Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a member of the TNF superfamily that selectively induces apoptosis in tumor cells without harming normal cells, making it an attractive agent for cancer therapy. TRAIL induces apoptosis by binding to and activating its death receptors DR4 and DR5. Several TRAIL-based treatments have been developed, including recombinant forms of TRAIL and its death receptor agonist antibodies, but the efficacy of TRAIL-based therapies in clinical trials is modest. In addition to inducing cancer cell apoptosis, TRAIL is expressed in immune cells and plays a critical role in tumor surveillance. Emerging evidence indicates that the TRAIL pathway may interact with immune checkpoint proteins, including programmed death-ligand 1 (PD-L1), to modulate PD-L1-based tumor immunotherapies. Therefore, understanding the interaction between TRAIL and the immune checkpoint PD-L1 will lead to the development of new strategies to improve TRAIL- and PD-L1-based therapies. This review discusses recent findings on TRAIL-based therapy, resistance, and its involvement in tumor immunosurveillance.

Immune checkpoint inhibitors (ICIs), including PD-L1 inhibitors, have been approved by the FDA for the treatment of cancers; however, only a small number of cancer patients benefit from these ICIs. Furthermore, the development of drug resistance to this type of treatment is often inevitable. The mechanisms of resistance to PD-L1 inhibitors can be attributed, in part, to an incomplete understanding of the regulation of PD-L1 protein expression. In this study, we identified the role of the E3 ligase GP78, also known as the Autocrine Motility Factor Receptor (AMFR), in the regulation of PD-L1 protein levels. We show that GP78 physically interacts with PD-L1, which is confirmed by IP and Western blotting and is supported by molecular modelling using AlphaFold2. Our modeling studies predict that the interface amino acids of the Ig1 domain of PD-L1 interact with the RING domain and a β-hairpin preceding the CUE domain of GP78. The crystal structure of the PD-1/PD-L1 complex reveals that the interaction with PD-1 is mediated by the Ig1 domain of PD-L1. Furthermore, proteasomal degradation of PD-L1 has been observed via GP78-mediated K48-linked ubiquitination, indicating a key regulatory role for GP78 in the downregulation of PD-L1. Because GP78 expression is inversely correlated with PD-L1 levels in cancer, these findings may have clinical implications for predicting tumor immune evasion and patient response to PD-1/PD-L1 blockade therapies. Taken together, these findings identify a previously unknown mechanism by which GP78 targets PD-L1 for ubiquitination and subsequent degradation in cancer cells, and suggest that blocking the interaction between PD-L1 and PD-1 by an E3 ligase is a novel strategy to improve immunotherapies for cancer patients.

Triple-negative breast cancer (TNBC) is an aggressive subtype characterized by chronic inflammation with limited therapeutic options. Although thrombin activation of protease-activated receptor 1 (PAR1) promotes ubiquitin-dependent signaling and inflammatory responses in endothelial cells, its contribution to inflammatory effects in TNBC remain poorly understood. Here, we show that loss of the deubiquitinase cylindromatosis (CYLD) markedly increases basal phosphorylation of p38 mitogen-activated protein kinase and p65 nuclear factor kappa B (NF-κB) subunit to levels comparable to thrombin stimulation, without further enhancement by thrombin, indicating that CYLD primarily restrains constitutive inflammatory activity. We also show that thrombin-activation of PAR1 promotes p38 signaling through an autophosphorylation-dependent mechanism in TNBC, leading to p65 NF-κB subunit phosphorylation and nuclear accumulation independent of canonical inhibitor of κBα degradation. This non-canonical signaling drives expression of pro-inflammatory interleukin (IL) cytokines including IL-6, IL-8, IL-1α, and IL-1β. Functionally, IL-6 and IL-8 contribute to thrombin-induced TNBC proliferation and migration, respectively. Together, these findings define a novel CYLD-regulated PAR1-p38-p65 signaling axis that promotes inflammatory cytokine production and tumor-associated phenotypes such as cell proliferation and migration, identifying multiple druggable nodes for therapeutic targeting in aggressive breast cancers.