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Biomedical subjects

H Koh

Publications and source records attributed to H Koh.

At least 19 recordsLinked to original sources
PubMed2006

Missing xy-band Fermi surface in 4d transition-metal oxide Sr2RhO4: effect of the octahedra rotation on the electronic structure.

Electronic structures of the 4d transition-metal oxide compound Sr2RhO4 are investigated by angle-resolved photoemission spectroscopy and density-functional electronic structure calculations. In the measured Fermi surfaces (FS) of Sr2RhO4, the xy-band FS sheet expected from the well-established results of the FS of Sr2RuO4 is missing, the volume of which should be different only by one additional electron for Sr2RhO4. The apparent contradiction is resolved by a careful analysis of the band structure where the rotation of octahedra results in the hybridization of e(g) and t(2g) states and thus plays a key role in the determination of the electronic structure near EF. The modification of the FS structure due to the distorted lattice is related to the charge transfer among the orbital states and suggested to be relevant to the metal-insulator transition in Ca(2-x)Sr(x)RuO4.

Journal Article↗
PubMed2005

JNK pathway mediates apoptotic cell death induced by tumor suppressor LKB1 in Drosophila.

Although recent progresses have unveiled the diverse in vivo functions of LKB1, detailed molecular mechanisms governing these processes still remain enigmatic. Here, we showed that Drosophila LKB1 negatively regulates organ growth by caspase-dependent apoptosis, without affecting cell size and cell cycle progression. Through genetic screening for LKB1 modifiers, we discovered the JNK pathway as a novel component of LKB1 signaling; the JNK pathway was activated by LKB1 and mediated the LKB1-dependent apoptosis. Consistently, LKB1-null mutant was defective in embryonic apoptosis and displayed a drastic hyperplasia in the central nervous system; these phenotypes were fully rescued by ectopic JNK activation as well as wild-type LKB1 expression. Furthermore, inhibition of LKB1 resulted in epithelial morphogenesis failure, which was associated with a decrease in JNK activity. Collectively, our studies unprecedentedly elucidate JNK as the downstream mediator of the LKB1-dependent apoptosis, and provide a new paradigm for understanding the diverse LKB1 functions in vivo.

AMP-Activated Protein Kinase Kinases↗
PubMed2005

Continuous tuning of electronic correlations by alkali adsorption on layered 1T-TaS2.

Angle-resolved photoemission spectroscopy shows that a Mott-Hubbard type metal-insulator transition occurs at the Rb adsorbed surface of the layered charge-density-wave compound 1T-TaS2. The transition is driven by adsorption induced modifications of the charge-density wave and of the interlayer coupling, leading to an increase of the on-site Coulomb correlation energy and a narrowing of the Ta 5d band perpendicular to the layers, respectively. The continuous rearrangement of spectral weight is measured live during the deposition process.

Journal Article↗
PubMed2005

Effect of the New York State cigarette fire safety standard on ignition propensity, smoke constituents, and the consumer market.

OBJECTIVES: This study examines empirical evidence from the New York experience testing tobacco industry arguments made in opposition to fire safety standards for cigarettes. DESIGN: Percentages of cigarettes exhibiting full length burns (FLBs), cigarette sales before and following the implementation of the New York standards, a sample of retail cigarette prices, brand availability, and selected smoke constituent yields were compared between cigarettes sold in New York and two other states. Cigarette paper analysis was conducted on cigarettes sold in New York. RESULTS: New York cigarette brands averaged 10.0% FLBs as compared to 99.8% for California and Massachusetts brands. Reduced ignition propensity (RIP) appears to have been achieved by cigarette paper banding. Cigarette sales, prices, and brand availability do not appear to have been affected by the New York standards. Yields of the majority of smoke constituents tested did not differ substantially between RIP cigarettes sold in New York as compared to the same brands sold in Massachusetts. Average yields of tar, carbon monoxide, and two compounds were slightly higher, the yields of seven compounds were higher for one brand only, and nicotine was lower, among New York brands tested. CONCLUSIONS: RIP cigarette brands have been designed to meet the New York fire safety standards. Their introduction has not affected cigarette sales or prices in New York. There is no evidence that the small increases in smoke constituent yields affect the already highly toxic nature of cigarette smoke. Data on smoking caused fires, deaths, and injuries dating from after the change in law are not yet available. Such data will be able to address the question of whether the demonstrated reduced ignition standards are associated with reduced fires and injuries. Based on the New York experience, prior industry objections to producing RIP cigarettes are unfounded. Other states and nations should adopt similar standards.

Commerce↗
PubMed2004

Mechanism of gap opening in a triple-band Peierls system: in atomic wires on Si.

One dimensional (1D) metals are unstable at low temperature undergoing a metal-insulator transition coupled with a periodic lattice distortion, a Peierls transition. Angle-resolved photoemission study for the 1D metallic chains of In on Si(111), featuring a metal-insulator transition and triple metallic bands, clarifies in detail how the multiple band gaps are formed at low temperature. In addition to the gap opening for a half-filled ideal 1D band with a proper Fermi surface nesting, two other quasi-1D metallic bands are found to merge into a single band, opening a unique but k-dependent energy gap through an interband charge transfer. This result introduces a novel gap-opening mechanism for a multiband Peierls system where the interband interaction is important.

Journal Article↗
PubMed2004

Fermi surface and quasiparticle dynamics of Na0.7CoO2 investigated by angle-resolved photoemission spectroscopy.

We present the first angle-resolved photoemission study of Na0.7CoO2, the host material of the superconducting NaxCoO2.nH(2)O series. Our results show a hole-type Fermi surface, a strongly renormalized quasiparticle band, a small Fermi velocity, and a large Hubbard U. The quasiparticle band crosses the Fermi level from M toward Gamma suggesting a negative sign of effective single-particle hopping t(eff) (about 10 meV) which is on the order of magnetic exchange coupling J in this system. Quasiparticles are well defined only in the T-linear resistivity (non-Fermi-liquid) regime. Unusually small single-particle hopping and unconventional quasiparticle dynamics may have implications for understanding the phase of matter realized in this new class of a strongly interacting quantum system.

Journal Article↗
PubMed2004

Electronic quasiparticle renormalization on the spin wave energy scale.

High-resolution photoemission data of the (110) iron surface reveal the existence of well-defined metallic surface resonances in good correspondence to band calculations. Close to the Fermi level, their dispersion and momentum broadening display anomalies characteristic of quasiparticle renormalization due to coupling to bosonic excitations. Its energy scale exceeds that of phonons by far, and is in striking coincidence with that of the spin wave spectrum in iron. The self-energy behavior thus gives spectroscopic evidence of a quasiparticle mass enhancement due to electron-magnon coupling.

Journal Article↗
PubMed2004

Occlusal adjustment for treating and preventing temporomandibular joint disorders.

To assess the effectiveness of occlusal adjustment (OA) for treating temporomandibular disorders (TMD) in adults and preventing TMD. The Cochrane Controlled Trials Register, MEDLINE and EMBASE were comprehensively searched using the Cochrane methods. Reports and review articles were retrieved. Unpublished reports or abstracts were considered from the SIGLE database. All randomized or quasi-randomized controlled trials comparing OA with placebo, reassurance or no treatment in adults with TMD. The outcomes were global measures of symptoms, pain, headache and limitation of movement. Data collection and analysis followed the Cochrane Oral Health Group's statistical guidelines. Results showed no difference between OA and control group in symptom-based outcomes for treatment or incidence of symptoms for prevention. There is no evidence that OA treats or prevents TMD. OA cannot be recommended for the management or prevention of TMD. Future trials should use standardized diagnostic criteria and outcome measures when evaluating TMD.

Adult↗
PubMed2003

Indium square root 7 x square root 3 on Si(111): a nearly free electron metal in two dimensions.

We present measurements of the Fermi surface and underlying band structure of a single layer of indium on Si(111) with square root 7 x square root 3 periodicity. Electrons from both indium valence electrons and silicon dangling bonds contribute to a nearly free, two-dimensional metal on a pseudo-4-fold lattice, which is almost completely decoupled at the Fermi level from the underlying hexagonal silicon lattice. The mean free path inferred from our data is quite long, suggesting the system might be a suitable model for studying the ground state of two-dimensional metals.

Journal Article↗
PubMed2003

Occlusal adjustment for treating and preventing temporomandibular joint disorders.

BACKGROUND: There has been a long history of using occlusal adjustment in the management of temporomandibular disorders (TMD). It is not clear if occlusal adjustment is effective in treating TMD. OBJECTIVES: To assess the effectiveness of occlusal adjustment for treating TMD in adults and preventing TMD. SEARCH STRATEGY: We searched the Cochrane Oral Health Group's Trials Register (April 2002); the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library Issue 2, 2002); MEDLINE (1966 to 8th April 2002); EMBASE (1980 to 8th April 2002) and handsearched journals of particular importance to this review. Additional reports were identified from the reference lists of retrieved reports and from review articles of treating TMD. There were no language restrictions. Unpublished reports or abstracts were considered from the SIGLE database. SELECTION CRITERIA: All randomised or quasi-randomised controlled trials (RCTs) comparing occlusal adjustment to placebo, reassurance or no treatment in adults with TMD. The outcomes were global measures of symptoms, pain, headache and limitation of movement. DATA COLLECTION AND ANALYSIS: Data were independently extracted, in duplicate, by two reviewers, Holy Koh (HK) and Peter G Robinson (PR). Authors were contacted for details of randomisation and withdrawals and a quality assessment was carried out. The Cochrane Oral Health Group's statistical guidelines were followed and relative risk values calculated using random effects models where significant heterogeneity was detected (P<0.1). MAIN RESULTS: Over 660 trials were identified by the initial search. Six of these trials, which reported results from a total of 392 patients, were suitable for inclusion in the review. From the data provided in the published reports, symptom-based outcomes were extracted from trials on treatment. Data on incidence of symptoms were extracted from trials on prevention. Neither showed any difference between occlusal adjustment and control group. REVIEWER'S CONCLUSIONS: There is an absence of evidence, from RCTs, that occlusal adjustment treats or prevents TMD. Occlusal adjustment cannot be recommended for the management or prevention of TMD. Future trials should use standardised diagnostic criteria and outcome measures when evaluating TMD.

Adult↗
PubMed2002

Stress relief as the driving force for self-assembled bi nanolines.

Bi nanolines self-assemble on Si(001) and are remarkable for their straightness and length-they are often more than 400 nm long, and a kink in a nanoline has never been observed. Through electronic structure calculations, we have found an energetically favorable structure for these nanolines that agrees with our scanning tunneling microscopy and photoemission experiments; the structure has an extremely unusual subsurface structure, comprising a double core of seven-membered rings of silicon. Our proposed structure explains all the observed features of the nanolines, and shows that surface stress resulting from the mismatch between the Bi and the Si substrates is responsible for their self-assembly. This has wider implications for the controlled growth of nanostructures on semiconductor surfaces.

Journal Article↗
PubMed2002

Smoking behaviours and attitudes among male restaurant workers in Boston's Chinatown: a pilot study.

OBJECTIVE: Asian American immigrants experience high rates of cigarette smoking. A community based survey was conducted to understand the smoking behaviours, knowledge, and attitudes of restaurant workers in Boston's Chinatown. DESIGN: Cross sectional survey in Chinese of a convenience sample of 54 restaurant workers recruited through extensive outreach activities. RESULTS: All 54 of the workers were male immigrants. 45 (83.3%) reported smoking cigarettes regularly, and the remaining nine were former smokers. 36 of the smokers (80.0%) started smoking before entering the USA. The workers were aware that cigarettes are addictive (98.1%), cause lung cancer (79.6%), and lead to heart disease (64.8%). However, a substantial number reported that smoking was relaxing (75.9%) and enhanced concentration (66.7%). Nearly half believed low tar and low nicotine cigarettes to be safer than standard brands. The vast majority of workers believed that smoking was not socially acceptable for women. Smokers reported they received information on quitting most commonly from friends (60%), newspapers (53.5%), and television (44.4%). The restaurant workers most often saw advertising against smoking in Chinese newspapers (63%). CONCLUSION: Despite high rates of smoking, Chinese American restaurant workers were generally aware of the health risks and were interested in quitting. Community based research can set the stage for targeted public health efforts to reduce smoking in immigrant communities.

Adult↗
PubMed2001

Elevated circulating level of ghrelin in cachexia associated with chronic heart failure: relationships between ghrelin and anabolic/catabolic factors.

BACKGROUND: Ghrelin is a novel growth hormone (GH)-releasing peptide, isolated from the stomach, that may also cause a positive energy balance by stimulating food intake and inducing adiposity. We sought to investigate the pathophysiology of ghrelin in the cachexia associated with chronic heart failure (CHF). METHODS AND RESULTS: Plasma ghrelin was measured in 74 patients with CHF and 12 control subjects, together with potentially important anabolic and catabolic factors, such as GH and tumor necrosis factor (TNF-alpha). Patients with CHF were divided into two groups, those with cachexia (n=28) and those without cachexia (n=46). Plasma ghrelin did not significantly differ between all CHF patients and controls (181+/-10 versus 140+/-14 fmol/mL, P=NS). However, plasma ghrelin was significantly higher in CHF patients with cachexia than in those without cachexia (237+/-18 versus 147+/-10 fmol/mL, P<0.001). Circulating GH, TNF-alpha, norepinephrine, and angiotensin II were also significantly higher in CHF patients with cachexia than in those without cachexia. Interestingly, plasma ghrelin correlated positively with GH (r=0.28, P<0.05) and TNF-alpha (r=0.31, P<0.05) and negatively with body mass index (r=-0.35, P<0.01). CONCLUSIONS: Plasma ghrelin was elevated in cachectic patients with CHF, associated with increases in GH and TNF-alpha and a decrease in body mass index. Considering ghrelin-induced positive energy effects, increased ghrelin may represent a compensatory mechanism under catabolic-anabolic imbalance in cachectic patients with CHF.

Adolescent↗
PubMed2001

Drosophila phosphoinositide-dependent kinase-1 regulates apoptosis and growth via the phosphoinositide 3-kinase-dependent signaling pathway.

Phosphoinositide-dependent kinase-1 (PDK-1) is a central mediator of the cell signaling between phosphoinositide 3-kinase (PI3K) and various intracellular serine/threonine kinases including Akt/protein kinase B (PKB), p70 S6 kinases, and protein kinase C. Recent studies with cell transfection experiments have implied that PDK-1 may be involved in various cell functions including cell growth and apoptosis. However, despite its pivotal role in cellular signalings, the in vivo functions of PDK-1 in a multicellular system have rarely been investigated. Here, we have isolated Drosophila PDK-1 (dPDK-1) mutants and characterized the in vivo roles of the kinase. Drosophila deficient in the dPDK-1 gene exhibited lethality and an apoptotic phenotype in the embryonic stage. Conversely, overexpression of dPDK-1 increased cell and organ size in a Drosophila PI3K-dependent manner. dPDK-1 not only could activate Drosophila Akt/PKB (Dakt1), but also substitute the in vivo functions of its mammalian ortholog to activate Akt/PKB. This functional interaction between dPDK-1 and Dakt1 was further confirmed through genetic analyses in Drosophila. On the other hand, cAMP-dependent protein kinase, which has been proposed as a possible target of dPDK-1, did not interact with dPDK-1. In conclusion, our findings provide direct evidence that dPDK-1 regulates cell growth and apoptosis during Drosophila development via the PI3K-dependent signaling pathway and demonstrate our Drosophila system to be a powerful tool for elucidating the in vivo functions and targets of PDK-1.

3-Phosphoinositide-Dependent Protein Kinases↗
PubMed2001

Cyclic AMP inhibits Akt activity by blocking the membrane localization of PDK1.

Akt is a protein serine/threonine kinase that plays an important role in the mitogenic responses of cells to variable stimuli. Akt contains a pleckstrin homology (PH) domain and is activated by phosphorylation at threonine 308 and serine 473. Binding of 3'-OH phosphorylated phosphoinositides to the PH domain results in the translocation of Akt to the plasma membrane where it is activated by upstream kinases such as (phosphoinositide-dependent kinase-1 (PDK1). Over-expression of constitutively active forms of Akt promotes cell proliferation and survival, and also stimulates p70 S6 kinase (p70S6K). In many cells, an increase in levels of intracellular cyclic AMP (cAMP) diminishes cell growth and promotes differentiation, and in certain conditions cAMP is even antagonistic to the effect of growth factors. Here, we show that cAMP has inhibitory effects on the phosphatidylinositol 3-kinase/PDK/Akt signaling pathway. cAMP potently inhibits phosphorylation at threonine 308 and serine 473 of Akt, which is required for the protein kinase activities of Akt. cAMP also negatively regulates PDK1 by inhibiting its translocation to the plasma membrane, despite not affecting its protein kinase activities. Furthermore, when we co-expressed myristoylated Akt and PDK1 mutants which constitutively co-localize in the plasma membrane, Akt activity was no longer sensitive to raised intracellular cAMP concentrations. Finally, cAMP was also found to inhibit the lipid kinase activity of PI3K and to decrease the levels of phosphatidylinositol 3,4,5-triphosphate in vivo, which are required for the membrane localization of PDK1. Collectively, these data strongly support the theory that the cAMP-dependent signaling pathway inhibits Akt activity by blocking the coupling between Akt and its upstream regulators, PDK, in the plasma membrane.

3-Phosphoinositide-Dependent Protein Kinases↗
PubMed2001

Retroperitoneoscopic radical nephrectomy by the posterior lumber approach for renal-cell carcinoma associated with chronic renal failure.

PURPOSE: To evaluate the efficacy and invasiveness of retroperitoneoscopic radical nephrectomy for renal-cell carcinoma (RCC) in patients with chronic renal failure (CRF), a group known to have relatively high surgical risk. PATIENTS AND METHODS: Between May 1996 and September 1999, six CRF patients maintained on hemodialysis underwent retroperitoneoscopic radical nephrectomy for clinically localized RCC by the posterior lumber approach. The excised kidneys were evacuated via a posterior skin incision (5 cm) between two port sites; the muscle layers were not incised. RESULTS: The procedure was completed in all patients with no major complications. The mean operative time was 162 (range 135-210) minutes, and the estimated blood loss was 58 (15-100) mL; none of the patients required a blood transfusion. Regular hemodialysis was restarted on postoperative day 2 or 3. CONCLUSIONS: This procedure seems to be minimally invasive and suitable for the treatment of small RCC in atrophic kidneys, especially in patients with CRF.

Atrophy↗
PubMed2001

Akt/PKB promotes cancer cell invasion via increased motility and metalloproteinase production.

The Akt/protein kinase B (PKB) serine/threonine kinase is well known as an important mediator of many cell survival signaling pathways. Here, we demonstrate for the first time a major role of Akt/PKB in the cell invasion properties of the highly metastatic cell line HT1080. Using confocal microscopic analyses of live samples, we found Akt/PKB to be localized in the leading edge membrane area of migrating HT1080 cells. This localization was dependent on phosphoinositide 3-kinase and required the lipid binding ability of the phosphoinositide binding pleckstrin homology domain of Akt/PKB. We examined the possible function of Akt/PKB in HT1080 invasion. Surprisingly, Akt/PKB potently promoted HT1080 invasion, by increasing cell motility and matrix metalloproteinase-9 (MMP-9) production, in a manner highly dependent on its kinase activity and membrane-translocating ability. The increase in MMP-9 production was mediated by activation of nuclear factor-kappaB transcriptional activity by Akt/PKB. However, Akt/PKB did not affect the cell-cell or cell-matrix adhesion properties of HT1080. Our findings thus establish Akt/PKB as a major factor in the invasive abilities of cancer cells.

Cell Adhesion↗