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Biomedical subjects

N Xu

Publications and source records attributed to N Xu.

At least 127 records · Page 7Linked to original sources
PubMed1994

Signaling through transforming G protein-coupled receptors in NIH 3T3 cells involves c-Raf activation. Evidence for a protein kinase C-independent pathway.

We have studied the role of Raf-1 in mitogenesis and cellular transformation induced by G protein-coupled receptors in NIH 3T3 cells transfected with the human m1 muscarinic receptor. We have observed that in m1-expressing NIH 3T3 cells, the cholinergic agonist carbachol induces a dose- and time-dependent shift in the electrophoretic mobility of p72Raf-1, equivalent to that observed when using phorbol esters or platelet-derived growth factor as stimulants. Phosphoamino acid analysis of slower mobility forms of p72Raf-1 revealed both phosphoserine and phosphothreonine. Carbachol potently induced c-Raf activity as judged by its in vitro phosphorylating activity using MEK as a substrate. However, induction of Raf-1 kinase activity by carbachol occurred much earlier than changes in its electrophoretic mobility. Raf-1 kinase activation followed a kinetic similar to that exhibited by an epitope-tagged ERK2 protein when coexpressed in the same cells. Conventional protein kinase C (PKC) inactivation by means of sustained phorbol ester treatment or by a new nontoxic PKC-specific inhibitor, GF 109203X, abolished p72Raf-1 mobility shift induced by carbachol or by phorbol esters. However, c-Raf and ERK2 enzymatic activity in response to carbachol was at least 50-80% PKC-independent. Furthermore, inhibition of PKC failed to affect DNA synthesis or focus formation induced by carbachol in cells expressing m1 receptors. In contrast, cotransfection of NIH 3T3 cells with the Raf-1 dominant negative mutant Raf-301 (K375W) drastically decreased the transforming ability of m1 receptors. Thus, our findings implicate Raf-1 activation in transformation by G protein-coupled receptors. In addition, our data suggest that activation of p72Raf-1 and ERK2 by G protein-coupled receptors involves PKC-independent pathways.

3T3 Cells↗
PubMed1994

Potent transforming activity of the G13 alpha subunit defines a novel family of oncogenes.

The finding of GTPase inhibiting mutations in genes for alpha subunits of Gs and Gi2 in certain endocrine tumors suggests that heterotrimeric G proteins might contribute to neoplasia. Expression of these activated forms of alpha s or alpha i2 in NIH 3T3 murine fibroblasts induces certain alterations in cell growth, but is weakly transforming. Mutationally activated forms of the alpha subunit of another G protein family, Gq, are fully oncogenic in NIH 3T3 cells, although with a very low potency. In contrast, we have recently shown that overexpression of the alpha subunit of a novel G protein, G12, is itself transforming, and an activated mutant of alpha 12 behaves as one of the most potent oncogenes known. In this study, we have explored whether another member of the G alpha 12 family, G alpha 13, harbors transforming potential. Our data demonstrate that G alpha 13 can behave as a potent dominant acting oncogene. These findings strongly suggest that the G12 family of G proteins represents a novel class of oncogenes.

3T3 Cells↗
PubMed1994

Ras-dependent activation of MAP kinase pathway mediated by G-protein beta gamma subunits.

Mitogen-activated protein kinases, MAP kinases or ERKs (extracellular signal-regulated kinases) are rapidly stimulated by growth-promoting factors acting on a variety of cell-surface receptors. In turn, ERKs phosphorylate and regulate key intracellular enzymes and transcription factors involved in the control of cellular proliferation. The tyrosine-kinase class of growth-factor receptors transmits signals to ERKs in a multistep process that involves Ras and a limited number of defined molecules. In contrast, ERK activation by G-protein-coupled receptors is poorly understood, as is the role of ras in this signalling pathway. We have explored in COS-7 cells the mechanism of ERKs activation by m1 and m2 muscarinic receptors, typical examples of receptors coupled through Gq proteins to induce phosphatidylinositol hydrolysis and to G(i) proteins to inhibit adenylyl cyclase, respectively. Here we present evidence that ERK activation is mediated by beta gamma subunits of heterotrimeric G proteins acting on a ras-dependent pathway.

Adenylate Cyclase Toxin↗
PubMed1994

Evidence for nicotine-induced skin flap ischemic necrosis in the pig.

There is clinical and experimental evidence to indicate that cigarette smoking may increase the risk of skin ischemic necrosis in flap surgery but the pathogenic mechanism remains unclear. The objectives of this project were to investigate the potential deleterious effects and mechanism of action of nicotine, a major by-product of cigarette smoking, in skin flap surgery in the pig. It was observed that 4-5 weeks of intramuscular nicotine injections (4 mg/kg; twice daily) significantly (p < 0.05) decreased the skin flap capillary blood flow and the length and area of skin flap viability in the pig. This nicotine treatment also induced a 1.6-fold increase in skin flap tissue content of norepinephrine compared with the saline-treated control. The estimated mean wet skin tissue content of norepinephrine (5 x 10(-7) M) was much higher than the circulating level of norepinephrine (1.8 x 10(-9) M) in nicotine-treated pigs. This level of norepinephrine (5 x 10(-7) M) was seen to induce a significant vasoconstrictor effect (75% increase over basal perfusion pressure) in isolated perfused pig skin flaps. It was also observed that the vasoconstrictor effect of norepinephrine was significantly (p < 0.05) enhanced in the presence of 10(-4) M N omega-monomethyl-L-arginine or NG-nitro-L-arginine, an endothelium-derived relaxing factor--nitric oxide (EDRF/NO) synthesis inhibitor. This vasoconstrictor effect was further enhanced in the presence of NG-nitro-L-arginine and 10(-5) M indomethacin, a cyclooxygenase inhibitor.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1994

Chylomicron-induced prothrombin activation and platelet aggregation.

The effects on platelet aggregation of native rat chyle chylomicrons, chylomicron remnants, and chylomicrons that had been preincubated with rat or human EDTA-plasma, serum, whole blood, or pure human prothrombin were examined. The native chyle chylomicrons did not induce platelet aggregation but decreased ADP- and thrombin-induced platelet aggregation and [14C]serotonin release. Chylomicron remnants also failed to induce platelet aggregation, but they potentiated the aggregation and the [14C]serotonin release induced by ADP and thrombin. Aggregation, after a lag phase of 15 to 20 minutes, was seen when platelets were incubated with chylomicrons that had been preincubated with plasma and then isolated as the top layer after a single centrifugation at d = 1.006. This aggregation was inhibited in a dose-dependent manner by an antiserum against prothrombin that also inhibited thrombin-induced platelet aggregation. After washing by centrifugation the plasma-preincubated chylomicrons did not induce platelet aggregation, but this effect could be restored by adding a small amount of prothrombin, which did not cause aggregation when added alone or together with native chyle chylomicrons. Addition of 2% (vol/vol) plasma, however, induced aggregation when added together with either native chyle chylomicrons or washed preincubated chylomicrons, but not when added alone. Binding of 125I-labeled prothrombin to native chyle chylomicrons was demonstrated by gradient ultracentrifugation. During incubation of washed plasma-preincubated chylomicrons with 125I-prothrombin and platelets, a significant conversion of 125I-prothrombin to 125I-prethrombin and 125I-thrombin occurred, as demonstrated by autoradiography after separation on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The interaction between chylomicrons and prothrombin, and possibly other coagulation proteins, thus enhances prothrombin activation in the presence of platelets.

Animals↗
PubMed1994

The non-catalytic domain of ras-GAP inhibits transformation induced by G protein coupled receptors.

We have studied the relationship between ras-GAP and G protein coupled receptors in a proliferative setting comprised of NIH3T3 expressing transfected muscarinic receptors (mAChRs). GAP expression plasmids were engineered to encode wild-type GAP, its carboxyl-terminal catalytic domain, a mutant lacking a portion of the catalytic domain, and an amino-terminal domain which contained the hydrophobic region as well as SH2-SH3 domains. Cotransfection of each GAP expression plasmid into NIH3T3 cells did not affect the transforming ability of the v-mos oncogene, but plasmids encoding wild-type GAP or the mutant consisting of an intact catalytic domain inhibited transformation induced by normal c-ras. Wild-type GAP also prevented transformation by m1 mAChRs, whereas the mutant consisting of only its catalytic domain lacked any demonstrable effect. In contrast, the N-terminal non-catalytic domain of GAP effectively prevented m1-induced focus-formation. Cell lines coexpressing m1 receptors and each of the GAP constructs revealed that GAP proteins do not affect m1 receptor density, receptor ligand binding characteristics or coupling to the PI-PLC signal transduction pathway. Thus, our findings suggest a role for the N-terminal non-catalytic domain of GAP in regulating biological functions mediated by G protein-coupled receptors.

3T3 Cells↗
PubMed1993

A mutant alpha subunit of G12 potentiates the eicosanoid pathway and is highly oncogenic in NIH 3T3 cells.

The discovery of GTPase-inhibiting mutations in genes for alpha subunits of Gs or G(i2) in certain human endocrine tumors has raised the possibility that heterotrimeric guanine nucleotide-binding regulatory proteins (G proteins) might contribute to neoplastic disease. Expression of GTPase-deficient alpha s or alpha i2 polypeptides in rodent fibroblasts increases or decreases cAMP, respectively, and induces certain alterations in cell growth but only a few of the phenotypic changes associated with cellular transformation. In contrast, an analogous mutation in the alpha subunit of Gq, which activates phosphatidylinositol (PI)-specific phospholipase C, is fully oncogenic. However, activated alpha q is cytotoxic and several orders of magnitude less potent as an oncogene than certain G protein-coupled receptors. Thus, G proteins other than those inducing PI hydrolysis might possess high transforming efficiency. In the present study, we explored the G12 family of G proteins for their oncogenic potential. Our results show that whereas overexpression of wild-type alpha 12 in NIH 3T3 cells is itself weakly transforming, an activated alpha 12 behaves as a remarkably potent oncogene. Transformation by alpha 12 correlates with alterations in the eicosanoid pathway but not with PI-specific phospholipase C or other G protein-linked second messengers.

3T3 Cells↗
PubMed1993

Pharmacologic intervention of skin vasospasm and ischemic necrosis in pigs.

Ischemic necrosis resulting from vasospasm is a common complication in skin flap surgery, and serotonin released by traumatized platelets is likely to play an important role in the pathogenesis of skin vasospasm in flap surgery. We studied the pathogenic role of serotonin and its pharmacologic intervention thereof in skin flap ischemic necrosis in pigs. We observed that serotonin caused a concentration-dependent (10(-8)-10(-5) M) increase in perfusion pressure in isolated perfused pig skin flaps. This vasoconstrictive effect of serotonin was blocked by S1C/2-serotonergic receptor antagonists LY53857 (10(-5) M) and ketanserin (10(-5) M), but not by an alpha 1-adrenoceptor antagonist (prazosin 10(-5) M), or a thromboxane A2 (TxA2)/endoperoxide receptor antagonist (SQ30741 10(-5) M). The vasoconstrictive effect of serotonin was more pronounced (p < 0.05) in the presence of an endothelium-derived nitric oxide (NO) synthesis inhibitor [N omega-monomethyl-L-arginine (L-NA) or NG-nitro-L-arginine (L-NMMA) 10(-5) M] but not a cyclooxygenase inhibitor (indomethacin 10(-5) M). In in vivo studies, serotonin infusion (5 micrograms/kg/min intravenously, i.v.) significantly (p < 0.05) decreased pig random pattern skin flap capillary blood flow. This in vivo vascular effect was also completely blocked in pigs pretreated with LY53857 (0.4 mg/kg i.v.). In a separate experiment without serotonin infusion, i.v. prazosin (2-8 micrograms/kg), dazmegrel (2-6 mg/kg), or SQ30741 (2-4 mg/kg) had no significant effect on skin flap capillary blood flow as compared with control. On the other hand, i.v. sergolexole or LY53857 significantly (p < 0.05) increased skin flap capillary blood flow in a dose-dependent manner.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenergic alpha-Antagonists↗
PubMed1993

[Effect of electroacupuncture at "taixi" point on plasma thromboxane A2 and prostacyclin in the rabbit with renal ischemia].

The purpose of this study is to observe the effect of electroacupuncture at "Taixi" point (KI-3) on the renal blood flow (RBF) under the condition of glycerol-induced renal ischemia and the changes of thromboxane A2 (TXA2) and prostacyclin (PGI2) during this course. The RBF, which is measured by hydrogen gas clearance method, was chosen as an index. The results are as follows: 1. After electroacupuncture at "Taixi" point, the RBF increased. 2. Under the condition of renal ischemia, the TXA2 increased, and the PGI2 decreased. 3. During the course of electroacupuncture at "Taixi" point, the TXA2 decreased and the PGI2 increased. These facts suggest that the effect of electroacupuncture at "Taixi" point is related to the PGI2 and the TXA2.

6-Ketoprostaglandin F1 alpha↗
PubMed1993

[Pharmacokinetics of ciprofloxacin in young healthy volunteers].

Pharmacokinetic studies on ciprofloxacin were carried out in 6 young healthy volunteers. Serum and urine levels of ciprofloxacin were measured by reversed-phase high performance liquid chromatography. The drug obeyed one-compartment model kinetics in serum. The mean peak serum concentration, time to peak concentration and the area under the curve were 4.17 +/- 1.05mg/L, 1.75 +/- 0.40h and 18.09 +/- 2.69mg/L.h respectively after oral administration of 500mg of ciprofloxacin tablet. The elimination half life was 2.34 +/- 0.81h and the apparent volume of distribution was 3.71 +/- 0.75L. The urinary rate of ciprofloxacin over 24h after dosing was 34.22 +/- 6.57%. These data indicate that ciprofloxacin is easily absorbed and its concentration reaches the peak level rapidly. There are high levels of drug both in serum and urine.

Adult↗
PubMed1992

SpCOUP-TF: a sea urchin member of the steroid/thyroid hormone receptor family.

Embryonic nuclear proteins from the sea urchin Strongylocentrotus purpuratus bind in vitro to a cis-acting element that lies upstream of the actin gene CyIIIb and consists of two direct repeats homologous to steroid hormone response elements. This sea urchin element is specifically recognized by transcription factor COUP-TF from HeLa cell nuclear extracts as well. A sea urchin gene homologous to the human COUP-TF1 gene was detected by blot hybridization to S. purpuratus genomic DNA. Screening of a genomic DNA library with a human COUP-TF1 cDNA probe produced overlapping genomic clones that carry the S. purpuratus gene. Our results indicate that the sea urchin homologue has a structure similar to the human COUP-TF1 gene, i.e., conserved intron positions characteristic of this subgroup of steroid hormone receptors. Sequencing of the exons that encode the DNA- and ligand-binding domains of the sea urchin protein revealed 96% and 92% amino acid identity to the domains of the human protein, respectively. Transcripts derived from the S. purpuratus COUP-TF homologous gene were detected in ovarian and embryonic RNAs from various stages.

Actins↗