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 mur ine mdr1b mRNA, transcribed from a multidrug resistance gene encoding a membrane protein which functions as a transporter of lipophilic cyto toxic agents, are low in nonpregnant, cycling UE, but drastically incr ease (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 t hose in the cycling UE cells. Levels of mdr Ib mRNA were elevated to l evels comparable to those observed during pregnancy, in the UE of ovar iectomized mice following 5-8 days of estrogen and progesterone admini stration. Withdrawal of these hormones resulted in a drastic reduction of mdr1b mRNA within 36 hr. These results suggested that steroid horm ones alone can account for increased mdr1b mRNA expression and do not require the presence of other placenta/embryo-derived factors. Moreove r, the hormonal effect on uterine mdr1b mRNA biosynthesis during pregn ancy apparently is a delayed phenomenon. Nuclear run-on assays demonst rated that the rate of mdr1b transcription in UE cells prepared from 1 5-day pregnant mice (d-15 UE cells) was about two- to three-fold highe r than that in nonpregnant UE cells. This increased transcription rate alone cannot account for mdr1b mRNA accumulation during pregnancy. md r1b mRNA expression was investigated in primary cultures of d-15 UE ce lls. mdr1b mRNA levels decayed by 50% within 3-4 hr of culture and rea ched a steady-state 0.5-2% of initial levels by 24 hr. The rate of mdr 1b mRNA decay in primary d-15 UE cells was decreased by treatment with alpha-amanitin or cycloheximide, suggesting that the decay pathway re quires both transcription and de novo protein synthesis. Our results s uggest that multiple mechanisms are involved in the maintenance of the high levels of mdr1b mRNA in pregnant UE cells. Furthermore, these da ta suggest that increased mRNA stability may contribute to the accumul ation of mdr1b transcript during pregnancy. (C) 1995 Wiley-Liss, Inc.