br Fig Molecular mechanism of autophagy
Fig. 1. Molecular mechanism of autophagy regulation in mammals.
Autophagic process consists of several phases such as initiation (A), nucleation (B), maturation (C), fusion and degradation (D). Same colours express the involvement of proteins or molecules in respective complexes or pathways.
this review, we mainly focus on autophagy and other major classes, CMA and microautophagy were discussed in detailed elsewhere (Kaushik and Cuervo, 2018; Oku and Sakai, 2018).
The ability to recycle macromolecules through autophagy gives cells an advantage for survival under stressful conditions such as nutrient starvation, oxidative stress, hypoxia, ER stress, metabolic stress etc. (Piacentini and Kroemer, 2015). Moreover, selective autophagy allows cells to control number of the organelles based on the requirement, eliminating dysfunctional compartments and disposing of pathogens by combining the ubiquitin-proteasome system (UPS) and autophagic machinery (Kocaturk and Gozuacik, 2018). However, under certain conditions excess or deregulated activity of autophagy may also lead cell death. Whether autophagy is an executioner or a savior is still a matter of debate and it is often determined in a context- and cell type-dependent manner (Liu et al., 2016).
In order to survive under stressful conditions within tumor such as hypoxia and/or nutrient deprivation or oxidative stress, cancer cells frequently exploit autophagy (Kenific and Debnath, 2015). Ad-ditionally, tumor cells could benefit from autophagy for Necrostatin-1 to metastasis for withstanding the environmental stress they face during the several steps of metastasis including migration into the systemic circulation, adherence to the vessel walls, extravasation and coloniza-tion (J. Su et al., 2015, Z. Su et al., 2015). Thus, recycling of cyto-plasmic materials by autophagy provides continuous supply of energy as well as essential ingredients for cancer cells to survive (J. Su et al., 2015, Z. Su et al., 2015) and promotes metastatic reocurrence of tumors (Vera-Ramirez et al., 2018).
2. Molecular mechanisms of autophagy
Autophagic process is initiated by the formation of double-mem-brane vesicles known as autophagosomes. Various cargos are engulfed into autophagosome and autophagosome eventually fuses with lyso-somes that forms autolysosomes. (Lamb et al., 2013). Engulfed mate-rials were degraded by the action of lysosomal hydrolases and newly generated building blocks (e.g., amino acids from protein degradation) are transferred back to cytosol for reuse (Fig. 1). A series of stimuli, including amino acid deprivation, serum starvation and growth factor deprivation, hypoxia, exposure to various chemicals and stress condi-tions are capable of activating autophagy.
Genetic studies in yeast provided initial discoveries of autophagy-related (ATG) genes and enlightened the details of molecular signaling pathway of autophagic process (Nakatogawa et al., 2007). The autop-hagic pathway can be divided into several diﬀerent phases: Initiation, nucleation, maturation, fusion and degradation (Fig. 1).
The target of rapamycin, TOR (mTOR in mammals), is an evolu-tionarily conserved serine/threonine kinase responsible for conveying a number of autophagy stimulating signals. In mammals, mTOR exists as two diﬀerent complexes: mTOR complex1 (mTORC1) and mTOR complex2 (mTORC2). mTOR complexes constitute a critical node for the integration of signaling pathways that regulate cellular energy homeostasis by coordinating anabolic and catabolic processes (Kroemer et al., 2010). PKB-AKT pathway can activate mTORC1 and suppresses autophagy (Dan et al., 2014; Zalckvar et al., 2009) (Fig. 1A). In con-trast, autophagy is activated by another kinase, AMP-activated protein kinase (AMPK), which has crucial role in sensing cellular energy and ATP levels (Garcia and Shaw, 2017; Xiao et al., 2011). Following de-crease in ATP, AMPK becomes activated through direct interaction with ADP or ATP resulting a conformational change. AMPK activation is also controlled by the two upstream kinases: LKB1 and calcium/calmodulin-dependent protein kinase kinase-β (CaMKKβ) (Hawley et al., 2005; Shaw et al., 2004). There has been cross-regulation between AMPK and mTOR activity. Low energy status activates AMPK, whereas this European Journal of Pharmaceutical Sciences 134 (2019) 116–137
activation leads inhibition of mTOR due to phosphorylation of TSC2 and RAPTOR (Gwinn et al., 2008; Inoki et al., 2003).
Under nutrient-rich conditions, mTORC1 complex suppresses au-tophagy by inactivation of ULK1/2 complex, which composed of ULK1 or ULK2 kinase, ATG13, FIP200 and ATG101. In response to nutrient deprivation, ULK1/2 complex is activated by dissociation of mTORC1 which in turn activates autophagy through class III phosphatidylinositol 3-kinase (PI3K) complex (Chen and Klionsky, 2011; Hosokawa et al., 2009).