Introduction

Mitochondria are the main sites of aerobic respiration of eukaryotic cells and the energy power stations of cells, and they are involved in indispensable functions of the cell, including adenosine triphosphate (ATP) production, calcium ions (Ca2+) regulation, reactive oxygen species (ROS) production and scavenging, regulation of apoptotic cell death, etc. Mitochondrial dysfunction is largely involved in aging, cancer, age-related neurodegeneration, and metabolic syndrome [1, 2]. Autophagy is an intracellular catabolic process of delivering cytosol and/or its specific content to the lysosomes for degradation [3]. Mitophagy is a special form of autophagy; autophagy lysosomes selectively remove damaged, folded, and redundant mitochondria, especially for damaged cytotoxic mitochondria, to maintain the balance of mitochondrial quality and quantity, which is essential for normal cellular physiology and tissue development [4]. When mitochondria are stimulated by stresses such as nutritional deficiencies, hypoxia, DNA damage, inflammation, and mitochondrial membrane depolarization, mitophagy is activated, and damaged mitochondria can be selectively cleared by cells through the mitophagy pathway, thereby maintaining the structural and functional integrity of mitochondria [5]. However, when the mitophagy function is abnormal, the damaged mitochondria cannot be cleared in time, and they generate a large amount of ROS. On the one hand, the ROS damage normal mitochondria and promote the inflammatory response; on the other hand, it can release apoptosis factors and induce apoptosis [6, 7]. Abnormal mitophagy can also cause mitochondrial dysfunction, which affects metabolic reprogramming in response to stress, changes the determination and differentiation of cell fate, and thus affects the occurrence and aetiology of diseases, including cancer [8] (Fig. 1).

Figure 1. The role of mitophagy in normal physiology and human disease. Mitophagy plays an important role in maintaining mitochondria homeostasis and various aspects of cellular function

In recent years, the regulatory mechanism of mitophagy and its impact on human physiology and pathology has been the focus of many researchers. Dissection of the functions of some of the regulators and molecular adaptors involved in targeting mitochondria to the autophagosome has increased our understanding of how mitophagy is initiated and executed. The most widely used among these mitophagy regulators are Parkin and PTEN-induced kinase 1 (PINK1), as well as BCL-2 adenovirus E1B 19 kDa-interacting protein 3 (BNIP3) and BNIP3-like (BNIP3/NIX), which plays a unique and non-overlapping role in promoting mitophagy. Other participants [such as multifunctional E3 ubiquitin ligase (Mul1) and FUN14 domain-containing 1 (FUNDC1)] are emerging and may become the focus of future research [9–11] (Fig. 2). Also, drugs can affect mitophagy by interfering with autophagosome formation, hypoxia-inducible factor (HIF) activation, autophagosome and lysosome fusion, and ubiquitination under hypoxia conditions [12].

Figure 2. The main mechanisms and related proteins of the typical mitophagy pathway are summarized. PTEN-induced kinase 1 (PINK1)/Parkin-mediated ubiquitination of mitochondrial proteins connects the autophagy cargo receptor sequestosome 1 (p62/SQSTM1) and optineurin (OPTN) to mitochondrial/autophagosome interactions. In addition, BCL-2 adenovirus E1B 19 kDa-interacting protein 3 (BNIP3), NIX, and FUN14 domain-containing 1 (FUNDC1) can directly bind to light chain 3 (LC3) molecules that modify autophagosomes, and the mechanism is regulated by their phosphorylation status

Cancer is a worldwide problem. How to prevent and treat cancer has always been the focus and difficulty of research. The impact of mitophagy on tumours has both beneficial and harmful aspects. Regulating mitophagy to inhibit the occurrence and development of tumours can provide new targets for tumour treatment in the clinic.

Mitophagy is the enemy of tumours: mitophagy can inhibit tumour development

Increasing evidence indicates that defects in the mitophagy machinery may play a role in the progression of cancer [13]. Parkin (PARK2) and PINK1 (PARK6) are key mediators of mitophagy; PINK1 is a kinase localized to mitochondria, whereas Parkin is an E3 ubiquitin ligase located in the cytosol [14, 15]. Mutations of PINK1 and Parkin are often detected in a variety of tumours, including lung cancer [16], ovarian cancer [17], glioblastoma/glioma [18], colorectal cancer [19], and breast cancer [20] (Tab. 1). It has been found that the PARK2 gene is significantly associated with adenomatous polyposis in human colorectal cancer, and PARK2 overexpression can inhibit the proliferation of colon cancer cells [19]. Also, after crossing PARK2 gene knockout mice with colorectal adenomatous polyposis, the development of intestinal adenomas in newborn mice was rapidly accelerated and polyp diversity increased, indicating that PARK2 is a tumour suppressor gene [13]. Another cancer often observed with abnormal Parkin is glioblastoma multiforme. Using data from the cancer genome atlas, it was found that about a quarter of glioblastoma multiforme specimens have PARK2 heterozygous or homozygous deletions [18]. In addition, in the absence of somatic mutations in PARK2, p53-mediated loss of Parkin transcription is considered to be a factor that promotes glioma development [21]. Neuroblastoma is a solid extracranial tumour that occurs in the sympathetic nervous system. Recently, mutations in mitophagy were found in patients with neuroblastoma, including in PARK6 [18]. A subtype of neuroblastoma known to have a poor prognosis is characterized by the oncogene MYCN amplification. Neuroblastoma cell line N-Myc and Parkin protein levels are negatively correlated [22]. Like glioblastoma, Parkin deletion is a common mutation in lung cancer. Due to autophagy inhibition, the upregulation of cyclin E138 and the accumulation of ROS are considered to be the drivers of lung cancer tumour progression [23]. The absence of mitophagy also affects the progression of breast cancer. The PARK2-containing FRA6E fragile region is commonly lost in some breast cancers, and changes in PARK2 copy numbers are relatively frequent in aggressive triple-negative tumours, supporting the role of Parkin as a tumour suppressor [20]. Mitophagy also plays a role in breast cancer by affecting adjacent stromal cells to alter the tumour microenvironment. Therefore, the ablation of PINK1/Parkin-mediated mitophagy and alternative forms of mitophagy can promote the progression of breast cancer [24]. Moreover, according to Biel and Rao, breast cancer cells use mitophagy to exert therapy resistance [25]. Biel and Rao highlight that anti-mitophagy molecules such as the mitochondrial translocator protein (TSPO) are overexpressed in aggressive forms of cancer. However, mitophagy is induced just in the aggressive breast cancer cell line MDA-MB-231 and not in the primary mammary epithelial cells (MCF-12A) [25]. Sublethal concentrations of TPP-conjugated redox molecules, including mitoapocynin (MitoA) and mitoquinone (MitoQ), exploit the increased ∆ψm reported in aggressive cancer cells leading to continued mitophagy [25]. Ovarian cancer shares some key features with neuroblastoma and breast cancer. N-Myc and cyclin E are both upregulated in these tumour subpopulations [26, 27]. N-Myc proved to be a transcription inhibitor of Parkin1, while cyclin E accumulated when Parkin was deleted [16, 22]. It is interesting to speculate that Parkin may indirectly act as a potential cell cycle regulator in ovarian cancer.

The hypoxic environment provides a good microenvironment for cancer stem cells, increasing the possibility of cancer progression and metastasis. Hypoxia will activate hypoxia-inducible factor la (HIF1a). HIF1a is the main transcription and regulatory factor for cell adaptation to hypoxia, which can regulate the expression of apoptosis related proteins BCL-2 and BNIP3. BNIP3 is a pro-apoptotic BH3-only protein associated with the pathogenesis of many diseases, including cancer and cardiovascular disease, it can enhance mitophagy by inhibiting damaged mitochondrial fusion and making damaged mitochondria easier to eliminate [29]. For example, in malignant glioma cells, BNIP3 expression is increased under the induction of ceramide, which further activates mitophagy and leads to cancer cell death [30]. Similarly, ethyl 3,4-dihydroxybenzoate activates mitophagy due to the high expression of BNIP3 when acting on oesophageal cancer cells, eventually leading to cancer cell death [31]. In a breast cancer aggressive mouse model, BNIP3 was also shown to inhibit tumour progression. In some cases, like Parkin loss, the loss of BNIP3 promotes tumour growth and metastasis by turning cells to tumourigenic glycolysis and increasing ROS production [32]. Mitochondrial outer-membrane protein, FUNDC1, is a mitophagy receptor that clears dysfunctional mitochondria in response to hypoxia and mitochondrial stresses; under hypoxic conditions, protein phosphatase 5 dephosphorylates FUNDCl, promoting advance FUNDCl to interact with microtubule-associated protein 1 light chain 3 (LC3) to promote mitophagy [33, 34]. Wenhui Li et al. found that the role of FUNDC1-mediated mitophagy in hepatocarcinogenesis (HCG) inhibited tumour initiation. In hepatocytes, due to the excessive activation of inflammatory bodies, the loss of FUNDC1 increases the susceptibility to HCG. However, in the later stages of tumour development, increased FUNDC1 expression is beneficial to tumour growth [35].

Divalent cation transporter 1 (DCT-1) is a transcription factor, a nematode homologue of nuclear factor erythroid 2-like 2 (NFE2L2); its expression is upregulated under mitophagy, and mediated by skinhead-1 (SKN-1), it is activated during oxidative stress to maintain mitochondrial homeostasis [36]. As a new regulatory factor of mitophagy, SKN-1 can stimulate the expression of core mitochondrial components and promote the assembly of new mitochondria and mitophagy regeneration [37]. It is worth noting that another mitophagy regulator, nuclear factor erythroid 2-related factor 2 (NRF2), is the main regulator of cellular redox homeostasis [38]. In recent years, it has been found that NRF2, as a part of cell protection gene expression regulator, affects mitochondrial function [39]. Studies using isolated mitochondria and cultured cells have shown that NRF2 deficiency can lead to impaired mitochondrial fatty acid oxidation, respiration, and ATP production. The small molecule activator of NRF2 supports mitochondrial integrity by promoting mitophagy and giving antioxidant stress-mediated osmotic transition [40]. The combined action of the above factors can also counteract the Warburg effect (the Warburg effect, also known as aerobic glycolysis, is defined as the propensity of cancer cells to take up high levels of glucose and to secrete lactate in the presence of oxygen ) to increase the stability of mitophagy and autophagy, thereby protecting mitochondrial metabolism from carcinogenic transformation [41]. In addition, the protective effect of NRF2 activation in preventing chemical and radiation-induced carcinogenesis has been fully affirmed [42, 43]. NRF2 can ensure the rapid enzymatic modification and excretion of chemical carcinogens, and through the expression of its target genes to quench ROS or repair oxidative damage to prevent cancer, this anti-cancer effect of NRF2 has been extensively reviewed elsewhere [44, 45].

Another major protein that mediates mitophagy and tumour is insulin/insulin-like growth factor (IGF). IGF-1 signalling through phosphoinositide 3-kinase/protein kinase B/mammalian target of rapamycin complex 1 (PI3K/AKT/mTORC1) signalling pathway has a wide range of effects in cell growth, oncogenic transformation, and cancer progression [46]. In addition, IGF-1/PI3K signalling can stimulate mitochondrial biogenesis and mitophagy [47], mainly through the induction of peroxisome proliferator-activated receptor g coactivator 1 (PGC-1) transcription activator and PGC-1 related coactivator (PRC), and the induction of mitophagy important mediator BNIP3 [32]. In this way, IGF-1 signal transduction enables cells to maintain a healthy mitochondrial pool to promote cell bioenergy efficiency and mitochondrial homeostasis. Amy Lyons et al. suggested that IGF-1-mediated mitochondrial protection plays a major role in metabolic reprogramming and resistance to targeted cancer treatments [48]. The continuously increasing IGF-1 receptor (IGF-1R) signal maintains the healthy growth of mitochondria while eliminating dysfunctional mitochondria, to prevent the accumulation of ROS, thereby improving the ability of cells to resist stress [48]. Specific targeting of insulin and IGF receptors in cancer cells is a potentially effective strategy for anti-cancer therapy. Because mitophagy confers some beneficial regulatory effects on low insulin signal transduction, increased insulin/IGF signal transduction can induce specific types of tumour mitophagy in mitochondria, thereby inhibiting tumour progression.

Mitophagy is the friend of tumours: mitophagy can promote tumour development

The beneficial effects of mitophagy are discussed in many pathological processes, including high-fat diet-mediated fatty liver, myocardial hypoxia-reoxygenation injury, oxidative stress-induced endothelial dysfunction, and acute kidney injury [49, 50]. The protective effect of mitophagy is mainly achieved by clearing ROS, reversing calcium balance, staining mitochondrial membrane permeability, and inhibiting mitochondria-dependent cancer death [51]. Tylichova et al. found that the use of short-chain and N-3 polyunsaturated fatty acids on human colon cancer HT29 and HCT116 cells could induce mitophagy, which had a protective effect on colon cancer cells and prevented them from being affected by drugs [52]. Mitophagy can avoid the differentiation of cancer cells induced by sodium butyrate and programmed death of cancer cells, and the damaged mitochondria can be digested, decomposed, and reused, and the mitophagy pathway can be used to promote the survival of tumour cells [52]. Studies have also found that hypoxia can promote autophagy by regulating the translation level of HCT116 in colon cancer cells. In this process, lysosomal glycoprotein prosaposin (PSAP) and lysosome-associated membrane protein-2 (LAMP2), which are highly conserved, induce mitophagy by upregulating the translation level, thus protecting tumour cells [53]. In addition, 12 kinds of mitochondrial-specific glycoproteins (MRPL36, MRPL12, MRP63, MRPL41, MRPS7, MRPS34, MRPS16, MRPL43, MRPL34, MRPS12, MRPL38, and MRPS26) can also promote mitophagy by up-regulating the translation level of colon cancer cells, which in turn is beneficial to the survival of HCT116 colon cancer cells [53].

Other factors that regulate hypoxia-induced mitophagy also play an important role in cancer cell homeostasis and tumour progression. The most common ones are Src and casein kinase 2 (CK2). Src family kinases (SFK) play a key role in cell adhesion, invasion, proliferation, survival, and angiogenesis during tumour development [54]. Overexpression or high activation of SFKs often occurs in tumour tissues, and they are the central mediators of many important signal pathways in tumourigenesis. SFK can affect cell proliferation through the Ras/ERK/MAPK pathway and can regulate gene expression through transcription factors such as STAT molecules [55]. CK2 is a serine/threonine-protein kinase that is highly conserved among eukaryotes and plays roles in many different cellular processes, such as cell survival, cell polarity, cell cycle regulation, stress responses, transcription and translation, and regulation of mitophagy by directly phosphorylating Atg32 [56]. The increased expression of CK2 in mice leads to the occurrence of various types of cancer (including solid and blood-related tumours, such as breast cancer, lymphoma, etc.), revealing its carcinogenic properties [57]. CK2 is in many key tumour-related organisms; it plays an important role in the process of learning, including cell apoptosis, DNA damage response, and cell cycle regulation. CK2 has become a potential anti-cancer target [57]. Different CK2 inhibitors have been developed for various cancers with anti-tumour properties.

Studies have shown that caveolin 1 (Cav-1) is a powerful biomarker of tumour progression and metastasis in the cell matrix. ROS released by cancer cells can induce the loss of Cav-1 in the matrix and further induce the metabolic changes of the tumour cell-matrix, leading to mitochondrial dysfunction and further enhancing mitophagy [58]. The loss of stromal Cav-1 not only predicts early tumour recurrence, lymph node metastasis, and poor survival rate, but also predicts the progression of ductal carcinoma in situ in patients to aggressive disease, suggesting that the loss of interstitial Cav-1 can regulate tumour progression [59]. There is a novel idea called the “autophagic tumour stroma model of cancer”, i.e. the loss of stromal cells Cav-1-induced autophagy/mitophagy in tumour stroma through oxidative stress; this creates a catabolic micro-environment in which the local accumulation of chemicals and circulating nutrients (such as amino acids and nucleotides) directly supplies cancer cells for survival and growth [60]. In addition, Cav-1 can negatively regulate transforming growth factor b (TGF-b), and the activation of TGF-b signalling is important for inducing mitophagy in tumour stromal cells [61]. In fact, in tumour stromal cells, the activation of TGF-b signalling that depends on paracrine or autocrine can induce abnormal metabolism of the tumour microenvironment, leading to oxidative stress, increased autophagy/mitophagy and glycolysis, and down-regulation of Cav-1. These metabolic changes can spread between neighbouring fibroblasts to maintain tumour cell growth [62, 63].

Breast cancer type 1 susceptibility protein (BRCA1) is a well-recognized tumour suppressor gene [64]. However, BRCA1-deficient ovarian cancer cells influence the metabolism of fibroblasts in the surrounding stroma by producing high levels of hydrogen peroxide, which activates nuclear factor-kappa B (NF-kB) signal transduction to induce mitophagy in tumour stroma cells, thereby promoting tumour growth [65]. Some cytokines or other bioactive factors, such as migration stimulating factor (MSF), as a genetically truncated N-terminal isoform of fibronectin that is highly expressed during mammalian development in foetal fibroblasts, and during tumour formation in human cancer-associated myofibroblasts, can activate the TGF-b and CD42-NF-kB in tumour stromal cells induced by mitophagy and promote tumour cell growth [66].

In general, mitophagy is activated by oncogenic signal transduction pathways, including the TGF-band NF-kB pathways, which regulate cancer cell metabolism to promote tumour cell growth. Therefore, further inhibition of mitophagy in tumour stroma may be a feasible strategy to inhibit tumour cell growth by neutralizing ROS or inhibiting metabolic uncoupling to inhibit TGF-b and NF-kB signal transduction levels in tumour cells and their surrounding stromal microenvironment. This provides useful hints for targeting mitophagy and a potential anticancer strategy.

Conclusions

Mitophagy is becoming a link between cells and physiology. Transcription factors related to mitophagy can also affect the development process of tumours, and the degree of mitophagy is crucial for the occurrence and development of tumours. Damage of mitophagy in healthy tissues can promote the formation and development of tumour cells, while mitophagy in hypoxic tumours can induce the survival of tumour cells. How to fully understand and utilize the regulatory mechanism of mitophagy to inhibit the development of tumours and avoid its beneficial side to tumours can be regarded as a research direction for the treatment of tumours.

Acknowledgements

This study was funded by Natural Science Foundation of Ningbo City (grant number 2021J031).

Ethics approval

Not applicable

Competing interests

Not applicable.

Authors’ contributions

F.H. contributed to all aspects of this study and article. L.L. contributed to the study conception, design, data analysis and interpretation, and the critical revision of the article. All authors read and approved the final version of the manuscript.

Funding

This study was funded by Natural Science Foundation of Ningbo City (grant number 2021J031).

Disclosure statement

The authors declare that they have no potential conflicts of interest related to this work.

Availability of data and materials

All data used and/or analysed during the present study are available from the corresponding author on reasonable request.