Biomedical subjects

I S Yuen

Publications and source records attributed to I S Yuen.

9 recordsLinked to original sources

PubMed1997

Inhalation of high concentrations of low toxicity dusts in rats results in impaired pulmonary clearance mechanisms and persistent inflammation.

This study was carried out to assess the time course of pulmonary clearance impairment and persistence of inflammation following high-dose inhalation exposures to titanium dioxide (TiO2) or carbonyl iron (CI) particles. Male rats were exposed to air, TiO2 or CI particles 6 hr/day, 5 days/week, for 4 weeks at concentrations of 5, 50, and 250 mg/m3 and evaluated at selected intervals through 6 months postexposure. Indices of pulmonary inflammation as well as alveolar macrophage clearance functions (i.e., morphology, in vivo and in vitro phagocytosis, and chemotaxis), cell proliferation, and histopathology endpoints were measured at several postexposure time periods through 6 months. In addition, amounts of TiO2 or CI in lungs and tracheobronchial lymph nodes were measured to allow an evaluation of particle clearance and translocation patterns. Four-week exposures to TiO2 or CI particles at concentrations of 250 mg/m3 resulted in lung burdens of 12 mg titanium and 17 mg iron, respectively, with particle retention half-times ranging from 68 days for 5 mg/m3 TiO2 to approximately 330 days for 250 mg/m3. The impact of this TiO2 dust load and similar lung burdens of CI particles produced a sustained pulmonary inflammatory response measured through a period of 3-6 months postexposure concomitant with increases in BrdU cell labeling of terminal airway and pulmonary parenchymal cells. The impairment of particle clearance mechanisms was accounted for by deficits in in vitro phagocytic and chemotactic potential of alveolar macrophages recovered from the lungs of high-dose, TiO2- or CI-exposed rats. Free granular pigment (TiO2 or CI) was present on the hypertrophic mucosal surfaces of bronchioles and bronchi, and particle-laden macrophages, found individually, were numerous throughout alveoli and within lymphoid tissues immediately after exposure. Aggregates of particle-laden macrophages were present within alveoli and alveolar ducts from 1 week postexposure through the entire 6-month recovery period. Macrophage accumulations increased in size and number from 1 week through 1 month postexposure and then appeared to remain constant through the remaining 5-month postexposure period. Minimal cellular hypertrophy and hyperplasia were evident at alveolar duct bifurcations adjacent to macrophage aggregates, and this effect was most prominent at 3 to 6 months postexposure. The results of this study clearly demonstrate that exposure to high dust concentrations of two different innocuous particle types produced sustained pulmonary inflammation, enhanced proliferation of pulmonary cells, impairment of particle clearance, deficits in macrophage function, and the appearance of macrophage aggregates at sites of particle deposition. In addition, the mass deposition rate determination appears to be a less sensitive indicator of "overload" when compared to biomarkers of pulmonary toxicity, such as macrophage function and cellular inflammation and proliferation indices.

Administration, Inhalation↗

PubMed1996

Subchronic inhalation of high concentrations of low toxicity, low solubility particulates produces sustained pulmonary inflammation and cellular proliferation.

Long-term inhalation exposures to high dust burdens can produce tumors or proliferative keratin cysts in the lungs of exposed rats. We hypothesized that dust burdens which overwhelm lung clearance mechanisms are associated with sustained cellular proliferation responses and pulmonary inflammation. Male rats were exposed to titanium dioxide (TiO2) or carbonyl iron (CI) particles for 4 weeks at concentrations of 5, 50 and 250 mg/m3. Following completion of exposure, the lungs of sham and dust-exposed animals were lavaged or assessed for cell proliferation or particle clearance immediately after, as well as 1 week, 1, 3 and 6 months postexposure. Exposures to TiO2 or CI at 250 mg/m3 produced persistent pulmonary inflammatory responses and increased BrdU labeling of terminal airway and pulmonary parenchymal cells. The results of this study clearly demonstrate that exposure to excessive dust concentrations of two low toxicity, low solubility particle-types produced sustained pulmonary inflammation, enhanced pulmonary cell labeling, impairment of particle clearance, and the development of pulmonary lesions.

Administration, Inhalation↗

PubMed1996

Time course of chemotactic factor generation and neutrophil recruitment in the lungs of dust-exposed rats.

The time course of neutrophil recruitment into the lung, neutrophilic chemotactic activity, and the gene expression of neutrophilic chemokines by lavaged cells was determined after intratracheal instillation of various particles. Low-toxicity, low-solubility dusts such as titanium dioxide (TiO2) particles, as well as fibrogenic crystalline silica and nonfibrogenic amorphous silica particles were instilled into the lungs of rats. Results showed that all three dusts induced neutrophilic inflammation as early as 5 h after exposure. Both crystalline and amorphous silica elicited higher degrees of pulmonary inflammation when compared with TiO2 particles. Maximal infiltration of neutrophils into the lungs occurred 5 to 6 h after intratracheal instillation of the dusts. The inflammatory response was transient for TiO2 and amorphous silica, i.e., evident at 2 days after exposure but not different from controls at 10 days after exposure. In contrast, inflammatory effects were sustained through a 10-day period following exposures to crystalline silica. Chemotactic activity for neutrophils was detected directly in bronchoalveolar lavage (BAL) fluids of dust-exposed rats within 2 h after exposure, but not in the BAL fluids of saline- or unexposed rats. The chemotactic activity was correlated with the influx and disappearance of neutrophils into alveolar regions of the lung in TiO2- and amorphous silica-exposed rats. The mRNA expression of two known neutrophil chemotactic cytokines in BAL cells, macrophage inflammatory protein-2 (MIP-2) and KC, also correlated with chemotactic activity and acute and pulmonary inflammatory responses. MIP-2 mRNA was expressed prior to the detection of chemotactic activity in BAL fluids. However, the mRNA expressions of MIP-2 and KC were transient for rats that were exposed to these dusts as KC and MIP-2 message were no longer detectable in BAL cells after 2 days of recovery. Although both neutrophilic chemotactic activity and inflammation remained prominent 10 days after exposure to crystalline silica, MIP-2 expression could not be detected in BAL cells. Thus, we conclude that MIP-2 is likely to be only one of several cytokines involved in mediating neutrophilic inflammation following a single instillation of crystalline silica.

Animals↗

PubMed1995

A density-sensing factor regulates signal transduction in Dictyostelium.

Dictyostelium discoideum initiates development when cells overgrow their bacterial food source and starve. To coordinate development, the cells monitor the extracellular level of a protein, conditioned medium factor (CMF), secreted by starved cells. When a majority of the cells in a given area have starved, as signaled by CMF secretion, the extracellular level of CMF rises above a threshold value and permits aggregation of the starved cells. The cells aggregate using relayed pulses of cAMP as the chemoattractant. Cells in which CMF accumulation has been blocked by antisense do not aggregate except in the presence of exogenous CMF. We find that these cells are viable but do not chemotax towards cAMP. Videomicroscopy indicates that the inability of CMF antisense cells to chemotax is not due to a gross defect in motility, although both video and scanning electron microscopy indicate that CMF increases the frequency of pseudopod formation. The activations of Ca2+ influx, adenylyl cyclase, and guanylyl cyclase in response to a pulse of cAMP are strongly inhibited in cells lacking CMF, but are rescued by as little as 10 s exposure of cells to CMF. The activation of phospholipase C by cAMP is not affected by CMF. Northern blots indicate normal levels of the cAMP receptor mRNA in CMF antisense cells during development, while cAMP binding assays and Scatchard plots indicate that CMF antisense cells contain normal levels of the cAMP receptor. In Dictyostelium, both adenylyl and guanylyl cyclases are activated via G proteins. We find that the interaction of the cAMP receptor with G proteins in vitro is not measurably affected by CMF, whereas the activation of adenylyl cyclase by G proteins requires cells to have been exposed to CMF. CMF thus appears to regulate aggregation by regulating an early step of cAMP signal transduction.

Animals↗

PubMed1994

Cell density-sensing in Dictyostelium by means of the accumulation rate, diffusion coefficient and activity threshold of a protein secreted by starved cells.

The simple eukaryote Dictyostelium discoideum grows as an amoeba on leaf and soil surfaces. When starved, the amoebae aggregate and differentiate. The amoebae can also be induced to differentiate as isolated cells submerged in buffer, if the buffer contains a sufficiently high concentration of a protein (CMF) secreted by starved cells. CMF is also necessary for aggregation and differentiation on surfaces. This indicated that CMF has either an autocrine function or is part of a density-sensing system. To distinguish between these two possible functions, we first examined the rate at which CMF is accumulated and the activity threshold of cells for CMF, since both parameters will affect whether a cell can provide enough CMF to self-stimulate. We find that CMF potentiates its own accumulation, and that otherwise the accumulation rate and activity threshold are affected very little by a variety of physiological conditions. We then use diffusion calculations to show that even after many hours of continuous secretion, the CMF concentration adjacent to an isolated starved cell on a leaf or soil surface will be too low to allow differentiation, whereas an extracellular concentration of CMF sufficiently high to allow differentiation will occur when starved cells are at high densities. We find a close match between the predicted and experimentally observed density necessary for differentiation. The theoretical and observed behavior of cells at different cell densities suggests that due to its accumulation rate, diffusion coefficient, and activation threshold, CMF can function as part of a cell density-sensing system which allows Dictyostelium cells in the wild to co-ordinate their development.

Animals↗

PubMed1992

Growing and starving Dictyostelium cells produce distinct density-sensing factors.

Prestarvation factor (PSF) and conditioned medium factor (CMF) are two autocrine factors produced by Dictyostelium cells. Although secreted at different times in the Dictyostelium life cycle (PSF by growing cells and CMF by starving cells), both factors are glycoproteins that are used by cells to measure their own density, and both are important in cell aggregation. To examine the relationship between PSF and CMF, a CMF antisense transformant was tested for the production of PSF during growth. Although this transformant produced extremely low levels of CMF, its production of PSF was essentially normal. We conclude that these two factors are not products of the same gene.

Animals↗

PubMed1992

A density-sensing factor controls development in Dictyostelium.

For an unknown reason, several genes expressed during Dictyostelium development are regulated by cell density. This is mediated by an 80-kD glycoprotein, conditioned medium factor (CMF), which is slowly secreted and simultaneously sensed by starved cells. To examine further this eukaryotic cell density-sensing mechanism, we have isolated a cDNA encoding CMF. The derived amino acid sequence of CMF shows no obvious similarity to any known protein and thus may represent a new class of eukaryotic intercellular signal. CMF antisense transformants do not aggregate, whereas normal development is restored by the addition of purified CMF protein. This suggests that CMF might synchronize the onset of development in Dictyostelium by triggering aggregation when a majority of the cells in a given area have starved, as signaled by CMF secretion.

Amino Acid Sequence↗

PubMed1991

A secreted 80 x 10(3) Mr protein mediates sensing of cell density and the onset of development in Dictyostelium.

In submerged monolayer culture, Dictyostelium cells can differentiate into prespore and prestalk cells at high cell densities in response to cAMP but not at low cell densities. However, cells at low densities will differentiate in medium taken from developing cells starved at a high density. The putative factor in the medium was designated CMF for conditioned medium factor (Mehdy and Firtel, Molec. cell. Biology 5, 705-713, 1985). In this report, we size-fractionate conditioned medium and show that the activity that allows low density cells to differentiate can be separated into high and low Mr (relative molecular mass) fractions. Interestingly, the two fractions both have the same activity and do not need to be combined to allow differentiation. The large conditioned medium factor is a protein, as determined by trypsin sensitivity, that can be purified to a single 80 x 10(3) Mr band on a silver-stained SDS-polyacrylamide gel, and has CMF activity at a concentration of approximately 4 pM (0.3 ng ml-1). Our results suggest that CMF is a secreted factor that functions in vivo as an indicator of cell density in starved cells. At high cell densities, the concentration of CMF is sufficient to enable cells to enter the multicellular stage of the developmental cycle. When present below a threshold concentration, cells do not initiate the expression of genes required for early development. This factor plays an essential role in the regulatory pathway necessary for cells to obtain the developmental competence to induce prestalk and prespore gene expression in response to cAMP.

Animals↗

PubMed1991

Regulation and processing of a secreted protein that mediates sensing of cell density in Dictyostelium.

During Dictyostelium development, the expression of some genes is dependent on cell density. This effect is mediated by soluble factors referred to as conditioned medium factors (CMFs) which the developing cells secrete at very low rates and simultaneously sense. There are at least two classes of CMFs: one is an 80 x 10(3) Mr glycoprotein and the other is a heterogeneous group of molecules, with relative molecular masses between 6.5 x 10(3) and 0.65 x 10(3). Interestingly, the two classes of molecules do not need to be combined for activity. We find that the 80 x 10(3) Mr CMF but not the small CMF is sequestered in vegetative cells. The 80 x 10(3) Mr CMF is then secreted by cells during early development, while the small CMF appears only during late development. Like the 80 x 10(3) Mr CMF, the small CMFs are trypsin-sensitive and contain N- and O-linked glycosylation. The breakdown products of a fraction containing 80 x 10(3) Mr CMF cochromatographed from a Sephadex G-50 column and a reverse-phase HPLC column with small CMFs. The specific activity of CMF increases roughtly 100-fold upon breakdown. The results suggest that, during differentiation, the slowly diffusing 80 x 10(3) Mr CMF is first produced from a precursor pool already present in vegetative cells, allowing differentiation of only those cells in the immediate vicinity of the aggregation center. The breakdown of 80 x 10(3) Mr CMF to a faster-diffusing, higher specific activity form then might enable cells farther from the aggregation center to differentiate.

Animals↗