Background PT-112 is a novel immunogenic small molecule1 under Phase II clinical development for cancer therapy.2–8 Besides mediating cytostatic and cytotoxic effects in numerous human and mouse cancer cells, PT-112 elicits various danger signals that are linked to immunogenic cell death (ICD) such as calreticulin exposure, as well as ATP and HMGB1 secretion.1 9–11 Accordingly, mouse cancer cells succumbing to PT-112 in vitro efficiently protect immunocompetent, tumor-naïve mice from challenge with living cancer cells of the same type.1 9 Moreover, PT-112 synergizes with PD-1 or PD-L1 blockade to control mouse tumors developing in immunologically competent hosts.1 9 This work focuses on elucidating the underlying mechanisms of PT-112-induced ICD.
Methods We harnessed a panel of human and mouse cell lines optionally engineered to lack specific genes involved in mitochondrial apoptosis (namely, Bcl2, Bax and Bak1) coupled with flow cytometry, immunoblotting, RT-PCR, immunofluorescence microscopy and clonogenic assays to determine the impact of reticular and mitochondrial events on the established ability of PT-112 to kill malignant cells in an immunogenic manner.1
Results In line with previous findings,10–13 PT-112 elicited eukaryotic translation initiation factor 2 subunit alpha (EIF2S1, best known as eIF2α) phosphorylation and mitochondrial dysfunction in malignant cells, a process that was accompanied by the release of interferogenic mitochondrial DNA (mtDNA)14 in the cytoplasm of PT-112 treated cells, and was differentially affected by the deletion of Bcl2, Bax, Bak1, or Bax plus Bak1. Similarly, the lack of Bcl2, Bax, Bak1, or Bax plus Bak1 had a differential impact on the ability of PT-112 to elicit early signs of mitochondrial apoptosis including reactive oxygen species (ROS) generation, mitochondrial transmembrane potential loss and ultimately plasma membrane permeabilization.
Conclusions ER stress and mitochondrial dysfunction appear to underlie the ability of PT-112 to drive ICD, the integrated stress response, and viral mimicry. This is in line with the well-established connections between ER stress and cytoplasmic nucleic acid sensing, which are pristine mechanisms of antiviral defense in mammalian cells, with the capacity of dying cells to emit immunostimulatory signals.15 Whether PT-112-driven stress also shifts the antigenic properties of cancer cells as a consequence of the accumulation of non-mutational neoantigens16 remains to be determined. Despite these and other open questions, PT-112 stands out as a powerful immunotherapeutic agent with promising clinical activity in patients with a variety of tumors1 under Phase II clinical development for cancer therapy.2–8
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