# The Exploitation of Lysosomes in Cancer Therapy with Graphene-Based Nanomaterials
## 5 The Role of Lysosomes in GNM-Based Cancer Therapy
### GNMs Induce Tumor Cell Death by Suppressing (Auto)Lysosomal Degradation  
In the process of autophagy, exhausted/damaged intracellular macromolecules and organelles are entrapped in double-membrane autophagosomes. Autophagosomes then fuse with lysosomes in autolysosomes where sequestered content is degraded by acid hydrolases. Since autophagy is primarily responsible for degrading intracellular cytoplasmic content, it is unclear how externally added GNMs are included in autophagic vacuoles [11]. We propose several mechanisms. (1) Small nanoparticles such as graphene sheets and graphene flakes can directly diffuse through the plasma membrane into the cytoplasm [41] and subsequently become sequestered in autophagosomes. (2) Endocytosed GNMs escape from endosomes into the cytoplasm, where they can be taken up into autophagosomes, similar to what has been shown for some microorganisms [74]. (3) Late endosomes containing GNMs can fuse with autophagosomes, as already demonstrated for gold nanoparticles [75]. The main role of autophagy is to protect cells from the accumulation of damaged proteins/organelles, energy deficiency, oxidative stress, and drug-induced stress. Consistent with its pro-survival role, the induction of complete autophagy has been shown in several studies to be protective for tumor cells treated with GNMs [76; 77; 78; 79]. On the other hand, several studies have shown that GNMs can induce tumor cell death by suppressing autophagic degradation. In the research conducted by [PERSON] and co-workers, GO induced oxidative stress, mitochondrial damage, and reduced energy production, thereby stimulating incomplete autophagy in human neuroblastoma cells SH-SYSY. The non-degradable GO accumulated in lysosomes, causing steric hindrance that elevated lysosomal pH and suppressed the degradative capacity of lysosomal enzymes such as acid phosphatase and cathepsin B. This resulted in the accumulation of dysfunctional mitochondria and ultimately contributed to mitochondria-dependent apoptosis [9]. [PERSON] et al. demonstrated that GO nanosheets induced LMP, impaired the acidity of lysosomes, and thus suppressed the activity of cathepsin B and acid phosphatase in PC12 rat pheochromocytoma-derived cells. The impairment of lysosomal degradation led to abnormal accumulation of the autophagic proteolysis substrate p62, which contributed in part to the induction of apoptosis [10]. Similarly, [PERSON] and coworkers demonstrated that GO induced lysosomal alkalization, inhibited cathepsin B, and induced p62 accumulation and apoptosis in F98 rat astrofiioma cells [11]. After internalization in A549 cells via clathrin-mediated endocytosis, GO-chloroquine nanocomposite (GO-Chl) was found to stimulate the synthesis of autophagosomes but suppress their fusion with lysosomes. This resulted in the blockade of the autophagy flux and activation of necrotic death, as demonstrated by [PERSON] et al. [53]. Conversely, through the selective degradation of mitochondria and cell survival proteins, prolonged and overactivated autophagy can cause apoptotic, necrotic, or necrotic death of tumor cells, or itself be an alternative mechanism of cell death (programmed cell death Type II). Consistent with the cytotoxic role of autophagy, autophagy was induced by the combined treatment with GO and the cisplatin-stimulated necrosis of colorectal carcinoma cells [80], whereas autophagy induced by photoactivated GQDs stimulated apoptosis in glioma cells [78]. The exact mechanisms of autophagy induction/suppression by GNMs, the role of autophagy in cell death and survival, and its medical significance are summarized in our previous review [81]. Therefore, either incomplete cytoprotective autophagy [9; 10; 11; 53] or autophagy that is itself cytotoxic [78; 80] could be useful in cancer therapy with GNMs. Moreover, it might be valuable to combine widely used chemotherapeutic agents that induce prov Survival autophagy with GNMs that block autophagy flux [9; 10; 11; 53].