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Biomedical subjects

L Ng

Publications and source records attributed to L Ng.

At least 37 records · Page 2Linked to original sources
PubMed1999

Mice devoid of all known thyroid hormone receptors are viable but exhibit disorders of the pituitary-thyroid axis, growth, and bone maturation.

Thyroid hormone (T3) has widespread functions in development and homeostasis, although the receptor pathways by which this diversity arises are unclear. Deletion of the T3 receptors TRalpha1 or TRbeta individually reveals only a small proportion of the phenotypes that arise in hypothyroidism, implying that additional pathways must exist. Here, we demonstrate that mice lacking both TRalpha1 and TRbeta (TRalpha1(-/-)beta-/-) display a novel array of phenotypes not found in single receptor-deficient mice, including an extremely hyperactive pituitary-thyroid axis, poor female fertility and retarded growth and bone maturation. These results establish that major T3 actions are mediated by common pathways in which TRalpha1 and TRbeta cooperate with or substitute for each other. Thus, varying the balance of use of TRalpha1 and TRbeta individually or in combination facilitates control of an extended spectrum of T3 actions. There was no evidence for any previously unidentified T3 receptors in TRalpha1(-/-)beta-/- mouse tissues. Compared to the debilitating symptoms of severe hypothyroidism, the milder overall phenotype of TRalpha1(-/-)beta-/- mice, lacking all known T3 receptors, indicates divergent consequences for hormone versus receptor deficiency. These distinctions suggest that T3-independent actions of T3 receptors, demonstrated previously in vitro, may be a significant function in vivo.

Age Factors↗
PubMed1998

A homologue of Drosophila aurora kinase is oncogenic and amplified in human colorectal cancers.

Genetic and biochemical studies in lower eukaryotes have identified several proteins that ensure accurate segregation of chromosomes. These include the Drosophila aurora and yeast Ipl1 kinases that are required for centrosome maturation and chromosome segregation. We have identified two human homologues of these genes, termed aurora1 and aurora2, that encode cell-cycle-regulated serine/threonine kinases. Here we demonstrate that the aurora2 gene maps to chromosome 20q13, a region amplified in a variety of human cancers, including a significant number of colorectal malignancies. We propose that aurora2 may be a target of this amplicon since its DNA is amplified and its RNA overexpressed, in more than 50% of primary colorectal cancers. Furthermore, overexpression of aurora2 transforms rodent fibroblasts. These observations implicate aurora2 as a potential oncogene in many colon, breast and other solid tumors, and identify centrosome-associated proteins as novel targets for cancer therapy.

3T3 Cells↗
PubMed1996

An adenovirus mutant that replicates selectively in p53-deficient human tumor cells.

The human adenovirus E1B gene encodes a 55-kilodalton protein that inactivates the cellular tumor suppressor protein p53. Here it is shown that a mutant adenovirus that does not express this viral protein can replicate in and lyse p53-deficient human tumor cells but not cells with functional p53. Ectopic expression of the 55-kilodalton EIB protein in the latter cells rendered them sensitive to infection with the mutant virus. Injection of the mutant virus into p53-deficient human cervical carcinomas grown in nude mice caused a significant reduction in tumor size and caused complete regression of 60 percent of the tumors. These data raise the possibility that mutant adenoviruses can be used to treat certain human tumors.

Adenovirus E1B Proteins↗
PubMed1996

Insulin-like growth factor-binding proteins (IGFBPs) and their regulatory dynamics.

The IGFBPs are a family of homologous proteins that have co-evolved with the IGFs and that confer upon the IGF regulatory system both functional and tissue specificity. IGFBPs are not merely carrier proteins for IGFs, but hold a central position in IGF ligand-receptor interactions through influences on both the bioavailability and distribution of IGFs in the extracellular environment. In addition, IGFBPs appear to have intrinsic biological activity independent of IGFs. The current status of research on IGFBPs is reviewed herein. Following a brief introduction to the entire IGF/IGFBP system, separate sections for each of the six cloned mammalian IGFBPs, the most extensive for IGFBP3, cover selected topics that emphasize the dynamics of IGFBPs--that is, their regulation in cells, their functionally important post-translational modifications, and their interactions in the cellular microenvironment--and how these dynamics influence physiological function.

Animals↗
PubMed1996

Thyroid hormone receptor beta is essential for development of auditory function.

Congenital thyroid disorders are often associated with profound deafness, indicating a requirement for thyroid hormone (T3) and its receptors in the development of hearing. Two T3 receptor genes, Tr alpha and Tr beta are differentially expressed, although in overlapping patterns, during development. Thus, the extent to which they mediate unique or redundant functions is unclear. We demonstrate that Tr beta-deficient (Thrb-/-) mice exhibit a permanent deficit in auditory function across a wide range of frequencies, although they show no other overt neurological defects. The auditory-evoked brainstem response (ABR) in Thrb-/- mice, although greatly diminished, displayed normal waveforms, which suggested that the primary defect resides in the cochlea. Although hypothyroidism causes cochlear malformation, there was no evidence of this in Thrb-/- mice. These findings suggest that Tr beta controls the maturation of auditory function but not morphogenesis of the cochlea. Thrb-/- mice provide a model for the human endocrine disorder of resistance to thyroid hormone (RTH), which is typically associated with dominant mutations in Tr beta. However, deafness is generally absent in RTH, indicating that dominant and recessive mutations in Tr beta have different consequences on the auditory system. Our results identify Tr beta as an essential transcription factor for auditory development and indicate that distinct Tr genes serve certain unique functions.

Animals↗
PubMed1995

Transforming growth factor-beta-induced cell growth inhibition in human breast cancer cells is mediated through insulin-like growth factor-binding protein-3 action.

Most estrogen receptor-negative breast cancer cells, including Hs578T cells, express mRNAs encoding insulin-like growth factor-binding protein (IGFBP)-3, as well as transforming growth factor (TGF)-beta receptors. Our previous studies (Oh, Y., Muller, H. L., Lamson, G., and Rosenfeld, R. G. (1993) J. Biol. Chem. 268, 14964-14971; Oh, Y., Muller, H. L., Pham, H. M., and Rosenfeld, R. G. (1993) J. Biol. Chem. 268, 26045-26048) have demonstrated a significant inhibitory effect of exogenous IGFBP-3 on Hs578T cell growth and existence of IGFBP-3-specific receptors that may mediate those direct inhibitory effect of IGFBP-3. TGF-beta is also a potent growth inhibitor in human breast cancer cells in vitro and regulates IGFBP-3 production in different cell systems, suggesting that IGFBP-3 is a major anti-proliferative factor and a key element for TGF-beta-induced growth inhibition in human breast cancer cells. In support of this hypothesis, we have demonstrated using Hs578T cells that: 1) TGF-beta stimulates IGFBP-3 gene expression and production prior to its inhibition of cell growth, 2) treatment with an IGFBP-3 antisense oligodeoxynucleotide selectively inhibits TGF-beta-induced IGFBP-3 synthesis and cell growth inhibition, and 3) treatment with IGF-II and IGF-II analogs diminish TGF-beta effects by blocking TGF-beta-induced binding of IGFBP-3 to the cell surface. These findings suggest that IGFBP-3 is a major anti-proliferative factor and a key element in TGF-beta-induced growth inhibition in human breast cancer cells.

Amino Acid Sequence↗
PubMed1995

Antiproliferative actions of insulin-like growth factor binding protein (IGFBP)-3 in human breast cancer cells.

A number of lines of evidence suggest that IGFs are important mitogens in human breast cancer: (1) IGFs are the most potent growth factor in human breast cancer cells; (2) estrogen stimulates expression of IGF-II and the type 1 IGF receptor; and (3) stromal cells express IGFs, which may act in a paracrine manner. Numerous studies have demonstrated that IGFBPs modulate the mitogenic effects of IGFs in the local environment. In particular, we have recently demonstrated that IGFBP-3 inhibits the growth of Hs578T and MDA-MB-231 human breast cancer cells in an IGF-independent manner. Further studies revealed the existence of cell surface-associated IGFBP-3 receptors. Receptor binding and the subsequent antiproliferative action of IGFBP-3 was inhibited by IGFs, owing to the formation of an IGF-IGFBP-3 complex that prevents the binding of IGFBP-3 to its receptors. In addition, exogeneously added soluble heparin or heparan sulfate inhibited the binding of IGFBP-3 to the cell surface in a dose-dependent manner. However, when heparin and heparan sulfate linkages of glycosaminoglycans on the cell surface were enzymatically remove, IGFBP-3 binding was only minimally affected. These data suggest that soluble heparin or heparan sulfate forms a complex with IGFBP-3, thereby inhibiting receptor binding of IGFBP-3, rather than competing with cell-surface glycosaminoglycans for binding of IGFBP-3. Additionally, the role of IGFBP-3 in the antiproliferative effects of transforming growth factor (TGF)-beta and retinoic acid (RA) is supported by our observations that: (1) inhibition of IGFBP-3 gene expression using an IGFNBP-3 antisense oligodeoxynucleotide not only blocks TGF-beta and RA simulation of IGFBP-3 production by up to 90%m but also inhibits their antiproliferative effects by 40-60%; and (2) treatment with IGF-II and IGF-II analogs diminish TGF-beta effects by blocking TGF-beta induced binding of IGFBP-3 to the cell surface. Taken together, our results support the hypothesis that IGFBP-3 is an important antiproliferative factor in human breast cancer, acting in an IGF-independent manner in addition to its ability to modulate the binding of IGF peptides to IGF receptors.

Breast Neoplasms↗
PubMed1995

Stimulation of two vascular smooth muscle-derived cell lines by angiotensin II: differential second messenger responses leading to mitogenesis.

1. We show here that angiotensin II (AII) and endothelin-1 (ET-1) stimulate [3H]-thymidine incorporation in a smooth muscle cell line derived from aortae of spontaneously hypertensive rats (SHR), but not in cells derived from normotensive controls (WKY). We have used the differential response of the two cell lines to investigate the relationship between second messenger systems and the mitogenic response. 2. AII produced an increase in accumulation of inositol 1,4,5-triphosphate which was greater in the SHR-derived cell line than in the WKY cells. 3. AII gave an increase in cytosolic Ca2+ in each of the cell lines, with both a larger peak (15-30 s) and plateau response (2 min) in the SHR cells. ET-1 gave an enhanced response in the SHR-derived cells with respect to the peak but not the plateau of cytosolic Ca2+. 4. Phospholipase D activity was studied by monitoring the formation of [3P]-phosphatidylbutanol in 32Pi prelabelled cells. AII stimulation gave a larger phospholipase D response in the SHR-derived cells, while ET-1 gave a larger response in WKY-derived cells. 5. Stimulation of SHR-derived cells with 100 nM AII for 1 h, followed by 19 h in the absence of agonist, stimulated [3H]-thymidine incorporation over the next 4 h. When the 1 h stimulation with AII was in the presence of increasing concentrations of butanol, which diverts the product of the phospholipase D pathway, there was a loss of stimulated [3H]-thymidine incorporation which was significant at 10 mM butanol and at 30-50 mM reached a maximum loss of 40%. 6. Contrasting with this there was no apparent loss of ET-l-stimulated thymidine incorporation when butanol was present at concentrations up to 40 mM.7. These results suggest that phospholipase D is one of several pathways in the mitogenic response of SHR-derived vascular smooth muscle cells to All.

Angiotensin II↗
PubMed1995

N-terminal variants of thyroid hormone receptor beta: differential function and potential contribution to syndrome of resistance to thyroid hormone.

The human syndrome of resistance to thyroid hormone (RTH) is associated with dominant mutations in the thyroid hormone receptor beta (TR beta) gene that generate mutant receptors with impaired binding for T3. Although the TR beta gene differentially expresses two N-terminal variant receptors, TR beta 1 and TR beta 2, functional analyses of RTH mutants have focused exclusively on TR beta 1. Since TR beta 2 is expressed in tissues that are malfunctional in RTH, the role of mutations in the context of TR beta 2 was examined. We compared the functional properties of corresponding RTH mutations in the common C-terminal domain of both TR beta 1 and TR beta 2. Wild type TR beta 1 and TR beta 2 bound similarly as homodimers and as heterodimers with retinoid X receptors to T3-responsive elements consisting of a direct repeat with 4-base pair spacing or an everted repeat. Homodimers, but not monomers or heterodimers, of both receptor subtypes were dissociated by the addition of T3. However, TR beta 2 formed at least 10-fold more stable homodimers than TR beta 1 on a palindromic repeat element, indicating that the N termini of TR beta 1 and TR beta 2 differentially influence dimerization on DNA. The RTH-like mutants of both TR beta 1 and TR beta 2 were equally insensitive to T3. They were defective in T3 binding but still bound DNA like their wild type counterparts except that the T3-dependent dissociation of homodimers from DNA was severely reduced. Wild type TR beta 1 and TR beta 2 mediated T3-inducible transactivation in cotransfection assays; this, however, was abolished in both mutants. TR beta 1 mediated more sensitive T3-dependent transcriptional suppression than TR beta 2 through the negative T3 response region of the TSH beta gene. Again, the mutation abolished T3-dependent suppression by both mutants. Furthermore, both mutants inhibited T3-inducible transcriptional activation by different wild type TR alpha and beta variants. These results indicate that both mutants have the potential to contribute to the pathogenesis of RTH and suggest that a reassessment of previous models of RTH is required to take into account the inhibitory activity of both TR beta 2 and TR beta 1 mutants.

3T3 Cells↗
PubMed1994

Identification of farnesol as the non-sterol derivative of mevalonic acid required for the accelerated degradation of 3-hydroxy-3-methylglutaryl-coenzyme A reductase.

The degradation of the microsomal enzyme 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase is highly regulated and is dependent on both a sterol and non-sterol derivative of mevalonic acid (MVA). We recently proposed that the non-sterol component is derived from farnesyl diphosphate (FPP), presqualene pyrophosphate, or squalene (Correll, C. C. and Edwards, P. A. (1994) J. Biol. Chem. 269, 633-638). In the current study, we have used digitonin-permeabilized cells to further define this MVA-derived non-sterol component required for the regulated degradation of HMG-CoA reductase. The addition of either FPP or farnesol to digitonin-permeabilized cells resulted in a rapid and dose-dependent degradation of HMG-CoA reductase. The effect of FPP, but not farnesol, was blocked by the phosphatase inhibitor sodium fluoride. The enhanced degradation of HMG-CoA reductase in permeabilized cells specifically required farnesol, since the addition of any of the structurally related isoprenoids geraniol, geranyl diphosphate, geranylgeranyl diphosphate, nerolidol, or all-cis-farnesol, or of the non-sterol squalene to the permeabilized cells did not stimulate enzyme degradation. The present studies demonstrate for the first time that the accelerated degradation of HMG-CoA reductase can be initiated in vitro. Further, since farnesol is shown to be specifically required for the enhanced degradation of the enzyme in vitro, we propose that this isoprenoid alcohol is important in this process in intact cells.

Animals↗
PubMed1994

Targeted disruption of NMDA receptor 1 gene abolishes NMDA response and results in neonatal death.

In vitro studies have suggested that the NMDA receptor consists of an essential subunit, NR1, and various modulatory NR2 subunits. To test this hypothesis directly in vivo, we generated mice carrying a disrupted NR1 allele. NMDA-inducible increases in intracellular calcium and membrane currents were abolished in neurons from homozygous null mutants (NR1-/-). Thus, NR1 has a unique role, which cannot be substituted by any other subunit, in determining the activity of the endogenous NMDA receptor. A concomitant reduction in levels of NR2B but not NR2A occurred in NR1-/- mice, demonstrating that there is an interdependence of subunit expression. NR1-/- mice died 8-15 hr after birth, indicating a vital neonatal function for the NMDA receptor. Although the NMDA receptor has been implicated in several aspects of neurodevelopment, overall neuroanatomy of NR1-/- mice appeared normal. Pathological evidence suggested that respiratory failure was the ultimate cause of death.

Animals↗
PubMed1993

Differential roles for Fos and Jun in DNA-binding: redox-dependent and independent functions.

The Fos and Jun family of transcription factors contain an invariant sequence motif lysine-cysteine-arginine (KCR) in the highly conserved DNA-binding region. Reduction of the cysteine residue is necessary to facilitate DNA-binding. Here, we examined the potential dual roles of the flanking lysine and arginine residues in influencing the redox reactivity of the cysteine and the DNA-binding activity of Fos and Jun. Two sets of Fos and Jun mutants were generated: the KCR and KSR series representing proteins capable of redox-dependent and redox-independent DNA-binding activity, respectively. Mutation of the lysine in Fos-Jun heterodimers had no obvious effect on the redox reactivity of the cysteine, suggesting that lysine is not essential in this respect. However, mutation of the arginine but not lysine, in both the KCR and the KSR series abolished DNA-binding activity. Thus, the arginine but not the lysine residue in the KCR motif is critical for both redox-dependent and redox-independent functions in DNA-binding. Surprisingly, the triple substitution, ISI, exhibited high levels of DNA-binding activity. This demonstrates that the effects of amino acid substitutions can be highly dependent on context and that non-basic amino acids can function efficiently in DNA-binding. Analysis of combinations of wild-type and mutant Fos and Jun proteins indicated that Fos was dominant in dictating the DNA-binding ability of Fos-Jun heterodimers. This suggests that the lysine and arginine residues in the KCR region of Fos are not equivalent to those in Jun and that they interact with DNA differently.

Animals↗
PubMed1993

Interaction of DNA polymerase delta, proliferating cell nuclear antigen, and synthetic oligonucleotide template-primers. Analysis by polyacrylamide gel electrophoresis-band mobility shift assay.

A polyacrylamide gel electrophoresis band-mobility shift assay was developed to study the binding of synthetic oligonucleotides by DNA polymerase delta (pol delta) and proliferating cell nuclear antigen (PCNA). As measured by this assay, neither calf thymus pol delta core enzyme nor PCNA alone bind DNA stably. However, mammalian PCNA but not Drosophila PCNA promotes the formation of a distinct pol delta.PCNA.template-primer complex. Appearance of this complex is primer-dependent but does not require Mg2+. Complex stability is also influenced by the presence or absence of individual dNTPs. A model for the ordered sequential interaction of pol delta, PCNA, and DNA template-primers is proposed.

Animals↗
PubMed1993

The Nottingham Adjustment Scale: a validation study.

The concept of adjustment to acquired sight loss is examined in the context of existing loss models. An alternative conceptual framework is presented which addresses the 'blindness experience', and which suggests that the depression so frequently encountered in those losing their sight can be understood better by recourse to cognitive factors than to psychoanalytically based theories of grieving. A scale to measure psychological status before and after rehabilitation is described, its factorial validity is demonstrated, and its validity in enabling changes to be measured. Practitioners are encouraged to adopt a similar perspective in other areas of acquired disability.

Adaptation, Psychological↗
PubMed1993

Shoulder antagonistic strength ratios during concentric and eccentric muscle actions in the scapular plane.

Reporting agonist/antagonist ratios is frequently more clinically applicable than reporting absolute strength values of single muscle groups. The purpose of this study was to measure shoulder abduction/adduction and external/internal rotation ratios in the functional scapular plane of shoulder movement. Ratios were calculated using peak and average torques during concentric and eccentric muscle actions, and ratios of healthy males and females were compared. Thirty-six, non-athletic subjects performed concentric-eccentric cycles at two angular velocities. No significant differences were observed in the ratios using peak and average torques as criterion measurements. Abduction/adduction ratios using peak torques were significantly greater during concentric than eccentric actions. External/internal rotation ratios using average torques were significantly greater during eccentric than concentric actions. External/internal rotation ratios at 180 degrees/sec were significantly greater than those at 90 degrees/sec. No significant differences were observed between male and female subjects. Ratios were higher than previously reported. This was attributed to the scapular plane testing position. The clinician should be aware that shoulder strength ratios are dependent on position and that the scapular plane may be advantageous in some cases for testing and treatment.

Adult↗
PubMed1993

Estimates of insulin action in normal, obese and NIDDM man: comparison of insulin and glucose infusion test, CIGMA, minimal model and glucose clamp techniques.

Many methods of varying complexity are available for the measurement of insulin resistance (action) in man. No study has previously compared several of these in the same subjects to establish which is the most appropriate for routine use. We have, therefore, compared six methods: the hyperinsulinemic eu(iso)-glycaemic clamp (Eu), insulin glucose infusion test (IGI), hyperglycaemic clamp (Hy), continuous infusion of glucose with model assessment (CIGMA), minimal model (Min) and modified minimal model (Mod). Nineteen subjects with varying degrees of glucose tolerance were studied. Eight normal (BMI 22.5 +/- 1.5 kg/m2), six obese (BMI 38 +/- 5 kg/m2) and five NIDDM subjects (BMI 27 +/- 3 kg/m2) were investigated, in a randomized fashion, on separate days. The ratio of metabolic clearance rate of glucose (MCRG) and Insulin (I) was used as the measure of insulin action during Eu, Hy and IGI. Si was calculated as the index of insulin sensitivity from (Min) and (Mod) and CIGMA was obtained as previously described. MCRG was converted to Si to allow for direct comparison with (Min). Methods requiring incremental endogenous insulin secretion (which was highly variable) to calculate an index of insulin action (Si Min, Si Hy and CIGMA) failed to find an overall difference between groups. Only Si Eu (p = 0.007) and Si IGI (p = 0.001) demonstrated a significant overall group difference when Si was used. When MCRG/I was used, Eu, IGI and Hy were able to distinguish a significant overall group difference. With the exception of CIGMA and SiHy all other methods found a significant difference in insulin action between normals and NIDDM subjects. Only Eu and IGI could distinguish obese from normal, while only Si Min, could distinguish obese from NIDDM subjects. Eu and IGI were the only methods to be significantly correlated: normals Rs = 0.75, p < 0.05, obese Rs = 0.9, p < 0.05, and NIDDM Rs = 1.0, p < 0.05. In conclusion we have demonstrated that: 1) The insulin-glucose infusion test and the eu(iso)glycaemic clamp were significantly correlated in normals, obese and NIDDM subjects. 2) Only the eu(iso)glycaemic clamp and insulin glucose infusion test could significantly separate obese from normal subjects. 3) the IGI appears to be a practical, simple and precise method for measuring in vivo insulin action in man and gives results closely similar to those found with the hyperinsulinemic eu(iso)glycaemic clamp.

Adult↗