Lysosomes and Cancer Progression: A Malignant Liaison

Abstract

During primary tumorigenesis isolated cancer cells may undergo genetic or epigenetic changes that render them responsive to additional intrinsic or extrinsic cues, so that they enter a transitional state and eventually acquire an aggressive, metastatic phenotype. Among these changes is the alteration of the cell metabolic/catabolic machinery that creates the most permissive conditions for invasion, dissemination, and survival. The lysosomal system has emerged as a crucial player in this malignant transformation, making this system a potential therapeutic target in cancer. By virtue of their ubiquitous distribution in mammalian cells, their multifaced activities that control catabolic and anabolic processes, and their interplay with other organelles and the plasma membrane (PM), lysosomes function as platforms for inter- and intracellular communication. This is due to their capacity to adapt and sense nutrient availability, to spatially segregate specific functions depending on their position, to fuse with other compartments and with the PM, and to engage in membrane contact sites (MCS) with other organelles. Here we review the latest advances in our understanding of the role of the lysosomal system in cancer progression. We focus on how changes in lysosomal nutrient sensing, as well as lysosomal positioning, exocytosis, and fusion perturb the communication between tumor cells themselves and between tumor cells and their microenvironment. Finally, we describe the potential impact of MCS between lysosomes and other organelles in propelling cancer growth and spread.

Keywords: cancer progression; lysosomal exocytosis; lysosomal membrane contact sites; lysosome movement; lysosome positioning.

FIGURE 1

Schematic drawing of the topology of the lysosome. The lysosomal membrane comprises several integral membrane proteins (i.e., LAMP1 and LAMP2A), ion (Ca²⁺) channels (i.e., TRPMLs and TPCs), traffic and fusion proteins (i.e., Rabs and the SNARE subunits, Synaptotagmin VII and VAMP7), lipid and amino acid transporters (NPC1 and SLC38A9). The lysosomal luminal domains of the LAMPs are heavily glycosylated/sialylated, forming a protective glycocalyx that ensures the integrity of the lysosomal membrane. The multimeric vacuolar H⁺ ATPase pump is essential for maintaining the acidic pH of the lysosomal lumen needed for the activity of all lysosomal hydrolases. A multiprotein complex (lysosomal nutrient sensing) assembled at the lysosomal membrane regulates mTORC1 activity.

FIGURE 2

Schematic representation of the components regulating the lysosomal nutrient sensing machinery upstream of mTORC1/TFEB/TFE3. (A) Under nutrient rich and high energy conditions, the Ragulator/LAMTOR complex bound to the amino acid transporter SLC38A9 at the lysosomal membrane together with the v-ATPase serve as scaffold for the Rag GTPases, RagA/B and RagC/D, which cycle between an active (RagA/B^GTP – RagC/D^GDP) or inactive (RagA/B^GDP – RagC/D^GTP) state. The GAP activity of the GATOR1 complex, tethered to the lysosomal membrane by KICSTOR, is inhibited by the GATOR2 complex, enabling RagA/B^GTP–mediated recruitment of mTORC1 to the lysosomal membrane. The FLCN/FNIP GAP activity towards RagC/D facilitates mTORC1 recruitment. The lysosome-anchored Rheb GTPase in its GTP-bound state mediates the activation of mTORC1 kinase that phosphorylates TFEB/TFE3 (TFEB/3), promoting their cytosolic retention and sequestration by the 14-3-3 proteins. In the nucleus, MYC/HDAC occupy the E-box/CLEAR binding site in the proximal promoters of lysosomal and autophagic genes, inhibiting their expression. (B) Under low nutrient and energy conditions the Rag GTPases are in an inactive state (RagA/B^GDP – RagC/D^GTP). Active GATOR1 converts RagA/B to their GDP-bound state, which inhibits mTORC1 recruitment to the lysosomal membrane. In addition, the Ragulator /v-ATPase become accessible to AMPK/LKB1/AXIN complex at the lysosomal membrane. AXIN inhibits the GEF activity of the Ragulator promoting mTORC1 dissociation. FLCN/FNIP complex, bound to the lysosomal membrane, inhibits the GEF activity of the Ragulator and switches the Rag GTPases to an inactive state, leading to dissociation of mTORC1 from the lysosomal surface and its inhibition. Dephosphorylated TFEB/3 is released from 14-3-3 and translocates to the nucleus. Inhibition of HDAC and acetylation of histones reduce c-MYC levels and allow for the binding of TFEB/3 to the E-boxes/CLEAR sequence, resulting in the transcriptional activation of lysosomal and autophagic genes.

FIGURE 3

Schematic representation of lysosomal positioning and lysosomal exocytosis. Retrograde movement of a pool of lysosomes to the perinuclear region occurs toward the minus-end of microtubules and is mediated by Rab7 and the dynein-dynactin complex. Anterograde movement of lysosomes to the cell periphery occurs toward the plus-end of microtubules and is mediated by kinesin motors. Close to the cell surface, lysosomes that engage in lysosomal exocytosis move along actin filaments via interaction with a motor myosin. During lysosomal exocytosis, the docking of lysosomes at the PM is mediated by LAMP1. The fusion of the lysosomal membrane with the PM depends on the Ca²⁺ sensing activity of SytVII and is mediated by v-SNARE and t-SNARE complexes.

FIGURE 4

Lysosomes redistribute to the cell periphery in aggressive cancer cells. Lysosomes marked with Lysotracker green in aggressive rhabdomyosarcoma cells (RH30) move to and redistribute at the cell periphery, whereas lysosomes in less aggressive rhabdomyosarcoma cells (RH41), cluster around the perinuclear region. The movement of a lysosome (white circle) to the cell periphery in RH30 cells was recorded in a movie and snapshots were taken at the indicated timepoints. The contours of the cells are demarcated with a white line. These images are adapted from a movie published in Machado et al. (2015).

FIGURE 5

In aggressive cancer cells an increased number of lysosomes accumulates at the PM, prior to undergoing lysosomal exocytosis. Total internal reflection (TIRF) microscopy shows the presence of lysotracker green marked lysosomes in the evanescence field underneath the PM of rhabdomyosarcoma cells. Aggressive rhabdomyosarcoma cells (RH30) show an increased number of clustered lysosomes juxtaposed to the PM, compared to the number seen in less aggressive rhabdomyosarcoma cells (RH41). The contours of the cells are demarcated with a white line.

FIGURE 6

Schematic representation of downstream effects of lysosomal exocytosis in cancer cells. Upon a Ca²⁺ spike lysosomes docked at the PM undergo fusion with the PM and secrete their content extracellularly, including active hydrolases and extracellular matrix (ECM) components that remodel and degrade the ECM; chemotherapeutic drugs that are weak-bases and tend to accumulate in the acidic lysosomes. Exosomes packed with invasive signaling molecules are released via lysosomal exocytosis by cancer cells and induce the transformation of resident fibroblasts and macrophages of the tumor microenvironment into cancer associated fibroblasts (CAFs) and tumor associated macrophages (TAMs).