Bladder cancer (BC) is a potentially life-threatening malignancy. Due to a high recurrence rate, frequent surveillance strategies and intravesical drug therapies, BC is considered one of the most expensive tumors to treat. As a fundamental evolutionary catabolic process, autophagy plays an important role in the maintenance of cellular environmental homeostasis by degrading and recycling damaged cytoplasmic components, including macromolecules and organelles. Scientific studies in the last two decades have shown that autophagy acts as a double-edged sword with regard to the treatment of cancer. On one hand, autophagy inhibition is able to increase the sensitivity of cancer cells to treatment, a process known as protective autophagy. On the other hand, autophagy overactivation may lead to cell death, referred to as autophagic cell death, similar to apoptosis. Therefore, it is essential to identify the role of autophagy in cancer cells in order to develop novel therapeutic agents. In addition, autophagy may potentially become a novel therapeutic target in human diseases. In this review, the current knowledge on autophagy modulation in BC development and treatment is summarized.
Bladder cancer (BC) is a potentially life-threatening malignancy that is considered one of the most expensive tumors in terms of treatment and medical care (
Autophagy, a fundamental evolutionary catabolic process, plays an important role in the maintenance of cellular environmental homeostasis by degrading and recycling damaged cytoplasmic components, including macromolecules and organelles (
Autophagy is a lysosomal degradation process in which damaged, long-lived cytoplasmic proteins and organelles are swallowed by double-membrane autophagic vesicles termed autophagosomes (
According to the mode of transport for intracellular components to the lysosome, the following three important subtypes of autophagy in mammals have been identified: Macroautophagy, microautophagy and chaperone-mediated autophagy (CMA) (
Autophagy is a dynamic process, and is referred to as autophagic flux. The complete process of autophagic flux can be divided into the following four steps: i) Activation and elongation, ii) maturation, iii) lysosome fusion and iv) degradation (
In addition, more than 30 autophagy-related genes and homologous proteins have been identified to be essential for autophagy (
Autophagy is induced under conditions of cellular stress, including hypoxia, oxidative stress, nutrient deprivation, organelle damage and radiotherapy or chemotherapy in order to meet cellular needs and promote cell survival. Moreover, autophagy levels are regulated by these cellular stresses through different signaling pathways. Subsequently, once these stresses are eliminated by autophagy upregulation, levels return to normal.
mTOR is a 300-kDa serine/threonine protein kinase present in mTOR complex 1 (mTORC1) (
The mitogen stimulation signal pathway is characterized by its dependence on serine/threonine kinase Akt. Class I PI3K is stimulated by multiple mitogen signals, including activated receptor tyrosine kinases, activated oncogene Ras and G protein-coupled receptors (
AMP-activated protein kinase (AMPK), a heterotrimer comprising of an α-catalytic subunit and regulatory β- and γ-subunits, is an evolutionarily conserved regulator of cellular energy homeostasis (
Amino acids are not only substrates of various metabolic pathways, but also signal molecules that regulate signal transduction pathways. The regulation of mTOR by amino acids is associated with the PI3K/Akt independent pathway. Mitogen-activated protein kinase kinase kinase kinase 3 (MAP4K3), a Ste20-related kinase, has been identified as an upstream regulator of mTORC1 in response to amino acids (
Numerous studies have shown that p53, an important tumor suppressor, plays an important role in regulating glucose homeostasis (
At present, mTOR and Beclin1 are considered significant signaling hubs in the context of autophagy. Beclin1, which is described as the mammalian homolog of yeast ATG6, plays an important role in the process of autophagosome nucleation. It recruits class III PIK3/vacuolar sorting protein-34 to form a regulated complex that generates phosphatidylinositol 3-phosphate [PI(3)P]. Subsequently, certain proteins, including ATG8 and ATG12 complex, bind with PI(3)P-binding domains to modulate autophagosome formation (
During tumor formation, autophagy plays a major role in suppressing tumor initiation and development by maintaining genomic integrity and preventing proliferation and inflammation (
The role of autophagy appears to be paradoxical in cancer therapy depending on the context. On one hand, inhibition of autophagy may be employed to increase the cytotoxic effect of treatments, including chemotherapy and radiotherapy (
BC is a malignant tumor associated with high morbidity and mortality, and a significant economic burden associated with it (
Autophagy may be inhibited at any stage of the autophagic flux. In the last decade, many studies involving autophagy mechanisms have been performed to identify chemical inhibitors of autophagy, including chloroquine (CQ) and 3-methyladenine (3-MA). Numerous studies have revealed that inhibition of protective autophagy via various approaches, including pharmaceutical inhibitors (
CQ, an anti-malarial drug, is the most frequently used and proficient agent for the inhibition of autophagy. Currently, CQ and its derivative hydroxychloroquine are the only clinically available autophagy inhibitors approved by the US Food and Drug Administration (
The second most widely used autophagy inhibitor is 3-MA, a class III PI3K inhibitor. PI3Ks are a diverse family of lipid kinases that play important roles in cellular processes, including cell proliferation, metabolism and autophagy regulation (
In addition to the pharmaceutical inhibitors listed in
An increasing number of studies focusing on RNA-interference have been performed to further explore the effects of protective autophagy inhibition on anticancer therapy. Kang
In summary, autophagy has been identified as a critical mechanism contributing to cancer therapy resistance. Moreover, autophagy may be regarded as a potential target for therapies involving autophagy inhibitors in combination with conventional therapeutics. The numerous studies mentioned above indicate that inhibition of protective autophagy may be able to increase the sensitivity of BC to chemotherapy or radiotherapy, providing important information on the effective regulation of autophagy during cancer treatment.
Autophagy appears to play a contradictory role in cancer therapy depending on the context. Apart from cytoprotective autophagy, the other primary and opposing form of autophagy, which may facilitate cell death either alone or in association with apoptosis, is cytotoxic autophagy. Functionally, cytotoxic autophagy is capable of decreasing the number of viable cells and/or reducing clonogenic survival upon treatment (
A better understanding of the potential role of vascular endothelial growth factor (VEGF) in tumor formation and progression has led to the addition of more effective agents to the therapeutic field of multiple tumor types. Among these anti-angiogenic agents, pazopanib, an oral tyrosine kinase inhibitor that targets the VEGF receptor, has been approved for metastatic renal cell carcinoma and soft tissue sarcoma (
ChlA-F, a novel derivative of Cheliensisin A isolated from
With the exception of pazopanib and ChlA-F, a number of alternative BC therapies have also been proven to provide anticancer effects through autophagy induction. One example is ubenimex, a broad-spectrum antitumor agent that has been used in adjuvant therapy. Aminopeptidase N (APN), known as the cell surface molecule CD13, is involved in several cell life activities, including cell survival, blood pressure regulation, angiogenesis, invasion and metastasis of tumor cells (
Salidroside, a bioactive tyrosine-derived phenolic compound isolated from
Although a large number of studies have been performed to confirm novel treatment methods for BC over the past few decades, the management and long-term survival rate of patients with BC have remained relatively stagnant without any significant improvement in clinical outcomes. Autophagy has been shown to be a complex cellular process with contrasting effects in the treatment of BC. Undoubtedly, the aforementioned findings prove that the application of autophagy activators and inhibitors provides further insight for the development of novel therapeutic options against human BC. To the best of our knowledge, since the multidisciplinary approach of surgery combined with radiotherapy, or chemotherapy is usually considered in patients with BC, autophagy inhibitors, including CQ and 3-MA, may be more beneficial in BC treatment due to their ability to increase cancer cell sensitivity to chemotherapy or radiotherapy. Therefore, a better understanding of the role of autophagy in BC treatment is crucial for the selection of effective drugs to target the autophagic pathway.
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This work was supported by Graduate Innovation Fund of Jilin University (grant no. 101832018C068).
The datasets used during the present study are available from the corresponding author upon reasonable request.
FL and HZ conceived and designed the study and prepared the manuscript. HG, MF and XR were responsible for the literature search, data visualization and analysis. YY and BL retrieved the relevant literature and revised the manuscript. All authors read and approved the final manuscript.
Not applicable.
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The authors declare that they have no competing interests.
Three main subtypes of autophagy. Long-lived and damaged cytoplasmic components are degraded via different autophagic processes. The diagram presents the three main subtypes: Macroautophagy, microautophagy and CMA. In macroautophagy, a double-membrane vesicle (phagophore) surrounds degradation components to form an autophagosome, which fuses with a lysosome for subsequent hydrolysis. In microautophagy, the lysosomal membrane alters its shape via invagination or bulging to engulf cytoplasmic components for degradation. In CMA, the adapter molecule HSC70 discerns and binds to the specific KFERQ motif of substrate proteins for translocation to the lysosome and ensures binding to Lamp2 for degradation. CMA, chaperone-mediated autophagy; HSC70, heat shock cognate protein of 70 kDa; Lamp2, lysosome associated membrane protein type 2.
Steps of the autophagic flux. Autophagy is activated in response to various cellular stress conditions. A double-membrane vesicle (phagophore) begins to form and elongate into an autophagosome in order to engulf intracellular degradation components, including mitochondria, damaged organelles and lipid droplets. The mature autophagosome with intracellular degradation components then fuses with the lysosome and forms an autolysosome, which provides an acidic environment for hydrolytic enzymes to hydrolyze the engulfed components.
Signaling pathways of autophagy. mTOR kinase is a pivotal molecule in the mTORC1 complex that plays an important role in the regulation of autophagy. Autophagy activation is triggered by decreased activity of the mTORC1 complex due to the activation of AMPK or p53 signaling. The decreased activity of mTORC1, an inhibitor of the mammalian ULK1 complex, leads to the increase the activity of the ULK1 complex, which subsequently initiates the formation of phagophore in conjunction with the PI3K complex. The elongation and maturation of the phagophore is dependent on two ubiquitin-like conjugation systems (ATG12 and ATG8), which involve multiple autophagy proteins, including ATG5, ATG16 and LC3. ATG, autophagy-related protein homolog; mTORC1, mTOR complex 1; AMPK, AMP-activated protein kinase; ULK1, uncoordinated-51-like protein kinase; LC3, microtubule-associated protein light chain 3; PE, phosphatidylethanolamine; TTI1, Tel2-interacting protein 1; TEL2, telomere length regulation protein TEL2; DEPTOR, DEP domain-containing mTOR-interacting protein; RAPTOR, regulatory-associated protein of mTOR; PRAS40, proline-rich Akt substrate of 40 kDa; MLST8, mTOR-associated protein LST8 homolog; MAPK, mitogen-activated protein kinase; FIP200, fusion-inhibiting peptide 200.
Autophagy inhibitors in bladder cancer.
Inhibitor | Mechanism of action | Treatments combined with inhibitor | Bladder cell line | (Refs.) |
---|---|---|---|---|
Chloroquine | Lysosomal lumen alkalizer | Cisplatin, radiotherapy, lapatinib or gefitinib | EJ, T24, RT-112, 5637, J82 | ( |
3-Methyladenine | PI3K inhibitor | Cisplatin, Fangchinoline, lapatinib or gefitinib | RT-112, T24, J82 | ( |
Icaritin | Protein synthesis inhibitor | Epirubicin | 5637, T24 | ( |
Frondoside A | Protein synthesis inhibitor | Cisplatin and gemcitabine | RT112 | ( |
shRNA | Knockdown of Beclin1 and |
Cisplatin | 5637, T24 | ( |
siRNA | Suppression of |
Lapatinib or gefitinib | T24, J82 | ( |
shRNA, short hairpin RNA; siRNA, small interfering RNA; ATG, autophagy-related protein.
Activators of autophagic cell death in bladder cancer.
Activator | Mechanism of action | Signaling pathways involved | Bladder cell line | (Refs.) |
---|---|---|---|---|
Pazopanib | Increasing cathepsin B activity | ERK1/2 | 5637, J82 | ( |
ChlA-F | Upregulating Sestrin-2 expression | Sestrin-2 | RT4, T24T, UMUC3 | ( |
Ubenimex | Akt agonist | Akt | 5637, RT112 | ( |
Salidroside | Suppressing PI3K and p-Akt | Autophagy/PI3K/Akt | T24 | ( |
Tetrandrine | Upregulating p-AMPK and downregulating p-mTOR | AMPK/mTOR | T24, 5637 | ( |
ChlA-F, Cheliensisine A-fluoride; AMPK, AMP-activated protein kinase; p-, phosphorylated.