br A class III PI K
A class III PI3K complex is mainly responsible for the nucleation of the autophagic membranes. Several proteins such as VPS34, Beclin-1, AMBRA1 and mATG9 were identified as novel regulator proteins in phagophore formation (Feng et al., 2016; Mehrpour et al., 2010; Papinski and Kraft, 2014; Park et al., 2016; Petherick et al., 2015; Russell et al., 2013). Beclin-1 is one of the key protein in membrane nucleation and its interaction with BCL2 inhibits autophagy (Fig. 1B). Conversely, disruption of this interaction allows Beclin-1 to bind with lipid kinase VPS34 and promote membrane nucleation (Pattingre et al., 2005). Alternatively, Beclin-1 diﬀerentially modulates membrane for-mation through interaction with diﬀerent mediators such as UVRAG (Liang et al., 2008), RUBICON, ATG14L (Matsunaga et al., 2009), AMBRA1 (Yazdankhah et al., 2014) and VMP1 (Molejon et al., 2013a, 2013b). VPS34-mediated enzymatic generation of phosphatidylinositol 3-phosphate (PtdIns3P) provides a platform for phosphatidylinositol 3-phosphate (PI3P)-binding domain-containing autophagy proteins, in-cluding WIPI1–4 and DFCP1 (Mauthe et al., 2011; Mercer et al., 2018).
Accumulation of PI3P-binding domain containing proteins at the membrane nucleation site resulted in binding of additional ATGs, which are required for AngiotensinI and closure of the autophagosome mem-brane. The two ubiquitin-like conjugation system regulate elongation of the isolation membrane (Fig. 1C). In one system, E1- and E2-like actions of ATG7 and ATG10 catalyze the covalent conjugation of ATG12 to the ATG5 protein. ATG5–12 conjugation is followed by the recruitment of ATG16L1 giving rise to the formation of ATG12-ATG5-ATG16L1 com-plex, which serves as an E3-like function to the second ubiquitin-like conjugation system (Mizushima et al., 2011; Shpilka et al., 2011; Tsuboyama et al., 2016).
The second system consists of the conjugation of LC3 protein to a lipid molecule, phosphatidylethanolamine (PE) (Tanida et al., 2004). LC3 precursor protein is cleaved by ATG4 and this cleavage allows exposure of the glycine residue from its carboxy-terminus that lead to PE conjugation. E1-like ATG7 and E2-like ATG3 proteins possess the LC3-PE conjugation which also known as LC3-II (an established marker for autophagosomes) (Hanada et al., 2007; Nakatogawa et al., 2007) (Fig. 1C).
2.4. Fusion and degradation
After formation of autophagosome membrane, autophagic vesicles are transported to lysosomes for degradation. Autophagosome-asso-ciated LC3 proteins become delipidated and recycled prior to fusion (Kriegenburg et al., 2018; Nakamura and Yoshimori, 2017). Several SNARE proteins, including STX17 and WAMP8 and lysosomal integral protein LAMP2 and RAB proteins play critical roles in autophaosome-lysosome fusion (Jager, 2004; Tanaka et al., 2000). Finally, autolyso-some is formed by fusion of autophagosomes with lysosomes where cargo is degraded by the lysosomal proteases (Fig. 1D). Thereafter, degredation products such as amino acids, fatty acids are redirected to cytosol for further reuse in various metabolic processes (Panda et al., 2015).
3. Autophagy-mediated cancer regulation
Beth Levine's group suggested a direct link between autophagy and cancer for the first time in 1999. They showed that monoallelic BECN1/ ATG6 gene deletions in human cells might contribute to malignancies both in vitro (Liang et al., 1999) and in in vivo (Qu et al., 2003). Currently, a vast number of studies indicate that ATGs and the related pathways can crosstalk with oncogenes and/or tumor suppressors. In-deed, accumulated data support the notion that the role of autophagy in malignant transformation is complicated and may have opposite con-sequences in a context and cell-type dependent manner (Galluzzi et al., 2015).
3.1. Autophagy as a tumor suppressor mechanism
Autophagy has been implicated as a favorable mechanism for sup-pression of cancer formation at multiple stages through its established roles in preservation of genomic stability; elimination of endogenous sources of reactive oxygen species (ROS); the maintenance of bioener-getic functions; degradation of oncogenic proteins and induction of immunresponse mechanisms against malignant transformations (Galluzzi et al., 2015).
In addition to Beclin-1, several other autophagy proteins have been described with their suppressive eﬀects on tumorigenesis. For instance, it has been proposed that ATG4C deficiency associated with increased tumorigenesis in mice (Marino et al., 2007). Similarly, ATG5 deletion in mice induced benign liver tumor formation (Takamura et al., 2011). Additionally, mutations in ATG2B, ATG4, ATG5, ATG12 and ATG9B were frequently observed in human cancers suggesting that autophagy plays a suppressive role in malignant transformation at several steps of tumorigenesis (An et al., 2011; Kang et al., 2009; Kim et al., 2011). Tumor suppressive function of autophagy is summarized in Fig. 2.