Kinetic comparison of the catalytic domains of SHP-1 and SHP-2

Intramolecular Interaction with the E6 Region Stabilizes the Closed Conformation of the N-SH2 Domain and Concurs with the Self-Inhibitory Docking in Downregulating the Activity of the SHP2 Tyrosine Phosphatase: A Molecular Dynamics Study

The localization and activity of the SHP2 tyrosine phosphatase across different cellular compartments to the target substrates are steered by the binding of phosphotyrosine (pY) peptides to the tandem SH2 domains. The most N-terminal domain (N-SH2) can also keep the enzyme inactive by intramolecular occlusion of the catalytic site. Enzyme activity can be recovered by an allosteric disruption of this self-inhibitory docking upon the binding of pY peptides to the N-SH2 domain. Prior to this, the N-SH2 domain must abandon the closed conformation because it impedes the access of pY peptides to the binding cleft. Although it cooperates with the self-inhibitory docking in the negative regulation of the phosphatase activity, the structural determinants of the stability of the closed conformation in the self-inhibited phosphatase are still elusive. To address this issue, a molecular dynamics simulation study is carried out. It is shown that the closed conformation is stabilized by the interaction of the N-SH2 domain with a conserved peptide portion in the region encoded by PTPN11 exon 6 (E6).

Cell cycle regulation by oncogenic tyrosine kinases in myeloid neoplasias: from molecular redox mechanisms to health implications

Neoplastic expansion of myeloid cells is associated with specific genetic changes that lead to chronic activation of signaling pathways, as well as altered metabolism. It has become increasingly evident that transformation relies on the interdependency of both events. Among the various genetic changes, the oncogenic BCR-ABL tyrosine kinase in patients with Philadelphia chromosome positive chronic myeloid leukemia (CML) has been a focus of extensive research. Transformation by this oncogene is associated with elevated levels of intracellular reactive oxygen species (ROS). ROS have been implicated in processes that promote viability, cell growth, and regulation of other biological functions such as migration of cells or gene expression. Currently, the BCR-ABL inhibitor imatinib mesylate (Gleevec) is being used as a first-line therapy for the treatment of CML. However, BCR-ABL transformation is associated with genomic instability, and disease progression or resistance to imatinib can occur. Imatinib resistance is not known to cause or significantly alter signaling requirements in transformed cells. Elevated ROS are crucial for transformation, making them an ideal additional target for therapeutic intervention. The underlying mechanisms leading to elevated oxidative stress are reviewed, and signaling mechanisms that may serve as novel targeted approaches to overcome ROS-dependent cell growth are discussed.

The neurite outgrowth multiadaptor RhoGAP, NOMA-GAP, regulates neurite extension through SHP2 and Cdc42

Neuronal differentiation involves the formation and extension of neuronal processes. We have identified a novel regulator of neurite formation and extension, the neurite outgrowth multiadaptor, NOMA-GAP, which belongs to a new family of multiadaptor proteins with RhoGAP activity. We show that NOMA-GAP is essential for NGF-stimulated neuronal differentiation and for the regulation of the ERK5 MAP kinase and the Cdc42 signaling pathways downstream of NGF. NOMA-GAP binds directly to the NGF receptor, TrkA, and becomes tyrosine phosphorylated upon receptor activation, thus enabling recruitment and activation of the tyrosine phosphatase SHP2. Recruitment of SHP2 is required for the stimulation of neuronal process extension and for sustained activation of ERK5 downstream of NOMA-GAP. In addition, we show that NOMA-GAP promotes neurite outgrowth by tempering activation of the Cdc42/PAK signaling pathway in response to NGF. NOMA-GAP, through its dual function as a multiadaptor and RhoGAP protein, thus plays an essential role downstream of NGF in promoting neurite outgrowth and extension.

Expression of p58.2 or CD94/NKG2A inhibitory receptors in an NK-like cell line, YTINDY, leads to HLA Class I-mediated inhibition of cytotoxicity in the p58.2- but not the CD94/NKG2A-expressing transfectant

Natural killer cytotoxicity is down-regulated by HLA Class I-specific inhibitory receptors classified as killer inhibitory receptors (KIRs) or C-type lectins. The regulation of their inhibitory signaling pathways is not completely understood. The YTINDY NK-like cell line was transfected to express p58.2 KIR (YT/C143 transfectant) or CD94/NKG2A C-type lectin (YT/CD94 transfectant); and YT/C143, but not YT/CD94, cytotoxicity was down-regulated by Class I. YT/C143 and YT/CD94 expressed equally low p56(lck) levels, suggesting that p56(lck) is not absolutely required for p58.2 signaling but may be required for CD94/NKG2A signaling. Lower SHP-1 levels and activity were observed in YT/CD94 compared to YT/C143. However, increasing SHP-1 to equivalent levels in YT/C143 did not restore inhibition in YT/CD94. Our results suggest that the combination of low p56(lck) and SHP-1 levels may be responsible for the absent inhibitory signal in YT/CD94. In addition, the possible expression of CD94/NKG2C activating receptor may override inhibitory signals transduced through CD94/NKG2A.

Relation between the flexibility of the WPD loop and the activity of the catalytic domain of protein tyrosine phosphatase SHP-1

The conserved WPD loop of protein tyrosine phosphatases play an important role in the catalytic activity and the invariant aspartate residue acts as a general acid/base catalyst in the dephosphorylation reaction. In our previous report, we have demonstrated that the catalytic activities of the PTPs are influenced by the flexibility and stability of the WPD loop in its active "open" conformation [Yang et al., 1998]. Phosphatases with a more flexible WPD loop generally have higher specific activity. In this report, we modify the WPD loop of SHP-1 by alanine-scan mutation of the residues flanking the loop and measure their effects on the catalytic activity of the phosphatase. We show that the S418A, V424A, S426A, E427A, and P428A mutants increase the phosphatase activity, possibly due to the increased flexibility of the WPD loop, whereas the L417A, L417G and P425A mutants decrease its phosphatase activity. In addition, we propose that the two-proline residues in the WPD loop (Pro(420) and Pro(425) in SHP-1) work as pivotal points through a conserved hydrophobic network and allows residues between the pivotal points to have maximum flexibility in enhancing the phosphatase activity. Furthermore, our data suggest that the hydrolysis of the phosphoryl-cysteine intermediate, not its formation, is the rate-limiting step with p-nitrophenyl phosphate as the substrate while both the steps are rate-limiting with phosphotyrosine as the substrate.

Assay of protein tyrosine phosphatases by using matrix-assisted laser desorption ionization time-of-flight mass spectrometry

A nonradioactive assay for protein tyrosine phosphatases (PTPs), employing a tyrosine-phosphorylated peptide as a substrate, has been developed and applied to analyze purified enzymes, cell extracts, and immunoprecipitates. The reaction was followed by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) in a linear and positive ion mode with delayed extraction. MALDI-TOF MS detects a loss of peptide mass by 80 Da as a result of dephosphorylation and, more importantly, it yields phospho-peptide to dephosphorylated product peak intensity ratios proportional to their concentration ratios. A strong bias of the MALDI-TOF MS toward detection of the non-phospho-peptide allows accurate detection of small fractions of dephosphorylation. The method is highly sensitive and reproducible. It can be applied to general assays of protein phosphatases with various phospho-peptides as substrates.

BCR/ABL inhibition by an escort/phosphatase fusion protein

Cellular transformation by the BCR/ABL oncogene depends on the ABL-encoded tyrosine kinase activity. To block BCR/ABL function, we created a unique tyrosine phosphatase by fusing the catalytic domain of SHP1 (SHP1c) to the ABL binding domain (ABD) of RIN1, an established binding partner and substrate for c-ABL and BCR/ABL. This fusion construct (ABD/SHP1c) binds to BCR/ABL in cells and functions as an active phosphatase. ABD/SHP1c effectively suppressed BCR/ABL function as judged by reductions in transformation of fibroblast cells, growth factor independence of hematopoietic cell lines, and proliferation of primary bone marrow cells. In addition, the leukemogenic properties of BCR/ABL in a murine model system were blocked by coexpression of ABD/SHP1c. Both the "escort" function provided by ABD and the inhibitor function provided by the phosphatase of SHP1c were necessary for effective BCR/ABL interference. Expression of ABD/SHP1c also reversed the transformed phenotype of K562, a human leukemia-derived cell line. These results have direct implications for leukemia therapeutics and suggest an approach to block aberrant signal transduction in other pathologies through the use of appropriately designed escort/inhibitors.

The BCR/ABL tyrosine kinase induces production of reactive oxygen species in hematopoietic cells

The BCR/ABL oncogene causes chronic myelogenous leukemia, a myeloproliferative disorder characterized by clonal expansion of hematopoietic progenitor cells and myeloid cells. It is shown here that transformation of the hematopoietic cell lines Ba/F3, 32Dcl3, and MO7e with BCR/ABL results in an increase in reactive oxygen species (ROS) compared with quiescent, untransformed cells. The increase in ROS was directly due to BCR/ABL because it was blocked by the ABL-specific tyrosine kinase inhibitor STI571. Oxidative stress through ROS is believed to have many biochemical effects, including the potential ability to inhibit protein-tyrosine phosphatases (PTPases). To understand the significance of increased production of ROS, a model system was established in which hydrogen peroxide (H(2)O(2)) was added to untransformed cells to mimic the increase in ROS induced constitutively by BCR/ABL. H(2)O(2) substantially reduced total cellular PTPase activity to a degree approximately equivalent to that of pervanadate, a well known PTPase inhibitor. Further, stimulation of untransformed cells with H(2)O(2) or pervanadate increased tyrosine phosphorylation of each of the most prominent known substrates of BCR/ABL, including c-ABL, c-CBL, SHC, and SHP-2. Treatment of the BCR/ABL-expressing cell line MO7/p210 with the reducing agents pyrrolidine dithiocarbamate or N-acetylcysteine reduced the accumulation of ROS and also decreased tyrosine phosphorylation of cellular proteins. Further, treatment of MO7e cells with H(2)O(2) or pervanadate increased the tyrosine kinase activity of c-ABL. Drugs that alter ROS metabolism or reactivate PTPases may antagonize BCR/ABL transformation.

Association of the N- and C-terminal domains of phospholipase D. Contribution of the conserved HKD motifs to the interaction and the requirement of the association for Ser/Thr phosphorylation of the enzyme

Rat brain phospholipase D1 (rPLD1) belongs to a superfamily defined by the highly conserved catalytic motif (H(X)K(X)(4)D, denoted HKD. rPLD1 contains two HKD domains, located in the N- and C-terminal regions. The integrity of the two HKD domains is essential for enzymatic activity. Our previous studies showed that the N-terminal half of rPLD1 containing one HKD motif can associate with the C-terminal half containing the other HKD domain to reconstruct wild type PLD activity (Xie, Z., Ho, W.-T. and Exton, J. H. (1998) J. Biol. Chem. 273, 34679-34682). In the present study, we have shown by mutagenesis that conserved amino acids in the HKD domains are important for both the catalytic activity and the association between the two halves of rPLD1. Furthermore, we found that rPLD1 could be modified by Ser/Thr phosphorylation. The modification occurred at the N-terminal half of the enzyme, however, the association of the N-terminal domain with the C-terminal domain was required for the modification. The phosphorylation of the enzyme was not required for its catalytic activity or response to PKCalpha and small G proteins in vitro, although the phosphorylated form of rPLD1 was localized exclusively in the crude membrane fraction. In addition, we found that the individually expressed N- and C-terminal fragments did not interact when mixed in vitro and were unable to reconstruct PLD activity under these conditions. It is concluded that the association of the N- and C-terminal halves of rPLD1 requires their co-expression in vivo and depends on conserved residues in the HKD domains. The association is also required for Ser/Thr phosphorylation of the enzyme.