EFFECTS OF ESTROUS-CYCLE STAGE AND PREGNANCY ON FOLLISTATIN GENE-EXPRESSION AND IMMUNOREACTIVITY IN RAT REPRODUCTIVE TISSUES - PROGESTERONE IS IMPLICATED IN REGULATING UTERINE GENE-EXPRESSION

Follistatin, a monomeric protein originally isolated from ovarian follicular fluid, is now believed to be a major local regulator of the multifaceted actions of activin by virtue of its activin-binding properties. In view of the ability of follistatin to stimulate progesterone production from granulosa cells and its presence in newly formed corpora lutea, the following study was conducted to determine the effects of cycle stage and pregnancy on follistatin gene expression and immunoreactivity in the rat ovary and uterus with the intent of gaining additional insights into the regulation of follistatin in these tissues. Decidua and placentas were also examined on days 15, 18, and 21 of pregnancy. Follistatin messenger RNA (mRNA) levels were quantified using a sensitive solution hybridization-RNase protection assay and values normalized to the amount of cyclophilin mRNA present in each sample. Levels of follistatin-like immunoreactivity (FLI) in serum and tissues were estimated using a homologous porcine follistatin RIA. Follistatin message levels in the ovary increased between proestrus and estrus with a return to proestrous values on both days of diestrus. In the nonpregnant uterus, mRNA levels on proestrus were similar to levels measured in uteri taken from hypophysectomized or ovariectomized rats. Interestingly, follistatin gene expression increased almost 3-fold between proestrus and estrus. An additional experiment demonstrated that this increase could be abated by treatment of proestrous rats with pentobarbital which blocks preovulatory rises in serum progesterone levels and could be restored by administration of progesterone to pentobarbital-treated proestrous rats. In pregnant rats, ovarian follistatin message levels on days 3 and 6 of pregnancy were identical to levels observed on day 2 of diestrus. However, an abrupt 4-fold increase in ovarian mRNA levels occurred between days 6 and 9 with a further 58% increase occurring by day 12. This marked increase in message levels was unaccompanied by changes in ovarian FLI levels. A precipitous decrease in transcript levels accompanied by a decline in FLI levels then followed with ovarian gene expression on days 15 through 21 being slightly higher than expression during the initial stages of gestation. Expression of the gene in the decidua and placenta did not vary between days 15 and 21 of pregnancy. Levels of FLI in serum also were invariant during the cycle and pregnancy. These data show that: 1) follistatin gene expression in the ovary varies as a function of cycle stage and day of pregnancy; 2) gene expression in the nonpregnant uterus (and perhaps decidual uterus) is regulated by progesterone; 3) the follistatin gene is expressed albeit to a limited extent in rat placenta; and 4) changes in the expression of this gene are often accompanied by invariant tissue FLI levels. These data should provide the basis for future studies to more clearly define the physiological role(s) of follistatin in these reproductive tissues.