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Biomedical subjects

R M Baker

Publications and source records attributed to R M Baker.

At least 37 records · Page 2Linked to original sources
PubMed1987

P-glycoprotein in human sarcoma: evidence for multidrug resistance.

Overexpression of an immunologically conserved, cell-surface glycoprotein (P-glycoprotein) is consistently associated with multidrug resistance in cell lines in vitro. A preliminary survey of specimens from 12 solid tumor types in our laboratories indicates significant overexpression of P-glycoprotein in some sarcomas. When tested by immunoblotting with monoclonal antibodies directed against P-glycoprotein; tumors from six of 25 sarcoma patients displayed elevated levels of P-glycoprotein. Three of the sarcoma patients exhibiting P-glycoprotein had not previously been exposed to chemotherapy, implying that overexpression of this marker and possible concomitant multidrug resistance may not depend only on selection during prior drug treatments. The P-glycoprotein overexpression in the sarcoma specimens is evidence for the presence of multidrug resistant cells in these tumors; thus, our data suggest that this mode of resistance may have clinical significance in sarcoma patients.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed1985

Chromosome-mediated transfer of murine alleles for hypoxanthine-guanine phosphoribosyl transferase (HGPRT) and ouabain resistance into human cell lines.

Genetic drug-resistance markers were transferred via purified metaphase chromosomes from mouse L cells into the human fibrosarcoma line HT1080 and HeLa S3 cells. Interspecific chromosome-mediated transfer of hypoxanthine-guanine phosphoribosyl transferase (HGPRT; EC 2.4.2.8) from mouse L cells into HGPRT- HT1080 cells occurred at a frequency of approximately 1 x 10(-7). The presence of the mouse allele for HGPRT in transferent isolates was confirmed by isoelectric focusing. Transfer of ouabain resistance from mouse L cells to HT1080 and HeLa S3 cells occurred at an average frequency of approximately 4 x 10(-7). Expression of the mouse trait in transferent isolates was confirmed by their ability to withstand doses of ouabain which would be lethal to spontaneous ouabain-resistant mutants of the human cells but not to mouse L cells, ouabain-resistant transferents of human cells showed 10(4)-to greater than 10(5)-fold enhanced drug resistance, characteristic of either wild-type or mutant alleles, respectively, from ouabain-resistant donor L cells. Unstable expression of the transferred phenotypes in the absence of selection was seen in some isolates, but expression was lost at slow rates.

Alleles↗
PubMed1983

Chromosome-mediated gene transfer of HPRT and APRT in an intraspecific human cell system.

Chromosome-mediated transfer of genes between human cell lines was accomplished using HeLa cells as chromosome donors and HT1080 fibrosarcoma lines as recipients. This report describes the intraspecific transfer of two genetic markers, hypoxanthine-guanine-phosphoribosyltransferase (HPRT+) and adenine phosphoribosyltransferase (APRT+). The isolation and characterization of the necessary enzyme-deficient (HPRT- and APRT-) recipient HT1080 cell lines are also described. The chromosome-mediated gene transfer was carried out using a modification of the procedure of Miller and Ruddle, including treatment of the donor chromosomes with calcium phosphate and subsequent exposure of the recipient cells of dimethyl sulfoxide. In experiments to optimize this procedure for HT1080 cell recipients, we found that a brief (2-min) exposure to high DMSO concentration (20%) was effective for enhancing transfer efficiencies in this system. Transfer frequencies (transferents per recipient cells assayed) averaged approximately 1 x 10(-6) for HPRT+ and were greater than 2 x 10(-6) for APRT+.

Adenine Phosphoribosyltransferase↗
PubMed1983

Identifying the content of family medicine for educational purposes: an empirical approach.

In this article, the authors present a method to assist in curriculum planning and report an application of the method at one institution. Through interviews of 40 selected subjects, the authors identified 27 content elements appropriate for inclusion in a family medicine curriculum for medical students. These elements were organized into four areas portraying family medicine as "a synthesis of content and process," "a field of inquiry," "a career and peer group," and "a value system." A questionnaire employing the method of pair-comparisons was subsequently completed by 38 of the interview subjects, and on the basis of these responses priorities were assigned to content elements within each area. For three of the four areas, there was statistically significant consensus about the priority orderings. Over three years, the content elements themselves and their priority orderings have proved beneficial to curricular planning at the University of North Carolina School of Medicine.

Communication↗
PubMed1983

Congruence between residents' and clinical instructors perceptions of teaching in outpatient care centres.

This paper describes a study to determine the extent of agreement about clinical teaching between clinical instructors and their house officers in selected family practice centres. Their perceptions were compared with regard to (1) clinical content emphasized, and (2) specific clinical teaching behaviours used by the instructors. Two versions of a questionnaire were used to assess views, and multivariate and univariate analyses of variance used to determine differences. The results suggest that the instructors differed from their house officers on certain dimensions and not on others. Those differences are discussed and strategies are suggested for increasing the agreement with the expectation that more agreement about educational preferences and expectations would enhance house officers' clinical learning in the centres.

Ambulatory Care Facilities↗
PubMed1983

Effects of nutrient deprivation on Vibrio cholerae.

Environmental and clinical strains of Vibrio cholerae were exposed to nutrient-free artificial seawater and filtered natural seawater microcosms for selected time intervals and examined for changes in cell morphology and number. Cells observed by transmission electron and epifluorescence microscopy were found to undergo gross alterations in cell morphology with time of exposure. The vibroid cells decreased in volume by 85% and developed into small coccoid forms surrounded by remnant cell walls. The initial number of cells inoculated into nutrient-free microcosms (culturable count and direct viable count) increased 2.5 log10 within 3 days, and even after 75 days the number of viable cells was still 1 to 2 log10 higher than the initial inoculum size. Nutrient-depleted coccoid-shaped cells were restored to normal size and assumed a bacillary shape within 3 h and began to divide within 5 h after nutrient supplementation. The increase in cell number and decrease in cell volume under nutrient-depleted conditions, as well as the rapid growth response after nutrient supplementation, may describe some of the survival mechanisms of V. cholerae in the aquatic environment.

Cell Division↗
PubMed1982

Characterization of dominant hamster cell mutants resistant to oxygenated sterols.

Stable mutants of Dede and CHO cells, resistant to suppression of cholesterogenesis by oxygenated sterols, have been isolated in a single step. Luria-Delbrück fluctuation analysis indicated a random occurrence of resistant at a rate of 1 x 10(-7) mutations/cell/generation. Cholesterol biosynthesis, 3-hydroxy-3-methylglutaryl coenzyme A reductase activity, and growth of the mutant cells were coordinately resistant to oxygenated sterols in the culture medium, and this resistance was expressed as a dominant trait in somatic cell hybrids of the wild-type and mutant cells. The dominant resistance was employed in the selection of various cells hybrids. There was complete additivity of reductase activities in mixed lysates of inhibited wild-type and uninhibited mutant cells, indicating that cytosolic (in)activation factors were not causative of this resistance. We suggest that oxygenated sterols are (co)repressors in suppression of the synthesis of the reductase and that the resistance mutant phenotypes result from altered regulatory loci.

Animals↗
PubMed1982

Resident preferences for the clinical teaching of ambulatory care.

The content and teaching behaviors of clinical teaching in ambulatory care and, in particular, in family practice centers are important to residents as they attempt to learn the varied aspects of patient care. This paper is based on a study of family medicine residents' preferences for the content areas of family medicine emphasized by those attending physicians perceived by the residents as the "best" clinical teachers in their respective centers. Also included in the study were the specific teaching behaviors of those same clinical teachers. A statistical analysis of the preferences of residents provided an indication of the dimensions of clinical teaching in resident education. Further, certain individual characteristics of residents were found to influence their perceptions. It was concluded that selected content areas should receive more emphasis and that the human aspects of clinical teaching are more important than technical teaching skills. These perceptions were helpful in suggesting ways of improving the teaching of residents and faculty development.

Ambulatory Care↗
PubMed1982

Family practice as a specialty choice: effect of premedical and medical education.

Identification of beneficial and detrimental parameters influencing the choice of family practice as a career was the object of the study reported here. Curriculum components and elements during or before medical school were examined. A questionnaire listing 18 factors that influenced specialty choice before residency training was completed by 134 first-year family practice residents. Participation in a family practice clerkship or preceptorship during the third and fourth years of medical school and association with family physicians during or before medical school were perceived as most beneficial. Generally, students' experiences during the first three years were perceived as detrimental to selecting family practice. Also, data indicated that 75 percent of the students select their specialty during years three and four. Medical schools wishing to increase the number of their graduates entering family practice should strongly consider requiring clinical rotations in family medicine and providing maximum exposure to family practice residents and faculty members as role models.

Career Choice↗
PubMed1981

Selection of mutant Chinese hamster ovary cells altered glycoproteins by means of tritiated fucose suicide.

Mutant Chinese hamster ovary cells altered in glycoproteins have been isolated by selecting for ability to survive exposure to [6-3H]fucose. Mutagenized wild-type cells were permitted to incorporate [3H]fucose to approximately 1 cpm of trichloroacetic acid-insoluble radioactivity per cell and then frozen for several days to accumulate radiation damage. The overall viability of the population was reduced by 5- to 50-fold. Four consecutive selection cycles were carried out. The surviving cells were screened by replica plating-fluorography for clones showing decreased incorporation of fucose into trichloroacetic acid-insoluble macromolecules. Considerable enrichment for cells deficient in fucose uptake or incorporation into proteins (or both) was found in populations surviving the later selection cycles. Two mutant clones isolated after the fourth selection cycle had the same doubling time as the wild type, but contained only 30 to 40% as much fucose bound to proteins as the wild type. Sialic acid contents of the mutants and the wild type were similar. The mutants differed quantitatively and qualitatively from the wild type and from each other with respect to total glycoprotein profiles as visualized by sodium dodecyl sulfate gel electrophoresis. Differences were also found in resistances to cytotoxicity of lectins such as concanavalin A and wheat germ agglutinin.

Animals↗
PubMed1980

Failure of the phorbol ester 12-O-tetradecanoylphorbol-13-acetate to enhance sister chromatid exchange, mitotic segregation, or expression of mutations in Chinese hamster cells.

The potent tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) was tested for its ability (a) to induce sister chromatid exchange, (b) to increase the rate of transition at the adenine phosphoribosyltransferase (apt) locus from the presumptive heterozygous state ((+/- to the homozygous state (-/ - or -), and (c) to enhance the frequency of mutations expressed after ultraviolet radiation mutagenesis. We have found no significant effect of TPA in any of these experiments. Sister chromatid exchange frequencies in both V79 and Chinese hamster ovary cells remained unchanged by TPA treatment under various conditions, a result inconsistent with the hypothesis that an important effect of TPA might be to increase the rate of chromosomal mitotic recombination (and hence segregation of recessive mutations) in a manner akin to increased chromatid recombination. We have also been unable to obtain evidence for mitotic recombination affecting the aprt locus in Chinese hamster ovary cells for which the rate of change to a high level of resistance to azaadenine was measured. The rate of 8.6 X 10(-7) mutation (and/or segregations) per cell generation assessed by fluctuation analysis was not increased by the continuous presence of TPA, 4 microgram/ml, in the medium. In the third set of experiments, mutant frequencies in Chinese hamster ovary cells after ultraviolet mutagenesis were measured for the markers ouabain resistance, thioguanine resistance, and azaadenine resistance, under conditions with and without pretreatment with TPA before mutant selection. No convincing enhancement in mutation expression was observed. In summary, these results argue that promotion by TPA does not proceed by a mechanism involving genetic recombination or the altered expression of newly mutated alleles.

Animals↗
PubMed1979

Characteristics of concanavalin A-resistant Chinese hamster ovary cells and certain revertants.

Clones of Chinese hamster ovary (CHO) cells were isolated by single-step selection for resistance to killing Concanavalin A (ConA) and certain cellular and membrane properties were examined. The ConA-resistant isolates were only about 2-fold more resistant than wild type cells to the selecting lectin, but exhibited pleiotropic temperature-sensitivity for growth, markedly altered morphology and adherence, and significant difference in susceptibility to other agents such as colchicine. Two revertants to full temperature-resistance were isolated from different ConA-resistant mutants. One revertant clone had reacquired wild type sensitivity to ConA while the other revertant remained ConA-resistant. The two series of wild typed, ConA-resistant, and temperature revertant clones were analyzed for altered mobility of cell surface glycoproteins using lactoperoxidase/125I and galactose oxidase/(3H) borohydride labelling procedures. The ConA-resistant clones showed increased mobility on polyacrylamide gels of three classes of labelled proteins, in the molecular weight ranges 225,000, 200,000, and 130,000 daltons. These changes persisted in the temperature-revertant that remained ConA-resistant, while two of the altered protein closses were restored to wild type mobility in the revertant that regained ConA-sensitivity. Cell hybridization experiments indicated that the temperature-sensitivity phenotypes of different ConA-resistant isolates are recessive and noncomplementing, implying that the same gene is affected in each case. The reversions to temperature resistance appear to be recessive suppressor mutation in different genes.

Animals↗
PubMed1979

HeLa cell variants that differ in sensitivity to monofunctional alkylating agents, with independence of cytotoxic and mutagenic responses.

Different strains of the established human cell line HeLa differ substantially in sensitivity to ethyl methanesulfonate (EtMes). The EtMes doses effective for either cytotoxicity or mutation induction in a line of HeLa S3 cells are about 1/10th those required in the CCL2 HeLa line of the American Type Culture Collection. By plating the sensitive HeLa S3 line in the presence of highly cytotoxic doses of EtMes, we obtained a clone (designated A6) that displays about 7-fold greater resistance to EtMes toxicity. This A6 isolate is also cross resistant to other simple monofunctional alkylating agents-exhibiting about 4-fold increased resistance to methyl methanesulfonate and 10- to 15-fold increased resistance to N-methyl-N'-nitro-N-nitrosoguanidine but is similar to the S3 parent in sensitivity to mitomycin C, UV radiation, and gamma-rays. In contrast to the results for cytotoxicity, the A6 variant and the S3 parent showed the same high susceptibility to EtMes induction of ouabain-resistant mutations. This is direct biological evidence that different alkylation lesions are normally responsible for mutagenic and cytotoxic effects. The S3 and A6 cell lines may differ in DNA repair capability specific to certain potentially lethal alkylation products. The comparative sensitivity of the A6 cells to alkylation mutagenesis may also prove useful in cell genetic studies by facilitating the generation of multiple mutants for recessive alleles and permitting exceptionally sensitive detection of specific mutagenic effects.

Cell Survival↗
PubMed1979

Functions of a family medicine preceptorship.

The preceptorship in family medicine, a required four-week experience for fourth-year medical students at the University of North Carolina, was studied during academic year 1976--77. Analysis of narrative comments and closed-ended scales indicated that the preceptorship was a "synthesis" experience valued by both students and preceptors, regardless of the students' choice of graduate specialty training. From these same data seven functions of the preceptorship, many of which are not official program goals, were identified. The authors recommend that preceptorships be designed and evaluated with these, often latent, functions in mind.

Career Choice↗