EXPRESSION OF CALBINDIN-D-9K IN THE EARLY PREGNANT RAT UTERUS - EFFECTS OF RU-486 AND CORRELATION TO ESTROGEN-RECEPTOR MESSENGER-RNA

Calbindin-D-9k (CaBP-9k) is a calcium binding protein expressed at high levels in the rat uterus. The CaBP-9k gene carries an estrogen response element which is involved in the steroid hormone regulation of the gene during the estrous cycle and gestation. The present study was aimed at determining expression of the gene during the first half of pregnancy and to assess the role of progesterone (P-4) and the estrogen receptor (ER). Expression of CaBP-9k mRNA was determined by Northern blot analysis during the first 10 days of pregnancy. On pregnancy day 1 (P1), CaBP-9k mRNA levels were relatively high. On P2, 3 and 5 CaBP-9k mRNA decreased to the detection limit using 10 mu g total RNA probed with a random primed cDNA. On P10, CaBP-9k transcripts began to reappear at levels of about 30% of P1. Expression of beta-actin mRNA displayed a continuous increase during this period with a rapid rise of 240% between P2 and P3. The typical increase of P-4 accompanied by moderate changes of estradiol (E(2)) was determined in serum of experimental groups. When RU 486 at 10 mg/kg was administered as a single s.c. injection on P3, the CaBP-9k down-regulation was rapidly interrupted and mRNA expression became extremely high. The effect was seen maximally at 24 h post injection and was maintained at 48 and 72 h. Expression of beta-actin mRNA was increased only moderately at 24 h and was unchanged at 48 and 72 h. Serum P-4 remained unaffected by the treatment and E(2) displayed a slight increase. When ER mRNA was quantified by reverse transcription/PCR techniques a more than 300% increase was detected in the RU 486 treated rats. These data implicate that P-4 in early pregnancy is responsible for down regulation of the CaBP-9k gene in uterus via an indirect effect involving the expression of the estrogen receptor.