Phosphorylation of the
tumor suppressor protein,
retinoblastoma (pRb), regulates the progression of the cell cycle. Previous work from this laboratory had shown that
estradiol (E(2)) regulates
tumor suppressor proteins, p53 and
retinoblastoma in
breast cancer cells. In the present study, we have examined the phosphorylation of pRB in T47D
breast cancer cells following treatments with
R5020 and antiprogestins. In growth medium containing serum depleted of endogenous
steroids by
charcoal treatment, pRb appeared mainly in its hypophosphorylated form. Addition of 10 nM
R5020 to the culture medium caused hyperphosphorylation of pRb within 24 h, but the hypophosphorylated form of pRb began to accumulate after 72 h. Upon prolonged
R5020 treatment (72-96 h), pRb was detected exclusively in its hypophosphorylated form. While treatment of cells with
R5020 caused a transient increase in the level of
cyclin D1, E(2) addition caused a sustained increase in the level of
cyclin D1 consistent with its role in stimulating pRb phosphorylation. Antagonists of both
estrogen receptor (ER) and
progesterone receptor (PR) blocked the E(2) and R5020-induced pRb phosphorylation, respectively. These results suggest that
R5020 induces pRb phosphorylation via a transient increased expression of
cyclin D1, whereas E(2) treatment results in sustained expression of
cyclin D1 and increased pRb phosphorylation. Furthermore,
R5020 effects on pRb phosphorylation appear PR-mediated as no cross-antagonism of pRb phosphorylation was observed: the
R5020 effects were blocked by
RU486 and ZK98299, but not by the pure ER antagonist, ICI 182, 780 (ICI).