What is autophagy?
Autophagy is a self-degrading catabolic response to stress in which cellular contents are recycled to maintain proficient cellular metabolism, promoting survival, where the degradation of intracellular components can be either non-specific or can target specific organelles (7). This self-degradating process is used for three key reasons and they are: 1) To recycle proteins and organelles that do not pass quality control or are in excess in an effort to maintain homestasis, 2) To sustain itself in times of need by degrading less-essential cellular components as an alternative fuel source, and 3) To defend itself against pathogens (1). The crucial process is typically a step prior to apoptosis in the cellular stress response, and apoptosis is induced upon over activation of autophaghy when self-consumption surpasses the cell's capacity for synthesis of the building blocks necessary for survival (12). Autophagy is influenced by several genes, but mainly controlled by the PI3K - AKT - mTOR pathway, which suppresses its actions (7).
The loss of the autophagic essential gene beclin1 (ATG6) is highly associated with breast, ovarian, and prostate cancers (7). This association is likely due to an increase in inflammation at tumour sites, resulting from the increased levels of necrotic cell death that stem from the loss of autophagy (7). Therefore, the existence of autophagy can prevent tumor growth my preventing inflammation, and also reduce the likelihood of mutation through this same mechanism (7). Additionally, decreased levels of autophagy are typically seen in tumor cells compared to normal cells in stress free conditions(8). Some treatment strategies have utilized this relationship; for example, TMZ is administered to induce autophagy in malignant glioma cells (8).
For remaining parts of this section in Chapter 11, the basis of autophagy will be discussed under the topic of carcinogensis. First, the role of autophagy in cancer will examined in order to determine the enigma behind its role as a cancer-promoting or cancer-inhibiting process. During this time, the underlying mechanisms within the self-degradating pathway will be throughly elaborated to discussed potential targets for cancer therapy.
Autophagy in Cancer
The definite role of autophagy in cancer cells has yet to be determined as it has been shown to have roles in both promoting tumorigenesis, as well as suppressing tumor formation. There are several hypotheses that have been proposed to reconcile the role of autophagy in both tumorigenesis and tumor suppression. Two theories that have been more readily accepted are: 1. Effects of autophagy on tumor growth may be tissue specific; and 2. The role of autophagy may change in relation to the stage of tumor growth. For instance, autophagy may initially play an inhibitive role in tumor growth before switching to supporting tumor growth at later stages of tumor development (2).
The role of autophagy in tumor suppression became evident when deletions in Beclin1, an essential protein in the autophagy pathway, were found in breast, ovarian and prostate cancer (3). Since then, other mechanisms have been proposed on the role of autophagy in tumour suppression.In addition to Beclin1, mutations in a few other proteins in the autophagy pathway including Atg2B, Atg5, Atg9B, Atg12, and UVRAG have been observed in cancer. In vitro studies involving the deletion of Atg5 and Atg7, for example, cause liver adenomas in mouse models, suggesting a tumor suppressor role for autophagy. To support this, it was also found that the reintroduction of Beclin1 into human breast cancer cells resulted in decreased cell growth and tumorigenicity when injected into mice. Furthermore, it was observed that overexpression of oncogenes results in decreased autophagy (4). Autophagy has been confirmed to be activated by an increase in reactive oxygen species (ROS) levels in multiple studies. ROS buildup occurs when cells are subject to metabolic stresses and can be harmful to DNA integrity. Increased ROS levels are commonly observed in cancer and can aid in cancer development by disrupting tumor suppressor genes or enhancing oncogene expression. Hence, the role of autophagy in decreasing ROS levels in cells is a way in which a cell can protect its DNA and prevent tumorigenesis (4).
The role for autophagy as a survival mechanism in tumour cells appears to contradict the observations indicated by loss-of-function mutations in autophagy pathways. When a cancer cell is under stressful conditions, such as hypoxia, autophagy is strongly induced (7). When this response occurs in an apoptotic deficient cancer cell, it is often sustained (7). In fact, autophagy in the absence of apoptosis appears to be a key mechanism exploited by tumor cells to survive prolonged stress conditions through dormancy; for example, in situations where anti-vascularization therapy is applied (7). Therapies to inhibit autophagy are currently under investigation for their therapeutic role, but this approach appears to still be in its infancy (7). The major findings that indicate autophagy as a tumour-promoting process is largely based on the observation that the cancer cells have a distinct dpendence on the catabolic pathway (7). One of this examples is observed in pancreatic cancers, where genetic or pharmacologic inhibition of autophagy results in the induction of reactive oxygen species. As a result, DNA damage is amplified along with decreased mitochondrial oxidative phosphorylation. Ultimately, the combination of these detrimental effects result in a significant growth repression of pancreatic cancer cells (7), indicuating that autophagy is required for tumorigenic growth in pancreatic cancer de novo. This basis surrounding autophagy's role will be investigated a potential clinical utility in treating cancers.
An explanation behind autophagy's role as a cancer-promoting pathway is that stimulation of necrotic cell death along with inflammation characteristics caused by apoptosis and autophagy provide cells a means for tumour promotion by expressing would-healing responses (7). A second explanation revolves around the concept that tumour cells recruit the use of autophagy as a means to manage metabolic stress and accumulation of deleterious mutations, where the mutations are the result of increasing oxidative stress due to amplified tumour growth. By maintaining an overall cellular viability, cancer cells are able to withstand the oxidative stresses that come with uncontrolled proliferation and cell growth.
Autophagy as a target in cancer therapy
Research is currently being conducted on the inhibition of autophagy as a potential therapy. Initial studies have shown that autophagy-defective cells are particularly sensitive to a range of cancer drugs. Autophagy-defective cells would lose an important way to cope with metabolic stresses, allowing them to be used in conjunction with other chemotherapeutic drugs. One example where autophagy inhibitors have been demonstrated to be successful in combination with a cancer drug is in chronic myelongenous leukemia (CML) with imatinib. While often very effective, patients who are imatinib-resistant due to mutations in the Abl kinase domain or whose CML stem cells are insensitive to imatinib are common. In vitro studies have shown that imatinib induces autophagy in CML cell lines and treatment of these cells with imatinib and an autophagy-inhibitor dramatically increase cell death (6).
A review by Mathew et. al. (2007) eloquently explains the available applications of autophagy modulation for cancer therapy (7). Tumours that possess a defective apoptotic pathway and as a result, are reliant on autophagy for survival can be treated using autophagy inhibitors. This process will result in acute necrotic cell death that may be in conjuncted with proteasome inhibition. Initial cancer treatment by either chemotherapy or radiotherapy is applied to remove the large bulk of the tumour. The resulting stressed environment within the cancer growth will then be more susceptible to autophagy inhibition, where tumour suppression can be achieved. Lastly, a counter-intuitive measure can be applied where autophagy stimulators such as Rapamycin is recruited to promote autophagic cell death. This method may also prevent further detrimental effects of autophagy deficiency, where mismanagement of metabolic stress yields tumour progression (7).
Difficulties in Targeting Autophagy
Autophagy is an essential cellular process, which enables cells to recycle damaged organelles, obtain extra energy during times of energy deprivation and maintain homeostasis. One of the main reasons for its duplicitous role in cancer is both up regulation and down regulation of autophagy can result in serious defects linked to carcinogenesis. In a recent study Rao, S. et al reported that specifically targeting ATG5, a gene necessary for autophagy, in the lungs, resulted in significant reductions in the progression of KRas-driven lung cancer. However, these autophagy deficient tumor cells often developed problems in mitochondrial homeostasis resulting in DNA mutations due to excessive amounts of free oxygen radicals. The study suggests loss of P53 function through this DNA damage could result in resumed proliferation. The onset of ATG5-mutant KRas-driven lung tumors is highly accelerated despite the overall improvement in survival. This suggests that autophagy plays a role in preventing early oncogenic mechanisms.(9) This dual role of autophagy in a single tumor has been empirically shown by demonstrating that combination therapy of pro- and anti- autophagy therapeutics actually increases survival compared to anti-autophagy therapies alone. The exact mechanism behind this process is not fully understood; scientist are unsure if this phenomenon is a result of the specific part of autophagy inhibited or an example of the dependence of cells of at least basal levels of autophagy to maintain cell function.(10)
Another interesting factor in the success of anti-autophagenic drugs is the effects they have on other cellular processes. For example, the anti-tumor drug APO866 has been shown to effectively reduce tumor size in a number of carcinomas by not only stimulating autophagy but also increasing caspase 3 activity. This pathway inhibited CAT, a compound that recognizes reactive oxygen species, which ultimately lead to apoptosis due to high levels of ROSs. Interestingly, providing exogenous CAT completely abolished the anti-tumorigenic potential of APO866. Clearly the different mechanisms involved in autophagy regulation need to be clarified in order to distinguish its dual role in cancer promotion and inhibition.(11)
1. Mathew, R., Karantza-Wadsworth, V., & White, E. (2007). Role of Autophagy in Cancer. Nature Reviews 7:961-967.
2. Lu, S., Harrison-Findik, D. D. (2013). Autophagy and Cancer. World J Biol Chem 4(3):64-70.
3. Liang, X.H., Jackson, S., Seaman, M., Brown, K., Kempkes, B., Hibshoosh, H., Levine, B. (1999). Induction of autophagy and inhibition of tumorigenesis by beclin 1. Nature 402:672-676.
4. Mah, L.Y., Ryan, K.M. (2011). Autophagy and Cancer. Cold Spring Harbor Perspectives in Biology 4:a008821.
5. Young A. R. , Narita M., Ferreira M., Kirschner K., Sadaie M., Darot J. F., Tavare S., Arakawa S., Shimizu S., Watt F. M. (2009). Autophagy mediates the mitotic senescence transition. Genes dev 23:798-803.
6. Chen, N., Debnath, J. (2010). Autophagy and Tumorigenesis. FEBS Lett 7:1427-1435.
7. Mathew, R., Karantza-Wadsworth, V., & White, E. (2007). Role of autophagy in cancer. Nature Reviews Cancer 7(12), 961-967.
8. Kondo, Y., Kanzawa, T., Sawaya, R., & Kondo, S. (2005). The role of autophagy in cancer development and response to therapy. Nature Reviews Cancer 5(9), 726-734.
9. Rao, S. et al. (2014) A dual role for autophagy in a murine model of lung cancer. Nat. Commun. 5:3056.
10. Rothe, K. (2014) Revisiting Autophagy Lecture Notes taken in MEDG421 at UBC on 27 March 2014.
11. Ginet V. et al. A critical role of autophagy in antileukemia/lymphoma effects of APO866, an inhibitor of NAD biosynthesis. Autophagy, 2014 (10); epub ahead of print date.