Autophagy and autophagy-related pathways in cancer

Abstract

Maintenance of protein homeostasis and organelle integrity and function is critical for cellular homeostasis and cell viability. Autophagy is the principal mechanism that mediates the delivery of various cellular cargoes to lysosomes for degradation and recycling. A myriad of studies demonstrate important protective roles for autophagy against disease. However, in cancer, seemingly opposing roles of autophagy are observed in the prevention of early tumour development versus the maintenance and metabolic adaptation of established and metastasizing tumours. Recent studies have addressed not only the tumour cell intrinsic functions of autophagy, but also the roles of autophagy in the tumour microenvironment and associated immune cells. In addition, various autophagy-related pathways have been described, which are distinct from classical autophagy, that utilize parts of the autophagic machinery and can potentially contribute to malignant disease. Growing evidence on how autophagy and related processes affect cancer development and progression has helped guide efforts to design anticancer treatments based on inhibition or promotion of autophagy. In this Review, we discuss and dissect these different functions of autophagy and autophagy-related processes during tumour development, maintenance and progression. We outline recent findings regarding the role of these processes in both the tumour cells and the tumour microenvironment and describe advances in therapy aimed at autophagy processes in cancer.

Fig. 1. Autophagy is regulated by cancer-associated factors and has multiple roles in tumour suppression.

Autophagy is initiated when nascent double membranes are formed from the endoplasmic reticulum and other sources forming the phagophore. The process is regulated by a complex containing the kinase UNC-51-like kinase 1 (ULK1), which works together with the class III PI3K kinase complex, containing beclin-1 and VPS34, to generate phosphatidylinositol-3-phosphate, thus facilitating expansion of the autophagosome membrane. ATG8 family members, including MAP1LC3 (microtubule-associated protein 1A/1B-light chain 3), commonly referred to as LC3, are converted to a lipidated form (LC3-II) by conjugation to phosphatidylethanolamine, via a complex containing ATG5, ATG12 and ATG16L1. They are then tethered in the phagophore membrane and regulate various steps of autophagosome biogenesis. During selective autophagy, lipidated ATG8 proteins also function in cargo selection, by associating with autophagy cargo receptors (ACRs; also known as selective autophagy receptors (SARs)) that recognize ubiquitylated cargo. The membranous structures grow to form an organelle termed an autophagosome, which ultimately fuses with lysosomes. Cargoes are then degraded by lysosomal hydrolases and the resulting constituent parts such as amino acids, lipids or sugars are transported into the cytosol for de novo biosynthesis or energy production. Autophagy serves to remove misfolded proteins and damaged organelles, which would otherwise lead to aberrant cellular functions, reactive oxygen species (ROS) imbalances, inflammation or defective antigen presentation, thus predisposing the cell to malignant transformation. In some cases, autophagy can facilitate tumour suppression by removing specific factors such as the ACR p62, elevated levels of which are found in many cancers and are thought to be tumour-promoting. Several cancer-associated factors, including the RAS oncoproteins and p53 tumour suppressor, have been shown to regulate autophagy and influence tumour initiation and development. For example, the nutrient-sensing mechanistic target of rapamycin complex 1 (mTORC1) is a repressor of autophagy, whereas the AMP-activated protein kinase (AMPK), which is activated in situations of energetic stress, is a promoter of autophagy. The regulation of autophagy by p53 is complex: at basal, unstimulated levels the tumour suppressor p53 has been reported to repress autophagy; however, when elevated and activated by cellular stress, p53 activates a panel of target genes (including those encoding damage-regulated autophagy modulator 1 (DRAM1) and AMPK through its subunit PRKAB1) that promote autophagy. Conversely, mutant RAS protein is considered to promote autophagy, but its inhibition can also promote autophagy, indicating that the control of autophagy by RAS is complex and probably context specific.

Fig. 2. Roles of autophagy in primary tumours and metastasis.

Autophagy can support tumour growth and survival through various paths. For example, autophagy has important roles during metabolic adaptation of tumour cells (for example, through the clearance of dysfunctional mitochondria) and escaping immune detection (for example, through NBR1-mediated degradation of major histocompatibility complex class I (MHC-I)). During metastasis, opposing roles have been described for autophagy. Autophagy can support resistance to detachment-induced cell death (anoikis) in delaminating or circulating tumour cells and can promote adaptation to nutrient limitations. However, autophagy has also been shown to be required to maintain tumour dormancy (for example, through the autophagic degradation of the glycolysis mediator PFKFB3) and genomic stability, leading to an increase in polyploid tumour cells following inactivation of autophagy. Thus, inhibition of autophagy can result in enhanced metastatic growth. Although the mechanisms underlying this tumour-suppressive activity of autophagy are largely unknown, they probably involve multiple autophagic targets, such as NBR1. In epithelial–mesenchymal transition, autophagy has both metastasis-promoting and metastasis-inhibitory effects (through degradation of the epithelial–mesenchymal transition master regulator TWIST1, not shown). ECM, extracellular matrix.

Fig. 3. Autophagy in host stromal cells supports a pro-tumorigenic microenvironment.

Studies of autophagy inhibition in host stromal cells, including cancer-associated fibroblasts (CAFs), have illuminated three principal non-cell-autonomous functions through which host cell autophagy impacts the tumour microenvironment. First, autophagy facilitates the production of diverse metabolites such as amino acids, which are released by stromal cells and subsequently used by the tumour cell compartment for growth and proliferation (centre). This metabolic exchange is particularly crucial for tumour cells as these often switch to a largely anabolic state and require high levels of essential amino acids, most notably alanine and asparagine, and non-essential amino acids (NEAA). Second, autophagy supports secretion of pro-inflammatory cytokines from CAFs, including IL-6, IL-8 and IL-1β. These promote tumorigenesis by directly facilitating tumour cell proliferation and modulating innate and adaptive immune cells to create a tumour-permissive immune microenvironment (left). In addition to cytokine secretion, autophagy-related processes, such as microtubule-associated protein 1A/1B-light chain 3 (LC3)-dependent extracellular vesicle (EV) loading and secretion (LDELS) and conjugation of ATG8 to single membrane (CASM), may promote biogenesis and secretion of EVs from both tumour cells and associated stromal cells. How such ATG-dependent EV subpopulations communicate with stromal elements to influence the tumour microenvironment remains unclear. Third, autophagy promotes procollagen proteostasis, which is necessary for type I collagen deposition and creates a stiff, desmoplastic extracellular matrix (ECM) that promotes neo-angiogenesis and primary tumour growth (right).

Fig. 4. Targeting autophagy for cancer therapy.

The autophagy pathway is a major contributor to tumour cell survival and as a result is considered a target for cancer therapy. To date, only a small number of autophagy modulators have been described, with the majority of studies focused on inhibition of the lysosomal degradation stage of autophagy, using agents such as hydroxychloroquine (HCQ) or the lysosomal autophagy inhibitor Lys05. Additional inhibitors targeting other stages of the process such as autophagosomal membrane elongation and closure as well as lysosomal fusion are currently largely lacking. Inhibitors against UNC-51-like kinase 1 (ULK1) and VPS34, which act on the initiation stage of autophagy, are showing promise in preclinical studies^–. Alternatively, autophagy can be targeted in cancer backgrounds that are particularly dependent on autophagy owing to oncogenic activation of signalling pathways. For example, the RAS–mitogen-associated kinase (MAPK) pathway is activated in a large proportion of cancers through overexpression or mutation of receptor tyrosine kinases (RTKs) and/or mutation of the downstream effectors RAS and RAF. Inhibitors of this pathway were designed to promote cell death in cases in which the pathway was activated. It was found that inhibition of RAS signalling pathway components causes activation of the kinase LKB1, resulting in the activation of AMP-activated protein kinase (AMPK) and leading to activation of autophagy, which in turn represses cell death and promotes cell survival^–. This has motivated interest in combining autophagy inhibitors with RAS–MAPK pathway inhibitors. Given the opposing roles of autophagy in cancer, a few studies have also indicated that promotion of autophagy may be beneficial for cancer therapy. For example, combination of the tricyclic antidepressant imipramine with the purinergic receptor inhibitor ticlopidine was found to cause excessive autophagy and cell death dependent on autophagy. The combination showed promising results in preclinical models of glioma. ACR, autophagy cargo receptor; ERK, extracellular signal-regulated kinase; LC3-II, lipidated microtubule-associated protein 1A/1B-light chain 3; MEK, mitogen-activated protein kinase kinase; ULK1i, inhibitor against ULK1; VPS34i, inhibitor against VPS34.