BTK mediated apoptosis, a possible mechanism for failure to generate high titer retroviral producer clones

Kinomics platform using GBM tissue identifies BTK as being associated with higher patient survival

BTK is a dominant bioactive kinase expressed within both cancer and immune cells of GBM tissue. Complex cell co-cultures might better model the impact of kinase inhibitors as therapeutics in GBM.

Better understanding of GBM signalling networks in-vivo would help develop more physiologically relevant ex vivo models to support therapeutic discovery. A “functional proteomics” screen was undertaken to measure the specific activity of a set of protein kinases in a two-step cell-free biochemical assay to define dominant kinase activities to identify potentially novel drug targets that may have been overlooked in studies interrogating GBM-derived cell lines. A dominant kinase activity derived from the tumour tissue, but not patient-derived GBM stem-like cell lines, was Bruton tyrosine kinase (BTK). We demonstrate that BTK is expressed in more than one cell type within GBM tissue; SOX2-positive cells, CD163-positive cells, CD68-positive cells, and an unidentified cell population which is SOX2-negative CD163-negative and/or CD68-negative. The data provide a strategy to better mimic GBM tissue ex vivo by reconstituting more physiologically heterogeneous cell co-culture models including BTK-positive/negative cancer and immune cells. These data also have implications for the design and/or interpretation of emerging clinical trials using BTK inhibitors because BTK expression within GBM tissue was linked to longer patient survival.

BTK modulates p73 activity to induce apoptosis independently of p53

Bruton’s tyrosine kinase (BTK) is a key component of B cell receptor signalling. Because of this, BTK plays an important role in cell proliferation and survival in various B cell malignancies. However, in certain contexts, BTK can also have tumour suppressor functions. We have previously shown that BTK activates the p53 transcriptional activity by binding to and phosphorylating p53, as well as acting on MDM2 to reduce its inhibitory effects. This results in increased p53 functions, including enhanced cell death. Here, we report that BTK can also induce cell death and increase responses to DNA damage independently of p53. This is concomitant to the induction of p21, PUMA and MDM2, which are classic target genes of the p53 family of proteins. Our results show that these p53-independent effects of BTK are mediated through p73. Similar to what we observed in the p53 pathway, BTK can upregulate p73 after DNA damage and induce expression of its target genes, suggesting that BTK is a modulator of p73 functions and in the absence of p53. This effect allows BTK to have pro-apoptotic functions independently of its effects on the p53 pathway and thus play an important role in the DNA damage-related induction of apoptosis in the absence of p53. This provides a novel role of BTK in tumour suppression and contributes to the understanding of its complex pleiotropic functions

BTK blocks the inhibitory effects of MDM2 on p53 activity

p53 is a tumour suppressor that is activated in response to various types of stress. It is regulated by a complex pattern of over 50 different post-translational modifications, including ubiquitination by the E3 ligase MDM2, which leads to its proteasomal degradation. We have previously reported that expression of Bruton’s Tyrosine Kinase (BTK) induces phosphorylation of p53 at the N-terminus, including Serine 15, and increases its protein levels and activity. The mechanisms involved in this process are not completely understood. Here, we show that BTK also increases MDM2 and is necessary for MDM2 upregulation after DNA damage, consistent with what we have shown for other p53 target genes. Moreover, we found that BTK binds to MDM2 on its PH domain and induces its phosphorylation. This suggested a negative regulation of MDM2 functions by BTK, supported by the fact BTK expression rescued the inhibitory effects of MDM2 on p53 transcriptional activity. Indeed, we observed that BTK mediated the loss of the ubiquitination activity of MDM2, a process that was dependent on the phosphorylation functions of BTK. Our data together shows that the kinase activity of BTK plays an important role in disrupting the MDM2-p53 negative feedback loop by acting at different levels, including binding to and inactivation of MDM2. This study provides a potential mechanism to explain how BTK modulates p53 functions.

BTK Modulates p53 Activity to Enhance Apoptotic and Senescent Responses

p53 is a tumor suppressor that prevents the emergence of transformed cells by inducing apoptosis or senescence, among other responses. Its functions are regulated tightly by posttranslational modifications. Here we show that Bruton's tyrosine kinase (BTK) is a novel modulator of p53. We found that BTK is induced in response to DNA damage and p53 activation. BTK induction leads to p53 phosphorylation, which constitutes a positive feedback loop that increases p53 protein levels and enhances the transactivation of its target genes in response to stress. Inhibiting BTK reduced both p53-dependent senescence and apoptosis. Further, BTK expression also upregulated DNA damage signals and apoptosis. We conclude that despite being involved in oncogenic signals in blood malignancies, BTK has antineoplastic properties in other contexts, such as the enhancement of p53's tumor suppressor responses. Along with evidence that BTK expression correlates with good prognosis in some epithelial tumors, our findings may encourage a reevaluation of the clinical uses of BTK inhibitors in cancer therapy. Cancer Res; 76(18); 5405-14. ©2016 AACR.

Differential evolutionary wiring of the tyrosine kinase Btk

BACKGROUND

A central question within biology is how intracellular signaling pathways are maintained throughout evolution. Btk29A is considered to be the fly-homolog of the mammalian Bruton's tyrosine kinase (Btk), which is a non-receptor tyrosine-kinase of the Tec-family. In mammalian cells, there is a single transcript splice-form and the corresponding Btk-protein plays an important role for B-lymphocyte development with alterations within the human BTK gene causing the immunodeficiency disease X-linked agammaglobulinemia in man and a related disorder in mice. In contrast, the Drosophila Btk29A locus encodes two splice-variants, where the type 2-form is the more related to the mammalian Btk gene product displaying more than 80% homology. In Drosophila, Btk29A displays a dynamic pattern of expression through the embryonic to adult stages. Complete loss-of-function of both splice-forms is lethal, whereas selective absence of the type 2-form reduces the adult lifespan of the fly and causes developmental abnormalities in male genitalia.

METHODOLOGY/PRINCIPAL FINDINGS

Out of 7004-7979 transcripts expressed in the four sample groups, 5587 (70-79%) were found in all four tissues and strains. Here, we investigated the role of Btk29A type 2 on a transcriptomic level in larval CNS and adult heads. We used samples either selectively defective in Btk29A type 2 (Btk29A(ficP)) or revertant flies with restored Btk29A type 2-function (Btk29A(fic Exc1-16)). The whole transcriptomic profile for the different sample groups revealed Gene Ontology patterns reflecting lifespan abnormalities in adult head neuronal tissue, but not in larvae.

CONCLUSIONS

In the Btk29A type 2-deficient strains there was no significant overlap between transcriptomic alterations in adult heads and larvae neuronal tissue, respectively. Moreover, there was no significant overlap of the transcriptomic changes between flies and mammals, suggesting that the evolutionary conservation is confined to components of the proximal signaling, whereas the corresponding, downstream transcriptional regulation has been differentially wired.

A novel reverse transduction adenoviral array for the functional analysis of shRNA libraries

Background

The identification of novel drug targets by assessing gene functions is most conveniently achieved by high-throughput loss-of-function RNA interference screening. There is a growing need to employ primary cells in such screenings, since they reflect the physiological situation more closely than transformed cell lines do. Highly miniaturized and parallelized approaches as exemplified by reverse transfection or transduction arrays meet these requirements, hence we verified the applicability of an adenoviral microarray for the elucidation of gene functions in primary cells.

Results

Here, we present microarrays of infectious adenoviruses encoding short hairpin RNA (shRNA) as a new tool for gene function analysis. As an example to demonstrate its application, we chose shRNAs directed against seven selected human protein kinases, and we have performed quantitative analysis of phenotypical responses in primary human umbilical vein cells (HUVEC). These microarrays enabled us to infect the target cells in a parallelized and miniaturized procedure without significant cross-contamination: Viruses were reversibly immobilized in spots in such a way that the seeded cells were confined to the area of the viral spots, thus simplifying the subsequent addressing of genetically modified cells for analysis. Computer-assisted image analysis of fluorescence images was applied to analyze the cellular response after shRNA expression. Both the expression level of knock-down target proteins as well as the functional output as measured by caspase 3 activity and DNA fractionation (TUNEL) were quantified.

Conclusion

We have developed an adenoviral microarray technique suitable for miniaturized and parallelized analysis of gene function. The practicability of this technique was demonstrated by the analysis of several kinases involved in the activation of programmed cell death, both in tumor cells and in primary cells.

Syk tyrosine kinase mediates Epstein-Barr virus latent membrane protein 2A-induced cell migration in epithelial cells

Although spleen tyrosine kinase (Syk) is known to be important in hematopoietic cell development, the roles of Syk in epithelial cells have not been well studied. Limited data suggest that Syk plays alternate roles in carcinogenesis under different circumstances. In breast cancer, Syk has been suggested to be a tumor suppressor. In contrast, Syk is essential for murine mammary tumor virus-mediated transformation. However, the roles of Syk in tumor migration are still largely unknown. Nasopharyngeal carcinoma, an unusually highly metastatic tumor, expresses Epstein-Barr virus LMP2A (latent membrane protein 2A) in most clinical specimens. Previously, we demonstrated LMP2A triggers epithelial cell migration. LMP2A contains an immunoreceptor tyrosine-based activation motif, which is important for Syk kinase activation in B cells. In this study, we explored whether Syk is important for LMP2A-mediated epithelial cell migration. We demonstrate that LMP2A expression can activate endogenous Syk activity. The activation requires the tyrosine residues in LMP2A ITAM but not YEEA motif, which is important for Syk activation by Lyn in B cells. LMP2A interacts with Syk as demonstrated by coimmunoprecipitation and confocal microscopy. Furthermore, LMP2A-induced cell migration is inhibited by a Syk inhibitor and short interfering RNA. Tyrosines 74 and 85 in the LMP2A immunoreceptor tyrosine-based activation motif are essential for both Syk activation and LMP2A-mediated cell migration, indicating the involvement of Syk in LMP2A-triggered cell migration. The LMP2A-Syk pathway may provide suitable drug targets for treatment of nasopharyngeal carcinoma.

HIV-mediated Expression of Btk in Hematopoietic Stem Cells is not Sufficient to Restore B Cell Function in X-linked Immunodeficient Mice

Mutations of Bruton's tyrosine kinase (Btk), which is critical for B cell development and function, cause X-linked agammaglobulinemia (XLA) in humans and X-linked immunodeficiency (xid) in mice. Although the severity of the clinical phenotype differs between the two species, xid mice are considered useful for evaluating treatment strategies for XLA patients. Hematopoietic stem cells (HSCs; 1 approximately 3 x 10(5))from xid mice were transduced with an HIV vector containing the human Btk (hBtk) gene under the control of the internal murine stem cell virus (MSCV) promoter and injected into 4-week-old xid mice. Thirty weeks later, the copy number of the integrated HIV vector was over 0.2 per cell in both bone marrow and spleen, but serum concentrations of IgM and IgG3 and the antibody response to nitrophenol (NP)-Ficoll challenge were not restored. The number of differentiated B cells (IgM(low)IgD(high)) was increased, while the peritoneal B1 cell count remained low. These results indicate that HIV-mediated expression of hBtk in bone marrow stem cells partially promotes B cell development, but is not sufficient for the restoration of B cell function in xid mice.

The role of tyrosine kinase Etk/Bmx in EGF-induced apoptosis of MDA-MB-468 breast cancer cells

Etk/Bmx, a member of the Tec family of tyrosine kinases, mediates various signaling pathways and confers several cellular functions. In the present study, we have explored the functional role of Etk in mediating EGF-induced apoptosis, using MDA-MB-468 cell line as a model. We first demonstrated that EGF treatment induces Etk tyrosine phosphorylation in both HeLa and MDA-MB-468 cells. Overexpression of Etk by recombinant adenovirus in MDA-MB-468 cells potentiates the extent of EGF-induced cell apoptosis. The observed Etk-enhanced MDA-MB-468 cell apoptosis is associated with the Stat1 activation, as demonstrated by electrophoresis mobility shift assays and reporter gene assays. By contrast, a kinase domain deletion mutant EtkDeltaK, functioning as a dominant-negative mutant, ameliorates EGF-induced Stat1 activation and apoptosis in MDA-MB-468 cells. To explore whether the activated Etk alone is sufficient for inducing apoptosis, a conditionally activated Etk (DeltaEtk-ER), a chimeric fusion protein of PH domain-truncated Etk and ligand-binding domain of estrogen receptor, was introduced into MDA-MB-468 cells. Upon beta-estradiol ligand activation, the DeltaEtk-ER could stimulate Stat1 activity and confer cell apoptosis independent of EGF treatment. Taken together, our findings indicate that Etk is a downstream signaling molecule of EGF receptor and suggest that Etk activation is essential for transducing the EGF-induced apoptotic signaling.

Interaction of Btk and Akt in B cell signaling

Reactive oxygen species (ROS) or reactive oxygen intermediates (ROIs) mediate complex signaling involving multiple pathways. In this report, we demonstrate for the first time that endogenous Bruton's tyrosine kinase (Btk) and Akt can interact with each other in DT40 chicken B cells and human Nalm6 B cells and that this interaction is inducible following H2O2 stimulation. This interaction is supported by visualizing the co-localization of Btk and Akt in the perinuclear region and membrane ruffles in COS-7 cells. We have also shown the involvement of phosphatidylinositol 3-kinase (PI 3-K) and Btk in the phosphorylation of Akt following stimulation by hydrogen peroxide (H2O2). Interestingly, Akt phosphorylation was found in the presence of Btk even in the absence of oxidative stress. In addition, we have investigated the involvement of PI 3-K in the MAPKs and ERK and JNK phosphorylation, in the presence or absence of Btk. Phosphorylation of both ERK and JNK increased when the PI 3-K pathway was inhibited and both pathways were modulated positively by Btk. Taken together, based on the study of endogenous conditions, we show the novel interaction of Btk and Akt in H2O2 signaling in B cells.

Expression profiling in transformed human B cells: influence of Btk mutations and comparison to B cell lymphomas using filter and oligonucleotide arrays

We have used both Clontech Atlas Human Hematology/Immunology cDNA microarrays, containing 588 genes, and Affymetrix oligonucleotide U95Av2 human array complementary to more than 12,500 genes to get a global view of genes expressed in Epstein-Barr virus (EBV)-transformed B cells and genes regulated by Bruton's tyrosine kinase (Btk). We compared EBV-transformed wild-type (WT) B cells from a healthy individual, WT1 and an X-linked agammaglobulinemia (XLA) patient cell line, XLA1, using the Clontech filters arrays. Eleven genes were > or =1.9-fold induced in absence of functional Btk. Furthermore, we analyzed a second patient cell line, XLA2, and compared this to two WT cell lines using oligonucleotide arrays. A total of 391 genes were found to be differentially expressed, including kinases and transcriptions factors. Furthermore, one expressed sequence tag and eight complementary DNA clones with unknown function were down-regulated in XLA2, indicating their biological role. Higher-fold inductions, Fyn (39.5), Hck (15.5) and Cyp1B1 (5.8), were observed using oligonucleotide array and were confirmed using real-time PCR for Fyn (20.8), Hck (6.7) and Cyp1B1 (10). Two genes, B cell translocation gene1 (BTG1) and B cell-specific OCT binding factor-1 (OBF-1) were induced > or =1.9-fold in both XLA1 and XLA2 analyzed by Atlas filter arrays andAffymetrix chips, respectively. Data from both filter and oligonucleotide arrays were compared to the gene clusters of a previously published lymphoma expression profile by linking to the UniGene transcript database. Our findings demonstrate for the first time the use of microarray to study the influence of Btk mutations and the use of functional annotation and validation of expression data by comparison of microarray analyses.

Gene therapy for primary immune deficiencies

Primary immunodeficiency diseases have been important targets of corrective gene transfer approaches since the very early days of gene therapy. The potential for selective survival advantage of gene-corrected cells over populations carrying the mutated, causative gene translates into the possibility of obtaining clinical meaningful results in patients with primary immunodeficiency diseases even if levels of gene transfer are low. This critical prospect has fueled the interest of researchers since the mid-1980s and has recently determined the success of a clinical trial of gene therapy for X-linked severe combined immunodeficiency.

Gene therapy for immunodeficiency

Since the early 1990s, primary immunodeficiency (ID) disorders have played a major role in the development of human gene therapy. Adenosine deaminase (ADA) deficiency was the first disease to be treated with a gene therapy approach in humans, and was also the first condition for which therapeutic gene transfer into the hematopoietic stem cell has been attempted in the clinical arena. A series of encouraging results obtained in chronic granulomatous disease (CGD) patients have followed these pioneer experiments and preceded the very recent and exciting reports of successful genetic correction procedures performed in patients affected with the X-linked form of severe combined immunodeficiency (XSCID). The technical progress made in the field of gene transfer in recent years is mostly responsible for these clinical advances, and will be critical for future development of gene therapy approaches for other forms of IDs.

Signaling network of the Btk family kinases

The Btk family kinases represent new members of non-receptor tyrosine kinases, which include Btk/Atk, Itk/Emt/Tsk, Bmx/Etk, and Tec. They are characterized by having four structural modules: PH (pleckstrin homology) domain, SH3 (Src homology 3) domain, SH2 (Src homology 2) domain and kinase (Src homology 1) domain. Increasing evidence suggests that, like Src-family kinases, Btk family kinases play central but diverse modulatory roles in various cellular processes. They participate in signal transduction in response to virtually all types of extracellular stimuli which are transmitted by growth factor receptors, cytokine receptors, G-protein coupled receptors, antigen-receptors and integrins. They are regulated by many non-receptor tyrosine kinases such as Src, Jak, Syk and FAK family kinases. In turn, they regulate many of major signaling pathways including those of PI3K, PLCgamma and PKC. Both genetic and biochemical approaches have been used to dissect the signaling pathways and elucidate their roles in growth, differentiation and apoptosis. An emerging new role of this family of kinases is cytoskeletal reorganization and cell motility. The physiological importance of these kinases was amply demonstrated by their link to the development of immunodeficiency diseases, due to germ-line mutations. The present article attempts to review the structure and functions of Btk family kinases by summarizing our current knowledge on the interacting partners associated with the different modules of the kinases and the diverse signaling pathways in which they are involved.