RAPID DOWN-REGULATION OF C-JUN PROTOONCOGENE TRANSCRIPTION BY PROGESTERONE IN THE AVIAN OVIDUCT

Previous work in this and other laboratories has shown that steroids rapidly regulate the expression of nuclear protooncogenes. In this present study, we have investigated the effect of progesterone (Pg) on the expression of c-jun in the avian oviduct system and its promoter activity in avian liver cells. Pg treatment of estrogen-withdrawn chickens brings about a decrease in the steady state mRNA level of the protooncogene c-jun within 30 min. This decrease is steroid dose dependent and gene specific. Using nuclear run-off transcription analyses, this rapid regulation was shown to occur at the level of gene transcription, as the rate of c-jun transcription decreases by more than 80% within 15 min after progesterone treatment. As expected, ovalbumin gene transcription is increased only after a lag period of 4 h following Pg treatment. In other studies, we have linked the c-jun promoter sequences between -1000 and +192 to chloramphenicol acetyltransferase reporter gene and cotransfected them into transformed avian liver cells along with the expression vector for the Pg receptor. Pg treatment of these cells causes a decrease in chloramphenicol acetyltransferase gene expression, albeit to a lesser extent than Pg inhibition of c-jun gene transcription. These results suggest that the 5'-domain of the chicken c-jun gene contains sequence elements that negatively regulate c-jun promoter activity in response to Pg.