Involvement of Ras in Bruton's tyrosine kinase-mediated JNK activation

G protein regulation of MAPK networks

G proteins provide signal-coupling mechanisms to heptahelical cell surface receptors and are critically involved in the regulation of different mitogen-activated protein kinase (MAPK) networks. The four classes of G proteins, defined by the G(s), G(i), G(q) and G(12) families, regulate ERK1/2, JNK, p38MAPK, ERK5 and ERK6 modules by different mechanisms. The alpha- as well as betagamma-subunits are involved in the regulation of these MAPK modules in a context-specific manner. While the alpha- and betagamma-subunits primarily regulate the MAPK pathways via their respective effector-mediated signaling pathways, recent studies have unraveled several novel signaling intermediates including receptor tyrosine kinases and small GTPases through which these G-protein subunits positively as well as negatively regulate specific MAPK modules. Multiple mechanisms together with specific scaffold proteins that can link G-protein-coupled receptors or G proteins to distinct MAPK modules contribute to the context-specific and spatio-temporal regulation of mitogen-activated protein signaling networks by G proteins.

Unimpaired activation of c-Jun NH2-terminal kinase (JNK) 1 upon CD40 stimulation in B cells of patients with X-linked agammaglobulinemia

X-linked agammaglobulinemia (XLA) is caused by mutations in the gene encoding the cytoplasmic Bruton's tyrosine kinase (Btk). Btk has been shown to play an essential role in the development of B1 (CD5+) and conventional circulating mature B cells (B2) in mouse and man. It has been shown in earlier studies that Btk is involved in both the BCR- and CD40-mediated signaling pathways. In this study, we analyzed the responsiveness of Epstein-Barr virus (EBV) transformed B cells from nine XLA patients to CD40 stimulation, particularly the CD40 induced activation of c-Jun N-terminal kinase (JNK). In eight XLA patients the JNK activation was unimpaired and in one case INK could not be activated by anti-CD40 stimulation. 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. Btk kinase activity was found in three cases and it was reduced in one and not detectable in five cases. Furthermore, in one female patient with an agammaglobulinemia, Btk expression and function as well as JNK activation by CD40 stimulation was unimpaired. Our findings demonstrate that INK activation via the CD40 signaling pathway is intact in EBV-transformed B cells of most if not all XLA patients, independent of the mutation and its effect on Btk expression and kinase activity. We suggest that Btk is not necessary for the activation of INK upon CD40 stimulation, at least in the B cell subpopulation we had studied. We cannot exclude that these B cells belong to a "leaky" B-cell subpopulation in which the CD40 signaling pathway has become independent of Btk function.

The Bmx tyrosine kinase is activated by IL-3 and G-CSF in a PI-3K dependent manner

Cytoplasmic protein tyrosine kinases play crucial roles in signaling via a variety of cell surface receptors. The Bmx tyrosine kinase, a member of the Tec family, is expressed in hematopoietic cells of the granulocytic and monocytic lineages. Here we show that Bmx is catalytically activated by interleukin-3 (IL-3) and granulocyte-colony stimulating factor (G-CSF) receptors. Activation of Bmx required phosphatidylinositol 3-kinase (PI-3K) as demonstrated by the ability of PI-3K inhibitors to block the activation signal. A green fluorescent protein (GFP) tagged Bmx was translocated to cellular membranes upon co-expression of a constitutively active form of PI-3K, further indicating a role for PI-3K in signaling upstream of Bmx. The expression of wild type Bmx in 32D myeloid progenitor cells resulted in apoptosis in the presence of G-CSF, while cells expressing a kinase dead mutant of Bmx differentiated into mature granulocytes. However, Bmx did not modulate IL-3-dependent proliferation of the cells. These results demonstrate distinct effects of Bmx in cytokine induced proliferation and differentiation of myeloid cells, and suggest that the stage specific expression of Bmx is critical for the differentiation of myeloid cells. Oncogene (2000) 19, 4151 - 4158

Tec family of protein-tyrosine kinases: an overview of their structure and function

The Tec family is a recently emerging subfamily of non-receptor protein-tyrosine kinases (PTKs) represented by its first member, Tec. This family is composed of five members, namely Tec, Btk. Itk/Emt/Tsk, Bmx and Txk/Rlk. The most characteristic feature of this family is the presence of a pleckstrin homology (PH) domain in their protein structure. The PH domain is known to bind phosphoinositides; on this basis, Tec family PTKs may act as merge points of phosphotyrosine-mediated and phospholipid-mediated signaling systems. Many Tec family proteins are abundantly expressed in hematopoietic tissues, and are presumed to play important roles in the growth and differentiation processes of blood cells. Supporting this, mutations in the Btk gene cause X chromosome-linked agammaglobulinemia (XLA) in humans and X chromosome-linked immunodeficiency (Xid) in mice, indicating that Btk activity is indispensable for B-cell ontogeny. In addition, Tec family kinases have been shown to be involved in the intracellular signaling mechanisms of cytokine receptors, lymphocyte surface antigens, heterotrimeric G-protein-coupled receptors and integrin molecules. Efforts are being made to identify molecules which interact with Tec kinases to transfer Tec-mediated signals in vivo. Candidates for such second messengers include PLC-gamma2, guanine nucleotide exchange factors for RhoA and TFII-I/BAP-135. This review summarizes current knowledge concerning the input and output factors affecting the Tec kinases.

Radiation-induced release of transforming growth factor alpha activates the epidermal growth factor receptor and mitogen-activated protein kinase pathway in carcinoma cells, leading to increased proliferation and protection from radiation-induced cell death

Exposure of A431 squamous and MDA-MB-231 mammary carcinoma cells to ionizing radiation has been associated with short transient increases in epidermal growth factor receptor (EGFR) tyrosine phosphorylation and activation of the mitogen-activated protein kinase (MAPK) and c-Jun NH(2)-terminal kinase (JNK) pathways. Irradiation (2 Gy) of A431 and MDA-MB-231 cells caused immediate primary activations (0-10 min) of the EGFR and the MAPK and JNK pathways, which were surprisingly followed by later prolonged secondary activations (90-240 min). Primary and secondary activation of the EGFR was abolished by molecular inhibition of EGFR function. The primary and secondary activation of the MAPK pathway was abolished by molecular inhibition of either EGFR or Ras function. In contrast, molecular inhibition of EGFR function abolished the secondary but not the primary activation of the JNK pathway. Inhibition of tumor necrosis factor alpha receptor function by use of neutralizing monoclonal antibodies blunted primary activation of the JNK pathway. Addition of a neutralizing monoclonal antibody versus transforming growth factor alpha (TGFalpha) had no effect on the primary activation of either the EGFR or the MAPK and JNK pathways after irradiation but abolished the secondary activation of EGFR, MAPK, and JNK. Irradiation of cells increased pro-TGFalpha cleavage 120-180 min after exposure. In agreement with radiation-induced release of a soluble factor, activation of the EGFR and the MAPK and JNK pathways could be induced in nonirradiated cells by the transfer of media from irradiated cells 120 min after irradiation. The ability of the transferred media to cause MAPK and JNK activation was blocked when media were incubated with a neutralizing antibody to TGFalpha. Thus radiation causes primary and secondary activation of the EGFR and the MAPK and JNK pathways in autocrine-regulated carcinoma cells. Secondary activation of the EGFR and the MAPK and JNK pathways is dependent on radiation-induced cleavage and autocrine action of TGFalpha. Neutralization of TGFalpha function by an anti-TGFalpha antibody or inhibition of MAPK function by MEK1/2 inhibitors (PD98059 and U0126) radiosensitized A431 and MDA-MB-231 cells after irradiation in apoptosis, 3-[4, 5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT), and clonogenic assays. These data demonstrate that disruption of the TGFalpha-EGFR-MAPK signaling module represents a strategy to decrease carcinoma cell growth and survival after irradiation.

Bruton's tyrosine kinase activity and inositol 1,4,5-trisphosphate production are not altered in DT40 lymphoma B cells exposed to power line frequency magnetic fields

Exposure of wild-type DT40 lymphoma B cells or Bruton's tyrosine kinase (BTK)-deficient DT40 cells reconstituted with the human btk gene to a 1-gauss 60-Hz electromagnetic field (EMF) has been reported to rapidly increase inositol 1,4,5-trisphosphate (Ins 1,4, 5-P3) production (1,2). Here we have used BTK-deficient DT40 B cells reconstituted with the human btk gene to evaluate the reproducibility of these findings. An experimental design with blinded exposures and anti-IgM treatment to induce Ins 1,4,5-P3 production as a positive control, showed no significant effect of a 1-gauss 60-Hz EMF on Ins 1,4,5-P3 production. Because recent work has shown that the activation of BTK was required for EMF-responsiveness (2), we also evaluated the reproducibility of this finding in wild-type DT40 cells. BTK was activated in a dose- and time-dependent manner by treatment with the tyrosine phosphatase inhibitor pervanadate. However, the ability to detect BTK activation, as measured by increased autophosphorylation by immune complex kinase assay, was dependent on the kinase buffer. Using cells from the original investigators, no evidence was obtained to support the hypothesis that exposure to a 1-gauss 60-Hz EMF had a causal effect on protein-tyrosine kinase activities affecting Ins 1,4,5-P3 production.