Inhibition of BCR-ABL

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.

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BackgroundAntigen (Ag)/IgE-mediated mast cell (MC) responses play detrimental roles in allergic diseases. MC activation via the high-affinity receptor for IgE (FcεRI) is controlled by the Src family kinase Lyn. Lyn-deficient (-/-) bone marrow-derived MCs (BMMCs) have been shown by various laboratories to exert stronger activation of the PI3K pathway, degranulation, and production of pro-inflammatory cytokines compared to wild-type (wt) cells. This mimics the phenotype of BMMCs deficient for the SH2-containing inositol-5’-phosphatase 1 (SHIP1). In this line, Lyn has been demonstrated to tyrosine-phosphorylate and activate SHIP1, thereby constituting a negative feedback control of PI3K-mediated signals. However, several groups have also reported on Lyn-/- BMMCs degranulating weaker than wt BMMCs.ResultsLyn-/- BMMCs, which show a suppressed degranulation response, were found to exhibit abrogated tyrosine phosphorylation of SHIP1 as well. This indicated that even in the presence of reduced SHIP1 function MC degranulation is dependent on Lyn function. In contrast to the reduced immediate secretory response, pro-inflammatory cytokine production was augmented in Lyn-/- BMMCs. For closer analysis, Lyn/SHIP1-double-deficient (dko) BMMCs were generated. In support of the dominance of Lyn deficiency, dko BMMCs degranulated significantly weaker than SHIP1-/- BMMCs. This coincided with reduced LAT1 and PLC-γ1 phosphorylation as well as Ca2+ mobilization in those cells. Interestingly, activation of the NFκB pathway followed the same pattern as measured by IκBα phosphorylation/degradation as well as induction of NFκB target genes. This suggested that Ag-triggered NFκB activation involves a Ca2+-dependent step. Indeed, IκBα phosphorylation/degradation and NFκB target gene induction were controlled by the Ca2+-dependent phosphatase calcineurin.ConclusionsLyn deficiency is dominant over SHIP1 deficiency in MCs with respect to Ag-triggered degranulation and preceding signaling events. Moreover, the NFκB pathway and respective targets are activated in a Lyn- and Ca2+-dependent manner, reinforcing the importance of Lyn for MC activation.Electronic supplementary materialThe online version of this article (doi:10.1186/s12964-016-0135-0) contains supplementary material, which is available to authorized users.

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 intensity and duration of endoplasmic reticulum (ER) stress converts the unfolded protein response (UPR) from an adaptive into a terminal response. The first regulates homeostasis, the latter triggers apoptosis. Cells that rapidly proliferate and possess developed secretory capabilities, such as leukemia cells, depend on an efficiently operating UPR to maintain proteostasis. Activation of terminal UPR by either blockade of adaptive UPR or exaggeration of ER stress has been explored as a novel approach in cancer therapy. For mast cell leukemia (MCL) the efficacy of both approaches, by utilizing the KIT V560G,D816V-positive MCL cell line HMC-1.2, was investigated. We show that HMC-1.2 cells display a tonic activation of the IRE1α arm of the UPR, which constitutively generates spliced XBP1. Inhibition of IRE1α by different types of inhibitors (MKC-8866, STF-083010, and KIRA6) suppressed proliferation at concentrations needed for blockade of IRE1α-mediated XBP1 splicing. At higher concentrations, these inhibitors triggered an apoptotic response. Blocking the proteasome by bortezomib, which confers an exaggerated UPR, resulted in a marked cytotoxic response. Bortezomib treatment also caused activation of the kinase JNK, which played a pro-proliferative and anti-apoptotic role. Hence, the combination of bortezomib with a JNK inhibitor synergized to induce cell death. In summary, the UPR can be addressed as an effective therapeutic target against KIT D816V -positive MCL.

Ischemic stroke is known to cause the accumulation of misfolded proteins and loss of calcium homeostasis leading to impairment of endoplasmic reticulum (ER) function. The unfolded protein response (UPR) is an ER-located and cytoprotective pathway that aims to resolve ER stress. Transmembrane BAX inhibitor-1 motif-containing (TMBIM) protein family member TMBIM3/GRINA is highly expressed in the brain and mostly located at the ER membrane suppressing ER calcium release by inositol-1,4,5-trisphosphate receptors. GRINA confers neuroprotection and is regulated by erythropoietin (EPO) after murine cerebral ischemia. However, the role of GRINA and the impact of EPO treatment on the post-ischemic UPR have not been elucidated yet. We subjected GRINA-deficient (Grina−/−) and wildtype mice to transient (30 min) middle cerebral artery occlusion (tMCAo) followed by 6 h or 72 h of reperfusion. We administered EPO or saline 0, 24 and 48 h after tMCAo/sham surgery. Oxygen–glucose deprivation (OGD) and pharmacological stimulation of the UPR using Tunicamycin and Thapsigargin were carried out in primary murine cortical mixed cell cultures. Treatment with the PERK-inhibitor GSK-2606414, IRE1a-RNase-inhibitor STF-083010 and EPO was performed 1 h prior to either 1 h, 2 h or 3 h of OGD. We found earlier and larger infarct demarcations in Grina−/− mice compared to wildtype mice, which was accompanied by a worse neurological outcome and an abolishment of EPO-mediated neuroprotection after ischemic stroke. In addition, GRINA-deficiency increased apoptosis and the activation of the corresponding PERK arm of the UPR after stroke. EPO enhanced the post-ischemic activation of pro-survival IRE1a and counteracted the pro-apoptotic PERK branch of the UPR. Both EPO and the PERK-inhibitor GSK-2606414 reduced cell death and regulated Grina mRNA levels after OGD. In conclusion, GRINA plays a crucial role in post-ischemic UPR and the use of both GSK-2606414 and EPO might lead to neuroprotection.

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