HPK1 is a Ste20-related serine-threonine kinase that inducibly associates with the adaptors SLP-76 and Gads after T cell receptor (TCR) signaling. Here, HPK1 deficiency resulted in enhanced TCR-induced phosphorylation of SLP-76, phospholipase C-gamma1 and the kinase Erk, more-persistent calcium flux, and increased production of cytokines and antigen-specific antibodies. Furthermore, HPK1-deficient mice were more susceptible to experimental autoimmune encephalomyelitis. Although the interaction between SLP-76 and Gads was unaffected, the inducible association of SLP-76 with 14-3-3tau (a phosphorylated serine-binding protein and negative regulator of TCR signaling) was reduced in HPK1-deficient T cells after TCR stimulation. HPK1 phosphorylated SLP-76 and induced the interaction of SLP-76 with 14-3-3tau. Our results indicate that HPK1 negatively regulates TCR signaling and T cell-mediated immune responses.

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Transforming growth factor ␤ (TGF-␤)-activated kinase (TAK1) is known for its involvement in TGF-␤ signaling and its ability to activate the p38-mitogen-activated protein kinase (MAPK) pathway. This report shows that TAK1 is also a strong activator of c-Jun Nterminal kinase (JNK). Both the wild-type and a constitutively active mutant of TAK1 stimulated JNK in transient transfection assays. Mitogen-activated protein kinase kinase 4 (MKK4)/stress-activated protein kinase/ extracellular signal-regulated kinase (SEK1), a dualspecificity kinase that phosphorylates and activates JNK, synergized with TAK1 in activating JNK. Conversely, a dominant-negative (MKK4/SEK1 mutant inhibited TAK1-induced JNK activation. A kinasedefective mutant of TAK1 effectively suppressed hematopoietic progenitor kinase-1 (HPK1)-induced JNK activity but had little effect on germinal center kinase activation of JNK. There are two additional MAPK kinase kinases, MEKK1 and mixed lineage kinase 3 (MLK3), that are also downstream of HPK1 and upstream of MKK4/SEK mutant. However, because the dominant-negative mutants of MEKK1 and MLK3 did not inhibit TAK1-induced JNK activity, we conclude that activation of JNK1 by TAK1 is independent of MEKK1 and MLK3. In addition to TAK1, TGF-␤ also stimulated JNK activity. Taken together, these results identify TAK1 as a regulator in the HPK1 3 TAK1 3 MKK4/ SEK1 3 JNK kinase cascade and indicate the involvement of JNK in the TGF-␤ signaling pathway. Our results also suggest the potential roles of TAK1 not only in the TGF-␤ pathway but also in the other HPK1/ JNK1-mediated pathways.To date, three related mitogen-activated protein kinase (MAPK) 1 cascades that propagate signals from the plasma membrane to the interior of mammalian cells have been identified (1). These three groups of the MAP kinase family include the extracellular signal-regulated kinases (ERKs) (2), p38-MAPKs (3), and the c-Jun N-terminal kinases (JNKs), also known as the stress-activated protein kinases (SAPKs) (4, 5). The ERKs are the central elements in mitogenic signal transduction downstream of Ras (2). p38-MAPKs seem to participate in the signaling pathways of pro-inflammatory cytokines and environmental stresses, such as interleukin-1, tumor necrosis factor ␣, and osmotic shock (1, 3). JNKs/stress-activated protein kinases play a crucial role in the responses stimulated by pro-inflammatory cytokines, environmental stresses, and apoptotic agents (4 -7). Considerable progress has been achieved in identifying the upstream kinases of the JNK cascade. JNK can be phosphorylated and activated by its immediate upstream kinase MAPK kinase 4/SEK1 which itself can be phosphorylated and activated by MEKK1 (8,9). Together with the recent findings from our laboratory that hematopoietic progenitor kinase-1 (HPK1) directly binds to and phosphorylates MEKK1, a simple JNK pathway paradigm can be drawn: HPK1 3 MEKK1 3 MAPK kinase 4/SEK1 3 JNK (10). However, the JNK cascade, like other MAPK cascades, is complicated by the fact that several other kinases su...

MAPKs are activated by a wide range of diverse stimuli and are essential for various cellular processes, such as stress responses, apoptosis, proliferation, differentiation, and early embryonic development (1-3). The prototypical MAPK signaling cascade is a three-kinase module, consisting of MAP kinase kinase kinase (MAP3K), MAP kinase kinase (MAP2K), and MAPK (1, 2, 4). The upstream molecules that link the MAPK module to extracellular stimuli include small G proteins and a group of mammalian Ste20-like kinases, including hematopoietic progenitor kinase 1 (HPK1), germinal center kinase (GCK), HKP1/germinal center kinase-like kinase, germinal center kinase-like kinase (GLK), and kinase homologous to Ste20 (KHS), which have been characterized as potential MAP kinase kinase kinase kinases (MAP4Ks) for the JNK pathway (1, 2, 4, 5). Within the three-kinase module, MAPKs are phosphorylated on both threonine and tyrosine residues within their signature sequence TXY motif by a dual specificity protein kinase MAP2K. These motifs include TEY in ERK, TPY in JNK, and TGY in p38. MAP2K are activated by phosphorylation of serine/threonine residues by MAP3Ks (1, 2, 4). In the case of ERK1/2, phosphorylation of the TEY motif also contributes to the dimerization and nuclear translocation of ERK1/2 in addition to mediating its activation (6). The extracellular stimuli-induced activation of MAPKs is transient under many conditions, and it has been well established that protein phosphatases play an essential role in the down-regulation of MAP kinases. A variety of classes of protein phosphatases, including tyrosine-specific protein phosphatases, serine/threonine protein phosphatases, and a family of dual specificity protein phosphatases (DSPs), have been implicated in the negative regulation of MAPKs (7-9). Among them, DSPs are the major group of phosphatases that contribute to the regulated inactivation of MAP kinases by dephosphorylating both phosphotyrosine and phosphothreonine residues within the TXY motif, thus also called MAP kinase phosphatases (MKPs) (7-9). Most MKPs identified so far consist of a conserved catalytic region and an extended regulatory region. However, some MKPs lack this regulatory region, such as VH1 (10) and VH1-related (VHR) phosphatase (11). The regulatory

Rhubarb is one of the most ancient, commonly used and important herbs in Chinese medicine. The modern researches of rhubarb clarified the efficacies, ingredients and mechanisms in a more scientific and rigorous way. The main chemical compositions of rhubarb include anthraquinones, anthrones, stilbenes, tannins, polysaccharides etc. These compositions show extensive pharmacological activities including regulating gastrointestinal, anticancer, antimicrobial, hepatoprotective, anti-inflammatory, protecting cardiovascular, cerebrovascular and so on. This paper reviews the recent studies on the active ingredients, pharmacological effects, clinical application and functional mechanism.