AJP - Endocrinology and Metabolism, Vol 249, Issue 6 589-E594, Copyright © 1985 by American Physiological Society
Occupied and unoccupied estrogen receptor during estrous cycle and pregnancy
W. W. Leavitt and W. C. Okulicz
Occupied and unoccupied forms of estrogen receptor (Re) were measured in
hamster uterus during the estrous cycle and pseudopregnancy with a new
assay method employing pyridoxal 5'-phosphate. In the cycle, occupied Re
increased in cytosol and nuclear fractions in parallel with increasing
serum estradiol during the follicular phase and then dropped sharply in
both compartments during preovulatory progesterone secretion. Unoccupied Re
occurred primarily in cytosol with low levels present in nuclei. It is
pertinent that similar concentrations of occupied Re accumulate in cytosol
and nuclear fractions, suggesting that occupied Re is distributed between
these compartments according to an equilibrium model. Progesterone disrupts
Re distribution through a selective action on the nucleus to downregulate
occupied Re as was shown by progesterone treatment of the proestrous
hamster. Re is chronically downregulated in the progesterone-dominated
uterus, and the occupied form of Re recovers rapidly after progesterone
withdrawal. Thus progesterone downregulates the Re system under
physiological conditions by a selective action on the occupied form of Re
in the target cell nucleus.
