Involvement of Bcl-XL deamidation in E1A-mediated cisplatin sensitization of ovarian cancer cells

Bcl-xL deamidation is regulated by multiple ion transporters and is intramolecularly catalyzed

In susceptible tumor cells, DNA-damaging antineoplastic agents induce an increase in intracellular pH during the premitochondrial stage of apoptosis. The rate of nonenzymatic deamidation of two asparagines in the anti-apoptotic protein Bcl-x_L is accelerated by this increase in pH. Deamidation of these asparagines is a signal for the degradation of Bcl-x_L, which is a component of the apoptotic response to DNA damage. It has previously been shown that the increase in pH is mediated by the ion transporter Na⁺/H⁺ exchanger 1 in some cells. Here we demonstrate that one or more additional ion transporters also have a role in the regulation of Bcl-x_L deamidation in at least some tumor cell lines and fibroblasts. As a second, independent finding, we report that there are histidines in close proximity to the Bcl-x_L deamidation sites that are highly conserved in land-dwelling species and we present evidence that deamidation of human Bcl-x_L is intramolecularly catalyzed in a manner that is dependent upon these histidines. Further, we present evidence that these histidines act as a pH-sensitive switch that enhances the effect of the increase in pH on the rate of Bcl-x_L deamidation. The conservation of such histidines implies that human Bcl-x_L is in essence "designed" to be deamidated, which provides further evidence that deamidation serves as a bona fide regulatory post-translational modification of Bcl-x_L.

Bcl-xL deamidation and cancer: Charting the fame trajectories of legitimate child and hidden siblings

Bcl-2 family proteins control programmed cell death through a complex network of interactions within and outside of this family, that are modulated by post-translational modifications (PTM). Bcl-x_L, an anti-apoptotic member of this family, is overexpressed in a number of cancers, plays an important role in tumorigenesis and is correlated with drug resistance. Bcl-x_L is susceptible to a number of different PTMs. Here, we focus on deamidation. We will first provide an overview of protein deamidation. We will then review how the apoptotic and autophagic functions of Bcl-x_L are modified by this PTM, and how this impacts on its oncogenic properties. Possible therapeutic outcomes will also be discussed. Finally, we will highlight how the specific case of Bcl-x_L deamidation provides groundings to revisit some concepts related to protein deamidation in general.

The anti-tumor activity of E1A and its implications in cancer therapy

The adenovirus type 5 E1A protein (E1A) plays a critical role in anti-cancer gene therapy and has been tested in clinical trials. The expression of E1A significantly reduces tumorigenesis, promotes cell death, and inhibits cancer cell mobility. Chemosensitization is one of the anti-tumor effects of E1A, increasing in vitro and in vivo sensitization of anti-cancer drugs, including cisplatin, gemcitabine, etoposide, doxorubicin, paclitaxel, and tumor necrosis factor-related apoptosis-inducing ligand and histone deacetylase inhibitors in different types of cancer cells. E1A also demonstrates anti-metastasis activity through various molecular mechanisms such as the repression of protease expression, suppression of HER2/neu and downregulation of microRNA (miR-520h). Moreover, E1A has been reported to reprogram transcription in tumor cells and stabilize tumor suppressors such as PP2A/C, p21 and p53. Because E1A plays a potentially significant role in anti-tumor therapy, there exists an urgent need to study the anti-cancer activities of E1A. This paper presents a review of our current understanding of the tumor-suppressive functions and molecular regulation of E1A, as well as the potential clinical applications of E1A.

Control of cellular Bcl-xL levels by deamidation-regulated degradation

Deamidation of two asparagines activates a conditional PEST sequence to target Bcl-xL for degradation.

The cellular concentration of Bcl-x_L is among the most important determinants of treatment response and overall prognosis in a broad range of tumors as well as an important determinant of the cellular response to several forms of tissue injury. We and others have previously shown that human Bcl-x_L undergoes deamidation at two asparaginyl residues and that DNA-damaging antineoplastic agents as well as other stimuli can increase the rate of deamidation. Deamidation results in the replacement of asparginyl residues with aspartyl or isoaspartyl residues. Thus deamidation, like phosphorylation, introduces a negative charge into proteins. Here we show that the level of human Bcl-x_L is constantly modulated by deamidation because deamidation, like phosphorylation in other proteins, activates a conditional PEST sequence to target Bcl-x_L for degradation. Additionally, we show that degradation of deamidated Bcl-x_L is mediated at least in part by calpain. Notably, we present sequence and biochemical data that suggest that deamidation has been conserved from the simplest extant metazoans through the human form of Bcl-x_L, underscoring its importance in Bcl-x_L regulation. Our findings strongly suggest that deamidation-regulated Bcl-x_L degradation is an important component of the cellular rheostat that determines susceptibility to DNA-damaging agents and other death stimuli.

Cellular levels of the pro-survival protein Bcl-x_L are an important determinant of cellular susceptibility to many death stimuli, including most cancer therapies. We previously showed that human Bcl-x_L undergoes deamidation – the conversion of two neutral asparaginyl side-chains into negatively charged aspartyl side-chains – a process that occurs spontaneously but is accelerated by the treatment of tumor cells with DNA-damaging agents. Here, we show that deamidation activates a hitherto undetected signal sequence within Bcl-x_L that targets it for degradation by a pathway involving the proteolytic enzyme calpain. This increased degradation of Bcl-x_L, and the consequent enhanced cellular susceptibility to programmed cell death, may contribute to the ability of DNA-damaging agents to kill tumors. We also demonstrate that deamidation of Bcl-x_L has likely been conserved from the simplest metazoans to humans, underscoring the importance of deamidation in the regulation of Bcl-x_L.

Non-enzymatic post-translational protein modifications and proteostasis network deregulation in carcinogenesis

Organisms are constantly challenged by stressors and thus the maintenance of biomolecules functionality is essential for the assurance of cellular homeostasis. Proteins carry out the vast majority of cellular functions by mostly participating in multimeric protein assemblies that operate as protein machines. Cells have evolved a complex proteome quality control network for the rescue, when possible, or the degradation of damaged polypeptides. Nevertheless, despite these proteostasis ensuring mechanisms, new protein synthesis, and the replication-mediated dilution of proteome damage in mitotic cells, the gradual accumulation of stressors during aging (or due to lifestyle) results in increasingly damaged proteome. Non-enzymatic post-translational protein modifications mostly arise by unbalanced redox homeostasis and/or high glucose levels and may cause disruption of proteostasis as they can alter protein function. This outcome may then increase genomic instability due to reduced fidelity in processes like DNA replication or repair. Herein, we present a synopsis of the major non-enzymatic post-translation protein modifications and of the proteostasis network deregulation in carcinogenesis. We propose that activation of the proteostasis ensuring mechanisms in premalignant cells has tumor-preventive effects, whereas considering that over-activation of these mechanisms represents a hallmark of advanced tumors, their inhibition provides a strategy for the development of anti-tumor therapies. This article is part of a Special Issue entitled: Posttranslational Protein modifications in biology and Medicine.

Adenovirus-mediated sensitization to the cytotoxic drugs docetaxel and mitoxantrone is dependent on regulatory domains in the E1ACR1 gene-region

Oncolytic adenoviruses have shown promising efficacy in clinical trials targeting prostate cancers that frequently develop resistance to all current therapies. The replication-selective mutants AdΔΔ and dl922–947, defective in pRb-binding, have been demonstrated to synergise with the current standard of care, mitoxantrone and docetaxel, in prostate cancer models. While expression of the early viral E1A gene is essential for the enhanced cell killing, the specific E1A-regions required for the effects are unknown. Here, we demonstrate that replicating mutants deleted in small E1A-domains, binding pRb (dl1108), p300/CBP (dl1104) and p400/TRRAP or p21 (dl1102) sensitize human prostate cancer cells (PC-3, DU145, 22Rv1) to mitoxantrone and docetaxel. Through generation of non-replicating mutants, we demonstrate that the small E1A12S protein is sufficient to potently sensitize all prostate cancer cells to the drugs even in the absence of viral replication and the E1A transactivating domain, conserved region (CR) 3. Furthermore, the p300/CBP-binding domain in E1ACR1 is essential for drug-sensitisation in the absence (AdE1A1104) but not in the presence of the E1ACR3 (dl1104) domain. AdE1A1104 also failed to increase apoptosis and accumulation of cells in G2/M. All E1AΔCR2 mutants (AdE1A1108, dl922–947) and AdE1A1102 or dl1102 enhance cell killing to the same degree as wild type virus. In PC-3 xenografts in vivo the dl1102 mutant significantly prolongs time to tumor progression that is further enhanced in combination with docetaxel. Neither dl1102 nor dl1104 replicates in normal human epithelial cells (NHBE). These findings suggest that additional E1A-deletions might be included when developing more potent replication-selective oncolytic viruses, such as the AdΔCR2-mutants, to further enhance potency through synergistic cell killing in combination with current chemotherapeutics.

Ling Zhi-8 mediates p53-dependent growth arrest of lung cancer cells proliferation via the ribosomal protein S7-MDM2-p53 pathway

Ling Zhi-8 (LZ-8), an immunomodulatory protein, is derived from and has been cloned from the medicinal mushroom Ganoderma lucidum (Reishi or Ling Zhi); this protein exhibits immunomodulating and antitumor properties. We investigated the effects of recombinant LZ-8 protein (rLZ-8) on the proliferation of A549 human lung cancer cells. Here, we showed that rLZ-8 inhibits cell growth and that this is correlated with increased G(1) arrest. The treatment of A549 cells with rLZ-8 activated p53 and p21 expression, and both the G(1) arrest and the antigrowth effect were found to be p53 dependent. It was further demonstrated that rLZ-8 inhibited tumor growth in mice transplanted with Lewis lung carcinoma cells. Interestingly, rLZ-8 treatment was found to lead to nucleolar stress (or ribosomal stress) as evidenced by inhibition of precursor ribosomal RNA synthesis and reduced polysome formation in A549 cells. These changes resulted in an increasing binding of ribosomal protein S7 to MDM2 and a decreased interaction between MDM2 and p53. Taking these results together, we have identified a novel rLZ-8 antitumor function that positively modulates p53 via ribosomal stress and inhibits lung cancer cell growth in vitro and in vivo. Our current results suggest that rLZ-8 may have potential as a therapeutic intervention for the treatment of cancers that contain wild-type p53 and high expression of MDM2.

Inhibition of the Bcl-xL deamidation pathway in myeloproliferative disorders

BACKGROUND

The myeloproliferative disorders are clonal disorders with frequent somatic gain-of-function alterations affecting tyrosine kinases. In these diseases, there is an increase in DNA damage and a risk of progression to acute leukemia. The molecular mechanisms in myeloproliferative disorders that prevent apoptosis induced by damaged DNA are obscure.

METHODS

We searched for abnormalities of the proapoptotic Bcl-x(L) deamidation pathway in primary cells from patients with chronic myeloid leukemia (CML) or polycythemia vera, myeloproliferative disorders associated with the BCR-ABL fusion kinase and the Janus tyrosine kinase 2 (JAK2) V617F mutation, respectively.

RESULTS

The Bcl-x(L) deamidation pathway was inhibited in myeloid cells, but not T cells, in patients with CML or polycythemia vera. DNA damage did not increase levels of the amiloride-sensitive sodium-hydrogen exchanger isoform 1 (NHE-1), intracellular pH, Bcl-x(L) deamidation, and apoptosis. Inhibition of the pathway was reversed by enforced alkalinization or overexpression of NHE-1, leading to a restoration of apoptosis. In patients with CML, the pathway was blocked in CD34+ progenitor cells and mature myeloid cells. Imatinib or JAK2 inhibitors reversed inhibition of the pathway in cells from patients with CML and polycythemia vera, respectively, but not in cells from a patient with resistance to imatinib because of a mutation in the BCR-ABL kinase domain.

CONCLUSIONS

BCR-ABL and mutant JAK2 inhibit the Bcl-x(L) deamidation pathway and the apoptotic response to DNA damage in primary cells from patients with CML or polycythemia vera.

DNA damage-induced Bcl-xL deamidation is mediated by NHE-1 antiport regulated intracellular pH

The pro-survival protein Bcl-x_L is critical for the resistance of tumour cells to DNA damage. We have previously demonstrated, using a mouse cancer model, that oncogenic tyrosine kinase inhibition of DNA damage–induced Bcl-x_L deamidation tightly correlates with T cell transformation in vivo, although the pathway to Bcl-x_L deamidation remains unknown and its functional consequences unclear. We show here that rBcl-x_L deamidation generates an iso-Asp⁵²/iso-Asp⁶⁶ species that is unable to sequester pro-apoptotic BH3-only proteins such as Bim and Puma. DNA damage in thymocytes results in increased expression of the NHE-1 Na/H antiport, an event both necessary and sufficient for subsequent intracellular alkalinisation, Bcl-x_L deamidation, and apoptosis. In murine thymocytes and tumour cells expressing an oncogenic tyrosine kinase, this DNA damage–induced cascade is blocked. Enforced intracellular alkalinisation mimics the effects of DNA damage in murine tumour cells and human B-lineage chronic lymphocytic leukaemia cells, thereby causing Bcl-x_L deamidation and increased apoptosis. Our results define a signalling pathway leading from DNA damage to up-regulation of the NHE-1 antiport, to intracellular alkalanisation to Bcl-x_L deamidation, to apoptosis, representing the first example, to our knowledge, of how deamidation of internal asparagine residues can be regulated in a protein in vivo. Our findings also suggest novel approaches to cancer therapy.

Cell survival and cell death (apoptosis) are controlled by a finely tuned ensemble of pro-survival and pro-apoptotic proteins. When the two types of protein are balanced, cells survive. But if the pro-survival proteins dominate, there is a danger that cells with damaged DNA will stay alive, leading to malignancy. One of the key pro-survival proteins, Bcl-x_L, acts by blocking the actions of pro-apoptotic proteins. We show here that DNA damage results in an important modification of Bcl-x_L. Specifically, when the amide groups are removed from two critical asparagine (amino acid) residues, Bcl-x_L can no longer block pro-apoptotic proteins, leading to cell death. Surprisingly, Bcl-x_L deamidation is catalysed not by an enzyme, but by increased pH inside the cell due to the up-regulation of an NHE-1 transporter that moves positive ions across the cell membrane. Indeed, artificially increasing pH causes Bcl-x_L deamidation and apoptosis in the absence of initial DNA damage. Exploring this novel pathway may ultimately suggest approaches to cancer therapy, especially when malignant cells are resistant to chemotherapy or radiotherapy.

Until now, the mechanisms and functional implications for DNA damage-induced Bcl-x_L deamidation were unknown. Here the authors provide important new insights into this phenomenon and its impact on cell survival.