Introduction
Macroautophagy (autophagy) is a tightly regulated self-digestive process in which cytoplasmic contents and organelles are sequestered within double-membrane vesicles, called autophagosomes, and delivered to the lysosome for degradation and recycling. Autophagy is regulated by a limited number of highly conserved genes called
ATGs (for AuTophaGy gene). Although originally identified in yeast [
1], there have been numerous recent breakthroughs in mammals demonstrating how autophagy critically regulates key physiological and pathological processes. Autophagy serves vital housekeeping functions in cells, including the turnover of damaged organelles and long-lived proteins [
2]. Furthermore, the bulk degradation of cellular material through autophagy allows cells to recycle both nutrients and energy during starvation and stress [
3]. Thus, autophagy is proposed to function as a “battery” that buys cells valuable time, allowing them to survive if the stressor is removed in a timely manner. The indispensable contribution of cellular autophagy to physiological homeostasis has been demonstrated by studies in which mice with genetic deletion of critical
ATGs die within a day after birth [
4,
5]. On the other hand, an excessive level of autophagy has been proposed to promote cell death due to the over-consumption of critical cellular constituents [
6]; this process has been termed ATG-dependent or type II-programmed cell death.
Overall, the malfunctioning of this cellular self-eating has been linked to a wide range of pathological conditions including neurodegenerative diseases, pathogen infection, and cancer [
7]. Autophagy is activated in response to multiple stresses during cancer progression, such as nutrient starvation, the unfolded protein response (UPR) (endoplasmic reticulum (ER) stress), and hypoxia; in addition, it is observed upon treatment of cancers with a wide spectrum of cytotoxic and targeted chemotherapeutic agents [
8,
9]. In this review, we will summarize the current understanding of the role of autophagy in tumorigenesis, the current status and opportunities of autophagy manipulation in cancer therapy and also discuss the need for identification of suitable markers of autophagy in clinical settings.
Regulation of autophagy by mammalian target of rapamycin
Significant progress has been made over the past years in determining the molecular regulators of autophagy. For detailed understanding of autophagic regulators, additional reading is recommended [
29]. Here, we principally focus on the role of the mTOR pathway in autophagy regulation.
The serine/threonine kinase mTOR plays a pivotal role in maintaining the balance between cell growth and autophagy in response to growth factor signaling, nutrient status, and stress. mTOR exists in two complexes: mTORC1 (consisting of mTOR, GβL, raptor, Deptor, PRAS40), which is sensitive to rapamycin, and mTORC2 (consisting of mTOR, GβL, rictor, Sin1, PRR5, Deptor) [
30]. Activated mTORC1 promotes mRNA translation by activating S6K and inhibiting 4EBP1; in addition, it also inhibits autophagy by phosphorylating the ATG13-ULK-FIP200 complex [
31]. Regulation of mTORC1 activity in response to diverse stimuli is primarily regulated by the tuberous sclerosis complex (TSC) proteins [
32]. TSC2, in a complex with TSC1, acts as a GTPase activating protein for the small GTPase Ras homolog enriched in brain (Rheb) that activates mTORC1 in its GTP-bound form [
33].
The role of mTOR in mediating growth factor signaling is primarily in response to the class I PI3K-protein kinase B (Akt) pathway [
34]. Activated growth factor receptor stimulates PI3K to convert phosphotidylinositol 4,5-bisphosphate to phosphotidylinositol 3,4,5-trisphosphate, leading to the recruitment of phosphatidylinositide-dependent protein kinase 1 (PDK1) and Akt to the plasma membrane followed by phosphorylation and activation of Akt by PDK1. Activated Akt then phosphorylates TSC2 and prevents TSC complex formation and thus leads to mTOR activation. Though the role of mTORC2 in autophagy is not clearly delineated, it is important to recognize that mTORC2 activates Akt; because Akt positively regulates mTORC1 activity, one can speculate that mTORC2 also negatively regulates autophagy [
35]. During tumorigenesis, constitutive activation of the PI3K pathway often leads to increased mTORC1 activation, presumably resulting in autophagy inhibition. In fact, inhibition of Akt kinase activity has been recently shown to induce autophagy levels [
36]. On the other hand, overexpression of tumor suppressor phosphatase and tensin homolog (PTEN), the negative regulator of PI3K/AKT pathway, induces autophagy [
37].
mTOR is also a critical sensor of cellular energy and nutrient status. Reduced ATP production during nutrient deprivation results in an elevated AMP/ATP ratio and activates the energy-sensing serine/threonine kinase 11 (LKB1)-adenosine monophosphate–activated protein kinase (AMPK) signaling axis. AMPK mediates the phosphorylation of TSC, leading to inactivation of mTORC1 and induction of autophagy [
38]. AMPK can also directly phosphorylate Raptor and thus regulate mTORC1 in a TSC-independent manner [
39]. Moreover, the cellular amino acid pool, especially branched chain amino acids, critically regulates mTOR activity. A recent study revealed that cellular uptake of L-glutamine by the transporter SLC1A5 followed by its rapid efflux via the bidirectional transporter SLC7A5/SLC3A2, which is responsible for simultaneous L-glutamine efflux and leucine import, results in mTORC1 activation. Thus, loss of SLC1A5 inhibits cell growth and activates autophagy presumably due to inhibition of leucine uptake into cells [
40].
Finally, mTOR also serves as an important sensor of cellular stress. Regulated in development and DNA damage response 1 (REDD1) is induced by hypoxia and leads to mTORC1 inhibition by regulating the TSC complex [
41]. Moreover, the tumor suppressor protein p53, which is induced in response to diverse genotoxic stress, has also been shown to promote transcription of various negative regulators of the mTOR pathway, such as AMPKβ, TSC2, and PTEN [
42,
43]. Furthermore, sestrin, which is induced in response to DNA damage and oxidative stress in a p53-dependent manner, was recently shown to inhibit mTORC1 activity via AMPK activation [
44]. Additional studies revealed that sestrin2 is indeed required for autophagy induction in response to various stress inducers, including nutrient starvation and rapamycin [
45], thereby further establishing the link between p53, mTORC1, and autophagy. However, it is important to recognize that the role of p53 in autophagy regulation is not straightforward, because it can also promote autophagy in an mTOR-independent manner via the transcriptional upregulation of its downstream target damage-regulated autophagy modulator (DRAM) [
46].
Though additional pathways exist, mTOR, which is positioned at the crossroads of various critical signaling pathways, serves as a focal point of autophagy regulation. Deregulation of mTOR activity due to perturbation in upstream signaling events during carcinogenesis can therefore have significant impact on autophagy levels and subsequent tumor outcome. Moreover, mTOR regulation of autophagy may significantly impact the efficacy of the growing number of anti-cancer agents targeting the PI3K/mTOR pathway [
47].
Autophagy in tumor suppression
Since the prototypic functions of autophagy are to recycle essential nutrients and provide energy for survival during starvation and stress, it initially seems counterintuitive that autophagy can act as a potential tumor suppressor mechanism. However, genetic evidence supports that autophagy can prevent tumor formation. The role of autophagy as a tumor suppressor was first broached through genetic studies of beclin1, the mammalian orthologue of yeast ATG6 [
51].
Beclin1 was mapped to a tumor susceptibility locus that is monoallelically deleted in a high percentage of human breast, ovarian, and prostate cancers; furthermore, analysis of human tissue samples revealed decreased beclin1 expression in human breast carcinomas compared to normal breast tissue [
51]. Moreover, oncogenes like Akt [
36], Ras, and ERK [
52] inhibit autophagy primarily by activating the mTOR signaling pathway. On the other hand, there is abundant evidence that tumor suppressors like p53 [
44,
46], PTEN [
37], and ARF [
53,
54] activate autophagy.
Although reduced autophagy is believed to promote tumor development, a minimal level is believed to be necessary for the survival of cancer cells. Moreover, increased autophagy is observed in transformed cells when exposed to diverse stress. Thus, it is increasingly appreciated that autophagy provides cancer cells with certain selective advantages to cope with stress, both in the primary tumor microenvironment as well as during dissemination and metastasis. In the following section, we overview three scenarios in which autophagy-mediated cell survival promotes tumor progression.
Manipulating autophagy for cancer therapy
Because autophagy plays a crucial role in tumor growth, one can predict that manipulation of autophagy levels will have significant effects on tumor outcome. A growing number of studies in preclinical settings indicate that both the cytoprotective as well as death-inducing properties of autophagy may be exploited for therapeutic purposes.
Detecting autophagy in human cancer tissues
Despite these exciting experimental findings in cell-based and mouse models, it is important to recognize that no direct evidence exists showing that reduced or defective autophagy is a common requirement for human cancer initiation. This is primarily because the direct and quantitative detection of autophagy levels in human tumor samples remains technically intractable. As the importance of autophagy in carcinogenesis is unveiled, there is a pressing need for better detection methods and the identification of suitable markers of autophagy in human pathology samples [
96]. Electron microscopy is still the most sensitive standard method for this purpose. The detection of autophagosomes (with intact cytoplasmic materials and organelles) and autolysosomes (with partially degraded materials) can be used to identify the different stages of autophagy by electron microscopy [
97]. However, subclassification of these autophagic ultrastructures is extremely subjective (e.g., distinction of autolysosomes from other membrane-enclosed cellular compartments) and thus requires extensive expertise [
98]. Moreover, due to the elaborate tissue processing steps involved, electron microscopy is labor-intensive and expensive, and its application is particularly unfeasible in clinical samples.
Therefore, even though electron microscopy remains the gold standard method for monitoring steady-state levels of autophagy, use of cell biological and biochemical methods are more practical approaches in detection and better quantification of autophagy and autophagic flux. Weakly basic dyes such as monodansylcadaverine and acridine orange were initially proposed as specific in vivo marker of autophagic vacuoles [
99]. However, these dyes were later shown to have high affinity for lysosomes [
100], and thus, their use is no longer recommended. As the molecular mechanism of autophagy is unraveling, better assays are being developed for reliable quantitative measurement of autophagy.
The biggest obstacle in developing methods to detect bona fide autophagy in clinical material arises from the fact that autophagy is a multi-step process characterized by the formation of an autophagosome, followed by its fusion to the lysosome and degradation of the autophagy substrates by lysosomal enzymes. As a result, an apparent increase in the number of autophagosomes observed within a cell can either mean an increase in the rate of autophagosome formation or alternatively, a decrease in the rate of autolysosome formation. Thus, the measurement of a dynamic process using static methods in clinical samples poses a fundamental challenge in data interpretation.
In summary, autophagy has emerged as an important regulator of tumorigenesis. In light of current evidence, it is hypothesized that autophagy maintains genomic integrity and thus prevents tumor initiation. On the other hand, in established tumors, the stress adaptive property of autophagy improves the cellular fitness of cancer cells and promotes their survival during disease progression and during anti-neoplastic therapy. In spite of these current dogmas, direct evidence establishing this stage-specific role of autophagy in carcinogenesis is missing. Thus, the development of mouse cancer models in which autophagy can precisely be manipulated is imperative to reveal important answers not only about the precise role of autophagy in different stages of carcinogenesis but also how autophagy levels can be regulated for optimal therapeutic benefits. Finally, there remains a need to identify robust tissue biomarkers for autophagy in human cancer specimens.
Acknowledgements
JD is supported by grants from the NIH (RO1CA126792; CA126792-S1 ARRA), the California Tobacco Related Disease Research Program (18XT-0106), and an HHMI Physician Scientist Early Career Award.