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T H Tan

Publications and source records attributed to T H Tan.

At least 73 records · Page 4Linked to original sources
PubMed1996

Molecular cloning and characterization of a novel protein kinase with a catalytic domain homologous to mitogen-activated protein kinase kinase kinase.

Mitogen-activated protein kinase (MAPK) signaling cascades include MAPK or extracellular signal-regulated kinase (ERK), MAPK kinase (MKK or MEK), and MAPK kinase kinase (MAPKKK or MEKK). MAPKK kinase/MEKK phosphorylates and activates its downstream protein kinase, MAPK kinase/MEK, which in turn activates MAPK. We report herein the isolation of a cDNA encoding a novel protein kinase designated MAPKKK5 from a human macrophage library. The nucleotide sequence predicts that MAPKKK5 encodes an open reading frame of 1374 amino acids with all 11 kinase subdomains. The putative catalytic domain of MAPKKK5 shows significant sequence homology to the kinase domains of the MAPKKK/MEKK level protein kinases from mouse MEKK2 and -3, Drosophila melanogaster PK92B, Saccharomyces cerevisiae STE11, and Schizosaccharomyces pombe BYR2. Northern blot analysis showed that MAPKKK5 transcript is abundantly expressed in human heart and pancreas. When transiently expressed in COS and 293 cells, MAPKKK5 markedly activated c-Jun N-terminal kinase or stress-activated protein kinase, but not MAPK/ERK. Furthermore, MAPKKK5 that was immunoprecipitated from transfected 293 cells was able to phosphorylate and activate MKK4 in vitro, suggesting that MAPKKK5 may be an upstream activator of MKK4 in the c-Jun N-terminal kinase pathway.

Amino Acid Sequence↗
PubMed1996

Adriamycin activates c-jun N-terminal kinase in human leukemia cells: a relevance to apoptosis.

We studied the activation of c-jun N-terminal kinase 1 (JNK 1) and extracellular signal-regulated kinases 1 and 2 (ERK 1/2) of mitogen-activated protein kinase (MAPK) family by adriamycin (ADR) in the human T cell leukemia line, H9. ADR caused an elevation of JNK1 activity at sublethal or lethal concentrations; however, at lower doses, ADR did not activate JNK1. The induction of JNK1 peaked at 4 h of treatment (about ten-fold over the control), and was sustained up to 5 h post-treatment. This induction preceded the onset of apoptosis, as determined by morphological features and internucleosomal degradation of DNA. Upon treatment of cells with JNK1-inducing doses, ADR caused an elevation of steady-state levels of c-jun and ATF3 mRNAs, as measured by RT-PCR. In contrast, the activity of ERK 1/2 remained unchanged throughout the treatments, indicating that members of MAPK family are differentially regulated in ADR-treated cells. A possible role of JNK1 activation in ADR-induced apoptosis is discussed.

Antibiotics, Antineoplastic↗
PubMed1996

Human HPK1, a novel human hematopoietic progenitor kinase that activates the JNK/SAPK kinase cascade.

The c-Jun amino-terminal kinases (JNKs)/stress-activated protein kinases (SAPKs) play a crucial role in stress responses in mammalian cells. The mechanism underlying this pathway in the hematopoietic system is unclear, but it is a key in understanding the molecular basis of blood cell differentiation. We have cloned a novel protein kinase, termed hematopoietic progenitor kinase 1 (HPK1), that is expressed predominantly in hematopoietic cells, including early progenitor cells. HPK1 is related distantly to the p21(Cdc42/Rac1)-activated kinase (PAK) and yeast STE20 implicated in the mitogen-activated protein kinase (MAPK) cascade. Expression of HPK1 activates JNK1 specifically, and it elevates strongly AP-1-mediated transcriptional activity in vivo. HPK1 binds and phosphorylates MEKK1 directly, whereas JNK1 activation by HPK1 is inhibited by a dominant-negative MEKK1 or MKK4/SEK mutant. Interestingly, unlike PAK65, HPK1 does not contain the small GTPase Rac1/Cdc42-binding domain and does not bind to either Rac1 or Cdc42, suggesting that HPK1. activation is Rac1/Cdc42-independent. These results indicate that HPK1 is a novel functional activator of the JNK/SAPK signaling pathway.

Amino Acid Sequence↗
PubMed1996

Phenethyl isothiocyanate, a natural chemopreventive agent, activates c-Jun N-terminal kinase 1.

Phenethyl isothiocyanate (PEITC) and other structurally related compounds are potent chemopreventive agents in a number of experimental models of cancer in animals. The mechanisms of cancer protection by these agents are not clear but may involve the regulation of gene expression, such as that by Phase II detoxifying enzymes. To unveil the upstream signaling events that lead to the potential transcriptional activation of genes, we studied the involvement of mitogen-activated protein kinase, c-Jun N-terminal kinase 1 (JNK1), and extracellular signal-regulated kinase 1 and 2 cascades, which have been shown to mediate numerous types of extracellular signals. On treatment of human ovarian HeLa cells with PEITC, JNK1 activity was strongly induced in a dose- and time-dependent manner, whereas the activation of extracellular signal-regulated kinase 1 and 2 was not substantial. Furthermore, activation of JNK1 by PEITC was inhibited by pro-oxidants hydrogen peroxide and diamide, although these two pro-oxidants by themselves had opposing effects on JNK1 activity. Pretreatment with an antioxidant, N-acetyl-L-cysteine, had no effects on PEITC activation of JNK1. When comparing the kinetics of JNK1 activation by different isothiocyanates, PEITC elicited a sustained activation, whereas 3-phenylpropyl isothiocyanate and 4-phenylbutyl isothiocyanate stimulated transient activations. The responsiveness of JNK1 to PEITC, 3-phenylpropyl isothiocyanate, and 4-phenylbutyl isothiocyanate suggests the involvement of JNK1 in the regulation of Phase II detoxifying enzyme gene expression. Furthermore, different patterns of JNK1 induction by these isothiocyanates may contribute to their distinct chemopreventive efficacies in some animal tumor model studies.

Anticarcinogenic Agents↗
PubMed1996

Interaction between c-Rel and the mitogen-activated protein kinase kinase kinase 1 signaling cascade in mediating kappaB enhancer activation.

The Rel family of transcription factors are important mediators of various cytokine stimuli such as interleukin (IL)-1, tumor necrosis factor (TNF)-alpha, and CD28 costimulation in T cell effector responses. These stimuli induce Rel family DNA-binding activity to the kappaB enhancer and CD28 response elements of many cytokine gene promoters leading to cytokine production. Consistent with the importance of Rel family induction during immune responses, c-Rel knockout mice exhibit profound defects in T cell functions including IL-2 secretion and T cell proliferative responses to CD28 plus T cell receptor costimulation. The novel protein kinases, c-Jun NH2-terminal kinases (JNKs)/stress-activated protein kinases, are also activated by TNF-alpha, IL-1, and CD28 costimulation. Because of the common regulation of c-Rel and JNK1 by these agents in T cells, we investigated the role of JNK1 in c-Rel activation. We found that MAP kinase kinase kinase (MEKK) 1, a JNK1 activator, induced transcription from the human immunodeficiency virus-1 long terminal repeat and IL-2R alpha promoters in a kappaB-dependent manner. Coexpression of IkappaBalpha, a c-Rel inhibitor, inhibited the MEKK1-induced transcriptional activity. JNK1 synergized with MEKK1 in activating transcription from a kappaB-driven heterologous promoter. Furthermore, JNK1 associated with c-Rel in vivo in Jurkat T cells by coimmunoprecipitation assays and bound directly to c-Rel in a yeast two-hybrid assay. c-Rel also competed with c-Jun in in vitro kinase assays. However, JNK1 did not phosphorylate c-Rel, NF-kappaB, and IkappaB alpha in vitro, indicating that c-Rel may serve as a docking molecule to allow JNK1 phosphorylation of certain Rel-associated proteins. Transactivation of the IL-2Ralpha and HIV-kappaB-driven promoters by c-Rel was augmented by coexpression of MEKK1. These results demonstrate the first significant role for the MEKK1 kinase cascade module in c-Rel-mediated transcription.

Calcium-Calmodulin-Dependent Protein Kinases↗
PubMed1996

Persistent activation of c-Jun N-terminal kinase 1 (JNK1) in gamma radiation-induced apoptosis.

The c-Jun N-terminal kinases (JNK) are activated by various stimuli, including UV light, interleukin-1, tumor necrosis factor-alpha (TNF-alpha), and CD28 costimulation. Induction of JNK by TNF-alpha, a strong apoptosis inducer, implies a possible role of JNK in the regulation of programmed cell death. Present studies show that lethal doses of gamma radiation (GR) induced JNK activities at the early phase of apoptosis in Jurkat T-cells. We demonstrate that JNK1 was activated by either the T-cell activation signals, anti-CD28 monoclonal antibody plus phorbol 12-myristate 13-acetate (PMA), or the apoptosis-inducing treatment, GR; however, the induction patterns were different. In contrast to the rapid and transient JNK1 activation caused by CD28 signaling plus PMA, GR induced a delayed and persistent JNK1 activation. This implies a distinct regulatory mechanism and specific function of JNK1 in irradiated cells. The nuclear and cytosolic JNK1 activities were simultaneously increased in the irradiated cells without an evident change in the protein levels. The abilities of GR to induce JNK1 activation and DNA fragmentation were correlated. Peripheral blood lymphocytes were more sensitive to GR than Jurkat cells in JNK1 induction. The responsiveness of JNK1 to GR suggests the involvement of JNK1 in the initiation of the apoptosis process.

Apoptosis↗
PubMed1996

Ruptured phaeochromocytoma--a rare differential diagnosis of acute abdomen.

Ruptured phaeochromocytoma may mimic other cause of acute abdomen and though it is rare, it must be remembered as a possible differential; clinical suspicion should be increased when there is lability in blood pressure. We report a patient whose presentation closely resembles that of a leaking aortic aneurysm. The ultrasound performed was non conclusive and was followed by a computerised axial tomography which showed a retroperitoneal mass. A laparotomy was performed and histology of the mass revealed a haemorrhagic phaeochromocytoma. A computerised axial tomography is a useful investigation to distinguish it from other causes of acute abdomen and in particular a leaking aortic aneurysm.

Abdomen, Acute↗
PubMed1995

RelA is a potent transcriptional activator of the CD28 response element within the interleukin 2 promoter.

T-cell activation requires two different signals. The T-cell receptor's recognition of a specific antigen on antigen-presenting cells provides one, and the second signal comes from costimulatory molecules such as CD28. In contrast, T cells that are stimulated with antigen in the absence of the CD28 costimulatory signal can become anergic (nonresponsive). The CD28 response element (CD28RE) has been identified as the DNA element mediating interleukin 2 (IL-2) gene activation by CD28 costimulation. Our previous work demonstrates that the Rel/NF-kappa B family proteins c-Rel, RelA (p65), and NFKB1 (p50) are involved in the complex that binds to the CD28RE. We also showed that c-Rel, but not NFKB1 (p50), can bind to the CD28RE and activate CD28RE-driven transcription in cotransfection assays. However, the role of RelA (p65) in CD28 signaling has not yet been addressed. We provide evidence that RelA (p65) itself bound directly to the CD28RE of the IL-2 promoter and other lymphokine promoters. In addition, RelA (p65) was a potent transcriptional activator of the CD28RE in vivo. We show that a RelA (p65)-c-Rel heterodimer bound to the CD28RE and synergistically activated the CD28RE enhancer activity. We also demonstrate that activated Raf-1 kinase synergized with RelA (p65) in activating the CD28RE enhancer activity. Interestingly, a soluble anti-CD28 monoclonal antibody alone, in the absence of other stimuli, also synergized with RelA (p65) in activating the CD28RE. Furthermore, we show that RelA (p65) activated expression of the wild-type IL-2 promoter but not the CD28RE-mutated IL-2 promoter. A combination of RelA (p65) and NFKB1 (p50) also activated the IL-2 promoter through the CD28RE site. These results demonstrate the functional regulation of the CD28RE, within the IL-2 promoter, by Rel/NF-kappa B transcription factors.

Base Sequence↗
PubMed1995

Ureteric pseudo-diverticula--its implications and a review of the literature on various ureteric diverticula.

Ureteric diverticula is classified as either congenital or acquired. The acquired form is further divided into false or ureter pseudo-diverticula. Of the various types, ureteric pseudo-diverticula have been associated with uro-epithelial tumour more often than expected. This report of a Chinese patient with ureteric pseudo-diverticula presenting with macroscopic haematuria is believed to be the first non-Caucasian to be reported in the literature.

Aged↗
PubMed1994

CD28 signaling causes a sustained down-regulation of I kappa B alpha which can be prevented by the immunosuppressant rapamycin.

CD28, an important T cell surface molecule, mediates a costimulatory signal in the activation of T cell immune responses. CD28 signaling is resistant to the immunosuppressant cyclosporin A (CsA) but sensitive to the immunosuppressant rapamycin. CD28 costimulation induces transcription from the interleukin (IL)-2 promoter via the CD28 response element. The levels of c-Rel, a CD28 response element-binding factor, were found previously to be increased by CD28 costimulation. Therefore, we focused our present study on the mechanism(s) of c-Rel up-regulation by CD28 signaling in Jurkat T cells. In this paper, we showed that CD28 costimulation accelerated the kinetics of nuclear translocation of c-Rel. We showed that CD28 signaling, distinct from other stimuli such as phorbol 12-myristate 13-acetate, IL-1, and tumor necrosis factor-alpha, caused a sustained down-regulation of the inhibitor I kappa B alpha in Jurkat T cells. However, the levels of two other c-Rel inhibitors, namely NFKB1 (p105) and NFKB2 (p100), were not affected. Remarkably, the CD28-mediated down-regulation of I kappa B alpha was prevented by rapamycin but not by CsA. The counter-regulation of I kappa B alpha by CD28 signaling and rapamycin observed in Jurkat T cells is also reproducible in primary T cells. In contrast, the phorbol 12-myristate 13-acetate/ionomycin-mediated down-regulation of I kappa B alpha was prevented by CsA but not by rapamycin. Our data suggest that I kappa B alpha is the down-stream target of both CD28 signaling and rapamycin; a continued down-regulation of I kappa B alpha by CD28 costimulation leads to enhanced nuclear translocation of c-Rel, which in turn causes a sustained up-regulation of IL-2 gene expression.

CD28 Antigens↗
PubMed1994

Expression of v-src in T cells correlates with nuclear expression of NF-kappa B.

NF-kappa B is a rapidly inducible transcriptional activator that responds to a variety of signals and influences the expression of many genes involved in the immune response. Protein tyrosine kinases transmit signals from cytokine and immune receptors. Very little information exists linking these two important classes of signaling molecules. We now demonstrate that v-src expression correlates with nuclear expression of a kappa B binding complex similar to that induced by phorbol ester and ionomycin, as detected by electrophoretic mobility shift assay using a variety of kappa B sites. This complex was blocked by the tyrosine kinase inhibitor, herbimycin A. The v-src-induced complex comprised the p50 and p65 components of NF-kappa B, as determined by supershift and immunoblot analysis. As a functional correlate of this finding, transient co-transfection of HIV-1 LTR reporter constructs in a different T cell line demonstrated that v-src activated this promoter in a kappa B-dependent manner. We found that transactivation of the HIV-1 LTR by v-src was more sensitive to mutations of the proximal, rather than the distal, kappa B element. The implications for T cell receptor signaling and HIV-1 gene expression are considered.

Base Sequence↗
PubMed1994

Long-term inositol phosphate release, but not tyrosine kinase activity, correlates with IL-2 secretion and NF-AT induction in anti-CD3-activated peripheral human T lymphocytes.

The cascade of events within the first few minutes of T cell stimulation has been well characterized. Although many second messengers have been shown to be necessary and sufficient for T cell activation in a number of model systems, the rate-limiting step in peripheral T cells has not been demonstrated. To model effective versus ineffective CD3-mediated stimulation in peripheral T cells, we used two anti-CD3 mAbs that differ in their ability to stimulate purified T cells: OKT3, which causes early second messenger generation but is unable to activate T cells without a second signal, and 64.1, which stimulates T cell proliferation on its own. We found that tyrosine kinase activity was similar for both mAbs over a period of hours. However, the inositol phosphate response was stronger for 64.1 than for OKT3. To tie these events to gene activation, we measured NF-kappa B and NF-AT activity in the nucleus after anti-CD3 stimulation. Both stimuli induced the appearance of the NF-kappa B components (c-Rel, p65 (RelA), and p50 (NF-kappa B1)) and NF-kappa B DNA binding activity in the nucleus. However, only 64.1 induced NF-AT in the nucleus, correlating with its ability to activate T cells. Thus, NF-AT induction and IL-2 secretion were correlated with the levels of inositol phosphate release but not with gross levels of tyrosine kinase activity induced late following the response. On the other hand, NF-kappa B induction and IL-2 receptor expression occurred even with the smaller second messenger response generated by OKT3.

Antibodies, Monoclonal↗
PubMed1994

Effect of CD28 signal transduction on c-Rel in human peripheral blood T cells.

Optimal T-cell activation requires both an antigen-specific signal delivered through the T-cell receptor and a costimulatory signal which can be delivered through the CD28 molecule. CD28 costimulation induces the expression of multiple lymphokines, including interleukin 2 (IL-2). Because the c-Rel transcription factor bound to and activated the CD28 response element within the IL-2 promoter, we focused our study on the mechanism of CD28-mediated regulation of c-Rel in human peripheral blood T cells. We showed that CD28 costimulation accelerated the kinetics of nuclear translocation of c-Rel (and its phosphorylated form), p50 (NFKB1), and p65 (RelA). The enhanced nuclear translocation of c-Rel correlated with the stimulation of Il-2 production and T-cell proliferation by several distinct anti-CD28 monoclonal antibodies. This is explained at least in part by the long-term downregulation of I kappa B alpha following CD28 signalling as opposed to phorbol myristate acetate alone. Furthermore, we showed that the c-Rel-containing CD28-responsive complex is enhanced by, but not specific to, CD28 costimulation. Our results indicate that c-Rel is one of the transcription factors targeted by CD28 signalling.

Base Sequence↗
PubMed1993

Oncogene activation of HIV-LTR-driven expression via the NF-kappa B binding sites.

The Raf-1 proto-oncogene product is a highly regulated serine/threonine kinase that functions in signal transduction downstream from growth factor receptors and upstream from nuclear proto-oncogene products. Using a transient cotransfection assay we have found that activated Raf-1 activates expression from the HIV-LTR. Analysis of a series of 5' deletion and point mutations revealed the NF-kappa B motifs as the Raf-responsive element in the HIV-LTR. Moreover, Raf-BXB activated expression from heterologous promoters driven by the HIV NF-kappa B binding sites. In addition to Raf, we show that v-Src, v-H-Ras and v-Mos activate HIV-LTR expression through the NF-kappa B binding sites and v-H-Ras-induced HIV-LTR expression is mediated by Raf-1. These findings may have implications for the involvement of the cellular homologues of these oncogenes in the switch from latent to productive infection by HIV in response to T-cell activation.

3T3 Cells↗
PubMed1993

The interleukin 2 CD28-responsive complex contains at least three members of the NF kappa B family: c-Rel, p50, and p65.

Optimal activation of T cells requires at least two signals. One signal can be delivered by the antigen-specific T-cell receptor, and the second signal is provided by the costimulatory molecule(s) delivered by the antigen-presenting cell. CD28 is a T-cell surface molecule and stimulation through this protein plays an important role in delivering the second activation signal. In this report, we show that in human peripheral blood T cells, CD28-mediated signal transduction involves the rel family proteins--c-Rel, p50, and p65. Treatment of peripheral blood T cells with phorbol 12-myristate 13-acetate (PMA) and anti-CD28 monoclonal antibody (mAb) results in augmentation of nuclear c-Rel, p50, and p65, and this augmentation can occur in the presence of the immunosuppressant cyclosporin A. It is also shown in this report that, in response to PMA/anti-CD28 mAb or anti-CD3/anti-CD28 mAb, c-Rel, p50, and p65 are associated with CD28-responsive element present in the promoter of the human interleukin 2 gene. The functional significance of c-Rel involvement in the CD28-responsive complex is demonstrated by transient transfection analysis, where cotransfection of c-Rel augments the level of expression of a chloramphenicol acetyltransferase reporter gene linked to the CD28-responsive element.

Antigens, CD↗
PubMed1992

The c-rel protooncogene product c-Rel but not NF-kappa B binds to the intronic region of the human interferon-gamma gene at a site related to an interferon-stimulable response element.

Interferon-gamma (IFN-gamma) is an important immunoregulatory protein that is expressed usually only in large granular lymphocytes and T cells. The gene encoding IFN-gamma was previously found to contain an intronic enhancer element that was not tissue-specific in its activity, despite the restricted expression of the intact IFN-gamma-encoding gene. Using nuclear extracts from the human T-cell line Jurkat, we have now identified two protein-binding regions in this intronic enhancer element. One of the protected regions has strong partial identify to the NF-kappa B site present in the promoter region of the human interleukin 2-encoding gene. Based on this observation and recent reports of the interaction of the c-rel protooncogene product (c-Rel) with NF-kappa B sites, we determined whether c-Rel could interact with the intronic enhancer element in the human IFN-gamma genomic DNA. Most surprisingly, gel-shift analysis, using c-Rel expressed in Escherichia coli established that c-Rel binds specifically to the IFN-gamma intronic DNA but not to the interleukin 2-like NF-kappa B site. Additional studies with antibodies prepared against c-Rel peptides verified specificity of the interaction of c-Rel with this binding site. In addition, using an affinity-purified p50 subunit of the NF-kappa B complex, we observed that the p50 protein did not bind to this additional c-Rel-binding site. Furthermore, nucleotide sequence analysis of this DNA region revealed a strong similarity of the additional c-Rel-binding site to a previously identified IFN-stimulable response element. These data show that c-Rel can interact with DNA regions distinct from that recognized by NF-kappa B and may, in fact, be involved in transcriptional regulation of the IFN-stimulable genes via the IFN-stimulable response element.

Base Sequence↗