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.