The Janus Face of Death Receptor Signaling during Tumor Immunoediting

Abstract

Cancer immune surveillance is essential for the inhibition of carcinogenesis. Malignantly transformed cells can be recognized by both the innate and adaptive immune systems through different mechanisms. Immune effector cells induce extrinsic cell death in the identified tumor cells by expressing death ligand cytokines of the tumor necrosis factor ligand family. However, some tumor cells can escape immune elimination and progress. Acquisition of resistance to the death ligand-induced apoptotic pathway can be obtained through cleavage of effector cell expressed death ligands into a poorly active form, mutations or silencing of the death receptors, or overexpression of decoy receptors and pro-survival proteins. Although the immune system is highly effective in the elimination of malignantly transformed cells, abnormal/dysfunctional death ligand signaling curbs its cytotoxicity. Moreover, DRs can also transmit pro-survival and pro-migratory signals. Consequently, dysfunctional death receptor-mediated apoptosis/necroptosis signaling does not only give a passive resistance against cell death but actively drives tumor cell motility, invasion, and contributes to consequent metastasis. This dual contribution of the death receptor signaling in both the early, elimination phase, and then in the late, escape phase of the tumor immunoediting process is discussed in this review. Death receptor agonists still hold potential for cancer therapy since they can execute the tumor-eliminating immune effector function even in the absence of activation of the immune system against the tumor. The opportunities and challenges of developing death receptor agonists into effective cancer therapeutics are also discussed.

Keywords: FAS (CD95); TNF-related apoptosis-inducing ligand (TRAIL); apoptosis; cancer; immune surveillance; necroptosis; pro-survival signaling.

Figure 1

Immune effector cells induce tumor cell death through apoptosis and necrotic-like cell lysis. Death ligands (FasL, TRAIL) presented by immune effector cell interact with their corresponding death receptors (DRs) on the surface of the tumor cell and activate the extrinsic apoptotic pathway. Ligand binding induces DR activation leading to the recruitment of the adaptor protein FADD and pro-caspase-8. Pro-caspase-8 is converted to its active form (active-C8), and it cleaves the effector caspase-3, -6, and -7 to their active forms, thus engaging the executioner caspase cascade. Active-C8 can also trigger the intrinsic apoptotic pathway through the conversion of the BH3-only protein Bid to its active form, tBid. tBid, in turn, induces the formation of Bax/Bak megachannels in the outer mitochondrial membrane-releasing cytochrome c (Cyt c) into the cytosol. In conjunction with Apaf-1, pro-caspase-9 and Cyt c assembles into the apoptosome, where pro-caspase-9 becomes activated (active-C9) and released. Active-C9 assists active-C8 in the induction of the executioner caspase cascade. Activation of the DRs may also induce necrosis-like cell death through DR-mediated assembly of the necrosome complex consisting of RIPK1, RIPK3, and MLKL. In the necrosome, MLKL gets phosphorylated by RIPK1/RIPK3 leading to its oligomerization and translocation into the plasma membrane where it triggers Ca²⁺ and Na⁺ influx driving cell lysis. Recognition of the tumor cell may also trigger the secretion of perforin and granzymes from lytic granules toward the target cell. Secreted perforin forms pores in the target cell causing direct cell lysis and enabling the entry of the serine proteases granzyme A and B (G_A and G_B) into the target cell. G_B can induce apoptosis by activating caspases through cleavage. G_B can also cleave Bid to tBid, thus engaging the mitochondrial apoptotic pathway. G_A can induce cell death in a caspase-independent manner by inducing DNA fragmentation and blocking DNA repair.

Figure 2

Key signaling modes of death receptors. (A) Apoptosis signaling mode. When pro-caspase-8 is abundantly expressed, binding of the death ligand (FasL or TRAIL, not shown) to its corresponding receptor induces the death domain (DD) of the receptor to complex with the DD of the adaptor protein FADD, allowing the recruitment, dimerization/oligomerization, and consequent activation of pro-caspase-8 (casp-8). Active caspase-8 inhibits RIPK1-mediated necroptosis and NF-κB activation by cleaving its kinase domain (KD) and intermediate domain (ID), driving apoptosis signaling. (B) NF-κB signaling mode-1. When cFLIP_L is highly expressed, together with pro-caspase-8, cFLIP_L is also recruited to the ligand-activated death receptor. Caspase-8 cleaves the small caspase homology domain of cFLIP_L (p12), generating cFLIP_p43. cFLIP_p43 and full-length caspase-8 form heterodimers that exhibit receptor-restricted and limited casapse-8 activity, not able to trigger the caspase cascade and apoptosis. The cFLIP_p43-caspase-8 complex cleaves the kinase domain of RIPK1, but not the intermediate domain, thus blocking RIPK1-mediated necroptosis and driving NF-κB activity. (C) NF-κB signaling mode-2. In the absence of caspase-8, the adaptor protein TRAF2 recruits the ubiquitin ligases cIAP1/2. cIAP1/2 polyubiquitinates the proteins in the receptor complex to which the linear ubiquitin chain assembly complex (LUBAC) will bind. LUBAC and cIAP1/2 polyubiquitinate RIPK1, thus creating the platform for the assembly of the NF-κB activating protein complex. (D) Necroptosis signaling mode-1. In the absence of caspase-8 and cIAP1/2, RIPK1 recruited to the death receptor escapes caspase-8-mediated cleavage. In the absence of cIAP1/2, RIPK1 does not get ubiquitinated, which enables it to associate with RIPK3. After sequential trans- and autophosphorylation of RIPK1 and RIPK3, MLKL can be recruited to RIPK1 and RIPK3 to form the necrosome, thus triggering necroptosis. (E) Necroptosis signaling mode-2. In the absence of caspase-8, even in the presence of cIAP1/2, necroptosis can be induced if the cIAP1/2-conjugated polyubiquitin chains are removed from RIPK1 by the deubiquitinating enzymes A20 and CYLD. Once RIPK1 is deubiquitinated, it will induce the formation of the necrosome, as in (D).

Figure 3

Prometastatic signaling mediated by death receptors. Activation of DR5 in TRAIL-resistant tumor cells can trigger activation of the tyrosine kinase Src in a RIPK1-dependent manner and phosphatidyl inositol 3 kinase (PI3K) in a RIPK1-independent manner. Activation of Scr induces signal transducer and activator of transcription 3 (STAT3) and focal adhesion kinase (FAK)-mediated motility, while PI3K promotes motility and invasive potential by activating Akt. In KRAS-driven tumor cells, KRAS-mediated inhibition of ROCK allows Rac-1 activation upon TRAIL receptor activation. Activation of Rac1 is mediated by the membrane proximal domain (MPD) of the TRAIL receptor independent of the death domain (DD). RAC-1 drives cell motility associated with mesenchymal morphology and invasive potential. Similar to TRAILR, in FasL-resistant cells, ligand binding by FAS can activate a Src family kinase, c-Yes. c-Yes activation is promoted by phospholipase C gamma (PLCγ) activated through the membrane proximal domain of FAS. PLCγ cleaves membrane phosphatidyl inositol bisphosphates (PIP2) into inositol trisphosphate (IP3). IP3 translocates to the endolasmic reticulum (ER) and opens the ER store calcium channel, IP3 receptor. The released calcium activates the plasma membrane Ca²⁺ channel Orai-1 causing a Ca²⁺ influx that induces a local Ca²⁺ signal in the vicinity of FAS and activates protein kinase C beta 2 (PKCβ2). PKCβ2 attenuates apoptosis signaling by phosphorylating FAS and triggers c-Yes activation that drives motility. FAS can also activate the small GTPase RhoA, but in contrast to DR5/TRAILR signaling events, it leads to the activation of ROCK and consequent amoeboid motility.

Figure 4

Tumor immune elimination converges on death ligand and perforin/granzyme-mediated cytotoxicity. All immune effector-mediated tumor elimination, regardless of the signal or the mechanism, converges and depends on the perforin and granzyme-induced cell lysis or death ligand-induced apoptosis or necroptosis. Tumor cells resistant to these cell death mechanisms escape tumor elimination. At the same time, effector cells can expose tumor cells to the full complement of cytotoxic molecules, thus maximizing the range of targetable tumor cells.