. The mutations that cause these debilitating syndromes are typically localized to tumor suppressor genes, including DDR genes. Examples of HCPSs that involve deficiencies in DDR are BRCA-associated breast and ovarian cancer predisposition syndrome, hereditary non-polyposis colorectal cancer, Nijmegen breakage syndrome, Turcot syndrome and Li–Fraumeni syndrome (10,81).

BRCA genes are involved in the repair of DSBs through HR. Loss of function of this critical repair protein results in higher prevalence of deletions and base substitutions (82). Hereditary non-polyposis colorectal cancer and Turcot syndrome both result from deficiencies in mismatch repair genes (75,80). Nijmegen break syndrome is caused by a loss-of-function mutation in the NBN gene. Patients who suffer from this cancer predisposition syndrome are predisposed to non-Hodgkin’s lymphoma (77). NBN is a critical component of the MRE11–RAD50–NBN (MRN) complex, regulating its nuclease activity and interactions with other DSBR proteins (83). The NBN protein is also involved in the process of IgG class-switching and gamma-irradiation repair in B lymphocytes (84), so it would be expected that loss-of-function mutations in this protein can lead to this subtype of lymphoma. Li–Fraumeni syndrome results from a loss in p53 function, consistent with the drastic predisposition to both rare and common neoplasms in multiple tissue types due to p53’s critical roles in cellular stress and DDRs (10,81). Current literature suggests that these syndromes are associated with mitochondrial dysfunction, indicating that abnormal nuclear to mitochondrial signaling may be implicated in tumorigenesis (76,85). For instance, deficiency in the MLH1 gene, which is a critical MMR gene, results in mitochondrial phenotypes such as diminished antioxidant response and diminished complex I activity (76). A lack of either BRCA1 or BRCA2 expression impairs mitophagy, and loss of BRCA1 in breast cancer cells induces hydrogen peroxide formation in both the cancerous epithelial cells and the surrounding stroma (52,73). BRCA1 deficiency is also associated with glycolytic flux in the neighboring stroma (73). A lack of NBN expression has been demonstrated to increase ROS formation following DNA damage and cause poly(ADP-ribose) polymerase (PARP) activation (78). In addition, NBS1-deficient cell lines exhibit diminished mitophagy following fractionated radiation exposure (79). However, further studies on these syndromes are required with regard to the specific mitochondrial defects that seem to be present in them. Further details surrounding these syndromes can be seen in Table 1.

DNA damage and repair play a significant mechanistic role in the relationship between stem cells and cancer (86,87). Variations in DDR have been highly implicated in cancer stem cells (CSCs) (87,88). Glioblastoma and lung CSCs can exhibit increased HR and non-homologous end joining efficiency, which may contribute to their observed resistance to conventional antineoplastic therapies (87,89–91). CSCs in glioblastomas can also express higher levels of PARP1, which is an additional method by which these cells can acquire radio resistance (87,92,93). Generally, through a number of possible mechanisms including increased DDR pathway efficiency, CSCs tend to adapt survival methods in the face of genotoxic stress (87). DDR may also play a role in tumorigenesis through adult stem cells (ASCs) (86,94,95). ASCs persist in a quiescent state until they become activated, allowing them to accrue mutations that have the potential to be tumorigenic (96). Hematopoietic stem cells from older mice, for instance, have been observed to exhibit diminished DDR and ATM activation following irradiation compared with young mice (97). ASCs are dependent on non-homologous end joining while in their quiescent state, which has been demonstrated to contribute to potentially tumorigenic chromosomal level abnormalities (94,98). The pathological relevance of the above mentioned DNA damage accumulation is corroborated by the observation that tissue-specific ASC mutational profiles have been shown to align with the mutational profiles seen in cancer driver genes (99). This suggests that the age-related accumulation of DNA damage within stem cells may increase their potential for transformation.

Premature aging syndromes are also associated with cancer predisposition, some of which are defined as HCPSs (10). Specific progeroid syndromes that exhibit cancer predisposition include WS, XP, Fanconi’s anemia and A-T (10). The persistence of DNA damage in the genomes of individuals suffering from these syndromes may lead to mutagenesis or tumorigenic chromosomal aberrations. This further highlights the critical role of DNA damage in both aging and cancer, suggesting that the accumulation of DNA damage with age may directly be contributing to the observed increase in cancer prevalence with age. As with the HCPSs, these DDR-related premature aging syndromes also have associated mitochondrial dysfunction (Table 1) (13,34). This indicates the possible role of nuclear to mitochondrial signaling in tumorigenesis with age. Potential mechanisms underlying the association between nuclear to mitochondrial signaling, aging and cancer will be discussed in later sections.

In contrast to these DNA repair-deficient, cancer-promoting diseases, remarkably, Cockayne syndrome (CS) patients do not display an increased risk of cancer development (100–102). CS patients are deficient in transcription coupled (TC)-NER but retain global genome (GG)-NER. Both XP patients and CS patients are photosensitive, and cells from both show cytotoxicity when exposed to UV. However, in a comparison of mutational frequency between XP-C- and CSB-deficient cells exposed to UVC, XP-C-deficient cells showed an ~5-fold greater UV-specific mutation rate than wild-type cells, while CSB-deficient cells exhibited the same UV-specific mutation rate as wild-type cells (103). This suggests that the loss of either TC-NER or GG-NER limits cell survival after UV exposure. CSB cells are prone to oxidative stress-related mutations, consistent with the increase in mitochondrial and cellular ROS seen in those cells (35,103,104). Given that CSB patients experience neurodegeneration, and other progeroid syndromes have associated cancer predisposition, it would be of interest to further evaluate the role of different forms of DNA repair and the mutational spectra associated with a loss in specific pathway function in cancer predisposition and neurodegeneration. For instance, further exploration of the dependence of particular tissues on specific DNA repair pathways for maintaining proper function may result in a more thorough understanding of cancer subtypes, which would be instrumental in the process of developing tissue-specific or targeted therapeutics.

Similarly, Hutchinson–Gilford progeria syndrome (HGPS) is a bona fide premature aging syndrome that is not commonly associated with carcinogenesis but demonstrates DNA repair defects. HGPS is a rare autosomal dominant disorder caused by mutations in the LMNA gene, encoding Lamin A/C (105,106). The most common mutation is c.1824C>T, which results in the accumulation of an aberrantly spliced Lamin A variant called progerin. Progerin aggregates, leading to chromatin changes, nuclear defects and constitutive DNA damage (107,108). A recent study shows that expression of progerin, in addition to decreased DSBR, skews the mechanism of DSBR from HR to non-homologous end joining, a more error-prone repair pathway (109). These repair deficits in other disorders are associated with increased cancer risk. In the case of HGPS, the short lifespan of patients may explain this paradox. However, as interventions become available that extend lifespan of HGPS patients, this may become an important consideration.

Telomeres in aging and cancer

Telomeres are genomic repeats present on the ends of chromosomes that are essential in compensating for the End Replication Problem, the loss of genetic material with every cellular division (14). In cells that have critically shortened or damaged telomeres, the DSBR response is initiated via ATM recruitment and activation at damaged telomere sites (17). ATM has recently been shown to be required for telomerase activation (110). WRN is also associated with the DDR at telomeres (111,112).

Poly-ADP ribosylation (PARylation) of telomeres occurs following telomeric DNA damage in a tankyrase-1-dependent manner, and NAD⁺ abundance as well as sirtuin function declines in states of telomere dysfunction in a p53-dependent manner (113,114). Inhibition of the NAD⁺-consuming enzyme, CD38, has been shown to mitigate the DDR at telomeres, further supporting the critical role of NAD⁺ and PARylation in ameliorating damage at the telomeres (115). Oxidative stress also contributes to telomere dysfunction and shortening, connecting age-related dysfunction in mitochondria to telomere damage in aging. Mitochondrial-derived ROS specifically cause telomere damage due to the susceptibility of telomeric GGG-repeats to oxidation by free radicals (116–118). Thus, increased levels of ROS resulting from the age-associated decline in mitophagy and accumulation of damaged mitochondria may also contribute to the role of telomeres in linking aging and cancer. In addition, the accumulation of the DNA damage and the concomitant erosion of telomeres during aging may both be implicated in the observed decline in NAD⁺ levels with age, decreasing a cell’s ability to repair DNA damage and carry out critical functions (119).

In cancer, telomere dysfunction has been shown to contribute to the formation of dicentric chromosomes, which can induce breakage–fusion–bridge cycles (120,121). Cancer cells can enter a state of telomere crisis, in which their telomeres are extremely degraded. One of the characteristic aspects of this cellular state is the formation of dicentric chromosomes. Once these aberrant chromosomes enter anaphase, they can be broken, generating two additional adhesive ends that can result in more fusion and subsequent breakage events (15). This can contribute to the formation of translocations, which can cause tumorigenesis. In addition, telomere crisis has also been demonstrated to contribute toward the processes of chromothripsis and kataegis in cancer (21). Chromothripsis is the localized shattering and rearrangement of chromosomal segments, and the break sites associated with these events are susceptible to base substitutions in an event called kataegis. As a result, telomere crisis may be contributing to genome instability in cancer, a critical factor in conferring malignancy. The mechanisms that relate telomere dysfunction to tumorigenesis are actively being researched.

Telomere shortening may contribute to the observed increase in cancer prevalence with age since age-related telomere shortening could lead to an increased number of senescent cells, which exacerbates the aging process by secreting inflammatory cytokines that induce systematic oxidative damage. SASP has also been studied extensively in the context of tumorigenesis (19,20). The mitochondria–telomere axis could also contribute to SASP induction with age, as a decline in mitophagy, the accumulation of damaged mitochondria and subsequently increased mitochondrial production of ROS can significantly damage telomeres (30–32,119). In addition, p53 induced by telomere dysfunction binds to and represses the promoters of genes involved in mitochondrial homeostasis such as PGC1A, exacerbating the aging process and tumorigenesis by disrupting mitochondrial function, health and inducing oxidative stress (122). More studies on telomeres in both aging and cancer are required before such mechanisms can be validated.

Mitochondrial dysfunction in aging and cancer

The human mitochondrial genome (mtDNA) is a small circular 16.5 kb molecule. It exists in multiple copies per mitochondria and its replication is regulated by several mitochondria-specific replicative proteins, such as mitochondrial polymerase gamma A (PolγA), mitochondrial transcription factor A (TFAM), TWINKLE and the mitochondrial single strand binding protein (mtSSB). This process is not cell cycle dependent but dynamic (123). Emerging evidence has revealed the accumulation of point mutations and deletions in mtDNA during aging and in age-associated disorders (124,125). A progressive decline in mitochondrial function has been associated with aging, concomitant with the appearance of several morphological alterations in the structure and number of mitochondria. Homozygous knock-in of PolγA lacking exonuclease activity in mice results in a mtDNA mutator phenotype, a shortened lifespan and premature onset of age-associated phenotypes such as weight loss, hair graying, hearing loss, reduced fertility, cardiomyopathy and osteoporosis (126). PolγA mutations have also been linked to several age-related pathologies, including Parkinson’s disease (127,128). There is increasing evidence for the role of mitochondrial fusion in aging. The mitofusins, Mfn1 and Mfn2, are located on the outer-mitochondrial membrane and play an important role in mitochondrial membrane fusion (129). Increasing evidence suggests a role of Mfn2 in age-associated neurodegenerative disorders. In particular, Mfn2 protein and mRNA levels are decreased in the frontal cortex of Alzheimer’s disease patients (130). Another study showed that loss of Mfn1 and Mfn2 in mice affects mtDNA stability in skeletal muscle tissue, suggesting that mitochondrial fusion proteins are guardians of mtDNA integrity (131). It has been reported that increased mitochondrial fusion confers longevity in C. elegans (132). The free radicals produced by mitochondria cause oxidative damage to lipids, proteins and DNA, and these oxidized molecules accumulate during aging as well as in age-related diseases like Alzheimer’s, Parkinson’s and cancer (116,133,134). Mitochondrial metabolism plays a crucial role in longevity by regulating nutrient sensing pathways and calorie restriction, such as insulin/IGF-1 and mTOR (target of rapamycin) signaling, which are linked to the regulation of lifespan (45,135,136). In summary, current studies suggest that mitochondrial DNA and metabolism play causal roles in aging and age-associated diseases.

MtDNA mutations and deletions have also been associated with cancer progression. Low copy number of mtDNA has been reported in many cancers including hepatocellular carcinoma, prostate cancer, breast and colon cancer (137–139). The TCGA database, a database that harbors vast amounts of molecular data from 20,000 primary cancers, has confirmed that many cancer types displayed significant reduction in mtDNA content, which is shown to be linked to decreased respiratory gene expression and increased expression of immune response and cell cycle-related genes (140). Increased ROS and oxidative stress products have been reported in aging and were also found in cancer. Consistent with this, increased ROS levels have been demonstrated to induce somatic mtDNA mutations and promote metastasis (138,141). Using a chemoptogenetic approach, an organelle-specific photosensitive technique, it was reported that mitochondrial ROS directly generated DNA DSBs in telomeres (116). This study revealed telomere communication via ROS. NADH dehydrogenase or complex I, part of oxidative phosphorylation (OXPHOS), has been shown to play important role in tumorigenesis. Mutations in complex I subunit ND6 increase metastatic potential in a mouse lung carcinoma cell line in vitro (141), and mutation of another complex I subunit, ND5, promoted tumorigenesis by Akt activation and oxidative stress (142). Emerging evidence clearly links altered mitochondrial morphology, metabolism and mtDNA depletion to cancer. However, the mechanisms underlying how altered mitochondrial morphology leads to tumorigenesis or cancer-like phenotypes remains to be elucidated.

Mitophagy—the missing piece

Mitophagy, or mitochondrial-targeted autophagy, is the mechanism by which dysfunctional mitochondria are targeted for degradation and their materials are recycled. This process is summarized in Figure 1. There are several stimuli that initiate this process, including membrane potential dissipation, differentiation and hypoxia. Ubiquitin-mediated mitophagy is dependent on membrane potential dissipation. In this situation, PINK1 is prevented from entering and being degraded in the mitochondria. This permits its autophosphorylation and the subsequent activation of Parkin. Parkin then ubiquitinates mitochondrial membrane proteins, which are phosphorylated by PINK1. These phosphorylated polyubiquitin chains are next recognized by autophagy adaptor proteins like p62 and OPTN, which are then bound by LC3 to mediate the formation of an autophagosome around the target mitochondrion. Receptor-mediated mitophagy involves receptors such as FUNDC1, Bcl-2-L13, BNIP3 and NIX. These receptors are all activated in distinct cellular scenarios to stimulate mitophagy. For instance, BNIP3 and NIX are upregulated by HIF-1α in scenarios of hypoxia. The activated versions of these receptors recruit LC3 and subsequently initiate the formation of the autophagosome (28,29).

Figure 1.

Shown below is a diagram displaying the two predominant forms of mitophagy and the general workflow of this quality control mechanism. PINK1/Parkin-mediated mitophagy, as displayed on the lower half of the above mitochondria, is induced by a decrease in mitochondrial membrane potential. This induces the autophosphorylation of PINK1, which is subsequently phosphorylates and activates Parkin. Finally, Parkin polyubiquitinylates mitochondrial membrane proteins and these ubiquitinylated proteins are recognized by LC3-II. Receptor-mediated mitophagy occurs when a stimulus, such as hypoxia, results in the overexpression or activation of receptors on the surface of mitochondria. This results in the recruitment of LC3-II and subsequent autophagosome formation. Finally, lysosomes fuse with autophagosomes to form autolysosomes and the mitochondria are degraded.

There is substantial evidence implicating this mitochondrial quality control mechanism in cancer, but its role in tumor formation and progression seems to be context dependent. It has been previously observed that Parkin is a tumor suppressor gene, and that Parkin loss is associated with the induction of the Warburg effect (143–146). In glioblastoma, it has been observed that the loss of PINK1 induces the Warburg effect by increasing ROS, stabilizing HIF-1α and reducing the activity of pyruvate kinase muscle isoenzyme 2 (147). A deficiency of BNIP3 has also been associated with increased glycolytic flux (148). Indeed, diminished BNIP3 expression is observed in the context of pancreatic and breast cancer progression (149,150). Further, Bcl-2-L13 downregulation is associated with poorer prognosis and therapy resistance in breast and rectal cancers, respectively (151,152).

Despite these observed tumor-suppressive effects of mitophagy machinery, there are a number of studies indicating that mitophagy levels are increased in certain cancers. For instance, BNIP3 is upregulated in breast cancer brain metastases (153). Combined with other reports indicating that BNIP3 is downregulated in breast cancer, this suggests that mitophagy levels may vary based on the types of tumor. This mitophagy receptor is also upregulated in other cancers, such as ovarian cancer and glioblastoma (154,155). FUNDC1 is consistently upregulated in breast, cervical and laryngeal cancer (36). Oncogene mutations have also been shown to result in increased mitophagy levels (156). These observations point to the complex role of mitophagy in cancer (36).

Mitophagy has been observed to decline with age, leading to the accumulation of damaged mitochondria (25). Although the molecular mechanism remains unclear, evidence implicates the age-associated decline in NAD⁺ as a key contributor to impaired mitophagy (24,37). NAD⁺, a vital cofactor for many cellular processes, is a key substrate in the DDR. Poly-ADP ribosylation is a key DNA damage recognition and repair step in which PARP1 utilizes NAD⁺ to produce long polymers of ADP-ribose that act as a signal for DNA repair proteins. DNA damage accumulation with age results in excessive PARP1 activation, leading to a decrease in the cellular NAD⁺ pool (37). This decrease seems to exacerbate pathology through diminished mitophagy in progeroid syndromes caused by deficiencies in DNA damage recognition and repair. For instance, the pathology of XP is associated with diminished levels of mitophagy resulting from increased PARP1 activity and a subsequent decrease in both NAD⁺ and sirtuin 1 (SIRT1) activity (34). The SIRT1 protein is an NAD⁺-dependent deacetylase that regulates a number of cellular functions, namely mitophagy. This is supported by the observation that SIRT1 is required for adenine monophosphate kinase activation, which ultimately activates ULK1 (157,158). The ULK1 protein is a critical mediator of the autophagy process. Significantly, inactivation of ULK1 results in the accumulation of p62 and the accumulation of damaged mitochondria (157). This observation may be explained molecularly by recent observations of this protein’s role in stimulating FUNDC1 and Bcl2-L-13-mediated mitophagy (159,160). Competition occurs between SIRT1 and PARP1 for cellular NAD⁺, which accounts for observation made in the context of XP (34). In line with this observation, supplementation with NAD⁺ rescues mitophagy in the context of XP (34).

Another area of active research is the changes in levels of NAD⁺-associated metabolites with age. These studies reveal the mechanisms underlying the decline in NAD⁺ levels with age, and how alterations associated with the NAD⁺ metabolome may also drive age-related pathologies. The decline in NAD⁺ with age is accompanied by increased NADH levels, which indicates that redox homeostasis may be hindered during aging causing the decline in NAD⁺ (161,162). In a recent human study, plasma levels of NADP⁺ and NAAD diminished, while levels of NAM and MeNAM increased with age (163). This study provides evidence that NAD⁺ synthesis pathways are also being impacted during the aging process. WRN-KD cells also exhibit diminished NAD⁺, NMN, MeNMN and NMNAT1 levels, suggesting that NAD⁺ synthesis pathways are impacted by age (41).

Though mitophagy levels vary considerably amongst cancer types, strong evidence suggests that the age-related decline in mitophagy may promote tumorigenesis. A decline in mitophagy can result in the formation of ROS and metabolic inefficiency via the accumulation of damaged mitochondria (30–32). The increased oxidative stress that occurs with diminished mitophagy may predispose cells to enter telomere crisis following tumor suppressor loss due to concomitant telomere degradation (119). Oxidative stress caused by ROS or other sources causes cells to enter senescence as a response to telomere degradation (164). Oxidative DNA damage has also been implicated in the formation of CCFs and the stimulation of senescence and SASP, which in turn contributes to tumorigenesis. Senescent cells contribute to tumorigenesis through the SASP response. Oxidative DNA damage has been implicated in the formation of CCFs (65,165). More directly, the accumulation of damaged mitochondria may also induce oxidative DNA damage in the genome, which can lead to increased tumorigenesis (166,167). More extensive experimentation is required to better elucidate the mechanisms that implicate mitophagy in the increase in cancer prevalence with age.

Conclusion

In this review, we discuss the age-related changes in cellular physiology that link aging with cancer (Figure 2). With age, the accumulation of DNA damage in the genome contributes to tumorigenesis by increasing the likelihood of oncogenic mutations. Telomere shortening with age also contributes to this increase by leading to the accumulation of senescent cells in tissues, increasing the inflammatory burden. Telomere dysfunction with age may also induce changes in mitochondrial homeostasis, which may cause cells to become neoplastic. Age-related mitochondrial dysfunction contributes to the age-related increase in cancer prevalence as a result of associated radical species and ROS formation. Specifically, the age-related decline in mitophagy that accompanies DNA damage accumulation promotes the tumorigenic potential of dysfunctional mitochondria, as they remain in the cellular environment for longer periods of time. The accumulation of DNA damage and telomere dysfunction with age seem to impact mitochondrial homeostasis and contribute to the age-related increased risk of cancer.

Figure 2.

This diagram displays a summary of the mechanisms we have proposed in this review as contributors to the increase in cancer risk with age. In young tissue, robust nuclear DNA repair results in a healthy genome, intact telomeres and abundant NAD⁺ for activation of sirtuins. Additionally, damaged mitochondria are readily recycled through mitophagy. In contrast, in aged tissue, there is increased nuclear DNA damage, leading to constitutive PARP activation and limiting levels of NAD⁺. Nuclear DNA damage results in cytoplasmic chromatin fragments, telomere attrition and nuclear to mitochondrial signaling. Loss of sirtuin activity due to diminished NAD⁺ inhibits mitophagy, resulting in persistent dysfunctional mitochondria. These conditions foster a tumorigenic environment.

Further exploration into how aging predisposes individuals to cancer will lead to more effective therapeutic developments to target proper pathways and prevent tumorigenesis with age. For instance, NAD⁺ is a cofactor that seems to play a critical role in the DDR response, telomere maintenance and mitochondrial homeostasis (114,115,168,169). Extensive studies have demonstrated the therapeutic potential of NAD⁺ supplementation in a variety of diseases, as well as the aging process, highlighting the strategy’s potential in ameliorating many of the symptoms of tumorigenesis and aging (34,45,115,170). Therefore, it would be of continued interest to explore the antitumorigenic potential of this and similar therapeutic strategies.

Funding

This work was supported by the Intramural Research Program, National Institute on Aging, NIH. J.P. was financially supported by NIH grants 8038673 and 8038919. E.K. was supported by NIH grant 8033483.

Conflict of Interest Statement: The Bohr laboratory receives material support from Chromadex.

Glossary

Abbreviations

ASC

adult stem cell

A-T

ataxia-telangiectasia

CCF

cytosolic chromatin fragment

CSC

cancer stem cell

DDR

DNA damage response

DSBR

double strand break repair

HCPS

hereditary cancer predisposition syndrome

HGPS

Hutchinson–Gilford progeria syndrome

HR

homologous recombination

NER

nucleotide excision repair

ROS

reactive oxygen species

SASP

senescence-associated secretory phenotype

WS

Werner syndrome

XP

xeroderma pigmentosum