Abstract
The effectiveness of chemotherapy is largely limited by drug resistance. In the past few decades, modulation of anticancer drug resistance plays little role in benefit of clinical outcomes due to complex drug resistance mechanisms. Mitophagy, an important mitochondrial quality control system, selectively degrades excessive or damaged mitochondria by autophagy. Accumulating reports are suggesting that dysregulation of mitophagy contributes to neoplastic progression and drug resistance in various types of tumors. Mitophagy was originally thought to be an onco-suppressor that maintains cellular homeostasis and prevents oncogenic transformation. On the other hand, mitophagy promotes cancer cell survival under cytotoxic stress by degrading damaged mitochondria and reducing mitochondrial reactive oxygen species. Therefore, induction and inhibition of mitophagy in cancer drug resistance are controversial. In this review, we summarize current knowledge on the dual role of mitophagy in cancer drug resistance.
Although chemotherapy is one of the most widely used strategies for cancer therapy, its effectiveness is limited by drug resistance (1, 2). Complex mechanisms mediate drug resistance in cancer, such as alterations in the drug target, activation of pro-survival pathways and ineffective induction of cell death (2). In the past few decades, modulation of anticancer drug resistance has played little role in the benefit of clinical outcomes (3). Therefore, there is an urgent demand for enhancing anticancer drug sensitivity via discovery of new appropriate targets.
Mitophagy, a selective autophagic process, has been shown to promote tumorigenesis and cell survival in various types of tumors by removing abnormal mitochondria (4, 5). The toxicity of most chemotherapeutic agents is partially attribute to the induction of mitochondrial dysfunction and oxidative stresses (6, 7). Thus, the rapid clearance of damaged mitochondria by mitophagy is thought to mediate drug resistance in cancer cells (5). In addition, excessive mitochondrial clearance may induce cell metabolic disorders and cell death (8). Therefore, mitophagy likely plays a dual role in cancer drug resistance depending on different conditions and cell types. In this review, we focus on current research efforts to identify and overcome underlying mechanisms of mitophagy-related drug resistance.
Biological Role of Mitophagy
Mitochondria are crucial organelles for energy metabolism, regulation of cell signaling and apoptosis in mammalian cells (9). In order to maintain cellular homeostasis, the cell has evolved complex systems for the quality control of mitochondria. One of these systems, mitophagy, selectively degrades excessive or damaged mitochondria by autophagy in response to various stresses (Figure 1) (10, 11). Autophagy is an essential catabolic pathway that degrades proteins or other cellular components within lysosomes (12). Differently from canonical autophagy, mitophagy selectively targets mitochondria which have been marked by mitophagy receptors. These multiple receptor systems mainly include Parkin/PTEN-induced putative kinase 1 (PINK1) receptor system, microtubule-associated proteins 1 light chain 3 (LC3)-interacting region (LIR)-containing receptor system [e.g. B-cell lymphoma 2 (BCL2)-interacting protein 3 (BNIP3) and BNIP3-like (BNIP3L)] and lipid-mediated system (e.g. ceramide) (13, 14).
Autophagy can act as a survival mechanism for cancer cells in response to various stresses (15-17), but the role of canonical autophagy in cancer remains unclear (5, 18, 19). Therefore, how about mitophagy in cancer? Mitophagy is causally linked to various physiological processes and diseases, for example defective mitophagy will impair survival of memory natural killer cells, conferring Parkinson's disease and cardiac defects (13, 20, 21). Furthermore, evidence indicates that dysfunction of mitophagy also promotes tumorigenesis and neoplastic progression (4, 5, 14). Guo et al. reported that the induction of mitophagy in macrophages prevents the progression of colitis-associated cancer (22). Moreover, tumorigenesis and metastasis are also promoted by mitochondrial reactive oxygen species (ROS) (23). According to Chourasia et al., BNIP3 loss, and ensuing defects in mitophagy, leads to ROS production and mammary neoplastic progression to metastasis (4).
Manipulating Mitophagy as a Potential Target for Cancer Therapy
Autophagy regulators (e.g. chloroquine, hydrochloroquine) have entered dozens of clinical studies on anticancer therapy (See https://clinicaltrials.gov/ct2/results?term=autophagy+and+cancer). There is great interest in manipulating mitophagy in order to improve cancer therapy, while these strategies are considerably confusing (5, 24, 25). The functional outcome of mitophagy-induced cell death or survival dependent on cancer therapy relies on different therapeutic treatments and cell types (Figure 2).
Induction of mitophagy increases cancer cell death and chemotherapy sensitivity. Ceramide, a central molecule of sphingolipid metabolism, is involved in the regulation of mitophagy (26). Studies show that CerS1/C18-ceramide selectively mediates lethal mitophagy, which is B-cell lymphoma 2-associated X (BAX)/BCL2 antagonist/killer 1 (BAK1)- and caspase-independent (24, 27). Ceramide induces the formation of lipidated LC3, which then binds ceramide on the mitochondrial membrane upon dynamin-related protein 1 (DRP1)-mediated mitochondrial fission, targeting autophagolysosomes to mitochondria and leading to lethal mitophagy in cancer cells (24, 27). Excessive mitochondrial clearance in the absence of mitochondrial biogenesis or metabolic plasticity likely induce cell metabolic disorders and death in cancer cells (8). Moreover, DRP1-mediated fission is also a prerequisite for mitophagy induced by BNIP3, which is a protective response activated by cardiac myocytes (28). Understanding the underlying mechanism for different effects of DRP1 between cancer cells and cardiac myocytes may help in better anticancer drug formulation.
FMS-like tyrosine kinase 3 (FLT3) is a membrane-bound receptor tyrosine kinase expressed by immature hematopoietic cells (29). Mutations of FLT3 have been detected in about 30% of patients with acute myelogenous leukemia (AML), and the most common activating FLT3 mutation is an internal tandem duplication (ITD) in the juxtamembrane domain (FLT3-ITD) (29). FLT3-ITD inhibitors, such as sorafenib, quizartinib (AC220), and crenolanib, showed efficacy for therapy in preclinical models of AML, but not in clinical trials because of the development of drug resistance (30). Interestingly, LCL-461, a mitochondria-targeted ceramide analog drug, is efficacious in attenuating crenolanib resistance by inducing lethal mitophagy in FLT3-ITD+ AML in vitro and in vivo (27). This lethal pathway of mitophagy induction by ceramide and ceramide analog drug is likely be a potential target in FLT3-ITD+ AML, and further studies will be needed to determine if ceramide will be effective in other cancer types.
Inhibition of mitophagy enhances drug sensitivity. A great number of studies have demonstrated that mitophagy can be inhibited through genetically or pharmacologically targeting different stages of the autophagic/mitophagic process, such as silencing of autophagic/mitophagic genes (e.g. autophagy-related 5 (ATG5), Parkin, PINK1, BNIP3 and BNIP3L) (5, 25, 31), blocking the formation of autophagosomes by phospho-inositide 3-kinase (PI3K) inhibitors (e.g. 3-methyladenine and LY294002) (25, 31) and inhibiting the fusion of autophago-somes with lysosomes or the degradation capacity of autophagolysosomes (e.g. chloroquine, bafilomycin A1, leupeptin and liensinine) (25, 31, 32).
Classical inhibitors of mitophagy. Doxorubicin, a DNA-damaging agent, is believed to cause toxicity by inducing mitochondrial dysfunction and enhancing superoxide formation (33, 34). Our previous study found that expression of BNIP3L, a regulator of mitophagy, significantly increased in colorectal cancer stem cells after doxorubicin treatment (5). The inhibition of mitophagy by BNIP3L silencing significantly enhanced doxorubicin sensitivity in colorectal cancer stem cells (5). But the mechanism for the regulation of BNIP3L in cancer stem cells is still unclear.
Salinomycin, an antibacterial and coccidiostat ionophore drug, also has a novel anticancer role through mitochondrial hyperpolarization (35). Mitochondrial dynamic fusion and fission mechanisms contribute to alterations in mitochondrial mass and maintenance of mitochondrial function in salinomycin-treated cells. The most important pro-survival mechanism is mitophagy, which eliminates salinomycin-induced dysfunctional mitochondria. Dysfunctional mitochondria may actually consume ATP, and generate excessive amounts of harmful ROS, thus contributing to loss of cellular homeostasis. When combined with mitophagy inhibition by ATG5 knockout, toxicity of salinomycin towards cancer cells and cancer stem cells was enhanced (35).
A variety of potential drugs are currently being tested for anticancer treatments. Mitophagy inhibition in combination with some potential anticancer drugs also increases cytotoxicity. UNBS1450 is a sodium channel antagonist potentially for the treatment of cancer. In stromal neuroblastoma SH-N-AS cell line, efficient mitophagy is the key mechanism causing the failure of activation of the apoptotic pathway, which increased resistance to UNBS1450 (32). Inhibition of autophagy by small inhibitory RNAs targeting ATG5, autophagy related 7 (ATG7) and Beclin-1 sensitized SK-N-AS cells by reactivating apoptosis (32).
The process of mitophagy in mammalian cells.
The controversial role of mitophagy in cancer drug resistance.
Novel inhibitors of mitophagy. Liensinine, an isoquinoline alkaloid, is extracted from the seed embryo of Nelumbo nucifera Gaertn. The mitophagy-inhibitory effects of liensinine enhance sensitivity of breast cancer cells to classical chemotherapeutic drugs (e.g. doxorubicin, paclitaxel, vincristine, and cisplatin) (25). Liensinine induced the accumulation of mitophagosomes by inhibiting autophagosome–lysosome fusion (25). Although the effect of liensinine was similar to that of chloroquine, hydroxychloroquine, and bafilomycin A1, the mechanism of liensinine-mediated blockade of autophagosome–lysosome fusion was different (25). The latter agents alkalinized the lysosomal pH to suppress the fusion between with autophagosomes and interfere with the action of lysosomal hydrolases (25). However, the lysosomal pH was not changed in response to liensinine treatment, suggesting that alteration of lysosomal pH is not necessary for inhibition of autophagosome–lysosome fusion and cathepsin maturation mediated by liensinine (25). The small GTP-binding protein ras-related protein 7A (RAB7A) has a role in the late endocytic pathway, lysosome biogenesis, and the final maturation of late autophagic vacuoles (36). Liensinine reduced recruitment of RAB7A to lysosomes, thus inhibiting the endocytic pathway-dependent cathepsin transport to lysosomes, and blocking autophagosome–lysosome fusion (25). It is likely one potential mechanism through which the inhibition of autophagy/mitophagy by liensinine sensitizes tumor cells to doxorubicin-induced apoptosis.
Conclusion and Perspectives
Although there was no consensus that mitophagy acts either as suppressor or inducer of cancer drug resistance in specific cancer types, the most appropriate treatment for each patient depending on the cancer type and anticancer drug can be selected. Whilst the induction of mitophagy by ceramide contributes to sensitivity to FLT3-ITD inhibitors in FLT3-ITD+ AML, further studies will be needed to determine if ceramide is effective in other cancer types. The inhibition of mitophagy is likely more suitable for combination with chemotherapeutic agents in various types of cancer. Further studies are warranted to elucidate how mitophagy is regulated in different cancer types and in anticancer drug treatments, which may enlighten the development of novel strategies for the treatment of cancer.
Acknowledgements
This study was supported, in part, by a Grant-in-Aid from the Ministry of Education, Culture, Sports, Science and Technology, Japan, and the Collaborative Research Program of the Atomic-bomb Disease Institute of Nagasaki University. The funder played no role in the study design, the data collection and analysis, decision to publish, or preparation of the article.
Footnotes
This article is freely accessible online.
Conflicts of Interest
The Authors declare no conflict of interest in regard to this article.
- Received November 2, 2017.
- Revision received November 21, 2017.
- Accepted November 23, 2017.
- Copyright© 2018, International Institute of Anticancer Research (Dr. George J. Delinasios), All rights reserved
