Biomedical subjects

J Y H Chan

Publications and source records attributed to J Y H Chan.

8 recordsLinked to original sources

PubMed2006

Cholinergic-receptor-independent dysfunction of mitochondrial respiratory chain enzymes, reduced mitochondrial transmembrane potential and ATP depletion underlie necrotic cell death induced by the organophosphate poison mevinphos.

Our current understanding of the nature of cell death that is associated with fatal organophosphate poisoning and the underlying cellular mechanisms is surprisingly limited. Taking advantage of the absence in an in vitro system of acetylcholinesterase, the pharmacological target of organophosphate compounds, the present study evaluated the hypothesis that the repertoire of cholinergic receptor-independent cellular events that underlie fatal organophosphate poisoning entails induction of mitochondrial dysfunction, followed by bioenergetic failure that leads to necrotic cell death because of ATP depletion. Pheochromocytoma PC12 cells incubated with the organophosphate pesticide mevinphos (0.4 or 4mumol) for 1 or 3h underwent a dose-related and time-dependent loss of cell viability that was not reversed by muscarinic (atropine) or nicotinic (mecamylamine) blockade. This was accompanied by depressed NADH cytochrome c reductase, succinate cytochrome c reductase or cytochrome c oxidase activity in the mitochondrial respiratory chain, reduced mitochondrial transmembrane potential, decreased ATP concentration, elevated ADP/ATP ratio, increased lactate dehydrogenase release and necrotic cell death. We conclude that Mev induces cholinergic receptor-independent necrotic cell death by depressing the activity of Complexes I to IV in the mitochondrial respiratory chain, eliciting reduction in mitochondrial transmembrane potential, depleting intracellular ATP contents and damaging cell membrane integrity.

Adenosine Triphosphate↗

PubMed2005

Phasic cardiovascular responses to mevinphos are mediated through differential activation of cGMP/PKG cascade and peroxynitrite via nitric oxide generated in the rat rostral ventrolateral medulla by NOS I and II isoforms.

The organophosphate insecticide mevinphos (Mev) acts on the rostral ventrolateral medulla (RVLM), where sympathetic vasomotor tone originates, to elicit phasic cardiovascular responses via nitric oxide (NO) generated by NO synthase (NOS) I and II. We evaluated the contribution of soluble guanylyl cyclase (sGC)/cyclic guanosine monophosphate (cGMP)/protein kinase G (PKG) cascade and peroxynitrite in this process. PKG expression in ventrolateral medulla of Sprague-Dawley rats manifested an increase during the sympathoexcitatory phase (Phase I) of cardiovascular responses induced by microinjection of Mev bilaterally into the RVLM that was antagonized by co-administration of 7-nitroindazole or Nomega-propyl-L-arginine, two selective NOS I inhibitors or 1-H-[1,2,4]oxadiaolo[4,3-a]quinoxalin-1-one (ODQ), a selective sGC antagonist. Co-microinjection of ODQ or two PKG inhibitors, KT5823 or Rp-8-Br-cGMPS, also blunted the Mev-elicited sympathoexcitatory effects. However, the increase in nitrotyrosine, a marker for peroxynitrite, and the sympathoinhibitory circulatory actions during Phase II Mev intoxication were antagonized by co-administration of S-methylisothiourea, a selective NOS II inhibitor, Mn(III)-tetrakis-(4-benzoic acid) porphyrin, a superoxide dismutase mimetic, 5,10,15,20-tetrakis-N-methyl-4'-pyridyl)-porphyrinato iron (III), a peroxynitrite decomposition catalyst, or L-cysteine, a peroxynitrite scavenger. We conclude that sGC/cGMP/PKG cascade and peroxynitrite formation may participate in Mev-induced phasic cardiovascular responses as signals downstream to NO generated respectively by NOS I and II in the RVLM.

Animals↗

PubMed2005

Blocking BRE expression in Leydig cells inhibits steroidogenesis by down-regulating 3beta-hydroxysteroid dehydrogenase.

Conversion of cholesterol to biologically active steroids is a multi-step enzymatic process. Along with some important enzymes, like cholesterol side-chain cleavage enzyme (P450scc) and 3beta-hydroxysteroid dehydrogenase/isomerase (3beta-HSD), several proteins play key role in steroidogenesis. The role of steroidogenic acute regulatory (StAR) protein is well established. A novel protein, BRE, found mainly in brain, adrenals and gonads, was highly expressed in hyperplastic rat adrenals with impaired steroidogenesis, suggesting its regulation by pituitary hormones. To further elucidate its role in steroidogenic tissues, mouse Leydig tumor cells (mLTC-1) were transfected with BRE antisense probes. Morphologically the BRE antisense cells exhibited large cytoplasmic lipid droplets and failed to shrink in response to human chorionic gonadotropin. Although cAMP production, along with StAR and P450scc mRNA expression, was unaffected in BRE antisense clones, progesterone and testosterone yields were significantly decreased, while pregnenolone was increased in response to human chorionic gonadotropin stimulation or in the presence of 22(R)OH-cholesterol. Furthermore, whereas exogenous progesterone was readily converted to testosterone, pregnenolone was not, suggesting impairment of pregnenolone-to-progesterone conversion, a step metabolized by 3beta-HSD. That steroidogenesis was compromised at the 3beta-HSD step was further confirmed by the reduced expression of 3beta-HSD type I (3ss-HSDI) mRNA in BRE antisense cells compared with controls. Our results suggest that BRE influences steroidogenesis through its effects on 3beta-HSD action, probably affecting its transcription.

3-Hydroxysteroid Dehydrogenases↗

PubMed2004

Differential contributions of NOS isoforms in the rostral ventrolateral medulla to cardiovascular responses associated with mevinphos intoxication in the rat.

The organophosphate poison mevinphos (Mev) elicits cardiovascular responses via nitric oxide (NO) produced on activation of M2 muscarinic receptors (M2R) in the rostral ventrolateral medulla (RVLM), where sympathetic vasomotor tone originates. This study further evaluated the contribution of nitric oxide synthase (NOS) isoforms at the RVLM to this process, using adult Sprague-Dawley rats. Bilateral co-microinjection into the RVLM of the selective NOS I inhibitor (250 pmol), 7-nitroindazole or N(omega)-propyl-L-arginine antagonized the initial sympathoexcitatory cardiovascular responses to Mev (10 nmol). Co-administration of a selective NOS II inhibitor, N6-(1-iminoethyl)-L-lysine (250 or 500 pmol) further enhanced these cardiovascular responses and reversed the secondary sympathoinhibitory actions of Mev. A potent NOS III inhibitor, N5-(1-iminoethyl)-L-ornithine (46 or 92 nmol) was ineffective. We also found that M2R co-localized only with NOS I- or NOS II-immunoreactive RVLM neurons. Furthermore, only NOS I or II in the ventrolateral medulla exhibited an elevation in mRNA or protein levels during the sympathoexcitatory phase, with further up-regulated synthesis of NOS II during the sympathoinhibitory phase of Mev intoxication. We conclude that whereas NOS III is not engaged, NO produced by NOS I and II in the RVLM plays, respectively, a sympathoexcitatory and sympathoinhibitory role in the cardiovascular responses during Mev intoxication.

Animals↗

PubMed2004

Contribution of cGMP but not peroxynitrite to negative feedback regulation of penile erection elicited by nitric oxide in the hippocampal formation of the rat.

We established previously that nitric oxide (NO) in the hippocampal formation (HF) participates actively in negative feedback regulation of penile erection. This study further evaluated whether this process engaged soluble guanylyl cyclase (sGC)/cGMP cascade or peroxynitrite in the HF. Intracavernous pressure (ICP) recorded from the penis in adult, male Sprague-Dawley rats anesthetized with chloral hydrate was employed as our experimental index for penile erection. Microinjection bilaterally of a NO-independent sGC activator, YC-1 (0.1 or 1 nmol) or a cGMP analog, 8-Bromo-cGMP (0.1 or 1 nmol), into the HF elicited a significant reduction in baseline ICP. Bilateral application into the HF of equimolar doses (0.5 or 1 nmol) of a sGC inhibitor, LY83583 or a NO-sensitive sGC inhibitor, ODQ significantly antagonized the decrease in baseline ICP induced by co-administration of the NO precursor, L-arginine (5 nmol), along with significant enhancement of the magnitude of papaverine-induced elevation in ICP. In contrast, a peroxynitrite scavenger, L-cysteine (50 or 100 pmol), or an active peroxynitrite decomposition catalyst, 5,10,15,20-tetrakis-(N-methyl-4'-pyridyl)-porphyrinato iron (III) (10 or 50 pmol), was ineffective in both events. These results suggest that NO may participate in negative feedback regulation of penile erection by activating the sGC/cGMP cascade in the HF selectively.

Aminoquinolines↗

PubMed2002

Contribution of peroxynitrite to fatal cardiovascular depression induced by overproduction of nitric oxide in rostral ventrolateral medulla of the rat.

We evaluated the contribution of peroxynitrite to the fatal cardiovascular depression induced by overproduction of nitric oxide (NO) after activation of inducible NO synthase (iNOS) in the rostral ventrolateral medulla (RVLM), the origin of sympathetic vasomotor tone. In Sprague-Dawley rats maintained under propofol anesthesia, microinjection of E. coli lipopolysaccharide (LPS) bilaterally into the RVLM elicited significant hypotension, bradycardia, reduction in sympathetic vasomotor tone and mortality. There was also a discernible elevation of iNOS expression in the ventrolateral medulla, followed by a massive production of nitrotyrosine, an experimental index for peroxynitrite. Co-administration bilaterally into the RVLM of the selective iNOS inhibitor, S-methylisothiourea (50, 100 or 250 pmol), an active peroxynitrite decomposition catalyst, 5,10,15,20-tetrakis- (N-methyl-4'-pyridyl)-porphyrinato iron (III) (10 or 50 pmol), a peroxynitrite scavenger, L-cysteine (5, 50 or 100 pmol), or a superoxide dismutase mimetic, Mn(III)-tetrakis-(4-benzoic acid) porphyrin (1 or 10 pmol), significantly prevented mortality, reduced nitrotyrosine production and reversed the NO-induced cardiovascular suppression after application of LPS into the RVLM. We conclude that the formation of peroxynitrite by a reaction between superoxide anion and NO is primarily responsible for the fatal cardiovascular depression induced by overproduction of NO after activation of iNOS at the RVLM.

Animals↗

PubMed2002

Down-regulation of basal Fos expression at nucleus tractus solitarii underlies restoration of baroreflex response after antihypertensive treatment in spontaneously hypertensive rats.

Antihypertensive therapy not only normalizes the elevated blood pressure but also restores the reduced baroreceptor reflex response associated with hypertension, although the underlying mechanism is not fully understood. We assessed the hypothesis that a reversal of the enhanced basal Fos expression seen during hypertension in nucleus tractus solitarii, the terminal site of baroreceptor afferents, underlies the restoration of baroreceptor reflex sensitivity after antihypertensive treatment. Male adult spontaneously hypertensive or normotensive Wistar-Kyoto rats received for 3 weeks captopril (100 mg/kg/day) added to their drinking water. Evaluated subsequently under pentobarbital anesthesia, captopril-treated spontaneously hypertensive rats exhibited significantly lowered systolic blood pressure and restoration of the sensitivity in baroreceptor reflex control of heart rate to levels comparable with Wistar-Kyoto rats. Reverse transcription-polymerase chain reaction analysis and immunohistochemical evaluation revealed concomitant down-regulation of basal expression in nucleus tractus solitarii of c-fos gene at both mRNA and protein levels. Captopril treatment, on the other hand, elicited no discernible effect on systolic blood pressure, cardiac baroreceptor reflex sensitivity or basal expression of Fos protein at the nucleus tractus solitarii of normotensive Wistar-Kyoto rats. From these findings we suggest that a down-regulation of basal Fos expression in nucleus tractus solitarii may contribute to the restoration of baroreceptor reflex sensitivity in spontaneously hypertensive rats that received antihypertensive treatment such as captopril.

Animals↗

PubMed2002

Differential expression of the suppressor PML and Ki-67 identifies three subtypes of human nasopharyngeal carcinoma.

The promyelocytic leukaemia (PML) gene, which encodes a transformation and growth suppressor, was found to regulate transcription and apoptosis. PML was first identified at the chromosomal translocation break-points t(15;17) of acute promyelocytic leukaemia and the gene product may mediate cell-cycle control and apoptosis. PML was found to interact with the co-transactivator CREB binding protein (CBP) and the apoptotic-modulator Bax. To determine if PML, CBP and Bax may be involved in solid tumours, such as the nasopharyngeal carcinoma (NPC), a rare neoplasia that is prevalent in Southern China, the expression of these proteins and the proliferation marker Ki-67 was analysed by immunohistochemical staining. Expression of PML in the PML-oncogenic domain (POD) or nuclear bodies in most NPC was inversely correlated with the expression of Ki-67. In addition, based on PML expression patterns in NPC three subtypes could be identified, namely, Subtype-1, with strong PML expression in POD structures and with low Ki-67 staining; Subtype-2, where PML was expressed in a homogeneously diffused pattern, but with a low intensity in the tumour cells; while Ki-67 was expressed in a moderate number of cells and Subtype-3, where the majority of tumour cells were PML-negative, while a considerable number of tumour cells were strongly labelled with Ki-67. Furthermore, CBP was present in most of the NPC cells with moderate-strong nuclear staining, while the expression in non-tumour cells were relatively weak. However, there was no direct correlation between PML and CBP expression in the NPC examined. In addition, there was low or no expression of Bax in the NP and NPC. This is, to our knowledge, the first report describing PML and CBP expression in NPC and our data strongly suggests that PML and CBP, but not Bax, may play a role in the transformed phenotypes of NPC.

CREB-Binding Protein↗