Targeting integrated stress response with ISRIB combined with imatinib treatment attenuates RAS/RAF/MAPK and STAT5 signaling and eradicates chronic myeloid leukemia cells.

The integrated stress response (ISR) facilitates cellular adaptation to unfavorable conditions by reprogramming the cellular response. ISR activation was reported in neurological disorders and solid tumors; however, the function of ISR and its role as a possible therapeutic target in hematological malignancies still remain largely unexplored. Previously, we showed that the ISR is activated in chronic myeloid leukemia (CML) cells and correlates with blastic transformation and tyrosine kinase inhibitor (TKI) resistance. Moreover, the ISR was additionally activated in response to imatinib as a type of protective internal signaling. Here, we show that ISR inhibition combined with imatinib treatment sensitized and more effectively eradicated leukemic cells both in vitro and in vivo compared to treatment with single agents. The combined treatment specifically inhibited the STAT5 and RAS/RAF/MEK/ERK pathways, which are recognized as drivers of resistance. Mechanistically, this drug combination attenuated both interacting signaling networks, leading to BCR-ABL1- and ISR-dependent STAT5 activation. Consequently, leukemia engraftment in patient-derived xenograft mice bearing CD34+ TKI-resistant CML blasts carrying PTPN11 mutation responsible for hyperactivation of the RAS/RAF/MAPK and JAK/STAT5 pathways was decreased upon double treatment. This correlated with the downregulation of genes related to the RAS/RAF/MAPK, JAK/STAT5 and stress response pathways and was associated with lower expression of STAT5-target genes regulating proliferation, viability and the stress response. Collectively, these findings highlight the effect of imatinib plus ISRIB in the eradication of leukemic cells resistant to TKIs and suggest potential clinical benefits for leukemia patients with TKI resistance related to RAS/RAF/MAPK or STAT5 signaling. We propose that personalized treatment based on the genetic selection of patients carrying mutations that cause overactivation of the targeted pathways and therefore make their sensitivity to such treatment probable should be considered as a possible future direction in leukemia treatment.

Cytoplasmic polyadenylation by TENT5A is required for proper bone formation.

Osteoblasts orchestrate bone formation through the secretion of type I collagen and other constituents of the matrix on which hydroxyapatite crystals mineralize. Here, we show that TENT5A, whose mutations were found in congenital bone disease osteogenesis imperfecta patients, is a cytoplasmic poly(A) polymerase playing a crucial role in regulating bone mineralization. Direct RNA sequencing revealed that TENT5A is induced during osteoblast differentiation and polyadenylates mRNAs encoding Col1α1, Col1α2, and other secreted proteins involved in osteogenesis, increasing their expression. We postulate that TENT5A, possibly together with its paralog TENT5C, is responsible for the wave of cytoplasmic polyadenylation of mRNAs encoding secreted proteins occurring during bone mineralization. Importantly, the Tent5a knockout (KO) mouse line displays bone fragility and skeletal hypomineralization phenotype resulting from quantitative and qualitative collagen defects. Thus, we report a biologically relevant posttranscriptional regulator of collagen production and, more generally, bone formation.

Immunoglobulin expression and the humoral immune response is regulated by the non-canonical poly(A) polymerase TENT5C.

TENT5C is a non-canonical cytoplasmic poly(A) polymerase highly expressed by activated B cells to suppress their proliferation. Here we measure the global distribution of poly(A) tail lengths in responsive B cells using a Nanopore direct RNA-sequencing approach, showing that TENT5C polyadenylates immunoglobulin mRNAs regulating their half-life and consequently steady-state levels. TENT5C is upregulated in differentiating plasma cells by innate signaling. Compared with wild-type, Tent5c-/- mice produce fewer antibodies and have diminished T-cell-independent immune response despite having more CD138high plasma cells as a consequence of accelerated differentiation. B cells from Tent5c-/- mice also have impaired capacity of the secretory pathway, with reduced ER volume and unfolded protein response. Importantly, these functions of TENT5C are dependent on its enzymatic activity as catalytic mutation knock-in mice display the same defect as Tent5c-/-. These findings define the role of the TENT5C enzyme in the humoral immune response.

Selective targeting of HDAC1/2 elicits anticancer effects through Gli1 acetylation in preclinical models of SHH Medulloblastoma.

SHH Medulloblastoma (SHH-MB) is a pediatric brain tumor characterized by an inappropriate activation of the developmental Hedgehog (Hh) signaling. SHH-MB patients treated with the FDA-approved vismodegib, an Hh inhibitor that targets the transmembrane activator Smoothened (Smo), have shown the rapid development of drug resistance and tumor relapse due to novel Smo mutations. Moreover, a subset of patients did not respond to vismodegib because mutations were localized downstream of Smo. Thus, targeting downstream Hh components is now considered a preferable approach. We show here that selective inhibition of the downstream Hh effectors HDAC1 and HDAC2 robustly counteracts SHH-MB growth in mouse models. These two deacetylases are upregulated in tumor and their knockdown inhibits Hh signaling and decreases tumor growth. We demonstrate that mocetinostat (MGCD0103), a selective HDAC1/HDAC2 inhibitor, is a potent Hh inhibitor and that its effect is linked to Gli1 acetylation at K518. Of note, we demonstrate that administration of mocetinostat to mouse models of SHH-MB drastically reduces tumor growth, by reducing proliferation and increasing apoptosis of tumor cells and prolongs mouse survival rate. Collectively, these data demonstrate the preclinical efficacy of targeting the downstream HDAC1/2-Gli1 acetylation in the treatment of SHH-MB.

Ipilimumab-dependent cell-mediated cytotoxicity of regulatory T cells ex vivo by nonclassical monocytes in melanoma patients.

Enhancing immune responses with immune-modulatory monoclonal antibodies directed to inhibitory immune receptors is a promising modality in cancer therapy. Clinical efficacy has been demonstrated with antibodies blocking inhibitory immune checkpoints such as cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) or PD-1/PD-L1. Treatment with ipilimumab, a fully human CTLA-4-specific mAb, showed durable clinical efficacy in metastatic melanoma; its mechanism of action is, however, only partially understood. This is a study of 29 patients with advanced cutaneous melanoma treated with ipilimumab. We analyzed peripheral blood mononuclear cells (PBMCs) and matched melanoma metastases from 15 patients responding and 14 not responding to ipilimumab by multicolor flow cytometry, antibody-dependent cell-mediated cytotoxicity (ADCC) assay, and immunohistochemistry. PBMCs and matched tumor biopsies were collected 24 h before (i.e., baseline) and up to 4 wk after ipilimumab. Our findings show, to our knowledge for the first time, that ipilimumab can engage ex vivo FcγRIIIA (CD16)-expressing, nonclassical monocytes resulting in ADCC-mediated lysis of regulatory T cells (Tregs). In contrast, classical CD14(++)CD16(-) monocytes are unable to do so. Moreover, we show that patients responding to ipilimumab display significantly higher baseline peripheral frequencies of nonclassical monocytes compared with nonresponder patients. In the tumor microenvironment, responders have higher CD68(+)/CD163(+) macrophage ratios at baseline and show decreased Treg infiltration after treatment. Together, our results suggest that anti-CTLA-4 therapy may target Tregs in vivo. Larger translational studies are, however, warranted to substantiate this mechanism of action of ipilimumab in patients.

Downregulation of BRCA1 protein in BCR-ABL1 leukemia cells depends on stress-triggered TIAR-mediated suppression of translation.

BRCA1 tumor suppressor regulates crucial cellular processes involved in DNA damage repair and cell cycle control. We showed that expression of BCR-ABL1 correlates with decreased level of BRCA1 protein, which promoted aberrant mitoses and aneuploidy as well as altered DNA damage response. Using polysome profiling and luciferase-BRCA1 3'UTR reporter system here we demonstrate that downregulation of BRCA1 protein in CML is caused by inhibition of BRCA1 mRNA translation, but not by increased protein degradation or reduction of mRNA level and half-life. We investigated 2 mRNA-binding proteins - HuR and TIAR showing specificity to AU-Rich Element (ARE) sites in 3'UTR of mRNA. BCR-ABL1 promoted cytosolic localization of TIAR and HuR, their binding to BRCA1 mRNA and formation of the TIAR-HuR complex. HuR protein positively regulated BRCA1 mRNA stability and translation, conversely TIAR negatively regulated BRCA1 translation and was found localized predominantly in the cytosolic stress granules in CML cells. TIAR-dependent downregulation of BRCA1 protein level was a result of ER stress, which is activated in BCR-ABL1 expressing cells, as we previously shown. Silencing of TIAR in CML cells strongly elevated BRCA1 level. Altogether, we determined that TIAR-mediated repression of BRCA1 mRNA translation is responsible for downregulation of BRCA1 protein level in BCR-ABL1 -positive leukemia cells. This mechanism may contribute to genomic instability and provide justification for targeting PARP1 and/or RAD52 to induce synthetic lethality in "BRCAness" CML and BCR-ABL1 -positive ALL cells.

The role of nibrin in doxorubicin-induced apoptosis and cell senescence in Nijmegen Breakage Syndrome patients lymphocytes.

Nibrin plays an important role in the DNA damage response (DDR) and DNA repair. DDR is a crucial signaling pathway in apoptosis and senescence. To verify whether truncated nibrin (p70), causing Nijmegen Breakage Syndrome (NBS), is involved in DDR and cell fate upon DNA damage, we used two (S4 and S3R) spontaneously immortalized T cell lines from NBS patients, with the founding mutation and a control cell line (L5). S4 and S3R cells have the same level of p70 nibrin, however p70 from S4 cells was able to form more complexes with ATM and BRCA1. Doxorubicin-induced DDR followed by cell senescence could only be observed in L5 and S4 cells, but not in the S3R ones. Furthermore the S3R cells only underwent cell death, but not senescence after doxorubicin treatment. In contrary to doxorubicin treatment, cells from all three cell lines were able to activate the DDR pathway after being exposed to γ-radiation. Downregulation of nibrin in normal human vascular smooth muscle cells (VSMCs) did not prevent the activation of DDR and induction of senescence. Our results indicate that a substantially reduced level of nibrin or its truncated p70 form is sufficient to induce DNA-damage dependent senescence in VSMCs and S4 cells, respectively. In doxorubicin-treated S3R cells DDR activation was severely impaired, thus preventing the induction of senescence.

Inhibition of PCAF by anacardic acid derivative leads to apoptosis and breaks resistance to DNA damage in BCR-ABL-expressing cells.

Acetylation of histones and nonhistone proteins is a posttranslational modification which plays a major role in the regulation of intracellular processes involved in tumorigenesis. It was shown that different acetylation of proteins correlates with development of leukemia. It is proposed that histone acetyltransferases (HATs) are important novel drug targets for leukemia treatment, however data are still not consistent. Our previous data showed that a derivative of anacardic acid - small molecule MG153, which has been designed and synthesized to optimize the HAT inhibitory potency of anacardic acid, is a potent inhibitor of p300/CBP associated factor (PCAF) acetyltransferase. Here we ask whether inhibition of PCAF acetyltransferase with MG153 will show proapoptotic effects in cells expressing BCR-ABL, which show increased PCAF expression and are resistant to apoptosis. We found that inhibition of PCAF decreases proliferation and induces apoptosis, which correlates with loss of the mitochondrial membrane potential and DNA fragmentation. Importantly, cells expressing BCR-ABL are more sensitive to PCAF inhibition compared to parental cells without BCRABL. Moreover, inhibition of PCAF in BCR-ABL-expressing cells breaks their resistance to DNA damage-induced cell death. These findings provide direct evidence that targeting the PCAF alone or in combination with DNA-damaging drugs shows cytotoxic effects and should be considered as a prospective therapeutic strategy in chronic myeloid leukemia cells. Moreover, we propose that anacardic acid derivative MG153 is a valuable agent and further studies validating its therapeutic relevance should be performed.

The PERK-eIF2α phosphorylation arm is a pro-survival pathway of BCR-ABL signaling and confers resistance to imatinib treatment in chronic myeloid leukemia cells.

Activation of adaptive mechanisms plays a crucial role in cancer progression and drug resistance by allowing cell survival under stressful conditions. Therefore, inhibition of the adaptive response is considered as a prospective therapeutic strategy. The PERK-eIF2α phosphorylation pathway is an important arm of the unfolded protein response (UPR), which is induced under conditions of endoplasmic reticulum (ER) stress. Our previous work showed that ER stress is induced in chronic myeloid leukemia (CML) cells. Herein, we demonstrate that the PERK-eIF2α phosphorylation pathway is upregulated in CML cell lines and CD34+ cells from CML patients and is associated with CML progression and imatinib resistance. We also show that induction of apoptosis by imatinib results in the downregulation of the PERK-eIF2α phosphorylation arm. Furthermore, we demonstrate that inactivation of the PERK-eIF2α phosphorylation arm decreases the clonogenic and proliferative capacities of CML cells and sensitizes them to death by imatinib. These findings provide evidence for a pro-survival role of PERK-eIF2α phosphorylation arm that contributes to CML progression and development of imatinib resistance. Thus, the PERK-eIF2α phosphorylation arm may represent a suitable target for therapeutic intervention for CML disease.

Increased acetylation of lysine 317/320 of p53 caused by BCR-ABL protects from cytoplasmic translocation of p53 and mitochondria-dependent apoptosis in response to DNA damage.

Chronic myeloid leukemia (CML) is a disorder of hematopoietic stem cells caused by the expression of BCR-ABL. Loss of p53 has not been implicated as important for the development of CML. Mutations in p53 protein are infrequent, however they correlate with the disease progression. The absence of p53 mutations does not exclude the possibility that other dysfunctions play an important role in CML pathology. Acetylation represents a very potent posttranslational mechanism regulating p53 stability, transcriptional activity and localization. In this study we have investigated whether the expression of BCR-ABL could influence the acetylation of p53, specifically at lysine 317/320 (K317/K320), which has been shown to regulate nuclear export and transcription-independent apoptotic activity of p53. We found that BCR-ABL expression increases K317 acetylation of p53 and is able to prevent a drop in acetylation observed upon DNA damage, followed by translocation of p53 to the cytoplasm and by Bax activation. We have shown that this site plays a crucial role in the regulation of p53 localization and p53-dependent, Bax-mediated apoptosis. Our study presents a novel BCR-ABL-dependent mechanism protecting from DNA-damage-induced cell death. It can, in addition to already known mechanisms, explain the resistance to p53-dependent apoptosis observed in CML cells expressing wt p53. We propose that the acetyltransferases regulating the p53 acetylation could be an interesting and potent target for therapeutic intervention.

Expression of oncogenic kinase Bcr-Abl impairs mitotic checkpoint and promotes aberrant divisions and resistance to microtubule-targeting agents.

Recent findings showed that BRCA1, in addition to its role in DNA damage response, acts as an upstream regulator of genes involved in the mitotic checkpoint regulation, thus protecting against promotion of aberrant divisions and aneuploidy. Moreover, there is also an indication that the BRCA1 protein is downregulated in chronic myeloid leukemia (CML) patients. We have investigated a possible functional relationship between BRCA1 and mitotic checkpoint competence in cells with the same genetic background expressing different levels of Bcr-Abl, an oncogene responsible for CML. Herein, we show that Bcr-Abl strongly downregulates the BRCA1 protein level, which is partially reversed on treatment with imatinib, an inhibitor of Bcr-Abl tyrosine kinase. Bcr-Abl leads to decreased expression of genes involved in the mitotic checkpoint activation--Mad2, Bub1, Bub3, and BubR1, resulting in mitosis perturbances, weakened mitotic checkpoint function, and mitotic slippage after nocodazole treatment. Furthermore, high Bcr-Abl-expressing cells showed also postmitotic checkpoint dysfunctions and inability to effectively arrest in the 4NG1 phase of the cell cycle, which was associated with limited p21 induction. These observations had significant biological consequences, as we found a high level of improper divisions, chromosomal missegregation, and generation of polyploid cells on mitotic checkpoint prolonged activation. Additionally, Bcr-Abl-expressing cells showed resistance to death activated by spindle defects, reversed by imatinib. Our study presents new facts and supports the hypothesis concerning the mutator nature of Bcr-Abl itself. The functional interaction between Bcr-Abl and mitosis dysfunctions, due to compromised mitotic checkpoints, may have important implications for the generation of aneuploidy and CML progression.