Mercuric chloride activates the Src-family protein tyrosine kinase, Hck in myelomonocytic cells

Protein phosphorylation profiling identifies potential mechanisms for direct immunotoxicity

Signaling networks are essential elements that are involved in diverse cellular processes. One group of fundamental components in various signaling pathways concerns protein tyrosine kinases (PTK). Various toxicants have been demonstrated to exert their toxicity via modulation of tyrosine kinase activity. The present study aimed to identify common cellular signaling pathways that are involved in chemical-induced direct immunotoxicity. To this end, an antibody array-based profiling approach was applied to assess effects of five immunotoxicants, two immunosuppressive drugs and two non-immunotoxic control chemicals on the phosphorylation of 28 receptor tyrosine kinases and 11 crucial signaling nodes in Jurkat T-cells. The phosphorylation of ribosomal protein S6 (RPS6) and of kinases Akt, Src and p44/42 were found to be commonly regulated by immunotoxicants and/or immunosuppressive drugs (at least three compounds), with the largest effect observed upon RPS6. Flow cytometry and Western blotting were used to further examine the effect of the model immunotoxicant TBTO on the components of the mTOR-p70S6K-RPS6 pathway. These analyses revealed that both TBTO and the mTOR inhibitor rapamycin inactivate RPS6, but via different mechanisms. Finally, a comparison of the protein phosphorylation data to previously obtained transcriptome data of TBTO-treated Jurkat cells resulted in a good correlation at the pathway level and indicated that TBTO affects ribosome biogenesis and leukocyte migration. The effect of TBTO on the latter process was confirmed using a CXCL12 chemotaxis assay.

Dual acylation and lipid raft association of Src-family protein tyrosine kinases are required for SDF-1/CXCL12-mediated chemotaxis in the Jurkat human T cell lymphoma cell line

Chemokines play pivotal roles in regulating a wide variety of biological processes by modulating cell migration and recruitment. Deregulation of chemokine signaling can alter cell recruitment, contributing to the pathogenic states associated with autoimmune disease, inflammatory disorders, and sepsis. During chemotaxis, lipid rafts and their resident signaling molecules have been demonstrated to partition to different parts of the cell. Herein, we investigated the role of lipid raft resident Src-family kinases (SFK) in stromal cell-derived factor 1/CXCL12-mediated chemotaxis. We have shown that Lck-deficient J.CaM 1.6 cells are defective in CXCL12-mediated chemotaxis in contrast to their parental counterpart, Jurkat cells. Ectopic expression of the SFK hematopoietic cell kinase (Hck) in J.CaM 1.6 cells reconstituted CXCL12 responsiveness. The requirement of lipid raft association of SFK was assessed using both isoforms of Hck: the dually acylated p59(Hck) isoform that is targeted to lipid rafts and the monoacylated p61(Hck) isoform that is nonraft-associated. We have shown using several gain and loss of acylation alleles that dual acylation of Hck was required for CXCL12-mediated chemotaxis in J.CaM 1.6 cells. These results highlight the importance of the unique microenvironment provided by lipid rafts and their specific contribution in providing specificity to CXCL12 signaling.

Regulation of p73 by Hck through kinase-dependent and independent mechanisms

Background

p73, a p53 family member is a transcription factor that plays a role in cell cycle, differentiation and apoptosis. p73 is regulated through post translational modifications and protein interactions. c-Abl is the only known tyrosine kinase that phosphorylates and activates p73. Here we have analyzed the role of Src family kinases, which are involved in diverse signaling pathways, in regulating p73.

Results

Exogenously expressed as well as cellular Hck and p73 interact in vivo. In vitro binding assays show that SH3 domain of Hck interacts with p73. Co-expression of p73 with Hck or c-Src in mammalian cells resulted in tyrosine phosphorylation of p73. Using site directed mutational analysis, we determined that Tyr-28 was the major site of phosphorylation by Hck and c-Src, unlike c-Abl which phosphorylates Tyr-99. In a kinase dependent manner, Hck co-expression resulted in stabilization of p73 protein in the cytoplasm. Activation of Hck in HL-60 cells resulted in tyrosine phosphorylation of endogenous p73. Both exogenous and endogenous Hck localize to the nuclear as well as cytoplasmic compartment, just as does p73. Ectopically expressed Hck repressed the transcriptional activity of p73 as determined by promoter assays and semi-quantitative RT-PCR analysis of the p73 target, Ipaf and MDM2. SH3 domain- dependent function of Hck was required for its effect on p73 activity, which was also reflected in its ability to inhibit p73-mediated apoptosis. We also show that Hck interacts with Yes associated protein (YAP), a transcriptional co-activator of p73, and shRNA mediated knockdown of YAP protein reduces p73 induced Ipaf promoter activation.

Conclusion

We have identified p73 as a novel substrate and interacting partner of Hck and show that it regulates p73 through mechanisms that are dependent on either catalytic activity or protein interaction domains. Hck-SH3 domain-mediated interactions play an important role in the inhibition of p73-dependent transcriptional activation of a target gene, Ipaf, as well as apoptosis.

Micromolar HgCl2 concentrations transitorily duplicate the ATP level in Saccharomyces cerevisiae cells

Low concentrations of HgCl2 elicited, in Saccharomyces cerevisiae, a transitory increase in the ATP level followed by a decrease of its concentration, until almost disappearance. At 1 microM HgCl2, the increase in ATP lasted for about 30 min, while at 10 microM the increase was only observed in the first 5 min of treatment. The initial burst of ATP was accompanied by a decrease in the level of hexose phosphates, whereas during the decrease of ATP an increase in the inosine and hexose phosphates levels took place. The treatment with HgCl2 inhibited the plasma membrane proton ATPase but not the activities of hexokinase or 6-phosphofructokinase.

The calcium-dependent activity of large-conductance, calcium-activated K+channels is enhanced by Pyk2- and Hck-induced tyrosine phosphorylation

Recent results showing that large-conductance, calcium-activated K+(BKCa) channels undergo direct tyrosine phosphorylation in the presence of c-Src tyrosine kinase have suggested the involvement of these channels in Src-mediated signaling pathways. Given the important role for c-Src in integrin-mediated signal transduction, we have examined the potential regulation of BKCachannels by proline-rich tyrosine kinase 2 (Pyk2), a calcium-sensitive tyrosine kinase activated upon integrin stimulation. Transient coexpression of murine BKCachannels with either wild-type Pyk2 or hematopoietic cell kinase (Hck), a Src-family kinase, led to an enhancement of BKCachannel activity over the range of 1–10 μM free calcium, whereas coexpression with catalytically inactive forms of either kinase did not significantly alter BKCagating compared with channels expressed alone. In the presence of either wild-type Pyk2 or Hck, BKCaα-subunits were found to undergo tyrosine phosphorylation, as determined by immunoprecipitation and Western blotting strategies. However, tyrosine phosphorylation of the BKCaα-subunit was not detected for channels expressed alone or together with inactive forms of either Pyk2 or Hck. Interestingly, wild-type, but not inactive, Pyk2 was also present in BKCachannel immunoprecipitates, suggesting that Pyk2 may coassociate with the BKCachannel complex after phosphorylation. Collectively, the observed modulation and phosphorylation of BKCachannels by Pyk2 and a Src-family kinase may reflect a general cellular mechanism by which G protein-coupled receptor and/or integrin activation leads to the regulation of membrane ion channels.

Physical and functional interaction between Hck tyrosine kinase and guanine nucleotide exchange factor C3G results in apoptosis, which is independent of C3G catalytic domain

The hematopoietic cell kinase Hck is a Src family tyrosine kinase expressed in cells of myelomonocytic lineage, B lymphocytes, and embryonic stem cells. To study its role in signaling pathways we used the Hck-SH3 domain in protein interaction cloning and identified C3G, the guanine nucleotide exchange factor for Rap1 and R-Ras, as a protein that associated with Hck. This interaction was direct and was mediated partly through the proline-rich region of C3G. C3G could be co-immunoprecipitated with Hck from Cos-1 cells transfected with Hck and C3G. C3G was phosphorylated on tyrosine 504 in cells when coexpressed with Hck but not with a catalytically inactive mutant of Hck. Phosphorylation of endogenous C3G at Tyr-504 was increased by treatment of human myelomonocytic THP-1 cells with mercuric chloride, which is known to activate Hck tyrosine kinase specifically. Coexpression of Hck with C3G induced a high level of apoptosis in many cell lines by 30-42 h of transfection. Induction of apoptosis was not dependent on Tyr-504 phosphorylation or the catalytic domain of C3G but required the catalytic activity of Hck. Using dominant negative constructs of caspases we found that caspase-1, -8, and -9 are involved in this apoptotic pathway. These results suggest that C3G and Hck interact physically and functionally in vivo to activate kinase-dependent and caspase-mediated apoptosis, which is independent of catalytic domain of C3G.

Redox-linked cell surface-oriented signaling for T-cell death

T-cell death, which occurs either for ontogenic T-cell selection or for activated T-cell elimination, is normally induced through binding of a specific ligand to cell-surface T-cell receptor for crosslinkage. Heavy metals and carbonyl compounds that bind to protein-reactive groups such as cysteine sulfhydryl groups and lysine epsilon-amino groups may also induce crosslinkage of cell-surface proteins, in part replacing or modifying the ligand-mediated action. This chemical event has been found to accompany clustering of membrane rafts, to which signal-transducing elements such as glycosylphosphatidylinositol-anchored proteins and Src family protein tyrosine kinases (PTKs) are attached, and to trigger the signal transduction for apoptotic T-cell death, inducing mitochondrial membrane potential reduction, caspase activation and DNA fragmentation. As signals potentially upstream of this signaling, activations of PTKs and mitogen-activated protein (MAP) family kinases and production of reactive oxygen species (ROS) were induced following the cell-surface event, and crucial roles of activation of c-Jun amino-terminal kinase and apoptosis signal-regulating kinase 1 by a redox-linked mechanism in the cell-death signaling were demonstrated. Intriguingly, ROS production as well as PTK/MAP family kinase activation occurred in a membrane raft integrity-dependent manner. The redox-linked and cell surface-oriented signal delivery pathway demonstrated here may play an important role in induction of immune disorders by protein reactive group-binding chemicals.