Hijacking homeostasis: Regulation of the tumor microenvironment by apoptosis

Abstract

Cancers are genetically driven, rogue tissues which generate dysfunctional, obdurate organs by hijacking normal, homeostatic programs. Apoptosis is an evolutionarily conserved regulated cell death program and a profoundly important homeostatic mechanism that is common (alongside tumor cell proliferation) in actively growing cancers, as well as in tumors responding to cytotoxic anti-cancer therapies. Although well known for its cell-autonomous tumor-suppressive qualities, apoptosis harbors pro-oncogenic properties which are deployed through non-cell-autonomous mechanisms and which generally remain poorly defined. Here, the roles of apoptosis in tumor biology are reviewed, with particular focus on the secreted and fragmentation products of apoptotic tumor cells and their effects on tumor-associated macrophages, key supportive cells in the aberrant homeostasis of the tumor microenvironment. Historical aspects of cell loss in tumor growth kinetics are considered and the impact (and potential impact) on tumor growth of apoptotic-cell clearance (efferocytosis) as well as released soluble and extracellular vesicle-associated factors are discussed from the perspectives of inflammation, tissue repair, and regeneration programs. An "apoptosis-centric" view is proposed in which dying tumor cells provide an important platform for intricate intercellular communication networks in growing cancers. The perspective has implications for future research and for improving cancer diagnosis and therapy.

Keywords: apoptosis; cancer; efferocytosis; extracellular vesicle; homeostasis; macrophage; tumor microenvironment.

FIGURE 1

Interrelationships between cell gain, cell loss, and population growth in tumors. Input (gain) represents mitosis and inward cell migration, while output (loss) represents cell death and outward cell migration. At normal homeostasis (left tank), the rates of cell gain and loss are equivalent. Tumor growth through aberrant homeostasis may be achieved by reduction of cell loss (center tank) and/or through increase of cell gain. Variation in growth rates may occur through changes in gain and/or loss rates during the course of tumor evolution (right tank, showing increased growth). Adapted from.

FIGURE 2

Overview of molecular pathways in apoptosis. Apoptosis is mediated by a series of caspases, cysteine proteases that only cleave after aspartate residues in the P1 position. Caspases, like apoptosis itself, are evolutionarily conserved, from worms to mammals. They are produced as zymogens and are themselves activated through proteolytic cleavage to form tetramers consisting of two large and two small fragments. In mammals, two pathways of caspase activation have been well defined: (Right) the extrinsic, or death receptor pathway and (Left) the intrinsic, or mitochondrial pathway. Prototypically, the extrinsic pathway is initiated in the plasma membrane by the clustering of transmembrane death receptors FAS, TNFR1, and DR4/5 following binding of their specific extracellular ligands, FASL, TNF‐⍺, and TRAIL, respectively. The death receptors consequently form death‐inducing signaling complexes which cleave initiator caspases 8 and 10 which, once activated, then cleave and activate executioner caspases 3 and 7. Crosstalk with the intrinsic pathway also serves to activate and amplify extrinsic apoptotic signaling. In the intrinsic pathway, multiple signals of cell stress and toxicity ranging from growth factor or nutrient deprivation to genotoxic damage, are able to initiate apoptosis by triggering mitochondrial outer membrane permeabilization (MOMP), which is controlled by the BCL2 family of apoptosis inducers and inhibitors. Among the inducers, the BH3 (BCL2 homology domain 3)‐only proteins such as tBID, PUMA, and NOXA act on the mitochondrial pore‐forming members BAX and BAK to stimulate MOMP. This allows the release of proteins, importantly cytochrome c (CYC), from the mitochondrial intermembrane space into the cytosol. Pore formation can be inhibited by the anti‐apoptotic BCL2‐family members BCL2 itself, BCL‐xL, BCLW, A1/BFL1, and MCL1. Once in the cytosol, CYC can interact with APAF1 to form the apoptosome, a heptameric cytosolic “scaffold” for the activation of caspase 9, the initiator caspase of the intrinsic pathway which goes on to activate the effector caspases 3 and 7. Additional regulation is provided by inhibitors of apoptosis (IAPs) such as XIAP and their antagonists OMI, SMAC, and ARTS. Key consequences of effector caspase activation include (lower left) rapid and sustained externalization of PtdSer through inhibition of the flippases ATP11A and C, together with activation of the phospholipid scramblase Xkr8‐BSG/NPTN; (lower center) opening of PANX1 channels enabling release of biologically active small molecules from the apoptotic cell; (lower right) induction of blebbing via MLCK phosphorylation initiated by effector caspase‐mediated activation of ROCK1, resulting in cell fragmentation/ApoEV production. (Additional outcomes of effector caspase activation of key relevance to the present perspective such as DNA cleavage and PGE₂ synthesis are not shown).

FIGURE 3

Cell‐autonomous and non‐cell‐autonomous effects of apoptosis on tumor growth. Decreased apoptosis or increased mitosis through cell‐autonomous programs (green arrows) have oncogenic implications, each leading to growth, which may also be facilitated through non‐cell‐autonomous, growth‐promoting effects of apoptosis such as AiP (apoptosis‐induced proliferation [aka compensatory proliferation], blue arrows). AiP may be induced directly by mitogens released from apoptotic cells or via interactions with other cells, notably efferocytosing tumor‐associated macrophages. Anti‐oncogenic increases in cell death (as induced, for example by anti‐cancer therapies) may also be mediated either by cell‐autonomous apoptosis or by non‐cell‐autonomous AiA (apoptosis‐induced apoptosis, red arrow).

FIGURE 4

Overview of the potential oncogenic effects of apoptosis in tumor tissues. Representation of intercellular signals from apoptotic cells in a growing tumor, which are mediated either through the apoptotic secretome, including small ApoEV production (lower center, red pathways), or via contact‐dependent means (upper center, dark blue pathways). Response signals are integrated into pro‐oncogenic effector programs (right) such as trophic apoptosis‐induced proliferation (AiP), angiogenesis, invasion, enhanced migration and metastasis, resolution and anti‐inflammatory signaling as well as suppression of pro‐inflammatory effects and of innate and adaptive anti‐tumor immunity. Recycling of digested apoptotic cell‐derived components (top left) also very likely helps support tumor growth. Macrophages probably play critical roles in many of these responses and comprise the major professional efferocytic cells of the TME and probably respond, independently of efferocytosis to contact‐mediated and secretory communication from apoptotic cells too. Notably, virtually any cell in the malignant tissue has the potential to respond to the various signals emanating from apoptotic cells, some acting as professional efferocytes. In this way, it is proposed that intricate, intercellular circuits (left), founded in apoptosis, can be established to support disease development. Illustrative, theoretical examples of such circuits are shown in which the endpoint in each case is pro‐oncogenic (green arrows). In circuit A (orange arrow), contactless stimulation of a non‐phagocyte (e.g., lymphocyte) of the TME is shown. In B (purple arrow), the response is that of an efferocytosing macrophage, while C is more complex (blue arrows) depicting communication between non‐professional and professional phagocytes. See text for molecular pathways that may contribute to these, and other, circuits.