Abstract
Autophagy is a cellular mechanism used to digest damaged cellular components into component residues, which may be recycled to generate essential macromolecules. All cells undergo autophagy, but the mechanism is up-regulated in stressed cells such as those with nutrient depletion and hypoxia; such stress is common in nutrient-deprived regions of solid tumors, and autophagy co-localizes with hypoxia in tumors (1). Autophagy involves the formation of autophagosomes, which have a double membrane enclosing cytoplasmic cellular components; this then fuses with an acidic lysosome to produce a mature autolysosome in which cellular proteins are degraded by cathepsins (2–3–4).
A series of autophagy-related proteins (known as ATGs) are responsible for the induction and regulation of autophagy. The human form of ATG8 is microtubule-associated protein light-chain 3B (LC3B), which exists in a cytosolic form as LC3B-I. Upon activation of autophagy, LC3B-I is cleaved and modified to LC3B-II, which then binds to the membrane of the autophagosome (5, 6). LC3B-II has been used widely as a marker of autophagy, although an increase in LC3B-II can be due to either increased LC3B-I processing from activation of autophagy or a buildup of membrane-bound LC3B-II following inhibition of lysosomal fusion. An additional marker of autophagy, p62/SQSTM1 (p62), is recruited with LC3B-II to autophagosomes but, unlike LC3B-II, is degraded within the mature autolysosome (6, 7); thus increased p62 is indicative of inhibition of lysosomal fusion to the autophagosome – i.e., inhibition of autophagy.
Autophagy is prognostic of poor outcome in multiple tumor types, including cancers of the breast, lung, and colon (8–9–10). High levels of autophagy have been associated with resistance to systemic therapy in several preclinical and clinical models, presumably because autophagy facilitates survival of stressed or damaged cells through recycling of cellular breakdown products (11). Recent studies in my laboratory have shown that treatment of cancer cells in vitro and of solid tumors in mice with a wide variety of anticancer agents induces autophagy, suggesting that it is a common survival mechanism for drug-damaged cells (especially those where autophagy is already up-regulated due to nutrient deprivation) and therefore an important cause of drug resistance.
Agents which inhibit endosomal acidification, including (hydroxy) chloroquine and proton pump inhibitors, can suppress autophagy. Proton pump inhibitors such as pantoprazole have been reported to sensitize cancer cells and experimental tumors to various chemotherapeutic agents (12–13–15). Although multiple mechanisms are likely involved, the dominant mechanism is probably inhibition of autophagy (15), either through increase of pH in endosomes, rendering cathepsins inactive, or inhibition of fusion of autophagosomes with acidic endosomes. Work in my laboratory has confirmed that pantoprazole inhibits autophagy in vitro and in vivo, and improves sensitivity of cultured cancer cells to multiple drugs. We showed marked effects of docetaxel and some other anticancer drugs to up-regulate autophagy, as indicated by increased levels of LC3B and reduced levels of p62 in all tumor regions, with opposite effects indicating inhibition of autophagy when chemotherapy was combined with pantoprazole. Pantoprazole enhances the activity of doxorubicin, docetaxel, and paclitaxel against human tumor xenografts, with improved distribution of drug toxicity throughout the tumors, as determined by the biomarkers γH2AX and cleaved caspases; it also leads to increased growth delay (15). We obtained further evidence for this being the main mechanism of action by using autophagy-deficient cells generated by shRNA knockdown of the autophagy proteins ATG7 and BECLIN1 (15).
Recognition that up-regulation of autophagy is a common mechanism of resistance to anticancer drugs identifies the process as an important therapeutic target. Some companies are now seeking to identify inhibitors of autophagy that are more effective than hydroxychloroquine and proton pump inhibitors. While such strategies will inhibit autophagy in all tissues, autophagy is already activated in hypoxic and nutrient-deprived regions of solid tumors, providing the potential for improvement in therapeutic outcome without a corresponding increase in toxicity to normal tissues.
Footnotes
Financial support: None.
Conflict of interest: None.