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PubMed2026

Molecular epidemiology of levofloxacin-resistant Klebsiella pneumoniae and the association of plasmid-mediated quinolone resistance genes with key biological phenotypes.

UNLABELLED: Klebsiella pneumoniae is a major opportunistic pathogen in China, yet the molecular epidemiology of quinolone resistance remains poorly characterized. This study analyzed 2,433 clinical isolates from 37 Chinese hospitals (2018-2022). The overall levofloxacin-non-susceptible (NS) rate was 53.60%, with urinary tract isolates showing higher resistance. Whole-genome sequencing identified 12 plasmid-mediated quinolone resistance (PMQR) genes. Among 1,304 NS strains, 74.54% carried at least one PMQR gene (mainly qnrS, qnrB, and aac(6')-Ib-cr), and 60.20% also had quinolone resistance-determining region (QRDR) mutations. Functional studies revealed diverse phenotypic impacts. Most PMQR genes conferred low-level resistance (minimum inhibitory concentration [MIC] = 1 mg/L), while qnrB52 and qnrB91 caused high-level resistance (MIC = 8-16 mg/L). Notably, qnrB91 reduced biofilm formation, indicating a trade-off between resistance and colonization. Growth assays showed that qnrB52, qnrB91, and qnrS1 inhibited normal growth, whereas qepA1 and qnrS1 enhanced growth under ethanol stress. Most PMQR genes (except qnrB6) attenuated bacterial adhesion. qepA1 promoted intracellular survival in macrophages, suggesting a role in chronic infection. Animal models confirmed that qnrB6, qnrB7, qnrVC6, and aac(6')-Ib-cr significantly enhanced virulence. This study is the first in China to report qnrVC6 and novel gyrA mutations (Ser83Ala/Val, Asp87Phe/His) in K. pneumoniae. It systematically reveals how PMQR genes influence infection by modulating resistance, immune evasion, and pathogenicity. These findings highlight that PMQR genes contribute not only to antibiotic resistance but also to virulence, suggesting that treatment strategies should consider specific PMQR genotypes. This research provides the largest-scale molecular epidemiological data and a theoretical basis for controlling quinolone-resistant K. pneumoniae in China. IMPORTANCE: Quinolone-resistant Klebsiella pneumoniae poses a serious threat to public health, yet the role of plasmid-mediated quinolone resistance (PMQR) genes beyond antibiotic resistance remains underexplored. In this largest-scale multicenter study in China, we analyzed 2,433 clinical isolates and discovered that PMQR genes do more than just confer drug resistance-they also influence bacterial growth, stress survival, biofilm formation, and the ability to evade or persist within host immune cells. Some PMQR genes even enhance virulence in an animal model. These findings challenge the traditional view of resistance genes as mere contributors to drug failure, revealing that they can also shape infection outcomes by altering bacterial behavior. Understanding these dual roles may guide more precise treatment strategies targeting specific PMQR genotypes.

Klebsiella pneumoniae↗
PubMed2005

Resistance to topoisomerase II inhibitors in human glioma cell lines overexpressing multidrug resistant associated protein (MRP) 2.

For understanding of the resistance to topoisomerase II inhibitors, 50 sublines were isolated as single clones from parental glioma cell lines by exposure to VP-16 or m-AMSA. The quantitative aspects of topoisomerase II alpha,multi drug resistant gene (MDR)-1, breast cancer resistance protein (BCRP), and multidrug resistant associated protein (MRP) 1-5 were studied by Northern blotting in 50 resistant cell lines. By understanding the function of MRP2, we picked up three drug resistant sublines (T98G-ml, T98G-m2, and gli36-VP1) that overexpressed MRP2, but did not overexpress MDR-1 or MRP1-5 except 2. Moreover, in the results of northern blot analysis of mRNA for topoisomerase II alpha identical results are observed in parental cell lines and their resistant cell lines, suggesting that alterations in topoisomerase II do not account for the resistance in these cells. To determine whether the cellular sensitivity to anticancer agents was closely associated with the cellular levels of MRP2, we established cell lines with the same levels of MRP2 as their parental cells by introducing the MRP2 antisense expression plasmid into resistant cells. Etoposide (VP-16) accumulation and efflux studies were carried out in the parental cell lines and their drug resistant cell lines. Decreases in the HS-VP-16 accumulation and increases in the efflux were observed in these drug resistant cell lines. In the cytotoxicity assay, these drug resistant cell lines were resistant to multiple topoisomerase II inhibitors with little cross resistance to vincristine, and display efflux of VP-16. We found that the resistant cells transfected with MRP2 antisense cDNA displayed increased cellular levels of VP-16 and enhanced sensitivities to topoisomerase II inhibitors. In this study on the T98G-ml, T98G-m2, and gli36-VP1 cell lines, we showed a high correlation between MRP2 mRNA and VP-16 efflux, suggesting that MRP2 could be a new transporter for topoisomerase II inhibitors.

Antineoplastic Agents, Phytogenic↗
PubMed1991

Cross-Resistance to Herbicides in Annual Ryegrass (Lolium rigidum) : II. Chlorsulfuron Resistance Involves a Wheat-Like Detoxification System.

Lolium rigidum Gaud. biotype SLR31 is resistant to the herbicide diclofop-methyl and cross-resistant to several sulfonylurea herbicides. Wheat and the cross-resistant ryegrass exhibit similar patterns of resistance to sulfonylurea herbicides, suggesting that the mechanism of resistance may be similar. Cross-resistant ryegrass is also resistant to the wheat-selective imidazolinone herbicide imazamethabenz. The cross-resistant biotype SLR31 metabolized [phenyl-U-(14)C]chlorsulfuron at a faster rate than a biotype which is susceptible to both diclofop-methyl and chlorsulfuron. A third biotype which is resistant to diclofop-methyl but not to chlorsulfuron metabolized chlorsulfuron at the same rate as the susceptible biotype. The increased metabolism of chlorsulfuron observed in the cross-resistant biotype is, therefore, correlated with the patterns of resistance observed in these L. rigidum biotypes. During high performance liquid chromatography analysis the major metabolite of chlorsulfuron in both susceptible and cross-resistant ryegrass coeluted with the major metabolite produced in wheat. The major product is clearly different from the major product in the tolerant dicot species, flax (Linium usitatissimum). The elution pattern of metabolites of chlorsulfuron was the same for both the susceptible and cross-resistant ryegrass but the cross-resistant ryegrass metabolized chlorsulfuron more rapidly. The investigation of the dose response to sulfonylurea herbicides at the whole plant level and the study of the metabolism of chlorsulfuron provide two independent sets of data which both suggest that the resistance to chlorsulfuron in cross-resistant ryegrass biotype SLR31 involves a wheat-like detoxification system.

Journal Article↗
PubMed1978

Acquired resistance to ticks. III. Cobra venom factor and the resistance response.

Guinea-pigs developed resistance to larvae of the ixodid tick, Dermacentor andersoni, after one infestation. Resistant hosts were characterized by allowing significantly fewer larvae to engorge than non-resistant hosts. Larvae engorging on non-resistant hosts had a mean weight six times that of larvae obtained from resistant hosts at the end of a 5-day infestation. This immunologically based resistance was previously shown to have a cell-mediated and a humoral component. In an attempt to ascertain the role of complement in the resistance response, cobra venom factor (CoF) was administered to guinea-pigs producing prolonged (85--95 per cent) depletion of complement titres. CoF was administered during an initial infestation with tick larvae to determine if complement depletion altered the acquisition of tick resistance. CoF was also administered to tick-resistant hosts in an attempt to determine if the expression of tick resistance and the development of the basophil-packed lesion, characteristic of the tick-attachment site in resistant hosts, could be altered by complement depletion. CoF did not alter the acquisition of resistance when complement levels were reduced during a primary infestation. However, complement depletion of an animal which had acquired tick resistance blocked the expression of that resistance during a challenge infestation. In addition to increased numbers and weights of larvae engorging on tick-resistant animals depleted of complement, the basophil packed lesion at the tick attachment site was greatly reduced. Complement plays an important role in the expression of tick resistance in guinea-pigs.

Animals↗
PubMed2004

The effect of initial drug resistance on treatment response and acquired drug resistance during standardized short-course chemotherapy for tuberculosis.

BACKGROUND: In Tomsk Oblast, Russian Federation, during the period of 1996-2000, most previously untreated patients with tuberculosis received standardized short-course chemotherapy, irrespective of drug-susceptibility testing results. A retrospective analysis was done to determine the effect of initial drug resistance on treatment outcome and acquired drug resistance in new patients receiving standardized short-course chemotherapy. METHODS: During the period of 1 November 1996 through 31 December 2000, a total of 2194 patients received a category 1 treatment regimen. Drug susceptibility test results for 1681 patients were available for analysis. Drug resistance patterns before and during treatment were compared for 73 patients whose culture results were persistently positive during treatment. Acquired resistance was defined as new drug resistance (during or at the end of treatment) that was not present at the beginning of treatment. RESULTS: Pretreatment drug resistance was strongly associated with treatment failure. In patients who had strains with pretreatment resistance patterns that included isoniazid or rifampin resistance, but not resistance to both, 17 (70.8%) of 24 cases involving treatment failures acquired new multidrug resistance. In patients with pretreatment pan-susceptible or streptomycin-monoresistant strains, 13 (41.9%) of 31 cases involving treatment failures acquired new multidrug resistance. CONCLUSIONS: Early diagnosis of drug-resistant tuberculosis and judicious use of second-line drugs is recommended to decrease transmission of drug-resistant strains and to prevent the creation of multidrug-resistant strains. Finally, if drug susceptibility tests are not available or results are delayed, physicians should recognize that patients who do not respond to directly observed empirical short-course chemotherapy are at high risk of having multidrug-resistant tuberculosis and should be treated accordingly.

Antitubercular Agents↗
PubMed2026

Narasin used as a feed additive in conventional rearing of broilers can co-select for vancomycin-resistant Enterococcus faecium through the NarAB ionophore resistance mechanisms.

OBJECTIVES: To investigate the role of the NarAB resistance mechanism in the selection of vancomycin-resistant Enterococcus faecium (VREfm) and assess the impact of ionophore feed additives, particularly narasin, on the emergence of VREfm in broiler chickens. MATERIALS AND METHODS: Three isogenic E. faecium strains with different antimicrobial resistance determinants were created by mutagenesis and conjugation and used in a controlled animal experiment. Ross 308 broiler chickens were inoculated with either a rifampicin-resistant, a rifampicin- and vancomycin-resistant or a rifampicin-, vancomycin- and narasin-resistant strain and fed diets supplemented with selected ionophores. Bacterial populations were analysed on selective Slanetz and Bartley agar to determine the presence and selection of VREfm and other vancomycin-resistant species. Bacterial inoculation strains and isolates were whole genome sequenced for species identification and to identify genetic resistance mechanisms. RESULTS: Narasin was shown to select for VREfm in broilers, with NarAB being essential for co-selection. Intrinsically vancomycin-resistant Pediococcus acidilactici and Enterococcus gallinarum were identified as part of the broilers' vancomycin-resistant resident microbiota. Notably, among the P. acidilactici isolates that were susceptibility tested, strains resistant to both vancomycin and narasin were only found in broilers fed narasin, supporting that narasin promotes the growth of narasin-resistant populations. CONCLUSION: Narasin use in broiler feed can co-select for vancomycin-resistant bacteria, including VREfm, through the NarAB mechanism. These findings emphasize the concerns associated with the use of particular ionophores in poultry and suggest that vancomycin and narasin resistance may be more widespread in the broiler microbiota than previously recognized. Further research is needed to understand the implications for antimicrobial resistance and human health.

Animals↗
PubMed2004

[Differentiation of resistance phenotypes among erythromycin-resistant streptococci].

Although macrolide antibiotics have proved to be a valuable alternative to beta-lactam antibiotics in the treatment of respiratory tract infections, resistance to these agents is now becoming established in streptococci, especially among Streptococcus pneumoniae isolates. Of particular concern is the emergence of cross-resistance to 14-, 15- and 16-membered macrolides, licosamides and group B streptogramins (MLSb phenotype). MLS resistance can be expressed either constitutively (cMLS phenotype) or inducibly (iMLS phenotype). MLS resistance is mediated by two classes of methylase genes--the conventional erm(B) and recently described erm(A) determinants. A new macrolide efflux mechanism has been described for streptococci, in which it is associated with a new resistance pattern (M phenotype) characterized by resistance to 14- and 15-membered macrolides, and susceptibility to 16-membered macrolides, lincosamides and streptogramin B. The recognition of the prevalence of M phenotype in streptococci has implications for sensitivity testing and may have an impact on the choice of antibiotic therapy in clinical practice. While M resistance is similar in S. pyogenes and S. pneumoniae being mediated by mef(A) and mef(E), respectively, MLS resistance in both species appears to be genotypically and phenotypically more varied. Differentiation of M and MLS phenotypes of erythromycin-resistant strains can be performed using the erythromycin-clindamycin double-disc method (ECDD). Distinguishing not only M resistance but also constitutively or inducibly MLS phenotype by ECDD in S. pyogenes is easily and reliably achieved. Inducible MLS phenotype S. pyogenes strain is genotypically and phenotypically heterogeneous and is further subdivided into three recently described subtypes, iMLS-A, iMLS-B and iMLS-C, by a triple-disk test with erythromycin plus clindamycin and josamycin. While distinguishing M from MLS resistance in S. pneumoniae by ECDD test is easily and reliably achieved, the differentiation between constitutive and inducible MLS resistance is by far more uncertain. The meaning of inducible MLS resistance appears to be different in S. pneumoniae from that in S. pyogenes. In order to easily differentiate, within erythromycin-resistant pneumococci, not only the strains of the M phenotype from those with MLS resistance but also among the latter, cMLS from iMcLS strains, a triple-disk test has been set up by adding a rokitamycin disk to the conventional

Drug Resistance, Bacterial↗
PubMed1992

Reduced topoisomerase II and elevated alpha class glutathione S-transferase expression in a multidrug resistant CHO cell line highly cross-resistant to mitomycin C.

We have isolated a multidrug-resistant derivative of Chinese hamster ovary CHO-K1 cells by exposure to progressively increasing concentrations of Adriamycin. This cell line, designated CHO-Adrr, was 27-fold more resistant than the parental line to Adriamycin and showed similar degrees of cross-resistance to several other topoisomerase II (topo II) inhibitors, including mitoxantrone, daunomycin and etoposide. CHO-Adrr cells showed a lower (4-fold) level of cross-resistance to vincristine and colchicine, drugs associated with the multidrug-resistant phenotype. While CHO-Adrr cells showed no enhanced resistance to several mono- and bi-functional alkylating agents or to UV and ionizing radiation, they were greater than 80-fold resistant to mitomycin C (MMC). There was a 5-fold decreased level of daunomycin accumulation in CHO-Adrr cells compared to CHO-K1 cells and this was associated with increased drug efflux. The resistant cells had amplified multidrug resistance gene (mdr) sequences and overexpressed (mdr) mRNA. Verapamil was able to completely reverse Adriamycin resistance but reversal of MMC resistance was only partial, with residual 23-fold resistance. CHO-Adrr cells expressed a 4-fold reduced level of topo II protein but overexpressed an alpha class (basic) glutathione S-transferase (GST). Analysis of cell hybrids showed that while the level of resistance to Adriamycin dropped by a factor of 3 in CHO-K1/CHO-Adrr hybrids compared to CHO-Adrr/CHO-Adrr hybrids, resistance to MMC dropped 10-fold. Thus, CHO-Adrr cells appear to exhibit simultaneously several different drug resistance mechanisms including MDR and GST overexpression, and topo II reduction.

Amsacrine↗
PubMed2005

Drug resistance of Enterococcus faecium clinical isolates and the conjugative transfer of gentamicin and erythromycin resistance traits.

Drug resistance and the transferability of resistance were examined in 218 Enterococcus faecium clinical isolates obtained from in-patients of a Japanese university hospital between 1990 and 1999. One hundred and sixty one isolates (73.9%) were drug-resistant and 127 (58.2%) isolates were resistant to two or more drugs. Vancomycin resistant E. faecium (VRE) was not isolated. The transferability of drug-resistance to an E. faecium strain was examined by broth or filter mating. Six (12.5%) of the 48 gentamicin resistance traits, and fifty (50%) of the 101 erythromycin resistance traits were transferred by filter mating. The gentamicin resistance traits of five isolates and the erythromycin resistance traits of four isolates were transferred to the recipient strains by both broth mating and filter mating at a frequency of about 10(-6) and 10(-5) per donor cell, respectively. The five gentamicin resistant strains were shown to harbor pMG1-like plasmids on the basis of their Southern hybridization with pMG1 (65.1 kbp, Gm(r)), which transfers efficiently between enterococci by broth mating. Each of the four erythromycin resistant transconjugants obtained by broth mating harbored a large conjugative plasmid (more than 100 kbp). The plasmids showed no homology with well-characterized enterococcal conjugative plasmids such as pAD1, pPD1, pAM(beta)1, pIP501 and pMG1 by Southern hybridization. Of the erythromycin resistance traits that transferred only by filter mating, it was found that the erythromycin resistance trait was conferred by a 47-kbp transposable element that transferred from the chromosome of the donor strain to different sites within the pheromone responsive plasmid pAD1 (60 kbp) of the recipient strain, suggesting that the erythromycin resistance trait was encoded on a conjugative transposon, which was named Tn950.

Anti-Bacterial Agents↗
PubMed2000

Cross-resistance in the 2',2'-difluorodeoxycytidine (gemcitabine)-resistant human ovarian cancer cell line AG6000 to standard and investigational drugs.

Gemcitabine (2'-2'-difluorodeoxycytidine; dFdC) is a deoxycytidine analogue which is effective against solid tumours, including lung cancer and ovarian cancer. dFdC requires phosphorylation by deoxycytidine kinase (dCK) for activation. In the human ovarian cancer cell line A2780 and its 30,000-fold dFdC-resistant variant AG6000 (P<0.001), we investigated the cross-resistance profile to several drugs. AG6000, which has a complete dCK deficiency, was approximately 1000-10,000-fold resistant to other deoxynucleoside analogues such as 1-beta-D-arabinofuranosyl cytosine, 2-chloro-deoxyadenosine, aza-deoxycytidine and 2', 2'-difluorodeoxyguanosine (dFdG) (P<0.001). dFdG can be activated by dCK and deoxyguanosine kinase (dGK), but the latter enzyme was not altered in AG6000 cells. Thus dFdG resistance was only due to dCK deficiency. AG6000 was 1.6- and 46.7-fold resistant to 5-fluorouracil (5-FU) and ZD1694, respectively (the latter was significant; P<0.01), which may be due to the 1.7-fold higher thymidylate synthase (TS) activity, but AG6000 cells were also 2. 7-fold resistant to the lipophilic TS inhibitor AG337 (P<0.05). Remarkably, AG6000 cells were 2.5-fold more sensitive to methotrexate (MTX) (P<0.01) than A2780 cells, but 1.6-fold more resistant to trimetrexate (TMQ) (P<0.10). However, no differences in reduced folate carrier activity, folylpolyglutamate synthetase (FPGS) activity and polyglutamation of MTX were found between the cell lines. AG6000 cells were approximately 2 to 7.5-fold more resistant to doxorubicin (DOX), daunorubicin (DAU), epirubicin and vincristine (VCR) (the latter was significant; P<0.02) and approximately 4-fold more resistant to the microtubule inhibitors paclitaxel and docetaxel (P<0.001). Fluorescent activated cell sorter (FACS) analysis revealed no P-glycoprotein (Pgp) or multidrug resistance-associated protein (MRP) expression, but less fluorescence of intercalated DAU in AG6000 cells. An approximately 2-fold resistance to the topoisomerase I and II inhibitors etoposide, CPT-11 and SN38 was found in AG6000 cells. Topoisomerase I and IIalpha RNA expression was decreased in AG6000 cells. AG6000 was 2.4, 2.4, 2.3 and 3.7-fold more resistant to EO9 (P<0.02), mitomycin-C (MMC) (P<0.05), cisplatin (CDDP) (P<0.10) and maphosphamide (MAPH), respectively. DT-diaphorase (DTD), which activates EO9, was 2.2-fold lower in AG6000 cells. CDDP resistance might be related to a reduced retention of DNA adducts in AG6000. However, glutathione levels were equal in A2780 and AG6000 cells. A 24 h exposure to DOX, VCR and paclitaxel at equimolar and equitoxic concentrations, resulted in more double-strand breaks (1.5- to 2-fold) in A2780 than in AG6000 cells. MAPH at 1120 nM and 17 nM of EO9 did not cause DNA damage in either cell line. In conclusion, AG6000 is a cell line highly cross-resistant to a wide variety of drugs. This cross-resistance might be related to altered enzyme activities and/or increased DNA repair.

Antineoplastic Agents, Phytogenic↗
PubMed1993

Changes in subcellular doxorubicin distribution and cellular accumulation alone can largely account for doxorubicin resistance in SW-1573 lung cancer and MCF-7 breast cancer multidrug resistant tumour cells.

Doxorubicin accumulation defects in multidrug resistant tumour cells are generally small in comparison to the resistance factors. Therefore additional mechanisms must be operative. In this paper we show by a quantitative approach that doxorubicin resistance in several P-glycoprotein-positive non-small cell lung cancer and breast cancer multidrug resistant cell lines can be explained by a summation of accumulation defect and alterations in the efficacy of the drug once present in the cell. This alteration of efficacy was partly due to changes in intracellular drug localisation, characterised by decreased nuclear/cytoplasmic doxorubicin fluorescence ratios (N/C-ratios). N/C-ratios were 2.8-3.6 in sensitive cells, 0.1-0.4 in cells with high (> 70-fold) levels of doxorubicin resistance and 1.2 and 1.9 in cells with low or intermediate (7.5 and 24-fold, respectively) levels of doxorubicin resistance. The change of drug efficacy was reflected by an increase in the total amount of doxorubicin present in the cell at equitoxic (IC50) concentrations. N/C ratios in highly resistant P-glycoprotein-containing cells could be increased with the resistance modifier verapamil to values of 1.3-2.7, a process that was paralleled by a decrease of the cellular doxorubicin amounts present at IC50. At the low to moderate residual levels of resistance, obtained with different concentrations of verapamil, a linear relationship between IC50 and cellular doxorubicin amounts determined at IC50 was found. This shows that at this stage of residual resistance, extra reversal by verapamil should be explained by further increase of drug efficacy rather than by increase of cellular drug accumulation. A similar relationship was found for P-glycoprotein-negative MDR cells with low levels of resistance. Since in these cells N/C ratios could not be altered, verapamil-induced decrease of IC50 must be due to increased drug efficacy by action on as yet unidentified targets. Although the IC50 of sensitive human cells cannot be reached with resistance modifiers, when using these relationships it can be shown by extrapolation that cellular and nuclear doxorubicin amounts at IC50 at complete reversal of resistance were the same as in sensitive cells. It is concluded that doxorubicin resistance factors for multidrug resistant cells can for a large part, and in the case of P-glycoprotein-containing cells probably fully, be accounted for by decreased amounts of drug at nuclear targets, which in turn is characterised by two processes only: decreased cellular accumulation and a shift in the ratio nuclear drug/cytoplasmic drug.

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMed2001

Pharmacological basis for cladribine resistance in a human acute T lymphoblastic leukaemia cell line selected for resistance to etoposide.

Cross-resistance between different classes of anti-neoplastic agents can jeopardize successful combination cancer chemotherapy. In this study, we observed an unexpected cross-resistance between the podophyllotoxine derivative etoposide (VP) and the nucleoside analogue cladribine (CdA) in CCRF-CEM cells developed for resistance to VP. The resistant cells also displayed 14- and twofold resistance to cytarabine (ara-C) and gemcitabine respectively. Closer analysis of these cells showed that they contained lower amounts of topoisomerase (topo) IIalpha (P < 0.001) and beta protein (P < 0.026), formed substantially lower amounts of the topo II-DNA complex, and had a markedly decreased level of Fas (CD95/APO-1)-ligand mRNA expression. Interestingly, Fas expression in the resistant cells did not differ from that in the parental cell line. No differences were observed in the accumulation/efflux of daunorubicin or in the gene expressions of P-glycoprotein, multidrug resistance-associated protein and the lung resistance-related protein. The activity of deoxycytidine kinase (dCK), responsible for activation of CdA and ara-C, was the same for resistant and wild-type cells. However, there was an increase in the activity of the cytosolic 5'-nucleotidases (5'-NT), responsible for deactivation of nucleotides, amounting to 206% (P < 0.001) for the high Km and 134% (P < 0.331) for the low Km 5'-NT in resistant cells. The high Km 5'-NT is probably responsible for the decreased amount of the active metabolite CdA 5'-triphosphate [40% decreased (P < 0.045)], as well as for other purine ribonucleosides and deoxyribonucleosides triphosphates in the resistant cells. In contrast, a significantly higher deoxycytidine triphosphate (dCTP) level (167%, P < 0.001) was observed in the resistant cells. Thus, this study suggests that the major cause of resistance to the nucleoside analogues CdA and ara-C in cells selected for resistance to VP is a result of metabolic alterations producing increased activity of 5'-NT and higher dCTP levels. Furthermore, these results indicate that there is a common factor in the regulation of nucleotide-degrading enzymes and DNA topoisomerases, which may be altered in cross-resistant cells.

5'-Nucleotidase↗
PubMed2001

Origin of multidrug resistance in cells with and without multidrug resistance genes: chromosome reassortments catalyzed by aneuploidy.

Cancer cells and aneuploid cell lines can acquire resistance against multiple unrelated chemotherapeutic drugs that are over 3,000-fold those of normal levels and display spontaneous resistances up to 20-fold of normal levels. Two different mechanisms were proposed for this phenotype: (i) classical mutation of drug metabolizing genes or (ii) chromosome reassortments, catalyzed by cancer- and cell line-specific aneuploidy, which generate, via new gene dosage combinations, a plethora of cancer phenotypes, including drug resistance. To distinguish between these mechanisms, we have asked whether three mouse cell lines can become drug resistant, from which two or three genes have been deleted, and on which multidrug resistance is thought to depend: Mdr1a, Mdr1b, and Mrp1. Because all three lines could acquire multidrug resistance and were aneuploid, whereas diploid mouse cells could not, we conclude that aneuploid cells become drug resistant via specific chromosome assortments, independent of putative resistance genes. We have asked further whether aneuploid drug-resistant Chinese hamster cells revert spontaneously to drug sensitivity in the absence of cytotoxic drugs at the high rates that are typical of chromosome reassortments catalyzed by aneuploidy or at the very low or zero rates (i.e., deletion) of gene mutation. We found that four drug-resistant hamster cell lines reverted to drug sensitivity at rates of about 2-3% per generation, whereas two closely related lines remained resistant under our conditions. Thus, the karyotypic instability generated by aneuploidy emerges as the common source of the various levels of drug resistance of cancer cells: minor spontaneous resistances reflect accidental chromosome assortments, the high selected resistances reflect complex specific assortments, and multidrug resistance reflects new combinations of unselected genes located on the same chromosomes as selected genes.

ATP Binding Cassette Transporter, Subfamily B↗
PubMedDate not supplied

Mechanisms of resistance and resistance transfer in anaerobic bacteria: factors influencing antimicrobial therapy.

The resistance of anaerobic bacteria to a number of antimicrobial agents has an impact on the selection of appropriate therapy for infections caused by these pathogens. Resistance to penicillin in Bacteroides fragilis has long been recognized. Most resistance is due to chromosomal beta-lactamases that are cephalosporinases. Two new enzymes that inactivate the ureidopenicillins and cefoxitin have been described in B. fragilis. The most common mechanisms of cefoxitin resistance is by the blocking of penetration of the drug into the periplasmic space. The transfer of beta-lactamase and penicillinase and of cefoxitin resistance has been demonstrated. Penicillin resistance in other Bacteroides is mediated by a penicillinase. Chloramphenicol resistance is mediated by a chloramphenicol acetyltransferase and by nitroreduction in anaerobic bacteria. Anaerobic bacteria are resistant to aminoglycosides because these organisms lack the oxidative transport system for intracellular drug accumulation. Metronidazole resistance, which is rarely encountered, is mediated by a decrease in nitroreduction of the compound to the active agent. Clindamycin-erythromycin resistance in B. fragilis is probably similar to macrolide-lincosamide-streptogramin resistance in aerobic bacteria. Two transfer factors, pBFTM10 and pBF4, which confer resistance to clindamycin have been described; the resistance determinant on them is widely distributed in nature. Tetracyline resistance in B. fragilis is mediated by a block in uptake of the drug. Transfer of tetracycline resistance is common; however, no transfer factor has been isolated. Transfer has been proposed to occur via a conjugal transposon. The special characteristics of the infected site influence the outcome of antimicrobial therapy, particularly in abscesses.(ABSTRACT TRUNCATED AT 250 WORDS)

Abscess↗
PubMed1989

Cross-resistance to the lipid-soluble antifolate trimetrexate in human carcinoma cells with the multidrug-resistant phenotype.

We are studying mechanisms of resistance to hydrophilic and lipophilic antifolates in cultured mammalian cells. We determined the cytotoxicity of methotrexate and the lipid-soluble antifolate trimetrexate to various human carcinoma cells and their doxorubicin-resistant sublines. These multidrug-resistant cells were 17-fold to 26-fold more resistant to trimetrexate but as sensitive to methotrexate as their parental cells. Verapamil and quinidine, which are known to modulate the degree of pleiotropic drug resistance, reversed the cross-resistance to trimetrexate but did not alter methotrexate toxicity in multidrug-resistant cells. By flow cytometry, we show that multidrug-resistant Chinese hamster ovary cells retained eightfold more fluorescein-methotrexate bound to dihydrofolate reductase upon competition with trimetrexate than the drug-sensitive cells did. However, methotrexate displaced fluorescein-methotrexate equally well in sensitive and multidrug-resistant cells. Furthermore, verapamil produced dose-dependent displacement of fluorescein-methotrexate from the multidrug-resistant cells in the presence of low concentrations of trimetrexate but had no significant effect on displacement of fluorescein-methotrexate from sensitive cells. Hamster cells that overproduce dihydrofolate reductase by 93-fold were 145-fold and 96-fold more resistant to methotrexate and trimetrexate, respectively, than their sensitive parental cells. This form of antifolate resistance was not altered by verapamil or quinidine. We conclude that the cross-resistance to trimetrexate in cells that do not overproduce dihydrofolate reductase is associated with the multidrug-resistant phenotype. Possible implications of trimetrexate resistance in cancer chemotherapy are discussed.

Animals↗
PubMed1995

Insulin resistant and non-resistant polycystic ovary syndrome represent two clinical and endocrinological subgroups.

We studied the clinical and endocrine features of 35 patients with polycystic ovary syndrome (PCOS) who are either insulin resistant or non-insulin resistant. The occurrence of insulin resistance was determined by measuring insulin and glucose concentrations following a standard 75 g oral glucose load. All patients were evaluated by anthropometric measurements: body mass index (BMI), percentage of body fat (BCF) and waist-to-hip ratio (W/H), degree of hirsutism (Ferriman-Gallwey method) and endocrine profile. Fourteen patients had insulin resistance of unknown origin whereas four were due to a type A insulin receptor mutation, and 17 were non-insulin resistant. The insulin resistant patients were significantly more obese (higher BMI P < 0.0001, BCF P < 0.002 and W/H ratio P < 0.005) and were more hirsute (P < 0.002) than the non-insulin resistant patients. Testosterone concentrations were significantly higher in the insulin resistant group than in the non-insulin resistant group (2.65 versus 1.37 nmol/l; P < 0.027), whereas sex hormone-binding globulin was lower in insulin resistant patients (30.61 versus 19.48 nmol/l; P < 0.02). Non-insulin resistant patients showed a high luteinizing hormone to follicle stimulating hormone ratio, while a normal ratio was found in the insulin resistant subpopulation (2.94 versus 1.34; P < 0.0001). We concluded that PCOS comprises two subpopulations, one with insulin resistance of different aetiologies and the other which has no insulin resistance. These two groups differ in their anthropometric and endocrine features. The diagnosis of insulin resistance in PCOS can be easily determined by the insulin response to an oral glucose tolerance test.

Adult↗
PubMed2003

Genotypic and phenotypic resistance: longitudinal and sequential analysis of hepatitis B virus polymerase mutations in patients with lamivudine resistance after liver transplantation.

OBJECTIVE: Lamivudine-resistant strains appear in 27-62.5% of liver transplant recipients treated with lamivudine for hepatitis B virus (HBV) recurrence, and may lead to failure of antiviral therapy. In an extension of our previous study, we investigated the molecular events associated with the emergence of lamivudine-resistant mutants in this population. METHODS: Sequential serum samples from 10 consecutive patients with lamivudine resistance after liver transplantation were analyzed for viral genotype, precore mutants, and viral polymerase gene mutants (L528M, M552V, M552I) using restriction fragment length polymorphism. Quantitative analysis of HBV DNA was performed using hybridization assay and polymerase chain reaction. RESULTS: Eight patients (80%) were infected with genotype D and two (20%) with genotype C. Polymerase mutants (genotypic resistance) were identified in all the patients. Phenotypic resistance (rise in serum HBV DNA titers above the detection limit of the hybridization assay) developed in five patients (50%); of the remainder, three (30%) did not have phenotypic resistance, and two were primary nonresponders. Genotypic resistance was detected earlier than phenotypic resistance (median 285 days [range 42-510] vs median 387 days [range 320-420], p = 0.055). In five patients (50%), the emergence of the YMDD mutants took over the wild type; in three (30%), the YMDD mutant took over the wild type, but the wild type re-emerged during lamivudine therapy; and in two (20%), the YMDD mutants were detected in a mixture with the wild type (in different percentages). The mean pretreatment serum ALT level was significantly lower in the patients who did not develop phenotypic resistance (p = 0.0002). The M552I pure viral population was found mainly in these patients, and all retained stable graft function (median follow-up 33 months). A high pretreatment HBV DNA level (>50 x 10(6) copies/ml) was highly statistically significantly correlated with the rapid occurrence of phenotypic resistance (r = -0.90, p = 0.04). CONCLUSIONS: We reached the following conclusions: 1) In our area, liver transplant recipients who develop resistance to lamivudine given for recurrent HBV infection seem to be mainly infected with genotype D. 2) Re-emergence of the wild type can occur during lamivudine therapy. 3) Genotypic resistance precedes phenotypic resistance, although phenotypic resistance does not always follow genotypic resistance. 4) Quantitative determination of viremia and analysis of polymerase gene mutants are recommended for monitoring antiviral therapy of liver transplant patients with HBV reinfection in the graft.

Adult↗
PubMed2001

Various levels of cross-resistance to Bacillus sphaericus strains in Culex pipiens (Diptera: Culicidae) colonies resistant to B. sphaericus strain 2362.

We studied the cross-resistance to three highly toxic Bacillus sphaericus strains, IAB-59 (serotype H6), IAB-881 (serotype H3), and IAB-872 (serotype H48), of four colonies of the Culex pipiens complex resistant to B. sphaericus 2362 and 1593, both of which are serotype H5a5b strains. Two field-selected highly resistant colonies originating from India (KOCHI, 17,000-fold resistance) and France (SPHAE, 23,000-fold resistance) and a highly resistant laboratory-selected colony from California (GeoR, 36,000-fold resistance) showed strong cross-resistance to strains IAB-881 and IAB-872 but significantly weaker cross-resistance to IAB-59 (3- to 43-fold resistance). In contrast, a laboratory-selected California colony with low-level resistance (JRMM-R, 5-fold resistance) displayed similar levels of resistance (5- to 10-fold) to all of the B. sphaericus strains tested. Thus, among the mosquitocidal strains of B. sphaericus we identified a strain, IAB-59, which was toxic to several Culex colonies that were highly resistant to commercial strains 2362 and 1593. Our analysis also indicated that strain IAB-59 may possess other larvicidal factors. These results could have important implications for the development of resistance management strategies for area-wide mosquito control programs based on the use of B. sphaericus preparations.

Animals↗