Disease relevance of BTK

• Fas-resistant DT-40 lymphoma B-cells rendered BTK-deficient through targeted disruption of the btk gene by homologous recombination knockout underwent apoptosis after Fas ligation, but wild-type DT-40 cells or BTK-deficient DT-40 cells reconstituted with wild-type human btk gene did not [1].
• Similarly, LFM-A13 inhibited recombinant BTK expressed in a baculovirus expression vector system with an IC50 value of 2.5 microM [2].
• BTK-deficient RAMOS-1 human Burkitt's leukemia cells underwent apoptosis after Fas ligation, whereas BTK-positive NALM-6-UM1 human B-cell precursor leukemia cells expressing similar levels of Fas did not [1].
• EMT is related to the B-cell progenitor kinase (BPK), which has recently been implicated in X-linked hypogammaglobulinemia, to the TECI mammalian kinase, which has been implicated in liver neoplasia, to the more widely expressed TECII mammalian kinase, and to the Drosophila melanogaster Dsrc28 kinase [3].
• In addition, a dominant negative form of Btk inhibited TLR4-mediated activation of a nuclear factor kappaB (NFkappaB)-dependent reporter gene in HEK293 cells as well as LPS-induced activation of NFkappaB in the astrocytoma cell line U373 and the monocytic cell line RAW264 [4].

High impact information on BTK

• The failure of carriers of XLA, XSCID, and WAS to show signs of their gene defects can be explained by the preferential use of the normal, nonmutant X as the active X in the cell lineages affected by the gene defects [5].
• We studied two families with autosomal recessive defects in B-cell development and patients with presumed X-linked agammaglobulinemia who did not have mutations in Btk [6].
• BACKGROUND: Most patients with congenital hypogammaglobulinemia and absent B cells are males with X-linked agammaglobulinemia, which is caused by mutations in the gene for Bruton's tyrosine kinase (Btk); however, there are females with a similar disorder who do not have mutations in this gene [6].
• Brief report: a point mutation in the SH2 domain of Bruton's tyrosine kinase in atypical X-linked agammaglobulinemia [7].
• We describe a novel cytoplasmic tyrosine kinase, termed BPK (B cell progenitor kinase), which is expressed in all stages of the B lineage and in myeloid cells [8].

Chemical compound and disease context of BTK

• Studies of BTK inhibitors showed that the leflunomide metabolite analog LFM-A13 inhibited BTK in leukemia and lymphoma cells [9].
• Expression of Bruton's agammaglobulinemia tyrosine kinase gene, BTK, is selectively down-regulated in T lymphocytes and plasma cells [10].
• The anti-leukemic Bruton's tyrosine kinase inhibitor alpha-cyano-beta-hydroxy-beta-methyl-N-(2,5-dibromophenyl) propenamide (LFM-A13) prevents fatal thromboembolism [11].
• Stimulation of Bruton's tyrosine kinase (BTK) and inositol 1,4,5-trisphosphate production in leukemia and lymphoma cells exposed to low energy electromagnetic fields [12].
• We have therefore successfully conferred a protective advantage against mitozolomide toxicity to cells by rAAV-mediated hAT expression [13].

Biological context of BTK

• Inhibition of BTK activity specifically induces apoptosis in BCR-ABL1+ leukemia cells to a similar extent as inhibition of BCR-ABL1 kinase activity itself [14].
• As opposed to full-length BTK, truncated BTK lacks kinase activity yet can bind to BCR-ABL1 through its SRC-homology domain 3 [14].
• Activated BTK is essential for survival signals that otherwise would arise from the pre-B cell receptor, including activation of PLCgamma1, autonomous Ca2+ signaling, STAT5-phosphorylation, and up-regulation of BCLX(L) [14].
• BTK-mediated tyrosine phosphorylation of ectopically expressed wild type (but not Tyr(694) mutant) STAT5A enhanced its DNA binding activity [15].
• The dimensions of this rectangle are approximately 18 x 8 x 9 x 17 A, and the thickness of the pocket is approximately 7 A. Advanced docking procedures were employed for the rational design of leflunomide metabolite (LFM) analogs with a high likelihood to bind favorably to the catalytic site within the kinase domain of BTK [2].

Anatomical context of BTK

• To determine whether a highly conserved 3.5-kb segment flanking the first exon of BTK contains transcriptional regulatory signals, we tested various portions of the segment for promoter and expression activity in several appropriate cell lines [16].
• These findings provide unprecedented experimental evidence that BTK plays a nonredundant and pivotal role in B cell antigen receptor-mediated STAT5A activation in B cells [15].
• Here, we provide unique biochemical and genetic evidence that BTK is an inhibitor of the Fas/APO-1 death-inducing signaling complex in B-lineage lymphoid cells [1].
• IkappaB alpha degradation activated by LPS was intact in macrophages from X-linked immunodeficiency (Xid) mice, which contain inactive Btk [17].
• Deficient oxidative burst and myeloid differentiation have been reported in the X-linked immunodeficiency mouse, but the precise mechanism and relevance of Btk activity in human monocytes is poorly understood [18].

Associations of BTK with chemical compounds

• To our knowledge, LFM-A13 is the first BTK-specific tyrosine kinase inhibitor and the first anti-leukemic agent targeting BTK [2].
• Treatment of the anti-Fas-resistant NALM-6-UM1 cells with the leflunomide metabolite analog alpha-cyano-beta-methyl-beta-hydroxy-N-(2, 5-dibromophenyl)propenamide, a potent inhibitor of BTK, abrogated the BTK-Fas association without affecting the expression levels of BTK or Fas and rendered them sensitive to Fas-mediated apoptosis [1].
• In accordance with the anti-apoptotic function of BTK, treatment of BTK+ B-lineage leukemic cells with LFM-A13 enhanced their sensitivity to ceramide- or vincristine-induced apoptosis [2].
• Accordingly, TaqMan quantitative PCR analysis of actinomycin D time courses presented in this work shows that overexpression of Btk is able to stabilize TNF, but not IL-6 mRNA [19].
• Multiple downstream signaling pathways were involved, including phosphatidylinositol 3-kinase (PI3K), Bruton's tyrosine kinase (Btk), and phospholipase C (PLC)-mediated release of Ca(2+) from inositol 1,4,5-trisphosphate (IP(3))-sensitive stores [20].

Physical interactions of BTK

• Purified recombinant BTK was capable of directly binding purified recombinant STAT5A with high affinity (K(d) = 44 nm), as determined by surface plasmon resonance using a BIAcore biosensor system [15].
• Our findings implicate Btk as a Toll/interleukin-1 receptor domain-binding protein that is important for NFkappaB activation by TLR4 [4].
• Transient overexpression of PKCmu deletion mutants as well as expression of selected PKCmu domains in 293T cells revealed that both the kinase domain and the regulatory C1 region are independently capable of binding to the Btk PH-TH domain [21].
• Tyrosine-phosphorylated SLP-65 assembles intracellular signaling complexes such as the Ca(2 +) initiation complex encompassing phospholipase C-gamma2 and Bruton's tyrosine kinase [22].
• Here, we demonstrate that G protein Gqalpha binds directly to the nonreceptor Bruton's tyrosine kinase (Btk) to a region composed of a Tec-homology (TH) domain and a sarcoma virus tyrosine kinase (Src)-homology 3 (SH3) domain both in vitro and in vivo [23].

Enzymatic interactions of BTK

• Ectopically expressed BTK kinase domain was capable of tyrosine-phosphorylating STAT5A both in vitro and in vivo [15].
• In addition, Btk is phosphorylated by SCF, which causes association of Btk with TRAIL-receptor1 [24].
• The protein product of the c-cbl protooncogene is phosphorylated after B cell receptor stimulation and binds the SH3 domain of Bruton's tyrosine kinase [25].

Regulatory relationships of BTK

• Bruton's tyrosine kinase is activated upon CD40 stimulation in human B lymphocytes [26].
• In addition, adenoviral overexpression of Btk in normal human monocytes enhanced TNF alpha production [27].
• Because so little is known about human Bright, we sought to determine where human Bright is expressed in normal B cell differentiation and whether it also forms complexes with Btk [28].
• Furthermore, we showed that the PH/TH module is required for Gbetagamma-induced membrane translocation of Btk [29].
• Early arrest in B cell development in transgenic mice that express the E41K Bruton's tyrosine kinase mutant under the control of the CD19 promoter region [30].

Other interactions of BTK

• Taken together, it seems that EMT is a member of a new family of intracellular kinases that includes BPK, TECI, and TECII [3].
• Our previous data suggested that Btk lies within a p38-dependent pathway that stabilizes TNF mRNA [19].
• Because B cells in these models are predominantly immature, experiments were also performed using mature B cells expressing low levels of Btk and BLNK [31].
• Bruton's tyrosine kinase as an inhibitor of the Fas/CD95 death-inducing signaling complex [1].
• Detailed analysis of the interaction between Btk and TLR8 demonstrates that the presence of both Box 2 and 3 motifs in the Toll/interleukin-1 receptor domain was required for the interaction [4].

Analytical, diagnostic and therapeutic context of BTK

• Introduction of wild-type BTK protein by electroporation rendered BTK-deficient DT-40 cells resistant to the apoptotic effects of Fas ligation [1].
• Large-scale comparative sequence analysis of the human and murine Bruton's tyrosine kinase loci reveals conserved regulatory domains [16].
• Btk protein expression was detectable by Western blotting in six cases, in one case Btk expression was drastically reduced, and in three cases no Btk expression could be observed [32].
• This work may provide clues about critical sites in the molecule and give support for gene therapy as a potential successful approach to XLA [33].
• A similar expression pattern was obtained when BTK protein was analyzed by immunoprecipitation and Western blotting [10].

References