Biomedical subjects

C M Tan

Publications and source records attributed to C M Tan.

At least 19 recordsLinked to original sources

PubMed2001

Tyrosine kinase-mediated serine phosphorylation of adenylyl cyclase.

Receptor tyrosine kinase (RTK) activation is associated with modulation of heptahelical receptor-stimulated adenylyl cyclase responses. The mechanisms underlying the RTK-mediated enhancement of adenylyl cyclase function remain unclear. In the present studies, we show that the tyrosine kinase-dependent enhancement of adenylyl cyclase isoform VI function parallels an enhancement in serine phosphorylation of the enzyme. This effect was mediated by both RTK activation, with IGF-1, and by tyrosine phosphatase inhibition, with sodium orthovanadate. This enhancement of adenylyl cyclase function was not attenuated by inhibitors of ERK, PKC, PKA, or PI3 kinase activity but was blunted by inhibition of endogenous p74(raf-1)() activity. To characterize the molecular site of this effect we identified multiple candidate serine residues in and adjacent to the adenylyl cyclase VI C1b catalytic region and performed serine-to-alanine site-directed mutagenesis using adenylyl cyclase VI as a template. Mutation of serine residues 603 and 608 or serine residues 744, 746, 750, and 754 attenuated both the tyrosine kinase-mediated enhancement of enzyme phosphorylation as well as the sensitization of function. Together, these data define a novel tyrosine kinase-mediated mechanism leading to serine phosphorylation of adenylyl cyclase isoform VI and the sensitization of adenylyl cyclase responsiveness.

Adenylyl Cyclase Inhibitors↗

PubMed2001

Arthroscopically-assisted reduction of intra-articular fractures and soft tissue management of distal radius.

Arthroscopy was used to help to reduce intra-articular fractures of the distal radius and treat soft tissue injuries in 33 acute cases. The fractures were treated by reduction under arthroscopic control and percutaneous fixation either with or without external fixation. The triangular fibrocartilage complex (TFCC) was torn in 18 of 33 patients (54%). All tears were peripheral and were repaired with arthroscopic procedures. Scapholunate (SL) ligament injuries prevailed in six (18%) patients; most of them exhibited instability in the SL joint. They received SL ligament debrided and transfixed with K-wires. Four (12%) of the patients suffered lunotriquetral (LT) ligament injuries; three of them also received transfixation with K-wires. Six (18%) of the patients exhibited chondral fractures. All fractures healed without measurable incongruity of joint surface and at follow-up (24 to 36 months), 11 patients displayed excellent results and 22 patients displayed good results according to the Mayo modified wrist score.

Adult↗

PubMed2000

Identification of a novel mechanism for endotoxin-mediated down-modulation of CC chemokine receptor expression.

In the present study, we explored the molecular mechanisms by which bacterial endotoxin (LPS) mediates the down-regulation of CCR2 receptors on human monocytes. We found that LPS induced a marked reduction in CCR2 cell surface protein levels which was blocked by pretreatment with the tyrosine kinase inhibitors genistein and herbimycin A. The effector mechanism underlying LPS-induced CCR2 down-modulation appears to involve the enzymatic activity of proteinases since Western blot analysis of LPS-stimulated monocytes revealed the degradation of a 38-kDa species corresponding to the CCR2B monomer. In RBL cells expressing the CCR2B-green fluorescent protein (GFP) fusion chemokine receptor, LPS stimulated the internalization and degradation of CCR2. The serine proteinase inhibitor N-alpha-p-tosyl-L-lysine chloromethyl ketone blocked LPS-induced down-modulation of CCR2 in monocytes and CCR2B-GFP in RBL cells. This work describes a previously uncharacterized mechanism for CC chemokine receptor down-modulation that is dependent upon tyrosine kinase activation and serine proteinase-mediated receptor degradation and may provide further insight into the mechanisms of leukocyte regulation during immunological and inflammatory responses.

Chemokine CCL2↗

PubMed2000

Localization of G-protein-coupled receptors in health and disease.

G-protein-coupled receptors (GPCRs) represent a superfamily of proteins, characterized by seven transmembrane alpha-helices, that signal through interactions with a family of heterotrimeric GTP-binding proteins, referred to as G proteins. The broad range of physiological functions associated with GPCRs indicates that a better understanding of these receptors and their regulation can provide a solid foundation for novel pharmacological interventions in a variety of disease states.

Animals↗

PubMed2000

Cleaning dental instruments: measuring the effectiveness of an instrument washer/disinfector.

PURPOSE: To analyze the cleaning effectiveness of one type of instrument washer available for use in a dental office. MATERIALS AND METHODS: Dental instruments were heavily contaminated with blood and specific test bacteria. They were placed in cleaning baskets or within instrument cassettes, processed through the instrument washer, and analyzed for remaining blood and viable bacteria. RESULTS: Information obtained indicated that the washer is an effective cleaning system for dental instruments.

Bacteria↗

PubMed1999

beta-arrestins regulate interleukin-8-induced CXCR1 internalization.

The functional role of neutrophils during acute inflammatory responses is regulated by two high affinity interleukin-8 receptors (CXCR1 and CXCR2) that are rapidly desensitized and internalized upon binding their cognate chemokine ligands. The efficient re-expression of CXCR1 on the surface of neutrophils following agonist-induced internalization suggests that CXCR1 surface receptor turnover may involve regulatory pathways and intracellular factors similar to those regulating beta2-adrenergic receptor internalization and re-expression. To examine the internalization pathway utilized by ligand-activated CXCR1, a CXCR1-GFP construct was transiently expressed in two different cell lines, HEK 293 and RBL-2H3 cells. While interleukin-8 stimulation promoted CXCR1 sequestration in RBL-2H3 cells, receptor internalization in HEK 293 cells required co-expression of G protein-coupled receptor kinase 2 and beta-arrestin proteins. The importance of beta-arrestins in CXCR1 internalization was confirmed by the ability of a dominant negative beta-arrestin 1-V53D mutant to block internalization of CXCR1 in RBL-2H3 cells. A role for dynamin was also demonstrated by the lack of CXCR1 internalization in dynamin I-K44A dominant negative mutant-transfected RBL-2H3 cells. Agonist-promoted co-localization of transferrin and CXCR1-GFP in endosomes of RBL-2H3 cells confirmed that receptor internalization occurs via clathrin-coated vesicles. Our data provides a direct link between agonist-induced internalization of CXCR1 and a requirement for G protein-coupled receptor kinase 2, beta-arrestins, and dynamin during this process.

Animals↗

PubMed1999

Vanadate stimulation of adenylyl cyclase: an index of tyrosine kinase vascular effects.

BACKGROUND: Beyond their mitogenic effects, hormones such as insulin, which activate receptor tyrosine kinases, regulate vascular tone. Further, we have demonstrated that receptor tyrosine kinase activation enhances adenylyl cyclase activation, a prominent mechanism that mediates vasodilation. However, whether tyrosine kinase-mediated human vascular responses parallel tyrosine kinase-mediated cellular effects on adenylyl cyclase activity is unknown. METHODS AND RESULTS: To assess tyrosine kinase-mediated vascular responses, vascular sensitivity to insulin was assessed with the dorsal hand vein linear variable differential transformer technique. Insulin infusion resulted in a dose-dependent relaxation in all subjects. Cellular responses were assessed by means of the insulinomimetic vanadate-mediated sensitization of vascular adenylyl cyclase activity. Vanadate stimulated a tyrosine kinase-dependent enhancement of adenylyl cyclase function in human and rat aortic vascular smooth muscle cells, human lymphocytes, and human aortic endothelial cells. Further, maximal insulin-mediated vasodilation was significantly positively correlated with maximal vanadate-mediated enhancement of human lymphocyte adenylyl cyclase activity. CONCLUSION: Insulin-mediated vasodilation is positively correlated with vanadate-mediated enhancement of adenylyl cyclase activity. Vanadate-mediated enhancement of adenylyl cyclase activity in lymphocytes may represent an index of tyrosine kinase-mediated vascular effects.

Adenylyl Cyclases↗

PubMed1999

G-protein-coupled receptor kinase expression in hypertension.

In human hypertension we have recently identified an increase in lymphocyte G-protein receptor kinase-2 (GRK-2) protein expression, the key protein regulating the interaction between G-protein-coupled receptors and activation of adenylyl cyclase. However, it was not known whether this increase in GRK-2 protein expression was attributable to regulation at the level of translation. Furthermore, the relationship between extent of GRK-2 expression, receptor activation of adenylyl cyclase, and blood pressure was unclear. We therefore studied lymphocytes from 7 young subjects with borderline hypertension and 14 young normotensive subjects. Immunodetectable GRK-2 protein expression in lymphocytes from subjects with hypertension was increased (155%+/-7% of normotensive subjects; P < .05). In addition, GRK-2 protein expression was positively correlated with blood pressure (r = 0.53; P = .013) and inversely correlated with beta-adrenergic-mediated adenylyl cyclase activity (r = -0.54, P = .012). However, lymphocyte GRK-2 messenger ribonucleic acid (mRNA) content was not altered (110%+/-13% of that observed in normotensive control subjects). Increased GRK-2 protein expression may be an important factor in the impairment of beta-adrenergic-mediated vasodilation, characteristic of the hypertensive state.

Adult↗

PubMed1997

G-protein-coupled receptor kinase activity is increased in hypertension.

Impaired vascular beta-adrenergic responsiveness may play an important role in the development and/or maintenance of hypertension. This defect has been associated with an alteration in receptor/guanine nucleotide regulatory protein (G-protein) interactions. However, the locus of this defect remains unclear. G-Protein-coupled receptor kinases (GRKs) phosphorylate serine/threonine residues on G-protein-linked receptors in an agonist-dependent manner. GRK activation mediates reduced receptor responsiveness and impaired receptor/G-protein coupling. To determine whether the impairment in beta-adrenergic response in human hypertension might be associated with altered GRK activity, we studied lymphocytes from younger hypertensive subjects as compared with older and younger normotensive subjects. We assessed GRK activity by rhodopsin phosphorylation and GRK expression by immunoblot. GRK activity was significantly increased in lymphocytes from younger hypertensive subjects and paralleled an increase in GRK-2 (beta ARK-1) protein expression. In contrast, no alterations in cAMP-dependent kinase (A-kinase) activity or GRK-5/6 expression were noted. GRK activity was not increased in lymphocytes from older normotensive subjects who demonstrated a similar impairment in beta-adrenergic-mediated adenylyl cyclase activation. These studies indicate that GRK activity is selectively increased in lymphocytes from hypertensive subjects. The increase in GRK activity may underlie the reduction in beta-adrenergic responsiveness characteristic of the hypertensive state.

Adenylyl Cyclases↗

PubMed1995

G protein alterations in hypertension and aging.

Defective vasodilator function could be important in the pathogenesis and/or maintenance of the hypertensive state and the predisposition of the elderly to hypertension. Impaired beta-adrenergic-mediated vasodilation and reduced lymphocyte beta-adrenergic activation of adenyl cyclase have been demonstrated both in aging and with hypertension. The cellular mechanisms responsible for these alterations remain unclear. To determine if these defects may be due to alterations in guanine nucleotide regulatory proteins (G proteins) that link receptor activation with effector function, we assessed (1) human lymphocyte adenyl cyclase activity, (2) stimulatory G proteins by cholera toxin-mediated [32P]ADP ribosylation and, in hypertensive subjects, with alpha s-specific and beta-subunit antisera, and (3) inhibitory G proteins by pertussis toxin-mediated [32P]ADP ribosylation and, in older subjects, with alpha i,1,2- and beta-subunit-specific antisera. Lymphocytes from older subjects and from hypertensive subjects demonstrated a comparable reduction in isoproterenol-stimulated adenyl cyclase. However, aluminum fluoride-stimulated activity was reduced only in lymphocytes from hypertensive subjects. Furthermore, aluminum fluoride-stimulated activity was inversely correlated with mean arterial pressure. In lymphocytes from younger hypertensive subjects, cholera toxin-mediated labeling was significantly increased. In contrast, inhibitory G protein labeling by immunodetection was unaltered. In lymphocytes from older subjects, cholera toxin-mediated labeling was not altered; however, pertussis toxin-mediated labelling was significantly increased. In contrast, inhibitory G protein labeling by immunodetection was unaltered. Overall, the study suggests alterations of G protein function of adenyl cyclase is impaired. However, these defects are associated with divergent alterations in stimulatory and inhibitory G proteins.

Adenylate Cyclase Toxin↗

PubMed1995

Oxidant stress enhances adenylyl cyclase activation.

The generation of oxygen-derived free radicals has been implicated in the disordered vascular regulation of inflammation and reperfusion. In the vasculature, oxygen-derived free radicals are vasodilatory. The mechanisms underlying this effect remain unclear. To examine the cellular processes involved, we studied the effects of hydrogen peroxide (H2O2) on adenylyl cyclase activity in A10 cells, a murine vascular smooth muscle cell line. Pretreatment with H2O2 caused a dose-dependent enhancement of forskolin-stimulated adenylyl cyclase activity (ED50, 44 mumol/L to a maximum of 166% of control activity; n = 4). This enhancement was attenuated by iron chelation with deferoxamine and by the intracellular hydroxyl scavenger dimethylthiourea and mimicked by preincubation with purine/xanthine oxidase either alone or in the presence of superoxide dismutase. The effects of H2O2 were completely blocked by the tyrosine kinase inhibitors genistein and tyrphostin A9 but not by its inactive analogue tyrphostin A1 (H2O2 alone, 149 +/- 13%; H2O2 + tyrphostin A9, 100 +/- 9%; H2O2 + tyrphostin A1, 171 +/- 21%; n = 4). H2O2 comparably enhanced adenylyl cyclase activity stimulated by isoproterenol (166 +/- 17% of control, n = 5) and sodium fluoride (177 +/- 18% of control, n = 5). Thus oxygen-derived free radicals enhance adenylyl cyclase activation, probably via tyrosine kinase-mediated effects on the catalytic subunit of adenylyl cyclase. Sensitization of adenylyl cyclase activation may be an important mechanism by which free radicals modulate hormone-mediated vasodilation.

Adenylyl Cyclases↗

PubMed1995

A modified Bristow procedure for recurrent anterior shoulder dislocation.

BACKGROUND: Dislocation of the shoulder is one of our earliest frequent injuries. Many procedures have been evaluated for treatment of the condition since the mid 1960s. The modified Bristow procedure was initially described in 1970 by May. Technically easy, the procedure is very effective in prevention of redislocation, and has gained popularity over two decades of use. METHODS: A retrospective study was done of 32 patients with recurrent anterior shoulder dislocation, who had received modified Bristow procedure during the years 1987 through 1991 in this Hospital. Average followup period was 26 months. RESULTS: At postoperative evaluation, average loss of external rotation and abduction as compared to the nonoperated side were 15 and 10 degrees, respectively. There was no limitation on daily activity, but some little restriction in extreme throwing sports was found. Postoperative complications included three patients who had bone block resorption and one patient who had superficial wound infection. There was no redislocation nor implant loosening in the study. CONCLUSIONS: This procedure is technically easy, had a low rate of complication, a high degree of patient satisfaction and very effectively prevents redislocation.

Adolescent↗

PubMed1993

Influence of context and preferred learning environments: approaches to studying physiology.

One hundred and twenty-eight medical students who had experienced a traditional-style preclinical curriculum completed three self-report questionnaires. Using factor analysis of students' responses this study explores interactions between study orientation, preferences for different kinds of learning environment, and evaluations of the physiology course. Such interactions can provide insight into the reasons why students fail to adopt effective learning strategies. Although many students had the intention to understand, they did not adopt a deep approach. Achievement motivation was strong, test anxiety high, and the course was perceived to be competitive. The meaning orientation merged with the achieving orientation; students were thus performance rather than task oriented. These students perceived the course to have been challenging, as did students within the reproducing orientation and who had 'surface' preferences. Students within the non-academic orientation had difficulty coping with the course. The findings suggest that conventional teaching and assessment methods are preventing students from developing appropriate criteria and internal standards for evaluating performance. An illusion of comprehension may prevent students from seeing the need to adopt more effective learning strategies and cause 'good' students with the ability to adopt a deep approach to abort the pursuit of deep understanding. Students' preferences and evaluations of teaching and assessment indicate that students within the different learning orientations have different educational needs. The implications for instruction and evaluation are discussed.

Attitude of Health Personnel↗

PubMed1991

Relative antigenicity of components of a vascularized limb allograft.

At present, the transplantation of vascularized limb-tissue allografts can be achieved only with generalized host immunosuppression, which results in significant systemic toxicity, thereby precluding their clinical use. A better understanding of the immunogenic mechanisms of these allografts may permit less toxic and thus clinically applicable means of host immunosuppression. In this study, individual vascularized limb tissues (skin, subcutaneous tissue, muscle, bone, and blood vessels) and a whole limb were transplanted microsurgically across a strong histocompatibility barrier in rats. The respective cell-mediated and humoral immune responses generated in the hosts were determined by means of mixed lymphocyte cultures by radioactive 51Cr release assays and compared. No single tissue predominated in the elicited immune response. Rather, the various tissue components interacted with the host immune system in a complex but predictable pattern with differing timing and intensity. Surprisingly, the whole-limb allograft elicited less immune response than did allografts of its individual components. The data presented here also serve as a foundation for further elucidation of the immunogenic mechanisms of vascularized limb-tissue allografts.

Animals↗

PubMed1991

Vascularized muscle allografts and the role of cyclosporine.

This study examined the fate of vascularized muscle allografts using a genetically defined rat model. Its purposes were (1) to analyze the histologic/immunologic responses, (2) to study the effect of cyclosporine on graft survival, and (3) to examine the possibility of inducing tolerance. In rats differing at a major histocompatibility locus, vascularized gastrocnemius muscle transplants were performed based on the sural branches of the femoral artery and vein. Forty-two animals studied were divided into three groups: Group 1, allografts, was treated without cyclosporine; Group 2, allografts, was administered continuous cyclosporine; and Group 3, allografts, was administered cyclosporine for 6 weeks only. Evaluation consisted of gross examination, H&E histology, and immunologic studies (MLC, CML, and complement-dependent 51Cr lysis assay). Lytic units (LU) were derived from the assays and served as the indicator of immune response. Group 1 animals had uniform rejection with intense cell-mediated response (LU 23 to 47) and low humoral response. Group 2 animals had viable allografts throughout with suppressed lytic unit values of 0 to 9 initially, which rose to 14 to 29 at 6 weeks despite continuous cyclosporine treatment. Group 3 animals showed rejection similar to the untreated animals. Autografts were performed as controls and survived indefinitely. Analysis of variance was significant at p less than 0.05. Using a reliable rat model for vascularized muscle allografts, we found that in transplantation across a major histocompatibility barrier, the initial immune response was primarily cell-mediated. Cyclosporine suppressed rejection only when given continuously, and short-term cyclosporine treatment did not induce a tolerant state. These data should be useful for future studies of vascularized muscle allografts.

Animals↗

PubMed1990

Academic standards and changing patterns of medical school admissions: a Malaysian study.

Changing social demands made it necessary for the Medical Faculty of the University of Malaya to accommodate students with a wider range of academic experience than before. However, teachers sought to achieve comparable academic standards to those in the West by striving to maintain a close resemblance to the Western model of medical education in other respects. As a result teachers failed to adapt their teaching methods, assessment techniques and curriculum design to meet the educational needs of the students, thus compromising academic standards. Many students lack basic academic skills and do not know how to learn effectively. In order to help students overcome their learning difficulties innovative teaching was required during the first year at university, designed to foster the joint development of knowledge and basic skills. In the case of less well-prepared students who lack self-confidence, a caring and supportive learning environment is crucial to the achievement of meaningful learning. Lecturers needed to become facilitators of learning rather than transmitters of knowledge. However, teachers' objective to retain international recognition of the degree, which presumably reflected the importance of teaching, was not operationalized in terms of its incentive structure such that teachers were constrained not to try to fill the new roles demanded of them. It was assumed that academic distinction accrued through scientific research was essential for the achievement of academic excellence. However, under the prevailing circumstances the two aims were mutually exclusive and incompatible and teaching quality deteriorated.

Curriculum↗

PubMed1990

The influence of laboratory experimental work on concept acquisition in a medical physiology course with implications for curriculum deliberations and instructional design.

Following revision of the curriculum the effectiveness of a traditional cookbook experiment, used in conjunction with an 'interpretation seminar', was evaluated. Curriculum revision had been predominantly concerned with an avoidance of overloading and provision of self-study periods. The preceding lectures were integrated with the experiment. The learning resulting from the practical experience was assessed using pre- and post-tests. The practical exercise was ineffective and did not facilitate conceptual understanding. Due to the central role of passive teaching methods the students adopted a surface approach to all learning, were teacher dependent and did not make effective use of their private study. Furthermore, owing to a broad-based entry into medical school many students lacked the basic skills essential to the achievement of meaningful learning. Clearly, for effective learning the curriculum and pedagogy must be geared to the background and educational needs of the students.

Curriculum↗

PubMed1989

[Assessment of present nutrition status with weight for height method in children of Guangxi].

According to the method of weight for height recommended by WHO in 1979. The present nutritional status of 9459 7-10 year old children in 5 prefectures and 3 cities of Guangxi was assessed. The results showed that the deficient nutrition rate was 19.94% and rate of obese children was 1.85%. The latter was lower than that reported by other provinces and foreign countries. The deficient nutrition rate (26.43%) of city children was higher than that of rural children (15.90%), but no statistically significant difference was found between male and female (P greater than 0.05).

Body Height↗