Lysosomes in Cancer-At the Crossroad of Good and Evil

Abstract

Although it has been known for decades that lysosomes are central for degradation and recycling in the cell, their pivotal role as nutrient sensing signaling hubs has recently become of central interest. Since lysosomes are highly dynamic and in constant change regarding content and intracellular position, fusion/fission events allow communication between organelles in the cell, as well as cell-to-cell communication via exocytosis of lysosomal content and release of extracellular vesicles. Lysosomes also mediate different forms of regulated cell death by permeabilization of the lysosomal membrane and release of their content to the cytosol. In cancer cells, lysosomal biogenesis and autophagy are increased to support the increased metabolism and allow growth even under nutrient- and oxygen-poor conditions. Tumor cells also induce exocytosis of lysosomal content to the extracellular space to promote invasion and metastasis. However, due to the enhanced lysosomal function, cancer cells are often more susceptible to lysosomal membrane permeabilization, providing an alternative strategy to induce cell death. This review summarizes the current knowledge of cancer-associated alterations in lysosomal structure and function and illustrates how lysosomal exocytosis and release of extracellular vesicles affect disease progression. We focus on functional differences depending on lysosomal localization and the regulation of intracellular transport, and lastly provide insight how new therapeutic strategies can exploit the power of the lysosome and improve cancer treatment.

Keywords: LMP; exocytosis; extracellular vesicles; lysosomal positioning; lysosome.

Figure 1

Lysosomal function. Lysosomes receive extracellular cargo via receptor-mediated endocytosis and uptake of bulk material via pinocytosis and phagocytosis (1). By utilizing receptor-mediated endocytic uptake of LDL particles, lysosomes participate in cholesterol homeostasis (2). Translocation of lysosomes to the plasma membrane and exocytosis of hydrolytic enzymes (3). mediates e.g., bone remodeling, degradation of the extracellular matrix and cell-to-cell communication. Lysosomal exocytosis is also important for plasma membrane repair where the lysosome donates its membrane to repair the lesion (3). Lysosomal processing of foreign and endogenous material allow antigen presentation on MHC-II molecules (4). Intracellular material is degraded in autolysosomes, formed by the fusion of a lysosome and an autophagosome (5). Damage to the lysosomal membrane results in release of lysosomal proteases to the cytosol and cell death induction (6). By acting as a central hub for nutrient sensing, the lysosome is involved in the regulation of gene expression and metabolic signaling (7). Image created with BioRender.com, adapted from [8].

Figure 2

Function of the major lysosomal membrane proteins. Lysosomal membrane proteins play crucial roles in maintaining the function, structure, and integrity of the organelle. They are involved in key processes such as metabolite transport, which includes proton pumping and acidification. Moreover, nutrient-sensing membrane proteins are involved in cell signaling and regulation of homeostasis and metabolism. Membrane contact sites coordinate with, for example lipid metabolism and Ca²⁺ signaling, while proteins associated with the membrane also regulate the dynamics of lysosomal fusion and fission, as well as lysosome trafficking along microtubule. Image created with BioRender.com, first published in [8].

Figure 3

Autophagic pathways. Three main routes of autophagy are identified. Large cytoplasmic material is mainly degraded via macroautophagy, where the material is sequestered by a phagophore, forming an autophagosome. The autophagosome fuses with a lysosome to form an autolysosome in which degradation takes place. Cytosolic proteins are degraded by chaperone-mediated autophagy (CMA), where the chaperone protein HSC70 recognizes a target motif on cytosolic proteins and facilitates its binding to the CMA-receptor, LAMP2a. The binding induces LAMP2a oligomerization and allows translocation of the target protein to the lysosomal lumen. During microautophagy, invaginations are formed in the lysosomal membrane to allow a direct uptake of cytoplasmic proteins and smaller structures into the lysosome. Image first published in [8].

Figure 4

The endocytic pathway. Soluble and membrane-bound material are taken up via endocytosis and sorted in early endosomes, where the majority is recycled back to the plasma membrane via recycling endosomes (orange arrows). Material to be degraded follows the endocytic route to the lysosome (grey arrows). During this process, delivery of newly synthesized lysosomal components from the trans-Golgi network (TGN) allows maturation of early endosomes into late endosomes and lysosomes. The delivery from TGN can occur via the secretory pathway (green arrows) where secreted components are taken up via endocytosis, or via a direct fusion of Golgi-derived vesicles with early and late endosomes (blue arrows). Endosomal maturation also includes accumulation of intraluminal vesicles to allow sorting and degradation of transmembrane cargo. Via retrograde transport, TGN-specific material is recycled from the endolysosomal compartments (red arrows). Transient and complete fusion events between endosomes and lysosomes generates endolysosomes and facilitates exchange of material and cargo degradation. Image created with BioRender.com, adapted from [8].

Figure 5

Participation of cathepsins in regulated cell death. Lysosomal membrane permeabilization (LMP) results in release of cathepsins to the cytosol and is associated with increased reactive oxygen species (ROS). Hyperactivation of autophagy during e.g., mitophagy and ER-phagy results in altered lipid metabolism and lysosomal membrane destabilization with ensuing cathepsin release and autophagy-dependent cell death. LMP and subsequent release of cathepsins can trigger necroptosis, a specific form of cell death with necrosis like morphology. Pyroptosis is the consequence of inflammatory processes where cathepsin-induced assembly of the inflammasome activates caspase-1. Cytosolic cathepsins can also induce cytochrome c release from the mitochondria to activate the intrinsic pathway to apoptosis. This is mediated by proteolytic activation of Bid or inactivation of anti-apoptotic Bcl-2 proteins, or a direct proteolytic processing of caspases. Mitochondrial outer membrane permeabilization can further amplify lysosomal damage by causing elevated levels of oxidative stress, and by inducing Bax oligomerization in the lysosomal membrane. Image created with BioRender.com.

Figure 6

Regulation of lysosomal transport. Retrograde transport towards the cell nucleus is mainly orchestrated by the dynein/dynactin motor protein complex. The GTPase Rab7 mediates lysosomal coupling to the dynein/dynactin complex with the aid of its effector RILP. Alternatively, TRPML1 mediated Ca²⁺ release, or starvation-induced transcription of the lysosomal transmembrane protein TMEM55B, induce the interaction with the adaptor proteins ALG2 and JIP4, respectively, to couple lysosomes to the dynein/dynactin complex. Anterograde movement to the cell periphery is instead mediated by kinesin proteins. The assembly of BORC, Arl8b and SKIP, or Rab7 and FYCO1, links lysosomes to microtubule. The transport is regulated by nutrient availability, TFEB activity and Ca²⁺ levels. Lysosomes adjacent to the nucleus are acidic and proteolytically active, and fuse with autophagosomes to allow degradation and nutrient generation. Peripherally located lysosomes are involved in lysosomal exocytosis and plasma membrane repair and induce mTORC activation. Image created with BioRender.com.

Figure 7

Routes of secretion from the endolysosomal system. Intraluminal vesicles (ILVs) originating from multivesicular endosomes (MVEs) are secreted as exosomes. Mature lysosomes secrete soluble proteins via lysosomal exocytosis or as ectosomes following fusion with the plasma membrane. While autophagosomes normally fuse with lysosomes to allow degradation of their cargo, they can also reroute to the plasma membrane and release soluble content or extracellular vesicles (EVs). Furthermore, autophagosome fusion with multivesicular endosomes forms a hybrid organelle termed amphisome, which can release autophagic degradation products and exosomes of both endosomal and autophagic origin. Image created with BioRender.com.

Figure 8

Lysosomal exocytosis. Lysosomal fusion with the plasma membrane is triggered by increased intracellular Ca²⁺ (iCa²⁺), originating from intracellular Ca²⁺ stores or via influx from the extracellular space. Ca²⁺ binds to and activates synaptotagmin VII (SytVII), resulting in transfer of lysosomes to the plasma membrane. After tethering to the plasma membrane, docking and merging of the phospholipid bilayers is performed by interaction between VAMP7, which is a lysosomal v-SNARE, and the t-SNAREs SNAP-23 and syntaxin 4 on the plasma membrane. Upon membrane fusion, the lysosomal content is released extracellularly. Image created with BioRender.com.

Figure 9

Cancer-promoting effects of lysosomal exocytosis. Several cancer-associated changes, such as TFEB upregulation and increased expression of Ca²⁺ permeable channels, increase lysosomal exocytosis and enhance release of both soluble content and ectosomes shedded from the plasma membrane. The released lysosomal content has been shown to mediate extracellular matrix (ECM) degradation, immunomodulation, and neovascularization. Lysosomal exocytosis induces TGF-β signaling to activate cancer-associated fibroblasts (CAFs) and promotes epithelial to mesenchymal transition (EMT). EMT is further stimulated by downregulation of adhesion molecules such as E-cadherin, facilitated through lysosomal degradation. Secreted cathepsins can promote release of exosomes from MVEs to further modulate the tumor microenvironment. By utilizing lysosomal membrane fusion with the plasma membrane, the cancer cell can elongate forming invadopodia to create breaches in the basement membrane and facilitate tumor invasion. Image created with BioRender.com.

Figure 10

Cancer-associated changes affecting lysosomal positioning. Peripheral positioning of lysosomes is Ca²⁺ dependent and increases lysosomal exocytosis and tumor invasiveness. Contrarily, tumor-induced perinuclear positioning is often associated with increased susceptibility to lysosomal membrane destabilization and lysosome-induced cell death. Lysosomal localization and membrane stability is determined by a variety of upregulated (red) and downregulated (blue) genes, including motor proteins and lysosomal membrane proteins. Cancer cells have relatively low levels of sphingomyelin (SM) compared to normal cells. SM is converted to ceramide on intraluminal vesicles (ILVs) in the lysosome by acid sphingomyelinase (ASMase). The action of ASMase is mediated by the negatively charged lipid BMP, an interaction that is stabilized by HSP70. Accumulation of SM causes lysosomal membrane permeabilization and cathepsin-induced cell death. Image created with BioRender.com.