Pharmacologic agents targeting autophagy

Abstract

Autophagy is an important intracellular catabolic mechanism critically involved in regulating tissue homeostasis. The implication of autophagy in human diseases and the need to understand its regulatory mechanisms in mammalian cells have stimulated research efforts that led to the development of high-throughput screening protocols and small-molecule modulators that can activate or inhibit autophagy. Herein we review the current landscape in the development of screening technology as well as the molecules and pharmacologic agents targeting the regulatory mechanisms of autophagy. We also evaluate the potential therapeutic application of these compounds in different human pathologies.

Figure 1. Small-molecule modulators of autophagy that target different steps of the autophagic machinery.

Autophagy can be activated by inhibitors against class I PI3K (LY294002), mTOR (rapamycin), AKT (perifosine), and IMPase (Li⁺) as well as activation of AMPK by metformin. While Ca²⁺ channel inhibitors (fluspirilene, verapamil) can stimulate autophagy, thapsigargin, an endoplasmic reticulum Ca²⁺ ATPase inhibitor, elevates Ca²⁺ and suppress autophagy. The initiation of autophagy is regulated by the activity of the class III PI3K complex, which can be inhibited by 3-MA and PT21 as well as by spautin-1 that can promote the degradation of the class III PI3K complexes. Once the phagophore formation is initiated, compounds such as verteporfin can interfere with LC3-interacting region motifs and potentially block the selective recruitment of cargos such as mitochondria. The trafficking of autophagosomes to the lysosome is facilitated by the cytoskeleton. Thus, microtubule destabilization by vinca alkaloids can block the maturation of autophagosomes, whereas stabilization by taxol may increase the fusion between autophagic vacuoles and lysosomes. On the other hand, paclitaxel was shown to inhibit autophagy by blocking the activation of the VPS34 complex by inducing inhibitory phosphorylation of VPS34 as a substrate of CDK1. Lysosomotropic agents that increase the lysosomal pH, such as CQ, NH₄Cl, and monensin, interfere with lysosomal function and block autophagy at a late stage. The final step of the autophagy pathway can also be blocked by inhibitors of lysosomal enzymes, such as E64d and pepstatin A, and by bafilomycin A1, a specific inhibitor of v-ATPase.