Immunoblot detection of P-glycoprotein in human tumors and cell lines

Expression of CYP3A isoforms and P-glycoprotein in human stomach, jejunum and ileum

1. CYP3A isoforms metabolise a diverse array of clinically important drugs and P-glycoprotein (P-gp), a transmembrane efflux pump, can extrude a wide variety of drugs from the cell. It has been suggested that the function of CYP3A4 is complementary to that of P-gp along the gastrointestinal (GI) tract, together forming a coordinated intestinal barrier against xenobiotics. Therefore, the expression of CYP3A4, CYP3A5, CYP3A7 and ABCB1 (P-gp) genes were quantified in five normal samples from the human stomach, seven from the jejunum and eight from the ileum by real-time reverse transcription-polymerase chain reaction and western blot analysis. 2. In the tissues examined, measurable mRNA expression of CYP3A was found in almost all samples from the stomach, jejunum and ileum. The rank order for CYP3A mRNA expression was CYP3A4 > CYP3A5 > CYP3A7 in the GI tract studied, whereas median mRNA CYP3A4 expression was highest in the small intestine and lowest in the stomach. Expression of ABCB1 mRNA was found in almost all samples and the median mRNA expression level was comparable in the jejunum and ileum, but lower in the stomach. Our data also show a significant correlation between all mRNA transcripts studied and a wide interindividual variation. 3. At the protein level, CYP3A4 was detected in all stomach and small intestine samples, the levels being substantially higher in the small intestine than in the stomach. P-Glycoprotein was detected in all GI samples, but no statistically significant difference was found along the GI tract considered. 4. Collectively, these results demonstrate that CYP3A4 is the main CYP3A expressed in the GI tract investigated, an extensive interindividual variability in the expression of the different CYP3A isoforms in all tissues examined and P-gp apoprotein levels similar in the stomach, jejunum and ileum.

Expression of Cytochromes P450 3A and P-Glycoprotein in Human Large Intestinse in Paired Tumour and Normal Samples

Our objective was to investigate the expression of different cytochromes P450 3A (CYP3A4, CYP3A5, and CYP3A7) and P-glycoprotein (ABCB1) genes along the human large intestine in paired tumour and normal samples. Real-time reverse transcriptase-polymerase chain reaction was used to measure CYP3A4-, CYP3A5-, CYP3A7- and ABCB1-specific mRNA expression, and Western blot analysis was used to measure membrane protein levels of CYP3A4/7, CYP3A5 and P-glycoprotein. Levels of mRNA and membrane protein fractions in the large intestine were compared with those of normal human liver. The mRNA expressions of CYP3A4, CYP3A5, CYP3A7 and ABCB1 in the large intestine were found to be highly variable, but overall the levels were significantly lower than those measured in liver (P < 0.0001, P < 0.001, P < 0.0001 and P < 0.01, respectively). At the membrane protein level, CYP3A4/7 was detected in all large intestine samples examined and the levels were substantially higher than those of the liver (P < 0.01). Although expression of CYP3A5 was detected in all large intestine samples, in most the levels were too low to allow quantification. P-glycoprotein was readily detected at levels slightly higher than those of liver (P < 0.05). Comparison between paired samples of normal and tumour in large intestine showed no significant differences in either the mRNA or membrane protein levels of these genes. In conclusion, this work suggests a potential role of the large intestine in the absorption and metabolism of xenobiotics and nutrients and no difference in the CYP3A and P-glycoprotein membrane protein fractions and mRNA expression between normal and tumour tissues.

Subclonal heterogeneity of the multidrug resistance phenotype in a cell line expressing antisense MDR1 RNA

A multidrug resistant (MDR) cell line was transfected with an antisense MDR1 expression vector and transfectant clones were analyzed for reversion of the MDR phenotype. Only one of 10 antisense-expressing transfectants showed a reduction in drug resistance, MDR1 mRNA and P-glycoprotein. Observations made using rhodamine-123, a fluorescent substrate for P-glycoprotein, revealed that dye retention in individual cells was highly variable within this antisense-expressing clone. Subpopulations were established from the original clone based on differences in rhodamine-123 retention. Rhodamine-123 retention varied inversely with levels of P-glycoprotein and MDR1 mRNA. All subpopulations expressed similar levels of antisense MDR1 RNA yet had dramatic differences in MDR1 mRNA levels. Analysis of vector integration site restriction fragment length polymorphisms confirmed that all populations originated from the same transfectant clone. Nuclear run-on analysis indicated that the mdr1 gene is transcribed at the same rate in all populations, suggesting that the reduction in MDR1 mRNA is mediated posttranscriptionally. Cells with the greatest reduction in MDR1 mRNA accumulate distinct antisense RNA transcripts in the nuclear RNA fraction, suggesting that antisense effectiveness in this system is associated with a nuclear event or process. These results reveal that antisense RNA activity is not necessarily distributed equally within a clonal population.

Identification of a distinctive P-glycoprotein-mediated resistance phenotype in human ovarian carcinoma cells after their in vitro exposure to fractionated X-irradiation

BACKGROUND

Clinical drug resistance is recognized in patients previously treated with radiotherapy and after chemotherapy. In vitro exposure of mammalian tumor cells to fractionated X-irradiation also resulted in the expression of drug resistance. Analysis of the resistance phenotype of irradiated Chinese hamster ovary sublines revealed P-glycoprotein overexpression, without any concomitant increase in P-glycoprotein messenger RNA, under posttranslational regulation. This study aimed to determine whether this distinctive resistance phenotype could also be identified in irradiated human tumor cells.

METHODS

Irradiated sublines established from two human ovarian tumor cell lines, SK-OV-3 and JA-T, which showed resistance to vincristine and to etoposide, were studied. Protein and RNA expression were quantitated by Western and Northern blotting or RNase protection assays. P-glycoprotein turnover was measured after immunoprecipitation of metabolically labelled cells.

RESULTS

Significant P-glycoprotein overexpression was detected using the C219 and C494 monoclonal antibodies in the two irradiated human ovarian tumor sublines. No concomitant increase in P-glycoprotein messenger RNA was detectable in the SK-OV-3/DXR10 subline, contrasting with the increased message characteristic of vincristine-selected SKVCR sublines. In addition, turnover of P-glycoprotein was significantly reduced in these DXR10 cells when compared with that measured in a vincristine-selected subline. These irradiated sublines showed reduced levels of epidermal growth factor receptors and unchanged levels of topoisomerase II, but they overexpressed c-erbB2 marginally and heat shock protein 27 significantly. These latter elevations in protein levels, however, were associated with concomitant increases in their respective messenger RNAs, implicating regulation at the transcriptional level.

CONCLUSIONS

Exposure of human ovarian tumor cells to fractionated X-irradiation in vitro resulted in the expression of a distinctive multiple drug resistance phenotype unusually involving posttranslational regulation of P-glycoprotein. Monitoring tumor biopsies for P-glycoprotein-associated drug resistance in patients treated with radiotherapy should evaluate protein levels rather than, or as well as, MDR1 mRNA expression.

P-glycoprotein expression in Crassostrea gigas and Mytilus edulis in polluted seawater

1. DNA-DNA hybridization showed that mussels (Mytilus edulis) and oysters (Crassostrea gigas) possess a highly conserved region constituting one or more genes of the ATP-dependent transmembrane transporter family to which belong the mammalian tumor cell genes responsible for multi-drug resistance. 2. Monoclonal antibody C219 visualized two proteins (220 and 240 kDa) which varied quantitatively in mussels depending on the level of organic pollution at the sampling sites. 3. The authors consider whether certain pollutants in the marine environment are capable of inducing pollutant-resistant gene expression.