Tibolone and its metabolites enhance tissue factor and PAI-1 expression in human endometrial stromal cells: Evidence of progestogenic effects

Tibolone is a highly effective postmenopausal hormone treatment that has its biological activity dependent on metabolism to 3α- and 3β-OH tibolone, which bind solely to the estrogen receptor. Despite the high levels of estrogen receptor-binding metabolites in the circulation, the endometrium becomes atrophic, suggesting inactivation of the estrogen response in this tissue which may be due to the progestogenic activity of tibolone and the Δ-4 tibolone metabolite. We evaluated the effects of tibolone and its metabolites on tissue factor (TF) and plasminogen activator inhibitor type 1 (PAI-1) expression in human endometrial stromal cells (HESCs). Since TF and PAI-1 exhibit long-term in vivo and in vitro up-regulation by progestin they serve as endpoints for assessing chronic effects of progestin exposure. Confluent HESCs were primed in serum-containing medium with vehicle control, 10−8 mol/L estradiol, 10−7 mol/L medroxyprogesterone acetate, or 10−8 to 10−6 mol/L tibolone or its metabolites, then switched to a defined medium with corresponding vehicle or steroids. After 24 h, ELISAs indicated that the progestin elevated TF (6.2-fold ±3.0; p < 0.05) and PAI-1 (eight-fold ±2.1; p < 0.05) levels, whereas the cells were refractory to estradiol exposure. Tibolone and Δ-4 tibolone (10−8 to 10−6 mol/L) were as effective as 10−7 mol/L medroxyprogesterone acetate in enhancing TF and PAI-1 output (p < 0.05). Unexpectedly, at the higher concentrations 3α- and 3β-OH tibolone also elevated TF and PAI-1 expression (p < 0.05). Western blotting confirmed the ELISA results. Our findings suggest that HESCs metabolize 3α- and 3β-OH tibolone to tibolone and subsequently to Δ-4 tibolone, which can both stimulate the progesterone receptor. Since TF and PAI-1 promote hemostasis by complementary mechanisms, our findings account for the reduced occurrence of abnormal uterine bleeding associated with tibolone therapy.