Therapeutic targeting of autophagy in disease: biology and pharmacology

Abstract

Autophagy, a process of self-digestion of the cytoplasm and organelles through which cellular components are recycled for reuse or energy production, is an evolutionarily conserved response to metabolic stress found in eukaryotes from yeast to mammals. It is noteworthy that autophagy is also associated with various pathophysiologic conditions in which this cellular process plays either a cytoprotective or cytopathic role in response to a variety of stresses such as metabolic, inflammatory, neurodegenerative, and therapeutic stress. It is now generally believed that modulating the activity of autophagy through targeting specific regulatory molecules in the autophagy machinery may impact disease processes, thus autophagy may represent a new pharmacologic target for drug development and therapeutic intervention of various human disorders. Induction or inhibition of autophagy using small molecule compounds has shown promise in the treatment of diseases such as cancer. Depending on context, induction or suppression of autophagy may exert therapeutic effects via promoting either cell survival or death, two major events targeted by therapies for various disorders. A better understanding of the biology of autophagy and the pharmacology of autophagy modulators has the potential for facilitating the development of autophagy-based therapeutic interventions for several human diseases.

Fig. 1.

The processes of autophagy. (A) Macroautophagy. Macroautophagy begins with engulfment of the cytoplasmic materials by the phagophore, which sequesters the materials into a double-membrane vesicle (i.e., autophagosome). The autophagosome fuses with a lysosome to form an autolysosome, and then the cytoplasmic materials are degraded by the lysosome. The initiation step of autophagosome formation requires the ULK1-Atg13-FIP200 complex and Beclin1-class III PI3K complexes. Two conjugation systems, Atg12-Atg5-Atg16 and Atg8-PE, are essential for the elongation and enclosure step of the autophagosome formation. Lipid conjugation leads to the conversion of the soluble form of LC3-I to the autophagic vesicle-associated form LC3-II, which is commonly used as a marker of autophagy. mTOR plays a critical role in regulating autophagy: under nutrient-rich conditions, mTOR is activated and inhibits autophagy through repression of ULK1 activity (the mammalian homologs of ATG1). Growth factors such as insulin or insulin-like growth factor can activate the class I PI3K-Akt/PKB pathway, which phosphorylates tuberous sclerosis complex (TSC2) and prevents the formation of an TSC1/2 protein complex, resulting in activation of mTOR. (B) Chaperone-mediated autophagy. During chaperone-mediated autophagy, the cytosolic proteins bind to the LAMP-2A receptor in an Hsc70 chaperone-dependent manner for translocation to the lysosomes, leading to their internalization and degradation. (C) Microautophagy. Microautophagy involves the direct sequestration of the cellular components by the lysosome through invagination of the lysosomal membranes.

Fig. 2.

The signaling pathways involved in autophagy regulation. Yellow ovals: autophagy stimulatory. Blue ovals: autophagy inhibitory. (1) Growth factors bind to their receptor, and activation of the receptor tyrosine kinase stimulates PI3K/Akt and Ras. Akt phosphorylates and inhibits the TSC1/2 complex, and dampens the inhibitory effect of TSC1/2 on Rheb, leading to mTORC1 activation and consequently to autophagy inhibition by affecting ULK1 complex formation. (2) PTEN inhibits PI3K/Akt/mTOR signaling and is an autophagy-promoting signal. (3) AMPK can be phosphorylated and activated by LKB1. Activation of AMPK phosphorylates and activates TSC1/2, leading to inactivation of mTOR and induction of autophagy. AMPK can also cause inactivation of mTOR by directly phosphorylating the mTOR binding partner Raptor. AMPK also regulates ULK1 and coordinates the induction of autophagy. (4) The Raf-1–MEK1/2–ERK1/2 signaling cascade causes the activation of autophagy. (5) Nuclear p53 stimulates autophagy in a transcription-dependent fashion by activating the expression of DRAM and sestrin. By contrast, cytoplasmic p53 is responsible for the inhibition of autophagy. (6) The Bcl-2 family antiapoptotic proteins interact with Beclin1 and exert inhibitory effects on autophagy. Conversely, proapoptotic proteins have stimulatory effects on autophagy by disrupting the association of antiapoptotic proteins with Beclin1. (7) Intracellular IP₃ negatively regulates autophagy via an mTOR-independent mechanism.

Fig. 3.

Autophagy can be targeted at multiple points on its regulatory pathways. (1) Activation of mTOR inhibits induction of autophagy, thus mTOR inhibitors are strong inducers of autophagy. (2) As the activity of mTOR is regulated by the class I PI3K-AKT pathway, the inhibitors of PI3K can stimulate autophagy. (3) EGFR is an upstream regulator of the class I PI3K/Akt pathway, so inhibition of EGFR activates autophagy through suppressing mTOR. (4) AMPK activates autophagy via suppressing mTOR or directly stimulating Ulk1 activity, thus activators of AMPK can induce autophagy. (5) The intracellular level of IP3 negatively regulates autophagy via an mTOR-independent mechanism; inhibiting this pathway can induce autophagy. (6) The antiapoptotic proteins Bcl-2 and Bcl-XL can interact with Beclin 1 through the Bcl-2 homology 3 (BH3) domain, thereby inhibiting Beclin1-dependent autophagy; thus, the small molecule mimetics of BH3 can activate autophagy by blocking the interaction between Bcl-2 and Beclin1. (7) Class III PI3K is critical for autophagy initiation, so its inhibitors can suppress autophagy by blocking the formation of autophagosome. (8) Inhibitors of the lysosomal enzymes and lysosomotropic agents that elevate the lysosomal pH can inhibit autophagy through blocking fusion of the autophagosome with the lysosome and degradation of the autolysosome.