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Results for “augmented feedback”

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PubMed2004

The cytosolic II-III loop of Cav2.3 provides an essential determinant for the phorbol ester-mediated stimulation of E-type Ca2+ channel activity.

There is growing evidence that E-type voltage dependent Ca(2+) channels (Ca(v)2.3) are involved in triggering and controlling pivotal cellular processes like neurosecretion and long-term potentiation. The mechanism underlying a novel Ca(2+) dependent stimulation of E-type Ca(2+) channels was investigated in the context of the recent finding that influx of Ca(2+) through other voltage dependent Ca(2+) channels is necessary and sufficient to directly activate protein kinase C (PKC). With Ba(2+) as charge carrier through Ca(v)2.3 channel alpha(1) subunits expressed in HEK-293 cells, activation of PKC by low concentrations of phorbol ester augmented peak I(Ba) by approximately 60%. In addition, the non-inactivating fraction of I(Ba) was increased by more than three-fold and recovery from short-term inactivation was accelerated. The effect of phorbol ester on I(Ba) was inhibited by application of the specific PKC inhibitor bisindolylmaleimide I. With Ca(2+) as charge carrier, application of phorbol ester did not change the activity of Ca(v)2.3 currents but they were modified by the PKC inhibitor bisindolylmaleimide I. These results suggest that with Ca(2+) as charge carrier the incoming Ca(2+) can activate PKC, thereby augmenting Ca(2+) influx into the cytosol. No modulation of Ca(v)2.3 channels by PKC was observed when an arginine rich region in the II-III loop of Ca(v)2.3 was eliminated. Receptor independent stimulation of PKC and its interaction with Ca(v)2.3 channels therefore represents an important positive feedback mechanism to decode electrical signals into a variety of cellular functions.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗

PubMed1993

Respiratory modulation of muscle sympathetic nerve activity in intact and lung denervated humans.

We determined the influences of breathing-induced changes in intrathoracic and intravascular pressures, central respiratory drive, and pulmonary vagal feedback on the within-breath variation in skeletal muscle sympathetic nerve activity (MSNA) in humans. MSNA (peroneal microneurography), arterial blood pressure (Finapres finger monitor), and tidal volume (VT) were recorded continuously in six normal subjects and four heart-lung transplant patients during: 1) spontaneous air breathing; 2) increased FICO2; 3) voluntary augmentation of VT with and without inspiratory resistance; and 4) positive pressure, passive mechanical ventilation. During conditions 3 and 4, which were performed under isocapnic conditions with a high MSNA background (either high resting activity or nonhypotensive lower body suction), subjects breathed at control or elevated VT with normal or prolonged inspiratory time (TI); breathing frequency was 12 breaths per minute. During control breathing in normal subjects there was a distinct within-breath pattern of MSNA, with approximately 70% of the activity occurring during low lung volumes (initial half of inspiration and latter half of expiration). This within-breath variation of MSNA was potentiated with increased VT breathing (> 85% of activity occurring during low lung volumes; p < 0.05 versus control breathing) and was similar during the voluntary and CO2-induced hyperpneas. MSNA decreased progressively and markedly from onset to late inspiration; fell slightly further, reaching its nadir at end-inspiration/onset-expiration; and rose sharply during mid-late expiration. Only the nadir of MSNA was associated with any change in arterial pressure. Resistive breathing, especially at elevated VT, caused a fall in arterial pressure and increased respiratory drive during inspiration, yet MSNA still declined as lung volume increased. Normal within-breath modulation of MSNA also was observed during control and elevated VT induced via positive pressure with passive ventilation, which reversed lung inflation/deflation-induced intrathoracic pressure changes and reduced or removed respiratory motor output. During control breathing in transplant patients the specific within-breath pattern of MSNA was somewhat different than that of the normal subjects, but on average, the overall low lung volume to high lung volume MSNA ratio was similar to normal subjects. In contrast to the normal subjects, however, there was no potentiation of the within-breath variation of MSNA with elevated tidal breathing. These findings indicate that during normal levels of tidal breathing most of the respiratory phase influence on muscle sympathetic outflow observed in normal conscious humans is independent of baroreceptor-sensed fluctuations in intrathoracic or intravascular pressures and of lung inflation-stimulated vagal afferent activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗

PubMed2006

Cooperativity between Rad51 and C/EBP family transcription factors modulates basal and Tat-induced activation of the HIV-1 LTR in astrocytes.

Transcription of the HIV-1 genome is a complex event that requires functional and physical communication of cellular proteins that recognize the LTR sequence with viral proteins, most notably, Tat. Moreover, studies have revealed the ability of Tat to induce transcription of a variety of cellular genes whose products can affect the status of cells, thus contributing to the pathogenesis of AIDS. Recently, we demonstrated that expression of Tat in astrocytes and other neural cells leads to upregulation of Rad51, a major component of DNA repair via homologous recombination. The unscheduled upregulation of Rad51, in turn, has an impact upon the extent of chromosomal abnormalities that are seen in Tat-producing cells. Here, we asked whether an elevation in Rad51 levels influences the extent of viral gene transcription in astrocytic cells. Our results demonstrate that ectopic expression of Rad51 enhances the basal- and the Tat-induced transcription of the LTR promoter. This event requires cooperativity from the C/EBP family of transcription factors including C/EBPbeta and C/EBPbeta homologous protein (CHOP). Similar to Tat, we showed that Rad51 interacts with C/EBPbeta and augments its interaction with the DNA motif spanning nucleotides -120 to -94 of the LTR. Interestingly, Tat exhibited the capacity to augment the synergism between Rad51 and C/EBPbeta. Our results also demonstrate that the level of activation of the LTR by CHOP and Tat, either alone or together, is elevated in the presence of the SW1/SNF1 chromatin remodeling complex. These observations unravel a new pathway for Tat activation of the LTR that includes the positive feedback loop involving Rad51 and C/EBPbeta family proteins.

Astrocytes↗

PubMed1984

Presynaptic serotonin receptors regulate the release of 3H-serotonin in hypothalamic slices of the rabbit.

Hypothalamic slices of the rabbit brain were incubated with 10(-7) M of 3H-serotonin (3H-5HT). After the incubation and an initial washout period, a nearly constant basal efflux of tritium was detected. This basal efflux was not significantly altered by Ca2+-free solution or by the 5HT-antagonist metitepin (10(-5) M), but was augmented by chlorimipramine (10(-5) M) and by unlabelled 5HT (10(-6) M); the acceleration caused by unlabelled 5HT was absent in presence of chlorimipramine (10(-5) M). Both electrical stimulation (4 Hz, 50 mA, 2 min) and high K+ (50 mM) induced an overflow of 3H. This overflow was nearly abolished in Ca2+-free solution. In presence of chlorimipramine (10(-5) M) both the tritium overflow evoked by electrical stimulation and that evoked by high K+ were augmented by metitepin (10(-5) M) and decreased in a concentration dependent manner by unlabelled serotonin (10(-8) - 10(-6) M); the latter effect was antagonized by metitepin (10(-6) M and 10(-5) M). These experiments suggest that in rabbit hypothalamic slices, the release of 3H-5HT is controlled by a negative feedback mechanism acting via presynaptic serotonin receptors.

Animals↗

PubMed1986

The role of GABA and excitatory amino acids in the development of the leptazol-induced epileptogenic EEG.

The developing epileptogenic electroencephalogram (EEG), seen during the slow intravenous infusion of leptazol, is sensitive to various anticonvulsant drugs, particularly those known to augment the function of gamma-aminobutyric acid (GABA), such as clonazepam and sodium valproate, which specifically prolong the earlier wave-like (pre-spiking) phases. Thus, whilst antagonism of GABA may be responsible for spiking, the early wave-like phases may be due to GABA released in the cortex as a feedback control to delay spiking. Intravenous infusion of the GABA antagonists, bicuculline and picrotoxin, produced a developing EEG with spiking the first abnormal feature noted and no wave-like phase, like that seen with leptazol. Cortical superfusion of GABA during the infusion of leptazol, enhanced kand prolonged the wave-like phase, whilst bicuculline reduced it. Cortical superfusion of leptazol, picrotoxin or larger concentrations of bicuculline produced spiking but no wave-like activity. When leptazol and GABA were superfused together they produced wave-like activity similar to that seen during infusions of leptazol. Of the excitatory amino acid antagonists, only those active at receptors for N-methyl-D-aspartate (NMDA) influenced the EEG changes induced by leptazol. It is suggested that leptazol produces waves in the EEG by stimulating subcortical pathways to release GABA in the cortex and that spiking occurs as the cortex is further stimulated by GABA antagonism and the release of excitatory amino acids.

Allylglycine↗

PubMed1998

[Inhibitory effects of nitric oxide on glutamate-induced neuronal activity of CA1 area in rat hippocampal slices].

Using extracellular recording technique, the effects of L-arginine (L-arg), SIN-1 and N-nitro-L-arginine (L-NNA) on glutamate-induced discharge of neurons in CA1 area of hippocampal slices were examined to define the role of L-arg:NO pathway in glutamate-induced discharge of hippocampal neurons and its possible underlying mechanism. The results obtained are as follows. (1) In response to the application of glutamate (0.5 mmol/L) into the superfusate for 1 min, the discharge rate of 12 neurons was increased markedly in an epileptiform pattern. (2) The increased discharge induced by glutamate (0.5 mmol/L) in 10 neurons was suppressed significantly by application of L-arg (10 mmol/L) into the superfusate for 2 min. (3) The glutamate-induced increase of discharge in 12 neurons was decreased markedly by superfusing the brain slice with NO donor SIN-1 (5 mmol/L) for 1 min. (4) As the discharge rate of 12 neurons was increased by pretreatment with glutamate (0.5 mmol/L), application of L-NNA (0.15 mmol/L) into superfusate for 2 min might further augment the discharge intensively and in some case eventually led to abrupt suppression of the discharge. Taken together, it is likely that glutamate binding with NMDA receptors in hippocampal neurons not only induces an increase in discharge, but also activates the L-arg: NO pathway to generate NO responsible for neuroprotection via negative feedback mechanisms.

Action Potentials↗

PubMed2000

Role of nitric oxide in the control of the hypothalamic-pituitary-adrenocortical axis.

The gaseous radical nitric oxide (NO) is catalyzed by conversion of L-arginine to L-citrulline by one cytokine inducible form (iNOS), which becomes active only within hours after the inducing event, and by two constitutively expressed forms, endothelial (eNOS) and neuronal (nNOS), which are regulated by the cytosolic concentration of free Ca2+. Brain nNOS is physiologically present in discrete populations of neurons, which are all excited by glutamate via the ionotropic N-methyl-D-aspartate (NMDA) receptor, which controls a Ca2+ channel. After its diffusion into the extraneuronal space, NO may activate in neurons, which as a rule do not stain for NOS, soluble guanylyl cyclase and formation of cGMP as an intracellular messenger. Beyond that, NO is important as a feedback regulator of glutamatergic excitation. NO as a nitrosylating agent enhances disulfide bonding of vicinal sulfhydryl (thiol) groups of the redox modulatory site of the NMDA receptor complex and thereby down-regulates its Ca2+ channel activity. Histochemical studies have revealed the presence of a large number of NOS containing neurons in the magnocellular and parvocellular subdivisions of hypothalamic nuclei. Numerous studies conform to the view that NO participates in the control of many different neurosecretory processes, especially of the corticotropin-releasing hormone (CRH) neurosecretory system. The redox-modulatory site of the NMDA receptor appears, therefore, as a critical structure in the control of the hypothalamic-pituitary-adrenocortical (HPA) axis. Moreover, glucocorticoids augment neuronal excitotoxicity by increasing the expression of glutamate receptors and inhibition of glutamate reuptake. In attempting to explain the many conflicting results obtained in studies with NO, it may be worthwhile to consider that the actual redox-environment of distinct loci of the brain may determine the final function of NO, acting either as a transmitter or neuromodulator or, in the worst case, causing neurodestruction. It seems likely that any kind of stress by altering the ratio of reduced vs oxidized thiols within the central nervous system influences neuronal excitability, with NO working either as an amplifier or as a feedback regulator of neuronal excitation or inhibition, which may alter acutely or chronically, among others, the homeostasis of a given neurosecretory system.

Animals↗

PubMed2005

IFN-gamma-mediated negative feedback regulation of NKT-cell function by CD94/NKG2.

Activation of invariant natural killer T (iNKT) cells with CD1d-restricted T-cell receptor (TCR) ligands is a powerful means to modulate various immune responses. However, the iNKT-cell response is of limited duration and iNKT cells appear refractory to secondary stimulation. Here we show that the CD94/NKG2A inhibitory receptor plays a critical role in down-regulating iNKT-cell responses. Both TCR and NK-cell receptors expressed by iNKT cells were rapidly down-modulated by priming with alpha-galactosylceramide (alpha-GalCer) or its analog OCH [(2S,3S,4R)-1-O-(alpha-D-galactopyranosyl)-N-tetracosanoyl-2-amino-1,3,4-nonanetriol)]. TCR and CD28 were re-expressed more rapidly than the inhibitory NK-cell receptors CD94/NKG2A and Ly49, temporally rendering the primed iNKT cells hyperreactive to ligand restimulation. Of interest, alpha-GalCer was inferior to OCH in priming iNKT cells for subsequent restimulation because alpha-GalCer-induced interferon gamma (IFN-gamma) up-regulated Qa-1b expression and Qa-1b in turn inhibited iNKT-cell activity via its interaction with the inhibitory CD94/NKG2A receptor. Blockade of the CD94/NKG2-Qa-1b interaction markedly augmented recall and primary responses of iNKT cells. This is the first report to show the critical role for NK-cell receptors in controlling iNKT-cell responses and provides a novel strategy to augment the therapeutic effect of iNKT cells by priming with OCH or blocking of the CD94/NKG2A inhibitory pathway in clinical applications.

Animals↗

PubMed1989

Environmental and interoceptive influences on chronic low back pain behavior.

The operant conditioning theory states that environmental stimuli greatly influence chronic pain behavior. In contrast, the hypochondriasis theory states that pain behavior is the result of an intensified pain perception which is part of a more general augmentation and amplification of normal bodily sensations. The operant theory predicts that pain behavior (operationalized as poorer endurance on the part of chronic low back pain patients as compared to the endurance of control subjects in a series of 6 working-to-tolerance treadmill tests) will decrease when no verbal or non-verbal feedback for treadmilling behavior is given. This hypothesis could not be confirmed in the present study. The hypochondriasis theory predicts that chronic pain patients will report more bodily sensations, both at rest and after treadmill exercises. This hypothesis was strongly supported by the findings of the present study.

Adult↗

PubMed2003

Incorporating robotics into an open-heart program.

The above described clinical series show that after a careful and thorough training program and stepwise introduction of surgical telemanipulation systems, application of telemanipulations is safe and shows acceptable results. Still, OR times are longer than for conventional procedures, and the operation is demanding, and expensive. The main shortcoming is that the procedure is only suitable for a highly selected patient population. However, despite all the clinical experience gathered in various centers, this technique is still evolving and in its beginning. There are some very promising developments that will improve the benefit of telemanipulators. For the first time, the separation of the surgeon from the surgical field facilitates training of surgeons on simulators. This might lead to a higher standard of surgical performance. Progress in sensor technology will make tactile-force feedback available, and new 3 D-visualization systems are designed to provide a better depth perception and higher resolution of the endoscopic image. Virtual stabilizing systems will enable robotic systems to operate on a virtual arrested heart without the need for CPB or mechanical stabilizers. These and other research topics summarized under the term augmented reality will enhance the natural senses and abilities of the surgeon. More and more, automatization will find its way into the OR. Preoperatively collected data about the patient's anatomy will be used to create safety margins, the robotic system will allow for the surgeon's movements, and instruments will be able to find their way to the surgical site without remote control. Because a stepwise approach has led to the clinical results that we and others have now achieved, it is the basis for further step-by-step development of the application of telemanipulation systems in coronary artery bypass grafting, and possibly other endoscopic procedures in cardiac surgery.

Animals↗

PubMed2001

The ancestral complement system in sea urchins.

The origin of adaptive immunity in the vertebrates can be traced to the appearance of the ancestral RAG genes in the ancestral jawed vertebrate; however, the innate immune system is more ancient. A central subsystem within innate immunity is the complement system, which has been identified throughout and seems to be restricted to the deuterostomes. The evolutionary history of complement can be traced from the sea urchins (members of the echinoderm phylum), which have a simplified system homologous to the alternative pathway, through the agnathans (hagfish and lamprey) and the elasmobranchs (sharks and rays) to the teleosts (bony fish) and tetrapods, with increases in the numbers of complement components and duplications in complement pathways. Increasing complexity in the complement system parallels increasing complexity in the deuterostome animals. This review focuses on the simplest of the complement systems that is present in the sea urchin. Two components have been identified that show significant homology to vertebrate C3 and factor B (Bf), called SpC3 and SpBf, respectively. Sequence analysis from both molecules reveals their ancestral characteristics. Immune challenge of sea urchins indicates that SpC3 is inducible and is present in coelomic fluid (the body fluids) in relatively high concentrations, while SpBf expression is constitutive and is present in much lower concentrations. Opsonization of foreign cells and particles followed by augmented uptake by phagocytic coelomocytes appears to be a central function for this simpler complement system and important for host defense in the sea urchin. These activities are similar to some of the functions of the homologous proteins in the vertebrate complement system. The selective advantage for the ancestral deuterostome may have been the amplification feedback loop that is still of central importance in the alternative pathway of complement in higher vertebrates. Feedback loop functions would quickly coat pathogens with complement leading to phagocytosis and removal of foreign cells, a system that would be significantly more effective than an opsonin that binds upon contact as a result of simple diffusion. An understanding of the immune response of the sea urchin, an animal that is a good estimator of what the ancestral deuterostome immune system was like, will aid us in understanding how adaptive immunity might have been selected for during the early evolution of the vertebrates and how it might have been integrated into the pre-existing innate immune system that was already in place in those animals.

Amino Acid Sequence↗

PubMed2005

PERIOD1-associated proteins modulate the negative limb of the mammalian circadian oscillator.

The clock proteins PERIOD1 (PER1) and PERIOD2 (PER2) play essential roles in a negative transcriptional feedback loop that generates circadian rhythms in mammalian cells. We identified two PER1-associated factors, NONO and WDR5, that modulate PER activity. The reduction of NONO expression by RNA interference (RNAi) attenuated circadian rhythms in mammalian cells, and fruit flies carrying a hypomorphic allele were nearly arrhythmic. WDR5, a subunit of histone methyltransferase complexes, augmented PER-mediated transcriptional repression, and its reduction by RNAi diminished circadian histone methylations at the promoter of a clock gene.

3T3 Cells↗

PubMedDate not supplied

[Changes in secretion of pituitary hormones after neuropeptide Y administration into the third cerebral ventricle].

Neuropeptide Y (NPY) administration into the third brain ventricle of normal and castrated male rats decreases prolactin and LH secretion almost not affecting FSH. Simultaneous augmentation of hypophyseal response to LH-RH is observed. NPY effect on LH release is more pronounced in castrated animals as compared to normal ones. It is concluded that in the CNS inhibitory action of NPY prevails as regards to neuroendocrine control of gonadotropin and prolactin secretion. The obtained results allow NPY participation in feedback regulation of LH secretion in males.

Animals↗

PubMed1980

The immunologic modulation of morbidity in schistosomiasis. Studies in athymic mice and in vitro granuloma formation.

These studies were designated to test the hypothesis that granulomatous hypersensitivity associated with schistosomiasis, its modulation, and subsequent morbidity are contingent on thymic-dependent lymphocyte function. Studies in athymic mice were integrated with in vitro studies on granuloma formation. Athymic animals tolerate low levels of infection well but are increasingly susceptible to morbidity as the challenge burden increases or becomes associated with simultaneous additional stress such as bacterial infection. In addition, the athymic animals produce smaller granulomas and fail to demonstrate accelerated granuloma formation, the spontaneous modulation of granulomas, or significant resistance to reinfection. These deficiencies could be reversed by thymic reconstitution. In vitro studies indicate that the response to egg antigens was due to the interaction of two subpopulations of T lymphocytes. Studies of "in vitro granuloma formation" augmented the in vivo findings, suggesting that the "in vitro granuloma" represents an excellent analog for delayed hypersensitivities; granuloma production was contingent on a macrophage function and a specific subpopulation of T lymphocytes defined as Ly1+. Granuloma modulation was due to two mechanisms, one contingent on direct suppression by ly2,3+ cells and the other by feedback inhibition requiring Ly,2,3+ cells. These studies suggest the feasibility of combined in vivo and in vitro technology to study the mechanisms of granuloma formation and subsequent morbidity.

Animals↗

PubMed2005

Adenosine diphosphate receptors on blood platelets: potential new targets for antiplatelet therapy.

Platelets play a key role not only in physiological haemostasis, but also under pathological conditions such as thrombosis. Platelet activation may be initiated by a variety of agonists including thrombin, collagen, thromboxane or adenosine diphosphate (ADP). Although ADP is regarded as a weak agonist of blood platelets, it remains an important mediator of platelet activation evoked by other agonists, which induce massive ADP release from dense granules, where it occurs in molar concentrations. Thus, ADP action underlies a positive feedback that facilitates further platelet aggregation and leads to platelet plug formation. Additionally, ADP acts synergistically to other, even weak, agonists such as serotonin, adrenaline or chemokines. Blood platelets express two types of P2Y ADP receptors: P2Y(1) and P2Y(12). ADP-dependent platelet aggregation is initiated by the P2Y1 receptor, whereas P2Y(12) receptor augments the activating signal and promotes platelet release reaction. Stimulation of P2Y(12) is also essential for ADP-mediated complete activation of GPIIb-IIIa and GPIa-IIa, and further stabilization of platelet aggregates. The crucial role in blood platelet biology makes P2(Y12) an ideal candidate for pharmacological approaches for anti-platelet therapy.

Animals↗

PubMed1992

Role of protein kinase C in the regulation of prostaglandin synthesis in human endothelium.

The present study specifically addresses the role of protein kinase C (PKC) activation in human endothelial cell Ca2+ mobilization, a response that is functionally coupled to the production of the potent arachidonate (AA) metabolite, prostacyclin (PGI2). Phorbol 12-myristate 13-acetate (PMA), alpha-thrombin, and sodium fluoride (NaF), a direct G-protein activator, produced a rapid and time-dependent translocation of PKC from the cytosol to the membrane. Activation of PKC by brief pretreatment of human umbilical vein endothelial cell (HUVEC) monolayers with PMA resulted in the inhibition of NaF-induced inositol phosphate increases and attenuation of both alpha-thrombin- and NaF-activated increases in intracellular Ca2+ (Ca2+i). Ca2+ mobilization induced by ionophore A23187 was not affected by PKC preactivation, suggesting PKC-dependent negative feedback inhibition of phosphatidylinositol (PI)-specific phospholipase C (PLC). Agonist-stimulated AA release and PGI2 synthesis in PMA-pretreated cultured human endothelial cells, however, was potentiated, and the enhanced PGI2 synthesis produced by A23187, NaF, and alpha-thrombin was dependent upon the dose of PMA. Treatment of HUVEC monolayers with an intracellular Ca2+ chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N'N'-tetraacetic acid-acetoxymethylester (BAPTA-AM), dramatically reduced alpha-thrombin-, NaF-, and A23187-induced PGI2 synthesis, demonstrating the importance of Ca2+i availability in PGI2 synthesis. BAPTA pretreatment did not inhibit PMA-induced PKC activation, and BAPTA-mediated inhibition of agonist-stimulated PGI2 synthesis was partially attenuated by prior PMA pretreatment. Staurosporine, a potent PKC inhibitor, at concentrations that inhibited PKC-induced phosphorylation of histone-1, augmented both alpha-thrombin- and NaF-induced production of inositol phosphates but markedly inhibited alpha-thrombin-, NaF-, and A23187-induced PGI2 synthesis. The downregulation of PKC activity by prolonged PMA treatment (18 h) produced similar inhibition of PGI2 synthesis by these agonists (approximately 50% inhibition). These studies indicate that the integrated phospholipase A2 and PLC activities are under complex regulation by factors that include both PKC activation and [Ca2+i]. PKC exerts dual effects on prostaglandin synthesis via negative regulation of Gp-coupled PI-specific PLC and positive feedback regulation of AA release and PGI2 synthesis. PKC is thus a critical determinant in the regulation of human endothelial cell prostaglandin synthesis by both receptor-mediated and G-protein-dependent cellular activation.

Aequorin↗

PubMed1985

Stimulation of hypothalamic LHRH levels and release by gonadal steroids.

The inhibitory feedback effects of steroids on pituitary LH release are believed to be mediated via steroidal effects on the hypothalamic LHRH activity. We have examined the direct effects of individual steroids (T, DHT and E2) on hypothalamic LHRH levels and on LHRH release in vitro. In castrated male rats, replacement of either steroid in physiological doses, resulted in augmentation of the MBH LHRH levels by steroidal action within the MBH. LHRH analyses of microdissected diencephalic nuclei revealed that this accumulation occurred exclusively in LHRH terminals in the ME. Careful examination of the time course of steroid action showed that whereas LH release was suppressed within hours of steroid treatment, the LHRH response occurred after 3-4 days of steroid exposure in 2-week castrated rats and 7-14 days in 8-week castrated rats. This temporal dichotomy in the LH and LHRH responses to steroid action was further substantiated by the differential effects of low, sub-physiological levels of steroids on these two responses. Very low levels of T or E2 evoked maximal accumulation of the MBH LHRH, but LH release in vivo and the rate of LHRH release in vitro were not affected. Surprisingly, physiological levels of T which suppressed LH release concomitant with elevations in LHRH levels, augmented the in vitro rate of LHRH release. In fact, the LHRH release rate was found to be correlated with LHRH concentrations in hypothalami of intact, castrated and castrated rats treated with T. Thus it appears that in the hypothalamo-pituitary axis there are different thresholds of responsiveness to steroids. Apparently, the LHRH neurons, particularly the processes involved in LHRH accumulation are most sensitive to low levels of steroids; however, higher physiological levels of steroids are required to suppress pituitary LH release as well as to promote LHRH release. On the basis of our cumulative data, it is reasonable to speculate that steroid-induced accumulation of LHRH in the ME may not be a consequence of decrease in LHRH release, but may involve synthesis of the neurohormone.

Animals↗

PubMed2002

Transcriptional regulation of the mdm2 oncogene by p53 requires TRRAP acetyltransferase complexes.

The p53 tumor suppressor regulates the cellular response to genetic damage through its function as a sequence-specific transcription factor. Among the most well-characterized transcriptional targets of p53 is the mdm2 oncogene. Activation of mdm2 is critical in the p53 pathway because the mdm2 protein marks p53 for proteosome-mediated degradation, thereby providing a negative-feedback loop. Here we show that the ATM-related TRRAP protein functionally cooperates with p53 to activate mdm2 transcription. TRRAP is a component of several multiprotein acetyltransferase complexes implicated in both transcriptional regulation and DNA repair. In support of a role for these complexes in mdm2 expression, we show that transactivation of the mdm2 gene is augmented by pharmacological inhibition of cellular deacetylases. In vitro analysis demonstrates that p53 directly binds to a TRRAP domain previously shown to be an activator docking site. Furthermore, transfection of cells with antisense TRRAP blocks p53-dependent transcription of mdm2. Finally, using chromatin immunoprecipitation, we demonstrate direct p53-dependent recruitment of TRRAP to the mdm2 promoter, followed by increased histone acetylation. These findings suggest a model in which p53 directly recruits a TRRAP/acetyltransferase complex to the mdm2 gene to activate transcription. In addition, this study defines a novel biochemical mechanism utilized by the p53 tumor suppressor to regulate gene expression.

Acetylation↗