Stromal estrogen receptors mediate mitogenic effects of estradiol on uterine epithelium

Abstract

Estradiol-17beta (E2) acts through the estrogen receptor (ER) to regulate uterine growth and functional differentiation. To determine whether E2 elicits epithelial mitogenesis through epithelial ER versus indirectly via ER-positive stromal cells, uteri from adult ER-deficient ER knockout (ko) mice and neonatal ER-positive wild-type (wt) BALB/c mice were used to produce the following tissue recombinants containing ER in epithelium (E) and/or stroma (S), or lacking ER altogether: wt-S + wt-E, wt-S + ko-E, ko-S + ko-E, and ko-S + wt-E. Tissue recombinants were grown for 4 weeks as subrenal capsule grafts in intact female nude mice, then the hosts were treated with either E2 or oil a week after ovariectomy. Epithelial labeling index and ER expression were determined by [3H]thymidine autoradiography and immunohistochemistry, respectively. In tissue recombinants containing wt-S (wt-S + wt-E, wt-S + ko-E), E2 induced a similar large increase in epithelial labeling index compared with oil-treated controls in both types of tissue recombinants despite the absence of epithelial ER in wt-S + ko-E tissue recombinants. This proliferative effect was blocked by an ER antagonist, indicating it was mediated through ER. In contrast, in tissue recombinants prepared with ko-S (ko-S + ko-E and ko-S + wt-E), epithelial labeling index was low and not stimulated by E2 despite epithelial ER expression in ko-S + wt-E grafts. In conclusion, these data demonstrate that epithelial ER is neither necessary nor sufficient for E2-induced uterine epithelial proliferation. Instead, E2 induction of epithelial proliferation appears to be a paracrine event mediated by ER-positive stroma. These data in the uterus and similar studies in the prostate suggest that epithelial mitogenesis in both estrogen and androgen target organs are stromally mediated events.

Figure 1

Labeling index of epithelium in uterine tissue recombinants (wt-S + wt-E, wt-S + ko-E, ko-S + wt-E and ko-S + ko-E). Grafts were grown for 1 month in female nude mouse hosts, and then hosts were ovariectomized. One week later, hosts were injected with either 100 ng of E₂ or vehicle alone. One group of hosts receiving E₂ also was treated with the anti-estrogen ICI 182,780 (1 mg/kg) on days 5–7 post-ovariectomy.

Figure 2

Thymidine ARG of tissue recombinants consisting of wt-S + wt-E (a and b), wt-S + ko-E (c and d), ko-S + wt-E (e), and ko-S + ko-E (f). Grafts were grown for 1 month in female nude mouse hosts, and then hosts were ovariectomized and injected 1 week later with either 100 ng of E₂ (a, c, e, and f) or oil vehicle (b and d).

Figure 3

Immunohistochemical localization of ER in various types of tissue recombinants.

Figure 4

Simultaneous assessment of ER status and [³H]thymidine labeling in wt-S + wt-E (a) and wt-S + ko-E (b) tissue recombinants from ovariectomized hosts injected with E₂.

Figure 5

Proposed mechanism of E₂-stimulated epithelial proliferation in the uterus. Based on mitogenic response of ER-negative UtE to E₂ when associated with ER-positive UtS and the ability of the anti-estrogen ICI 182,780 to antagonize this effect, mitogenic effects of E₂ on epithelial proliferation appear to be mediated through stromal ER.