Evidence for nuclear processing of progesterone receptors in rat placenta

The distribution of the progesterone receptor (Rp) in cytosolic and nuclear compartments of placenta has been studied in intact and ovariectomized (Ovx) rats on the 14th day of pregnancy. Removal of estradiol (E) and progesterone (P) by Ovx caused a 50% decrease in progesterone receptors from cytosolic and nuclear compartments. Estradiol replacement restored binding to intact levels. Progesterone, given 19 h after E, induced an additional 3-fold increment in the number of cytosolic and nuclear binding sites 1 h later. Four hours after progesterone the number of receptor sites in the placenta fell 60%, signifying processing. This was followed 4 h later by reversal of processing mechanisms leading to full recovery of nuclear and cytoplasmic binding sites. Actinomycin D (0.6 mg/ani) was found to have no influence on these events. On the other hand cycloheximide (0.5 mg/ani) completely prevented processing of binding sites when administered at the same time as progesterone or 2 h before, but did not influence the unmasking of nuclear sites which occurred 1 h after a progesterone challenge. The cycloheximide block to processing was partial when given 2 or 3 h after progesterone (61 and 43% complete, respectively). The full complement of receptors was processed when cycloheximide treatment was delayed 3.75 h after progesterone administration. These findings have led to the view that processing represents rapid and reversible changes in binding properties of the receptor rather than a gain or loss of receptor protein per se. The findings of this study suggest that a hypothetical substance, "processin", whose production is blocked by cycloheximide binds to the receptor and in some undefined manner inhibits ligand-receptor interaction within 4 h after an in vivo progesterone challenge. Nuclear accumulation of receptor induced by progesterone was not accompanied by cytoplasmic depletion of receptor nor was the apparent loss of processed nuclear receptor due to recycling of receptor to cytoplasm. We propose that nuclear receptors continually recycle within the nucleus in masked and unmasked states regulated by delicate interplay between progesterone and processin.