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Biomedical subjects

K Rhee

Publications and source records attributed to K Rhee.

33 records · Page 2Linked to original sources
PubMed1995

Distinct patterns of expression of the D-type cyclins during testicular development in the mouse.

The three D-type cyclins have been shown to be differentially expressed in a number of isolated cell types and cell lines, suggesting distinct roles in cell cycle regulation in particular cell lineages. The testis provides unique opportunities to study genes involved in cell cycle regulation, since it contains cells in both mitosis and meiosis as well as differentiated cells with little proliferation activity. Major transcripts of 4.2 kb, 6.8 kb, and 2.3 kb were detected in the adult mouse testis by Northern hybridization analyses for cyclin D1, cyclin D2, and cyclin D3, respectively. Additional transcripts of 1.8 and 2.7 kb were detected by Northern hybridization for cyclin D3 in the testis, but not in other tissues, and these transcripts were limited to germ cells. Northern and in situ hybridization analyses of normal and germ cell-deficient testes showed the surprising result that cyclin D1 was expressed in a pattern consistent with expression in the non-dividing Sertoli cells. Cyclin D2 levels appeared slightly enriched in germ cell-deficient testes as compared to intact testis, but in situ hybridization analysis did not reveal any distinct cellular localization. Also surprising was the observation that cyclin D3 expression was highest in the non-dividing, haploid, round spermatids. The possible roles of these cyclins in the events of spermatogenesis are discussed.

Animals↗
PubMed1995

Genetic control of mitosis, meiosis and cellular differentiation during mammalian spermatogenesis.

Gametogenesis in both the male and female mammal represents a specialized and highly regulated series of cell cycle events, involving both mitosis and meiosis as well as subsequent differentiation. Recent advances in our understanding of the genetic control of the eukaryotic cell cycle have underscored the evolutionarily-conserved nature of these regulatory processes. However, most of the data have been obtained from yeast model systems and mammalian cell lines. Furthermore, most of the observations focus on regulation of mitotic cell cycles. In the present paper: (i) aspects of gametogenesis in mammals that represent unique cell-cycle control points are highlighted; (ii) current knowledge on the regulation of the germ cell cycle, in the context of what is known in yeast and other model eukaryotic systems, is summarized; and (iii) strategies that can be used to identify additional cell cycle regulating genes are outlined.

Animals↗
PubMed1995

Glucocorticoid regulation of G1 cyclin-dependent kinase genes in lymphoid cells.

These experiments were undertaken to study cell cycle-dependent regulation of expression of genes encoding cyclin-dependent kinases (Cdks). P1798 T-lymphoma cells were studied as a model system, since these cells undergo reversible G0 arrest within 24 h after addition of 0.1 microM dexamethasone to mid log phase cultures. G0 arrest is associated with inhibition of expression of several Cdks. The mRNAs encoding Cdk1 and Cdk4 decreased by 80-90% within 24 h. Fifty % inhibition of Cdk4 mRNA occurred within about 4 h, and 50% inhibition of Cdk1 mRNA was observed within 12-14 h. There was a slight decrease (< 50%) in the abundance of the mRNAs encoding Cdk2 and Cdk5. Cdk6 mRNA did not decrease in glucocorticoid-treated cells. Cdk1 and Cdk2 protein levels were reduced by no more than 50-70% within 24 h after the addition of dexamethasone, and the amounts of Cdk5 and Cdk6 protein did not change. However, the amount of Cdk4 protein decreased by > 90% under these circumstances. P1798 cells enter S phase in a synchronous fashion within 16-20 h after removal of dexamethasone. Cdk1, Cdk2, and Cdk5 mRNAs and proteins increased at or after the time that cells began to enter S phase. The mRNA encoding Cdk4 increased much more rapidly after removal of glucocorticoids, and a 5-fold increase in Cdk4 mRNA abundance was observed within 8 h after removal of the steroid. A corresponding increase in Cdk4 protein was observed, indicating that inhibition of Cdk4 expression is more proximal to the glucocorticoid-induced blockade in G1 progression.(ABSTRACT TRUNCATED AT 250 WORDS)

Cyclin-Dependent Kinase 4↗
PubMed1994

Gene for an extracellular matrix receptor protein from Pneumocystis carinii.

An initial and crucial step in the establishment of many microbial infections is the attachment of the pathogen to the host cells. Thus, adherence of Pneumocystis carinii (Pc) to type I pneumocytes is believed to be important in the induction of Pc pneumonia. Little is known about the nature of the attachment of Pc to type I cells, although extracellular matrix (ECM) proteins, such as fibronectin and laminin, have been implicated in the process. We report here the isolation of a Pc gene encoding a receptor protein that binds both fibronectin and laminin in vitro. A cDNA clone encoding the Pc ECM receptor was isolated from a Pc cDNA library and identified on the basis of sequence homology to the human colon carcinoma laminin receptor. Southern blot analysis of Pc genomic DNA confirmed that the cDNA was of Pc origin. Northern blot analysis of Pc total RNA showed a predominant mRNA of approximately 1400 nucleotides that hybridized to the ECM receptor gene. The ECM receptor predicted from the cDNA sequence is 295 amino acid residues long, with a molecular mass of 32.8 kDa. The C-terminal third of the polypeptide is highly negatively charged, whereas the N-terminal two-thirds contains hydrophobic segments that may play a role in membrane association. Sequence analysis and alignment of the N terminus with the laminin receptor cDNA sequence of human colon carcinoma support the conclusion that the Pc ECM receptor cDNA clone is a full-length clone. A Western blot of the overexpressed ECM receptor protein bound both laminin and fibronectin in vitro. Antibodies raised to the overexpressed receptor protein interacted with a 33-kDa protein in total Pc cell lysates. These findings raise the possibility that the Pc ECM receptor protein may mediate the organism's attachment to type I pneumocytes and, thus, may play a crucial role in Pc pathogenesis.

Amino Acid Sequence↗
PubMed1994

The macromolecular state of the transcription factor E2F and glucocorticoid regulation of c-myc transcription.

Glucocorticoids inhibit transcription of the proto-oncogene c-myc in lymphoid cells of thymic origin. To determine if this effect is associated with changes in the properties of the transcription factor E2F, extracts were prepared from control and glucocorticoid-treated P1798 murine T lymphoma cells, and the macromolecular state of E2F was assessed by gel-mobility shift. Control extracts exhibit two predominant gel-mobility shift entities of which one corresponds to "free" E2F. A second entity, complex C, has properties similar to those described for the complex containing E2F, p107, cyclin A, and Cdk2. Complex C disappears after addition of dexamethasone and is replaced by complex D. The mobility of this complex and its sensitivity to SV40 T antigen suggest that complex D corresponds to an E2F-p105Rb-1 complex. Extracts from control and glucocorticoid-treated cells yield identical DNase I protection patterns on the c-myc P2 promoter. Furthermore, such extracts transcribe the c-myc P2 promoter in vitro with equal activity. The relative abundance of the E2F complexes was measured after addition of dexamethasone. Complex C disappears as cells withdraw from S phase, and complex D appears at this time. The genes encoding thymidine kinase (Tk-1) and p34cdc2 (cdc2) are regulated with kinetics similar to those observed for changes in the macromolecular state of E2F. However, regulation of c-myc expression occurs long before any change in E2F. The macromolecular state of E2F may regulate expression of genes at the G1/S boundary. However, the data are not consistent with the hypothesis that association of E2F with tumor suppressor gene products such as p107 or p105Rb-1 is relevant to glucocorticoid regulation of c-myc transcription.

Animals↗
PubMed1994

Analysis of thymidine kinase gene expression in preimplantation mouse embryos.

Thymidine kinase (TK) activity was examined during the development of preimplantation mouse embryos. TK activity was increased approximately 20-fold from day 2 embryos (2-cell) to day 5 embryos (late blastocyst). TK activity did not change along with the progression into S-phase of the first and the second cell cycles but increased sharply at S-phase of the third cell cycle. Analysis of TK mRNA with a reverse transcription-polymerase chain reaction (RT-PCR) method showed that the level of TK mRNA was low in ovulated eggs and 1-cell embryos and was hardly detectable in day 2 embryos (2-cell), but sharply increased in day 3 embryos (mixture of 5- to 8-cell and morula). The functional role of 5'-flanking sequence of TK gene was also investigated in preimplantation embryos after microinjection with the DNA construct of 5'-flanking sequence of TK (2.4 kb) linked to bacterial lacZ gene (TK2.5lacZ) into the pronucleus of 1-cell and subsequently by histochemical staining with X-gal. beta-Galactosidase activity was first detected in day 3 embryos (8-cell), and 30% of embryos were stained with X-gal in day 4 and day 5 embryos, respectively. These results show that an increase in TK activity occurred after 2-cell stage, and this increase was primarily due to the embryonic activation of TK gene expression. Also, it appears that the 5'-flanking sequence of TK may directly regulate the TK gene expression at the transcriptional level during preimplantation murine development.

Animals↗
PubMed1994

A study of the wall shear rate distribution near the end-to-end anastomosis of a rigid graft and a compliant artery.

The development of intimal hyperplasia in the anastomotic region of a vascular graft which does not match the compliance of the parent artery may be related to altered wall shear rates near the anastomosis. The purpose of this study is to determine the effect of radial wall motion and the phase angle between pressure and flow waves (impedance phase angle) on the wall shear rate distribution near an end-to-end vascular graft anastomosis model incorporating a rigid graft and a compliant artery. The wall shear rate is determined from near-wall velocity profiles obtained by a flow visualization method using a photo-chromic dye for different locations near the anastomosis. The results show that the mean wall shear rate under pulsatile flow conditions is 15-30% lower than under steady flow conditions at the same mean flow rate. The effect of the impedance phase angle on the mean wall shear rate is shown to be small, but its effect on the amplitude of the wall shear rate is not negligible. For our anastomosis model which has well-matched diameters at the mean pressure, the mean shear rates at the distal sites are lower than at the proximal sites by 15-23%. We suppose that the differences in the mean wall shear rate between the proximal and distal sites are related to the convergent/divergent geometry caused by the mismatch of the compliance. Since the distal side of the anastomosis is more prone to intimal hyperplasia, lower shear rates near the distal anastomosis favor the hypothesis that low/oscillatory wall shear rates lead to intimal hyperplasia.

Anastomosis, Surgical↗
PubMed1994

Expression of c-Myc in glucocorticoid-treated fibroblastic cells.

Glucocorticoids inhibit proliferation of L929 fibroblastic cells in culture. Inhibition of proliferation is reversible and is not associated with changes in the plating efficiency of the cells. Flow cytometric analysis indicates that glucocorticoid-treated cells exhibit a decrease in the percentage of cells with DNA content > 2 N. Thymidine kinase expression is inhibited as cells with 2 N DNA content accumulate. These observations indicate that glucocorticoids arrest proliferation of L929 cells in the G1 phase of the cell cycle. The abundance of c-Myc mRNA does not decrease in glucocorticoid-treated cells, and c-Myc protein content in dexamethasone-treated cells is approximately the same as that detected in mid-log phase cells. Nuclear run-on transcription of c-Myc is not inhibited by glucocorticoids. These observations indicate that glucocorticoid regulation of fibroblastic cell proliferation does not involve inhibition of c-Myc transcription. Although regulation of c-Myc expression is central to the mechanism whereby glucocorticoids regulate proliferation of lymphoid cells, it is clear that different mechanisms must be involved in glucocorticoid regulation of fibroblastic cell proliferation.

Animals↗
PubMed1994

[Genetic control of cellular differentiation and proliferation during gametogenesis in mammals].

Understanding the genetic program that controls the regulation of cells progressing through the mitotic and meiotic cell cycles, whether in response to growth factors, other extracellular signals, or intrinsic programs, is critical to problems in infertility and in gonadal neoplasias. In this study, we present a consideration of important stages of cell cycle control in mammalian germ cells and data illustrating the experimental approaches that can be used to determine the pattern of such genes which may be involved in these events.

Animals↗
PubMed1993

Glucocorticoid regulation of thymidine kinase (Tk-1) expression in L929 cells.

Glucocorticoids regulate the proliferation of mouse L cells. Incorporation of [3H]thymidine is inhibited by 70-90% within 24 h after addition of 0.1 microM dexamethasone. This effect on L cells is completely reversible. The expression of the thymidine kinase gene (Tk-1) has been examined in L cells that have been treated with 0.1 microM dexamethasone for 24 h. Dexamethasone inhibits thymidine kinase activity 70-90% after 24 h. This is associated with a 90-95% decrease in Tk mRNA abundance. The decrease in Tk mRNA is not caused by a decrease in transcription of Tk-1, as shown by nuclear run-on transcription assays. Transient expression of the CAT (chloramphenicol acetyltransferase) gene, driven by the Tk-1 promoter, was not affected by dexamethasone, and transcription of stably integrated Tk minigenes in LMTk- cells was not affected by dexamethasone. This effect was observed regardless of whether Tk cDNA was fused to the simian virus 40 promoter or the mouse Tk-1 promoter region. Conversely, expression of thymidine kinase was inhibited when stable Tk+ transformants of LMTk- cells were exposed to glucocorticoids; and inhibition of expression was observed irrespective of the promoter that was used to drive transcription of the Tk minigenes. These data indicate that glucocorticoid regulation of Tk-1 in mouse L cells is, within the limits of detection of the assays used in these studies, entirely due to a posttranscriptional mechanism.

Animals↗
PubMed1992

Red 25, a protein that binds specifically to the sterol regulatory region in the promoter for 3-hydroxy-3-methylglutaryl-coenzyme A reductase.

A protein that binds to the sterol regulatory region of the hamster promoter for 3-hydroxy-3-methylglutaryl-coenzyme A reductase has been identified. All of the DNA bases crucial to the binding of this protein were previously shown to be essential for sterol regulation of the intact promoter in cultured cells. This low abundance protein, called Red 25, has been purified from nuclear extracts of hamster liver by a series of standard chromatographic techniques coupled with a DNA affinity step. Its size has been estimated as approximately 42 kDa by gel electrophoresis, size exclusion chromatography, and protein-DNA cross-linking studies. Furthermore, it binds to its target site with a Kd = 6 x 10(-11) M. Red 25 does not bind to the sterol regulatory regions of the LDL receptor or 3-hydroxy-3-methylglutaryl-coenzyme A synthase. This is consistent with recent studies that show there is a unique site for sterol regulation in the reductase promoter. The identification and purification of this protein represents a significant step in the study of feedback regulation by cholesterol.

Animals↗
PubMed1976

Health food users in two Texas cities. Nutritional and socioeconomic implications.

Health food buyers--both casual purchasers and confirmed customers of health food stores--were surveyed in Dallas and Houston. In both groups, users were generally in the middle class economically and in the active productive age brackets. Users in lower income groups spent larger proportions of their food budget in health food stores. The majority of confirmed users did so in the belief that such foods would provide health benefits, as health food proponents claim. Sixty-six per cent of the general public reported using vitamin and mineral supplements, compared with 93% of the confirmed health food users; the latter usually bought their supplements in health food stores, where megavitamins are likely to be promoted. Users among the general public, on the other hand, purchased supplements in drug stores and supermarkets as a rule.

Adolescent↗
PubMedDate not supplied

Effects of radial wall motion and flow waveform on the wall shear rate distribution in the divergent vascular graft.

Among the hemodynamic factors influencing intimal hyperplasia in the anastomotic region of a vascular graft, wall shear rate is believed to be one of the most important. We would like to study the effects radial wall motion on the wall shear rate distribution in the end-to-end anastomosis model of an artery and a divergent graft. Rigid and elastic models are constructed and the wall shear rate distributions are measured along the anastomosis using photochromic flow visualization method for carotid and femoral flow waveform. The mean and peak of shear rate decrease along the divergent graft, and the decreases are more significant in the elastic model. The shear rate waves are decomposed using the Fourier transform in order to separate the effects of radial wall motion and geometry. The percentage reductions of mean wall shear rates compared to steady shear rates at mean flow are calculated, and additional 8% (carotid) and 22% (femoral) reductions are observed in the elastic models near the end of the divergent graft. Also radial wall motion decreases the amplitudes of higher harmonics of wall shear rates in the elastic models. Since radial wall motion may affect the flow field differently for different geometry, wall elasticity should be considered in studying arterial hemodynamics.

Anastomosis, Surgical↗
PubMedDate not supplied

Left ventricular assist using a jet pump.

A simple, effective, cardiac assist device was developed using a jet pump, a device that performs pumping by energy transfer from a high speed jet to low speed surrounding fluids. This jet pump is inserted retrograde through the aorta and placed in the left ventricle transvalvularly. The jet of oxygenated venous blood entrains blood inside the left ventricle and pumps into the aorta through the aortic valve. Jet velocity is kept below the hemolytic threshold of 1000 cm/sec. The device was placed in a mock circulatory system that stimulates the left ventricle and vascular system by generating a pressure wave (120/75 mmHg) with a 4 L/min cardiac output (CO). A bypass loop (from the venous reservoir to aorta using a Biomedicus pump, Biomedicus Inc., Eden Prairie, MN) was set up, and the jet pump was installed. When the jet pump is turned on, bypass flow rate (BF) is 2.5 L/min, entrainment pumping 1.5 L/min, and peak ventricular pressure (VP) falls below aortic pressure (AP), while maintaining the mean AP. Time tension index (TTI) is decreased 31%. This result, when compared with simple bypass at differing BF, shows more than a 20% reduction in TTI. This simple jet pump provided significant unloading of the left ventricle and may be potentially useful as a left ventricular assist device.

Blood Pressure↗