IRE1, PERK, and ATF6 are the three transducers of the mammalian canonical unfolded protein response (UPR). GSK2606414 is a potent inhibitor of PERK, while KIRA6 inhibits the kinase activity of IRE1. Both molecules are frequently used to probe the biological roles of the UPR in mammalian cells. In a direct binding assay, GSK2606414 bound to the cytoplasmic domain of KIT with dissociation constants ( K d ) value of 664 ± 294 nM whereas KIRA6 showed a K d value of 10.8 ± 2.9 µM. In silico docking studies confirmed a compact interaction of GSK2606414 and KIRA6 with KIT ATP binding pocket. In cultured cells, GSK2606414 inhibited KIT tyrosine kinase activity at nanomolar concentrations and in a PERK-independent manner. Moreover, in contrast to other KIT inhibitors, GSK2606414 enhanced KIT endocytosis and its lysosomal degradation. Although KIRA6 also inhibited KIT at nanomolar concentrations, it did not prompt KIT degradation, and rescued KIT from GSK2606414-mediated degradation. Consistent with KIT inhibition, nanomolar concentrations of GSK2606414 and KIRA6 were sufficient to induce cell death in a KIT signaling-dependent mast cell leukemia cell line. Our data show for the first time that KIT is a shared target for two seemingly unrelated UPR inhibitors at concentrations that overlap with PERK and IRE1 inhibition. Furthermore, these data underscore discrepancies between in vitro binding measurements of kinase inhibitors and inhibition of the tyrosine kinase receptors in living cells.

The integrated stress response (ISR) converges on eIF2α phosphorylation to regulate protein synthesis. ISR is activated by several stress conditions, including endoplasmic reticulum (ER) stress, executed by protein kinase R-like endoplasmic reticulum kinase (PERK). We report that ER stress combined with ISR inhibition causes an impaired maturation of several tyrosine kinase receptors (RTKs), consistent with a partial block of their trafficking from the ER to the Golgi. Other proteins mature or are secreted normally, indicating selective retention in the ER (sERr). sERr is relieved upon protein synthesis attenuation and is accompanied by the generation of large mixed disulfide bonded complexes, including ERp44. sERr was pharmacologically recapitulated by combining the HIV-protease inhibitor nelfinavir with ISRIB, an experimental drug that inhibits ISR. Nelfinavir/ISRIB combination is highly effective to inhibit the growth of RTK-addicted cell lines and hepatocellular (HCC) cells in vitro and in vivo. Thus, pharmacological sERr can be utilized as a modality for cancer treatment.

The unfolded protein response (UPR) is an adaptive signaling pathway activated in response to endoplasmic reticulum (ER) stress. The effectors of the UPR are potent transcription activators; however, some genes are suppressed by ER stress at the mRNA level. The mechanisms underlying UPR‐mediated gene suppression are less known. Exploration of the effect of UPR on NK cells ligand expression found that the transcription of NK group 2 member D (NKG2D) ligand major histocompatibility complex class I polypeptide‐related sequence A/B (MICA/B) is suppressed by the inositol‐requiring enzyme 1 (IRE1)/X‐box binding protein 1 (XBP1) pathway of the UPR. Deletion of IRE1 or XBP1 was sufficient to promote mRNA and surface levels of MICA. Accordingly, NKG2D played a greater role in the killing of IRE1/XBP1 knockout target cells. Analysis of effectors downstream to XBP1s identified E2F transcription factor 1 (E2F1) as linking UPR and MICA transcription. The inverse correlation between XBP1 and E2F1 or MICA expression was corroborated in RNA‐Seq analysis of 470 primary melanoma tumors. While mechanisms that connect XBP1 to E2F1 are not fully understood, we implicate a few microRNA molecules that are modulated by ER stress and possess dual suppression of E2F1 and MICA. Because of the importance of E2F1 and MICA in cancer progression and recognition, these observations could be exploited for cancer therapy by manipulating the UPR in tumor cells.—Obiedat, A., Seidel, E., Mahameed, M., Berhani, O., Tsukerman, P., Voutetakis, K., Chatziioannou, A., McMahon, M., Avril, T., Chevet, E., Mandelboim, O., Tirosh, B. Transcription of the NKG2D ligand MICA is suppressed by the IRE1/XBP1 pathway of the unfolded protein response through the regulation of E2F1. FASEB J. 33, 3481–3495 (2019). http://www.fasebj.org

The B7 family member, B7H6, is a ligand for the natural killer cell receptor NKp30. B7H6 is hardly expressed on normal tissues, but undergoes upregulation on different types of tumors, implicating it as an attractive target for cancer immunotherapy. The molecular mechanisms that control B7H6 expression are poorly understood. We report that in contrast to other NK cell ligands, endoplasmic reticulum (ER) stress upregulates B7H6 mRNA levels and surface expression. B7H6 induction by ER stress requires protein kinase R-like ER kinase (PERK), one of the three canonical sensors of the unfolded protein response. PERK phosphorylates eIF2α, which regulates protein synthesis and gene expression. Because eIF2α is phosphorylated by several kinases following different stress conditions, the program downstream to eIF2α phosphorylation is called the integrated stress response (ISR). Several drugs were reported to promote the ISR. Nelfinavir and lopinavir, two clinically approved HIV protease inhibitors, promote eIF2α phosphorylation by different mechanisms. We show that nelfinavir and lopinavir sustainably instigate B7H6 expression at their pharmacologically relevant concentrations. As such, ER stress and ISR conditions sensitize melanoma targets to CART cells directed against B7H6. Our study highlights a novel mechanism to induce B7H6 expression and suggests a pharmacological approach to improve B7H6-directed immunotherapy.

Synthetic receptors targeted to the secretory pathway often fail to exhibit the expected activity due to post-translational modifications (PTMs) and/or improper folding. Here, we engineered synthetic receptors that reside in the cytoplasm, inside the endoplasmic reticulum (ER), or on the plasma membrane through orientation adjustment of the receptor parts and by elimination of dysfunctional PTMs sites. The cytoplasmic receptors consist of split-TEVp domains that reconstitute an active protease through chemically-induced dimerization (CID) that is triggered by rapamycin, abscisic acid, or gibberellin. Inside the ER, however, some of these receptors were non-functional, but their activity was restored by mutagenesis of cysteine and asparagine, residues that are typically associated with PTMs. Finally, we engineered orthogonal chemically activated cell-surface receptors (OCARs) consisting of the Notch1 transmembrane domain fused to cytoplasmic tTA and extracellular CID domains. Mutagenesis of cysteine residues in CID domains afforded functional OCARs which enabled fine-tuning of orthogonal signalling in mammalian cells.