Ataxia telangiectasia mutated down-regulates phospho-extracellular signal-regulated kinase 1/2 via activation of MKP-1 in response to radiation.

Ataxia telangiectasia mutated (ATM) kinase plays a crucial role in the cellular response to DNA damage and in radiation resistance. Although much effort has focused on the relationship between ATM and other nuclear signal transducers, little is known about interactions between ATM and mitogenic signaling pathways. In this study, we show a novel relationship between ATM kinase and extracellular signal-regulated kinase 1/2 (ERK1/2), a key mitogenic stimulator. Activation of ATM by radiation down-regulates phospho-ERK1/2 and its downstream signaling via increased expression of mitogen-activated protein kinase phosphatase MKP-1 in both cell culture and tumor models. This dephosphorylation of ERK1/2 is independent of epidermal growth factor receptor (EGFR) activity and is associated with radioresistance. These findings show a new function for ATM in the control of mitogenic pathways affecting cell signaling and emphasize the key role of ATM in coordinating the cellular response to DNA damage.

Down-regulation of the c-Jun N-terminal kinase (JNK) phosphatase M3/6 and activation of JNK by hydrogen peroxide and pyrrolidine dithiocarbamate.

Oxidative stress activates the c-Jun N-terminal kinase (JNK) pathway. However, the exact mechanisms by which reactive oxygen species (ROS) activate JNK are unclear. We found that the ability of hydrogen peroxide (H(2)O(2)) to induce JNK activation varied in different cell types. Pyrrolidine dithiocarbamate (PDTC), a presumed antioxidant, induced JNK activation on its own and enhanced JNK activation by H(2)O(2) in many cell types, including Jurkat, HEK293, and LNCaP and Tsu-Pr1 prostate cancer cells. The activation of JNK by PDTC, in the presence or absence of exogenous H(2)O(2), was dependent on its chelating ability to metal ions, most likely copper ions. Despite the strong JNK-activating ability, H(2)O(2) plus PDTC did not induce significant activation of the upstream kinases, SEK1/MKK4 and MKK7. However, the JNK inactivation rate was slower in cells treated with H(2)O(2) plus PDTC compared with the rate in cells treated with ultraviolet C (UV-C). Treatment of H(2)O(2) plus PDTC significantly decreased the expression levels of a JNK phosphatase, M3/6 (also named hVH-5), but not the levels of other phosphatases (PP2A and PP4). In contrast, UV-C irradiation did not cause the down-regulation of M3/6. These results suggest that JNK activation by H(2)O(2) plus PDTC resulted from the down-regulation of JNK phosphatases. Our data also reveal a necessity to carefully evaluate the pharmacological and biochemical properties of PDTC.

Redox regulation of signal transduction in mammalian cells.

This mini-review addresses the mechanism of ultraviolet-light-induced activation of receptor tyrosine kinases. The experimental approach into this mechanism revealed the existence of redox regulation of signal transduction in mammalian cells. It is postulated that, in addition to responsiveness to oxidative attacks from outside, redox regulation of specific redox-sensitive proteins likely represents an important physiological mechanism.

Bowman-Birk protease inhibitor reduces the radiation-induced activation of the EGF receptor and induces tyrosine phosphatase activity.

PURPOSE: To investigate the molecular action of the radioprotective Bowman Birk protease inhibitor (BBI) on radiation-induced tyrosine kinase activity. MATERIALS AND METHODS: Radiation-induced activation of tyrosine kinases and phosphatases was measured in normal human skin fibroblasts by in vitro kinase assays after pre-incubation with BBI. RESULTS: Pre-incubation with BBI resulted in a time-dependent block of the radiation-induced activation of tyrosine kinases. Whilst radiation-induced pp60c-Src activity was not modulated due to BBI pre-treatment, activation of epidermal growth factor receptor (EGFR) was inhibited. Additionally, pre-incubation with BBI resulted in enhanced tyrosine specific phosphatase (PTP) activity. CONCLUSIONS: BBI might exert its radioprotective activity by stabilizing specific tyrosine-phosphatases that interfere with EGFR activation in response to radiation exposure.

The stress response to ionizing radiation involoves c-Abl-dependent phosphorylation of SHPTP1.

c-Abl is a nonreceptor tyrosine kinase that is activated by certain DNA-damaging agents. The present studies demonstrate that nuclear c-Abl binds constitutively to the protein tyrosine phosphatase SHPTP1. Treatment with ionizing radiation is associated with c-Abl-dependent tyrosine phosphorylation of SHPTP1. The results demonstrate that the SH3 domain of c-Abl interacts with a WPDHGVPSEP motif (residues 417-426) in the catalytic domain of SHPTP1 and that c-Abl phosphorylates C terminal Y536 and Y564 sites. The functional significance of the c-Abl-SHPTP1 interaction is supported by the demonstration that, like c-Abl, SHPTP1 regulates the induction of Jun kinase activity following DNA damage. These findings indicate that SHPTP1 is involved in the response to genotoxic stress through a c-Abl-dependent mechanism.

Wip1, a novel human protein phosphatase that is induced in response to ionizing radiation in a p53-dependent manner.

Exposure of mammalian cells to ionizing radiation (IR) induces a complex array of cellular responses including cell cycle arrest and/or apoptosis. IR-induced G1 arrest has been shown to depend on the presence of the tumor suppressor p53, which acts as a transcriptional activator of several genes. p53 also plays a role in the induction of apoptosis in response to DNA damage, and this pathway can be activated by both transcription-dependent and -independent mechanisms. Here we report the identification of a novel transcript whose expression is induced in response to IR in a p53-dependent manner, and that shows homology to the type 2C protein phosphatases. We have named this novel gene, wip1. In vitro, recombinant Wip1 displayed characteristics of a type 2C phosphatase, including Mg2+ dependence and relative insensitivity to okadaic acid. Studies performed in several cell lines revealed that wip1 accumulation following IR correlates with the presence of wild-type p53. The accumulation of wip1 mRNA following IR was rapid and transient, and the protein was localized to the nucleus. Similar to waf1, ectopic expression of wip1 in human cells suppressed colony formation. These results suggest that Wip1 might contribute to growth inhibitory pathways activated in response to DNA damage in a p53-dependent manner.

Ionizing radiation and TNF-alpha stimulate gene expression of a Thr/Tyr-protein phosphatase HVH1 and inhibitory factor IkappaB alpha in human squamous carcinoma cells.

Exposure of cells to ionizing radiation (IR) or tumor necrosis factor-alpha (TNF-alpha) results in the stimulation of the DNA binding activities of transcription factors, AP-1 and NF-kappaB. HVH1/CL100, a dual specificity protein phosphatase, may attenuate the AP-1 response by dephosphorylating a key upstream element, mitogen-activated protein kinase (MAPK). The members of IkappaB family of proteins regulate the NF-kappaB response. We examined the effects of IR and TNF-alpha on HVH1 and IkappaB alpha gene expression. Our data demonstrate that IR or TNF-alpha treatment of head and neck squamous carcinoma cells (PCI-04A) increased the steady-state levels of HVH1 and IkappaB alpha mRNAs; however, the induction patterns were different. TNF-alpha treatment led to a relatively prolonged stimulation of HVH1 and IkappaB alpha mRNAs lasting at least 7 h, while IR caused a transient stimulation of these mRNAs and the expression returned to basal levels within 6 h post-IR treatment. Treatment of cells with cycloheximide did not prevent the IR orTNF-alpha-inducible expression of HVH1 and IkappaB alpha genes, indicating that these responses were independent of the new protein synthesis. These data imply that protein phosphatase HVH1 and regulatory factor IkappaB alpha may play important roles in cellular response to IR and TNF-alpha. In addition, the kinetics of responsiveness indicates that the mechanisms of IR and TNF-alpha-induced signalling are distinct.

Mutation of the hematopoietic cell phosphatase (Hcph) gene is associated with resistance to gamma-irradiation-induced apoptosis in Src homology protein tyrosine phosphatase (SHP)-1-deficient "motheaten" mutant mice.

To determine the role of Src homology protein tyrosine phosphatase (SHP-1) in the ionizing radiation-induced stress response, we analyzed the apoptotic response and cell cycle function in irradiated spleen cells of motheaten (me/me) mice. The defect in me/me mice has been attributed to mutations of the HCPH: gene, which encodes SHP-1. Homozygotes develop severe systemic autoimmune and inflammatory disease, whereas heterozygotes live longer and develop hematopoietic and lymphoid malignance. Spleen cells from C57BL/6 (B6)-me/me and B6-+/+ controls were analyzed after gamma-irradiation from a (137)Cs source. B6-me/me cells were significantly more resistant than B6-+/+ cells to gamma-irradiation-induced apoptosis exhibiting a higher LD(50). The defective apoptosis response of the B6-me/me cells was exhibited by T and B cells and macrophages. Of the Bcl-2 family members analyzed, a significant difference was observed in the transcription of Bax mRNA, which was up-regulated early after irradiation in B6-+/+ cells, but not B6-me/me cells. Analysis of 3,3'-dihexyloxacarbocyanine iodide revealed resistance to the gamma-irradiation-induced mitochondrial transmembrane permeability transition in the B6-me/me cells. The blocking of the cell cycle in the G(0)/G(1) phase characteristic of the irradiated B6-+/+ cells was not observed in the B6-me/me cells. There was decreased phosphorylation of p38 mitogen-activated protein kinase and increased phosphorylation of p53 from spleen cell lysates of irradiated B6-me/me mice compared with wild-type mice. These data suggest that SHP-1 plays an important role in regulation of apoptosis and cell cycle arrest after a gamma-irradiation-induced stress response.