br Activation of AMPK through mitochondria
Activation of AMPK through mitochondria-derived ROS suggests a direct link between the oxidative stress response and autophagy acti-vation. Indeed, oxidation of glutathione was able to induce autophagy even in the absence of any autophagic stimulus (Desideri et al., 2012). Oxidation of Cys residues post-translationally suggested an important mechanism in terms of both structure and function of the modified protein (de Duve et al., 1955). Therefore, proteins containing oxidized residues are readily responsive to oxidative stress conditions. Several autophagy regulating proteins such as the ATG7-ATG3 and ATG7-ATG10 systems, some members of RAB GTPase (e.g., RAB33b), and the phosphatase and tensin homologue deleted (PTEN) (Filomeni et al., 2015) are found to act as Cys residues. Furthermore, p62 was shown to contain a cysteine-rich zinc-finger motif, which could be regulated by redox and protein homeostasis (Carroll et al., 2018). Deficiency in au-tophagy genes, such as Beclin-1, UVRAG, ATG5 and ATG7 led to DNA damage accumulation (Eliopoulos et al., 2016). In line with these, suppression of FIP200 impaired DNA damage response (DDR) and sensitized cancer PKC412 to ionizing radiation-induced oxidative stress (Bae and Guan, 2011). ATM, one of the major DNA repair proteins, established another link between DDR and autophagy (Hurley and Bunz, 2007). ATM, through the LKB1/AMPK pathway, acted on TSC2 and inhibited mTORC1 in response to ROS-induced cellular damage (Alexander et al., 2010). Autophagy is considered to delay apoptotic cell death upon DNA damage by providing the energy required for DNA repair processes, which play a role in development of chemoresistance mechanisms in cancers (Abedin et al., 2007; Vessoni et al., 2013; Yoon et al., 2012). Additionally, removal of dysfunctional mitochondria under oxidative or nitrosative stress conditions reduce the level of DNA-
damage stress to allow cancer cells escape from cell death mechanisms.
3.2.3. Autophagy promotes angiogenesis, metastasis and invasion during tumorigenesis
Metastasis has been described as the distribution of tumor cells from the primary tumor to surrounding tissues and even to distant organs (Valastyan and Weinberg, 2011). Proliferation and the metastatic spread of cancer cells require adequate level of oxygenation and con-tinuous nutrient supply. In order to reach these demands, tumors de-velop new blood and lymphatic vessels through a process called an-giogenesis. Angiogenesis is controlled via the angiogenic activators and inhibitors whose levels correlate with the aggressiveness of tumor cells (Zhou et al., 2013). Autophagy is also implicated in development of vasculature in tumor tissues. ATG5 was shown to modulate angiogen-esis in endothelial cells, which is proposed to occur through the high mobility group box 1 (HMGB1) pathway (G.J. Du et al., 2012, J. Du et al., 2012). HMGB1 induced autophagy by binding to Beclin-1 (Kang et al., 2011). Additionally, HMGB1 contributed to angiogenesis and tumor cell survival by mediating the crosstalk between endothelial cells and tumor cells (Yang et al., 2014). Autophagy is also reported to play an important role during angiogenesis in bovine aortic endothelial cells. Evidence suggests that induction of autophagy can also promote VEGF-induced angiogenesis (G.J. Du et al., 2012, J. Du et al., 2012). Ad-ditionally, mTOR inhibitors influenced tumor angiogenesis in malig-nant and apoptosis-inhibited lung cancer cells (Shinohara et al., 2005). Rather than tumor suppressive role of Beclin-1, it has been also shown that in a context dependent manner Beclin-1 promoted tumorigenesis. For example, it has been shown that in triple negative breast cancer cells complete knockout of Beclin-1 resulted in impaired tumor growth through promoting G0/G1 cell cycle arrest and impaired migration capacity in collaboration with reverse signal of EMT (Wu et al., 2018).
Throughout the process of metastasis tumor cells acquire pheno-typic changes allowing them to gain the ability to enter/exit the vas-culature, survive the immune attack and the demanding conditions within the circulatory system. Additionally, metastatic cells should also be able to extravasate at distant capillary beds, attach to distant tissues and proliferate in a foreign microenvironment (Kenific and Debnath, 2015). Anoikis is a special form of apoptosis, which occurs upon de-tachment of cells from the extracellular matrix (ECM) (Paoli et al., 2013). Therefore, resistance to anoikis favored for metastasis by al-lowing cancer cells to survive stress conditions associated with ECM detachment (Wang et al., 2017). Autophagy promoted the survival of metastatic cells during ECM detachment and components of the ECM were also able to regulate autophagic activity in cervical cancer cells (Tuloup-Minguez et al., 2011). Autophagy is rapidly induced cell sur-vival during anoikis when cells are grown under low ECM attachment conditions in MCF10A mammary epithelial cells (Debnath et al., 2002). Accordingly, siRNA-mediated knockdown of ATG5, ATG6 and ATG7 was found to suppress matrix detachment-induced autophagy (Fung et al., 2008).