Multidrug resistance in cancers of childhood: clinical relevance and circumvention

Enhanced xenobiotic transporter expression in normal teleost hepatocytes: response to environmental and chemotherapeutic toxins

Many aquatic organisms are resistant to environmental pollutants, probably because their inherent multi-drug-resistant protein extrusion pump (pgp) can be co-opted to handle man-made pollutants. This mechanism of multixenobiotic resistance is similar to the mechanism of multidrug resistance exhibited in chemotherapy-resistant human tumor cells. In the present study, a variety of techniques were used to characterize this toxin defense system in killifish (Fundulus heteroclitus) hepatocytes. The cellular localization and activity of the putative drug efflux system were evaluated. In addition, in vitro and in vivo studies were used to examine the range of expression of this putative drug transporter in the presence of environmental and chemotherapeutic toxins. The broad range of pgp expression generally observed in transformed mammalian cells was found in normal cells of our teleost model. Our findings suggest that the expression of the pgp gene in the killifish could be an excellent indicator of toxin levels or stressors in the environment.

Identification and characterization of a hepatoma cell-specific enhancer in the mouse multidrug resistance mdr1b promoter

The expression of multidrug resistance/P-glycoprotein genes mdr1b(mdr1) and mdr1a(mdr3) is elevated during hepatocarcinogenesis. To investigate the regulation of mdr1b gene expression, we used transient transfection expression assays of reporter constructs containing various 5'-mdr1b flanking sequences in hepatoma and non-hepatoma cells. We found that nucleotides -233 to -116 preferentially enhanced the expression of reporter gene in mouse hepatoma cell lines in an orientation- and promoter context-independent manner. DNase I footprinting using nuclear extracts prepared from hepatoma and non-hepatoma cells identified four protein binding sites at nucleotides -205 to -186 (site A), -181 to -164 (site B), -153 to -135 (site C), and -128 to -120 (site D). Further analyses revealed that, while site B alone played a major part for the enhancer function, sites A and B combined conferred full enhancer activity. Site-directed mutagenesis results also supported these results. Gel retardation experiments using oligonucleotide competitors revealed that the site B contains a dominant binding protein. This is the first report demonstrating a cell type-specific enhancer in the mdr locus. The role of this enhancer in the activation of mdr1b gene during hepatocarcinogenesis is discussed.

Regulation of multidrug resistance gene mdr1b/mdr1 expression in isolated mouse uterine epithelial cells

The mammalian uterine epithelium (UE) undergoes drastic physiological and morphological changes during pregnancy. Steady-state levels of murine mdr1b mRNA, transcribed from a multidrug resistance gene encoding a membrane protein which functions as a transporter of lipophilic cytotoxic agents, are low in nonpregnant, cycling UE, but drastically increase (about 1,500- to 2,000-fold) at day 8 of gestation. At day 16 of gestation, levels of mdr1b mRNA are 2,500- to 3,000-fold higher than those in the cycling UE cells. Levels of mdr1b mRNA were elevated to levels comparable to those observed during pregnancy, in the UE of ovariectomized mice following 5-8 days of estrogen and progesterone administration. Withdrawal of these hormones resulted in a drastic reduction of mdr1b mRNA within 36 hr. These results suggested that steroid hormones alone can account for increased mdr1b mRNA expression and do not require the presence of other placenta/embryo-derived factors. Moreover, the hormonal effect on uterine mdr1b mRNA biosynthesis during pregnancy apparently is a delayed phenomenon. Nuclear run-on assays demonstrated that the rate of mdr1b transcription in UE cells prepared from 15-day pregnant mice (d-15 UE cells) was about two- to three-fold higher than that in nonpregnant UE cells. This increased transcription rate alone cannot account for mdr1b mRNA accumulation during pregnancy. mdr1b mRNA expression was investigated in primary cultures of d-15 UE cells. mdr1b mRNA levels decayed by 50% within 3-4 hr of culture and reached a steady-state 0.5-2% of initial levels by 24 hr. The rate of mdr1b mRNA decay in primary d-15 UE cells was decreased by treatment with alpha-amanitin or cycloheximide, suggesting that the decay pathway requires both transcription and de novo protein synthesis. Our results suggest that multiple mechanisms are involved in the maintenance of the high levels of mdr1b mRNA in pregnant UE cells. Furthermore, these data suggest that increased mRNA stability may contribute to the accumulation of mdr1b transcript during pregnancy.

Continuous-infusion verapamil with etoposide in relapsed or resistant paediatric cancers

This study evaluates the use of a multidrug resistance (MDR) modulator (verapamil) in combination with a standard dose of single-agent etoposide in relapsed or refractory paediatric malignancy. A total of 20 patients (median age 6.5 years) were treated with an infusion of verapamil (loading dose 0.1 mg kg-1, followed by continuous infusion 0.15 mg kg-1 h-1) for 72 h. Etoposide was given daily (150 mg m-2 day-1) for three doses (each over 1 h); the first dose was given 12 h into the verapamil infusion. Cardiovascular toxicity was monitored by ECG and 2 hourly blood pressure and pulse recordings. Verapamil and norverapamil plasma concentrations were measured daily. Disease response was assessed after two courses. A total of 29/35 treatment courses were given at the desired verapamil dose; five courses required a dose reduction owing to cardiovascular toxicity. No patient required intensive monitoring. All patients who developed cardiovascular toxicity were over 14 years old. There was no correlation between plasma verapamil or norverapamil concentrations and toxicity. There were six partial responses (three rhabdomyosarcoma, three neuroblastoma) after two courses, but because of variation in the dose and schedule of etoposide these cannot be unequivocally contributed to MDR reversal. In conclusion, a regimen using a continuous infusion of verapamil combined with divided-dose etoposide is tolerable in children, and this strategy may be effective in refractory neuroblastoma and rhabdomyosarcoma.