Dioxin inhibition of estrogen-induced mouse uterine epithelial mitogenesis involves changes in cyclin and transforming growth factor-beta expression.

A single dose of dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin or TCDD; 5 microg/kg, ip) inhibits 17beta-estradiol (E2)-induced uterine epithelial mitogenesis, apparently through disruption of stromal-epithelial interactions. To understand if TCDD alters early uterine (Ut) responses to E2, young adult C57BL/6J mice were ovariectomized and given (i.p.) either oil or 5 microg/kg TCDD. After 24 h, TCDD-treated mice received E2, and oil-treated mice were given E2 or oil. Body and Ut weights were collected 6 and 18 h later. Ut were flash-frozen at 6 h. E2 increased Ut weight (p < 0.0001) and Ut/body weight ratio (p < 0.0001), compared to mice given oil alone. Ut cyclin expression was assessed by an RNase protection assay. E2 increased mRNA expression for cyclin A2 and B1 (p < 0.05), in addition to D1, D2, and D3 (p < 0.001), while cyclin C was unchanged from oil controls and cyclins A1 and B2 were undetectable. In contrast, TCDD completely abolished E2-induced cyclin A2, which has been associated with S phase initiation, and reduced B1 and D2 (p < 0.05). Interestingly, TCDD did not alter E2-induced Ut weight increases at 6 h, but inhibited E2-induced Ut weight gain at 18 h. A 10-microg/kg TCDD dose was necessary for attenuation of the early E2-induced Ut weight increases (p < 0.01). Since TGF-beta regulates cyclins, Ut TGF-beta was also assessed in TCDD + E2-treated and control mice. TGF-beta mRNA levels were increased after TCDD compared to E2 alone (p < 0.01), suggesting a possible mechanism for TCDD inhibition of Ut cyclin A2. Thus, TCDD alters specific E2-regulated Ut G(1) phase activities and may inhibit E2-induced Ut epithelial mitogenesis by disrupting specific cell signaling mechanisms necessary for S phase initiation in vivo.