Strange attractors: DAMPs and autophagy link tumor cell death and immunity

Abstract

Resistance to 'apoptotic' cell death is one of the major hallmarks of cancer, contributing to tumor development and therapeutic resistance. Damage-associated molecular patterns (DAMPs) are molecules released or exposed by dead, dying, injured, or stressed non-apoptotic cells, with multiple roles in inflammation and immunity. Release of DAMPs not only contributes to tumor growth and progression but also mediates skewing of antitumor immunity during so-called immunogenic tumor cell death (ICD). Autophagy is a lysosome-mediated homeostatic degradation process in which cells digest their own effete organelles and macromolecules to meet bioenergetic needs and enable protein synthesis. For tumor cells, autophagy is a double-edged sword. Autophagy, in balance with apoptosis, can function as a tumor suppressor; autophagy deficiency, associated with alterations in apoptosis, initiates tumorigenesis in many settings. In contrast, autophagy-related stress tolerance generally promotes cell survival, which enables tumor growth and promotes therapeutic resistance. Most anticancer therapies promote DAMP release and enhance autophagy. Autophagy not only regulates DAMP release and degradation, but also is triggered and regulated by DAMPs. This interplay between autophagy and DAMPs, serving as 'strange attractors' in the dynamic system that emerges in cancer, regulates the effectiveness of antitumor treatment. This interplay also shapes the immune response to dying cells upon ICD, culling the least fit tumor cells and promoting survival of others. Thus, DAMPs and autophagy are suitable emergent targets for cancer therapy, considering their more nuanced role in tumor progression.

Figure 1

Extrinsic and intrinsic apoptosis. There are two intracellular pathways to control apoptosis. A third pathway, mediated by cytolytic effectors including T and NK cells, delivers pro-apoptotic granzymes through perforin-mediated pores, culling the least fit cells, and promoting survival of the remaining cells through autophagy. The extrinsic pathway is mediated by DRs including tumor necrosis factor family members such as Fas/CD95 and dependence receptors like deleted in colorectal carcinoma or unc-5 homolog A. The intrinsic mitochondrial pathway is controlled by Bcl-2 family proteins such as Bax and Bak. When lethal signals prevail, mitochondrial outer membrane permeabilization occurs and leads to release of the mitochondrial proteins such as Cyt C, SMAC, HTRA2, endonuclease G (ENDOG), apoptosis-inducing factor (AIF), and HTRA2. Among these, Cyt C, SMAC, and HTRA2 contribute to caspase-dependent apoptosis, whereas ENDOG, AIF, and HTRA2 contribute to caspase-independent apoptosis

Figure 2

Pathways of necrosis. Necrosis can be elicited by a wide range of stimuli. Activation of DRs such as Fas and TNFR or cellular stresses induces the interaction and activation of the kinases RIP1 and RIP3. Necrotic death stimuli can also activate PARP-1, which can potentially induce necrosis either through activation of the RIP kinases or another signaling pathway. Activation of necrosis by RIP1/RIP3 and PARP-1 is thought to occur through cathepsin release, increased ROS production, ATP depletion, and calpain activation. Necrostatin 1 is a small molecule inhibitor of RIP1

Figure 3

The process and types of autophagy. (a) Autophagy is an intracellular bulk degradation system through which cytoplasmic components are delivered to lysosomes to be degraded. The main process of autophagy includes formation and maturation of the phagophore, autophagosome, and autolysosome. LC3, a mammalian homolog of yeast Atg8, is localized in autophagosome membranes after processing to LC3-II and can be degraded by the autolysosome. Autophagy provides nucleic, amino, and fatty acids for the synthesis of DNA/RNA, protein, and ATP. (b) Types of selective autophagy. Autophagy can also target selective cargo for degradation such as organelles, proteins, microbes, and RNA

Figure 4

The process of immunogenic cell death (ICD). Cancer cells responding to ICD inducers expose calreticulin and HSP70/HSP90 on the outer leaflet of their plasma membranes at a preapoptotic stage and secrete ATP during apoptosis and autophagy. In addition, cells undergoing ICD release HMGB1 during necrosis. Consequently, these DAMPs interact with receptors on immature DCs such as CD91, P2X7, TLR4, and with unknown surface molecules. These DAMPs cause maturation of DCs, characterized by cell-surface upregulation of MHC-II, CD86, CD83, and CD80 and a distinctly pro-inflammatory cytokine pattern as indicated. Altogether, these processes result in IL-1β-dependent activation of IL-17-producing γδT cells and increased proliferation of IFN-γ-producing CD4⁺ or CD8⁺ αβ T cells, which eventually can lead to the eradication of therapy-resistant tumor cells

Figure 5

Calreticulin structure and exposure. (a) Linear representation of calreticulin domains is shown. The protein contains an N-terminal amino-acid signal sequence, N-domain, P-domain, C-domain, and a C-terminal KDEL ER retrieval signal (not shown). Repeats A (amino-acid sequence PXXIXDPDAXKPEDWDE) and B (amino-acid sequence GXWXPPXIXNPXYX) are located in the P-domain. The N- and P-domains of calreticulin are responsible for the chaperone function of the protein. The C-terminal C-domain contains a large number of negatively charged amino acids and is involved in high-capacity Ca²⁺ storage. In addition, the P-domain is responsible for binding to ERp57. Calreticulin contains three redox-sensitive cysteine residues (C106, C137, and C163), which are important for calreticulin function. (b) A summary of the mechanism of calreticulin exposure

Figure 6

HMGB1 structure and release. (a) Linear representation of HMGB1 domains is shown. The protein contains two homologous DNA-binding domains (box A and box B) and a negatively charged C-terminal domain. Residues 150–183 are responsible for binding to RAGE, whereas residues 89–108 and residues 7–74 are responsible for binding to TLR4 and p53 transactivation domains, respectively. Two nuclear localization signals (NLS1 and NLS2) and two nuclear emigration signals (NES1 and NES2; not shown) control the nuclear transport of HMGB1. In addition, HMGB1 contains three redox-sensitive cysteine residues (C23, C45, and C106), which are important for HMGB1 activity. (b) A summary of the mechanisms of HMGB1 release

Figure 7

The interplay between HMGB1 and autophagy. HMGB1 has important nuclear, cytosolic, and extracellular roles in the regulation of autophagy. Autophagy may also have a central role in the regulation of cellular traffic, secretion, and degradation of HMGB1. Although the connection between HMGB1 and autophagy is well recognized, the fully explicated molecular basis for it remains elusive