Estrogen stimulates expression of p21Waf1/Cip1 in mouse uterine luminal epithelium

p21^Waf1/Cip1 was originally identified as an inhibitor of the cell cycle. Recent evidence suggests that it can act as a positive regulator of the cell cycle under the influence of some growth stimulators. We investigated the effects of ovarian steroids on the expression of p21, DNA synthesis, and mitosis in the uterus. Capsules containing 17β-estradiol (E₂) were subcutaneously implanted in ovariectomized mice that were sacrificed on different days. Their uteri were collected for p21 immunohistochemical staining. To study mitosis and DNA synthesis, colchicine and bromodeoxyuridine (BrdU) were injected into mice 3 or 5 h before sacrifice. The results showed that p21 expression, BrdU incorporation, and the mitotic index in uterine luminal epithelium increased 1 to 2 d after E₂ stimulation and then declined to basal levels between d 3 and 6. Furthermore, cotreatment with progesterone (P₄) and E₂ suppressed both p21 expression and the DNA synthesis stimulated by E₂ alone in uterine epithelial cells. Our results show that estrogen stimulates p21 expression and cell proliferation in uterine luminal epithelium and that cotreatment with P₄ prevents both effects, suggesting that p21 may act as a positive cell-cycle regulator.